Long Alkyl Side Chains Simultaneously Improve Mechanical Robustness and Healing Ability of a Photoswitchable Polymer

Abstract

Mechanical robustness and healing ability are important properties of polymeric materials. However, the development of mechanically robust yet rapidly healable polymers is challenging. Herein, we demonstrate that long alkyl side chains simultaneously improve mechanical properties and the healing rate of a photoswitchable polymer. The photoswitchable polymer is a polyacrylate. Its side chain contains a hexamethylene spacer, a photoresponsive azobenzene group, and a long n-hexadecyl tail. The n-hexadecyl tail increases the decomposition temperature and glass transition temperature (T g ) of the polymer by 92 and 40 °C, respectively. Such an increase of T g is opposite to textbook knowledge, which makes the polymer robust even at elevated temperatures. The photoswitchable polymer, which has increased T g and improved mechanical properties, is a hard solid under normal usage conditions. It exhibits healing ability because it can be reversibly switched between a solid and a liquid via photoisomerization of the azobenzene groups. The polymer in the liquid state has significantly reduced viscosity for rapid healing. Importantly, the n-hexadecyl tail increases the healing rate of the polymer by 470 times. The introduction of long alkyl side chains is a new strategy for simultaneously improving mechanical properties and the healing rate of polymeric materials.

Cucurbit[7]uril Complexation of Near-Infrared Fluorescent Azobenzene-Cyanine Conjugates

“…In water, aCy7 self-aggregates, which produces a blue-shifted cyanine absorption band (cyanine H-aggregate) that is nonfluorescent. At the same time, there is a smaller red shift in absorption for the azobenzene component, indicating an end-to-end alignment of azobenzenes within the aggregate (azobenzene J-aggregate) [ 43 , 44 ]. When CB7 is added to the sample, it encapsulates the protonated azobenzene component (azonium tautomer), which converts the azobenzene absorption band at 430 nm into a peak at 534 nm and converts the cyanine H-aggregate peak at 635 nm into a monomer absorption peak at 786 nm that emits NIR fluorescence.…”

Visible-Light-Induced Reversible Photochemical Crystal–Liquid Transitions of Azo-Switches for Smart and Robust Adhesives

“…Photochemical solid ↔ liquid transitions are interesting phenomena that couple reversible photochemical transformations with thermophysical phase transitions. They have been found in organic materials based on molecular photoswitches, generally azo-switches (azobenzene and its heteroaryl analogue), because the large geometrical difference between the trans and cis states of azo units may lead to a significant change in melting points or glass transition temperatures of the whole molecular or polymeric materials. − Since the first observation of photoinduced melting phenomena in macrocyclic azobenzenes in 2010, a variety of azo derivatives showing photoinduced reversible solid ↔ liquid transitions have been developed ,− and they have received great attention in many applications, e.g., athermal patterning, , self-healing, , remote actuation, − and energy storage. − For the currently developed azo materials, UV and visible light irradiations are generally used to achieve photomelting and photosolidification, respectively. However, the use of UV light is not beneficial to practical applications, considering its low penetration depth, high cost, and possible damage to materials and the human body. − Therefore, it is desirable to develop azo materials that achieve visible-light-controlled photochemical solid ↔ liquid transitions; i.e., photomelting and photosolidification processes occur under two different wavelengths of visible lights.…”

Effects of photo-isomerizable side groups on the phase and mechanical properties of main-chain nematic elastomers

“…This is essentially different from previously studied polymers with azobenzene units tethered at the middle of the side chain. 47 Thus, the long axis of the trans form hardly becomes parallel to n , preferentially disturbing the nematic order. The effect of the photo-isomerization of this new tethering type on the nematic order then remains a non-trivial problem.…”