Segmental Mobility of Chain Ends in Poly(Methyl Acrylate)-d3
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Better control of polymeric materials can be achieved with a thorough understanding of the dynamics of their constituents. In the present study, we consider polymer chains as composed of chain middles and chain ends. Even though chain ends do not comprise much of the sample by mass, they may play a crucial role in the ultimate properties of the polymers. Although chain ends have been assigned a higher mobility, as compared to chain middles, there have not been a large number of experimental studies that directly probe their mobility. Among those, the studies of Kitahara et al.1 and Miwa et al.2 demonstrated the higher mobility of chain ends using ESR for polyethylene and polystyrene (PS), respectively. They observed that the transition temperatures for the onset of rapid molecular motion at the chain ends were 5 K lower than those inside the chains. A molecular mass dependence was also observed for the transition for the chain ends. A specular neutron reflectivity (SNR) study3 indicated that segments in the center sections of PS have a lag in mobility across a welded interface, as compared to chain ends. Previous deuterium NMR studies,4,5 by our group, have shown the a molecular-mass dependence on segmental dynamics through the glasstransition region. More heterogeneous segmental dynamics were observed in the NMR spectra of lower molecular mass poly(methyl acrylate) (PMA) compared to those of higher molecular mass. This heterogeneity was attributed to the presence of a higher concentration of more-mobile chain ends. In this study, the segmental dynamics of the PMA samples, with methyl-groupdeuterated chain ends, was studied using the 2H quadrupole-echo NMR technique. These results provided significant insight into the role of chain ends in the glass transition region of polymers.