Preparation And Properties Of Polyimide-based Dielectric Nanocomposites

Polyimides (PI) are widely used in the fileld of microelectronics and aerospace industry because of their favorable electric and mechanical properties, superior thermal stability and high strength that can not be compared by other polymer materials. With the materials continuously developing towards performance-enhancing multi functions and cost degradation, pure PI with some shortcomings of its own (such as moisture absorption and thermal expansion coefficient is higher) does not meet market requirement of low water absorption, slightly higher (or lower) dielectric constant and good mechanical properties. In order to improve application performance and expand the applications of polyimide materials, the PI matrix composites for the development and application in academic and industry has been a hot research focus. The purpose of this paper is to improve the preparation method of PI/inorganic nanocomposites. In this paper, the structure, morphology and properties of the composites influenced by the levels of inorganic nanoparticles were investigated. It shoule offer reference for prepare and properties study of PI/inorganic composites.In this paper, the methods of in situ polymerization with nanoparticle and in situ prepared by sol-gel process have been adopted to prepared composites of PI/SiO2, FPI/SiO2, PI/MWNTS, FPI/MWNTs, PI/TiO2and PI/TNTs. The relationship between properties of resulting materials and their structure, morphology have been further studied.In this paper, PI/SiO2nanocomposites were prepared by two methods, i.e., in situ prepared with nanopartocle and in situ prepared by sol-gel process. Study results indicated that composite film in situ prepared by sol-gel process,compareing to method of in situ prepared with nanoparticle, had better thermal stability, light transmittance, resistance and dielectric properties (dielectric constant less than3.0), higher storage modulus(ε) and glass transition temperature(Tg).5wt%mass loss temperature of composite thin films prepared by two methods all above540℃, this can be good to meet requirements of materials used as a dielectric material in nurmuous devices.FPI/SiO2composites in situ prepared by sol-gel process. The composite films maintained good thermal stability, and the water absorption rate decreased with the incorporation of silica nanoparticles increasing. The dielectric constants of composite films increased and then slightly decreased with the increased of silica contents, reaching to maximum when the content of silica reached to10wt%. All the composite films beared low dielectric constants.Other two PI composites, PI/MWNTs and FPI/MWNTs, successfully prepared by two-step method. The storage modulus and Tg continuously increased with increasing of MWNTs level. Tg of PI/MWNTs and FPI/MWNTs composites were above280℃and360℃, respectively, and decomposition temperature (Td), above550℃and about500℃. Because of MWNTs itself has excellent thermal stability and after modified hold good compatibility with PI matrix,PI/MWNTs possesed favorable dynamic mechanical properties. The dynamic mechanical properties improved with increasing of the mass fraction of MWNTs. The value of storage modulus was2.03GPa at50℃when the fraction of MWNTs was10wt%. Compared with that of pure PI, the storage modulus increased23.1%. The dielectric properties enhanced sharply with the increasing of mass fraction of MWNTs. For the nanocomposites containing10wt%of MWNTs, the dielectric constant reached to66.7at1MHz, which came up to18.6times of3.6for pure polyimide, indicating the dielectric of PI composites can be increased greatly by adding small amount of MWNT. Therefore, the PI/MWNTs is a kind of nanocomposite material with favorable properties of thermal properties, dynamic mechanical properties, dielectric properties.PI/TiO2and PI/TNTs films were prepared. The inorganic reinforcing phase TiO2or TNTs dispersed in the PI matrix homogeneously as amorphous state which caused transmittance of composite films decreased. The water absorption rate of two types of composite films had different trends with or without irradiation of UV. Water absorption rate of PI composite films decreased with increasing TiO2content without UV irradiation. Water absorption rate of the20wt% TiO2/PI film decreaed0.3percentage points compared to that of pure PI film, water absorption rate increased with the increase of TiO2content with UV irradiation (λ=365nm). PI/TNTs composite films had the same water absorption feature with PI/TiO2, and PI/TNTs had higher water absorption than PI/TiO2with or without UV irradiation.Massive Hydroxy (-OH) existed on exine of TNTs can act with matrix PI, then crosslinking between them was produced. Compered with PI/TiO2, PI/TNTs composites had better properties of thermal and mechanical properties. Temperature of5%weight loss of PI composite film with3wt%TNTs reched to535.4℃, increasing64.2℃compared to that of pure PI. Effect of TNTs and TiO2on the conductivity of composite film was relatively little.Compared to pure PI, the resistivities of composite films fell by only3orders of magnitude. Dielectric constants of the composite films had been only slightly improved but had a responsiveness to the UV. Under UV irradiation with a wavelength of365nm the dielectric constant of composite film, especially in PI/TNTs film, had a certain increase.