Structure-property behaviour of soluble polyimide-polydimethylsiloxane segmented copolymers

New, high-molecular-weight, randomly coupled poly(siloxane imide) soluble segmented copolymers have been synthesized from bis(aminopropyl) polydimethylsiloxane oligomers of controlled molecular weights and a series of dianhydride and diamine monomers. A solution imidization procedure was used to convert quantitatively the poly(siloxane amic acid) intermediates to the corresponding polyimides. The solution imidization procedure is based upon the co-amide solvents N-methylpyrrolidinone and cyclohexylpyrrolidinone and is conducted at milder temperatures than the classical bulk imidization. The bulk imidization technique requires temperatures of approximately 300°C, in slight excess of the ultimate glass transition. The solution imidization technique, on the other hand, is conducted at approximately 160°C. The poly(siloxane imide) segmented copolymers are flexible, tough, transparent and soluble, and are possible candidates for environmentally stable composite matrix resins, coatings and structural adhesives. Upper glass transition temperatures of many copolymers approach those of the unmodified controls, indicating good microphase separation. Lap shear strengths achieved against surface-treated titanium showed that relatively good adhesion could be achieved under practical bonding conditions. Moreover, the values were basically unchanged as a function of siloxane content at low siloxane levels. Photoelectron spectroscopy demonstrated that the top surface (∼ 10 Å) is dominated by the siloxane. Significant reductions in the dielectric constant were obtained by incorporating fluorinated dianhydrides and hydrophobic aromatic diamines into the polyimide structure. Lower moisture sorption may also be obtained via siloxane modification and by the use of more non-polar reactants.