Morphology-property relationships for simultaneously biaxially stretched films and heat-set with fixed dimensions in the temperature range of 100-240-degrees-C have been studied. The observed transition in various properties at 180-degrees-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.