The influence of poly(ether imide) (PEI) on the structural ordering process in heat setting from the preoriented state of poly(ethylene 2,6-naphthalene dicarboxylate) (PEN)/PEI blend films was investigated with the two-color online birefringence system. The addition of small amounts of noncrystallizable PEI enhanced the crystallization from the preoriented state. This was attributed to the loosening of the structure with the addition of lower density and bulkier PEI at small concentrations. With a maximum effect around 15% PEI, a further increase in the concentration resulted in a dilution effect that spatially separated the crystallizable PEN chains. This slowed down the structural rearrangement process during heat setting. At elevated temperatures, 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 relaxation due to melting followed by crystallization and partial reorientation of crystalline regions in the original stretching direction.
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