Glass transition temperature and the nature of the amorphous phase in semicrystalline polymers: Effects of drawing, annealing and hydration in polyamide 6

The influence of the organization of the amorphous chains segments on the glass transition temperature (T_g) in semicrystalline polymers is analyzed by studying the effects of drawing, annealing and hydration in polyamide 6 fibers. We consider the role of three of the features of the amorphous phase: orientation (configurational entropy), density (free volume) and confinement (segmental mobility). Three classes of amorphous phases are identified; two of these are constrained in the intercrystalline regions, at the fold surfaces (between the lamellae within the lamellar stack) and at the stem surface (growth surface of the lamellae or between the fibrils). The third species is the bulk amorphous phase outside the lamellar stacks, and constitutes a large fraction of the amorphous phase especially at low crystallinities. Because the small fraction of the amorphous chain segments in the intercrystalline regions, and because they are in confined spaces, we suggest that these interlamellar and the interfibrillar components do not contribute significantly to the observed major glass transition peak. Rather, it is the amorphous region outside the lamellar stack that determines the T_g. T_g increases upon drawing and decreases upon annealing (heat setting). Our data suggest that orientation has a direct influence on T_g and can easily be measured whereas the influence of crystallinity is more complex. The influence of orientation of T_g can be understood in terms of a two T_g model in which the oriented amorphous component has a higher T_g than the unoriented component.