  We study the spontaneous out-of-plane bending of a planar untwisted ribbon
composed of nematic polymer networks activated by a change in temperature. Our
theory accounts for both stretching and bending energies, which compete to
establish equilibrium. We show that when equilibrium is attained these energy
components obey a \emph{complementarity} relation: one is maximum where the
other is minimum. Moreover, we identify a \emph{bleaching} regime: for
sufficiently large values of an activation parameter (which measures the
mismatch between the degrees of order in polymer organization in the reference
and current configurations), the ribbon's deformation is essentially
independent of its thickness.
