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| Paper IPM / P / 15478 |
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Poly(methyl-methacrylate), PMMA, is a disubstituted vinyl polymer whose properties depend significantly
on its tacticity. Here we present a detailed study of the structure, conformation, and dynamics of
syndiotactic, atactic, and isotactic PMMA melts at various temperatures (580, 550, 520, and 490 K) via
all-atom molecular dynamics simulations. The calculated volumetric properties are close to
experimental data. The T g and chain dimensions of PMMA model systems are found to depend
strongly on tacticity in agreement with experimental findings. The backbone bonds in trans (t), diads in
tt, and inter-diads in t - t torsional states are the most populated for all PMMA stereo-chemistries and
their fractions increase with the number of syndiotactic sequences. Also, the effective torsional barrier
heights for the backbone, ester side group, and a-methyl group are larger for syndiotactic PMMA
compared to the isotactic one. The structure of the PMMA chains is studied by computing the intra-
and inter-chain static structure factors, S(q), and the radial pair distribution functions. In the first peak
of S(q), both intra- and inter-chain components contribute, whereas the second and third peaks
mainly come from inter- and intra-chain parts, respectively. For all PMMA stereo-isomers a clear
tendency of ester-methyl groups to aggregate is observed. The local dynamics are studied by
analyzing torsional autocorrelation functions for various dihedral angles. A wide spectrum of
correlation times and different activation energies are observed for the motions of different parts of PMMA chains. The stereo-chemistry affects the backbone, ester side group, and a-methyl motions,
whereas the ester-methyl rotation remains unaffected. The dynamic heterogeneity of the PMMA
DOI: 10.1039/c7sm02008b chains is also studied in detail for the different stereo-chemistries via the temperature dependence of the stretching exponent. Furthermore, the reorientational dynamics at the chain level and translational
rsc.li/soft-matter-journal dynamics for monomer and chain centers-of-mass are analyzed.
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