Molecular Dynamics of Polyisoprene/Polystyrene Oligomer Blends: The role of self-concentration and fluctuations on blend dynamics

The effect of self-concentration and intermolecular packing on the dynamics of polyisoprene (PI)/polystyrene (PS) blends is examined by extensive atomistic simulations. Direct information on local structure of the blend system allows a quantitative calculation of self- and effective composition terms at various length scales that are introduced to proposed models of blend dynamics. Through a detailed statistical analysis, the full distribution of relaxation times associated with reorienation of carbon-hydrogen bonds was extracted and compared to literature experimental data. A direct relation between relaxation times and local effective composition is found. Following an implementation of a model involving local composition as well as concentration fluctuations the relevant length scales characterizing the segmental dynamics of both components were critically examined. For PI the distribution of times becomes narrower for the system with the lowest PS content and then broadens as more PS is added. This is in contrast to the slow component (PS), where an extreme breadth is found for relaxation times in the 25/75 system prior to narrowing as we increase PI concentration. The chain dynamics was directly quantified by diffusion coefficients as well as the terminal (maximum) relaxation time of each component in the mixed state. Strong coupling between the friction coefficients of the two components was predicted that leads to very similar chain dynamics for PI and PS, particularly for high concentrations of PI. We anticipate this finding to the rather short oligomers (below the Rouse regime) studied here as well as to the rather similar size of PI and PS chains. The ratio of the terminal to the segmental relaxation time, τterm/τseg,c, presents a clear qualitative difference for the constituents: for PS the above ratio is almost independent of blend composition and very similar to the pure state. In contrast, for PI this ratio depends strongly on the composition of the blend; i.e. the terminal relaxation time of PI increases more than its segmental relaxation time, as the concentration of PS increases, resulting into a larger terminal/segmental ratio. We explain this disparity, based on the different length scales characterizing dynamics. The relevant length for the segmental dynamics of PI is about 0.4-0.6 nm, smaller than chain dimensions which are expected to characterize terminal dynamics, whereas for PS associated length scales are similar (about 0.7-1.0 nm) rendering a uniform change with mixing

Local Structure and Dynamics of Trans-polyisoprene oligomers

Mono- and poly-disperse melts of oligomers (average length 10 monomers) of trans-1,4-polyisoprene are simulated in full atomistic detail. The force-field is developed by means of a mixture of ab initio quantum-chemistry and an automatic generation of empirical parameters. Comparisons to NMR and scattering experiments validate the model. The local reorientation dynamics shows that for C$-$H vectors there is a two-stage process consisting of an initial decay and a late-stage decorrelation originating from overall reorientation. The atomistic model can be successfully mapped onto a simple model including only beads for the monomers with bond springs and bond angle potentials. End-bridging Monte Carlo as an equilibration stage and molecular dynamics as the subsequent simulation method together prove to be a useful method for polymer simulations.Comment: 25 pages, 15 figures, accepted by Macromolecule

Self-Association of Models of Transmembrane Domains of ErbB Receptors in a Lipid Bilayer

Association of transmembrane (TM) helices is facilitated by the close packing of small residues present along the amino-acid sequence. Extensive studies have established the role of such small residue motifs (GxxxG) in the dimerization of Glycophorin A (GpA) and helped to elucidate the association of TM domains in the epidermal growth factor family of receptors (ErbBs). Although membrane-mediated interactions are known to contribute under certain conditions to the dimerization of proteins, their effect is often considered nonspecific, and any potential dependence on protein sequence has not been thoroughly investigated. We recently reported that the association of GpA is significantly assisted by membrane-induced contributions as quantified in different lipid bilayers. Herein we extend our studies to explore the origin of these effects and quantify their magnitude using different amino-acid sequences in the same lipid environment. Using a coarse-grained model that accounts for amino-acid specificity, we perform extensive parallel Monte Carlo simulations of ErbB homodimerization in dipalmitoyl-phosphatidylcholine lipid bilayers. A detailed characterization of dimer formation and estimates of the free energy of association reveal that the TM domains show a significant affinity to self-associate in lipid bilayers, in qualitative agreement with experimental findings. The presence of GxxxG motifs enhances favorable protein-protein interactions at short separations. However, the lipid-induced attraction presents a more complex character than anticipated. Depending on the interfacial residues, lipid-entropic contributions support a decrease of separation or a parallel orientation to the membrane normal, with important implications for protein function

Potential of mean force between two nanometer-scale particles in a polymer solution

This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder