Graduated Segmental Mobility in Polymer Layers on Silica
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The behavior of thin polymer layers on oxide substrates has gotten a great deal of attention as the size of materials, and hence polymer films, has gotten smaller. Indeed, the properties of adsorbed polymers are such that they can be different from those of bulk polymers, especially when the films are very thin. A wide variety of studies have been made on adsorbed polymers and sometimes the results seem contradictory. One of the properties often measured in thin films is the apparent glasstransition temperature, Tg. In principle, many techniques are sensitive to phenomenological changes occurring around Tg and, therefore, can be compared to each other. For polymer thin films there are two surfaces (often different) to consider. For example, a thin polymer film on a solid substrate, such as silica, has a polymer-silica interface and a polymer-air interface. In principle, these two interfaces would be expected to have different properties. In addition, it is not always clear if either of these would dominate the properties measured. In this paper, we provide support for the existence of graduated segmental mobility in thin polymer films. As examples we cite the behavior observed from both deuterium NMR of poly(methyl acrylate)-d3 (PMA-d3) and modulated differential scanning calorimetry (MDSC) of poly(methyl methacrylate) (PMMA), both on silica powder. In Figure 1, we depict a surface with which the polymer has a favorable interaction. We propose that the segments at the polymer-air interface have enhanced mobility while those at the polymer-silica interfect have reduced mobility.