Molecular dynamics simulations on heterogeneity and percolation of epoxy nanofilm during glass transition process

• Published in: Materials chemistry and physics 2018-07, Vol.213, p.239-248
• Main Authors: Wang, Zhikun, Lv, Qiang, Chen, Shenghui, Faller, Roland, Li, Chunling, Sun, Shuangqing, Hu, Songqing
• Format: Article
• Language: English
• Subjects: Asynchronous; Atomic mobilities; Atoms & subatomic particles; Computer simulation; Domains; Dynamic percolation; Glass; Glass transition; Glass transition temperature; Graphene; Heterogeneity; Molecular dynamics; Percolation; Polymer nanofilm; Temperature dependence
• ISSN: 0254-0584; 1879-3312
• DOI: 10.1016/j.matchemphys.2018.04.040

This study uses an all-atom computational model to investigate the temperature dependent heterogeneity and percolation in a nanofilm system of short linear epoxy chains on a solid graphene surface. The heterogeneity, which indicates having physical characters that vary within the nanofilm, is mainly manifested in distributions of volume, energy and the dynamic properties. Local glass transition temperatures, Tgs, from above properties depend largely on the separation to the graphene surface, and the glass transition of the nanofilm is asynchronous along the film normal. Distinct Tg increases and decreases are particularly observed in the solid and free interfaces, respectively, compared with the bulk. From the dynamic heterogeneity, percolation effect, which indicates the connectivity of mobile and immobile domains, of the nanofilm is also observed during glass transition by plotting internal atomic mobility distribution diagrams. A multi-stage percolation mechanism based on the glass transition state and the connection state of immobile and mobile domains of the nanofilm is developed. A relatively immobile domain near the graphene surface is observed, even at temperatures much higher than Tg, and it initiates the dynamic percolation. The interconnection of immobile domains after percolation accelerate the transition from the rubbery to the glassy state. [Display omitted] •Film heterogeneity manifests in overall and local volume, energy and dynamic.•Solid and free interfaces foster asynchronous glass transition of the nanofilm.•Percolation of immobile domains relates glass transition to film heterogeneity.•Immobile domains initiate and accelerate film percolation during glass transition.