Effect of sparse long-chain branching on the step-strain behavior of a series of well-defined polyethylenes

The effect of sparse long chain branching, LCB, on the shear step‐strain relaxation modulus is analyzed using a series of eight high‐density polyethylene (HDPE) resins. Strains of 1 to 1250% are imposed on materials with LCB content ranging from zero to 3.33 LCB per 10,000 carbon atoms. All materials are observed to obey time–strain separation beyond some characteristic time, τk. The presence of LCB is observed to increase the value of τk relative to the linear resin. The behavior of the relaxation modulus at times shorter than τk is investigated by an analysis of the enhancement seen in the linear relaxation modulus, G0(t), as a function of strain and LCB content. This enhancement is seen to (1) increase with increasing strain in all resins, (2) be significantly larger in the sparsely branched HDPE resins relative to the linear HDPE resin, and (3) increase in magnitude with increasing LCB content. The shape and smoothness of the damping function is also investigated. The finite rise time to impose the desired strain is compared to the Rouse relaxation time of linear HDPE resins studied. Sparse LCB is found to increase the magnitude of the relaxation modulus at short times relative to the linear resin. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers