Synthesis and characterization of polycarbonate-polydimethylsiloxane block copolymers
Two methods for the preparation of bisphenol-A polycarbonate-polydimethylsiloxane multi-block or segmented copolymers have been investigated.
The first is a solution reaction which utilized the interaction of a preformed, α,ω-dimethylamino terminated polydimethylsiloxane with a preformed, α,ω-hydroxyl terminated polycarbonate. This synthesis produces perfectly alternating block copolymers wherein the block molecular weights are equal to the number average molecular weights of the respective well-characterized oligomeric precursors. This silylamine-hydroxyl reaction is clean, efficient and can be utilized to produce copolymers which consist of a variety of block sizes ranging from approximately dimeric to 25,000-30,000 g/mole. Intrinsic viscosities of 1.0 di/g. are easily attained and mechanical properties range from being elastomeric to rigid depending on the overall bulk composition. However, these copolymers do incorporate the relatively hydrolytically unstable [the molecular geometry of the bonds between Si—O—C] moiety as the link between the block structures.
Thermal analysis of these perfectly alternating block copolymers reveals their two-phase nature even at block molecular weights as low as ~ 2000 g/mole. Surface analysis of the copolymer films performed by X-ray photoelectron spectroscopy (known as ESCA) indicates preferential surface migration of the siloxane component in all cases studied. In addition, preliminary ESCA studies of blends of these materials with a commercial polycarbonate homopolymer show siloxane on the surface even when bulk percentage of siloxane is as low as 0.05 wt.%. A method for estimating the percentage of the surface area sampled by ESCA which is comprised of siloxane was developed. Results derived from this procedure indicate the particular importance of the siloxane block size in determining surface composition.
The second synthetic method investigated was a phase transfer catalyzed reaction which produced a more randomly coupled block copolymer. Preformed hydroxypropyl, hydroxybutyl, or carboxypropyl terminated siloxane oligomers were used whereas the polycarbonate blocks were produced in situ. These reactions, in general, were found to be less efficient than the "silylamine reaction." However, promising results were obtained using the hydroxyl-terminated siloxanes in two cases: 1, when siloxanes of ≃ 5000 g/mole were used and, 2, when the siloxane oligomers were capped with the dichloroformate of bisphenol-A prior to the interfacial step. When these phase transfer catalyzed reactions were carried out utilizing the carboxyfunctional oligomeric siloxanes, reasonably high molecular weight copolymers could be obtained. The best results were achieved when an anhydrous "pre-phosgenation" step was utilized. In contrast to the perfectly alternating copolymer systems previously described, these randomly coupled block copolymers contain the ≡SiC≡ bond which may be preferable in some applications (e.g. biomaterials).
One problem which must be addressed for the synthesis of block copolymers derived from preformed oligomers is the separate synthesis of those oligomers. The techniques for the preparation of hydroxybutyl and carboxypropyl terminated siloxane oligomers were developed in this research. Moreover, a novel, facile method for the production of polycarbonate oligomers of well-controlled number average molecular weights was also devised. This procedure involves monofunctional capping of a calculated fraction of the phenolic end-groups prior to oligomerization by the direct phosgenation route. The hydroxyl end-groups can then be regenerated by selective hydrolysis of the protecting groups. Trimethylsilyl chloride, trifluoroacetic acid and trifluoroacetic anhydride were shown to be suitable capping reagents.
Additional information pertaining to the physical characteristics of these novel copolymers was derived from collaborative studies with other colleagues at this university and at the University of Akron.