Polyamide, aromatic melting temperature

With DMA the effect of temperature on the modulus can be studied. By increasing the temperature from -150 to 300°C, one encounters several transitions in PA (Fig. 3.1). There is a transition at about —120°C, the y-transition, which is due to the mobilization of methylene units. There is also a transition at —30°C, which is present in wetted aliphatic PA this is due to non-H-bonded amide units and is termed the /J-transition. At about 50°C the glass Uansition (Tg) (a-transition) of the aliphatic polyamides PA-6 and PA-6,6 occurs. At this Uansition, the modulus is lowered considerably. For partially aromatic PA, the Tg occurs above 100°C. The last transition is the flow temperature, at which temperature the material melts the flow temperature and the melt temperature, as measured by DSC, correspond well. The modulus is a measure of dimensional stability and increases with crystallinity and filler content (Fig. 3.12). 

Semicrystalline partial aromatic polyamides, 139 Semicrystalline polyesters, 45 Semicrystalline polymers, melting temperatures of, 33 Semirigid foams, 203 tests for, 244 Sensitization, 246... 

The preparations of thermotropic polyester-amides from comparable monomers as those of thermotropic polyesters were a logical extension of a series of studies in thermotropic polyesters and lyotropic polyamides. (4.) However the inclusion of carbonates had rarely been explored.(5) Because of the flexibility of carbonate compared to substituted aromatic rings, it should be an even more effective approach in lowering the melting temperatures of the unmodified all aromatic polyesters into the easily processable range. 

The dimensional stability of Kevlar is outstanding It shows essentially no creep or shrinkage as high as 200°C. In view of the high melting temperatures of the aromatic polyamides and their poor solubility in conventional solvents, special techniques are required to produce the fibers. For example, Kevlar is wet spun from a solution in concentrated sulfuric add. 

Partially aromatic, melt processable, polyamides are produced as random copolymers, which do not crystallize and are therefore transparent, but are still capable of high-temperature use because of their high Tg values. Several commercial polymers of this type that have glass-like clarity, high softening point, and oil and solvent resistance have been developed. For example, Trogamid T (Dynamit Nobel) contains repeat units of... 

Most aromatic polyamides cannot be made by a melt polymerization process because the polymer melt temperature exceeds the decomposition temperature. Singh developed a unique procedure for preparing certain aromatic polyamides by a melt process using an internal plasticizer generated in-situ during the polymerization [82]. The following reaction scheme was used to prepare aromatic polyamides in the absence of a solvent (Equation 13.24). 

A semi-crystalline and partial aromatic polyamide with a high melting temperature has been described... 

Melt glues are amorphous and/or partially crystalline polymers above their glass transition temperatures or melt temperatures. Their viscosities should not be too high so that they can wet surfaces well, and not too low so that they do not flow away from where they are applied. Best results are obtained for viscosities of about 10-1 000 Pa s. Poly(ethylenes), poly(ethylene-co-vinyl acetate), poly(vinyl butyrals), versamides, polyamides, aromatic copolyesters, polyurethanes, bitumens, and asphalts, for example, are used as melt glues. The adhesive effect is produced by solidification of the melt glue. 

Typically rigid-rod systems such as the aromatic polyamides or the poly-heterocyclic polymers do not exhibit a glass transition, because their amorphous fraction is negligibly small, and they decompose before their melting temperature. By comparison, many of the crystalline random coil polymers, such as the aliphatic nylons, exhibit normal melting phenomena, as well as glass transitions. Thus the properties of the new rigid-rod polymers are quite different from those of random coil polymers. 

The introduction of ring stmctures into a linear chain substantially raises the melting temperature relative to that of the aliphatic chain. This would be expected because of the decrease in conformational entropy of the melt that results. Striking examples of this phenomenon are found on comparing the melting temperatures of the aliphatic and aromatic polyesters and polyamides. 

Direct amidation is generally carried out ia the melt, although it can be done ia an iaert solvent starting from the dry salt (46). Because most aUphatic polyamides melt ia the range of 200—300°C and aromatic-containing polyamides at even higher temperatures, the reactants and products must be thermally stable to be polymerized via this method. 

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