Assemblies of Nanoparticles as 3D Scaffolds for New Materials Design: from Polymer Crosslinked Aerogels to Polymer Matrix Composites
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From a materials perspective, nanotechnology furnishes materials with useful macroscopic properties by manipulating matter in the 1-100 nm size regime. Improvements in performance in terms of strength, modulus and wetability are accomplished by, for example, introducing nanoparticles as fillers in plastics . Two issues that usually interfere with optimal materials performance are agglomeration of the nanoparticles and materials compatibility. Agglomeration cancels the advantage of using nanoparticulate matter as dopant, while lack of materials compatibility introduces a discontinuity at the polymer/dopant interface from where failure may begin. Agglomeration is encountered with surfactants that keep nanoparticles dispersed, while materials compatibility is improved by chemical bonding of the filler with the polymer. Overall the criterion for success is enhancement of the materials properties beyond what is obtained by simple mixing nanoparticles in the matrix. Silica is the most common dopant in use as a filler in plastics. Silica derived through a base-catalyzed sol-gel process consists of interconnected string of nanoparticles dispersed randomly in the 3D space, leaving up to >99% empty mesoporous space between the nanoparticle network. If we consider providing those mesoporous surfaces with functional groups capable of covalent bonding with a polymer formed from monomers introduced in the mesopores, then we can achieve two extreme structures with distinct materials properties: (a) at one end, we may deposit only a thin conformal polymer layer on the nanoparticle network; while, (b) at the other end, we may grow enough polymer to fill the mesopores completely. The first kind of structure emphasizes the materials properties deriving from the porosity, that is lightweight, low thermal conductivity and dielectric constants, and high acoustic impedance. The second kind of structure refers to nanoparticle/matrix polymer composites tackling both issues of dispersion and covalent bonding between matrix and dopant all at once.