Primitive chain network simulations for elongational viscosity of bidisperse polystyrene melts
In spite of the industrial significance, molecular mechanism of the strain hardening saliently observed in bidisperse polymeric liquids has not been elucidated yet. In this study, the multi-chain slip-link simulation (called primitive chain network simulation) was performed for the bidisperse polystyrene blends for which experimental data for elongational viscosity have been reported earlier. The simulation reasonably reproduced linear viscoelasticity and transient and steady uniaxial elongational viscosities. It has been confirmed that the long chain stretch dominates the stress at the strain hardening as already demonstrated earlier via the tube model. The molecular analysis employing the decoupling approximation revealed for the first time that there exist two molecular mechanisms to induce strain hardening in bidisperse blends. The mechanism switches depending on the Weissenberg number with respect to the Rouse relaxation time of the long chain, WiRL. At WiRL < 1, the simultaneous increase of the long chain orientation and stretch with increasing WiRL lifts the viscosity beyond the Trouton’s viscosity. At WiRL ≥ 1, the isotropic short chain suppresses the stretch/orientation-induced reduction of friction to enhance the stretch of long chain.
Comparison among Slip-Link Simulations of Bidisperse Linear Polymer Melts
Recent simulations of entangled polymer dynamics are based on slip-link models pioneered by Hua and Schieber (J. Chem. Phys. 1998, 109, 10018). In this study, we compare different slip-link models in their prediction of the linear viscoelasticity of bidisperse linear polymers to examine how effectively such models account for constraint release. We compare the DT model by Doi and Takimoto (Philos. Trans. R. Soc. London, Ser. A 2003, 361, 641), the NS model by Nair and Schieber (Macromolecules 2006, 39, 3386), and the primitive chain network (PCN) model by Masubuchi et al. (J. Chem. Phys. 2001, 115, 4387). It is found that DT and PCN models are in better quantitative agreement with literature data than is the NS model. This indicates that renewal of entanglements along the chain due to constraint release, explicitly accounted for in DT and PCN, is not equivalent to spatial fluctuations of slip-links, as included in the NS mean field approach.
Ideological Quietism? Ideology and Party Politics in Britain
Ideology and political parties are frequently depicted as disparate entities, with scholars citing a range of exogenous and endogenous changes to demonstrate the decreasing relevance of ideology to party politics. This article moves away from such accounts by looking at the role of actors, and specifically party leaders, in contributing to perceptions of ideological decline. Through an examination of the rhetoric of Labour and Conservative Party leaders in Britain between 1946 and 1997, this article contends that politicians have engaged in, what is termed here, ‘ideological quietism’. In this sense, parties have not abandoned ideology but have made rhetorical shifts indicative of ideological decline.