Liquid crystalline melts

Since the earliest discoveries of polymeric liquid crystalline melts and solutions a large number of such systems have been reported and this continues to be a vigorous field of research. 

Commercial hydroxypropyl cellulose has been melt-spun from a liquid-crystalline melt at 180 °C to produce fibres with an orientation factor (fc) of 0.6 even at zero take-up stress and 0.7 at higher take-up stresses, a tensile strength of 80 MPa, Young s modulus of 3 GPa and extension at break of 2 to 5 % 71). 

McFarlane, F. E., Nicely, V. A., Davis, T. G. Liquid crystal polymers. II. Preparation and properties of polyester exhibiting liquid-crystalline melts, in Contemporary Topics in Polymer Science, Vol. 2 (ed.) Pearse, E. M., Schaefgen, J. R., p. 109, London, Plenum 1977... 

The first fibers from a thermotropic liquid crystalline melt whose properties were reported were spun from a copolyester of para-hydroxybenzoic acid (PHB) and PET by workers at Tennessee Eastman Co. The preparation of the copolymer proceeds in two stages. First, / ara-acetoxybenzoic acid is reacted with PET in an acidolysis step to give a copolyester prepolymer, which in the second step is condensed further to a higher degree of polymerization suitable for fiber formation. 

When the mol. percent of PHB in the copolymer exceeds about 30-40 percent, a liquid-crystalline melt is obtained. Up to about 60 mol. percent, order in the melt increases and melt viscosity decreases. Compositions containing about 60 mol. percent PHB can be melt-spun into fibers using standard extrusion techniques. It is the unusual combination of properties that makes this class of materials valuable for the formation of high-strength fibers and plastics. 

Among melt-spun fibers, those based on thermotropic liquid-crystalline melts have the highest strength and rigidity reported to date, and appear comparable to polyamides spun from lyotropic liquids-crystalline solutions. This was a very active field of research in the 1970s and later, and many comonomers have been reported. Obviously, these compositions must contain three components at a minimum, but many have four or five com-... 

Various process steps were used to determine their Influence on the morphological nature of liquid crystalline copolyester films. Compression molding was used to form quiescent films, while extenslonal deformation above and below the onset of fluidity, as well as shear deformation above the onset of fluidity was used to make non-quies-cent films. It Is a basic result that molecular orientation can only be achieved when the deformation is done while the polymer is In a liquid crystalline melt state. Experimental details are given In the subsection Materials and Processing, while an interpretation is offered in the discussion in the subsection Morphological and Process Consideration. ... 

From a biologic viewpoint, of greatest interest are the transitions that involve the physiologically important lamellar liquid-crystalline phase, namely, the gel- liquid-crystalline (melting) transition, and the lamellar- nonlamellar mesomorphic transitions. 

One of the most surprising observations was that the X-ray diffraction pattern of the azoxy polymers showed crystalline-like d-spacings in the liquid crystalline melt while in the isotropic region the polymers showed the expected amorphous halo. A possible explanation of such a highly ordered mesophase was the retention of much of the order of the solid in the molten state. A highly structured mesophase occurs in the smectic E state where directional and layered order are maintained in the melt... 

A very interesting area of investigation, where X-ray diffraction coud be useful, would be in the measurement of spacer extension in the liquid crystalline melt. The study of polymers forming the smectic mesophase would be the most informative, since a measure of the interlayer spacing should answer the question of whether or not the spacer is fully extended in the mesophase. To date, however, only a few polymers forming smectic mesophases have been studied by X-ray diffraction. 

Other polymers with oligo ethylene oxide) spacers have been observed. The polymers of limura and coworkers, with mesogenic structures 10 and 12 of Table 1 and spacers of di-, tri- and tetraethylene oxide, had X-ray patterns in the melt that were virtually identical to those of the solid This result was also observed for Polymer 37 with a tetraethylene oxide spacer prepared by us Such patterns are consistent with a smectic E mesophase which maintains much of the solid state order in the liquid crystalline melt. 

It is clear that to differentiate between the two proposed mechanisms, in-depth studies of the structure present in the liquid crystalline melt must be performed. DSC studies in which the melt structure has been quenched might be helpful. The only difficulty is that the transitions observed in the DSC are often very difficult to detect. 

All of the polymers described In Table 4 and In Tables 5 and 6 yield liquid crystalline melts. 

A wide variety of copolymers which give liquid crystalline melts were prepared. Three are shown in Table 6. In the third example a high proportion of hexamethylene units greatly depressed the melting point, yet the melted polymer was liquid crystalline. 

As was stated earlier, the aromatic polyazomethlnes described form liquid crystalline melts. The presence of liquid crystallinity can be determined by observing the high light transmittance of the melt and Its texture on the hot stage of a microscope with crossed polarizers. For a more quantitative examination the microscope can be equipped with a photometer and a recorder for plotting the transmitted light and the temperature of the sample on the hot stage (15). Because of the tendency of the polyazomethlnes to... 

In conclusion then, we have synthesized a series of extended-chain, aromatic polyazomethlnes under on-degradatlve conditions. Fusible, tractable polymers were obtained by use of unsymmetrlcally placed substituents, copolymerization, and/or limited proportions of flexible chain units. Many of the polymers yield liquid crystalline melts which were spun Into oriented, high tenacity, high modulus fibers. The fibers were further strengthened by heat treatment. The ease of preparation of the aromatic polyazomethlnes and the outstanding tenacity and modulus range of the fibers make these products excellent candidates for use as reinforcing fibers In resins and rubber. 

McFarlane and coworkersL have synthesized other polymers containing other moities that are also known to lead to liquid crystallinity in non-polymeric forms. Thus, a wide variety of high molecular weight copolymers exhibiting liquid crystalline melts may be prepared. Structural shapes consisting of a high degree of molecular orientations and chain extensions may be easily fabricated from such liquid crystalline melts. 

X-ray diffraction patterns of fibers spun from liquid crystalline melts of p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) show a high degree of axial orientation. Several meridional maxima are detected which are aperiodic and also change in position and number with the monomer composition. The positions of these maxima can be predicted by calculating the theoretical scattering of random copolymer chains, in which the residues are represented by points separated by the monomer lengths. Both peak positions and intensities are reproduced when intraresidue interferences are allowed for in an atomic model for the random chains. This procedure also allows determination of the stiff-chain persistence (or correlation) length from the breadth of the maximum at d=2.lA which increases from 9 to 13 residues as the HBA content is increased from 25 to 75%. 

Crystalline-1 Crystalline-2 Liquid crystal-1 Liquid crystalline melt-2... 

The authors conclude that "in anisotropic, liquid crystalline melts where domains of local orientation are formed, shearing will not orient molecules if domains are stable. However, in elonga-tional flow, each domain will be stretched out and all the molecules become aligned to the flow direction, yielding indistinct boundaries between domains."... 

FIGURE 17.18 X-ray PF8 polymer fiber diffraction patterns, reported by Grell et al. (fiber c-axis vertical) after the following processes (a) fiber drawn from liquid crystalline melt (b) same fiber as (a), after thermalcrystallization (140 C for 1 h) (c) fiber exposed to saturated toluene vapor (40 C for 3 d) (d) same fiber as (c) after subsequent annealing (135 C for 1 h). (Reprinted from Grell, M., Bradley, D.D.C., Ungar, G., Hill, J., and Whitehead, K.S.,... 

F.E. McFarlane, V.A. Nicely, and T.G. Oavis, "Liquid Crystal Polymers. 11. Preparation and Properties of Polyesters Exhibiting Liquid Crystalline Melts," Contemp. Topics Polym. Sci., 2, 109-138 (1977). 

R.E. Jerman and D.G. Baird, "Rheological Properties of Copolyester Liquid Crystalline Melts. 1. Capillary Rheometry," J. Rheol., 25, 275-292 (1981). 

Also in 1972 (20), Carbomndnm researchers described a family of aromatic copolyesters that were recognized later to form liquid-crystalline melts. The polymers are based on a bisphenol monomer. In 1976, in a patent assigned to Carborundum, a hydroxybenzoic acid-terephthalic acid-bisphenol system, modified and softened with isophthalic acid, was reported to be melt spinnable to prodnce fiber (21). 

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