Morphology spherulitic

It should be re-emphasized that although our block copolymers do not display spherulitic morphology when they are compression molded, they are nevertheless crystalline. Hence, this indicates that under this mode of film preparation, aggregation into well developed superstructure is apparently kinetically limited. 

Figure 3. Plot of linewidth, against temperature at 67.9 MHz for two linear PE samples having a spherulitic morphology but differing levels of crystallinity degree of cryHallinity 0.78, (O) degree of crystallinity 0.50, (9).

Slow cooling has also been reported to enhance the formation of non-spherulitic morphologies in stareh gels (Nordmark and Ziegler, 2002). 

Fanta, G. F., Felker, F. C., Shogren, R. L., Saleh, J. H. (2008). Preparation of spherulites from jet cooked mixtures of high amylose starch and fatty acids. Effect of preparative conditions on spherulite morphology and yield. Carbohydrate polymers, 71, 253-262. 

The importance of spherulites and spherulitic morphology for the mechanical response of semicrystalline thermoplastics rests on experimental evidence that... 

Our findings in regard to the effects of deformation on PTFE accord in some respects with observations on the deformation of polyethylene " and polypropylene, although PE and PP usually have a spherulitic morphology before drawing. 

Most undrawn crystalline polymers possess spherulite morphology with a radial arrangement of fibrils which are complex aggregates of crystallites and amorphous regions. 

Fig. 2. Spherulitic morphology of thermoplastic elastomer where the heavy lines represent polyester segments (184).

Polymerization on heterogeneous catalysts differs from other catalytic reactions in the sense that the product remains on the catalyst. Several techniques can be used to study the polymer product after reaction. Figures 9.29 and 9.30 show several examples of polymer that was formed at 160 °C (i.e., above its melting point), and subsequently cooled to room temperature. During cooling, polyethylene crystallizes and is expected to develop its well-known spherulite morphology [101]. 

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...

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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