Effect of self-generated thermal field on spatio-temporal growth of hierarchical structures during polymer crystallization

The spatio-temporal evolution of non-conserved crystal order parameters pertaining to polymer crystallization has been investigated theoretically in the context of a phase field model based on a time dependent Ginzburg-Landau equation (known as TDGL model A), by coupling with a temperature field. In the description of the total free energy, an asymmetric double-well local free energy density signifying metastability of crystal ordering is combined with a non-local free energy term representing an interface gradient. The potential for metastability in polymer crystallization has been analyzed in relation to the formations of faceted single crystals to nonfaceted hierarchy structures. The thermal effect was described by the heat conduction equation that generated the latent heat along the moving crystal growth front. Of particular importance is that the model field parameters can be linked directly to the material parameters and experimental conditions; therefore free parameters can be minimized or eliminated.; Firstly, the phase field model was successfully applied to unidirectional solidification of low molecular weight materials such as succinonitrile. Five possible interfacial morphologies were captured including dendrite, seaweed, cellular, planar and degenerate patterns. The main controlling parameters were the external temperature gradient field and the anisotropy of interfacial energy. Later, the emergence of polymer single crystals and spherulites were studied with the modified phase field model. The numerical calculations confirmed the occurrence of various single crystal forms including faceted hexagonal patterns, nonfaceted snowflakes and lamellar branching morphology of isotactic polystyrene in the self-generated temperature field. Subsequently, the landscape of these morphological textures has been established as functions of anisotropy of surface energy and thermodynamic driving force, viz. supercooling. In the case of the polymer spherulitic growth, the nucleus is anisotropic with lamellar stacks that transform to sheaf-like and eventually to lamellar branching morphology with dual eye-pocket-like texture at the core. It is apparent that the released latent heat drives the curved crystal surfaces anisotropically to lamellar branching. The transcrystalline morphology (epitaxial crystallization) was also studied under external applied temperature gradient; the simulations revealed the emergence of structural gradient with epitaxial lamellar growth at the chilled wall and spherulites in the bulk.; Finally, the spatio-temporal growth of spherulites in isothermal crystallization of poly(vinylidene fluoride) in its blends with ethylene vinyl acetate copolymer has been investigated theoretically. The simulation captures the emergence of the target and spiral patterns undergoing fragmentation at a very shallow supercooling. It was found that the supercooling not only affected the domain size, but also exerted profound influence on the types of spherulitic morphologies formed.