Compressive stress relaxation of metallocene-based polyolefin foams: Effect of gamma-ray-induced crosslinking

Metallocene-based polyolefin (MPO) foams possess a closed-cell structure wh ich is in contrast to the open-celled structure of polyurethane (PU) foams. In this study, we investigate the effects of gamma-irradiation on the mech anical behavior of MPO foams using PU foam behavior as a basis. Compressive step-strain experiments reveal a two-step relaxation process in MPO foams, dominated by polymer chain relaxation at short times and gas diffusion fro m the closed cells at longer times. On the other hand, the relaxation in PU foams is similar to fully crosslinked polymers with the relaxation modulus reaching an equilibrium value after an initial decay. The closed-celled st ructure of MPO foams lends to rapid stress relaxation and low structural re coverability upon application of compressive loads. Exposure to gamma radia tion induces crosslinking in MPO foams and improves their resilience and re coverability. Stress relaxation tests reveal that nonradiated MPO foams sho w complete relaxation and structural loss at high temperatures. In contrast , radiated MPO foams show a significant retardation in relaxation kinetics and structural stability attributed to radiation-induced crosslinking. Dyna mic rheology and solvent-extraction studies also support the results obtain ed from stress-relaxation experiments. (C) 1999 John Wiley & Sons, Inc.