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 γSL value 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 T gs 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-T gs effect seen in cylindrical nanopores.

New Insights from Nonequilibrium Kinetics Studies on Highly Polar S-Methoxy-PC Infiltrated into Pores

“…Interestingly, the combination of dielectric and calorimetric data revealed that the temperature at which the change in τ α occurs agrees with the calorimetric glass transition temperature assigned to the vitrification of the interfacial molecules (adsorbed on the pore walls, T g,interfacial ) . According to this hypothesi,s below T g,interfacial , immobilized adsorbed molecules generate pseudoisochoric conditions, where free volume barely varies, resulting in a change in the temperature dependence of structural relaxation times that mimics τ α ( T ) evolution at constant volume. , Finally, one should also mention recent studies that clearly showed that roughness-induced variation in the mobility of adsorbed molecules strongly affects the overall dynamics of polymer films, which agrees with the results of molecular dynamics simulation studies. − Nevertheless, surprisingly, the chemical modification of the inner pore walls (leading to the change in the strength of interfacial interactions) does not influence T g,interfacial . This data increases doubt about the vitrification of the interfacial layer being responsible for a change in the molecular dynamics of confined materials.…”

“…This indicates that the behavior of the highly polar liquid infiltrated into porous membranes is not governed by wettability but rather by other factors, probably the density packing and the lower extent of guest–host interactions. Importantly, similar effects have been reported for poly­(methylphenylsiloxane) (PMPS) within AAO templates …”

“…Importantly, similar effects have been reported for poly-(methylphenylsiloxane) (PMPS) within AAO templates. 23 Furthermore, complementary dielectric measurements performed on the bulk sample showed the presence of the two relaxation processes: (i) the dc conductivity and (ii) the αrelaxation (related to the glass transition phenomenon) at low and high frequencies, respectively (inset in Figure 1d). In the case of infiltrated materials, apart from the charge transport and the structural mode, also the Maxwell−Wagner (MW) polarization at the middle-frequency range can be seen (Figure 1d).…”

“… 20 − 22 Nevertheless, surprisingly, the chemical modification of the inner pore walls (leading to the change in the strength of interfacial interactions) does not influence T g,interfacial . 23 This data increases doubt about the vitrification of the interfacial layer being responsible for a change in the molecular dynamics of confined materials.…”

“…Importantly, similar effects have been reported for poly(methylphenylsiloxane) (PMPS) within AAO templates. 23 …”