Polyamide Solutions

The polyamides are soluble in high strength sulfuric acid or in mixtures of hexamethylphosphoramide, /V, /V- dim ethyl acetam i de and LiCl. In the latter, compHcated relationships exist between solvent composition and the temperature at which the Hquid crystal phase forms. The polyamide solutions show an abmpt decrease in viscosity which is characteristic of mesophase formation when a critical volume fraction of polymer ( ) is exceeded. The viscosity may decrease, however, in the Hquid crystal phase if the molecular ordering allows the rod-shaped entities to gHde past one another more easily despite the higher concentration. The Hquid crystal phase is optically anisotropic and the texture is nematic. The nematic texture can be transformed to a chiral nematic texture by adding chiral species as a dopant or incorporating a chiral unit in the main chain as a copolymer (30). 

Sufficiently concentrated polyamide solutions as needed for physical measurements (e.g., viscosimetry, opytical tests) must be prepared in two steps At first, synthesis of the polyamide by use of precipitation polymerization and secondly, dissolving the carefully washed and dried polymer in concentrated sulfuric acid. 

The use of polyamide solutions for the preparation of graft copolymers is restricted to monomer and initiator systems able to be dissolved and stand strongly polar media such as phenols, organic or inorganic strong adds, or aqueous salt solutions, which are common solvents for the backbone polymer. 

Acrylic acid can be used simultaneously as a monomer and as the solvent, the polyamide solution being frozen at dry ice temperature and ground in a vibration mill. The graft copolymer is said to be formed by mechanosynthesis and was confirmed by nitrogen analysis and solubility as well as swelling characteristics (5). 

The patents 9 47) filed by Richter and Hoebn in du Pont include many aromatic and heterocyclic condensed polymers for asymmetric reverse osmosis membranes, for example, the condensed polymers 4 from 3-arninobcnzhydrazide, 4-amino-benz-hydrazide (5), isophthaloyl chloride, and terephthaloyl chloride (6). The membranes were cast from polyamide solutions containing 0 3 % LiCl in dimethylacetamide,... 

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. 

Figure 6. Scanning electron micrographs of membrane cross-sections prepared from polyamide solutions of different concentration and by different precipitation procedures ...

Figure 1.18 Scanning electron micrographs of membrane cross sections prepared from different polyamide solution concentrations and by different precipitation procedures (a) 10% polyamide in DMAc precipitated by water vapor (b> 10% polyamide in DMAc precipitated in a water bath (c) 22% polyamide in DMAc precipitated in a water bath.

Conio et al. studied the fiber formation of PpBA from an organic solvent, viz. JV,JV -dimethylacetamide containing 3% LiCl, but in these experiments an air gap was hardly used [86]. This may be the reason that they arrived at somewhat different conclusions concerning the spinning of para-aromatic polyamide solutions. 

H. Aoki, J.L. White, and J.F. Fellers, "A Rheological and Optical Properties Investigation of Aliphatic (Nylon 66, P BLG) and Aromatic (Kevlar, Nomex) Polyamide Solutions," J. Appl. Polym. Sci. 23, 2293-2314 (1979). 

Values of k and a in this equation have been measured for several aromatic polyamide solutions in 96% sulfuric acid (18,19). 

Viscosities for polyamide solutions in dimethyl acetamide (DMA) solvent were measured with an Ubbelohdc viscometer. The time required for the air-solution boundary to pass between two marks on the capillary was... 

Carboxyl group concentration is determined by titration of polyamide solutions in benzyl alcohol or a phenol-methanol mixture with 0.1 M aqueous potassium hydroxide solution, using phenolphthalein as indicator. The amino group concentration is evaluated by titration of the polyamide solution with 0.1 M aqueous hydrochloric acid. This method is not applicable to the analysis of polyamides with M higher than about 10 which cannot be dissolved in benzyl alcohol or phenol-methanol mixture with 0.1 M aqueous potassium hydroxide solution. In order to overcome this serious drawback, a new method has been proposed [24]. In this, a mixture of CaCh and methanol with potassium hydroxide as a solvent and HCl solution alone as titrating reagent are used to analyse the carbonyl and the amino groups concurrently. 

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