Thermal Degradation of Adsorbed Bottle-Brush Macromolecules: Molecular Dynamics Simulation

TL;DR

This study uses molecular dynamics simulations to investigate how adsorption on substrates accelerates the thermal degradation and bond scission of bottle-brush macromolecules, revealing key dependencies on chain length and side chain size.

Contribution

It provides new insights into the degradation kinetics of bottle-brush polymers on substrates, highlighting the impact of adsorption and molecular architecture on bond scission behavior.

Findings

Bond lifetime decreases significantly upon substrate adsorption.

Mean bond lifetime scales inversely with chain length and side chain size.

Fragment length distribution matches experimental data.

Abstract

The scission kinetics of bottle-brush molecules in solution and on an adhesive substrate is modeled by means of Molecular Dynamics simulation with Langevin thermostat. Our macromolecules comprise a long flexible polymer backbone with $L$ segments, consisting of breakable bonds, along with two side chains of length $N$, tethered to each segment of the backbone. In agreement with recent experiments and theoretical predictions, we find that bond cleavage is significantly enhanced on a strongly attractive substrate even though the chemical nature of the bonds remains thereby unchanged. We find that the mean bond life time $<\tau>$ decreases upon adsorption by more than an order of magnitude even for brush molecules with comparatively short side chains $N=1 \div 4$. The distribution of scission probability along the bonds of the backbone is found to be rather sensitive regarding the interplay between length and grafting density of side chains. The life time $<\tau>$ declines with growing contour length $L$ as $<\tau>\propto L^{-0.17}$, and with side chain length as $<\tau>\propto N^{-0.53}$. The probability distribution of fragment lengths at different times agrees well with experimental observations. The variation of the mean length $L(t)$ of the fragments with elapsed time confirms the notion of the thermal degradation process as a first order reaction.