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.

We have studied the structure and defects of fully syndiotactic polypropylene [sPP] in its dominant (t^h phase using X-ray and electron diffraction and molecular modeling. In agreement with our previous proposal from much less syndioperfect sPP (77 %), we find the chains to pack in a fully antichiral cell along both the a and b axes, which is thus doubled from its currently accepted dimensions. Cell parameters are a = 1.45 nm, b = 1.12 nm, c = 0.74 nm, and space group Ibca. Crystallization at temperatures below ca. 130 °C causes incorporation of packing defects along the b axis, which we have simulated using molecular modeling. At high crystallization temperatures, the defects consist of individual rows of chains displaced by b/4 and identified by narrow streaks superimposed upon sharp h20 spots in electron-diffraction patterns from tilted crystals. At lower crystallization temperatures, entire groups of such chains are displaced by b/4, forming discrete domains and leading to streaking of the individual h20 spots. No evidence for large-scale row or point vacancies is found. In addition, we find evidence for new intermolecular packing defects along the chain direction, which may arise from conformational defects and may involve small-scale (ca. 5-10% of c) axial displacements of the macromolecules.

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.