Light matters: The photoresponsive properties of chitosan(s)

Prof. Dr. Linus Stegbauer, Technische Universität Bergakademie Freiberg

Recently, we have shown, that low molecular weight chitosan with the azobenzene derivative  TEGABS shows photomodulation of the mechanical properties and photoactuation. Herein, we  present a procedure to manufacture several UV-light-responsive chitosan films with different degree  of deacetylation (DD, 70-95%) and polymerization (DP, 200-3500) by incorporating the azobenzene  derivative Methoxy diethylene glycol azobenzene sulfonate (TEGABS). The films are investigated  by Powder X-ray diffraction (PXRD), UV-VIS spectroscopy, Raman spectroscopy, FTIR  spectroscopy and nanoindentation. With increasing DD at near constant DP and increasing amount  of TEGABS the films show a decrease in efficiency of photoisomerization (E→Z) of TEGABS and  in the half-life of thermal relaxation after UV irradiation. This behavior is related to an increasing  phase separation: Starting with solid-solutions of TEGABS in the chitosan matrix (low DP and low  DD) TEGABS precipitates into nanocrystals with increasing DD and DP, which compromises the  responsive properties.   The films not only show an increase of stiffness and hardness while irradiated with UV light, they  also show a light-induced removal of residual moisture. The TEGABS/Chitosan films were fabricated  into membranes to test water vapor permeabilities with and without UV irradiation. The water vapor  permeability decreases during irradiation with UV light. we show the potential of biogenic  polysaccharides in the design of humidifying membranes with a light-adjustable permeability by DP  and DD.