  The emergent photoactive materials through photochemistry make it possible to
directly convert photon energy to mechanical work. There is much recent work in
developing appropriate materials and a promising new system is semi-crystalline
polymers of the photoactive molecule azobenzene. We develop a phase field model
with two order parameters for the crystal-melt transition and the trans-cis
photo-isomerization to understand such materials, and the model describes the
rich phenomenology. We find that the photo-reaction rate depends sensitively on
temperature: at temperatures below the crystal-melt transition temperature,
photoreaction is collective, requires a critical light intensity and shows an
abrupt first order phase transition manifesting nucleation and growth; at
temperatures above the transition temperature, photoreaction is independent and
follows first order kinetics. Further, the phase transition depends
significantly on the exact forms of spontaneous strain during the crystal-melt
and trans-cis transitions. A non-monotonic change of photo-persistent cis ratio
with increasing temperature is observed accompanied by a reentrant
crystallization of trans below the melting temperature. A pseudo phase diagram
is subsequently presented with varying temperature and light intensity along
with the resulting actuation strain. These insights can assist the further
development of these materials.
