Blend samples spherulites

Figure 10.5 shows scanning electron micrographs of blend samples that were prepared as described in the Experimental Section . The etchant preferentially attacks polyethylene, producing a topography in which the polystyrene-rich domains are raised above the polyethylene domains. The interlamellar amorphous material provides a location for styrene to penetrate and polymerize. A considerable amount of polystyrene is present in the center of the spherulites. This is due either to amorphous polyethylene that is present in these locations or to voids that develop during crystallization... 

For pure iPP and PB-1 homopolymers and their respective blends, the spherulite radius increases linearly with time t for all T, investigated. For all samples, the isothermal radial growth rate G was calculated at different as G = dR /dt. Generally, the G values decrease an increase in the values and with increase in the amount of noncrystallizable component in the blend. As shown in Fig. 6.1, where the relative G values of the iPP-based blends are reported for = 125°C, the depression of the G values was more pronounced for the blends prepared with HOCP as the second component. 

Polarizing light microscopy employs crossed polarizers to view the sample. With isotropic specimens, the field of view is dark, while anisotropic, birefringent samples or areas of a sample will appear bright. Polarizing microscopy is employed to view spherulitic structure [66-68] and deformation morphologies (crazes, shear banding) [69] in polymer blends. Samples for... 

Blends of polymers show complicated spherulite patterns. For example, in the blend sample of PVDF with atactic poly (methyl methacrylate) (PMMA), PVDF is in the crystalline state and PMMA is in the amorphous state [32]. For polymer-diluent systems, the diluent molecules are expelled out of the lamellae and the spherulite because of their thermal mobility thus, the diluent concentration in the system changes in front of the growing spherulite. For polymer-polymer blend systems, the thermal mobility of the polymers is not very high thus, the amorphous polymers remain trapped between the lamellae of the crystalline polymer component. For example, in the crystallization of PVDF70/PMMA30... 

The crystallization of poly(L-lactic acid) (PLLA) occurs slowly, and is not very high when crystallized from the melt. We can add nucleating agents such as talc or clay to enhance crystallization. Adding a small amount of poly(glycolic acid) (PGA) enhances crystallization even when the content of PGA is only 0.1 wt%. Figure 5.8 shows POM snapshots taken during crystallization of neat PLLA and a PLLA/PGA blend sample. The presence of PGA increases the number of PLLA spherulites per unit area [33]. 

Microscopic examination of these samples show for the PET-rich blends the presence of very small spherulites. The PBT-rich blends show very low birefringence, low turbidity, and no organized structures. The SALS patterns are circularly symmetrical. 

Microscopic examination of the samples crystallized at 130°C shows very low turbidity and birefringence for the PBT samples the turbidity in the blends increased, and small spherulites were present for PET. The samples crystallized at 110°C again showed small spherulites for PET, and no organized structures were observed in the blends of intermediate composition although their turbidity was quite high with samples of very high PBT composition, the turbidity was lost. 

The presence of HOCP considerably slows down the melt crystallization process of PB-1. Therefore, the adopted values, lowered by increasing the HOCP fraction, provided similar rates of crystallization for pure PB-1 and blends. Previous calculations from the spherulite growth rate and from the overall kinetic rate constant showed that the number of nuclei per unit volume was similar for samples crystallized at equal undercoolings. Had we used a constant value of T, there would have... 

The spheruhte dimension, at constant T, increases with increasing concentration of noncrystallizable component. The spherulite radius R increases linearly with crystalhzation time for pure iPP and iPP/PB-l/HOCP blends for all investigated. For all samples, the isothermal radial growth rate, G = dR/dt, calculated at different Tc, is reported in Table 6.11. With the increase in the T, the G values appear to decrease for all investigated compositions. The blends prepared with the same fraction of iPP show G values that decrease with increasing of HOCP fraction at constant Tc value. 

The morphology and the isothermal radial growth rate of PEO spherulites in the blends were studied on thin films of these samples using a Reichert polarizing microscope equipped with a Mettler hot stage. The films were first melted at 85° C for 5 minutes, following which they were rapidly cooled to a fixed crystallization temperature T and the radius of the growing spherulites was measured as a function of time. 

It should be mentioned that several homopolymers (of which polyethylene is probably the best known sample) also exhibit a complex melting behavior. Branched polyethylenes (LDPE, LLDPE, and VLDPE) show multiple melting endotherms, due to the presence of fractions with different branching contents (Schouterden et al. 1985 Defoor et al. 1993). This was clearly illustrated by Defoor et al. who fractionated LLDPE with respect to the short-chain branching content and blended the fractions with the highest and the lowest branching content. It was shown that they both crystallized and melted separately. Both fractions determined the spherulitic morphology in a cooperative way. 

The observation that spherulites were space-filling at high PCL contents means that the PVC in the blend must be contained within the spherulites (in-terfibrillar or interlamellar). At 50 wt %, spherulites were found to be not quite space-filling and the observation of a single Tg in the partially crystalline samples means that the PVC and residual PCL were in a single homogeneous amorphous phase within and between the spherulites. 

None of the models considered could explain the behaviour of the blends containing 50 wt % PCL which contained some crystalline PCL as spherulites. At low elongations the spheruhtes did not break up but deformed. Spherulites started to break up when samples were extended beyond their yield points. 

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