Investigation of glass transition and dielectric behavior of nanoporous PEI (polyetherimide)

Nanoporous polymers are of great interest for their potential applications as low dielectric constant materials in the microelectronics industry. In general, nanoporous polymers can be viewed as nanocomposite materials, which consist of three parts: polymer matrix, nanopores and the interface region surrounding the nanopores. Evidence has been shown that the interface region has unique properties different from the polymer matrix, which would cause property changes in the polymer nanocomposites. However, the understanding of the interface region and its influence on properties are still limited. Therefore, the main objectives of this work were to create a controlled nanoporous polymer structure and to investigate the interface effects on glass transition temperature and dielectric constant in the nanoporous polymer system.; Nanoporous polyetherimide (PEI) was developed based on a blend of PEI and polycaprolactone-diol (PCLD). Because PEI and PCLD are immiscible, dispersed PCLD phase domains were formed in the PEI matrix. Nanoporous PEI was created by immersing the phase-separated samples in acetone, which dissolved the PCLD phase only. The pore size in spin-coated samples was found to be in the nanometer scale due to the fast phase separation process. In this case, the phase separation was far from thermal equilibrium, and kinetic factors played an important role in the process. The pore structure was characterized by microscopy methods. The pore size was found to depend on the processing time and the initial PCLD phase content (PCLD is the minor phase) due to the kinetics of phase coarsening.; The glass transition temperature and dielectric constant of the nanoporous PEI system were investigated with various means. The relationship between the properties and the pore structure was studied by controlling the pore volume fraction through varying the initial minor phase content in the blend. It was found that the glass transition temperature of nanoporous PEI would decrease with increasing pore volume fraction, which was due to the effects of the interface region where polymer chains have higher mobility than the bulk. Because of the low dielectric constant of the pores, the effective dielectric constant of the nanoporous PEI was found to decrease by the addition of pores. The interface region also has an effect on the effective dielectric constant. It was found that the dielectric constant of the interface region was lower than that of the polymer matrix, which would further decrease the effective dielectric constant of the nanoporous PEI system.