Stimuli-responsive polymers are an important class of advanced materials that have lots of applications in biochemistry, biomedicine, optical memory, and cell labeling. The physical and chemical properties of these materials and also their surrounding media could be varied in response to environmental excitation. Temperature-responsive polymers are a significant category of smart polymers, displaying phase separation and cloud points in response to temperature variations in the solution state. Controlling such responses with light has become an interesting research area, because of the advantages of light-responsive materials for the reversible control of the physical and chemical properties of media in systems with tunable intensities. Photochromic compounds are some of the most important light-responsive materials used for the light-control of temperature-responsivity in light- and temperature-responsive polymers. The incorporation of photochromic compounds, such as spiropyran, azobenzene, diarylethene, and spiroxazine, in temperature-responsive polymers results in reversible light-induced variations of lower critical solution temperatures, upper critical solution temperatures, and phase separation temperatures in such systems. The incorporation of o-nitrobenzyl ester photoactive compounds in temperature-responsive polymers can result in irreversible light-induced variations of these characteristics. These variations originate from light-induced polarity changes in the systems resulting from the responses of the light-responsive compounds. Controlling the thermo-responsivity of dual light- and temperature-responsive polymers using light illumination is the main focus of this study, which is beneficial for the development of new dual- or multi-responsive polymers with potential applications in drug- and gene-delivery systems.

Free and nanocrystalline cellulose (NCC) attached homopolymers of N-isopropylacrylamide and acrylic acid (AA) as temperature- and pH-sensitive materials and also their dual-sensitive copolymers with different contents of AA were synthesized by RAFT polymerization. NCCs were obtained from microcrystalline cellulose by an acid hydrolysis process. Then, the surface of NCC was chemically modified by 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) as chain transfer agent. In situ synthesis of polymers in the presence of 1 wt% of NCC was carried out at 70 °C via the R-group approach. Successful attachment of DDMAT and polymer chains on the backbone of NCC was studied by X-ray photoelectron (XPS), Fourier transform infrared, proton nuclear magnetic resonance, and Raman spectroscopies. Elemental analysis, thermogravimetric analysis, and XPS were also used to evaluate the grafting of DDMAT and polymers. Thermal behaviour of the NCC-attached polymers was also studied by differential scanning calorimetry. The lower critical solution temperatures (LCST) of polymers as phase separation temperatures were measured by the cloud point method using dynamic light scattering. Addition of NCC, AA content, and pH at higher pH values results in increase of LCST. At low pH values, increase of LCST occurred by an increase of NCC and also pH value. The morphology and crystalline structure of polymer-grafted NCCs were examined by transmission electron microscopy and X-ray diffraction respectively.