Confinement effect of inter-arm interactions on glass formation in star polymer melts

Abstract

We utilized molecular dynamic simulation to investigate the glass formation of star polymer melts in which the topological complexity is varied by altering the number of star arms (f). Emphasis was placed on how the “confinement effect” of repulsive inter-arm interactions within star polymers influences the thermodynamics and dynamics of star polymer melts. All the characteristic temperatures of glass formation were found to progressively increase with increasing f, but unexpectedly the fragility parameter KVFT was found to decrease with increasing f. As previously observed, stars having more than 5 or 6 arms adopt an average particle-like structure that is more contracted relative to the linear polymer size having the same mass and exhibit a strong tendency for intermolecular and intramolecular segregation. We systematically analyzed how varying f alters collective particle motion, dynamic heterogeneity, the decoupling exponent ζ phenomenologically linking the slow β- and α-relaxation times, and the thermodynamic scaling index γt. Consistent with our hypothesis that the segmental dynamics of many-arm star melts and thin supported polymer films should exhibit similar trends arising from the common feature of high local segmental confinement, we found that ζ increases considerably with increasing f, as found in supported polymer films with decreasing thickness. Furthermore, increasing f led to greatly enhanced elastic heterogeneity, and this phenomenon correlates strongly with changes in ζ and γt. Our observations should be helpful in building a more rational theoretical framework for understanding how molecular topology and geometrical confinement influence the dynamics of glass-forming materials more broadly.

Glass Formation in Ring Polymer Melts Having Variable Knot Complexity and Molecular Mass

“…Moreover, we may expect a progressive reduction of fragility with increasing m c and, indeed, we see this trend, as shown in Section 3.4. The same trends are observed in many- arm star polymer melts with increasing f at a fixed polymer molecular mass, 61 consistent with the correspondence between knotted rings and star polymers in solution. 59 We now consider another property that is closely related to S(0).…”

“…58,59 We find as an overall effect that increasing m c in knotted ring melts leads to a substantial contraction of the relative size of the ring polymers and an increasing average sphericity with respect to linear polymers, an effect that increases progressively with m c , as observed before in solution properties of knotted ring polymers. 59 The same trend is found for star polymers in solution 60 and in the melt 58,61 with increasing f. The densification effect induced by chain topology, in association with changes in average molecular shape and size in the knotted ring melts, directly leads to a reduction in the fragility of glass formation when the topological constraint becomes sufficiently large. We have observed exactly the same trends in many-arm star polymer melts with increasing f in our previous work.…”

“…A previous work 60 comparing the solution properties of knotted rings and star polymers showed that varying m c had a similar influence on solution properties of ring polymers as varying the number of star arms f, and we anticipated that the same correspondence should hold also in the melt, and the present work, among other things, provides a check of this hypothesis. These general considerations motivated us to study the same thermodynamics and segmental dynamics properties of knotted ring melts at fixed values of m c and polymer mass M, and to compare these results with many-arm star polymers having variable f and fixed M. 61 We also consider how variable mass influences the nature of glass formation in knotted ring As we anticipated, the general trends that we observe for the thermodynamic and segmental dynamic properties of our model knotted ring polymer melts with variable m c and fixed M are confirmed to be rather similar to those observed for manyarm star polymer melts with variable f and fixed M. Specifically, the radius of gyration R g of the ring polymers becomes likewise progressively reduced with increasing topological complexity (m c ) and their average shape, as quantified by the eigenvalues of the radius of gyration tensor, becomes more spherical with increasing m c . Evidently, the correspondence between the knotted ring polymers and many-arm star polymers identified by Vargas-Lara et al 60 extends at least qualitatively to the polymer melt regime.…”

“…This section focuses on the thermodynamic properties that have been shown by both the GET 26,27 and simulation 35,121 to be relevant to the understanding of polymer glass formation. Because we did not find anything particularly unexpected in the T dependence of the thermodynamic properties of ring polymer melts, which our previous studies have discussed in detail for linear polymers, 35,61,121 we focus on their variations with m c and M at fixed T to illustrate the importance of topological rigidification in determining these properties.…”

Unexpected Stress Overshoot in Extensional Flow of Star Polymer Melts

“…We note that for star polymers, the entanglements on the arms near the core of the star must be less compared to that of the outer part, especially with larger arm number f and larger arm length M a . The differences of relaxations between the segments near the free-ends and those near the branch point signify an intramolecular heterogeneous behavior as already reported in literature. ,,− During the stretching of chains, the dynamical heterogeneities of stars could be amplified, since even linear entangled polymers may exhibit heterogeneities in nonlinear tension as shown in molecular dynamics simulations . Therefore, as illustrated in Figure b, our hypothesis is that the not-well-entangled part of the arms near the core are initially highly stretched in extensional flow due to the entanglements of the outer part which act as ties.…”