Polyimide Composite Films Research Articles

Coordination of Ultralow Permittivity and Higher Thermal Conductivity of Polyimide Induced by Unique Interfacial Self‐Assembly Behavior

AbstractUpgrading the available dielectric materials is the most effective approach to solve the poor quality of signal transmission and heat buildup caused by high density integration. In this work, a design strategy for multilayer 3D porous composite networks is proposed, relying on the self‐assembly effect of “crystal‐like phase” to achieve the synergistic optimization of low permittivity and high thermal conductivity of polyimide. The obtained three‐layer porous polyimide composite film (PSLS) features an ultralow permittivity of 1.89, an in‐plane thermal conductivity as high as 13.58 W m−1 K−1, and maintains well electrical insulating property. Inspiringly, the first digital thermoacoustic generator with wide frequency response has been designed based on PSLS film. It achieves sound pressure levels up to 60.1 dB in the 20–100 kHz range and integrates the efficient sound generation of an ultrasonic generator with real‐time display. This work will provide a novel concept material for the smart electronics and electrical fields.

Dual covalent bond induced high thermally conductive polyimide composite films based on CNT@CN complex filler

Dual covalent bond induced high thermally conductive polyimide composite films based on CNT@CN complex filler

Mechanically strong, transparent polyimide composite thin films with a low dielectric constant

Aromatic polyimide is widely used in microelectronics as dielectric packaging layers. There is a high demand to reduce its dielectric constant to meet the fast development of communication technology. Here we report a simple and eco-friendly approach to preparing polyimide films with a lowered dielectric constant by adding silica hollow nanospheres with an average size of 100 nm in an aqueous solution of poly(amic acid) salt, and subsequent film casting and thermal imidization. The silica hollow nanospheres present a uniform distribution in polyimide matrix after surface modification with 3-aminopropyltriethoxysilane. The dielectric constant of the resulting composite films decreases linearly with particle loading till 15 wt% and reaches the lowest value of 2.5. The surface hardness and elastic modulus of the films improve by adding hollow nanospheres, while maintaining high optical transparency, high flexibility and a low coefficient of thermal expansion.

Porous polyimide composite films containing mesoporous hollow silica nanospheres with ultralow dielectric constant and dissipation loss

Porous polyimide hybrid films with ultralow dielectric constant and dissipation loss were synthesized during chemical imidization and rapid evaporation of polymer solutions. Polyimide (PI) of bis(4-aminophenyl)-1,4-diisopropylbenzene (BISP) and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) were synthesized in N-methylpyrrolidinone (NMP) to yield the corresponding poly(amic acid)s (PAA)s, which were then chemically imidized. To reduce the dielectric properties of the porous PI films, amine-functionalized, surface-modified hollow silica nanoparticles (AHNS) with various sizes from 100 nm to 1 μm, prepared via sol-gel process, were added with PAA at 3, 5, or 10 % weight content, followed by chemical imidization to produce the porous PI/AHNS films. To avoid the high-volume shrinkage of wet gels and maintain the high porosity of the PI with low polarizability, AHNS nanoparticles were added to the porous structure of the PI xerogels as a physical crosslinker. The polyimide hybrid film with 10 nm AHNS nanoparticles at 10 % weight content showed a dielectric constant of 1.151 and a dissipation rate of 0.001 at a frequency of 10 GHz.

Silane modified Cr2O3/polyimide nanocomposite films with excellent surface insulation performance for space applications

The exploration of deep space significantly increases the probability of spacecraft failures due to surface electrostatic discharge, which imposes higher vacuum insulation protection requirements on polyimide (PI), the external insulation material of spacecrafts. To address this challenge, this study proposes using silane coupling agent KH550 for organic grafting treatment of Cr2O3nanoparticles, which are then used to dope and modify PI to enhance the vacuum surface insulation of PI films. The KH550 grafting improves the interface strength between the fillers and the matrix, allowing the fillers to be uniformly dispersed in the matrix. Compared to pure PI films, the prepared PI-Cr2O3@KH550 composite films exhibit significantly enhanced vacuum surface flashover voltage, improved surface/volume resistivity, and dielectric properties. The results demonstrate that PI composite films with 0.8% by mass of Cr2O3@KH550 show the most notable performance improvement, with the DC flashover voltage and impulse flashover voltage in vacuum increasing by 20.7% and 27.8%, respectively. The doping of chromium oxide nanoparticles introduces more deep traps into the PI films and reduce the surface resistivity. The higher deep trap density inhibits charge migration, thereby alleviating secondary electron emission and surface electric field distortion. Simultaneously, the lower surface resistivity facilitates dissipating surface charges and improves the surface insulation. These findings are of significant reference value for promoting the enhancement of aerospace insulation performance.

Low dielectric polyimide microsphere/polyimide composite films based on porous polyimide microsphere

AbstractA low dielectric polyimide/polyimide microsphere (PI/PM) composite film was constructed by thermal imidization of polyamic acid from pyromellitic dianhydride (PMDA) and 4,4′‐oxydianiline (ODA) in the presence of porous PM. The PM particles with particle size of about 2 μm were prepared via the solvothermal method using 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA) and ODA as monomers through thermal imidization. Due to the favorable compatibility between the porous PM and PI matrix, the mechanical properties, thermal stability, and dielectric properties of the obtained composite films were significantly improved. The PI/PM composite films had a tensile strength of 44.18–64.32 MPa, and the corresponding elongation at break of 6.21%–11.7%. Furthermore, the thermogravimetric temperatures of T5% were 538.9–563.7°C. The dielectric constants of the composite films at 1 MHz were 2.59–3.68, and the corresponding dielectric loss were only 0.0119–0.00405. Thus, the combination of excellent mechanical properties, high thermal stability, extremely low dielectric constant, and dielectric loss make the composite films ideal for deployment as high‐performance materials for 5G applications.Highlights Low dielectric polyimide composite film was prepared by thermal imidization of polyamic acid in the presence of porous polyimide microspheres. Porous polyimide microspheres were prepared by thermal using the imidization solvothermal method. Low dielectric polyimide composite film with good comprehensive properties.

High‐performance cross‐linked SiCOH/polyimide composite film with an ultralow dielectric constant at high frequency

AbstractUltralow‐k materials with promising comprehensive performance to meet the increasing requirements of high‐frequency communication are highly desired. Here, a cross‐linked porous polymer SiCOH, synthesized via simple sol–gel method, was used as the filler to prepare SiCOH/PI composite films by physically blending, which demonstrates ultralow dielectric constant at high frequency. Notably, only 2.5% of SiCOH in the composite can significantly reduce the dielectric constant of PI from 3.0 to an ultralow value of 1.7 in the 9.5–12.4 GHz, together with a low dielectric loss of 0.029. Additionally, the SiCOH/PI composite maintains a high tensile strength of 82.4 MPa and a tensile modulus of 1.82 GPa, which is comparable to pure PI film. While the glass transition temperature (Tg) increases to 370°C and the linear expansion coefficient is reduced to 30.7 ppm/°C, both of which are superior to those of pure PI film. Moreover, by introducing the hydrophobic SiCOH into PI, the contact angle of the composite films increased to 92.0°–99.0°. This simple and low‐cost method not only effectively lowers the dielectric constant of PI but also improves the thermal stability, mechanical properties, and hydrophobic ability, greatly boosts the practical application of PI in high‐frequency communication in the future.

In-situ amino-functionalized and reduced graphene oxide/polyimide composite films for high-performance triboelectric nanogenerator

As a promising sustainable power source in intelligent electronics, Triboelectric Nanogenerators (TENGs) have garnered widespread interest, with various strategies explored to enhance their output performance. However, most optimization methods for triboelectric materials have focused solely on tuning chemical compositions or fabricating surface microstructures. Here, we have prepared amino-functionalized reduced graphene oxide (FRGO)/polyimide (PI) composite films (PI-FRGO) via in-situ polymerization, aimed at enhancing PI materials’ nanotribological power generation performance. By varying the doping levels of amino groups and controlling the FRGO proportion during synthesis, we can explore the optimal FRGO/PI composite film ratio. At a p-Phenylenediamine: reduced Graphene Oxide (PPDA: RGO) ratio of 1:1 and an FRGO addition of 0.1 %, the output electrical performance peaks with a voltage of 58 V, a charge of 33 nC and a current of 12 μA, nearly 2 times that of a pure PI film. We have fabricated a TENG with an optimally formulated PI-FRGO composite to explore its application potential. Under a 10 MΩ external load resistance, the TENG can deliver a power density of 3.5 mW/m2 and can be powering small devices. This work presents new effective strategies to significantly enhance TENG output performance and promote their widespread application.

Covalent bond-assisted continuous interface structure for high thermal conductive polyimide composite films

Polyimide (PI) is widely used in electronic, electrical and communication fields because of its outstanding mechanical properties, electrical insulation and thermal stability. However, the lower thermal conductivity limits their application. Here, PI composite films with high thermal conductivity are obtained by utilizing reactive groups on the carbon nitride nanosheets (CNNS) surface to form covalent bond with PI matrix. The thermal conductivity of CNNS/PI composite films can reach 5.84 W∙m−1∙K−1, which is about 10 times higher than that of pure PI (0.56 W∙m−1∙K−1). This is attributed to the covalent bond between CNNS and PI matrix acting as a “thermal bridge” to connect the discontinuities at the interface, thereby reducing the interfacial thermal resistance between CNNS and PI. The heat transfer mechanism of composite film is further revealed by finite element analysis. This work provides a viable solution to improve the thermal conductivity of PI, allowing for a wide range of potential applications for PI in electronic devices.

Laser-assisted synthesis of MnOx and 3D graphene composites for wide-voltage flexible planar microcapacitors

Herein, we propose a simple and eco-friendly strategy based on laser direct writing technology to regulate the valence state of Mn-based oxides nanoparticles anchored on three-dimensional(3D) framework of laser-induced graphene (LIG) for the application of high-performance flexible planar interdigital microsupercapacitors (MSCs). Under the laser radiation, a polyimide (PI) and MnO2 composite film was directly converted into 3D graphene and MnOx (MnO, MnO2 and Mn3O4 mixture). Furthermore, the results reveal that LIG/MnOx MSCs device exhibits a high specific capacitance of 23.3 mF cm−2, which is about 29 times higher than that of LIG MSCs (0.8 mF cm−2). Moreover, LIG/MnOx MSCs possess a high energy density of 7.28 μWh cm−2, outstanding cycle stability with the capacitance retention rate of 97.4 % after 10,000 cycles, and excellent mechanical flexibility. Therefore, this preparation strategy of LIG/MnOx provides a reference for the performance regulation of Mn-based oxides and the low-cost construction of MSCs devices with high energy density, which carries significant research implications for diverse flexible wearable electronic applications in the forthcoming days.

Caffeic-Acid-Functionalized MWCNTs and PEDOT:PSS Formed Composite Flexible Films with "Reinforced Concrete" Structure for Electrical Heating and EMI Shielding.

Nowadays, flexible multifunctional composites are attracting much attention and are practically being used in various emerging electronic devices. However, most composites suffer from the disadvantages of high loadings of conductive fillers, complicated preparation processes, and low energy conversion efficiency. In this article, Caffeic acid-modified multiwalled carbon nanotubes (C-MWCNTs)/poly(3,4-ethylene dioxythiophene):polystyrene sulfonic acid (PEDOT:PSS)/polyimide (PI) composite films (CPFs) were prepared using a simple layer-by-layer deposition method. The "reinforced concrete" structure of the C-MWCNTs/PEDOT:PSS layer ensures high electrical conductivity of the film, while the PI layer provides excellent mechanical properties (72.69 MPa). The composite film exhibits excellent electrothermal response and thermal stability up to approximately 125 °C at 5 V. In addition, the good conductivity of the film provides its electromagnetic shielding effectiveness (32.69 dB). With these advantages, we expect that flexible CPFs will be widely utilized in wearable devices, electromagnetic interference (EMI) shielding applications, and thermal management of personal or electronic devices.

UiO-66 uniformly assembled foldable polyimide films with high transmittance and excellent ultraviolet resistance

As a protective layer for the flexible solar cell, a flexible protective material should be of high transmittance, excellent ultraviolet (UV) resistance and mechanical property. Herein, a foldable transparent polyimide (PI) composite film assembled with the zirconium-based metal-organic frameworks (UiO-66-X, X = H, NH2, NO2) is prepared by the direct dispersion or in-situ growth methods. The results show that the transmittances of UiO-66-X@PI films (U@PI, NU@PI and OU@PI) prepared with the in-situ growth are higher than those of UiO-66-X/PI films prepared with the direct dispersion, and the former's mechanical properties are also better than those of the latter. This may be because that the UiO-66-X in the former are embedded into the PI matrix to construct multipoint bonding network. The U@PI exhibits stronger UV-resistance than the Pristine PI, NU@PI and OU@PI after 5000 equivalent sun hours (ESHs) UV irradiation. The attenuations of average transmittance and tensile strength for U@PI are 2.19 % and 18.68 %, respectively, which are only one half and one-third of those for the Pristine PI (4.59 % and 56.58 %), respectively. And the average transmittance of U@PI has been over that of the Pristine PI after 5000 ESHs UV irradiation. This may be because that the porous UiO-66 increases the UV transmission path and the electron-hole recombination center, thereby inhibits the destructive effect of UV.

Preparation of MoS2@PDA-Modified Polyimide Films with High Mechanical Performance and Improved Electrical Insulation.

Polyimide (PI) has been widely used in cable insulation, thermal insulation, wind power protection, and other fields due to its high chemical stability and excellent electrical insulation and mechanical properties. In this research, a modified PI composite film (MoS2@PDA/PI) was obtained by using polydopamine (PDA)-coated molybdenum disulfide (MoS2) as a filler. The low interlayer friction characteristics and high elastic modulus of MoS2 provide a theoretical basis for enhancing the flexible mechanical properties of the PI matrix. The formation of a cross-linking structure between a large number of active sites on the surface of the PDA and the PI molecular chain can effectively enhance the breakdown field strength of the film. Consequently, the tensile strength of the final sample MoS2@PDA/PI film increased by 44.7% in comparison with pure PI film, and the breakdown voltage strength reached 1.23 times that of the original film. It can be seen that the strategy of utilizing two-dimensional (2D) MoS2@PDA nanosheets filled with PI provides a new modification idea to enhance the mechanical and electrical insulation properties of PI films.

Enhanced Mechanical and Thermal Properties of Polyimide Films Using Hydrophobic Fumed Silica Fillers.

Polyimide (PI) composite films with enhanced mechanical properties were prepared by incorporating modified fumed silica (FS) particles while preserving their optical and thermal characteristics. The PI matrix was synthesized using a fluorinated diamine, a fluorinated dianhydride, and a rigid biphenyl dianhydride via chemical imidization. Commercially available FS particles, including unmodified FS particles (0-FS) and particles modified with dimethyl (2-FS), trimethyl (3-FS), octyl (8-FS), octamethylcyclotetrasiloxane (D4-FS), and polydimethylsiloxane (PDMS-FS) were used. Scanning electron microscope images and nitrogen adsorption-desorption isotherms revealed well-defined porous structures in the FS particles. The water contact angles on the composite films increased compared to those of the pristine PI films, indicating improved water resistance. The PI/0-FS films exhibited a typical trade-off relationship between tensile modulus and elongation at break, as observed in conventional composites. Owing to the poor compatibility and agglomeration of the PDMS-FS particles, the PI/PDMS-FS composite films exhibited poor mechanical performance and diminished optical characteristics. Although the longer-chained FS particles (8- and D4-FS) improved the tensile modulus of the PI film by up to 12%, a reduction of more than 20% in toughness was observed. The PI composite films containing the methylated FS particles (2- and 3-FS) outperformed 8- and D4-FS in terms of mechanical properties, with PI/3-FS films showing an over 10% increased tensile modulus (from 4.07 to 4.42 GPa) and 15% improved toughness (from 6.97 to 8.04 MJ/m3) at 7 wt. % silica loading. Except for the PI/PDMS-FS composites, all composite film samples exhibited more than 86% transmittance at 550 nm. Regarding thermal properties, the glass transition temperature (Tg) and thermal stability remained stable for most composite films. In addition, PI/3-FS films demonstrated enhanced dimensional stability with lower coefficients of thermal expansion (from 47.3 to 34.5 ppm/°C). Overall, this study highlights the potential of incorporating specific modified FS particles to tailor the mechanical, optical, and thermal properties of PI composite films.

Research on the heat resistance and electrical properties of fluorinated polyimide/nano-Al2O3/nano-Si3N4 composite materials

Polyimide (PI) refers to a polymer containing an imide ring characteristic structure on the main chain. It has excellent thermal stability, insulation properties and other characteristics. The Tesla transformer is the core component of the primary pulse source of the high-current electron beam accelerator. It is also an essential component of the energy storage pulse power supply. Therefore, it has broad application prospects in the fields of national defense and civil industry. If the insulation performance of the turn-to-turn coils of the Tesla transformer does not meet the standards, it will break down during high-voltage corona discharge, causing transformer failure. Therefore, it is crucial to improve the insulation performance of Tesla transformer inter-turn wrapping materials. The incorporation of fluorine-containing groups has proven to be a transformative strategy that significantly enhances the physical, chemical, optical, and electrical properties of PI films. In addition, numerous studies have demonstrated that introducing inorganic fillers into the polyimide matrix can substantially enhance the insulating properties of polyimide films. This paper specifically concentrates on elevating the insulating and thermal attributes of polyimide. Initially, a range of polyimide films was synthesized for performance assessment by employing various fluorinated dianhydride and diamine components, utilizing them as substrates. Then, we used nano-silicon nitride (nano-Si3N4) and nano-alumina (nano-Al2O3) as doping phases to prepare PI composite films, which reduced the dielectric constant and increased the breakdown strength.

Carbon nitride/polyimide porous film via an NIPS method with advanced dielectric and hydrophobicity properties.

Herein, an ultra-low dielectric porous polyimide (PPI) composite film was fabricated by non-solvent induced phase separation (NIPS). High-performance carbon nitride nanosheets grafted by heptadecafluoro-1,1,2,2-tetradecyl-trimethoxysilane (CNNF) were incorporated into the PPI film to enhance thermomechanical and hydrophobic properties. The effects of non-solvent and filler content on the porous morphology, dielectric properties, hydrophobicity and thermomechanical properties of films were investigated. The porous morphology of the CNNF/PPI film changed from the coexistence of pipe-like and spongy structure via H2O, to a tightly-stacked porous structure via MeOH as non-solvent. The dielectric constants ε' of 0.5 wt%-CNNF/PPI(H2O) and 0.5 wt%-CNNF/PPI(MeOH) were 1.56 and 1.69 at 1 MHz, respectively, which were ∼50% lower than that of the original PI film (ε' = 3.33). With the introduction of CNNF, the water contact angle (WCA) of CNNF/PPI(H2O) increased from 66° to 107° and that of CNNF/PPI(MeOH) increased from 92° to 120°. Simultaneously, the storage modulus E' of 2 wt%-CNNF/PPI(MeOH) reached its highest value of ∼881 MPa, which was ∼350 MPa higher than that of PPI(MeOH), together with an enhancement in Tg. This method confirmed a promising prospect for the utilization of porous PI substrates in integrated circuits and microelectronic devices.

Dual-Effect Coupling for Superior Dielectric and Thermal Conductivity of Polyimide Composite Films Featuring "Crystal-Like Phase" Structure.

To match the increasing miniaturization and integration of electronic devices, higher requirements are put on the dielectric and thermal properties of the dielectrics to overcome the problems of delayed signal transmission and heat accumulation. Here, a 3D porous thermal conductivity network is successfully constructed inside the polyimide (PI) matrix by the combination of ionic liquids (IL) and calcium fluoride (CaF2 ) nanofillers, motivated by the bubble-hole forming orientation force. Benefiting from the 3D thermal network formed by IL as a porogenic template and "crystal-like phase" structures induced by CaF2 - polyamide acid charge transfer, IL-10 vol% CaF2 /PI porous film exhibits a low permittivity of 2.14 and a thermal conductivity of 7.22 W m-1 K-1 . This design strategy breaks the bottleneck that low permittivity and high thermal conductivity in microelectronic systems are difficult to be jointly controlled, and provides a feasible solution for the development of intelligent microelectronics.

Preparation and characterization of colorless and transparent semi‐alicyclic fluoro‐containing polyimide nanocomposite films with enhanced high‐temperature dimensional stability via incorporation of colloidal silica nanofillers

AbstractColorless and transparent semi‐alicyclic fluoro‐containing polyimide (PI) nanocomposite films with enhanced high‐temperature dimensional stability and sustained optical transparency were designed and prepared via incorporation of nano‐sized colloidal silica (SiO2) fillers into the PI matrix derived from hydrogenated pyromellitic dianhydride (HPMDA) and 2,2′‐bis[(4‐aminophenoxy)phenyl]hexafluoropropane (BDAF). The derived PI composite films, abbreviated as 6FCPI‐X in which X stands for the weight percent of the SiO2 in the composite films, exhibited superior thermal and comparable optical properties to the pristine 6FCPI‐0 (6FDA‐BDAF) matrix film. For example, as for the thermal properties, 6FCPI‐35 composite film showed a glass transition temperature (Tg) of 279.1°C and a residual weight ratio at 750°C (Rw750) of 66.1%, which were apparently higher than those of 6FCPI‐0 matrix (Tg = 272.2°C; Rw750 = 47.1%). More importantly, the 6FCPI/SiO2 nanocomposite films showed obviously lower linear coefficients of thermal expansion (CTE) values as compared with that of the 6FCPI‐0 matrix. 6FCPI‐35 composite film showed a CTE value of 36.1 × 10−6/K in the temperature range of 50–250°C, which was lower than half of that of the 6FCPI‐0 matrix (CTE = 77.9 × 10−6/K) in the same range of temperature. In addition, incorporation of nano‐sized colloidal SiO2 particles basically maintained the intrinsically good optical properties of the pristine 6FCPI‐0 matrix film. 6FCPI‐35 composite film showed the optical transmittances of 81.7%, 85.3%, and 87.2% at the wavelength of 400 nm (T400), 450 nm (T450), and 500 nm (T500), respectively, which were in the same level with those of the 6FCPI‐0 film (T400 = 84.0%; T450 = 86.3%; T500 = 87.6%). The 6FCPI series of nanocomposite films also exhibited similar CIE Lab color parameters, including the yellow indices (b*) lower than 2.0 and haze values lower than 1.0%.

Preparation and properties of mesoporous SiO2/polyimide composite films

AbstractAlthough polyimide films have a low dielectric constant, with the advent of the 5G era, the dielectric properties of the material are required to be further improved. Therein, we prepared controlled hollow mesoporous SiO2 nanomicrospheres by the hard template method to make mesoporous low dielectric polyimide films. The composite membrane exhibits better thermal stability, mechanical properties, and hydrophobicity compared to the pure membrane. Hollow mesoporous SiO2 dispersed in polyimide can reduce the inhomogeneity of charge distribution between molecules and reduce the interaction between polar chemical bonds and aromatic rings, thus improving the dielectric properties of the whole film. The minimum dielectric constant of the composite films was 2.80 when the hollow mesoporous SiO2 was added at 1.0%. These results fully indicate that the addition of hollow mesoporous SiO2 is beneficial to the multifaceted performance of the polyimide films.Highlights Preparation of hollow mesoporous SiO2 microspheres by using different polystyrene microsphere solutions and TEOS ratios. Synthesis of composite films with different contents of hollow mesoporous SiO2 as functional fillers. The effect of hollow mesoporous SiO2 content on the mechanical properties and thermal stability of PI composite films was investigated. Addition of hollow mesoporous SiO2 reduces the dielectric constant of polyimide composite films.

Mechanism analysis of PI/PEEK interface bonding properties enhanced by atmospheric plasma treatment: Experiment and DFT calculation

Due to the increase of service temperature, the bond strength of polyimide composite film will decrease significantly, the continuity of the lapped structure is broken. Therefore, this study proposed a double-layer structure consisting of a polyimide (PI) substrate and Polyetheretherketone (PEEK) coating. The PI/PEEK interface adhesion was improved by using atmospheric plasma treatment. The surface wettability, surface chemical structure, surface element analysis, surface morphology and surface roughness of PI films were studied, respectively. The Materials Studio (MS) was used to model the active groups' grafting mechanism after plasma treatment. The results shown that the surface free energy increased obviously from 27.65 mJ/m2 to 66.93 mJ/m2. Reactive groups containing oxygen, such as C = O, C–O, were generated on the surface of PI film due to etching and addition reactions during the plasma process. The mechanical properties of PI films had little change after plasma treatment. The inorganic particles on the surface of the PI film were exposed by plasma etching, which would increase the adhesion between the PI film and the PEEK coating. MS simulated the sequence, relative energy change, and energy barrier of the three functional groups, revealing that the reactivity sequence of the three groups was: –OH > –COOH > –CO–.