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Either of the two endothermic melting peaks of nylon66 by differential thermal analysis can be converted to the other by appropriate choice of annealing conditions. The two peaks are considered due to the melting of two morphological species, forms1 and forms 2.forms 1 is relatively fixed in melting temperature, while  the melting temperature of form 2 varies with annealing conditions. It has been shown that form 1 results from rapid cooling from the melt, and form 2 results from slow cooling. Form 1 appears to be kinetically favored, while form 2 is thermodynamically preperred. The variability in the form 2 melting point is attributed to varible crystal size and/or perfection.