Lyotropic polyamides

Similady, hquid-crystal polymers exhibit considerable order in the hquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (atamid) with the repeating unit designated as (2). It is spun into... 

An increase in rod-like arrangement of the macromolecules can also arise by stretching a polymer either in its solid state, either in the melt or even in solution (for polymers leading to lyotropic liquid crystals such as aromatic polyamides). This is the basis of the development of synthetic fibres including high modulus polyethylene Dyneema , polyamide Nylons and Kevlar , polyester Tergal or Dacron fibres. 

Lyotropic polymeric LC, formed by dissolving two aromatic polyamides in concentrated sulphuric acid, have been studied using variable-director 13C NMR experiments.324 The experimental line shapes at different angles w.r.t the external field were used to extract macromolecular order and dynamic in these ordered fluids. An interesting application of lyotropic LC is for the chiral discrimination of R- and S-enantiomers, and has recently been demonstrated by Courtieu and co-workers.325 The idea was to include a chiral compound 1-deutero-l-phenylethanol in a chiral cage (e.g., /1-cyclodextrin) which was dissolved and oriented by the nematic mean field in a cromolyn-water system. Proton-decoupled 2H NMR spectrum clearly showed the quad-rupolar splittings of the R- and S-enantiomers. The technique is applicable to water-soluble solutes. 

Since Robinson [1] discovered cholesteric liquid-crystal phases in concentrated a-helical polypeptide solutions, lyotropic liquid crystallinity has been reported for such polymers as aromatic polyamides, heterocyclic polymers, DNA, cellulose and its derivatives, and some helical polysaccharides. These polymers have a structural feature in common, which is elongated (or asymmetric) shape or chain stiffness characterized by a relatively large persistence length. The minimum persistence length required for lyotropic liquid crystallinity is several nanometers1. 

A major impetus was given to work, both academic and industrial, in the field of lyotropic systems by the development by duPont of commercial fibres having exceptionally high tensile strength and modulus through use of nematic anisotropic solutions of relatively rigid-chain aromatic polyamides. The earliest product to appear, Fibre B, was based upon poly (p-benzamide) (I)10), but was replaced by the fully commercial product, Kevlar, based upon poly (p-phenylene terephthalamide) (II) U). Arenka, from Akzo, also has the latter chemical repeating unit. 

Both lyotropic and thermotropic liquid-crystalline synthetic polymers have been widely studied. Aromatic polyamides constitute the most important class forming liquid-crystalline solutions the solvents are either powerfully protonating acids such as 100% sulphuric acid, chloro-, fluoro- or methane-sulphonic acid, and anhydrous hydrogen fluoride, or aprotic dipolar solvents such as dimethyl acetamide containing a small percentage, usually 2-5 %, of a salt such as lithium chloride or calcium chloride. Such solutions constitute a nematic phase within certain limits. Some criteria for formation of a nematic instead of an isotropic phase are ... 

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

Introduction of ring-opening metathesis as a versatile polymerisation technique (ROMP) by Chauvin and Herisson Nobel Prize Chemistry to Paul J. Flory for his fundamental achievements, both theoretical and experimental, in the physical chemistry of the macromolecules Fully aromatic polyamides developed Aramids, being lyotropic liquid crystalline polymers of high strength, due to extended molecular chains (Morgan and Kwolek)... 

Solutions of the aromatic polyamides (PpBA, PpPTA and PmPTA), the polybenzazoles (PBT and PBO), poly(benzyl glutamate) (PBG) and hydroxypropylcellulose (HPC) are the most studied main chain lyotropic systems and our understanding of the behaviour of lyotropics is based on investigations of this relatively small number of materials (Moldenaers, 1996). They form main chain liquid crystals because of their rigid molecular structure in the appropriate solvents. Two kinds of solvents are used (Collyer, 1996) ... 

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-... 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

One of the characteristic properties of rod-like polymers is that their concentrated solutions form lyotropic liquid crystals. Such examples among synthetic polymers are polyamides -and polyisocyanates " which form cholesteric or nematic liquid crystals in selected solvents. 

The range of polymers which were found to be able to form liquid crystalhne systems has been considerably extended. Poly(Y-benzyl-L-glutamate) and its analogs, as well as para-aromatic polyamides, exhibit this property in solutions, which served the basis for relating them to lyotropic liquid crystals (see Sect. 2.1). Subsequently, the classes of polymers were found which exhibited such a transition during a change of temperature thermotropic liquid crystals). 

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... 

Recent developments in the substitution of completely aromatic LC polyesters have produced polymers which show improved solubilities and reduced transition temperatures (29). The presence of these side groups provides a method for producing polymers that are compatible with other similarly modified polymers. In this way, blends of rigid and flexible polymers can be prepared. Substituents have included alkyl, alkoxy (30) and phenyl alkyl groups (21), some of which lead to mesophases that have been reported as being "sanidic" or board-like. This approach has been used with both polyesters and polyamides and has lead to lyotropic and thermotropic polymers depending on the particular composition used. Some compositions even show die ability to form both lyotropic and thermotropic mesophases (22). 

The preparations of thermotropic polyester-amides from comparable monomers as those of thermotropic polyesters were a logical extension of a series of studies in thermotropic polyesters and lyotropic polyamides. (4.) However the inclusion of carbonates had rarely been explored.(5) Because of the flexibility of carbonate compared to substituted aromatic rings, it should be an even more effective approach in lowering the melting temperatures of the unmodified all aromatic polyesters into the easily processable range. 

Solubilities. Rogers et al. (16-18) have prepared and reported on a large number of aromatic polyesters and polyamides containing the 2,2 -disubstituted 4,4 -biphenylene moiety. Some of the polymers were soluble in common solvents such as tetrahydrofuran and acetone, with solubility as high as 50% in one case. The solubilities of the amide monomers synthesized in this work are shown in Table I. These compounds exhibited virtually no solubility in common solvents such as acetone but displayed fairly high solubilities in several amide solvents, with and without added salts. Lyotropic liquid crystallinity was not observed although solubilities were as high as 40% (w/w) in some solvents. 

A variable-director NMR method was used to investigate the order and dynamics of aromatic polyamides in lyotropic liquid crystalline phases. " In... 

The asymmetric polyamide membranes were prepared using a casting solution consisting of 15 g polymer, 85 g DMAC and 35 wt 2 LiNO, (bop) as the lyotropic salt. Salt concentrations in the casting solution are conveniently expressed as percent based on polymer (BOP). The doped polyamide solution was cast on a Pyrex glass plate using a 25-mil doctor knife. The film was then partially dried to obtain a gel membrane of polymer/solvent/salt. The air side (skin side) was identified and the gel film immersed in ice water. After an hour in ice water, the membrane was transferred to distilled water and stored until it was tested. 

In addition to DMAC, the solvents DMF, DMSO and UMP can be used for casting aromatic polyamide membranes. Lyotropic salts that give good polyamide membranes are those with lithium, calcium and magnesium as the cation and chloride, bromide, iodide, nitrate, thiocyanate and perchlorate as the anion. 

Whang and Wu [3] have described the liquid crystalline state of polyimide precursors and shown that certain polyamic acids derived from pyromellitic anhydride exhibit lyotropic behaviour. Liquid crystal phases have also been observed by Wenzel et al. [4] in polyimides derived from pyromellitic anhydride and 2,5-di-n-alkoxy-1,4-phenyl ene diisocyanate. Dezern [5] has disclosed a synthesis for linear polyamide-imides derived from benzophenone dianhydride but the occurrence or otherwise of mesophases is not mentioned. 

Within the extensive literature on this subject, there are many examples of the synthesis of thermotropic polyesters, polyesteramides, polycarbonates, polyethers, polyurethanes and polyester-imides. Until recently, the main omissions had been thermotropic polyamides and polyimides however, many examples of polyamides that show lyotropic behaviour have been known for a long time. 

A series of lyotropic poly(imide-amide)s were synthesised [39], in which an ordered unit comprising trimellitimide condensed with p-aminobenzoic acid andp-phenylene diamine (PPD) was copolymerised in varying ratios (0,50 and 80%) with a terephthalic acid/PPD polyamide. The MI predicts all three polymers to be LC, but lyotropic behaviour was observed in only the first one (Table 13). This is particularly surprising because incorporating increasing quantities of the known lyotropic and more linear TPA/PPD units would be expected to produce definite lyotropic behaviour. Similarly, a copoly(amide-imi-de) of 3,3 -dimethyl benzidene and 4-amino benzoic trimellitimide was predicted by the MI to possess a mesophase but none was detected. These apparent failures may be due to the fact that the polymers which did show lyotropic behav-... 

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