Enhanced Tg-Confinement Effect in Cross-Linked Polystyrene Compared to Its Linear Precursor: Roles of Fragility and Chain Architecture

Abstract

The glass transition temperature (Tg) of cross-linked polystyrene (PS) is directly compared to that of its linear precursor polystyrene-co-vinylbenzocyclobutene (PS–VBCB) in bulk and confined states. The VBCB units incorporated into a linear PS precursor cross-link with one another upon annealing at 250 °C. Bulk Tg (Tg,bulk) is characterized by differential scanning calorimetry (DSC) and ellipsometry, with Tg,bulk and Tg breadth increasing with increased cross-linking. The Tg-confinement effect is characterized by ellipsometry in supported PS–VBCB films before and after cross-linking; Tg decreases with decreasing nanoscale thickness in both supported linear and cross-linked polymer films. The magnitude of the confinement effect is greater in cross-linked PS compared to linear precursors; e.g., with PS-VBCB containing 8.5 mol % VBCB, Tg – Tg,bulk = −2 to −3 °C for a 23 nm thick film of linear polymer whereas Tg – Tg,bulk = −8 to −9 °C after cross-linking. The larger Tg reduction upon confinement in cross-linked PS is correlated with increased bulk fragility after cross-linking as measured by DSC and ellipsometry. Neat linear PS–VBCB copolymers provide an extension to lower fragility of the correlation between the Tg-confinement effect and bulk fragility observed previously [Evans Macromolecules 2013, 46, 6091] for neat linear polymers lacking attractive polymer–substrate interactions; i.e., the strength of the Tg-confinement effect increases with increasing bulk fragility. Both cross-linked PS and doped PS films deviate from the relationship for neat linear polymers, the former being weaker and the latter stronger. These results indicate that chain architecture and dopant content modify the relationship between the strength of the Tg-confinement effect and bulk fragility observed in many neat, linear polymers.