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Despite the many controversial discussions about the nanometric confinement effect on the properties of ultra-thin films, much remain not understood and/or experimentally unproven. Here, a combination of Broadband Dielectric Spectroscopy (BDS) and Specific Heat Spectroscopy (SHS) employing AC nanochip calorimetry were utilized to investigate the glassy dynamics of ultra-thin films of a low MW Poly (vinyl methyl ether) (PVME) (thicknesses: 7 nm – 160 nm). For BDS measurements, a recently developed nano-structured electrode sample arrangement is employed; where ultra-thin films are spin-coated on an ultra-flat highly conductive silicon wafer, sandwiched between a wafer with nanostructured SiO2 nano-spacers with heights between 35 nm and 70 nm. For films with thicknesses up to 50 nm, BDS measurements showed two relaxation processes, which can be analyzed for these film thicknesses in details. The process located at higher frequencies coincidence in both, its position and temperature dependence, with the -relaxation of bulk PVME and is therefore assigned to the -relaxation of a bulk-like layer. The temperature dependence of the relaxation rate of this process in independent of film thickness. This is further confirmed by the SHS investigations, which superimpose in its temperature dependence with the BDS results; independent of film thickness. The second process is located at lower frequencies, where it shows a different temperature dependence and ascribed to the relaxation of polymer segments adsorbed at the substrate. The interaction of PVME with SiO2 was further confirmed by contact angle investigations. This adsorbed layer further undergoes a confinement effect that results in a lower Vogel temperature than that of the bulk-like layer. A detailed analysis of the dielectric strengths of both processes reveals that the thickness of the adsorbed layer decreases with increasing temperature, while that of the bulk-like layer increases. As a main conclusion, BDS showed that the glassy dynamics of the bulk-like and the adsorbed layer are thickness independent, which is in agreement with the SHS results. To our knowledge, this is the first probing of the segmental dynamics of an adsorbed layer in ultrathin films.
Thermal and dynamic glass transition of ultrathin films of homopolymers and a miscible polymer blend
(2017)
Despite the decade long controversial discussion on the effect of nanometer confinement on the glass transition temperature (Tg) of ultrathin polymer films, there is still no consistent picture. Here, the dynamic calorimetric glass transition of ultrathin films of a blend, which is miscible in the bulk, is directly investigated by specific heat spectroscopy. By a self-assembling process, a nanometer thick surface layer with a higher molecular mobility is formed at the polymer/air interface. By measuring the dynamic calorimetric Tg in dependence on the film thickness, it was shown that the Tg of the whole film was strongly influenced by that nanometer thick surface layer, with a lower Tg. Since the observed thickness dependence of the dynamic Tg is similar to the thickness dependence of the Tg for thin films of homopolymers, it is concluded that also for homopolymer a highly mobile surface layer is relevant for the widely observed Tg depression.