Synthesis Of Novel Polyimides With Large Volume Pendant Groups

It is necessary to use interlayer materials with lower dielectric constant(k)for the development of ultra large scale integrated circuit.Based on the excellent performances including thermal stability,mechanical and electrical properties,polyimide(PI)materials have been widely used in the microelectronics and optoelectronics industry.The k of the conventional polyimide is about 3.5.Fluorine-containing or porous PIs can decrease k to less than 2.0,but leading to some disadvantages.Therefore,it has become an important issue in the study on low dielectric constant polyimides to balance dielectric properties and others.4,4'-Diamino-3''-phenoxytriphenylmethane(DPTM)with triphenylmethane and bulky side groups was synthesized and added as the third monomer to PI backbones.A series of copolyimides were obtained.These PI films had good thermal properties,the glass transition temperatures(T_gs)were over 268°C,and the 5%weight loss temperatures were all over520°C.The dielectric constant of the PI film decreased as the content of DPTM increased,and DPTM-PMDA film(PI-1)had the lowest dielectric constant(k=2.58,1 MHz),which also had the best solubility in most organic solvents.The twisted structure of the main chain and the bulky side groups were favorable for the larger free volume of molecules,and for the lower dielectric constant.An aliphatic diamine p-xylylenediamine(p-XDA)was used to synthesize polyimide with light color and transparence.p-XDA reacted with acetic acid to form a salt,and then dianhydride was added to prepare a viscous polyamic acid(PAA)solution for thermal imidization.p-XDA-based PIs showed light-colored and transparent,and possessed good thermal stability and mechanical properties.The film prepared from p-XDA and3,3',4,4'-biphenyltetracarboxylic dianhydride(BPDA)had transmittance over 80%at 400 nm.Methylene in p-XDA interrupts the conjugation and reduces the formation of charge-transfer complex(CTC).Furthermore,p-XDA exhibits a highly distorted molecular conformation in PI backbone,which leads to a looser packing and a larger free volume of PI molecules.The amino groups attached to the methylene of p-XDA have strong basicity and can react with the carboxyl group on PAA molecules.Therefore,as a cross-linking agent,p-XDA was introduced to the precursor of PI-1 to prepare crosslinked PI films(CPIs).The thermal and mechanical properties of CPIs increased with the cross-linking degree.When the degree of cross-linking reached 18.8%,the T_g,5%thermal decomposition temperature and Young's modulus of CPI-2.5 film were increased by 11.6°C,20.1°C and 46.5%,respectively,compared to the uncross-linked PI.