Atactic crystal phase

DSC melting peak, was used as an indieator of how the two crystal phases were present in the PVA blend. To caleulate FWHM, the peak value of the DSC trace is located. The full width of the melting peak is then evaluated at a distance halfway between the peak value and the DSC traee baseline. In Tanigami s work, the melting peak FWHM increased to 25-3 5 C when the atactic-rich and the syndiotaetic-rieh phases coexisted, whereas it remained approximately 10°C when either of the two erystal phases was in exeess. The FWHM for our nanocomposite combined/dual melting peaks is plotted in Fig. 8.15(a), as a funetion of the silieate loading (i mmt)- The full width inereases sharply from about 13°C to above 25 C as the silicate eomposition erosses the pereolation... 

The details of the interchain packing determine whether there will be any crystalline phase at all, what the maximum possible crystallinity is, and what the rate of crystallization is under a given set of conditions [30]. It is, for example, entirely possible and very common for a polymer to be anticipated to have a very high Tm but to be completely amorphous because it has irregular chains which do not pack well in a crystalline lattice. For example, (a) the absence of stereoregularity makes crystalline packing impossible in atactic polymers with a vinyl-type... 

Discussion of the effect of the polymer backbone in Sec. 3.4 of this chapter already provided examples of highly isotactic po-ly(f )-endo,exo-5,6-di [n-[4 -(4"-methox-yphenyl)phenoxy]alkyl]carbonyl bicy-clo[2.2.1]hept-2-ene s [190] and syndiotactic poly n-[4 -(4"-methoxyphenyl)phen-oxyjalkoxy methacrylate s [42, 44] which crystallize and form more ordered meso-phases than those of the corresponding atactic polymers (Fig. 15). Although more flexible backbones are more able to achieve the conformation necessary to order, the side chains are evidently already attached to the polymer backbone of these tactic polymers with the proper configuration to order, which obviates the need to distort their conformation for such purposes. 

Fully amorphous polyethylene is rather difficult to make because of its fast crystallization. By superfast quenching it was, however, possible to make small amounts of polyethylene glass of phase area 11 [6]. Glassy polymers are much more easily available for macromolecules that crystallize slowly, such as poly(ethylene terephthalate) and poly(oxy-l, 4-phenyleneoxy-1,4-phenylenecarboxy-1,4-phenylene) or macromolecules that have structural irregularities which prohibit crystallization completely, such as atactic polystyrene andpoly(methyl methacrylate). More details about the special properties of the transition 11— are discussed in Sect. 6.3. 

LL phase separation may play a key role in the gelation of some noncrystalline polymers such as atactic polystyrene (a-PS) [34-36]. For a-PS, the physical junctions are not crystals and there are no specific interchain interactions, yet a-PS can form gels. Amauts and Berghmans were the first to explain the gelation of a-PS in terms of LL phase separation combined with vitrification [36]. In this case, vitrification acts as the agency responsible for arrest of the LL phase separation, and locks the system in a metastable state. 

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