Study On The Sol-gel Phase Transition And Wetting Behavior Of Azobenzene Hydroxypropyl Cellulose

Hydroxypropyl cellulose ?HPC? is an important cellulose derivative, which has attracted the attention of many scientists because of its biodegradability and biocompatibility. Azo hydroxypropyl cellulose ?azo-HPC? is an interesting novel functional derivative of HPC. The introduction of azo group gives HPC new features, functions and application fields, such as optical response intelligent materials, optical information storage and liquid crystal materials[1]. The main research and findings are listed below.?1? A series of 2-azobenzenoxy-ethoxy-hydroxpropylcelluloses ?azo-EHPC? and 4-azobenzenoxy-butoxy-hydroxpropylcelluloses ?azo-BHPC? were synthesized by etherification reaction with 2-bromoethoxy-azobenzene ?BEA?/ 4-bromobutoxy-azobenzene ?BBA? and HPC. Their chemical structure and thermal property were characterized by gel permeation chromatography ?GPC?, proton nuclear magnetic resonance ?¹H-NMR?, fourier transform infrared spectroscopy ?FT-IR?, differential scanning calorimetry ?DSC? and thermogravimetric analysis ?TGA?. It was found that the molecular interation and thermal stability of azo-HPC decreased with increasing degree of substitution ?DS? of azo group. The DS_azo of water soluble azo-EHPCs and azo-BHPCs were less than 0.066 and 0.045, respectively.?2? azo-EHPC and azo-BHPC showed reversible sol-gel transition behavior in their aq. solution. Due to the similarity of azo-EHPC and azo-BHPC, we studied the lower critical solution temperature ?LCST? of azo-EHPC as a representative. The clear azo-EHPC aq. solution became turbid when the solution temperature surpassed the LCST. The sol-gel transition phenomenon was investigated by optical microscopy and turbidimetric measurement. It was found that the LCST was related to the cis-/trans-conformation of the azobenzene side group, the type of cyclodextrin ?CD?, concentration of azo-EHPC, and NaCl concentration. The LCST of azo-EHPC was lower than that of HPC ?36.6 ?? by at most 13.6 ?, and the LCST of trans-azo-EHPC was less than that of cis-azo-EHPC by ca.3 ?. Additionally, the presence of CD in solutions displayed a positive effect on the LCST, i.e. increasing the LCST by 3-5 ?. And this impact was more profound on the azo-EHPC with higher DS values. The LCST reduced with increasing concentration of salt or polyacrylic acid ?PAA?. It is because that they form hydrogen bonds with water through competitive action, which results in the ability of azo-EHPC combining with water was reduced, and the hydrophobic interaction between azo-EHPCs was enhanced. The thermoreversible phase transition mechanism was discussed. We proposed that the effect of DS_azo, conformation of azobenzene group, azo-EHPC concentration, type of salt, salt concentration, and CD on the LCST of azo-EHPCs was a rearrangement of the hydrophilic/hydrophobic interaction between side azobenzene groups and water molecules.?3? The azobenzene moiety on the azo-HPCs exhibited a reversible cis-trans isomerization transition upon irradiation with Ultra Violet ?UV? and visible light in both solution and solid film states. The reversible trans-cis photoisomerization led to change of wettability of azo-HPC films. Surface energy estimation suggests that the tunable wettability was attributed to the change of dipole moment of the azobenzene moieties on the film surface. The variation of water contact angle depended on the DS_azo of azo-HPC derivatives. A higher DS_azo corresponded to a larger variation of water contact angle upon UV irradiation under the same conditions. This work offers a versatile method to manipulate the surface wetting property of cellulose.