Effect of Surface Modification on the Glass Transition Dynamics of Highly Polar Molecular Liquid S-Methoxy-PC Confined in Anodic Aluminum Oxide Nanopores

Abstract

In the presence of nanoscale confinement, the role of surface effects come to the fore in determining the glass-transition dynamics of the molecular liquids and polymers. Therefore, by playing with the surface chemistry it is possible to understand better the mechanism which governs the behavior of glass-forming systems at the nanoscale. In this work, we have combined dielectric and calorimetric data to study surface and confinement effects for highly polar glass-forming liquid S-Methoxy-PC constrained within anodic aluminum oxide membranes of different pore sizes. The inner surface of the pores was modified either by silanization or atomic layer deposition (ALD) coatings. For the tested substance in native nanopores, we have observed two glass transition events in the calorimetric response accompanied by a characteristic deviation of the temperature dependence of the α-relaxation time from the Vogel–Fulcher–Tamman law. We found that depending on the hydrophobicity of the ALD layer the glass transition temperature of the interfacial layer tends to decrease, the α-relaxation peak broadens, and the molecular mobility slows down compared to the native pores. These changes are more visible with increasing hydrophobicity of the surface. Silanization was also found to eliminate at least partially the effects caused by nanopore confinement. However, in this study the most pronounced effects were observed only for pores with large diameters.

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.…”

“…Recent studies on polymer thin films and nanopore confinement highlight the important role of nonequilibrium phenomena in determining the behavior of confined glass-forming systems at the nanometer length scale. ,,,,,, In the presence of spatial restrictions comparable in size with the coil dimensions, the polymer chains cannot approach an equilibrium conformation within the time typically allowed for processing or stabilization of a new condition. As a result of the frustration in the density, deviations in segmental mobility is observed in a confined space.…”

“…It is often claimed that glass-transition dynamics in the confined state is the outcome of the counterbalance between finite size and surface effects. ,,, In addition, in some cases, with the reduction of the thickness, the influence of the finite size effect can be overpowered by the substrate interaction. For this reason, understanding how the surface conditions affect the behavior of polymers and glass-forming systems at the nanoscale is an active research topic. ,,,− …”

“…However, the surface modification strategies do not involve only silanization; there are other methods, for example, the use of ODPA (octadecylphosphonic acid), which may affect the crystallization and local dynamics of nanopore-confined poly­(ε-caprolactone), as well as control the phase behavior of liquid-crystalline materials . Apart from that an atomic-layer-deposition technique can be utilized to fine-tune the surface properties of the confined nanomaterials …”

“…53 Apart from that an atomic-layer-deposition technique can be utilized to fine-tune the surface properties of the confined nanomaterials. 31 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).…”