The melting of crystalline polymers

Abstract

It has been shown that the thermograms of many 66 nylon samples contain two million endotherms. White originally reported this phenomenom for drawn fibres. It has now been observed with precipitated and annealed 66 nylon and with other suitably annealed crystalline polymers. The new endotherm always appeared at a lower temperature than the original single melting endotherm of the untreated ploymer. On annealing, the development of the new endotherm was usually dependent on both the time and temperature of annealing. With 66 nylon, as the annealing time at a given temperature increased, the area of the new endotherm increased slightly. Prolonged annealing at 242 degrees C resulted in a single melting endotherm at a temperature lower than that of the unnaealed polymer. Subsequent quantative annealing experiments confirmed these observations. The crystallisation of molten 66 nylon at 242 degrees C produced polymer with two melting endotherms. These were shown to have resulted from an annealing process, rather than the primary crystallisation. Further experiments on annealed 66 nylon, including quantitative and X-ray diffraction measurements showed that changes in the polymer occurred during annealing. These however could not be attributed to changes in the molecular weight distribution or in the visual morphology of the polymer, during the annealing process. Slow, rather than fast precipitation of 66 nylon favoured the formation of polymer with two melting endotherms. Other properties were also shown to depend on the precipitation method used. The apparatus used, together with the experimental techniques and methods of sample preparation are described. The results obtained and the possible causes of the double melting endotherms are discusses in relation to the current concepts of polymer morphology