Thermal inverse phase transition of azobenzene hydroxypropylcellulose in aqueous solutions

Abstract

Water soluble 2-azobenzenoxy-ethoxyhydroxpropylcelluloses (azo-EHPC) were synthesized by etherification reaction of bromoethoxy-azobenzene (BEA) with hydroxypropylcellulose (HPC) to study their phase transition behavior in aq. solution. The degree of substitution (DS) of the water soluble azoEHPCs was less than 0.066. Their chemical structure and thermal property were characterized by proton nuclear magnetic resonance ( 1 H-NMR), fourier transform infrared spectroscopy (FT-IR), and differential scanning calorimetry. The azo-EHPC showed a reversible sol-gel transition behavior in its aq. solution, i.e. a clear azo-EHPC aq. solution became turbid when the solution temperature surpassed a lower critical solution temperature (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-/transconformation 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°C) by at most 13.6°C, and the LCST of trans-azo-EHPC was less than that of cis-azo-EHPC by ca. 3°C. Additionally, the presence of CD in solutions displayed a positive effect on the LCST, i.e. increasing the LCST by 3-5°C. And this impact was more profound on the azo-EHPC with higher DS values. The thermoreversible phase transition mechanism was discussed. We proposed that the effect of DS, conformation of azobenzene group, azo-EHPC concentration, 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.

Photoswitchable assembly of long-lived azobenzenes in water using visible light

“…[63] The 4F-AZO-PEO exhibits a minimum of CP at 5 mM which can be considered as the lower critical solution temperature (LCST). A shift of LCST upon cis/trans conversion is expected, as already reported for many (co)polymers bearing traditional azobenzenes and dissolved in water [64][65][66][67][68][69] or in ionic liquids. [70,71] Generally, the cis form exhibits higher LCST value than the trans one due to the lower hydrophobicity (higher dipole moment) of the azobenzene group in cis configuration.…”

Self-Assembled Nanogels of Cholesterol-Bearing Hydroxypropyl Cellulose: A Thermoresponsive Building Block for Nanogel Tectonic Materials

“…This aggregation difference between deionized water and PBS was explained by the salting out effect in which the water of hydration of HPC polymers decreased in PBS compared to that of deionized water. 28 The particle sizes of Ch-HPC nanogels in deionized water were also determined by DLS. Ch-HPC0.7 nanogels showed similar thermoresponses to HPC polymers.…”

“…In the case of HPC polymer, the hydrodynamic diameters increased from 20 to 270 nm in deionized water at 60 °C whereas that increased to over 2300 nm in PBS. This aggregation difference between deionized water and PBS was explained by the salting out effect in which the water of hydration of HPC polymers decreased in PBS compared to that of deionized water . The particle sizes of Ch-HPC nanogels in deionized water were also determined by DLS.…”

Azo-Dyes-Grafted Oligosaccharides—From Synthesis to Applications

“…Azobenzenes can also be covalently linked to hydroxypropyl cellulose (HPC) Then, AZO-EHPC [92] or AZO-HPHPC (Figure 14) can give access to a degree of az zene substitution in the range of 0.6-1.8 by adjusting the mole ratio of azobenzene to They also had the property of a very low Tg of approximately 20-40 °C being thermot liquid crystals and also fully reversible with a trans-cis-trans transition upon altern irradiation of UV and visible light. However, the mixing of heat/cooling and UV il nation were not tested, maybe because of the thermal instability of the metastable c obenzene.…”