High fracture toughness and high modules silicone resins

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2000.Includes bibliographical references (leaves 159-163).Rigid silicone resins, generally referred to as polyalkylsilsesquioxanes, are an important class of hybrid thermosetting polymers with both inorganic and organic characteristics. They have superior thermal stability, heat resistance, fire resistance, and still can be easily processed. Silicone resins recently have attracted much interest as low dielectric constant materials replacing silicon dioxide as interlevel dielectrics. Unfortunately, poor mechanical properties, such as brittleness and low strength, limit their broader acceptance and applications. Efforts to toughen them date back to the 1970's, but little success has been achieved until now. Conventional polymer toughening techniques, such as incorporating second phase particles directly into the resins, typically do not work. Neither does decreasing the crosslink density of the resin network, which can compromise modulus and other properties. High fracture toughness and modulus addition cure rigid silicone resins are obtained in this study by a combination of intrinsic toughening and extrinsic toughening techniques. The addition cure silicone resin comprises two components: polyphenylsilsesquioxane oligomers containing silicon vinyls and low molecular weight silane crosslinkers containing silicone hydrides. The resin is cured by hydrosilylation between the two. Intrinsic toughening improves the plasticity and rigidity of the resin by choices of crosslinkers and polyphenylsilsesquioxane oligomers. The characteristics of molecular structure of the crosslinker and polyphenylsilsesquioxane oligomers that contribute to high fracture toughness and modulus are identified.(cont.) Extrinsic toughening with rubber particles and rigid inorganic fillers further utilized the improved plastic deformation capability from intrinsic toughening This approach not only increases the fracture toughness from 0.3MPam⁰ç⁵ to as high as 1.44 MPam⁰ç⁵, but also the modulus from 1.03GPa to 1.90GPa.by Zhongtao Li.Ph.D