Glass Transition Dynamics of Poly(phenylmethylsiloxane) Confined within Alumina Nanopores with Different Atomic Layer Deposition (ALD) Coatings

Abstract

Dielectric spectroscopy (DS) and differential scanning calorimetry (DSC) were employed to study the effect of changes in the surface conditions on the segmental dynamics of poly(phenylmethylsiloxane) confined in alumina nanopores (AAO). The inner surface of the pores was modified by using the atomic layer deposition technique. Coated membranes include 5 nm thick layers of hafnium oxide, titanium oxide, and silicon oxide, exhibiting different wetting properties. Modification of the surface conditions dramatically affects the interfacial interactions between the polymer and confining surface. The interfacial energy calculations indicate a decrease of γSLvalue from 18.7 mN/m in SiO2-coated to 0.5 mN/m in HfO2-coated nanopores. The results of the dielectric relaxation study demonstrate that the segmental relaxation time of confined PMPS 2.5k depends on the thermal treatment protocol and the hydrophilic/hydrophobic character of the pore walls. From calorimetric measurements, we found that the two glass transitions events are still observed, even in the absence of strong interfacial interactions. Values of both Tgs do not depend strongly on the chemical nature of the surface. In this way, changes in the glass-transition behavior of the tested polymer confined in ALD-coated nanopores cannot be rationalized in terms of the polymer/substrate interfacial energy. Eliminating strongly adhered surfaces does not eliminate the puzzling two-Tgs effect seen in cylindrical nanopores.