Siloxane modified engineering polymers: synthesis and characteristics

TR Number
Journal Title
Journal ISSN
Volume Title
Virginia Polytechnic Institute and State University

Novel, polyand weight, randomly coupled, poly(imide siloxane) segmented copolymers were prepared and characterized. The copolymers were synthesized in two steps, the first involving the generation of soluble poly(amic acid) intermediates through reaction of various aromatic dianhydrides and aromatic diamines with a series of bis(aminopropyl)polydimethylsiloxane oligomers. These difunctional siloxane oligomers were prepared through the anionic equilibration of octamethylcyclotetrasiloxane with bis(3-aminopropyl)tetramethyldisiloxane in the presence of a siloxanolate catalyst. The aromatic monomers and siloxane oligomers were quantitatively reacted in a cosolvent system.

The poly(amic acid) intermediates were then cyclodehydrated employing two different thermal treatments to afford the imidized homo- and copolymers. The first imidization process, conducted on solution cast poly(amic acid) films, employed stepwise heating cycles to 300°C. Quantitative thermal imidization was also achieved in solution at temperatures in the 140 to 170°C range. This novel method of imidization used a coamide solvent system. Kinetic studies employing FTIR indicated that the imidization process could be described by first order kinetics. An activation energy of 26 kcal/mole (109 kj/mole) was derived.

The homopolymers and siloxane modified copolymers were characterized as a function of chemical composition and imidization method. Polymer solubility and processability greatly improved upon siloxane oligomer incorporation and the use of the solution imidization procedure. Regardless of the method of imidization, all homo- and copolymers possessed excellent thermal, mechanical, and adhesive properties. These properties were found to be a function of siloxane content and siloxane oligomer molecular weight.

In all segmented copolymer systems, a two-phase microstructure developed at relatively low block molecular weights. X-ray photoelectron spectroscopy (XPS) results indicated that the surface of copolymer films was largely dominated by siloxane. Because of this, the siloxane modified copolymers advantageously displayed lower water uptake and much improved resistance to oxygen plasma degradation. XPS and SEM studies showed that the stabilization mechanism involved a siloxane to silicate transformation under an oxygen plasma environment.