Synergies among the self-assembled β-nucleating agent and the sheared isotactic polypropylene matrix
Abstract The impacts of both shear temperature and shear rate on both the self-assembled β-nucleating agent (β-NA) in the hosting isotactic polypropylene (iPP) matrix and the subsequent crystallization of the hosting iPP matrix were investigated by polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS) measurements, respectively. Under static condition, β-NA could self-assemble into snowflake-like aggregate. Once shear was applied, three kinds of self-assembled aggregates could be developed prior to the crystallization of matrix depending on the applied shear temperatures, that is, snowflake-like self-assembled aggregate at 200 °C, deformed snowflake-like one with smaller branches at 180 °C, tiny and dense needle-like one if shear temperature was further decreased to 160 °C. Since β-NA with various self-assembled aggregates can act as nucleating templates for the subsequent nucleation of iPP matrix, it is conceivable that the crystallization behavior of the iPP matrix can be altered. As expected, β-phase crystallinity, long period and lamellar thickness are steadily dependent on the shear temperature and shear rate. That is, when shear was applied at 200 °C, β-phase crystallinity, long period and lamellar thickness keeps almost constant regardless of the shear rate. If melt was sheared at 180 °C, β-phase crystallinity was decreased while long period and lamellar thickness remained unchanged with elevating shear rate. At lower shear temperature (i.e., 160 °C), β-phase crystallinity, long period and lamellar thickness were further decreased with increasing the shear rate. These results are of practical significance to understand the long-term ignored issue: apart from the loading of β-NA, different self-assembled aggregates formed in flow field are another crucial factor in determining the microstructure development of matrix.
Importance of superior dispersion versus filler surface modification in producing robust polymer nanocomposites: The example of polypropylene/nanosilica hybrids
Abstract With polymer nanocomposites, achieving highly effective dispersion of agglomerated nanofiller and major or optimal property enhancements remain challenges. A commonly posited solution is to improve the polymer-filler surface thermodynamic compatibility; this approach has led to significant improvements in some cases, but it has not provided a general solution. We address the question of whether achieving a metastable, well-dispersed state is better than compatibilization in attaining the goal of major property enhancements. We use solid-state shear pulverization to produce well-dispersed polypropylene (PP) nanocomposites with up to 8 or 9 wt% pristine nanosilica ( p -NS) or organically modified nanosilica ( m -NS). Microscopy shows that as-received, tens-of-micron-sized p -NS and m -NS agglomerates undergo very good dispersion, with ∼10–100 nm size-range nanofiller in hybrids. Rheology is consistent with very good dispersion, with only 92/8 wt% PP/ p -NS indicating incipient nanofiller network formation. The PP/ p -NS hybrids have superior Young's modulus and tensile strength. Relative to PP, modulus increases by 22% and 12% and tensile strength by 19% and 14% for 99/1 wt% PP/ p -NS and 99/1 wt% PP/ m -NS, respectively. The PP/ p -NS hybrids have the largest increases in modulus (46% at 8 wt% p -NS) and tensile strength (22% at 6 wt% p -NS). Upon melting and crystallization, both PP/ p -NS and PP/ m -NS result in PP β-crystal formation at 1 wt% nanosilica, with p -NS having a greater effect. The PP/ p -NS hybrid shows larger increases in thermal stability and nucleating efficiency for PP crystallization. Thus, with very good dispersion, unmodified nanofiller in a metastable dispersed state can result in more robust nanocomposites than when modified nanofiller is used to compatibilize the polymer–nanofiller interface.
Anomalous Annealing Behavior of Arsenic Implanted Silicon as a Function of Dose and Energy
Anomalous annealing behavior of damage in As implanted Si has been investigated by channeling effect measurements. Room temperature As implants with doses from 1014 to 2 × 1016cm-2 at energies ranging from 50 to 250 keV into Si were annealed over the range of 600°C to 1100°C. Anneal temperatures ≥ 950°C reduce the disorder for ion doses of 0.5 to 1 × 1016 cm-2 at energies between 50 and 250 keV. Anomalously high residual disorder was found for the intermediate doses of 2 × 1014 to 2 × 10/cm2 implanted above 100 keV. For this intermediate dose, annealing of implants performed at liquid nitrogen temperatures did not show significant reduction in this damage. However implants at 200° to 300°C indicate a lower amount of damage compared to the room temperature implants.