Forming a Homeotropic SmA Structure of Liquid Crystalline Epoxy Resin on an Amine-Modified Surface

The molecular orientation of a liquid crystalline (LC) epoxy resin (LCER) on silane coupling surfaces of amorphous soda-lime-silica glass substrates was investigated. The LC epoxy monomer on the silane coupling surfaces of the substrates was revealed to form a smectic A (SmA) phase with planar alignments because of the relatively low surface free energy. An LCER with a curing agent, however, formed a homeotropically aligned SmA structure by curing on a substrate surface modified using a silane coupling agent with amino groups. This formation of homeotropic alignment was considered due to the attribution of the reaction between the amino group on the surface of the substrate and the epoxy group of the LCER. The homeotropic alignment had a relatively high orientation parameter of 0.95. Therefore, it is expected to possess high thermal conductivity and be applied as high-thermal-conductivity adhesives or packaging materials for electrical and electronic devices.


INTRODUCTION
Epoxy resin (ER) has been widely adopted to a diverse range of applications because of its thermal resistance, electrical insulation, moldability, and adhesiveness.These properties meet the component requirements of electric and electronic devices.Among ERs, liquid crystalline (LC) ERs have been intensively synthesized 1−10 and investigated because of their outstanding properties such as high thermal conductivity, 11−22 fractural toughness, 23−26 and moisture resistance, 27 in addition to the properties of general LCs or amorphous ERs.Particularly, the high thermal conductivity of LCERs has gathered attention because electrical and electronic devices progress in downsizing, integrating, and multi-functionalizing, and the heat accumulated in the devices has become a critical issue.
The property of LCERs depend much on their molecular orientation.For instance, the thermal conductivity of an aligned LCER in the direction of the molecular chains is much higher than that in the transverse direction of the molecular chains. 12hus, forming a uniaxial alignment of molecular chains is effective in improving thermal conductivity in the direction of the molecular chains.Although general thermoplastic polymers can be molecularly oriented by stretching, it is difficult for thermosets.Alternatively, applying electric 28 or magnetic 12,29,30 fields during cure is effective for a LCER to form a uniaxial alignment.
Whereas a homeotropic alignment for typical LC molecules is generally formed by a surface effect.According to Creagh's conception, 31 a homeotropic alignment is induced on the surface of a substrate that possesses lower surface free energy than that of a LC [surface free energy of substrate (γ S ) < surface free energy of liquid (γ L )], and a planar alignment is induced on a higher energy surface (γ S > γ L ).However, most of these were the results regarding the typical alkyl-terminated LC molecules, and exceptions also occurred. 32In our previous study, a LC epoxy monomer was also an exception to Creagh's conception and homeotropically oriented on a substrate with high γ S . 18The thermal conductivity of the cross-linked LCER forming a homeotropically aligned smectic A (SmA) structure on a substrate with high γ S was 0.81−5.8W m −1 K −1 . 18From wideangle X-ray diffraction and grazing-incidence small-angle X-ray scattering (GISAXS) measurements, this outstanding thermal conductivity resulted from the uniaxially aligned SmA structure with 0.70−0.75 of the orientation parameters. 18,19hemical treatment has been widely applied to the ceramic filler surfaces of a composite to increase the adhesiveness of the filler−matrix interfaces and increase the mechanical property, 33−37 thermal conductivity, 38−41 and so forth 42−44 of the composites.However, when chemically treating with a coupling agent to ceramic surfaces, the effect of the functional group on the molecular ordering of a LCER has not been fully elucidated.
In this study, we investigated the molecular orientation of a LC epoxy monomer and conventional LC molecules on a ceramic surface modified using silane coupling agents.The LC epoxy monomer was cured with a curing agent on the surface of the substrates modified with the silane coupling agents.The orientational order of the cured LCER on their surfaces was then investigated using GISAXS.As a result, the cured LCER formed homeotropic alignment with 0.95 of the orientation parameter on the ceramic surface modified with the amino group, even though the surface had a low γ S .
The contact angles of water and hexadecane on G1, C1/G1, C2/G1, C3/G1, C4/G1, and C5/G1 were obtained at ambient temperature with a goniometer applying the 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 Owens−Wendt 45 further expanded the equation using the terms of the dispersive and polar components of the surface free energy with the geometric mean method The surface free energy components of the liquids are listed in Table 1, 46 and the resulting contact angles were assigned to eq 2.
To investigate the relationship between the polarity of a LC molecule and its orientational order, R1 and R2 were also observed with a POM under crossed polarizers on G1, C1/G1, C2/G1, C3/G1, C4/G1, and C5/G1.The chemical structures of R1 and R2 are shown in Figure 2b,c  ). with a thickness of 150−200 μm.GISAXS measurements of the cured droplets were carried out at room temperature.TM/DAN on G1 had not been prepared in this study because GISAXS measurements of it had already been reported in our previous paper. 18,19 RESULTS AND DISCUSSION 3.1.Surface Free Energy of Silane-Treated Glass Substrates.The contact angles of water droplets as a polar liquid and hexadecane droplets as a nonpolar liquid on the surfaces of G1, C1/G1, C2/G1, C3/G1, C4/G1, and C5/G1 are shown in Figure 3.The contact angles of water droplets on these surfaces of C1/G1, C2/G1, C3/G1, C4/G1, and C5/G1 were all higher than those of G1.This indicated that G1 was the most hydrophilic among them.It is also obvious by the comparison of γ S p of the substrates in Table 2 estimated using eq 2.

Characteristics of LC Molecules.
The differential scanning calorimetry (DSC) heating and cooling curves of R1 and R2 are shown in Figure 4.The curves of R1 showed an endothermic peak at 34 °C in the heating trace, derived from the N-Iso transition, and an exothermic peak at 33 °C in the cooling trace.However, R2 showed a monotropic N phase.An endothermic peak at 41 °C derived from the Cr-Iso transition appeared in the heating trace, while two exothermic peaks at 16 and 37 °C, indicating the Cr−N and N-Iso transitions, respectively, appeared in the cooling trace.The loop-shaped exothermic peak at 16 °C in Figure 4b is thought to be caused by the temperature rise due to the large amount of heat generated with crystallization.DSC curves of TM were reported in our previous paper, 17 and TM shows phase sequences on heating Cr (97 °C) SmA (140 °C) Iso phase and on cooling Iso (139 °C) N (138 °C) SmA (50 °C) Cr phase.
3.3.Characteristics of LC Molecules between Silane-Treated Glass Substrates.The POM observations under crossed polarizers revealed that TM sandwiched between pairs of each C1/G1, C2/G1, C3/G1, C4/G1, and C5/G1, which possess relatively low γ S , showed a fan-shaped texture of a planaraligned SmA phase, as shown in Figure 5b−f, while TM sandwiched between a pair of G1 that possesses high γ S showed dark fields at 130 °C after cooling from the Iso phase, as shown in Figure 5a.The dark fields were thought to be derived from a homeotropically aligned SmA phase. 18These results were consistent with our previous report, in which homeotropic alignments were induced on high-energy substrates and planar alignments were induced on low-energy substrates. 18,19The homeotropically alignments of TM were considered to be attributed to the formation of hydrogen bonds between the hydroxyl-terminated surfaces of G1 and the epoxy groups of TM.
The POM regarding R1, which possesses a polar group on one side of a molecular termination, sandwiched between pairs of each C1/G1, C3/G1, and C4/G1 that possess middle γ S , showed schlieren textures of a planar aligned N phase at 30 °C after cooling from the Iso phase, as shown in Figure 5h,j,k, while R1 sandwiched between pairs of G1 that possesses the highest γ S and R1 sandwiched between pairs of each C2/G1 and C5/G1 that possess relatively low γ S showed dark fields at 30 °C after cooling from the Iso phase, as shown in Figure 5g,i,l.The dark fields were thought to be derived from a homeotropically aligned N phase.The homeotropically alignment N of R1 on G1 was considered to be attributed to the polar interaction between the hydroxyl-terminated surfaces of G1 and the cyano group of R1, while the homeotropically aligned N of R1 on C2/G1 or C5/G1 was considered to be attributed to the nonpolar interaction between the alkyl surfaces of C2/G1 or C5/G1 and the terminal alkyl group of R1.For a LC molecule with no polar terminations, R2 sandwiched between pairs of each G1, C1/G1, C3/G1, C4/ G1, and C5/G1 that possess high γ S showed schlieren textures of a planar aligned N phase, as shown in Figure 5m,n,p−r, while R2 sandwiched between a pair of C2/G1 that possesses the lowest γ S showed dark fields derived from a homeotropically aligned N phase at 30 °C after cooling from the Iso phase, as shown in Figure 5o.The homeotropic alignment of R2 on C2/G1 was   also considered to be attributed to the nonpolar interaction between the alkyl surfaces of C2/G1 and the terminal alkyl group of R2, as well as that of R1 on C2/G1 or C5/G1.The relationship between γ S and textures of TM, R1, and R2 sandwiched between substrates pairs is shown in Figure 6.
Homeotropic alignments were induced on relatively high γ S surfaces for TM, on relatively low or high γ S surfaces for R1, and on relatively low γ S surfaces for R2.Since R1 and R2 are N and TM is Sm, the mechanism of phase transition is thought to be different.However, these homeotropic alignments were all considered to be derived from polar or nonpolar interactions between a molecular termination and substrate surface in this situation.

Characteristics of TM/DAN Mixtures on Chemically Treated Glass
Substrates.The surface effects on the molecular orientations of TM/DAN droplets cured on C1/G1, C2/G1, C3/G1, C4/G1, and C5/G1 were investigated using GISAXS measurements.The evident spots in the GISAXS patterns of the TM/DAN droplets cured on C5/G1, which corresponded to Sm layers with a periodicity of approximately 22 Å in the vertical direction to C5/G1, were observed (Figure 7e).Whereas a half-ring appeared in the GISAXS patterns of the TM/DAN droplets cured on C1/G1, C2/G1, C3/G1, and C4/ G1.The half-ring was slighter in the vertical direction to the substrates than that being in the transverse direction (Figure 7a−d).This was also obvious compared with the GISAXS intensity of β scans, as shown in Figure 8.These results indicate that TM/DAN is more likely to form the homeotropically aligned SmA domains on the C5/G1 surface modified with amino groups, however, its surface had low γ S .This homeotropic alignment formation on C5/G1 is considered to be derived from the reaction between the amino group on the surface and the epoxy group of TM.
The affinity between TM and C5/G1 is thought to be lower than that between TM/DAN and C5/G1 because amine is not included in TM.Also, the temperature of POM observations of 130 °C was too low to react between an epoxide of TM and an amino group on C5.For these reasons, it is considered that TM alone sandwiched between pairs of C5/G1 did not form a homeotropic alignment (Figure 5f).
The S of the induced homeotropically aligned Sm layers in the TM/DAN droplets cured on C5/G1 was calculated from the β scan of the GISAXS pattern (Figure 8e) using eq 3, referring to calculations by Benicewicz et al. 29  where I and α express the SAXS intensity and 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 the Lorentzian function (Figure 9); the graphs using the calculations are shown in Figure 10 for further information.As a result, the orientation parameter was estimated to be 0.95 for the homeotropically aligned TM/DAN droplets cured on C5/G1.This value was remarkable, and it was superior to 0.73  and 0.75 for the homeotropically aligned TM/DAN cured on untreated or UV-treated glass substrates. 18Moreover, the orientational order parameter of TM/DAN was expected to further increase when TM/DAN was sandwiched between a pair of C5/G1 but not a droplet on the substrate because of no interfaces between the TM/DAN droplet and air that may generate disorder.

CONCLUSIONS
The relationship between the molecular orientation of a LCER and the functional groups on a glass surface modified using chemical surface treatments was investigated.A LC epoxy monomer TM was revealed not to form homeotropic alignment    on a chemically treated substrate surface because of its relatively low γ S .However, a TM/DAN mixture formed a homeotropically aligned SmA structure by curing on a substrate surface modified with amino groups.This formation of homeotropic alignment was considered due to the attribution of the reaction between the amino group on the surface and the epoxy group of TM.The homeotropic alignment had a relatively high orientation parameter of 0.95.Therefore, it is expected to possess high thermal conductivity and be applied as high thermal conductivity adhesives or packaging materials for electrical and electronic devices.In addition, a homeotropic SmA LCER is also expected to be developed as a transparent high-thermalconductivity material because homeotropic SmA LCs are known to be highly transparent.

2 . 3 .
Chemical Structure and LC Behavior of LC Epoxy Monomer and Typical LC Molecules.The chemical structures of TM, which we used, are shown in Figure 2a.TM exhibits a phase sequence on heating the crystalline (Cr) (97 °C) SmA (140 °C) isotropic (Iso) phase and on cooling Iso (139 °C) nematic (N) (138 °C) SmA (50 °C) Cr phase.

Figure 4 .
Figure 4. DSC heating and cooling curves of (a) R1 and (b) R2 measured at rate of 10 °C/min.

Figure 6 .
Figure 6.Relationship between γ S and textures of TM, R1, and R2 sandwiched between pairs of each G1, C1/G1, C2/G1, C3/G1, C4/ G1, and C5/G1 observed using a POM.Open and closed circles show fan-shaped textures and dark fields in the Sm phase, respectively.Open and closed triangles show schlieren textures and dark fields in the N phase, respectively.

Figure 9 .
Figure 9. GISAXS intensities depending on angle α for homeotropically aligned TM/DAN mixture cured on C5/G1.Black solid and red dash lines indicate experimental results and Lorentzian fitting curves, I(α) = h/(1 + (α − u) 2 /w 2 ) + b, which were calculated using solver function in Microsoft Excel.Experimental results of maximum intensities, angles at maximum intensities, full at half-maximum, and background intensities were assigned to initial values of h, u, w, and b for the calculations, respectively.

Table 1 .
a (γ L p and γ L h in ref 46 were summarized as γ L p