(488aa) Isothermal Crystallization in the Nematic Phase of Poly(Trimethylene Terephthalate) Fibers by Molecular Dynamic Simulations

Atomistic molecular dynamic simulations are used to study isothermal and stress-induced crystallization of poly(trimethylene terephthalate) (PTT) at various temperatures and various degree of applied stresses. While extensive experimental and theoretical efforts are made to verify and explain the molecular mechanism in the growth of nuclei in polymeric materials, the primary stage of polymer crystallization is less well understood. Our atomistic simulations reveal the formation and growth of an orientationally ordered phase (nematic phase) upon quenching a melted semi-rigid polymer. This work we focus on the crystallization of both isotropic and nematic phases respectively at the early stage.

Our simulations show that the structural order and the torsions of polymer are affected by the different degree of orientation in the nematic phase. The nematic phase was first created by applying an uniaxial stress to an amorphous model system, containing 32 PTT chains, each having 27 repeating units. Subsequently, isothermal crystallization took place as the sample was maintained at 400 K for 20 ns, at which temperature the rate of structural ordering was found to be highest. It is found that the torsional conformation, trans-trans-gauche-trans (ttgt), of segment O-CH2-CH2-CH2-O continues to develop and is at a higher rate in the nematic phase (compared to the isotropic phase). Furthermore, the rate of the torsional structure development strongly depends on the orientation of the system. The rapid development of ttgt conformation could potentially facilitate the formation of crystalline tggt conformations.