posted on 2012-03-28, 00:00authored byQuirina Ferreira, Paulo A. Ribeiro, Osvaldo N. Oliveira, Maria Raposo
Optical memories with long-term stability at high temperatures
have long been pursued in azopolymers with photoinduced birefringence.
In this study, we show that the residual birefringence in layer-by-layer
(LbL) films made with poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene
sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) alternated with poly(allylamine
hydrochloride) (PAH) can be tuned by varying the extent of electrostatic
interactions with film fabrication at different pHs for PAH. The dynamics
of both writing and relaxation processes could be explained with a
two-stage mechanism involving the orientation of the chromophores
per se and the chain movement. Upon calculating the activation energies
for these processes, we demonstrate semiquantitatively that reduced
electrostatic interactions in films prepared at higher pH, for which
PAH is less charged, are responsible for the longer stability at high
temperatures. This is attributed to orientation of PAZO chromophores
via cooperative aggregation, where the presence of counterions hindered
relaxation.