EFFECT OF AGGREGATION STRUCTURE ON NONLINEAR DYNAMIC VISCOELASTIC CHARACTERISTICS OF ORIENTED HIGH-DENSITY POLYETHYLENES UNDER CYCLIC FATIGUE

Non-linear viscoelastic properties under cyclic fatigue for oriented h igh-density polyethylenes (HDPEs) with different molecular aggregation states were discussed in terms of the non-linear viscoelastic paramet er (NVP). Non-linear viscoelasticity of the oriented HDPEs became domi nant with an increase of imposed strain amplitude, and fatigue strengt h decreased with an increase in the magnitude of NVP. Also, in the cas e of the same magnitude of NVP, the fatigue strength of the oriented H DPE drawn at the crystalline relaxation temperature, T-alpha c (drawin g temperature, T-d = 353 K) was greater than that of the oriented HDPE s drawn at the other temperatures, because PE drawn at T-alpha c had t he most stable aggregation structure. Higher-order structural change d uring the fatigue process by cyclic straining for the oriented HDPE at T-d = 353 K was not so apparent compared with that for the oriented H DPEs prepared at the other temperatures. In the case of the oriented H DPE at T-d = 300 K, composed of crystallites with small dimensions, th e larger the magnitude of imposed strain amplitude, the greater the in crease in crystallite size and/or the orientation of molecular chains to the direction of cyclic deformation occurring with cyclic straining . Also, that became more dominant with an increase in the magnitude of imposed strain amplitude. In the case of the HDPE drawn at a higher t emperature than T-alpha c (T-d = 383 K), the HDPE was composed of crys tallites with large dimensions, and the crystalline disordering accomp anying the decomposition of lamellar crystals into the small fragments occurred at the initial stage of cyclic fatigue. The magnitude of NVP for the oriented HDPEs increased with an increase in the degree of st rain concentration in the amorphous and/or crystallite boundary region s in the case of fatigue experiments at 300 K. Thus, it is reasonable to conclude that the non-linear viscoelasticity of the oriented HDPEs under cyclic fatigue at 300 K mainly originated from the deformation o f the amorphous and/or crystallite boundary regions. Also, it was clar ified that the appearance of non-linear viscoelasticity remarkably red uced the fatigue strength. (C) 1997 Elsevier Science Ltd.