In order to confirm the structural features in the Brill transition of aliphatic nylons, molecular dynamics calculation has been performed at the various temperatures for nylon 10/10 crystal as a model. With an increase in temperature, the torsional motions around the methylene-amide and methylene-methylene bonds were activated. These motions became more remarkable above 420 K and the interconversion between trans and gauche forms occurred frequently, resulting in the disordered conformation of the methylene sequences and the pseudo-hexagonal packing of these parts. During such a drastic structural disordering, however, the hydrogen bonds were kept alive although the bond strength became weaker. These calculations were found to be consistent with the experimental results by the temperature-dependent Xray diffraction and infrared spectral measurements. The Young's modulus along the chain axis was also calculated as a function of temperature: it decreased remarkably from 250 GPa at 0 K to 180 GPa at 300 K due to a small contraction of the skeletal chain by only about 0.2-0.5% through the torsional motion of the skeletal chains, and the modulus decreased furthermore to 80 GPa at 550 K because of the larger conformational disordering of the skeletal chains. The Young's modulus in the direction perpendicular to the chain axis was also found to decrease remarkably in parallel to the change of the chain packing mode. (C) 2004 Elsevier Ltd. All rights reserved.