Melting and Crystallization Processes in Strained Rubber: WAXS, NMR-D, Surface Calorimetry Studies

The role of crystallites on the mechanical properties of crystallizing rubbers is analyzed by WAXS, NMR-D and by surface calorimetry (SC). Stress induced crystallization (SIC) and melting (SIM) have been widely studied by WAXS since 60 years. Non crystalline swollen networks (with deuterated chains and/or probes) have been extensively studied by NMR-D since 20 years. This technique has been used only recently to determine the state of amorphous chains in crystallizing rubbers during SIC and SIM, in particular to verify the predictions of Flory, published in 1947. Surface calorimetry is a new technique for studying phase transitions (crystallization, melting) appearing in rapidly deformed rubbers; surface temperature of the material during deformation is determined by IR Laser, acquisition time 10 to 125 ms. By SC the crystallization and melting processes is studied during alternative deformations, e = e₀ + e₁ sin wt . These three techniques (with different acquisition times) gives complementary information on the state of the crystalline and amorphous chains (crystallinity and orientation). The role of the crystallites on the strain hardening, stress relaxation, mechanical hysteresis, geometrical instability (inverse yielding), amplification factor (in reinforced materials) will be discussed.