Curing of polyimides

The imidization of films of the polyamic acid obtained from reaction of pyromellitic dianhydride (PMDA) with oxydianiline (ODA) was studied by dynamic mechanical thermal analysis (with the Polymer Labs DMTA), Two process regions are observed. The first process is characterized by decomplexation of the NMP/polyamic acid complex, plasticization, and imidization. The heating rate determines the character of the first process. At high temperatures (above 300°C) the second process becomes apparent. It is connected to the increase in chain mobility at these temperatures which allow molecular packing processes (densification, ordering) to occur. The ordering leads to an increase of the storage modulus. The particular transition temperature increases with annealing time.     

Characterization of alicyclic polyimides based on cyclobutane ring dianhydride and aromatic diamine

Films of alicyclic polyamic acid and polyimide containing cyclobutane ring in dianhydride moiety and aromatic ring with p- or m-linkages in diamine moiety were characterized by infrared (IR) spectroscopy, dynamic viscoelasticity, differential-scanning calorimetry (DSC), density, and wide-angle X-ray diffraction analyses. Partially and fully imidized polyimides were obtained by varying the imidization temperature, e.g., 150°C, 250°C, and 350°C. It was found from the results of IR spectra, dynamic viscoelasticity, and DSC measurements that the imidization of alicyclic polyamic acid was reduced at about 150°C and needed a higher imidization temperature than aromatic polyamic acid. Alicyclic polyimide with m-linkage in the diamine moiety had a higher density and a much more ordered structure than with p-linkage. © 1994 John Wiley & Sons, Inc.     

Effect of film formation process on residual stress of poly(p-phenylene biphenyltetracarboximide) in thin films

Soluble poly(p-phenylene biphenyltetracarboxamine acid) (BPDA-PDA PAA) precursor, which was synthesized from biphenyltetracarboxylic dianhydride and p-phenylene diamine in N-methyl-2-pyrrolidone (NMP), was spin-cast on silicon substrates, followed by softbake at various conditions over 80–185°C. Softbaked films were converted in nitrogen atmosphere to be the polyimide films of ca. 10 μm thickness through various imidizations over 120–400°C. Residual stress, which is generated at the polymer/substrate interface by volume shrinkage, polymer chain ordering, thermal history, and differences between properties of the polymer film and the substrate, was measured in situ during softbake and subsequent imidization processes. Polymer films imidized were further characterized in the aspect of polymer chain orientation by prism coupling and X-ray diffraction. Residual stress in the polyimide film was very sensitive to all the film formation process parameters, such as softbake temperature and time, imidization temperature, imidization step, heating rate, and film thickness, but insensitive to the cooling process. Softbaked precursor films revealed 9–42 MPa at room temperature, depending on the softbake temperature and time. That is, residual stress in the precursor film was affected by the amount of residual solvent and by partial imidization possibly occurring during softbake above the onset of imidization temperature, ca. 130°C. A lower amount of residual solvent caused higher stress in the precursor film, whereas a higher degree of imidization led to lower stress. Partially imidized precursor films were converted to polyimide films revealing relatively high stresses. After imidization, polyimide films exhibited a wide range of residual stress, 4–43 MPa at room temperature, depending on the histories of softbake and imidization. Relatively high stresses were observed in the polyimide films which were prepared from softbaked films partially imidized and by rapid imidization process with a high heating rate. The residual stress in films is an in-plane characteristic so that it is sensitive to the degree of in-plane chain orientation in addition to the thermal history term. Low stress films exhibited higher degree of in-plane chain orientation. Thus, residual stress in the film would be controlled by the alignment of polyimide chains via the film formation process with varying process parameters. Conclusively, in order to minimize residual stress and to maximize in-plane chain orientation, precursor films should be softbaked for 30 min-2 h below the onset imidization temperature, ca. 130°C, and subsequently imidized over the range of 300–400°C for 1–4 h by a two-step or multi-step process with a heating rate of ? 5.0 K min⁻¹, including a step to cover the boiling point, 202°C, of NMP. In addition, the final thickness of the imidized films should be <20 μm. © 1997 Elsevier Science Ltd.     

Physicochemical characterization of alloy of polyimide with varying degree of crosslinking through diisocyanates

Polyimide alloys are prepared by blending the crosslinked and uncrosslinked polyamic acid components and followed by thermal imidization. The blend components can be synthesized by the reaction of polyamic acid with the varying concentration of crosslinker [here methylene bis (4-phenyl isocyanate or MDI)] from 1.54 × 10^?2 mol/L (i.e. hypothetically calculated critical crosslinker concentration or CCC) to 1.54 × 10^?6 mol/L. This communication discusses the synthesis and characterization of polyimide (PI) blends and alloys prepared by varying degrees of crosslinking introduced via isocyanate-amic acid reaction. The polyimides were prepared by thermally imidizing the polyamic acid blends at different curing temperatures from 50°C to 350°C. The degree of imidization and residual solvent content for blends having varying mole fractions of crosslinked (or branched) and uncrosslinked components and two extreme conditions and at specified temperature-time profiles have been studied. The resultant PI-MDI blends have exhibited synergism on mechanical properties. The improvement in mechanical properties, however, was significantly higher at the lower imidization temperature (i.e. 50°C to 150°C). The feasibility of preparing polyimide alloys with synergistic combinations of crosslinked and uncrosslinked polyimide components was inferred.     

Preparation and properties of polyimide solvent‐resistant nanofiltration membrane obtained by a two‐step method

This study investigated the preparation of polyimide solvent‐resistant nanofiltration membranes by a two‐step method (casting the membrane first and then crosslinking by the thermal imidization method). The influences of polymer concentration, thickness of membranes, temperature of the imidization, phase inversion time and thermal imidization procedure were studied. The membranes with the highest rejection rate of Fast Green FCF (molecular weight 808.86 g mol^?1) were prepared in the following conditions: polymer concentration 13 wt%, phase inversion time 1 h, membrane thickness 150 µm and thermal imidization procedure 200 °C for 2.5 h, 250 °C for 2 h and 300 °C for 2 h in a vacuum environment; the heating rate was 5 °C min^?1. The membrane was stable in most of the solvents tested and the fluxes of some common solvents were equal to or higher than a number of commercial solvent‐resistant nanofiltration membranes. A much higher rejection of dyes in water than in methanol was observed in the filtration experiments and a new way to explain it was developed. Copyright © 2011 Society of Chemical Industry     

Fabrication and electrochemical characterization of polyimide-derived carbon nanofibers for self-standing supercapacitor electrode materials

We report the electrochemical performance of aromatic polyimide (PI)-based carbon nanofibers (CNFs), which were fabricated by electrospinning, imidization, and carbonization process of poly(amic acid) (PAA) as an aromatic PI precursor. For the purpose, PAA solution was electrospun into nanofibers, which were then converted into CNFs via one-step (PAA-CNFs) or two-step heat treatment (PI-CNFs) of imidization and carbonization. The FTIR and Raman spectra demonstrated a successful structural evolution from PAA nanofibers to PI nanofibers to CNFs at the molecular level. The SEM images revealed that the average diameter of the nanofibers decreased noticeably via imidization and carbonization, while it decreased slightly with increasing the carbonization temperature from 800 °C to 1000 °C. In case of PI-CNF carbonized at 1000 °C, a porous structure was developed on the surface of nanofibers. The electrical conductivity of PI-CNFs, which was even higher than that of PAA-CNFs, increased significantly from 0.41 to 2.50 S/cm with increasing the carbonization temperature. From cyclic voltammetry and galvanostatic charge/discharge tests, PI-CNF carbonized at 1000 °C was evaluated to have a maximum electrochemical performance of specific capacitance of ~126.3 F/g, energy density of ~12.2 Wh/kg, and power density of ~160 W/kg, in addition to an excellent operational stability. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47846.     

The retarding effects and structural evolution of a bio-based high-performance polyimide during thermal imidization

The thermal imidization evolution of a bio-based high-performance polyimide, namely adenine-containing polyimide (API), was investigated by thermogravimetric analysis (TGA), in situ Fourier transform infrared spectroscopy (in situ FTIR), and wide-angle X-ray diffraction (WAXD), in contrast to an adenine-free 4,4′-oxydiphthalic anhydride (ODPA)/4,4′-oxydianiline (ODA) PI. The influence derived from adenine was focused. At precursor stage of API (polyamic acid, PAA), the H-bonding interaction of PAA–PAA type as well as the especial interaction between the secondary amine of adenine and solvent (dimethylacetamide, DMAc) was discovered. Structural evolution of API was traced by in situ FTIR and multistage WAXD from PAA stage to PI stage. Compared with OPI, the retarding effects were found in the process of thermal imidization of API, partly due to the formation of H-bonding derived from the extra secondary amine of adenine moieties, which complicated the H-bonding form in API. Finally, a hypothesis of evolution of thermal imidization process about API molecule was proposed in contrast with adenine-free ODPA ODA PI. Compared with the consistency of both API and ODPA ODA PI in PAA stage, API possessed a more delicate thermal imidization process. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46953.     

Processable conductive blends of polyaniline/polyimide

By using camphorsulfonic acid (CSA) to protonate polyaniline (PANI), the counterion enabled the PANI–CSA complex processable as a solution phase. So camphorsulfonic acid (CSA)-doped polyaniline/polyimide (PANI/PI) blend films were prepared by the solvent casting method using N-methylpyrrolidinone (NMP) as a cosolvent followed by thermal imidization. The conductivity of the PANI–CSA/PAA (50 wt % PANI content) is greater than that of the pure PANI sample at room temperature. As the thermal imidization proceeded, molecular order of polymer chain structure was improved in the resulting PANI–CSA/PI film due to the annealing effect of PANI chain, and this PANI–CSA/PI film showed higher conductivity than PANI–CSA and PANI–CSA/PAA film. PANI–CSA/PI blend films had a good thermal stability of conductivity at high temperature. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1863–1870, 1998     

Kinetics of imidization of poly(amic acid) in miscible and immiscible polymer blends

Blends of commercial poly(amic acid), LARC-PAA polymer were prepared with thermoplastic polyimide P84. The blends were shown to be miscible over a certain composition range. Chemical imidization of LARC-PAA in the blends leads to phase separation. Thermal imidization of LARC PAA in the blends was carried out in the solid state. Miscible compositions of the blend remained miscible after imidization and showed a single glass transition for each blend. Immiscible compositions after imidization remained immiscible and showed two glass transition temperatures in the DSC scans. The kinetics of thermal imidization of PAA to polyimide (PI) in the solid state were studied using FTIR. In the immiscible blend the kinetics remained unaffected while in the miscible state the rate of imidization became faster than that of the pure PAA.     

Studies of polyimide/copper interface and its improvement by a two-component primer

The inhibition of the imidization of polyamic acid, a precursor of polyimide, in the presence of Cu, was confirmed by the incorporation of Cu²⁺ ions in polyamic acid films. It was found that the imidization reaction was incomplete below 300°C but decomposition took place when heating above 300°C. Pretreating the Cu surface with a two-component primer solution containing polybenzimidazole and 2-mercaptobenzimidazole can make the imidization proceed without retardation and avoid the decomposition above 300°C. By choosing appropriate surface treatment systems, one can achieve the improvement of adhesion between PI and Cu substrate.     

Effect of isothermal aging on post-imidization and glass transition temperature of LaRC-IA polyimide resin

The isothermal aging of partilly imidized NASA Langley Research Center, LaRC-IA polymide resin containing 70 wt% N-methyl pyrrolidone, NMP was performed in a vacuum oven at 65, 95, 135, 165 and 200°C for 0.5, 1.0, 1.5 and 2 h. The weight loss and chemical changes that occurred during aging was determined gravimetrically and by FTIR spectroscopy, respectively. The imide absorption peak at 1778 cm⁻¹, increased in intensity as the aging temperature was increased from 65 to 200°C. The expulsion of NMP (70 wt%) was completed after ∼2.5 h of aging at 135°C. Additional weight loss ≤4 wt%, after the expulsion of NMP, was attributed to post-imidization. The imide carbonyl peak absorption at 1721 and 1778 cm⁻¹, respectively, were broadened after aging at 200°C. The broadening of the imide absorption peaks was marked by the disappearance of the amide peak near 1660 cm⁻¹ and is attributable to post-imidization of the partially imidized polyamic-acid. Dissolution of the polyimide aged at T ≥ 165°C in dimethyl formamide, DMF, was unsuccessful even after long times of stirring (∼12 h) at elevated temperature (T ∼ 85°C). The DSC thermogram for the LaRC-IA resin showed a series of broad endothermic peaks between 150–180°C and narrow endothermic peaks at 210°C. The low temperature endotherm disappeared after aging at T ≥ 135°C for t ≥ 1 h. The high temperature endotherm decreased with increased aging temperature and time. The glass transition temperature of the polyimide increased with increased aging temperature and time.     

Preparation and characterization of high‐temperature resistance polyimide foams

A new high‐temperature resistance polyimide foam was synthesized from 2,3,3′,4′‐biphenyltetracarboxylic dianhydride (α‐BPDA) and p‐phenylenediamine (p‐PDA). The structures and foaming process of polyimide precursor powders were characterized by wide‐angle X‐ray diffractometer (WXRD) and the self‐made visualization device, respectively. The imidization degree, thermal mechanical properties and thermal stability of the polyimide foams with different post‐treatment temperatures were also measured by fourier transform infrared spectrometer spectrum (FTIR), dynamic thermal mechanical anaylsis (DMTA), and thermogravimetric analysis (TGA). Results showed that the inflation onset temperatures of polyimide precursor powders ranged from 122 to 135°C with varying the heating rate. And the increase in the imidization degree, glass transition temperatures (T_g) and temperatures for 5 wt% mass loss of high‐temperature resistance polyimide foams can be achieved with increasing post‐treatment temperature. It was quite surprising to find that T_g of high‐temperature resistance polyimide foam post‐treated at 420°C was up to above 450°C, and the char yield at 800°C was more than 60%. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

静电纺聚酰亚胺非织造布的制备与表征

以N,N-二甲基乙酰胺(DMAc)为溶剂,3,3',4,4'-二苯醚四甲酸二酐(ODPA)和4,4'-二氨基二苯醚(ODA)为单体,利用高压静电纺丝技术,制备了聚酰胺酸(PAA)和聚酰亚胺(PI)非织造布,并采用扫描电镜(SEM)对PAA及PI非织造布的表面形态进行表征,研究了PI非织造布的力学性能。结果表明:经300℃热亚胺化处理得到的PI非织造布,纤维平均直径减小到500nm以下,纤维的带状形貌与PAA明显不同,并且出现了收缩、弯曲等现象。静电纺丝法制得的PI非织造布的力学性能仍然比较优越。     

Porous Polyimide Membranes Prepared by Wet Phase Inversion for Use in Low Dielectric Applications

A wet phase inversion process of polyamic acid (PAA) allowed fabrication of a porous membrane of polyimide (PI) with the combination of a low dielectric constant (1.7) and reasonable mechanical properties (Tensile strain: 8.04%, toughness: 3.4 MJ/m³, tensile stress: 39.17 MPa, and young modulus: 1.13 GPa), with further thermal imidization process of PAA. PAA was simply synthesized from purified pyromellitic dianhydride (PMDA) and 4,4-oxydianiline (ODA) in two different reaction solvents such as γ-butyrolactone (GBL) and N-methyl-2-pyrrolidinone (NMP), which produce M_w/PDI of 630,000/1.45 and 280,000/2.0, respectively. The porous PAA membrane was fabricated by the wet phase inversion process based on a solvent/non-solvent system via tailored composition between GBL and NMP. The porosity of PI, indicative of a low electric constant, decreased with increasing concentration of GBL, which was caused by sponge-like formation. However, due to interplay between the low electric constant (structural formation) and the mechanical properties, GBL was employed for further exploration, using toluene and acetone vs. DI-water as a coagulation media. Non-solvents influenced determination of the PAA membrane size and porosity. With this approach, insight into the interplay between dielectric properties and mechanical properties will inform a wide range of potential low-k material applications.     

Simulation of polyimide fibers with trilobal cross section produced by dry‐spinning technology

As one type of high performance fibers, polyimide fibers can be prepared from polyamic acid (PAA) solution by dry‐spinning technology. The transformation from the precursor of polyamic acid to polyimides via thermal cyclization reaction in the dry‐spinning process is a main distinguishing feature, which is very different from other fibers produced by dry‐spinning such as cellulose acetate fiber and polyurethane fiber. In this report, the dry‐spinning formation of polyimide fibers with trilobal cross section from PAA solution in N,N‐dimethylacetamide is simulated via a one‐dimensional model based on a viscoelastic constitutive equation, combined with profile degree equation of cross section and imidization kinetics equation. The glass transition temperature, imidization degree and profile degree of the filament along the spinline are predicted by the model, as well as relative parameters such as solvent mass fraction and temperature. As a simulated result, solidification of polyimide fibers take place about 150 cm from the spinneret which is farther than for cellulose acetate fiber (70 cm). Moreover, the final profile degree of fiber is affected by many spinning parameters, such as spinning temperature, surface tension, spinning solution concentration, major, and minor axis length of the spinneret hole. POLYM. ENG. SCI., 55:2148–2155, 2015. © 2015 Society of Plastics Engineers     

Curing behavior,thermal, and mechanical properties of epoxy/polyamic acid based on 4,4′-biphtalic dianhydride and 3,3′-dihydroxybenzidine

Different synthesis routes were studied to obtain 4,4′-biphtalic dianhydride/3,3′-dihydroxybenzidine polyimide precursors (polyamic acids [PAAs]) with different inherent viscosities (IVs) and imidization degrees. The synthesized PAAs were introduced as a thermoplastic modifier into an epoxy (EP) resin. Different loadings of PAA were used to investigate the curing behavior, heat resistance, and mechanical properties. The onset curing temperature of the EP by adding 20 wt% PAA diminished by around 15°C. Thermogravimetric analysis revealed that the initial and 10 wt% weight loss temperature for EP with 5 wt% PAA improved by 13°C and 7.7%, respectively. Further, the results of tensile and plane-strain fracture toughness tests indicated that as the amount of PAA increased, the strength and toughness of EP decreased. These improvements were due to the high heat resistance and mechanical properties of PI precursor introduced into the EP, which formed a three-dimensional structure together. The interlaminar shear strength (ILSS) of the system experienced a reduction; however, after adding 2 phr nanosilica to the system containing PAA with average IV and imidization degree, ILSS showed 4.4% increment.     

Effect of isothermal aging on the imidization of PMR-15

The imidization of polymerizable reactive mixtures, PMR-15 has been performed in a vacuum oven at isothermal aging temperatures ranging from 65 to 200°C for aging periods of 0.5 to 2.5 h. The weight loss of the resin and chemical changes that occurred as a result of aging were monitored gravimetrically and by FT-IR spectroscopy. Differential scanning calorimetry was used to determine the temperature at which imidization took place. Imidization was observed to commence at 65°C after long aging times, t ≥ 2.5 h and at ∼95°C at a shorter time, t ∼0.5 h. At higher aging temperatures of 135 to 165°C, extensive imidization occurred. This was shown by the dramatic increase in imide absorption bands at 1780 and 1380 cm⁻¹. Beyond 165°C, there were no significant changes in the imide absorption bands, suggesting that imidization was nearly complete. The activation energy for isothermal aging was determined from the slope of the log of the rate of weight loss vs 1/T curve to be ∼4.5 kJ/mol and is lower than the average activation energy for imidization ∼43 kJ/mol obtained from the plot of the log of the rate of increase of the imide carbonyl peak absorption at 1780 cm⁻¹ vs 1/T.     

CO2 capture efficiency in post‐combustion using MWNT‐PAA in a packed bed column

The adsorption capacity of polyaspartamide (PAA) and multi‐wall carbon nanotubes with polyaspartamide (MWNT‐PAA) was investigated through a packed bed column with the flowing of flue gas composed of 15 % CO₂, 5 % O₂ and the balance N₂. The adsorption performed at 25 °C, 110 kPa and inlet gas flow rate of 60 mL/min resulted in high CO₂ adsorption capacity of 5.70 and 10.20 mmol‐CO₂/g for PAA and MWNT‐PAA, respectively. The adsorption kinetics was very high, so 7 min were enough for the effluent gas to reach the breakthrough after saturation. The consistency of adsorbents in recurring regeneration was successful through a continuous TSA system of 10 cycle adsorption‐desorption with temperatures of 25–100 °C. The evaluation of heat through differential scanning calorimetry (DSC) resulted in exothermic adsorption with heat release of 45.14 kJ/mol and 124.38 kJ/mol for PAA and MWNT‐PAA, respectively. The heat release was found favourable to promote the desorption as the temperature could rise after adsorption. This is an advantage for energy efficiency, as it depicts the potential of energy recovery. Thus, both adsorbent PAA and MWNT‐PAA were demonstrated to be promising for CO₂ adsorption capture in post‐combustion.     

聚酰亚胺纤维的制备及其结构研究

将均苯四甲酸二酐(PMDA)和4,4’-二氨基二苯醚(ODA)在N-甲基吡咯烷酮(NMP)中进行溶液聚合得到聚酰胺酸(PAA)溶液,并用该溶液进行干湿法纺丝得到PAA纤维,分别用化学酰亚胺化法和热酰亚胺化法得到聚酰亚胺(PI)纤维。研究了凝固浴组成和工艺条件对PAA形态结构和纤维性能的影响,以及不同酰亚胺化方法对PI纤维形态结构和性能的影响。结果表明:以甲醇为凝固浴制备的PAA初生纤维,无孔致密,最高拉伸强度和初始模量分别为2．21 cN/dtex和40．73 cN/dtex;采用化学酰亚胺化法制得的PI纤维中存在少许孔洞缺陷,其强度较低,热酰亚胺化法制得的PI纤维无孔致密,其强度和模量分别达到2．83 cN/ dtex和43．4 cN/dtex。     

聚酰胺酸酰亚胺化条件及其对聚酰亚胺力学性能的影响

采用均苯四酸二酐(PMDA)和4,4'-二氨基二苯醚(ODA)为单体,N-甲基吡咯烷酮(NMP)为溶剂,合成了黏度为1.87 dL/g的聚酰胺酸(PAA)。对聚酰胺酸分别进行了热酰亚胺化和化学酰亚胺化处理,研究了完全酰亚胺化所需的条件以及不同酰亚胺化方式对聚酰亚胺(PI)纤维条断裂强度的影响;对PAA初生纤维条进行拉伸和酰亚胺化处理的顺序不同,所得到的聚酰亚胺(PI)纤维条的力学性能不同,采用先酰亚胺化再拉伸的方法能得到力学性能更优异的聚酰亚胺(PI)纤维条。