Spherulitic morphologies of nylon 6 9 as an example of an even-odd nylon were studied by optical microscopy. A well-defined dependence on crystallization temperature was found. In particular, positively birefringent spherulites were characteristic for temperatures higher than 232 degrees C, low birefringence spherulites developed between 232 and 225 degrees C, positive ringed spherulites were found between 225 and 220 degrees C, and finally negative fibrillar spherulites were formed at temperatures lower than 220 degrees C. These optical properties were clearly different from those observed with even even nylons (e.g., negative and positive birefringence for high and low temperatures, respectively), and may derive from the peculiar crystalline structures determined for even-odd nylons. Furthermore, low birefringence spherulites were characterized by a flat-on lamellar disposition and reversibility of the birefringence sign in the 80-120 degrees C temperature interval (positive and negative values at high and low temperatures, respectively).; Real time WAXD profiles taken during heating and cooling processes demonstrated that different crystalline structures (named I, II and III) developed depending on the temperature and crystallization procedure of samples (e.g., from solution or from the melt state). Crystalline structures were characterized by the formation of hydrogen bonds along two crystalline directions in all cases, a peculiar arrangement that may account for the development of positive and negative spherulites in a more simple way than formulated for conventional polyamides having a single hydrogen bonding direction. DSC and FTIR data also showed a complex structural behavior with structural transitions in the 80-120 degrees C range, a region that corresponds to birefringence sign reversibility. (C) 2015 Elsevier Ltd. All rights reserved.