Banded spherulite

Macrospherulites of polyethylene glycols that have diameters of up to about 2 cm (Figure 8) have been reported (88). When viewed between crossed Polaroids , the characteristic Maltese cross patterns and spherulitic banding that are seen in microscopic spherulites are apparent in these spherulites and can be seen with the naked eye (Figure 9). 

Figure 4.12 Spherulites of poly( 1-propylene oxide) observed through crossed Polaroid filters by optical microscopy. See text for significance of Maltese cross and banding in these images. [From J. H. MaGill, Treatise on Materials Science and Technology, Vol. lOA, J. M. Schultz (Ed.), Academic, New York, 1977, with permission.]...

A larger number of smaller spherulites are produced at larger undercoolings, a situation suggesting nucleation control. Various details of the Maltese cross pattern, such as the presence or absence of banding, may also depend on the temperature of crystallization. 

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.

When the substrate surface is curved or spherical, various textures of polycrystalline aggregate appear through geometrical selection. Spherulites will be formed when a sand grain or spherical polycrystalline aggregate formed at the early stage of nucleation acts as a substrate, and wavy banding parallel to the sub-... 

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... 

Figure 5.15 Top, images of the center of a PEG spherulite obtained by plotting the 1343 cm-1 band from data obtained using either parallel (right) or perpendicular (left) radiation. Bottom, a dichroic ratio image obtained by ratioing the top images. Reproduced form figure 2 of Ref. 31, with permission.

Significant variation of the ultimate mechanical properties of poly(hexamethylene sehacate), HMS, is possible by con-trol of thermal history without significant variation of percent crystallinity. Both banded and unbanded spherulite morphology samples obtained by crystallization at 52°C and 60°C respectively fracture in a brittle fashion at a strain of r O.Ol in./in. An ice-water-quenched specimen does not fracture after a strain of 1.40 in./in. The difference in deformation behavior is interpreted as variation of the population of tie molecules or tie fibrils and variation of crystalline morphological dimensions. The deformation process transforms the appearance of the quenched sample from a creamy white opaque color to a translucent material. Additional experiments are suggested which should define the morphological characteristics that result in variation of the mechanical properties from ductile to brittle behavior. 

Figure 2. Optical micrographs of the (a) banded and (b) unhanded spherulitic morphologies obtained under different crystallization conditions...

Figure 3. Engineering stress-strain curves for HMS banded and unbanded spherulitic specimens crystallized at 52°C and 60°C respectively...

HMS can crystallize in either banded or nonbanded spherulitic morphologies as illustrated in Figures 2a and 2b respectively (14). Banding is typical of HMS crystallized below 56°C whereas nonbanded spherulites are formed by crystallization above 56°C (14). Banding of polymer spherulites is thought to be related to the periodic twisting of... 

Both banded (Tc = 52°C) and unbanded (Tc = 60°C) spherulitic morphologies had essentially identical stress-strain curves despite a difference in crystallinity of 8% and variations in spherulite size for these two crystallization conditions. These changes in crystallinity and spherulite size might compensate sufficiently to allow similar bulk deformation behavior. However, the sample crystallized at 52 °C should have smaller spherulites and thinner lamellae than the sample crystallized at 60 °C because of a greater probability of tie molecules. This, combined with its lower crystallinity, should allow more ductile behavior for the 52° C crystallized sample. The fact that both specimens deform similarly indi-... 

In conclusion, the deformation behavior of poly(hexamethylene sebacate), HMS, can be altered from ductile to brittle by variation of crystallization conditions without significant variation of percent crystallinity. Banded and nonbanded spherulitic morphology samples crystallized at 52°C and 60°C fail at a strain of 0.01 in./in. whereas ice-water-quenched HMS does not fail at a strain of 1.40 in./in. The change in deformation behavior is attributed primarily to an increased population of tie molecules and/or tie fibrils with decreasing crystallization temperature which is related to variation of lamellar and spherulitic dimensions. This ductile-brittle transformation is not caused by volume or enthalpy relaxation as reported for glassy amorphous polymers. Nor is a series of molecular weights, temperatures, strain rates, etc. required to observe this transition. Also, the quenched HMS is transformed from the normal creamy white opaque appearance of HMS to a translucent appearance after deformation. 

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