| Two aspects of novel polysiloxane chemistry have been explored in this work: (1) the synthesis and characterization of polysiloxanes functionalized with terminal cyanate ester groups for use as low flammability impact modifiers for commercial phenolic cyanate ester resins (CERs) and (2) the synthesis and characterization of polydiethylsiloxane and polycyclotetramethylenesiloxane.; CERs have been proposed as a potentially useful matrix material for commercial aircraft manufacture. However, commercial CERs do not currently have the appropriate fracture toughness for practical applications. Prior studies have shown that siloxanes offer excellent flame suppression and can improve the CERs impact properties. In this work, a series of new siloxanes terminated with cyanate-reactive groups, including bis[1,3-(4--cyanatophenyl)]-1,1,3,3-tetramethyldisiloxane (11), alpha,o-(4-cyanatophenyl) oligodimethylsiloxane (12), and alpha,o-(4-cyanatophenyl) oligo(dimethyl- co-diphenylsiloxane) (13), have been synthesized from corresponding diphenols. The intermediates and products were characterized by 1H NMR, 13C NMR, FTIR, GC-MS, and GPC. To synthesize the diphenols, a new route to synthesize disiloxane terminated with THP protected diphenols (dimer) was established. The synthesized dimer was used to equilibrate with D4 and/or D4'' to make desired siloxane oligomers. The mechanism of the equilibration of the dimer with D4 were studied. The synthesized CER monomers were polymerized by a cyclotrimerization reaction. The cured compound 11 is a brittle solid with Tg of 274°C. The cured 12 (oligodimethyl-siloxane) and 13 (oligodimethylsiloxane- co-diphenylsiloxane) are elastomers with Tg of -110°C and -8°C, respectively. The curing processes and intermediates were studied by dynamic and isothermal differential scanning calorimetry, GPC, IR and 13C NMR. The synthesized CER monomers were used as impact modifiers to blend and co-cure with the commercial CERs, Primaset PT-30. SEM shows that the cured blends of 12 and 13 with PT-30 are multiphase while the cured blend of 11 with PT-30 has no phase separation.; In the second part of this work, the synthetic methods for preparing polydiethylsiloxane (PDES) and polycyclotetramethylenesiloxane were explored by both anionic and cationic ring-opening polymerization and by condensation of hydroxyl end-groups in alpha,o-dihydroxy terminated siloxanes. The intermediates and products were characterized by GPC, IR GC-MS and 1H NMR. The process of polycondensation of PDES was followed by GPC and IR. |
