Structure and dynamical intra-molecular heterogeneity of star polymer melts above glass transition temperature

Abstract

Structural and dynamical properties of star melts have been investigated with molecular dynamics simulations of a bead-spring model. Star polymers are known to be heterogeneous, but a systematic simulation study of their properties in melt conditions near the glass transition temperature was lacking. To probe their properties, we have expanded from linear to star polymers the applicability of Dobkowski's chain-length dependence correlation function [Z. Dobkowski, Eur. Polym. J. 18, 563 (1982)]. The density and the isokinetic temperature, based on the canonical definition of the laboratory glass-transition, can be described well by the correlation function and a subtle behavior manifests as the architecture becomes more complex. For linear polymer chains and low functionality star polymers, we find that an increase of the arm length would result in an increase of the density and the isokinetic temperature, but high functionality star polymers have the opposite behavior. The effect between low and high functionalities is more pronounced for short arm lengths. Complementary results such as the specific volume and number of neighbors in contact provide further insights on the subtle relation between structure and dynamics. The findings would be valuable to polymer, colloidal, and nanocomposites fields for the design of materials in absence of solution with the desired properties.

Self-assembly of polymer-grafted nanoparticles in solvent-free conditions

“…1(b), where the size of the GNP is D / σ = 1; the particular system corresponds to a star polymer melt. 52,53 If this liquid-like structure is maintained for the whole temperature range while the material is in the liquid state, then the material is called a “star-liquid”; see Fig. 1.…”

“…The glass transition temperature is also approximately outlined based on star polymers having f = 8. 53,54 …”

Communication: When does a branched polymer become a particle?

“…Despite the good agreement between the correlation function (see the supplementary material for details 23 ) and the density results, a subtle feature is apparent. We see an inverted behavior for the stars with f in the sense of the direction with which the density deviates from its bulk average value for large M. 37 The crossover between these two extremes occurs for star polymers of f = 6, where ρ b is nearly independent of arm mass M (also total star mass M w ). It is also striking that stars with f = 6 exhibit a similar behavior to ring polymers, both in terms of the value of ρ b as well as the lack of a ρ b 's variation with M w (see Fig.…”

“…According to the free chainends rationale, stars should pack space rather differently than linear and ring polymers since increasing the number of arms (free chain-ends) should increase the "free volume" (decrease ρ b ). Instead, a recent computer simulation study 37 demonstrated that high functionality stars pack more efficiently, resulting in a higher ρ b . An extension of these results is presented in Fig.…”

“…For star melts, the T g dependence on both M and f is complex due to the intra-molecular heterogeneous behavior in the structure and the dynamics, 37 and so, here, we focus (see Fig. 2) on stars with f ≤ 8.…”