Thermochromic Polymer Nanocomposites for the Heat Detection System: Recent Progress on Properties, Applications, and Challenges

Reversible thermochromic polymers have emerged as compelling candidates in recent years, captivating attention for their application in heat detection systems. This comprehensive review navigates through the multifaceted landscape, intricately exploring both the virtues and hurdles inherent in their integration within these systems. Their innate capacity to change colour in response to temperature fluctuations renders reversible thermochromic nanocomposites promising assets for heat detection technologies. However, despite their inherent potential, certain barriers hinder their widespread adoption. Factors such as a restricted colour spectrum, reliance on external triggers, and cost considerations have restrained their pervasive use. For instance, these polymer-based materials exhibit utility in the domain of building insulation, where their colour-changing ability serves as a beacon, flagging areas of heat loss or inadequate insulation, thus alerting building managers and homeowners to potential energy inefficiencies. Nevertheless, the limited range of discernible colours may impede precise temperature differentiation. Additionally, dependency on external stimuli, such as electricity or UV light, can complicate implementation and inflate costs. Realising the full potential of these polymer-based materials in heat detection systems necessitates addressing these challenges head-on. Continuous research endeavours aimed at augmenting colour diversity and diminishing reliance on external stimuli offer promising avenues to enhance their efficacy. Hence, this review aims to delve into the intricate nuances surrounding reversible thermochromic nanocomposites, highlighting their transformative potential in heat detection and sensing. By exploring their mechanisms, properties, and current applications, this manuscript endeavours to shed light on their significance, providing insights crucial for further research and potential applications.

The "Tethered Solvent" Effect - H-Bonding-Controlled Thermo-Halochromism of a Push-Pull Azo Chromophore via Its Secondary Amidoalkyl Acrylamide Side Chain

The fascinating field of thermo-halochromism of azo chromophores still astounds with unexplored facets nourished by the intricate relationship between molecular structure variations and their spectroscopic signatures. In this respect, we investigated the thermally dependent absorption behaviour of acrylamide derivatives of o-methyl red, characterised by two secondary amide linkages with hydrogen bonding-active protons in the pendant alkyl substituent. The systems were studied by a combination of UV-vis, derivative, and difference, as well as 2D-NMR (Nuclear Overhauser Effect Spectroscopy, NOESY) spectroscopy. These experiments show that the thermo-halochromism is specifically influenced by hydrogen bonding interaction of the secondary amidoalkyl acrylamide side chain with the azobenzene core in dependence of the spacer length. Apparently, the substituent acts like a solvent, which is directly tethered to the chromophore and where the tether length determines the interaction by conformational freedom. We refer to this novel phenomenon as "H-bonding-controlled thermo-halochromism".

Photomotion of Hydrogels with Covalently Attached Azo Dye Moieties—Thermoresponsive and Non-Thermoresponsive Gels

The unique photomotion of azo materials under irradiation has been in the focus of research for decades and has been expanded to different classes of solids such as polymeric glasses, liquid crystalline materials, and elastomers. In this communication, azo dye-containing gels are obtained by photocrosslinking of non-thermoresponsive and lower critical solution temperature type thermoresponsive copolymers. These are analysed with light microscopy regarding their actuation behaviour under laser irradiation. The influences of the cloud-point temperature and of the laser power are investigated in a series of comparative experiments. The thermoresponsive hydrogels show more intense photoactuation when the cloud-point temperature of the non-crosslinked polymer is above, but closer to, room temperature, while higher laser powers lead to stronger motion, indicating a photothermal mechanism. In non-thermoresponsive gels, considerably weaker photoactuation occurs, signifying a secondary mechanism that is a direct consequence of the optical field-azo dye interaction.