(581e) Ellipsometry Measurements of Glass Transition Breadth in Gradient, Block and Random Copolymer Bulk and Thin Films

Gradient copolymers are a class of polymers that exhibit a gradual change in composition along the entire chain from mostly A-monomer to mostly B-monomer. Theoretical work has predicted that gradient copolymers organize into lamellar nanostructures with sinusoidal composition profiles rather than the step-like profiles seen for block copolymers. Such profiles should lead to the presence of a wide range of material compositions and therefore extremely broad glass transition temperature (Tg) responses compared to single-phase materials. These responses have the potential to be further broadened by confinement to the nanoscale, following the effects observed in homopolymer systems. We apply ellipsometry analysis techniques - previously developed by Kawana and Jones [Phys. Rev. E, 63, 021501 (2001)] in their studies of increasing Tg breadth arising from confinement effects - to study our gradient copolymer materials in both bulk and ultrathin film geometries in comparison to homopolymers, random and block copolymers. In bulk gradient cases, extremely broad Tgs could be observed, up to 5-6 times broader than the transitions seen for single-phase materials e.g. homopolymers and random copolymers. By confining the gradient copolymers in ultrathin films, even broader Tg regions could be observed, as both free surface effects and substrate interaction effects act upon different regions of the phase segregated gradient material to increase the heterogeneity in segmental mobility even further. For block copolymers, the drive to form island and hole structures at the film surface led to more noise and complexity in the expansivity response with decreasing thickness compared to the gradient materials.