Molecular Dynamics Study on the Temperature Response of a Chitosan-Based Graft Polymer with Different Grafting Densities of Oligo(ethylene glycol) Methacrylate Side Chains

Chitosan-based thermoresponsive graft polymers serve as promising functional materials for sustainable and intelligent applications, while their temperature-induced phase transition mechanism associated with the two components of the backbone and side chain remains elusive. Here, we perform molecular dynamics simulations from 300 to 390 K to explore the lower critical solution temperature (LCST) behavior of a graft polymer (CMCS-g-OEGMA) with carboxymethyl chitosan (CMCS) as the backbone and oligo(ethylene glycol) methyl ether methacrylate (OEGMA) as the side chain. The simulations show that a high grafting density (>0.5, especially when all structural units are fully occupied) is essential for LCST transition, at which a structural change of single-chain extended-to-collapsed and multichain dispersed-to-aggregated, a conformational transition from syn to anti, and a solvation structure disruption in cage-like first and second shells happen above LCST. The driving effect for this transition comes from the hydrophobic interaction of OEGMA brushes, energetically diminishing the original stabilizing interactions of the exo-anomeric effect, intramolecular hydrogen-bond networks, and steric solvation shells along the backbone, consequently enabling the phase transition stabilized by reformed intramolecular interactions in the rearranged collapsed chain structure.