Nitrogen-Free Tetrafunctional Epoxy and Its DDS-Cured High

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A nitrogen-free tetrafunctional epoxy and its DDS-cured high performance matrix for aerospace applications Tuan Liu, Liangdong Zhang, Ruoshi Chen, Liwei Wang, Bing Han, Yan Meng, and Xiaoyu Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00096 • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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Industrial & Engineering Chemistry Research

A nitrogen-free tetrafunctional epoxy and its DDScured high performance matrix for aerospace applications Tuan Liu†1,, Liangdong Zhang†2, Ruoshi Chen1, LiweiWang2, Bing Han3, Yan Meng*1, and Xiaoyu Li*2

1. Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, P.R. China 2. State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, P.R. China 3. Department of Orthodontics, Peking University School and Hospital of Stomatology & National Engineering Laboratory for Digital and Material Technology of Stomatology, 22 Zhongguancun South Ave., Haidian District, Beijing 100081, P.R. China

KEYWORDS Epoxy, Composite, Glass transition, Interfacial strength, Water absorption, Aerospace, Carbon fiber

ACS Paragon Plus Environment

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ABSTRACT

Tetrafunctional epoxy is an indispensable matrix for the aerospace industry, high-temperature adhesives, and encapsulation materials, where high service temperatures (> 220 °C) are required. N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenyl methane (TGDDM) has long been the dominant candidate in those applications; however, fully cured TGDDM epoxy materials suffer from poor toughness, unwanted side reactions, and inadequate moisture resistance. A novel tetrafunctional epoxy (TFTE) is synthesized to address those issues which have not been resolved for decades. TFTE can be prepared through a simple three-step procedure using readily available raw materials. Each step shows high yield (>90%) and involves only mild reaction conditions. When TFTE is mixed with DGEBA and cured with DDS, the cured epoxy shows a Tg of 252 °C, a tensile strength of 80.0 MPa, and more importantly, a higher toughness (29.8 kJ/m2) and better moisture resistance than TGDDM/DDS system. In addition, the interfacial strength, thermal stability, and processability of TFTE/DGEBA are comparable to those of TGDDM. These excellent properties and processability makes TFTE a potential replacement for TGDDM. INTRODUCTION Epoxy materials are widely used as matrix materials in composites, adhesives, coatings and electrical materials, because of the balanced mechanical performance, processability, versatility, chemical resistance, low shrinkage, good wetting and adhesion to fibers and other substrates.1-3 The widely used diglycidyl ether of bisphenol A (DGEBA) epoxy can hardly achieve a glass transition temperature (Tg) higher than 200 °C.4 In aerospace/aircraft applications, where high service temperature (>220°C) is required, multifunctional epoxy (f≥3) have to be used. Tg of


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cured epoxy depends on many factors, including backbone stiffness, functionality of reactants, stoichiometry, and cure cycle. If backbones of both epoxy and curing agent are stiff, the fully cured epoxy can yield high crosslink density and Tg, which can be used in high temperature applications.5,6 Typical multifunctional epoxies include epoxy novolacs, aromatic amine based tetrafunctional epoxy (e.g., TGDDM), and tetrafunctional epoxies based on amino phenol. In theory, Tg is expected to increase with functionality; however, epoxies with excessive functionalities (f>4) tend to suffer from incomplete cure due to steric hindrance and topological constrains.7,8 Thus, a compromise has to be made between higher functionality and high extent of cure. As a result, tetrafunctional epoxies are often used in high temperature applications. Structures of typical tetrafunctional epoxies are shown in Scheme 1. It is worth noting that most tetrafunctional epoxies are derived from diamines; among them, N,N,N',N'-tetraglycidyl-4,4'diaminodiphenyl methane (TGDDM) has been the main choice.9 In particular, 4,4’-diamino diphenylsulfone (DDS) cured TGDDM system has remained the dominant structural matrix in aerospace/aircraft industry for decades. Numerous papers have addressed various aspects of the TGDDM/DDS curing systems, including curing kinetics, toughening, thermal degradation, and effects of moisture.10-12 However, major issues persist in TGDDM and other amine-based multifunctional epoxy curing systems, which will be detailed below.


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Scheme 1. Structures of commercial multifunctional epoxy resins: 1. TGDDM; 2. TGDDE; 3. N,N,N',N'-tetraglycidyl-m-xylenediamine (TGMXD); 4. Tetraglycidyl ether tetraphenyl ethane (TGETPE); 5. Tetrafunctional epoxy resin of bisphenol-C (EBCF).

The first issue is associated with the nitrogen atom (N) in their structures and the high postcure temperature used in order to achieve high degree of conversion. When one epoxy group is reacted, the other epoxide group attaching to the same N may react with the formed -OH group (especially at high elevated temperatures) and form an intramolecular ring7 (Scheme S1 in the Supporting Information) which is detrimental to mechanical and thermal properties. The N atoms in TGDDM lead to two disadvantages. On one hand, the polar N atom increases water absorption which compromises thermal stability, dielectric properties, and other properties. This is one reason why cresol novolac epoxy is preferred over TGDDM in high temperature encapsulation materials14 despite of its lower Tg and toughness. On the other hand, the tertiary amine structure could promote uncontrollable homopolymerization of epoxide groups at elevated temperatures. The second issue is that when TGDDM is cured with aromatic amines (e.g., DDS, DDM, or DETDA in Scheme 2), those high Tg products (>250 °C) show low toughness and elongation at break, which have to be toughened when used as the structural matrix. Epoxies with medium crosslink densities can be easily toughened by elastomers, core-shell particles, plastics, liquid crystals, and rigid fillers.15-21 However, highly crosslinked TGDDM/DDS is notoriously difficult to toughen even at costs of Tg and processability. Thus, designing a high Tg epoxy with intrinsically higher impact strength and elongation at break is more desirable. The third issue is the impurities in TGDDM. Aside from isomers, commercial TGDDM contains impurities which


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has 15-20% less epoxide groups13, which not only reduce the crosslink density but also is unfavorable for structural control and maintaining good reproducibility of cured products.13

Scheme 2. Chemical structures of DDS, DDM and DETDA. In order to address those issues, developing a tetrafunctional epoxy with less impurity, less side reactions, better impact strength, higher elongation at break, and lower moisture resistance is crucial. Because amine-based epoxies are prone to side reactions and moisture absorption, the Nfree tetrafunctional epoxy with far-apart epoxide groups is structurally favorable. Although a handful of N-free tetrafunctional epoxies have been reported22,23 (Scheme 1), they are not able to avoid major intramolecular side reactions due to the proximity of adjacent epoxide groups. Thus, a novel structure is still needed, which will be detailed in this paper. The paper is organized as follows. In the first part, the synthesis of TFTE is described. Effects of reaction condition on structure and yield were investigated. In the second part, a high performance DDS-cured hybrid TFTE/DGEBA epoxy is demonstrated. The cured epoxy shows a high Tg (>250°C), improved strength, low moisture uptake, adequate fiber-matrix interfacial strength, and more importantly, a much improved toughness (compared with TGDDM/DDS). Our results suggest that the mass-producible TFTE can be a potential replacement for TGDDM in aerospace applications. EXPERIMENTTAL SECTION Materials


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4,4’-Diaminodiphenyl sulfone (DDS, 98%) was purchased from Energy Chemical reagent company. Tetrabutylammonium bromide (TBAB, 98%), p-toluenesulphonic acid (PTSA, 99%), and epichlorohydrin (ECH) were purchased from Tianjin Fuguang reagent Co. 1,4Dibromobutane (98%) and 4-hydroxylbenzaldehyde (PHBA, 98%) were obtained from Zhongsheng Huateng Reagent Co. Dimethylsulphoxide-d (CD3SOCD3), was acquired from Beijing InnoChem Science & Technology Co. Diglycidyl ether of bisphenol A (DGEBA) was purchased from Yueyang Resin Factory (EEW=190.04 g equiv.-1). All other solvents and reagents were purchased from Beijing reagent Co. Ltd. Characterization Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AV-600 spectrometer (600 MHz). Chemical shifts of 1H and 13C-NMR are reported in ppm, and CD3SOCD3 or CDCl3 was used as the solvent in all NMR measurements. Fourier transform infrared spectroscopy (FTIR) spectra were collected on a Bruker Tensor37 spectrophotometer using the potassium bromide (KBr) disc technique. The epoxy equivalent weights (EEW) were determined by titration using the HCl-acetone method.24,25 Molecular weights (MWs) were determined by the matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) method using a Shimadzu KOMPACT MALDIII. The purity was estimated using a Waters ACQUITY Ultra Performance Liquid Chromatography with a BEH C-18 column (2.1×50mm, 1.7 µm). The mobile phase was a mixture of 0.1wt% formic acid aqueous solution and acetonitrile (V:V =10:90). The flow rate was 0.3 mL/min, and the solution concentration is 5 ppm. Density functional theory (DFT) calculations were used to simulate the structures of TFTE and TGDDM. Molecular geometries of two epoxies were optimized in a trans-conformation, at the B3LYP/6-31G(d, p) level of theory in vacuum using the Gaussian 09 program package.


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Dynamic scans were performed at a heating rate of 10 °C min-1 using a DSC-1 (MettlerToledo, Switzerland) differential scanning calorimeter (DSC) equipped with an intra-cooler under nitrogen atmosphere. The thermal stability under nitrogen atmosphere was evaluated using a PerkinElmer Pyris1 thermo-gravimetric analyzer (TGA) from 50 to 800 °C at a heating rate of 10 K min-1. Linear coefficients of thermal expansion (LCTE) were determined by a TMA/SDTA841e thermal mechanical analyzer (Mettler-Toledo) during cooling at 2 °C min-1. Dynamic mechanical properties at 1 Hz were measured using a TA Q800 dynamic mechanical analyzer (DMA) in the single cantilever mode; and measurements were performed from 50 to 300 °C at 5 °C min-1 using a sample size of 35.0 mm×12.8 mm×3.2 mm. Tensile strengths were characterized using an Instron 1185 test machine according to ISO527:1993, and 1BA type specimens were used. Unnotched impact strength tests were performed on a Ceast Resil impact tester according to ISO 179:1982. For each composition, at least five samples were measured. Before tests, all samples were conditioned in an air-circulating oven at a temperature higher than their Tgs in order to remove thermal histories. After impact tests, fracture surfaces were coated with gold and imaged by scanning electron microscopy (SEM; JEOL JSM-6700). All measurements were performed at 23±2 °C unless otherwise stated. Synthesis of tetrafunctional epoxy resin (TFTE) The tetrafunctional epoxy resin (TFTE) was synthesized using a three-step procedure (Scheme 3), and details are given below.


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Scheme 3. Three steps involved in the synthesis of TFTE.

Step 1: Synthesis of 1,4-bis(4-formylphenoxy)butane Under mechanical stirring, 4-hydroxylbenzaldehyde (18.3 kg, 150 mol), 1,4-dibromobutane (12.9 kg, 60 mol) and industrial ethanol (95%, 50 L) were charged into a 100-liter reactor and heated to 80 °C. After 4-hydroxylbenzaldehyde was dissolved, K2CO3 (16.6 kg, 120 mol) and KI (1 kg, 6 mol) were added and reacted for 5 h. After cooling to room temperature, the mixture was kept for 12 h in order for the crude product to crystallize. The mixture was filtered, washed with water, recrystallized in ethanol, and dried under vacuum at 80 °C. The product is a light yellow solid (92% yield).

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H-NMR (600 MHz, (dimethyl sulfoxide)-d6,δ): 1.88-1.94 (m, 4H,

OCH2(CH2)2CH2O), 4.16 (t, 4H, OCH2-), 7.11 (d, 4H, Ph-H), 7.85 (d, 4H, Ar-H), 9.85 (s, 2H, PhCHO).

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C-NMR (600 MHz, (dimethyl sulfoxide)-d6, δ): 25.11, 67.63, 114.88, 129.54,

131.77, 163.56. Step 2: Synthesis of the tetra-phenolic compound.


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1,4-bis(4-formylphenoxy) butane (3.2 kg, 10.4 mol) and hot phenol (7.8 kg, 83.2 mol, 80 °C) were charged into a 20-liter reactor under stirring. After a homogenous solution was obtained, ptoluenesulfonic acid (447.2 g, 2.6 mol), and ZnCl2 (348.0 g, 2.6 mol) were added. The mixture was kept at 40 °C for 24 h under mechanical stirring. After reaction, the mixture was washed with hot water (>70 oC) three times to remove residual salts. Most unreacted phenol was removed by reduced pressure distillation at 150 oC. The obtained crude product was dissolved in ethanol and precipitated into water under vigorous stirring. The precipitate was collected and dried under vacuum at 100 oC to give a red solid (90% yield). It is noted that the crude product can be used directly in the next step without purification, because byproducts and unreacted monomers can be automatically removed in the post treatments of step 3. 1H-NMR (600 MHz, (dimethyl sulfoxide)-d6, δ): 1.78-1.87 (m, 4H, OCH2(CH2)2CH2O), 3.97 (t, 4H, OCH2-), 5.27 (s, 2H, Ph3H), 6.64-6.97 (24H, Ph-H).

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C-NMR (600 MHz, (dimethyl sulfoxide)-d6, δ): 25.46,

53.53, 67.00, 114.03, 114.88, 129.71, 134.97, 136.92, 155.40, 156.72. Step 3: Epoxidation of the tetra-phenolic compound. Under mechanical stirring, the tetra-phenolic compound (1.9 kg, 3 mol) and ECH (10.2 kg, 111 mol) were charged into a 20-liter reactor and heated to 110 °C. After the tetra-phenolic compound was completely dissolved, TBAB (193.5 g, 0.6 mol) was added. The mixture was allowed to react for 5 h at 110 °C. When the mixture was cooled to 50 °C, water-NaOH solution (NaOH 600 g, water 1.4 kg) was then added dropwise within 10 h. The resultant mixture was kept at 50 °C for another 5 h. After cooling to room temperature, the mixture was washed with water three times to remove residual salts. Most unreacted ECH was removed and recycled through vacuum using distillation. The obtained solution was washed with ethanol at least twice and dried under vacuum at 90 °C. The final product, TFTE, is a highly viscous yellow liquid


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(90% yield).

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H-NMR (600 MHz, (dimethyl sulfoxide)-d6, δ): 1.77-1.87 (m, 4H,

OCH2(CH2)2CH2O), 2.68 (q, 4H, OCH2-), 2.82 (q, 4H, OCH2-), 3.27-3.33 (m, 4H, OCH-), 3.78 (q, 4H, OCH2-), 3.91-4.01 (br, 4H, OCH2-), 4.26 (dd, 4H, OCH2-), 5.41 (s, 2H, Ph3H), 6.75-7.03 (24H, Ph-H);13C-NMR (600 MHz, (dimethyl sulfoxide)-d6, δ): 25.10, 25.46, 43.72, 49.69, 53.39, 66.99, 68.86, 114.15, 114.22, 129.79, 129.84, 136.32, 136.87, 156.49, 156.88. Synthesis of model compounds M1 and M2 In order to help to characterize TFTE, two model compounds, M1 and M2, which mimic structures of the main and minor products in step 2, were synthesized, and the synthetic route is shown in Scheme 4.26,27 The first step is the synthesis of 4-(4-bromo-butoxy)-benzaldehyde. Under mechanical stirring, 4-hydroxylbenzaldehyde (6.1 g, 0.05 mol), 1,4-dibromobutane (43.2 g, 0.2 mol), K2CO3 (13.8 g, 0.1 mol), and 250 mL ethanol were added into a three-necked flask and refluxed for 8 h. After cooling to room temperature, the mixture was filtered, and ethanol was removed using a rotary evaporator. The crude product was purified using column chromatography with CH2Cl2/petroleum ether (1:1) as the eluent. The obtained product is a light yellow crystal-like solid (84% yield).

Scheme 4. The synthesis route of model compounds M1 and M2.

In the second step, 4-(4-bromo-butoxy)-benzaldehyde (2.58 g, 0.01 mol) and hot phenol (0.05 mol, 4.7 g, >80 oC) were added into a three-necked flask under mechanical stirring. After stirring for 30 min, ZnCl2 (0.14 g, 1 mmol) and p-toluenesulfonic acid (0.19 g, 1 mmol) were added, and


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the mixture was heated to 45 oC for 24 h and then washed twice with hot water (>70 oC) to remove residual salts. After evaporation at 140 oC, most phenol was removed, and the crude product was purified using column chromatography with 1:5 ethyl acetate/petroleum ether as the eluent. Due to the different polarity, M1 and M2 can be easily separated. Compound M1 is a red solid with a yield of 44%. 1H-NMR (600 MHz, (dimethyl sulfoxide)d6, δ): 1.78-1.84 (m, 2H, OCH2CH2(CH2)2Br), 1.91-1.98 (m, 2H, O(CH2)2CH2CH2Br), 3.59 (t, 2H, O(CH2)3CH2Br), 3.94 (t, 2H, OCH2(CH2)3Br), 5.27 (s, 1H, CHPh3), 6.66 (d, 4H, C6H4O), 6.83 (d, 2H, C6H4O), 6.85 (d, 4H, C6H4O), 6.95 (d, 2H, C6H4O), 9.21 (s, 2H, PhOH); 13C-NMR (600 MHz, (dimethyl sulfoxide)-d6, δ): 27.91, 29.59, 35.29, 54.03, 66.94, 114.53, 115.39, 130.21, 130.26, 135.46, 137.50, 155.91, 157.13. Compound M2 is a red solid with a yield of 21%. 1H-NMR (600 MHz, (dimethyl sulfoxide)d6, δ): 1.78-1.84 (m, 2H, OCH2CH2(CH2)2Br), 1.93-1.98 (m, 2H, O(CH2)2CH2CH2Br), 3.59 (t, 2H, O(CH2)3CH2Br), 3.95 (t, 2H, OCH2(CH2)3Br), 5.64 (s, 1H, CHPh3), 6.66 (d, 4H,C6H4O), 6.83 (d, 2H, C6H4O), 6.85 (d, 4H, C6H4O), 6.95 (d, 2H, C6H4O) 9.21 (s, 2H, PhOH);13C-NMR (600 MHz, (dimethyl sulfoxide)-d6, δ): 27.47, 29.15, 34.87, 47.36, 66.47, 113.97, 114.85, 118.56, 126.92, 129.62, 129.87, 131.00, 134.30, 136.38, 154.57, 155.39, 156.62. Preparation of TFTE/DGEBA curing system Under mechanical stirring, TFTE and DGEBA were mixed at different mass ratios (e.g. 10/90, 20/80, 30/70, and 40/60) at 90 °C. After homogeneous mixtures were obtained, stoichiometric amounts of DDS were added at 120 °C under mechanical stirring. When DDS was fully dissolved, the mixture was poured into silicon rubber molds and cured in a MEMMERT aircirculating oven. All samples were cured using a four-step cure scheme: 120 °C for 2 h, 160 °C


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for 3 h, 200 °C for 1 h, and 230 °C for 1 h. After curing, samples were cooled naturally to room temperature. Water uptake measurements The samples were first dried in vacuum oven at 100 °C for 48 h to remove any residual water. Dried samples were weighed (W0) and immersed in boiling water bath, which is kept at 100 °C under 1 atm. After regular time intervals, samples were taken out and weighed (Wt) after the surface water has been removed by filter paper. The water absorption (WA) is calculated based on WA=(Wt/W0-1)×100% Microdroplet pull-out test The interfacial strength between carbon fiber and matrix was characterized using an MDT-5000 microdroplet test machine. A T800 carbon fiber was used to prepare the microdroplet-bond specimens. A drop of epoxy/curing agent mixture was wetted onto a carbon fiber to form a microdroplet surrounding the T800 carbon fiber. The microdroplet was then cured in an aircirculating oven. Samples were cured at 160 °C for 1 h and then cured at 220 °C for 1 h. After a cured microdroplet was formed, the fiber was fixed on the microdroplet test machine. The center of the fiber was fixed at the center of the gap. The microdroplet pull-out tests was carried out at a displacement rate of 0.02 mm/min. At least 10 specimens were tested for each formulation.

RESULTS AND DISCUSSIONS Synthesis TFTE was synthesized using a three-step procedure (Scheme 3), and NMR spectra for the products in each step are shown in Figures 1 and 2. In step 2, although both ortho- and para-


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positions of phenol are active,28 the para-substitution product is the main product due to the smaller steric hindrance. Compounds M1 and M2, which mimic structures of the main and minor products, were synthesized (Scheme 4) in order to help determining corresponding chemical shifts in NMR spectra (Figure 3). Chemical shifts at ~5.27 and ~5.63 ppm represent Ph3H protons in ortho- and para-substitutions, respectively. By integrating areas of corresponding peaks, the ratio between ortho-and para-substitutions is found to be ca. 0.15:1. It is worth noting that dimmers may form if two reactive centers in one phenol molecule are reacted simultaneously (Scheme 5). MALDI-TOF was used to detect possible dimmers. As shown in Figure4, aside from the expected m/z values, another peak appears at higher m/z value corresponding to that of the dimmer. The formed dimmer, however, is not an issue, because it can be automatically removed in the post treatments of step 3.

Figure 1.1H NMR spectra of (a) 1,4-bis(4-formylphenoxy)butane, (b) tetra-phenolic compound and (c) the tetrafunctional epoxy, TFTE.


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Figure 2.13C NMR spectra of (a) 1,4-bis(4-formylphenoxy)butane, (b) tetra-phenolic compound and (c) the tetrafunctional epoxy, TFTE.

Scheme 5. Possible products during the second step.


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Figure 3. 1H NMR spectra of model compounds with well-defined o-substituent (a) and psubstituent (b), respectively; (c) 1H NMR spectrum of tetra-phenolic compound.

Step 3 is the epoxidation of the tetra-phenolic compound. The epoxidation was carried out using a 10-time excess of epichlorohydrin (with respect to the hydroxyl group) in order to prevent chain extension side reactions. MALDI-TOF results of TFTE are shown in Figure 4b. Clearly, dimmers formed in step 2 are not detected, and only one main peak and its ionized peak are obvious. The absence of dimmers in the final product may be due to the steric-hindranceinduced low reactivity of the ortho-substituted phenol (the boxed region in Scheme 5). The incompletely epoxidized dimmers cannot precipitate in ethanol during post treatments; thus, it can be easily removed in the precipitate collection process. As a result, the final TFTE product


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has an epoxide equivalent weight (EEW) of 241 g/equiv., which is very close to the theoretical value, and the structure has also been confirmed by NMR (Figures 1 and 2). 2D-NMR data were also supplied in Figures S1 and S2 in Supporting Information. MALDI-TOF and titration results of the final product suggest that almost all four phenolic groups in the tetra-phenolic compound (product of step 2) are successfully converted to epoxide groups. Thus, TFTE does not contain similar impurities as in TGDDM which have less epoxide groups13 , which is an advantage over TGDDM. However, it is worth noting that although almost all final products have four terminal epoxide groups, isomers which contain one or more ortho-substituted epoxide groups (compared to the ideal target structure in which all four epoxide groups are in para-positions) do exist. High performance liquid chromatography results (Figure S3 in the Supporting Information) shows that aside from the ideal target product, other isomers exit. By comparing the peak area again the calibration curves, HPLC results suggest that ~66% of the final products have the ideal structure in which all four epoxide groups are in the para positions.

Figure 4. MALDI-TOF results of (a) the tetra-phenolic compound and (b)TFTE.

Processability


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DDS-cured tetrafunctional epoxy system can achieve a Tg as high as 260 °C when cured at high temperatures;29 however, high cure temperature can lead to unwanted side reactions, residual stresses, and even thermal degradation. For high temperature epoxies, excessive high Tg (>250 °C) is unnecessary because toughness, thermal degradation, and processability are also important considerations. Mixing solid TFTE with solid DDS directly is not easy, however dissolving TFTE in liquid DGEBA can overcome this issue. In fiber- or particulate-reinforced composites, the processing window, i.e. the temperature range where the viscosity becomes lower than 1 Pa·s, is critical.30,31 Variations of complex viscosities with temperature for TFTE and typical epoxies are shown in Figure 5a. At room temperature, TGDDM and DGEBA are liquid; whereas TFTE is a solid. At 120 °C, the viscosity of TFTE is still high for many applications. The high Tg of fully cured trifunctional epoxy (TGAP) suggests that a curing system composed of 100% tetrafunctional epoxy may not be necessary in achieving a Tg of 250 °C.4 In practice, mixing TFTE with appropriate amounts of liquid epoxy can improve processability while still keeping a high Tg. In this paper, DGEBA which is the most commonly used liquid epoxy is thus chosen as the reactive diluent.32 The temperature-dependent viscosity of TFTE/DGEBA hybrid epoxy at different mixing ratios are shown in Figure 5b. The hybrid containing 40 wt% TFTE shows a similar viscosity to that of commercial TGDDM. Thus, in later sections, all results are reported based on hybrid epoxies containing ≤40wt% TFTE.


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Figure 5. (a) Temperature-dependent complex viscosities of neat TFTE, TGDDM and DGEBA resins; (b) Temperature-dependent complex viscosities of hybrid epoxies with different TFTE weight fractions. High conversions in TFTE/DGEBA hybrid curing system FTIR spectra of the stoichiometric mixture of the hybrid epoxy (containing 40wt% TFTE) and DDS, before and after cure, are shown in Figure 6a. Unreacted mixture shows a band at 912 cm-1 corresponding to epoxide groups. When epoxide groups react with DDS, steric hindrance can lead to incomplete cure. When TGDDM is cured with a primary amine, steric hindrance often lead to incomplete cure and leave some secondary amines unreacted. In contrast, in our hybrid curing systems, a complete cure is reached as suggested by the disappearance of 912 cm-1 band. The absence of residual peaks in cured TFTE/DGEBA/DDS system (Figure 6b) also suggests a high ultimate conversion. In contrast, an exothermic peak (~240 °C) is observed in TGDDM/DDS curing system, suggesting an incomplete cure; unreacted epoxide groups can lower crosslink density and create dangling chain (defects) in the cured network, which compromises thermal and mechanical properties. In addition, unreacted groups may trigger degradation or attract water during service, which is not favorable for structural matrix and


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encapsulation materials. Thus, the complete cure in TFTE/DGEBA/DDS system is another advantage over the TGDDM/DDS system.

Figure 6. (a) FTIR spectra of cured and uncured hybrid resins with 40wt% TFTE loading; (b) the first heating runs of cured TGDDM/DDS and cured 40wt%TFTE/DDS loading hybrid using DSC method. DMA characterization The temperature dependent storage moduli (E′) and loss tangent (tanδ) for neat DGEBA system and for hybrid systems (containing 10, 20, 30, and 40 wt% TFTE) are shown in Figures 7a and 7b, respectively. For each composition, only one single step is observed in E' and only one peak is observed in tanδ, suggesting that no phase separation occurs. When TFTE loading increases, Tg increases monotonically. As expected, the rubbery plateau modulus (Er) increases with TFTE loading (Table 1). The average crosslinking density could be estimated from rubbery plateau modulus (Er) based on the rubber elasticity theory. The completely cured TFTE/DGEBA/DDS system yields higher crosslink density, which is consistent with the higher Er and Tg values.


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Figure 7. Effects of TFTE fraction on storage modulus E’ (a) and tan δ (b) of cured hybrids. Table 1. Effects of TFTE loading on TGA, DMA and viscosity results. Td5a

Tgb

Er

η40 °C

(°C)

(°C)

(MPa)

(Pa•s)

Neat DGEBA

418.2

227

36.9

1.76

10 wt% TFTE

413.8

234

42.7

3.19

20 wt% TFTE

407.9

242

53.2

6.57

30 wt% TFTE

404.1

252

65.6

11.19

40 wt% TFTE

402.5

260

78.6

31.5

357.2

263

Samples

TGDDM a

15.7

b

The peak temperature at tanδ; The temperature at 5 wt% decomposition.

Tg of fully cured epoxy depends largely on the backbone stiffness and average functionality of reactants. At first glance, Tg of TFTE/DGEBA/DDS should be smaller than that of TGDDM/DDS considering the lower average epoxide functionality as well as softer-C4H8- units in TFTE. However, at 40wt% TFTE loading, the fully cured hybrid reaches a Tg of 260 °C, which is almost as high as the that of TGDDM system (263°C). Molecular simulation of TGDDM (Scheme 6) suggests that two adjacent epoxy groups attaching to the same N atom are close enough to allow inter- and intramolecular cyclizations or lead to incomplete cure (due to


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steric hindrance). The network with higher crosslink density and less defects is more readily formed when two epoxy groups attaching to the same N atom react with different amines, which is unlikely in TGDDM as well as other amine-derived epoxies. In contrast, the epoxide groups in TFTE are all connected to different benzene rings. The six non-coplanar benzene rings (Scheme 6) ensure that epoxy groups attaching to the same molecule are far enough to eliminate unwanted cyclization side reactions. As a result, the epoxide groups in TFTE can reacted more completely and lead to a network with higher crosslink density and Tg.

Scheme 6. Molecular simulation results of (a) TFTE and (b) TGDDM. Thermal stability of cured hybrids. TGA curves of cured hybrids are shown in Figure 8, and the temperature corresponding to 5% weight loss (Td5) are tabulated in Table 1. All hybrids show a Td5 higher than 400 °C, and Td5 decreases slightly with TFTE loading. Td5 values of all hybrids are ~50 °C higher than that of TGDDM/DDS. This implies that although TGDDM/DDS system has higher average functionality, the incomplete cure and cyclizations impair the thermal stability. The decrease in


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Td5 with TFTE loading can be explained by the dilution or blending effects of -(CH2)4- units despite the higher average crosslink density. In addition, the char yield at 750 °C increases with TFTE loading, which could be explained by the rigid triphenyl structures. Thus, in terms of thermal stability, DDS cured TFTE hybrid also outperforms cured TGDDM/DDS.

Figure 8. TGA curves of cured hybrids with different TFTE fractions TMA characterization of cured hybrids Based on the free volume theory, the fractional free volume at temperature T (fT) can be expressed as fT = fg + ∆αv (T-Tg), where ∆αv=αv,r-αv,g is the difference between volumetric thermal expansions (CTE) coefficients in the rubbery and glassy states, respectively, and fg is the fractional free volume at Tg. ∆αv can be related to the fractional free volume, which has been confirmed by positron annihilation lifetime spectroscopy (PALS) measurements.33,44 For isotropic materials, the linear coefficient of thermal expansions (LCTE) is 1/3 of CTE. As shown in Table 2, when the TFTE loading increases, LCTE in both glassy (αg) and rubbery (αr) state increase, and ∆α also increases. For typical epoxy materials, higher crosslink density often accompanies with lower ∆α; however, this is not the case in TFTE/DGEBA/DDS. The higher ∆α


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could be related to the non-coplanar structure of TFTE. Compared with coplanar DGEBA, addition of TFTE can incorporate more free volume and lead to higher ∆α. Table 2. Effects of TFTE loadings on Linear Coefficient of Thermal Expansion Sample

ag

∆α = αr-αg

αr -6

-1

-6

-11

(×10 ·K )

(×10 ·K )

(×10-6·K-1)

Neat DGEBA

87.5

172.2

84.7

10wt% TFTE

88.4

178.8

90.4

20wt% TFTE

89.7

187.7

98.0

30wt% TFTE

90.0

190.5

100.5

40wt% TFTE

91.4

211.7

120.3

Mechanical properties and morphology of fractured surfaces The tensile and impact strengths of cured hybrids are shown in Figure 9. Tensile strength (squares) increase slightly at 10% and 20 wt% TFTE loadings and then decreases at 30 wt%. The impact strength (triangles) reaches the maximum at 20 wt% and then decreases at higher loadings. Although the 20% formula has higher strength and toughness, the hybrid containing 30wt% TFTE show optimum balanced performance if we also take Tg into consideration. Compared with TGDDM/DDS, the 30 wt% hybrid system shows much higher toughness, strength and Td5 (Table 3). The fracture surfaces are shown in Figures 10a (TGDDM) and 10b (hybrid with 30 wt% TFTE). Dimple-like structures and stress-whitened zones are observed in the vicinity of cracks in hybrids, whereas the fracture surface of TGDDM is relatively smooth. At the end of the crack (Figure 10b3), fibrils which can absorb energy during impact are clearly seen, which parallels with the higher elongation at break. In contrast, in TGDDM/DDS, no fibrils are observed (Figure 10a3). In epoxy, improved toughness is often associated with decreases in Tg and yield stress. However, simultaneous enhancements in Tg, strength, and toughness are


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achieved in our hybrid systems. Explanations based on blending effects and molecular structure considerations will be discussed below.

Figure 9. Effects of TFTE loading on tensile and impact strengths.

Table 3. Effects of TFTE loading on mechanical properties of cured hybrids Sample

TGDDM

0wt%

10wt%

20wt%

30wt%

40wt%

Tg (°C)

263

227

234

242

252

260

Tensile Strength (MPa)

73.0

82.9

83.4

84.4

80.0

68.2

Elongation at Break (%)

5.7

14.1

11.8

12.5

11.5

9.2

Impact Strength (kJ/m2)

13.2

25.5

25.0

30.7

29.8

25.2


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Figure 10. SEM images of fracture surfaces at different positions. For the TGDDM system: (a1) the starting point of the crack, (a2) the middle region, and (a3) the end zone. For the hybrid with 30 wt% TFTE: (b1) the starting point of crack, (b2) the middle region, and (b3) the end zone.

Dilution of TFTE with DGEBA not only ensures better mixing but also improves processability and toughness at marginal cost of Tg.35 It is worth pointing out that even when DGEBA is cured with aromatic diamine, it cannot reach full conversion (~92% for DGEBA/mPDA); when tetrafunctional epoxy (e.g. TGDDM) is cured with aromatic diamine, an even lower conversion is often expected due to stronger steric hindrance and possible vitrification. As a result, when a tetrafunctional epoxy is cured with aromatic diamine, highly cross-linked network with less defects could be achieved by mixing due portions of liquid epoxy (e.g. DGEBA). This practice could maintain a high enough Tg as long as the fraction of TFTE is higher enough. It is also interesting to compare colors of different curing systems. The cured TGDDM/DDS is dark black, whereas the cured hybrid epoxies show a much lighter color


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(Figure 11). This also implies that our hybrid epoxies do not have those side reactions which generate carbon-carbon double bonds and give it a dark look.

Figure 11. Photos showing colors of cured neat system and hybrids; (a) neat TGDDM, (b) neat DGEBA, (c) 10wt% TFTE, (d) 20wt% TFTE, (e) 30wt% TFTE, (f) 40wt% TFTE.

To our knowledge, epoxy material with high Tg, much improved toughness, high strength, and high elongation at break has not been reported. Recently, it is shown that highly branched hyperbranched epoxy with appropriate backbone structure and molecular weights could simultaneously improve Tg, strength, and impact strength in DGEBA curing systems,28-31 and insitu toughening mechanisms, i.e., higher free volume and deformation of cavities, are invoked to explain the improved toughness and elongation at break. Similarly, addition of non-coplanar TFTE also increases fractional free volume (Table 2). In our hybrids, notable increase in fractional free volume is observed, thus in-situ toughening mechanism may also play a role in TFTE/DGEBA/DDS system. For hyperbranched epoxy with relatively low molecular weights, optimum toughening results are often observed at approximately 5 wt% loading.28-31 Adding excessive hyperbranched epoxy or adding hyperbranched epoxy with excessive terminal epoxide groups could lead to incomplete cure, which is not favorable. In contrast, TFTE has lower functionality and thus have less steric effects, which is beneficial to achieving higher conversions. As a result, an optimum mechanical performance is observed at much higher loadings (ca. 20~40wt%). Water absorption


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In epoxy materials, absorbed water not only act as plasticizers but also trigger degradation, which can lower Tg and mechanical performance. In fiber-reinforced composite, absorbed water can migrate to the interface and weaken the matrix-fiber interfacial strength. Thus, for high temperature epoxy, it is crucial to reduce water absorption. As shown in Figure 12, water absorption of all samples show rapid increase up to 150 h and then level off at 150-300 h. The saturation water absorption of cured TGDDM is 30% higher than those of cured DGEBA and TFTE/DGEBA systems. As mentioned in the introduction, the poor moisture resistance of TGDDM is related to the N atom, impurity, and incomplete cure. Because TFTE does not contain N atom, the water uptake of TFTE/DGEBA hybrids only increase slightly with TFTE loading. The higher absorption may be related to the higher hydroxyl (-OH) concentration and higher fractional free volume in cured hybrids.

Figure 12. Water absorption (in boiling water) of cured hybrids as a function of time.

Interfacial shear strength In high performance composites, tetrafunctional epoxy is often used as composites, in which epoxy is reinforced with carbon fibers. It is necessary to evaluate the fiber-matrix interfacial strength in order to ensure the performances of composites.36 The interfacial shear strength


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between T800 carbon fiber and DDS-cured TFTE/DGEBA was evaluated using the microdroplet test method, and results are shown in Figure 13. The interfacial shear strength shows a continuous increase with TFTE loading. At 40 wt% TFTE loading, the interfacial shear strength is higher than TGDDM/DDS. The good interfacial strength of hybrids can be explained as follows. The hydroxyl group concentration (due to converted epoxide groups) increases with TFTE loading, which can enhance hydrogen bonding and possible reactions between the fiber and matrix. At low TFTE loadings, the average functionality and resultant –OH concentration is relatively low, thus the interfacial strength is lower than that of TGDDM. However, at 40wt% loading, the hybrid has enough average functionality and, more importantly, it can achieve higher degree of conversion than TGDDM, giving a higher -OH concentration. In addition, after cure, the hybrid network has less defects due to the higher conversion, which enhances the stress transfer at interface and thus improves interfacial strength.

Figure 13. Droplet specimens (a) before and (b) after fractured by loading through the pin-holed steel plate. (c) Effects of TFTE fractions on the Fiber/epoxy interfacial shear strength.

CONCLUSIONS A novel high performance tetrafunctional epoxy (TFTE) is synthesized using a three-step procedure. In each step, the reaction condition is mild, and the yield is high. Although some


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isomers exist in TFTE, almost all of them carry four epoxide groups, either in para- or orthopositions. TFTE can be made from well accessible raw materials, which are suitable for mass production. Moreover, the N-free nature and well-separated epoxide groups exclude major cyclization side reactions, which commonly exist in TGDDM and other tetrafunctional aminederived epoxy. When the weight fraction of TFTE is lower than 40wt%, TFTE/DGEBA hybrid shows a lower viscosity than TGDDM. The optimized TFTE/DGEBA/DDS system shows a high Tg and balanced mechanical performance. At 30wt% TFTE, Tg, impact strength and tensile strength are 252 °C, 29.8 kJ/m2, and 80 MPa, respectively. Compared with the TGDDM/DDS, TFTE/DGEBA/DDS shows much improved toughness and elongation at break as well as 30% lower water absorption. In addition, the cured hybrid also shows good tensile strength and interfacial shear strength with carbon fiber. The novel TFTE system shows better balanced mechanical performance, good processability and thermal stability, making it a potential replacement for TGDDM.

AUTHOR INFORMATION Corresponding Authors Y. Meng, E-mail: [email protected], Tel:86-10-64419631, Fax: (+86)10-64452129; X. Li, [email protected], Tel: 86-10-64423162. Author Contributions † Tuan Liu and Liangdong Zhang contribute equally to this work. All authors have given approval to the final version of the manuscript.


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Funding Sources This work is financially supported by the National Natural Science Foundation of China (No. 51173012) and Joint funds of Equipment Pre-research Foundation and Ministry of Education (No. 6141A02022207).

Supporting Information Possible side reactions in TGDDM/DDS curing systems, 2D-NMR spectra of tetra-functional epoxy, and HPLC results are supplied.

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Table of Contents


34

Nitrogen-Free Tetrafunctional Epoxy and Its DDS-Cured High

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