# Many-Chain Effects on the Co-nonsolvency of Polymer Brushes in a Good Solvent Mixture

## TL;DR

This study uses molecular dynamics simulations to explore the complex effects of many-chain interactions on the co-nonsolvency phenomenon in polymer brushes within good solvent mixtures, revealing new scaling laws and structural behaviors.

## Contribution

It introduces a detailed analysis of many-chain effects on co-nonsolvency, including a novel scaling law for brush height and a phase diagram based on structural parameters.

## Key findings

- Brush height scales as h ∼ σ_g^0.71 in co-nonsolvent mixtures
- Formation of intermolecular and intramolecular bridging structures occurs
- Grafting density influences looping, thickness, and orientation of polymers

## Abstract

Polymer brushes normally swell in a good solvent and collapse in a poor solvent. An abnormal response of polymer brushes, so-called co-nonsolvency, is the phenomenon where the brush counter-intuitively collapses in a good solvent mixture. In this work, we employed molecular dynamics simulations to investigate the structural properties of the grafted polymers on the occurrence of co-nonsolvency. Brushes with various grafting densities were considered to study the effect of topological excluded volumes on the co-nonsolvency. We found that the brush height follows a novel scaling behavior of the grafting density $h \sim \sigma_{\text g}^{0.71}$ in the co-nonsolvent mixture. Using the scaling exponent and Alexander-de Gennes theory, an analytic function that predicts the monomer density was obtained. The many-chain effects in the co-nonsolvent lead to the formation of both intermolecular and intramolecular bridging structures. Increasing the grafting density entails lower looping events occuring because of the intermolcular bridging, causing diverse structural properties. We report how the average thickness, the polymer orientation, and the looping probability varies as the grafting density increases. Based on these observations, we constructed a phase diagram of the polymer brush system using the average thickness and orientation as order parameters. Our simulations and analytical results reveal the nature of co-nonsolvency in polymer brushes in an explicit way and will help to provide practical guidelines for applications such as drug delivery and sensor devices.