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.

The effect of composition on the true mechanooptical properties of thermoplastic poly-(urethane urea)s was investigated by selectively varying the type and content of soft and hard segments. Real-time stress-strain-birefringence data together with off-line wide-angle X-ray scattering measurements revealed that soft segment and chain extender play dominant roles on the chemical structures of the poly(urethane urea)s. All poly(tetramethylene oxide) glycol-based samples showed the same crystal structure. The samples containing ethylenediamine as the chain extender showed enhanced crystallizability as compared to those with 1,6-diaminohexane no matter which soft segment was used. In general, samples with lower fraction of hard segment exhibited higher crystallizability than their high hard segment counterparts. Long-term holding of poly(ethylene oxide) samples in stretched state was found to increase crystallinity. The strain-induced crystallization in low hard segment content poly(tetramethylene oxide)based samples was only observed at very high deformation levels. On the other hand, crystallization in the samples containing high hard segment was found to evolve gradually over large deformation range. The strain rate has a considerable effect on the crystallization behavior of poly(tetramethylene oxide)based samples. While the low hard segment content poly(tetramethylene oxide) sample experiences decreasing crystallizability as the strain rate increases, its counterpart containing higher fraction of hard segments exhibits opposite behavior. We have investigated linear and nonlinear stress optical behavior and observed that the span of the initial linear stress optical region varied primarily with composition (slope ranging from 0.1 to 2.2 GPa -1 ) and secondarily with the deformation rate. Hysteresis experiments show that there is a considerable loss of energy in cyclic loading of these materials, and hysteresis increases as the chain extender is changed from 1,6-diaminohexane to ethylenediamine.

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