Influence of Particle Volume Fraction on Packing in Responsive Hydrogel Colloidal Crystals

Abstract

We describe the influence of particle concentration on the formation of crystals from soft colloidal particles. Specifically, laser scanning confocal and differential interference contrast microscopies are used to directly observe the packing in entropic crystals formed from submicron-sized particles composed of the thermoresponsive polymer poly- N -isopropylacrylamide (pNIPAm) and the thermoresponsive/pH-responsive copolymer poly- N -isopropylacrylamide- co -acrylic acid (pNIPAm-AAc). Close-packed crystals are observed when either the pNIPAm and pNIPAm-AAc particles are concentrated by centrifugation. At that point the particle−particle spacing is smaller than the hydrodynamic diameter as determined by photon correlation spectroscopy, reflecting compression of the soft particles. Upon dilution of the pNIPAm crystals, the melting point of the assembly is reached when the repulsive particle interactions are no longer sufficient to dictate long-range order in the assembly. However, excellent crystallization is observed even after a 9-fold dilution of the acrylic acid-containing particles, whereupon the particle size and hence the center-to-center particle spacing is observed to be more than twice that of the original sample. These results are not explainable using a simple hard-sphere packing model, which would predict crystal melting upon dilution, and are therefore interpreted in terms of particle compressibility (for higher concentration samples) as well as an attractive pair- or multibody-potential (for low-concentration samples).