Anomalous Swelling Behavior of Graded Rubber

ABSTRACT This study demonstrated the anomalous swelling behavior of graded rubber in toluene at 25°C. Ethylene‐propylene rubber with a graded crosslink density in the thickness direction was prepared using a compression molding machine, under a temperature gradient (upper temperature: 200°C, lower temperature: 150°C). The crosslink densities of each surface of graded rubber corresponded to those of the homogeneous samples, which were 3.39 × 10 −6 mol/cm 3 and 3.74 × 10 −7 mol/cm 3 for samples crosslinked at 200°C and 150°C, respectively. The swelling behavior of graded rubber occurred in two distinct states: (1) after 20 min (short period), the rubber bent concavely toward the low‐crosslink side, and (2) after 540 min (long period), it bent toward the high‐crosslink side. According to Flory–Rehner's theory, the low‐crosslink side is expected to be on the outer side. However, in the state (1), the high‐crosslink side became the outer side. The possible mechanism can be explained by the theory of gel under constrained swelling. The unswollen region restricts in‐plane expansion of the low‐crosslink region, causing inhomogeneous stress that bent graded rubber toward the low‐crosslink side in state (1). These findings introduce a novel method for developing soft robotic actuators and self‐shaping materials.