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Article Cite This: ACS Omega 2018, 3, 3562−3570

Highly Oriented Liquid Crystalline Epoxy Film: Robust High ThermalConductive Ability Shingo Tanaka,*,†,§ Fusao Hojo,† Yoshitaka Takezawa,‡ Kiyoshi Kanie,§ and Atsushi Muramatsu§ †

Research & Development group, Hitachi, Ltd., 7-1-1 Omika, Hitachi, Ibaraki 319-1292, Japan Advanced Technology Research & Development Center, Hitachi Chemical Co., Ltd., 48 Wadai, Tsukuba, Ibaraki 300-4247, Japan § Institute of Multidisciplinary Research for Advanced Material, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan ‡

S Supporting Information *

ABSTRACT: The molecular orientation effect of a liquid crystalline (LC) epoxy resin (LCER) on thermal conductivity was investigated, with the thermal conductivity depending on the surface free energy of amorphous soda−lime−silica glass substrate surfaces modified using physical surface treatments. The LC epoxy monomer was revealed to form a smectic A (SmA) phase with homeotropic alignments on the surfaces of substrates that possess high surface free energies of 71.3 and 72.7 mN m−1, but forming a planar alignment on the surface of a substrate that possesses a relatively low surface free energy of 46.3 mN m−1. The optical microscopy observations and the X-ray analyses revealed that the LC epoxy monomer also induced a homeotropically aligned SmA structure due to cross-linking with a curing agent on the high-free-energy surface. The orientational order parameter of the resulting homeotropic SmA structure was calculated from the grazing incidence small-angle X-ray scattering patterns to be 0.73−0.75. The thermal conductivity of the cross-linked LCER forming a homeotropically aligned SmA structure was also estimated to be 2.0 and 5.8 W m−1 K−1 for the average and maximum in the direction of the Sm layer normal. The value of the thermal conductivity was remarkable among the thermosetting polymers and ceramic glass, and the LCER could be applied for high-thermal-conductive adhesives and packaging materials in electrical and electronic devices. W m−1 K−1 in the transverse direction of the oriented columnar axis. Geibel et al.12 also reported a 5.2 W m−1 K−1 uniaxitially oriented nematic (N) LC acrylate polymer, which was photopolymerized from the LC diacrylate monomer due to UV irradiation on the rubbed polyimide film. Among polymers, epoxy resin (ER) has been applied to diverse applications because of its abundance of properties, such as high thermal resistance, electrical insulation, moldability, and adhesiveness. These properties suit the requirements of components in electric/electronic devices. Therefore, highthermal-conductive ERs are desirable for heat dissipation in such applications. Up to date, various LCERs have been synthesized13−22 and intensively investigated because of their abundance of outstanding properties, such as high thermal conductivity,23−28 fractural toughness,29−32 and moisture resistance,33 in addition to the properties of general LCs or amorphous ERs. These remarkable properties are considered to depend on the macroscopically formed polydomain structures and the microscopically formed anisotropic LC structures in the domains. The LC epoxy monomers can spontaneously form LC ordering derived from the mesogenic core in the molecules. The

1. INTRODUCTION Because electric and electronic devices are rapidly being downsized and their power energy being enhanced, the heat density generated in such devices is continuously increasing. Although the metallic parts of heat spreaders and heat sinks have widely been used in the devices, they have not yet been enough to address the heat generated in advanced devices. Therefore, polymer components in the devices-mold resin, adhesives, coating materials, and so forth also need to possess high thermal conductivity. The thermal conductivity of typical amorphous polymers (approximately 0.1−0.5 W m−1 K−1) was around 2−3 orders of magnitude lower than that of metal and ceramic materials.1 Generally, high-thermal-conductive ceramic particles, such as alumina (Al2O3),2,3 boron nitride,4−7 and aluminum nitride,8−10 have added into polymers to increase the thermal conductivity. However, these additives probably lead to a lack of some properties, such as adhesiveness and lightness, in filling a high amount of ceramic particles. Therefore, increasing the thermal conductivity of the polymers themselves is essential. Kang et al.11 reported a high-thermal-conductive liquid crystalline (LC) polymer, which was photopolymerized using a thiol−ene reaction between disk-shape (discotic) liquid crystal (DLC) molecules. The thermal conductivity of the photopolymerized DLC polymer with a uniaxial orientation was estimated to be 3.0 © 2018 American Chemical Society

Received: December 30, 2017 Accepted: March 15, 2018 Published: March 27, 2018 3562

DOI: 10.1021/acsomega.7b02088 ACS Omega 2018, 3, 3562−3570

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treatments were carried out with a Mettler FP90 central processor with a FP82 HT hot stage. UV/ozone treatments were conducted with a SEN Lights photo surface processor PL21-200 equipped with a low-pressure mercury lamp EUV200GS-14. The radiant exposure was measured using an Ushio UIT-150 UV radiometer equipped with a UVD-S254 photodetector. Water was deionized for use. Diiodomethane and hexadecane (Wako Pure Chemicals, 136-07812 and 084-03683) were used as received. Their contact angles were measured by utilizing a Kyowa Interface Science CA-D goniometer, applying a sessile drop method. A tri-mesogenic LC epoxy monomer (Sumitomo Chemical, hereinafter called TM25,28) and 1,5-diaminonaphthalene (Sun Chemical, hereinafter called DAN) were used as received. Their molecular structures, in which the potential energies were minimized, were determined by molecular mechanical MM2 method with a ChemBio3D Ultra 13.0 (PerkinElmer). The measurements of the phase transition temperatures and determinations of LC phases were carried out with a Nikon Optiphot-2 Pol. The POM was equipped with a Mettler FP82 HT hot stage. Conoscopic observations were also carried out with the POM. Sekisui Chemical Micro Pearl SP-210 spherical spacers with an average diameter of 10.0 ± 0.5 μm were used to fabricate 10 μm gap glass cells. The thermal characterization was performed with a TA Instruments Q-2000 differential scanning calorimetry (DSC). The GISAXS measurements were carried out by a Rigaku Nano-viewer system using a Cu Kα radiation (40 kV, 30 mA) equipped with a 5-axis stage. A PILATUS-100K detector was used for the measurements. The WAXD measurements were also performed using a Rigaku RINT2500HL X-ray diffractometer with a Cu Kα radiation (50 kV, 250 mA). The Suzuki Optical White Glasses (the average thickness was 30 μm) and Matsunami Micro Cover Glasses (the average thickness was 145 μm) were used for the effective thermal conductivity measurements. A TM/DAN mixture with a 8−33 μm thickness was cured between a pair of glass substrates. The area of each layer in the three-layer structures was 10 mm square. The thicknesses of the glass substrates and each layer of the threelayer structures were measured by a micrometer and confirmed by measuring on the optical micrographs of the cross sections of the three-layer structures. The thermal diffusivity, density, and specific heat capacity were measured using a NETZSCH Instruments LFA-447 Nanoflash system, a Mirage Trading SD200L densitometer, and a TA Instruments Q-2000 DSC, respectively. 2.2. Surface Treatments and Estimation of Surface Free Energy of Glass Substrates. Amorphous soda−lime−silica glass substrates that possess three kinds of different surface states were prepared. One was untreated (hereinafter called G1), another was thermally treated at 250 °C for 10 min in an atmosphere (hereinafter called G2), and the other was UV/ ozone treated for 10 min (4.1 J cm−2) after the thermal treatment (hereinafter called G3). The contact angles of water, diiodomethane, and hexadecane on the prepared G1−G3 substrates were obtained at ambient temperature with a goniometer applying a sessile drop method. The equilibrium relationship between the vectors of the three-phase interfaces regarding a liquid droplet on a flat solid surface can be described by Young’s equation

LC ordering is maintained or developed during their crosslinking with a curing agent, which can suppress phonon scattering to improve the thermal conductivity.23 Although the thermal conductivity of LCERs varied corresponding to their chemical structures, LC orders, and so forth, those values (0.29− 0.96 W m−1 K−123,24,28) were much higher than those of conventional amorphous ERs (0.17−0.21 W m−1 K−1). Later, Song et al. reported25 that the thermal conductivity of an LCER could increase to 1.16 W m−1 K−1 by forming spherical domains with diameter more than 80 μm. One reason for this increase in the thermal conductivity was considered to be a reduction in the domain boundaries. The other reason should be an increase in the volume fraction of the LCER molecules aligned parallel to the direction of heat conduction, which is supported by the fact that the thermal conductivity of the aligned LCER is relatively higher in the direction parallel to the molecular chains than in the transverse direction.24 In the case of adhering two surfaces of substrates with LCER as an adhesive, the thermal conductivity of the adhesion layer should vary depending on the direction of the molecular alignment at the interface between the surfaces of the substrates and the adhesive layer of the LCER. The LCER adhesive layer was expected to improve the thermal conductivity in the direction of the layer thickness by forming a homeotropic alignment with the two surfaces of the substrates. Although electric,34 magnetic,24,35,36 and mechanical fields were effective to form a homeotropic alignment, we aimed to obtain one without those fields using the surface effects of the substrates to exert a high thermal conductivity more easily and commonly. On typical LC molecules, the physicochemical interactions, such as hydrogen bonds and dipole−dipole interactions between the LC molecules and the surfaces of substrates, have been considered to determine the orientation of the molecules planar or homeotropic to the surfaces of the substrates. According to the Creagh’s conception,37 a homeotropic alignment is induced on the surface of a substrate that possesses a lower surface free energy than that of an LC (surface free energy of substrate (γS) < surface free energy of liquid (γL)) and a planar alignment is induced on a higher energy surface (γS > γL). However, these were the results of utilizing the typical alkyl-terminated LC molecules, and exceptions also occurred.38 In the case of the diepoxides, the relationship between the molecular orientation of the LCER and the surface free energy of solid surfaces has not been fully elucidated. In this study, we first investigated the relationship between the molecular orientation of the LCER and the surface free energy of the glass surface varied by utilizing physical surface treatments. Next, an LC epoxy monomer was cured with a curing agent on the surface of the substrate modified by the surface treatment and oriented by the surface effect without the electric and magnetic fields. Then, the effect of the resultant alignment of the LCER on the thermal conductivity was also investigated. As a result, the thermal conductivity of the cross-linked LCER forming homeotropically aligned SmA structure on the high-free-energy surface was estimated to be 5.8 W m−1 K−1 for the maximum in the direction of the Sm layer normal.

2. EXPERIMENTAL SECTION 2.1. General. Amorphous soda−lime−silica glass substrates (Matsunami Micro Slide Glasses) were used for polarized optical microscope (POM) observations and contact angle, grazing incidence small-angle X-ray scattering (GISAXS), and wideangle X-ray diffraction (WAXD) measurements. Thermal

γL cos θ + γLS = γS 3563

(1) DOI: 10.1021/acsomega.7b02088 ACS Omega 2018, 3, 3562−3570

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ACS Omega Owens−Wendt39 further expanded the equation using the terms of the dispersive and polar components of the surface free energy with the geometric mean method

mixtures were also observed between pairs of G1−G3 substrates under crossed polarizers during curing at 150 °C. Then, the droplets of TM/DAN mixtures were cured on the prepared G1− G3 substrates at 150 °C for 2 h. The GISAXS and WAXD measurements of the cured droplets were carried out at room temperature. 2.5. Effective Thermal Conductivity Measurements. The thermal diffusivity, density, and specific heat capacity of the cured TM/DAN mixture and glass substrates were measured at a constant pressure using a Xe flash apparatus, a densitometer, and a DSC apparatus, respectively. Additionally, pairs of G1−G3 substrates were adhered with a TM/DAN mixture at 150 °C for 2 h. The effective thermal diffusivity (αeff) of the three-layer materials composed of pairs of G1−G3 substrates and the cured TM/DAN layer were also measured at constant pressure, respectively. Next, the thermal conductivity of the glass substrates (λsubstrates) and the effective thermal conductivity of the three-layer materials (λeff) were respectively estimated by

γ = γd + γ p γL(1 + cos θ ) = 2 γLdγSd + 2 γLpγSp

(2)

This equation was transformed to slope-intercept formula to incorporate results of the contact angles for more than two liquids at once40 γL(1 + cos θ ) 2

γLd

=

γSp ·

γLp γLd

+

γSd (3)

The surface free energy components of the liquids as described in Table 141 and the resulting contact angles were assigned to eq 3. Table 1. γLd, γLp, and γL of Water, Diiodomethane, and Hexadecanea

λ i = αi ρi C Pi (i = substrates and eff)

surface free energy (mN m )

a

liquid

γLd

γLp

γL

hexadecane diiodomethane water

27.6 46.8 29.1

0.0 4.0 43.7

27.6 50.8 72.8

(4)

where ρeff and CP,eff of the three-layer materials were calculated by42

−1

ρeff = ρresin ϕv + ρglass (1 − ϕv )

(5)

and

The γLp and γLh in the ref 41 were summarized as γLp.

C P,eff = C P,resinϕw + C P,glass(1 − ϕw )

The surface free energy components of the surfaces of the prepared G1−G3 substrates were calculated from the linear regression coefficients. 2.3. Chemical Structure and LC Behavior of LC Epoxy Monomer. The chemical structures and molecular structures in which potential energies were minimized for TM and DAN, which we used as the LC epoxy monomer and curing agent, are shown in Figure 1. TM exhibits a phase sequence on heating Cr

(6)

and ϕv and ϕw are the volume and mass fraction of TM/DAN, respectively.

3. RESULTS AND DISCUSSION 3.1. Surface Free Energy of the G1−G3 Substrates. The thermal treatment of the prepared G2 substrate was intended to reduce the surface free energy. This was supported by the fact, which Zhuravlev reported,43 that a H2O monolayer is completely removed and the total OH groups on the surface of amorphous silica significantly decrease, although the Si−O−Si bonds increase at over 190 °C in vacuo. In contrast, the UV/ozone treatment is a common practice for the increasing silanol groups on the glass surface;44,45 therefore, the surface of the prepared G3 substrate was intended to possess a high surface free energy compared with the G2 substrate. The contact angles of water droplets as a polar liquid on the surfaces of the prepared G1 and G3 substrates were significantly lower than that of the G2 substrate, whereas those of hexadecane droplets as a nonpolar liquid on the surfaces of the G1 and G3 substrates were much higher than that on the G2 substrate, as exhibited in Table 2. This indicated that the polarities of the surfaces of the G1 and G3 substrates were higher than that of the G2 substrate, which is also obvious by the comparison of γSp of the substrates in Table 3 estimated from the linear regression coefficients described in Figure S1 (the Supporting Information).

Figure 1. Chemical and molecular structures of (a) a tri-cyclic-type mesogenic epoxy monomer TM and (b) a curing agent DAN.

(97 °C) SmA (140 °C) Iso and on cooling Iso (139 °C) N (138 °C) SmA (50 °C) Cr.28 Textures of TM between pairs of G1− G3 substrates with 10 μm gaps were observed with a POM under crossed polarizers at 130 °C. 2.4. Preparation of LC Epoxy Monomer/Diamine Mixtures. A TM/DAN mixture in a 1:1 stoichiometric ratio shows a phase sequence on heating Cr (92 °C) SmA (112 °C) Iso.28 The curing behavior of the TM/DAN mixture was investigated using a DSC at the isothermal temperatures of 130, 140, 150, 160, 170, and 180 °C. The textures of the TM/DAN

Table 2. Contact Angles of Water, Diiodomethane, and Hexadecane Droplets for the G1−G3 Substrates contact angles of prepared glass substrates (deg)

3564

liquid

G1

G2

G3

hexadecane diiodomethane water

17 43 9

3 39 56

32 36 6


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ACS Omega Table 3. γSd, γSp, and γS of the prepared G1−G3 substrates

3.3. Characteristics of the TM/DAN Mixtures on the G1−G3 Substrates. Figure 3a,b shows the isothermal DSC curves of the TM/DAN mixtures at various temperatures and the enlarged view of the curve curing at 150 °C. The heat flow increased from the start of heating, which derived from the reaction of the active hydrogen atoms of DAN and the epoxides of TM. After approximately 6−8 min of elapsed time, the peak deriving from an Iso−Sm transition was generated in addition to the heat of the reaction due to the cross-linking.13 The TM/DAN mixtures were also observed by using a POM between the pairs of the G1−G3 substrates during curing at 150 °C under crossed polarizers, as shown in Figure 4. First, the mixtures melted into the Iso phase (Figure 4a,d,g). Second, the Sm domains formed in the Iso phase after approximately 8 min had elapsed (Figure 4b,e,h). The duration time required for the appearance of Sm structures was almost consistent with the DSC peak deriving from an Iso−Sm transition at 150 °C. Next, the molecules forming the Sm structures reoriented homeotropically or planarly depending on the surface free energy of the surfaces of the substrates. The resulting alignments of the TM/DAN mixtures cured between pairs of the G1−G3 substrates (Figure 4c,f,i) corresponded to those of TM between pairs of the G1−G3 substrates (Figure 2a,b,c), respectively. The Maltese crosses found in the textures by conoscopic observations could be identified as homeotropic alignments, as shown in Figure 4c,i insets. The mechanism to form a homeotropic alignment of TM/ DAN was considered to be different from TM because TM/ DAN can be influenced by a chemical reaction when it formed a homeotropic alignment. However, the surface free energy was dominant to determine the alignments of the LC epoxy thermosets in this case, even though it was under curing. The surface effects on the molecular orientations of TM/DAN droplets cured on the G1−G3 substrates were also investigated by utilizing the GISAXS measurements. The evident spots in the GISAXS patterns of TM/DAN droplets cured on the G1 and G3 substrates, which corresponded to Sm layers with a periodicity of approximately 23 Å in the vertical direction to the G1 and G3 substrates, were observed (Figure 5a,c). However, a relatively weak half-ring appeared in the GISAXS pattern of a TM/DAN droplet cured on the G2 substrate (Figure 5b). The half-ring was more slightly in the vertical direction to the G2 substrate rather than in the transverse direction. This was also obvious in a comparison of the GISAXS intensity of qz, qx, and azimuthal (β) scans, as shown in Figure 6. The narrow peaks in the qz scans were only detected from TM/DAN droplets cured on the G1 and G3 substrates (Figure 6a), although a narrow peak in the qx scans was detected from that on the G2 substrates (Figure 6b). These results also indicate that the TM/DAN is more likely to

surface free energy of prepared glass substrates (mN m−1) γSd γSp γS

G1

G2

G3

23.2 48.0 71.3

28.1 18.2 46.3

22.8 50.0 72.7

Our thermal treatment at 250 °C was revealed to be effective in decreasing the polar groups on glass surfaces, even though the treatments were executed in an atmosphere. The increase in the surface free energy by the UV/ozone treatment was also consistent with what occurred in preceding studies.44,45 The duration time dependent for UV/ozone treatment for the contact angles and surface free energy with respect to the surfaces of G2 substrates was also measured and is expressed in Figure S2 (the Supporting Information). 3.2. Characteristics of TM between the G1−G3 Substrates. Orthoscopic observations under crossed polarizers revealed that TM sandwiched between a pair of G2 substrates that possess relatively low γS showed a fan-shaped texture of a planar-aligned SmA phase (Figure 2b), whereas TM sandwiched between pairs of G1 and G3 substrates that possess high γS showed dark fields at 130 °C as shown in Figure 2a,c, respectively. The dark fields were revealed to be derived from a homeotropically aligned SmA phase but were not isotropic because Maltese crosses appeared in the textures during conoscopic observations46 (Figure 2a,c insets). These results were inconsistent with the Creagh’s conception, where denoted homeotropic alignments are induced on low energy substrates and planar alignments are induced on high-energy substrates. Park et al. have reported47 that UV-treated 4-cyano-4′pentylbiphenyl (5CB) induced a homeotropic alignment on hydroxyl-terminated surface, which was considered to possess a high γS. This result was considered to originate from the formation of hydrogen bonds between the hydroxyl-terminated surface of a substrate and 4-cyano-4′-pentylcarboxylic acid generated by the photochemical decomposition of 5CB. Coinciding with this report, the homeotropically alignments of TM were considered to be attributed to the formation of hydrogen bonds between the hydroxyl-terminated surfaces of G1 or G3 substrates and the epoxy groups of TM. It was also supported by the fact that epoxy and hydroxyl groups can form hydrogen bonds, which have been well researched in the preceding studies regarding graphene oxides utilizing simulations48 and experiments49,50 and regarding composites composed of silica and epoxidized natural rubbers.51−53

Figure 2. Orthoscopic optical micrographs showing (a, c) dark fields and (b) a fan-shaped texture at 130 °C of TM between pairs of (a) G1, (b) G2, and (c) G3 substrates taken under crossed polarizers. Insets in (a) and (c) show conoscopic optical micrographs. The scale bar in (c) is common for all images. 3565


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Figure 3. (a) Isothermal DSC curves for TM/DAN mixtures at 130, 140, 150, 160, 170, and 180 °C and (b) the enlarged view of the curve curing at 150 °C.

Figure 4. Orthoscopic optical micrographs showing textures during isothermal curing of a TM/DAN mixture at 150 °C between pairs of (a−c) G1, (d− f) G2, and (g−i) G3 substrates taken under crossed polarizers. Textures were observed at (a), (d), (g) 0, (b), (e), (h) 8, (c), (f), and (i) 12 min from the start of cure at 150 °C. Insets in (c) and (i) show conoscopic optical micrographs. The scale bar in (i) is common for all images.

form the homeotropically aligned SmA domains on the high γS surfaces but not on the relatively low γS surfaces. The POM images of the TM/DAN droplets cured on the G1−G3 substrates are also shown in Figure 7. These textures were similar to those of the TM/DAN mixtures cured between the pairs of the G1−G3 substrates (Figure 4c,f,i). However, the focal-conic and oily-streak defects were partially observed in the homeotropically aligned TM/DAN droplets cured on the G1 and G3. The appearance of these defects was considered to be due to the anchoring at the droplet and the air interface.

Then, the orientational order parameter (S) of the induced homeotropically aligned Sm layers in the TM/DAN droplets cured on the G1 and G3 were calculated from the β scans of GISAXS patterns (Figure 6c) using the following equations, referring to the calculations by Benicewicz et al.35 and Li et al.36 2

⟨cos α⟩ =

∫0

π /2

I(α) sin α cos2 α dα

∫0 S= 3566

π /2

, I(α) sin α dα

1 (3⟨cos2 α⟩ − 1) 2

(7) DOI: 10.1021/acsomega.7b02088 ACS Omega 2018, 3, 3562−3570

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substrates but not a droplet on the substrate because of no interfaces between the TM/DAN droplet and air that may generate a disorder. The WAXD measurements were also carried out for further information, as shown in Figure S5 (the Supporting Information); the first-, second-, third-, and fourth-order diffraction peaks corresponding to the Sm layers with a periodicity of approximately 23 Å in the vertical direction to the G1 and G3 substrates were obviously detected from the droplets cured on their substrates, whereas the first- and slight second-order diffraction peaks were only detected from the droplet cured on the G2 substrate. Broad peaks, which expressed the average distance of 4.7 Å between rodlike mesogens in the plane of Sm layers, were also detected from the TM/DAN droplets cured on the G1−G3 substrates. 3.4. Thermal Conductivity of the TM/DAN Mixtures Cured between the G1−G3 Substrates. Aiming to investigate the effect of the orientation of the Sm layer on the thermal conductivity, we estimated the effective thermal conductivities of the three-layer structures fabricated by curing the TM/DAN mixtures sandwiched between pairs of the prepared G1−G3 substrates, respectively, as shown in Figure 8. Here, the density and specific heat capacity of the cured TM/ DAN (ρresin and CP,resin) were assumed to be constant regardless of the molecular direction. The effective thermal conductivities of the three-layer structures fabricated using pairs of the G1 and G3 substrates, shown as open circles and squares in Figure 8, respectively, were all higher than those fabricated using pairs of the G2 substrates, shown as closed squares. All the samples were also confirmed by utilizing POM observations, where the TM/ DAN mixtures of adhesion layers formed homeotropic alignments between pairs of the G1 and G3 substrates, respectively, whereas they formed planar alignments between pairs of the G2 substrates, as shown in Figure S6 (Supporting Information). The effective thermal conductivity of the three-layer structure was also predicted by utilizing the following equation, based on the inverse rule of mixtures,54 with the thickness (l) and the thermal conductivity of each layer

Figure 5. GISAXS patterns for the TM/DAN droplets cured on (a) G1, (b) G2, and (c) G3 substrates.

where I and α express the SAXS intensity and the angle between the Sm layer plane and the substrate surface, respectively. The α can be calculated from cos α = cos χ cos θ, where θ is the Bragg angle for the scattering and χ is β + 90°. Then, I(α) was obtained by fitting with Lorentzian function (Figure S3, Supporting Information); the graphs used in the calculations are expressed in Figure S4 (Supporting Information) for further information. As a result, the orientation parameters were estimated to be 0.73 and 0.75 for the homeotropically aligned TM/DAN droplets cured on the G1 and G3 substrate, respectively. These values were remarkable and superior to 0.4 for an LCER oriented by curing under a 9.4 T magnetic field, which was reported by Li et al.,36 and they were close to the value of 0.8 for an LCER cured under a 12 T magnetic field by Benicewicz et al.35 The orientational order parameter of TM/DAN was expected to increase more when TM/DAN was sandwiched between a pair of G1 or G2

lsum l l = resin + substrates λeff λresin λsubstrates

(8)

where the adhesion layers of 0.41, 0.61, 0.81, and 5.8 W m−1 K−1 were assigned. The predicted curves are also displayed in Figure 8. The thermal conductivities of the planar and homeotropically aligned Sm layers of TM/DAN were found to be in the range of 0.41−0.61 and 0.81−5.8 W m−1 K−1, respectively, and their

Figure 6. (a) qz, (b) qx, and (c) β scans of GISAXS intensities for the TM/DAN droplets cured on the G1, G2, and G3 substrates, respectively. 3567


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Figure 7. Orthoscopic optical micrographs for the TM/DAN droplets cured on the (a) G1, (b) G2, and (c) G3 substrates taken under crossed polarizers. The focal-conic and oily-streak defects partially observed in the homeotropically aligned TM/DAN droplets cured on (a) G1 and (c) G3.

conductivity. Therefore, the alignment control also makes the design of the thermal conductive pass in the devices possible on demand.

4. CONCLUSIONS The relationship between the molecular orientation of an LCER and the surface free energy of a glass surface varied using physical surface treatments was investigated. An LC epoxy monomer TM was revealed to form a homeotropically aligned SmA phase on a substrate surface that possesses a high surface free energy, but it formed a planar alignment on a substrate surface that possesses a relatively low surface free energy. A TM/DAN mixture also formed the homeotropically aligned SmA structure on a substrate surface that possesses a high surface free energy under curing. These formations of the homeotropic alignments were considered because of the attribution of the hydrogen bonds formation between the hydroxyl-terminated surfaces and the epoxy groups of TM. The thermal conductivity in the direction of the molecular chains of a cross-linked LCER that induced a homeotropically aligned SmA structure was estimated to be 5.8 W m−1 K−1 for the maximum, and it was considered to be derived from the relatively high orientation parameter of 0.73−0.75. The thermal conductivity was remarkable among epoxy resins and also amorphous glasses. Therefore, a combination of the LCER and the surface treatments can be applied as high-thermal-conductive adhesives or as packaging materials for electrical and electronic devices. Moreover, this alignment control of an LCER by the surface effect enables control of the anisotropy of its thermal conductivity and designing of the thermal conductive pass in the devices on demand.

Figure 8. Effective thermal conductivities of the three-layer structures fabricated by curing of the TM/DAN mixture between the pairs of G1− G3 substrates at 150 °C, respectively, and those of the predicted value curves from eq 8. The horizontal axis expressed lresin/lsum. Open circles, closed squares, and open squares express pairs of G1−G3 substrates, respectively. A closed triangle indicates the average thermal conductivity of the glass substrates. Adhesion layers of 0.41, 0.61, 0.81, and 5.8 W m−1 K−1 were assigned in eq 8.

average values were 0.44 and 2.0 W m−1 K−1 in the calculations using eq 8. The scatter was considered due to the differences in the order parameter, cross-linking density, volume of defects, and so on, among the samples. Thus, the thermal conductivity in the vertical direction was revealed to be increased approximately 4.5 times for the average by changing the orientation from a planar to a homeotropic. The maximum thermal conductivity of the homeotropically aligned Sm layer of the TM/DAN was estimated to be 5.8 W m−1 K−1, which is higher than the thermal conductivity of 5.2 W m−1 K−1 for the photopolymerized LC diacrylate forming the N orientation.12 The improvement in the thermal conductivity in the direction of the layer normal was attributed to the homeotropically aligned SmA structures to the surfaces with orientation parameter of 0.73−0.75 induced by the effect of the high-free-energy surface. Consequently, a glass substrate could be adhered to another glass substrate with a neglective thermal resistance using the LC epoxy thermosets without including any filler and also without a magnetic or electric field. Thus, the combination of the LCER and the physical surface treatments for increasing the surface free energy to the substrates was expected to be applied as a high-thermalconductive adhesive or as packaging materials for electrical and electronic devices. This alignment control of the LCER by the surface effect enables control of the anisotropy of its thermal

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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/acsomega.7b02088. Linear regression lines to calculate surface free energy; duration time dependent for UV/ozone treatments; Lorentizian fitting curves to obtain I(α); graphical exhibitions to determine orientational order parameters; WAXD profiles for the TM/DAN droplets on the G1−G3 substrates; polarized optical micrographs of three-layer structures are available (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 3568


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Shingo Tanaka: 0000-0001-7373-6105 Notes

The authors declare no competing financial interest.

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