Local Dynamics of Syndiotactic Poly(methyl methacrylate) Using Molecular Dynamics Simulation Chunxia Chen ² and Janna K. Maranas* Department of Chemical Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 Victoria Garcı ´a-Sakai ‡ NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562 ReceiVed May 10, 2006; ReVised Manuscript ReceiVed September 20, 2006 ABSTRACT: The local dynamics of syndiotactic poly(methyl methacrylate) (PMMA) are investigated by explicit atom molecular dynamics (MD) simulations and quasielastic neutron scattering at temperatures well above the glass transition temperature. Using MD, we are able to isolate specific local motions well above T g . These include rotations of the R-methyl and ester methyl, rotations of the entire ester side group and segmental motion of the chain backbone. This capacity is unique to simulation as proton motion at high temperatures necessarily involves multiple motions. The force field used is validated by direct comparison to structural and dynamic neutron scattering measurements, and by comparison via temperature extrapolation of activation energies and rotational times for methyl group rotations. We find that the rotation of the ester side group is consistent with the -relaxation at low temperatures: the activation energy closely matches that assigned from dielectric spectroscopy (DS), and relaxation times are also consistent with these measurements. Although the ester protons rotate continuously with no preferred spatial orientation, the rotation of the ester oxygen around the C 1 -C bond [O-C-C 1 -C 3 ] does appear to be a 2-fold jump as observed in NMR experiments. The R-relaxation is associated with the motion of the main chain. Relaxation times for these protons are not Arrhenius, but rather begin to diverge as the temperature is lowered. Rotation of the slower R-CH 3 group occurs with rates similar to the R- and -relaxations in the temperature range we investigate. Both this rotation and that of the ester side group are more prominent at smaller scales and explain why neutron scattering measurements on PMMA reveal the R-relaxation as the spatial scale is increased. I. Introduction The dynamic processes of glass forming systems have been widely investigated using a variety of techniques including NMR, 1 dielectric spectroscopy (DS), 2-4 and light 5 and neutron 4,6 scattering. The R-relaxation, which arrests at the glass transition temperature T g , is commonly associated with the segmental relaxation of the main chain. Polymer molecules, especially those with side groups, have other relaxation processes known as secondary relaxations. 7-9 Poly(methyl methacrylate) [PMMA] is one such polymer and its repeat unit is illustrated in Figure 1. Secondary processes in PMMA include rotations of the ester and R-methyl groups and the -relaxation, which has been linked with the reorientation of the entire ester side group around the C-C bond linking it to the backbone. At temperatures well above T g all processes are active; below T g the segmental relaxation is frozen and only secondary relaxations are active. As a result, at temperatures above T g , secondary motions may merge with those of the main chain making individual assess- ment difficult. The -relaxation is normally prominent in dielectric measure- ments for PMMA. 2D and selective-excitation 3D exchange NMR techniques 10 have been applied to explore the molecular motion underlying the -relaxation in PMMA below T g . These experiments show that the OCO plane of the side group undergoes 180° flips which are related to the -relaxation. This side-group flip is accompanied by a main chain rearrangement. Recently our group initiated a quasi-elastic neutron scattering [QENS] study of the segmental dynamics of PMMA and how they are influenced by blending with poly(ethylene oxide) [PEO]. 11 This neutron measurement showed that pure PMMA displays different relaxation behavior at different spatial scales. At spatial scales larger than the interchain peak, relaxation times increase rapidly as the temperature is decreased toward the glass transition temperature: typical for the R-relaxation. At intrachain spatial scales the temperature dependence of relaxation times is Arrhenius with an activation energy of 118 kJ/mol, consistent with the merged R process. These experiments were performed above T g and thus potentially include contributions from all motions, including secondary relaxations in PMMA. Rotation of the methyl groups is the most straightforward motion in PMMA. This rotation is often approximated by an effective one-dimensional single particle potential, and is usually represented by instantaneous jumps between three equidistant sites on a circle. 12,13 The resulting rotational motion often falls * Corresponding author. E-mail: jmaranas@psu.edu. ² E-mail: cqc10@psu.edu. ‡ Email address: vicky@nist.gov. Figure 1. Chemical structure of the repeat unit of PMMA. 9630 Macromolecules 2006, 39, 9630-9640 10.1021/ma0610562 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/19/2006