Many studies have established a major effect of nanoscale confinement on the glass transition temperature (T) of polystyrene (PS), most commonly in thin films with one or two free surfaces. Here, we characterize smaller yet significant intrinsic size effects (in the absence of free surfaces or significant attractive polymer-substrate interactions) on the T and fragility of PS. Melt infiltration of various molecular weights (MWs) of PS into anodic aluminum oxide (AAO) templates is used to create nanorods supported on AAO with rod diameter (d) ranging from 24 to 210 nm. The T (both as T and fictive temperature) and fragility values are characterized by differential scanning calorimetry. No intrinsic size effect is observed for 30 kg/mol PS in template-supported nanorods with d = 24 nm. However, effects on T are present for PS nanorods with M and M ≥ ∼175 kg/mol, with effects increasing in magnitude with increasing MW. For example, in 24-nm-diameter template-supported nanorods, T - T = -2.0 to -2.5 °C for PS with M = 175 kg/mol and M = 182 kg/mol, and T - T = ∼-8 °C for PS with M = 929 kg/mol and M = 1420 kg/mol. In general, reductions in T occur when d ≤ ∼2R, where R is the bulk polymer radius of gyration. Thus, intrinsic size effects are significant when the rod diameter is smaller than the diameter (2R) associated with the spherical volume pervaded by coils in bulk. We hypothesize that the T reduction occurs when chain segment packing frustration is sufficiently perturbed by confinement in the nanorods. This explanation is supported by observed reductions in fragility with the increasing extent of confinement. We also explain why these small intrinsic size effects do not contradict reports that the T-confinement effect in supported PS films with one free surface exhibits little or no MW dependence.

We used differential scanning calorimetry (DSC) to study the effect of confinement on the glass transition temperature (T g) of poly(methyl methacrylate) (PMMA) nanotubes supported in anodic aluminum oxide (AAO) templates. We created nanotubes by wetting templates with polymer melts and developed a design equation relating tube thickness (t tube) with bulk radius of gyration (R g): (t tube ≈ 2 R g + 9 nm). The results indicate that t tube depends on overall conformation and size of the polymer coils and can be tuned at the nanoscale by polymer molecular weight. The T g of AAO template-supported PMMA nanotubes increases with decreasing t tube , with T g,tube-T g,bulk = 12 K in 18-nm-thick nanotubes; we attribute the T g increase to hydrogen bonds between PMMA ester side groups and hydroxyl groups on the surface of the γ-Al 2 O 3 templates. Using ellipsometry, we characterized T g-confinement effects for PMMA films supported on Si/SiO x , sputtered Al 2 O 3 and sapphire (α-Al 2 O 3). Films supported on substrates with higher concentrations of surface hydroxyl groups (α-Al 2 O 3 > sputtered-Al 2 O 3 > Si/SiO x) exhibit larger T g-confinement effects. The DSC-determined T g enhancements for nanotubes supported in γ-Al 2 O 3 templates fall between the ellipsometry-determined T g enhancements determined for PMMA films on α-Al 2 O 3 and those for films on sputtered-Al 2 O 3. These results show that molecular weight provides for tunability of polymer nanotube thickness in AAO templates, that there is excellent agreement in confinement effects measured by DSC and by ellipsometry, and that T g can be tuned by modulating the levels of interfacial, polymer-substrate interactions by using surfaces with different chemical or crystallographic properties.