Partly Imidized Polyamic Acid and Its UniaxialStretched Polyimide Films

Partly imidized polyamic acid(PAA) has been used to prepare high performance polyimide films. The behaviors of two polyamic acids derived from pyromellitic dianhydride(PMDA)/4,4′-oxydianiline(ODA) and 3,3′,4,4′-biphenyltetracarboxylic diahhydride(BPDA)/paraphenylenediamine(PPD) containing dehydrating agents composed of acetic anhydride and a tertiary amine as the catalyst were investigated. The gel point was dependent on imidization degree in despite of temperature and the molar ratio of catalyst to acetic acid. Imdization content was about 35% for PMDA/ODA and about 22% for BPDA/PPD. The effect of catalyst on imidization possessed an order of triethylamine>3-methylpyridine>pyridine>isoquinoline>2-methylpyridine. The stretching of the films greatly reduced the coefficient of linear thermal expansion(CTE) either in the longitudinal direction or transversal direction. Compared to the film from polyamic acid, the partly imidized film had greater stretching ratio, so that the uniaxial stretched polyimide film from partly imidized PAA had higher tensile strength and tensile modulus, but lower elongation in the stretching direction.     

Spontaneous molecular orientation of polyimides induced by thermal imidization (6). Mechanism of negative in-plane CTE generation in non-stretched polyimide films

The mechanism of negative coefficient of thermal expansion (CTE) generation for non-stretched polyimide (PI) films is proposed in this work. Negative CTE behavior was observed in some miscible binary blend films composed of a major fraction of a rod-like semi-crystalline PI derived from pyromellitic dianhydride (PMDA) with p-phenylenediamine (PDA) and flexible PIs based on 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA) whereas homo PMDA/PDA PI film shows a considerably low but a positive CTE value. The results suggest that the negative CTE generation is related to not only a considerably high extent of in-plane orientation of the PMDA/PDA chains but also to the crystallinity of the blends. The present work revealed that some other PIs, a poly(ester imide), and a polybenzoxazole system also display negative CTE and these systems also possess extremely high extents of in-plane chain orientation without exception. In addition to CTE, the morphologies were monitored as a function of imidization temperature for two PI systems, PMDA/2,2′-bis(trifluoromethyl)benzidine and PMDA/m-tolidine by wide-angle X-ray diffraction, FT-IR spectroscopy, birefringence, and film density measurements. The results suggested that the negative CTE phenomenon occurs when PI films possess very high extents of in-plane orientation and a less crystalline morphology simultaneously, thereby significant thermal expansion can be allowed to the thickness direction.     

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.     

Dielectric properties of oxydianiline‐based polyimide thin films according to the water uptake

For polyimide thin films, the dielectric properties were investigated with the capacitance and optical methods. The dielectric constants of the 4,4′‐oxydianiline (ODA)‐based polyimide thin films varied from 2.49 to 3.10 and were in the following decreasing order: 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA)–ODA > 1,2,4,5‐benzenetetracarboxylic dianhydride (PMDA)–ODA > 4,4′‐hexafluoroisopropylidene diphthalic dianhydride (6FDA)–ODA. According to the absorption of water, the diffusion coefficients in the films varied from 4.8 × 10^?10 to 7.2 × 10^?10 cm²/s and were in the following increasing order: BPDA–ODA < PMDA–ODA < 6FDA–ODA. The dielectric constants and diffusion coefficients of the polyimides were affected by the morphological structures, including the molecular packing order. However, because of the water uptake, the changes in the dielectric constants in the polyimide thin films varied from 0.49 to 1.01 and were in the following increasing order: BPDA–ODA < 6FDA–ODA < PMDA–ODA. Surprisingly, 6FDA–ODA with bulky hexafluoroisopropylidene groups showed less of a change in its dielectric constant than PMDA–ODA. The total water uptake for the polyimide thin films varied from 1.43 to 3.19 wt % and was in the following increasing order: BPDA–ODA < 6FDA–ODA < PMDA–ODA. This means that the changes in the dielectric constants in the polyimide thin films were significantly related to the morphological structure and hydrophobicity of hexafluoroisopropylidene groups. Therefore, the morphological structure and chemical affinity in the polyimide thin films were important factors in controlling the dielectric properties. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2190–2198, 2002     

含酰胺结构超低膨胀聚酰亚胺薄膜的热膨胀行为

采用联苯二酐与3种含酰胺结构二胺制备了具有不同取代基团的聚酰胺-酰亚胺薄膜, 考察了酰胺结构对薄膜力学、 耐热及尺寸稳定性的影响, 研究了聚集态结构与薄膜热膨胀行为的关系和规律. 该系列薄膜具有超高强度和优异的耐热性能, 拉伸强度高达280.5 MPa, 玻璃化转变温度在389~409 ℃, 并在30~300 ℃温度范围内表现出超低负膨胀, 热膨胀系数(CTE, ppm/℃, 即10 ⁶ cm·cm ^-1·℃ ^-1)在-3.05~-1.74 ppm/℃之间. 聚集态分析结果表明, 酰胺结构使分子链间形成了强氢键相互作用, 分子链在薄膜面内方向高度有序取向, 并在膜厚方向堆积更为紧密, 使薄膜表现出热收缩现象. 通过不同体积大小的取代基团进一步调控分子链间相互作用及排列堆积, 可实现薄膜在高温下近乎零尺寸形变, 为设计制备超低膨胀聚合物基板材料提供了新思路.     

Structural and compositional effects on thermal expansion behavior in polyimide substrates of varying thicknesses

Polyimide films with thicknesses ranging from 6 μm to 80 μm were prepared with a solvent casting method to explore film thickness effects on the in-plane thermal expansion coefficient (CTE). In the case of polyimide films composed of bulky and flexible molecular units, CTE is consistent regardless of film thickness. In contrast, films with rigid and planar molecular structure show CTE increase according to the increase of film thickness up to 40–50 μm, which then plateau for thicker films. It is apparent that the film thickness dependent thermal expansion originates from complex effects of molecular orientation, charge transfer complex formation, and crystal formation as a function of film thicknesses, through characterization on UV–Vis absorption, crystalline structure, glass transition behavior, and optical retardation. These results provide insight into the design of polymer structures for flexible display substrates that require appropriate CTE values.     

Preparation and characterization of a polyimide membrane

Two different polyimide semipermeable membranes have been prepared from two base polyimides of PMDA/ODA and BTDA/ODA by incorporation of lithium chloride with the respective poly(amic acids) and subsequently leaching out the maximum possible lithium chloride by water at slightly elevated temperature. The water and various organic vapor permeability of the lithium chloride modified films has been found to be better compared to the respective control films. BTDA/ODA based polyimide films show overall lower permeability.     

Effects of chemical structure and precursor on optical properties,thermo-mechanical properties,and molecular orientation and order of thin films of stiff aromatic polyimides

Thin films of rigid poly(p-phenylene pyromellitimide) (PMDA-PDA) and semi-rigid poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), prepared by thermal imidization of the respective poly(amic acid) and poly(amic ethyl ester) precursors, were characterized with respect to their optical, thermomechanical and structural properties. Both polyimides exhibit an unusually large anisotropy between the in-plane and out-of-plane refractive indices, with n ranging from 0.198 to 0.216 for PMDA-PDA and from 0.230 to 0.242 for BPDA-PDA, nearly independent of the nature of the initial polyimide precursor, film thickness, and film preparation method. PMDA-PDA films exhibit low coefficients of thermal expansion (CTE's) of 6.5 and 8.2 ppm/C for the acid-derived and the ester-derived polyimides, respectively. In comparison, the BPDA-PDA films show CTE values of 4.3 and 18.0 for the acid-derived and ester-derived samples, respectively, despite the small differences in their optical anisotropies. Wide-angle x-ray diffraction patterns obtained in reflection and transmission for the various samples reveal a strong in-plane chain orientation for both PMDA-PDA and BPDA-PDA polyimides, with somewhat better intermolecular packing order for the ester-derived polyimide films. These effects of chemical structure and precursor on properties and structures of the polyimide films are discussed in light of recent theoretical considerations of semiflexible polymers.Dedicated to Prof E. W. Fischer on the occasion of his 65th birthday     

Extruder‐made TPO nanocomposites. II. Effect of maleated poly(propylene)/organoclay ratio on morphology and thermal expansion behavior

Coefficients of linear thermal expansion (CTE) for poly(propylene)/ poly(propylene)‐grafted‐maleic anhydride/montmorillonite ethylene‐co‐octene elastomer (PP/PP‐g‐MA/MMT/EOR) blend nanocomposites were determined as a function of MMT content and various PP‐g‐MA/organoclay masterbatch ratios. The nanocomposites were prepared in a twin‐screw extruder at a fixed 30 wt % elastomer, 0–7 wt % MMT content, and various PP‐g‐MA/organoclay ratio of 0, 0.5, 1.0, and 1.5. The organoclay dispersion facilitated by the maleated PP helps to reduce the size of the dispersed phase elastomer particles in the PP matrix. The elastomer particle size decreased significantly as the PP‐g‐MA/organoclay ratio and MMT content increased; the elastomer particles viewed // to flow direction (FD) are smaller and less deformed compared to those viewed // to transverse direction (TD). The elastomer particle shape based on the view along the three orthogonal directions of the injection molded sample is similar to a prolate ellipsoid. The CTE decreased significantly in the FD and TD, whereas a slight increase is observed in the normal direction in the presence of MMT and PP‐g‐MA. The Chow model based on a two population approach showed better fit to experimental CTE when the effect of MMT and elastomer are considered individually. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B Polym. Phys. 2013 , 51, 952–965     

Effect of the presence of silica nanoparticles in the coefficient of thermal expansion of LDPE

The effect of the presence of alumina microparticles and silica nanoparticles on the coefficient of thermal expansion (CTE) of films of low density polyethylene (LDPE) based composites was investigated. A new method based on the use of an atomic force microscope (AFM) is proposed for measuring nano-thermal expansion of films to finally obtain the CTE in polymer based materials. Nanocomposites based on silica nanoparticles and LDPE were prepared by mixing those constituents by high energy ball milling (HEBM). Pure alumina microparticles come from the milling tools used to mix the components of the composites. When silica nanoparticles are used as nanofiller of LDPE the effectiveness on reducing the CTE (about a 40% of CTE reduction) is higher than that obtained when high amount of alumina microparticles are present in the LDPE. Only when high amount of silica nanoparticles and low amount of alumina microparticles are present, the reduction of CTE expected from the Levin model is in accordance with the experimental results. This effect was associated to the high surface to volume ratio of nanoparticles considering uniform dispersions of them within the polymer. The region of polymer between particles must be so thin (few nanometers) that constraint effects must play an important role on reducing the chain mobility and therefore the thermal expansion.     

Carbon nanocones/discs – a new type of filler to improve the thermal and mechanical properties of polymer films

The mechanical characteristics and thermal properties of composite films based on the thermally stable aromatic polyimide (PI) (PMDA‐ODA) and carbon nanocones/discs (CNC) were studied. The introduction of CNC to PMDA‐ODA leads to the substantial increase of film stiffness. The Young's modulus values of the composite films are somewhat higher than those of the previously characterized composite films of this PI filled with nanoclay, carbon nanofibers, and asbestos‐like hydrosilicate nanotubes. The introduction of CNC into PMDA‐ODA (concentrations of CNC were up to 15 vol%) does not cause any marked aggregation of nanoparticles. The presence of CNC in the PI matrix does not affect the glass transition temperature of the polymer but hinders chain mobility at temperatures above T_g. This behavior makes it possible to increase the working temperature range of the composite films containing more than 5 vol% of CNCs, up to the temperature of thermal decomposition. The introduction of CNC into PMDA‐ODA leads to dramatic (～12 orders of magnitude) increase of active electrical conductivity of the material. Copyright © 2011 John Wiley & Sons, Ltd.     

High performance polymer films 4. Mechanical behavior

Tensile properties of the polyimide and copolyimide films based on two dianhydrides, pyromellitic dianhydride (PMDA) and 3,3^′,4,4^′-benzophenonetetracarboxylic dianhydride (BTDA) and two diamines, 4,4^′-oxydianiline (ODA), and a proprietary aromatic diamine (PD) have been described. The tensile strength of the films containing higher proportions of BTDA or PMDA and PD is much higher (except the fully rigid film based on PMDA-PD which is brittle in nature) than the films containing higher proportion of ODA moiety. The films containing PD as the diamine moiety exhibit high initial moduli than the films containing exclusively or mainly ODA as the diamine moiety. The films having higher concentration of the -O- linkage originated from diamine ODA are found to exhibit higher elongation values. There is found to be no direct correlation between η_inh of the precursor casting solutions and mechanical properties of structurally different polyimide/copolyimide films. For a particular polyimide or copolyimide film, the tensile strength value is found to be less sensitive than the elongation to the variation of η_inh value of the precursor poly(amic acid) or copoly(amic acid). Tensile strength and elongation of the film, basically rigid in nature, may be improved by post-curing at 360°C/370°C. While Kapton H film retains 78% and 63.5% of its tensile strength and % elongation at break (% E_b) respectively after hot-wet mechanical test, the film based on BTDA 80, PMDA 20 and PD shows an increase of about 27% and 22% in its tensile strength and % E_b respectively.     

Pretilt angle for BTDA,PMDA and CPDA series polyimides with various side chain lengths

Polyamic acid precursors were prepared by mixing dianhydride of 3,3',4,4'-benzophenone-tetracarboxylic dianhydride (BTDA), 1,2,3,4-benzene-tetracarboxylic dianhydride (pyrromellitic dianhydride PMDA), cis-1,2,3,4-cyclopentane-tetracarboxylic dianhydride (CPDA), the diamine (alkyl 3,5-diaminobenzoate) with side chain, and 4,4'-oxydianiline (ODA) without side chain. Copolyimide films with various side chain lengths were prepared by thermal imidization of polyamic acid precursors. The roughness of rubbed polyimide surface increased with increase in the side chain length. The pretilt angle for the BTDA and PMDA series polyimide (PI) increased exponentially with increase in side chain length. Various pretilt angles were obtained on the synthesized polyimides. In the case of CPDA series PI, the pretilt angle was nearly constant at 0 until the alkyl side chain length reached 12 (C12) and then increased markedly at C18. Models of pretilt angle generation were tested.     

含磷聚酰亚胺纤维的制备及其力学与阻燃性能

采用廉价的三苯基氧膦和混酸合成了一种含磷二胺单体,二（3-氨基苯基）苯基膦氧(DAPPO)。在4,4′-二胺基二苯醚（ODA）、3,3′,4,4′-联苯四酸二酐（BPDA）和均苯四酸二酐（PMDA）聚合体系中引入该单体,制备含磷聚酰亚胺纤维。热失重分析（TGA）结果表明,聚酰亚胺纤维的热稳定性随含磷量的增加而明显提高。当n（DAPPO）：n（ODA）为7：93时,纤维的极限氧指数达到了43,说明纤维的阻燃性能显著提高。     

A novel type of Si-containing poly(urethane-imide)s: synthesis, characterization and electrical properties

A novel type of a Si-containing poly(urethane-imide) (PUI) was prepared by two different methods. In the first method, Si-containing polyurethane (PU) prepolymer having isocyanate end groups was prepared by the reaction of diphenylsilanediol (DSiD) and toluene diisocyanate (TDI). Subsequently the PU prepolymer was reacted with pyromellitic dianhydride (PMDA) or benzophenonetetracarboxylic dianhydride (BTDA) in N-methyl pyrolidone (NMP) to form Si-containing modified polyimide directly. In the second method, PU prepolymer was reacted with diaminodiphenylether (DDE) or diaminodiphenylsulfone (DDS) in order to prepare an amine telechelic PU prepolymer. Finally, the PU prepolymer having diamine end groups was reacted with PMDA or BTDA to form a Si-containing modified polyimide. Cast films prepared by second method were thermally treated at 160 °C to give a series of clear, transparent PUI films. Thermogravimetric analysis indicated that the thermal degradation of PUI starts at 265 °C which is higher than degradation temperature of conventional PU, confirming that the introduction of imide groups improved the thermal stability of PU.To characterize the modified polyimides and their films, TGA, FTIR, SEM and inherent viscosity analyses were carried out. The dielectrical properties were investigated by the frequency-capacitance method. Dielectric constant, dielectric breakdown strength, moisture uptake and solubility properties of the films were also investigated.     

Soluble and transparent polyimides from unsymmetrical diamine containing two trifluoromethyl groups

A new unsymmetrical diamine monomer containing two trifluoromethyl (CF₃) groups is prepared from 2‐bromo‐5‐nitro‐1,3‐bis(trifluoromethyl)benzene. The monomer is polymerized with typical dianhydrides, in this case PMDA, BPDA, BTDA, ODPA, and 6‐FDA, using a one‐pot synthetic method to obtain corresponding polyimides. All of the prepared polyimides are readily soluble in many organic solvents and can be solution‐cast into transparent, flexible, and tough films. These films have a UV–vis absorption cut‐off wavelength at 340–375 nm and light transparencies of 87–91% at a wavelength of 550 nm. Incorporation of two CF₃ groups unsymmetrically into rigid polyimides improves their solubility and transparencies without decreasing their physical properties. The polymers exhibit high thermal stability with 5% weight loss at temperatures ranging from 534 to 593 °C in nitrogen and from 519 to 568 °C in air, and high glass transition temperatures (T_g) above > 300 °C depending on their molecular structures. They also have a coefficient of thermal expansion (CTE) value of 46–69 ppm/°C. In addition, they show low refractive indices in the range of 1.535–1.602 at a wavelength of 633 nm due to the unsymmetrical incorporation effect of the two CF₃ groups. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4413–4422     

柔性OLED用高耐热聚酰亚胺薄膜的制备与性能

聚酰亚胺(PI)薄膜作为柔性有机发光显示(OLED)基板材料应用时, 需要满足玻璃化转变温度(T_g)大于450 ℃和热膨胀系数(CTE)在0~5×10^-6 K^-1之间. 为了提高PI薄膜的热性能, 本文合成了2,7-占吨酮二胺 (2,7-DAX), 并将其与均苯四甲酸二酐(PMDA)和2-(4-氨基苯基)-5-氨基苯并噁唑(BOA)共聚制备了一系列新型PI薄膜. 研究了PI薄膜的聚集态结构、 耐热性能、 尺寸稳定性和力学性能. 结果表明, 占吨酮结构和苯并噁唑结构提高了PI分子链的刚性与线性, 使分子链在平面内紧密堆积与取向, 制备的PI薄膜综合性能优异, 玻璃化转变温度高于408 ℃, CTE在-5.0×10^-6~8.1×10^-6 K^-1之间, 拉伸强度大于140 MPa, 拉伸模量大于4.2 GPa, 断裂伸长率为7.1%~20%, 5%热失重分解温度(T_5%)在601~624 ℃之间. 其中, PI-50和PI-60薄膜具有超高玻璃化转变温度和超低热膨胀系数, T_g高于450 ℃, CTE分别为2.1×10^-6 K^-1和1.6×10^-6 K^-1. 制备的系列PI薄膜作为柔性OLED基板材料有潜在应用前景.     

Mechanical properties of BPDA-ODA polyimide fibers

An aromatic polyimide was synthesized via a one-step polycondensation reaction between biphenyltetracarboxylic dianhydride (BPDA) and 4,4′-oxydianiline (ODA) in p-chlorophenol. The polyimide (BPDA-ODA) solution dopes were spun into fibers by means of dry-jet wet spinning. The as-spun fibers were drawn and treated in heating tubes for improving the mechanical properties. The thermal treatment on the fibers resulted in a relatively high tensile strength and modulus. Thermal mechanical analysis (TMA) was employed to study the linear coefficient of thermal expansion (CTE). Thermal gravimetry analysis (TGA) spectra showed that the BPDA-ODA fibers possessed an excellent property of thermo-oxidative degradation resistance. The sonic modulus E_s of the polyimide fibers was measured.     

Spontaneous molecular orientation of polyimides induced by thermal imidization (5). Effect of ordered structure formation in polyimide precursors on CTE

Six poly(amic acid) (PAA) systems based on pyromellitic dianhydride (PMDA) formed some ordered structures with optical anisotropies clearly detectable on an optical polarizing microscope (POM) in N-methyl-2-pyrrolidone (NMP) at room temperature at high solute concentrations (15-25 wt.%) with complete sol-gel transition reversibility, whereas PAA systems based on 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) with a variety of diamine components showed no optical anisotropy in solution. However, a fluorescence probe technique combined with solution viscosity measurements suggested that a PAA derived s-BPDA with 1,4-phenylenediamine (PDA), i.e., PAA(s-BPDA/PDA) forms some ordered structure with a POM-undetectable very local scale during prolonged storage in NMP at room temperature. The introduction of the biphenyldiimide (BPDI) units at 33% into the PAA(s-BPDA/PDA) main chains by copolymerization allowed the formation of optically anisotropic gels with a smectic liquid crystal-like ordered structure by cooling the NMP solution at −20 °C. PI films derived from s-BPDA with PDA, i.e., PI(s-BPDA/PDA) were prepared upon thermal imidization of the BPDI-containing PAA films dried at 40 °C for 2.5 h. An increase in the BPDI content caused a gradual decrease in the linear coefficient of thermal expansion (CTE) of the PI films. This can be interpreted as a result of an intensified pre-orientation at the stage of the PAA cast films by incorporation of the BPDI units. When the BPDI-containing PAA solutions were heated at 70 °C for 4 min prior to the drying process at 40 °C, the ordered structures can be cancelled without imidization, and the CTE values of the resulting PI films appreciably increased compared to the case without heating at 70 °C. A similar effect was observed even in the BPDI-free original s-BPDA/PDA system. The results suggest the presence of a POM-undetectable very locally ordered structure in the PAA cast films, which promotes the pre-orientation of the PAA chains in the cast films and consequently can contribute to a further decrease in the CTE of the PI(s-BPDA/PDA) films.     

Polyimides with low coefficient of thermal expansion derived from diamines containing benzimidazole and amide: Synthesis,properties, and the N-substitution effect

Three novel diamines, incorporating benzimidazole and amide moieties, namely 4-amino-N-(5-amino-benzimidazol-2-yl)-benzamide (6a), 4-amino-N-(5-amino-1- methyl-benzimidazol-2-yl)-benzamide (6b), and 4-amino-N-(5-amino-1-phenyl -benzimidazol-2-yl)-benzamide (6c), were designed and synthesized. A series of poly(benzimidazole-amide-imide) (PBIAI) films were prepared from the resulting diamines and 4,4-biphthalic dianhydride (BPDA). These flexible polyimides (PIs) showed high glass transition temperatures (T_g = 353–379°C), low coefficients of thermal expansion (CTE = 3.7–12.3 ppm K⁻¹) and good mechanical properties (σ = 152–207 MPa and E = 4.5–7.7 GPa), promising candidates for applications in flexible-display substrates. Furthermore, the data guided a feasible method to enhance T_g and reduce CTE by introducing benzimidazole and amide units into PI main chains, and the effect of different N-substituents on performance was revealed.