Polymers without Liquid Crystalline Phases

As a consequence of this almost perfect alignment of molecule structures, such polyamides are able to orientate in solution and to form liquid crystalline phases (see Sect. 1.3.4). Out of these solutions one obtains fibers of poly(p-phenylene terephthalamide) (PPTA) having 5-10-fold higher values for stiffness and strength as the all-mefa linked polymers. In addition, PPTA crystallizes, whereupon the fibers achieve an extraordinary temperature resistance in a nitrogen atmosphere they decompose at temperatures above 550 °C without melting. 

We see that the properties of the intramolecular liquid-crystalline phase in the long macromolecules are universal, i.e. not sensitive to the polymer chain models (compare the results of this section for the models shown in Figs. 7b-d). It is due to this universality that it has become possible to establish the main properties of the large globules using only general considerations and without referring to the special formalism. 

The same authors increased the complexity of their systems by introducing in a polyester chain both ionic and chiral chain segments. The series containing both the isosorbide chiral units and the ionic moieties yielded chiral smectic C (SmC ) and chiral smectic B (SmB ) liquid-crystalline phases, exhibiting broken focal-conic texture and schlieren texture. Not surprisingly, the analogous polymer without the chiral units exhibited only the nonchiral SmC mesophase. On the other hand, in this case, the effect of ionic units on the phase behavior was negligible [91]. 

Very recently, Schmidt et al. synthesized novel polyamides 51 by using arylsubstituted terephthalic acids moieties such as para- or ortfto-terphenyI-2,5-dicarboxylic acids in combination with substituted and non-coplanar diamines [70]. Those polyamides 51 from substituted diacids, diamines or non-coplanar diamines showed high solubility in DMAc, and in most cases without addition of inorganic salts (LiCl). In DMAc (LiCl), polyamide 51ca (rjinh = 1-63 dl/g) forms a lyotropic liquid crystalline phase at > 8 wt % at room temperature, and at > 5 wt % at 110 °C for polyamide 51ae (T)i h = 3.58 dl/g). On the other hand, with the copolyamides 52 the critical concentrations of liquid crystal formation are around 40-45% at room temperature. For these copolyamides, concentrated solutions in DMAc/LiCl with polymer concentration up to 50wt% could be prepared for polymers with x > 0.6. 

As long as the crosslinking density is low, the liquid-crystalline phases of the corresponding uncrosslinked polymer are retained for the networks [94-97], with the same structure and without large shifts of the transition temperatures (see Chap. V of this Volume). As the crosslinking density increases, the smectic phases should disappear (Table 9) for the benefit of a less-ordered nematic state [98] then all the liquid-crystalline phases should be destroyed for the most crosslinked networks, at least for materials crosslinked in an isotropic state [99-101]. Degert et al. [98] pointed out that this evolution in the mesophase stability also depends on the nature (mesogenic or aliphatic) of the crosslinks. 

The irradiation of natural rubber in the presence of a vinyl monomer thus leads primarily to a synthesis of graft copolymers, but some block copolymer is certainly always present. Irradiation syntheses may be carried out in solution, either in contact with liquid monomer (with or without a diluent) or in contact with monomer in the vapor phase, or in emulsion or suspension. The rubber may be preirradiated in the absence of air to produce free radicals for later monomer addition, but the life of these radicals is short as a result of mobility within the rubber matrix. Irradiation at very low temperatures makes it possible to use the trapped radicals technique for a variety of natural and synthetic rubbers. Plastics and polymers with a crystalline phase are more readily preirradiated to initiate later grafting by trapped radicals. Irradiation may also be carried out in air to introduce peroxide groupings ... 

Besides being at the origin of lyotropic phases, cellulose derivatives can also originate thermotropic liquid crystalline phases without solvent. This behavior is an indication that lateral chains act as solvent, or plasticizer, increasing the mobility of the polymer backbone. 

Indeed, PBA and PPTA, which are soluble in concentrated sulfuric acid, display simple lyotropic behavior, closely obeying the simple relation (2). Figure 3 shows a plot of the minimum polymer concentration for the onset of the formation of an anisotropic, liquid crystalline phase versus the inherent viscosity of PPTA. The data in Figure 3, taken from ref. 3, reveal the systematic, inverse relation between the minimum concentration and the polymer viscosity. The latter quantity scales with the molecular weight M approximately as M l+ (12). The ordered phase of the present lyotropic aramids is nematic, and consist of arrays of approximately parallel chains, presumably without regular arrangement of the end groups or chain units. 

Figure 9.22 shows the imusual concentration dependence of the solution viscosity of lyotropic liquid crystalline polymers. With increase in polymer concentration, the viscosity first increases. However, the viscosity decreases rapidly after the concentration reaches a critical value. This is because at low concentrations, the liquid crystalline phase has not been formed and the polymer solution is isotropic, without ordered distribution of polymer chains. The viscosity of the isotropic solution increases with increasing polymer concentration. When the... 

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