Is there a relationship between wettability and rates of equilibration of the H-bonded oligomer PMMS under confinement?

TL;DR

This study explores how the wettability of confined PMMS influences its relaxation and equilibration rates, revealing that surface interactions significantly affect the material's thermal and dielectric behavior under nanoscale confinement.

Contribution

It demonstrates a correlation between wettability changes and the activation energy of equilibration in confined PMMS, linking surface properties to molecular dynamics.

Findings

Relaxation times deviate from bulk behavior near glass transition.

Annealing can restore bulk-like relaxation behavior.

Activation energy decreases with smaller pore sizes and altered wettability.

Abstract

In this paper, we investigated the annealing experiments of poly(mercaptopropylmethylsiloxane, PMMS) confined within two types of porous templates (anodic aluminium oxide, AAO, and silica) characterized by different pore diameter, d= 8-120nm, using different thermal protocols (varying significantly in cooling/heating rate) by means of Broadband Dielectric Spectroscopy (BDS) supported by the complementary Differential Scanning Calorimetry (DSC) and temperature-dependent contact angle, {\theta}, measurements. It was found that relaxation times obtained from routine temperature-dependent dielectric investigations deviate from the bulk behavior when approaching the glass transition temperature. Importantly, this confinement induced effect can be easily removed by the annealing experiments performed at some specific range of temperatures. The analysis of the dielectric data collected during isothermal experiments of confined samples that was beforehand cooled with different rates revealed that (i) constant rates of annealing gets longer with cooling and weakly depend on the rate of cooling, and (ii) activation energy of the equilibration process, E_a, varies with the reduction of the pore diameter and material the porous template is made of. In fact, there is significant reduction in E_a from ~62 to ~23 kJ/mol obtained for the annealing process carried out in AAO (d= 10 nm) and silica (d= 8 nm) membranes, respectively. Such significant change in E_a can be explained taking into account temperature-dependence of {\theta} of PMMS indicating a notable change in wettability between both surfaces upon cooling. As a consequence, one can expect that the mass exchange between interfacial and core molecules as well as adsorption-desorption processes occurring at the interface at lower temperatures must be affected.