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

Abstract

Broadband dielectric spectroscopy and differential scanning calorimetry were used to study the effect of changes in the surface conditions on the segmental dynamics of poly­(phenylmethylsiloxane) confined in alumina nanopores. Functionalization was done using highly polar propyl phosphoric units separated by the assumed concentration of triethoxysilane groups (from N = 0 to N = 24). By adjusting the proportion between polar units and nonpolar spacers, it was possible to control the surface polarity. Modification of the surface conditions does not inhibit the formation of the adsorbed layer, as revealed by the presence of two T g’s in calorimetric results. However, changes in the surface polarity will prevent the growth of the additional interlayer in between the core volume and the interfacial layer. Finally, we also found that the changes in the surface polarity affect the equilibration kinetics and can be used to control the time scale of the structural recovery toward the equilibrium state.

Glass Transition Dynamics of Poly(phenylmethylsiloxane) Confined within Alumina Nanopores with Different Atomic Layer Deposition (ALD) Coatings

“…Then, with decreasing the temperature, the characteristic deviation in τ α ( T ) appears. This observation agrees with our previous results for PMPS 2.5k confined in AAO nanopores with chemically modified surfaces. , Similar behavior was also reported for many other glass-forming substances under nanoconfinement. ,,− ,, Such deviation of the τ α ( T ) from the bulk is often related to the presence of two fractions of molecules, i.e., the interfacial and core layers. In each layer, the molecules have different mobility.…”

“…As demonstrated in the Supporting Information, the correction, which includes the contribution coming from the matrix and incomplete pore filling/air gaps (∼10%) with the tested polymer, modifies only the intensity of the dielectric permittivity while not the position of the segmental relaxation or either its shape. In the previous study, we have also ended up with the same finding. , Nevertheless, one should bear in mind that such correction is mandatory before evaluating the conductivity behavior of ionic liquids in nanopore confinement or when the air-gap contribution is more dominant.…”

“…Former experimental work confirms the hydrophilic nature of silicon and titanium oxides , and the hydrophobic properties of hafnium oxide coatings. − In our previous study, functionalization was prepared by adjusting the proportion between highly polar phosphoric units and nonpolar spacers. In such a case, we have discovered that the hydrophobic properties of the surface increase with increasing the number of nonpolar units per single polar group . Furthermore, literature data suggest that the glass transition dynamics under nanoconfinement might depend on the hydrophilicity/hydrophobicity of the inner pore walls. ,,,, …”

“…This strategy can involve, for example, the separation of highly polar phosphoric units by a controlled number of nonpolar spacers. Interestingly, our data indicate that in such a case precise control over surface polarity might prevent the formation of the interlayer between the adsorbed layer and core volume observed in native alumina nanopores …”

“…The glass transition dynamics of the polymer materials subjected to nanometric confinement isin many casesnotably different from the bulk. It has been recognized that this behavior depends on several factors, which include the finite size effect, interfacial interactions, conformational changes, − free volume, , negative pressure effects, , and surface properties (including the impact of the roughness, silanization, or polarity). Thus, lowering the pore diameter or modifying the surface interactions typically changes the glass transition temperature value. − In addition, the thermal history of the sample also influences the glass transition dynamics. − For example, it can broaden the shape of the segmental relaxation and decrease its dielectric strength. ,, Apart from that, the properties of the confined material can change due to interactions with the constraining environment. ,− …”