The glass transition and interfacial dynamics of single strand fibers of polymers

We investigate the glass transition and interfacial dynamics of single strand fibers of flexible polymers by employing molecular dynamics (MD) simulations along with a coarse grained model. While the polymer fiber has drawn significant attention due to its applicability in tissue engineering and stretchable electronics, its dynamic properties, especially the glass transition temperature (T_g), are yet to be understood at the molecular level. For example, there has been a controversy on the effect of the polymer fiber radius (R) on T_g: T_g decreased with a decrease in R for some polymer fibers, whereas T_g of other polymer fibers was not sensitive to R. In this article, we estimate the bond relaxation time of polymers and evaluate both T_g and fragility (m) as a function of R. We illustrate that T_g of the polymer fiber decreased with a decrease in R monotonically and also that the values of T_g follow faithfully the empirical equation proposed by Keddie et al. as a function of R, which was successfully employed to fit the values of T_g of both polyvinyl alcohol (PVA) fibers and polyethylene (PE) fibers. We also find that the dynamics of polymers at the interface between a polymer fiber and air is faster than that of polymers at the center. By employing Adam-Gibbs theory, we show that the fast interface dynamics of polymer fibers should influence the cooperative motion of monomers, which should be responsible for the decrease in T_g for smaller values of R. Near the interface there are more mobile monomers that participate in the cooperative motions of polymers. Interesting is that due to the curved surface (unlike flat polymer films) the cooperative motion of monomers is anisotropic in polymer fibers.