Miscible Polyisoprene/Polystyrene Blends: Distinct Segmental Dynamics but Homogeneous Terminal Dynamics

The effect of chemical constitution on polyisoprene dynamics

Polyisoprene (PI) melts have been studied, with most reports focusing on systems with high 1,4-cis content. In contrast, 1,4-trans PI homopolymers or random copolymers have seldom been examined, despite a handful of investigations suggesting a distinct dynamic behavior. Herein, we employ all-atom simulations to investigate the effect of chemical architecture on the dynamics of cis and trans-PI homopolymers, as well as copolymers. We examine the thermodynamic, conformational, and structural properties of the polymers and validate the performance of the models. We probe chain dynamics, revealing that cis-PI presents accelerated translation and reorientation modes relative to trans as recorded by the mean square displacement of the chain center-of-mass as well as by the characteristic times of the lower modes in a Rouse analysis. Interestingly, progressing to higher modes, we observe a reversal with trans units exhibiting faster dynamics. This was further confirmed by calculations of local carbon-hydrogen vector reorientation dynamics, which offer a microscopic view of segmental mobility. To obtain insight into the simulation trajectories, we evaluate the intermediate incoherent scattering function that supports a temperature-dependent crossover in relative mobility that extends over separations beyond the Kuhn-length level. Finally, we analyzed the role of non-Gaussian displacements, which demonstrate that cis-PI exhibits increased heterogeneity in dynamics over short-timescales in contrast to trans-PI, where deviations persist over times extending to terminal dynamics. Our all-atom simulations provide a fundamental understanding of PI dynamics and the impact of microstructure while providing important data for the design and optimization of PI-based materials.

Reorientational dynamics of highly asymmetric binary non-polymeric mixtures – a dielectric spectroscopy study

Two separated relaxations α₁ and α₂ with different temperature dependences are identified in the mixtures. They are attributed to the dynamics associated with the high-T_g (α₁) and the low-T_g component (α₂) with distinct T_g concentration dependences.

Segmental dynamics in miscible polymer blends: recent results and open questions

In this short review we summarize the outcome of the large amount of effort made during the past decade from both the experimental and the theoretical point of view in order to understand the effect of blending on the segmental dynamics in polymers. Each of the two families of models proposed-one based on thermally activated concentration fluctuations, the other on chain connectivity effects-account for each of the two main experimental observations: the broadening of the component response with respect to that of the homopolymer and the dynamic heterogeneity, respectively. The complementarity of these approaches, their main achievements and failures, are critically revised. We also include recent results on blends of components with very different mobilities. In the neighbourhood of the glass-transition of the slow polymer, the dynamics of the other component seem to be confined within the frozen chains. We suggest possible ingredients and new routes to be considered in order to elaborate more predictive theoretical frameworks for all these phenomena.

Phase separation in polyisoprene/polystyrene blends by a systematically coarse-grained model

The authors have successfully developed a structurally coarse-grained 1,4-cis-polyisoprene-atactic polystyrene blend model by systematic mapping between a detailed atomistic model and a mesoscale model. This is to their best knowledge the first time that a chemically specific polymer blend model has been used to study the phase separation morphology and kinetics in a blend. A structurally optimized force-field model has many advantages over simple bead-spring models in terms of representing the chain microstructure. It keeps the identity of the polymers, particularly the structure through radial distribution functions. Starting from randomly mixed initial configurations, the blends show a clear phase separation for chain lengths around 10 monomers and this separation becomes more pronounced with the increase of chain length. The ensuing morphology is lamellar at equiweight concentrations and cylindrical or spherical at unbalanced concentrations. Morphologies are validated to be stable under increasing system sizes and further characterized quantitatively by density profiles.

Proteins: is the folding process dynamically encoded?

Recent findings for nano-structured polymeric systems support the existence of dynamic heterogeneities in glass-forming materials. The implications for structure formation in proteins are considered. The high speed and efficiency of the folding process might be explainable based on differences in the slow segmental mobility along the protein chain.