Structure and dynamics of stereo-regular poly(methyl-methacrylate) melts through atomistic molecular dynamics simulations

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 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 α-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 α-methyl motions, whereas the ester-methyl rotation remains unaffected. The dynamic heterogeneity of the PMMA 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 dynamics for monomer and chain centers-of-mass are analyzed.

Acrylic Paints: An Atomistic View of Polymer Structure and Effects of Environmental Pollutants

“…The variation of T g from difference force fields results from small changes in density, structure, and packing of polymer chains due to differences in parameters and becomes more pronounced as the side chain of the polymer becomes longer. While these T g values are higher compared to those measured in experiments, i.e., 333–387 K 53 , 58 − 60 for PMMA, 249 K 61 or 231 K 62 for PEA, and 223 K 59 for P( n BA), there is good qualitative agreement between our simulations and the experimental data where we observe a decrease in the T g with growing length of polymer side chains. This trend of decreasing T g with increasing side chain length is often discussed in terms of the “plasticization effect”, 63 , 64 but in our case the side groups (methyl, ethyl, and butyl) are quite short to decide whether it is a packing or plasticization effect.…”

Gradient of Segmental Dynamics in Stereoregular Poly(methyl methacrylate) Melts Confined between Pristine or Oxidized Graphene Sheets

“…Then, the τ bulk seg values of each stereoisomer were fitted with a Vogel-Fulcher-Tammann (VFT) relation (τ = τ ∞ exp(B/(T − T 0 ))). The parameters of the fitted VFT curves are: τ ∞ = 10 −13 sec and B = 2800 K for the three model PMMA stereoisomers and T 0 = 262 K, 287 K, and 298 K for i-, a-, and s-PMMA, respectively (because of the extensions of the runs and recalculations of the relaxation times, the T 0 values are very slightly different from our previous results [44,49]). The bulk segmental relaxation times and the fitted VFT curves are shown in Figure 6.…”

“…For PMMA, an all atom force field has been employed that fairly describes the bulk melts of stereoregular PMMA [42][43][44]. The PMMA force field is provided in Table S2.…”

“…The simulations were conducted using the open source package GROMACS [45]. The model PMMA stereoisomers (confined between PG, RGO, or GO) have been studied at three different temperatures, 580 K, 550 K, and 520 K, whereas a-PMMA/RGO and a-PMMA/GO systems were simulated at 580 K. As reference systems, the corresponding bulk PMMA melts were also simulated at 580 K, 550 K, 520 K, and 490 K. These temperatures are well above the bulk T g of the model PMMA chains (≈T > T g +100 K) [31,44]. Performing the simulations at temperatures well above T g is a consequence of the computational limitations of the atomistic simulations.…”

“…For the layer resolved calculation of TACF, confined films are divided into five layers: the closet layer to the surface ([0-7] Å) corresponds to the adsorption layer that was observed in f (d)/ f • curves (Figure 2b). At the second layer ([7-15] Å), PMMA chains show slight perturbations of local density (particularly i-PMMA chains) and the other three layers ( [15][16][17][18][19][20][21][22][23][24][25] Å, [25][26][27][28][29][30][31][32][33][34][35] Å, and [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] Å) concern regions where the interfacial layering effects are almost negligible (Figure 2b). Each dihedral angle contributes to the TACF of a layer based on the its position (minimum distance from the middle point between the central atoms of the dihedral angle to the surface atoms) at the time origin.…”