Molecular dynamics of polyisoprene/polystyrene oligomer blends: The role of self-concentration and fluctuations on blend dynamics

Abstract

1The 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, τ term /τ seg,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.2

Structural and Dynamical Properties of Polyethylene/Graphene Nanocomposites through Molecular Dynamics Simulations

“…This has been recently recognized also in other hybrid polymeric systems, such as miscible polymer blends [32]. This will be the subject of a future work.…”

Regimes of streaming potential in cylindrical nano-pores in presence of finite sized ions and charge induced thickening: An analytical approach

“…31,32 Miniaturized devices have been employed for a wide variety of applications ranging from chemical separation, [33][34][35][36][37][38][39][40] mixing, [41][42][43][44][45][46][47][48][49] analysis, 35,50 DNA hybridization, [51][52][53][54][55] etc. Realizing the importance and wide-range applicability of these phenomena, there have been several attempts to theoretically model the coupled electrical and hydrodynamic phenomena.…”

Multiscale Molecular Simulations of Polymer-Matrix Nanocomposites

“…Specifically, when a chain makes a single contact, dynamics is only decelerated to a small extent. The second important finding was that, as train segments grow in length along the surface, the dynamics of the repeat units becomes progressively slower, with the findings implying that, similar to chain-ends in bulk dynamics, [ 246 , 247 ] the ends of train segments contribute to increased dynamic heterogeneities on the surface. However, the former are only significant for short chains, the latter were present for any chain length studied.…”

“…In addition, short train segments can be more pronounced around surfaces with higher curvature [ 16 , 159 ]. Finally, a PI specific result was the asymmetry present along a train segment originating from the methyl group, much alike findings on bulk dynamics along the chain backbone [ 247 ].

Fig.

…”