Spherulites ring-banded

It was noted in early session that sphemlitic structure of a polymer affects the mechanical properties of the polymer. Micrographs of PHB and PHB12 V crystals stractrrre, revealed using POM, are shown in Figure 12. The PHB V and PHBHHx show fine fibrillar structure of spherulites while PHB shows circular ring banded spherulites, both types of crystals display Maltese cross. 

Fig. 7. Optical micrographs (under crossed polars) of PCL spherulites in pure PCL and blends of PCL with PVC numerals define the weight fraction of PCL in the blends. The micrographs show normal spherulites in pure PCL and ring-banded spherulites in the blends the periodicity in the blends increases with PCL content taken from [51]...

The ring-banded structure of spherulites (Fig. 7) is often seen in blends in which the amorphous components are miscible, i.e. both components are present in the melt at the growth surface, and it is frequently considered that such features are only seen in these situations. Detailed information on ring-banded spherulites is exempHfied in subsequent sections deaUng with individual blend types. To date there is no agreed explanation for the origin of this feature i.e. there is no explanation as to why the fibrils should twist in a regular... 

Fig. 18. Variations in average widths of the extinction rings with PCL content in ring-banded spherulites in PCL-PVC blends, calculated as ( ) (O) where n,...

Fig.20a-d. Optical micrographs showing the distinction between ring banded spherulites in PCL-PVC blends and non-banded structures in blends of PCL with CPE-56.3 taken from [99]... 

Defieuw et al. [99] also observed the textures of spherulites found in different blends. Ring-banded spheruhtes were found only in systems with low CPE contents ( 10 wt % CPE-49.1). At high CPE contents (20 wt % CPE-49.1) the spher-uHtes were more disordered and had a dendritic texture. At high PCL contents (at least 70 wt %) spheruhtes were found to be space-filling and the CPE was necessarily contained in the amorphous regions of the spherulites. For higher CPE contents (of CPE-49.1) spheruhtes were found to be separated by a CPE-rich amorphous phase. Details were stated to depend on the detailed experimental procedures, e.g. on the crystalhsation temperatnres which inflnenced rates of crystalhsation and of molecnlar diffiision. 

Fig. 29. Variation in periodic distances in ring-banded spherulites in PCL/SAN blends with (T .-Tg) for different compositions data taken from [107]...

Kummerlowe and Kammer recently further reported on the nature of ring-banded spherulites in PCL/SAN blends. Having suggested that the periodicity L in these structures is given by Eq. (19), they reported data in support of this relationship. For blends of PCL with SAN copolymers of different composition and of different proportions of PCL and SAN, they plotted data which are in accord with Eq. (19), Fig. 35 [52]. 

Samples containing a copolymer having 87 wt % e-caprolactone gave small spherulites after crystallisation for 12 h at a variety of crystallisation temperatures only the samples crystallised at the highest temperature (48 °C) showed any indication of forming ring-banded spherulites [123]. 

Ring-banded spherulites were more commonly seen in blends of PCL with polymers with which there was miscibility in the amorphous phase (cf. PVC and chlorinated polyethylenes. Sects. 8 and 9). The initial copolymers were immiscible with PCL but it was considered that miscibility was enhanced by the formation of different copolymer structures on heating during processing. 

These copolymers were also reported to generate ring-banded spherulites over a wide range of volume fractions of PCL. B16 copolymer alone was found to have a lamellar morphology and B7 was assumed to be lamellar also. 

Fig. 2.7 (a) Spherulites in a thin film of polypropylene (Elaine Ann Perkins, University of Reading, UK, unpublished work, 1985), (b) spherulites in polyhydroxybutyrate-co-valerate (Reprinted with permission from Macromolecules, 2010, 43 4441-4444 Wang Z, Li Y, Yang J, Gou Q, Wu Y, Wu X, Liu Pand Gu Q (2010) Twisting of Lamellar Crystals in Poly (3-hydroxybutyrate-co-3-hydroxyvalerate) Ring-Banded Spherulites Copyright (2010) American Chemical Society, (c) Schematic of sphaulite twisted orientations. Redrawn from Hsieh et al. (2014)... 

Crystalline lamellae can be arranged in different ways, from a parallel arrangement in stacks or bundles, sheaf-like structures, up to spherulites with a radial arrangement of lamellae (typical of HDPE, (high-density polyethylene), or polypropylene (PP)) or twisted lamellae, arranged in concentric rings (banded spherulites, typical of LDPE (low-density polyethylene), as in Fig. 1.9). Depending on the polymer and thermal treatment, the spherulites can reach diameters of up to several tens or hundreds of microns. 

Wang W, Jin Y, Yang XN, Su ZH. Chain orientation and distribution in ring-banded spherulites formed in poly(ester urethane) multiblock copolymer. J Polym Sci B 2010 48 541-547. 

Xiao Q, Yan S, Rogausch KD, Petermann J, Huang Y. Ring-banded spherulites in poly(e-caprolactone) blended with hydroxyethyl cellulose acetate as an indication for partial miscibility. J Appl Polym Sci 2001 80 1681-1686. 

Woo EM, Wu PL, Wu MC, Yan KC. Thermal behavior of ring-band versus Maltese-cross spherulites, case of monomotphic polyfethylene adipate). Macromol Chem Phys 2006 207 2232-2243. 

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