Evolution of crosslinking structure in vulcanized natural rubber during thermal aging in the presence of a constant compressive stress

The evolution process of the crosslinking structure in sulfur-cured natural rubber (NR) under the combined effect of compressive stress and temperature was investigated using a homemade constant-stress device. The aged NR samples were characterized by infrared spectroscopy, chemical probe method, differential scanning calorimetry, and mechanical analysis. The results revealed that compressive stress promoted the breakage of sulfur bonds (mainly low-energy ones) between NR chains, leading to a decline in crosslink density and an increase in mesh size. According to the Arrhenius equation, a compressive stress of 1.78 MPa reduced the apparent activation energy for the breakage of polysulfide bonds from 56.3 kJ/mol to 48.2 kJ/mol, resulting in a 14.4% decrease. The sulfur-containing side groups formed after the breakage of polysulfide bonds not only participated in post-crosslinking reactions after stress release but also increased the hysteresis loss of NR under large deformation.