Returning Materials to a Youthful Form

It is not uncommon to snap and break a plastic knob in an older car. The properties of plastics change slowly over time. Plastics can become brittle as the long molecules--the fundamental building blocks of plastics and polymeric materials more generally--degrade. Increasing the temperature of the material until it becomes molten and cooling it back down until it returns to a solid, can reverse the aging process. But can a mechanical deformation do the same thing?

To answer this question, researcher Dan Lacks and student Greg Chung used a computer to simulate polystyrene molecules under shape-altering stress. Polystyrene appears in many applications including plastic drinking glasses and eating utensils. Lacks and Chung found that mechanical deformation can lead to macroscopic properties more like those of the plastic in its youth. But examination at the microscopic level shows that in contrast to the age reversing effects of temperature cycling, the mechanically rejuvenated material differs in a fundamental way--it is in a state that cannot be achieved by changing temperature alone. The insight gained from further simulations may lead to ways of overcoming aging, leading to better polymer materials.

The results of this research contradict the precise notion of mechanical rejuvenation. In contrast to previouswork that focused on changes in macroscopic properties of the material, this work examines deformation-induced changes to the structure of the material at the molecular level. The molecules in the material assume configurations that are not reached by processes where only the temperature is changed.

This work may lead to computationally inspired ways to design materials, particularly polymeric materials that have superior aging properties.