Effect of chemical structure on the crystallization kinetics of triple polymorphic high-sulfur-content polythioethers

This work studies how the chemical structure of relatively similar high-sulfur alternating polythioether homopolymers (DMDS-alt-DVE, DMDS-alt-TEGDVE, and DMDS-alt-BDDVE) affects their structural properties, morphology, polymorphism, and crystallization kinetics. Differential scanning calorimetry (DSC) and polarized light optical microscopy (PLOM) experiments revealed a complex crystallization for the samples in which up to three different polymorphic phases were identified: a very low melting crystal form (VL-Tm form), a low melting crystal form (L-Tm form) and a high melting crystal form (H-Tm form), characterized by their corresponding melting temperature ranges and confirmed via Wide-Angle X-ray Scattering (WAXS). A coexistence of negative and positive spherulites was found, and their origin was revealed by atomic force microscopy (AFM), which showed how the lamellar arrangement varied in the samples from predominantly radial to a cross-hatched morphology. Additionally, the overall isothermal crystallization kinetics were investigated using a combination of PLOM, DSC, and Fast Scanning Calorimetry (FSC) experiments in which DMDS-alt-BDDVE exhibited the fastest overall crystallization kinetics among the three homopolymers, followed by DMDS-alt-DVE and then DMDS-alt-TEGDVE. These results were explained by molecular dynamics simulations in which the diffusion capabilities of each homopolymer along their density, characteristic ratio, and chain diffusion effects were simulated to understand the polymer backbone flexibility and chain mobility, revealing differences among them. DMDS-alt-BDDVE demonstrated the fastest diffusion in the melt, potentially explaining its exceptionally rapid crystallization kinetics.