Introduction. Cubic silsesquioxanes (see Figure 1) are unique molecules that combine three-dimensional cubic symmetry with single nanometer diameters and a core that is the smallest single crystal of silica. Symmetry places a functional group on each vertex in a different octant in Cartesian space providing the opportunity to form covalent bonds accordingly, such that the potential exists to construct materials in 1-, 2-, or 3-dimensions nanometer by nanometer. In principle, this permits manipulation of global properties by tailoring structures at nanometer length scales, allowing the finest control possible. It also provides access to materials with highly reproducible properties and the potential to predict and design them for specific applications. [1][2][3][4][5][6][7][8][9][10] Results and Discussion. We recently began exploring the chemistries and properties of epoxy resins and polyimides made with octaaminophenylsilsesquioxane, [NH 2 PhSiO 1.5 ] 8 , OAPS. [11][12][13][14] In early studies we demonstrated that global silsesquioxane nanocomposite properties can be tailored by controlling the structure of the organic tether linking cube vertices, at nanometer length scales. [15][16][17][18][19] We report here efforts to develop single-phase materials that offer control of the coefficients of thermal expansion (CTE) of silsesquioxane epoxy resins over an order of magnitude. Control of CTE is of considerable importance in multiple materials applications (e.g., coatings that offer resistance to abrasion, corrosion, photooxidation, hydrophobicity, staining, etc.) where the polymer coating is applied to glass, ceramic, or metal substrates with quite dissimilar CTEs. In such instances, thermal cycling often leads to loss of adhesion followed by coating failure via chemical and/or mechanical mechanisms. 20 CTE mismatches are also quite problematic in electronic applications, for example, in interlayer dielectrics and flip-chip underfills. 21 In the latter case, the underfill epoxy must match the CTEs of silicon-based ICs (CTEs of 2-3 µm/°C) with substrates (CTEs of 20-40 µm/°C) to ensure good thermal management. Current epoxy materials require silica fillers to adjust CTEs to g20 µm/°C. Such CTEs are intermediate between substrates and silicon to minimize fatigue at solder joints. These fillers raise resin viscosities to levels near 50 000 mPa‚s, making processing very difficult. Likewise, corrosionresistant epoxy resin coatings on Al alloys for aircraft bodies must minimize environmental corrosion and offer good abrasion resistance and curing at temperatures <50 °C but also have CTEs close to those of the alloys, typically 22-24 µm/°C. Such values were heretofore unknown for simple epoxy systems and especially for primer coats on aircraft fuselages that are typically DGEBA/DDM materials (60-70 µm/°C). 22 Epoxy resin thermosets studied here were produced from a series of epoxys (see Table 1 and Figure 2) formulated using OAPS as the curing agent. The formulations chosen were made according to our original model...