Enhanced Thermal Conductivity of Liquid Crystalline Epoxy Resin

Sep 14, 2018 - A powerful strategy to enhance the thermal conductivity of liquid crystalline epoxy resin (LCER) by simply replacing the conventional a...
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Letter Cite This: ACS Macro Lett. 2018, 7, 1180−1185

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Enhanced Thermal Conductivity of Liquid Crystalline Epoxy Resin using Controlled Linear Polymerization Akherul Md. Islam,†,§ Hongjin Lim,† Nam-Ho You,‡ Seokhoon Ahn,† Munju Goh,‡ Jae Ryang Hahn,*,§ Hyeonuk Yeo,*,∥ and Se Gyu Jang*,†

ACS Macro Lett. Downloaded from pubs.acs.org by WASHINGTON UNIV on 09/14/18. For personal use only.

†

Functional Composite Materials Research Center and ‡Carbon Composite Materials Research Center, Institute of Advanced Composites Materials, Korea Institute of Science and Technology, Wanju, Jeonbuk 55324, Republic of Korea § Department of Chemistry and Department of Bioactive Material Sciences and Research Institute of Physics and Chemistry, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea ∥ Department of Chemistry Education, Kyungpook National University, Daegu 41566, Republic of Korea S Supporting Information *

ABSTRACT: A powerful strategy to enhance the thermal conductivity of liquid crystalline epoxy resin (LCER) by simply replacing the conventional amine cross-linker with a cationic initiator was developed. The cationic initiator linearly wove the epoxy groups tethered on the microscopically aligned liquid crystal mesogens, resulting in freezing of the ordered LC microstructures even after curing. Owing to the reduced phonon scattering during heat transport through the ordered LC structure, a dramatic improvement in the thermal conductivity of neat cation-cured LCER was achieved to give a value ∼141% (i.e., 0.48 W/mK) higher than that of the amorphous amine-cured LCER. In addition, at the same composite volume fraction in the presence of a 2-D boron nitride filler, an approximately 130% higher thermal conductivity (maximum ∼23 W/mK at 60 vol %) was observed. The nanoarchitecture effect of the ordered LCER on the thermal conductivity was then examined by a systematic investigation using differential scanning calorimetry, polarized optical microscopy, X-ray diffraction, and thermal conductivity measurements. The linear polymerization of LCER can therefore be considered a practical strategy to enable the cost-efficient mass production of heat-dissipating materials, due to its high efficiency and simple process without the requirement for complex equipment. composites with fillers and also to achieve the same level of thermal conductivity with less quantities of fillers. Liquid crystalline epoxy resin (LCER) has been spotlighted as a promising candidate for an efficient thermally conducting matrix due to the molecular alignment of mesogens in its structure, which significantly enhances the mechanical and physical properties, such as the phonon-based thermal transport efficiency.8−17 Interestingly, it has been found that the thermal conductivities of such resins are highly dependent on their chemical structures, domain sizes, and the aligned molecular directions of the LCERs, which are directly related to phonon scattering.8,18,19 However, the well-ordered mesogen structure in LCERs, which is generally a nematic or smectic phase, can be distorted by cross-linking with conventional diamine hardeners due to the formation of a restricted bond angle between the amine and epoxy moieties (i.e., ∼108°). Although the high

Due to the generation of high heat densities by miniaturized devices, the service lifetimes of highly integrated circuits have been shortened. This has led to an increasing demand for efficient heat-dissipating materials for application in high-power electronic devices and their related applications.1,2 In this context, polymeric composites with high thermally conducting inorganic fillers have been considered as a promising solution for heat sinks and thermal interface materials due to their good insulating properties, lightweight nature, moldability, flexibility, and mechanical durability.3−6 However, these advantages are often lost upon the addition of fillers with high volume contents, thereby leading to processing difficulties and poor mechanical properties. Furthermore, recent studies into the thermal conductivities of composite materials have revealed that the thermal conductivity enhancement of composites is marginal even at high filler contents when the thermal conductivity of the filler is significantly higher than the one of matrix (i.e., >100×).6,7 Therefore, the development of polymeric resins with thermal conductivity of higher than 2 W/mK, which is 1/ 100 of thermal conductivity of typical fillers ranging from 30 to 200 W/mK, is desirable to maximize the thermal conductivity of © XXXX American Chemical Society

Received: June 20, 2018 Accepted: September 5, 2018

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DOI: 10.1021/acsmacrolett.8b00456 ACS Macro Lett. 2018, 7, 1180−1185

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ACS Macro Letters

Scheme 1. Schematic Illustration of the Different Microstructures of Liquid Crystalline Epoxy Resin (LCER) Obtained Using the Two Curing Methodsa

a

In contrast to the use of amine hardeners, which disturb the molecular alignment of the mesogens, linear polymerization of the epoxy groups using a cationic initiator resulted in a resin cured with minimum disturbance to the prealigned mesogens, thereby giving in a higher thermal conductivity.

conductivity measurements. The thermal conductivities of the BPH-cured LCER will be compared to that of the DDS-cured LCER, and the influence of 2-dimensional (2-D) hexagonal boron nitride (h-BN) platelets will be examined.25 Figure 1 showed the chemical structures of the LCER and the cationic initiator employed herein. More specifically, the LCER is a bifunctional epoxy resin composed of a biphenyl (BP) mesogen moiety, namely, 4,4′-diglycidyloxybiphenyl. The epoxy equivalent weight (EEW) of the synthesized LCER was determined by 1H NMR spectroscopy to be ∼190 g/equiv, which reflects the fact that the LCER is a mixture of monomers and oligomers. Although the liquid crystallinity of BP-based LCERs has been a topic of debate,11,26 the synthesized resin showed a clear birefringence under POM characterization at 180 °C, as indicated in Figure 1b,c. Combination of the POM and DSC results indicated that the LC-forming temperature window of our LCER was between 154 and 190 °C (Figure S1 in Supporting Information). Thus, to achieve successful curing of the LCER within the LC-forming temperature window, a latent thermal cationic initiator (BPH, Figure 1d) with an initiation temperature of ∼155 °C was selected to initiate the linear polymerization process. Following initiation, the benzyl cation generated thermally from BPH rapidly cured the LCER through linear and interchain polymerization of the epoxy groups. For a detailed investigation of the thermal curing behavior, the LCERs cured using varying quantities of BPH (i.e., 1−5 wt %) were characterized by DSC and compared to the LCER cured using DDS (Figure 2a). In contrast to the DDS-cured LCER, which exhibited an endothermic peak centered at 140 °C due to the melting of DDS, the BPH-cured LCER gave an endothermic peak at ∼157 °C, which constitutes a similar thermal behavior to the neat LCER. In addition, curing of the LCER by BPH was initiated within the LC-forming temperature window range, as observed by an exothermic reaction beginning at ∼170 °C, and the reaction was allowed to continue until the temperature reached 250 °C. Interestingly, regardless of the BPH quantity employed, the cumulative reaction heats from the dynamic DSC curves were similar (i.e., ∼397 J/g), but slightly higher than that

thermal conductivity of LC-based resins has been demonstrated via cross-linking under a strong magnetic field to align the molecules in a uniaxial direction,14,16,20,21 the high operational costs, complex equipment setup, and limited processing scale hinder the application of this method in mass production. A simple and cost-efficient protocol that retains the molecular alignment of mesogens even following curing is therefore required. Thus, we herein demonstrate a simple strategy for the curing of LCER with minimum disturbance to the liquid crystalline molecular alignment. For this purpose, we propose a linear polymerization of the epoxy groups, which we expect will lead to an enhanced thermal conductivity (Scheme 1). Even in the case where curing takes place at the temperature range of LC formation,22 the use of a conventional hardener with a rigid aromatic diamine structure (e.g., 4,4′-diaminodiphenylsulfone, 4,4′-DDS) should disturb the ordered mesogen structure, resulting in the formation of an amorphous molecular structure. In contrast, we expect that the ordered stacking of mesogens can be successfully frozen by simply altering the hardener from a diamine to a cationic initiator (i.e., N-benzyl pyrazinium hexafluoroantimonate, BPH).23,24 The use of a linear polymerization process in the presence of a cationic initiator is hoped to have several advantages over the use of conventional amine hardeners.23,24 For example, the aligned LC molecular structure could potentially be frozen by weaving the epoxy groups, which would lead to reduced phonon scattering. In addition, the initiation temperature, which is crucial for freezing the LCER in the temperature window of LC formation, could be easily adjusted by careful selection of the latent cationic initiators. Furthermore, rapid curing during the linear polymerization process would be expected to shorten the processing time required for composite fabrication, which is essential in the context of mass production. We will also conduct a systematic study into the relationship between the enhanced thermal conductivity and the cross-linked LCER microstructure using differential scanning calorimetry (DSC), polarized optical microscopy (POM), X-ray diffraction (XRD), and thermal 1181

DOI: 10.1021/acsmacrolett.8b00456 ACS Macro Lett. 2018, 7, 1180−1185

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Figure 1. (a) Chemical structure of the LCER (4,4′-diglycidyloxybiphenyl) employed herein. Polarized optical microscopy (POM) images taken at 180 °C with magnifications of (b) ×10 and (c) ×50. Scale bars are 50 μm. (d) Schematic illustrations of the initiation and propagation steps of the linear polymerization process in the presence of the cationic initiator, N-benzyl pyrazine hexafluoroantimonate (BPH).

Figure 2. Comparison of (a) the dynamic thermal behaviors, and (b) the thermal conductivities of the LCERs cured by the conventional diamine (DDS) and varying quantities of the cationic initiator (BPH).

specimens (2 cm diameter, >3 mm height) and thermal conductivities of at least three pairs of specimens were measured for each composition. (see Supporting Information S4 for detailed theory related to measurements) In contrast to the similar thermal behaviors obtained for the LCER cured using various quantities of BPH, variations in the thermal conductivity were observed for BPH contents of 1, 2, 3, and 5 wt %, with average values of 0.43 ± 0.02, 0.47 ± 0.01, 0.48 ± 0.02, and 0.45 ± 0.03 W/mK being calculated, respectively (Figure 2b). In addition, compared to the thermal conductivity of the DDScured LCER (0.34 ± 0.02 W/mK), the conductivities of the BPH-cured LCER were enhanced up to 41%, which is significantly higher than the values reported for conventional thermosetting resins (i.e., ∼0.2 W/mK).6 This increase in the thermal conductivity was mainly attributed to the reduced phonon “structure scattering” due to harmonic molecular lattice vibrations through the ordered LC structure.27 To confirm the presence of an ordered LC structure in the cured resin, dynamic POM characterization using a microscope equipped with a heat stage was conducted between 150 and 250 °C with a temperature ramp of 50 °C/min. Thus, Figure 3 shows the POM images of the LCERs mixed with DDS (Figure 3a−d) and BPH (Figure 3e−h). During the temperature ramp, the birefringence of the LCER mixed with DDS decreased, finally disappearing at 250 °C due to a slow amine−epoxy reaction and disturbance of the LC structure by the diamine hardener. In contrast, the birefringence pattern remained relatively constant when the LCER was heated in the presence of BPH under equal

of the DDS-cured resin (i.e., ∼370 J/g). In addition, from the results of dynamic mechanical analysis (DMA), the Tg of the BPH-cured LCER was determined to be ∼165 °C, which is high compared to those of other epoxy resins. Furthermore, the crosslink density of the BPH-cured LCER (∼0.1 mol/cm3) calculated from the DMA data, revealing that the higher storage modulus at the rubbery plateau, was significantly higher than that of DDScured LCER (i.e., ∼0.01 mol/cm3, Supporting Information S2 and Figure S2). These observations confirm that the use of cationic polymerization to give the BP-based LCER produces an excellent thermosetting resin with good mechanical properties and a high cross-link density. To evaluate the effect of linear polymerization on the thermal conductivity of the LCER, coin-shaped (2 cm diameter, >3 mm height) neat LCER samples cured using BPH and DDS were prepared by sequential homogeneous mixing and hot pressing under 20 MPa at 170 °C.22 The curing temperature was carefully chosen based on the isothermal DSC results obtained between 155 and 230 °C and the corresponding calculated degrees of conversion (40 min at 170 °C for full curing, see discussion in Supporting Information S3 and Figures S3−S5). Thermal conductivities were measured using an isotropic mode of a Hot Disk TPS 2500 S model with #7531 or #7577 Kapton sensors (Hot Disk, Sweden) according to the ISO standard 22007−2. Briefly, the Kapton sensors were sandwiched by the coin-shaped 1182

DOI: 10.1021/acsmacrolett.8b00456 ACS Macro Lett. 2018, 7, 1180−1185

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Figure 3. Dynamic POM images taken at various temperatures between 150 and 250 °C during heating (ramp speed = 50 °C/min) of the LCERs mixed with (a−d) DDS and (e−h) BPH.

conditions. This indicates that the ordered LC structure is efficiently frozen via the cation-induced rapid polymerization of the epoxy groups. To further confirm the presence of an ordered microstructure in the cured LCER, both cured samples were characterized by XRD (Figure 4a). In contrast to the DDS-cured LCER, which gave a broad amorphous peak, the BPH-cured LCER exhibited sharp peaks at 2θ = 19, 20, 23.2, 27.7, and 28.4° superposed on a broad amorphous peak. These sharp peaks reveal the presence of regular intersegmental spacing (3.1 to 4.6 Å) between the mesogens of the cured LCER.28 Further XRD characterization of LCER (without hardener) was also carried out between 160 and 190 °C, as shown in Figure 4b. The dashed black lines indicating the sharp peak positions of the BPH-cured LCER in Figure 4a correspond with the LCER peaks observed between 160 and 190 °C, thereby revealing that the LC molecular structure was successfully frozen. Recently, Kawamoto et al. reported the formation of a spherulite-like crystal structure from the same BP-based LCER cured using 4,4′-diaminodiphenylmethane (DDM).29 The crystalline peaks observed in this case (2θ = 19, 22, and 27°) were also observed in the diffraction pattern of our LCER powder at 130 °C (i.e., below the phase transition temperature in Figure 4b).27,29 Importantly, the diffraction peaks from the BPH-cured LCER could be clearly distinguished from those of the spherulite-like crystals, meaning that the birefringence of the BPH-cured LCER originates from the LC rather than from the crystal (or soft-crystal) of the LCER. From these observations it is reasonable to conclude that the LC structure was successfully

frozen by linear polymerization, and the higher thermal conductivity of the BPH-cured LCER compared to the DDScured resin could be attributed mainly to the efficient weaving of the ordered LC structure, which in turn resulted in reduced phonon scattering through the resin. To exploit the full potential of the BPH-cured LCER as a high thermally conducting matrix, composite samples with various contents of the 2-D h-BN fillers (∼30 μm diameter, ∼1 μm thickness, 10−60 vol %) were prepared by the dry homogeneous mixing and hot pressing method. For all composite samples filled with h-BN in quantities ranging from 30 to 60 vol %, a homogeneous distribution of the h-BN platelets was observed in the BPH-cured LCER matrix with low void fractions (130% were achieved for the thermal conductivity, with a maximum value of ∼23 W/mK (at 60 vol %) being obtained for the hexagonal boron nitride platelet-containing composites. We could therefore conclude that the advantages imparted by the cation-induced curing of LCER, including the higher thermal conductivity of the neat resin and composite, faster curing than amine hardeners, and a simple fabrication process without the requirement for complex equipment (e.g., a high-power magnetic field) may contribute to the commercialization of LCER as a highly efficient industrial heat dissipating material.

Figure 4. (a) Plot indicating the different XRD spectra obtained for the LCERs cured by DDS (red line) and BPH (black line). (b) XRD spectra for the neat LCER at a range of temperatures (without curing agents). The dashed black lines indicate the peak positions observed for the BPH-cured LCER in (a) (2θ ∼ 19, 20, 23.2, and 28.4°), which correspond with those of the neat LCER (160−190 °C).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00456. Materials and methods, DSC data showing thermal phase transitions of LCER, DMA data and discussion on data analysis, isothermal DSC data and discussion on curing of LCER by cationic initiator, a brief theoretical description of Hot Disk thermal constants analysis, a table showing measured void fractions of composite samples, crosssectional SEM images of composite samples, and theoretical analysis on thermal conductivity of h-BN/ LCER composites (PDF).

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Figure 5. Experimental thermal conductivities of the h-BN/LCER composites cured with BPH (red filled circles) and DDS (blue filled triangles) and theoretical fitting of the experimental data with the theoretical thermal conduction model by Nielson. Black open circles and blue open triangles represent the predicted thermal conductivity values calculated with matrix thermal conductivities of 0.34 and 0.48 W/mK, respectively.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

significant increase of ∼130%, showing good agreement with the theoretical prediction by Nielson model (see Supporting Information S7 for detailed equation and calculation),30 was observed on average for the thermal conductivity compared to that of the DDS-cured LCER (maximum value = 23.2 ± 0.5 W/ mK at 60 vol %). Considering the enhancement of thermal conductivity of the neat LCERs (141% over the one of DDScured LCERs), the improvement of 130% seems to be small. In addition, at 60 vol %, lower thermal conductivity compared to the predicted value was observed. This is mainly due to the

Nam-Ho You: 0000-0001-8886-226X Munju Goh: 0000-0002-6061-8625 Hyeonuk Yeo: 0000-0003-2629-4353 Se Gyu Jang: 0000-0002-9969-7236 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. J.R.H., H.Y., and S.G.J. planned and designed the experiments and prepared the manuscript. H.Y. and A.M.I. 1184

DOI: 10.1021/acsmacrolett.8b00456 ACS Macro Lett. 2018, 7, 1180−1185

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(13) Jahromi, S.; Kuipers, W. A. G.; Norder, B.; Mijs, W. J. Liquid Crystalline Epoxide Thermosets. Dynamic Mechanical and Thermal Properties. Macromolecules 1995, 28 (7), 2201−2211. (14) Tanaka, S.; Hojo, F.; Takezawa, Y.; Kanie, K.; Muramatsu, A. Highly Oriented Liquid Crystalline Epoxy Film: Robust High ThermalConductive Ability. ACS Omega 2018, 3 (3), 3562−3570. (15) Douglas, E. P.; Langlois, D. A.; Benicewicz, B. C. Synthesis, Phase Behavior, and Curing Studies of Bisacetylene Rigid-Rod Thermosets. Chem. Mater. 1994, 6 (11), 1925−1933. (16) Benicewicz, B. C.; Smith, M. E.; Earls, J. D.; Priester, R. D.; Setz, S. M.; Duran, R. S.; Douglas, E. P. Magnetic Field Orientation of Liquid Crystalline Epoxy Thermosets. Macromolecules 1998, 31 (15), 4730− 4738. (17) Harada, M.; Ando, J.; Yamaki, M.; Ochi, M. Synthesis, Characterization, and Mechanical Properties of a Novel Terphenyl Liquid Crystalline Epoxy Resin. J. Appl. Polym. Sci. 2015, 132 (1), 41296. (18) Song, S.-h.; Katagi, H.; Takezawa, Y. Study on High Thermal Conductivity of Mesogenic Epoxy Resin with Spherulite Structure. Polymer 2012, 53 (20), 4489−4492. (19) Hansen, D.; Bernier, G. A. Thermal Conductivity of Polyethylene: The Effects of Crystal Size, Density and Orientation on the Thermal Conductivity. Polym. Eng. Sci. 1972, 12 (3), 204−208. (20) Tan, C.; Sun, H.; Fung, B. M.; Grady, B. P. Properties of Liquid Crystal Epoxy Thermosets Cured in A Magnetic Field. Macromolecules 2000, 33 (17), 6249−6254. (21) Li, Y.; Rios, O.; Kessler, M. R. Thermomagnetic Processing of Liquid-Crystalline Epoxy Resins and Their Mechanical Characterization using Nanoindentation. ACS Appl. Mater. Interfaces 2014, 6 (21), 19456−19464. (22) Yeo, H.; Islam, A. M.; You, N.-H.; Ahn, S.; Goh, M.; Hahn, J. R.; Jang, S. G. Characteristic Correlation between Liquid Crystalline Epoxy and Alumina Filler on Thermal Conducting Properties. Compos. Sci. Technol. 2017, 141, 99−105. (23) Kim, Y. C.; Park, S.-J.; Lee, J.-R. Effects of N-Benzylpyrazinium Hexafluoroantimonate Concentration on Rheological Properties in Cationic Epoxy Cure System. Polym. J. 1997, 29, 759−765. (24) Park, S. J.; Seo, M. K.; Lee, J. R. Isothermal Cure Kinetics of Epoxy/Phenol-Novolac Resin Blend System Initiated by Cationic Latent Thermal Catalyst. J. Polym. Sci., Part A: Polym. Chem. 2000, 38 (16), 2945−2956. (25) Wu, Y.; Xue, Y.; Qin, S.; Liu, D.; Wang, X.; Hu, X.; Li, J.; Wang, X.; Bando, Y.; Golberg, D.; Chen, Y.; Gogotsi, Y.; Lei, W. BN Nanosheet/Polymer Films with Highly Anisotropic Thermal Conductivity for Thermal Management Applications. ACS Appl. Mater. Interfaces 2017, 9 (49), 43163−43170. (26) Li, Y.; Kessler, M. R. Liquid Crystalline Epoxy Resin Based on Biphenyl Mesogen: Effect of Magnetic Field Orientation During Cure. Polymer 2013, 54 (21), 5741−5746. (27) Choy, C. L. Thermal Conductivity of Polymers. Polymer 1977, 18 (10), 984−1004. (28) Kim, Y.; Yeo, H.; You, N.-H.; Jang, S. G.; Ahn, S.; Jeong, K.-U.; Lee, S. H.; Goh, M. Highly Thermal Conductive Resins Formed from Wide-Temperature-Range Eutectic Mixtures of Liquid Crystalline Epoxies Bearing Diglycidyl Moieties at the Side Positions. Polym. Chem. 2017, 8 (18), 2806−2814. (29) Kawamoto, S.; Fujiwara, H.; Nishimura, S. Hydrogen Characteristics and Ordered Structure of Mono-Mesogen Type LiquidCrystalline Epoxy Polymer. Int. J. Hydrogen Energy 2016, 41 (18), 7500−7510. (30) Hill, R. F.; Supancic, P. H. Thermal Conductivity of PlateletFilled Polymer Composites. J. Am. Ceram. Soc. 2002, 85 (4), 851−857.

synthesized LCER. A.M.I. and H.L. measured thermal conductivity of samples and carried out XRD characterizations. S.G.J. synthesized BPH. H.Y. and A.M.I. carried out DMA experiment and data analysis. N.Y., S.A., and M.G. gave valuable comments on experimental design and provided useful discussion on XRD analysis. S.G.J. performed theoretical calculation. The corresponding authors J.R.H., H.Y., and S.G.J. supervised this work. All authors discussed the results and implications and commented on the manuscript at all stages. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the Materials and Components Technology Development Program of MOTIE/ KEIT, Republic of Korea (10076464, Development of lightweight and high heat dissipating bioinspired composites for printed circuit board with thermal conductivity of 20 W/mK), Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4905619), and the Graphene Materials/Components Develpment Project (10044366) through the Ministry of Trade, Industry, and Energy (MOTIE), Republic of Korea.

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REFERENCES

(1) Sirringhaus, H. 25th Anniversary Article: Organic Field-Effect Transistors: The Path beyond Amorphous Silicon. Adv. Mater. 2014, 26 (9), 1319−1335. (2) Moore, A. L.; Shi, L. Emerging Challenges and Materials for Thermal Management of Electronics. Mater. Today 2014, 17 (4), 163− 174. (3) Vadivelu, M. A.; Kumar, C. R.; Joshi, G. M. Polymer Composites for Thermal Management: A Review. Compos. Interfaces 2016, 23 (9), 847−872. (4) Tiwari, N.; Agarwal, N.; Roy, D.; Mukhopadhyay, K.; Prasad, N. E. Tailor Made Conductivities of Polymer Matrix for Thermal Management: Design and Development of Three-Dimensional Carbonaceous Nanostructures. Ind. Eng. Chem. Res. 2017, 56 (3), 672−679. (5) Kaltenbrunner, M.; Sekitani, T.; Reeder, J.; Yokota, T.; Kuribara, K.; Tokuhara, T.; Drack, M.; Schwodiauer, R.; Graz, I.; Bauer-Gogonea, S.; Bauer, S.; Someya, T. An Ultra-Lightweight Design for Imperceptible Plastic Electronics. Nature 2013, 499 (7459), 458−463. (6) Huang, X.; Jiang, P.; Tanaka, T. A Review of Dielectric Polymer Composites with High Thermal Conductivity. IEEE Electrical Insulation Magazine 2011, 27 (4), 8−16. (7) Nielsen, L. E. The Thermal and Electrical Conductivity of TwoPhase Systems. Ind. Eng. Chem. Fundam. 1974, 13 (1), 17−20. (8) Miyuki, H.; Mitsukazu, O.; Masayuki, T.; Tohru, K.; Tsukasa, I.; Naoyuki, S.; Hisashi, A. Thermal-Conductivity Properties of LiquidCrystalline Epoxy Resin Cured under A Magnetic Field. J. Polym. Sci., Part B: Polym. Phys. 2003, 41 (14), 1739−1743. (9) Zongyong, G.; Yingfeng, Y.; Yuanze, X.; Shanjun, L. Synthesis and Characterization of a Liquid Crystalline Epoxy Containing Azomethine Mesogen for Modification of Epoxy Resin. J. Appl. Polym. Sci. 2007, 105 (4), 1861−1868. (10) Masaki, A.; Yoshitaka, T. Study of High Thermal Conductive Epoxy Resins Containing Controlled High-Order Structures. J. Appl. Polym. Sci. 2003, 89 (9), 2464−2467. (11) Li, Y.; Badrinarayanan, P.; Kessler, M. R. Liquid Crystalline Epoxy Resin Based on Biphenyl Mesogen: Thermal Characterization. Polymer 2013, 54 (12), 3017−3025. (12) Langlois, D. A.; Benicewicz, B. C.; Douglas, E. P. Liquid Crystalline Bispropargyl Thermosets. Chem. Mater. 1998, 10 (11), 3393−3399. 1185

DOI: 10.1021/acsmacrolett.8b00456 ACS Macro Lett. 2018, 7, 1180−1185