Blend samples crystallization

Thus, for the investigation of buried polymer interfaces, several techniques with molecular resolution are also available. Recently NMR spin diffusion experiments [92] have also been applied to the analysis of a transition zone in polymer blends or crystals and even the diffusion and mobility of chains within this layer may be analyzed. There are still several other techniques used, such as radioactive tracer detection, forced Rayleigh scattering or fluorescence quenching, which also yield valuable information on specific aspects of buried interfaces. They all depend very critically on sample preparation and quality, and we will discuss this important aspect in the next section. 

The kinetics of the crystallization process can be followed using the FT-IR technique 290). A particularly interesting example comes from the study of the polymer blends of PVDF/PMMA where the crystallization of the alpha and beta forms have been followed during heating of the blend samples which had been quenched from the melt and crystallized by heating at 2 °K/min in the spectrometer. Wien the blend has 70 wt % PVDF the beta crystal form is obtained directly but for other compositions the alpha form is dominant or unique. 

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... 

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 next sections of this paper contain the results of solid state structure investigations of these blend samples. The suggestion which will be advanced is that ideas concerning traditional polymer blend systems are more applicable for providing an understanding of these results than are ideas concerning small molecule liquid crystals. 

Figures 4 and 5 contain the DSC traces which are obtained upon second heating for each of the component materials. The small endotherm observed is typical of these materials. This is attributed to the small entropy change which occurs at the crystal-nematic transition temperature (15). It should also be noted that the two polymers display transition temperatures which differ by about forty degrees. Thus, in the blend samples, if two endotherms are present, there should be no problem in discerning them.

A typical cooling curve for a blend sample is shown in Figure 6. There is typically an undercooling of about forty degrees which is observed for these samples. In all cases, only a single crystallization exotherm is noted, suggesting that cocrystallization is occurring. 

In contrast, in DSC and WAXD studies on HDPE/LDPE blends cooled both rapidly and slowly, Datta and Birley (61) found that both the rapidly and slowly cooled blend samples exhibited two distinct endothermic peaks in the DSC measurements, indicating that the HDPE and LDPE components in the blends crystallize separately. However, no data were given on the molecular characteristics of LDPE. 

Figure 5.1 (a) DSC thermograms and (b) the temperature dependence of infrared absorbance estimated for the D and H infrared crystalline bands in the cooling process from the melt measured for a series of DHDPE/LLDPE(2) blend samples. The starting temperature of crystallization is slightly different between these two curves probably because of the difference in the cooling rate, the monitoring point of temperature, and so on. But essential behavior is the same. 

Figure 5.6 Wide-angle neutron scattering profiles measured at the various temperatures for (c) DHDPE/LLDPE(2) and (d) DHDPE/LLDPE(3) blend samples in comparison with the profiles of (a) the amorphous state and (b) the crystalline state calculated for the pure DHDPE and the blend of D and H chain components. In (c), the low angle broad scattering is detected at any temperature due to the homogeneous mixing of D and H species in the same crystallite state as well as in the melt state. In (d), the low angle scattering can be seen only in the molten state, while it becomes lower when the sample crystallizes into the separated phases of D and H chains. These observations are consistent with the simulated results of (a) and (b).

In order to investigate the cocrystallization and phase segregation behaviors of the above-mentioned PE blend samples, we performed the experiments for both the isothermal and nonisothermal crystallizations. In the nonisothermal crystallization, the temperature is changed gradually and the WAXD, SAXS, or infrared spectra are collected as a function of temperature. In the isothermal crystallization, the... 

Figure 5.17 Comparison of crystallization rate between the pure components and their blend sample in DHDPE/LLDPE(2) system in the isothermal crystallization process from the melt. (Erom Reference 38 with permission from the American Chemical Society.)...

In the present review, the crystallization behavior has been described for the D and H chains in the blend samples of DHDPE and LLDPE with various degrees of side... 

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... 

Table 9.5, the K-values of blend samples are lower than that of iPP, indicating that blending of a rubbery EHR suppresses the formation of j6-form crystals of iPP, irrespective of the miscibility in the molten state. 

In Fig. 3.25 Hoffman-Weeks plot for PCL/PBT blends is compared to pure PBT sample. A nonlinear extrapolation procedure was applied for the determination of the equilibrium melting temperature of the crystallizing phase. The linear extrapolation as proposed initially by Hoffman-Weeks neglects the contribution of the increment of the lamellar thickness. Note that the PCL is miscible with PBT only when the PCL molecular weight is equal or lower than MW = 1,250. The blend samples having a PCL molecular weight of 10,000 or 50,000 form immiscible mixture for which the crystallization behavior of pure PBT is recovered. The T ° of... 

Figure 2.2 WAXD spectrum of the PLLA/PDLA blend after crystallization in the second cycle of Figure 2.1 (the sample was taken out at 140 °C for WAXD measurements).

Blends with copolymers having lower PCL contents were less miscible with PCL. Glass-transition temperatures of quenched blends were almost independent of composition and close to those of the copolymers themselves blend TgS changed by about 10 °C over the composition range 0-90 wt % PCL. Thus, it is seen that PCL was phase-separated from the copolymers at high temperatures. In such blends the crystal-melting temperatures of samples crystallised as above were very close to those of pure PCL and heats of fusion per gram of PCL... 

Tsuji (2002) conducted an investigation into the hydrolysis of an amorphous form of PLA, to determine the effects of L-lactide content, tacticity and enantiomeric polymer blends. In this work four samples were prepared—poly(D,L-lactide), poly(L-lactide), poly(D-lactide) film and the blend sample of poly(L-lactide) and poly(D-lactide). The results are sununa-rized in Table 7.4, which also covers a complementary study that explored the effects of hydrolysis in terms of molecular weight and its distribution, glass transition temperature, crystallization temperature, melting temperature and mechanical properties. 

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