Narrow Results

A external microbeam system using polyimide windows has been developed for PIXE (Particle Induced X-ray Emission) analysis in air. A beamline consists of a magnetic quadrupole-doublet lens, cylindrical objective slits and double-deflection scanning coils. Polyimide films with a thickness of 3.5 μm or 7.5 μm were used for the exit windows. Spot sizes in air were measured by beam-induced fluorescence of CsI scintillation plates. A minimum spot size obtained was about 10 μm for a 1.5 MeV proton beam. PIXE-mapping images of a Cu grid were successfully obtained in air.

We studied the dynamic secondary electron emission (SEE) characteristics of a polyimide sample in negative charging process under electron bombardment. The time evolution of secondary electron yield (SEY) has been measured with a pulsed electron gun. The dynamic SEY, as well as the surface potential have been analyzed using a capacitance model. The shift in surface potential caused by the negative charge accumulation on the sample reduces the landing energy of the primary electrons (PEs), which in turn alters the SEY. The charging process tends to be stable when the landing energy of PEs reaches the secondary crossover energy where the corresponding SEY is 1. The surface potential has an approximately negative exponential relationship with the irradiation time. The total accumulated charge at the stable state is found to be proportional to the product of the sample capacitance and the difference between initial incident energy and the secondary crossover energy. The time constant of the exponential function is proportional to the ratio of final accumulated charge to the incident current.

In this paper, the distribution of the charge trap levels in polyimide (PI) films was investigated by using photo-stimulated discharge (PSD) technique, which was to explore the effect of photoconductivity on photo-stimulated discharge current. The PSD spectra show that the photo-stimulated discharge current of the PI films is mainly distributed in the range of 649–320 nm, but there are two significant current peaks when the wavelength is 300 nm and 244 nm, respectively. By studying the reason of generating two current peaks, it is concluded that the two current peaks are not generated by trapped charge de-trapping but generated by photoconductivity of charge-transfer complex in the PI films. According to the research, it is concluded that the trap levels in PI films are mainly distributed in the range of 1.92 eV–3.88 eV.

Nominally undoped ZnO thin films were grown on polyimide (PI) substrates at various temperatures by using radio-frequency magnetron sputtering. Atomic force microscopy images showed that the root mean squares of the average surface roughnesses for the ZnO thin films grown on the PI substrates at 27°C, 100°C, 200°C, and 300°C were 4.08, 4.50, 4.18, and 3.89 nm, respectively. X-ray diffraction patterns showed that the crystallinity of the ZnO films had a preferential (0001) direction and that the full width at half-maxima for the (0002) ZnO diffraction peak for the ZnO thin films grown on the PI substrates at 27°C, 100°C, 200°C, and 300°C were 0.22, 0.22, 0.22, and 0.23, respectively. The average optical transmittances in the visible ranges between 550 and 750 nm for the ZnO/PI heterostructures grown at 27°C, 100°C, 200°C, and 300°C were 87%, 83%, 87%, and 78%, respectively.

Photorefractive composites sensitive to 1064 nm on a base of an aromatic polyimide containing J-aggregates of a thiacarbocyanine dye are presented. The molecules of the dye form the nanocrystalline J-aggregates that are responsible for photoelectric sensitivity at 1064 nm and nonlinear third-order optical properties. The net gain coefficient 266 cm^-1 and the response time 0.09 s were achieved at an external electric field of about 15 V/μm.

Intercalation nonlinear-optical (NLO) polyimide and polyimide/silica hybrid material were synthesized by the sol–gel technique. The results showed that the interpenetrating hybrid polymer networks were formed and showed excellent thermal stability. The electro-optic property suggests that this hybrid material has potential for applications in high performance optical devices.

A wide range of new materials for many applications can be formed by controlling the composition and order of constituents at the molecular level. For systems thus engineered, ensuring chemical, thermal and mechanical robustness is a major challenge. Consequently, polyimides and other imide-containing materials are attractive as matrices for functional materials. We investigate the construction of functional nanostructures in organic/polymeric matrices with clearly demonstrated chemical, thermal and mechanical stability. Surface functionalization, layer-by-layer (LBL) assembly in various media (including supercritical), incorporation of functional moieties, molecular orientation, and interfacial reactions are areas of interest. We demonstrate the robustness of ultrathin film structures containing polyimides and oligoimides formed by LBL molecular assembly with inter-layer covalent links. Covalent bonding between the layers provides strength, while utilizing a supercritical medium for the processing, results in the deployment of a solvent-free environment and avoids problems related to residual solvent, thereby improving film quality when compared to conventional films.

An aromatic diamine monomer, 4,4′-bis(3-amino-5-trifluoromethyl phenoxy)-biphenyl (TFBPDA), was synthesized via the nucleophilic displacement reaction of 3,5-dinitrobenzotrifluoride and 4,4′-biphenol. The monomer was reacted with various aromatic dianhydrides via the high temperature polycondensation procedure to provide a series of polyimides. The polyimides, PI-1 to PI-4, show good solubility not only in aprotic solvents, such as N-methyl-2-pyrrolidinone and N,N-dimethylacetamide, but also in many common solvents, such as m-cresol, chloroform and cyclopentanone. PI-4, derived from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride and TFBPDA, was even soluble in toluene. Moreover, PI films exhibit good thermal stability, outstanding transparency in the visible light region and acceptable mechanical and electrical properties. The excellent combined properties of the polyimides make them as a good candidate for fabricating microelectronics.

Polyimide-alumina hybrid films were synthesized via in situ polymerization and thermal imidation process from a solution of polyimide precursor and nanosized alumina in N,N-dimethylacetamide, and the microstructure of the hybrid films was characterized by transmission electron microscope (TEM) and infra-red (IR) spectrometry. The dependence of thermal stability, tensile properties, dielectric properties and degradation endurance under corona on the nano-Al₂O₃ content of polyimide-alumina hybrid films was studied. The results show that with the increase of Al₂O₃ content, the thermal stability and the dielectric properties of the hybrids increase, while the tensile properties decrease. Better corona resistance can be achieved if the PI film is filled with α-Al₂O₃ nanometric particle.

Five kinds of polyimides were synthesized using five dianhydrides (including 2,2-bis[4-(3,4-dicarboxyphenoxy)-phenyl] propane dianhydride (BPADA), 3,3′,4,4′-diphenylsulfone-tetracarboxylic dianhydride (DSDA), 4,4′-(hexafluoroisopropylidene)-diphthalic anhydride (6FDA), 1,4-bis(3,4-dicarboxyphenoxy) benzene dianhydride (HQDPA), and 4,4′-oxydiphthlic dianhydride (ODPA)) and 2,2-bis[4-(4-aminophenoxy)phenyl] hexafluoropropane (BDAF) via the two-step method that included polyaddition to form the polyamic acid and subsequent chemical imidization at ambient temperature. The structures of polyimides were characterized by FTIR and NMR. The thermal properties were characterized by DSC and TGA. All five kinds of polyimides showed good thermal properties and solubility in organic solvents such as DMF, DMAc, NMP and THF at room temperature. The pervaporation (PV) experiments of polyimides for toluene/n-heptane mixture were carried out, and all the polyimides showed selective permeation towards toluene. The fluxes of 6FDA-BDAF, DSDA-BDAF, HQDPA-BDAF and ODPA-BDAF at 80°C were 1.08, 0.96, 1.77 and 0.10 kg·μm/(m²·h), and the separation factors were 5.44, 1.64, 1.28 and 11.44, respectively. The increasing feed temperature resulted in higher flux and lower separation factor of the 6FDA-BDAF membrane.

There is a demand for composite films with excellent hydrophobic properties in inertial confinement fusion (ICF) physics experiments. In this paper, we prepared fluorinated polyimide hydrophobic films using spinning and plasma etching methods. The experimental results indicate that the water contact angle for the perfluorodecyltrichlorosilane (PFTS) treatment polyimide (PI) film is 112.0${}^{\circ }$, which is larger than the pure PI film $(69.0^{\circ })$. The rap oil contact angle is 84.2${}^{\circ }$, which is also much larger than the contact angle of PI film $(12.0^{\circ })$. Moreover, the surface roughness of the prepared films was measured by white light interferometry (WLI). The surface roughness (Ra) of pure PI is 9.79nm, but with the application of FSiO₂ particles, the Ra of the films increases to 65.05nm. After plasma treatment, the Ra of the PI/FSiO₂ composite film increases to 186.71nm because plasma treatment can scratch the film surface and increase its roughness. However, treating the PI/FSiO₂ composite film with the plasma and PFTS, the Ra is only 88.90nm. This decrease in Ra is due to the PFTS, which is able to reduce the surface roughness. The development of composite films, compared to pure PI films, could prove to be an extremely valuable material in ICF experiments.

Fluorinated graphene oxide (FGO) was successfully prepared in this paper, and the chemical structure of FGO was characterized. A series of polyimide (PI) composite films with different content of FGO was prepared by in situ polymerization. Because of the homogeneous dispersion of FGO and the strong interfacial interaction between FGO and the PI matrix, the resulting composite films that contained FGO exhibited effects of mechanical properties, thermal properties and dielectric properties that were even better than the pure PI. Furthermore, due to the hydrophobic properties of FGO, the water absorption of the composite films were reduced. The result showed when the 0.6wt.% FGO was filled into the PI, the tensile strength was increased about 66.3%, the decomposition temperature was increased from 540^∘C to 570^∘C, and the dielectric constant (D_k) decreased with the increasing content of FGO, and a D_k value of 2.28 was achieved when the content of FGO was 1.0wt.%.

Organic transistors are crucial components in future flexible electronics due to their excellent properties and ease of circuit integration. Previously, we demonstrated that flexible organic (polyimide) thermal transistors could be prepared using commercial graphite paper as the substrate. These materials exhibited excellent temperature sensitivity, linearity and recoverability due to the intrinsically high thermal conductivity of graphite. In this study, boron nitride (BN) sheets/polyimide hybrid dielectric layers were synthesized for the fabrication of flexible organic transistors using a commercial graphite paper. Under test, the results showed that the introduction of BN sheets was beneficial in improving the mobility and transistor characteristics of the device, as well as enhancing the overall stability. The as-fabricated transistors virtually exhibited no hysteresis at all BN contents.

Interlaminar delamination and brittle fracture of matrix have been a dilemma that fiber-reinforced composites have been faced with. Herein, the polyimide (PI) nanofiber-toughened glass fiber fiber-reinforced epoxy composites were prepared by electrospinning method and subsequent vacuum assistant resin transfer molding. The effect of spinning parameters including PI concentration, applied voltage, collector distance, jet speed and ambient humidity on the resultant fiber diameter and its distribution was systematically evaluated. The surface properties of obtained PI nanofibers were characterized by FT-IR, TG-DSC and water contact angle. The effect of PI concentration on tensile strength of PI membranes was also studied. The mode I (G${}_{\mathrm{\text{Ic}}})$ and mode II (G${}_{\mathrm{\text{IIc}}})$ interlaminar fracture toughness were measured. The results indicated that G_Ic and G_IIc increased by 127.69% and 85.33%. The improvement of interlaminar fracture toughness may be attributed to the bridging effect of PI nanofibers.

Polyimide (PI) is an important engineering material, but its poor thermal conductivity limits its application in the field of electronic packaging. In this work, we prepared boron nitride nanosheets (BNNSs) and boron nitride nanosheets/silver nanoparticles (BNNSs/AgNPs) hybrid fillers which combined successfully with the PI matrix by in-situ polymerization to produce high-thermal-conductivity PI composites. The results showed that when the filler addition was 20wt.%, the composites had the best overall performance, and the thermal conductivities of the BNNSs/PI and BNNSs/AgNPs/PI composite films were 0.863W$\cdot$m${}^{-1}\cdot {\mathrm{\text{K}}}^{-1}$ and 1.175W$\cdot$m${}^{-1}$$\cdot$K${}^{-1}$, respectively, which improved 506% and 726%, respectively, compared with pure PI. Furthermore, the volume resistivity of the composites is higher than 10${}^{17}\mathrm{\Omega }\cdot \mathrm{\text{cm}}$, indicating an excellent insulating property. At the same time, when used as the thermal interface material (TIM) for light-emitting diode (LED) chips, the composites have significantly improved heat dissipation effect and excellent mechanical properties at 75^∘C, which are expected to be widely used in the field of electronic packaging.

A three-phase composite film was produced by inserting multi-walled carbon nanotubes (MWCNTs) and BaTiO₃ nanoparticles into polyimide (PI). The combination of in-situ polymerization and water-based preparation involved in the experiment ensured fillers’ homogeneous dispersion in the matrix, which led to flexible shape of the composite films. The dielectric properties of composite films as a function of the frequency and the volume fraction of MWCNTs were studied. Such composite film displayed a high dielectric constant (314.07), low dielectric loss and excellent flexibility at 100Hz in the neighborhood of percolation threshold (9.02 vol%) owing to the special microcapacitor structure. The experimental results were highly consistent with the power law of percolation theory.