Defect Generation Surrounding Nanoparticles in a Cross-Linked Hydrogel Network

Abstract

A detailed understanding of polymer-nanoparticle interactions is a key element in demystifying the reinforcement mechanism for nanocomposites. To decouple the effects of the polymer-nanoparticle interactions from the particle distribution, we utilized polymerized crystalline colloidal arrays based on a thermosensitive hydrogel, poly(N-isopropylacrylamide) (pNIPAAm). First, the hydrogel network structure in the vicinity of the nanoparticles was investigated by the deswelling behavior of particle-filled hydrogels. The addition of nanoparticles led to an increased rate of deswelling when the particle-filled hydrogel was heated beyond the lower critical solution temperature (32 degrees C). To interpret this observation, we have suggested that the polymer network has a significant increase in defects (e.g., dangling chain ends) in the vicinity of the nanoparticles. The apparent percolation threshold associated with the interaction of the nanoparticles was about 20 times smaller than the theoretical percolation threshold of spherical particles. As a consequence, we have determined the thickness of this defect zone to be about 85 nm. This is much larger than the size of the unperturbed linear pNIPAAm chains, suggesting that the polymers that play a role in the adsorption are not constrained segments of polymers bound between cross-link junctions but relatively free chains. This finding enabled us to emulate the adsorption behavior of pNIPAAm hydrogels on the particles by simply adding linear pNIPAAm chains to the particle suspensions. We then prepared silica and polystyrene suspensions with free pNIPAAm chains at a concentration much lower than the overlap concentration c*. A rheological study was conducted to determine the adsorption thickness of linear polymer chains on both silica and polystyrene nanoparticles. No significant adsorption was observed on silica, whereas the resultant thickness of the polymer was 8 nm on polystyrene.