Preparation And Characterization Of Polyimide Ultrafine Fiber Membranes Enabled By Electrospinning

Aromatic polyimide with its special chemical structure has many advantages in terms of dielectric, thermal stability, mechanical, chemical resistance, etc, which had been widely used in electronic, automobile, space, military, bio-medical, communications, and high performance parts. Followed on Moore's law, the speed of computer chips has doubled every 18 months, which requested a low loss and low dielectric substrates in the industry. In order to realize this goal, three methods were introduced in industry in order to achieve the low dielectric constant: (1) Using Flour substrated monomers; (2) filled in xerogels to form nanoporous film: (3) introducing low dielectric side chain such as, silicone group.In this thesis, we would like to introduce a novel way to prepare the low dielectric constant polyimide membranes through electrospinning. The major advantages of this preparation are (1) produced ultra thin (nano grade) polyimide fabric materials, (2) the non-woven formed polyimide fiber membranes has many nanoporous structures with a lager surface area, which has many potential industrial applications such as, filtration, insulation, packaging, etc., (3) achieved low dielectric constant of 1.5, which is even lower than the polyimide nanoporous film (2.5) and floured polyimide (2.8), and polyimide film (3.4).We used two-steps synthesis method to make polyimide (PI), i.e., preparation of polyimide precursor and imidization. Several polyimide precursors, i.e., polyamic acids (PAA), were prepared through the combinations of diamine monomers of ODA, p-TPEQ, and dianhydride monomers of PMDA, s-BPDA, BPADA, and ODPA. Then, electrospinning was applied to those polyimide precursors. We selected several monomer combination as standards to conduct the process study such as, PMDA//ODA (Kapton-H), s-BPDA//ODA (Upilex-R), and 90%PMDA/10% BPDA// ODA.As for spinning process condition studies, we had optimized the spinning concentration of polyamic acid as well as different co-solvent combination. We also conducted used FTIR to optimize the curing temperature of polyimide. SEM was used to analyze the morphology changes from PAA to PI. As for PMDA//ODA type polymer, its morphology almost has no changes during the imidization. This is because the polymer has a high Tg of 385oC (by DSC) and its polymer chain is very rigid. However, a polymer networks were formed after the imidization on s-BPDA//ODA polymer. This is because the polymer has a low Tg of 285oC (by DSC) and its polymer chain is more flexible comparing to PMDA//ODA based polymer. As for the 90%PMDA/10% BPDA// ODA polymer, it formed a"branched-tree truck"structure after the imidization. This is because the 10% BPDA has changed the polymer chain regularity, therefore, formed more amorphous structure.We had obtained an ultra low dielectric constant (DK) of ~1.5, which is significantly lower than the standard polyimide film (DK~3.4). This is because the ultra thin (nano grade) polyimide fabrics formed non-woven membranes created many nanoporous structures. This material should be very interested in several industrial areas such as, low loss and low DK substrates for high frequency electronic packaging; low DK substrates for IC packaging; high temperature stable with nanoporous membranes for filtration; High temperature stable with larger surface areas membranes for insulation, etc.