Deuterium NMR of Adsorbed Poly(Methyl Acrylate)-d₃

Frank D. Blum, Missouri University of Science and Technology
Wuu-Yung Lin

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The dynamics of polymers at interfaces are important in a variety of applications, especially where those applications depend on the mechanical or surface properties. Unfortunately, the surface properties of interfacial species like polymers used in composites are not easily measured macroscopically. Microscopic techniques, especially spectroscopic ones, have been successful. In our laboratory we have been successful in applying NMPR to determine the behavior of absorbed polymers. NMR has several advantages for this kind of study, the principal ones being that it can be used on opaque materials, different nuclei can be used as probes, and interfacial material can sometimes be selectively probed. The chief disadvantage is that the inherent insensitivity of NMR is compounded by filling the sample with the substrate. Deuterium NMR has been the nucleus of choice in our laboratory. It has a moderate quadrupole moment and, for polymers, the spectra for deuteron bonded to a carbon can be representative of the behavior of the reorientation of the C-D bond. Surface selectivity can also be obtained through the use of a deuteron label, if the label is at the interface. We have applied these techniques to poly(vinyl acetate)-d3 adsorbed on silica and postulated that a motional gradient existed within a single molecule on the surface. In that case, polymer segments near the polymersilica interface (in or close to trains) move more slowly than polymer segments near the air-polymer interface (loops and tails). More recently we have extended these studies to poly(methyl acrylate)-d3 (PMA) on silica and found that a similar trend had been found for that polymer, although its detailed nature depended on the relative amount of polymer adsorbed. In the present work, we report studies of the dependence of the dynamics of PMA-d3 on both the amount of polymer adsorbed and also the molecular weight of the polymer used. We find that both affect the behavior of the adsorbed polymer probably due to the conformation of the polymer on the surface.