PPTA/sulfuric acid system concentration

The key invention for aramid spinning was the discovery of the liquidPPTA-sulfuric acid systems at polymer concentrations above 10%. Morgan (Monsanto) was the first to describe spinning of fully aromatic polymers from concentrated solutions (in organic solvents), but did not report their liquid-crystalline behavior [11]. Kwolek (DuPont) patented optically anisotropic solutions of PPTA in sulfuric acid [12] and Blades (DuPont) further specified the use of an air gap in the spinning process [13]. 

The change in the viscous properties of solutions and consequently the torque (power consumed) on the mixer shaft is directly correlated with the phase transitions in the PPTA-sulfuric acid system (Fig. 10.3) [24]. In passing through a critical concentration, i.e, from iso- to anisotrqiic solutions, an extreme dependence of the viscosity on the concentration with a maximum for phase inversion where the LC phase becomes a continuous matrix is obs ed. There is a sharp decrease in the viscosity, whose basic cause is the anisotropy of the viscosity (cf. ChaptCT 9), with a further increase in the concentration of the polymer. 

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. 

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 dependence of the heat of activation of flow on the concentration of copolymer based on PPTA in sulfuric acid is shown in Fig. 9.25. An increase in the steepness in the pretransition region and a sharp decrease in for c> c are characteristic of this dependence. The value of E decreases by approximately 3 times in the range of concentrations of =1% in the region of the transition from the isotropic to the LC state. Not only the heat of activation but also generally the temperature dependence of the viscosity are the same for different concentrations for LC solutions. This can indicate the preservation of the same kinetic unit of flow on the segment of quasi-Newtonian flow (at the limit the molecule or segment), which is characteristic of systems with a continuous LC phase. 

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