Narrowly distributed poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-co-AA) nanoparticles with different particle sizes were synthesized and used as building blocks to form crystalline polymer gels. It was found that PNIPAM-co-AA nanoparticles can self-assemble into crystalline arrays in organic solvent as indicated by their iridescent colors and by the scattering peak in UV-vis spectra caused by Bragg diffraction. These crystalline structures were stabilized in acetone using epichlorohydrin to cross-link neighboring particles at -90 degrees C. The resultant opals had much higher polymer concentration than that of similar hydrogels. Due to their higher polymer contents, these opals had much better mechanical strength and could undergo the solvent exchange from an organic solvent to water without being broken. Kinetics of the solvent exchange was measured and explained in terms of the volume phase transition of the PNIPAM in mixed solvents. Shear modulus of the opal was measured in the linear stress-yield ranges for the same gel crystals in both acetone and water. (C) 2007 Elsevier Ltd. All rights reserved.