Memory of Filled Rubbers

Memory or information storage is often associated with computer chips, CD-devices, and recording media that retain and recall information or data for some interval of time, or referred to an organism's mental ability to store information. It would sound strange if someone tells you that your tire rubbers can act like computer chips that may memorize and then tell the information such as where you have driven or where you have been. Our recent studies, however, shows this is probably true that a particle-filled elastomer can actually remember and precisely record its past loading history. Storage of the information can be conducted by simply shearing the rubber in an oscillation manner at very small amplitudes, such as 1% strain. This operation can often produce a dynamic spectrum drop or mark in its dissipation spectrum and the material in fact becomes harder than its original hardness at that amplitude, but not at other amplitudes. This behavior is very different from the classical Mullins effect in which the material becomes softened after successive stretching, and takes place at large strains (i.e., >20%). This newly discovered memory is non-volatile, which means that no external force or power is needed to maintain the information stored in the rubber. The recorded events can last for more than 10 days without noticeable sign of disappearing. Refreshing of this rubber is simple, as it can be carried out by simply shearing the rubber up to certain higher strains. After that, all the previous memories or deformation histories will be erased completely. At present stage of technologies, it would be difficult to imagine that computers someday will use these elastomeric chips that are made from tire rubbers. However, the presence of this memory character in filled rubbers would potentially affect the performance of tires whenever they are running on a road or after they have been parked in a garage. Even though there are many unsolved technical questions, the physical origin of this phenomenon is fascinating for it may connect to the physics of jamming. We have previously reported some results that suggest the phenomenon depends on the microscopic interactions between the dispersed particles (or fillers) in the polymer matrix, particularly, on the heterogeneity. It is the purpose of this contribution to show the details of how the memory of a filled rubber substantially depends on the strain amplitude, loading duration, test frequency, and filler types, and to address this memory character of filled rubbers from a much broader concept --- the structure pinning in a condensed frustrated system.