Strain-Induced Crystallization and Mechanical Properties of Rubber.: 3.Strain-Induced Crystallization under Static and Dynamic Condition

Strain-induced crystallization(SIC)of rubber has been studied under static and dynamic condition. The static condition means that the sample is kept at a certain strain and the dynamic condition means that the sample is being stretched under certain speed. The difference of both condition should be considered. 1.Under the static condition, lowering temperature decreases stress and increases SIC crystal fraction significantly. The stress decreases linearly with temperature. The inclination of stress decrease is larger than T/T0(T: absolute temperature, T0: reference absolute temperature). The inclination of T/T0 means the decrease of micro-Brownian motion due to the decrease of temperature. Therefore, the larger inclination suggests that the network density also decreases with the lowering of temperature. The SIC crystal fraction increases linearly with lowering temperature. Therefore, the creation of SIC crystal decreases the number of amorphous chains. 2. Under the static condition, elevating temperature increases stress and decreases SIC crystal fraction. The stress increases linearly with elevating temperature. The inclination of stress increase is T/T0. It means that the micro-Brownian motion increases with elevating temperature, but the network density does not change by the decrease of SIC crystal fraction. Beyond 80 °C, the stress shows the maximum and decreases with temperature. Because the SIC crystal fraction decreases with elevating temperature linearly, the stress behavior has no direct relation with SIC. 3. Under the dynamic condition, un-vulcanized IR does not perform SIC beyond 25 °C but does perform SIC below 0 °C. Because un-vulcanized IR is polymer melt, SIC is close to FIC(flow induced crystallization)of semi-crystalline plastics. Un-vulcanized NR perform SIC from +100 °C, +75 °C to -25 °C, -50 °C. The onset strain of SIC increases with temperature. 4. Under the dynamic condition, un-vulcanized NR and vulcanized NR perform SIC at from -50 °C to +100 °C. The onset strain of SIC increases with elevating temperature(except -50 °C). The stress at large strain decreases with elevating temperature, but the decrease of stress does not seem to be simple because the inclinations of the decrease of stress from -25 °C to +25 °C and from +25 °C to +100 °C are significantly different. It seems that the stress and tensile strength do not have direct relation with SIC. 5. Network density and network structure do not change onset strain of SIC. The onset strain of SIC of un-vulcanized NR is smaller than that of vulcanized NR. Therefore, a pivot point to orient and align rubber chains into SIC seems to be not a network point but an entanglement.