(250b) Transient Recovery after Extensional Flow of Elastomeric Random Copolymers

Long chain branching in polymers may be of two distinct types. The type observed in low density polyethylene is profuse and tree type with multiple interconnected branches on a single backbone. A different type of branched structure involving sparse, isolated long chain branches on a backbone (sparse LCB) is observed in metallocene based ethylene alpha-olefin random copolymers. The sparse LCB disappears at the highest comonomer contents. The two different types of branches are known to lead to different trends in strain hardening with strain rate ? increasing with increasing strain rate for the tree-type branched structure and decreasing with increasing strain rate for the sparse LCB structure. The elastic recovery of such components and their blends after elongation has not received much attention although Langouche and Debbaut [2] have studied linear HDPE melts from this perspective. The objective of this paper is to examine the strain hardening and recovery of stretched copolymer melts with and without sparse LCB and of their blends, in a Rheometrics Melt Extensional (RME) Rheometer.

Two elastomeric random ethylene-octene copolymers (EO1 and EO3) with and without sparse long chain branched structure have been tested at 150°C in elongational flow and during strain recovery with the help of the RME instrument after stopping the elongation and cutting the sample at one end. The octene content is 20 wt. % in EO1 (ENGAGE 8480) and 38 wt. % in EO3 (ENGAGE 8100); both have melt flow indices of 1.0. Linear viscoelastic data on the EO samples as well as nonlinear shear creep recovery curves have been presented in a recent paper by Patham and Jayaraman [1]. VanGurp-Palmen plots presented in this paper reveal that EO1 has sparse long chain branching while EO3 does not and is linear. The molecular weight distributions were very similar with EO1 having higher Mw than EO3; the characteristic relaxation times were 22 s and 19 s respectively.

The elongational viscosity results for the sparse long chain-branched EO1 displayed a decreasing strain hardening ratio as the strain rate was increased. The linear EO3 showed no strain hardening at the lower strain rates (0.01 to 0.1 s^-1) but at higher strain rate of 0.5 to 1 s^-1, strain hardening was observed.

The recovery of the elastomers was recorded with a video camera after it had been stretched to a Hencky strain of 2.9. The recovery factor or the ratio of the length of the stretched sample (when the flow is stopped) to the recorded length of the strand was plotted over time from analysis of video images. Two features have been identified on the recovery curves obtained with the elastomers: (1) above a certain strain rate, the curves switch from concave up to concave down and (2) the curves at different strain rates start to converge after a certain time. These features are significant in that they may be related to separation of the elastic recovery time scale and the surface tension associated time scale. These trends are compared with computed trends using the Wagner integral model.

(1)Patham, B. and K. Jayaraman, J Rheol., 49: 989-999 (2005) (2)Langouche, F. and B. Debbaut, Rheol Acta, 38: 48-64 (1999)