A novel approach to obtain high performance layered silicate thermoset polyimide matrix nanocomposites

The use of exfoliated layered silicates as reinforcing filler in polymer composites shows great potential for drastically improving the material properties. Much work has been done with thermoplastic systems showing improved composite properties from exfoliated clay reinforcement. More recently, thermoset systems have been increasingly studied, however little progress has been made in achieving full clay exfoliation in high performance thermoset polyimides, which otherwise possess desirable processability, thermal, and mechanical properties. Literature has shown that attaining necessary levels of clay exfoliation and dispersion in these materials is a challenge using current technology. This study explores a novel method of clay exfoliation, which involved prior intercalation of organoclay with lower molecular weight PMR-5 oligomer and incorporation of clay into higher molecular weight PMR-15 resin. In the process, the elastic force within clay galleries grows at a higher rate and pushes the clay platelets apart to yield an exfoliated clay structure. Process of clay intercalation by lower molecular weight PMR monomer is studied under conditions of low and high shear mixing and sonication, in order to determine effect of processing on properties. In addition, this work sought to determine an optimum organic treatment for nanoclay to be compatible with the method stated above. Specifically, an organic treatment should promote PMR-5 intercalation of clay and be stable at very high curing temperature, ∼315°C. In this work four surfactants were used to treat clay, including hydrochloride salts of aliphatic and aromatic diamines and, reactive and non-reactive amines. The resultant mechanical, thermal, and rheological properties were evaluated as functions of clay type, degree of clay exfoliation, and clay intercalation strategies and compared with those available in literature.; It was found that method of mixing plays a key role in achieving high degree of exfoliation, and a 1:3 ratio by weight of APND and C12 combination of surfactants are optimum to achieve maximum clay exfoliation and improvements in thermal and mechanical properties of PMR-15 composites. The resultant composites showed increased barrier properties, improved thermal stability, and increased flexural strength and strain in carbon fiber laminate composites compared to composites of neat PMR-15 resin. The new and novel method of clay exfoliation in PMR-15 resin explored in this work was met with success.