Influence of stereochemistry on the thermal properties of partially cycloaliphatic polyamides

The effects of the partial substitution of 1,4‐disubstituted cyclohexane monomers for linear aliphatic monomers in polyamides are discussed. More specifically, the relation between the stereochemistry of the cycloaliphatic residues and the thermal properties [melting temperature (Tm) and crystallization temperature (Tcr)] was investigated. For this purpose, two different types of copolyamides were synthesized: in polyamides 12.6, the adipic acid residues were partially replaced by cis/trans‐1,4‐cyclohexanedicarboxylic acid (1,4‐CHDA), whereas in polyamides 4.14, the 1,4‐diaminobutane residues were partially substituted with cis/trans‐1,4‐diaminocyclohexane (1,4‐DACH). For both systems, increasing the degree of substitution of cycloaliphatic residues for linear aliphatic residues resulted in a rise of both Tm and Tcr. This points to the isomorphous crystallization of the linear and cycloaliphatic residues. In contrast to the use of 1,4‐DACH as a comonomer, 1,4‐CHDA residues showed isomerization upon thermal treatment of the polyamides. This isomerization of the cyclohexane residues influenced the thermal properties of the copolyamides. The use of a nonisomerizing cis–trans mixture of 1,4‐DACH exhibited the large influence of the stereochemistry of the cycloaliphatic residues on the Tm of the copolyamides. For both the 1,4‐CHDA‐ and 1,4‐DACH‐based copolyamides, differential scanning calorimetry analysis revealed that recrystallization occurs during melting. This exothermal effect becomes less pronounced with an increasing content of rigid cycloaliphatic residues.