PPTA/sulfuric acid system

Although the presence of solvate phases has been established and qualitative phase diagrams have been published, to our knowledge, a detailed model for a pol3nner solvate and its phase behavior has not been presented. At this time we would like to present a partial phase diagram for the poly(p-phenylene terephthalamide) (PPTA)/sulfuric acid system and a model for the crystal solvate formed. In addition the structure of a model complex will be described. 

Differential Scanning Calorimetry (DSC) of the PPTA/ sulfuric acid system was carried out in order to establish the melting behavior of the system. Samples (5-10 mg) were sealed in gold-coated aluminum pans under N, and cooled at 10 C/min to -150 C. Subsequently the samples were heated to 100 C at 10 C/min and the melting thermograms were recorded. 

GARDNER ETAL. PPTA Sulfuric Acid System... 

The key invention for aramid spinning was the discovery of the liquidpolymer 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. 

Figure 6 shows the diffraction pattern of a 22% PPTA/sulfuric acid solution as a function of temperature. The diffraction patterns can be seen to consist of two components -sharp diffraction peaks superimposed on a broad component. This pattern is consistent with a two phase system containing semicrystalline PPTA/sulfuric acid solvate and disordered components. As the temperature is raised the portion of the diffraction patterns attributable to the solvate phase decreases and finally disappears at temperatures consistent with the melting endotherm observed by DSC. This diffraction pattern agrees wj.t that previously reported for the PPTA/sulfuric acid solvate. ... 

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. 

The temp ture dependence of the viscosity of sulfuric acid solutions of PPTA is very important. Anomalous behavior can be obsmed in the case of anisotropic solutions, manifested by an increase in the viscosity with an increase in the temperature due to the transition of the LC system into the isotropic state where a structural network characterized by higher resistance to flow appears [25]. 

In the PANI/PPTA system, PPTA comes out of sulfuric acid at a water content of about 10% arid PANI comes out at a water content of about 25%. Thus, when spinning fibers into a water coagulating bath, the PPTA comes out first, forms a mechanically connected network. That structure may serve to assemble the conducting PANI along the pre-existing connected paths, thereby leading to the kind of fibrillar conducting pathways that would decrease the percolation threshold to fp 0.16. 

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