Nanofibrillar Networks in Poly(ethyl methacrylate) and Its Silica Nanocomposites Elizabeth A. Wilder, † Michael B. Braunfeld, § Hiroshi Jinnai, | Carol K. Hall, † David A. Agard, § and Richard J. Spontak* ,†,‡ Departments of Chemical Engineering and Materials Science & Engineering, North Carolina State UniVersity, Raleigh, North Carolina 27695, Departments of Biochemistry & Biophysics and the Howard Hughes Medical Institute, UniVersity of California, San Francisco, California 94143, and Department of Polymer Science & Engineering, Kyoto Institute of Technology, Kyoto 606-8585, Japan ReceiVed: April 24, 2003; In Final Form: August 12, 2003 Recent advances in polymer materials design seek to incorporate functionality, enhance existing properties, and reduce weight without compromising mechanical properties or processability. While much attention has been drawn to the development of organic/inorganic hybrid nanocomposites modified with discrete siliceous nanoparticles (such as fumed/colloidal silica or organoclays), other opportunities exist for comparably enlightened materials design. Dibenzylidene sorbitol (DBS) is a sugar derivative that is capable of self- organizing into a 3D nanofibrillar network at relatively low concentrations in a wide variety of organic solvents and polymers. In this work, we explore the morphological characteristics and properties of DBS in poly(ethyl methacrylate) (PEMA) and PEMA nanocomposites with colloidal silica. Transmission electron microscopy and microtomography reveal that the DBS molecules form highly connected networks, with nanofibrils measuring ca. 10 nm in diameter and ranging up to several hundred nanometers in length. Dynamic mechanical property analysis reveals that, while DBS has little effect on glassy PEMA, it serves to increase the elastic modulus in molten PEMA. Introduction Polymer nanocomposites generally refer to organic/inorganic materials designed so that the matrix consists of a macromol- ecule to which an inorganic nanoscale particle is physically added or in which an inorganic species is grown under tightly controlled conditions to retain nanoscale dimensions and minimize aggregation. 1,2 Incorporation of such particles provides a versatile and efficient route to multifunctional materials possessing enhanced properties such as electrical conductivity, 3,4 nonlinear optics, 5,6 mechanical toughness, 7 catalytic activity, 2 separation selectivity, 8 and magnetism. 9 In this work, we only consider those nanocomposites prepared by the addition of inorganic particles, such as fumed or colloidal silica, to a polymer matrix. Colloidal silica has been widely used in the production of polymer nanocomposites due to its ability to improve mechanical stability at elevated temperatures, 10 electri- cal conductivity, 11 and reverse selectivity. 12 One of the chal- lenges that plagues nanocomposite design is the integration of new functionality without compromising the inherent properties, e.g., optical clarity, mechanical toughness, and facile process- ability, of the polymer matrix. Fumed silica particles, for instance, may aggregate as a polymer nanocomposite consisting of PDMS ages, 1 leading to the undesirable deterioration of optical clarity. One strategy to eliminate dispersion problems arising from inorganic nanofillers is to immobilize, and therefore stabilize, the particles within a continuous nanoscale network. 1,3:2,4-Dibenzylidene-D-sorbitol (DBS) is a derivative of the natural sugar alcohol D-glucitol 13 and can be synthesized by a condensation reaction between benzaldehyde and sorbitol. 14 In its native state, DBS is a crystalline solid with a melting point of about 220 °C. 15 This amphiphilic molecule is depicted in Scheme 1 and is often described as “butterfly-like” with a sorbitol body and two benzylidene wings. The hydrophobic phenyl rings facilitate DBS dissolution in a wide variety of organic media, 16 and, together with its ether linkages and pendant hydroxyl groups, endow DBS with a unique ability to self-organize into nanofibrils that, at surprisingly low concentra- tions, form a 3D nanoscale network and ultimately induce physical gelation. 17 Since the pioneering gelation studies 18,19 of DBS in organic solvents, numerous efforts have demonstrated that this low-molar-mass organic gelator (LMOG) can form stable networks in, and consequently gel, a wide variety of macromolecules including polyolefins such as isotactic polypro- pylene (iPP), 20 polyethers such as poly(propylene glycol) (PPG) 21-23 and poly(ethylene glycol) (PEG), 24 and silicones such as poly(dimethylsiloxane) (PDMS). 25 Moreover, DBS has been found to gel small-molecule liquid crystals 26 and block copoly- mers 27,28 derived from PPG, PEG, and/or PDMS, all of which can exhibit mesomorphic behavior on their own. Since DBS can promote gelation in some organic systems at concentrations as low as 0.1 wt %, 29 it is particularly attractive for technologies requiring uncompromised physical and/or chemical properties of a polymer melt in a thermally reversible and thixotropic elastic solid. * Corresponding author. E-mail: Rich_Spontak@ncsu.edu. † Department of Chemical Engineering, North Carolina State University. ‡ Department of Materials Science & Engineering, North Carolina State University. § University of California. | Kyoto Institute of Technology. SCHEME 1 11633 J. Phys. Chem. B 2003, 107, 11633-11642 10.1021/jp035113u CCC: $25.00 © 2003 American Chemical Society Published on Web 09/27/2003