(441g) 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 linear PS (see J. Chem. Phys. 2017, 146, 203323). 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 ≥ ~ 175 kg/mol, with effects increasing in magnitude with increasing MW. For example, in 24-nm-diameter template-supported nanorods, T_g,rod – T_g,bulk = -2.0 to -2.5 °C for PS with M_n = 175 kg/mol and M_w = 182 kg/mol, and T_g,rod – T_g,bulk = ~ -8 °C for PS with M_n = 929 kg/mol and M_w = 1420 kg/mol. In general, reductions in T_g occur when d ≤ ~ 2R_g, where R_g is the bulk polymer radius of gyration. Thus, intrinsic size effects are significant when rod diameter is smaller than the diameter (2R_g) associated with the spherical volume pervaded by coils in bulk. We hypothesize that the T_g 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 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. This point can be understood by comparing template-supported nanorod and thin film T_g reductions at a similar characteristic length scale, t*. For PS nanorods with d_m = 63 nm or t* = 15.8 nm, as measured by DSC there is no reduction in T_g relative to bulk response when M_n and M_w ≤ ~500 kg/mol, and T_g,rod – T_g,bulk = ~ -2 °C for PS with M_n = 929 kg/mol and M_w = 1420 kg/mol. As determined by ellipsometry, supported PS films with thickness of 16 nm (t* = 16 nm) exhibit T_g,film – T_g,bulk = ~ -19 °C (Europhys. Letter.1994, 27,59). Thus, at MWs of PS most commonly used in studies of T_g-confinement effects in the presence of a free surface, intrinsic size effects contribute nothing within error to the measured effect; at extremely high PS MW, the contribution of intrinsic size effects is only a very small percentage of the reported T_g reduction.

The characterization of intrinsic size effect on T_g is being extended to a crosslinked, supported PS nanorod system. Linear precursor polystyrene-co-vinylbenzoycyclobutene (VBCB) nanorods with M_n = 30 kg/mol exhibit no intrinsic size effect on T_g, where d > 2R_g even in template with 24-nm pore diameter. Upon crosslinking the VBCB units, the network structure exhibit effectively infinite MW. The comparison of T_g-confinement effects in linear and crosslinked PS nanorods is being studied and will be discussed.