  Rubber composites containing multi-walled carbon nanotubes have been
irradiated with near infrared light to study their reversible photo-mechanical
actuation response. We demonstrate that the actuation is reproducible across
differing polymer systems. The response is directly related to the degree of
uniaxial alignment of the nanotubes in the matrix, contracting the samples
along the alignment axis. The actuation stroke depends on the specific polymer
being tested, however, the general response is universal for all composites
tested. We conduct a detailed study of tube alignment induced by stress and
propose a model for the reversible actuation behavior, based on the
orientational averaging of the local response. The single phenomenological
parameter of this model describes the response of an individual tube to
adsorption of low-energy photons; its experimentally determined value may
suggest some ideas about such a response.
