Multiple melting in nylon 66

Abstract

Abstract Either of the two endothermic melting peaks found by differential thermal analysis of nylon 66 may be converted to the other by appropriate choice of annealing conditions. The two peaks are considered due to the melting of two morphological species, forms I and II. Form I is relatively fixed in melting temperature, while the form II melting temperature varies with annealing conditions and can be either above or below form I. The two forms can be distinguished by whether or not the conversion I → II takes place; if the sample is in form II no change in the thermogram is observed under suitable conversion conditions. The conversion of form I to form II also takes place during cold drawing. It has been previously shown that form I results from rapid cooling from the melt, and form II results from slow cooling. Form I appears to be kinetically favored, while form II is thermodynamically preferred. The variability in the form II melting point is attributed to variable crystal size and/or perfection.