As most organic materials age, there are changes to their microstructure. This is mainly due to exposure to the environment and loading conditions. An easily attainable organic material is rubber. Rubber is composed of many hydrocarbon chains that are entangled and interacting with each other. Some chains will have chemical bonds to other chains (called a cross-link). The number of cross-links determine how stiff and strong the rubber is. Over time new cross-links will form and old cross-links will break (called scission). As cross-links break and form, the mechanical properties of the rubber will change, and if enough occur, the rubber component will no longer behave as it was designed. 

Most of the knowledge about the behavior of rubber was determined from experiments on uni-axial specimens (stretching the rubber in one direction). Performing multi-axial experiments (stretching the rubber in multiple directions) is difficult to do well but provides new and interesting information on the evolution of mechanical properties under realistic conditions that rubber specimen may undergo. Also, rubber can be considered a very simplified model for the response of biological materials under similar conditions. Thus experimental techniques studying the degradation of rubber and elastomeric materials can be applied to biological specimens rather easily.

One method of creating multi-axial deformations is to inflate a rubber membrane. This creates equal bi-axial stretching at the pole (top) of the membrane and a number of different stretch states as you go from the pole to the clamped edge. To accelerate the cross-link and scission process experiments can be performed at elevated temperature as well as at high stretches. Below you can see an image of a piece of rubber that was initially flat, and was inflated at high temperature for a number of hours. After removing the pressure, the rubber membrane retained its inflated shape due to the breakdown of the original cross-links and the formation of new cross-links in this inflated state. The little dots printed on the surface were used to determine the actual deformation of the rubber. (See the  Experimental Mechanics page if you want more information on that).

There are research opportunities in AML in elastomer degradation. Please contact Dr. Alan Jones for more information.