Effect of molecular weight on vitrification kinetics and molecular mobility of a polymer glass confined at the microscale

Highlights: • Vitrification temperature is strongly reduced for micrometric PtBS samples with respect to the bulk. • The α relaxation remains bulk-like for all micrometric PtBS samples and all molecular weights. • Vitrification kinetics of PtBS is markedly decoupled from the α relaxation. • This decoupling exists for all investigated molecular weights. • Frenkel–Kobeko–Reiner analysis indicates that the decoupling is more marked for high molecular weight PtBS. -- Abstract: We employ fast scanning calorimetry (FSC) to investigate how varying the molecular weight interferes in the vitrification kinetics and molecular mobility of micrometric poly(4-tert-butyl styrene) (PtBS). In line with recent results, we find marked negative deviations of the fictive temperature T_f, that is, the metric employed to characterize vitrification kinetics, with the sample size and for all molecular weights. In contrast, the molecular mobility remains size independent and bulk-like. This implies that vitrification kinetics and molecular mobility are decoupled. Furthermore, quantifying the magnitude of such decoupling through the Frenkel–Kobeko–Reiner approach, indicates that this is more pronounced in the high molecular weight samples than in the low molecular weight PtBS employed in the present work.