Liquid crystalline, chain folding

Thin polymer films may also be investigated by TEM and high resolution images are obtained for e.g. thin films of liquid crystalline polymers [64]. Usually thin microtome cuts from bulk samples are investigated, but also epitaxial growth of polyoxymethylene on NaCl [152], chain folding of polyethylene crystals [153], epitaxial crystallization of polypropylene on polystyrene [154] or monomolecular polystyrene particles [155] are observed. The resolution is, however, in most cases not comparable to STM. 

It is now generally accepted that folding is universal for spontaneous, free crystallisation of flexible polymer chains. It was first of all found in crystallisation from very dilute solutions, but it is beyond doubt now, that also spherulites, the normal mode of crystallisation from the melt, are aggregates of platelike crystallites with folded chains, pervaded with amorphous material. "Extended chain crystallisation" only occurs under very special conditions in the case of flexible chains for rigid polymer chains it is the natural mode ("rigid rod-crystallisation" from the melt in case of thermotropic polymers, and from solution in case of the lyotropic liquid-crystalline polymers both of them show nematic ordering in the liquid state). 

For all copolymers studied (copolymers containing between 18 % and 83 % of polypeptide] the liquid crystalline structures are always lamellar and are very similar to the structure of saccharide-peptide block copolymers. In the lamellar structure of copolymers with a polyvinyl block and a hydrophobic polypeptide block, each sheet of thickness d results from the superposition of two layers one of thickness d/ formed by the polyvinyl chains in a more or less random coil conformation, the other of thickness dg formed by the polypeptide chains, in an a helix conformation, arranged in an hexagonal array, and generally folded ( 7, 8]. ... 

Percec and cowoikers [107—110] have synthesized liquid crystalline cyclic oligomeric polyethers based on 1 -(4-hydroxy-4 -biphenyl)-2-(4hydroxyphenyl)butane with dibromoalkanes. Rings varying from 2 to 5 monomer units were prepared and show isotropic-nematic transition. The nematic order is modeled to arise from the collapse of the rings in the form of a folded chain stracture, as shown schematically in Fig. 1.4. This is similar to the case of chain folded crystallization of cycUc alkanes (with 34-288 CH2 groups) and cycUc urethanes [111-114]. 

For a side chain liquid crystalline polymer [8] with a poly(acrylate) main chain and a 2- or 6-fold aliphatic spacer (Fig. 21.13) one finds three relaxation processes The -relaxation, the 8-relaxation, and the dynamic glass... 

Extended chain crystals represent the most thermodynamically stable state. However, kinetically, it is very difficult. Extended chain crystals can be grown only under extreme conditions in flexible chain polymers, such as high pressure and high temperature [9]. Nevertheless, the polymer chains of most thermotropic liquid crystalline polymers are rigid. It is unlikely for rigid, rod-like molecules to form chain-folded crystallization. Macromolecules in rigid chain polymers may take extended chain crystallization [10]. 

It must be emphasized that after the solvent is removed, the polymer crystallizes. Thus the aromatic polyamide fibers, while existing in liquid crystal mesophases during preparation, are highly crystalline when fuUy formed. Again, because of the lack of chain folds and other imperfections, the fibers have higher moduli and higher strength. 

---