A polymer that changes shape in response to a variety of temperatures could be used in a wide range of applications from temperature sensors to weather-responsive public art scientists claim.

Polymers that return to a previous conformation after stimulation are called shape memory polymers. Normally they have one critical temperature where the shape change begins and a different shape memory polymer is needed get responses at different temperatures. Now a team led by Patrick Mather at Syracuse University in the US has overcome this problem. 'You can use one material with one base formulation' says Mather. 'It's the first time that someone's graded the response of a shape memory polymer.'

Mather made the polymers by treating a commercially available liquid photopolymer with UV light while carefully controlling the temperature - a process known as curing. As it cures, a photo-crosslinking reaction takes place which changes it from a rubbery to glassy state at the surrounding temperature.

The material is then placed on a surface with a temperature gradient and cured under UV light again. This time the polymer experiences different temperatures along its length and the temperature at which it changes to a glassy state mimics the temperature gradient on the surface. 'The process should be applicable to other photo-cured glassy shape memory polymers' claims Mather.

Indents created in the polymer were monitored using polar optical microscopy. The images disappear on heating, indicating that the polymer recovers its original shape.

'While the first generation of this new family of materials is based on a rather simple temperature gradient, the work beautifully demonstrates the power of the design. It paves the way to the next generation of smarter shape memory polymer objects that can be taught to do tricks that so far have not been possible' explains Christoph Weder an expert in stimuli-responsive polymers at the University of Fribourg, Switzerland.

Mather says the next steps are 'to make more interesting things than a linear gradient, such as patterned or 2D gradients.'

Functionally graded shape memory polymers 'could be used as cheap, passive temperature sensors in packaging to monitor the maximum temperature experienced by packages,' he adds. Eventually the functionally graded shape memory polymers 'could be used to create public art that responds to temperature or the sun,' he suggests.

Russell Johnson 

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