To account for spherulitic crystallization from the melt, one must explain the origins (i) of fibrous crystal habits in the absence of appreciable temperature gradients and (ii) of profuse noncrystallographic branching. Attention is drawn to properties held in common by spherulite-forming melts of various types and, in particular, to the facts that (a) they are multicomponent systems, (b) they exhibit small coefficients of self-diffusion, and (c) they crystallize slowly. It is shown that a consequence of these properties is that a plane crystal face cannot grow without suffering an instability of profile. Analagous instabilities lead in metal crystals to a cellular interface but, because of unusual growth kinetics, instability in spherulite-forming melts gives rise to a drastic modification of crystal habit. Bundles of discrete fibers are formed whose widths are determined by δ = D/G, D being the coefficient of self-diffusion and G being the growth rate. δ is generally small in these systems and commensurate with the scale of crystalline disorder in the fibers. It is this circumstance that allows noncrystallographic branching to occur.

A survey has been made of spherulites formed by the isothermal crystallization of polypropylene from the melt in the temperature range 110°-148°C. Four types of spherulite have been distinguished and their structural morphology, optical properties, melting behavior, and growth rates have been examined.

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Circumstances leading to twisting crystallographic orientation in banded polymer spherulites are analyzed, both from the standpoint of qualitative morphology and of more quantitative measurement and calculation. Particular attention is paid to linear polyethylene, for which the most extensive information is available, and to α-poly(vinylidene fluoride), which behaves in a closely similar manner. In polymers that exhibit vigorous twisting (band spacings extending down to 10 μm or less), chiral factors of two distinct kinds can be recognized. One is enantiomorphism, and the other is chain tilt (molecular stems in chain-folded crystals not being codirectional with lamellar normals). Quantitative evidence strongly suggests that the latter produces twisting orientation by generating surface forces in lamellae. It is also responsible for formation of dislocations that are isochiral, although these generally contribute relatively little to twisting. Banding in various classes of polymer is reviewed, and it is emphasized that there are several patterns of behavior, most still in need of elucidation; suggestions are made for continuing research.

A systematic study of impurity segregation in spherulitic crystallization is described. The experiments deal principally with high polymers, in which the role of "impurities" is fulfilled by stereo irregular molecules. or by molecules of low molecular weight. It is shown that these species are rejected preferentially by growmg crystals and that their diffusion plays a vital part in governing over-all morphology. In particular openness of texture is related to the concentration of impurity present; and coarseness of texture, which is ~ measur~ of. the. "diamet~rs" of crystalline fibers between which impurities become concentrated during crystalhzatlOn, IS determmed by o=D/G, where D is the diffusion coefficient in the melt and G is the radial growth rate of the spherulite. Results provide substantial support for a theory of spherulitic crystallization proposed by the authors.