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JIEC-2507; No. of Pages 8 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
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Effect of backbone moiety in diglycidylether-terminated liquid crystalline epoxy on thermal conductivity of epoxy/alumina composite Thanhkieu Giang, Jinhwan Kim * Department of Polymer Science and Engineering, Sungkyunkwan University, Suwon 440-746, Gyeonggi-do, South Korea
A R T I C L E I N F O
Article history: Received 18 March 2015 Received in revised form 1 May 2015 Accepted 5 May 2015 Available online xxx Keywords: Liquid crystalline epoxy Alumina Composite Thermal conductivity Mesophase
A B S T R A C T
Three diglycidylether-terminated liquid crystalline epoxy (LCE) structures based on azomethine mesogen, 40 40 -bis(4-hydroxybenzylidene)-diaminodiphenylether diglycidylether (LCE-DPE), 40 40 -bis(4hydroxybenzylidene)-diaminophenylene diglycidylether (LCE-DP), and terephthalylidene-bis-(4-aminophenol) diglycidylether (LCE-TA) were synthesized in an attempt to investigate the effect of backbone moiety in epoxy on the thermal conductivity of LCE/alumina (Al2O3) composite. The synthesized species were characterized by 1H-NMR, FT-IR, Difference scanning calorimetry (DSC), Thermogravimetric analysis (TGA), and Optical microscope (OM). The liquid crystalline properties of three LCE resins themselves and the epoxy systems cured with an identical curing agent, 4,40 -diaminodiphenylsulfone (DDS) were examined using DSC and OM. The results show that all three LCE resins exhibit typical liquid crystalline behaviors: clear solid crystalline state below its melting temperature (Tm), sharp crystalline melting at Tm, transitions into either nematic or smectic mesophase above Tm, and consequent isotropic phase above isotropic temperature (Ti). The thermal conductivity was measured by laser flash method and the results were compared with the theoretical model. Experimental data fit well with the wellknown Agari–Uno’s theoretical model. It was found that the thermal conductivity of LCE/Al2O3 composite is strongly dependent on the backbone structure of LCE and the more the ordering in LCE backbone the higher the thermal conductivity. ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Since electric and electronic devices in today operation show a consistent tendency towards increasing radiant heat, one of the most important issues for such devices is to how to dissipate heat and what material would be the most desirable. The most commonly used materials for this purpose are composites that are comprised of organic polymer and inorganic filler. One noticeable approach is to improve thermal conductivity (l) of the organic polymer having lower l compared to that of filler, thus being able to apply diminished amount of inorganic filler. Epoxy resin is one of the polymeric materials widely used for this purpose. In our previous report [1], the thermal conductivities of amorphous, crystalline, and LCE systems filled with Al2O3 were investigated and the results clearly indicated that the thermal conductivity value of LCE composite is greater than those of
* Corresponding author. Tel.: +82 31 290 7283; fax: +82 31 290 7309. E-mail address: [email protected] (J. Kim).
crystalline and amorphous epoxies at the same Al2O3 loading. A highly ordered structure is crucial for obtaining high thermal conductivity of composite by suppressing phonon scattering. It is known that LCE species bearing various mesogenic units in structure, such as azo [2,3], stilbene [4–12], biphenyl [13–16], binaphthyl [17,18], azine [19], ester [20–25], and azomethine [26–30], are widely used as thermosetting resins as well as liquid crystalline polymers. Many research groups have reported the synthesis, curing behavior, and mechanical properties of epoxy containing liquid crystalline structures, but only a few articles have been published on the thermal conductive property of LCE based on above mentioned mesogens. Most of studies have focused on biphenyl or ester based epoxies [31–33] and azomethine mesogenic unit containing LCE resin was reported to exhibit high thermal conductivity [34]. Harada at al. reported that the mesogenic unit in LCE backbone is able to form self-ordered structure during curing process and is believed to be responsible for high thermal conductivity. In this study, the effect of backbone moiety in azomethine based LCE resin on the thermal conductivities of LCE/alumina
http://dx.doi.org/10.1016/j.jiec.2015.05.004 1226-086X/ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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Table 1 Characteristics of LCE resins employed in this study. Structure
composites was investigated. For this purpose, three different LCE structures bearing different Schiff-base backbone moieties (designated as LCE-DPE, LCE-DP, and LCE-TA) were synthesized. The chemical structures and characteristics of materials in terms of Tm and epoxy equivalent weight (EEW) are given in Table 1. Al2O3 of a commercial source was applied as an inorganic filler. The thermal conductivities of composites were experimentally determined and the results were compared with values predicted by a well-known Agari-Uno’s theoretical model. Experimental Materials Epichlorohydrin, tetrabutylammonium bromide (TBAB), 4-hydroxybenzaldehyde, 4,40 -oxydianiline, p-phenylenediamine, 4-aminophenol, terephthalaldehyde, and 4,40 -diaminodiphenyl sulfone (DDS) were purchased from Aldrich. N,N-dimethylacetamide, methanol, ether, acetone, and other basic chemicals were purchased from Samchun Chemical Company, Korea. 2-Methylimidazole (2MI) and Al2O3 with a mean diameter of 5 mm of commercial grade were provided by LG Innotek, Korea. The Al2O3 had a close spherical shape and irregular in shape. All chemicals were used without further purification.
Scheme 1. Synthetic schemes for LCE-DPE and LCE-DP employed in this study.
Name
Yield (%)
EEW (g/eq)
Tm (8C)
LCE-DPE
71
350
240
LCE-DP
56
249
194
LCE-TA
66
241
206
Synthesis of liquid crystalline epoxies Three LCE species containing various backbone moiety units were synthesized. The synthetic schemes are presented in Schemes 1 and 2. The yield, EEW, and Tm are presented in Table 1. The EEW of LCE resins was determined by analyzing the experimental 1H-NMR results following the procedures described in the literatures [35,36], (theoretical EEW values: 260 g/eq for LCE-DPE and 219 g/eq for LCE-DP and LCE-TA). The success of synthesis was confirmed by 1H-NMR and FT-IR. The results for LCETA are given in Figs. 1 and 2. Synthesis of 40 40 -bis(4-hydroxybenzylidene)-diaminodiphenylether (DPE) and 40 40 -bis(4-hydroxybenzylidene)-diaminophenylene (DP) 4-hydroxybenzaldehyde (3.67 g, 0.03 mol) was dissolved in methanol, and added into a 250 ml three-neck round-bottom flask at 30 8C. 4,40 -oxydianiline (3.0 g, 0.015 mol) or p-phenylenediamine (1.62 g, 0.015 mol) dissolved in methanol was added dropwise with vigorous stirring at 30 8C. The solution was stirred at that temperature for 18 h. The precipitated yellow solid was filtered, and sequentially washed several times with methanol and ether. The product was dried at 50 8C in a vacuum to give a yield of 85% and a melting point of 240 8C by DSC (heating rate of 10 8C/ min) for DPE; and a yield of 90% and a melting point of 271 8C by DSC (heating rate of 10 8C/min) for DP. 1H-NMR (DMSO-d6, ppm): DPE; d = 10.11 (s, 2H), d = 8.47 (s, 2H), d = 7.70–7.77 (m, 4H), d = 7.24–7.27 (m, 4H), d = 7.02–7.06 (m, 4H), and d = 6.86–6.89
Scheme 2. Synthetic scheme for LCE-TA.
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Synthesis of terephthalylidene-bis-(4-aminophenol) (TA) A 250 ml three-neck round-bottom flask was equipped with a temperature controller, reflux condenser, 100 ml addition funnel, heating oil bath, and mechanical stirrer. 4-aminophenol (3.27 g, 0.03 mol) was dissolved in methanol, and added into the flask at 30 8C. Terephthalaldehyde (2.01 g, 0.015 mol) dissolved in methanol was added dropwise with vigorous stirring at 30 8C. The solution was stirred at that temperature for 18 h. The precipitated yellow solid was filtered, and sequentially washed several times with methanol and ether. The product was dried at 50 8C in a vacuum to give a Yield of 96% and a melting point of 274 8C by DSC (heating rate of 10 8C/min). 1H-NMR (DMSO-d6, ppm): d = 9.58 (s, 2H), d = 8.65 (s, 2H), d = 7.98 (s, 4H), d = 7.22–7.27 (m, 4H), and d = 6.79–6.84 (m, 4H). Fig. 1. 1H-NMR spectra of synthesized LCE-TA.
(m, 4H). DP; d = 10.12 (s, 2H), d = 8.51 (s, 2H), d = 7.76–7.79 (m, 4H), d = 7.26–7.27 (m, 4H), and d = 6.87–6.90 (m, 4H). Synthesis of 40 40 -bis(4-hydroxybenzylidene)-diaminodiphenylether diglycidyl ether (LCE-DPE) and 40 40 -bis(4-hydroxybenzylidene)diaminophenylene diglycidyl ether (LCE-DP) A mixture of DPE (5.0 g, 0.012 mol), DMAc (50 ml), and epichlorohydrin (48.0 ml, 0.61 mol) was added into a 250 ml three-necked round-bottom flask equipped with a temperature controller, a reflux condenser, a heating oil bath, and a mechanical stirrer at room temperature. The mixture was slowly heated to 100 8C, and TBAB was added as a catalyst. The solution was stirred at that temperature for 6 h. After cooling to room temperature, the reaction mixture was poured into methanol. A light yellow solid was collected and continuously washed with deionized water and methanol. The product was dried at 40 8C in a vacuum. 1H-NMR (CDCl3-d6, ppm): d = 8.57 (s, 2H), d = 7.85–7.89 (m, 4H), d = 7.29– 7.31 (m, 4H), d = 7.06–7.11 (m, 4H), d = 4.41–4.44 (m, 2H), d = 3.91– 3.95 (m, 2H), d = 3.35–3.38 (m, 2H), 2.86–2.88 (m, 2H), and d = 2.73–2.75 (m, 2H). FT-IR: 2920, 2876 cm1 (CH2); 1606 cm1 (C5 5N); 1027 cm1 (ether); 916 cm1 (epoxy ring). Similar synthesis procedure was employed for the preparation of LCEDP with DP (5.0 g, 0.016 mol), DMAc (50 ml), and epichlorohydrin (62.0 ml, 0.79 mol). Yellow solid was obtained. 1H-NMR (CDCl3-d6, ppm): d = 8.43 (s, 2H), d = 7.84–7.87 (d, 4H), d = 7.24 (s, 4H), d = 6.96–7.02 (d, 4H), d = 4.29–4.31 (m, 1H), d = 4.00–4.04 (m, 1H), d = 3.36–3.39 (m, 1H), 2.92–2.94 (m, 1H), and d = 2.77–2.79 (m, 1H). FT–IR: 2933, 2882 cm1 (CH2); 1606 cm1 (C5 5N); 1027 cm1 1 (ether); 913 cm (epoxy ring).
Synthesis of terephthalylidene-bis-(4-aminophenol) diglycidyl ether (LCE-TA) TA (5.0 g, 0.016 mol), DMAc (50 ml), and epichlorohydrin (62.0 ml, 0.79 mol) were added into a three-necked round-bottom flask equipped with a temperature controller, reflux condenser, heating oil bath and mechanical stirrer at room temperature. The mixture was slowly heated to 100 8C, and TBAB was added as a catalyst. The solution was stirred at that temperature for 6 h. After cooling to room temperature, the reaction mixture was poured into methanol. A light yellow solid was collected and washed continuously with deionized water and methanol. The product was dried at 40 8C in a vacuum. 1H-NMR (CDCl3-d6, ppm): d = 8.52 (s, 2H), d = 7.98 (s, 4H), d = 7.25–7.27 (d, 4H), d = 6.96–6.98 (d, 4H), d = 4.24–4.27 (m, 2H), d = 3.98–4.01 (m, 2H), d = 3.36–3.39 (m, 2H), 2.91–2.93 (m, 2H), and d = 2.77–2.78 (m, 2H). FT-IR: 2930, 2873 cm1 (CH2); 1622 cm1 (C5 5N); 1030 cm1 (ether); 919 cm1 (epoxy ring). Measurements and sample preparation Spectroscopic analysis 1 H-NMR was performed on a Varian Unity Inova 500NB spectrometer using CDCl3-d6 as a solvent and tetramethylsilane (TMS) as a reference. The infrared (IR) spectra were obtained by means of a Nicolet 380 FT-IR spectrometer using KBR pellets. Thermal analysis TGA was performed on 2 to 10 mg samples under a nitrogen atmosphere at a heating rate of 10 8C/min using a TA instruments 2050 thermogravimetric analyzer. DSC was performed on 3 to 5 mg samples under a nitrogen atmosphere at a heating rate of 10 8C/min using a TA instruments 2910 DSC analyzer. Morphology observation Morphological features of the starting components and cured composites were observed with an OM equipped with crossed polarizers in transmitted light mode. Wide-angle X-ray diffraction Wide-angle X-ray diffraction (WAXD, D8 discover) with Cu(Ka) radiation was used to determine liquid crystalline characteristics and d-spacing. The tube source was operated at 1.6 kW with scanning speed of 58/min and sample interval of 0.058.
Fig. 2. FT-IR spectra of synthesized LCE-TA.
Sample preparation for thermal conductivity measurement A mixture of epoxy, curing agent, and catalyst at the designated composition was dissolved in N,N-dimethylacetamide. The mixture was shaken by a paste mixer machine. Then, a designated amount of Al2O3 was added and the mixture shaken again. The solvent was removed by placing the sample in a vacuum oven at a pre-determined temperature for 24 h. Samples were molded by a
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hot press with a pressure of 2500 psi. LCE-DPE/DDS system was cured at 220 8C for 30 min. LCE-DP/DDS and LCE-TA/DDS systems were cured at 190 8C for 30 min. The specimen had a diameter of 12.7 mm and thickness of 1.0 mm. Thermal conductivity measurement The thermal conductivity (l) was obtained from the formula:
l ¼ a d Cp where a, d, and Cp represent the thermal diffusivity, density, and heat capacity of the test specimen at 25 8C, respectively. The thermal diffusivity was measured by laser flash method employing a LFA 447 flash diffusivity instrument (NETZSCH-Gera¨tebau GmbH). Cp was determined separately by DSC in a temperature range from 0 to 100 8C at a heating rate of 10 8C/min.
phase transition of LCE-TA can be referred to as C 206 S 234 N 250 I. Similar results were reported in the literatures [37,38]. It should be noticed that the temperature range at which mesophase remains stable is broad for LCE-TA and LCE-DP while very narrow range is observed for LCE-DPE. One more thing that should be noted from the DSC thermograms of three LCE resins is that there are very profound exothermic reactions above 250 8C, which are related to the homopolymerizations of LCE molecules at isotropic phase. The degradation behaviors of three LCE resins were investigated and the results are shown in Fig. 5. Three LCE resins decompose thermally in single step with an onset degradation temperature of over 300 8C and leave high proportion of residual chars. This
Results and discussion Characteristics of liquid crystalline structures of synthesized LCE resins The thermal behaviors and morphologies of three synthesized LCE resins were examined using DSC, TGA, and OM. Fig. 3 shows DSC thermograms of the LCE resins recorded under dynamic conditions. It is clear that LCE-DPE has very broad endothermic peak at 240 8C (Tm), which corresponds to the melting of this compound (melting enthalpy (DHm) = 42.8 kJ mol1 and melting entropy (DSm) = 83.4 J mol1 K1). However, LCE-DPE does not show clear endotherm above Tm since the high temperatures at which the nematic mesophase is formed lead to homopolymer reactions and crosslinking. Thus, the mesophase range of LCE-DPE is quite narrow and the nematic phase shown in Fig. 4a, which is observed by optical microscope, becomes isotropic phase in a short period of heating. The DSC curve of LCE-DP shows an distinct endothermic peak centered at 194 8C due to melting of crystalline structure, which corresponds to the change of crystalline solid into liquid crystalline state (DHm = 35.5 kJ mol1 and DSm = 76.0 J mol1 K1), and the nematic phase is still preserved up to 220 8C. The optical microscope picture given in Fig. 4b shows direct evidence for the nematic phase of LCE-DP having disclination brushes which can be characterized as Schlieren texture. This phase disappears above 220 8C, which related to the transition (Ti) of liquid crystalline structure to isotropic phase. Thus, it is concluded that LCE-DP exhibits a nematic mesophase between 194 and 220 8C (C 194 N 220 I). LCE-TA shows two peaks: the first one centered at 206 8C (DHm = 32.8 kJ mol1 and DSm = 68.5 J mol1 K1) is due to the melting of crystalline solid and above that temperature the smectic phase is observed as presented in Fig. 4c; the second small peak observed at 234 8C concerns the transition to the nematic phase (Tn). Therefore, the
Fig. 3. DSC thermograms of synthesized LCE resins.
Fig. 4. Morphologies of (a) LCE-DPE at 230 8C, (b) LCE-DP at 200 8C, and (c) LCE-TA at 210 8C when observed by an optical microscope.
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Table 2 Thermal properties related to the curing of LCE resins with DDS.
Fig. 5. TGA thermograms of synthesized LCE resins.
confirms that the LCE resins are thermally stable in the mesophase range. Curing of epoxies with a curing agent (DDS) Three LCE resins are cured with an identical curing agent, DDS and their curing behaviors are investigated by dynamic and isothermal DSC experiments. DSC thermograms of LCE-DPE/DDS, LCE-DP/DDS, and LCE-TA/DDS mixtures isothermally cured at 210 8C are shown in Fig. 7. The curing of mixture takes place quite fast enough and can be accomplished within 30 min. Fig. 6 shows the dynamic curing behaviors of LCE/DDS systems. Generally, the reaction between the oxirane ring in epoxy and the amine in DDS proceeds by a nucleophilic substitution reaction [39]. For the experiments, an identical curing agent at an equimolar ratio was employed and the same amount (3 mol%) of 2MI was added as a catalyst. The epoxy/amine curing reaction is initiated by the reaction of epoxy ring reactant with the primary amine to generate the secondary amine, then the other epoxy groups further react with the secondary amine [40]. Since the same amine is used as a curing agent, the same curing mechanism applies to the curing reaction of LCE-DPE/DDS, LCE-DP/DDS, and LCE-TA/DDS mixtures. However, the dynamic curing behaviors shown in Fig. 6 and the characteristic curing properties summarized in Table 2 are quite different for three systems. The first thing to note in Fig. 6 is the onset temperature (Tonset) and the peak temperature (Tpeak) of the curing reaction. The Tonset values of the LCE-DPE/DDS and LCE-TA/DDS systems are quite lower than Tm values of neat LCE resins. The LCE-DP/DDS and LCETA/DDS systems show typical exothermic curing peaks above
System
Tonset (8C)
Tpeak (8C)
Heat of curing (J/g)
LCE-DPE/DDS LCE-DP/DDS LCE-TA/DDS
228 186 191
238 211 206
37 256 244
Tonset. The Tpeak of LCE-DP/DDS is slightly higher than that of LCETA/DDS. This is easily understandable when considering that the curing proceeds faster in the initial stages of the curing reaction for LCE-DPE/DDS and LCE-TA/DDS systems. In Fig. 3, the Tm is found to be 194 and 206 8C for LCE-DP and LCE-TA, respectively. Small endothermic peaks at about 177 and 180 8C observed in Fig. 6 are assigned to the melting of DDS in the mixtures, indicating that crosslinking between LCE and DDS occurs prior to the melting of LCE-DP and LCE-TA. This means that the curing reaction for LCEDP/DDS and LCE-TA/DDS mixtures starts in the solid state. Hence, the curing reactions of LCE-DP/DDS and LCE-TA/DDS proceed immediately after the melting of DDS, followed by fast exothermic curing reaction up to Tpeak = 211 8C for LCE-DP/DDS and Tpeak = 206 8C for LCE-TA/DDS, which are closely related the Tm values of neat LCE resins. Then the curing reaction decreases slowly and reaches a minimum at about 250 8C, which temperature is believed to be related to the T i of LCE-DP and LCE-TA. LCE-DP and LCE-TA display isotropic phase at temperature above T i and crosslinking is possible above this temperature. It is believed that the partial crosslinking of LCE-DP resin with DDS at nematic mesophase and LCE-TA with DDS at both smectic and nematic mesophase. The second exothermic peak observed above 250 8C is believed to be related to homopolymerization of LCE at isotropic phase. Similar finding is reported by other researchers [41]. However, more study is needed to support this assertion. On the other hand, the curing heat of LCE-DPE/DDS is surprisingly lower compared to those of LCE-DP/DDS and LCETA/DDS mixtures. The main reason is assumed to be related to its mobility during the curing process. The electron-withdrawing aromatic azomethine group may accelerate the nucleophilic substitution reaction between the oxirane ring in epoxy and the aromatic amine in DDS. Therefore, the curing rate of LCE/DDS should depend on the electron-withdrawing tendency of the azomethine group present in LCE resin. Since the same curing conditions (curing agent, catalyst, and curing temperature) are employed for all the systems, only the difference in LCE structure would affect the curing rate. LCE-DPE should exhibit faster curing rate than LCE-DP and LCE-TA because the electron-withdrawing tendency of diphenylether (–C6H4–O–C6H4–) group is stronger compared with that of phenylene (–C6H4–) group. However, the opposite result is observed in Fig. 6. Then, this may be due to the difference in stereo structure of the three LCE resins. The –C6H4–O– C6H4– unit present in LCE-DPE is much bulkier than the –C6H4– unit in LCE-DP and LCE-TA and this steric hindrance effect leads to higher Tm for LCE-DPE and consequently suppressed curing for LCE-DPE/DDS system. At 210 8C, LCE-DPE remains as solid crystal because its Tm is 240 8C. When LCE-DPE is cured at this temperature, very limited curing is possible with already molten curing agent, DDS. On the contrary, LCE-DP and LCE-TA exhibit liquid crystalline mesophase at 210 8C, enabling curing to proceed in liquid state. The isothermal DSC result shown in Fig. 7 strongly supports this explanation. Morphologic observation for cured LCE/DDS mixtures
Fig. 6. Dynamic DSC thermograms of LCE/DDS systems.
An optical microscope is employed for the cured specimens to observe directly the differences in the morphology of the cured
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Fig. 7. Isothermal DSC thermograms of LCE/DDS systems at 210 8C.
mixtures and the results are given in Fig. 8. The mixture of each LCE and DDS at the equivalent molar ratio with the same catalyst amount was cured by following the curing schedules presented in Fig. 8. The curing schedules are chosen by analyzing the curing behaviors shown in Fig. 6 and employed to prepare the specimens for the thermal conductivity measurements, which results are given in Fig. 10. For the sake of clear observation, optical microscopic pictures under cross polarized light are taken for the specimens containing no filler at all. In Fig. 8a, birefringence regions and black isotropic phase are observed for the cured LCE-DPE/DDS system, indicating that it is a biphase of nematic and isotropic phases. On the other hand, well distinct liquid crystalline morphologies are observed for the cured LCE-DP/DDS and LCETA/DDS systems. The LCE-DP/DDS mixture shows well reserved ordered Schlieren texture while the LCE-TA/DDS mixture displays smectic liquid crystalline structure after curing as can be seen in Fig. 8b and c, respectively. According to theory, the smectic state is more solid-like than the nematic and greater alignment with the director is possible in the semectic phase than in the nematic phase [42]. Therefore, it is concluded that the cured LCE-DPE/DDS, LCE-DP/DDS, and LCE-TA/DDS mixtures are quasi-isotropic, nematic, and smectic liquid crystal, respectively and it can be stated that the higher ordered network structure is obtained for cured LCE-TA/DDS than for cured LCE-DP/DDS and LCE-DPE/DDS. To investigate the ordered network structure for the cured systems, the wide-angle X-ray diffraction (WAXD) patterns was measured at room temperature for the cured specimens and the results are presented in Fig. 9. A broad WAXD peak at 2u = 18.78 is observed for cured LCE-DPE/DDS, suggesting that there exists a layered molecular organization of network [43]. LCE-DP/DDS system shows broad peak at 2u = 19.98, which is an indication of the lateral diffraction of mesogenic unit. The d-spacing is calculated to be 4.5 A˚, which is a characteristic of a nematic mesophase. Comparing the lateral distances between LCE-DPE/ DDS and LCE-DP/DDS, the latter has a shorter space and that is more aligned structure. The cured LCE-TA/DDS system displays the typical WAXD pattern of a layered structure with two major peaks: the first one at 2u = 20.88, which corresponds to the lateral diffraction of mesogenic unit with the d-spacing of 4.3 A˚; the other sharp peak at 2u = 3.08 in the small-angle region, which represents the d-spacing 29.4 A˚ of the rigid-rod unit between LCE-TA containing cross-link sites. LCE-TA/DDS system shows the lowest lateral distance, indicating that the highest ordered mesogenic structure is present. The X-ray results are in accordance with DSC and OM experimental results given previously.
Fig. 8. Morphologies of cured (a) LCE-DPE/DDS, (b) LCE-DP/DDS, and (c) LCE-TA/DDS systems when observed by an optical microscope.
Thermal conductivity of LCE/DDS/Al2O3 composite The thermal conductivities of various LCE/DDS/Al2O3 composites as a function of alumina loading are experimentally determined and the results are presented in Fig. 10. The results clearly show that the thermal conductivity of composite linearly increases with filler loading as expected. The most important thing to pay attention to in Fig. 10 is that the thermal conductivities of composites are strongly dependent on the type of LCE employed. At the same filler loading, the thermal conductivity of composite is lowest for the LCE-DPE/DDS system
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Fig. 9. WAXD patterns of cured LCE/DDS systems.
and highest for the LCE-TA/DDS system. That of the LCE-DP/DDS system falls in between. Therefore, it is concluded that the thermal conductivity is governed by the LCE structure. The thermal conductivity of a solid depends strongly on phonon diffusion [44] and it is well known that phonons dominate heat conduction in insulators and semiconductors [45]. The thermal conductivity of LCE-DPE/DDS/Al2O3 determined in this study is 2.83 W/m K at 50 vol% loading of Al2O3. The thermal conductivity of 3.04 W/m K is achieved for the LCE-DP/DDS system at the same filler loading. From these results, it can be clearly stated that the thermal conductivity of LCE containing –C6H4– backbone is higher than that containing – C6H4–O–C6H4– backbone, implying that the phasic nematic structure of oriented phenylene groups has a positive effect on suppressing phonon scattering and consequently improves the thermal conductivity of the composite; and that an aligned crystalline structure is needed to increase the efficiency of the phonon’s diffusion. The thermal conductive value of LCE-TA/DDS/ Al2O3 composite is 3.59 W/m K at the same 50 vol% of Al2O3 loading and this value is 1.27 and 1.18 times greater than that of the LCEDPE/DDS/Al2O3 and LCE-DP/DDS/Al2O3 systems, respectively. From these results, it is concluded that the thermal conductivity of LCE containing –N5 5CH–C6H4–CH5 5N– is higher than that containing – CH5 5N–C6H4–N5 5CH–. However, the reason for this finding is not clear at this moment and further study is needed. Comparison with the theoretical model
7
Fig. 11. Thermal conductivity plots of LCE/DDS/Al2O3 composites as a function of filler content: (symbols and full lines) experimental; (broken lines) calculated by adopting Agari-Uno model.
inorganic filler. In our previous work [1], two models were discussed: the Lewis-Nielsen [46,47] and the Agari-Uno [48,49] model. The results showed that for all cases, the Agari-Uno model provides a better estimate compared to the Lewis-Nielsen model. In this study, the experimentally determined thermal conductivity values discussed previously are only compared with the values predicted by the Agari-Uno model. Fig. 11 shows the comparison between experimental thermal conductivity data for the LCE/DDS systems and the values calculated by adopting the Agari-Uno model. For all systems, the Agari-Uno model provides good estimates. For LCE-DPE/DDS, the theoretical values fit the experimental data very well when 0.30 W/m K is used for the thermal conductivity of epoxy resin itself. To obtain reasonable fits with the experimental data, 0.35 and 0.45 W/m K should be used for the thermal conductivity of LCE-DP/DDS and LCE-TA/DDS, respectively. This very clearly supports the objective of this study in that the thermal conductivity of the cured LCE containing network depends strongly on the nature of liquid crystalline backbone epoxy and the degree of ordering in the cured network. It has been reported in literatures that the thermal conductivity values of neat polymers range from 0.1 to 0.6 W/m K. The value of 0.19 W/m K was reported for a most commonly used epoxy resin, diglycidyl ether of bisphenol A [50] and a higher value was achieved for neat liquid crystalline epoxy [51].
Various models have been proposed to predict the thermal conductivity of composite comprised of organic polymer and
Conclusions
Fig. 10. Thermal conductivities of various LCE/DDS/Al2O3 composites as a function of filler content.
The effects of backbone moiety in three synthesized LCE resins and of the mesophase of curing systems on the thermal conductivities of LCE/Al2O3 composites were investigated. All of the synthesized LCE resins themselves exhibit typical liquid crystalline behaviors above their high melting temperatures. And the crystalline structures are well retained when cured with the curing agent, which is believed to contribute high thermal conductivity of composite with Al2O3 filler. Cured LCE-TA/DDS system displays a smectic mesophase while cured LCE-DP/DDS system exhibits a nematic mesophase and LCE-DPE/DDS system shows a biphasic mixture of nematic mesophase and isotropic phase. Ordering of mesogenic unit is greatest for cured LCE-TA/DDS as clearly demonstrated by the lowest lateral spacing from WAXD results. The broad peak in the wide-angle region was observed at 2u = 18.78, 19.98, and 20.88 for the cured LCE-TA/DDS, LCE-DP/DDS, and LCE-DPE/DDS, respectively. At the same Al2O3 loading of 50 vol%, the thermal conductivities for these mixtures were 2.83,
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3.04, and 3.59 W/m K, respectively. It indicates clearly that the thermal conductivity is directly affected by the ordering tendency of mesogenic unit. It was found that the thermal conductivity values calculated by adopting the Agari-Uno model fit the experimental data fairly well when 0.30 W/m K, 0.35 W/m K, and 0.45 W/m K are used for the thermal conductivity of LCE-DPE, LCE-DP, and LCE-TA, respectively. The more ordered structure, the higher thermal conductivity. This clearly supports the objective of this study in that the thermal conductivity of LCE is a key factor to govern the thermal conduction of LCE containing thermosets. Acknowledgments The authors appreciate the financial support from LG Innotek. This work was also supported by the grant funded by the Korea Government Ministry of Trade, Industry and Energy (10040860) and by the R&D program funded by the Ministry of Science, ICT & Future Planning (Grant number: 2013M3C8A3075845). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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JIEC-2507; No. of Pages 8 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
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Effect of backbone moiety in diglycidylether-terminated liquid crystalline epoxy on thermal conductivity of epoxy/alumina composite Thanhkieu Giang, Jinhwan Kim * Department of Polymer Science and Engineering, Sungkyunkwan University, Suwon 440-746, Gyeonggi-do, South Korea
A R T I C L E I N F O
Article history: Received 18 March 2015 Received in revised form 1 May 2015 Accepted 5 May 2015 Available online xxx Keywords: Liquid crystalline epoxy Alumina Composite Thermal conductivity Mesophase
A B S T R A C T
Three diglycidylether-terminated liquid crystalline epoxy (LCE) structures based on azomethine mesogen, 40 40 -bis(4-hydroxybenzylidene)-diaminodiphenylether diglycidylether (LCE-DPE), 40 40 -bis(4hydroxybenzylidene)-diaminophenylene diglycidylether (LCE-DP), and terephthalylidene-bis-(4-aminophenol) diglycidylether (LCE-TA) were synthesized in an attempt to investigate the effect of backbone moiety in epoxy on the thermal conductivity of LCE/alumina (Al2O3) composite. The synthesized species were characterized by 1H-NMR, FT-IR, Difference scanning calorimetry (DSC), Thermogravimetric analysis (TGA), and Optical microscope (OM). The liquid crystalline properties of three LCE resins themselves and the epoxy systems cured with an identical curing agent, 4,40 -diaminodiphenylsulfone (DDS) were examined using DSC and OM. The results show that all three LCE resins exhibit typical liquid crystalline behaviors: clear solid crystalline state below its melting temperature (Tm), sharp crystalline melting at Tm, transitions into either nematic or smectic mesophase above Tm, and consequent isotropic phase above isotropic temperature (Ti). The thermal conductivity was measured by laser flash method and the results were compared with the theoretical model. Experimental data fit well with the wellknown Agari–Uno’s theoretical model. It was found that the thermal conductivity of LCE/Al2O3 composite is strongly dependent on the backbone structure of LCE and the more the ordering in LCE backbone the higher the thermal conductivity. ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Since electric and electronic devices in today operation show a consistent tendency towards increasing radiant heat, one of the most important issues for such devices is to how to dissipate heat and what material would be the most desirable. The most commonly used materials for this purpose are composites that are comprised of organic polymer and inorganic filler. One noticeable approach is to improve thermal conductivity (l) of the organic polymer having lower l compared to that of filler, thus being able to apply diminished amount of inorganic filler. Epoxy resin is one of the polymeric materials widely used for this purpose. In our previous report [1], the thermal conductivities of amorphous, crystalline, and LCE systems filled with Al2O3 were investigated and the results clearly indicated that the thermal conductivity value of LCE composite is greater than those of
* Corresponding author. Tel.: +82 31 290 7283; fax: +82 31 290 7309. E-mail address: [email protected] (J. Kim).
crystalline and amorphous epoxies at the same Al2O3 loading. A highly ordered structure is crucial for obtaining high thermal conductivity of composite by suppressing phonon scattering. It is known that LCE species bearing various mesogenic units in structure, such as azo [2,3], stilbene [4–12], biphenyl [13–16], binaphthyl [17,18], azine [19], ester [20–25], and azomethine [26–30], are widely used as thermosetting resins as well as liquid crystalline polymers. Many research groups have reported the synthesis, curing behavior, and mechanical properties of epoxy containing liquid crystalline structures, but only a few articles have been published on the thermal conductive property of LCE based on above mentioned mesogens. Most of studies have focused on biphenyl or ester based epoxies [31–33] and azomethine mesogenic unit containing LCE resin was reported to exhibit high thermal conductivity [34]. Harada at al. reported that the mesogenic unit in LCE backbone is able to form self-ordered structure during curing process and is believed to be responsible for high thermal conductivity. In this study, the effect of backbone moiety in azomethine based LCE resin on the thermal conductivities of LCE/alumina
http://dx.doi.org/10.1016/j.jiec.2015.05.004 1226-086X/ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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Table 1 Characteristics of LCE resins employed in this study. Structure
composites was investigated. For this purpose, three different LCE structures bearing different Schiff-base backbone moieties (designated as LCE-DPE, LCE-DP, and LCE-TA) were synthesized. The chemical structures and characteristics of materials in terms of Tm and epoxy equivalent weight (EEW) are given in Table 1. Al2O3 of a commercial source was applied as an inorganic filler. The thermal conductivities of composites were experimentally determined and the results were compared with values predicted by a well-known Agari-Uno’s theoretical model. Experimental Materials Epichlorohydrin, tetrabutylammonium bromide (TBAB), 4-hydroxybenzaldehyde, 4,40 -oxydianiline, p-phenylenediamine, 4-aminophenol, terephthalaldehyde, and 4,40 -diaminodiphenyl sulfone (DDS) were purchased from Aldrich. N,N-dimethylacetamide, methanol, ether, acetone, and other basic chemicals were purchased from Samchun Chemical Company, Korea. 2-Methylimidazole (2MI) and Al2O3 with a mean diameter of 5 mm of commercial grade were provided by LG Innotek, Korea. The Al2O3 had a close spherical shape and irregular in shape. All chemicals were used without further purification.
Scheme 1. Synthetic schemes for LCE-DPE and LCE-DP employed in this study.
Name
Yield (%)
EEW (g/eq)
Tm (8C)
LCE-DPE
71
350
240
LCE-DP
56
249
194
LCE-TA
66
241
206
Synthesis of liquid crystalline epoxies Three LCE species containing various backbone moiety units were synthesized. The synthetic schemes are presented in Schemes 1 and 2. The yield, EEW, and Tm are presented in Table 1. The EEW of LCE resins was determined by analyzing the experimental 1H-NMR results following the procedures described in the literatures [35,36], (theoretical EEW values: 260 g/eq for LCE-DPE and 219 g/eq for LCE-DP and LCE-TA). The success of synthesis was confirmed by 1H-NMR and FT-IR. The results for LCETA are given in Figs. 1 and 2. Synthesis of 40 40 -bis(4-hydroxybenzylidene)-diaminodiphenylether (DPE) and 40 40 -bis(4-hydroxybenzylidene)-diaminophenylene (DP) 4-hydroxybenzaldehyde (3.67 g, 0.03 mol) was dissolved in methanol, and added into a 250 ml three-neck round-bottom flask at 30 8C. 4,40 -oxydianiline (3.0 g, 0.015 mol) or p-phenylenediamine (1.62 g, 0.015 mol) dissolved in methanol was added dropwise with vigorous stirring at 30 8C. The solution was stirred at that temperature for 18 h. The precipitated yellow solid was filtered, and sequentially washed several times with methanol and ether. The product was dried at 50 8C in a vacuum to give a yield of 85% and a melting point of 240 8C by DSC (heating rate of 10 8C/ min) for DPE; and a yield of 90% and a melting point of 271 8C by DSC (heating rate of 10 8C/min) for DP. 1H-NMR (DMSO-d6, ppm): DPE; d = 10.11 (s, 2H), d = 8.47 (s, 2H), d = 7.70–7.77 (m, 4H), d = 7.24–7.27 (m, 4H), d = 7.02–7.06 (m, 4H), and d = 6.86–6.89
Scheme 2. Synthetic scheme for LCE-TA.
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Synthesis of terephthalylidene-bis-(4-aminophenol) (TA) A 250 ml three-neck round-bottom flask was equipped with a temperature controller, reflux condenser, 100 ml addition funnel, heating oil bath, and mechanical stirrer. 4-aminophenol (3.27 g, 0.03 mol) was dissolved in methanol, and added into the flask at 30 8C. Terephthalaldehyde (2.01 g, 0.015 mol) dissolved in methanol was added dropwise with vigorous stirring at 30 8C. The solution was stirred at that temperature for 18 h. The precipitated yellow solid was filtered, and sequentially washed several times with methanol and ether. The product was dried at 50 8C in a vacuum to give a Yield of 96% and a melting point of 274 8C by DSC (heating rate of 10 8C/min). 1H-NMR (DMSO-d6, ppm): d = 9.58 (s, 2H), d = 8.65 (s, 2H), d = 7.98 (s, 4H), d = 7.22–7.27 (m, 4H), and d = 6.79–6.84 (m, 4H). Fig. 1. 1H-NMR spectra of synthesized LCE-TA.
(m, 4H). DP; d = 10.12 (s, 2H), d = 8.51 (s, 2H), d = 7.76–7.79 (m, 4H), d = 7.26–7.27 (m, 4H), and d = 6.87–6.90 (m, 4H). Synthesis of 40 40 -bis(4-hydroxybenzylidene)-diaminodiphenylether diglycidyl ether (LCE-DPE) and 40 40 -bis(4-hydroxybenzylidene)diaminophenylene diglycidyl ether (LCE-DP) A mixture of DPE (5.0 g, 0.012 mol), DMAc (50 ml), and epichlorohydrin (48.0 ml, 0.61 mol) was added into a 250 ml three-necked round-bottom flask equipped with a temperature controller, a reflux condenser, a heating oil bath, and a mechanical stirrer at room temperature. The mixture was slowly heated to 100 8C, and TBAB was added as a catalyst. The solution was stirred at that temperature for 6 h. After cooling to room temperature, the reaction mixture was poured into methanol. A light yellow solid was collected and continuously washed with deionized water and methanol. The product was dried at 40 8C in a vacuum. 1H-NMR (CDCl3-d6, ppm): d = 8.57 (s, 2H), d = 7.85–7.89 (m, 4H), d = 7.29– 7.31 (m, 4H), d = 7.06–7.11 (m, 4H), d = 4.41–4.44 (m, 2H), d = 3.91– 3.95 (m, 2H), d = 3.35–3.38 (m, 2H), 2.86–2.88 (m, 2H), and d = 2.73–2.75 (m, 2H). FT-IR: 2920, 2876 cm1 (CH2); 1606 cm1 (C5 5N); 1027 cm1 (ether); 916 cm1 (epoxy ring). Similar synthesis procedure was employed for the preparation of LCEDP with DP (5.0 g, 0.016 mol), DMAc (50 ml), and epichlorohydrin (62.0 ml, 0.79 mol). Yellow solid was obtained. 1H-NMR (CDCl3-d6, ppm): d = 8.43 (s, 2H), d = 7.84–7.87 (d, 4H), d = 7.24 (s, 4H), d = 6.96–7.02 (d, 4H), d = 4.29–4.31 (m, 1H), d = 4.00–4.04 (m, 1H), d = 3.36–3.39 (m, 1H), 2.92–2.94 (m, 1H), and d = 2.77–2.79 (m, 1H). FT–IR: 2933, 2882 cm1 (CH2); 1606 cm1 (C5 5N); 1027 cm1 1 (ether); 913 cm (epoxy ring).
Synthesis of terephthalylidene-bis-(4-aminophenol) diglycidyl ether (LCE-TA) TA (5.0 g, 0.016 mol), DMAc (50 ml), and epichlorohydrin (62.0 ml, 0.79 mol) were added into a three-necked round-bottom flask equipped with a temperature controller, reflux condenser, heating oil bath and mechanical stirrer at room temperature. The mixture was slowly heated to 100 8C, and TBAB was added as a catalyst. The solution was stirred at that temperature for 6 h. After cooling to room temperature, the reaction mixture was poured into methanol. A light yellow solid was collected and washed continuously with deionized water and methanol. The product was dried at 40 8C in a vacuum. 1H-NMR (CDCl3-d6, ppm): d = 8.52 (s, 2H), d = 7.98 (s, 4H), d = 7.25–7.27 (d, 4H), d = 6.96–6.98 (d, 4H), d = 4.24–4.27 (m, 2H), d = 3.98–4.01 (m, 2H), d = 3.36–3.39 (m, 2H), 2.91–2.93 (m, 2H), and d = 2.77–2.78 (m, 2H). FT-IR: 2930, 2873 cm1 (CH2); 1622 cm1 (C5 5N); 1030 cm1 (ether); 919 cm1 (epoxy ring). Measurements and sample preparation Spectroscopic analysis 1 H-NMR was performed on a Varian Unity Inova 500NB spectrometer using CDCl3-d6 as a solvent and tetramethylsilane (TMS) as a reference. The infrared (IR) spectra were obtained by means of a Nicolet 380 FT-IR spectrometer using KBR pellets. Thermal analysis TGA was performed on 2 to 10 mg samples under a nitrogen atmosphere at a heating rate of 10 8C/min using a TA instruments 2050 thermogravimetric analyzer. DSC was performed on 3 to 5 mg samples under a nitrogen atmosphere at a heating rate of 10 8C/min using a TA instruments 2910 DSC analyzer. Morphology observation Morphological features of the starting components and cured composites were observed with an OM equipped with crossed polarizers in transmitted light mode. Wide-angle X-ray diffraction Wide-angle X-ray diffraction (WAXD, D8 discover) with Cu(Ka) radiation was used to determine liquid crystalline characteristics and d-spacing. The tube source was operated at 1.6 kW with scanning speed of 58/min and sample interval of 0.058.
Fig. 2. FT-IR spectra of synthesized LCE-TA.
Sample preparation for thermal conductivity measurement A mixture of epoxy, curing agent, and catalyst at the designated composition was dissolved in N,N-dimethylacetamide. The mixture was shaken by a paste mixer machine. Then, a designated amount of Al2O3 was added and the mixture shaken again. The solvent was removed by placing the sample in a vacuum oven at a pre-determined temperature for 24 h. Samples were molded by a
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hot press with a pressure of 2500 psi. LCE-DPE/DDS system was cured at 220 8C for 30 min. LCE-DP/DDS and LCE-TA/DDS systems were cured at 190 8C for 30 min. The specimen had a diameter of 12.7 mm and thickness of 1.0 mm. Thermal conductivity measurement The thermal conductivity (l) was obtained from the formula:
l ¼ a d Cp where a, d, and Cp represent the thermal diffusivity, density, and heat capacity of the test specimen at 25 8C, respectively. The thermal diffusivity was measured by laser flash method employing a LFA 447 flash diffusivity instrument (NETZSCH-Gera¨tebau GmbH). Cp was determined separately by DSC in a temperature range from 0 to 100 8C at a heating rate of 10 8C/min.
phase transition of LCE-TA can be referred to as C 206 S 234 N 250 I. Similar results were reported in the literatures [37,38]. It should be noticed that the temperature range at which mesophase remains stable is broad for LCE-TA and LCE-DP while very narrow range is observed for LCE-DPE. One more thing that should be noted from the DSC thermograms of three LCE resins is that there are very profound exothermic reactions above 250 8C, which are related to the homopolymerizations of LCE molecules at isotropic phase. The degradation behaviors of three LCE resins were investigated and the results are shown in Fig. 5. Three LCE resins decompose thermally in single step with an onset degradation temperature of over 300 8C and leave high proportion of residual chars. This
Results and discussion Characteristics of liquid crystalline structures of synthesized LCE resins The thermal behaviors and morphologies of three synthesized LCE resins were examined using DSC, TGA, and OM. Fig. 3 shows DSC thermograms of the LCE resins recorded under dynamic conditions. It is clear that LCE-DPE has very broad endothermic peak at 240 8C (Tm), which corresponds to the melting of this compound (melting enthalpy (DHm) = 42.8 kJ mol1 and melting entropy (DSm) = 83.4 J mol1 K1). However, LCE-DPE does not show clear endotherm above Tm since the high temperatures at which the nematic mesophase is formed lead to homopolymer reactions and crosslinking. Thus, the mesophase range of LCE-DPE is quite narrow and the nematic phase shown in Fig. 4a, which is observed by optical microscope, becomes isotropic phase in a short period of heating. The DSC curve of LCE-DP shows an distinct endothermic peak centered at 194 8C due to melting of crystalline structure, which corresponds to the change of crystalline solid into liquid crystalline state (DHm = 35.5 kJ mol1 and DSm = 76.0 J mol1 K1), and the nematic phase is still preserved up to 220 8C. The optical microscope picture given in Fig. 4b shows direct evidence for the nematic phase of LCE-DP having disclination brushes which can be characterized as Schlieren texture. This phase disappears above 220 8C, which related to the transition (Ti) of liquid crystalline structure to isotropic phase. Thus, it is concluded that LCE-DP exhibits a nematic mesophase between 194 and 220 8C (C 194 N 220 I). LCE-TA shows two peaks: the first one centered at 206 8C (DHm = 32.8 kJ mol1 and DSm = 68.5 J mol1 K1) is due to the melting of crystalline solid and above that temperature the smectic phase is observed as presented in Fig. 4c; the second small peak observed at 234 8C concerns the transition to the nematic phase (Tn). Therefore, the
Fig. 3. DSC thermograms of synthesized LCE resins.
Fig. 4. Morphologies of (a) LCE-DPE at 230 8C, (b) LCE-DP at 200 8C, and (c) LCE-TA at 210 8C when observed by an optical microscope.
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Table 2 Thermal properties related to the curing of LCE resins with DDS.
Fig. 5. TGA thermograms of synthesized LCE resins.
confirms that the LCE resins are thermally stable in the mesophase range. Curing of epoxies with a curing agent (DDS) Three LCE resins are cured with an identical curing agent, DDS and their curing behaviors are investigated by dynamic and isothermal DSC experiments. DSC thermograms of LCE-DPE/DDS, LCE-DP/DDS, and LCE-TA/DDS mixtures isothermally cured at 210 8C are shown in Fig. 7. The curing of mixture takes place quite fast enough and can be accomplished within 30 min. Fig. 6 shows the dynamic curing behaviors of LCE/DDS systems. Generally, the reaction between the oxirane ring in epoxy and the amine in DDS proceeds by a nucleophilic substitution reaction [39]. For the experiments, an identical curing agent at an equimolar ratio was employed and the same amount (3 mol%) of 2MI was added as a catalyst. The epoxy/amine curing reaction is initiated by the reaction of epoxy ring reactant with the primary amine to generate the secondary amine, then the other epoxy groups further react with the secondary amine [40]. Since the same amine is used as a curing agent, the same curing mechanism applies to the curing reaction of LCE-DPE/DDS, LCE-DP/DDS, and LCE-TA/DDS mixtures. However, the dynamic curing behaviors shown in Fig. 6 and the characteristic curing properties summarized in Table 2 are quite different for three systems. The first thing to note in Fig. 6 is the onset temperature (Tonset) and the peak temperature (Tpeak) of the curing reaction. The Tonset values of the LCE-DPE/DDS and LCE-TA/DDS systems are quite lower than Tm values of neat LCE resins. The LCE-DP/DDS and LCETA/DDS systems show typical exothermic curing peaks above
System
Tonset (8C)
Tpeak (8C)
Heat of curing (J/g)
LCE-DPE/DDS LCE-DP/DDS LCE-TA/DDS
228 186 191
238 211 206
37 256 244
Tonset. The Tpeak of LCE-DP/DDS is slightly higher than that of LCETA/DDS. This is easily understandable when considering that the curing proceeds faster in the initial stages of the curing reaction for LCE-DPE/DDS and LCE-TA/DDS systems. In Fig. 3, the Tm is found to be 194 and 206 8C for LCE-DP and LCE-TA, respectively. Small endothermic peaks at about 177 and 180 8C observed in Fig. 6 are assigned to the melting of DDS in the mixtures, indicating that crosslinking between LCE and DDS occurs prior to the melting of LCE-DP and LCE-TA. This means that the curing reaction for LCEDP/DDS and LCE-TA/DDS mixtures starts in the solid state. Hence, the curing reactions of LCE-DP/DDS and LCE-TA/DDS proceed immediately after the melting of DDS, followed by fast exothermic curing reaction up to Tpeak = 211 8C for LCE-DP/DDS and Tpeak = 206 8C for LCE-TA/DDS, which are closely related the Tm values of neat LCE resins. Then the curing reaction decreases slowly and reaches a minimum at about 250 8C, which temperature is believed to be related to the T i of LCE-DP and LCE-TA. LCE-DP and LCE-TA display isotropic phase at temperature above T i and crosslinking is possible above this temperature. It is believed that the partial crosslinking of LCE-DP resin with DDS at nematic mesophase and LCE-TA with DDS at both smectic and nematic mesophase. The second exothermic peak observed above 250 8C is believed to be related to homopolymerization of LCE at isotropic phase. Similar finding is reported by other researchers [41]. However, more study is needed to support this assertion. On the other hand, the curing heat of LCE-DPE/DDS is surprisingly lower compared to those of LCE-DP/DDS and LCETA/DDS mixtures. The main reason is assumed to be related to its mobility during the curing process. The electron-withdrawing aromatic azomethine group may accelerate the nucleophilic substitution reaction between the oxirane ring in epoxy and the aromatic amine in DDS. Therefore, the curing rate of LCE/DDS should depend on the electron-withdrawing tendency of the azomethine group present in LCE resin. Since the same curing conditions (curing agent, catalyst, and curing temperature) are employed for all the systems, only the difference in LCE structure would affect the curing rate. LCE-DPE should exhibit faster curing rate than LCE-DP and LCE-TA because the electron-withdrawing tendency of diphenylether (–C6H4–O–C6H4–) group is stronger compared with that of phenylene (–C6H4–) group. However, the opposite result is observed in Fig. 6. Then, this may be due to the difference in stereo structure of the three LCE resins. The –C6H4–O– C6H4– unit present in LCE-DPE is much bulkier than the –C6H4– unit in LCE-DP and LCE-TA and this steric hindrance effect leads to higher Tm for LCE-DPE and consequently suppressed curing for LCE-DPE/DDS system. At 210 8C, LCE-DPE remains as solid crystal because its Tm is 240 8C. When LCE-DPE is cured at this temperature, very limited curing is possible with already molten curing agent, DDS. On the contrary, LCE-DP and LCE-TA exhibit liquid crystalline mesophase at 210 8C, enabling curing to proceed in liquid state. The isothermal DSC result shown in Fig. 7 strongly supports this explanation. Morphologic observation for cured LCE/DDS mixtures
Fig. 6. Dynamic DSC thermograms of LCE/DDS systems.
An optical microscope is employed for the cured specimens to observe directly the differences in the morphology of the cured
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Fig. 7. Isothermal DSC thermograms of LCE/DDS systems at 210 8C.
mixtures and the results are given in Fig. 8. The mixture of each LCE and DDS at the equivalent molar ratio with the same catalyst amount was cured by following the curing schedules presented in Fig. 8. The curing schedules are chosen by analyzing the curing behaviors shown in Fig. 6 and employed to prepare the specimens for the thermal conductivity measurements, which results are given in Fig. 10. For the sake of clear observation, optical microscopic pictures under cross polarized light are taken for the specimens containing no filler at all. In Fig. 8a, birefringence regions and black isotropic phase are observed for the cured LCE-DPE/DDS system, indicating that it is a biphase of nematic and isotropic phases. On the other hand, well distinct liquid crystalline morphologies are observed for the cured LCE-DP/DDS and LCETA/DDS systems. The LCE-DP/DDS mixture shows well reserved ordered Schlieren texture while the LCE-TA/DDS mixture displays smectic liquid crystalline structure after curing as can be seen in Fig. 8b and c, respectively. According to theory, the smectic state is more solid-like than the nematic and greater alignment with the director is possible in the semectic phase than in the nematic phase [42]. Therefore, it is concluded that the cured LCE-DPE/DDS, LCE-DP/DDS, and LCE-TA/DDS mixtures are quasi-isotropic, nematic, and smectic liquid crystal, respectively and it can be stated that the higher ordered network structure is obtained for cured LCE-TA/DDS than for cured LCE-DP/DDS and LCE-DPE/DDS. To investigate the ordered network structure for the cured systems, the wide-angle X-ray diffraction (WAXD) patterns was measured at room temperature for the cured specimens and the results are presented in Fig. 9. A broad WAXD peak at 2u = 18.78 is observed for cured LCE-DPE/DDS, suggesting that there exists a layered molecular organization of network [43]. LCE-DP/DDS system shows broad peak at 2u = 19.98, which is an indication of the lateral diffraction of mesogenic unit. The d-spacing is calculated to be 4.5 A˚, which is a characteristic of a nematic mesophase. Comparing the lateral distances between LCE-DPE/ DDS and LCE-DP/DDS, the latter has a shorter space and that is more aligned structure. The cured LCE-TA/DDS system displays the typical WAXD pattern of a layered structure with two major peaks: the first one at 2u = 20.88, which corresponds to the lateral diffraction of mesogenic unit with the d-spacing of 4.3 A˚; the other sharp peak at 2u = 3.08 in the small-angle region, which represents the d-spacing 29.4 A˚ of the rigid-rod unit between LCE-TA containing cross-link sites. LCE-TA/DDS system shows the lowest lateral distance, indicating that the highest ordered mesogenic structure is present. The X-ray results are in accordance with DSC and OM experimental results given previously.
Fig. 8. Morphologies of cured (a) LCE-DPE/DDS, (b) LCE-DP/DDS, and (c) LCE-TA/DDS systems when observed by an optical microscope.
Thermal conductivity of LCE/DDS/Al2O3 composite The thermal conductivities of various LCE/DDS/Al2O3 composites as a function of alumina loading are experimentally determined and the results are presented in Fig. 10. The results clearly show that the thermal conductivity of composite linearly increases with filler loading as expected. The most important thing to pay attention to in Fig. 10 is that the thermal conductivities of composites are strongly dependent on the type of LCE employed. At the same filler loading, the thermal conductivity of composite is lowest for the LCE-DPE/DDS system
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Fig. 9. WAXD patterns of cured LCE/DDS systems.
and highest for the LCE-TA/DDS system. That of the LCE-DP/DDS system falls in between. Therefore, it is concluded that the thermal conductivity is governed by the LCE structure. The thermal conductivity of a solid depends strongly on phonon diffusion [44] and it is well known that phonons dominate heat conduction in insulators and semiconductors [45]. The thermal conductivity of LCE-DPE/DDS/Al2O3 determined in this study is 2.83 W/m K at 50 vol% loading of Al2O3. The thermal conductivity of 3.04 W/m K is achieved for the LCE-DP/DDS system at the same filler loading. From these results, it can be clearly stated that the thermal conductivity of LCE containing –C6H4– backbone is higher than that containing – C6H4–O–C6H4– backbone, implying that the phasic nematic structure of oriented phenylene groups has a positive effect on suppressing phonon scattering and consequently improves the thermal conductivity of the composite; and that an aligned crystalline structure is needed to increase the efficiency of the phonon’s diffusion. The thermal conductive value of LCE-TA/DDS/ Al2O3 composite is 3.59 W/m K at the same 50 vol% of Al2O3 loading and this value is 1.27 and 1.18 times greater than that of the LCEDPE/DDS/Al2O3 and LCE-DP/DDS/Al2O3 systems, respectively. From these results, it is concluded that the thermal conductivity of LCE containing –N5 5CH–C6H4–CH5 5N– is higher than that containing – CH5 5N–C6H4–N5 5CH–. However, the reason for this finding is not clear at this moment and further study is needed. Comparison with the theoretical model
7
Fig. 11. Thermal conductivity plots of LCE/DDS/Al2O3 composites as a function of filler content: (symbols and full lines) experimental; (broken lines) calculated by adopting Agari-Uno model.
inorganic filler. In our previous work [1], two models were discussed: the Lewis-Nielsen [46,47] and the Agari-Uno [48,49] model. The results showed that for all cases, the Agari-Uno model provides a better estimate compared to the Lewis-Nielsen model. In this study, the experimentally determined thermal conductivity values discussed previously are only compared with the values predicted by the Agari-Uno model. Fig. 11 shows the comparison between experimental thermal conductivity data for the LCE/DDS systems and the values calculated by adopting the Agari-Uno model. For all systems, the Agari-Uno model provides good estimates. For LCE-DPE/DDS, the theoretical values fit the experimental data very well when 0.30 W/m K is used for the thermal conductivity of epoxy resin itself. To obtain reasonable fits with the experimental data, 0.35 and 0.45 W/m K should be used for the thermal conductivity of LCE-DP/DDS and LCE-TA/DDS, respectively. This very clearly supports the objective of this study in that the thermal conductivity of the cured LCE containing network depends strongly on the nature of liquid crystalline backbone epoxy and the degree of ordering in the cured network. It has been reported in literatures that the thermal conductivity values of neat polymers range from 0.1 to 0.6 W/m K. The value of 0.19 W/m K was reported for a most commonly used epoxy resin, diglycidyl ether of bisphenol A [50] and a higher value was achieved for neat liquid crystalline epoxy [51].
Various models have been proposed to predict the thermal conductivity of composite comprised of organic polymer and
Conclusions
Fig. 10. Thermal conductivities of various LCE/DDS/Al2O3 composites as a function of filler content.
The effects of backbone moiety in three synthesized LCE resins and of the mesophase of curing systems on the thermal conductivities of LCE/Al2O3 composites were investigated. All of the synthesized LCE resins themselves exhibit typical liquid crystalline behaviors above their high melting temperatures. And the crystalline structures are well retained when cured with the curing agent, which is believed to contribute high thermal conductivity of composite with Al2O3 filler. Cured LCE-TA/DDS system displays a smectic mesophase while cured LCE-DP/DDS system exhibits a nematic mesophase and LCE-DPE/DDS system shows a biphasic mixture of nematic mesophase and isotropic phase. Ordering of mesogenic unit is greatest for cured LCE-TA/DDS as clearly demonstrated by the lowest lateral spacing from WAXD results. The broad peak in the wide-angle region was observed at 2u = 18.78, 19.98, and 20.88 for the cured LCE-TA/DDS, LCE-DP/DDS, and LCE-DPE/DDS, respectively. At the same Al2O3 loading of 50 vol%, the thermal conductivities for these mixtures were 2.83,
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3.04, and 3.59 W/m K, respectively. It indicates clearly that the thermal conductivity is directly affected by the ordering tendency of mesogenic unit. It was found that the thermal conductivity values calculated by adopting the Agari-Uno model fit the experimental data fairly well when 0.30 W/m K, 0.35 W/m K, and 0.45 W/m K are used for the thermal conductivity of LCE-DPE, LCE-DP, and LCE-TA, respectively. The more ordered structure, the higher thermal conductivity. This clearly supports the objective of this study in that the thermal conductivity of LCE is a key factor to govern the thermal conduction of LCE containing thermosets. Acknowledgments The authors appreciate the financial support from LG Innotek. This work was also supported by the grant funded by the Korea Government Ministry of Trade, Industry and Energy (10040860) and by the R&D program funded by the Ministry of Science, ICT & Future Planning (Grant number: 2013M3C8A3075845). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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