IntroductionStructure formation in crystallizable polymers under the in¯uences of heat transfer and¯ow is a subject of major interest in polymer processing and in the development of`t ailored resins''. The ®rst author of this paper observed in 1982 ± to his own surprise ± that no serious attempt had been made to understand the underlying processes. So it is also not surprising that the required mathematical tools had not been developed. For instance, the path was not paved for a description of the overall crystallization speed in terms of the increasing surface of the crystalline areas on which further growth takes place. In order to make use of the developments in mathematics, one needs several physical parameters, which ± if one wants to avoid speculations ± can only be determined by experiment. Traditionally, the rate of primary nucleation and the growth speed of spherulites, both as functions of temperature, were required in a wide range of temperatures for a description of structure development during solidi®cation even for the simplest case of a quiescent melt.The pertinent experimental diculties, however, are tremendous, because of the fact that the relevant undercoolings are considerable. The experiments had to be extended over quite a large temperature range of undercooling. For i-PP a range of no less than 127°C could be covered in our laboratory. In a recent review [1] the results of this extended research are compiled up to early 1995. Some more recent results were published afterwards in separate papers (growth speed measurements with four independent methods [2] and further ®ndings in shear-induced crystallization [3,4]).The reader will understand that for all these experiments, in which heat transfer problems play a dominant role, some guidelines with respect to expected phenomena are rather desirable. In other words, a classi®cation of the temperature range between the Abstract The lower temperature limit for sporadic``thermal'' nucleation, as estimated in a previous paper (Janeschitz-Kriegl (1997) Colloid Polym Sci), can be veri®ed by a variety of earlier experiments which are compiled in this paper. Below the said limit only``athermal'' nuclei are of importance in samples containing no large numbers of hetero-nuclei. The number of athermal nuclei increases tremendously (by many decades) with decreasing temperature. Why these nuclei are dormant and``awake'' only at`t heir'' temperature is explained in terms of their growth conditions, which depend on the length of the incorporated (helical) sequences. Also the sluggishness of molecular processes, occurring in the temperature range of metastable conditions, where sporadic nucleation can occur is demonstrated by some of the recalled experiments. Except for very fast industrial processes, where¯ow plays a dominant role, this sluggishness makes a noticeable in¯uence of the sporadic nucleation on structure formation very improbable.

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Following some early rudimentary results on shear‐induced crystallization of polybutene‐1 (1, 2), the present paper contains more detailed results. In the course of this work the origins of the highly oriented crystalline surface layers, as well known from injection molded samples, are more closely investigated. For the purpose, a special extrusion experiment is used, in which melts of various degrees of undercooling are moved through a duct of large aspect ratio. When, after the release of the pressure at the die entry, a quench of the duct to a temperature far below the melting point is delayed, a relaxation phenomenon is observed, in accordance with the experiences with i‐PP. From these experiments one learns that the leading parameter of the process is something like the mechanical work done per unit volume, and that the relaxation time increases with decreasing temperature much faster than the viscosity of the melt. The results are qualitatively in excellent agreement with our previously obtained results for polypropylene.

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