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

The spherulite growth rate, GII, was measured for three monodisperse linear polyethylenes filled with up to 4 vol. % of SiO2 nanoparticles in the crystallization regime II of small undercooling, ΔT. The fumed SiO2 used did not exhibit any measurable nucleation activity. The GII scaled with the number average molecular weight, Mn, as Mnν 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, logGII was plotted against the logarithm of the number of effective entanglements per chain, logNeff. The Neff was the sum of the number of “true” entanglements in the neat resin of a given Mn and the number of apparent “temporary” entanglements due to adsorption/desorption of segments of PE chains onto SiO2 nanoparticles with their inter-particle distance equal or shorter than the average entanglement length. Adding 2 vol % and 4 vol. % SiO2, respectively, resulted in an increase of the Neff 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 logGII vs. logNeff dependence was independent of ΔT and varied from −2.13 to −2.24, similarly to the slope of the logGII vs. logMn dependence. This supported the conclusion that the effects of increasing the Mn and/or adding the non-nucleating nanometer sized SiO2 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/SiO2 nanocomposites investigated.