Effect of Surface Chemistry on the Glass-Transition Dynamics of Poly(phenyl methyl siloxane) Confined in Alumina Nanopores.

Roksana Winkler,Wenkang Tu,Karolina Adrjanowicz,Lukasz Laskowski

doi:10.1021/acs.langmuir.0c01194

Abstract

Broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC) are combined to study the effect of changes in the surface chemistry on the segmental dynamics of glass-forming polymer, poly(methylphenylsiloxane) (PMPS), confined in anodized aluminum oxide (AAO) nanopores. Measurements were carried for native and silanized nanopores of the same pore sizes. Nanopore surfaces are modified with the use of two silanizing agents, chlorotrimethylsilane (ClTMS) and (3-aminopropyl)trimethoxysilane (APTMOS), of much different properties. The results of the dielectric studies have demonstrated that for the studied polymer located in 55 nm pores, changes in the surface chemistry and thermal treatment allows the confinement effect seen in temperature evolution of the segmental relaxation time, τα(T) to be removed. The bulk-like evolution of the segmental relaxation time can also be restored upon long-time annealing. Interestingly, the time scale of such equilibration process was found to be independent of the surface conditions. The calorimetric measurements reveal the presence of two glass-transition events in DSC thermograms of all considered systems, implying that the changes in the interfacial interactions introduced by silanization are not strong enough to inhibit the formation of the interfacial layer. Although DSC traces confirmed the two-glass-transition scenario, there is no clear evidence that vitrification of the interfacial layer affects τα(T) for nanopore-confined polymer.

Highlights

When going down with the size of a soft matter to a nanometre size, its physical and chemical properties drastically change
We investigate the effect of changes in the surface chemistry on the glassy dynamics of polymethylphenylsiloxane (PMPS) confined in anodic aluminum oxide nanopores
We demonstrate the presence of two glass-transition events in differential scanning calorimetry (DSC) thermograms of all considered systems, which means that the changes in the interactions between the polymer and silanized pore walls are not strong enough to inhibit in any case the formation of the interfacial layer

Summary

Introduction

When going down with the size of a soft matter to a nanometre size, its physical and chemical properties drastically change. Scientists are incessantly interested in obtaining novel nanomaterials with remarkable morphologies or structures that, in many cases, cannot be achieved by any other means at the macroscale. Such nanomaterials can find numerous promising applications in drug delivery systems, coatings, sensors, electronic devices, and many others.[1−6]. One of the very useful strategies allowing for the study of the properties of polymers and molecular liquids at the nanoscale level is by constraining them within solid interfaces of nanometer size in either one- (thin films), two- (nanochannels/cylindrical pores), or three-dimensions (suspended nanoparticles or droplets). Depending on the boundary conditions, one can discriminate between soft and hard confinement. In the case of hard confinement, the sample is located within the rigid porous matrix/nanochannels or supported on hard substrate (e.g., aluminum or silicon).[7−9]

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Conclusion