Molecular dynamics simulations of uniaxial deformation of thermoplastic polyimides.

The results of atomistic molecular-dynamics simulations of mechanical properties of heterocyclic polymer subjected to uniaxial deformation are reported. A new amorphous thermoplastic polyimide R-BAPO with a repeat unit consisting of dianhydride 1,3-bis-(3',4,-dicarboxyphenoxy)diphenyl (dianhydride R) and diamine 4,4'-bis-(4''-aminophenoxy)diphenyloxide (diamine BAPO) was chosen for the simulations. Our primary goal was to establish the impact of various factors (sample preparation method, molecular mass, and cooling and deformation rates) on the elasticity modulus. In particular, we found that the elasticity modulus was only slightly affected by the degree of equilibration, the molecular mass and the size of the simulation box. This is most likely due to the fact that the main contribution to the elasticity modulus is from processes on scales smaller than the entanglement length. Essentially, our simulations reproduce the logarithmic dependence of the elasticity modulus on cooling and deformation rates, which is normally observed in experiments. With the use of the temperature dependence analysis of the elasticity modulus we determined the flow temperature of R-BAPO to be 580 K in line with the experimental data available. Furthermore, we found that the application of high external pressure to the polymer sample during uniaxial deformation can improve the mechanical properties of the polyimide. Overall, the results of our simulations clearly demonstrate that atomistic molecular-dynamics simulations represent a powerful and accurate tool for studying the mechanical properties of heterocyclic polymers and can therefore be useful for the virtual design of new materials, thereby supporting cost-effective synthesis and experimental research.

A coarse-grained molecular dynamics study of segmental structure and mobility in capped crosslinked copolymer films.

We present results from molecular-dynamics simulations of a generic bead-spring model of copolymer chains confined between solid walls and report on the glass-transition temperature and segmental dynamics as a function of film thickness and mesh size (the end-to-end distance of the subchains in the crosslinked polymer networks). Apparently, the glass-transition temperature displayed a steep increase for mesh-size values much smaller than the radius of gyration of the bulk chains, otherwise it remained invariant to mesh-size variations. The rise in the glass-transition temperature with decreasing mesh size and film thickness was accompanied by a monotonic slowing-down of segmental dynamics on all studied length scales. This observation is attributed to the correspondingly decreased width of the bulk density layer that was obtained in films whose thickness was larger than the end-to-end distance of the bulk polymer chains. To test this hypothesis, additional simulations were performed in which the crystalline walls were replaced with amorphous or rough walls. In the amorphous case, the high polymer density close to the walls vanished, but the dynamic response of the film was not affected. The rough walls, on the other hand, only slightly decreased the density close to the walls and led to a minor slowing-down in the dynamics at large length-scales.

Glassy boundary layers vs enhanced mobility in capped polymer films.

Molecular-dynamics simulations have been carried out for a coarse-grained model of a random AB-copolymer confined between two crystalline substrates. The strength of substrate-polymer interactions, and the distance between the two substrates have been varied in a wide range. For thick films the film-averaged segmental mobility decreases for intermediate adsorption strengths, but start to increase for very high substrate-polymer attraction strength. We saw that this non-monotonic behavior is caused by a very strong heterogeneity of the segmental dynamics above the glass-transition temperature: the segmental mobility slows down drastically close to adsorbing substrates, but strongly increases in the middle part of the film. This effect, and its sensitive dependence on film thickness, are explained by finite-size effects in confinement, in combination with glassy boundary layers. It is demonstrated that film-averaged mobility as often measured cannot be understood without resolving local mobility in space and time.

Mechanical properties and local mobility of atactic-polystyrene films under constant-shear deformation.

We have performed molecular-dynamics simulations of atactic polystyrene thin films to study the effect of shear rate, pressure, and temperature on the stress-strain behaviour, the relevant energetic contributions and non-affine displacements of polymer chains during constant-shear deformation. Under this deformation sliding motion is observed at high shear rates between the top substrate and top polymer layer, which disappears when the shear rate decreases. At low shear rates stick-slip motion of the whole film with respect to the bottom substrate takes place. We found that at low shear rates the yield stress logarithmically depends on the shear rate; this behaviour can be explained in terms of the Eyring model. It was also observed that an increase in the normal pressure leads to an increase in the yield stress in agreement with experiments. The contributions to the total shear stress and energy are mainly given by the excluded-volume interactions. It corresponds to a local translational dynamics under constant shear in which particles are forced to leave their original cages much earlier as compared to the case of the isotropic, non-sheared film. Moreover, it was observed that under constant-shear deformation the polymer glass is deformed non-affinely. As a result, the middle part of the film is much more deformed than the layers close to the supporting substrates, meaning that the well-known effect of shear-banding occurs.

Shear-induced effects in hyperbranched-linear polyelectrolyte complexes.

Static and dynamic properties of complexes formed by hyperbranched polymers with linear polyelectrolytes are studied under the influence of steady shear flow by means of Brownian dynamics simulations. Models of peripherally charged hyperbranched molecules bearing two extreme topological structures and different molecular weights complexed with linear neutralizing chains are subjected to a range of shear rates starting from a low-shear regime toward the complex-breaking point. Examination of the stability limit, shape and mass distribution parameters, and dynamics in different lengths and timescales is performed as a function of the applied shear. The results described illustrate features of the generic behavior that should be expected from such systems under conditions of steady shear flow.

Effects of topology and size on statics and dynamics of complexes of hyperbranched polymers with linear polyelectrolytes.

Brownian dynamics simulations with explicit hydrodynamic interactions have been employed to study generic effects of size and topology in noncovalent (Coulombic-driven) complexes formed by irregular-shaped hyperbranched polymers and linear polyelectrolytes. The behavior of the complexes was explored in detail in terms of static and dynamic properties, both in local and in the entire complex scale. The results were compared to previous studies on perfect dendrimers and other hyperbranched molecules where available. It was found that both molecular weight and structure may impart significant changes to key factors known to be associated with the ability of these systems to take part in relevant nanoscale applications.

Local polymer dynamics under strong connectivity constraints: the dendrimer case.

The characteristics of local motion are explored by molecular dynamics simulations in a series of AB(2)-type dendrimer melts. Systems of generations 3-5 were simulated in a wide temperature range, allowing the assessment of effects associated with molecular size, proximity to the detected glasslike transitions, and the strong connectivity constraints imposed by the dendritic topology. Investigation of the mechanisms involved in local motion at short temporal and spatial scales revealed the connection between the non-Gaussian nature of monomer displacements to alpha-relaxation and the caging/decaging process under different degrees of confinement. In the latter mechanism, two characteristic localization lengths were identified: at the low temperature limit spatial localization was realized within approximately 10% of the nearest neighbor distance while at temperatures higher than the glass transition, the existence of an analogous length scale is ascribed to the geometric constraints due to the dense connectivity pattern. As the results from this study are discussed in comparison to the behavior observed in linear polymers and supercooled liquids, new insight is provided on the universal/specific mechanisms involved in local dynamics of different glass-forming systems.