Development of crystalline order during hot-drawing of poly(ethylene terephthalate) film: influence of strain rate

Abstract

We have studied the evolution of crystalline order during drawing of poly(ethylene terephthalate) at 90°C and at strain rates in the range 0.01–2.1 s −1. The onset of crystallization always occurs at the inflection point in the stress-strain relationship, which shifts to higher strain levels and lower stress levels as strain rate decreases. Crystallinity develops in two regimes: in the low stress regime (regime 1) crystallinity increases more rapidly than in the high stress regime (regime 2). The onset of regime 2 occurs in the region of the sharp upturn in the stress-strain relationship, which shifts to higher strain levels as strain rate decreases, and at a characteristic level of crystallinity, which is independent of strain rate. We suggest that regime 1 involves the formation of a crystallite network which, at the characteristic level of crystallinity, has sufficiently crosslinked and reinforced the polymer that further deformation generates large stresses. WAXS determinations of lateral crystallite dimensions, normal to the 010 and 100 planes, reveal that crystallization in regime 2 involves modest growth of small crystallites (∼2.5−3.5 nm), with preferential growth occurring normal to 010. A preliminary investigation of the influence of molecular weight indicates that a lower rate of molecular relaxation at higher molecular weight reduces the strain-rate dependent shift in the onset of crystallization.