Self-organization of gel networks formed by block copolymer stars

Abstract

The equilibrium properties of block copolymer star networks (BCS) are studied via computer simulations.We employ both molecular dynamics and multiparticle collisional dynamics simulations to investigate the self-organization of BCS with f = 9 functionalized arms close to their overlap concentrations under conditions of different fractions of functionalization and varying attraction strength. We find three distinct macroscopic self-organized states depending on fraction of attractive end-monomers and the strength of the attraction. At weak attractions, ergodic, diffusive liquids result, with short-lived bonds between the stars. As the attraction strength grows, the whole system forms a percolating cluster, while at the same time the individual molecules are diffusive. Finally, arrested gels emerge when the attractions become strong. The conformation of the BCS in these solutions is found to be strongly affected by the concentration, with the stars assuming typically spherical, open configurations in seeking to maximize inter-star associations as opposed to the inter-star collapse that results at infinite dilution, giving rise to strongly aspherical shapes and reduced sizes.

Aggregation shapes of amphiphilic ring polymers: from spherical to toroidal micelles

“…Moreover, here and in what follows, it can be seen that the micellar core often features a tight packing of the attractive beads, a feature which, combined with rigidity, leads to visually recognizable crystalline arrangements. Though we have not attempted to perform a detailed analysis of the crystallinity of these domains, we point out that the phenomenon is common in other micelle forming systems, such as the recently investigated moderate functionality block copolymer stars [22]. It should further be mentioned that a temperature change also affects the persistence length of the rings and thus their apparent flexibility, despite the fact that κ is kept fixed; nevertheless, the major temperature effects observed are due to the influence of the same on the relative attraction strength of the solvophobic monomers.…”

Dynamical Properties of Concentrated Suspensions of Block Copolymer Stars in Shear Flow

“…The reason for this choice is based on our previous equilibrium studies on BCS network formation. 13 For f = 9, N = 30, and the α-s considered, single isolated stars are only partially self-assembled, indicating that there are some arms clustered together by their attractive segments while others are unbounded. We also note that although the model employed here is the same as the one used by Rovigatti et al, 5 the single-star configuration there is different for f = 9 and λ = 1.2 because stars with much longer arms ( N > 120) are considered in that work.…”

“…We simulate BCSs through a bead-spring model with monomers being soft spheres of diameter σ and mass M . Following our previous work, 13 simulations were performed on BCSs of functionality f = 9 and a polymerization degree N = 30 for each arm. The arms are all connected to a central bead (anchor) of diameter σ a = 1.5σ and mass M .…”

“… 5 On the other hand, changes in the same parameters in dilute solutions of BCSs may lead the system to microphase separation, that is, the assembly of micelle-like aggregates, macrophase separation, that is, demixing, or the formation of percolating networks. 12 , 13 …”

“…Because it is expected that higher-functionality BCSs facilitate the formation of stiffer networks than lower-functionality BCSs and similar mass, 6 the aim of this work is to complement our previous in-equilibrium study of network-forming BCSs of moderate functionality. 13 We systematically analyze the effect of shear flow on the global structure and molecular conformations of the BCSs in reversible percolating systems, which are not too dense yet topologically interesting. For these systems, our results suggest that the orientation and elongation of the BCSs induced by the shear flow give rise to reorganization of both the patchiness and network connectivity, leading to shear thinning behavior with the thinning exponent growing in magnitude with the concentration of the solution.…”