Amorphous polyamide

Only a few commercial uses for TDA per se have been found. In epoxy curing appHcations, 2,4- I DA has been used as a component of a eutectic mixture with short chain aUphatic glycidal ether resins (46) as well as by itself (46,47) TDA (46) and single isomers (47) are also used as amine curatives. TDA can be used as a chain extender in polyurethanes (48,49). TDA is cited as a monomer in making aromatic polymers with unique properties, eg, amorphous polyamides (50), powdered polyamides (51), and low melting, whoUy aromatic polyamides (52). 

Polyamides (nylons) The main types of nylon are oil and petrol resistant, but on the other hand susceptible to high water absorption and to hydrolysis. There are a few solvents such as phenol, cresol and formic acid. Special grades include a water-soluble nylon, amorphous copolymers and low molecular weight grades used in conjunction with epoxide resins. Transparent amorphous polyamides are also now available. 

Semi-aromatic polyamides, polyphthalamide (PPA), polyarylamide (PAA), transparent amorphous polyamide (PA-T)... 

Table I. Amorphous Polyamides Based on Terephthalic Acid by Gabler et al.

The glass temperatures of all products fall within the range 130°-160 °C. The partially aromatic polycarbonate has a glass temperature of 150 °C. the partially aromatic PETP has a glass temperature of 75 °C. To determine the ratio of the aromatic to the aliphatic content, we assume four chain links for the benzene rings. Then the polycarbonate has the ratio aromatic/aliphatic links = 4 6, the polyester has a ratio 4 2, and the polyamides (Table I) have a ratio 4 10. Despite the lower aromatic content the glass temperatures of some of the amorphous polyamide are... 

Lee and Char [93] studied the reinforcement of the interface between an amorphous polyamide (PA) and polystyrene with the addition of thin layers of a random copolymer of styrene-maleic anhydride (with 8% MA) sandwiched at the interface. After annealing above the Tg of PS, they found significantly higher values of Qc for samples prepared with thinner layers of SMA than for the thicker ones. They initially rationalized their results by invoking the competition between the reaction rate at the interface and the diffusion rate of the SMA away from the interface. For very thick layers, and therefore also for pure SMA, the reaction rate was much faster than the diffusion rate away from the interface and favored therefore a multiple stitching architecture, as shown schematically in Fig. 50. Such an interfacial molecular structure does not favor good entanglements with the homopolymer and is mechanically weak. 

The use of other materials as the central layer of the barrier laminate is, of course, feasible. Obvious candidates for this application include the high barrier amorphous polyamides (Chapter 5 (50)) and the liquid crystalline polyesters (Chapter 3(57)) which either develop slightly improved barriers under elevated relative humidity conditions or at least do not lose barrier properties. No reports are yet available concerning the performance of such structures. 

The structure of an amorphous polyamide prepared from hexamethylenediamine and isophthalic/tere-phthalic acids was modified in order to determine the effect of chemical structure on the oxygen permeation properties. The greatest increase in permeation was obtained by lengthening the aliphatic chain. Placement of substituents on the polymer chain also led to increased permeation. Reversal of the amide linkage direction had no effect on the permeation properties. Free volume calculations and dielectric relaxation studies indicate that free volume is probably the dominant factor in determining the permeation properties of these polymers. 

Selar PA, poly(hexamethylene isophthalamide/terephthalamide) or 6-I/T (the diamine components are listed first, then the diacid components), is an amorphous polyamide which is marketed by Du Pont. As shown in Figure 1, it has unique properties for a barrier resin in that the oxygen barrier properties actually... 

Effect of RH on OPV. It was also of interest to determine the factors which lead to a decrease in OPV with increasing RH in amorphous polyamides. As noted above, this behavior is unique for commercial oxygen barrier materials. This phenomena, however, appears to be general for amorphous polyamides, so the discussion which follows will assume that the OPV decrease is caused by the same effect in all cases. 

Through systematic modification of the polymer backbone, the effects of chemical structure upon the oxygen permeation properties of aliphatic-aromatic amorphous polyamides were determined. In this class of polymers, the greatest effects were obtained by alteration of the chain length and disruption of the amide hydrogen bonding by N-alkylation. It is remarkable that reversal of the amide linkage has no effect whatsoever on the permeation properties of the examples studied. 

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