Advanced Course on Rubber Science and Technology
Strain-Induced Crystallization and Mechanical Properties of Rubber.
4. Crystal and Crystallites Structure
2018 Volume 91 Issue 4 Pages 129-134
Details
2018 Volume 91 Issue 4 Pages 129-134
1. Unit cells of strain-induced crystals of NR, vulcanized NR, IR, vulcanized IR are the same because cis-1, 4-polyisoprene makes crystal units with exclusion of entanglements and network points. Unit cell of cis-1, 4-polyisoprene is composed of four chains with P212121 symmetry.
2. Crystallites of NR, vulcanized NR, IR and vulcanized IR are cubic figure with the size of several nm being composed of around 100 chains. The sizes depend on temperature but do not increase with strain. The crystalline fraction increases with strain due to increase of the number of crystallites. Crystallites could be considered as a giant crosslink point, but many crosslink points are bound together at the upper and lower outside of crystallites. Therefore, crystallites do not increase necessarily the number of amorphous molecules which contribute to the elasticity of rubber.
3. WAXD patterns are affected by the size of crystallites, the orientation of crystallites and the completeness of crystal. We developed a simulation program to analyze the size, orientation and completeness of crystallites. The X-ray scattering peaks of NR and vulcanized NR are crescent figure, on the other hand the ones of IR and vulcanized IR are round. The differences are caused by the degree of orientation of crystallites, the crystallites of NR and vulcanized NR do not orient more than the ones of IR and vulcanized IR. The crystallites of NR and vulcanized NR are less degree of completeness of crystal than IR and vulcanized IR, although the stereo-regularity of NR is higher than the one of IR. Pseudo networks of NR and vulcanized NR may suppress a crystallizing process of strain-induced crystallization.
4. Higher order structure of crystallites such as the lamella structure is not observed well although NR show slight two points SAXS pattern only at 0 °C at strain 6.0. During stress relaxation after deformation, density fluctuation is observed due to orientation. It is not considered to be caused by strain-induced crystallization.
