INFLUENCE OF BRANCH LENGTH ON THE DYNAMIC FATIGUE PROPERTIES OF SHORT-CHAIN BRANCHED POLYETHYLENES

An investigation of the influence of the short-chain branch length on the dynamic fatigue behavior of short-chain branched polyethylenes (SB PE) is reported. The number of fatigue failure cycles (N-f) increases significantly with increasing branch length at temperatures above the beta-transition of SBPE polymers. In addition, an examination of the f racture surfaces of these specimens indicates a transition to a more d uctile-type behavior when specimens are associated with larger branch lengths. Detailed characterization shows that these SBPEs have approxi mately the same molecular weight, molecular weight distribution, branc h frequency, crystallinity or microstructure but different branch leng th. The difference in branch length is suggested to be responsible of the dramatic change in N-f and in the morphology of the fracture surfa ce at temperatures above the beta-transition of SBPEs. In contrast, no significant change in N-f and in the morphology of the fracture surfa ce were found with increasing branch length at temperatures near or be low the beta-transition. This dramatic change in the effect of short-c hain branches on N-f and on the morphology of the fracture surface are attributed to the change in the molecular relaxation motion at temper atures near the beta-transition of SBPE polymers.