Elongational flow birefringence of ethylene–tetracyclododecene copolymer

Simultaneous measurements of transient tensile stress σ(ϵ̇0;t) and birefringence Δn(ϵ̇0;t) were conducted under uniaxial elongation with constant Hencky strain rate ϵ̇0 on new olefin-copolymer of ethylene (E) and tetracyclododecene (TD) of three different TD content from 20 to 41 mol%. For the low TD content copolymer, the extentional viscosity ηE(ϵ̇0;t)(≡σ(ϵ̇0;t)/ϵ̇0) showed only moderate increase with elongation time t or Hencky strain ϵ(=ϵ̇0t) and the stress optical coefficient C(ϵ̇0;t)(≡Δn(ϵ̇0;t)/σ(ϵ̇0;t)) was nearly constant for the whole range of σ examined. On the other hand, for other samples of high TD content (≥30%), we observed strong uprising of ηE often referred to as strain-induced hardening, and rapid decrease in C with increasing σ. As the copolymer chains were stretched out from the entangled state, the larger amount of the bulky TD groups in the latter may suppress the flow-induced conformational change to take place and suppress the disentangling of the chains, leading to the increase in σ and decreasing of C.