Heat Transfer Organic Materials: Robust Polymer ... - ACS Publications

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Heat Transfer Organic Materials: Robust Polymer Films with the Outstanding Thermal Conductivity Fabricated by the Photopolymerization of Uniaxially Oriented Reactive Discogens Dong-Gue Kang,† Minwook Park,† Dae-Yoon Kim,† Munju Goh,‡ Namil Kim,*,§ and Kwang-Un Jeong*,† †

BK21 Plus Haptic Polymer Composite Research Team & Department of Polymer Nano Science and Technology, Chonbuk National University, Jeonju 561-756, Korea ‡ Carbon Composite Materials Research Center, Korea Institute of Science and Technology, Jeonbuk 565-905, Wanju-gun, Korea § Smart Materials R&D Center, Korea Automotive Technology Institute, Cheonan 330-912, Korea S Supporting Information *

ABSTRACT: For the development of advanced heat transfer organic materials (HTOMs) with excellent thermal conductivities, triphenylene-based reactive discogens, 2,3,6,7,10,11-hexakis(but-3-enyloxy)triphenylene (HABET) and 4,4′,4″,4‴,4⁗,4⁗′(triphenylene-2,3,6,7,10,11-hexaylhexakis(oxy))hexakis(butane-1-thiol) (THBT), were synthesized as discotic liquid crystal (DLC) monomers and cross-linkers, respectively. A temperature−composition phase diagram of HABET-THBT mixtures was first established based on their thermal and microscopic analyses. From the experimental results, it was realized that the thermal conductivity of DLC HTOM was strongly affected by the molecular organizations on a macroscopic length scale. Macroscopic orientation of self-assembled columns in DLC HTOMs was effectively achieved under the rotating magnetic fields and successfully stabilized by the photopolymerization. The DLC HTOM polymer-stabilized at the LC phase exhibited the remarkable thermal conductivity above 1 W/mK. When the DLC HTOM was macroscopically oriented, the thermal conductivity was estimated to be 3 W/mK along the in-plane direction of DLC molecule. The outstanding thermal conductivity of DLC HTOM should be originated not only from the high content of two-dimensional aromatic discogens but also from the macroscopically oriented and self-assembled DLC. The newly developed DLC HTOM with an outstanding thermal conductivity as well as with an excellent mechanical sustainability can be applied as directional heat dissipating materials in electronic and display devices. KEYWORDS: discotic liquid crystal, reactive mesogen, thiol−ene photopolymerization, thermal conductivity, anisotropic heat transfer

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between the polymeric matrix and the fillers. The introduction of conductive nanoparticles is very effective in maximizing the thermal conducting properties of composite systems at low filler contents, but the uniform dispersion with reduced interfacial resistance remains a big challenge. Another approach is based on the use of an advanced heat transfer organic material (HTOM) as a continuous matrix. The polymeric matrix with high order tends to possess a higher thermal conductivity through the effective lattice vibration and the suppression of phonon scattering. Therefore, the development of novel HTOM has been considered as a breakthrough for enhancing the thermal conductivity of composite systems.1 To take full advantages of the high thermal conductivity of ordered polymeric matrix, the molecular building blocks should be specifically arranged in a two-dimensional (2D) or 3D superstructure. Among numerous building blocks, reactive liquid crystal (LC) molecules, often called reactive mesogens

INTRODUCTION Rapid advancements in electronic devices and energy storage systems require high powers and fast processing times within the limited spaces. Since the amount of heat accumulation is proportional to the power density, the effective heat management is of paramount importance to improve the reliability and lifetime of devices.1−4 In this regard, most electronic packages contain a metallic heat spreader or heat sink. Although metallic materials are favorable ones in terms of heat removal, they are intrinsically heavy and unsuitable for the complicated geometry control especially in the confined areas. Polymer-based conductive materials can be substituents to overcome the intrinsic drawbacks of metals. So far, a significant amount of research has been conducted for the development of polymer composites by incorporating thermal conductive fillers, such as ceramics, metals, and carbon-based additives, where the thermal conducting performances mainly depend on the properties of fillers and their distributions.5−10 Nevertheless, these polymer composites often show low thermal conductivities despite the large amount incorporation of thermal conductive fillers, mainly due to the occurrence of phonon scattering at the interface © 2016 American Chemical Society

Received: August 15, 2016 Accepted: October 20, 2016 Published: October 20, 2016 30492

DOI: 10.1021/acsami.6b10256 ACS Appl. Mater. Interfaces 2016, 8, 30492−30501

Research Article

ACS Applied Materials & Interfaces

Figure 1. Chemical structures and thermal behaviors of reactive discotic building blocks: (a) molecular chemical structures and self-assembled columnar structures of HABET and THBT; (b) first heating DSC thermograms of HABET-THBT mixtures; (c) temperature−composition phase diagram of HABET-THBT mixtures.

plane for phonon transfer.40,41 Upon reducing temperature below isotropic (I) phase, the aspect ratio of self-assembled nanocolumns suddenly increases due to the formation of strong π−π interactions between aromatic cores and the nanophase separation between rigid discotic cores and flexible tails. The self-assembled nanocolumns can organize into the superstructures with long-range molecular orientational or positional orders.42−45 Because of the high interfacial affinity between DLC cores and polyaromatic structure of carbon-based thermal conducting fillers, DLC HTOMs can be effectively applied as organic matrices in the carbon-based heat transfer composites. Therefore, the discotic RMs can play a key role to overcome such severe problems of heat accumulation in electronic and electric devices. In order to develop novel HTOMs, we newly synthesized two triphenylene-based discotic RMs (2,3,6,7,10,11-hexakis(but-3-enyloxy)triphenylene (HABET) and 4,4′,4″,4‴,4⁗,4⁗′(triphenylene-2,3,6,7,10,11-hexaylhexakis(oxy))hexakis(butane1-thiol) (THBT)) as discotic vinyl monomer and thiol crosslinker, respectively.46 To find the optimum content of HABET in the DLC HTOM, the temperature−composition phase diagram of HABET-THBT mixtures was first established based on their thermal and microscopic analyses. The photopolymerization conditions were determined from the physical, chemical and thermal properties of DLC HTOM. Utilizing the rotating magnetic fields, well-oriented columnar structures were effectively obtained on the macroscopic length scale for the directional heat transfer. DLC HTOMs prepared with and without the rotating magnetic fields were studied with infrared camera, and their experimental results were further analyzed with computer simulations.

(RM), can be promising candidates because the LC molecules show the positional and directional orders with high fluidity at the mesophases.11−16 Typical LC monomers include rodshaped (calamitic) and disc-shaped (discotic) LCs.17,18 By the polymerization of ordered LC molecules on the macroscopic length scale, we can obtain the robust polymeric superstructures without disturbing the initial ordered states.19−22 Among the various polymerizations, the photopolymerization using thiol−ene click chemistry is fairly straightforward for construction of a robust LC network.23−25 Especially, it is easy to obtain the monodomain morphology with the photopolymerization of RMs due to the relatively low viscosity of the small molecules in comparison to the analogous LC polymers. Up to now, the thermal conductive properties of LC systems have been mainly observed using calamitic LC molecules. Calamitic LC HTOMs with high molecular orientations exhibited higher thermal conductivities than conventional epoxy resins. Ordered calamitic LC in the HTOM can provide outstanding thermal conductivity up to 0.7−0.8 W/mK.26−32 However, the development of discotic liquid crystal (DLC) HTOM is quite limited in spite of the remarkable self-assembly performance. Numerous discogens have been extensively studied for scientific interests since the first discovery in 1977.33−36 The triphenylene-based DLCs with C3 rotational symmetry form the self-assembled nanocolumns with remarkable electronic and thermoelectric characteristics. Cutting out a 2D carbon graphene sheet into disc shapes on a few nanometer length scales can provide the discotic triphenylene core, often abbreviated as “nanographene”.37−39 The nanographene has a hexagonal lattice structure from polyaromatic hydrocarbon, similar to the carbon-based thermal conducting additives such as graphite and graphene. However, those rigid nanographenes themselves are difficult to dissolve and melt. The processability of nanographene is dramatically enhanced by attaching the flexible alkyl or alkoxy tails at the periphery of rigid core, resulting in the evolution of DLC mesophases. The honeycomb-like structure of DLC core can offer an effective lattice

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RESULTS AND DISCUSSION Phase Transition Behaviors of Reactive Discogens. Chemical structures and purities of HABET and THBT reactive discogens are confirmed with proton (1H) NMR, carbon-13 (13C) NMR, and MALDI-ToF spectra (Figures S1− S4 in the Supporting Information). The HABET and THBT 30493


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Figure 2. (a) POM images and their corresponding macroscopic images (insets) of DLC HTOM films polymerized at different temperatures. (b) Optical transmittance spectra of DLC HTOM films with different crystalline domain sizes. (c) Macroscopic image of the free-standing flexible DLC HTOM film polymerized at 80 °C.

19.6° are due to the lateral organization of self-assembled THBT discogens, while the diffractions at 2θ = 21.5° and 25.6° are associated with the π−π interaction between THBT cores. No distinctive change is noticed in the temperature range between 25 and 80 °C. Upon increasing temperature above 90 °C, two broad and diffused amorphous halos are only discerned at 2θ = 5.5° and 21.5°. The cross-polarized optical microscopy (POM) images taken during the 2.5 °C/min heating process confirm the evolution of a strong birefringence in the temperature range from 25 to 85 °C (Figure S5c). When the temperature is increased above 70 °C, the grain boundary of the texture starts to melt and transforms to a completely isotropic (I) dark state. The phase transition behaviors of HABET-THBT mixture with different compositions are also investigated with DSC (Figure 1b). Pure HABET exhibits a strong crystal melting point at 68 °C with heat of transition of 49.7 kJ/mol. Minor endothermic peaks at 131 °C (4.0 kJ/mol) and 163 °C (19.4 kJ/mol) correspond to the tilted hexagonal columnar LC− hexagonal columnar LC (t-colh−colh) and hexagonal columnar LC−isotropic (colh−I) phase transition, respectively.11 Pure THBT shows a single melting peak at 74 °C (59.7 kJ/mol).46 The characteristic peaks of HABET and THBT are shifted to lower temperatures with reduced intensity by increasing the amount of counterpart. The DSC thermogram of the 50HABET-50THBT mixture (1:1 molar ratio) obtained at a

molecules containing the vinyl and thiol functional groups at the periphery of triphenylene cores are polymerizable to fix their self-assembled nanocolumns at specific temperature ranges (Figure 1a). Pure HABET exhibits a typical columnar LC phase in a temperature range between 68 and 163 °C.11 From the DSC result of pure THBT, it is realized that THBT discogen reveals the dual exothermic peaks during the cooling process, i.e., a strong peak in the temperature range of 75−78 °C and a weak peak at lower temperature, with the total heat of transition of −36.2 kJ/mol (Figure S5a).46 The subsequent heating runs exhibit three endothermic peaks between 45 and 67 °C with the total heat of transition of 36.3 kJ/mol which corresponds to the exothermic transition during the previous cooling process. In our previous report, we have identified the molecular packing structure of HABET using 2D wide-angle Xray diffraction (WAXD) of oriented samples. There are three ordered structures below the isotropization temperature: a columnar hexagonal LC phase (colh), a tilted columnar hexagonal LC phase (t-colh), and a highly ordered columnar oblique crystal phase (Cr).11 To investigate the phase evolutions of THBT, 1D WAXD experiments are conducted at different temperatures. Figure S5b represents a set of 1D WAXD powder patterns obtained at a heating rate of 1.0 °C/ min. At 25 °C, several diffraction peaks at 2θ = 4.8°, 5.8°, 9.6°, 11.1°, 14.5°, 15.9°, 19.6°, 21.5°, and 23.1° are clearly detected. The diffraction peaks observed between 2θ = 4.8° and 2θ = 30494


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ACS Applied Materials & Interfaces cooling rate of 10 °C/min exhibits two exothermic peaks at 47 and 116 °C (Figure S6a). When the mixtures are cooled below 120 °C, several anisotropic dendrites start to appear. The emerged morphologies correspond to the LC ordered hexagonal columnar phase of HABET. When temperature reaches 47 °C, the whole microscopic view becomes completely anisotropic due to the crystallization of THBT. As shown in Figure 1c, the experimental phase diagram of HABET-THBT mixtures is established on the basis of the DSC and POM results. Various coexistence regions including crystal + crystal (Cr1 + Cr2), tilted hexagonal columnar LC + crystal (t-Colh1 + I2), and hexagonal columnar LC + isotropic (Colh1 + I2) are identified upon increasing temperature at the intermediate composition. a broad Cr1 + Cr2 region indicates that the emerged HABET and THBT crystals are rather pure. The single phase crystal (Cr1, Cr2) and hexagonal columnar phase (t-Colh1, Colh1) regions are observed at the compositions >90% and kT

(3)

where the N indicates the number of molecules in a domain. As temperature suppresses from the isotropic melting state to the LC state, the discogens coalesce into a mesophase domain. Therefore, the magneto static free energy becomes sufficient to surpass the thermal disordering effects due to an additive effect of anisotropy.63−66 As described in Figure 1c and Figure S6a, a 50HABET-50THBT mixture is slowly annealed from the isotropic state (140 °C) to the LC state (80 °C) under 1.5 T of magnetic field and then photopolymerized. The aligned samples are prepared in three ways; without any magnetic field, with the static magnetic field, and with the rotating magnetic field (100 rpm). By means of the magnetic field, a DLC HTOM film containing uniaxial columnar order is successfully constructed. It is worthwhile mentioning that the DLCs have the large diamagnetic anisotropy of the aromatic core. As a result, when external magnetic field is applied to the DLCs, the discotic columns are edge-on aligned to the field direction.62 Even though the columnar axes of DLCs align perpendicularly to the field direction of a static magnetic, the columnar axes are not aligned in the one direction but distributed azimuthally. Fortunately, this problem is clearly solved with sample rotation in the static magnetic field and the uniaxially aligned columnar DLCs are successfully achieved over the centimeter length scale.66 As shown in Figure 4 and Figure S11, the DLC HTOMs obtained in the absence of magnetic force exhibit the multidomain structures due to random orientations. First, structural and morphological analyses are investigated for the DLC HTOM which columns are randomly distributed (Figure 4a). The POM observation confirms the formation of the randomly oriented birefringent domains (Figure 4d). As shown in Figure 4i, the 2D WAXD pattern of DLC HTOM shows two diffraction peaks at 2θ = 5.6° and 25.6° as azimuthally dispersed spots, indicating that molecular axes in mesophase domains are randomly stood. In contrast, the DLC HTOM obtained under the static magnetic field (Figure 4b) reveals the partial orientation of the columnar LC state. As shown in Figure 30497


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DLC HTOM film in which the columnar axis is parallel to the heat flow direction.68,69 The value of S of the aligned DLC HTOMs under magnetic field with rotation and without rotation are 0.97 and 0.64, respectively. Heat (100 °C) from the bottom is dissipated along the longitudinal direction of film, where the transfer rate is affected by the thermal conductivity of each film (Figure 6). As illustrated in Figure S14, the calculated heat transfer behaviors are in good agreement with the experimental results.

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CONCLUSIONS Triphenylene-based reactive discotic liquid crystals (DLC), HABET vinyl monomer and THBT thiol cross-linker, were specifically synthesized and applied for the development of advanced heat transfer organic materials (HTOM) with an excellent thermal conductivity. Based on the temperature− composition phase diagram of HABET-THBT mixtures and the physical, chemical, and thermal properties of DLC HTOM, the optimum content of HABET was determined to be 50 wt % and the DLC HTOMs were polymer-stabilized by irradiating 365 nm UV light at 80 °C for 1 h. By polymerizing DLC HTOM under a rotating magnetic field (1.5 T), the uniaxially oriented DLC HTOMs with excellent mechanical, chemical, and thermal stabilities were successfully prepared on the macroscopic length scale. The DLC HTOM prepared at tilted hexagonal columnar LC + isotropic (t-colh1 + I2) coexistence region exhibited the highest thermal conductivity value: 1.09 W/mK (TPS measurement) and 2.33 W/mK (LFA measurement). Based on our own knowledge, this value is the highest one achieved with organic materials. The outstanding thermal conductivity of DLC HTOM should be due to the high content of 2D aromatic discogens as well as the macroscopically oriented and self-assembled DLC. The DLC HTOMs with outstanding thermal conductivities as well as with excellent mechanical and chemical stabilities can be applied as heat dissipating materials in electronic, automobile, and display industries.

Figure 5. (a) Schematic illustrations of the DLC HTOM films with different molecular orientations with respect to the long axis of films. The columnar axes of discotic assemblies are (i) randomly distributed, (ii) along the transverse direction of the film plane, and (iii) along the longitudinal direction of the film plane, respectively. (b) In-plane thermographic images of the DLC HTOM films taken at different times.

6) to further investigate the effect of anisotropic thermal conductivity on the heat transfer behavior. Temporal heat

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EXPERIMENTAL SECTION

Preparation of DLC HTOMs. The reactive discogens HABET and THBT were prepared according to the synthetic procedures reported in the references and the detail processes are described in the Supporting Information.11,46 In order to prepare a free-standing film using the discotic RMs, a capillary mold was manufactured by sandwiching of two glass substrates and the cell gap was controlled to be 0.1 mm using tape spacers. A homogeneous 50HABET-50THBT mixture containing 5 wt % 2,2-dimethoxy-2-phenylacetophenone (DMPA) as photoinitiator was injected into the mold by capillary action at 140 °C and vacuum was applied to the mold for 30 min in order to eliminate microvoids. The mold was then annealed at different target temperatures (60, 80, 100, 120, and 140 °C, respectively). The uniaxially aligned DLC HTOM was prepared using an electromagnet system (vibrating sample magnetometer 7407, LakeShore) with a field strength of 1.5 T. The specimen was slowly cooled from 140 to 80 °C under a constant magnetic field with rotation (100 rpm) about a perpendicular axis of the magnetic field. Photopolymerization at each annealed temperature was triggered by irradiating ultraviolet (UV, 365 nm) light at an intensity of 20 mW/ cm2 using UV light generator (SP-9 spot cure, Ushio). After the UV exposure for 1 h, the glass mold was etched by dipping into concentrated hydrofluoric acid. Characterization. Chemical structures and purities of HABET and THBT were identified using proton (1H) and carbon-13 (13C) nuclear magnetic resonance (NMR, JNM-EX400, JEOL) techniques in deuterated chloroform (CDCl3) or dimethyl sulfoxide (DMSO-d6).

Figure 6. Computer-simulated images of heat transfer behaviors of the DLC HTOM films with time. The columnar axes of discotic assemblies are (i) randomly distributed, (ii) along the transverse direction of the film plane, and (iii) along the longitudinal direction of the film plane, respectively.

evolution of the DLC HTOM is acquired by solving the combined heat transfer and Navier−Stokes equation using the material parameters including heat capacity (Cp = 1,521 J/(kg K)) and density (ρ = 1,188 kg/m3).67 The thermal conductivity of randomly oriented DLC HTOM is evaluated to be 1.09 W/ mK as measured by TPS method. Based on the order parameters (S) demonstrated from a Gaussian approximation of the 2D WAXD results, the directional conductivity of uniaxially oriented DLC HTOMs is assumed to be 3.0 W/mK for the aligned film in which the DLC columnar axis is along the transverse direction of the film plane and 0.6 W/mK for the 30498


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ACS Applied Materials & Interfaces Chemical shifts were quoted in part per million (ppm) with tetramethylsilane (TMS) as a reference. Molecular weights of HABET and THBT were confirmed by matrix-assisted laser desorption ionization−time-of-flight mass spectroscopy (MALDIToF MS, Voyager DE PRO Spectrometer, PerSeptive Biosystems). The phase transition behaviors of HABET and THBT were determined using differential scanning calorimetry (DSC, Q20, TA Instruments), and the sample weights were held to be 4.0 mg. The heating scan always preceded the cooling scan in order to eliminate previous thermal histories, while the cooling and heating rates were always kept identical. Morphological change of the ordered phases depending on the temperatures was observed using cross-polarized optical microscopy (POM, ECLIPSE LV100 POL, Nikon) coupled with a temperature controller (FP 90, Mettler Toledo). Mechanical property of DLC HTOM was evaluated by dynamic mechanical analysis (DMA, Q800, TA Instruments). The specimen size was prepared to be a rectangle of 7.5 mm by 5.3 mm with 0.1 mm of thickness. The test was proceeded in the temperature range from −30 to 200 °C at a heating rate of 2 °C/min. Morphological observation on a sub-micrometer length scale was conducted by scanning electron microscopy (SEM; JSM 5900, JEOL) using the gold sputtered specimens, and the instrument was operated at 20 kV accelerating voltage. The lattice structures of DLC HTOMs were investigated by two-dimensional wide-angle X-ray diffraction (2D WAXD; D8 Discover 3 kW, Bruker AXS) technique. The peak positions and widths were calibrated using a silver behenate in the low-angle region and silicon crystal standard in the wide-angle region. Variation of light transmittance of DLC HTOMs depending on mesophase sizes was monitored by a visible light transmittance analyzer (LCMS-200, Sesim Photonics Technology). In-plane thermal conductivity of the DLC HTOMs obtained by curing at different temperatures was measured and compared at room temperature by the transient plane source (TPS) technique (TPS 500S, HotDisk) and the laser flash analysis (LFA) method (LFA 447 MicroFlash, Netzsch). The samples with a dimension of 12 mm in diameter and 0.1 mm in thickness were used. Anisotropic heat evolution of aligned DLC HTOMs was observed by infrared camera (T200, FLIR). The effect of thermal conductivity of the aligned DLC HTOMs on heat transfer was further examined by conducting numerical calculation using a computational simulation software (Comsol Multiphysics).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10256. 1 H NMR, 13C NMR, MALDI-ToF MS, DSC, POM, SEM, and WAXD results and the experimental processes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.K.). *E-mail: [email protected] (K.-U.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was mainly supported by BRL (Grant 2015042417), MOTIE-KDRC (Grant 10051334), the Mid-Career Researcher Program (Grant 2016R1A2B2011041), and KIST Institutional Program (Grant 2Z04750) of Korea.

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REFERENCES

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