Deuterium NMR Studies of Segmental Dynamics of Anopore-Adsorbed Poly(Methyl Acrylate)

Macduff O. Okuom
Frank D. Blum, Missouri University of Science and Technology

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Studies of segmental dynamics of polymers at interfaces provide a basis for understanding the properties of composite materials. Interfacial phenomena in multi-phase systems need to be understood as devices made from multi-phase polymeric systems become smaller. Various techniques have been used to investigate interfacial polymers at the air-polymer-solid interface; for example, ellipsometry,1 X-ray,2 and neutron reflectometry.3 These techniques typically characterize the structure (often the thickness) of the polymer layer and are normally used to determine the glass-transition temperature (Tg) through a break in the thicknesstemperature curve. There have been a few reports where the dynamics of polymers at the surface-air interface has been probed directly or inferred by NMR4 or ESR5. Using deuterium quadrupolar echo techniques on poly(vinyl acetate)-d3, Blum et al.6 showed that a deuterated methyl group could be used to probe the dynamics of adsorbed polymers. Lin and Blum7,8 observed that molecular mass affected segmental dynamics in both bulk and silica-adsorbed poly(methyl acrylate) (PMA-d3). As temperature increased, both high and low molecular mass samples showed increased motional rates, with significantly faster motional rates in the high molecular mass samples. They also reported that lower adsorbed amounts exhibited relatively slower motional rates than higher adsorbed amounts did. Anopore is an inorganic membrane that is produced by anodic oxidation of aluminum.9 Since the process is electrochemical, the conditions can be controlled and reproducible pore structures with narrow pore-size distributions can be obtained. They are available in 4.7 cm discs, 60 um thick, with 0.2 or 0.02 um diameter pore sizes. The anopore membranes are usually used for filtration purposes. Anopore has been used as a confining substrate in studies of polymer adsorption.10-12 However, not much is known about the adsorption mechanism or the state of the polymer in them. In this paper, we examine the adsorption of PMA-d3 on anopore by 2H NMR. We look at the process of adsorption, the effect of adsorbed amounts, and the effect of pore size. The results show little difference in the effect of pore size, or adsorbed amounts on the mobilities of PMA-d3 segments.