Contraction of Entangled Polymers After Large Step Shear Deformations in Slip-Link Simulations

Although the tube framework has achieved remarkable success to describe entangled polymer dynamics, the chain motion assumed in tube theories is still a matter of discussion. Recently, Xu et al. [ACS Macro Lett. 2018, 7, 190–195] performed a molecular dynamics simulation for entangled bead-spring chains under a step uniaxial deformation and reported that the relaxation of gyration radii cannot be reproduced by the elaborated single-chain tube model called GLaMM. On the basis of this result, they criticized the tube framework, in which it is assumed that the chain contraction occurs after the deformation before the orientational relaxation. In the present study, as a test of their argument, two different slip-link simulations developed by Doi and Takimoto and by Masubuchi et al. were performed and compared to the results of Xu et al. In spite of the modeling being based on the tube framework, the slip-link simulations excellently reproduced the bead-spring simulation result. Besides, the chain contraction was observed in the simulations as with the tube picture. The obtained results imply that the bead-spring results are within the scope of the tube framework whereas the failure of the GLaMM model is possibly due to the homogeneous assumption along the chain for the fluctuations induced by convective constraint release.

Direct Measurement of Topological Interactions In Polymers under Shear Using Neutron Spin Echo Spectroscopy

<div><div><div><div><p>We present in-situ neutron spin echo measurements on an entangled polydimethylsiloxane melt under shear and demonstrate the ability to monitor nano-scale dynamics in flowing liquids. We report no changes in the topological interactions of the chains for shear rates approaching the inverse longest relaxation time. Further experiments following along this line will allow to systematically test the predictions of theories, like e.g. convective constraint release.</p></div></div></div></div>

Direct Measurement of Topological Interactions In Polymers under Shear Using Neutron Spin Echo Spectroscopy

<div><div><div><div><p>We present in-situ neutron spin echo measurements on an entangled polydimethylsiloxane melt under shear and demonstrate the ability to monitor nano-scale dynamics in flowing liquids. We report no changes in the topological interactions of the chains for shear rates approaching the inverse longest relaxation time. Further experiments following along this line will allow to systematically test the predictions of theories, like e.g. convective constraint release.</p></div></div></div></div>

3D Simulations and Experimental Validation Using the Molecular Strain Function Model With Convective Constraint Release

The Molecular Strain Function model with Convective Constraint Release has been demonstrated by Wagner to fit elongational and shear viscosities, and First and Second Normal stress differences for a variety of polymer melts, when used with a Convective Constraint Release mechanism [J. Rheol. 45 (2001), 1387]. A modification to the CCR mechanism was shown to give more accurate representation of corner vortices in an abrupt contraction flow [JNNFM 135 (2006), 68] for both planar and axisymmetric contraction flows. It is highly desirable to assess the model against 3D flows. A primary advantage of 3D simulation in assessing a constitutive model is that, experimentally, it is very difficult to produce truly 2D data; the side walls of a finite die affect stress birefringence measurements (since this is a ‘line of sight’ cumulative measurement), and also induce significant 3D motion into the flow. The existing 2-dimensional code has been extended to fully 3-dimensional flows using 27-node ‘brick’ elements, and using a number of developments to deal with tracking and storage problems inherent in 3D time-integral solution. The 3D code is assessed against known 2-dimensional solutions to verify its accuracy; the constitutive model is then assessed against experimental data for a 4:1 contraction ratio die, which has finite width (5:3 depth to inlet height), inducing 3D effects. Stress birefringence, vortex size, and cross-sectional flow rate data at a number of flow rates are compared. The model is shown to give good accuracy against this flow.