Properties of Aramid Fibers

Aromatic polyamides are characterized by high melt temperatures, excellent thermal stability and flame resistance, and poor solubility in many inorganic and organic solvents. An important aspect here is the effect of polymer composition on physical properties. This can be detected by comparing homopolymers vs. copolymers, unsubstituted vs. substituted polymers, and para- vs. meta-oriented polymers.

DENSITY

Unsubstituted polymers, such as PPD-T, PPD-I, MPD-T, and MPD-I, were reported to exhibit densities of 1.43-1.46 g cm-3. Substituted polymers as a group give considerably lower density of 1.2-1.4 g cm-3.

GLASS TRANSITION TEMPERATURE

PPD-T was reported to have a Tg of >375 OC, while polymers containing pendant substitutions and meta-oriented phenylene segments showed Tgs in the range of 255-260 OC.

MELT TEMPERATURE

PPD-T, PPD-I, MePPD-T, and MeMPD-T failed to show any melting point. However, melt temperatures of 530 and 518 OC for PPD-T and MePPD-T, respectively. The discrepancy may be attributed to differences in polymer molecular weight and molecular weight distribution.

DECOMPOSITION TEMPERATURE

Aromatic polyamides are generally very stable at high temperatures. The decomposition temperature (Td) is often determined by the thermo gravimetric analysis (TGA). PPD-T is thermally stable to about 550 OC. Most unsubstituted polyamides in the literature were stable at 400-500 OC. Chloro, methyl, and other substituted polyamides were reported to be stable at 300-400 OC.

SOLUBILITY

Wholly aromatic polyamides with rigid chains are soluble in only a few solvents such as concentrated sulfuric acid, hydrofluoric acid, and methane sulfonic acid. They are insoluble in formic acid and m-cresol, which are good solvents for aliphatic polyamides. Wholly aromatic polyamides dissolve very slightly in amide solvents like hexamethyl phosphoramide, N-methyl-2-pyrrolidone and N, N'-dimethyl acetamide, often in the presence of lithium chloride, calcium chloride, and other metal halides. The solubility in organic solvents is improved only by polymer modification such as copolymerization and substitution.