Lyotropic solutions

Lyotropic solution stable more than 2 months... 

Fibers and films obtained from lyotropic solutions of rigid polymers exhibit high tensile modulus and strength and are therefore of interest for structural applications. Poly(p-phenylene benzobisthiazole) (PBT) is a rigid polymer from which high... 

Liquid crystalline compounds are remarkable because of their ability to show spontaneous anisotropy and readily induced orientation in the liquid crystalline state. When polymers are processed in the liquid crystalline state, this anisotropy may be maintained in the solid state and can readily lead to the formation of materials of great strength in the direction of orientation. A particularly important example of the use of this property for polymers is in the formation of fibers from aromatic polyamides which are spun from shear oriented liquid crystalline solutions Solutions of poly(benzyl glutamate) also show characteristics of liquid crystalline mesophases, and both of these types of polymers are examples of the lyotropic solution behaviour of rigid rod polymers which was predicted by Flory... 

Isotropic polymeric systems as well as particulate systems might also show time-dependent moduli after cessation of flow. As long as the shear does not induce structure growth, the moduli always increase with time after flow. An increase of the moduli upon cessation of flow has also been reported for thermotropic PLCs (18) as well as for lyotropic solutions of hydroxy propyl cellulose in water (19) and in acetic add (20). The possibility of changing in either direction seems to be characteristic for mesomorphic materials. A fundamental theory for describing complex moduli does not exist for such materials. The present results, combined with the information about optical relaxation mentioned above, could be explained on the basis of reorientation of domains or defects. The different domains orient differently, even randomly, at rest whereas flow causes an overall orientation. Depending on the molecular interaction the flow could then cause an increase or decrease in moduli as recently suggested by Larson (21). 

As shown in Fig. 9, the stress relaxation curves of all AEC/AA solutions collapse into one curve when the solutions were presheared with the same rate. Because the stress relaxation is at the molecular level and the chiro-optical properties reflect the suprastructural level, it is expected that the lyotropic solutions with different chiro-optical properties have the same stress relaxation behavior in both the tumbling and flow-align regions. 

Navard, P. Haudin, J.M. Rheological behavior of isotropic and lyotropic solutions of cellulose. In Cellulose Structure, Modification, and Hydrolysis, Young, R.A., Rowell, R.M., Eds. Wiley New York, 1985 247-261. 

Lizaso, I. Munoz, M.E. Santamaria, A. Transient rheological behavior of lyotropic solutions of ethyl cellulose in m-cresol. Rheol. Acta 1999, 38 (2), 108-116. 

We have found that in dry DMF, the PA-R polymers indeed form lyotropic solutions with a strong light scattering effect. The preliminary studies for PA-OCi(R is methoxy in PA-R) have shown that the scattering is observable at concentrations of about 0.2 wt%. It becomes stronger... 

Millaud and co-workers (12) have examined several of these polymers, particularly the polymer from 2-methyl-l,4-phenylenediamine and terephthaldehyde, for viscosity-light scattering molecular weight relationships, persistence length, light absorption spectra, and the formation of lyotropic solutions in sulfuric acid. 

Other structures capable of forming lyotropic solutions are ... 

The same structural modification concepts, which were utilized to modify the properties of para-linked aromatic LC polyesters, have also been applied to aromatic polyamides. Para-linked aromatic polyamides are an important class of LC polymers. In contrast to thermotropic LC polyesters, para-linked aromatic polyamides form lyotropic solutions. Due to the formation of intermolecular hydrogen bridges, these polymers are in most cases unable to melt below their thermal decomposition temperature. Infusibility and limited solubility of unsubstituted para-linked aromatic polyamides are characteristic properties which limit synthesis, characterization, processing, and applications. 

The solution behavior has been significantly enhanced by the same structural modifications as reported previously for aromatic LC polyesters. For example poly-(p-phenylene terephthalamide) has been modified by bulky, stiff substituents [32], flexible alkyl side chains [33], the incorporation of kinked and double kinked comonomers, and comonomers of different lengths [34], as well as the use of noncoplanar bipheny-lene monomers [35]. To develop high performance materials, modifications that increase the solubility while maintaining the rod-like character, high glass transition temperatures, and the temperature stability are of particular interest. The solubility and the chain stiffness are critical factors in achieving lyotropic solutions. 

Observed structures of a lyotropic material are classified into three categories nematic, smectic, and cholesteric. Nematic and cholesteric mesophases can be readily identified by microscopic examination. The existence of a smectic mesophase is not well defined and is only suggested in some cases. Solvent, solution concentration, polymer molecular weight, and temperature all affect the phase behavior of lyotropic polymer solutions. In general, the phase transition temperature of a lyotropic solution increases with increasing polymer molecular weight and concentration. It is often difficult to determine the critical concentration or transition temperature of a lyotropic polymer solution precisely. Some polymers even degrade below the nematic isotropic transition temperature so that it is impossible to determine the transition temperatures. Phase behavior is also affected by the polymer molecular conformation and intermolecular interactions. 

For fiber preparation, a lyotropic solution is best processed at a solids concentration near the minimum solution viscosity and at a temperature close to its anisotropic transition temperature (Figure 13.2). These conditions maximize solution ordering prior to spinning. 

Lyotropic solutions generally exhibit viscoelastic behavior. They are pseudoplastic and exhibit shear thinning with increasing shear rate. For polymers of near-linear chain conformation, their lyotropic solutions are known to give less die swell and are less tractable than isotropic solutions. The PPTA-H2SO4 solution was the first to be used commercially and has been studied most extensively. 

Work by Suto et al [26] demonstrated that HPC T- m-cresol forms a lyotropic solution with a peak in viscosity vs. concentration at around 20 wt% however no indication was given as to the presence of negative N, Navard and Hauden [28] examined HPC + acetic acid and found... 

An interesting experimental observation was noted by Magda et al, [32] regarding the effect of smoothness of the rheometer plates. For a solution of flexible polymers, two sets of plates gave identical linear normal stress profiles, as required to extract from a combination of point normal force measurements and total thrust measurements. However for the lyotropic solutions, only the smoother plate yielded a linear profile. They concluded that although it is possible to impose... 

Doi [23] had already noted that his theory was restricted to a monodomain or textureless sample. The extension by Marucci and Maffettone [68] retains that restriction. This issue was addressed by Larson and Doi [72], who proposed a model for the rheology of textured lyotropic solutions in the tumbling regime. In the linear Larson-Doi polydomain model the response of the material is expressed in terms of a variable / proportional to the defect density. The defect density is proportional to the shear rate, so that texture refinement is a feature of this model. The steady state predictions for the order parameter S are independent of shear rate. 

Up to this point we have spoken loosely of the band texture as consisting of striations perpendicular to the shearing direction, with width determined by composition, rate of prior shear and time duration after cessation of shear. The disposition of the molecules within the bands has until now not been mentioned. The most detailed investigations have been carried out on dried films of sheared lyotropic solutions, since the structure is obviously stabilized and amenable to careful examination after evaporation of the solvent. 

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