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Predicting the Self-Assembled Morphology and Mechanical Properties of

Zhenyu Shou, Gavin A. Buxton and Anna C. Balazs
Chemical Engineering Department, University of Pittsburgh, US

Keywords: computer simulation, self-assembly, filled copolymers, nanoparticles, mechanical properties

Predicting the Self-Assembled Morphology and Mechanical Properties of Mixtures of Diblocks and Nano-rods Zhenyu Shou, Gavin A. Buxton and Anna C. Balazs (speaker) Chemical and Petroleum Engineering Department University of Pittsburgh, Pittsburgh, PA 15261 Phone: 412-648-9250, Fax: 412-624-9639, E-mail: The blending of rod-like particles and block copolymers can potentially yield a system where the rods effectively form ?columns?, which provide significant reinforcement of the soft matrix material. For example, if the rods were preferentially wetted by one block of an AB diblock, the microphase separation of the copolymer could promote the localization of rods into cylindrical cores or lamellar layers that extend throughout the material. The extensive particle-filled domains can enhance the mechanical behavior of the entire system. In order to design such self-reinforcing materials, it is important to be able to predict both the morphology of the copolymer/particle mixture and the macroscopic behavior of that specific composite. In this paper, we present a methodology for calculating the structure of rod-filled diblock copolymers, and couple this method with a technique for determining the micromechanical behavior of composites. We thereby develop a scheme for predicting both the structure and mechanical properties of hybrid materials. The method for predicting the mesoscale morphology of the diblock/rod mixture is based on our recently developed SCF/DFT model (1-3), which integrates a self-consistent field theory (SCFT) for copolymers with a density functional theory (DFT) for hard particles. In previous studies, we used this method to examine the self-assembly of diblocks and spherical particles (1-3). We now modify the DFT component of the SCF/DFT to model the behavior of nanoscopic, rectangular particles (which, for simplicity, we refer to as ?rods?) and focus our studies on the interactions between relatively long, narrow fillers and AB diblocks. These rod-like fillers can represent carbon fibers, ceramic whiskers or nanotubes. The output of these SCF/DFT calculations serves as the direct input to a three-dimensional Lattice Spring Model (LSM) (4). In the LSM, the material is represented by a network of springs and heterogeneous materials are modeled by assigning different force constants to different regions of the network. Through this micromechanical model, we apply a virtual deformation to the self-assembled diblock/rod system and determine the elastic response of the material. In this manner, we can obtain insight into the effect that the morphology and the mechanical characteristics of the constituents have on the macroscopic behavior of the composite. In this talk, we begin by describing the modified SCF/DFT model and detailing the LSM technique. We then describe the effects that varying the rod aspect ratio and the rod-block interaction energies have on the self-assembly of the hard and soft components and on the mechanical properties of the resultant hybrid system. By integrating the morphological and mechanical models, we can isolate how specific modifications in the geometry or stiffness of the components affect not only the self-assembly of the material but also, the macroscopic behavior of the composite. Thus, we can establish relationships between the physical characteristics of the components, the mesoscale structure of the material and the ultimate performance of the system. Such studies can contribute to the efficient development of materials with optimal mechanical properties for specific applications. Acknowledgements ACB gratefully acknowledges financial support from the ARO, DOE and NSF. References 1.Lee, J. Y.; Thompson, R.; Jasnow, D.; Balazs, A. C. Macromolecules 2002, 35, 4855. 2.Thompson, R.; Ginzburg, V.; Matsen, M.; Balazs, A. C. Macromolecules 2002, 35, 1060. 3. Thompson, R.; Ginzburg, V.; Matsen, M.; Balazs, A. C. Science 2001, 292, 2469. 4. Buxton, G. A.; Car, C.M; Cleaver, D. J. Modelling Simul. Mater. Sci. Eng. 2001, 9, 482 and references therein.

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