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

Abstract

We used differential scanning calorimetry (DSC) to study the effect of confinement on the glass transition temperature (Tg) 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 (ttube) with bulk radius of gyration (Rg): (ttube ≈ 2 Rg + 9 nm). The results indicate that ttube depends on overall conformation and size of the polymer coils and can be tuned at the nanoscale by polymer molecular weight. The Tg of AAO template-supported PMMA nanotubes increases with decreasing ttube, with Tg,tube − Tg,bulk = 12 K in 18-nm-thick nanotubes; we attribute the Tg increase to hydrogen bonds between PMMA ester side groups and hydroxyl groups on the surface of the γ-Al2O3 templates. Using ellipsometry, we characterized Tg-confinement effects for PMMA films supported on Si/SiOx, sputtered Al2O3 and sapphire (α-Al2O3). Films supported on substrates with higher concentrations of surface hydroxyl groups (α-Al2O3 > sputtered-Al2O3 > Si/SiOx) exhibit larger Tg-confinement effects. The DSC-determined Tg enhancements for nanotubes supported in γ-Al2O3 templates fall between the ellipsometry-determined Tg 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 Tg can be tuned by modulating the levels of interfacial, polymer-substrate interactions by using surfaces with different chemical or crystallographic properties.