Bulk physical aging behavior of cross-linked polystyrene compared to its linear precursor: Effects of cross-linking and aging temperature

Abstract

The bulk physical aging behavior of cross-linked polystyrene (PS) is directly compared to that of its linear precursor polystyrene-co-vinylbenzocyclobutene (PS-VBCB) with 8.5 or 12.5 mol% VBCB content. The VBCB units incorporated into a linear PS precursor cross-link with one another upon annealing at 250 °C. Physical aging rates of bulk PS-VBCB films before and after cross-linking are characterized using ellipsometry by monitoring the decrease in thickness during isothermal annealing at a specific aging temperature (Ta) below the glass transition temperature (Tg). Physical aging rates show strong dependences on Ta values, with neat PS, linear PS-VBCB precursors with 8.5 or 12.5 mol% VBCB, and cross-linked PS with 8.5 mol% VBCB exhibiting maxima at Ta - Tg = −40 to −30 °C. In contrast, cross-linked PS with 12.5 mol% VBCB exhibits a maximum aging rate at Ta - Tg = −65 to −45 °C. The distribution of Ta-dependent aging rates broadens in cross-linked PS samples compared to linear precursors, which is correlated with increased Tg breadth or heterogeneity after cross-linking. Comparisons of aging rates of cross-linked PS with linear precursors demonstrate that cross-linking can lead to an increased, decreased, or unchanged aging rate depending on the values of Ta and Ta - Tg: e.g., when Ta - Tg > −60 °C, both cross-linked PS samples have a lower aging rate compared to their linear precursors at the same Ta - Tg whereas the cross-linked PS exhibits a similar or even greater aging rate compared to the linear precursor when Ta - Tg < −60 °C. These results help to reconcile previous seemingly contradictory observations regarding effects of cross-linking on physical aging rates. To the best of our knowledge, cross-linking induced enhancement of physical aging rates of cross-linked polymers relative to their linear precursors at the same Ta - Tg value is being reported for the first time here.