Molecular weight scaling of the spherulite growth rate in isothermally melt crystallized polyethylene nanocomposites

摘要

The spherulite growth rate, G_(II), was measured for three monodisperse linear polyethylenes filled with up to 4 vol. % of SiO_2 nanoparticles in the crystallization regime II of small undercooling, ΔT. The fumed SiO_2 used did not exhibit any measurable nucleation activity. The G_(II) scaled with the number average molecular weight, M_n, as M_n~ν with the scaling exponent, ν, equal to (2.2 ± 0.1). This corresponds to the reptation controlled surface self-diffusion of loop-train adsorbed chains with the contour length fluctuation (CLF) and the chain constraint release (CR) contributions. In order to verify the hypothesis of the chain reptation as the molecular mechanism responsible for the chain transport, logG_(II) was plotted against the logarithm of the number of effective entanglements per chain, logN _(eff). The N_(eff) was the sum of the number of "true" entanglements in the neat resin of a given M_n and the number of apparent "temporary" entanglements due to adsorption/desorption of segments of PE chains onto SiO_2 nanoparticles with their inter-particle distance equal or shorter than the average entanglement length. Adding 2 vol % and 4 vol. % SiO_2, respectively, resulted in an increase of the N_(eff) by 40% and 80% of apparent "temporary" entanglements, respectively. When plotted against logNeff, all the experimental logGII data for a given undercooling, ΔT, collapsed to a single line. The slope of the logG _(II) vs. logNeff dependence was independent of ΔT and varied from -2.13 to -2.24, similarly to the slope of the logG_(II) vs. logM_n dependence. This supported the conclusion that the effects of increasing the M_n and/or adding the non-nucleating nanometer sized SiO_2 on the spherulite growth rate were additive in nature and their effect can be superimposed. The retarded reptation of the chains to the growing crystal front was identified as the primary molecular mechanism of chain transport controlling the reduction of the spherulite growth rate in the model PE/SiO_2 nanocomposites investigated.