Is chain end retardation responsible for anomalous dependence of crystal growth rate on molecular weight?

We present results on a robust anomaly in the crystallisation of blends of poly(ethylene oxide) (PEO) of differing molecular weight, M$_w$. Previous studies probing PEO blends have mainly focused on blends of low M$_w$, for which integral chain folding is important, with very high M$_w$. The PEO samples used in this study consist of a blend with both M$_w$'s well above the integral chain folding limit. In general, one would expect that such blends should show a monotonic decrease in spherulite growth rate, G, as the blend contains more high-M$_w$ component. Our results however show a clear non-monotonic G, with a minimum in a plot of G as a function of the volume fraction. Surprisingly, blending a small amount of lower- M$_w$ PEO into a higher- M$_w$ PEO slows the growth kinetics. We present a model that attributes this behavior to a chain end retardation mechanism.