Star Block-Copolymers in Shear Flow

Abstract

Star block-copolymers (SBCs) have been demonstrated to constitute self-assembling building blocks with specific softness, functionalization, shape, and flexibility. In this work, we study the behavior of an isolated SBC under a shear flow by means of particle-based multiscale simulations. We systematically analyze the conformational properties of low-functionality stars, as well as the formation of attractive patches on their corona as a function of the shear rate. We cover a wide range of system parameters, including functionality, amphiphilicity, and solvent quality. It is shown that SBCs display a richer structural and dynamical behavior than athermal star polymers in a shear flow [ Ripoll Phys. Rev. Lett. , 2006 , 96 , 188302 ], and, therefore, they are also interesting candidates to tune the viscoelastic properties of complex fluids. We identify three factors of patch reorganization under shear that lead to patch numbers and orientations depending on the shear rate, namely, free arms joining existing patches, fusion of medium-sized patches into bigger ones, and fission of large patches into two smaller ones under high shear rates. Because the conformation of single SBC is expected to be preserved in low-density bulk phases, the presented results are a first step in understanding and predicting the rheological properties of semidilute suspensions of this kind of polymers.

Single-Chain Nanoparticles under Homogeneous Shear Flow

“…28 We have characterized the dependence of several observables of interest measuring the size, orientation and intrinsic viscosity, on the applied shear rate. The obtained power-laws have characteristic exponents that are clearly different from those found in other architectures (linear chains, rings, stars, dendrimers [28][29][30][31][32][33][34][35][36][37][38] ). Thus, SCNPs constitute a novel class of macromolecules with distinct response to shear.…”

Rotation Dynamics of Star Block Copolymers under Shear Flow

“…On the other opposite limit, at high α and λ (α 0.6 and λ 1.1), the macromolecule acquires cylindrical symmetry around its principal axis, since it self-assembles into dumbbell-like structures with one or two massive patches (Case 3). At intermediate values of α and λ, the SBCs form a number of patches that can break-up and/or merge as a consequence of shear (Case 2) [9][10][11]. These three tendencies can be also observed from the dynamical point of view, as displayed in Fig.…”

“…We focus on the following three particular sets of parameters: { f , α, λ} = {12, 0.3, 1.0} (Case 1), {15, 0.5, 1.1} (Case 2) and {18, 0.7, 1.1} (Case 3). According to our previous study, these parameters represent the typical trends found in regard to the patchiness of the SBCs, namely, no patches are formed, several patches are formed having a small population, and few (one or two) bulky patches are formed [11]. For each run, a preparation cycle of 5 × 10 6 MD steps was executed in first place, which was long enough for the SBC to reach its stationary state and then, a production cycle of 1.5 × 10 7 MD steps took place.…”

“…The dynamics of the case 2 is richer; although this system features four patches in average [11], the shear causes those patches to break and to cluster over and over again. Therefore, here the rotational frequency results from the average of the tank-treading motion of free and clustered arms.…”

“…In particular, the formation of attractive patches on the SBC corona 2 of 20 as a function of the shear rate was analysed. Three mechanisms of patch reorganization under shear were identified, which determine the dependence of the patch numbers and orientations on the shear rate, namely, free arms joining existing patches, fusion of medium-sized patches into bigger ones, and fission of large patches into two smaller ones at high shear rates [11].…”