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Commercially available cycloaliphatic tetracarboxylic dianhydrides (two monomers shown in the bottom are those prepared by the author's group).
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This paper reviews the development of new high-temperature polymeric materials applicable to plastic substrates in image display devices with a focus on our previous results. Novel solution-processable colorless polyimides (PIs) with ultra-low linear coefficients of thermal expansion (CTE) are proposed in this paper. First, the principles of the co...
Citations
... In many applications, a particular rate of contraction with respect to temperature is normally required to fine tune the operation of the components such as valves. Polymers with relatively low CTEs would work effectively to avoid development of stress concentrations caused by thermal mismatches between metals [36,37]. While in systems operating dynamically over the cryogenic temperature range, those polymers with higher thermal contraction or expansion can be installed initially under interference fit [38] to ensure after thermo-mechanical loading down to low temperature they continue to seal effectively [39]. ...
Cryogenic fuels, such as liquid hydrogen and liquid natural gas, emerge as versatile and sustainable energy carriers that are revolutionising various industries including aerospace, automotive, marine, and power generation. Thermoplastic polymers can be a suitable alternative to metal seals in cryogenic fuel systems. However, there is limited study about the behaviours of thermoplastics at cryogenic temperatures, especially at liquid hydrogen temperature of 20 Kelvin (K). This paper measured the tensile properties and coefficient of thermal expansion of three popular thermoplastics: PTFE, PEEK and UHMWPE at room temperature (RT), 77 K and 20 K and at four strain rates. Further microscopic analysis was also conducted to understand the failure mechanisms occurring when combining reduced temperature with varying strain rate. The tensile strength of each polymer increased from RT to 77 K and decreased from 77 K to 20 K. Elastic modulus tended to increase, and the strain recorded at failure decreased when reducing temperature from RT to 20 K. From microscopic observation of PEEK and UHMWPE, a reduction in temperature from 77 K to 20 K resulted in a larger instantaneous fracture, with multi-faceted fracture surfaces containing many small mirror like and opaque or misty sub-regions within the fracture zone. For PTFE, the surface morphology exhibited an insensitivity to the increase in strain rate at cryogenic temperatures, and the microscopy showed how the size of dimples found within the fracture interface became smaller when temperature was reduced from 77 K to 20 K. Finally, PEEK was found to contract much less than PTFE and UHMWPE at 20 K, in agreement to it having the highest glass transition temperature of the three polymers, which is normally a good indicator when attempting to identify polymers that will tend to exhibit smaller contraction at cryogenic temperatures.
... However, if the main chain of the polymer has an aromatic structure with high thermal stability or the entire polymer chain is rigid and straight, heat transfer is hindered, and expansion does not occur easily. 48,49 Structure I is a rigid rod-type straight para-structure that is difficult to change with heat treatment. In addition, although structure II has an m-structure, it has excellent dimensional resistance because it contains a rigid biphenyl moiety and exhibits high thermal stability. ...
Six poly(amic acid)s (PAAs) were synthesized by reacting bis(3-aminophenyl) sulfone with various dianhydride monomers such as pyromellitic dianhydride, 4,4′-biphthalic anhydride, dicyclohexyl-3,4,3′,4′-tetracarboxylic dianhydride, 4,4′-oxidiphthalic anhydride, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride, and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride. These PAAs were then converted to polyimide (PI) films by thermal imidization at various temperatures. To obtain colorless and transparent PI (CPI), the dianhydride monomer used in this study had an overall bent structure, a structure containing a strong electron-withdrawing −CF3 substituent or an alicyclic ring. In addition, some monomers contained ether or ketone functional groups in their bent structures. The thermomechanical properties, optical transparency, and solubility of CPI films with six different dianhydride monomer structures were investigated, and the correlation between the monomer structure and CPI film properties was clarified. Overall, CPI with an aromatic main chain structure or a linear structure had excellent thermal and mechanical properties. In contrast, CPI with a bent structure containing functional groups or substituents in the main chain exhibited excellent optical transparency and solubility.
... Meanwhile, the APPFP-based film had a lower elastic modulus and tensile strength of 4.1 GPa and 81 MPa, respectively, and the highest value of elongation at break (38%), indicating a less rigid chain. These values are comparable to those of the colorless and transparent polyimide and polyamideimide films reported so far [19][20][21]. For example, fluorinated polyimide obtained from TFMB and 4,4 ′ -(hexafluoroisopropylidene)diphthalic anhydride (6FDA) has an elastic modulus of 3.1 GPa and a tensile strength of 120 MPa. ...
In this study, various diamine monomers were used to synthesize aramid polymer films via a low-temperature solution condensation reaction with diacid chloride. For diamines with relatively high basicity, the reaction system became opaque because amine salt formation inhibited polymer synthesis. Meanwhile, low-basicity diamines with strong electron-withdrawing groups, such as CF3 and sulfone, were smoothly polymerized without amine salt formation to provide highly viscous solutions. The acid byproduct HCl generated during polymerization was removed by adding propylene oxide to the reaction vessel and converting the acid into highly volatile inert substances. The resulting solutions were used as varnishes without any additional purification, and polymer films with an excellent appearance were easily obtained through a conventional casting and convection drying process. The films neither tore nor broke when pulled or bent by hand; furthermore, even when heated up to 400 °C, they did not decompose or melt. Moreover, polymers prepared from 2,2-bis(trifluoromethyl)benzidine (TFMB) and bis(4-aminophenyl)sulfone (pAPS) did not exhibit glass transition until decomposition. The prepared polymer films showed a high elastic modulus of more than 4.1 GPa and a high tensile strength of more than 52 MPa. In particular, TFMB-, pAPS-, and 2,2-bis(4-aminophenyl)hexafluoropropane-based polymer films were colorless and transparent, with very high light transmittances of 95%, 96%, and 91%, respectively, at 420 nm and low yellow indexes of 2.4, 1.9, and 4.3, respectively.
... There are several approaches to reduce CTE. One promising method is the incorporation of amide groups in PIs [9], [10]. In this study, PSPI with low CTE and excellent adhesion strength have been developed by selection of appropriate precursor and photo-sensible group. ...
In advanced packaging schemes, such as fan-out integration technology, photosensitive polyimide (PSPI) is the key material to the fabrication of panel level redistribution-layer (RDL). However, a large mismatch of coefficient of thermal expansion (CTE) between silicon (Si) and PSPI will cause serious warpage issue. Furthermore, polyimide deformation may occur under external heat and pressure, leading to deterioration of RDL reliability. In this work, PSPI with low CTE and excellent adhesion strength on different substrate was developed and evaluated by lithography test, adhesion test, and reliability test, showing the high feasibility for the application in advanced packaging process.
... Meanwhile, the APPFP-based film had a lower elastic modulus and tensile strength of 4.1 GPa and 81 MPa, respectively, and the highest value of elongation at break (38%), indicating a less rigid chain. These values are comparable to those of the colorless and transparent polyimide and polyamideimide films reported so far [14][15][16]. For example, fluorinated polyimide obtained from TFMB and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) has an elastic modulus of 3.1 GPa and a tensile strength of 120 MPa. ...
In this study, various diamine monomers were used to synthesize aramid polymer films via a low-temperature solution condensation reaction with diacid chloride. For diamines with relatively high basicity, the reaction system became opaque because amine salt formation inhibited polymer synthesis. Meanwhile, low-basicity diamines with strong electron-withdrawing groups, such as CF3 and sulfone, were smoothly polymerized without amine salt formation to provide highly viscous solutions. The acid byproduct HCl generated during polymerization was removed by adding propylene oxide to the reaction vessel and converting the acid into highly volatile inert substances. The resulting solutions were used as varnishes without any additional purification, and polymer films with an excellent appearance were easily obtained through a conventional casting and convection drying process. The films neither tore nor broke when pulled or bent by hand; furthermore, even when heated up to 400°C, they did not decompose or melt. Moreover, polymers prepared from 2,2-bis (trifluoromethyl) benzidine (TFMB) and bis (4-aminophenyl) sulfone (pAPS) did not exhibit glass transition until decomposition. The prepared polymer films showed a high elastic modulus of more than 4.1 GPa and a high tensile strength of more than 52 MPa. In particular, TFMB-, pAPS-, and 2,2-bis (4-aminophenyl) hexafluoropropane-based polymer films were colorless and transparent, with very high light transmittances of 95%, 96%, and 91%, respectively, at 420 nm and low yellow indexes of 2.4, 1.9, and 4.3, respectively.
... Actually, semi-alicyclic PIs have been widely investigated as the colorless and transparent PI (CPI) films in the past decades [21][22][23][24][25]. However, the lack of crystalline regions in the molecular chains of semi-alicyclic PIs caused by the poorly conjugated molecular skeleton and the relatively low polarity of the molecular structures might deteriorate the melting processability of the polymers as TPI applications. ...
Two semi-alicyclic thermoplastic polyimides (PI) have been prepared from stereoisomeric hydrogenated pyromellitic dianhydrides, including 1S,2R,4S,5R-hydrogenated pyromellitic dianhydride (ccHPMDA or H-PMDA) and 1R,2S,4S,5R-hydrogenated pyromellitic dianhydride (ctHPMDA or Hʹʹ-PMDA) and a fluorine-containing diamine, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (BDAF), respectively. The derived PI-1 (ccHPMDA-BDAF) and PI-2 (ctHPMDA-BDAF) resins were easily soluble in polar aprotic solvents, such as N -methyl-2-pyrrolidinone (NMP) and N,N -dimethylacetamide (DMAc) and meanwhile they also exhibited good thermoplasticities. Thus, the PI films (thickness: ~25 µm) and sheets (thickness: ~3 mm) were separately prepared from the PI solutions in DMAc and the PI powders, respectively. The stereoisomeric effects in the ccHPMDA-derived PI-1 and the ct-HPMDA-derived PI-2 apparently affected the melt-flowability of the polymers. PI-2 exhibited lower melting viscosities than that of the PI-1 counterpart. The PI sheets exhibited obviously reduced optical transparency compared with those of the PI films; however better optical transparency than those of the common thermoplastic PI sheets. For example, PI-2 sheet showed the optical transmittance value at the wavelength of 760 nm (T 760 ) of 69.6%, which was 19.5% lower than that of the analogous PI-2 film (T 760 = 89.1%), however 50.3% and 65.7% higher than those of the commercially available PI-ref1 sheet (T 760 = 19.3%) derived from 3,3ʹ,4,4ʹ-oxydiphthalic dianhydride (ODPA) and 4,4ʹ-oxydianiline (ODA) and the PI-ref2 sheet (T 760 = 3.9%, Aurum® PL450C, Mitsui Chem. Co, Ltd., Japan), respectively. Meanwhile, the PI-1 and PI-2 sheets showed the heat deflection temperatures (HDT) of 261.0 o C and 265.0 o C, respectively, which were obviously higher than those of the referenced PIs. At last, the PI sheets exhibited good mechanical properties, including high flexural strength (130.0 ~ 132.0 MPa), flexural modulus (2.87 ~ 2.96 GPa), compression strength (124.4 ~ 149.3 MPa), compression modulus (1.49 ~ 1.59 GPa), and impact strength (129.0 ~ 139.6 KJ/m ² ).
... PI is an aromatic heterocyclic polymer compound with imide group chains in its molecular structure, which has the advantages of being non-toxic, having high insulation and strength, and having a low thermal expansion coefficient. 9,10 PI is an ideal humidity sensitive material for its excellent waterabsorbing quality. 7,8 Dual parameter measurement has been realized by cascading two LPFGs, one of which coated with PI and graphene quantum dots shows a humidity sensitivity of ∼78 pm/%RH, 6 which is still not sensitive enough, and it is high cost and difficult to prepare. ...
... LPFG coated with polymer composite film shows an ultra-high humidity sensitivity of ∼2485 pm/%RH, 8 but the fabrication process is complex and it can only used for sensing humidity. 10 Optical fiber Mach-Zehnder interferometers (MZIs) are widely used for sensing gas pressure, 11 temperature, 12-16 and humidity. 12,[14][15][16][17][18] Tapered fiber is the most popular method for the fabrication of MZIs with a mode field diameter of micro or nano magnitude, which enhances the evanescent field and then greatly improves the sensitivity and response speed. ...
Two different structures, long period fiber grating (LPFG) and a Mach–Zehnder interferometer (MZI), were cascaded to fabricate a sensor for sensing humidity and temperature simultaneously. Due to the humidity sensitive characteristic of polyimide (PI) and the temperature sensitive characteristic of polydimethylsiloxane (PDMS), the LPFG was coated with PI and the MZI was wrapped in PDMS to improve the humidity and temperature sensitivities, respectively. Humidity and temperature experiments and the stability and repeatability of the proposed sensors were performed. The sensor shows humidity and temperature sensitivities of ∼164 pm/%RH and 183 pm/°C in the humidity and temperature increasing experiments, respectively. Combining increasing, decreasing, and repeatable experiments of temperature and humidity, the proposed sensor shows stable performance in humidity sensitivity of ∼159 pm/%RH and temperature sensitivity of ∼175 pm/°C, with good linearity of ∼0.994. The structure of the proposed sensor has the advantages of low implementation cost, easy fabrication, and good stability.
... The PI body must then be dried; however, this operation takes place at significantly lower temperatures and mild vacuum compared to thermal imidization conditions [38,46]. The PI preparation process using chemical imidization also has the advantage of increasing the visible transmittance of the resulting PI film compared to the conventional two-step process based on thermal imidization [47,48]. These benefits make the chemical imidization a very promising method for the preparation of PIs on an industrial scale, and further efforts to find more efficient catalysts are important for broadening industrial applications. ...
Polyimides are a group of polymers with excellent thermal and mechanical properties. However, generally, they exhibit brown coloration due to charge transfer interactions between diamine donor and dianhydride acceptor moieties. In this study, semi-alicyclic polyimides derived from cyclohexane tetracarboxylic dianhydride and 4,4′-oxydianiline were prepared using a two-step method, where the imidization step proceeded via a chemical route. The effects of the catalyst type (quinoline, triethylamine, and 1,4-diazabicyclo(2.2.2)octane (DABCO)) and reaction temperature (120, 140, and 160 °C) on the imidization process were evaluated using infrared spectroscopy and nuclear magnetic resonance spectroscopy. The prepared polyimides' thermal, mechanical, and optical properties were compared with those synthesized via standard thermal imidization. Polyimide films possessing favorable mechanical properties, thermal stability, and good optical transparency were prepared via a simple preparation route based on chemical imidization using the triethylamine catalyst at the reaction temperature of 160 °C, which was shown to be a promising way for the PI preparation on an industrial scale.
Graphical abstract
... The intense coloration of wholly aromatic PI films can be effectively eliminated by replacing either aromatic tetracarboxylic dianhydrides or aromatic diamines with aliphatic ones (usually cycloaliphatic (alicyclic) monomers) to impart heat resistance to the resultant PIs [18,. Figure 1 shows the molecular structures of the typical aliphatic diamine monomers. However, the use of aliphatic diamines causes a serious problem, namely, the formation of poorly soluble salts between the COOH groups of the initially formed low-molecularweight amic acids (AAs) and the unreacted NH 2 groups of the added diamine and disturbs the smooth progress of polyaddition [38]. The poor solubility of the salts is likely related to its cross-linked structure, as schematically depicted in Figure 2. When non-rigid structures of aliphatic diamines (e.g., MBCHA, IPDA, and ALDA in Figure 1) were used, the initially formed salts gradually dissolve and participate in the polymerization during prolonged stirring at room temperature. ...
... This product also showed a very sharp endothermic peak at 384 • C for melting by differential scanning calorimetry (DSC), as in the sublimated product ( Figure S2), confirming a very high purity. An amide-type fluorinated diamine (AB-TFMB) was synthesized according to the procedures described in our previous studies [38,43]. The analytical data are as follows. ...
... In principle, simultaneously achieving low CTE and high optical transparency is not always the trade-off. However, in fact, this issue is not easy to solve because the use of cycloaliphatic monomers to ensure film transparency often increases CTE, suggesting the importance of their steric structures in reducing the CTE of the resultant PI films [30][31][32]38,45,47]. On the other hand, there is the trade-off between low CTE and excellent solution processability because the molecular design for ensuring a low CTE (i.e., to enhance main-chain linearity/rigidity) often results in a significant decrease in solubility, and vice versa [61,62]. ...
In this paper, novel colorless polyimides (PIs) derived from 5,5′-bis(2,3-norbornanedicarboxylic anhydride) (BNBDA) were presented. The results of single-crystal X-ray structural analysis using a BNBDA-based model compound suggested that it had a unique steric structure with high structural linearity. Therefore, BNBDA is expected to afford new colorless PI films with an extremely high glass transition temperature (Tg) and a low linear coefficient of thermal expansion (CTE) when combined with aromatic diamines with rigid and linear structures (typically, 2,2′-bis(trifluoromethyl)benzidine (TFMB)). However, the polyaddition of BNBDA and TFMB did not form a PI precursor with a sufficiently high molecular weight; consequently, the formation of a flexible, free-standing PI film via the two-step process was inhibited because of its brittleness. One-pot polycondensation was also unsuccessful in this system because of precipitation during the reaction, probably owing to the poor solubility of the initially yielded BNBDA/TFMB imide oligomers. The combinations of (1) the structural modification of the BNBDA/TFMB system, (2) the application of a modified one-pot process, in which the conditions of the temperature-rising profile, solvents, azeotropic agent, catalysts, and reactor were refined, and (3) the optimization of the film preparation conditions overcame the trade-off between low CTE and high film toughness and afforded unprecedented PI films with well-balanced properties, simultaneously achieving excellent optical transparency, extremely high Tg, sufficiently high thermal stability, low CTE, high toughness, relatively low water uptake, and excellent solution processability.
... Fortunately, many methodologies are available that could be applied to reducing the CTE values of the semi-alicyclic CPI films. Related studies have been widely reviewed in the literature [35,36]. ...
Polyimide (PI) optical films with high glass transition temperatures (high-Tg), high optical transparency, and low optical retardations (low-Rth) are highly desired in advanced optoelectronic applications. However, the standard PI films usually suffer from deep colors, high optical anisotropies and limited Tg values. In the current work, a series of semi-alicyclic colorless and transparent PI (CPI) films were developed from hydrogenated pyromellitic dianhydride stereoisomers, 1S,2R,4S,5R-hydrogenated pyromellitic dianhydride and 1R,2S,4S,5R-hydrogenated pyromellitic dianhydride, and fluorene-containing diamines, including 9,9-bis(4-aminophenyl)fluorene and 9,9-bis(3-fluoro-4-aminophenyl)fluorene, respectively. The derived CPI films showed Tg values higher than 420 °C according to differential scanning calorimetry measurements. In addition, the fluorene-based CPI film showed optical transmittances higher than 80% at the wavelength of 400 nm, with yellow indices in the range of 0.60~1.01 and haze values below 3.0%. The CPI films showed average refractive indices from 1.5407 to 1.6309, extremely low birefringence at the level of minus fourth power of ten, and further exhibited quite low optical retardations below 10 nm.
