Heat-setting Effect on the Morphology and Phase Structures of PPS Nonwovens

Thermal shrinkage has for long been a critical issue in nonwoven industrial applications. A dimensionally stable nonwoven web with excellent mechanical property was achieved by heat treatment with tension [taut annealed (TA)] instead of annealing under free state [free annealed (FA)], while the original high porosity was not affected. To better understand the fundamental structure–property relationship, these two different heat treatments’ effects on the thermal behaviors, morphology, and phase structures of poly(phenylene sulfide) (PPS) nonwovens were elucidated via a combination of different experimental techniques such as temperature-modulated differential scanning calorimetry (DSC), two-dimensional wide-angle X-ray diffraction, two-dimensional small-angle X-ray scattering, and polarized Raman spectroscopy. It is evidenced that no obvious online stress-induced crystallization occurs during the melt-blowing process, making virgin PPS fibers mostly in the amorphous state. However, experimental results confirm that a certain amount of mesophase or constrained amorphous exists in the virgin PPS fibers without any presence of crystallites. After annealing, the DSC melting curves of both TA and FA PPS nonwovens exhibit endothermic annealing peaks, the origin of which is interpreted as the relaxation and devitrification of the rigid amorphous phase. However, the annealing peak position of the FA sample is always higher than that of the TA sample, which is reported for the first time. As compared with the FA sample annealed at the same temperature, TA PPS nonwovens reveal lower crystallinity and slightly larger lamellar long period (Lp) but a higher level of macroscopic fiber alignment and molecular chain orientation. These morphology and microstructure differences provide a good explanation for the superior mechanical property of TA PPS nonwovens with higher elastic modulus and tensile strength. The results are helpful in deepening the understanding of essential physical pictures of PPS nonwovens and promoting the development of the high-temperature nonwoven industry.