Strain-induced crystallization and ultimate properties

Abstract

As already mentioned in Chapter 2, large amounts of crystallization in an undeformed network interfere with rubberlike behavior because they suppress the mobility of the network chains. They can also enhance the degree of intermolecular correlations. A relatively small number of very small crystallites present in an undeformed network are no problem, however, and can in fact function as temporary cross links. Even better are polymers with melting points a little below room temperature, since these materials can undergo strain-induced crystallization at typical temperatures of utilization (Mark, 2004a, b). This type of crystallization occurs because the melting point T_m = ΔH_m/ΔS_m is elevated by the decrease in entropy of the stretched network chains. Specifically, ΔS_m = S_amorp − S_cryst (12.1). It is S_amorp that is decreased by the stretching, and, since S_cryst = 0 (at least for perfect order), ΔS_m is decreased as well, and T_m increases. The melting point of very carefully annealed natural rubber is as high as 28 °C, but under most conditions, the melting point T_m in the unstretched state is below room temperature. Thus unstretched natural rubber generally is totally amorphous at room temperature. Stretching a network of the polymer, however, increases T_m considerably. The extent to which it exceeds the test temperature (for example, room temperature) is called supercooling. Crystallization now occurs (is "induced"), as is readily shown by X-ray diffraction and scattering measurements (Toki et al., 2004, 2005; Trabelsi et al., 2004), and by NMR spectroscopy (Lin et al., 2004).