Alkaline-developable positive-type photosensitive polyimide with high mechanical strength and high resolution based on chain extendable poly(amic acid), thermally degradable cross-linker and photoacid generator

A positive-type photosensitive polyimide ( PSPI ) based on a chain extendable poly(amic acid) ( PAA ), a thermally degradable cross-linker 1,3,5-tris[(2-vinyloxy)ethoxy]benzene ( TVEB ), and a photoacid generator (PAG) (5-propylsulfonyloxyimino-5H-thiophene-2-ylidene)-(2-methylphenyl)acetonitrile ( PTMA ) has been developed. The chain extendable PAA was prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride ( BPDA ) and 4,4′-oxydianiline ( ODA ) and end-capped with di-tert-butyl dicarbonate ( DIBOC ) in N-methyl-2-pyrrolidone (NMP), which has a controlled molecular weight for developing in a 2.38 wt% tetramethyl ammonium hydroxide aqueous solution ( TMAH _aq ) and undergoes a chain extending reaction during curing stage. The photosensitive resist solution was formulated with the polymerization solution (30 wt% in NMP), TVEB (15 wt% for the polymer), and PAG (4.5 wt% for the polymer). The PSPI showed a sensitivity of 47 mJ cm⁻² and a contrast of 5.8 when exposed to 365-nm light, followed by postexposure baking at 90 °C for 10 min and development with the 2.38 wt% TMAH _aq at room temperature. A fine-positive image with 3-μm line-and-space patterns was obtained on a 3-μm thick film exposed to UV light at 365 nm in the contact-printed mode. After thermal curing at 350 °C for 1 hr, the resulting PSPI features excellent mechanical strength and elongation.     

Synthesis of polyimides with lower H2O-absorption and higher thermal properties by incorporation of intramolecular H-bonding

To introduce intramolecular hydrogen bonds in the polyimides (PIs), 5(6)-amino-2-(5-aminopyridin-2-yl)-benzimidazole (PyPABZ) were designed and synthesized. The intramolecular interaction was indicated by Fourier transform infrared when different copolyimides were prepared with 4,4′-oxydianiline and PyPABZ. These modified poly(benzimidazole imide)s eliminate the problem of high water absorption for benzimidazole (BI)-containing PIs in the materials applications. Moreover, the high packing coefficient and rigidity of these copolyimides caused by the strong intramolecular interaction from the H-bonding and the resulting PIs exhibited outstanding thermal properties such as high glass-transition temperature (T_g) and low coefficient of thermal expansion.     

Synthesis and properties of novel colorless and thermostable polyimides containing cross-linkable bulky tetrafluorostyrol pendant group and organosoluble triphenylmethane backbone structure

A colorless and high temperature resistant fluorinated polyimide (PI) film based on a novel cross-linkable diamine monomer 4,4′-diamino-4″-(2,3,5,6-tetrafluoro-4-ethylenephenoxy) triphenylmethane (DFPTM) was successfully prepared. The special triphenylmethane backbone and the bulky tetrafluorostyrol pendant group in DFPTM gave the PI material eximious solubility in organic solvents, high tensile properties, good thermal stability, and excellent optical transparency (cutoff wavelength for 331 nm and light transmittance above 94% at 450 nm). Furthermore, the styryl could initiate self-crosslinking reaction to form a dense three-dimensional network structure by thermal curing. Hence, the cross-linked PI film showed superior combination property (including excellent resistance to solvents). It's T_g increased by 57.1–306.0°C, the 5% thermal weight-loss temperature improved by 10.2% to 500.0°C, and the tensile strength enhanced by 17.5% to 39.7 MPa. Moreover, after thermal curing, the cross-linked PI attained a lower elongation at break from 8.1 to 3.9%, and the thermal expansion coefficient reduced from 82.2 to 68.8 ppm/°C without obvious loss of optical transparency (cutoff wavelength for 347 nm and light transmittance remained 93.0% at 450 nm).     

Alkaline-developable and negative-type photosensitive polyimide with high sensitivity and excellent mechanical properties using photo-base generator

An alkaline developable and negative-type PSPI with a high sensitivity and excellent mechanical properties based on a poly(amic acid) (PAA) and a photo-base generator has been developed. The PAA was prepared by the polycondensation of p-phenylenediamine (PDA) with an equimolar of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6-FDA) and converted thermally to the corresponding polyimide, PI(PDA-BPDA/6-FDA). PI(PDA-BPDA/6-FDA) showed the high thermal and mechanical properties and the dimensional stability such as the thermal decomposition temperature of 530°C, glass transition temperature of 369°C, linear coefficient of thermal expansion of 28 ppm/K, ultimate tensile strength of 148 MPa, elongation at break of 25% and dielectric constant of 2.8. The PSPI was formulated directly from PAA(PDA-BPDA/6-FDA) with a photo-base generator (PBG), (E)-3-(2-hydroxy-4-methoxyphenyl)-1-(piperidin-1-yl)prop-2-en-1-one (HMPP) (10 wt% to PAA) and the optimized parameters for photolithographic process were investigated including the PBG content, post-exposure bake (PEB) temperature, and PEB time. The PSPI based on PAA(PDA-BPDA/6-FDA) and HMPP (10 wt% to PAA) showed a sensitivity of 114 mJ/cm² and contrast of 1.29 when exposed to 365-nm light (i-line), post-exposure baked at 160°C for 5 min, and developed with an aqueous solution of 2.38 wt% tetramethylammonium hydroxide and iso-propanol. A clear negative 8-μm features pattern was obtained by contact-printing and converted into the PI pattern upon heating at 250°C, confirming by scanning electron microscopy and infrared spectroscopy.     

Dynamic fluid–structure interaction of an elastic capsule in a viscous shear flow at moderate Reynolds number

The unsteady behavior of a 2-D circular elastic capsule was investigated in three viscous shear flows. An immersed boundary method (IBM) has been used to solve the dynamic fluid-structure interaction of the capsule. Computations were carried out in finite parameter ranges where the Reynolds number is Re=1-40 and the capillary number is Ca=0.0005-0.05, which is the ratio of the external viscous shear stress to the resistant elastic tensions of the membrane. For the simple shear flow, the effect of inertia on the transient behavior of the capsule was studied. For the pulsatile shear flow, two values of the peak fluid strain, T_f=1 and 5, were considered for the quasi-steady capsule mechanics. The capsule shows a cyclic structural response that includes subharmonics as the Reynolds number is elevated to 10 and 40. The capsule dynamic response includes a phase lag, which is a function of the capillary number, the Reynolds number, and the peak fluid strain. Finally, the capsule flowing in the Couette flow shows lateral migration due to the transient lift force, which is higher for lower Ca and higher Re. When capsules with diverse elasticity are dispersed along the velocity gradient, the capsule with a hard membrane experienced greater lift than the one with a soft membrane.     

On-line measurement of the RMS radius of gyration and molecular weight of polyimide precursor fractions eluting from a size-exclusion chromatograph

The recent introduction of multiangle light-scattering detectors for size-exclusion chromatography has made possible the measurement of the root mean square radius of gyration (R_g) and molecular weight (M) of polymer fractions eluting from a size-exclusion chromatography column. The characterization of the dimensions of a polymer may be accomplished with only a few milligrams. The dimensions of a polyimide precursor prepared by the condensation of the meta-diethyl ester of pyromellitic dianhydride with para-phenylene diamine have been measured with this technique. The dependence of R_g on M across the distribution is compared with that predicted for a freely rotating chain, and with other similar polymers measured with hydrodynamic techniques.     

Stress in polyimide coatings

The effect of film thickness on in-plane molecular orientation and stress in polyimide films prepared from pyromellitic dianhydride with 4,4′-oxydianline was investigated using a prism coupling technique to measure the refractive index. Film thickness was controlled by varying both solution concentration and spinning conditions. Birefringence, the difference between the in-plane and out-of-plane refractive indices, was used to characterize the in-plane molecular orientation. The observed birefringence is a combination of the birefringence resulting from molecular orientation and the birefringence induced by the residual stress present in the films. The birefringence decreases with increasing film thickness over the range of thicknesses studied (3–20 μm) indicating that the molecular orientation decreases with increasing film thickness. The in-plane coefficient of linear thermal expansion (CTE), controlled by the level of orientation in the film, increases from 18 to 32 × 10^?6/°C over the same thickness range. The birefringence of free-standing films was lower than that of adhered films due to the release of residual stress in the film once the film is removed from the substrate. The residual film stress arises primarily from the mismatch in CTEs between the polyimide film and the substrate to which the film is adhered. Thus, since the film anisotropy decreases with increasing thickness, the film stress increases with increasing thickness. Residual stress calculated by integrating the product of the film modulus and the CTE mismatch assuming temperature-dependent properties is comparable to experimentally measured film stress. Ignoring the temperature dependence of the film properties leads to an overestimation of stress. Moisture uptake was used to study the stress dependence of the optical properties. Moisture uptake increases both the in-plane and out-of-plane refractive indices by equal amounts in free-standing films due to an isotropic increase in the polarizability. In adhered films, an increase in moisture uptake leads to a decrease in the birefringence due to a swelling-induced decrease in the residual film stress. © 1994 John Wiley & Sons, Inc.     

Study on the enhancement of the interface performance between CF and polyimide by triethylamine grafting nano-TiO2 particles

The TiO₂ is grafted by triethylamine and is introduced on the surface of the carbon fiber, which improves the wettability and activity of the carbon fiber surface and at the same time strengthens the carbon fiber/polyimide composite. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed the triethylamine grafting successfully on TiO₂. Scanning electron microscope (SEM) results show that when the trimethylamine concentration is 1.0%, a uniform coating is formed on the surface of the carbon fiber. The interlaminar shear strength (ILSS) and interfacial shear strength (IFSS) were increased by 32% and 69%, respectively.     

Heat-resistant polyimides with low CTE and water absorption through hydrogen bonding interactions

A novel diamine, 1H,1′H-(2,2′-bibenzimidazole)-5,5′-diamine (DPABZ), containing bisbenzimidazole unit was successfully synthesized, and used to prepare a series of copolyimides BPDA:(ODAm/DPABZn) by polycondensation with 4,4-diaminodiphenyl ether (ODA) and 4,4-biphthalic anhydride (BPDA). For comparison, a series of copolyimides BPDA:(ODAm/PABZn) based on another benzimidazole diamine 5-amino-2-(4-aminobenzene)-benzimidazole (PABZ) was also prepared. As a result, with the increase of PABZ or DPABZ content, the heat resistance (T_g and T_d) and mechanical properties (σ and E) of the resulting polyimide (PI) films increased, while the coefficient of thermal expansion (CTE) decreased. Overall, the DPABZ-based PIs showed higher T_g values and much lower CTE values than PABZ. As the content of PABZ increased, the water absorption of PABZ-based PIs increased obviously, but no significant change in DPABZ-based PIs. The intramolecular hydrogen bonding in DPABZ-based PIs caused by the diamine DPABZ was believed to be the reason for the aforementioned differences. The BPDA: DPABZ film with low-water adsorption of 2.1%, high-T_g value of 436°C and low-CTE value of 5.4 ppm/°C could be a promising new generation of flexible display substrates.