Single crystal, chain-folded morphology

Melt Crystallized.—Spherulites are observed to consist of radiating fibrils or lamellae which in melt-crystallized polymers are 100—500 A thick, depending on crystallization temperature, and /um in breadth. The c axis, i.e, the chain direction, coincides with the thickness of the lamellae and, by analogy to single crystal morphology, the molecular chain folds backwards into the lamellae. Amorphous polymer fills the regions between lamellae and can account for a substantial amount of the content. Painter et al. have shown that these inter-lamellar regions are very similar to melt-crystallized polymer in their i.r. spectrum. 

FIG. 19.1 Morphological models of some polymeric crystalline structures. (A) Model of a single crystal structure with macromolecules within the crystal (Keller, 1957). (B) Model of part of a spherulite (Van Antwerpen, 1971) A, Amorphous regions C, Crystalline regions lamellae of folded chains. (C). Model of high pressure crystallised polyethylene (Ward, 1985). (E) Model of a shish kebab structure (Pennings et al., 1970). (E) Model of paracrystalline structure of extended chains (aramid fibre). (El) lengthwise section (Northolt, 1984). (E2) cross section (Dobb, 1985). 

The morphology which is illustrated with Fig. 5.78 is a rare example of a truly macroscopic isometric single crystal, that of a polyacetylene. To avoid chain folding. 

A well studied example is given by the poly(oxyethylene-Z locfc-styrene). In case of atactic sequences of polystyrene, only the poly(oxyethylene), POE, can crystallize. A typical morphology of the POE is shown in Fig. 5.55. Single crystals of the copolymer can be grown from a common solvent which keeps both components mobile up to the time of crystallization of the POE-component. Figure 7.53 illustrates a growth spiral out of poly(oxyethylene-fclocfe-styrene), grown at 293 K from a solution of ethylbenzene (AB diblock, 28 wt-% oxyethylene block with a molar mass of about 10,000 Da). The crystal is comparable to the lamellar crystals of Fig. 5.55, i.e., the poly(oxyethylene) crystals are chain-folded with about 2.5 nm amorphous polystyrene layers at the interfaces. 

Fig. 24. DTA melting peak temperature as a function dt heating rate for poly-ethylenes of different crystal morphologies. Curve 1, attended diain crystak curve 2, slowly cooled melt crystallized spherulites curve 3, fast cooled mdt crystallized spherulites curve 4, solution grown folded chain single crystals.

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