Anisotropic mechanical responses of composites having water microchannels

Tough, Self-Recoverable, Spiropyran (SP3) Bearing Polymer Beads Incorporated PAM Hydrogels with Sole Mechanochromic Behavior

Spiropyran-containing hydrogels that can respond to external stimuli such as temperature, light, and stress have attracted extensive attention in recent years. However, most of them are generally dual or multiple stimuli-responsive to external stimuli, and the interplay of different stimulus responses is harmful to their sensitivity. Herein, spiropyran bearing polymer beads incorporated PAM (poly(AM–co–MA/DMSP3)) hydrogels with sole mechanochromic properties were synthesized by emulsion polymerization of acrylamide (AM) and methyl acrylate (MA) in the presence of spiropyran dimethacrylate mechanophore (DMSP3) crosslinker. Due to the hydrophobic nature of MA and DMSP3, the resultant hydrogel afforded a rosary structure with DMSP3 bearing polymer beads incorporated in the PAM network. It is found that the chemical component (e.g., AM, MA, and DMSP3 concentrations) significantly affect the mechanical and mechanoresponsive properties of the as-obtained poly(AM–co–MA/DMSP3) hydrogel. Under optimal conditions, poly(AM–co–MA/DMSP3) hydrogel displayed high mechanical properties (tensile stress of 1.91 MPa, a tensile strain of 815%, an elastic modulus of 0.67 MPa, and tearing energy of 3920 J/m²), and a good self-recovery feature. Owing to the mechanoresponsive of SP3, the hydrogels exhibited reversible color changes under force-induced deformation and relaxed recovery states. More impressive, the poly(AM–co–MA/DMSP3) hydrogel showed a linear correlation between tensile strain and chromaticity (x, y) as well as a stain and resting time-dependent color recovery rate. This kind of hydrogel is believed to have great potential in the application of outdoor strain sensors.

Temperature-Responsive On-Off Control over Water Evaporation Achieved via Sweat-Gland-Mimetic Composites

Responsive cooling materials that mimic sweat glands have gained popularity because they are efficient and do not require artificial energy sources. Temperature-responsive hydrogels sweat above their volume transition temperature through the release of water and exhibit excellent cooling ability. However, thus far, practical applications have not been possible because the water in these materials cannot be preserved in cool environments. To address this issue, this paper presents a simple composite of poly(N-isopropylacrylamide) and polydimethylsiloxane that offers excellent on-off control over water evaporation and can be used repeatedly; the proposed composite features an evaporation rate of 2.97 g/h above the lower critical solution temperature (LCST) and 0.08 g/h below the LCST. This 35.7-fold change in the water evaporation rate is comparable to that in mammalian sweat glands. The responsive on-off control relies on the structures of the composite and the dry layers formed on the surface of the composite in cool environments. The proposed material effectively regulates water evaporation and offers a novel, low-cost cooling strategy suitable for numerous applications.