Lyotropic LCP

Lyotropic LCP s are processed from a solution, thermotropic ones from the melt. In both cases the flow pattern provides the necessary orientation of the stiff chains. 

Perhaps the most widely utilized (and studied) lyotropic LCP is poly j -phenylene terephthalamide (PPTA), more commonly known as Kevlar (see Figure 1.70). Kevlar belongs to the class of aramids that are well known for their LCP properties. Because these polymers are crystalline in solution, they are often spun into filaments, from which the solvent is subsequently removed in order to retain the aligned polymer structure. The result is a highly oriented, strong filament that can be used for a wide variety of structural applications. Most thermotropic LCPs are polyesters or copolymers that can be melted and molded into strong, durable objects. 

A well-known example of a lyotropic LCP is the aramide fibre (Twaron or Kevlar), an aromatic polyamide with the structure ... 

B) Flory s lattice theory (1956) has received most attention. It is an extension of Onsager s model to higher concentrations. It is ideally suited for lyotropic LCPs consisting of solvent and rigid rods. It predicts that at a phase transition an isotropic and a nematic phase coexist with respective volume fractions... 

The general description of Eq. (16.60) is in agreement with experiment for a variety of lyotropic LCPs. A consequence of the theory is that for a polydisperse system of rods fractionation will occur longer rods will become part of the anisotropic phase and the smaller ones of the isotropic phase. The major drawback to his lattice theory is that it is athermal, just like the Onsager model and therefore also fails to predict the occurrence of the clearing temperature. 

For lyotropic LCPs, there is a hiphasic window of concentrations over which nematic and isotropic phases coexist, t he polymer concentrations in the coexisting isotropic and nematic phases are designated by C (or = Trd Lv /A) and Cj (or (pj = jrd Lv2/A), respectively. There is also a theoretical concentration C, at which the isotropic phase becomes unstable to orientational fluctuations. According to the Onsager theory, 02/0 = 1.047 and 02/0i = 1.27 (see Section 2.2.2). Thermotropic LCPs often have a biphasic window of temperatures over which isotropic and nematic phases coexist. This biphasic window exists in nominally single-component thermotropics because of polydispersity the nematic phase is typically enriched in the longer molecules relative to the coexisting isotropic phase (D Allest et al. 1986). 

In lyotropic LCPs, the shear-rate range over which Region I exists is sensitive to polymer concentration (Walker and Wagner 1994). In one batch of PBLG, Region I appears suddenly as the polymer concentration is increased above about 37% (see Fig. 11-9). In other PBLG solutions. Region I behavior has been reported at much lower concentrations... 

The microstructure and polymer-solvent interactions of lyotropic cellulosic mesophases can be derived from rheological studies. The lyotropic LCP solution is a complicated system and a wide range of unusual rheological phenomena have been observed. 

Flexible polymers Lyotropic LCP Main-chain type LCP Mesogen-jacketed LCP... 

The fairly good quality of the fits validates both Leadbetter s assumptions and the Maier-Saupe distribution function. However, the values of S obtained and even the quality of the fits obviously depend on the odd or even number of (CH2) groups in the flexible spacer. This odd-even effect is widespread and well known in the field of main-chain LCPs and will be discussed later in this article. The nematic order parameter of main-chain LCPs may reach values as high as 0.85 which demonstrates the very high orientation of the nematic phase of these polymers. Such a large orientation is undoubtedly responsible for the good mechanical properties of this type of materials. The treatment described above therefore provides a very easy way of characterizing the orientational order of a nematic phase. It has also been tested for thermotropic side-chain LCPs and found to be satisfactory as well [15]. Unfortunately, it has not been used yet in the case of lyotropic LCPs except for some aqueous suspensions of mineral ribbons (Sect. 5) which are not quite typical of this family of materials. 

In this section, we consider the case of solutions of rigid or semi-flexible polymers which display one or several liquid crystalline phases in a given range of concentration. The main control parameter is not flie temperature as is the case for thermotropic LCPs but rather the concentration of polymer in the solvent. There are many different kinds of lyotropic LCPs. Some are synthetic like Kevlar which has become a very important structural material with mechanical properties comparable to those of steel. Some are natural like the Tobacco Mosaic Virus and like DNA which shows a nematic and a hexagonal phase. Some are mineral like the vanadium pentoxide ribbons. In the next section, we shall first describe the lyotropic system which is probably best known, namely the tobacco mosaic virus. 

Lyotropic LCPs are often used as structural materials and are therefore processed in industrial facilities. These processes usually involve flow so that it is extremely important to understand the rheological properties of these materials. These properties are of course related to the molecular organization which can be probed by X-ray scattering. Unfortunately, the flow behavior of LCPs is very complicated both from the theoretical and the experimental points of view. Therefore, we shall only briefly review this topic. 

Through the study of the different topics considered in this article, it was shown how X-ray scattering is a useful tool to characterize the most salient features of the mesophases of LCPs. For instance, a simple procedure can be used to measure the nematic order parameter and it is so far valid for all kinds of LCPs based on rod-like moieties. In the case of main-chain polymers, useful information about the conformation of the repeat unit can also be deduced from the diffuse scattering. In the case of side-chain polymers, not only the smectic period but also the amplitude and shape of the smectic modulation can be derived from the measurement of the smectic reflection intensities. Moreover, fluctuations and localized defects may be detected through their contribution to the diffuse scattering. The average distance between lyotropic LCPs can be measured as a function of concentration which tells us the kind of local packing of the particles. 

Among lyotropic LCPs, there are at least two rod-like polymers with excellent thermal and mechanical properties. These two polymers are poly(p-phenylenebenzobisoxazole) (PBO)... 

Since the discovery of Kevlar, scientists world-wide have attempted to synthesize many thermotropic LCPs that the liquid crystalline state can be formed in a melt instead of in a solution as lyotropic LCPs. Generally, two kinds of thermotropic LCPs can be prepared depending on the position of mesogens (J9.) ... 

Ordered behavior is also observed in solutions of some liquid crystal polymers (lyotropic LCPs). Unlike flexible polymers that assume a random coil conformation in solution, the rigid polymers being... 

Lyotropic LCPs exhibit quite characteristic viscosity behavior in solution as the polymer concentration in solution is changed. Typically the viscosity follows the trend shown in Figure 5.6. As more and more polymer is added to the solvent, the viscosity increases while the solution remains isotropic and clear. At a critical concentration (which depends on the polymer and the solvent) the solution becomes opaque and anisotropic and there occurs a sharp fall in viscosity with further increase in the polymer concentration. This results from the formation of oriented nematic domains in which the chains are now aligned in the direction of flow, thereby reducing the frictional drag on the molecules. 

Many unique LCP characteristics come from this structure. LCP is also classified into thermotropic and lyotropic LCP, just as conventional polymers, namely thermoplastics and thermoset. Thermotropic LCP corresponds to thermoplastics because it reveals its liquid crystal nature in the molten stage. 

Aramid fiber (lyotropic LCP) is known as a super high modulus fiber. Recently, a fiber of thermotropic LCP has been introduced. This fiber is made by polyester and exhibits high modulus as that of aramid. (Table 5)... 

The second and third sections deal with thermotropic and lyotropic LCP systems. Papers are grouped together by structure of the polymer chain rather than by the nature of properties investigated. The reader can thus compare research on similar polymers approached from different perspectives. 

Although in many studies the viscoelastic behaviour of thermotropic and lyotropic LCPs in bulk, with their good mechanical properties and processability, has been described, only a few publications deal with the viscoelastic properties of their thermoreversible gels because, in generaL it is difficult to dissolve them on a molecular scale, and immediate applications are unUkely. 

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