Effect of supramolecular structure on polymer nanofibre elasticity

Abstract

Polymer materials of reduced size and dimensionality, such as thin films, polymer nanofibres and nanotubes, exhibit exceptional mechanical properties compared with those of their macroscopic counterparts. We discuss here the abrupt increase in Young's modulus in polymer nanofibres. Using scaling estimation we show that this effect occurs when, in the amorphous (non-crystalline) part of the nanofibres, the transversal size of regions consisting of orientation-correlated macromolecules is comparable to the nanofibre diameter, thereby resulting in confinement of the supramolecular structure. We suggest that in polymer nanofibres the resulting supramolecular microstructure plays a more dominant role in the deformation process than previously thought, challenging the commonly held view that surface effects are most significant. The concept we develop also provides a way to interpret the observed--but not yet understood--temperature dependence of Young's modulus in nanofibres of different diameters.

Electrospun polymer nanofibers: Mechanical and thermodynamic perspectives

“…13) has been observed when their diameters dropped below 80-100 nm. A similar dependence was observed for electrospun polystyrene 27 and nylon 6,6 11,12 nanofibers (see Figs. 14 and 15), while in the latter case, the crossover to size-dependent behavior was already seen at 500-800 nm.…”

“…11 At first glance, one may attribute the additional jump in entropy to polymer crystal orientation, which results in a decrease of the overall entropy of the fiber below the melting temperature. However, this assumption turned out to be wrong as the melting temperature of ordinary (macro-) fibers is no different than that of bulk, despite prominent crystal orientation in macrofibers.…”

“…Nanotechnol., 2007, 2, 59-62, V C Nature Nanotechnology, reproduced by permission. 11 FIGURE 27 A sketch of the arrangement of crystallites and surrounded oriented amorphous matrix in polymer nanofiber, where, L cor is the orientation correlation length within the amorphous polymer portion, and l K is the length of the Kuhn segment. From Arinstein et al, Nat.…”

“…Nanotechnol., 2007, 2, 59-62, V C Nature Nanotechnology, reproduced by permission. 11 segment concentrations exceeding the critical value, C cr ! 1/ (al 2 K ) ¼ 1/(a 3 n 2 K ), corresponding to that of a specific volume, h cr !…”

“…8 Polymer nanofibers are intrinsically different from common bulk in that they demonstrate size-dependent behavior, a wellknown phenomenon frequently observed in nano-objects. Experimental studies have demonstrated the effect of size on the mechanical and thermodynamic properties of nano-objects, as seen with the elastic moduli of hollow fibers 9 and electrospun nanofibers, [10][11][12][13][14] which sharply increase below a certain fiber diameter, as well as shifts in object melting temperatures. 15,16 Similarly, thickness and surface interactions of ultrathin polymer films (the film thickness is in the order of 2R g of a polymer chain, or less) highly influence their glass transition and melting temperatures, [17][18][19] polymer dynamics in the glassy state, 20 crystallization kinetics and degree of crystallinity, [21][22][23] phase behavior, 24 and morphology.…”