N-[(heptaisobutyl-POSS)propyl]-3,5-diaminobenzamide (DABA-POSS) and a common aromatic diamine, 4,4′-oxydianline (ODA) and the aromatic dianhydride, pyromellitic dianhydride (PMDA) by a two-step thermal imidization procedure. The POSS-containing filler, trisilanolphenyl POSS (TSP-POSS) was added with the fixed proportion of 20 wt% in the final films. Incorporation of TSP-POSS additive apparently improved the thermal stability, but decreased the high-temperature dimensional stable nature of the PI composite films. The 5% weight loss temperature (T5%) of POSS-PI-20 with 20 wt% of DABA-POSS is 564 °C, and its coefficient of linear thermal expansion (CTE) is 81.0 × 10−6/K. The former is 16 °C lower and the latter was 20.0 × 10−6/K higher than those of the POSS-PI-10 film , respectively. POSS components endowed the PI composite films excellent AO resistance and self-healing characteristics in AO environments. POSS-PI-30 exhibits the lowest AO erosion yield (Es) of 1.64 × 10−26 cm3/atom under AO exposure with a flux of 2.51 × 1021 atoms/cm2, which is more than two orders of magnitude lower than the referenced PI (PMDA-ODA) film. Inert silica or silicate passivation layers were detected on the surface of the PI composite films exposed to AO.For the development of spacecraft with long-servicing life in low earth orbit (LEO), high-temperature resistant polymer films with long-term atomic oxygen (AO) resistant features are highly desired. The relatively poor AO resistance of standard polyimide (PI) films greatly limited their applications in LEO spacecraft. In this work, we successfully prepared a series of novel AO resistant PI composite films containing nanocaged polyhedral oligomeric silsesquioxane (POSS) components in both the PI matrix and the fillers. The POSS-containing PI matrix film was prepared from a POSS-substituted aromatic diamine,  PMDA-ODA, trademark: Kapton®, Dupont, DE, USA), poly  have been widely used in spacecraft due to their excellent combined properties, including good thermal resistance, excellent radiation resistance, good mechanical properties, and so on [−24 cm3/atom in LEO space environments [Research on the improvement of the atomic oxygen (AO)-resistant properties of standard polyimide (PI) films have become one of the most important topics in the development of high-performance polymer materials for low earth orbit (LEO) spacecraft ,2,3. Theronments ,6. This ronments ,8.After several decades of basic and applied studies for the AO erosion mechanisms of standard PI films, there have been establishing some effective procedures for enhancing the AO-resistant abilities of the PI films. These procedures can roughly be classified as the passive and active ones. Both procedures obey the same mechanism and all achieve the AO protection via the formation of inorganic passivation layers ,10. The −26 cm3/atom (Fluence: 4.0 × 1020 atoms/cm2) [® films. Song, et al. also reported the synergistic improvement of AO resistance of PI films by introduction of both phosphorus and silicon elements [Considering the individual disadvantages for these two procedures, a combination of the passive and active pathways was proposed in recent years. In our previous work, the AO-resistant POSS fillers were used in combination with the AO-resistant phosphorus-containing matrix, so that the obtained composite films had excellent AO resistance. Due to the synergistic effect of phosphorus and silicon elements, the AO erosion yield of the derived composite films was as low as 3.1 × 10oms/cm2) . The lowelements . Althoug® PI film (MC-POSS-Kapton) and side-chain substituted one (SC-POSS-Kapton) [® films by combining POSS units into the PI films via copolymerization. Especially, the SC-POSS-Kapton® film exhibited relatively lower cost and comparable AO resistance compared with the main chain one. Thus, SC-POSS-Kapton® showed great application future in long-term LEO exploration. In addition, POSS compounds have also been used as the fillers to develop PI composite films with improved AO resistance [® matrix and the trisilanolphenyl POSS (TSP-POSS) additives [® matrix film. The AO erosion yields of TSP-POSS/Kapton composite films are comparable to those of the MC-POSS-Kapton and SC-POSS-Kapton films.In recent years, polyhedral oligomeric silsesquioxane (POSS) have been paid increasing attention as a promising component to enhance the AO resistant properties of the PI films due to the great potential ability to form silicon-containing passivation layers when reacted with AO . Various-Kapton) . Both POsistance ,28,29. Idditives . TSP-POSIn our previous work, the effects of the side-chain substituted POSS units on the optical, thermal, mechanical, and AO erosion behaviors of PI (PMDA-ODA) films were systemically reported . In the N-methyl-2-pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF) were purchased from Sinopharm Chem Reagent Co. Ltd.  without purification.TSP-POSS was obtained from Hybrid Plastics, Co. Ltd., . DABA-POSS was prepared according to our previous work . Pyromel−1. The wide-angle X-ray diffraction (XRD) patterns of PI composite films were determined with Rigaku D/max-2500 X-ray diffractometer  using Cu-Kα1 radiation at room temperature between 3 °C and 80 °C. X-ray photoelectron spectroscopy (XPS) data were measured with ESCALab220i-XL electron spectrometer . XPS spectra were obtained using MgKα X-ray source with a power of 300 W. The base pressure of vacuum chamber was set at 3 × 10−9 mbar. The binding energies were 284.8 eV derived from the adventitious C1s as a reference. Field emission scanning electron microscopy (FE-SEM) was carried out using a Technex Lab Tiny-SEM 1540  with an accelerating voltage of 15 KV for imaging. Pt/Pd was sputtered on each film before the SEM measurements. The atomic force microscopy (AFM) images of PI composite films were measured in tapping mode on a Bruker Multimode 8 AFM microscope .The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of PI composite films were obtained using a Bruker Tensor-27 FT-IR spectrometer  with the scan range of wavenumber from 4000 to 400 cmFor the transparency measurements, ultraviolet-visible (UV-Vis) spectra of PI composite films were recorded on a Hitachi U-3210 spectrophotometer  at room temperature. PI samples were dried at 100 °C for 1 h to remove any absorbed moisture prior to tests. Yellow index (YI) and haze values of the PI films (thickness: 20 µm) were measured using an X-rite Ci7800 spectrophotometer .3. The coefficients of linear thermal expansion (CTE) values of composite films were recorded in the range of 50–200 °C.Thermogravimetric analysis (TGA) was performed with a Perkin-Elmer TGA4000 thermal analysis system . The thermal scanning mode ranges from 50 to 750 °C at a heating rate of 10 K/min in nitrogen atmosphere with a gas flow of 20 mL/min. Similarly, thermo-mechanical analysis (TMA) was recorded on a TA-Q 400 thermal analysis system . The thermal scanning mode ranges from 50 to 400 °C at a heating rate of 10 K/min in nitrogen atmosphere. The size of the film samples was 10 × 5 × 0.025 mm® reference sample, whose erosion yield (EKapton) was 3.0 × 10−24 cm3/atom. The fluence could be calculated from the etch depth by the following Equation (1):F = AO fluence (atoms/cm2); ΔHKapton = erosion depth of the Kapton® reference sample (cm); EKapton = erosion yield of Kapton® reference sample (3.0 × 10−24 cm3/atom).The AO erosion behaviors of PI composite films were tested in a ground-based AO effect simulation facility . The AO 21 atoms/cm2. In order to study the etching depth, the samples were covered with stainless steel meshes to produce the etched and pristine areas, which were used to measure the step height using a profilometer. The erosion yield of the sample, Es, is calculated through the following Equation (2) [Es = erosion yield of the sample (cm3/atom); ΔHs = erosion depth of the sample (cm).According to the equation above, the fluence of AO in this experiment is calculated to be 2.51 × 10tion (2) :(2)Es=ΔHThe preparation of poly(amic acid) (PAA) composite solutions was achieved by the combination of copolymerization and physical blending of POSS-containing components, which could be illustrated by the synthesis of POSS-PAA-30 containing 30 wt% of DABA-POSS and 20 wt% of TSP-POSS. The experimental device is equipped with a mechanical stirrer, an ice-cold bath, and a 500-mL three-necked round-bottom flask with nitrogen inlet. First, ODA , DABA-POSS , TSP-POSS (12.63 g) and ultra-dry DMAc solvent (182.5 g) were added into the flask. A clear solution was obtained after stirring at 5–10 °C for 2 h under nitrogen flow. Then, PMDA  and the additional DMAc (70.0 g) were added to the reaction mixture obtained above. The solid content of the reaction mixture was adjusted to be 20 wt% by this procedure. The cold bath was removed and the reaction mixture was stirred at room temperature for another 22 h. The deep-brown viscous solution obtained was filtered through a 0.45-µm polytetrafluoroethylene (PTFE) syringe filter. Then the prepared POSS-PAA-30 solution was stored in a glass bottle at −18 °C before use.The degassed POSS-PAA-30 solution was warmed to room temperature before use, and then blade-coated onto a clean glass substrate. The thickness of the wet PAA films was controlled by the height of the slit. The glass substrates were then placed in an oven with flowing nitrogen gas. The POSS-PAA-30 was gradually thermally imidized with the following curing procedure: 80 °C/3 h, 120 °C/1 h, 150 °C/1 h, 180 °C/1 h, 250 °C/1 h, 300 °C/1 h, and 350 °C/0.5 h. Then, the glass substrate was immersed in warm deionized water to obtain the free-standing POSS-PI-30 film.The other PAA solutions and the corresponding PI films, including POSS-PI-10, POSS-PI-15, POSS-PI-20, and POSS-PI-25 were prepared according to a similar procedure. The difference was the contents of DABA-POSS were different. The pristine PI (PMDA-ODA) film was also prepared in a similar way.A serious of POSS-PAA composite varnishes were first prepared by the combination of copolymerization and physical blending procedures according to the chemical reaction equation shown in −1 and 1717 cm−1, respectively, and the stretching vibration of C-N bonds are at 1375 cm−1. These are the characteristic absorptions of imide rings. The characteristic absorptions of C=C bonds at 1499 cm−1 in phenyl units were observed in all of the PI systems in the spectra. Moreover, the characteristic absorptions of Si-O-Si in both of the DABA-POSS and the TSP-POSS were all detected at 1086 cm−1. In addition, the peaks at 2953 cm−1 could be assigned to the absorptions of saturated C-H bonds in isobutyl groups in DABA-POSS units. The absorptions of Si-OH in the TSP-POSS filler at the wavenumber around 3300–3500 cm−1 were not observed in the spectra, which are in consistence with the other PI/TSP-POSS system [The successful incorporation of POSS units into the composite films, on one hand could be directly proven by the structure characterization, such as ATR-FTIR and XPS measurements, and on the other hand could be indirectly reflected by the XRD and optical properties evaluation. S system . This miThe XRD patterns of the POSS-PI composite film were shown in cut) between 411 nm and 431 nm. Incorporation of the TSP-POSS additives into the films slightly increased the yellow indices (b*) and haze values of the composite films. Obviously, the haze values of the PI composite films increased from 7.19% to 15.82% when the contents of DABA-POSS increased from 10 to 30 wt% in the films. Furthermore, the haze values of the pristine DABA-POSS-PI films and the composites films are compared in The good miscibility of the additives with the DABA-POSS-PI matrix could be further revealed by the optical characteristics of the composite films. The optical data are tabulated in T5%) of the composite films decreased with the increasing contents of DABA-POSS in the polymers. Basically, the T5% values of the film decrease by 5–8 °C for every 5% increase of the DABA-POSS contents in the film. What’s more, addition of TSP-POSS apparently improved the thermal decomposition temperatures of the PI composite films. The T5% values of the composite films were about 40 °C higher than those of the pristine films without TSP-POSS additives. For example, POSS-PI-30 film showed a T5% value of 554 °C, which was 42 °C higher than that of the PI-30 film without TSP-POSS [Rw750) values of the composite films were all higher than 70.0% in nitrogen, while the un-filled films showed the Rw750 values below 60.0%. The increase of the thermal stability of the composite films might be owing to the enhanced interactions and miscibility between the TSP-POSS additives and the POSS-containing PI matrix. According to the DTG plots in Tmax) for the PI composite films were all higher than 600 °C and also decreased with the increase of the DABA-POSS contents in the composite films.In order to evaluate the thermal stability of the PI composite films, TGA and TMA measurements was used, and the data obtained are summarized in −6/K, which was obviously higher than the CTE of the analogous PI film without TSP-POSS additives (CTE = 56.1 × 10−6/K). It is possible that the TSP-POSS additive act as a “plasticizer”, which made the matrix easy to move at elevated temperatures.In our previous work, it has been found that the introduction of side-chain-substituted POSS units deteriorated the high-temperature dimensional stability of the derived PI films because of the easy movement of the latent POSS groups at elevated temperatures . In the Es) of the PI films were tabulated in 21 atoms/cm2. Es values of the POSS-PI films together with the representative appearances and SEM images of the POSS-PI-10 and POSS-PI-30 films after AO erosion, respectively. It can be clearly observed that as the POSS contents in the composite films are higher, the Es values are lower. The Es value of the POSS-PI-30 film was 1.64 × 10−26 cm3/atom, which was nearly an order of magnitude lower than the Es value of the PI sample without TSP-POSS filler (Es = 11.1 × 10−26 cm3/atom) and was only 0.55% of the referenced Kapton® film (Es = 300 × 10−26 cm3/atom). The lowest Es value of the novel PI composite films developed in the current work is more than two orders of magnitude lower than that of the standard Kapton® film. The big difference in the Es values of these two series of PI films is mainly due to the synergistic effects of the nanocaged POSS units in both matrix and the additives for the newly developed PI composite films. The POSS units effectively decreased the AO erosion of the PI composite films. In addition, AO exposure deteriorated the optical transparency of the PI composite films, and a relatively dense layer could be observed on the surface of the POSS-PI-30 film through SEM detection. The surface elemental compositions of the PI composite films before and after AO exposure were then detected by XPS. The plots were shown in x or silicate layers could be confirmed to form on the surface of the AO-eroded PI samples. Undoubtedly, these inert passivation layers provided efficient protection for the under-layer films.The ground-simulated AO facility was used to investigate the AO erosion behaviours of the composite films. And the experimental erosion yields (Rt). Basically, the surface roughness caused by the AO exposure decreased gradually with the increasing contents of POSS components in the films due to the formation of compact passivation layer. For instance, the Rt values of the composite films decreased from the initial value of −286.3–285.2 nm of POSS-PI-10-AO to −19.0–41.2 nm of POSS-PI-30-AO. This result indicates that the distribution of passivation layer onto the surface of the composite films progressively became dense and compact, thus providing much efficient protection for the underlying films and apparently reducing the AO erosion yield of the composite films.The formation of the passivation layers caused by AO exposure could also be verified by inspecting the surface roughness of the AO-attacked surface of the composite films. 21 atom/cm2, the composite films exhibited the erosion yield as low as 1.64 × 10−26 cm3/atom, which was ranked as one of the lowest values for the PI films reported in the literature. The good miscibility and compatibility of the POSS-substituted PI matrix and the POSS-containing additives achieved the molecular-level combination. This feature endowed composite films good uniformity, by which the active AO protection via DABA-POSS in the matrix and the passive AO protection via TSP-POSS in the fillers achieved good synergistic effects. The main disadvantage for the PI composite films might be the deteriorated high-temperature dimensional stability due to the internal plasticization effects of the POSS components. Fortunately, the relatively mild temperature cycle in LEO space environments might provide some practical applications for the composite films developed in this study. It is foreseeable that the excellent AO resistance of the POSS-containing PI composite films might provide long-term AO protection for LEO spacecraft in the future.A series of PI composite films with POSS components in both the matrix and the fillers were designed and synthesized. This structural design reduced the AO erosion yields of the PI composite films. After an AO erosion with the fluence of 2.51 × 10


A star polymer with a polyhedral oligomeric silsesquioxanne (POSS) core and poly(ethylene glycol) (PEG) vertex groups is incorporated in a polyurethane with flexible hard segments in-situ during the polymerization process. The blends are studied in terms of morphology, molecular dynamics, and charge mobility. The methods utilized for this purpose are scanning electron and atomic force microscopies , X-ray diffraction (XRD), differential scanning calorimetry (DSC), and to a larger extent dielectric relaxation spectroscopy (DRS). It is found that POSS reduces the degree of crystallinity of the hard segments. Contrary to what was observed in a similar system with POSS pendent along the main chain, soft phase calorimetric glass transition temperature drops as a result of plasticization, and homogenization of the soft phase by the star molecules. The dynamic glass transition though, remains practically unaffected, and a hypothesis is formed to resolve the discrepancy, based on the assumption of different thermal and dielectric responses of slow and fast modes of the system. A relaxation α′, slower than the bulky segmental α and common in polyurethanes, appears here too. A detailed analysis of dielectric spectra provides some evidence that this relaxation has cooperative character. An additional relaxation g, which is not commonly observed, accompanies the Maxwell Wagner Sillars interfacial polarization process, and has dynamics similar to it. POSS is found to introduce conductivity and possibly alter its mechanism. The study points out that different architectures of incorporation of POSS in polyurethane affect its physical properties by different mechanisms. Organic groups R attached on its siliceous vertices (vertex groups) form a shell which typically has a size of the same order of magnitude. The whole structure spans thus a total of 1–3 nm, and POSS moieties are often referred to as nanoparticles. Indeed, POSS have been observed to behave as conventional nanoparticles, however this is usually an effect of their self-assembly to nano-sized aggregates [A standard cubic siliceous core of a polyhedral oligomeric silsesquioxane  Glass transition of polyurethane is known to be affected significantly by the degree of microphase separation. In fully separated systems by definition the soft phase consists only of polyol segments and exhibits a similar gT. As more hard segments blend in, the soft phase is not pure anymore and, consequently, its gT increases moderately following mixing models such as that of Fox [POSS moieties have been incorporated covalently in polyurethane matrices in various ways using standard urethane or urea chemistry. They have been built-in as pendent groups ,25,26,27t of Fox . (iii) PgT dropped slightly more than expected by Fox’s equation, indicating possible development of additional free space in the soft phase. In a previous work, we studied systems where PEG-POSS moieties were blended into a model polyurethane with polytetramethylene ether glycol (PTMG) as soft phase, and hard segments consisting of the aromatic, rigid methylene diphenyl diisocyanate (MDI) and butanediol . In thisgT was practically unaffected, whereas the dynamic glass transition as studied by dielectric spectroscopy was slowed down. In order to resolve the discrepancy we put forward the hypothesis that the slow modes that emerged have a dielectric footprint but small to negligible thermal response, much like the immobilized polymer fraction observed in crystalline or nanocomposite systems [Traditionally, the diisocyanate contains aromatic groups, however this is not favourable for applications where biocompatibility and non-toxicity are important. Hence, interest has grown in the development of polyurethanes with aliphatic diisocyanates . In past systems ,37. The In this work, we have introduced PEG-POSS star polymers, in-situ during the synthesis of the matrix with the flexible diisocyanate. The soft segments consist of polytetramethylene ether glycol (PTMG) with molar mass ~1000 g/mol. The hard segments consist of the linear hexamethylene diisocyanate (HDI) and butanediol (BD). We study the influence of PEG-POSS on the morphology with X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Thermal transitions, with emphasis on the glass transition and the complex order-disorder process are studied by differential scanning calorimetry (DSC). Finally, we conduct a very detailed investigation of the molecular and charge mobility by means of dielectric relaxation spectroscopy (DRS). We will show that POSS has a moderate effect on the microphase separation, as well as in the glass transition dynamics, although again a discrepancy occurs between the calorimetric and dielectric response. Charge mobility is greatly enhanced as a result of the addition of PEG-POSS in the soft phase.Scanning electron microscopy images were recorded on the surface of the elastomers, and an energy dispersive x-ray spectroscopy (EDX) mapping was performed at the spectral line of silicon, in order to study the dispersion of POSS .Conventional secondary electron images for the 2 wt% blend, show scarce bright objects of size less than μm. More and bigger ones are present at a higher POSS content (8 wt%), with the largest of them having a size of 1–2 μm. EDX mapping confirms that these objects are POSS aggregates. In the case of the 8 wt% blend, except for the visible objects, EDX mapping shows more siliceous objects, presumably lying below the surface. The silicon mapping of the 2 wt% blend shows a rather homogeneous distribution of the element.Atomic force microscopy (AFM) phase images  shed morThe diffractograms in The two main features are a complex halo including three Bragg reflections in the region 15–30° and a broad amorphous halo, centred roughly around 40°. The complex peak at small angles is attributed to average distances of atoms within the hard microdomains, whereas the halo at wider angles corresponds to the soft phase ,42,43. Bθ ~10 and 20°. No further peaks or halos are observed at wider angles, which were not accessible by the experiment in ref [PEG-POSS is known to have a quite rich diffractogram, consisting of three amorphous halos, with the main one appearing at smaller angles  than those typically accessible by conventional XRD setups . This ret in ref .Interestingly, there are no qualitative changes in the diffractograms of the blends with increasing POSS content. No new features emerged, and the existing peaks do not seem to change position. This indicates that POSS does not change radically the micromorphology of the PU matrix at nanometer level, accessible by conventional XRD. Only changes in intensity are observed. In order to quantify these changes, we fitted the diffractograms with pseudo-Voigt peaks . Two broAs a measure of the degree of crystallinity of the hard microdomains, we calculated the ratio:odomains . It is nThe differential scanning calorimetry (DSC) curves follow the paradigm of standard segmental polyurethanes .Starting from low temperatures, the step-like event around −70 °C corresponds to the glass transition of the soft phase. Around −10 °C, an endothermic peak corresponds to the melting of PEG-POSS . In the gT drops significantly with increasing POSS content starting at 4 wt%  consists of polyol segments and diluted hard segments with various lengths. In that respect, the inhomogeneity, even in the nanoscale, is enormous. With increasing temperature, this inhomogeneity decreases. With increasing POSS content the exponent a shows a weak decreasing trend indicative of slightly more inhomogeneous systems [The strength Δansition a. This i systems .g relaxation faster than α but slower than MWS was observed at relatively low temperatures .Discussing in detail the dependence of Cole-Cole parameters on temperature is beyond the scope of this article. Nevertheless, we would like to show their dependence on POSS content at the representative temperature of 60 °C . Neitherε″ data, in the low frequency region, but for frequencies higher than the onset of electrode polarization in ε′ spectra. A term of the form: In order to calculate dc conductivity The traces of dc conductivity of the polyurethanes on the Arrhenius plot have a non-standard shape, presumably due to the thermal event at ~50 °C, which is identified as the peak I of microphase mixing procedure ,44,57 anAt temperatures below this point addition of the much more conductive POSS increases remarkably the dc conductivity. The trend saturates at high POSS contents, presumably due to the incomplete mixing of POSS in the PU matrix. With the exception of the matrix where we don’t have enough data points, the traces show a concave behaviour, characteristic of cooperative processes. In order to quantify this, we fitted a VFT-like equation: changes . HoweverAbove 55 °C, conductivity shows a slight downwards deviation from its initial VFT behaviour, possibly because hard segments dilute in the soft phase and inhibit the overall charge mobility. Nevertheless, the overall trend is increasing with POSS content with the exception of the 4 wt% blend which exhibits particularly high values. This might also be related to the change of conduction mechanism which was hypothesized to take place at intermediate POSS loadings.The morphology of the polyurethane systems at hand is in accordance to that of typical segmented polyurethanes, with hard microdomains distributed in a soft phase ,35,58,59POSS seems to disrupt the development of crystallinity in the hard microdomains . It is uThe dynamic glass transition (α relaxation) of the system at hand is accompanied by a slower contribution α′, as has been observed in several polyurethane systems with PTMG as soft segment and various types of hard segments ,29,50,61gT by a few degrees, resembling what has been observed in a similar system with rigid hard microdomains [gT that increased with the calorimetric one being relatively constant [gT increased with addition of POSS, the onset of the step remained practically stable, and the end decreased by quite a lot, accompanied by a significant drop of the Addition of PEG-POSS seems to reduce the calorimetric odomains . Interesodomains ,63. It wconstant . Meanwhi the ΔcP . This ing, accompanies the Maxwell Wagner Sillars (MWS) relaxation, being faster, broader, and by one order of magnitude weaker. The fact that g and MWS have similar dynamics in terms of activation energy points to the hypothesis that g is also related to charge mobility. Such a double MWS relaxation has been observed in a system with multiple interfaces [A new relaxation referred to as terfaces . In the ®), 1,4-butanediol and Sn(Oct)2 as catalyst was obtained from Sigma-Aldrich and PEG-POSS was purchased from Hybrid Plastics Ltd. . Polyether diol, and PEG-POSS were dried under vacuum at 80 °C, chain extender was dried under vacuum at 40 °C for 24 h before the reaction.Hexamethylene diisocyanate  was purchased from Sigma-Aldrich  and used as received. Poly(tetramethylene glycol) (PTMG) with molar mass 1000 g/mol was acquired from Invista  was then added in the reaction mixture. The reaction was allowed to proceed under stirring for 2 min, and the mixture was poured on pre-heated steel moulds, and placed in a vacuum oven. After 30 min of degassing, the temperature was increased to 100 °C and allowed to cure for 24 h. Materials with 0 to 10 wt% PEG-POSS were prepared. Mass fraction of hard segments with respect to polyurethane component was always 50 wt%. Batches of ~30 g were prepared.The two-step prepolymer method was utilized in a fashion similar to this described in ref. . In the Scanning electron microscopy images were recorded with a JSM-6010LA microscope  equipped with an energy dispersive X-ray analysis (EDS) detector. Images where taken on cryofractured samples, coated with gold. Secondary electron images at 10 kV, as well as EDS mappings on the Si spectral line were taken to study the distribution of POSS particles in the matrix.Atomic force microscopy images were recorded on selected samples on the surfaces produced by the blade of a microtome. A Veeco  Innova microscope was used in tapping mode. A tip with resonance frequency ~300 kHz was used. Scans were performed on 1 × 1 μm canvas with resolution 512 × 512 pixels. Scanning frequency was 1 line per second.Diffractograms were recorded with a Bruker D2 phaser diffractometer, on the surfaces of the as produced polyurethane sheets.Differential scanning calorimetry curves were recorded with a Q20 calorimeter  purged with He and calibrated with Indium and sapphire standards. Samples were cooled down to −150 °C at 10 K/min and subsequently heated up to 200 °C at K/min to study thermal transitions on the as received materials. The melt was then cooled down to 0 °C at 10 K/min to observe how POSS affect the phase separation process. Specimens of 5–8 mg were used, placed in standard aluminium pans.gT and heat capacity steps Glass transition temperatures −1–106 Hz. The temperature was controlled by a Quatro cryosystem of the same manufacturer. Spectra were recorded in the range −120 to 100 °C in heating steps of 5 or 10 K. As-received specimens of thickness ~1mm were placed between polished brass electrodes of 20 mm diameter.Dielectric function spectra were recorded with an Alpha Analyzer  in the range 10A POSS variant with oligomeric PEG vertex groups was blended in a segmental polyurethane matrix with PTMG soft phase and flexible hard segments, based on the aliphatic hexamethylene diisocyanate. No significant changes are observed in the microphase separation. The apparent glass transition temperature seems to decrease and become significantly narrower, however by elimination only of the slow (high temperature/long relaxation time) components. On the contrary, the dynamic glass transition is hardly affected in terms of time scale and it becomes rather broader. A hypothesis towards the resolution of this discrepancy is that POSS arrests the thermal response of slow modes of glass transition but enhances their dielectric one. Slow modes appear to have stronger dielectric but weaker thermal response than the fast ones. The slow modes seem to be affected more by POSS.The star-polymers with a siliceous core affect the morphology and molecular mobility of the matrix with different mechanisms as compared to POSS pendent on the hard segments . The forA relaxation referred to as g, not typically observed in such systems, accompanies the Maxwell Wagner Sillars relaxation and follows its dynamics. It is proposed that it actually involves two contributions from interfaces: (i) between amorphous and crystalline hard domains or soft phase and hard domains and (ii) between polyurethane and POSS-rich domains. Clarification of its origin would be interesting in future work.


Increasing concerns about the rising rates of antibiotic therapy failure and advances in single-cell analyses have inspired a surge of research into antibiotic persistence. Bacterial persister cells represent a subpopulation of cells that can survive intensive antibiotic treatment without being resistant. Several approaches have emerged to define and measure persistence, and it is now time to agree on the basic definition of persistence and its relation to the other mechanisms by which bacteria survive exposure to bactericidal antibiotic treatments, such as antibiotic resistance, heteroresistance or tolerance. In this Consensus Statement, we provide definitions of persistence phenomena, distinguish between triggered and spontaneous persistence and provide a guide to measuring persistence. Antibiotic persistence is not only an interesting example of non-genetic single-cell heterogeneity, it may also have a role in the failure of antibiotic treatments. Therefore, it is our hope that the guidelines outlined in this article will pave the way for better characterization of antibiotic persistence and for understanding its relevance to clinical outcomes. Antibiotic persistence contributes to the survival of bacteria during antibiotic treatment. In this Consensus Statement, scientists working on the response of bacteria to antibiotics define antibiotic persistence and provide practical guidance on how to study bacterial persisters. Bigger realized that the small number of bacteria that manage to survive intensive antibiotic treatments are a distinct subpopulation of bacteria that he named ‘persisters’.More than 70 years ago, Hobby3. Several approaches have independently emerged to define and measure persistence. Research groups following seemingly similar procedures may reach different results, and careful examination of the experimental procedures often reveals that results of different groups cannot be compared. During the European Molecular Biology Organization (EMBO) Workshop ‘Bacterial Persistence and Antimicrobial Therapy’ (10–14 June 2018) in Ascona, Switzerland, which brought together 121 investigators involved in antibiotic persistence research from 21 countries, a discussion panel laid the main themes for a Consensus Statement on the definition and detection procedure of antibiotic persistence detailed below. In light of the potential role that antibiotic persistence can have in antibiotic treatment regimens, it is our hope that clarification and standardization of experimental procedures will facilitate the translation of basic science research into practical guidelines.Fuelled in part by increasing concerns about antibiotic resistance but also by technological advances in single-cell analyses, the past 15 years have witnessed a great deal of research on antibiotic persistence by investigators with different backgrounds and perspectives. As the number of scientists that tackle the puzzles and challenges of antibiotic persistence from many different angles has profoundly increased, it is now time to agree on the basic definition of persistence and its distinction from the other mechanisms by which bacteria survive exposure to bactericidal antibiotic treatmentsWe adopt here a phenomenological definition of antibiotic persistence that is based on a small set of observations that can be made from experiments performed in vitro and that does not assume a specific mechanism. We focus on the differences and similarities between antibiotic persistence and other processes enabling bacteria to survive exposure to antibiotic treatments that could kill them, such as resistance, tolerance and heteroresistance. We identify different types of persistence that should be measured differently to obtain meaningful results; therefore, the definition of these types goes beyond semantics. For the more mathematically oriented readers, we provide a mathematical definition of the various terms based on a widely used phenomenological model for survival under drug exposure in Box Box 1 Mathematical distinctions between antibiotic resistance, tolerance, persistence and heteroresistance54. Common phenomenological models for the relationship of the survival, S, with the concentration of the drug, c, or duration of treatment, t, are the Zhi function55, Emax or Hill model56. In these frameworks, the killing rate, ψ, is described by three main parameters that represent distinct underlying physicochemical mechanisms: the MIC; the minimum duration to kill 99% of the population, MDK99; and the Hill coefficient for the steepness of the concentration dependence, k.Predictive models of the survival of microorganisms exposed to cidal drugs show that measuring the minimum inhibitory concentration (MIC) is not enough to characterize the behaviour, although it is widely usedmax. Note that the common notation of the model uses the following:This general function predicts how the concentration of the antibiotic and its duration will affect the growth or death of a strain with growth rate without antibiotic, ψ99. Thus far, the parameters describe a uniform population. When the population is heterogeneous, it means that at least one of the parameters is heterogeneous.In the framework of this model, resistance is defined as an increase in the MIC, whereas tolerance is defined as an increase in the MDK57.Heteroresistance entails that subpopulations of cells have a higher MIC than the majority of the population. In typical reports of heteroresistance, it is also assumed that the heritability of the increased MIC is long enough to create detectable colonies99 than the majority of the population. If we assume that the fraction of persisters is α, the survival can be written as the sum of the survival of two subpopulations with different killing rates:Antibiotic persistence  entails that a subpopulation of cells have a higher MDK* with a longer MDK99.The killing rate of the normal population is ψ, as in equation 5  or mutations  that confer the resistance phenotype to the bacterial population12.‘Resistance’ is the ability of bacteria to replicate and not just survive in the presence of a drug Box . The mos13. Fourth, in contrast to resistant cells, persister bacteria cannot replicate in the presence of the drug any better than the non-persister cells but are killed at a lower rate than the susceptible population from which they arose. This property also distinguishes persistence from heteroresistance, a phenomenon in which a small subpopulation transiently displays a substantially (more than eightfold) higher MIC14 , inactivation of the antibiotic or target modification to reduce drug binding. Although mechanisms of resistance are diverse, they typically achieve the same result — reduced antibiotic binding to the target15. However, persistence has the added attribute of affecting only a subpopulation of cells, whereas tolerance is the general ability of a population to survive longer treatments, for example, by having a lower killing rate  the molecular mechanism of tolerance that enables the persister bacteria to survive, for example, a reduction in their metabolism, and (2) the mechanism that generates heterogeneity in the population17, for example, nonlinear mechanisms leading to bimodality by amplifying stochasticity19. Finally, several persister subpopulations may coexist; therefore, a multimodal killing curve may occur.‘Tolerance’ and persistence are similar phenomena of increased survival in the presence of an antibiotic without an increase in the MIC. In studies that focus on only a qualitative understanding of the molecular mechanisms, the two terms are often interchangeablesee Fig. , but witBox 3 DefinitionsAntibiotic resistant cellAn antibiotic resistant cell is a cell that survives antibiotic treatment by carrying a resistance factor . Resistance factors enable resistant bacteria to grow at antibiotic concentrations that would prevent the growth of more susceptible bacteria.Antibiotic tolerant cellAn antibiotic tolerant cell is a cell that survives treatment with an antibiotic, without carrying a resistance factor, and that can regrow after removal of the antibiotic. Often, tolerant cells are non-growing before antibiotic exposure, but not necessarily. Tolerance factors enable bacteria to survive the duration of treatment that would kill more susceptible bacteria. These tolerance factors can be environmental or genetic.Antibiotic persistenceAntibiotic persistence is a population-level phenomenon that historically has been derived from the observation of biphasic killing curves, indicating the presence of two subpopulations, consisting of cells that are killed fast by the antibiotic and tolerant cells that may survive. By definition, the term antibiotic persistence is always connected with a heterogeneous population, in which only a part of the population consists of tolerant cells.ToleranceTolerance is a population-level phenomenon that enables the population to survive the duration of a transient antibiotic treatment several times above the minimum inhibitory concentration (MIC) without a resistance mechanism.Persister cellA persister cell is a tolerant cell originating from a population that displays antibiotic persistence.Dormancy60. However, tolerance and persistence may arise without dormancy.Dormancy reflects the state of a bacterium that does not grow and has decreased activity when compared with growing cells or even typical stationary phase cells. This term is often also used for single cells that are viable but do not grow despite environmental conditions that support growth. Dormant bacteria are often tolerant to many antibiotics because of their growth arrest or their decreased metabolismBecause the definition of antibiotic persistence is anchored in the heterogeneity of the response to antibiotics in the population, it is a population-level phenotype. However, tolerance can be the attribute of a whole population that is killed at a slow rate as well as of a single cell that manages to survive an extensive treatment  by mutations. Although within a clonal population, persister and non-persister bacteria are typically isogenic, mutations have been identified that are able to increase the level of persistence or tolerance. For example, one of the first identified high persistence mutations, the We stress the fact that the definition of persistence presented here is not directly linked to a specific mechanism or to a physiological state of the bacteria. Rather, it is defined by the time-kill assay; therefore, we outline below some important considerations for increasing the reproducibility and reliability of persistence detection. Typical microbiology procedures developed for measuring uniform bulk phenomena need to be carefully re-evaluated when measuring survival that is dominated by a small subpopulation of cells.35 and anticancer treatments36.Although we focus here on antibiotic persistence of bacteria, we believe that the definitions and guidelines should be relevant for heterogeneous responses to other drugs such as antifungalsThe starting point for identifying persistence is the time-kill assay, which measures survival of bacteria at different time points during exposure to the antibiotic. Survival is defined as the ability to regrow when the antibiotic is removed. The hallmark of persistence is the bimodal  killing curve Fig. . However37. If resistant mutants were responsible for the slower killing rate in the first killing curve, the second assay will show reduced killing of a much higher proportion of the population than in the first assay.First, a bimodal killing curve may be due to resistant mutants. To rule out this effect, surviving bacteria that are clearly in the tail of the survival curve should be regrown in the same conditions and exposed again to the same antibiotic treatment. Persistence requires that the killing curve remains the same as in the initial inoculation14, the killing curve should be performed at high antibiotic concentration, at least several times the MIC, to efficiently kill bacteria that have a higher MIC than the rest of the population. The killing rate should only weakly depend on the antibiotic concentration. Strong dependence on the antibiotic concentration  reflects phenomena linked to resistance.Second, in order to distinguish persistence from transient modes of resistance such as heteroresistance38, where the antibiotic may not efficiently kill them.Third, experimental pitfalls that may result in bimodal killing should be ruled out. One of the most common reasons for a decrease in the killing rate is degradation of the drug with time. Therefore, it is important to test that the killing efficacy of the drug itself does not decrease with time because of natural degradation of the drug, uptake by bacteria or changes in the medium. Another reason for survival of some bacteria may be their adhesion to the walls of the culture vessel39. In addition, bacteria recovering from an antibiotic treatment may have a delayed regrowth either because they are in the tail of the lag time distribution40 or because of the post-antibiotic effect42, which results in the delayed growth of bacteria after treatment. Therefore, evaluating the survival by counting colonies should be done not only after the typical appearance time of colonies but also several days later.Finally, in addition to clearly stating in which conditions the time-kill assay is performed, care should be given to the recovery conditions, when bacteria are allowed to grow after removal of the antibiotic. The precise conditions for the evaluation of survival after treatment should be described, such as the washing out of the antibiotics, the medium in which the bacteria are recovered and the time that has passed from the exposure to the antibiotics until the exposure to the recovery conditions. For example, it has been shown that keeping the bacteria in non-growing conditions after treatment may increase their survival40. Typically, to obtain reproducible results, the time between the trigger for persistence and the exposure to antibiotics should be minimized to avoid the uncontrolled loss of persister bacteria that switch back to normal cells.As the trigger is an integral part of triggered persistence, the trigger duration, intensity and exact conditions should be clearly mentioned and kept the same between experiments. For example, one of the most common triggers of persistence is starvation. Many reports used an ‘overnight culture’ as inoculum. This overnight culture has been exposed to several stress signals during starvation, such as high cell density, stringent response and altered pH, that may trigger persistence, and therefore inevitably still contains bacteria that experienced a trigger. In this example of triggered persistence, the persistence level will depend strongly on several parameters, including the size of the inoculum, the time that has elapsed since the inoculum was regrown and the duration of starvation during the previous overnight culture44, or a dissection of the molecular mechanism that allows the bacteria to respond to the drug by activating a stress response31. Earlier attempts to characterize drug-induced tolerance or drug-induced persistence made use of the minimum bactericidal concentration (MBC)38. The MBC is the concentration required to kill bacteria. Some drugs may arrest the growth at the MIC but require a higher concentration to kill. If the drug itself induces persistence at the MIC, a higher concentration may be required to reach killing. Drug-induced persister bacteria have a higher MBC than the rest of the population, but their MIC is unchanged.The conditions for measuring drug-induced persistence are the same as for measuring spontaneous persistence, namely, steady-state growth, as the trigger is the drug itself and should be applied in steady-state conditions to avoid stationary-phase-induced persistence. Without further characterizations, the spontaneous and drug-induced persistence are difficult to distinguish. In this case, direct observation of single cells as they respond to the antibiotics is needed37 before performing the time-kill assay, making sure to dilute the inoculum to below the persistence level24. As spontaneous persistence is a steady-state phenomenon, care should be taken to evaluate whether the culture remains in steady-state growth, also after the inoculum influence has been ruled out. For example, a common pitfall is to perform the time-kill assay without diluting and subculturing the bacteria for enough time to eliminate the persister bacteria triggered by past stationary phase growth. Another common pitfall is to perform the time-kill assay when the culture is too close to the next stationary phase, which again may trigger the formation of persister cells. Even if the culture seems to be growing exponentially, it may no longer be in balanced growth and persister formation may be already triggered at a cell density that is ten times lower than the maximal density45. The spontaneous persistence fraction should remain constant with time in steady-state growth conditions. A simple way to test that the results do not depend on the cell density is to perform the same experiment at a twofold lower density and verify that the persistence fraction remains the same.In contrast to triggered persistence, which is determined by the history of the culture, the rare spontaneous persistence should be measured in conditions of steady-state  growth so as to avoid the effect of the past growth conditions. This measurement can be achieved in a chemostat or by subdiluting the culture several timesAn inherent part of the persistence phenomenon is the ability of persisters to eventually resume growth. As evidenced by the low killing rate displayed in the second phase of biphasic killing curves Fig. , persist30 to illustrate that bacteria have survived exposure to the drug before those can be dubbed persisters.Single-cell observation often shows non-growing cells that remain intact during exposure to the drug. However, regrowth must be documented47 and can be followed by the evolution of resistance48 suggests that persistence may be evolving quickly in the host as well. It has been suggested that the presence of antibiotic persister cells is responsible, at least partly, for lack of clearance of pathogenic bacteria by antibiotic treatment. Indeed, even in the absence of any antibiotic resistance, many bacterial infections are hard to treat and tend to relapse . The underlying reasons are most likely multifactorial, with suboptimal pharmacodynamics in the host probably playing a major role in some instances. However, it is also clear that non-growing bacteria29 and high-persister-forming mutants are selected over time in patients exposed to repeated doses of antibiotics50. Further work is needed to evaluate the possible impact of persister cells on the treatment outcome of bacterial infections and to find ways to fight them15. As seen above, many pitfalls exist even for in vitro analysis of persistence, and controlling the experimental conditions is crucial. The understanding of persistence in the host, in which our knowledge of the conditions is scarce, is orders of magnitude more challenging52. It is our hope that the guidelines outlined in this article will enable a consensus on in vitro measurements and pave the way for designing protocols adapted to the clinical evaluation of antibiotic persistence.There has been a sharp increase in the interest for antibiotic persistence in the past years in the background of growing concerns about antimicrobial resistance. The observation that triggered persistence evolves fast in vitro


Today, it is already foreseeable that additive manufacturing of mortar and concrete has groundbreaking potential and will revolutionize or at least fundamentally change the way we build. In recent years, 3D concrete printing (3DCP) with extrusion-based deposition methods has been pushed forward by a growing research community. Albeit being regarded one of the most promising innovations in construction industry, a consistent characterization methodology for assessing the constitutive behavior of 3D printed, hardened cementitious materials is missing, so far, which hinders its widespread use in engineering practice. The major objective of this paper is to fill this gap by developing a new experimental framework that can thoroughly describe the mechanical properties of 3D printed cementitious materials. Based on both a review of state-of-the-art test setups and a comprehensive experimental campaign, the present paper proposes a set of easy-to-use experimental methods that allow us to assess flexural, tensile, shear and compressive strength as well as fracture energy of 3D printed concretes and mortars in a reliable and reproducible manner. The experimental results revealed anisotropic material behavior for flexural, tensile, shear and compressive loading. Furthermore, they confirm that interval time (time gap between deposition of subsequent layers) has a crucial effect on investigated material properties leading to a severe reduction in strength and fracture energy for longer interval times. Additive manufacturing (AM) or 3D-printing of concrete is an emerging technology that has high potential to contribute to further automation in the construction sector and eventually lead to fully digital fabrication of concrete structures. An increasing number of universities and companies is working on the development of extrusion-based printing systems, printable cementitious materials and concrete mixtures as well as methods for integration of reinforcement into the printing process.Lighthouse projects as the printed cyclist bridge in Eindhoven , the DFAToday, there is already a large number of viable digital production methods available. However, the construction sector in Europe and other countries is regulated by a strict set of rules aligned to prevent injury or damage to persons and property caused by the built environment. The restrictive set of rules and the conservative design approaches resulting from this design philosophy are in stark contrast to the extremely progressive and forward-thinking research efforts in the field of digital production with concrete. This contrast is exemplified by the following fact: While many new digital construction methods, such as extrusion-based 3D concrete printing, rely entirely on the adhesive (tensile) strength of the layered connection along the interfaces, the design concept propagated in Europe and Western countries does not allow us, or only within narrow limits, to consider this tensile strength of concrete for calculating structural resistances. Obviously, verification of structural integrity and safety of printed structures is impossible within this current set of regulations. This conflict of objectives is further intensified by the fact that, due to the layering of filaments, it is in the nature of additively manufactured members to have a non-homogenous internal structure. The interfaces lead to an anisotropic behavior of hardened printed concrete on a macroscale, which needs to be additionally considered in designing printed members ,21,22,23In order to allow for making widespread use of groundbreaking successes made in the field of digital fabrication, we obviously need new impulses to reform our design strategy. The basis for developing new design concepts is formed by reproducible and reliable standard test methods for determining the mechanical properties of printed structures. Up to now, standard test methods known from concrete construction have been adopted without adapting them to the special boundary conditions of printed components. However, the layered structure of the material resulting from the new production methods requires the development of new experimental methods. This paper examines to what extent the adaption of well-known standard tests, e.g., for the investigation of tensile and bending behavior or fracture energy, is reasonable and adequate and which deficits arise when using these setups for printed concrete. Additionally, new test setups  are proposed and assessed for their suitability.Finally, the idea of a completely new testing method based on the use of a combined torsion-axial testing unit TORAX is presented, which allows us a systematical and reproducible determination of all relevant material properties of printed components by use of one single test setup .The term 3D printed concrete or digital manufactured concrete is widely used in the ongoing research publications and industry. However, a closer look reveals that due to the propagated aggregate sizes, several printable concrete mixtures from the literature should be actually specified as mortars. The tests setups for printed cementitious materials, presented in this paper, are intended for analyzing both mortars and concretes with small aggregate sizes (<8 mm). For the reason of simplicity, the term “printed concrete” is thus used throughout the whole paper for all cementitious materials (concretes and mortars) with maximum aggregate diameter of 8 mm. For statements that are explicitly pointed towards cementitious materials with maximum aggregate size smaller than 4, the term mortar is used..In the recent literature several test methods for printed concrete were presented and anisotropic mechanical properties were found for different loading directions . Initially, a new coordination system was introduced by Bos et al.  to take Figure 1Compression tests in different directions showed only a slight anisotropic behavior, but a reduction of the compressive strength by 10%  and 25% Further shear tests were conducted on the interface region by Rahul et al.  as shownFor the determination of the tensile strength either a tensile splitting test or direct tensile test is used as shown in Even though most researchers used similar experimental principles to assess the material strength, the studies came to significantly different results. These discrepancies may be related to different test setup dimensions and small deviations in loading and supporting conditions. In addition, the scatter in observed mechanical properties is amplified by different printing parameters. Especially the fresh concrete production scheme may influence the mechanical properties . When miBesides strength, also the toughness of concrete interfaces plays a major role to ensure sufficient bond and structural resistance of 3D printed structures . Debondit. Wolfs  performet. Wolfs ,30. Howet. Wolfs ,31 also t. Wolfs . The watt. Wolfs ,32. For t. Wolfs ,27.Further, the surface moisture content plays a major role in the inter-layer bonding strength. If the interfaces are not protected against drying, significant strength losses occur after 10 min, whereas protected interfaces are stable up to 90 min . Wolfs eObviously, there is a strong need for objective and coherent test methods that enable a reproducible experimental determination of the complex material behavior of 3D printed concrete. The described developments indicate the complexity of testing hardened 3D printed concrete. Until now, there is no unified testing strategy that can fully characterize all relevant properties. There is a strong need for refined experimental methods focused on individual stress transfer effects within both, the printed interfaces and the layered composite material. Besides established test methods for the characterization of concrete behavior  the focus of this paper is on proposing entirely new characterization methods, that overcome the drawbacks of current ones. To this end, the current paper is organized as follows. After presenting the used machinery, materials, and production methods, different test setups and experimental results are explained in detail. Then, strengths and weaknesses of testing methods are evaluated with regard to suitability for characterization of 3D printed concrete, to finally propose an enhanced test method that allows characterizing several different mechanical properties of hardened 3D printed concrete in a unique, universal test setup.The experimental investigations on 3D printed concrete specimen described in the following were conducted at the Institute of Structural Concrete, RWTH Aachen University and Materials and Constructions group, KU Leuven. The temperature during the production, storing and testing was constant at 22 °C with a relative humidity of 50%. To investigate the mechanical properties of printed interface regions, a printing system and a printable concrete were initially developed, to achieve comparable results. The basis for the system is a Putzmeister S5 grout pump, which can run with different rotor and stator sizes. In order not to be limited by the pump when choosing the maximum grain size, a D8-1.5 soft auger was used. With this auger a maximum grain size of 5 mm and up to 40 L/min can be transported. As the handling system, a computerized numerical control (CNC) machine IMATEC FBF 2000 was used. The CNC can move in three axes. In the Y- and Z-axis the machine head is moving and in the X-axis the printing bed is moving. Structures up to 1500 mm length, 900 mm width and 1000 mm height can be realized with a maximum machine velocity of 1.5 m/min. The dry concrete components were mixed in advance and stored in dry and sealed buckets. At the beginning of a printing event, water is added to the dry mixture, which is then mixed for 90 s followed by a 30 s break and another 30 s of mixing. Then, the fresh concrete is filled into the reservoir of the pump, where a screw conveyer constantly moves the material and transports it to the auger. The concrete is transported from the pump around 2.5 m to the CNC machine in a 1-inch diameter hose.Generally, the mechanical properties of printed concrete are influenced by different time dependent effects. One effect is the age of the fresh concrete mixture, another effect is the interval time between the deposition of subsequent concrete layers. To reasonably limit the number of tests presented in this paper, both effects have not been varied independently, but in a constant ratio: for all tests, the concrete age at application was chosen equal to the interval time.Based on the machine setup, a printable mix was designed with a maximum grain size of 4 mm. To verify the fresh concrete properties, a Hägermann flow table (HFT) test according to DIN EN 1015-3  was condVicat test [Vicat apparatus. The setting time was reached after 135 min, which describes the end of the processability of the mixture. In addition, the standard stiffness was determined to be 8.25 mm in two separate measurements.The behavior of fresh concrete over time, especially the balancing of pumpability and buildability is crucial in 3D printing of concrete. In order to quantify the fresh concrete properties of the proposed mix design over time, HFT  and Vicacat test  accordinTo practically test the presented 3D printing system and materials an exemplary wall segment was printed, as illustrated in The goal of the following investigations is to systematically assess the suitability of different test setups for determination of interface properties of printed concrete with variable interval time between deposition of subsequent layers. An overview of testing methods, parameters investigated, and dimensions of specimens is given in Distinction is made between fully printed specimen and hybrid specimen, in which only the investigated filament was printed, while bulk material of the specimen was made of casted concrete.The production of fully printed specimen was done using a 45 mm nozzle, as shown in The concrete age was chosen equal to the interval time. Compared to e.g., Li et al.  this leaAfter curing the specimens were cut by a diamond saw into the desired specimen dimensions, listed in In addition, hybrid specimens with printed filament and casted bulk material have been produced for determination of fracture energy in larger 3-point bending tests. The intention was to reduce the production effort through just printing the investigated interface regions of 150 × 150 mm² with the respective interval times and connecting them to casted concrete to form the outer parts of the beam specimen. The flexural strength in orientation W and V is determined according to European standard EN 196-1 . This reHybrid printed/casted specimen with 150 mm height, 150 mm width and 700 mm length  leading to a rather uniform, constant shear stress distribution in the tested interface. With this test setup, we may reasonably assume to obtain average shear capacities that are close to the physical shear strength of the printed interface.Before performing the tests, the drill cores are grouted into plane steel pipes with a gap of 4 mm. Inside the testing area the relevant interface is arranged with the respective interval times between 1 and 40 min as well as casted concrete. Through the plane steel pipes the specimens can be precisely mounted to the test rig. After inserting the specimen into the test rig, it is positioned by the semi-circular steel parts, which prevent a rotation (bending) of the specimens but allow free horizontal movement of the specimens  and thus avoid restraining forces in the sheared-off interface.An Instron 5582 universal tester was used as the testing device. The tests were driven displacement controlled with a rate of 0.2 µm/s. The test setup can be assumed stiff and the high size accuracy results in a normal force free support.ct,flf was determined from the measured bending moment. The results of the bending tests are shown in In the following, the results of flexural bending tests of printed interfaces are presented. Perpendicular to the interface (W-orientation), only a slight decrease in flexural interface strength is found for 1 min interval time. After 10 min, a strength reduction of approx. 25% is observed and, from 20 to 40 min, the strength reduces severely to less than 25%, compared to casted concrete. The reduction curve can be described with a good fit by an exponential function, shown in This finding is also corroborated through visual analysis of fracture surfaces from 1 min to 40 min interval time in Through the different colors of the layers, it was also possible to analyze blending of concrete in subsequent layers as well as corrugation and undulation of the interfaces, 2 and an increase of around 16% compared to casted concrete. Results from Wolfs [The observations made in tests loaded in V-direction  are in hard contrast to results in W-orientation. The development of flexural strength over time shown in om Wolfs  authentiom Wolfs  found a The characterization of the bending tensile strength with unnotched specimens is a frequently used standard test in several studies on digitally fabricated concrete. However, it is interesting to note that the flexural strength is not a material property in the strict sense, but describes a property dependent on both the mode I concrete behavior (strength and fracture energy) and the specimen geometry. This is why the flexural strength is only suitable for comparing results of specimens with identical geometry. A direct use of the flexural strength in the design of printed specimens is not useful, as this value is not objective but always influenced by the test specimen’s geometry.Further bending tests with hybrid specimen were conducted to characterize the mechanical properties of printed interfaces. Some tests were performed displacement controlled, which leads to slowly growing cracks in the interface region as illustrated in the test setup in in In addition, uniaxial tensile tests were conducted in a closed-loop deformation-controlled test setup to assess tensile strength and fracture energy of the printed interfaces.Here, the fracture energy is determined through integration of post-peak load-displacement curve which corresponds to the load-crack opening response (without elastic elongation of concrete) ,45. FiguIn order to assess the fracture energy for plain concrete a very stiff test setup is needed, as stated by Hillerborg . FurtherIn the following, a brief overview on the results and the strengths and weaknesses of the conducted test methods (direct tensile tests/notched bending test) regarding the determination of fracture energy mode I is given. In general, the average values of fracture energy for casted concrete reveal comparable results to similar concrete mixtures from literature. The notched flexural bending test and the uniaxial tensile test showed an overall good accordance and are both suitable for the determination of fracture energy as In contrast to casted concrete, where a tortuous crack path through aggregates and matrix can be observed, microcracks develop along the interface with a given, predefined direction, as shown in As both test setups lead to comparable results, the decision to choose either the one or the other setup should be based on the test infrastructure at hand and on the effort of specimen production. Due to their hybrid nature, bending test specimens composed of casted and printed regions are rather difficult to produce. However, such tests are robust and less sensitive to inadvertent eccentricities. As the bending load induces a rather stable crack growth and propagation process in the notched specimens, these tests can be done in a setup with conventional deformation control. These tests do not necessarily have to be performed with a closed-loop testing procedure and there are no specific requirements regarding the stiffness of the test setup.By contrast, the direct tensile tests have to be performed in a closed-loop testing procedure and in a very stiff testing frame to ensure a proper measurement in the post-peak branch. Proper installation without eccentricities is of crucial importance. However, it is an important advantage of these tests, that they can be performed either at easy-to-produce specimens  or at small samples extracted from larger 3D printed structures.Furthermore, shear tests were conducted using the test setup presented in In addition to the aforementioned tests, also the effect of interval time (1 min to 40 min) has been investigated. However, these tests did not deliver reliable results. Due to problems in the concrete mixing procedure, fresh concrete was added during the printing event  as well as different test setups fulfilling specific requirements each  need to be consulted. This is time- and cost-intensive and bears the potential for errors. In addition, the coherence and comparability of the results obtained in different test rigs is not easy to prove. A uniform testing strategy based on a universal specimen format and test setup, eligible to characterize all relevant mechanical properties of hardened 3D printed concrete, is lacking, so far.In addition to coherence and reproducibility, an important requirement that has received little attention in the past is the scalability of the results. In conventional shear tests with finite shear face length non-uniform shear stress distributions with stress peaks at both edges of the ligament occur leading to size effects a. This icoherent characterization of all relevant mechanical properties of 3D printed concrete and mortar ;use of a uniform test specimen with standardized geometry and production process/ printing procedure;testing with a high degree of reproducibility and minimized scatter;as well as avoidance of inadvertent size effects to gain objective test results.Obviously, there is a strong need for a universal test setup, that allows:To overcome the shortcomings of current test setups and fulfill these requirements, the authors propose a new testing procedure that can simultaneously load a specimen in torsional and axial directions b. The geThe developed experimental method exploiting the macroscopically continuous strain and stress profiles in the circumferential direction of a tubular specimen is ideally suited for the characterization of 3D printed concrete specimen. It provides a fine resolution control over the dosage of either shear, tension or compression within a tested zone and can thus be used to closely monitor the local strain and stress redistribution within the material structure during the loading. This unique possibility shall be systematically exploited in the future to obtain a deep and general insight into the phenomenology of damage and failure within the material structure of 3D printed concrete.Within the scope of this paper, special testing methods designed towards the requirements of 3D printed concrete have been developed and evaluated. By initially implementing a printing system for additive manufacturing of concrete, different specimen types were produced. By the help of different testing setups, the influence of concrete age and interval time on the mechanical properties were investigated. Therefore, different testing series with interval times from 1 to 40 min were conducted. It should be noted that the investigated interval times were set equal to the concrete age after mixing of concrete at the beginning of a printing event. It is obvious that continuous mixing improves the material properties and ensures consistent fresh concrete properties, but also requires enhanced machinery. Therefore, the findings made in this paper are a conservative (lower bound) approximation of mechanical properties of hardened 3D printed concrete and mortars.Despite using different specimen dimensions and testing methods, the performance of interfaces with regard to strength and fracture energy showed a uniform phenomenological behavior.Strong anisotropy of printed concrete has been observed for all different loading cases . This anisotropic behavior has to be taken into account in the development of design and engineering models.The interface properties under mode I (tensile and bending) and mode II (shear) action decrease significantly with increasing interval times as dehydration of interface area impedes adhesion of subsequent filaments. The reduction in strength can be well approximated by an exponential curve.The fracture energy reacts sensitive to the presence of interfaces, too. Reduced aggregate content in interface area and dehydration due to increasing interval times heavily reduce fracture energy and consequently the ductility and toughness of printed concrete. The decrease in fracture energy can be approximated by a log-function.By contrast, the interval time did not significantly affect the performance of 3D printed concrete under compressive loading. Here, still the anisotropic behavior is of crucial importance .Especially determination of fracture energy is a tricky task. It can be determined either by uniaxial tensile tests or bending tests. Experimental procedures to be applied should be selected based on the testing infrastructure at hand. If only displacement-controlled testing setups (without closed-loop control) are available, a 3-point bending test provides stable results. Such tests are not very sensitive to eccentricities but require rather large test specimen. In contrast, uniaxial tensile tests need a very stiff test setup with closed-loop control and accurate installation without eccentricities. These tests allow determination of fracture energy and tensile strength at small (and easy-to-produce) specimen.For printed concrete shear stress is solely transferred via adhesive properties of cement matrix which results in reduced properties compared to casted concrete. Further investigations on the influence of interval time on shear strength are required.A new uniform testing procedure based on combined torsion-axial force (TORAX) test setups have been proposed for coherent characterization of all relevant mechanical properties of 3D printed concrete and mortar .Based on the gained results, following conclusions on testing scheme and mechanical properties of printed concrete are made:Although current concrete design models do not explicitly use tensile strength and fracture energy as input values, this should not obscure the fact that these models implicitly rely on the existence and presence of a specific amount of both mechanical properties. Therefore, the results of the presented test series, showing a severe reduction of this mechanical properties, should be taken into account, when adapting current or developing new design tools for 3D printed concrete structures.


Concrete is prone to spalling when exposed to fire. During fire tests, the strains of concrete elements are hard to identify, both from the fire-exposed face and the non-exposed face. This paper presents a field experiment which employed the original CivEng digital image correlation (DIC) method developed at the Cracow University of Technology to measure the strain fields of elements exposed to heating by a pin-point gas burner. The paper presents experimental proof that it is possible to analyze the deformation of both unheated and fire-exposed sides of heated concrete by the DIC method. The strain fields, crack patterns and modes of crack development are presented. The study shows the encouraging results of employing the DIC method to test concrete behavior under fire attack. The thermal instability of concrete is one of the most important phenomena that take place during fire exposure on concrete structures. Concrete disintegrates explosively or gradually in the first 15 min of fire in the temperature range between 150 and 300 °C. To describe this phenomenon, the fire spalling term is used frequently by the civil engineering community. Experimental tests on concrete’s sensitivity to fire spalling are becoming more complex and still delivering new data for analytical and numerical models ,3. One oWith regards to the measures taken during and after the fire test, the accurate and reliable monitoring of the deformation of the concrete element due to fire exposure is of high importance, especially for further use in numerical models. The methods of monitoring concrete deformation at an elevated temperature can be divided into those requiring direct contact with the tested element and those that monitor deformation without contact. Le et al.  presenteLinear voltage displacement transducers (LVDTs): LVDTs are used to measure the vertical deformation of the unheated part of the concrete. They are mounted on the opposite side of the heated surface, in several places and in two directions. They are commonly used to monitor displacement toward and away from the source of fire . Miah  investigStrain gauges/high-temperature strain gauges: strain gauges are commonly used to measure concrete deformation on a horizontal plane (displacement at the concrete surface), in structural and material testing. In the case of the testing of concrete under fire exposure, the bond between concrete and gauge is weakened by evaporating water, flames, and an elevated temperature, even on the unexposed side. Standard gauges may work up to ca. 100 °C, while high-temperature strain gauges are described as withstanding ca. 1000 °C, but the severity and change of the condition mean that the concrete–gauge bond cannot be considered reliable. The measuring grid is highly prone to any change in temperature or moisture state .In terms of the contact methods that are more frequently used while testing concrete under fire action, we can distinguish between them as follows:Non-contact measurement methods are used to expand the recording capabilities using inductive sensors or strain gauges. By using them, it is possible to register the full-field displacement of the surface of the tested specimen while maintaining a high resolution and the stability of the measurement.One of the most widely used non-contact measuring techniques is the digital image correlation (DIC) method. Nowadays, the DIC method is widely used to determine the full deformation fields of various materials or composites, which can be used to better characterize and describe the mechanisms of the deformation of these materials. The DIC method is based on the mutual correlation of digital images of the specimen surface taken during the test. From the images taken during the test, the region of interest (ROI) is separated from the image background. When generating full-displacement or deformation fields, the ROI is divided into smaller grid elements. Elements of such a grid are referred to as subsets or markers in the shape of squares or rectangles. Visualization of the displacements of grid elements can be conducted using the vector field or displacement paths assigned to each grid element. Based on the obtained displacements of the grid elements, in the first place, a full displacement field can be generated, based on which a full deformation field can be determined. Nevertheless, digital methods present strict requirements to ensure proper data acquisition. Particularly, they require the researcher to ensure enough disk volume for data storage or to avoid environmental instability; for example, moving shadows, excessive sunlight, or wind.In the case of testing concrete behavior under fire exposure, the DIC method was only previously used for the small-scale preliminary studies conducted by Le et al. . In the fu is the intensity of the reference subset and gu is the intensity of the target subset form  used in this research was previously developed by Tekieli and Słoński at Cracow University of Technology ,13. In tset form . A detaiWithin the framework of the presented research, tests on the fire spalling of concrete were carried out. To present the usefulness and feasibility of the DIC measurements while testing concrete’s susceptibility to fire spalling, concrete specimens of two different sizes were employed. The flame from a propane-butane torch acted on the concrete element from one side while the DIC measurement was taken from both the fire-exposed and unexposed surfaces. The presented results show that DIC is a reliable method that can be used in such a number of the measurement points that may represent the displacement field for the entire surface. Moreover, the development of cracks during the fire test may be tracked.In this study, high strength concrete (HSC) made with basalt aggregate was tested. The detailed composition is given in The small testing specimens that were 0.3 m × 0.3 m × 0.2 m in size were manufactured and cut into two sizes: 0.2 cm × 0.2 cm × 0.1 m and 0.3 m × 0.3 m × 0.1 m. The specimens were cut 30 days before carrying out the fire exposure tests. The specimens were placed in plastic molds for 24 h and for the next 7 days were protected from drying with a plastic cover. Subsequently, the specimens were stored until the test in air-drying conditions.The material properties of the manufactured concrete were determined after 90 days of curing in air-drying conditions. The physical properties ) and the mechanical properties (compressive strength and modulus of elasticity) were tested using standard concrete specimens. The values obtained for the initial properties are presented in The experimental test method was inspired by the reaction to fire tests—part 2: single-flame source test, according to ISO 11925-2b . The sinIn the case of a fire test with the use of a small torch, it is possible to take pictures in front of the fire-exposed side. The test stand employed is presented in To track the deformation of the tested specimen, the DIC method was used. In this case, the DIC method relied on the correlation of the subsequent digital pictures taken while exposing the concrete to fire from two surfaces: the one that was directly exposed to fire and the one on the opposite side to the flames.Each specimen was observed using two DSLR cameras connected by one automatic trigger: one set on the heated side and one located on the opposite side . The picAfter the test, the grid of subsets was defined. Due to the spalling events, the front side of the specimen could only be analyzed in the area unaffected by spalling, while the entirety of the opposite side could be analyzed.Sub-pixel interpolation was employed to improve the resolution of the optical measurement. The sub-values were calculated by cubic spline interpolation, which enabled the measurement resolution level to be increased to 1/100 pixel. The displacements of the subset were computed by converting px to mm. Before the test, the transition factor was determined with the use of the calibration form.Since the central part of the specimen was very quickly destroyed by the falling pieces of concrete and the random pattern was degraded, the masking procedure was used on the deformation maps so that only the effects around the damaged fragment were visible .In the case of the medium specimen, 345 pictures were taken during the fire exposure period, while in the case of the large specimen, 441 pictures were collected. The six pictures presented for the medium  and largMoreover, the deformation fields of the exact spalling event show that the very early spalling event in the form of aggregate popcorn spalling provided slightly enhanced deformations in at least one direction —11/441. Finally, explosive spalling took place in the larger specimen just after the picture 276/441. It can be seen that the deformation observed in the neighboring pictures (271/441 and 281/441) was stable, which confirms that explosive spalling resulted from internal water pore pressure rather than thermal stress .Due to the severe conditions of the test as caused by the high temperature and the almost unpredictable behavior of the concrete surface that was exposed to fire, the use of standard strain sensors was impossible at the heated side. Moreover, tracking the deformation of almost the entire surface was not possible using any contact method of measurement. Hence, the DIC method was effective and produced new results in our research on the spalling of concrete due to fire exposure.The unheated side of a specimen is the most frequently analyzed element during a spalling test. This is mostly caused by the configurations of experimental setups that let only the unheated part of an investigated specimen be observed . In the The CivEng DIC method was successfully employed to measure the deformation of concrete exposed to fire at the unexposed side. The method allowed not only the deformation field across the entire surface and at any time of the test to be measured but also provided a reliable method to measure the development of cracking, including micro cracks.Generally, in standard tests, the development of cracks and their width is mostly measured by an extensometer or LVDT. However, the crack location while testing is hard to predict and the crack pattern is impossible to assume before the test. The location of cracks differs from one test to another. An experienced researcher can predict the possible pattern of cracks, but the exact location is impossible to point out before the cracks start to develop. What is more, visual inspection is only possible when the crack starts to be visible with the naked eye, which occurs long after the creation of a crack. Finally, with the use of a contact method, it is impossible to capture the exact length, width, and number of cracks as the points to be analyzed are limited. Therefore, all of the contact methods for measuring crack width may be burdened with a high level of inaccuracy.What is more, the standard method sometimes relies on the measurement of cracks as the tension of other devices, so any slip of gauges glued to the concrete surface is counted as the crack width. In the DIC method, the concrete surface itself is measured, so the accuracy of this method is only limited by the resolution of the digital camera.With the use of the DIC method, the cracking process is estimated during the post-processing stage. As a result, the exact number, pattern, and development of cracks within the entire test can be presented. Moreover, with the use of a virtual extensometer, the evolution of the crack width can be measured. This is obtained by measuring the changing distance between two points that are near enough to both sides of the crack development . The conIn Three cracks for both specimens were seen to have developed during the tests. Therefore, for each specimen, three virtual extensometers were employed to measure their evolution.As can be seen in the diagrams, at least one crack started to develop in the first few minutes of fire exposure. It can be seen that the sensitivity of the measurement method was very high. Not only the process of crack creation but also the end of this process were made visible by the virtual extensometer. It can also be observed that the development of cracks increased when the more frequent surface spalling events stopped. Finally, as the diagram for the larger specimen shows, explosive spalling (no. 276/441) did not affect crack development.Finally, the DIC method also allows any two points of the tested specimen to be linked and the change in the distance between them to be analyzed. The authors proposed the calculation for the total expansion of the specimen along its height during the test. For this purpose, two markers were linked together: one from the left edge and one from the right edge (the scheme of this procedure is shown in Such measurements may be helpful during an analysis of the uniformity of the distributed fire activity across the surface of a specimen or they may confirm if the boundary conditions were kept the same across the sample. In the presented case, the bottom part of the concrete was insulated with ceramic wool and was also the site that supported the specimen. As a result, the total expansion of the specimen could not be symmetrical along the specimen height.The DIC method is a reliable method for analyzing the deformation of fire-exposed concrete. The authors did not encounter any obstacles when using the CivEng DIC method successfully during the tests—neither evaporated water nor heated air hindered the production of sharp pictures.It was experimentally proven that it is possible to analyze the deformation of both the unheated and fire-exposed sides of concrete. What is more, the major advantage of the DIC method is that it can be used to analyze crack patterns as well as their development. This is a definite advantage that distinguishes the DIC method from other known contact methods.Such measurement may be employed in further tests on a larger scale to analyze the behavior of concrete under fire exposure.