T_g-confinement effects in strongly miscible blends of poly(2,6-dimethyl-1,4-phenylene oxide) and polystyrene: Roles of bulk fragility and chain segregation

The glass transition temperature (T_g)-confinement effects in single-layer, supported films of strongly miscible poly(2,6-dimethyl-1,4-phenylene oxide)/polystyrene (PPO/PS) blends are characterized using ellipsometry and fluorescence, both of which report a single, reduced blend T_g upon confinement. These results indicate that PPO/PS blends exhibit strong miscibility under conditions of nanoscale confinement and that there is a lack of strong attractive polymer-substrate interactions in PPO/PS films supported on Si/SiO_x. The strength of the T_g-confinement effect obtained via ellipsometry scales with bulk blend fragility in PPO/PS blends and exhibits a weaker dependence on bulk fragility than that observed previously [Evans et al. Macromolecules 2013, 46, 6091] in supported films of linear homopolymers lacking attractive polymer-substrate interactions. We further compare T_g results obtained from fluorescence of both pyrenyl labels (covalently attached to PS) and dopants (freely dispersed in PPO/PS blends) along with those from ellipsometry. In the confined state, pyrenyl dopant fluorescence reports blend T_g values that agree relatively well with T_g values reported by ellipsometry; in contrast, pyrenyl label fluorescence reports blend T_g values that are significantly lower than those reported by both ellipsometry and pyrenyl dopant fluorescence. We attribute this difference to the combined effects of the distribution of pyrenyl dye across the film and underlying surface segregation. Our results indicate that the combination of ellipsometry and fluorescence provides a powerful methodology to study interface-induced chain segregation in nanoconfined miscible polymer blends, whether in film geometry or in nanocomposites.