Preparation of Photoresponsive Azo Polymers Based
on Lignin, a Renewable Biomass Resource
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Abstract
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, <sup>1</sup>H nuclear magnetic
resonance, and ultraviolet–visible spectroscopy. Results show
that only the 2-(4-nitrophenyl azo)
phenol lignin-modified polymer (AL-azo-NO<sub>2</sub>) 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-NO<sub>2</sub> 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