Molecular dynamics simulation of size-dependent structural and thermal properties of polymer nanofibers

We present the results of molecular dynamics (MD) simulations of amorphous polymer nanofibers to study their size-dependent properties. The fibers consist of chains that mimic the prototypical polymer polyethylene, with chain lengths ranging between 50 and 300 carbons (C50 to C300). These nanofibers,have diameters in the range 1.9 to 23.0 nm. We analyzed these nanofibers for signatures of emergent behavior in their structural and thermal properties as a function of diameter. The mass density at the center of all fibers is constant and comparable to that of the bulk polymer. The surface layer thickness ranges from 0.78 to 1.39 nm for all fibers and increases slightly with fiber size. The calculated interfacial excess energy is 0.022 +/- 0.002 J/m(2) for all of the nanofibers simulated. The chains at the surface are more confined compared to the chains at the center of the nanofiber; the latter acquire unperturbed dimensions in sufficiently large nanofibers. Consistent with experiments and simulations of amorphous polymer films of nanoscale thickness, the glass transition temperature of these amorphous nanofibers decreases with decreasing fiber diameter, and is independent of molecular weight over the range considered.