Preparation And Properties Of PI/ZrW₂O₈ Composites With Low Thermal Expansion

Inorganic/organic composites have attracted extensive interests due to their unique properties. The composites combine the advantages of the inorganic materials and organic polymers, achieveing extraordinary properties which are different from the single component.Aromatic polyimide is a kind of high performance polymer material. It has excellent thermal stability, high mechanical strength and low dielectric constant, but high thermal expansion coefficient. While zirconium tungstate, ZrW₂O₈, is an excellent isotropic negative thermal expansion material. The aim of this work is to control the thermal expansion coefficient of PI/ZrW₂O₈ composites through combining the excellent properties of polyimide and ZrW₂O₈. The preparation technics, the relationship between the structure and properties (especially the thermal expansion property) were investigated.First, ZrW₂O₈ powders were prepared by step by step solid state reaction method. The linear thermal expansion coefficient of ZrW₂O₈ which was measured by thermal expansion dilatometer is -5.08×10^-6 K^-1. The structure and properties of PI/ZrW₂O₈ composites, prepared by in-situ polymerization, were investigated by FTIR, SEM, TG and the thermal expansion dilatometer analysis. A series of PI/ZrW₂O₈ composites with different ZrW₂O₈ concentration and different coupling agent (KH550) concentration were prepared. FTIR spectra confirmed the conversion from PAA into polyimide in composites. SEM images of cross section of the PI/ZrW₂O₈ composites showed that ZrW₂O₈ particles were very small and uniformly dispersed in the composites. TG analysis indicated that the addition of ZrW₂O₈ particles could improve the thermal stability of PL Compared with the pure PI, the Td increased from 492.58℃to 570℃when the concentration of ZrW₂O₈ reached 25wt%.Thermal expansion coefficient (CTE) of PI/ZrW₂O₈ composites containing 0-50 wt % (0-22 vol %) ZrW₂O₈ were measured by the NETZSCH DIL 402EP thermal expansion instrument and the results show that the thermal expansion coefficient decreased with the increasing of ZrW₂O₈ content. A 22 vol % ZrW₂O₈ loading gives a 30%(14×10^-6 K^-1) reduction of CTE. With ZrW₂O₈ content increasing, the dielectric constants of the PI/ZrW₂O₈ composites increased from 3.76 to 6.05, which showed a controllable possibility in certain scope. Rom, Kerner and Turner model were used to predict the coefficient of thermal expansion of PI/ZrW₂O₈ composites. The results showed that our experimental results were close to Kerner model.Moreover, we also tried to prepare PI/ZrW₂O₈ composite films and a series of PI/ZrW₂O₈ composites films with different ZrW₂O₈ and KH550 concentration have been made by the spin-coating method. The obtained films were investigated by FTIR, SEM and TG analysis.