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

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, τ α (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 τ α (T) for nanopore-confined polymer.

Effect of the Surface Polarity, Through Employing Nonpolar Spacer Groups, on the Glass-Transition Dynamics of Poly(phenyl methylsiloxane) Confined in Alumina Nanopores

“…Experimental evidence demonstrates that the hydrophobic and hydrophilic properties of the confining surfaces might significantly influence the glass-transition dynamics. ,,, Therefore, in the next step, we have investigated changes in the segmental dynamics of the confined polymer induced by changing the surface polarity. From the previous work, , we know that in the dielectric spectra of bulk PMPS 2.5k one can detect three relaxation processes of different molecular origins: α′, α, and β-relaxations. The former one is identified as the sub-Rouse mode, the second one is due to segmental mobility, and the latter one represents more local motions, though it is strongly coupled to the structural relaxation.…”

“…Depending on the silanization procedure, which is used to modify the surface of the nanopores, the formation of the adsorbed layer, as well as the segmental mobility, might be quite different. This we have demonstrated recently for poly­(phenylmethylsiloxane) confined in alumina membranes with surface modification using two different silanization agents, chlorotrimethylsilane (ClTMS) and (3-aminopropyl)­trimethoxysilane (APTMOS) . ClTMS was responsible for replacing the native hydroxyl groups with trimethylsilane units, while APTMOS provides aminopropylsilane species at the pore surface.…”

“…On the other side, the surface effects also have an enormous impact on the polymer’s dynamics. ,− Strong attractive interactions can lead to the adsorption of the polymer segments on the surface by forming hydrogen bonds. This produces a gradient in dynamics across the film or nanopores, resulting in more than one glass-transition event in a confined geometry.…”

“…In such conditions, it can reside for a very long time . The effects seen in confined geometry depend on the thermal history or the processing conditions. − When provided enough time, confined material can reach the equilibrium configuration and therefore reduce or even eliminate some of the striking features seen only in constrained geometry. ,− ,− In thin films, upon prolonged annealing, the conformation of the polymer chains located close to the substrate is changing due to increased irreversible adsorption. ,− Prolonged annealing can slow down initially faster dynamics as observed in thin films of poly­(methylmethacrylate) and poly­(vinyl acetate) . Plus, it can restore some of the bulk sample’s properties, such as T g and dielectric strength. ,− Likewise, in nanopore confinement, one can observe out-of-equilibrium phenomena. − One of them is the ability to recover by the system some of the bulk properties, such as the temperature evolution of the α-relaxation time. , Upon annealing, the polymer chain packing density changes and slows down the segmental dynamics. , …”

Influence of Tacticity on the Glass-Transition Dynamics of Poly(methyl methacrylate) (PMMA) under Elevated Pressure and Geometrical Nanoconfinement

“…The increased pressure causes changes in the glass-transition dynamics. The percentage of the free volume available for the molecular movements decreases under the influence of the elevated pressure, which induces shifts in the glass-transition temperature. − Also, the properties of the polymer glass formers under geometrical nanoconfinement can differ significantly from the bulk behavior. − ,− The most frequently observed effects are the faster dynamics and changes in the glass-transition temperature for both thin films − and nanopores-confined systems. − …”