  The mechanical response of rubbers has been ubiquitously assumed to be only a
function of the imposed strain. Using innovative X-ray measurements capturing
the three-dimensional spatial volumetric strain fields, we demonstrate that
rubbers and indeed many common engineering polymers, undergo significant local
volume changes. But remarkably the overall specimen volume remains constant
regardless of the imposed loading. This strange behaviour which also leads to
apparent negative local bulk moduli is due to the presence of a mobile phase
within these materials. Combining X-ray tomographic observations with
high-speed radiography to track the motion of the mobile phase we have revised
classical thermodynamic frameworks of rubber elasticity. The work opens new
avenues to understand not only the mechanical behaviour of rubbers but a large
class of widely used engineering polymers.
