Properties of PAN Fibers Solution Spun into a Chilled Coagulation Bath at High Solvent Compositions
In this work, multifilament, continuous polyacrylonitrile (PAN) fiber tow was solution spun mimicking industrial processing at the small pilot scale (0.5 k tow), while carefully altering the composition of the coagulation bath, in order to determine the effect on the resulting fiber shape, density, orientation, and tensile properties at varying points in the spinning process. Novel here are the abnormally high coagulation bath solvent compositions investigated, which surpass those often reported in the literature. In addition, the coagulation bath was maintained at a slightly chilled temperature, contrary to reported methods to produce round fibers. Further, by altering the composition of the bath in a step-wise fashion during a single spinning run, variations in all other process parameters were minimized. We found that with increasing solvent composition in the coagulation bath, the fibers not only became round in cross section, but also became smaller in diameter, which persisted down the spin line. With this decrease in diameter, all else equal, came an accompanying increase in apparent fiber density via a reduction in microvoid content. In addition, molecular orientation and tensile properties also increased. Therefore, it was found that inadequate understanding of the coagulation bath effects, and spinning at low coagulation bath solvent compositions, can hinder the ability of the fiber to reach optimum properties.
Preparation of High-Quality Polyacrylonitrile Precursors for Carbon Fibers Through a High Drawing Ratio in the Coagulation Bath During a Dry-Jet Wet Spinning Process
Abstract Excellent poly(acrylonitrile-co-itaconic acid) (99/1) (PAI) nascent fibers, which have an important role in preparing high-quality precursors for carbon fibers, were prepared by a dry-jet wet spinning process. Their structures were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM) and an ultrasound solvent etching method, as well their properties being determined by a strength and extension meter and a fineness meter, both designed specifically for fibers. When a high drawing ratio, over 300%, was applied to the fibers in the dry-jet wet spinning coagulation bath, the molecular chains were easy to orient and regularly arrange, resulting in the relative crystallinity, crystal size and amorphous orientation degree of the nascent fibers being improved. The fibrils with large diameter were formed, increasing the bulk density with the overall porosity and pore numbers decreasing. Therefore, the nascent fibers had smaller diameters, higher strength, higher rupture elongation and smaller coefficients of variation. The optimum high performance PAI precursor fibers, with 0.59dtex in titer, 7.51cN/dtex in tensile strength, 7.9% in rupture elongation and the final carbon fiber with 5.54GPa in tensile strength, were obtained through a post-spinning treatment in which they were subjected to a high coagulation bath draw ratio and carbonization.
Submicron polymer structures with X-ray lithography and hot embossing
This article describes the process chain for replication of submicron structures with varying aspect ratios (AR) up to 6 in polymethylmethacrylate (PMMA) by hot embossing to show the capability of the entire LIGA process to fabricate structures with these dimensions. Therefore a 4.7 μm thick layer of MicroChem 950k PMMA A11 resist was spin-coated on a 2.3 μm Ti/TiOx membrane. It was patterned with X-ray lithography at the electron storage ring ANKA (2.5 GeV and λc ≈ 0.4 nm) at a dose of 4 kJ/cm3 using a Si3N4 membrane mask with 2 μm thick gold-absorbers. The samples were developed in GG/BDG and resulted in AR of 6–14. Subsequent nickel plating at 52°C resulted in a 200 μm thick nickel tool of 100 mm diameter, which was used to replicate slit-nozzles and columns in PMMA. Closely packed submicron cavities with AR 6 in the nickel shim were filled to 60% during hot embossing.