Morphology–property relationship in oriented PET films: Microstructural reorganization during heat treatment

Abstract Morphology–property relationships for simultaneously biaxially stretched films and heatset with fixed dimensions in the temperature range of 100–240°C have been studied. The observed transition in various properties at 180°C can be explained on the basis of microstructural changes caused by competition among several processes, such as crystallization, solid‐state thickening, melting, and molecular relaxation as well as by melting and recrystallization. The resulting structures and, thereby, the properties are different in temperature Regime‐II ( T g to T max ) and Regime‐III ( T max to T m ). In Regime‐II, the high rate of crystallization compared to the rate of molecular relaxation develops a constrained amorphous phase, whereas the predominant melting and recrystallization process in Regime‐III generates the relaxed amorphous phase. The structural reorganization during heat treatment is almost the same for uniaxially oriented film, fibers, and biaxially oriented films prepared under similar processing conditions. © 1994 John Wiley & Sons, Inc.