Chemistry: Separation a thousand-fold faster may lead to new composite materials

Inhomogeneous blends of polymers with other polymers or liquid crystals are widely used in industrial applications -- in LCD displays, gas-flow sensors, optical memories and other devices. Researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw analysed the behaviour of such blends in alternating external electric field. „We managed to determine precisely the conditions permitting even a thousand-fold acceleration of component separation process in the blends under study," says Prof. Robert Hołyst.

With time, many blends separate spontaneously into their components, usually at a very low rate. It has been known since long that the separation can be accelerated when an inhomogeneous liquid is placed in an external alternating electric field with adequately tuned frequency. It is generally accepted that the acceleration of separation is due to ions -- natural constituents of such mixtures.

The researchers from the Institute of Physical Chemistry of the PAS studied blends of a polymer with another polymer or liquid crystal. In the presence of an alternating electric field with the strength of several million volts per meter the ions of the component with higher conductivity start to move freely towards the electrode with the opposite charge. Having reached the phase interface with a non-conductive material on the other side they are strongly hampered. „Under these conditions, an additional force appears at the interface. With electric field alternating at appropriate frequency the ions start to yank the interface. Due to the yanking, the droplets of a component merge with each other significantly more efficiently than in the normal case, thus leading to a faster separation of both phases," says Natalia Ziębacz, a PhD student at the IPC PAS.

The separation efficiency of studied blends into their components is strongly dependent on the frequency of the applied electric field. Optical measurements carried out at the IPC PAS have shown that under optimal conditions, at frequencies up to the kilohertz range, the separation takes place even thousand-fold faster. Too low or too high frequencies of the electric field do not result in significant movements of ions and the separation occurs at a regular, low rate. The physical mechanism of the phenomenon suggests that similar effect can be expected in all blends contaminated with ions and containing components with different charge conductivities.

Controlling the rate of separation process over so long time range, extending over three orders of magnitude, opens the way to interesting applications. The separation process can be carried out very quickly, and then virtually stopped at a precisely selected stage. The structure of the blend so obtained can be then fixed, for instance by changing the temperature. Thus, the method to control separations of blends of polymers and liquid crystals using electric field turned out to be an excellent tool for the development of new materials. A patent application for the method has been filed.