Effect of thermal history during drying and curing process on the chain orientation of rod-shaped polyimide

Chain orientation in polyimide (PI) film is influenced by the thermal history during drying and curing process. The amount of residual solvent and the degree of imidization, among other factors, play a major role in determining the chain orientation during the process. In the present study, poly(amic acid), the precursor of PI, coated on the glass substrate was imidized to PI through different drying and curing protocols. On the way of complete imidization, the residual solvent concentration and the degree of imidization were characterized using confocal Raman spectroscopy. The poly(amic acid) began to imidize quickly while retaining more solvent in the film as the initial drying temperature increased. The degree of in-plane chain orientation in fully imidized PI film made by different process protocols was compared using polarized Raman spectroscopy. The fully imidized PI showed the lowest degree of in-plane chain orientation when it was processed by the protocol with the highest drying temperature. The difference in the degree of in-plane chain orientation among different PI films significantly influenced the in-plane thermal expansion coefficient, while no significant change in crystallinity or glass transition temperature was observed.     

Multi-walled carbon nanotube/polyimide composite film fabricated through electrophoretic deposition

Multi-walled carbon nanotube (MWCNT)/polyimide composite films were fabricated through electrophoretic deposition (EPD) of MWCNT-polyamic acid colloidal suspension which was derived from carboxylated-MWCNTs and poly(pyromellitic dianhydride-co-4,4′-oxydianiline) (PMDA-ODA). Under electric field, both negatively charged MWCNTs and PMDA-ODA colloid particles migrate onto a positively charged anode simultaneously, and are converted to a coherent MWCNT/polyimide composite film in the ensuing imidization reaction. Uniform dispersion of MWCNTs in the composite film was observed using transmission electron microscopy. The thickness of the prepared composite film can be tuned by varying processing conditions such as deposition time and anode conductivity. The electrical conductivity of the composite film increased with increasing the concentration of MWCNTs in EPD suspension. The mechanical reinforcement of polyimide using MWCNTs was evaluated by tensile testing and nanoindentation testing.     

Molecular mechanism of temporal physico/chemical changes that take place during imidization of polyamic acid: Coupled real-time rheo-optical and IR dichroism measurements

Multiple overlapping physical and chemical changes often take place during casting/drying and imidization from PMDA-ODA polyamic acid precursors from cast solutions. To shed light into details of these complex phenomena, we designed a unique real time measurement system that combines true stress, true strain, in-plane birefringence and temperature with polarized ultra-rapid scan FT-IR spectrometry (URS-FT-IR). At the early stages of heating (21°C–130 °C), initially isotropic solution cast film was observed to develop stress and birefringence as the solvent decomplexed and evaporated without showing any imidization as it was held in uniaxially constrained state. At a temperature around 130 °C, the onset of imidization reaction was detected while the stress went through a maximum. Beyond this stage, the evaporation of bound solvent and chemical conversion was observed to take place simultaneously and this is accompanied by a steady increase in birefringence. As the majority of the bound solvent evaporated, the stress and birefringence values started leveling off at long times.     

Ordering-induced micro-bands in thin films of a main-chain liquid crystalline chloro-poly(aryl ether ketone)

Micro-banded textures developed from thin films of a main-chain thermotropic liquid crystalline chloro-poly(aryl ether ketone) in the melt were investigated using transmission electron microscopy (TEM), selective area electron diffraction, and atomic force microscopy techniques. The micro-banded textures were formed in the copolymer thin films after annealing at temperatures between 320 and 330 °C, where a highly ordered smectic crystalline phase is formed without mechanical shearing. The micro-banded textures displayed a sinusoidal-like periodicity with a spacing of 150 nm and an amplitude of 2 nm. The long axis of the banded texture was parallel to the b-axis of an orthorhombic unit cell. In the convex regions, the molecular chains exhibited a homeotropic alignment, i.e. the chain direction was parallel to the film normal. In the concave regions, the molecular chains possessed a tilted alignment. In addition to the effects of annealing temperatures and times, the thickness of the film played a vital role in the formation of the banded texture. A possible formation mechanism of this banded texture was also suggested and discussed. It was suggested that the micro-bands were formed during cooling.     

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.     

Influence of Film Casting Condition and Thermal Treatment on Order Structure of Polyurethane Block Copolymer

An attempt has been made to investigate the influence of film casting temperature and thermal ageing on order structure formation of segmented polyurethane (SPU) block copolymer. For this purpose films were casted at three different temperature of 60, 80, or 100°C and thermal aged at 100, 120, 130, or 150°C. The structure of films were investigated by Fourier-transform infrared (FT-IR), wide angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). Film casted at slow solvent evaporation has more regular structure than the film casted at rapid solvent evaporation. Crystaline structure decreases with increase in casting temperature due to chain entanglement, which makes crystallization process tougher. The effect of ageing temperature on crystalline structure formation remains unclear. Decrease of ordered structure was observed when the samples were heat treated at 100 or 120°C, however, slight increase of ordered structures were observed for the samples heat treated at 130 or 150°C. Therefore, the influence of thermal ageing has been explained in terms of order-disorder temperature of block copolymer.     

Thermal imidization of poly(amic acid) on Si(100) surface modified by plasma polymerization of glycidyl methacrylate

The plasma polymerization of glycidyl methacrylate (GMA) on pristine and Ar plasma-pretreated Si(100) surfaces was carried out. The epoxide functional groups of the plasma-polymerized GMA (pp-GMA) could be preserved, to a large extent, through the control of the glow discharge parameters, such as the radio-frequency (RF) power, carrier gas flow rate, system pressure, and monomer temperature. The pp-GMA film was used as an adhesion promotion layer for the Si substrate. The polyimide (PI)/pp-GMA-Si laminates, formed by thermal imidization of the poly(amic acid) (PAA) precursor poly(pyromellitic dianhydride-co-4,4′-oxydianiline) (PMDA-ODA) on the pp-GMA-deposited Si surface (the pp-GMA-Si surface), exhibited a 180°-peel adhesion strength as high as 9.0 N/cm. This value was much higher than the negligible adhesion strength for the PI/Si laminates obtained from thermal imidization of the PAA precursor on both the pristine and the argon plasma-pretreated Si(100) surfaces. The high adhesion strength of the PI/pp-GMA-Si laminates was attributed to the synergistic effect of coupling the curing of epoxide functional groups in the pp-GMA layer with the imidization process of the PAA, and the fact that the plasma-deposited GMA chains were covalently tethered onto the Si(100) surface. The chemical composition and structure of the deposited films were characterized, respectively, by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy, while the surface morphology of the deposited films was characterized by atomic force microscopy (AFM).     

Relationship between microstructure of lamellar graphene sheets and properties of polyimide/graphene nanocomposites film under different imidization stages

Herein, polyimide/graphene sheets (PI/GS) nanocomposite films with different GS distribution structures have been successfully obtained by controlling the imidization degrees, and the effect of the lamellar structure on the properties of PI film has been investigated. The results show that GS are gradually parallel to the surface of PI nanocomposite film with the increase of the imidization temperature, and 150 °C is the critical temperature, where the imidization rate is the fastest and the lamellar structure begins to form. Furthermore, with the drying temperature increasing, the corresponding thermal, electrical and mechanical properties of PI/GS nanocomposite films are significantly improved compared with that of pure PI films, which are ascribed to both the higher imidization degree and the lamellar GS structure. It is noteworthy that the formation process of the lamellar structure at different imidization stages can be directly observed by scanning electron microscope. Based on these results, a model has been proposed to explain the relationship between the lamellar structure and properties of PI composite film under different imidization stages, and the confinement of the thickness may be the most important factor for the formation of lamellar GS structure. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43575.     

Cyclodehydration of Poly (amic acid) to Polyimide

Abstract

It was observed that the cyclodehydration of PMDA-ODA or BTDA-ODA poly (amic acid) to polyimide in presence of ε-caprolactam takes place at a faster rate at lower temperature compared to the control film. It appears that the said lactam performed some catalytic function in the curing process and its influence was less in case of the PMDA based film than the BTDA based films. Also the rate of cyclodehydration of BTDA-ODA based poly (amic acid) to the corresponding polyimide is higher than the PMDA-ODA based poly (amic acid).     

PHASE SEPARATION OF POLY(AMIC ACID-CO-IMIDE) SOLUTION

Poly(amic acid-co-imide) (PA-I) is an intermediate for preparation of polyimide from polyamic acid (PAA). Phase separation does not appear for the solution until a certain imidization degree. The experimental results of a rotation viscometer and FT-IR analysis showed that the critical point of phase separation (CPPS) was related to the imidization methods (thermal or chemical imidization) and the backbone structures (flexible ODPA-ODA or semi-rigid PMDA-ODA). Phase separation time is shorted with the increase of the initial PAA concentration, acetic anhydride amount, or temperature. When phase separation occurs, the solution viscosity increases, while the imidization degree is almost constant as the initial PAA concentration rises. The greater the chain mobility, the higher the imidization degree at phase separation point. At CPPS, higher imidization degree and lower solution viscosity are obtained in the thermal imidization than in the chemical imidization; ODPA-ODA system exhibits higher imidization degree and better solubility than that of PMDA-ODA system.     

Solvent effect on the curing of polyimide resins

The effect of the solvent 1-methyl-2-pyrrolidinone (NMP) on the curing of polyimide resins synthesized from pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) has been investigated. Three polyimide precursors, i.e., the polyamic acid (PAA), with controlled amount of NMP were prepared. The study was aimed first to independently investigate the decomplexation process, which involved the evolution of hydrogen-bonded NMP from PAA, without interference from imidization. This was accomplished by TGA at varying heating rates using different solvent content in PAA. The observed one-stage decomplexation process suggested that the complex formation of NMP and PAA was not the same as the model compound studied by others. An average value of 150 kJ/mol for the activation energy of the decomplexation process was obtained. The study then sought to identify the effect of the decomplexation on the imidization kinetics by employing DSC at several drying temperatures and also varying heating rates. This allowed one to control the extent of plasticization that occurred to facilitate the imidization process. Our DSC data showed that over-drying PAA resulted in prolonged imidization due mainly to the lack of plasticization by decomplexed NMP. The estimated enthalpy of imidization and that of decomplexation were 114 KJ/mol and 53 kJ/mol NMP, respectively. Finally, the imidization kinetics was independently investigated using FTIR, without the interference from decomplexation process. The results indicated that there were four stages during the entire imidization process. Up to a temperature of 150°C, less than 20% of amide groups had reacted to give imide groups and the reaction was slow. Most of the imidization took place between 150 and 180°C with conversion as high as 90%. The imidization process was completed after the temperature was further raised to 250°C. Above 250°C, the reverse reaction became more significant (due probably to configurational and packing preference) and resulted in a lowering of final conversion back to 80%. © 1992 John Wiley & Sons, Inc.     

An ellipsometric study of polymer film curing: 2,6-Bis(3-aminophenoxy) benzonitrile/4,4′oxidiphthalic anhydride poly(amic acid)

The curing of 2,6-bis(3-aminophenoxy)benzonitrile/4,4′oxidiphthalic anhydride ((β-CN) APB/ODPA) has been investigated using spectroscopic ellipsometry on films with various degrees of imidization. Results indicate that much of the film imidization is accomplished at 200 °C and above. Three absorption peaks have been observed (4.1, 5, and 6 eV) which correspond to intra- and inter-molecular optical transitions. A comparison of the film optical constants for the pristine poly(amic acid) and the fully cured polyimide shows film densification upon imidization. A curing timeline has been obtained using in situ real-time spectroscopic ellipsometry, and ellipsometry is shown to serve as a general technique for studying organic film curing.     

The effect of curing history on the residual stress behavior of polyimide thin films

The effect of curing history on the residual stress behaviors in semiflexible structure poly(4,4′‐oxydiphenylene pyromellitimide) (PMDA–ODA) and rigid structure poly(p‐phenylene biphenyltetracarboximide) (BPDA–PDA) polyimide was investigated. Depending upon the curing history and different structures of polyimide, the residual stress behaviors and the morphology of polyimide thin films were detected in situ by using a wafer bending technique and wide angle X‐ray diffraction (WAXD), respectively. For the rigid structure BPDA–PDA polyimide, the residual stress and the slope decreased from 11.7 MPa and 0.058 MPa/°C to 4.2 MPa and 0.007 MPa/°C as the curing temperature increased, and the annealing process is done. However, for the semiflexible structure PMDA–ODA, the change of the residual stress and the slope was relatively not significant. In addition, it was found that the cured polyimide prepared at a higher temperature with a multistep curing process showed a higher order of chain in‐plain orientation and packing order than does the polyimide film prepared at a lower temperature with a one‐step curing process. These residual stress behaviors of polyimide thin films show good agreement with WAXD results, such as polyimide chain order, orientation, and intermolecular packing order, due to curing history. Specifically, it shows that the effect of curing history on residual stress as well as morphological change was significant in rigid BPDA–PDA polyimide but, not in semiflexible PMDA–ODA polyimide. Therefore, it suggests that the morphological structure depends upon curing history, and the polyimide backbone structure might be one of important factors to lead the low residual stress in polyimide thin films. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3287–3298, 1999     

Adhesion and interface studies between copper and polyimide

The adhesion and interface structure between copper and polyimide have been studied. Polyimide films were prepared by spinning a polyamic acid solution (Du Pont PMDA-ODA) in an NMP solvent onto a Cu foil, followed by thermal curing up to 400°C. The adhesion strength was measured by a 90° peel test. The peel strength of 25 μm thick Cu foil to 25 μm thick polyimide substrate was about 73 g/mm with the peel strength decreasing with increasing polyimide thickness. Cross-sectional TEM observation revealed very fine Cu-rich particles distributed in the polyimide. Particles were not found closer than 80-200 nm from Cu boundary. These Cu-rich particles were formed as a result of reaction of polyamic acid with Cu during thermal curing. We attribute the high peel strength to interfacial chemical bonding between Cu and polyimide. This behavior is in contrast to vacuum-deposited Cu onto fully cured polyimide.     

Characterization and tribological properties of two polyimide thin films sputtered onto a copper substrate

Two kinds of polyimide (PI), pyromellitic dianhydride-oxydianiline (PMDA-ODA) and biphenyl dianhydride-p-phenylene diamine (BPDA-PDA), thin films were sputtered onto a copper substrate by conventional RF sputtering with argon. These PI thin films were characterized, and their adhesion and tribological properties were evaluated. Elemental compositions and chemical bonding states of these thin films were analyzed with X-ray photoelectron spectroscopy (XPS). Oxygen and nitrogen concentrations in these thin films were less than those in bulk PIs. In addition, the amounts of C—O and C—N moieties in these PI thin films decreased as compared to the bulk PIs. Contact angles of water and methylene iodide on these PI thin films were higher than those on the bulk PIs. Surface energies of these PI thin films were calculated by measuring contact angles of water and methylene iodide. Surface energies of these PI thin films were lower than those of the bulk PIs and the polar components of these PI thin films were one-third of those of the bulk PIs. Friction coefficients of these thin films were almost the same as those of the bulk PIs. The abrasion durability of PMDA-ODA thin film was higher than that of BPDA-PDA thin film. The adhesion strength between the PMDA-ODA thin film and copper substrate was also higher than that between BPDA-PDA thin film and copper substrate.     

Transition between crystallization and microphase separation in PS-b-PEO thin film Influenced by solvent vapor selectivity

In this paper, the effect of solvent selectivity on the transition between crystallization and microphase separation of the semicrystalline diblock copolymer polystyrene-b-poly(ethylene oxide) (PS-b-PEO) thin films was investigated. Square-shaped crystals formed due to lower barrier of crystalline nucleation in both poor and good solvent vapor for PEO. However, in poor solvent (cyclohexane) vapor for PEO, crystalline structure changed to microphase separated structure in the square platelets due to the high mobility of PS blocks. Then breakout crystals dominated the morphology of the film. While, in good solvent (water) vapor for PEO, competition between nucleation and dissolution of crystallization caused the formation of imperfect crystals. Then imperfect crystals dissolved due to the high mobility of PEO blocks, and microphase separation dominated the morphology of the film. The gain of free volume of soluble block and the low swelling of crystalline block are keys for microphase separation and crystallization, respectively.     

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     

聚酰胺酸薄膜热环化过程热分析

用差热扫描量热仪(DSC)和傅立叶红外光谱仪(FTIR),考察了梯度升温过程中聚酰胺酸PAA[由4,4-二胺基二苯醚(ODA)和3,3,4,4-二苯醚四酸二酐(ODPA)制备]薄膜环化度和玻璃化温度(Tg)随反应温度的变化。结果表明,随着温度的升高,聚合物薄膜的亚胺化程度和Tg不断增大,且各恒温点的薄膜Tg均高于反应温度。另外,由亚胺化程度与Tg的关系曲线可见,在环化程度低时,薄膜Tg增长缓慢;随着亚胺化程度继续增高,薄膜的Tg迅速增大。用热环化过程中分子活动性的变化解释了酰亚胺化反应过程中环化速率的变化。     

Spin coating of thin and ultrathin polymer films

The spin coating of thin (> 200 nm thick) and ultrathin (< 200 nm thick) polymer films is examined in several solvents of varying volatility over a broad range of polymer solution concentrations and spin speeds. Experimentally measured film thicknesses are compared with a simple model proposed by Bornside, Macosko, and Scriven, which predicts film thickness based on the initial properties of the polymer solution, solvent, and spin speed. This model is found to predict film thickness values within 10% over the entire range of conditions explored, which gave film thicknesses from 10 nm to 33 μ:m. The model underpredicts film thickness for cases in which a very volatile solvent is used or the initial concentration of polymer is high, while overpredicting film thickness for cases in which a low volatility solvent is used or the initial polymer concentration is very low. These deviations are a consequence of how the model decouples fluid flow and solvent evaporation.     

Chemical resistance of waterborne epoxy/amine coatings

Waterborne epoxy/amine coatings, compared to solvent-based, show considerably lower chemical resistance. This fact is often blamed on hydrophilic emulsifiers or crosslinking agents remaining inside the coating after curing. To judge this assertion, the influence of surfactants and hardeners, as well as various other parameters were investigated. Surfactants appear just to influence the solvent resistance slightly. The main responsibility for the poor resistance to other chemicals – especially acids – lies with unreacted amine curing agents and/or water trapped inside the coating, and, most important, an inhomogeneous film structure due to insufficient coalescence during the curing process. Incremental improvements are possible by using excess epoxy, increasing the film thickness, or curing at elevated temperatures. The latter forces water out of the coating, drives the epoxy/amine reaction to completion, and also somewhat improves the coalescence. However, the resistance can still not be enhanced to the level of a solvent-based system. But although there seem to be inherent limitations to the chemical resistance of waterborne epoxy systems, for many practical applications it is adequate.