Comparative study on the molecular chain orientation and strain-induced crystallization behaviors of HNBR with different acrylonitrile content under uniaxial stretching

The Role of Lanthanum Stearate on Strain-Induced Crystallization and the Mechanical Properties of Whole Field Latex Rubber

Natural rubber (NR) is extensively utilized in numerous industries, such as aerospace, military, and transportation, because of its exceptional elasticity and all-around mechanical qualities. However, commercial NR made using various techniques typically has distinct mechanical characteristics. For instance, whole field latex rubber (SCR-WF) cured with accelerator 2-Mercaptobenzothiazole exhibits poor mechanical properties. This work attempts to enhance the mechanical property of SCR-WF via the addition of lanthanum stearate (LaSt). The influence of LaSt on strain-induced crystallization (SIC) and the mechanical properties of SCR-WF were investigated. The results of crosslinking density measured by the equilibrium swelling method demonstrate that the presence of LaSt significantly increases the crosslinking density of SCR-WF with lower loading of LaSt. The results of the mechanical properties show that the introduction of LaSt can enhance the tensile strength and fracture toughness of SCR-WF. To reveal the mechanism of LaSt improving the mechanical properties of SCR-WF, synchrotron radiation wide-angle X-ray diffraction (WAXD) experiments were used to investigate the SIC behaviors of SCR-WF. We found that the LaSt leads to higher crystallinity of SIC for the strain higher than 3.5. The tube model indicates the contribution of LaSt in both crosslinking and topological constraints. This work may provide an instruction for developing SCR-WF with superior mechanical properties.

Strain hardening in biaxially stretched elastomers undergoing strain-induced crystallization

We reveal strain hardening due to strain-induced crystallization (SIC) in both cross-linked natural rubber (NR) and its synthetic analogue (IR) under planar extension, a type of biaxial stretching where the rubber is stretched in one direction while maintaining the dimension in the other direction unchanged. Utilizing a bespoke biaxial tensile tester, planar extension tests were conducted on geometrically designed and optimally shaped sheet specimens to achieve a uniform and highly strained field. Evident strain hardening due to SIC was observed in both stretching (x) and constrained (y) directions when the stretch (λx) exceeded a critical value λx,c. The λx,c value aligned with the onset stretch of SIC in planar extension, as determined by wide-angle X-ray scattering measurements. Interestingly, the nominal stress ratio between the constrained (σy) and stretching (σx) axes as a function of λx exhibited a distinct minimum near λx,c. This minimum signifies that the increment of σx induced by an increase in λx surpasses that of σy before strain hardening (λx < λx,c), while the relationship is reversed in the strain hardening region (λx > λx,c). The λx,c value in planar extension (4.7 for IR and 4.5 for NR) was slightly lower than that in uniaxial extension (5.7 for IR and 5.2 for NR). This difference in λx,c values can be explained by considering a single mechanical work required for strain hardening, owing to the relatively small dissimilarities between the two stretching modes. This investigation contributes significantly to the understanding of SIC phenomena in biaxial stretching, and provides valuable insights for predicting the mechanical response of SIC rubber under various deformation conditions.