Low dielectric constant and highly intrinsic thermal conductivity fluorine‐containing epoxy resins with ordered liquid crystal structures

Epoxy resins with a high dielectric constant and low intrinsic thermal conductivity coefficient cannot meet the current application requirements of advanced electronic and electrical equipment. Therefore, novel fluorine‐containing liquid crystal epoxy compounds (TFSAEy) with fluorinated groups, biphenyl units, and flexible alkyl chains are first synthesized via amidation and esterification reactions. Then, 4,4′‐diaminodiphenylmethane (DDM) is used as a curing agent to prepare the corresponding fluorine‐containing liquid crystal epoxy resins. The obtained dielectric constant (ε) and dielectric loss (tan δ) values of TFSAEy/DDM at 1 MHz are 2.54 and 0.025, respectively, which are significantly lower than those of conventional epoxy resins (E‐51/DDM, 3.52 and 0.038). Additionally, the intrinsic thermal conductivity coefficient (λ) of TFSAEy/DDM is 0.36 W/(m·K), 71.4% higher than that of E‐51/DDM (0.21 W/(m·K)). Meanwhile, the corresponding elastic modulus, hardness, glass transition temperature, and heat resistance index of TFSAEy/DDM are 5.73 GPa, 0.35 GPa, 213.5°C, and 188.7°C, respectively, all superior to those of E‐51/DDM (3.68 GPa, 0.27 GPa, 107.2°C, and 174.8°C), presenting potential application in high‐heating electronic component packaging and printed circuit boards.


INTRODUCTION
2][3] As electronic information technology advances rapidly, especially in the context of high-frequency, high-power and high-density electronic components, the rapid buildup of heat within epoxy resins significantly undermines the reliability and stability of internal electronic components. 4,5However, conventional epoxy resins (e.g., E-51) have high dielectric constant (ε, 3.4-3.6)and low intrinsic thermal conductivity coefficient (λ, around 0.20 W/(m⋅K)), which are insufficient to meet the urgent application requirements of high frequency, high flux information transmission and high thermal conductivity. 6Under this background, the development of novel epoxy resins with low ε and high intrinsic λ values is instrumental in significantly enhancing the service life of related microelectronic products and reducing the ecological impact, also in favor of enhancing the sustainable development in the field of microelectronics. 7o our knowledge, the methods to reduce the ε of epoxy resins primarily involve physical blending and chemical structural modification.Physical blending modification is to directly incorporate inorganic nanofillers, such as the hollow microcapsules, 8 glass microbeads, 9 and porous ceramic particles, 10 into the epoxy matrix to obtain relatively lower ε.However, the nanofillers usually introduce new interfaces inside epoxy resins, against the reduction of ε value. 11Chemical structural modification is to synthesize and introduce the compounds with hollow structures (e.g., polysiloxane, 12,13 adamantane 14,15 ) to decrease the number of polarized molecules in the per unit volume of epoxy resins, and to incorporate lowpolarity molecules or groups (e.g., C-Si, 16 O-Si, 17 C-F 18 ) to reduce the molecular polarization of the epoxy curing network.Na et al. synthesized a kind of fluorine-containing epoxy monomer, 1,5-bis(4-fluorobenzoyl)−2,6-diglycidyl naphthalene (DGENF), followed by mixed with methyl hexahydro phthalic anhydride (MeHHPA) as a curing agent to prepare DGENF/MeHHPA cured resins.The ε of DGENF/MeHHPA cured resins was 2.97 (1 MHz), which was significantly lower than that of bisphenol A epoxy resin (BPA/MeHHPA, 3.87), bisphenol F epoxy resin (BPF/MeHHPA, 4.08) and 3,3′−5,5′-tetramethylbiphenyl diglyceryl ether epoxy resin (TMBPER/MeHHPA, 3.84). 19ong et al. synthesized two kinds of epoxy monomers containing cycloaliphatic hydrocarbons, 1,4-bis(4-(N,Ndiglycidylamine)phenoxy) cyclohexane (CyhEy) and 1,3-bis(4-(N,N-diglycidylamine) phenoxy) adamantane (AdaEy).The ε of AdaEy/MeHHPA cured resins was 3.39 (1 MHz), lower than that of CyhEy/MeHHPA (3.63), primarily due to the adamantane structure with low molecular polarizability. 20However, the introduction of hollow-structure inner epoxy resins does not favor the close packing of molecular chains and efficient phonon transmission against enhancing the intrinsic thermal conductivity. 21here are two primary methods for enhancing the thermal conductivities of epoxy resins.One is to incorporate highly thermally conductive fillers into the epoxy matrix, while the other is to reduce phonon scattering within the epoxy curing networks by increasing microordering degree. 22,23Liu et al. prepared boron nitride nanosheet (BNNS)/epoxy resin composites by extrusionhot pressing forming method.When the amount of BNNS was 40 wt%, the λ of BNNS/epoxy composites reached 5.86 W/(m⋅K), which was significantly higher than that of pure epoxy resin (0.21 W/(m⋅K)) and simply blended BNNS/epoxy composites (3.03 W/(m⋅K)) with the same amount of BNNS. 24Wu et al. used 4,4′-bis(4-hydroxybenzyl) diaminophenyl diglycidyl ether (SBPDAE), cationic initiator (BMIM), and spherical BN (s-BN) fillers to prepare the corresponding s-BN/SBPDAE composites.When the amount of s-BN was 60 vol%, the λ of s-BN/SBPDAE composites was 9.36 W/(m⋅K), 45.8 times higher than that of pure epoxy resin (0.20 W/(m⋅K)). 25owever, the introduction of excessive thermally conductive fillers would lead to high interfacial thermal resistance, resulting in severe phonon scattering, which is not conducive to improving the λ value. 26Additionally, due to relatively high ε value (>4.0) of the thermally conductive fillers, the final ε values of the epoxy composites are often high.Recent researches have indicated that the dependence of final λ for polymer composites on the intrinsic λ of polymer matrix is much larger than that of the λ of thermally conductive fillers. 27,280][31] Tokushige et al. synthesized two kinds of aromatic ester-type liquid crystal epoxy monomers (nematic phase and smectic phase, respectively) via cationic polymerization.The λ values of the aromatic ester-type liquid crystal epoxy resins were 0.31 and 0.43 W/(m⋅K), respectively, higher than that of conventional epoxy resins (0.20 W/(m⋅K)). 32Giang et al.
F I G U R E 1 1 H nuclear magnetic resonance (NMR) (A and C) and 13 C NMR (B and D) spectra of TFSA, TFGA, TFSAEy, and TFGAEy.synthesized three kinds of methylene amine liquid crystal epoxy monomers (LCE-DPE, LCE-DP, and LCE-TA) and prepared the corresponding epoxy resins using diaminodiphenyl sulfone (DDS) as a curing agent.Herein, LCE-TA/DDS exhibited the highest λ value of 0.45 W/(m⋅K). 33n our previous work, Gu et al. synthesized fluorinecontaining liquid crystal compounds (LCFEs) to prepare the corresponding modified BPA-type cyanate (LCFE-BADCy) resin.When the mass fraction of LCFE was 10 wt%, the ε of LCFE-BADCy resin was 2.61, lower than that of pure BADCy resin (3.06), and the corresponding λ was 0.33 W/(m⋅K), which was also higher than that of pure BADCy resin (0.25 W/(m⋅K)). 34n this work, fluorine-containing liquid crystal epoxy compounds with fluorinated groups, biphenyl units and flexible alkyl chains, are synthesized via amidation and esterification reactions.Then, 4,4′diaminodiphenylmethane (DDM) is used as a curing agent to prepare the corresponding fluorine-containing liquid crystal epoxy resins.The molecular structures of different fluorine-containing liquid crystal compounds are characterized by Fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS).The liquid crystal weaving, liquid crystal behavior and curing behavior of the fluorine-containing liquid crystal epoxy resins are characterized by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WXRD), in situ X-ray diffraction (XRD), small-angle X-ray diffraction (SAXD) and polarization microscopy (POM).In addition, the effects of molecular structures for fluorinecontaining liquid crystal compounds on the dielectric and insulation properties, thermal conductivities, mechanical properties, and heat and wear resistances of the cured epoxy resins are investigated in detail.

Molecular structures of fluorine-containing liquid crystal compounds
Figure 1 shows 1 H NMR and 13 C NMR spectra of the fluorinated liquid crystal curing agents (TFSA and TFGA) and the corresponding fluorinated liquid crystal epoxy monomers (TFSAEy and TFGAEy, Scheme 1).Chemical shifts at 7.21, 7.68, and 8.10 ppm in TFSA (Figure 2A) correspond to the protons on biphenyl structures.Signals at 2.52, 10.36, and 12.11 ppm correspond to the protons on the -CH 2 -of flexible alkyl chains, amido groups, and terminal carboxyl groups, respectively.And the ratio of the integration for these peaks (a:b:c:d:e:f+g) is 1.06:1.00:1.00:1.01:1.04:4.10,consistent with the chemical formula of TFSA.In comparison to TFSA, the proton peak of -CH 2 -of TFSAEy shifts to 2.74 ppm, which can be attributed to the strong electron-absorbing ability of the oxygen atom in the ether bond (stronger electronegativity of O atom than that of Br atom). 35The significant increase in intensity of the above peak is due to the coincidence of absorption peak of protons in the epoxy groups and the absorption peak on alkyl chain.Additionally, the ratio of the integration for these peaks (a:b+c+d:e+f+g+h+i) is 0.24:1.00:3.11,consistent with the chemical formula of TFSAEy.Compared with the 13 C NMR spectra of TFDB (Figure 1B), the carbon atoms on alkyl chains appear at 29.09 and 31.56 ppm, and the number of absorption peaks is consistent with the number of carbon atoms in TFSA.Compared with TFSA, the characteristic peaks of carbon atoms in the epoxy groups of TFSAEy appear at 44.13, 49.45, and 65.40 ppm, and the absorption peaks of carboxyl carbon atoms in the ester bonds at 177.17 ppm shift to the low field.This indicates that the epoxy groups are successfully integrated into the molecular structure of TFSAEy.
Similarly, peaks at 7.19, 7.71, and 8.08 ppm are attributed to hydrogen atoms in three different chemical environments in the biphenyl structure of TFGA (Figure 1C).Signals at 1.74, 2.22, and 2.33 ppm correspond to the characteristic peaks of methylene on the flexible carbon chain, and peaks at 10.26 and 12.04 ppm correspond to the amide and carboxyl groups, respectively.The ratio of the integration for these peaks (a:b:c:d:e:f:g:h) is 1.04:1.01:1.00:1.02:1.00:2.09:2.09:2.14, consistent with the chemical formula of TFGA.Compared with TFGA, chemical shifts at 3.1-4.5 ppm correspond to the protons in epoxy groups of TFGAEy, and the peak of methylene connected with the carboxyl group moves to the low field.This indicates that the epoxy groups have been successfully integrated into the molecular structure of TFGAEy.The ratio of the integration for these peaks (a:b:c:d:e:f+g:h 1 +h 2 :i:j) is 0.86:1.02:1.00:1.04:2.05:4.13:1.80:0.93:1.87,consistent with the chemical formula of TFGAEy.Meanwhile, peaks at 20.7, 33.35, and 35.88 ppm (Figure 1D) are attributed to carbon atoms in methylene on the flexible carbon chain, and signals at 171.92 and 174.63 ppm can be attributed to carbon atoms in the amide and ending carboxyl groups of TFGA, respectively.After the esterification reaction, the characteristic peaks of epoxy groups appear at 44.27, 49.44, and 65.24 ppm, and the chemical shift of carbonyl groups in the formed ester groups moves to the low field, indicating the successful preparation of TFGAEy.
Figure 2A,B shows the FT-IR spectra of TFDB, TFSA, TFGA, TFSAEy, and TFGAEy.Compared with TFDB, TFSA, and TFGA show the stretching vibration peaks of methylidene (3000-3200 cm −1 ) and carbonyl groups (1723 cm −1 ).Compared with TFSA and TFGA, TFSAEy and TFGAEy have stretching vibration peaks of epoxy groups at 875 cm −1 , and the intensity of absorption peaks for carboxyl groups and amino groups at 3500 cm −1 is significantly reduced.In addition, the molecular ion peaks with mass-charge ratios (m/z) have the highest relative intensity (Figure 2C

Liquid crystal behaviors of fluorine-containing liquid crystal compounds
DSC curves of TFSA, TFGA, TFSAEy and TFGAEy are shown in Figure 3A-C, and the corresponding POM images are shown in Figure 3G-J.There are two endothermic peaks during the heating process of TFSA and TFGA.Before the first endothermic peak, the observed region is the deep yellow (Figure 3G1,G2,H1,H2), indicating that TFSA and TFGA are semi-crystalline compounds with anisotropy.With the increase in temperature, the observed region becomes bright and uniform (Figure 3G3-G5,H3-H5), indicating that TFSA and TFGA have fluidity and anisotropy, presenting typical characteristics of thermotropic liquid crystal compounds.After the appearance of the second endothermic peak, the color of observation area gradually disappears, and the field of view is finally completely transparent (Figure 3G6,H6), indicating that TFSA and TFGA are isotropic liquids.Therefore, it can be concluded that both TFSA and TFGA are thermotropic liquid crystal compounds.Among them, the DSC baselines of TFSA and TFGA in heating processing are slightly uneven due to the thermal history and other unfavorable factors.The first endothermic peak (peak temperature of crystal melting, T 1 ) corresponds to the transition from the crystal state to the liquid crystal state, and the second endothermic peak (peak temperature of clearing point, T 2 ) corresponds to the transition from the liquid crystal state to the isotropic liquid state.T 1 of TFSA and TFGA are 190.3 • C and 170.0 • C, and the corresponding T 2 are 195.5 • C and 172.3 • C, respectively.Compared with TFGA, TFSA has a relatively higher T 1 value and a larger yellow bright spot area, demonstrating the stronger crystallization ability of TFSA.The reason is that the length of flexible chain of TFGA is larger, the flexibility of molecular structure is higher, and the steric hindrance of π-π stacking of biphenyl units is increased, resulting in decreased crystallization capacity.Besides, there are also two exothermic peaks in the cooling process of TFSA and TFGA, corresponding to the two-phase transition processes from the amorphous liquid state to the liquid crystal intermediate state and from the liquid crystal intermediate state to the solid state, respectively.The above results indicate that both TFSA and TFGA are bidirectional liquid crystals.In contrast, there is only an exothermic peak at 196.0 • C during the heating process of TFSAEy.This is because the temperature of the liquid crystal phase transition for TFSAEy is higher than the reaction temperature between the amino groups and the epoxy groups, gradually generating the cross-linking curing reaction of TFSAEy during the heating process. 36Therefore, the observation region of TFSAEy under POM shows yellow bright spots (Figure 3I1-I3) before the exothermic peak.When the temperature is further increased, the color of the observed region gradually deepens, and the yellow bright spots do not disappear (Figure 3I4-I6).This can be attributed to the local ordered structure formed by π-π stacking of biphenyl units after the cross-linking reaction between the amide groups and epoxy groups of TFSAEy. 37Compared with TFSAEy, there are two endothermic peaks and one exothermic peak during the heating process of TFGAEy.This can be attributed to the phase transition (from the crystalline state to the liquid state and from the liquid state to the amorphous liquid state), and the curing reaction of amide groups and epoxy groups of TFGAEy.The peak temperatures of T 1 , T 2 , and the curing reaction of TFGAEy are 113.5 • C, 127.2 • C, and 177.5 • C, respectively.This is because the length of flexible alkyl chains for TFGAEy is longer than that of TFSAEy, in favor of decreasing the liquid crystal phase transition temperature.Meanwhile, there are yellow bright spots (Figure 3j1,j2), indicating that the local area has various anisotropies.With the increase in temperature, the compounds gradually melt and yellow bright spots appear (Figure 3J3), presenting characteristics of liquid crystal compounds.After the second endothermic peak appears, the yellow bright spots disappear (Figure 3J4).When the temperature is further increased to the temperature range of exothermal peak, the color of the droplets is gradually deepened (Figure 3J5,J6), mainly attributed to the more compact molecular structure in solid state. 38esides, the brightness of yellow bright spots of TFGAEy is significantly lower than that of TFSAEy, mainly attributed to the excessive length of flexible chains in TFGAEy, resulting in the reduction of the order degree. 39igure 3D shows the WXRD spectra of TFDB, TFSA, TFGA, TFSAEy, and TFGAEy at room temperature.TFDB shows a sharp diffraction peak, due to the typical crystal characteristics.After the introduction of flexible alkyl chains, TFSA, TFGA, TFSAEy, and TFGAEy also have the sharp diffraction peaks, mainly attributed that all of them are semi-crystalline compounds with anisotropy. 40eanwhile, the crystallinities of TFSA, TFGA, TFSAEy, and TFGAEy compounds fitted by JADE software are 58.3%, 40.3%, 37.5%, and 21.6%, respectively. 41,42Compared with TFSAEy, the peak width of TFGAEy increases significantly and the crystallinity of TFGAEy decreases, mainly attributed to the introduction of excessive flexible chains, resulting in influencing the tight stacking of biphenyl units to a certain extent.Besides, the 2θ values of diffraction peaks for TFSA, TFGA, TFSAEy, and TFGAEy are basically the same, indicating that the introduction of flexible alkyl chains and epoxy groups does not change the π-π stacking mode of biphenyl units.Specifically, TFSA, TFGA, TFSAEy and TFGAEy all have crystal surface spacings of 16.8 • (the crystal plane spacing [d] = 0.309 nm), 20.8 • (d = 0.252 nm), and 23.3 • (d = 0.226 nm), proving that the liquid crystal structure of above compounds is close to the smectic F-phase. 43On the other hand, the sharp diffraction peak at 2θ = 25.8 • (d = 0.206 nm) indicates the formation of π-π packing structure.The dislocation migration of the crystal surface forms the "side step" structure, which is beneficial to promoting the intermolecular π-π stacking of biphenyl units. 44eanwhile, the SAXD spectra of TFSA, TFGA, TFSAEy, and TFGAEy (Figure S1) show strong diffraction peaks at 2θ = 0.656 • (grain size of 15.64 nm), 2θ = 0.734 • (grain size of 13.98 nm), 2θ = 0.687 • (grain size of 14.93 nm), and 2θ = 6.736 • (grain size of 1.524 nm), respectively, further confirming the formation of long-range ordered biphenyl stacking structures with large grains. 45Compared with TFSA, the excess flexible alkyl chains in TFGA have a larger steric hindrance, which would affect the microscopic arrangement of biphenyl crystalline elements and the formation of long-range ordered region, resulting in the significant decrease in grain size from 15.64 to 13.98 nm.Compared to TFSA and TFGA, the flexibility of molecular structures for TFSAEy and TFGAEy are further increased, resulting in a decrease in the long-range ordered structure.Compared with TFSAEy, the proportion of flexible chains in TFGAEy is too large, and the grain size significantly decreases from 14.93 to 1.54 nm, resulting in a shift from the long-range ordered structure to short-range ordered structure. 46Compared with TFSA, TFGA, and TFSAEy, the continuity of yellow bright spots for TFGAEy (Figure 3J) is significantly reduced, and the brightness of corresponding spots is also descended.
Besides, the sharp diffraction peaks of TFSA and TFGA gradually weaken and disappear with increasing temperature, and the disappearing temperature of the diffraction peaks is consistent with characterization results of DSC and POM.In contrast, sharp diffraction peaks of TFSAEy exist with increasing temperature, indicating that the ordered structure does not disappear, mainly attributed to the cross-linking curing reaction between the amide groups and epoxy groups.With the increasing temperature, the diffraction peaks of TFGAEy gradually disappear, indicating that TFGAEy changes from the anisotropic crystal state to the liquid crystal intermediate state and then to the isotropic liquid state.DSC, POM, WXRD, SAXD, and in situ XRD analyses indicate that TFSA, TFGA, TFGAEy are the typical thermotropic liquid crystal compounds, and the  The reason is that the hydrogen bonds in TFSA and TFGA make the melting points significantly higher than that of DDM, and the carboxyl and amide groups in TFSA and TFGA have lower reactivity than that amino groups in DDM. 47Similarly, the peak temperature of the curing reaction for TFSAEy/DDM and TFGAEy/DDM is also higher than that of E-51/DDM, mainly attributed to the epoxy groups in TFSAEy and TFGAEy are constrained by biphenyl units, resulting in lower reactivity than epoxy groups in E-51. 48In comparison, the peak temperatures of curing reaction for E-51/TFGA and TFGAEy/DDM are higher than those of E-51/TFSA and TFSAEy/DDM.This is because the lengths of flexible chains in TFGA and TFGAEy are larger, which can enhance the molecular thermal motion and further reduce the effective collision probability between the reaction groups, so that the curing reaction needs to be carried out at relatively higher temperature.Additionally, it is necessary to cure the resins in the range of liquid crystal phase transition temperatures to retain the ordered structure formed by the liquid crystal state. 49Therefore, the curing temperatures of E-51/TFSA, E-51/TFGA, TFSAEy/DDM, and TFGAEy/DDM are selected to be 190 • C, 170 • C, 190 • C, and 180 • C, respectively.Figure 4B shows the FT-IR spectra of E-51/DDM, E-51/TFSA, E-51/TFGA, TFSAEy/DDM, and TFGAEy/DDM cured at the selected temperature after 6 h.Peaks at 1699-1724 cm −1 can be attributed to the stretching vibration of amide and carbonyl groups in the curing networks.Besides, the carbonyl peaks in TFSA/DDM and TFGA/DDM shift from 1718 and 1711 cm −1 of the carboxyl groups to 1724 and 1725 cm −1 of the formed ester groups, respectively, indicating that the active hydrogen in the terminal carboxyl groups of TFSA and TFGA have reacted with the epoxy group in E-51. 50Simultaneously, the characteristic peak of epoxy groups at 875 cm −1 completely disappears, which indicates their full curing.

Dielectric properties of fluorine-containing liquid crystal epoxy resins
Compared with E-51/DDM, ε and tan δ of E-51/TFSA, E-51/TFGA, TFSAEy/DDM, and TFGAEy/DDM are significantly decreased (Figure 5A-D) at the same frequency (1 MHz).TFSAEy/DDM obtains the lowest ε and tan δ of 2.54 and 0.025, respectively, which are significantly lower than those of E-51/DDM (3.52 and 0.038), E-51/TFSA (2.78 and 0.032), E-51/TFGA (2.86 and 0.036), and TFGAEy/DDM (2.66 and 0.030).This is because the molecular structures of TFSA, TFGA, TFSAEy and TFGAEy contain the low polarizability -CF 3 groups, which can decrease the electronic and atomic polarizability under the external electric field. 51Additionally, the introduction of biphenyl units in TFSA, TFGA, TFSAEy, and TFGAEy can increase the order degree of curing networks, which is beneficial to reducing the dipole polarization.Compared with TFSA, the length of flexible alkyl chain in TFGA is too large, which is not conducive to the ordered arrangement in the liquid crystal phase.The cross-linking point spacing of E-51/TFGA is larger than that of E-51/DDM, which would weaken the dipole binding effect of the cured epoxy networks, increasing ε and tan δ. 52 TFSAEy/DDM with a relatively higher content of biphenyl crystalline elements shows a relatively higher degree of micro-order (Figure 5D4), in favor of decreasing ε and tan δ.Compared with TFSAEy, the liquid crystal temperature (113.5 • C-127.2 • C) of TFGAEy does not match the temperature of the curing reaction between TFGAEy and DDM (160.2 • C-198.7 • C), which would destroy the ordered π-π stacking structure of biphenyl units by the strong molecular thermal motion, increasing ε and tan δ. 53 Besides, the corresponding ε and tan δ decrease with increasing frequency.This is because the electron, atomic and orientation polarization can keep up with changes in the external electric field at low frequencies.With the increasing frequency, only electron and atomic polarization with short relaxation time can keep up with the change in the electric field, to decrease ε. 54 Meanwhile, ε and tan δ of E-51/DDM, E-51/TFSA, E-51/TFGA, TFSAEy/DDM, and TFGAEy/DDM gradually increase with the increasing ambient temperature, mainly attributed to the stronger thermal motion of the internal molecules, resulting in an increase in ε and tan δ.Additionally, ε and tan δ of E-51/DDM, E-51/TFSA, and E-51/TFGA present sudden increase at the temperature range of 100 • C-120 • C, mainly attributed to the movement of polymer segments at the high temperature, resulting in a significant increase in internal molecular polarizability.In contrast, ε and tan δ of TFSAEy/DDM and TFGAEy/DDM have good temperature stability, mainly attributed to the high density of biphenyl units, increasing the rigidity of curing networks and the interaction of molecular chains. 55ompared with conventional epoxy resins and other modified systems in this work, TFSAEy/DDM cured resins with a higher degree of micro-order show the lowest ε and tan δ values.

Insulation properties of fluorine-containing liquid crystal epoxy resins
Figure 5E,F shows the insulation properties of the fluorinecontaining liquid crystal epoxy resins.TFSAEy/DDM possesses the highest breakdown strength and volume resistivity of 26.7 kV/mm and 5.3 × 10 15 Ω⋅cm, respectively, 11.7% and 10.4% higher than those of E-51/DDM (23.9 kV/mm and 4.8 × 10 15 Ω⋅cm) and higher than E-51/TFSA (24.5 kV/mm and 5.0 × 10 15 Ω⋅cm), E-51/TFGA (25.2 kV/mm and 4.7 × 10 15 Ω⋅cm), and TFGAEy/DDM (25.4 kV/mm and 5.1 × 10 15 Ω⋅cm).The large electronegativity of F atoms can weaken the carrier density in epoxy resins, which is conducive to the improvement of insulation properties.Additionally, the π-π interaction between biphenyl units can improve the carrier binding effect of epoxy curing networks, in favor of increasing the insulation properties. 56Compared with TFSA, TFGA increases the cross-linking point spacing of curing networks, which is not conducive to the close packing between the molecular chains, decreasing the breakdown strength and volume resistivity of E-51/TFGA.Compared with E-51/TFSA, TFSAEy/DDM with the higher density of biphenyl units can improve the rigidity of curing networks, which is beneficial to increasing the resistance of carriers' directional migration.As the liquid crystal temperature of TFGAEy is lower than the curing reaction temperature of TFGAEy/DDM, the crystalline structure of biphenyl units is destroyed by the molecular thermal motion, resulting in a reduction in the insulation properties of TFGAEy/DDM.Thus, TFSAEy/DDM cured resins also exhibit optimal insulation properties compared to those of conventional epoxy resins.

Intrinsic thermal conductivity of fluorine-containing liquid crystal epoxy resins
The λ values of fluorine-containing liquid crystal epoxy resins are shown in Figure 6A.The λ values of E-51/TFSA, E-51/TFGA, TFSAEy/DDM, and TFGAEy/DDM are all higher than that of E-51/DDM.Among them, TFSAEy/DDM obtains the highest λ value of 0.36 W/(m⋅K), 71.4% higher than that of E-51/DDM (0.21 W/(m⋅K), also higher than E-51/TFSA (0.29 W/(m⋅K)), E-51/TFGA (0.27 W/(m⋅K)), and TFGAEy/DDM (0.30 W/(m⋅K)).This is because the introduction of biphenyl units can improve the ordering of curing networks, which is conducive to the efficient conduction of phonons to increase λ values. 57Compared with TFGA, TFSA with shorter flexible chains is conducive to the accumulation of biphenyl crystalline units, resulting in a higher estimated crystallinity and order degree of E-51/TFSA curing networks (Figure 4D2), which is conducive to the heat transfer along the molecular chains.Then, the stacking effect of the biphenyl units in TFSAEy/DDM is better than that of E-51/TFSA, which can further inhibit the random orientation of molecular chains, resulting in an increasing density of the curing networks (the free volume of TFSAEy/DDM cured resin is significantly lower than that of E-51/DDM and E-51/TFSA, Figure S2). 58On the one hand, the grain size (Figure S1) and estimated crystallinity (Figure 4D2) of TFGAEy are significantly smaller than those of TFSAEy, which is not conducive to the extension of phonon propagation path, resulting in a low λ value. 59On the other hand, the liquid crystal temperature of TFGAEy/DDM does not match its curing temperature, so there is no micro-ordered structure in the curing networks of TFGAEy/DDM, resulting in a decrease in the λ value.
Additionally, the λ values of E-51/DDM, E-51/TFSA, E-51/TFGA, TFSAEy/DDM, and TFGAEy/DDM are 0.23, 0.29, 0.26, 0.37, and 0.31 W/(m⋅K) at 125 • C (Figure 6B), respectively, slightly higher than the corresponding λ values at 25 • C.This is because the molecular thermal motion in epoxy resins gradually increases with the increasing temperature, which is conducive to enhancing the heat transfer efficiency.On the other hand, when the external ambient temperature further increases, the internal polymer segments begin to move to weaken the order of the curing networks, increasing the phonon scattering. 60verall, the λ values of the above cured resins vary little with the changing temperature, showing excellent temperature stability.
Figure 6C,D shows the temperature rising curves and the corresponding infrared thermal images of the fluorinecontaining liquid crystal epoxy resins under flat heat source heating.The surface temperatures of the epoxy resins all increase with the extension of heating time.At the same heating time, the surface temperature and heating rate of TFSAEy/DDM are the highest, and the surface temperature and heating rate of E-51/DDM are the lowest, which is consistent with λ values.Compared with E-51/DDM, the introduction of fluorine-containing groups and the crystalline ordered structure in TFSAEy would decrease the molecular polarizability of cured networks, and synchronously increase the phonon transport efficiency (Figure 6E), thus endowing the higher λ values.in Figure 7B.The elastic modulus and hardness of E-51/TFSA, E-51/TFGA, TFSAEy/DDM, and TFGAEy/DDM are higher than those of E-51/DDM.This is because the introduction of biphenyl units in TFSA, TFGA, TFSAEy, and TFGAEy increases the proportion of rigid structures in the curing networks, which is beneficial to improving the hardness and elastic modulus.TFSAEy/DDM obtains the highest elastic modulus and hardness, which are 5.73 GPa and 0.35 GPa, 55.3% and 27.5%, respectively, higher than those of E-51/DDM (3.69 and 0.27 GPa) and higher than those of E-51/TFSA (4.64 and 0.29 GPa), E-51/TFGA (4.58 and 0.26 GPa), and TFGAEy/DDM (5.22 and 0.32 GPa).This is because E-51/TFSA has a smaller cross-link distance and higher density of curing networks than that of E-51/TFGA, which is conducive to increasing the elastic modulus and hardness.Compared with E-51/TFSA, TFSAEy/DDM with a higher content of biphenyl units can be beneficial to improving the elastic modulus and hardness. 61As the curing networks of TFGAEy/DDM do not retain the ordered structures, the elastic modulus and hardness of TFGAEy/DDM are lower than those of TFSAEy/DDM.However, the large number of biphenyl units still increases the rigidity of curing networks for TFGAEy/DDM, endowing higher elasticity modulus and hardness than those of E-51/DDM.Besides, TFSAEy/DDM cured resins also show outstanding mechanical properties compared to those of conventional epoxy resins (E-51/DDM).

2.4.5
Heat resistances of fluorine-containing liquid crystal epoxy resins Figure 7C-E shows the TGA and DSC curves of the fluorine-containing liquid crystal epoxy resins.The endothermal peak (Figure 7E) of the cured resins (sample 0-4) is mainly attributed to the movement of chain segments in curing networks under the range of glass transition temperature (T g ).The corresponding heat resistance index 62 (T HRI , Table S1) and T g (peak temperature of the above endothermal peaks) are summarized in Figure 7F.).This is because the introduction of rigid biphenyl units and the formation of the ordered structures can limit the thermal movement of molecular chains, in favor of improving the heat resistance.Compared with TFSA, the flexible alkyl chain of TFGA is more prone to thermal degradation (Figure 7C), reducing the T g and T HRI of E-51/TFGA.Compared with E-51/TFSA, the content of biphenyl units in TFSAEy/DDM and TFGAEy/DDM is higher, which is beneficial to constraining the thermal motion of molecules, endowing higher T g and T HRI .Compared with TFSAEy, TFGAEy with a larger length of flexible chain would increase the cross-linking point spacing of curing networks, resulting in the decrease in T g and T HRI .TFSAEy/DDM cured resins also show excellent heat resistances compared to those of E-51/DDM resins.

2.4.6
Wear resistances of fluorine-containing liquid crystal epoxy resins Figure 8A-C shows the friction coefficient, wear volume and wear rate of the fluorine-containing liquid crystal epoxy resins, and the corresponding laser scanning confocal microscope images of the wearing area are shown in Figure 8D.Compared with E-51/DDM, the friction coefficient, wear volume and wear rate of E-51/TFSA, E-51/TFGA, TFSAEy/DDM, and TFGAEy/DDM are significantly reduced.Among them, TFSAEy/DDM has the lowest friction coefficient, wear volume, and wear rate of 0.470 N⋅m, 1.44 × 10 6 μm 3 , and 3.07 × 10 6 μm 3 /(N⋅m), 19.9%, 71.3%, and 64.0%lower than those of E-51/DDM (0.587 N⋅m, 5.01 × 10 6 μm 3 , and 8.54 × 10 6 μm 3 /(N⋅m)), also lower than E-51/TFSA (0.491 N⋅m, 2.61 × 10 6 μm 3 , and 5.31 × 10 6 μm 3 /(N⋅m)), E-51/TFGA (0.506 N⋅m, 3.11 × 10 6 μm 3 , and 6.14 × 10 6 μm 3 /(N⋅m)), and TFGAEy/DDM (0.485 N⋅m, 1.80 × 10 6 μm 3 , and 3.71 × 10 6 μm 3 /(N⋅m)).The wearing surface of E-51/DDM is relatively rough, and the surface is scattered with a large amount of abrasive dust (Figure S3a). 63And the surface of E-51/DDM has obvious melt marks, failing to form a relatively complete transfer layer.The wearing types of E-51/DDM are mainly fatigue wearing and adhesive wearing, and the corresponding friction mechanism can be seen in Figure S3f.In addition, due to the low hardness of E-51/DDM, abrasive dust accumulates in the middle of wearing area, resulting in the middle area of the three-dimensional image showing red (Figure 8D1).Compared with E-51/DDM, the introduction of biphenyl units in TFSA increases the interaction force of molecular chains and the rigidity of curing networks, which can improve the hardness of the E-51/TFSA, further enhancing the wear resistances.Under the same wearing conditions, the wearing area of E-51/TFSA is significantly reduced (Figure 8D2), and the friction surface becomes flat and smooth (Figure S3b).Moreover, fluorinated groups in TFSA can significantly decrease the surface energy (Figure 8E), thus reducing the friction coefficient between the E-51/TFSA and the GCr15 steel ball. 64Additionally, the decrease in water absorption is also conducive to improving the hydrothermal stability (Figure S4).Compared with TFSA, the flexible chain length of TFGA further increases to decrease the hardness of E-51/TFGA, which is beneficial to decreasing the resistance of friction loading.In comparison, the curing networks of TFSAEy/DDM with higher content of biphenyl units can improve the wearing resistances (Figure 8D4).Meanwhile, the formation of crystalline structures in TFSAEy/DDM can further inhibit the relative slip between molecular chains, which is beneficial to further enhancing the wear resistances (Figure S3d).Compared with TFSAEy, the introduction of excessive flexible chains in TFGAEy would decrease the cross-linking spacing of curing networks, resulting in relatively lower hardness and wear resistance.And the TFGAEy/DDM possesses a lower relative content of F atoms than TFSAEy/DDM, leading to a decrease in surface energy and an increase in the friction coefficient against the improvement of wear resistances.
Besides, TFSAEy/DDM has excellent flame retardance (Figure S5), and the corresponding limiting oxygen index (LOI) is 27.8%, 16.8% higher than that of E-51/DDM (23.8%), which is also higher than that of E-51/TFSA (26.7%),E-51/TFGA (25.3%), and TFGAEy/DDM (27.1%).This is because the molecular structure of TFSAEy contains a large number of fluorinated groups, which can be beneficial to shielding the oxygen transport by generating the hydrogen fluoride. 65And the π-π conjugation of biphenyl units and the formation of an ordered crystalline structure in TFSAEy/DDM can effectively prevent heat transfer during combustion, which is also beneficial to further improving flame-retardant properties. 66In conclusion, TFSAEy/DDM cured resins have the optimal comprehensive properties, such as a low ε value, highly intrinsic λ value, excellent mechanical properties, outstanding heat resistance, wear resistance, and flame retardancy, which would replace conventional epoxy resins in the high-heating electronic component packaging and printed circuit boards.

Syntheses of fluorine-containing liquid crystal epoxy compound
First, the carboxy-terminated fluorinated liquid crystal intermediates (TFSA and TFGA) were prepared from 2,2′bis(trifluoromethyl)−4,4′-diaminobiphenyl (TFDB) and anhydride monomers (succinic anhydride and glutaric anhydride).Then, the fluorine-containing liquid crystal epoxy compounds (TFSAEy and TFGAEy) with different molecular structures were synthesized by esterizing the hydroxyl groups in glycidyl.The corresponding synthesis processes were described as follows (Scheme 1): TFDB (3.20 g, 10 mmol) and succinic anhydride (2.10 g, 21 mmol) were dissolved in 250 mL of EtOH and reacted at room temperature for 24 h.Then, EtOH was removed by a rotary evaporator.The obtained solid was washed with deionized water at 70 • C five times to obtain the intermediate product of TFSA after vacuum drying.Next, TFSA (2.60 g, 5 mmol), Glycidol (GI, 1.17 g, 15 mmol), 1-(3dimethylaminopropyl)-3-ethylcarbodiimide (EDCI, 2.87 g, 15 mmol), and 4-dimethylaminopyridin (DMAP, 0.29 g, one-tenth of EDCI amount) were dissolved in 250 mL of DCM.The reaction system was naturally heated to room temperature and continued for 24 h.Then, DCM was removed by rotary evaporation, and the obtained solid mixtures were washed with deionized water to obtain the target product of TFSAEy after high-speed centrifugation (7000 rpm) and dried in a vacuum oven at 40 • C. The synthesis route of TFGA and TFGAEy was basically the same as that of TFSA and TFSAEy except that the succinic anhydride was replaced by glutaric anhydride.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

S C H E M E 1
Schematic diagram of the synthesis route for TFSA, TFGA, TFSAEy, and TFGAEy.F I G U R E 2 Fourier transform infrared (FT-IR) spectroscopy (A and B) and high-resolution mass spectrometry (HRMS) (C and D) spectra of TFSA, TFGA, TFSAEy, and TFGAEy.

Figure
Figure 4A shows the DSC curves of the fluorine-containing liquid crystal epoxy resins.The peak temperatures of curing reaction for E-51/DDM, E-51/TFSA, E-51/TFGA, TFSAEy/DDM, and TFGAEy/DDM are 141.1 • C, 171.3 • C, 196.4 • C, 183.1 • C, and 188.4 • C, respectively.Compared with E-51/DDM, the peak temperatures of curing reaction for E-51/TFSA and E-51/TFGA are significantly increased.The reason is that the hydrogen bonds in TFSA and TFGA make the melting points significantly higher than that of DDM, and the carboxyl and amide groups in TFSA and TFGA have lower reactivity than that amino groups Figure 4C,D shows WXRD spectra and POM images of the fluorine-containing liquid crystal epoxy resins.E-51/DDM shows a wide dispersion peak and a uniform transparent image (Figure 4D1), indicating an isotropic curing network.E-51/TFSA, E-51/TFGA and TFSAEy/DDM show the sharp diffraction peaks and yellow bright spots (Figure 4D2-D4), indicating the anisotropic biphenyl stacking structures in cross-linking networks.And the corresponding crystallinities of E-51/TFSA, E-51/TFGA, and TFSAEy/DDM are 21.3%, 13.2%, and 25.4%, respectively, lower than those of TFSA, TFGA, and TFSAEy compounds.This is because both E-51 and DDM are the amorphous compounds, decreasing the order degree of curing network structures for epoxy resins.The flexible chain length of TFSA is shorter than that of TFGA, resulting in stronger crystal diffraction peak intensity and yellow bright spots and relatively higher crystallinity.Compared with E-51/TFSA, TFSAEy/DDM has a higher stack density of biphenyl units, enhancing the order degree of curing network and resulting in higher distribution density of yellow bright spots and crystallinity.Compared with TFSAEy, the temperature of liquid crystal phase transition for TFGAEy is 113.5 • C-127.2 • C, lower than the curing reaction temperature of TFGAEy/DDM (164.2 • C-198.7 • C).Due to the isotropic state of TFGAEy at the curing temperature, the WXRD spectra of TFGAEy/DDM present a wide dispersion peak, and the corresponding POM image does not show yellow bright spots (Figure 4D5).DSC and FT-IR analyses indicate that the E-51/TFSA, E-51/TFGA, TFSAEy/DDM, and TFGAEy/DDM resins have been cured completely, and WXRD and POM analyses confirm that the curing networks of E-51/TFSA, E-51/TFGA, and TFSAEy/DDM resins contain the ordered structure formed by the stacking of biphenyl elements.And the TFGAEy/DDM cured resins present an isotropic state.

Figure
Figure7Ashows the elastic modulus and hardness of the fluorine-containing liquid crystal epoxy resins, and the corresponding load-displacement curves are shown Figure7C-E shows the TGA and DSC curves of the fluorine-containing liquid crystal epoxy resins.The endothermal peak (Figure7E) of the cured resins (sample 0-4) is mainly attributed to the movement of chain segments in curing networks under the range of glass transition temperature (T g ).The corresponding heat resistance index62 (T HRI , TableS1) and T g (peak temperature of the above endothermal peaks) are summarized in Figure7F.T g and T HRI of E-51/TFSA, E-51/TFGA, TFSAEy/DDM, and TFGAEy/DDM are higher than those of E-51/DDM.Among them, TFSAEy/DDM has the highest T g and T HRI , 213.5 • C and 188.7 • C higher than those of E-51/DDM (107.2 • C and 174.8 • C), E-51/TFSA (114.0 • C and 166.4 • C), E-51/TFGA (122.8 • C and 162.4 • C), and E-51/TFGAEy (206.1 • C and 182.1 • C).This is because the introduction of rigid biphenyl units and the formation of the ordered structures can limit the thermal movement of molecular chains, in favor of improving the heat resistance.Compared with TFSA, the flexible alkyl chain of TFGA is more prone to thermal degradation (Figure7C), reducing the T g and T HRI of E-51/TFGA.Compared with E-51/TFSA, the content of biphenyl units in TFSAEy/DDM and TFGAEy/DDM is higher, which is beneficial to constraining the thermal motion of molecules, endowing higher T g and T HRI .Compared with TFSAEy, TFGAEy with a larger
DDM, TFSA, and TFGA were mixed with E-51 resins at the predetermined temperature (ratio of the molar fraction for active hydrogen in the carboxyl groups and the amino groups to epoxy groups of 1:1).Then, the above mixtures were poured into the preheated mold for 6 h to obtain E-51/DDM (sample 0, 150 • C), E-51/TFSA (sample 1, 190 • C), and E-51/TFGA (sample 2, 170 • C) resins, and the corresponding schematic diagrams of curing reaction for E-51/DDM and E-51/TFSA are shown in Scheme 2A,B.Similarly, according to the same calculation method for the mass fraction of epoxy resins and curing agent, TFSAEy/DDM (sample 3) and TFGAEy/DDM (sample 4) were obtained after curing at 190 • C and 180 • C for 6 h with the same process, and a schematic diagram of curing reaction for TFSAEy/DDM is shown in Scheme 2C.A C K N O W L E D G M E N T S The authors are grateful for the support and funding from the Foundation of National Natural Science Foundation of China (51973173) and the Fundamental Research Funds for the Central Universities.X.R. Fan thanked the Undergraduate Innovation and Business Program in Northwestern Polytechnical University.Z. Liu thanked the Innovation Foundation for Doctor's Dissertation of North-western Polytechnical University (CX2023026).This work is also financially supported by the Polymer Electromagnetic Functional Materials Innovation Team of Shaanxi Sanqin Scholars.We would like to thank the Analytical and Testing Center of Northwestern Polytechnical University for SEM, XRD, and TGA tests.
and TFGAEy) were successfully synthesized.Compared with those of E-51/TFSA, E-51/TFGA, and TFGAEy/DDM, TFSAEy/DDM has optimal comprehensive properties, such as low ε, highly intrinsic λ, excellent mechanical properties, and heat and wear resistance.At 1 MHz, ε and tan δ values of TFSAEy/DDM are 2.54 and 0.025, respectively, which are significantly lower than those of E-51/DDM.