Online monitoring of birefringence development in heat-setting polymer films with a fast dual-wavelength optical technique. II. Uniaxially oriented PEN/PEI films

The influence of poly(ether imide) (PEI) on the structural ordering process in heat setting from the preoriented state of poly(ethylene 2,6-naphthalen e dicarboxylate) (PEN)/PEI blend films was investigated with the two-color online birefringence system. The addition of small amounts of noncrystalliz able PEI enhanced the crystallization from the preoriented state. This was attributed to the loosening of the structure with the addition of lower den sity and bulkier PEI at small concentrations. With a maximum effect around 15% PEI, a further increase in the concentration resulted in a dilution eff ect that spatially separated the crystallizable PEN chains. This slowed dow n the structural rearrangement process during heat setting. At elevated tem peratures, the real-time birefringence data obtained from the developed two -color birefringence system temporally located the relaxation that followed with an increase in birefringence. This was associated with molecular rela xation due to melting followed by crystallization and partial reorientation of crystalline regions in the original stretching direction. (C) 2001 John Wiley & Sons, Inc.