Polymer spherulites, banding

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... 

A dispersion of spherulitic liquid crystalline particles in brine exists between 0.8 gm/dl NaCl (Figure 2(a), first sample on the left) and 1.2 gm/dl. As the salinity is increased to about 1.4 gm/dl NaCl, the amount of liquid crystals as well as the birefringence increase, and the texture observed using PLS is intermediate between those of the spherulite (S) and lamellar (L) structures. The aqueous solution is a homogeneous lamellar phase between 1.6 and 1.8 gm/dl NaCl. The surfactant molecules form bilayers with their polar heads toward the brine. Figure 3(a) shows the lamellar structure as observed by polarized microscopy at 1.6 gm/dl salt and without any polymer. The bands represent "oily streaks" in a planar background. 

It is clear that uncompensated chemical (configurational) chirality causes lamellar twist manifested as banded polymer spherulites. A particular enantiomer of a particular chiral polymer almost always has lamellar twist of one hand only, but R enantiomers of different... 

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. 

Fig, 37a—d. Structure of shear fracture surface a SEM-micrograph of a shear fracture in PP 1120 (T = —80 °C) b secondary crack formation (white arrow) in one of the shear bands of type B in fine spherulitic PB-l (T = -196 °C) c traces of shear bands B containing fibrillated polymer substance on a shear fracture plane in fine spherulitic PP 1120 (T = —196 °C) d preferred shear fracture along spherulite boundaries (SB) of coarse spherulitic PP 1120 (T = —80 °C)... 

Fig. 38, Mode Il-fracture toughness, vs. band initiation stress, Cj, for various polymers at T = —80 °C (open symbols fine spherulitic, dark symbols coarse spherulitic morphology)...

The formation of shear bands under compression is found in crystalline polymers when loaded at temperatures lower than 0.75 T. Under such a condition the shear bands interact with certain morphological features such as spherulite boundaries or lamellar arrangements inside the spherulites. The band initiation stress, ct, increases and the strain at break, Cp, decreases with decreasing temperature and increasing stiffness of the tested polymer, i.e. increasing degree of crystallinity. 

The sensitivity of Raman spectroscopy to crystallinity is due to the conformational changes in polymers occurring during the transformation of polymer chains from amorphous domains to that of three-dimensional ordered, crystalline domains [33]. pRS could be used to quantify the crystalline fractions of spherulites by analyzing the characteristic Raman bands in the region of interest. 

Fig. 15.6 Raman spectra recorded along the diameter of an iPP spherulite during isothermal crystallization from the melt at Tj, = 130°C. The variation in Raman bands at 809 cm and 841 cm indicating the amorphous and crystalline content of iPP are matched -with their positions on the polymer (Reprinted from [38])...

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. 

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... 

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