The effects of the molar ratio of adipic acid to total diacid on the phase morphology and thermal properties of nylon copolymers containing dimer acid, adipic acid, and hexamethylenediamine were investigated. A significant structural transition from the c form to a and beta mixture forms occurred with increasing molar ratio. Meanwhile, the content of the crystals increased markedly. A sigma-pi* hyper conjugation effect was present in all of the as-prepared samples. A strong sigma-pi* hyper conjugation effect could decrease the average distance between inter-chain amide groups and suppress the scission of the CAN bond through the formation of highly combining aza double-bond C=N groups, which would, consequently, make the nylon copolymer much more stable. Differential scanning calorimetry-thermogravimetric analysis results indicate that the low thermal stability of the nylon copolymers with higher molar ratios of adipic acid to total diacid was simply due to the formation of cyclopentanone and caprolactam at 350-4208 degrees C, whereas for the nylon copolymers with lower molar ratios, the scission of the C-C bond at 430-5008 degrees C directed the pyrolysis of the main chain. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 2511-2516, 2011