Stimuli-responsive polymers derived from a visible light photoswitchable species

Abstract

The incorporation of photoswitches into polymeric materials is a favoured method for providing a degree of external control over certain macroscopic properties. Popularly used switches include: azobenzenes, spiropyrans and diarylethenes. However, species like these most often require damaging, high energy UV light for activation, as such visible light activated systems are highly sought after. Donor-Acceptor Stenhouse Adducts (DASAs) are a relatively new class of visible light photoswitches, that exhibit negative photochromism upon activation. These new switches are synthetically accessible and producible under mild conditions with relatively inexpensive reagents. Their large changes in polarity and the ability to almost completely decolourise when exposed to visible light, make them a highly desirable alternative to other more traditional photoswitches. Herein, stimuli-responsive materials were produced through the doping of Sn-catalysed crosslinked polydimethylsiloxane (PDMS) networks with recently discovered visible light photoswitches, DASAs, and precursory species, known as Meldrum's activated furan (MAF). MAF reacts readily with secondary amines to produce a DASA, this synthetic route was exploited by the doping of a crosslinked PDMS network with the precursor molecule (PDMS-MAF). The amine-detection responses were evaluated through UV-Vis spectroscopy, an image analysis technique and changes in RFID sensing values. The applicability of the PDMS-MAF architecture as a colourimetric sensing device was demonstrated by simulation of a food packaging environment. The amine-sensing material was sealed in a container for 24 hours, along with a cod fillet cutting - the presence of decomposition product amines was indicated by a colour change in the material. The doping of a crosslinked PDMS network with a DASA culminated in a photo-responsive material (PDMS-DASA). Crystallisation of DASA at higher concentrations was apparent, encouraging the curing of PDMS-DASA networks in a sealed, solvent-rich environment. Powder X-ray diffraction provided evidence to suggest that the sealed atmosphere curing (SAC) reduced the concentration of crystallisation. The photoswitching kinetics of PDMS-DASA samples were examined using UV-Vis spectroscopy, with a focus on the effect of variation in DASA concentration. Furthermore, initial investigations into photoswitching fatigue, the effect of solvent swelling of photoswitching kinetics and RFID-sensing applicability were conducted. Additionally, other investigations into the covalent incorporation of DASAs for stimuli-responsive polymers are detailed. These further investigations included various polymeric systems, such as polyethyleneimine, poly(methylhydrosiloxane-co-dimethylsiloxane) and building from a polyurea-based scaffold.