Transient network, thickening

This chapter is devoted to the molecular rheology of transient networks made up of associating polymers in which the network junctions break and recombine. After an introduction to theoretical description of the model networks, the linear response of the network to oscillatory deformations is studied in detail. The analysis is then developed to the nonlinear regime. Stationary nonhnear viscosity, and first and second normal stresses, are calculated and compared with the experiments. The criterion for thickening and thinning of the flows is presented in terms of the molecular parameters. Transient flows such as nonhnear relaxation, start-up flow, etc., are studied within the same theoretical framework. Macroscopic properties such as strain hardening and stress overshoot are related to the tension-elongation curve of the constituent network polymers. 

For a constant recombination rate a, a similar calculation was done by Fuller et al. [47] to study the effect of entanglements in the polymer melts. They found that the number density of entanglement junctions is a decreasing function of the flow rate, and that shear thinning and elongational thickening are two major characteristics shared by all transient networks that fall into this category. 

Wang, S. Q., Transient network theory for shear-thickening fluids and physically cross-linked systems. Macromolecules, 25, 7003-7010 (1992). 

The formation of micelles results in a three dimensional transient network in which the latex particles serve as the branch points and the thickener molecules act as associative crosslinks. These crosslinks resist the stretching that must occur when shear is applied to the system. In practice, this means improved brush drag properties. The transient nature of the crosslinks, due to the dynamic equilibrium of the micelle which is continually forming, breaking and re-forming allows the system to flow under low shear resulting in excellent levelling. 

---