Non-Isothermal Crystallization Behavior and Crystal Morphology of EPDM/PP Thermoplastic Vulcanizates Containing TAC and OV-POSS Coagents

Ethylene propylene diene monomer rubber/polypropylene (EPDM/PP) thermoplastic vulcanizates containing triallyl cyanurate (TAC) or octavinyl polyhedral oligomeric silsesquioxane (OV-POSS) coagents were prepared by dynamic vulcanization. The effects of coagent type, loading level, and EPDM/PP ratio on non-isothermal crystallization were investigated using differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD). Crystallization kinetics were evaluated using the Avrami, Ozawa, and combined Avrami-Ozawa models, while nucleation activity and apparent activation energies were assessed using the Dobreva-Gutzow, Kissinger, Takhor, and Friedman approaches. Both coagents, particularly OV-POSS, shifted crystallization onset toward higher temperatures, demonstrating enhanced heterogeneous nucleation. The decreasing Avrami-derived half-times t0.5 with increasing cooling rate were qualitatively consistent with the experimental DSC half-times t1/2. The Ozawa model did not adequately describe the investigated systems, while the combined Avrami-Ozawa model provided satisfactory correlations. The influence of the coagents on the overall crystallization rate depended on composition, loading, and cooling rate. In particular, high OV-POSS loading promoted earlier nucleation but did not consistently shorten t0.5, especially in the EPDM-rich TPVs. POM revealed earlier crystal formation, increased nucleation density, and smaller spherulites in the coagent-containing samples. DSC analysis revealed an increase in PP-normalized crystallinity upon OV-POSS incorporation, while XRD confirmed an increased crystalline fraction, predominantly associated with the α-crystalline form. This effect was particularly pronounced in the PP-rich formulations. Although OV-POSS promoted heterogeneous nucleation and crystalline structure development, its influence on the overall crystallization rate remained dependent on blend composition, coagent loading, and cooling conditions, suggesting a complex interplay between nucleation and subsequent crystal growth.