Strings in sheared polymer blends

Many advanced polymers are composed of two or more components—like rubber and nylon—that do not mix but are forced into intimate contact, often by a shearing process. By varying the components, researchers can alter the properties of the resultant material. For example, polystyrene is very brittle, but when rubbery particles are incorporated, it can withstand large impacts. Ordinarily, such a blend has droplets of one polymer dispersed within a matrix of the other, and the final product (such as a car bumper) is much larger than the micron size of an individual droplet. In a new experiment at NIST in Gaithersburg, Maryland, Kalman Migler looked at sheared blending in a system whose size is just tens of microns across. He found that, as the shear rate decreases, the droplets first grow, then line up like a pearl necklace, and finally coalesce, as shown here, into very stable strings or ribbons that can be 10 cm long. According to Migler, potential microscale applications of string components could include conductive plastic wires, ultrathin composite materials, and tissue engineering. (K. Migler, Phys. Rev. Lett. 86, 1023, 2001.)http://dx.doi.org/10.1103/PhysRevLett.86.1023