Helical conformation crystalline polymers

In concluding this discussion, it is important to point out that crystalline polymers can be polymorphic because of slight differences in the conformation of the helical disposition of stereoregular polymer chains the polymorphism is attributable to differences in the weak intermolecular bonds. This abstruse phenomenon (which does not have the same centrality in polymer science as it does in inorganic materials science) is treated by Lotz and Wittmann (1993). 

We examine briefly some specific instances starting with syndiotactic polypropylene (sPP). Aside from the already discussed hexagonal mesophase which can be obtained both drawing fibers and under quiescent conditions, this polymer presents four crystalline forms phases I [73] and II [74-76] where chains adopt the (T2G2)n helical conformation, forms III [30] and... 

Table 3 Data relative to selected crystalline polymers for which only chiral crystalline phases, characterized by chiral helical conformations, are known... 

Crystallization of polymers in chiral crystals, even in the case of achiral polymers, is quite frequent and strictly related to the occurrence of helical conformations of the chains. The crystallizable polymer consists of a regular sequence of a chemical repeating unit which can be chiral if it presents an asymmetric center or achiral. On the contrary, helical conformations assumed by the polymer chains in the crystalline state are intrinsically chiral, even though the chemical repeat is achiral. Three possible cases can be distinguished ... 

Figure U. 3/l, 7/2, and 4/1 helical conformations of isotactic polymers in the crystalline state. From ref. (166) Copyright Soc. Chim. llal.

Is a crystalline polymer in which the chains are present in helical conformations, with three repeating units per turn. Because of the added volume which is left between the pendant methyl groups, this opaque polymer has a low specific gravity (0.90) as cited in Table 112. Its index of refraction is 1.49. 

X-ray diffraction techniques can be used to establish the structure of crystalline polymers. Measurements are typically made on crystalline lamellar platelets grown from dilute solution, fibres or stretched films. Such methods have been applied to several different inorganic polymers. For example, based on measurements on stretched samples of silicone rubber, poly(dimethylsiloxane) (Me2SiO) has been shown to possess a helical conformation (Figure 8.4). ... 

In the presence of chiral lithium alkoxides as initiators, chloral forms completely isotactic, crystalline polymers that are insoluble in all solvents (Scheme 71) (166). The polychloral film displays rotation values as high as 3000-5000°. The stereoregular helicity comes about at the stage of the trimer (167). A pure enantiomer of a tert-butoxy-initiated, acetate end-capped pentamer of chloral, which has 4r-helical conformation in both chloroform solution at 35 °C and in the crystalline state, can be obtained by HPLC resolution on a chiral stationary phase (168). 

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

In this section we will discuss the molecular structure of this polymer based on our results mainly from the solid-state 13C NMR, paying particular attention to the phase structure [24]. This polymer has somewhat different character when compared to the crystalline polymers such as polyethylene and poly(tetrameth-ylene) oxide discussed previously. Isotactic polypropylene has a helical molecular chain conformation as the most stable conformation and its amorphous component is in a glassy state at room temperature, while the most stable molecular chain conformation of the polymers examined in the previous sections is planar zig-zag form and their amorphous phase is in the rubbery state at room temperature. This difference will reflect on their phase structure. 

In an attempt to investigate the phase structure of this sample, the line shape analysis of the CH2 resonance line in the DD/MAS spectrum at 87 °C that is shown in Fig. 25 was examined. The result is shown in Fig. 26-(a). The elementary line shape of the crystalline phase was obtained as the line shape of the longest Tic component by Torchia s pulse sequence [53]. It was a doublet and was represented approximately by two down- and upheld Lorentzians with an intensity ration of 2 1 (Spectrum A shown by dotted line in Fig. 26). Since all methylene carbons in the a-crystalline form of this polymer are equivalent in the intramolecular helical conformation, the origin of the doublet could be attrib-... 

As pointed out above with relation to the data at 87 °C, the Tic of the crystalline-amorphous interphase is appreciably longer than that of the amorphous phase, suggesting the retention of the helical molecular chain conformation in the interphase. We also note that a Tic of 65-70 s for the crystalline phase is significantly shorter than that for other crystalline polymers such as polyethylene and poly-(tetramethylene oxide), whose crystalline structure is comprised of planar zig-zag molecular-chain sequences. In the crystalline region composed of helical molecular chains, there may be a minor molecular motion in the TiC frame, with no influence on the crystalline molecular alignment that is detected by X-ray diffraction analyses. Such a relatively short TiC of the crystalline phase may be a character of the crystalline structure that is formed by helical molecular chain sequences. 

Rod-like 6, adopting an almost 73 helical conformation, belongs to a unique set of stiff polymers, exhibiting both TchLC and lyotropic liquid crystallinity. Indeed, experiments demonstrated that solutions of 6 became cholesteric at high concentrations [99]. The isotropic-biphasic phase boundary concentration increases as the molecular weight is increased. This increase has been described theoretically using the molecular parameters determined from dilute solution data. 

Isotactic poly(x-olcfin)s crystallise in a helical conformation, and, in the case of polypropylene, with three units per turn [4,5], Isotactic polypropylene has a melting point of 175°C and does not dissolve in boiling n-heptane [6,7], Note that, depending upon the configuration of the tertiary carbon atom of the polymer main chains, the poly(x-olefin) helices will be characterised by right-handedness or left-handedness. It should be mentioned that the helical structure of the poly(x-olcfin) chain per se is sufficient for the appearance of chirality of such a macromolecule [8], Figure 3.3 presents the helical conformation of chains of isotactic poly(a-olefin)s in the crystalline state (with three units per turn - the case of polypropylene) [5],... 

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