Influence of PDMS Molecular Weight on Isothermal Crystallization in PDMS and PDMS–Zeolite Composites: A DSC and ESR Spin Probe Study

This study investigates the influence of polydimethylsiloxane (PDMS) molecular weight on crystallization kinetics and molecular dynamics in pristine PDMS and PDMS–zeolite composites. Three PDMS samples with molecular weights of 4,000, 28,000, and 63,000 were examined. Zeolite A with spherical particle morphology was used as a filler, and composites containing 30 wt% zeolite were prepared. Crystallization kinetics and thermal behavior were investigated by differential scanning calorimetry (DSC), while changes in the local molecular dynamics of the amorphous phase during isothermal crystallization at −85 °C were monitored by electron spin resonance (ESR) using the spin probe method. Both DSC and ESR results demonstrate that the presence of zeolite accelerates isothermal crystallization, regardless of PDMS molecular weight, with the most pronounced effect observed for the intermediate molecular weight (28,000). Avrami analysis further confirms the strong influence of molecular weight on crystallization kinetics, with higher molecular weight PDMS crystallizing more rapidly in both pristine and zeolite-filled systems. The close correspondence between the characteristic times determined by DSC and ESR indicates a strong relationship between crystal formation and changes in the molecular dynamics of the remaining amorphous phase. Subsequent DSC heating scans revealed differences in melting behavior, with strongly overlapping melting endotherms for pristine low molecular weight PDMS and the corresponding zeolite composite, and clearly resolved double melting peaks for the higher molecular weight samples, suggesting differences in the stability and organization of the crystalline regions.