Azobenzene Molecular Trigger Controlling Phase Transitions of PNIPAm in Ionic Liquids and Light-Controlled Adhesiveness

Abstract

Photomoieties are generally covalently attached to thermoresponsive polymers to prepare photo/thermoresponsive materials, but the bonded thermo- and photomoieties prevent us from obtaining insights into the effect of the individual response. To clarify the synergetic effect, azobenzene molecules, structurally free from poly­(N-isopropylacrylamide) (PNIPAm), are utilized as photoresponsive triggers to tune upper critical solution temperature (UCST) behaviors of PNIPAm in ionic liquids (ILs). PNIPAm in an IL becomes soluble upon trans-to-cis isomerization of azobenzene under UV irradiation and aggregates again under visible light illumination. It is revealed that cis-azobenzene promotes the interaction between PNIPAm and anions but suppresses hydrogen bonding between PNIPAm chains, contributing to lower clouding temperatures under UV irradiation. Furthermore, the light-switchable macroscopic response of PNIPAm/azobenzene/IL is first exploited for an application of photoinduced reversible adhesives. This work sheds light on the intrinsic relationship between the phase transition of PNIPAm and isomerization of azobenzene in multicomponent systems.

Advances in Application of Azobenzene as a Trigger in Biomedicine: Molecular Design and Spontaneous Assembly

“…The azobenzene in the polymer acts as a light‐controlled switch and adjusts the interaction between polymer molecules by switching its cis – trans configuration to achieve macroscopic behavior changes, such as light switchable adhesion. [ 290 ] Such methods have advantages and disadvantages based on the polymers chosen, and they provide a means of exploring the synergy of thermo‐ and photoresponse at the molecular level along with their influence on macroscopic morphology.…”

Polarization of ionic liquid and polymer and its implications for polymerized ionic liquids: An overview towards a new theory and simulation

“…The addition of ILs to polymer mixtures alters the phase behavior, 97,[185][186][187][188] often more drastically than polymers dissolved in aqueous systems. 180,181 In the case of ILs, both LCST 97,177,181,182,185,[189][190][191] and UCST 192,193 for phase separation were reported. Among other things, Lee et al found a peculiar phase behavior in that the location of the critical point for a mixture of IL and PEO cannot be understood by the standard version of the Flory-Huggins theory.…”

“…For example, a structure-independent photoisomer (azobenzene) in a polymer/IL (PNIPAm/[C 2 mim][NTf 2 ]) system gave a higher T cp under visible light and gave a lower T cp under UV irradiation. 192 Morphology and ionic conductivity: The relationship between morphology and conductivity in polymer membranes is critical for the rational design of electrochemical devices. Jung et al investigated the conductivities of phosphonated block copolymers containing ILs at high temperatures.…”

“…Importantly, the photosensitive trigger can even be added individually in the monomeric units without modifying the entire polymer block. For example, a structure‐independent photoisomer (azobenzene) in a polymer/IL (PNIPAm/[C 2 mim][NTf 2 ]) system gave a higher T cp under visible light and gave a lower T cp under UV irradiation 192 …”

Photo-Controlled Adhesives Based on Photoinduced Solid-to-Liquid Transition of an Azobenzene Compound

“…The design of smart surfaces with switchable adhesive properties has remained a challenge in adhesion science and materials engineering. 5 Lightcontrolled adhesives 6 have some advantages, such as noncontact stimulus response, cleanliness, and convenience of light energy, which will be the trend for the development of adhesives. 7 Azobenzene is a switchable compound with reversible cis-trans photoisomerization abilities.…”