This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Article Cite This: ACS Omega 2018, 3, 4295−4305
Using Dicyclopentadiene-Derived Polyarylates as Epoxy Curing Agents To Achieve High Tg and Low Dielectric Epoxy Thermosets Chia-Min Lin,† Chien-Han Chen,† Ching-Hsuan Lin,*,† Wen Chiung Su,‡ and Tzong-Yuan Juang§ †
Department of Chemical Engineering, National Chung Hsing University, Taichung 40227, Taiwan Chung Shan Institute of Science and Technology, Lungtan, Tauyuan 32546, Taiwan § Department of Cosmeceutics, China Medical University, Taichung 40402, Taiwan ‡
S Supporting Information *
ABSTRACT: To achieve high-Tg and low-dielectric epoxy thermosets, four dicyclopentadiene-derived polyarylates (26-P, 26-M, 236-P, and 236-M) were prepared from 2,6-dimethyl (or 2,3,6-trimethyl) phenol-dicyclopentadiene adduct with terephthaloyl (or isophthaloyl) chloride by high-temperature solution polymerization. The resulting polyarylates, exhibiting active ester linkages (Ph− O−(CO)−) are found to be reactive toward a commercial dicyclopentadiene phenol epoxy (HP7200) in the presence of some lone-pair electron-containing compounds. Five compounds including 4-dimethylaminopyridine (DMAP), imidazole, 2-methylimidazole, triphenylphosphine, and triphenylimidazole have been evaluated as a catalyst for the curing reactions. We found that DMAP, with the smallest pKb among them, is the best catalyst according to differential scanning calorimetry, infrared, and thermal analyses. The thermal and dielectric properties of the polyarylate/HP7200 thermosets are evaluated. We found that they exhibit a high Tg characteristic (e.g., Tg is 238 °C for DMAP-catalyzed, 236P/HP7200 thermoset). Furthermore, because of the hydrophobic methyl and cycloaliphatic moieties, and the secondary hydroxyl-free structure, polyarylate/HP7200 thermosets show a relative low-dielectric constant of around 2.75 U. The detailed structure−properties relationship is discussed in this work. of crown ether with organic and inorganic salts;19,20 and (3) studied thermocrosslinking reactions of polymers containing pendant epoxy with various active esters.21−23 Nelson and Jackson,24 Hiroaki,25 Wang et al.,26,27 have synthesized low-dielectric hydrocarbon-containing epoxy thermosets. Among these hydrocarbons, dicyclopentadiene (DCPD) and higher oligomers of cyclopentadiene are the most commonly used starting materials because of their availability (DCPD is a byproduct of C5 streams in oil refineries) and low cost. Taking advantage of DCPD, Dainippon Ink and Chemicals Corporation has commercialized EPICLON HPC-8000, a phenol-DCPD adduct-based active ester for preparing low-dielectric epoxy thermosets.28 In addition to HPC-8000, DC808, a phenol novolac-based active ester, has been commercialized by Japan Epoxy Resins for preparing low-dielectric epoxy thermosets.29 The commercial active ester-type curing agents such as EPICLON HPC-8000 and DC808 are oligomer types. They are generally prepared from the esterification of (a) phenol-DCPD adduct, or (b) naphthol-DCPD adduct, or (c) phenolformaldehyde novolac, or (d) biphenol A-formaldehyde novolac (Figure 1). It is worth noticing that main-chain-type
1. INTRODUCTION Because of the good chemical resistance, adhesion, dimensional stability, and insulation after thermal curing, epoxy resins have been widely applied in the electronic field such as encapsulation and printed circuit boards. Generally, epoxy resins are cured by reacting the epoxy groups with active hydrogen-containing curing agents, such as phenol novolac, diamines, and thiols.1−11 The curing reaction leads to a highly polar secondary alcohol linkages, which will increase the dielectric constant of epoxy thermosets. Therefore, compared with other low-dielectric thermosets,12 epoxy thermosets show a moderate-to-high dielectric constant. Using active ester-type epoxy curing agents (curing agents with Ph−O−(CO)− structure) is a solution to prepare secondary alcohol-free epoxy thermosets.13−15 (Note that the active esters mentioned in this work are different from those reviewed by Das and Theato for postpolymerization modification).16 Nakamura and Arima prepared a triphenol and its active ester as epoxy curing agents,15 and they found that epoxy thermosets cured by the active ester exhibit better dielectric properties than those cured by the triphenol. They also (1) cured copolymers of phenyl methacrylate (contains active ester moiety after polymerization of the CC bond) by polyfunctional epoxy;17,18 (2) prepared polyethers with pendant ester by the condensation of p-phenylene dibenzoate with diepoxy in the presence of quaternary salts or the mixture © 2018 American Chemical Society
Received: February 12, 2018 Accepted: April 3, 2018 Published: April 18, 2018 4295
DOI: 10.1021/acsomega.8b00256 ACS Omega 2018, 3, 4295−4305
Article
ACS Omega
active hydrogens show higher Tg than those cured by active ester-type curing agents. This is the penalty for epoxy thermosets cured by active ester-type curing agents. However, our previous work36−38 indicates that using polymeric-type curing agent can result in epoxy thermosets with a high Tg characteristic. In this work, combining the concept of (1) active ester, (2) polymeric-type curing agent, and (3) hydrophobic DCPD and methyl groups, we prepare four DCPD-derived polyarylates (26-P, 26-M, 236-P, and 236-M) for preparing high-Tg, lowdielectric epoxy thermosets. The resulting polyarylates, all with active ester Ph−O−(CO)−linkages, are reactive toward epoxy resin in the presence of some lone-pair electroncontaining compounds. To the best of our knowledge, no DCPD-derived polyarylates have been applied as curing agents. Five compounds including 4-dimethylaminopyridine (DMAP), imidazole (Imi), 2-methylimidazole (2-MI), triphenylphosphine (TPP), and triphenylimidazole (TPI) have been evaluated as a catalyst for the curing reactions. We found that the basicity of the compound has a strong influence on curing reactions and thermal and dielectric properties of the resulting epoxy thermosets. The synthesis of polyarylates and the influence of the catalyst on curing reaction and properties of thermosets will be discussed in this work.
Figure 1. Structure of phenolic oligomers (the precursor of active esters) (a) phenol-DCPD adduct, (b) naphthol-DCPD adduct, (c) phenol-formaldehyde novolac, and (d) biphenol A-formaldehyde novolac.
epoxy curing agents show a series of advantages,30−32 such as improvement in the thermal and mechanical properties of the epoxy thermosets. For example, Francis et al. prepared a hydroxyl-terminated poly(ether ether ketone) with different molecular weights and used them as epoxy curing agents.33−35 They found that the mechanical properties of thermosets increase with the increase in the molecular weight of the curing agents. In our previous work,36 we have successfully prepared a phenol-functionalized phosphinated polyether (P1). P1, with phenol linkages, can react with cresol novolac epoxy to get a flexible and transparent of an epoxy thermosetting film with a Tg value as high as 250 °C. We have also demonstrated that high-MW polybenzoxazine precursors can lead benzoxazine thermosets to a much higher Tg than those cured by monomertype benzoxazines.37,38 The secondary alcohol resulting from the curing reaction of active hydrogen and epoxy can provide intermolecular interactions, so epoxy thermosets cured by
2. RESULTS AND DISCUSSION 2.1. Synthesis of 2,6-Phenol-DCPD and 2,3,6-PhenolDCPD. 2,6-Phenol-DCPD and 2,3,6-phenol-DCPD were prepared from the alkylation of DCPD with 2,6-dimethyl phenol or 2,3,6-dimethyl phenol, respectively, in the presence of AlCl3 (Scheme 1). The reaction involves two steps. In the first step, DCPD reacted with AlCl3 to form carbocations, which then reacted with 2,6-dimethyl phenol or 2,3,6-dimethyl phenol in the second step. Figure S1 shows the 1H NMR spectra of 2,6-phenol-DCPD and 2,3,6-phenol-DCPD. There
Scheme 1. Synthesis of 2,6-Phenol-DCPD and 2,3,6-Phenol-DCPD and Four Polyarylates
4296
DOI: 10.1021/acsomega.8b00256 ACS Omega 2018, 3, 4295−4305
Article
ACS Omega
Figure 2. 1H NMR spectra of polyarylates (a) 26-P, (b) 26-M, (c) 236-P, and (d) 236-M.
are no unsaturated −HCCH− signals of DCPD at 5.4 and 5.9 ppm in both spectra, indicating a complete reaction. The signals of aliphatic hydrogens and methyl in the range of 1.2− 3.6 ppm, the characteristic peaks of Ar-H at 6.6−7.0 ppm, and the OH peaks at 4.43 and 4.46 ppm support the structure of 2,6-phenol-DCPD and 2,3,6-phenol-DCPD. There are multiple constitutional isomers in both 2,6-phenol-DCPD39 and 2,3,6phenol-DCPD, so the peak patterns are not as simple as a pure compound. Figure S2 shows the infrared (IR) spectra of DCPD, 2,6-phenol-DCPD, and 2,3,6-phenol-DCPD. The characteristic peak of unsaturated HCCH signals at 3053 and 750 cm−1 disappeared, and characteristic absorption of the aromatic ring at 1490−1450 cm−1 and the OH absorption at 3400−3230 cm−1 support the structure of 2,6-phenol-DCPD and 2,3,6-phenol-DCPD.
2.2. Synthesis of Polyarylates. Polyarylates were prepared from 2,6-phenol-DCPD or 2,3,6-phenol-DCPD with terephthaloyl chloride (TPC) or isophthaloyl chloride (IPC) by hightemperature solution polymerization in o-dichlorobenzene (Scheme 1). Figure 2 shows the 1H NMR of the four polyarylates. The Ar-H from the terephthalic structure was clearly observed at 8.4 ppm for 26-P and 236-P. The Ar-H from the isophthalic structure was clearly observed at 7.8, 8.5, and 9.2 ppm for 26-M and 236-M. Figure S3 shows the IR spectra of the four polyarylates. The broad and strong OH absorption at around 3500 cm−1 disappeared, and the ester absorption at 1735 cm−1 was clearly observed. Figure S4 shows the differential scanning calorimetry (DSC) thermograms of synthesized polyarylates. The Tg values are 224, 199, 187, and 177 °C for 26-P, 26-M, 236-P, and 236-M, respectively. The Tg values depend on the structure of polyarylates. 4297
DOI: 10.1021/acsomega.8b00256 ACS Omega 2018, 3, 4295−4305
Article
ACS Omega
Figure 3. DSC heating scans of polyarylate/HP7200 blends with different types of catalysts. (a) 26-P/E-X blend, (b) 26-M/E-X blend, (c) 236-P/EX blend, and (d) 236-M/E-X blend. X is the catalyst: DMAP, 2MI, Imi, TPP, and TPI, respectively. The catalyst used is marked in the figure.
Figure 4. pKb value and the structure of catalysts used in this work.
stronger basicity displays a better catalytic effect, and DMAP is found to be the best catalyst among them. 2.4. Influence of Catalysts on Carbonyl Absorption of IR Spectra. In our previous work, we have designed a model reaction of glycidyl phenyl ether and phenyl acetate at 120 °C in a molten state in the presence of 2.0 wt % (based on the weight of glycidyl phenyl ether) DMAP.40 After careful evaluation of the product’s peaks, we found the reaction product is 1,3-diphenoxy-2-acetoxypropane (Figure 5). The carbonyl absorption changes from Ar−O−CO− to R−O− CO−. According to the IR absorption principle, aromatic ester with Ar−O−(CO) has higher absorption than that of aliphatic ester with R−O−(CO).41 Therefore, we can use the
Polyarylates 26-P and 236-P, with the para-substituted structure, are more rigid than those (26-P and 236-P) with the meta-substituted structure, leading to higher Tg values. Furthermore, the 236-series polyarylates (236-P and 236-M) have lower Tg values than 26-series polyarylates (26-P and 26M), probably because of the fact that the three methyl substitutions hinder the molecular packing. 2.3. Influence of Catalysts on the Exothermic Peak of DSC Thermograms. Figure 3 shows DSC heating scans of polyarylate/HP7200 blends using different types of catalysts. Note that the word blend means polyarylate/HP7200 has not been thermally cured before DSC scans. The order of the exothermic peak temperature is DMAP-catalyzed < 2MIcatalyzed < Imi-catalyzed systems, suggesting that DMAP is the best catalyst among them. No exothermic peak was found for TPP- and TPI-catalyzed systems, indicating that no reaction occurs in both systems. Figure 4 lists the pKb and structure of catalysts used in this work. According to Figures 3 and 4, we found that a catalyst with smaller pKb has the lower exothermic peak temperature. These results indicate that a catalyst with a
Figure 5. Model reaction of glycidyl phenyl ether and phenyl acetate in the presence of DMAP. 4298
DOI: 10.1021/acsomega.8b00256 ACS Omega 2018, 3, 4295−4305
Article
ACS Omega
Figure 6. Enlarged FTIR spectra of polyarylate and polyarylate/HP7200 thermosets catalyzed by different catalysts. (a) 26-P and 26-P/E-X, (b) 26M and 26-M/E-X, (c) 236-P and 236-P/E-X, and (d) 236-M and 236-M/E-X. X is the catalyst: DMAP, 2MI, Imi, TPP, and TPI, respectively. The catalyst used is marked in the figure.
substitution of the phenoxy anion of polyarylate on the carbon of the C−N bond of (II), forming intermediate (III) and releasing DMAP that can repeat step 1. Another way to obtain (III) is through steps 4 and 5. The fourth step is the nucleophilic addition of the phenoxy anion of polyarylate on the epoxy group of HP7200, forming intermediate (IV) with an alkoxy anion. The fifth step is the nucleophilic substitution of the alkoxy anion of (IV) on the ester group of polyarylate, forming (III) and polyarylate with a phenoxy anion that can repeat step 4. 2.6. Solubility of Polyarylate/HP7200 Thermosets. Table 1 lists the solubility of the polyarylates and polyarylate/HP7200 thermosets. With 10 wt % concentration, the four polyarylates are soluble in tetrahydrofuran (THF), NMP, toluene, N,N-dimethyl acetamide (DMAc), and CHCl3 at room temperature, and are soluble in methyl ether ketone (MEK) on heating. The multiple methyl groups and multiple constitutional isomers of 2,6-dimethyl (or 2,3,6-trimethyl) phenol-DCPD adduct make them soluble. The good solubility of polyarylates makes them easy for solution processing for future application in copper clad laminate. In contrast to the good solubility of polyarylates, the thermosets are insoluble, suggesting that network structures are formed. 2.7. Thermomechanical Property. To discuss the influence of the catalyst on Tg of a thermoset, we evaluate polyarylate/HP7200 thermosets catalyzed by different catalysts by dynamic mechanical analysis (DMA, Figure 8). The data are listed in Table 2. With the same polyarylate, the order in the Tg
shift of CO absorption to monitor the reaction of polyarylate and HP7200 in the presence of a catalyst. Figure 6 shows enlarged Fourier transform infrared (FTIR) spectra of polyarylates and polyarylate/HP7200 thermosets catalyzed by different catalysts. Note that the word thermoset means polyarylate/HP7200 has been thermally cured. The C O absorption value of epoxy thermosets is listed in Figure 7. For DMAP-, Imi-, and 2MI-catalyzed system, the shift is obvious from around 1737 cm−1 to around 1725 cm−1. The order of IR shift is DMAP-catalyzed > 2MI-catalyzed > Imicatalyzed systems, suggesting that DMAP is the best catalyst among them. No shift in carbonyl absorption was found for TPP- and TPI-catalyzed systems, indicating that no reaction occurs in both systems. The results indicate that a catalyst with a stronger basicity has a better catalytic effect, and DMAP is found to be the best catalyst among them. This result is consistent with the DSC thermogram in Figure 3. 2.5. Proposed Mechanism. According to the conclusion of the model reaction of glycidyl phenyl ether and phenyl acetate in the presence of DMAP40 and IR spectra in Figure 6, we propose a curing mechanism of polyarylate (236-P) and HP7200 in the presence of DMAP in Scheme 2. The first step is the attack of DMAP on the epoxy group of HP7200, forming intermediate (I) with an alkoxy anion. The second step is the nucleophilic substitution of the alkoxy anion of (I) on the ester group of polyarylate, forming intermediate (II) and polyarylate with a phenoxy anion. The third step is the nucleophilic 4299
DOI: 10.1021/acsomega.8b00256 ACS Omega 2018, 3, 4295−4305
Article
ACS Omega
Figure 7. CO absorption values of polyarylate and polyarylate/HP7200 thermosets catalyzed by different catalysts. (a) 26-P and 26-P/E-X, (b) 26-M and 26-M/E-X, (c) 236-P and 236-P/E-X, (d) 236-M and 236-M/E-X. X is the catalyst: DMAP, 2MI, Imi, TPP, and TPI, respectively.
Scheme 2. Proposed Reaction Mechanism of Polyarylates (236-P) and HP7200 in the Presence of DMAP
catalytic effect, and DMAP is found to be the best catalyst among them. According to the IR spectra (Figure 7), the shift of the CO signal strongly depends on the basicity of the catalyst.
value is DMAP-catalyzed > 2MI-catalyzed > Imi-catalyzed systems. For example, for 26-P/E-X, Tg values are 239, 222, and 217 when X is DMAP, 2MI, and Imi, respectively. The results indicate that a catalyst with a stronger basicity has a better 4300
DOI: 10.1021/acsomega.8b00256 ACS Omega 2018, 3, 4295−4305
Article
ACS Omega Table 1. Solubility of Polyarylate and Polyarylate/HP7200 Thermosetsa,b sample
THF
MEK
NMP
toluene
DMAc
CHCl3
26-P 26-P/E-Xc 26-M 26-M/E-Xc 236-P 236-P/E-Xc 236-M 236-M/E-Xc
+ − + − + − + −
+h − +h − +h − +h −
+ − + − + − + −
+ − + − + − + −
+ − + − + − + −
+ − + − + − + −
Table 2. Thermal Properties of Polyarylates/HP7200 Thermosets sample 26-P/E-DMAP 26-M/E-DMAP 236-P/E-DMAP 236-M/E-DMAP 26-P/E-2MI 26-M/E-2MI 236-P/E-2MI 236-M/E-2MI 26-P/E-Imi 26-M/E-Imi 236-P/E-Imi 236-M/E-Imi
a
Solubility was tested with a 5 mg in 0.5 mL of solvent. b+, soluble at room temperature; +h, soluble on heating, −, insoluble. cAll of the thermosets are insoluble regardless of X (the catalyst: DMAP, 2MI, or Imi).
Therefore, the difference in Tg is attributed to the different curing degrees. To discuss the influence of polyarylate on Tg of the thermoset, DMA thermograms of polyarylate/HP7200 thermosets cured by different polyarylates are shown in Figure S5. With the same catalyst, the order in Tg values is generally 26-P/E-X > 236-P/E-X > 26-M/E-X > 236-M/E-X. For example, the Tg values are 239, 238, 225, and 209 °C for 26P/E-DMAP, 236-P/E-DMAP, 26-M/E-DMAP, and 236-M/EDMAP, respectively. Thermosets based on the para-substituted polyarylate (26-P and 236-P) are more rigid than those based on the meta-substituted polyarylates (26-M and 236-M). The Tg values in this work are in the range of 209−239 °C, which are much higher than that (165 °C, DMA data) of the DMAP-
Tg Tg (°C)a (°C)b 239 225 238 209 222 213 220 213 217 212 212 209
197 188 195 179 187 176 181 175 186 173 176 173
CTE (ppm/ °C)c
Td,onset (°C)d
Td5% (°C)e
CY (%)f
57 52 59 80 68 74 78 70 75 49 68 54
402 401 401 400 404 399 402 399 404 401 401 400
416 415 407 406 414 412 407 411 412 412 406 407
14 10 12 11 10 10 11 10 12 15 12 13
Measured by DMA at a heating rate of 5 °C/min. bMeasured by TMA at a heating rate of 5 °C/min. cCTE is recorded from 50 to 150 °C. dOnset decomposition temperature, recorded by thermogravimetry at a heating rate of 20 °C/min. e5 wt % decomposition temperature, recorded by thermogravimetry at a heating rate of 20 °C/ min. fResidual wt % at 800 °C in N2 atmosphere. a
catalyzed EPICLON HPC-8000/HP-7200 thermoset.42 This work demonstrates that the epoxy thermoset cured by polymeric-type curing agents with active ester in the main chain, such as polyarylates in this work, displays higher Tg than those cured by oligomeric-type curing agents with active ester in the side chain, such as HPC-8000.
Figure 8. DMA thermograms of polyarylate/HP7200 thermosets catalyzed by different catalysts. (a) 26-P/E-X, (b) 26-M/E-X, (c) 236-P/E-X, and (d) 236-M/E-X. X is the catalyst: DMAP, 2MI, and Imi. The catalyst used is marked in the figure. 4301
DOI: 10.1021/acsomega.8b00256 ACS Omega 2018, 3, 4295−4305
Article
ACS Omega
Figure 9. TMA thermograms of polyarylate/HP7200 thermosets catalyzed by different catalysts. (a) 26-P/E-X, (b) 26-M/E-X, (c) 236-P/E-X, and (d) 236-M/E-X. X is the catalyst: DMAP, 2MI, and Imi. The catalyst used is marked in the figure.
To further discuss the influence of the catalyst on Tg of a thermoset, we evaluate polyarylate/HP7200 thermosets catalyzed by different catalysts by thermomechanical analysis (TMA) (Figure 9). The data are listed in Table 2. With the same polyarylate, the order in the Tg value is DMAP-catalyzed > 2MI-catalyzed > Imi-catalyzed systems. For example, for 26P/E-X, Tg values are 197, 187, and 186 when X is DMAP, 2MI, and Imi, respectively. The relation between the type of the catalyst and Tg is the same as those observed in DMA data. However, no obvious trend between structure and the coefficient of thermal expansion (CTE) was observed. To discuss the influence of polyarylate on Tg of a thermoset, TMA thermograms of polyarylate/HP7200 thermosets cured by different polyarylates are shown in Figure S6. With the same catalyst, the order in Tg values is 26-P/E-X > 236-P/E-X > 26M/E/X > 236-M/E-X. For example, Tg values are 197, 195, 188, and 179 °C for 26-P/E-DMAP, 236-P/E-DMAP, 26-M/EDMAP, and 236-M/E-DMAP, respectively. The relation between the structure of polyarylates and Tg is the same as those observed in DMA data. 2.8. Thermal Stability. Figure S7 shows the thermal gravimetric analysis (TGA) thermograms of the thermosets in a nitrogen atmosphere. Table 2 lists the thermal stability data of polyarylate/HP7200 thermosets. The onset degradation temperature in a nitrogen atmosphere is around 400 °C, and the 5 wt % degradation temperatures (Td5) in nitrogen atmosphere range from 406 to 416 °C, and char yields are in the range of 10−15%. These values are slightly higher than typical epoxy thermosets, suggesting the benefit in thermal
stability using polyarylates as the curing agent. However, no obvious structure−thermal stability relationship was found. 2.9. Dielectric Property. Table 3 lists the dielectric constant and dissipation factor of the polyarylate/HP7200 Table 3. Dielectric Properties of DCPD Series Cured Epoxy Thermosets sample
thickness (μm)
26-P/E-DMAP 26-M/E-DMAP 236-P/E-DMAP 236-M/E-DMAP 26-P/E-2MI 26-M/E-2MI 236-P/E-2MI 236-M/E-2MI 26-P/E-Imi 26-M/E-Imi 236-P/E-Imi 236-M/E-Imi
403 384 377 371 238 211 203 198 202 217 233 200
Dk (U, 1 GHz) 2.85 2.83 2.75 2.74 2.88 2.83 2.83 2.76 2.87 2.84 2.81 2.79
± ± ± ± ± ± ± ± ± ± ± ±
0.002 0.002 0.001 0.003 0.002 0.001 0.002 0.003 0.01 0.03 0.01 0.02
Df (U, 1 GHz) 0.0148 0.0136 0.0135 0.0101 0.0151 0.0143 0.0140 0.0125 0.0154 0.0146 0.0143 0.0129
± ± ± ± ± ± ± ± ± ± ± ±
0.0002 0.0003 0.0002 0.0003 0.0001 0.0002 0.0001 0.0003 0.0001 0.0002 0.0001 0.0003
thermosets. The dielectric constant ranges from 2.74 to 2.88 U, and the dissipation factor ranges from 10.1 to 15.4 mU. With the same catalyst, meta-substituted polyarylate/HP7200 thermosets show better dielectric properties than parasubstituted polyarylate/HP7200 thermosets. For example, the dielectric constant (dissipation factor) is 2.88 U (15.1 mU) and 2.83 U (14.3 mU) for 26-P/E-2MI and 26-M/E-2MI, respectively; and it is 2.83 U (14.0 mU) and 2.76 U (12.5 4302
DOI: 10.1021/acsomega.8b00256 ACS Omega 2018, 3, 4295−4305
Article
ACS Omega
lead to a better dissipation factor, probably because of the better conversion that will transform highly polar epoxy groups to less polar ester groups. The dielectric constant of epoxy thermosets ranges from 2.74 to 2.88 U which is much smaller than that (around 3.5) for common epoxy thermosets. The Tg values corresponding to the dielectric constant of 2.74−2.88 are as high as 209−239 °C, demonstrating that high-Tg, lowdielectric epoxy thermosets can be achieved through this facile approach. To the best of our knowledge, this is the first example using DCPD-derived polyarylates as epoxy curing agents to achieve high-Tg and low-dielectric epoxy thermosets.
mU) for 236-P/E-2MI and 236-M/E-2MI, respectively. In addition, epoxy thermosets cured by trimethyl-substituted polyarylates (236-P and 236-M) show better dielectric properties than those cured by dimethyl-substituted polyarylates (26-P and 26-M). For example, the dielectric constant (dissipation factor) is 2.85 U (14.8 mU) and 2.75 U (13.5 mU) for 26-P/E-DMAP and 236-P/E-DMAP, respectively; and it is 2.83 U (13.6 mU) and 2.74 U (10.1 mU) for 26-M/E-DMAP and 236-M/E-DMAP, respectively. Hougham et al. reported that the dielectric constant can be reduced by increasing the molecule’s hydrophobicity and free volume and by decreasing polarization.43 The fact that the meta-substituted polyarylates provide higher free volume than the para-substituted polyarylates might be responsible for the better dielectric property of meta-substituted polyarylate/ HP7200 thermosets. The hydrophobic methyl group might be the reason for the better dielectric property of trimethylsubstituted polyarylate/HP7200 thermosets. As listed in Table 3, with the same polyarylate, the order of the dissipation factor is generally DMAP-catalyzed < 2MI-catalyzed < Imi-catalyzed systems. For example, the dissipation factor is 14.8, 15.1, and 15.4 mU for 26-P/E-DMAP26-P/E-2MI and 26-P/E-Imi; and it is 10.1, 12.5, and 12.9 mU for 236-M/E-DMAP, 236-M/E2MI, and 236-M/E-Imi, respectively. Previous data (Figure 3, 6−9) show that the order of the catalytic effect is DMAP > 2MI > Imi, so the higher conversion (with less residual high-polar epoxy) might be the reason for the order of the dissipation factor. The dielectric constant is in the range of 2.74−2.88 U, which is much smaller than that (around 3.5)44−48 for common epoxy thermosets. We think that the hydrophobic methyl and cycloaliphtic moiety and the secondary alcohol-free structure of the thermosets are responsible for the good dielectric properties. Moreover, the Tg corresponding to the dielectric constant of 2.74−2.88 is as high as 209−239 °C (Tables 2 and 3), demonstrating that highTg, low-dielectric epoxy thermosets can be achieved through this facile approach.
4. EXPERIMENTAL SECTION 4.1. Materials. DCPD (from Sigma), aluminum(III) chloride (AlCl3, from Alfa), TPC (from TCI), IPC (from TCI), 2,6-dimethylphenol (from Acros), 2,3,6-dimethylphenol (from Acros), and phenol DCPD novolac epoxy with an EEW of 250 g/equiv were kindly supplied by Dainippon Ink and Chemicals Corporation under the commercial name of HP7200. 4-Dimethylaminopyrdine (DMAP, from Alfa), Imi (from Acros), 2-MI (from Aldrich), TPP (from Alfa), and 2,4,5-TPI (from Alfa) were used as received without purification. oDichlorobenzene (from Acros), N-methylpyrrolidone (NMP from Tedia), DMAc (from Tedia), MEK (from Acros), and other solvents used for the solubility test are HPLC or ACS grade and were used as received without purification. 4.2. Synthesis of 2,6-Dimethyl Phenol-DCPD Adduct (2,6-Phenol-DCPD). 2,6-Dimethyl phenol (0.7143 mol) and AlCl3 (0.015 mol) were added to a four-necked roundbottomed flask equipped with a nitrogen inlet, magnetic stirrer, thermocouple, and temperature controller. The reaction mixture was gradually heated to 120 °C. DCPD 0.1 mol was added gradually over a period of 2 h. The solution was kept at 120 °C for another 4 h. After the reaction was completed, 0.06 mol of 5 wt % NaOH(aq) was added, and the mixture was stirred for 1 h. The reaction mixture was filtered, and the filtrate was washed three times with water. Next, the organic phase was separated and distilled in a rotary evaporator to remove excess 2,6-dimethyl phenol. The crude product was dissolved in toluene and extracted with water for several times. The organic phase was distilled to remove toluene, and a deep-brown 2,6phenol-DCPD was obtained in quantitative yield. 1H NMR (ppm, CDCl3): δ 1.05−2.80 (26H, aliphatic-H), 6.60−7.10 (4H, aromatic-H), 4.43−4.76 (2H, OH). FT-IR (KBr, cm−1): 3468 (OH), 2929, 2487 (CH3), 1200 (C−O). HR-MS (FAB+) m/z: calcd for C26H32O2, 376.24; anal., 375.2326. 4.3. Synthesis of 2,3,6-Trimethyl Phenol-DCPD Adduct (2,3,6-Phenol-DCPD). 2,3,6-Phenol-DCPD was prepared by the reaction of 2,3,6-trimethyl phenol and DCPD in the presence of AlCl3. The preparation procedure is the same as that of 2,6-phenol-DCPD. 1H NMR (ppm, CDCl3): δ 1.21− 2.40 (32H, aliphatic-H), 6.81−7.00 (2H, aromatic-H), 4.45− 4.80 (2H, OH). FT-IR (KBr, cm−1): 3468 (OH), 2929, 2487 (CH3), 1200 (C−O). HR-MS (FAB+) m/z: calcd for C28H36O2, 404.27; anal., 403.2368. 4.4. Synthesis of Polyarylate (26-P). 2,6-Phenol-DCPD (8 mmol), TPC (8 mmol), and 30 mL of o-dichlorobenzene were added to a four-necked round-bottomed flask equipped with a nitrogen inlet, magnetic stirrer, thermocouple, and temperature controller. The solution was heated to reflux temperature for 30 h. Then, the solution was poured into methanol to precipitate the polyarylate. After filtration, the filter cake was washed two times with methanol/water (1/1, v/v)
3. CONCLUSIONS We have successfully prepared four DCPD-derived polyarylates. The resulting polyarylates, exhibiting active ester linkages (Ph− O−(CO)−), are found to be reactive toward epoxy (HP7200) in the presence of some lone-pair electroncontaining compounds. Among the compounds, DMAP, with the lowest pKb, has the lowest exothermic peak for the polyarylate/HP7200 blend in the DSC thermogram (Figure 3), the highest CO shift of the thermoset in the IR spectrum (Figure 6), and the highest Tg of thermosets (Figures 8 and 9). Tg values of thermosets cured by the four polyarylates are in the range of 209−239 °C, which are much higher than that (165 °C) of the DMAP-catalyzed commercialized EPICLON HPC8000/HP-7200 thermoset. This result demonstrates the advantage of polyarylates over commercialized oligomerictype curing agents in thermal properties. With the same catalyst, thermosets based on the para-substituted polyarylate (26-P and 236-P) display higher Tg than those based on the meta-substituted polyarylates (26-M and 236-M) (Table 2). Thermosets based on meta-substituted and trimethyl-substituted polyarylates (236-M) show the best dielectric properties. With the same polyarylate, the value of the dissipation factor is generally DMAP-catalyzed < 2MI-catalyzed < Imi-catalyzed systems (Table 3), which is the inverse order of reactivity of the catalyst. In other words, a catalyst with a better reactivity will 4303
DOI: 10.1021/acsomega.8b00256 ACS Omega 2018, 3, 4295−4305
Article
ACS Omega
with PerkinElmer Pyris 1 at a heating rate of 20 °C/min in a nitrogen atmosphere. Dielectric properties were determined with Agilent E4991A at 1 GHz with a sample size with 2.5 cm in diameter and thickness at least 200 μm. TMA was performed by a SII TMA/SS6100 at a heating rate of 5 °C/min. The CTE was recorded at the temperature range of 50−150 °C. GPC was performed by Hitachi L2400 in N-methyl-2-pyrrolidone (NMP) (Hitachi, Tarrytown, NY, Tarrytown, USA), using polystyrene as the standard.
and dried at 60 °C in a vacuum oven to obtain a brown polyarylate (26-P). The yield is quantitative. 1H NMR (ppm, CDCl3): δ 1.21−2.40 (26H, aliphatic-H), 6.81−7.20 (4H, aromatic-H), 8.20−8.62 (4H, aromatic-H). FT-IR (KBr, cm−1): 2929, 2487 (CH3), 1735 (CO), 1200 (C−O). Gel permeation chromatography (GPC): number-average molecular weight 12 600 g/mol, weight-average molecular weight 34 500 g/mol. 4.5. Synthesis of Polyarylate (26-M). Polyarylate (26-M) was prepared by 2,6-phenol-DCPD and IPC. The preparation procedure is the same as that of 26-P. 1H NMR (ppm, CDCl3): δ 1.05−2.80 (26H, aliphatic-H), 6.70−7.20 (4H, aromatic-H), 7.58−7.8 (1H, aromatic-H), 8.28−8.64 (2H, aromatic-H), 8.86−9.16 (1H, aromatic-H). FT-IR (KBr, cm−1): 2929, 2487 (CH3), 1736 (CO), 1200 (C−O). GPC: number-average molecular weight 13 900 g/mol, weight-average molecular weight 35 900 g/mol. 4.6. Synthesis of Polyarylate (236-P). Polyarylate (236P) was prepared by 2,3,6-phenol-DCPD and TPC. The preparation procedure is the same as that of 26-P. 1H NMR (ppm, CDCl3): δ 1.21−2.40 (32H, aliphatic-H), 6.61−7.20 (4H, aromatic-H), 8.17−8.57 (4H, aromatic-H). FT-IR (KBr, cm−1): 2929, 2487 (CH3), 1733 (CO), 1200 (C−O). GPC: number-average molecular weight 21 000 g/mol, weightaverage molecular weight 49 100 g/mol. 4.7. Synthesis of Polyarylate (236-M). Polyarylate (236M) was prepared by 2,3,6-phenol-DCPD and IPC. The preparation procedure is the same as that of 26-P. 1H NMR (ppm, CDCl3): δ 1.21−2.40 (32H, aliphatic-H), 6.7−7.2 (4H, aromatic-H), 7.58−7.8 (1H, aromatic-H), 8.28−8.60 (2H, aromatic-H), 8.86−9.16 (1H, aromatic-H). FT-IR (KBr, cm−1): 2929, 2487 (CH3), 1737 (CO), 1200 (C−O). GPC: number-average molecular weight 11 400 g/mol, weightaverage molecular weight 28 900 g/mol. 4.8. Curing Procedure for the Polyarylate/Epoxy Thermoset. The DCPD-phenol epoxy, HP7200, was thermally cured by the four polyarylates. The molar ratio of the ester moiety to oxirane (epoxy group) was controlled at one. Polyarylate, HP7200, and a catalyst (DMAP, Imi, 2MI, TPP, or TPI) 0.5 wt % (based on the weight of epoxy) were dissolved in NMP to make a solution with 30 wt % solid content. The viscous solution was cast on a glass by an automatic film applicator. The resulting thin films were dried at 80 °C for 12 h. Then, under nitrogen atmosphere, the curing process was done for 140 °C (2 h), 160 °C (2 h), 180 °C (2 h), and 200 °C (2 h). The thermosets are named in the format of “polyarylate/EX”. X is the catalyst. For example, the thermoset cured from 26P, HP7200, and DMAP is named 26-P/E-DMAP and that cured from 236-M, HP7200 and Imi is named 236-M/E-Imi and so on. 4.9. Characterization. DMA was performed with a PerkinElmer Pyris Diamond DMA with a sample size of 5.0 cm × 1.0 cm × 0.2 cm, and the test was performed using a tension mode with an amplitude of 25 μm. The storage modulus and tan(δ) were determined as the sample subjected to the temperature scan mode at a programmed heating rate of 5 °C/min with a frequency of 1 Hz. NMR measurements were performed using a Varian Inova 600 NMR in DMSO-d6 or CDCl3. The chemical shift of 1H NMR was calibrated by setting the chemical shift of a trace amount of DMSO-d5 in DMSO-d6 as 2.49 ppm. IR spectra were obtained in the standard wavenumber range of 400−4000 cm−1 by a PerkinElmer RX1 IR spectrophotometer. TGA was performed
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00256. NMR and IR spectra of synthesized monomers; IR spectra and DSC thermograms of synthesized polyarylates; and DMA, TMA, and TGA thermograms of polyarylate/HP7200 thermosets cured by different catalysts (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 886-4-22850180, +886-22840510. Fax: 886-4-22854734. ORCID
Ching-Hsuan Lin: 0000-0003-0381-2736 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Financial supports on this work from Chung San Institute of Science and Technology (Taiwan, grant no. NCSIST-1246V401(106)) are highly appreciated.
■
REFERENCES
(1) Endo, T.; Sudo, A. Development and application of novel ringopening polymerizations to functional networked polymers. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 4847−4858. (2) Liu, J.-g.; Ueda, M. High refractive index polymers: fundamental research and practical applications. J. Mater. Chem. 2009, 19, 8907− 8919. (3) Konuray, A. O.; Fernández-Francos, X.; Ramis, X. Analysis of the reaction mechanism of the thiol-epoxy addition initiated by nucleophilic tertiary amines. Polym. Chem. 2017, 8, 5934−5947. (4) Fernández-Francos, X.; Konuray, A.-O.; Belmonte, A.; De la Flor, S.; Serra, À .; Ramis, X. Sequential curing of off-stoichiometric thiolepoxy thermosets with a custom-tailored structure. Polym. Chem. 2016, 7, 2280−2290. (5) Jin, K.; Wilmot, N.; Heath, W. H.; Torkelson, J. M. PhaseSeparated Thiol−Epoxy−Acrylate Hybrid Polymer Networks with Controlled Cross-Link Density Synthesized by Simultaneous Thiol− Acrylate and Thiol−Epoxy Click Reactions. Macromolecules 2016, 49, 4115−4123. (6) Jones, B. H.; Wheeler, D. R.; Black, H. T.; Stavig, M. E.; Sawyer, P. S.; Giron, N. H.; Celina, M. C.; Lambert, T. N.; Alam, T. M. Stress Relaxation in Epoxy Thermosets via a Ferrocene-Based Amine Curing Agent. Macromolecules 2017, 50, 5014−5024. (7) Min, Y.; Huang, S.; Wang, Y.; Zhang, Z.; Du, B.; Zhang, X.; Fan, Z. Sonochemical Transformation of Epoxy−Amine Thermoset into Soluble and Reusable Polymers. Macromolecules 2015, 48, 316−322. (8) Jin, F.-L.; Li, X.; Park, S.-J. Synthesis and application of epoxy resins: A review. J. Ind. Eng. Chem. 2015, 29, 1−11. 4304
DOI: 10.1021/acsomega.8b00256 ACS Omega 2018, 3, 4295−4305
Article
ACS Omega (9) Zhao, S.; Abu-Omar, M. M. Synthesis of Renewable Thermoset Polymers through Successive Lignin Modification Using LigninDerived Phenols. ACS Sustainable Chem. Eng. 2017, 5, 5059−5066. (10) Zhao, S.; Abu-Omar, M. M. Renewable thermoplastics based on lignin-derived polyphenols. Macromolecules 2017, 50, 3573−3581. (11) Zhang, S.; Yang, P.; Bai, Y.; Zhou, T.; Zhu, R.; Gu, Y. Polybenzoxazines: Thermal Responsiveness of Hydrogen Bonds and Application as Latent Curing Agents for Thermosetting Resins. ACS Omega 2017, 2, 1529−1534. (12) Volksen, W.; Miller, R. D.; Dubois, G. Low Dielectric Constant Materials. Chem. Rev. 2010, 110, 56−110. (13) Nishikubo, T.; Kameyama, A. Addition reactions of cyclic ethers with various carbonyl compounds and their application for polymer synthesis. Prog. Polym. Sci. 1993, 18, 963−995. (14) Nakamura, S.; Saegusa, Y.; Yanagisawa, H.; Touse, M.; Shirai, T.; Nishikubo, T. Thermal analysis of epoxy curing using polyfunctional active esters as curing agents. Thermochim. Acta 1991, 183, 269−277. (15) Nakamura, S.; Arima, M. Characterization of the Network Structure o Epoxy Resins Cured with Active Esters. Int. J. Polym. Anal. Charact. 1995, 1, 75−86. (16) Das, A.; Theato, P. Activated ester containing polymers: opportunities and challenges for the design of functional macromolecules. Chem. Rev. 2015, 116, 1434−1495. (17) Nishikubo, T.; Saita, S. New thermo-crosslinking reactions of copolymers of phenyl methacrylates by use of polyfunctional epoxy compounds. Macromol. Chem. Phys. 1987, 188, 799−809. (18) Nishikubo, T.; Shimokawa, T.; Hirano, T.; Shiina, A. A novel insertion reaction of epoxy compounds into phenyl ester linkages in polymer chains. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 2519− 2530. (19) Nishikubo, T.; Tanaka, K. A novel reaction of epoxy resins with polyfunctional active esters. J. Appl. Polym. Sci. 1987, 33, 2821−2831. (20) Nishikubo, T.; Shimokawa, T. A novel insertion reaction of epoxy compounds into the pendant ester linkage of poly [4-(4nitrobenzoyloxy) styrene]. Macromol. Rapid Commun. 1986, 7, 179− 181. (21) Nishikubo, T.; Saita, S.; Fujii, T. New thermo-crosslinking reactions of polymers containing pendant epoxide groups with various polyfunctional active esters. J. Polym. Sci., Part A: Polym. Chem. 1987, 25, 1339−1351. (22) Nishikubo, T.; Iizawa, T.; Takahashi, E.; Nono, F. Study of photopolymers. 26. Novel synthesis of self-sensitized photosensitive polymers by addition reactions of poly (glycidyl methacrylate) with nitroaryl cinnamate. Macromolecules 1985, 18, 2131−2135. (23) Nishikubo, T.; Iizawa, T.; Saita, S. Addition reactions of pendant epoxide group in poly (glycidyl methacrylate) with various active esters. J. Polym. Sci., Part A: Polym. Chem. 1986, 24, 1685−1695. (24) Nelson, D. L.; Jackson, L. Phenolic hydroxy-containing compositions and epoxy resins prepared therefrom. U.S. Patent 4,390,680, 1983. (25) Hiroaki, N. Japanese Patent 8,713,5,539, 1987. (26) Wang, C. S.; Bogan, G. W. Epoxy resins prepared from trisphenols and dicyclopentadiene. U.S. Patent 4,663,400, 1987. (27) Wang, C. S.; Chen, W. N.; Bowden, R. L.; Berman, J. R. U.S. Patent 4,684,400, 1987. (28) Sukeaki, U.; Satoshi, D.; Katsuji, T. Epoxy resin composition for low-dielectric material. JP 2002−012650 A, 2002. (29) Satoshi, D.; Masao, Y.; Koichi, F.; Katsuji, T.; Minoru, T.; Kenichi, K. Epoxy resin composition, and sheet, preperg-like material, sheet with metal foil, laminated sheet, electrical insulating material and resist material obtained therefrom. JP 2004277460 A, 2004. (30) Vijayan, P. P.; Puglia, D.; Jyotishkumar, P.; Kenny, J. M.; Thomas, S. Effect of nanoclay and carboxyl-terminated (butadiene-coacrylonitrile) (CTBN) rubber on the reaction induced phase separation and cure kinetics of an epoxy/cyclic anhydride system. J. Mater. Sci. 2012, 47, 5241−5253. (31) Vijayan, P. P.; Puglia, D.; Kenny, J. M.; Thomas, S. Effect of organically modified nanoclay on the miscibility, rheology, morphol-
ogy and properties of epoxy/carboxyl-terminated (butadiene-coacrylonitrile) blend. Soft Matter 2013, 9, 2899−2911. (32) McGrath, L. M.; Parnas, R. S.; King, S. H.; Schroeder, J. L.; Fischer, D. A.; Lenhart, J. L. Investigation of the thermal, mechanical, and fracture properties of alumina−epoxy composites. Polymer 2008, 49, 999−1014. (33) Francis, B.; Thomas, S.; Jose, J.; Ramaswamy, R.; Lakshmana Rao, V. Hydroxyl terminated poly(ether ether ketone) with pendent methyl group toughened epoxy resin: miscibility, morphology and mechanical properties. Polymer 2005, 46, 12372−12385. (34) Francis, B.; Poel, G. V.; Posada, F.; Groeninckx, G.; Lakshmana Rao, V.; Ramaswamy, R.; Thomas, S. Cure kinetics and morphology of blends of epoxy resin with poly (ether ether ketone) containing pendant tertiary butyl groups. Polymer 2003, 44, 3687−3699. (35) Francis, B.; Lakshmana Rao, V.; Jose, S.; Catherine, B. K.; Ramaswamy, R.; Jose, J.; Thomas, S. Poly(ether ether ketone) with pendent methyl groups as a toughening agent for amine cured DGEBA epoxy resin. J. Mater. Sci. 2006, 41, 5467−5479. (36) Lin, C. H.; Wang, Y. R.; Feng, Y. R.; Wang, M. W.; Juang, T. Y. An approach of modifying poly(aryl ether ketone) to phenolcontaining poly(aryl ether) and its application in preparing highperformance epoxy thermosets. Polymer 2013, 54, 1612−1620. (37) Lin, C. H.; Chang, S. L.; Shen, T. Y.; Shih, Y. S.; Lin, H. T.; Wang, C. F. Flexible polybenzoxazine thermosets with high glass transition temperatures and low surface free energies. Polym. Chem. 2012, 3, 935−945. (38) Wang, M. W.; Jeng, R. J.; Lin, C. H. The robustness of a thermoset of a main-chain type polybenzoxazine precursor prepared through a strategy of A-A and B-B polycondensation. RSC Adv. 2016, 6, 18678−18684. (39) Lin, C. H.; Chiang, J. C.; Wang, C. S. Low dielectric thermoset. I. Synthesis and properties of novel 2,6-dimethyl phenol-dicyclopentadiene epoxy. J. Appl. Polym. Sci. 2003, 88, 2607−2613. (40) Chen, C. H.; Gu, Z. C.; Lin, C. H. Identification of the reaction mechanism between phenyl methacrylate and epoxy and its application in preparing low-dielectric epoxy thermosets with flexibility. Polymer 2018, 140, 225−232. (41) Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Vyvyan, J. A. Introduction to Spectroscopy, 4th ed.; Cengage Learning, 2008; p 68. (42) Zeng, X. Epoxy resin composition and prepreg and copper clad laminate manufactured by using the same. U.S. Patent 20,140,342,161 A1, 2014. (43) Hougham, G.; Tesoro, G.; Shaw, J. Synthesis and Properties of Highly Fluorinated Polyimides. Macromolecules 1994, 27, 3642−3649. (44) Tao, Z.; Yang, S.; Ge, Z.; Chen, J.; Fan, L. Synthesis and properties of novel fluorinated epoxy resins based on 1,1-bis(4glycidylesterphenyl)-1-(3′-trifluoromethylphenyl)-2,2,2-trifluoroethane. Eur. Polym. J. 2007, 43, 550−560. (45) Zhao, X.-Y.; Liu, H.-J. Review of polymer materials with low dielectric constant. Polym. Int. 2010, 59, 597−606. (46) Lin, C. H.; Jiang, Z. R.; Wang, C. S. Low dielectric thermoset. II. Synthesis and properties of novel 2,6-dimethyl phenol−dipentene epoxy. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 4084−4097. (47) Lin, C. H.; Hsiao, C. N.; Li, C. H.; Wang, C. S. Low dielectric thermoset. IV. Synthesis and properties of a dipentene-containing cyanate ester and its copolymerization with bisphenol A dicyanate ester. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 3986−3995. (48) Cai, S. X.; Lin, C. H. Flame-retardant epoxy resins with high glass-transition temperatures from a novel trifunctional curing agent: Dopotriol. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 2862−2873.
4305
DOI: 10.1021/acsomega.8b00256 ACS Omega 2018, 3, 4295−4305
