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 (SBPE) 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 fracture surfaces of these specimens indicates a transition to a more ductile-type behavior when specimens are associated with larger branch lengths. Detailed characterization shows that these SBPEs have approximately the same molecular weight, molecular weight distribution, branch frequency, crystallinity or microstructure but different branch length. The difference in branch length is suggested to be responsible of the dramatic change in N-f and in the morphology of the fracture surface at temperatures above the beta-transition of SBPEs. In contrast, no significant change in N-f and in the morphology of the fracture surface were found with increasing branch length at temperatures near or below the beta-transition. This dramatic change in the effect of short-chain branches on N-f and on the morphology of the fracture surface are attributed to the change in the molecular relaxation motion at temperatures near the beta-transition of SBPE polymers.