A constitutive analysis of the extensional flows of nearly monodisperse polyisoprene melts

Here two particular formulations [M.H. Wagner, S. Kheirandish, O. Hassager, Journal of Rheology 49 (6) (2005) 1317-1327; H.K. Rasmussen, Q. Huang, Rheologica Acta 53 (3) (2014) 199-208; G. Marrucci, G. Ianniruberto, Macromolecules 37 (10) (2004) 3934-3942] of the 'interchain pressure' [39], incorporated into the molecular stress function method [M.H. Wagner, S. Kheirandish, O. Hassager, Journal of Rheology 49 (6) (2005) 1317-1327], are used to assess the extensional [J.K. Nielsen, O. Hassager, H.K Rasmussen, G.H. McKinley, Journal of Rheology 53 (6) (2009) 1327-1346; G. Liu, H. Sun, S. Rangou, K. Ntetsikas, A. Avgeropoulos, S.-Q. Wang, Journal of Rheology 57 (1) (2013) 89-104] and shear viscosities [D. Auhl, J. Ramirez, A.E. Likhtman, P. Chambon, C. Fernyhough, Journal of Rheology 52 (3) (2008) 801-835] of narrow molecular weight distributed (NMMD) polyisoprene melts. These two formulations are expected to represent the highest [M.H. Wagner, S. Kheirandish, O. Hassager, Journal of Rheology 49 (6) (2005) 1317-1327, G. Marrucci, G. Ianniruberto, Macromolecules 37 (10) (2004) 3934-3942] and lowest level [H.K. Rasmussen, Q. Huang, Rheologica Acta 53 (3) (2014) 199-208] of the 'interchain pressure'. The needed Rouse times are here defined as tau(R)/tau(max) alpha (M/M-e)(-1.4) with a proportional factor of 1.4, achieved based on the viscosity measurement. tau(max) is the maximal relaxation time, M the molecular weight and the entanglement molecular weight M-e = (4/5)rho RT/G(N)(0) [M. Doi M, S.F. Edwards, The Theory of Polymer Dynamics; Clarendon Press: Oxford (1986)]. rho is the density, R the gas constant, T the temperature and G(N)(0) the plateau modulus. The method by [M.H. Wagner, S. Kheirandish, O. Hassager, Journal of Rheology 49 (6) (2005) 1317-1327, G. Marrucci, G. Ianniruberto, Macromolecules 37 (10) (2004) 3934-3942] predicts start- up of extensional viscosities significantly below the measured value. The formulations by [H.K. Rasmussen, Q. Huang, Rheologica Acta 53 (3) (2014) 199-208] seem to be in agreement with both the start-up of extension as well as the shear flow of all NMMD polyisoprenes. Potential non-isothermal effects were addressed computationally using the pseudo time principle, assuming the most critical case of adiabatic heating. (C) 2016 Elsevier Ltd. All rights reserved.