Molecular weight dependence of the intrinsic size effect on Tg in AAO template-supported polymer nanorods: A DSC study

Many studies have established a major effect of nanoscale confinement on the glass transition temperature (T-g) 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-g 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-g (both as T-g, onset 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-g are present for PS nanorods with M-n and M-w >= similar to 175 kg/mol, with effects increasing in magnitude with increasing MW. For example, in 24-nm-diameter template-supported nanorods, T-g,T-rod - T-g,T-bulk = -2.0 to -2.5 degrees C for PS with M-n = 175 kg/mol and M-w = 182 kg/mol, and T-g,T-rod - T-g,(bulk) = similar to -8 degrees C for PS with M-n = 929 kg/mol and M-w = 1420 kg/mol. In general, reductions in T-g occur when d <= similar to 2R(g), where R-g is the bulk polymer radius of gyration. Thus, intrinsic size effects are significant when the rod diameter is smaller than the diameter (2R(g)) associated with the spherical volume pervaded by coils in bulk. We hypothesize that the Tg 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-g-confinement effect in supported PS films with one free surface exhibits little or no MW dependence. Published by AIP Publishing.