Poly , PPTA

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. 

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. 

In concluding this section, we should touch upon phase boundary concentration data for poly(p-benzamide) dimethylacetamide + 4% LiCl [89], poly(p-phenylene terephthalamide) (PPTA Kevlar)-sulfuric acid [90], and (hydroxy-propyl)cellulose-dichloroacetic acid solutions [91]. Although not included in Figs. 7 and 8, they show appreciable downward deviations from the prediction by the scaled particle theory for the wormlike hard spherocylinder. Arpin and Strazielle [30] found a negative concentration dependence of the reduced viscosity for PPTA in dilute Solution of sulfuric acid, as often reported on polyelectrolyte systems. Therefore, the deviation of the Ci data for PPTA in sulfuric acid from the scaled particle theory may be attributed to the electrostatic interaction. For the other two systems too, the low C] values may be due to the protonation of the polymer, because the solvents of these systems are very polar. 

The zero-shear viscosity r 0 has been measured for isotropic solutions of various liquid-crystalline polymers over wide ranges of polymer concentration and molecular weight [70,128,132-139]. This quantity is convenient for studying the stiff-chain dynamics in concentrated solution, because its measurement is relatively easy and it is less sensitive to the molecular weight distribution (see below). Here we deal with four stiff-chain polymers well characterized molecu-larly schizophyllan (a triple-helical polysaccharide), xanthan (double-helical ionic polysaccharide), PBLG, and poly (p-phenylene terephthalamide) (PPTA Kevlar). The wormlike chain parameters of these polymers are listed in Tables... 

We compare Eq. (74) with the experimental results for two more stiff-chain polymers, PBLG and poly(p-phenylene terephthalamide) (PPTA). Since avail-... 

In most of the previous work with polyimide fibers, the fibers were spun from poly(amic acid) precursors, which were thermally imidized in the fiber form. However, high degrees of imidization were not achieved. Thus, tensile properties of these polymers were not as good as those of high-performance fibers. Work in our laboratories has shown that when the fibers are spun directly from preimidized polymers, it is possible to achieve tensile properties that are as good or even better than those of poly(p-phenyleneterephthalamide) (PPTA or Kevlar ) fibers. For example, fibers have been prepared from m-cresol solutions of BPDA-PFMB using a dry-jet wet-spinning method. The as-spun fibers were then extensively drawn and annealed above 400°C to achieve excellent mechanical properties. 

PdCI . deposited on poly(p-phenylenc terephthalamide) (PPTA)... 

Sometimes in practice, we may have no choice of solvent for a given polymer. For example, poly(l,4-phenyleneterephthalamide) (PPTA or Kevlar) is only soluble in very strong acids which are viscous. In such cases, ultracentrifugation instead of filtration has to be used to remove dust particles from the solution [14-16,25]. As for copolymers, the selection of proper solvents is even more difficult, because at least two solvents which satisfy the above mentioned three criteria are needed. For this reason, reported characterization of copolymers is quite limited [26-29]. 

Poly-aramides (ARAMIDS) PPTA nh 0 nh ° 0 ° f Kevlar Twaron ... 

Hgurc 4.14 (a) A typical phase diagram for a poly (p-phenylene terephthala-mide)-sulfuric acid system, a solution that is used to make Kevlar fiber (after Kikuchi, 1982). (b) Viscosity versus polymer concentration in PPTA/HjSO solution. Note the sharp drop in viscosity at about 20% which corresponds to a transition between isotropic and nematic liquid crystal phase. 

Many solid-state NMR studies of oriented polymer fibers or film other than silk have been described. Orientation-dependent chemical shielding tensors especially serve as probes with which the relative orientations of specific bond vectors can be determined [10]. This analytical method can be applied to obtain structural information from oriented polyamide fibers such as poly (p-phenylene terephthalamide) (PPTA) [11], poly(m-phenylene isophthalamide) (PMIA) and poly(4-methyl-m-phenylene terephthalamide) (P4M-MPTA) fibers without isotope labeling of the samples [12] (Chapter 12). Oriented carbonyl carbon labeled poly (ethylene terephthalate) (PET) films have also been analyzed with this method [13] (Chapter 14). Especially, more quantitative structural information will be obtained for a locally ordered domain which has been recognized as an amorphous domain in X-ray diffraction analysis in heterogeneous polymer samples. 

Fig. 8.8. Solid-state NMR spectra of the oriented block samples of natural abundance B. mori silk fibroin, poly(p-phenylene terephthal amide) (PPTA), and poly(7-benzyl L-glutamate) (PBLG).

Some recent examples demonstrating the molecular dispersion of rod polymer molecules in coil polymer matrices due to ionic interactions were given by Parker et al. (1996). These systems were based on three types of ionic PPTA s (Figure 5.4) and polar polymers, such as poly(4-vinylpyridine) (PVP), poly(vinyl chloride) (PVC), poly(ethylene oxide) (PEO), and poly(styrene-co-acrylonitrile) (S-AN). Due to the ionic-dipole interactions the rod-coil polymer pairs formed molecular composites as revealed by optical clarity, polarized microscopy, Tg measurements, as well as TEM observations. More significantly the molecular composites based on amorphous matrix polymers (e.g., PVP) were all transparent and showed no phase separation upon heating. Therefore they are melt-processible. As would expected, the mechanical properties of the molecular composites were... 

Poly(p-phenylene terephthalamide) (PPTA) with PA-6 or PA-66 in methanesulfonic acid coagulation resulted in MC that upon thermal treatment phase separated Chuahefa/., 1989... 

PPTA PS poly(p-phenylene terephtalamide) polystyrene V volume fraction of interface... 

Some three decades ago, scientists from the Du Pont company developed polycondensation methods which allowed the preparation of high molecular weight wholly aromatic polyamides. The first commercially produced wholly aromatic polyamide fibre was poly(m-phenyleneisophthalamide) (Nomex, Du Pont, 1967) [la, c]. Some years later, development of the preparation and processing of poly(p-phenyleneterephthalamide) (PPTA) led to the commercialization of the para product Kevlar (Du Pont) in the early seventies [lb, c]. While Nomex shows excellent thermal stability and flame-retardance, and indeed is referred to as a heat and flame resistant aramid fibre, Kevlar fibre also has similar properties, but in addition it has exceptional tensile strength and modulus, and is referred to as an ultra-high strength, high modulus aramid fibre. 

Recently, Reynolds [68] reported the preparation of poly(p-phenyl-eneterephthalamide)propanesulphonate 48 made from the PPTA polyanion and 1,3-propanesultone (Scheme 13). This polyelectrolyte can be solubilised in water to greater than 18 wt % when the amide is alkylsulphonated to 66%, but no lyotropic behaviour is observed. 

Thiophenes have also been used to make liquid crystalline polymers. Polyamides (196) are reported as potentially useful high performance polymers which compare to poly(p-phenylene terephthalamide) (PPTA or Kevlar) <9IPP213,92PP1194>. The polyesters (197) were investigated to examine the effects of nonlinearity on mesophase stability <92MM563>. The insertion of these... 

It is now well established that certain aromatic polyamides form ordered complexes with their solvents (i.e., crystal solvates). A summary of the occurrence of these crystal solvates h s been compiled in a recent review article by lovleva and Papkov. Among the crystal solvates that have been identified jre poly(m-phenylene isophthalamide) with N-methyl-pyrrolidone and with hex methylphosphortriamide poly(p-benz-amide) with sulfuric acid and poly(p-phenylene terephthalamide) (PPTA) with sulfuric acid and with hexamethylphosphortriamide. These solvates are all characterized by a discrete melting point and a crystalline diffraction pattern. 

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