Overall crystalline fraction

The overall crystalline fraction Xc can be estimated experimentally from Wide Angle X-ray Scattering measurements (WAXS). 

Thus, a certain fraction of amorphous material is associated with the crystalline material and the resulting diffraction pattern is the sum of an ordinary pattern with Bragg peaks and a continuous pattern with one or more broad maxima. To illustrate this, x-ray diffraction patterns of some amorphous polymers are given in Fig. 5. The size of the amorphous fraction relative to the crystalline fraction (amorphous to crystalline ratio) does influence the overall spectrum considerably (Fig. 6). 

Fig. 25 Increases in the overall crystallinity with time, at 390 K ( ), at 370 K (A), at 350 K (A), at 330 K (o), and at 320 K ( ). The crystallinity is defined as the fraction of chain atoms that belong to the ordered domains...

In common cases, the grain-boundary thickness is close to a few nanometers the overall volume fraction of the boundaries is very small. The opposite situation is observed in nano crystalline solids, with profound effects on defect - thermodynamics and transport. 

Cruz et al. [21] estimated crystallinities of samples from DTA data. Figure 45 shows how the crystalline fraction of PCL increased with overall PCL content from zero at about 25 wt % PCL to reach a plateau at about 80 wt %, comparable with that found in bulk PCL. Jonza and Porter obtained similar data [126] PCL crystallisation increased from about 8%, in 50 50 (w/w) blends, to about 60% in blends with 70% PCL. These latter workers also studied PCL crystallisation kinetics and found equilibrium crystallisation was achieved (at 37 °C) in 10 min or less for samples containing 70% or more PCL. Crystallisation times in excess of 40 min were required for samples with less PCL (Fig. 46) [126]. Similar measurements indicated that PC crystallised over the same composition range, i.e. PC... 

The overall crystallinity of a polymer can be simply assessed from heats of fusion, and this must be the best quality-control method available. The prime requirement is a knowledge of A//o the heat of fusion per gram (or kilogram) of crystal phase. Then determination of the heat of fusion per gram of the actual sample AHs allows determination of the weight fraction crystallinity Xy,) via ... 

Few results have been reported on the effect of temperature on stereoelection. Propylene oxide was polymerized at various temperatures. It was found that the overall stereoelectivi-ty is not modified when the temperature is changed from +80 to -8° [13]. However the rate of polymerization is decreased (by a factor of 10 ) and the distribution of different fractions in the polymer modified. With a lowering of the temperature the proportion of crystalline fraction is increased up to 70% (Figure 11). The behaviour of propylene sulfide is similar to that of propylene oxide, r being not modified between +30° and -30°, while the crystallinity was increased in the latter case [24]. 

Perez and Vanderhart [60] investigated the morphological partitioning of chain ends and methyl branches in ethylene copolymers by C-NMR in the solid state. The PE samples, which were crystallised from the melt, varied in molecular weight, polydispersity, crystallisation rate, and comonomer content. For the limited set of samples considered, the ratio of crystal to overall end concentration is independent of those variables. This ratio takes the values of 0.75 and 0.60 for the methyl and vinyl ends, respectively. When the crystalline fraction of these samples is taken into account, 50-75% of the total saturated ends and 42-63% of the total vinyls reside in the crystal. For an ethylene/propylene copolymer, 21-27% of the methyl branches were determined to be in the crystal. This level of incorporation puts methyl branches in a position intermediate between chain ends and ethyl branches. 

Figure 5.3. Effects of soil pH on the Zn amount bound to the Fe oxide fraction (amorphous/crystalline Fe oxide and overall Fe oxide bound fractions) in soils from China with pH 3.73-8.1 and 0-14.7% CaC03 (after Han et al., 1995. Reprinted from Geoderma, 66, Han F.X., Hu A.T., Qi H.Y., Transformation and distribution of forms of zinc in acid, neutral and calcareous soils of China, p 128, Copyright (1995), with permission from Elsevier)...

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