Copolyamides 2.14/piperazine.14 with variable built-in ratios of 1,2-ethyl-enediamine (1,2-EDA) and piperazine (pip) were synthesized by solution polycondensation. The built-in ratio of both diamine comonomers was determined with solution C-13 NMR analysis. The gradual replacement of 1,2-EDA units by cycloaliphatic pip units in polyamide 2.14 resulted in a progressively decreased melting (T.) and crystallization temperature of the obtained copolyamides. Apparently, the T-m raising effect of the incorporation of rigid cycloaliphatic moieties is overruled by the simultaneous T. reduction caused by a decreasing hydrogen-bond density. Indications for cocrystallization of 2.14 and pip.14 repeating units were obtained by the thermal analysis of copolyamides 2.14/pip.14 and of a blend of both homopolyamides. A preliminary wide-angle X-ray diffraction study pointed to the same conclusion. Solid-state NMR spectroscopy was used to investigate the influence of the composition on the percentage of the rigid phase of the copolyamides and delivered additional indications for cocrystallization. (C) 2003 Wiley Periodicals, Inc.