Syndiotactic crystal phase

However, the syndiotactic segments of extended chains (613 cm"1) are obviously situated in a different local environment. These more rigid segments form into microcrystalline phases which orient as units to much higher degrees than the amorphous isotactic sequences. The C-Cl peak at 613 cm"1 arises mainly from seqeunces of four or more trans conformations in syndiotactic repeat units. With syndiotactic sequences of four or more repeat units, crystallinity can form (17,18). This ease of crystallization has been ascribed to the strong dipole-dipole interaction between the C-Cl bonds. 

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. 

Above the the rate of polymer chain relaxation is faster than the diffusion of CO2, and hence Fickian diffusion is to be expected. The diffusion of CO2 is believed to occur within the amorphous domains of the polymer matrix, and for this reason the diffusion in semi-crystalline polymers may be more complex than it in the case for glassy polymers. In the case of semi-crystalline polymers, CO2 is not soluble in the crystalline domains, and therefore the degree of crys-taUinity and hence the amorphous fraction available for CO2 molecules may influence the diffusion characteristics. Furthermore, C02-induced crystallization is likely to lead to an increase in the tortuosity factor, and thus the diffusion path length may increase as a function of time. Syndiotactic polystyrene and poly(4-methyl-l-pentene) [45] are semi-crystalline polymers which have crystalline phases (helical in the case of sPS) with lower densities than that of the amorphous phase and are exceptions, as CO2 access is not restricted to the amorphous domains, in fact CO2 diffuses faster in the helical sPS than in the amorphous polymer [46]. 

Keywords epitaxy, isotactic polypropylene, syndiotactic polypropylene, crystal structure, crystal polymorphism, helical hand, nucleating agents, AFM, a-phase, 3-phase, lamellae. 

Rodriguez-Arnold,., Zhengzheng, B. and Cheng, S.Z.D. (1995) Crystal structure, morphology, and phase transitions in syndiotactic polypropylene. MS Rev. Macromol. Chem. Phys., C35(l), 117-154. 

Finally, a few comments about the uniqueness of polymer crystal structures and phase space localization are warranted. Almost all crystallizable polymers exhibit polymorphism, the ability to form different crystal structures as a result of changes in thermodynamic conditions (e.g., temperature or pressure) or process history (e.g., crystallization conditions) [12]. Two or more polymorphs of a given polymer result when their crystal structures are nearly iso-energetic, such that small changes in thermodynamic conditions or kinetic factors cause one or another, or both, to form. Polymorphism may arise as a result of competitive conformations of the chain, as in the case of syndiotactic polystyrene, or as a result of competitive packing modes of molecules with similar conformations, as in the case of isotactic polypropylene. In some instances, the conformational change may be quite subtle isotactic polybutene, for example, exhibits... 

Thermoreversible gelation, as a consequence of crystallization from dilute solutions of random copolymers has been observed in a variety of mixtures. These include, among others, poly(vinyl chloride) in dioctyl phthalate,(55) poly-(acrylonitrile) in dimethyl formamide,(56) nitrocellulose in ethyl alcohol,(272) methyl cellulose in water,(273) ethylene copolymers,(274) syndiotactic isotactic and atactic poly(styrene),(275-279) and random copolymers of ethylene terephtha-late with isophthalate.(280) Flory and Garrett (281) have shown that the classical thermoreversible gelation system, gelatin in water, is the result of a crystal-liquid transformation. The gelation or dissolution can be treated as a first-order phase transition. 

Fig. 6.14 Pseudo phase diagram of syndiotactic poly(styrene) in trans decalin. O crystallization of helix conformation crystallization of zigzag conformation melting of helix conformation melting of zigzag conformation. (From Deberdt and Berghmans (292))...

Syndiotactic polypropylene (s-PP) presents a very complex polymorphic behavior (the first part of this section on s-PP modifications is reported in the publication [203-212]). s-PP chains crystallize to various polymorphic crystalline phases and,... 

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