Surface-relief-grating formation behavior of two hyperbranched azo polymers

In this work, surface-relief-grating formation behavior was studied by using two hyperbranched azo polymers. The hyperbranched polymers containing pseudo-stilbene type azo chromophores throughout the hyperbranched structure were synthesized by step-growth polycondensation of AB₂ type monomers. The AB₂ monomer, 4-(4′-(bis(2-chloroethyl)amino)-phenylazo)benzoic acid (BAA), was prepared through azo-coupling reaction between N,N′-bis(2-chloroethyl)aniline and 4-aminobenzenic acid. The another AB₂ monomer, 4-(4′-(bis(2-chloroethyl)amino)phenylazo)-3-nitro-benzoic acid (BANA), was prepared through azo-coupling reaction between N,N-bis(2-chloroethyl)aniline and 3-nitro-4-aminobenzenic acid. The hyperbranched polymers (PBAA and PBANA) were prepared through nucleophilic substitution reaction of BAA and BANA, respectively. The polymers synthesized were characterized by using spectroscopic methods and thermal analysis. The photoinduced dichroism and photo-induced surface-relief-grating (SRG) formation of the hyperbranched polymers were investigated upon irradiation with Ar⁺ laser at 488 nm. PBAA shows typical photoinduced dichroism SRG formation behavior. On the contrary, PBANA does not show the photoresponsive properties. The results indicate that the nitro at the ortho position of azo group of PBANA shows the effect of inhibiting the photoinduced variations. The effect can be attributed to the blockage of the trans-cis isomerization of the azobenzene moieties by the steric hindrance.