Templating, Structure and Transport within Nanostructured Block Copolymer Hydrogels

Lynn M. Walker, Carnegie Mellon University

Self-assembled block copolymer templates are used to control the nanoscale structure of materials that would not otherwise order in solution. In this work, we have developed a technique to use close-packed cubic and cylindrical mesophases of a thermoreversible block copolymer (PEO-PPO-PEO) to impart spatial order on dispersed nanoparticles. The thermoreversible nature of the template allows for the dispersion of particles synthesized outside the template. This feature extends the applicability of this templating method to many particle-polymer systems and also permits a systematic evaluation of the impact of design parameters on the structure and mechanical properties of the nanocomposites.  The criteria for forming co-crystals has been fully characterized using contrast-matching small-angle neutron scatting (SANS) and the mechanical properties of these soft crystals determined. Numerous crystal structures have been reported for the block copolymer system and we have taken advantage of several to generate soft co-crystals.  The interstitial spaces of these micellar crystals are used to template nanoparticles with hydrodynamic diameters ranging from 4-7 nm. The result of this templating is spatially ordered nanoparticle arrays embedded within the block copolymer nanostructure. In this work, the dynamics of nanoparticles within the nanostructured material are characterized with fluorescence recovery after photobleaching (FRAP). Properties such as interstitial spacing and proximity to sol-gel transition temperatures are dependent on polymer concentration, and may account for the differences in particle mobility for such nanostructured systems.  Characterization of transport of nanoparticles and globular proteins in these hydrogels is important as the gels are proposed as matrices for storage of nanoparticulate material without aggregation.