Segments of 4-aminomethylcyclohexane carboxylic acid (AMCC) inserted into the polyamide 6 (nylon 6, N6) chain are distributed in both the crystalline lattice and the amorphous regions. Thermal, spectroscopic and X-ray diffraction data indicate that the N6/AMCC copolymer crystallizes in an otherwise thermodynamically unfavorable gamma crystalline form of N6. The chain-axis repeat or the helical pitch of 15.7 Angstrom of this copolymer is the shortest ever reported for N6. This is accompanied by an expansion of the lattice in the equatorial plane. This short pitch was observed at comonomer concentrations as low as 10 mol%, and the lattice remains otherwise essentially same as that of N6 up to about 30 mol% AMCC. Conformational constraints imposed by interchain hydrogen bonding between N6 and AMCC with chain-axis repeat distance of 17.2 and 15.8 Angstrom, respectively, are thought to be the reason for this shortened helical pitch. This short pitch (length scale similar to 10 Angstrom) results in a lower chain-modulus. Differences in the H-bond strength are invoked to explain the higher glass transition temperature in the dry copolymer, i.e. the increased stiffness of the multiple chain segments (length scales > 10 Angstrom) and the differing response to guest molecules such as water. Although the mechanical properties (tensile strength, shrinkage and creep) of the dry copolymer and the homopolymer were similar, moisture had a dramatically different effect on the two polymers. This behavior is attributed to the differences in lamellar morphology (length scales similar to 100 Angstrom), especially the contribution of the amorphous regions, which determine the bulk properties. (C) 2003 Elsevier Ltd. All rights reserved.