Morphology cylindrites

Neat isotactic polypropylene (iPP) crystallized from melt exhibits spherulitic morphology of the crystalline phase (72,73). In some cases and under very specific conditions, cylindrites, axialites, quadrites, hedrites, and dendrites may be formed of iPP (74). In general, crystallization from quiescent melts results in spherulitic morphology, whereas crystallization fi-om melts subjected to mechanical loads results in cylindrites (75). Crystalline supermolecular structure caused by oriented crystal growth from heterogeneous surfaces is commonly termed transcrystallinity (76). 

The morphology of high-modulus carbon-fiber (HM-CF) reinforced iPP was investigated by AFM using chemically etched specimens [7]. The images exhibit typical features of a-transcrystalline morphology for samples which were crystallized from quiescent melts, and nucleated on HM-CF. In melts sheared by fiber pulling, ap-cylindritic columnar... 

Fig. 18. SFM image of the a, 8-cylindritic columnar morphology of transcrystallized isotactic PP induced by pulling of a carbon fiber. On the basis of these observations, Vancso and co-workers concluded that the lamellar growth occurs during epitaxial crystallization on bundles of extended isotactic PP chains, which form during shearing of the polymer matrix by fiber pull. Reprinted with permission from Ref 183. Copyright (1997) Steinkopff Verlag.

In isotactic polypropylene [73,76], the smectic phase or the monoclinic a phase can be obtained according to cooling conditions. Furthermore, the a phase may, in certain conditions, exhibit a bimodal crystalline texture, that is, two populations of crystals with their c or their a-Sixis along the fiber axis, respectively (a is the axis of the reciprocal lattice related to a). This is revealed by additional arcs or spots on the 110 and 130 reflections, while the 040 reflections remain unchanged [79] (Fig. 15.21). This can be interpreted in terms of a cylindritic morphology with two types of lamellae classical radial lamellae and tangential ones obtained by epitaxial growth, which is a particular feature of polypropylene crystallization (see Chapter 8). 

When highly branched polyethylene samples, either (dendritic) low density polyethylene or (comblike) very low density polyethylene, crystallize from oriented melts they do not form cylindrites because they contain insufficient linear chain segments to generate microfibrillar nuclei. In such cases the relatively slow crystallization kinetics and low crystallization temperature permit the molecules a relatively long time to relax prior to solidification. The lamellae that form under these circumstances are well separated from one another and do not share a common axis. The resultant semicrystalline morphology is similar to that of low density samples crystallized from an isotropic melt. 

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