Winner of the Y2003 Society of Rheology Publication Award

Reversible shear-induced breakdown of polymer networks

Previous work at the University of Minnesota MRSEC led to the discovery of polymeric bicontinuous microemulsions - a new class of polymeric materials exhibiting interpenetrating networks of immiscible polymer segments. These materials can impact a variety of technologies, including membranes for chemical separations and energy production, conducting polymer films for the microelectronics industry, catalysts for chemical reactions, and porous materials capable of sequestering therapeutic drugs. Almost all polymer materials, however, are processed as liquids into usable forms, and understanding how these microemulsions respond to imposed flow is crucial to their future application. Investigators at the Minnesota MRSEC and the Northwestern University MRSEC have now discovered a remarkable phenomenon that ensues when these microemulsions are subjected to shear forces. Specifically, a low-viscosity phase in which the bicontinuous network breaks down is observed under shear. This behavior promises improved economics of processing because lower viscosity means less energy, and less time is expended in forming parts by extrusion, injection, molding, or coating. Importantly, the polymer system recovers its appealing bicontinuous network structure when the flow stops, thereby recovering the material properties of the parent form. The manuscript describing this work was recently named the recipient of the 2003 Society of Rheology Publication Award. [K. Krishnan, B. Chapman, F. S. Bates, T. P. Lodge, K. Almdal and W. R. Burghardt Journal of Rheology 46, 529-554 (2002)]

Technical Summary

Effects of shear flow on a polymeric bicontinuous microemulsion: Equilibrium and steady state behavior. We have investigated the effects of shear flow on a polymeric bicontinuous microemulsion using neutron scattering, light scattering, optical miscroscopy, and rheology. The microemulsion consists of a ternary blend of poly(ethyl ethylene) (PEE), poly(dimethyl siloxane) (PDMS), and a PEE-PDMS diblock copolymer. At equilibrium, the microemulsion contains two percolating microphases, one PEE rich and the other PDMS rich, separated by a copolymer-laden interface; the characteristic length scale of this structure is 80 nm. Low strain amplitude oscillatory shear measurements reveal behavior similar to that of block copoymer lamellar phases just above the order-disorder transition. Steady shear experiments expose four distinct regimes of response as a function of the shear rate. At low shear rates (regime I) Newtonian behavior is observed, whereas at intermediate shear rates (regime II) development of anisotropy in the morphology leads to shear thinning. When the shear rate is further increased, there is an abrupt breakdown of the bicontinuous structure, resulting in flow-induced phase separation (regime III). Rheological measurements indicate that the shear stress is almost independent of the shear rate in this regime. Light scattering reveals a streak-like pattern, and correspondingly a string-like morphology with micron dimensions is observed with video microscopy. Upon a further increase of the shear rate (regime IV), the sample resembles an immiscible binary polymer blend with the block copolymer playing no significant role; the stress increases strongly with the shear rate. In some respects these results resemble those from other weakly structured complex fluids (sponge phases, liquid crystals, worm-like micelles, block copolymers, polymer blends, and polymer solutions), yet in important ways this system is unique. © 2002 The Society of Rheology. [K. Krishnan, B. Chapman, F. S. Bates, T. P. Lodge, K. Almdal and W. R. Burghardt Journal of Rheology 46, 529-554 (2002)]

Fig. A. Schematic diagram of the Small Angle Neutron Scattering (SANS) experimental setup, which produces scattering patterns corresponding to the blend microstructure. The shear cell has Couette flow geometry with a rotating inner cylinder. The beam was directed both in the radial and tangential directions.

Fig. B. Schematic representation of the morphology of the polymer blend in the four regimes: I = static structure (100 nm lengthscale), II = same blend elongated under gentle flow, revealing the development of anisotropy, III = low viscosity, phase-separated state (micron length scale) under strong shear, and IV = low viscosity, highly perturbed phase-separated state (micron-sized strings) at high shear.