Chemical Compatibility and Reaction-Induced Exfoliation in Photopolymerizable Clay Nanocomposites Kwame Owusu-Adom and C. Allan Guymon* Department of Chemical & Biochemical Engineering, UniVersity of Iowa, 4133 Seamans Center, Iowa City, Iowa 52241 ReceiVed July 26, 2008; ReVised Manuscript ReceiVed October 29, 2008 ABSTRACT: Exfoliation of clay nanoparticles is a critical step in achieving unique properties associated with clay-polymer nanocomposites. However, a rational means of designing exfoliated nanocomposites, especially in photopolymerizable systems, remain elusive to date. This study investigates the influence of monomer-clay dispersant interactions on organoclay dispersion behavior in photopolymerizable systems. The dispersion behavior of a nonpolar, nonpolymerizable organoclay in a range of monomers with different sizes and polarity was investigated utilizing X-ray scattering and electron microscopy. Results show that increasing chemical similarity between monomer and organoclay dispersant as well as enhanced polar/nonpolar interaction facilitates exfoliation. Organoclays modified with nonpolar dispersants only intercalates in polar polymers. Conversely, exfoliation of the nonpolar organoclay is facilitated in increasingly nonpolar polymers. A set of quaternary ammonium surfactants modified with methacrylate or thiol functionalities were used as dispersants for new organoclays. These polymerizable organoclays exfoliate more readily in a number of monomer systems as compared to nonpolym- erizable organoclays. For polymerizable organoclays, the position of the reactive functional group and the type of functionality influence the degree of exfoliation. Methacrylate functionalized organoclays in which the reactive moiety is located away from the clay surface exfoliates before photopolymerization. Thiol functionalized organoclays disperse in a mixture of intercalated and exfoliated domains but exfoliate to a larger degree due to copolymerization of thiol and acrylate species in the clay galleries. Polymerizable organoclay systems exhibit higher storage modulus, glass transition temperatures, and enhanced photopolymerization behavior when compared to the nonpolymerizable analogues. Introduction Since the discovery of remarkable improvements in physical properties of polymeric materials with addition of small amounts of clay, 1-3 researchers have placed heavy emphasis on the synthesis of polymer-clay nanocomposites. 4-6 Such enhance- ments in properties have been attributed to the nanoscale dimensions of clay particles which allow significant polymer-clay interaction. Many unique aspects including improved mechan- ical, flame retardancy, and gas barrier properties have been observed in recent years. 4-9 The most significant enhancements have been observed with complete delamination of the clay platelet (exfoliation). The clay particles may also disperse in aggregates that retain their ordered morphology (intercalated state) with polymer embedded between clay layers. The most frequently observed morphology is a mixture of intercalated and exfoliated domains. To aid clay dispersion and exfoliation in organic media, hydrophilic clay particles are typically modified with quaternary ammonium surfactants. Even with incorporation of quaternary ammonium surfactants, complete exfoliation of clay particles is usually difficult. Different mechanical and chemical means have been utilized to facilitate exfoliation to varying degrees of success. Exfoliated nanocomposites have been produced through application of shear force over long periods in melt-blended clay nanocom- posites 10-13 as well as through solvent-aided exfoliation. 14,15 Other novel techniques have examined chemically induced delamination through surface-initiated polymerization tech- niques. 16,17 Initiating polymerization from clay surfaces allows growth of polymer that could aid exfoliation as the polymer chains grow in size, breaking apart the clay aggregates. To date, no substantive approach has been successfully utilized to exfoliate clays in different systems. Since the ability to induce exfoliation depends on interactions between monomer and organoclay surface, especially for systems formed in situ, a fundamental understanding of this relationship is important to successfully design exfoliated nanocomposite materials. It would be expected, for instance, that clay particles would exfoliate more readily in systems in which the clay surface modifiers are chemically similar to the monomer. This premise is indeed the principal reason for utilizing dispersants in organoclay-polymer composite systems. Hence, understanding such interactions is important to the design of exfoliated nanocomposite systems that could access advanced properties not attainable with traditional composites. While such knowledge could be useful in designing exfoliated nanocomposites in thermoplastic materials obtained through melt-blending or solvent-induced processing, it is especially critical in developing cross-linked polymer materials. Nano- composites formed by in situ processes such as photopolym- erization are at a disadvantage since network formation occurs rapidly, thus limiting monomer diffusion into clay galleries that would aid exfoliation. On the other hand, photopolymers are an important class of materials that may benefit significantly from the novel properties imparted by nanoparticles. Photo- polymerization continues to expand rapidly due to unique advantages such as ultrafast reactions, solvent-free reaction media, and temporal and spatial control of initiation. 18 Decker et al. and others have successfully developed clay photopolymer materials with some enhancement in properties. 6,7,19 However, few studies have investigated the impact of constituent interac- tions in order to optimize the nanocomposite properties through clay particle exfoliation. The lack of information regarding photopolymerizable nano- composites may also be a result of potential difficulties in generating exfoliated nanoclays due to the photopolymerization mechanism. The degree to which clay particles exfoliate in a * Corresponding author: Ph 319-335-5015; Fax 319-335-1415, e-mail cguymon@engineering.uiowa.edu. 180 Macromolecules 2009, 42, 180-187 10.1021/ma801688q CCC: $40.75  2009 American Chemical Society Published on Web 12/10/2008