(649b) Mechanism of Plasticization and Antiplasticization in Chitosan-Based Systems

Plasticization is often utilized to modify the thermomechanical properties of polymeric materials for numerous applications. While the effects of plasticizers have been well characterized for a wide array of polymer-additive combinations, the molecular mechanism of plasticization is not fully understood. We employ molecular dynamics simulations to characterize the mechanism of plasticization and antiplasticization in chitosan-water systems with a focus on nanoscale polymer-additive interactions and their effects on the thermomechanical properties of the material. We evaluate the validity of three proposed mechanisms on the plasticization phenomena described by the free volume, lubricity, and gel theories. We reveal a strong correlation between material free volume and polymer segment mobility indicated by the Debye-Waller factor. We analyze the polymer-water hydrogen-bonding characteristics and highlight significant differences in water orientation profiles at various concentrations. We highlight specific chemical groups that strongly influence the plasticization and antiplaticization mechanism via interactions with water molecules. Furthermore, we analyze the evolution of nano- and macroscale properties at the antiplasticization to plasticization crossover concentration to establish a structure-property relationship. We support our findings with the analysis of system dynamics at each concentration. We highlight the stability of hydrogen bonds formed by water molecules connecting two polymer chains, resulting in slower chain dynamics. This study helps understand the effect of additives in polymer materials stemming from various molecular interactions. Our analysis allows more informed design of plasticized systems for applications including pharmaceutics, packaging, and agriculture.