Effect of high energy radiation on the stress–relaxation of ultra‐high molecular weight linear polyethylene

摘要

AbstractStress–relaxation behavior was studied in ultra‐high and normal molecular weight linear polyethylenes (UHMWPE and NMWPE) as a function of radiation dose over the range 0–128 Mrad. Irradiation up to 16 Mrad raises the crystallinity in both types of PE, as demonstrated previously,1and thus increases the initial modulus. Also, the initial modulus of NMWPE is higher than that of UHMWPE because the former has a higher crystallinity. Consequently, the initial stress at a constant imposed strain of 1 varies greatly between the two materials. To eliminate the effect of this initial difference on relaxed stress, the stress–relaxation data were normalized with respect to the initial stress and plotted as the fraction, retained stress after timet/initial stress. The normalized plots show no significant difference between NMWPE and UHMWPE in their stress–relaxation behavior. For both materials stress retention improves progressively with increasing radiation dose, the percentage improvement being greatest at long times (50 at 50 h and 64 Mrad, compared with 6 at 10−2h). These results are interpreted to indicate that radiation crosslinking in the amorphous phase is independent of molecular weight and preferentially retards those molecular motions responsible for short relaxation times. The motions in question could involve molecular flow in the amorphous phase or “pull out” of tie molecules from the cryst