Anisotropic Thermal Interface Materials: Directional Heat Transfer in

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Surfaces, Interfaces, and Applications

Anisotropic Thermal Interface Materials: Directional Heat Transfer in Uniaxially Oriented Liquid Crystal Networks Dong-Gue Kang, Hyeyoon Ko, Jahyeon Koo, Seok-In Lim, Jin Soo Kim, Yeontae Yu, Cheul-Ro Lee, Namil Kim, and Kwang-Un Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09982 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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ACS Applied Materials & Interfaces

Anisotropic Thermal Interface Materials: Directional Heat Transfer in Uniaxially Oriented Liquid Crystal Networks Dong-Gue Kang,† Hyeyoon Ko † Jahyeon Koo,† Seok-In Lim,† Jin Soo Kim,‡ Yeon-Tae Yu,‡ Cheul-Ro Lee,‡ Namil Kim,*,§ and Kwang-Un Jeong,*,† †

BK21 Plus Haptic Polymer Composite Research Team & Department of Polymer Nano Science and Technology, Chonbuk National University, Jeonju 54896, Republic of Korea ‡

Division of Advanced Materials Engineering, Chonbuk National University, Jeonju 54896, Republic of Korea

§

Smart Materials R&D Center, Korea Automotive Technology Institute, Cheonan 31214, Republic of Korea

KEYWORDS: thermal interface materials, anisotropic heat transfers, thermal conducting liquid crystals, reactive mesogens, liquid crystal networks

ABSTRACT For the development of anisotropic thermal interface materials (TIMs), a rod-shaped reactive monomer PNP-6MA is newly designed and successfully synthesized. PNP-6MA reveals a smectic A (SmA) mesophase between crystalline (K) and isotropic (I) phases. PNP-6MA can be oriented under a magnetic field (B = 2 T), and its macroscopic orientation can be robustly stabilized by in situ polymerization. Even without macroscopic orientations, the fabricated thermal conducting liquid crystal (TCLC) films show the outstanding thermal conductivity of 1.21 W/mK, which is higher than conventional organic materials. The thermal conductivity of uniaxially and macroscopically oriented TCLC films can be 2.5 W/mK along the long axis of mesogenic core. The newly developed TCLC film can be used as a TIM between a high-power light emitting diode (LED) and a heat sink.

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INTRODUCTION To manage the heat of a high-power light emitting diode (LED), there has been a constant demand for the development of a highly efficient heat dissipation material.1-4

Heat sinks are

generally used to dissipate the heat generated by the LED and the accumulated heat in the LED must be transferred to the heat sink immediately to prevent device failure.5,6 Due to the incomplete contact of the surface, an insulating air gap is inevitably created at the interface between the heat source and the heat sink, so that the thermal energy from the heat source cannot be efficiently transferred to the heat sink.7-9 The contact resistance at the interface can be reduced by introducing a thermal interface material (TIM) that fills the air gap between the heat source and the heat sink.10,11 Ideal TIMs should have not only high thermal conductivity but also electrical insulation, good processability, long-term thermal stability, and high interfacial affinity to heat sources and heat sinks.10,11 Conventional thermal conducting materials made of epoxy thermosets filled with thermal conducting ceramic, metal or carbon filler have limitations in application to TIMs.12-18 Because of the inherent low thermal conductivity of the epoxy resin, a large amount of thermal conducting filler should be used to increase the thermal conductivity of the composite.16 However, the viscosity of material increases in proportion to the amount of filler, resulting in poor processability.16 Therefore, it is necessary to develop a polymeric resin showing not only high thermal conductivity but also electrical insulation. Among the building blocks for polymeric networks, liquid crystal (LC) molecules with reactive functional groups can be effectively used as thermal conducting and electric insulating materials.19-27 Thermotropic and lyotropic LCs exhibit high degree of positional and directional orders as well as benign fluidity at certain temperature range or concentration.28-31 Selfassembled LC monomers can be chemically stabilized in a three-dimensional (3D) polymeric


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network in which the initial mesophase is permanently retained.28-31 Polymeric LC networks exhibit high thermal conductivity because the ordered mesophase enhances the crystallinity of the network as well as the periodicity of the atomic lattice structure that extends the phonon transfer pathway in the polymeric networks.22 The directional order of LC molecules can be easily adjusted by external stimuli such as electric field, mechanical force and magnetic field, so that a polymeric LC network with a high degree orientation can be obtained in the macroscopic regions.32-34 Since the mesogenic core of the LC molecule has structural anisotropy, the directional heat flow can be maximized in a particular direction by tuning the orientation of LC molecule.22 Due to the advantages of LCs, thermal conducting liquid crystal (TCLC) films with high thermal conductivity can be developed with calamitic and discotic LCs.19-27 However, a small amount of research on TCLC materials has been reported compared to conventional epoxy-based heat dissipating materials. In this research, a mesogenic monomer, PNP-6MA, was synthesized and its thermal conductivity was investigated in terms of molecular structure. To determine the physical and chemical properties of PNP-6MA, thermal and structural analyses were conducted. The isotropic phase and anisotropic mesophase of PNP-6MA were successfully stabilized into the 3D polymeric networks via photopolymerization so that the free-standing TCLC films can be obtained. Moreover, the directional order of LC molecule inside the TCLC film was controlled to expedite the heat flow in a direction perpendicular to the film plane by performing in situ photopolymerization under magnetic field (B = 2T). The possibility of applying the TCLC films to the TIMs for high-power LED device was confirmed by comparing hear dissipation behavior using infrared (IR) imaging camera.


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RESULT AND DISCUSSION Chemical and Physical Analyses of Liquid Crystal Monomer The calamitic LC monomer PNP-6MA was synthesized to serve as the polymerizable building block of TCLC film (Figure S1). PNP-6MA mainly consists of three parts; thermal conductive mesogenic core, flexible alkyl chain which can induce mesophase, and polymerizable functional group. As a rigid mesogenic core, 2-phenylnaphthalene group was selected because it has a highly ordered carbon atomic lattice structure which provides the enhanced phonon transfer. Methacrylate functional group is attached at the end of PNP-6MA molecule and the mesogenic core and the polymerizable group are connected by flexible alkyl chain. Chemical structure and purity of PNP-6MA were confirmed by nuclear magnetic resonance (NMR) and mass spectroscopy (Figure S2-S4). The phase transition behavior of PNP-6MA was studied using differential scanning calorimetry (DSC) and polarized optical microscopy (POM) (Figure 1). The DSC thermogram of PNP-6MA exhibits dual exothermic transitions at 93 °C (-4.8 kJ/mol) and 7 °C (-24.5 kJ/mol) during the cooling process at -1 °C/min (Figure 1b). Subsequent heating at the same scanning rate represents two endothermic peaks at 27 °C and 93 °C. The transition heat (29.3 kJ/mol) during the heating process corresponds to the heat of the exothermic peaks during the previous cooling process. At a scanning rate above 5 °C/min, an endothermic transition at 7 °C (1.5 kJ/mol) appears, subsequently followed by an exothermic transition at 9 °C (-1.5 kJ/mol). This rate-dependent transition can be due to the recrystallization of alkyl chains.35 To identify the morphologies at each phase, POM images were taken during the cooling process from 100 °C at -1 °C/min (Figure 1c). Since PNP-6MA is isotropic at 100 °C, the POM image displays a dark appearance. When the temperature drops below 90 °C, a LC phase with a strong birefringence appears due to the molecular self-assembly of PNP-6MA. The emerged fan-


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shaped texture indicates that PNP-6MA forms a smectic LC phase. Upon further cooling to 0 °C, PNP-6MA crystallizes and exhibits strong birefringence. To investigate the molecular packing structure of PNP-6MA, 2D wide-angle X-ray diffraction (WAXD) experiment was conducted by varying the temperature (Figure S5). The macroscopically oriented PNP-6MA samples were obtained by mechanical shearing at 60 °C and subsequent quenching to 0 °C. As shown in Figure S5b, the 2D WAXD pattern obtained at 0 °C shows several diffractions at 2θ = 7.56° (1.17 nm), 10.42° (0.85 nm), 12.01° (0.74 nm), 14.87° (0.60 nm), 15.71° (0.56 nm), 18.48° (0.48 nm), 21.92° (0.40 nm), 22.77° (0.39 nm), 23.61° (0.38 nm) and 24.70° (0.36 nm) indicating the formation of a crystal structure. The set of several intense diffractions indicates the formation of long-range ordered crystalline structure of PNP6MA below which corresponds to the DSC thermograms and POM observation. Increasing the temperature to 60 °C removes all diffraction peaks of the crystal disappear and instead a new intense diffraction emerges at 2θ = 2.50° (3.53 nm) on the equator (Figure S5c). Since the length of energy-minimized PNP-6MA is estimated to be 3.61 nm, the strong diffraction at 2θ = 2.50° and the broad amorphous halo in the wide-angle region represent the formation of the smectic A LC phase (SmA). When the temperature rises above 100 °C, the diffraction suddenly disappears and only the weak halo remains due to the isotropization of PNP-6MA. Morphological and Physical Characters of Photopolymerized TCLC films Based on the temperature-dependent phases of PNP-6MA, photopolymerization was carried out at 60 °C (SmA phase) and 120 °C (isotropic phase) to chemically stabilize each phase of the TCLC film. The network morphology of the TCLC films were observed on microscopic and macroscopic length scales (Figure 2). Although the PNP-6MA monomer tends to crystalize at


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room temperature, TCLC films cured in the isotropic state maintain the optically isotropic phase even after cooling down to room temperature (Figure 2b). When the curing temperature is lowered to the LC phase, the birefringent domains cover the entire area that maintains the SmA phase (Figure 2a). TCLC films with the LC state are optically opaque even to the naked eye, but TCLC films cured in the isotropic state are transparent (insets in Figure 2a and Figure 2b). The light transmittance of the TCLC films cured in the isotropic and SmA states is 89% and 8% in the visible light region, respectively. Because the POM observation is insufficient to identify the crystal structure, the lattice structure of TCLC film was confirmed by 1D WAXD technique. As shown in Figure S6, the TCLC film cured at 60 °C shows major diffraction peaks at 2θ = 2.50° and 19.67°. The diffraction peak at 2θ = 2.50° (corresponding to d-spacing = 3.53 nm) represents the layer structure of SmA phase, while the diffraction at 2θ = 19.67° (0.45 nm) corresponds to the ordered aliphatic chain packing of cross-linked network. Upon raising the curing temperature above 120 °C, TCLC films shows only shallow and broad amorphous halos without any noticeable diffraction peaks. Based on the results, it is apparent that the initial isotropic and anisotropic

phases

of

the

monomeric

units

can

be

effectively

frozen

after

the

photopolymerization process. The scanning electron microscopy (SEM) image of the TCLC film cured in the LC phase reveals the formation of the LC domains in the polymeric network, indicating that the mesophase is confined in the network (Figure 2c). On the other hand, SEM images of isotropic TCLC films exhibit smoother cross-sectional morphology without any identifiable domains (Figure 2d). Viscoelastic properties of free-standing TCLC films were investigated using dynamic mechanical analysis (DMA). As shown in Figure 2e, TCLC films show glassy properties below 0 °C. As the temperature rises, the films undergo a single relaxation process. The glass transition


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temperatures (Tg) of LC and isotropic TCLC films are 62.7 °C and 68.2 °C, respectively (Table 1). The tanδ graph shows the α-transition peaks for LC and isotropic TCLC films at 106.2 °C and 131.4 °C, respectively. The coefficient of thermal expansion (CTE, α) of the TCLC film was determined using thermomechanical analysis (TMA). As shown in Figure S7, both LC and isotropic TCLC films were proportionally expanded as temperature increases with the α values of 275.9 ppm/°C and 203.6 ppm/°C, respectively. The thermo-mechanical properties of the TCLC films are summarized in Table 1. A uniaxially oriented TCLC film in the macroscopic region was fabricated by in situ polymerization of PNP-6MA under magnetic field (Figure 3).34 As the thermal fluctuation energy of the isotropic state of a mesogenic molecule is much higher than the magnetic coupling energy, the oriented TCLC cannot be obtained from the isotropic state.34 In case of the LC mesophase, the mesogenic molecules gather into form a mesophase domain due to the strong intermolecular interaction between each mesogen and the molecules in the domain act like a single unit.34 Consequently, the magnetic coupling energy of the domain exceeds the thermal disordering energy and thus the mesogenic domain can be aligned by the magnetic field.34 As described in Figure 3a, monomeric PNP-6MA was slowly annealed from the isotropic state (120 °C) to the SmA state (60 °C) under magnetic field (B = 2 T). The magnetically oriented PNP-6MA was photopolymerized for the preparation of a uniaxially oriented TCLC film having mesogenic cores aligned perpendicular to the film plane. After chemical stabilization, the internal molecular structure of the oriented TCLC film was evaluated by 2D WAXD analysis. As shown in Figure 3b, 2D WAXD pattern exhibits a strong diffraction at 2θ = 2.50° (3.53 nm) which is equal to the layer distance of monomeric PNP-6MA in the SmA phase and the diffraction is located in the meridian, which is the same direction as the direction of the magnetic


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field. In the wide-angle region, a relatively shallow diffraction (2θ = 19.67°, d-spacing = 0.45 nm) is detected in the equator as a result of the alkyl chain packing. This anisotropic diffraction pattern clearly demonstrates that the long axis of PNP-6MA is uniformly aligned parallel to the direction of the magnetic field while maintaining the SmA structure within the matrix. By azimuthal scanning of the diffraction peaks, the degree of orientation of the mesogens in the aligned film was determined (Figure 3c). Based on the full-width half-maximum (FWHM = 44.8°) value of the low-angle diffraction, the order parameter (S) of the aligned TCLC film was calculated (Equation 1).35-37 S = (3cos2ϕ-1)/2

(1)

Here, ϕ represents the average angle distribution between the director of mesogen and the orientational axis (half value of FWHM). The S value of the aligned TCLC film was calculated to be 0.78, indicating that the aligned TCLC film exhibits a high degree of uniaxial orientation. Thermal Conducting Properties of TCLC films Depending on the Molecular Orientation The thermal conductivity of the TCLC film was measured by the transient plane source (TPS) method. The thermal conductivity of polymerized TCLC films in the SmA state is 1.21 W/mK, which is higher than that of conventional polymeric materials (Table 1). The thermal conductivity of the isotropic TCLC film (1.10 W/mK) is slightly lower than that of the TCLC film in the SmA state, but still high. The higher thermal conductivity of the LC state than the isotropic state is due to the highly ordered structure of SmA phase. The hierarchically assembled structure of mesogenic molecules elongates the phonon transfer pathway than the isotropic state, so that a continuously oriented structure promotes heat transfer. The thermal conductivity of the isotropic TCLC film is superior to conventional materials due to the high periodicity of the


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carbon atomic lattice of mesogenic core that enhances lattice vibration. This means that PNP6MA itself has high thermal conducting properties. To compare the thermal conductivity of the TCLC film with molecular orientation, the thermal dissipation along the normal direction of the TCLC film was monitored with an IR camera (Figure 4). A disc-shaped specimen of 20 mm in diameter and 0.5 mm in thickness was placed on a heat source maintained at 50 °C and then the temperature of specimen was monitored over time (Figure 4a). According to the time-lapsed IR image, the surface temperature of the conventional epoxy film took 20 seconds to reach 50 °C, while the TCLC films in the isotropic and SmA states reached within 12 seconds as short as half of the epoxy film (as indicated by ‘I’ and ‘LC’ in Figure 4). For the TCLC film in which the mesogens were homeotropically aligned within the film (sign ‘LC(H)’ in Figure 4), the surface temperature reached 50 °C within 8 seconds, faster than the TCLC films in the isotropic and un-oriented LC states. This result indicates that TCLC materials with high degree of atomic lattice order exhibit improved heat dissipation performances over conventional epoxy materials. In addition, the molecular orientation control in the TCLC film can promote heat transfer along the long axis of the mesogenic core. To determine the directional thermal conductivity of the homeotropically oriented TCLC film, computer calculations were conducted based on experimental data (Figure 4d). The temporal heat evolution of the TCLC films was obtained by solving the combined heat transfer and Navier-Stokes equation using the physical properties of the material such as density (ρ), specific heat capacity (Cp) and thermal conductivity (κ) (Table 1).38 The simulated heat transfer behavior of the TCLC film is in good agreement with the IR camera observation. By comparing the calculated data with the experimental results, the directional conductivity of uniaxially aligned TCLC film is estimated to be 2.5 W/mK along the long axis of PNP-6MA.


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TCLC Films as Thermal Interface Materials (TIMs) for High-power LED The feasibility of heat management of TCLC films as TIMs was tested with real LED modules (Figure 5). The temporal variation of the surface temperature of the LED modules with various TIMs was recorded by an IR camera.39 TCLC films having different degree of order were placed between the LED chip (10 W) and the water-circulating heat sink, and thermal grease (κ = 0.84 W/mK) was applied to each side of the surface to prevent the unexpected air insulation (Figure 5a). While the surface temperature of an LED module using a conventional epoxy film as a TIM increases to a maximum of 48 °C, the TCLC TIMs exhibit much lower temperature rises regardless of the molecular orientation. When the TCLC films (I and LC) are used as TIMs, the equilibrium temperature of the LED modules is about 38 °C. Note that the LED module with the TCLC film whose mesogens are aligned normal to the film plane (LC(H)) presents the lowest equilibrium temperature (33 °C). Moreover, when the TCLC films are used as TIMs, the temperature rising rates in the early stage of LED operation do not change more rapidly than the use of epoxy film. When the epoxy film is used as the TIM, the temperature rising rate in the linear temperature rise section (0 ~ 50 s) before reaching the equilibrium temperature is 0.52 °C/s. On the other hand, when using the TCLC films as the TIMs, the temperature rising rates are 0.43 °C/s (I) and 0.39 °C/s (LC), respectively. In the case of the use of TCLC film (LC(H)) as the TIM, not only the equilibrium temperature is lower than the conventional epoxy TIM, but also it takes longer time to reach the equilibrium temperature (120 s). The temperature rising rate of the LC(H) film is 0.13 °C/s, which is slower than the epoxy TIM as well as the unoriented


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TCLC TIMs. These experimental results indicate that the uniaxially aligned TCLC film effectively dissipates detrimental heat from the LED module onto the heat sink to protect the device from extreme temperature increases. CONCLUSIONS Liquid crystal (LC) monomer PNP-6MA was synthesized and applied to advanced thermal interface materials (TIMs). From the thermal conductivity of thermal conducting liquid crystal (TCLC) films photopolymerized in LC phase (1.21 W/mK) and isotropic phase (1.10 W/mK), it was realized that the thermal conductivity of TCLC films is mainly determined by the packing and orientation of PNP-6MA. Compared to conventional epoxy materials, the thermal conductivity of TCLC films is excellent because the high periodicity of the carbon atomic lattice of the mesogenic core improves the lattice vibration. Uniaxially and macroscopically oriented TCLC film were prepared by in situ photopolymerization under a magnetic field (B = 2T). Based on the experimental results and theoretical calculations, the directional thermal conductivity of the uniaxially oriented TCLC film was estimated to be 2.5 W/mK along the director of the mesogenic core. Direct thermal imaging demonstrated that TCLC films can be used very effectively as TIMs between high-power light emitting diodes (LED) and heat sinks. With excellent thermal conductivity and heat-guiding ability, TCLC film can be applied as an advanced thermal conducting material for sophisticated thermal management systems.

ASSOCIATED CONTENT


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Supporting Information. Synthetic procedure of PNP-6MA, preparative method for TCLC films, 1H NMR,

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C NMR, GC/MS/MS, WAXD, and TMA results were represented and

discussed. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (N. Kim), [email protected] (K.-U. Jeong). ACKNOWLEDGMENT This work was supported by BRL2015042417, MOTIE-KDRC (10051334), and Mid-Career Researcher Program (2016R1A2B2011041) of Republic of Korea.

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and

Circular-polarizing

Chirophotonic

Crystal

Reflectors:

Photopolymerization of Helical Nanostructures. ACS Nano 2016, 10 (10), 9570-9576. (29) Kim, D.-Y.; Lee, S.-A.; Jung, D.; Koo, J.; Kim, J. S.; Yu, Y.-T.; Lee, C.-R.; Jeong, K.U. Topochemical Polymerization of Dumbbell-shaped Diacetylene Monomers: Relationship between Chemical Structure, Molecular Packing Structure, and Gelation Property. Soft Matter 2017, 13 (34), 5759-5766. (30) Kang, D.-G.; Kim, D.-Y.; Park, M.; Choi, Y.-J.; Im, P.; Lee, J.-H.; Kang, S.-W.; Jeong, K.-U. Hierarchical Striped Walls Constructed by the Photopolymerization of Discotic Reactive Building Blocks in the Anisotropic Liquid Crystal Solvents. Macromolecules 2015, 48 (4), 898-907. (31) Im, P.; Kang, D.-G.; Kim, D.-Y.; Choi, Y.-J.; Yoon, W.-J.; Lee, M.-H.; Lee, I.-H.; Lee, C.-R.; Jeong, K.-U. Flexible and Patterned Thin Film Polarizer: Photopolymerization of Perylene-based Lyotropic Chromonic Reactive Mesogens. ACS Appl. Mater. Interfaces 2015, 8 (1), 762-771. (32) Park, M.; Yoon, W.-J.; Kim, D.-Y.; Choi, Y.-J.; Koo, J.; Lim, S.-I.; Kang, D.-G.; Hsu, C.-H.; Jeong, K.-U. Pyrene-Based Asymmetric Supramolecule: Kinetically Controlled Polymorphic Superstructures by Molecular Self-Assembly. Cryst. Growth Des. 2017, 17 (4), 1707-1715.


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(33) Kim, D.-Y.; Kang, D.-G.; Lee, M.-H.; Kim, J.-S.; Lee, C.-R.; Jeong, K.-U. A PhotoResponsive Metallomesogen for An Optically and Electrically Tunable Polarized Light Modulator. Chem. Commun. 2016, 52 (87), 12821-12824. (34) Feng, X.; Tousley, M. E.; Cowan, M. G.; Wiesenauer, B. R.; Nejati, S.; Choo, Y.; Noble, R. D.; Elimelech, M.; Gin, D. L.; Osuji, C. O. Scalable Fabrication of Polymer Membranes with Vertically Aligned 1 nm Pores by Magnetic Field Directed Selfassembly. Acs Nano 2014, 8 (12), 11977-11986. (35) Kim, D.-Y.; Park, M.; Lee, S.-A.; Kim, S.; Hsu, C.-H.; Kim, N.; Kuo, S.-W.; Yoon, T.H.; Jeong, K.-U. Hierarchical Superstructures from A Star-shaped Molecule Consisting of A Cyclic Oligosiloxane with Cyanobiphenyl Moieties. Soft Matter 2015, 11 (1), 5868. (36) Ghosh, S. A Model for the Orientational Order in Liquid Crystals. Nuovo Cimento D 1984, 4 (3), 229-244. (37) Collings, P. J.; Hird, M. Introduction to Liquid Crystals: Chemistry and Physics, CRC Press: 1997. (38) Nordström, J.; Berg, J. Conjugate Heat Transfer for the Unsteady Compressible Navier– Stokes Equations using A Multi-block Coupling. Comput. Fluids 2013, 72, 20-29. (39) Chen, J.; Huang, X.; Sun, B.; Wang, Y.; Zhu, Y.; Jiang, P. Vertically Aligned and Interconnected Boron Nitride Nanosheets for Advanced Flexible Nanocomposite Thermal Interface Materials. ACS Appl. Mater. Interfaces 2017, 9 (36), 30909-30917.


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Figure 1. (a) Molecular structure of PNP-6MA. Thermal transition behavior of PNP-6MA: (b) DSC thermograms of PNP-6MA at different scanning rates; (c) POM observation of PNP-6MA upon cooling from 100 °C to 0 °C at -1 °C/min.


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Figure 2. POM and their corresponding macroscopic images (insets) of TCLC films polymerized at (a) LC and (b) isotropic states, respectively. SEM cross-sectional observations of TCLC films polymerized at (c) LC and (d) isotropic states, respectively. (e) DMA plot of TCLC films polymerized at different phases.


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Table 1. Thermal and physical properties of TCLC films cured at LC and isotropic phases, respectively.

Density (ρ, g/cm3)a

Specific heat capacity (Cp, J/gK)b

Thermal conductivity (κ, W/mK)c

Tg (°C)d

Coefficient of thermal expansion (α, ppm/°C)e

LC phase

1.17

1.241

1.21

62.7

275.9

Isotropic phase

1.15

1.121

1.10

68.2

203.6

a

calculated from ρ = m/V, using mass and dimension of disc-shaped samples. bmeasured by DSC. cmeasured by TPS method. dmeasured by onset point of storage modulus of DMA analysis. e measured by TMA analysis.


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Figure 3. (a) Schematic illustration of the uniaxially aligned PNP-6MA molecules under a magnetic field (B = 2 T). (b) 2D WAXD pattern of the uniaxially aligned TCLC film prepared by in situ polymerization under a magnetic field (B = 2 T) and the magnified image of the low angle diffraction (inset). (c) Azimuthal scans of the 2θ-angle between 2.3° and 2.7° and between 18° and 21°, respectively.


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Figure 4. (a) Schematic illustration of experimental setup for the observation of heat transfer behaviors of TCLC films. (b) Surface temperature variations of TCLC films with time. (c) IR thermal images of TCLC films with time. (d) Computer-simulated heat transfer images for the TCLC films with time. The signs I, LC, and LC(H) indicate the TCLC film with isotropic, LC state and LC state with homeotropic alignment, respectively.


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Figure 5. (a) Schematic illustration of experimental setup for the observation of heat transfer behaviors of LED module with TCLC films as TIMs. (b) Temporal heat transfer images for the surfaces of LED modules with different TIMs. The signs I, LC, and LC(H) indicate the TCLC film with isotropic, LC state and LC state with homeotropic alignment, respectively. (c) Surface temperature variations of LED surfaces with time.


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Table of Contents


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Anisotropic Thermal Interface Materials: Directional Heat Transfer in

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