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Paper   IPM / Physic / 15478
School of Physics
  Title:   Structure and dynamics of stereo-regular poly(methyl-methacrylate) melts through atomistic molecular dynamics simulations
  Author(s): 
1.  A.F. Behbahani
2.  S.M. Vaez Allaei
3.  G.H. Motlagh
4.  M. Eslami
5.  V.A. Harmandaris
  Status:   Published
  Journal: Soft Matter
  Vol.:  14
  Year:  2018
  Pages:   1449
  Supported by:  IPM
  Abstract:
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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