Large Scale Structures in Nanocomposite Hydrogels

The colloidal properties of nanocomposite polymerclay gels and solutions have received considerable attention in the literature. Unusual properties are induced by the physical presence of the nanoparticle and by the interaction of the polymer with the particle and the state of dispersion. Approaches developed range from manipulation of individual particles to exploitation of self-assembly in colloids.1,2 The large aspect ratio of platelets promotes a supramolecular organization similar to other mesoscopic systems such as liquid crystalline polymers, surfactants, or block copolymers. Poly(ethylene oxide) (PEO) absorbs onto clay platelets. Low molar mass polymers thus inhibit aggregation by classic steric hindrance, while higher molar mass polymers, particularly at higher concentrations of clay, bridge between particles and lead to the formation of large clusters3 or smart gels with novel properties.4-7 Recent small-angle neutron scattering (SANS) studies describe the adsorption of PEO polymer chains onto Laponite clay platelets at low polymer and clay concentrations using contrast variation methods to separate the contributions from bulk and adsorbed polymer chains.8,9 These studies showed that the adsorbed polymer layer thickness is about 1.5 nm on each face independent of polymer molecular weight. Even more recently, SANS studies made with no excess polymer confirm these results but also find that the edges of the platelets have polymer density on them that is much thicker than on the face.10 Other research groups have used neutron diffraction to look at the local segmental, salt, and water structure around each clay platelet and to understand the mechanism of bridging flocculation.11,12 Below the threshold for complete saturation of clay particles by sufficiently large molecular weight polymers, “shake gels” can be generated which undergo a dramatic shear thickening when subjected to vigorous shaking.7 Above this threshold we showed in our previous work that the systems form strong shear thinning gels. In that case, we postulated a dynamic adsorptiondesorption equilibrium of the entangled polymer chains with the clay particles to form “permanent” networks.4-6 Those preliminary data suggest that it is possible to tune gelation as well as kinetics; however, more studies are necessary to quantify results. The purpose of this communication is to describe and correlate microscopic and nanoscopic structures in a polymer clay hydrogel. Several methods have been applied to examine the polymer-clay interactions. Among them, microscopy and light, X-ray, and neutron scattering have proven to be powerful techniques for revealing their structure and providing a measure of size, shape, and interfacial polymer conformation. Here we use a combination of small-angle neutron scattering and microscopy techniques to provide a more complete physical picture of the large scale structure and explain some of the unique behavior observed previously. In this study aqueous solutions of the synthetic hectorite type clay (Laporte Industries Ltd.), Laponite RD (LRD), and poly(ethylene oxide) (PEO) were used. The clay particles are composed of platelets of high purity and relatively uniform size (30 ( 5 nm in diameter and ca. 1 nm thick) and have a specific surface of 370 m2/g as estimated by the supplier. PEO with a Mw ) 100 × 103, 300 × 103, 600 × 103, and 106 g/mol was purchased from Polysciences Inc. For all samples in this study a pH value of 10 and a NaCl concentration of 10-3 mol/L were used. The results reported here were obtained from gels containing mass fractions of 3% LRD and 2% PEO at room temperature. At equilibrium, the clay surfaces are covered by adsorbed PEO segments with the diffuse polymer chains serving to “bridge” neighboring clay particles, leading to a macroscopically homogeneous transparent hydrogel with clay relatively complete exfoliation as evidenced by a lack of peaks at high q in SANS.4 SANS and ultrasmall-angle neutron scattering (USANS) measurements were performed on instruments at the Center for Neutron Research of the National Institute of Standards and Technology (NIST). Both instruments are described well in the literature.13,14 The scattering data were corrected for background and parasitic scattering and placed on an absolute level using NIST standard procedures. Circularly averaged SANS data yield the scattered intensity I(q) as a function of the wave vector q. Small-angle and ultrasmall-angle scattering investigations are excellent tools in providing quantitative information concerning structures on the 1 to ca. 20 000 nm length scales. A combination of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) provides direct visualization of the nanometer and micron scale structures and can help to clarify their distribution in the system. TEM samples from the polymer clay gel were prepared by the freeze fracture (FF) technique, resulting in a carbon-platinum replica of the fractured sample surface. TEM from a pure Laponite clay solution (<0.1 wt % of clay) is from a vitrified solution and imaged in the frozen state by Cryo-TEM. Environmental scanning electron microscopy, providing a controlled humidity sample environment, was performed with a FEI Quanta 200 environmental SEM instrument. Careful attention was paid to minimize effects from solvent evaporation and condensation. All SEM samples were freeze fractured in Freon to avoid ice crystals formation and the fractured surface imaged directly without use of replicas. Our current understanding of present and previous results is that the polymer chains are in a dynamic adsorption/desorption equilibrium with the clay particles to form a network. Since there is more polymer † Louisiana State University. ‡ National Institute of Standards and Technology. § TechnionsIsrael Institute of Technology. * Corresponding author. E-mail gudrun@lsu.edu. 2047 Macromolecules 2005, 38, 2047-2049