Poly(methyl methacrylate) nanotubes in AAO templates: Designing nanotube thickness and characterizing the T_g-confinement effect by DSC

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_tube2R_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 α-Al₂O₃ templates. Using ellipsometry, we characterized T_g-confinement effects for PMMA films supported on Si/SiO_x, sputtered Al₂O₃ and sapphire (α-Al₂O₃). Films supported on substrates with higher concentrations of surface hydroxyl groups (α-Al₂O₃ sputtered-Al₂O₃ Si/SiO_x) exhibit larger Tgconfinement effects. The DSC-determined T_g enhancements for nanotubes supported in α-Al₂O₃ templates fall between the ellipsometry-determined T_g enhancements determined for PMMA films on α- Al₂O₃ and those for films on sputtered-Al₂O₃. 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.