Multistage Structural Ordering and Crystallization of Poly(trimethylene Terephthalate) During Sub-Tg Stretching: Synergetic Effects of Chain Orientation and Conformational Transition

Crystallizable glassy polymers can undergo multi-stage structural evolutions and form multiple types of ordered phases (e.g., mesomorphic and crystalline phases) during stretching below their glass transition temperatures (T-g). However, the structural evolution of polymers during sub-T-g stretching and the associated molecular mechanisms have not been well understood. In this work, we investigated the multistage structural ordering of glassy polymers and the related mechanisms during stretching below T-g by studying poly(trimethylene terephthalate) (PTT) as a model system. Roles of chain orientation and conformational transition on the multistage structural evolution of melt-quenched PTT were elucidated through a loading-unloading experiment. When stretched below T-g (e.g., 25 degrees C), the orientation of polymer chains and the stacking of phenylene rings lead to an intermolecular ordering, which induces the generation of a smectic C-like mesomorphic phase. The orientation degree of chains in such a mesomorphic phase increases with further stretching. This highly oriented mesomorphic phase transforms into a triclinic crystalline phase when the applied stress is unloaded. Stretching at a higher temperature (e.g., 40 degrees C) results in the formation of a more ordered mesomorphic phase and facilitates the induced crystallization due to the enhanced chain mobility. Fourier transform infrared results demonstrated that the unloading-induced crystallization of stretched PTT is originated from the trans-gauche conformational transition.