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Viscoelastic silicone rubber (VSR) is a remarkable shape-memory solid. Thematerial's polymer network retains a memory of its shape history, so itscurrent and future shapes depend strikingly on its past shapes. Although VSR'smemory fades gradually and it has a permanent (cured-in) shape to which it willeventually return when left alone, VSR can be taught new shapes and retain themfor significant lengths of time.  To examine VSR's ability to learn, remember, and recover shapes, this workfocuses on a simple experiment. A VSR that has relaxed into its permanent shapeis suddenly compressed to about 80% of its original height. After a specificperiod of compression, the VSR is released and allowed to return to itspermanent shape. Having learned a new shape during the compression period,however, the VSR is reluctant to return and takes seconds, minutes, or hours todo so, depending on how long it was compressed.  In addition to observing these behaviors experimentally in VSR, we show thatthose behaviors are well-described by the Fractional Zener model. That modelinvolves time derivatives of non-integer order and those generalized timederivatives have memory. Both VSR and the Fractional Zener model describing itare acutely aware of the past.  The model's mathematical machinery make it possible to design VSR behaviorsbased on physical parameters, although some of the model's relationships arenot yet known in closed form. VSR's existence as a practical material meansthat devices can be designed and produced that use a memory of past shapes todo things that would otherwise be difficult or impossible to make.

Shape Recovery in Viscoelastic Silicone Rubber and the Fractional Zener Model