Frank D. Blum
Department of Chemistry and Materials Research Center
University of Missouri-Rolla
Rolla, MO 65409

        The properties of composite systems are dictated by the properties of their components, plus the structure and dynamics of the interfacial molecules between the different components.  Both the structure and dynamics of interfacial species are very important to properties, but the latter is often neglected and seldom studied.  In our laboratory, we have focused our research efforts on the molecular motion of molecules at or near a solid surface.  Most of these studies have used nuclear magnetic resonance (NMR) plus other techniques such as modulated differential scanning calorimetry (MDSC) to probe the mobility of polymers near solid surfaces.

        We have found that polymers from the acrylate, methacrylate and related families are well suited for interfacial studies.  Polymers from these families have been adsorbed on silica and studied on silica surfaces.  With specifically-labeled poly(methyl acrylate)-d3 (PMA-d3), we found the bulk polymer could classified as "homogeneous", while the surface adsorbed polymer could be characterized as "heterogeneous" with respect to segmental dynamics in different parts of the sample.  More careful examination revealed that a motional gradient existed in the polymer on the silica surface.  This gradient was such that those segments at the polymer-air interface were more mobile and those at the polymer-solid interface were less mobile than those in bulk.  More mobile segments could be eliminated through over-layering with unlabeled polymer compression molded on top of the surface polymer. The effects of molecular weight were also probed.  For example, a lower molecular weight polymer was found to be less mobile on the surface than the higher molecular weight one.  This is in opposition to their behavior in bulk.  We have verified this motional heterogeneity through the use of MDSC of poly(methyl methacrylate) (PMMA) on silica.  MDSC showed an increase in temperature and breadth of the glass transition region for the surface polymer.