Dynamic Equivalence between Soft Star Polymers and Hard Spheres

TL;DR

This paper demonstrates that soft star polymers exhibit dynamics similar to hard spheres when their osmotic compressibility is matched, revealing a dynamic equivalence despite their complex interactions.

Contribution

It introduces a compressibility equivalence condition to evaluate soft colloids and shows their long-time diffusivity behavior maps onto hard-sphere dynamics.

Findings

Long-time self-diffusivity of star polymers can be mapped onto hard-sphere behavior.

The compressibility equivalence provides a practical way to compare soft and hard colloids.

Soft colloids exhibit dynamic behavior similar to hard spheres at high concentrations.

Abstract

Understanding the dynamics of soft colloids, such as star polymers, dendrimers, and microgels, is of scientific and practical importance. It is known that the excluded volume effect plays a key role in colloidal dynamics. Here, we propose a condition of compressibility equivalence that provides a simple method to experimentally evaluate the excluded volume of soft colloids from a thermodynamic view. We apply this condition to survey the dynamics of a series of star polymer dispersions. It is found that as the concentration increases, the slowing of the long-time self-diffusivity of the star polymer, normalized by the short-time self-diffusivity, can be mapped onto the hard-sphere behavior. This phenomenon reveals the dynamic equivalence between soft colloids and hard spheres, despite the apparent complexity of the interparticle interaction of the soft colloids. The methods for measuring the osmotic compressibility and the self-diffusivities of soft colloidal dispersions are also presented.