✍ Scribed by Keith, H. D.
No coin nor oath required. For personal study only.
A paper recently presented to the American Chemical Society dealt in a general way with mechanisms of spherulitic crystallization. Attention was drawn to the fact that spherulites of remarkably similar morphology are found in polymeric and nonpolymeric materials, and to the implication that growth mechanisms of widespread validity are involved. It was shown that the grosser details of this morphology can be explained on a sufficiently broad basis by appealing to the observation that spherulitic crystallization is almost always found in multicomponent systems. The crucial point is that some of these components—which we call crystallization‐rejected species or, simply “impurities”—are rejected preferentially by growing crystals and have a drastic influence on crystal habit. Our subsequent purpose was to describe in greater detail how this interpretation would apply to the specific case of high polymers^2^.
In the crystallization of a polymer melt there are principally two types of crystallization‐rejected species to be considered. On one hand, stereo‐irregular molecules of various kinds—atactic, stereoblock or heavily branched molecules—may be present, and these will tend to be rejected on the grounds that their structures are less compatible with crystalline ordering than those of their stereoregular isomers. On the other hand, crystallization is accompanied by fractionation and molecules of low molecular weight will also tend to be rejected. In the melt, the concentration of these “impurities” will be greatest at growing crystal faces and the crystals will be surrounded by impurity‐rich layers. In general, molecules which crystallize readily must first diffuse through these layers in order to reach growth sites, and the crystals are unlikely to grow indefinitely without developing a fibrous habit. As shown previously,^1^ a fuller analysis of crystal growth under these conditions in melts of appreciable viscosity yields a rational explanation for most of the observed features of spherulitic morphology.
For present purposes, we may summarize the results as follows. As the surface of a growing spherulite advances into the melt, most of the readily crystallizable molecules are built into the advancing tips of radial crystalline fibers. Rejected impurities, on the other hand, diffuse away from the tips and are left behind to accumulate in the interstices between the fibers. The widths of the fibers should be about D/G, where D is the self‐diffusion coefficient of the melt and G is the radial growth rate of the spherulite. On this view, one may easily deduce the likely dependence of crystalline texture upon such variables as temperature of crystallization, average molecular weight, molecular weight distribution, and stereoregularity. Experimental studies confirm our expectations, and the results of these investigations will be outlined^1,2^.
Measurements have been made of the influence of impurity concentration upon the radial growth rates of spherulites. These assist us in analyzing the kinetics of bulk crystallization, by allowing estimates to be made of rates of crystallization in the “impure” melt left behind after the radial fibers have grown. Regardless of whether the impurities are stereoirregular molecules or molecules of low molecular weight, these rates will be shown to be relatively slow in almost all cases, corresponding to commonly observed departures from Avrami isotherms at large values of t, the time of crystallization. The data can also be analyzed to demonstrate the role of self‐diffusion in the melt in determining rates of crystallization, thus providing a basis for understanding variations in crystallization kinetics with molecular weight.
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