CCFE - Center for Complex Fluid Engineering - Carnegie Mellon University

Three-dimensional spatial organization of inorganic nanoparticles or nanometer-sized biomolecules is particularly important in emerging nanotechnology applications involving optical materials, controlled-release of therapeutic proteins and separation of biomolecules in gel electrophoresis. The self-assembly of block-copolymers in a selective solvent is used to drive the organization of dispersed nanoparticles into arrays with three-dimensional periodicity. We are currently using water-soluble triblock copolymers (PEO-PPO-PEO) to provide highly organized templates with typical dimensions of tens of nanometers. These materials show reversible gelation (gel formation on increasing temperature) that allows the use of pre-made silica and gold nanoparticles as well as globular proteins. The order-disorder transition of the polymer arises from a difference in dehydration temperatures between the middle PPO block and the two PEO end blocks. This drives the formation of micelles that have a dehydrated PPO core and an extended and hydrated PEO corona. At sufficiently high polymer concentrations, these micelles pack into organized structures with long-range crystalline order. The diagram below shows a schematic of the templating method.

Using rheology and small-angle neutron scattering (SANS), we have characterized the influence of nanoparticle loading on both the macroscopic sample properties and on the local structure of the gel. We also evaluate the influence of nanoparticle properties (size, interaction, monodispersity) and polymer properties (micelle size, shape, concentration) on the overall nanocomposite order. By using solvents containing isotope mixtures in SANS we are able to separately study the structure of the polymer template and the templated particles through the variation of the neutron scattering contrast. Using SANS, we demonstrate that the template approach is feasible as nanoparticles are incorporated into the block copolymer gel without destroying the ordered structure. The figure below shows the agreement between the scattering from the Face Centered Cubic polymer template (red) and the templated Bovine Serum Albumin (BSA) protein particles (black). The proteins are located in the interstitial spaces of the cubic crystal.
The addition of protein particles in numbers that exceed the number of available template sites, is found to have a direct impact on the mechanical structure of the gel. The following figure shows the maximum modulus of nanostructured gels as a function of protein concentration. The figure shows significant gel weakening (lowering of the modulus) and increased fluidity (increasing phase angle) with the addition of proteins. The outcome of this research is expected to guide the design of nanocomposite materials for new applications. We are currently exploring the diffusion of particles through the template, the effects of intercalated particles on template shear alignment and the effect of spatial confinement on the thermal stability of the templated proteins.