Thermosensitive two-dimensional arrays of hydrogel particles

We prepared two-dimensional (2D) arrays composed of nanoscale hydrogel particles as a model of suprastructured assemblages. Poly(N-isopropylacrylamide) (PNIPAM) hydrogel particles and the core-shell particles, which possess a rigid polystyrene core encased in a PNIPAM hydrogel shell, were selected as components for arrays. Below and above 32°C, these two kinds of particles swelled and shrank, respectively, due to temperature-dependent solubility of a PNIPAM chain. The particle dispersion was spread and then compressed to produce particle monolayers closely packed at the air-water interface. To study thermal behavior of 2D arrays, surface pressure and surface area for obtained 2D arrays were measured at 40 and 25°C with leaving the monolayer on the water surface. Both of them varied in response to temperature, indicating that each particle of 2D arrays swelled and shrank at the water surface. Although the pressure and the area for the 2D array of PNIPAM hydrogel particles exhibited great responses at the beginning, they gradually decreased and eventually disappeared with repeating the temperature cycle, due to particle coalescence. On the other hand, the 2D array of core-shell particles maintained the thermosensitive response without relaxation though its variation between 40 and 25°C was still small. By assembling the mixture of PNIPAM and core-shell particles into the monolayer, the 2D array with large and steady responses could be obtained.