Supercritical fluid applications in polymer nanocomposites David L. Tomasko * , Xiangmin Han, Dehua Liu, Weihong Gao Department of Chemical Engineering, The Ohio State University, Columbus, OH 43220, USA Received 6 October 2003; received in revised form 14 October 2003; accepted 14 October 2003 Abstract Supercritical fluids have been used to synthesize and foam a variety of polymer nanocomposite materials. There have been significant advances in developing and characterizing nanoscale structures and using supercritical fluids to alter properties at the nanoscale. In this work we summarize these advances and discuss foam properties generated using supercritical carbon d well as relevant fundamental properties of the system. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Nanocomposite; Supercritical; Carbon dioxide; Foam adsorption; Rheology Major recent advances Nanoscale reinforced polymeric materials (aka poly- mer nanocomposites) provide an opportunity for com- bining the features of supercritical fluids with traditional polymer processing. Recent work emphasizes reducing the domain size of the dispersed component to achieve true nanoscalefeatures.Advances include surface modification to exfoliate nanoclays, controlled synthesis in confined matrices, and better understanding of the role of supercritical CO 2 in rheology and interfacial properties. 1. Introduction The Ôtop down’ approach to nanotechnology signifies the modification of microscale processing techniques to produce nanoscale features. Much of the recent work in using supercritical fluids (SCFs) for polymer composite materials falls into this vein and is the primary focus of this review. The term polymer nanocomposite is broadly defined to include a material consisting of two immis- cible phases of which one is a polymeric material and one is present as a dispersed phase (or domain) with a nanoscale feature in at least one dimension. For exam- ple, the seminal work from Toyota R&D [*1,2]dem- onstrated a nanoclay platelet dispersed in a polymer matrix to improvethe mechanical propertiesof the composite material. The nanoclay is a layered silicate which can be exfoliated into platelets with nanometer thickness and aspect ratios on the order of 10 3 . The application of supercritical fluids to the prepa- ration of composites is through one of two approaches: in-situ polymerization of the dispersed phase in a SCF- swollen polymermatrix,or dispersive mixing of the nanoscale phase into the polymer matrix using the SCF to reduce the matrix viscosity and/or nucleate nanoscale bubbles in the material. While the basic feasibility of the in-situ processes were demonstrated in the mid-1990s [**3,*4,**5,6], recent work has concentrated on reduc- ing the dispersed phase domain size and isolating the nanoscale features in the composite materials. 2. In situ polymerization for polymer/polymer and poly- mer/inorganic composites In situ processes for producing nanocomposites apply a two-step procedure. Initially,the polymer substrates are infused with the precursor reactants dissolved in the SCF and then a reaction is induced (typically thermally) to generate a new dispersed phase. The SCF functions as a swelling agent for the polymer to enhance diffusion, a solvent for the precursor reactants and byproducts, and nonsolventfor the reaction products. The resulting nanoparticle or nanophase products are restrained from agglomerating by the polymeric continuous phase. * Corresponding author. Tel.: +1-614-292-4249; fax: +1-614-292- 3769. E-mail address: tomasko.1@osu.edu (D.L. Tomasko). 1359-0286/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2003.10.005 Current Opinion in Solid State and Materials Science 7 (2003) 407–412