Predicting polymer nanofiber interactions via molecular simulations

Physical and functional properties of nonwoven textiles and other fiberlike materials depend strongly on the number and type of fiber-fiber interactions. For nanoscale polymeric fibers in particular, these interactions are governed by the surfaces of and contacts between fibers. We employ both molecular dynamics (MD) simulations at a temperature below the glass transition temperature T(g) of the polymer bulk, and molecular statics (MS), or energy minimization, to study the interfiber interactions between prototypical polymeric fibers of 4.6 nm diameter, comprising multiple macromolecular chains each of 100 carbon atoms per chain (C100). Our MD simulations show that fibers aligned parallel and within 9 nm of one another experience a significant force of attraction. These fibers tend toward coalescence on a very short time scale, even below T(g). In contrast, our MS calculations suggest an interfiber interaction that transitions from an attractive to a repulsive force at a separation distance of 6 nm. The results of either approach can be used to obtain a quantitative, closed-form relation describing fiber-fiber interaction energies U(s). However, the predicted form of interaction is quite different for the two approaches, and can be understood in terms of differences in the extent of molecular mobility within and between fibers for these different modeling perspectives. The results of these molecular-scale calculations of U(s) are used to interpret experimental observations for electrospun polymer nanofiber mats. These findings highlight the role of temperature and kinetically accessible molecular configurations in predicting interface-dominated interactions at polymer fiber surfaces, and prompt further experiments and simulations to confirm these effects in the properties of nonwoven mats comprising such nanoscale fibers.