Tremendous reinforcing,pore‐stabilizing and response‐accelerating effect of in situ generated nanosilica in thermoresponsive poly(N‐isopropylacrylamide) cryogels

The tremendous reinforcing and pore‐stabilizing effect of in situ formed nanosilica in a highly porous temperature‐responsive poly(N‐isopropylacrylamide) (PNIPA) matrix is demonstrated. A very weakly crosslinked semi‐liquid hydrogel can be reinforced to the point that it displays a fast, extensive and nearly symmetric temperature‐responsiveness in combination with an acceptable modulus. In soft but solid porous PNIPA, only 0.6 wt% of the nanofiller is sufficient to stabilize the pores against collapse upon de‐swelling, thus enabling ultrafast responsiveness. A spectacular effect is achieved with dried porous PNIPA (matrix is glassy, T_g ≈ 140 °C), which in the case of optimal nanosilica amounts can re‐swell in only 3 min. The key importance of efficient hydrogen bridging between PNIPA and SiO₂ is demonstrated by comparing in situ formed nanosilica with similarly sized commercial Ludox particles, the surface of which is saturated with ammonia (for stabilization). Synthesis parameters like the amount of crosslinker and of nanosilica were varied in a wide range, in order to achieve the fastest possible responsiveness of the hydrogels in combination with a high modulus. The porosity, nanosilica distribution, moduli, temperature‐dependent swelling as well as the swelling kinetics of the gels were determined as functions of contents of crosslinker and nanosilica. © 2017 Society of Chemical Industry