Abstract
The melting behavior of nylon 6 yarns was studied by three kinds of DSC (differential scanning calorimetry) techniques; zero-entropy-production (z-e-p), conventional, and constraint melting techniques.
The introduction of cross-links into the amorphous part by irradiating the samples with γ-rays in gaseous acetylene proved an effective technique for suppressing reorganization of the imperfect crystals occurring during the DSC heating. The melting curve thus obtained was demonstrated to represent the melting point distribution inherent to the original crystal in the sample, i. e., what Wunderlich calls z-e-p melting point. Effects of drawing and annealing conditions of the yarns to the z-e-p melting were investigated in detail.
Secondly, origin of the double melting peaks of the drawn yarn observed frequently with a conventional DSC technique was studied by combining nonisothermal annealing simulating the DSC heating process and the above z-e-p technique. It was concluded that the double peaks are the result of superposition of three processes which occur successively during heating; perfection of the original crystal, melting of the perfected crystal concurrently with recrystallization, and melting of the recrystallized one.
Finally, the melting of the samples prevented from shrinking during the heating was studied. The DSC curve exhibiting a single peak at a higher temperature instead of the double peaks described above is explained quantitatively in terms of the perfecting of the original crystal followed by monotonic melting of the perfected crystal. The absence of recrystallization is the main reason for the appearance of the single peak. The melting temperature of the constrained sample increases linearly with draw ratio, being independent of the z-e-p melting point. The elevation of the melting temperature can be explained by one of the entropical superheating models drawn theoretically by Zachmann. This strongly supports the drawn fiber structure model that the oriented crystals are connected tightly by lots of tie molecules.
