Real-time FTIR and WAXS studies of drawing behavior of poly(ethylene terephthalate) films

The development of molecular orientation and crystallization was studied du ring uniaxial drawing of poly(ethylene terephthalate) (PET) films, which wa s immediately followed by subsequent taut annealing at the drawing temperat ure. The behavior was monitored in real time throughout the drawing and ann ealing using dynamic FTIR spectroscopy and in situ WAXS measurements using the Daresbury Synchrotron Radiation Source. Films were drawn at 80 and 85 d egreesC at varying strain rates (0.001-0.7 s(-1)). The true stress-strain b ehavior was determined at each of the drawing conditions and the density an d optical anisotropy of unloaded samples was measured. The IR spectra were analyzed using curve reconstruction procedures developed previously, and th ey showed that orientation of the phenylene groups and the trans glycol con formers occurred before significant gauche-trans conformational changes cou ld be seen. The onset of crystallization, defined as the point that the cry stalline 105 reflection could be first observed using WAXS, was not found t o correlate with any specific change in the proportions of trans and gauche isomers nor with any feature on the stress-strain curve. However, it was c lear that, for these comparatively low strain rates, crystallization occurr ed during the drawing process while the crosshead was moving and the draw r atio was increasing. The orientation of the crystallites was calculated fro m the (1) over bar 05 reflection observed in a tilted film, transmission ge ometry. The crystallites were found to form at a draw ratio of about 2.5 wi th high orientation values (P-2> 0.8) that increased during drawing and ann ealing to P-2 values of 0.95, irrespective of the drawing conditions. Semiq uantitative measurements of crystallinity showed that the fraction of cryst alline material that developed during drawing decreased with increasing str ain rate. (C) 2001 John Wiley & Sons, Inc.