Atomistic Simulations of Glassy Polystyrenes with Realistic Chain Conformations

Abstract

The detailed structure of glassy polymers is still a subject of debate. One of the main accepted views is Flory's hypothesis, 1-3 according to which the chain conformation in the glassy state is close to that in the melt or in solutions under θ conditions. However, other structural models with higher degrees of local order have been put forward (see Sperling 4 for a collection of references). Until recently, the structure of glassy polymers has mainly been studied by scattering techniques; in most studies, experimental scattering intensities are compared to those calculated from atomistically detailed chain and bulk models. [5][6][7][8] In the past few years, solid-state NMR has proven to be a powerful tool to investigate the structure of glassy solids; this technique has revealed many details about chain conformation and chain packing in glassy polymers. [9][10][11][12] On the basis of Flory's hypothesis, Theodorou and Suter introduced an approximate algorithm to generate atomistic structures of bulk glassy polymers, known as the "amorphous-cell" method. 13 Following that work, other algorithms to generate atomistic packings of glassy polymers have been developed. 14-17 Although most of these algorithms aim at generating chain conformations close to those found in melts and specified in rotational-isomeric-state (RIS) models, 2,3 the conformations of the generated chains often depart from that target. The discrepancies are very pronounced in polymers with bulky side chains such as atactic polystyrene (a-PS). 5,8,12 Solid-state NMR measurements showed that in a-PS these large deviations from the RIS conformations are mainly artifacts of the simulations and do not occur in the bulk polymer. 12 This contribution shows that atomistic simulations of glassy polymers with conformations close to RIS models can be generated by a suitable algorithm even if the polymer chains bear bulky side groups.To generate atomistic structures of bulk glassy polymers with periodic boundary conditions and chain conformations close to those found in RIS models, Mü ller et al. 18 developed a new algorithm, "PolyPack", that minimizes the nonbonded interactions within the subspace of chain conformations close to those of a given RIS model. This is in contrast to the amorphous-cell algorithm, where the conformations result from a tradeoff between the interactions due to the local conformation within a chain segment and those due to the packing of different segments. 13 Especially at the end of the chain-growing procedure used in the amorphous-cell algorithm, when the last monomer units are squeezed into the box, the interactions between different chain segments largely dominate and lead to large deviations from the RIS conformations. In this contribution, we present atomistic simulations of bulk a-PS and amorphous isotactic polystyrene (i-PS) with chain conformations close to the RIS models. These structures were generated with PolyPack and, in contrast to previous amorphous-cell simulations, agree well with solid-state NMR measurements sens...

Local Segmental Dynamics and Stresses in Polystyrene–C60 Mixtures

“…NMR measurements on atactic PS have helped gain insight into the conformations adopted by its chains. Suter and collaborators [51][52][53][54] The fusion of trans and gauche states is apparently produced by the atomistic potential at this high temperature, since it is also present in the single-chain system which has been equilibrated by MD. Comparing the curves in Figures Figure 5 and Figure 6 we observe that torsion angle distributions in the reverse-mapped structures are extremely close to those obtained from the 80mer structure that was directly equilibrated by MD at the atomistic level, without intervention of any coarse-graining and reverse mapping.…”

Magnetic orientation of diamagnetic amorphous polymers

“…Then, these samples would exhibit isotropic magnetic susceptibility and, at first, would show no preferential orientation when immersed in a magnetic field. In this regard, experimental and theoretical studies on a ‐PS in bulk have provided evidence of only short range order (∼6 Å) 28–31. However, studies in a ‐PS very thin films (i.e., thickness less than 500 nm) have shown that the phenyl groups at an interface air‐polymer tend to be oriented with the carbon in p‐position pointing away from the surface and the plane defined by the ring parallel to the surface normal,32–34 whereas at a solid–polymer interface the ring planes are oriented perpendicular to the surface normal 32.…”

Molecular Simulation Via Connectivity‐altering Monte Carlo and Scale‐jumping Methods: Application to Amorphous Polystyrene

“…Spyriouni et al were successful in preparing PS in the melt with correct global chain conformations; on the more local level, discrepancies were observed between their distributions of dyad conformations and those suggested by NMR data of Robyr et al [14][15][16] . This justifies renewed attempts, such as ours, to prepare well-equilibrated PS.…”

“…This justifies renewed attempts, such as ours, to prepare well‐equilibrated PS. Comparison to the NMR data of Robyr et al14–16 is performed in the separate publication 12. In general, our algorithm, which is applicable to other polymers than PS as well, is very useful when more detail than provided by 1:1 models is required.…”