Concentration-dependent shear response of multi-chain amphiphilic block copolymer self-assemblies

TL;DR

This study uses Brownian dynamics simulations to explore how multi-chain amphiphilic block copolymer assemblies respond to shear across different concentrations, morphologies, and hydrophobic fractions, revealing universal rheological behaviors.

Contribution

It systematically characterizes the shear response and morphology evolution of diblock and triblock copolymer assemblies in dilute and semi-dilute regimes, highlighting concentration-dependent structural transitions.

Findings

Shear induces morphology transitions from spherical to sheet-like structures.

Diblocks have higher equilibrium viscosity; triblocks maintain higher viscosity under flow.

Universal power-law scaling governs micellar relaxation dynamics across conditions.

Abstract

Amphiphilic block copolymers self-assemble into diverse nanoscale morphologies with significant implications for drug delivery. This work presents systematic Brownian dynamics simulations of multi-chain diblock and triblock copolymers across dilute and semi-dilute unentangled regimes, hydrophobic fractions, f of 0-1, and shear rates of 0-0.1 1/ns. In the dilute regime, quiescent conditions yield spherical micelles evolving to cigar-like structures at shear rate ~0.01 1/ns and fragmenting at higher shear; varying f produces dispersed chains (f=0), cigar-like (f=0.25), short cylindrical (f=0.5), and gnarled or worm-like (f=0.75) micelles, culminating in sheet-like phase-separated structures (f=1). While, in the semi-dilute regime, shear drives collective reorganisation toward sheet-like morphologies at moderate rates before fragmentation; the f-dependent progression yields cigar-like (f=0.25), sheet-like (f=0.5), and necklace micelles (f=0.75), with larger phase-separated domains at f=1. Rheological characterisation reveals a universal architectural inversion between equilibrium and flow conditions: diblocks show higher equilibrium viscosity while triblocks maintain superior viscosity under flow via bridging networks. Aggregation number scaling exponents of alpha=0.833 in dilute, consistent with star-to-crew-cut bounds of 0.8 to 1.0, and alpha=1.07 in semi-dilute confirm the concentration-driven transition between regimes. Viscoelastic analysis establishes universal non-terminal power-law scaling across all conditions, governed by micellar relaxation dynamics independent of concentration or topology. These findings provide valuable insights into tailoring the injectability and flow behaviour of block copolymers in drug delivery formulations.