Preparation of Photoresponsive Azo Polymers Based on Lignin, a Renewable Biomass Resource

Lignin-based azo polymers are prepared from alkali lignin, a byproduct in spent liquor from the pulping and papermaking industry, and their structures and photochromic effects are characterized by elemental analysis, Fourier transform infrared, H-1 nuclear magnetic resonance, and ultraviolet visible spectroscopy. Results show that only the 2-(4-nitrophenyl azo) phenol lignin-modified polymer (AL-azo-NO2) shows a significant photochromic effect, and its photoresponsive behavior is evidently slower than that of the synthetic polymer with a similar azo chromophore. For the 2-(4-methoxyphenyl azo) phenol lignin-modified polymer, its photoisomerization behavior was expected to be similar to that of azobenzene-type molecules, but its photoresponse is not obvious. The abnormal photochromic effect of AL-azo polymers is related to strong steric hindrance of lignin backbones. With addition of water (poor solvent), AL-azo-NO2 shrinks gradually, which prevents azobenzene groups from isomerizing and results in a lower isomerization efficiency at higher water contents. Preparation of lignin-based azo polymers offers a novel source of azo polymers and provides a green and sustainable pathway for value-added utilization of lignin biomass recovered from the pulping industry.