Mesomorphic form

All solid forms lacking order to a substantial degree and ordered liquids (smectic, nematic) are called at times mesomorphic forms. 

In this subsection some examples are presented of polymorphic polymers, for which one or more forms are solid mesomorphic, in the sense described in the Sect. 2.1. Let us recall that in this review we will not deal with liquid mesomorphic forms. 

A conformationally disordered mesomorphic form is present, for instance, in the high-temperature phase I of PTFE. In this form, a long-range 3-D order is present only in the periodic pseudohexagonal placement of the chain axes [49]. In fact intramolecular helix reversals would produce the conformational disorder [50-52] and a complete intermolecular rotational disorder would be also present [49, 52,53]. 

Differently from the cases of ETFE, PTFE, 1,4-trans-polybutadiene, and PE, in the mesomorphic form of i-PP it is believed that there is a nearly complete conformational order, in the chains packed with parallel axes, (intrachain long range 3-D order) and only short-range lateral order in the positioning of the chain axes [60, 61]. 

It is, possibly, also worthy to cite the relevance of some mesomorphic forms in the optimization of the drawing conditions to get highly oriented manufacts although this is, possibly, mainly related to the peculiar morphological organization of these solid mesomorphic forms. 

Analogously, for polyethyleneterephtalate, drawing procedures in multiple steps, with preliminary drawings at room temperature generating a mesomorphic form [127], have been suggested by several authors [128-130], in order to get high modulus and high tenacity fibers. 

Crystalline forms presenting large amounts of disorder of the kind (ii) or (iii) are generally called mesomorphic modifications (Section 3.6), in analogy with the ordered liquids (smectic and nematic). In these cases the lack of periodicities in one or two dimensions (e.g., along the chain axes or along the directions normal to the chain axes) prevents the definition of a unit cell. Typical features in the X-ray diffraction patterns of mesomorphic forms are diffuse halos on the equator or on the layer lines depending on the kind of disorder present. 

Examples of mesomorphic forms characterized by disorder in the conformation of the chains have already been described in Section 2.6. For instance, a mesomorphic form is present in the high-temperature form I of polytetrafluoro-ethylene.106,107 In this phase the chains are in disordered conformation due to the presence of helix reversals along the chains.108-110 Moreover, intermolec-ular disorder is also present due to the random rotations of the chains around the chain axes.109 A long-range three-dimensional order is present only in the pseudohexagonal placement of the chain axes.107,109... 

Another example of a conformationally disordered mesomorphic form is the high-temperature phase of trans-1,4-poly(1,3-butadiene).111112 As discussed in Section 2.6, in this phase the disorder corresponds to a statistical succession... 

Mesomorphic forms characterized by conformationally ordered polymer chains packed in lattices with different kinds of lateral disorder have been described for various isotactic and syndiotactic polymers. For instance, for iPP,706 sPP,201 sPS,202 syndiotactic poly(p-methylstyrene) (sPPMS),203 and syndiotactic poly(m -methylstyrene),204 mesomorphic forms have been found. In all of these cases the X-ray fiber diffraction patterns show diffraction confined in well-defined layer lines, indicating order in the conformation of the chains, but broad reflections and diffuse haloes on the equator and on the other layer lines, indicating the presence of disorder in the arrangement of the chain axes as well as the absence of long-range lateral correlations between the chains. 

In the mesomorphic form of iPP, chains in the ordered 3/1 helical conformation are arranged in small domains where only the first neighboring chains... 

Ziabecki, A. Uber die mesomorphe Form von Polycaproamid und ihre Umwandlung in die kristalline Form. Kolloid-Z. 167, 132—141 (1959). 

The collection of diffraction data of polymers in a solid mesophase for the structural analysis is generally performed on imiaxially oriented samples (fibers) [152-154]. Oriented fibers of high polymers in a mesomorphic form may be easily obtained by extrusion procedures from solution or melt and/or by cold stretching procedures. 

The discussed examples of random rotational and translation displacement disorder of chains around their axes have important implications for the interpretation of the X-ray fiber diffraction patterns of poly(tetrafluoro-ethylene) oriented samples in the mesomorphic forms stable at high temperatures (Sect. 5.7). 

Two possible models of the chains of l,4-trans-poly(l,3-butadiene) in the mesomorphic form II are shown in Fig. 3B, as an example. 

The chain periodicity of nylon 6 in the form cp = 8.35 A) [24] is about one-half that of nylon 6 in the y form (Cy = 16.9 A) [172] and shghtly shorter than one-half the chain periodicity of nylon 6 in the a form (c = 17.2 A) [23]. This indicates that the chains in the mesomorphic form present highly disordered, nearly extended conformations, with a periodicity along the chain axis defined by the average distance along the c axis between the center of mass of consecutive monomeric units [24]. A structural model of form... 

Rg.7 Possible packing model for nylon 6 in the mesomorphic form. The chain axes are placed at the nodes of a bidimensional hexagonal lattice. The concomitant formation of H-bonds along lines parallel to [100], [010] and [iTo] lattice direction is indicated. (Reprinted with permission from [163]. Copyright 1997 by the American Chemical Society)... 

Fig. 16 (a) Experimental X-ray diffraction intensity distribution along the first layer line (/ = 1) of a stretched sample of EP copolymer in the pseudo-hexagonal mesomorphic form solid line). The dashed line indicates the contribution of amorphous phase, and 18° are also present, indicating the presence of (b)-(d) X-ray diffraction intensity distribution along the first layer line calculated for small aggregates of EP copolymer chains, where consecutive boCo layer of 4-6 chains of kind shown in Fig. 15, are faced along the o lattice according to orthorhombic-like (Fig. 15A), monocliniclike (Fig. 15B,B ) and triclinic-like (Fig. 15C,C ) models, with probabilities po, pm and pt, respectively. (Reprinted with permission from [195]. Copyright 1996 by the American Chemical Society)...

Both crystalline forms of ETFE copolymers may be considered as mesomorphic forms [250-253]. In the low-temperature form, the chains are in the ordered trans planar conformation and are packed in a polyethylenelike orthorhombic lattice [250,251]. However, intermolecular order is present... 

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