THE INFLUENCE OF DRYING ON THE STRUCTURE AND MECHANICS OF POLY (P-PHENYLENE BENZOBISTHIAZOLE) FIBERS (HIGH MODULES/STRENGTH, DRY-JET WET SPINNING, PERFORMANCE, PROCESSING)
A study of the influence of the drying process in developing fiber properties during the dry-jet wet spinning of poly (p-phenylene benzobisthiazole) is undertaken. The fiber's structure and mechanical characteristics from the coagulated state through the drying process to the as-spun and heat treated state are investigated herein.
The wet fiber is composed of a swollen macro and microfibrillar network of highly oriented molecules with lateral but no longitudinal order and exhibits a high degree of mechanical anisotropy. The structural and mechanical features of the as-spun fiber appear to develop during coagulation.
The drying process during drying is characterized by a distinct radial contraction and axial elongation due to the porous and anisotropic nature of the coagulated fiber. Alterations in the structure and mechanics of PBT fibers are observed upon drying which is rationalized in terms of the collapse process. The drying process is a fiber modification phase.
High temperature and tension are employed to optimize the driving force toward property enhancement during the drying stage. Tension during drying straightens the wet fibrillar network while offsetting axial compressive internal stresses whereas the use of high temperature increases the extent of the lateral molecular order and increases molecular orientation resulting in enhanced tensile properties.
The compressive strength and shear modulus increase after drying suggesting that removal of water may serve to enhance lateral interactions within the swollen fibrillar network inhibiting shear and compressive deformation and the onset of buckling instabilities. Further increases in the compressive strength through high tension and temperature drying are not achieved since further enhancements in intermicrofibrillar and fibrillar associations are speculated to be minimal.
A force-temperature (F-T) technique is implemented to study thermal expansion characteristics of these rigid rod aromatic fibers through examination of the reversible F-T behavior. Subsequent mass loss and microstructural changes during the F-T experiment (an effective post processing heat treatment) is detected from irreversible F-T effects. An assessment of the effectiveness of the initial drying and heat treatment processing to induce thermal stability and material alterations is obtained through this F-T technique.