Crystalline PEO

Fig. 5.3 Comparison of the temperature-dependent conductivities for amorphous PE0 NH4SCN (circles) and crystalline PEO NH4SCN (squares).

The large drop in modulus for the blends containing more than 46% PEO occurs when the crystalline PEO melts, and the specimens are then composed of... 

Fig. 5.21 The effect of PEO and PBO block lengths on the extent of chain folding of the PEO block in the equilibrium states of crystalline PEO,.PBO copolymers (o) unfolded, ( ) one fold, ( ) two folds, ( ) three folds (Mai et al. 1997). The dashed curves approximately delineate the four regions. The data points at m = 0 indicate the equilibrium state (unfolded) of low-molar-mass poly(oxyethylene)s.

It is not know what is the precise function of polymer crystallinity in the complex release mechanism where there are several time and release dependent changing parameters but the release, particularly in the early stages, from crystalline PEO hydrogels is more constant than the release from amorphous PEO hydrogels. However, the degree of crystallinity is related to the... 

Fig. 35. Variation of the thickness dpEo (°) of the crystalline PEO layer, the average degree of crystallinity r (o) and the average number of folds v of the PEO chains of the copolymer SEO.9 as a function of the concentration C4...

Evidence for the effect of chain ends on interfacial energy is provided by a number of experiments. The melting points of short PEO-containing diblocks [26] and triblocks [27,28], Tm=47-51 °C,is low compared to perfectly crystalline PEO (Tm=76 °C). This is due to the positive free energy of formation for the block copolymers of the amorphous layer from the melt, and of the crystal-line/amorphous interface. The end interfacial theory can be analyzed in terms of the theories for melting points of low molecular weight polymers [29,30]. 

Model C is mostly found for a blend of crystalline and amorphous polymers. In general, the miscibility for the crystalline/amorphous blends would be better in an amorphous component-rich system than that of a crystalline-rich system. For example, when the crystalline PEO composition is more than 60 wt% in PEO/amorphous PVPh, PEO in the blend showed two Tip relaxation times (Table 10.2) [34]. One of the two Tip agrees well with Tip... 

The difference of relaxation times in different domains makes it possible to observe the spectrum of one of the domains. Figure 10.23(a), shows the Ti-selected spectrum of PVPh/PEO = 40/60 [34]. Since the Ti of crystalline PEO ( 15 s) is much longer than that of the amorphous phase (—0.1 s), it is possible to observe the spectrum of crystalline PEO selectively (indicated by arrow in Fig. 10.23(a)). On the other hand, for the miscible PVPh-rich blend (PVPh/PEO = 58/42), the crystalline-PEO peak is not appreciable. This is in agreement with the above-mentioned results (Table 10.2). The signals of mobile domains/component polymers can be observed selectively by utilizing the weaker dipolar interaction between H. To name a few examples, the dipolar dephasing [128,131,152], the cross-polarization-depolarization [152] and the pulse saturation transfer [151] techniques have been applied. 

Fig. 10.23. (a) Pulse sequence for selective measurement of the crystalline domain of PEO, and the resulting CP/MAS spectra for (b) PVPh/PEO = 58/42 and (c) 40/60. The signal from the crystalline PEO is indicated by an arrow. (Reprinted with permission from Ref. [34]. 1992 American Chemical Society, Washington, DC.)... 

DSC studies on the MEEP-PEO composite system clearly suggests a multiphase character in the composite with both amorphous MEEP-like and crystalline PEO-like phases being present. 7Li-NMR studies based on Ti measurements and line widths have been helpful in distinguishing lithium ions present in the crystalline and amorphous phases. Thus Li+ ion in a crystalline phase is associated with a longer Ti than that in an amorphous phase [207]. Further, it is expected that enhanced ionic mobility (Li+) in polymer electrolytes leads to a line narrowing in the 7Li NMR spectrum of the corresponding ion. Arrhenius plots of 7Li NMR... 

The melting temperatures T of the isothermally crystallized blends were measured both by DSC and optical microscopy by heating the samples directly from T to with heating rates of 20° and l°/min. respectively. The mass crystallinity of the blends was calculated for various T by the ratio between the apparent enthalpy of fusion of DSC endotherms and the enthalpy of fusion of 100% crystalline PEO. 

In any case, the general transition behavior is quite consistent with the notion of separation into two phases one, crystalline PEO the other, glassy PS. Further, the separation must be essentially complete, for the glass phase obviously exists in the presence of the molten crystalline phase (7 > 7 ). (It should be pointed out that a case in which 7 > has never been observed for homopolymers.) The particular finding here, of course, reflects the existence of two phases, each behaving nearly independently with respect to transition temperatures. 

In this context, the term crystal refers to a whole platelet, which in this case contains segments of both crystalline PEO and amorphous PS (see Figure 6.10). 

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