Banded spherulites

Figure 11 Left Spherulites of a Ziegler-Natta isotactic poly(propylene) with Mw = 271,500 g/mol and mmmm — 0.95, isothermally crystallized at 148°C. Right Banded spherulites of a linear polyethylene with Mw = 53,600 g/mol slowly cooled from the melt.

Fig. 16a-c Polarised photomicrographs of different linear polyethylenes showing a non-banded spherulites, b banded spherulites (from [115] with permission from Elsevier, UK) and c axialites. Scale bars represent 20 pm... 

Fig. 17 Schematic (simplified) representation of twisting lamella radiating out from the centre of a banded spherulite...

The twisting lamellar structure of banded spherulites has been debated for decades without obtaining any satisfactory answer until recently. The nature of the isochiral (certain uniform handedness) lamellar twisting and the synchronic character of the twisting of a group of adjacent dominant lamellae both require an explanation. The permanganic etching technique provided... 

Fig. 5.15 Banded spherulites growing in a thin film of polyhydroxybutyrate. (Courtesy of Dr P. G. Klein.)...

Nucleation density reduced by addition of PVB large banded spherulites... 

Fig. 3 Atomic force microscope (AFM) topograph of a PBS banded spherulite...

Fig. 4 AFM images of the lamellae in a PBS banded spherulite. The left colum and the right column shows the height and the phase images, respectively...

Lamellae within adjacent bands of a banded spherulite of linear polyethylene crystallized at 125 C. The specimen was cut open after crystallization, and lamellae are revealed by permanganic etching of the cut surface. The electron micrograph is of a carbon replica of the etched cut surface (after 0. C. BassetO. Scale bar=f 0 fim. 

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. 

PLLA-Z)-PEG diblock copolymers exhibit banded spherulites when iso-thermally crystallized at rc>HO Yang et proposed that the ex-... 

PLLA generally form non-banded spherulites from the melt that exhibit negative birefringence. Banding can appear after special annealing conditions or in blends and block copolymers. 

Figure 2 SEM micrographs of an etched banded -spherulite (a) crystallized at Tj = 130°C and its lamellar structure (b) [9).

When PHB is crystallized from the melt it forms large banded spherulites. Because of its biological origin and the extensive purification process used to separate it from cell debris, the polymer does not contain inorganic catalyst residues or other impurities which could act as heterogeneous nucleation centres. As a result it is very easy to obtain samples of PHB with low nucleation density that form massive spherulites on cooling from the melt. With a modicum of care, spherulites of several millimetres diameter that are visible to the naked... 

Fig. 11. Portion of a banded spherulite of linear polyethylene (Sclair 2907). This sample has been annealed at 0-5 GPa and 230°C. Replica of an etched...

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... 

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