Photoinduced dichroism and surface-relief-gratings of hyperbranched azo polymers synthesized by azo-coupling reaction

This article reports the synthesis and photores-ponsive properties of a hyperbranched azo polymer contain-ing electron donor-acceptor type azo chromophores both in backbone and peripheral positions.For the synthesis,a hyperbranched azo precursor polymer was synthesized by step-growth polymerization of an AB2 monomer,bis[2-(N-ethylanilino)ethyl]5-aminoisophthalate,through azo-coupling reaction.The hyperbranched precursor polymer was then reacted with the diazonium salt from 4-cyanoaniline to introduce donor-acceptor azo chromophores in the peripheral groups.The resulting azo polymers were characterized by the spectroscopic methods and thermal analysis.The photo-induced dichroism of the hyperbranched polymer was studied and its orientation order parameter was estimated to be 0.063.Thin films of the hyperbranched azo polymers were used to fabricate surface-relief-gratings(SRGs)by exposing them to an interference pattern of Ar^(+)laser beam at modest intensities(150 mW/cm^(2)).The azo chromophores introduced by post-azo-coupling reaction at the para-positions of the peripheral azobenzenes shows a significant effect on the SRG inscrip-tion rate.The hyperbranched azo polymers can potentially be used for applications such as reversible optical data storage,photoswitch,sensors,and other photo-driven devices.

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.

Hyperbranched azo polyurethane synthesized through A_(2)+B_(3) scheme

A hyperbranched azo polyurethane was synthe-sized by one-step polymerization of an A_(2) type monomer diphenylmethane-4,4p-diisocyanate and a B_(3) type monomer 4-(N,Np-bis(2-hydroxyethyl))amino-2p-hydroxyethoxyl-4p-nitro-azobenzene.The azo polymer was characterized by 1H nuclear magnetic resonance(NMR),ultraviolet-visible(UV-Vis)spectrum and thermal analysis.The l max of the poly-mer in dimethylformamide(DMF)solution is 488 nm.The number average molecular weight(M n)determined by GPC is 9,300 with a polydispersion index 1.9.The glass transition temperature(T g)of the polymer is 131℃ observed from DSC thermogram.The results show that the azo polyurethane has been successfully synthesized through this scheme.Surface-relief-gratings(SRGs)were fabricated on the poly-mer film after being irradiated by interference pattern of Ar+laser beams for 1,000 s.The surface modulation depth and the grating space period measured by AMF are 67 and 770 nm,respectively.