Preparation and Properties of Electrospun Phenylethynyl—Terminated Polyimide Nano-Fibrous Membranes with Potential Applications as Solvent-Free and High-Temperature Resistant Adhesives for Harsh Environments

Abstract

High-temperature-resistant polymeric adhesives with high servicing temperatures and high adhesion strengths are highly desired in aerospace, aviation, microelectronic and other high-tech areas. The currently used high-temperature resistant polymeric adhesives, such as polyamic acid (PAA), are usually made from the high contents of solvents in the composition, which might cause adhesion failure due to the undesirable voids caused by the evaporation of the solvents. In the current work, electrospun preimidized polyimide (PI) nano-fibrous membranes (NFMs) were proposed to be used as solvent-free or solvent-less adhesives for stainless steel adhesion. In order to enhance the adhesion reliability of the PI NFMs, thermally crosslinkable phenylethynyl end-cappers were incorporated into the PIs derived from 3,3’,4,4’-oxydiphthalic anhydride (ODPA) and 3,3-bis[4-(4-aminophenoxy)phenyl]phthalide (BAPPT). The derived phenylethynyl-terminated PETI-10K and PETI-20K with the controlled molecular weights of 10,000 g mol−1 and 20,000 g mol−1, respectively, showed good solubility in polar aprotic solvents, such as N-methyl-2-pyrrolidinone (NMP) and N,N-dimethylacetamide (DMAc). The PI NFMs were successfully fabricated by electrospinning with the PETI/DMAc solutions. The ultrafine PETI NFMs showed the average fiber diameters (dav) of 627 nm for PETI-10K 695 nm for PETI-20K, respectively. The PETI NFMs showed good thermal resistance, which is reflected in the glass transition temperatures (Tgs) above 270 °C. The PETI NFMs exhibited excellent thermoplasticity at elevated temperatures. The stainless steel adherends were successfully adhered using the PETI NFMs as the adhesives. The PI NFMs provided good adhesion to the stainless steels with the single lap shear strengths (LSS) higher than 20.0 MPa either at room temperature (25 °C) or at an elevated temperature (200 °C).

Methylethynyl-Terminated Polyimide Nanofibrous Membranes: High-Temperature-Resistant Adhesives with Low-Temperature Processability

“…The results were consistent with the rheological test results shown in Figure 8 . Compared with our previous work [ 26 ], the melting temperatures of METI NFMs were greatly decreased, which was mainly attributed to the lower crosslinking temperature of flexible propynyl groups compared with the rigid phenylethynyl end-cappers. The processing temperatures for the final bonding procedures were then set as follows: METI-5K: 290 °C; METI-10K: 310 °C; and METI-20K: 330 °C.…”

“…Furthermore, the bonding temperature for the METI-10K samples was only 310 °C. Compared with our previous work [ 26 ], the processing temperature of the current METI-10K decreases nearly 70 °C lower when compared to the analogous PETI-10K sample (380 °C). The bonding strength was comparable to those of the PETI ones.…”

“…The rheological behaviors of the METI NFMs were further compared with the phenylethynyl-terminated (PETI) analogous materials [ 26 ] and the results are shown in Figure 8 b. On one hand, the T min values of the METIs were apparently lower than those of the PETI counterparts with the same designed molecular weights.…”

“…To further explore the effects of processing temperature on the melting flowability of the METI NFMs, hot-pressing experiments were performed. The detailed evaluation procedure was reported in our previous work [ 26 ]. The double-layer fibrous membranes were placed into a heat-sealing machine, and the pressure was set to 0.5 MPa.…”

“…Thus, it could be anticipated that electrospun PI NFMs might find wide applications as high-temperature-resistant adhesives. In our previous work, a series of phenylethynyl-terminated PI (PETI) NFMs were prepared and characterized as adhesives [ 26 ]. Although the developed PI NFM adhesives exhibited good adhesion properties, adhesion temperatures over 370 °C limit their wide application in practice.…”