Crystalline polymers background

Some polymers manifest liquid crystalline ordering, which does not have the full long-range three-dimensional periodicity of crystallinity but is far more ordered than amorphicity. Since many excellent books and articles have been published on such polymers and the author does not have much that is new to add to this background information, very little will be said about polymer liquid crystallinity in this book. Van Krevelen [3] has reviewed liquid crystallinity in polymers in a readable manner and discussed its effects on properties for which quantitative structure-property relationships are available. Adams et al [41] have published a valuable compendium of articles covering the theory, synthesis, physical chemistry, processing and properties of liquid crystalline polymers. Woodward [42] has discussed and illustrated liquid crystallinity in polymers with many beautiful micrographs. 

Figure Z,5. Schematic x-ray diffraction pattern of a partially crystalline polymer. The x-ray crystallinity can be calculated by equation (2,3) after separation of the diffraction pattern into background, and amorphous and crystalline parts as indicated. (I, = la and Q — 4n sin OlA)...

The period under review, January 1977 to December 1978, has seen much activity in the study of polymer crystallization and the properties of partially crystalline polymers. This partly reflects the impact of new and sophisticated developments in conventional spectroscopic techniques, and also developments in synthesis, fractionation, and characterization of polymers and copolymers. The subject is in a state of flux but many of today s apparent diiflculties and inconsistencies will be reconciled by the impact of these various and different techniques. Any attempt to achieve a definitive formulation is premature instead an attempt has been made to highlight some of the controversies which have engaged most interests in this period, supplying some background information to explain the difficulties in each case. 

The density of a crystalline phase must be determined indirecdy in polymers. Examination of crystalline polymers by X-ray difiEraction shows periodicity analogous to that found in simple crystals, but also reveals a background of diffuse scattering arising from the presence of disordered material. Electron and light microscopy fre-quendy show amorphous material between the crystalline aggr ates. The preferred method of determination of the crystalline density in the presence of this amorphous phase is by analysis of the X-ray diffraction pattern. 

Against this background of infusible conducting polymers, the development of the soluble polythiophenes is most interesting. Glass transition temperatures have been reported as 48 °C for poly(3-butylthiophene) and 145 °C for poly(3-methyl-thiophene) 261). These polymers also show crystallinity in films and can be crystallized from solution. Solution studies indicate that there are two chain conformations 262) and the availability of a non-conjugated conformation may be a key to the low transition temperatures and solubility, when compared to the stiff-chain conjugated polymers. 

Nevertheless, when we carry out x-ray crystallinity measurements on textile fibers, we must consider distortions that always affect crystalline material. Even in a completely crystalline material, the scattered x-ray intensity is not located exclusively in the diffraction peaks. That is because the atoms move away from their ideal positions, owing to thermal motion and distortions. Therefore, some of scattered x-rays are distributed over reciprocal space. Because of this distribution, determinations of crystallinity that separate crystalline peaks and background lead to an underestimation of the crystalline fraction of the polymer. In this paper, we attempt to calculate the real crystallinity for textile fibers from apparent values measured on the x-ray pattern. This is done by taking into account the factor of disorder following Ruland s method (3). 

There is overwhelming evidence that the aramide fibres possess a radially oriented system of crystalline supramolecular structure (see Fig. 19.1). The background of the properties, the filament structure, has been studied by Northolt et al. (1974-2005), Baltussen et al. (1996-2001), Picken et al. (2001), Sikkema et al. (2001, 2003), Dobb (1977-1985) and others. The aramid fibres (and the "rigid" extended chain fibres in general) are exceptional insofar as they were - with the rubbers - the first polymer fibres whose experimental stress-strain curve can very well be described by a consistent theory. 

Fig. 6. Liquid crystals of Pt-D -Pd-D1 polymer in a trichloroethylene solution observed between crossed polarizers at about 200X (left) droplets of a liquid-crystalline phase in the dark background of the isotropic phase (right) thin layer of a large liquid-crystalline phase...

A dispersion of spherulitic liquid crystalline particles in brine exists between 0.8 gm/dl NaCl (Figure 2(a), first sample on the left) and 1.2 gm/dl. As the salinity is increased to about 1.4 gm/dl NaCl, the amount of liquid crystals as well as the birefringence increase, and the texture observed using PLS is intermediate between those of the spherulite (S) and lamellar (L) structures. The aqueous solution is a homogeneous lamellar phase between 1.6 and 1.8 gm/dl NaCl. The surfactant molecules form bilayers with their polar heads toward the brine. Figure 3(a) shows the lamellar structure as observed by polarized microscopy at 1.6 gm/dl salt and without any polymer. The bands represent "oily streaks" in a planar background. 

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