Pulling polymers into line.

Pulling polymers into line

One way to line up the tangled strands of a polymer is to stretch the material. This alignment often strongly influences the polymer's properties. Now some researchers have found a way to use magnetic fields to align polymer molecules quickly and efficiently. "Using a magnetic field adds a new dimension to the processing of materials,' says polymer scientist Samuel I. Stupp of the University of Illinois at Urbana-Champaign.

Stupp and his colleagues start with a liquid crystal polymer, such as an aromatic polyester. Like any polymer, this type of material consists of a large number of repeating units arranged in a long molecular chain. Unlike many other polymers, however, these special polymer molecules, when in the liquid state, tend to organize themselves into a loose pattern characteristic of liquid crystals.

To make the liquid crystal polymers more responsive to magnetic fields, small units of organometallic compounds such as copper complexes are added. These additives, which are paramagnetic and interact strongly with an external magnetic field, find places within the polymer chain or attach themselves loosely to the polymer's backbone. Thus, when the liquid polymer is under the field's influence, the polymer molecules are pulled into line. When the material solidifies, the induced pattern is frozen into place.

The insertion of paramagnetic segments greatly speeds the alignment process so that it takes only minutes. Without such units dispersed along the polymer chains, magnetic alignment could take hours or days. The new method also overcomes some of the problems associated with using electric fields to achieve the same goal.

Stupp has discovered that magnetic alignment can significantly change a polymer's properties, particularly in certain directions. In one experiment, alignment increased a material's electrical conductivity in one direction by about 100 times. This process also alters a polymer's optical and mechanical properties.

Aligning polymer molecules in certain directions may turn out to be useful for strengthening thin plastic filmes often used as coatings. Normally, the molecules in a thin film line up so that they stand upright like a brush's bristles, stretching from the lower to the upper surface of the film. A magnetic field could pull these molecules so that they lie flat, parallel to the film's surfaces.