Polymer Brushes Respond to Heat

MACROMOLECULAR SCIENCE

A polymer brush material consisting of water-soluble and temperature-responsive polyacrylamide bristles grafted onto an electrically conducting polythiophene backbone promises a wide range of applications, according to the team of chemists at Louisiana State University, Baton Rouge, who synthesized the material.

The unique optical and electronic properties that result from combining the two polymers could find use in bioelectronics, biosensors, actuators, fluorescent thermometers, and supramolecular materials, suggest chemistry professor Robin L. McCarley and coworkers (Angew. Chem. Int. Ed, 2005, 44, 4872).

The team synthesized the material—poly(thiophene-g-NIPAAm)—using a transition-metal-catalyzed controlled radical polymerization method, known as atom transfer radical polymerization (ATRP), to grow poly(N-isopropylacrylamide) chains on the 3-positions of polythiophene.

"Growth of the polyacrylamide chains from each polythiophene repeat unit under ATRP conditions allows the synthesis of polythiophene with densely grafted polyacrylamide chains," McCarley says. "The material is exceedingly soluble in water, which bodes well for green chemical approaches for applications of conducting polythiophenes, including spray coatings and sensing in aqueous media."

Poly(thiophene-g-NIPAAm) also exhibits a reversible color change and becomes insoluble in water when the material is heated above a critical temperature (30–32 ^oC).

The color of the material is related to the conjugation length of the polythiophene backbone. Below the critical temperature, the polyacrylamide side chains are fully hydrated, coiled, and extended. Above this temperature, the side chains are collapsed in a globular conformation that shortens the conjugation length of the polythiophene backbone.

"This work nicely demonstrates how the combination of ATRP of NIPAAm with an electronically active polythiophene can lead to an efficient thermoresponsive sensor," says Krzysztof Matyjaszewski, professor of natural sciences at Carnegie Mellon University, Pittsburgh. "It will be interesting to extend this approach to gels and explore the behavior of poly(NIPAAm) copolymers grafted onto a regioregular polythiophene backbone."