The crystallization behavior after partial or complete melting of the α phase of iPP is examined by combined differential scanning calorimetry (DSC) and optical microscopy: calorimetric results are directly correlated with corresponding morphologies of microtome sections of DSC samples. On partial melting at various temperatures (hereafter referred to as Ts) located in a narrow range (4°C) below and near Tm, the number of nuclei increases (as in classical self‐nucleation experiments), by several orders of magnitude; on subsequent cooling, the crystallization peak is shifted by up to 25°C. After partial melting in the lower part of the Ts range and recrystallization, the polymers display a prominent morphology “memory effect” whereby a phantom pattern of the initial spherulite morphology is maintained. After partial melting in the upper part of the Ts range the initial morphology is erased and self‐nucleation affects only the total number of nuclei. The present experimental procedures make it possible to define, under “standard” conditions, the crystallization range of the polymer and in particular, the maximum crystallization temperature achievable when “ideally” nucleated. © John Wiley & Sons, Inc.

A simple, convenient and reliable calorimetric efficiency scale is proposed for the evaluation of nucleating additives for polymers. The scale is based on conventional differential scanning calorimetry cooling runs and makes use of a crystallization range determined in self‐nucleation experiments. It can be correlated with spherulite sizes, and indicates the potential range of improvement of nucleating additives. Typical nucleating agents for isotactic polypropylene are evaluated; at best they rate at 60 to ca. 70% on this efficiency scale. © 1993 John Wiley & Sons, Inc.

The metastable phase of isotactic polypropylene (βiPP) is crystallized epitaxially on two specific nucleating agents:  γ-quinacridone and dicyclohexylterephthalamide (DCHT). The resulting thin films are investigated by electron microscopy and atomic force microscopy. Epitaxial crystallization yields a biaxially oriented sample of βiPP, an orientation which cannot be achieved by mechanical means due to the β → α phase transformation on stretching. Electron diffraction indicates that the βiPP (110) plane is the contact face involved in both epitaxies. AFM investigation with methyl group resolution reveals a lateral periodicity of 19 Å in that (110) contact face, which corresponds to the distance between three chains and is a trademark indicator of the frustrated packing of βiPP. AFM further indicates some variability of the surface pattern, suggesting that two different frustrated structures may coexist in the surface layer; this, in turn, suggests some type of surface reconstruction. Structural requirements that efficient βiPP nucleating agents must meet are analyzed.