Thermoresponsive microgel-based materials.

With the continued development of thermoresponsive colloidal hydrogel particles, a number of groups have begun to exploit their properties to create dynamic materials self-assembled from those components. The fundamental details of how those building blocks are assembled, the component functionality, and the geometry or length-scales present in the assemblies contribute to the behavior of the resultant material. In this tutorial review, we examine recent progress in the assembly of responsive hydrogel colloids in two and three dimensions, highlighting their potential applications, especially in the domain of biotechnology.

Local control over phase transitions in microgel assemblies.

Poly( N-isopropylacrylamide) microgels were coassembled with Au nanoparticles into disordered glassy phases and irradiated with a tightly focused laser (lambda = 532 nm) to study crystallization dynamics following a localized photothermal annealing process. The degree of crystallization produced by the annealing process is dependent upon heat flux into the sample at the site of irradiation, the length of irradiation time, and the temperature of the surrounding bulk assembly that functions as a quenching bath. Control over these sample conditions provides a method by which to probe the dynamics of crystallization over a set of microgel concentrations. The mobility, and thus the crystallization, of particles is shown to be frustrated as the microgel concentration is increased. This is in contrast with equilibrium experiments that have shown an increase in particle mobility with microgel concentration that is manifest as an increase in the freezing transition of the bulk assembly with increasing packing density.

Coadsorption of sodium dodecyl sulfate and a polyanion onto poly(ethylenimine) in multilayered thin films.

Mixed surfactant-polyelectrolyte multilayer films were fabricated by both ionic self-assembly and spin assembly. A polycation [PEI = poly(ethylenimine)] was deposited from a dilute solution, while a polyanion (PAZO = poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]) was deposited from a mixture containing a fixed concentration of polyanion and various concentrations of the anionic surfactant sodium dodecyl sulfate (SDS). Coadsorption of SDS and PAZO onto PEI layers was observed using both deposition methods and attributed to strong PEI-SDS interactions and entropic factors. Increasing the concentration of SDS resulted in films containing progressively less adsorbed PAZO. No further reduction in the amount of adsorbed PAZO was observed above the SDS critical micelle concentration. We attribute the film growth behavior to a fast adsorption of SDS onto PEI, followed by a slower adsorption of PAZO onto the remaining unoccupied binding sites. We observe that SDS interpenetrates throughout the PAZO and PEI layers, increasing the surface hydrophobicity of both. We observed similar behavior for both ionically self-assembled and spin-assembled systems.