J. Phys. Chem. C 2008, 112, 53-60
53
Tuning Mechanical Properties of Mesoporous Silicas Using Associating Homopolymers/ Block Copolymer Blends as Templates Xinxin Li, Lingyan Song, and Bryan D. Vogt* Department of Chemical Engineering, Arizona State UniVersity, Tempe, Arizona 85284 ReceiVed: August 5, 2007; In Final Form: October 16, 2007
Ordered mesoporous films are synthesized from the vapor-phase condensation of tetraethyl orthosilicate within preformed amphiphilic template films consisting of a blend of poly(ethylene oxide)-b-poly(propylene oxide)b-poly(ethylene oxide) (Pluronic F127) and poly(4-hydroxystyrene) (PHOSt). By modification of the Pluronic F127:PHOSt ratio in the template film, the long range order of the synthesized mesoporous silica can be tuned with the largest grains observed at approximately 15 wt % PHOSt. This increase in ordering and orientation of the mesopores is accompanied by a narrowing of the pore size distribution, a slight increase in the average mesopore radius, and a substantial increase in the Young’s modulus of the film. Deviations from this 15 wt % PHOSt composition to higher or lower PHOSt loadings result in a decay in the long range ordering and subsequently a decrease in the film modulus. The modulation in the mechanical properties through modification in the PHOSt concentration (1-50 wt %) is nearly 1 order of magnitude without significantly altering the porosity. Moreover, samples with nearly indistinguishable morphology as determined through TEM micrographs can exhibit a difference in modulus by a factor of 3. This work demonstrates the importance of long range order for fabricating mechanically robust porous films.
Introduction Templated synthesis of mesoporous materials, introduced first by scientists at Mobil,1,2 has emerged as a facile route to the design of nanostructured materials that can be tuned for specific applications. Synthesis of mesoporous metal oxides has been demonstrated for a host of different templates and frameworks through solution-phase chemistry to yield powders and thin films.3-7 These methods generally yield highly ordered periodic mesoporous materials with pore sizes dictated by the template and swelling agents (where applicable), whose morphology can be predicted by the surfactant phase diagram.8-10 More recently alternative synthetic approaches have been developed that utilize preformed template films and introduce the reactive precursors either through vapor11-13 or supercritical fluid14,15 phases. These synthetic routes are thought to have potential advantages over the traditional solution-phase synthesis as self-assembly and inorganic network formation can be fully decoupled yielding improved control on the long range order and orientation of the mesopores.14 The importance of this morphological control is highly application dependent. Potential applications for mesoporous films include photovoltaic devices,16 catalysis,17 separation membranes,18 sensors,19 and a low-k dielectric for microelectronics.20 In this later application, the orientation and ordering of the pores do not appear critical for implementation in interlayer devices. Rather, these materials are generally limited by their weak mechanical properties, which lead to layer failure during chemical mechanical planarization (CMP), and large coefficient of thermal expansion, which leads to failure from stresses developed during processing due to mismatch in thermal expansion between layers.21 Additionally, the pore size distribution is critical to integration; since ultralow-k is predicted for * To whom correspondence should be addressed. E-mail: bryan.vogt@ asu.edu.
implementation at the 45 nm node, all pores must be significantly smaller for proper feature definition (preferably
