Sonochemical Transformation of Epoxy–Amine Thermoset into

Article pubs.acs.org/Macromolecules

Sonochemical Transformation of Epoxy−Amine Thermoset into Soluble and Reusable Polymers Yuqin Min, Shuyun Huang, Yuxiang Wang, Zhijun Zhang, Binyang Du, Xinghong Zhang,* and Zhiqiang Fan MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: The degradation and reuse of epoxy thermosets have significant impact on the environments. We report that an epoxy−amine thermoset embedded with Diels−Alder (DA) bonds was transformed into soluble polymers via sonochemistry under mild temperature (ca. 20 °C) for the first time. Sonication could effectively induce the position-oriented cleavage of DA bonds (i.e., retro-DA) of the fully swelled epoxy thermoset in dimethyl sulfoxide (DMSO), leading to the soluble polymers. Of importance, such sonochemical process could be regulated on demand via switching on-and-off of the sonication. The obtained soluble polymers could be recured to form epoxy−amine thermosets via DA reaction. This sonochemical method might provide an unprecedented and efficient way to the controlled degradation and recycling of the epoxy thermosets containing the dynamic covalent bonds likes DA groups.

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of 104−106 g/mol) via position-oriented cleavage of the special chemical bonds in the matrix of the thermoset, namely, the controlled degradation of the thermoset. As a consequence, the thermoset can be recycled to be a new polymer. Dynamic covalent bonds,5,6 which are unique in the sense that they combine the characteristics of the covalent and noncovalent bonds, meet the above requirement. The dynamic covalent bonds, such as imine linkage,7 trithiocarbonate,8 acylhydrazone,9 disulfide,10 and Diels−Alder (DA) bonds,11 can reversibly form and break under certain conditions.12 Recently, the so-called covalent adaptable epoxy thermosets containing dynamic covalent bonds have been developed as bulky selfhealing or malleable materials.13−15 However, it is not yet reported that the 3-D network of the epoxy thermoset could be effectively transformed into soluble and reusable polymers. Aiming to transform the epoxy thermoset into the soluble polymers under mild temperature, we incorporated DA bonds into the epoxy thermoset in the present work (Scheme 1). In chemistry, DA bond is formed by [4 + 2] cycloaddition of furan and maleimide group at 60−80 °C, while retro-DA (r-DA)

INTRODUCTION Among of all commercial thermosets, epoxy thermosets present outstanding chemical, heat-resistant, and mechanical properties and excellent insulation due to their irreversible threedimensional (3-D) cross-linked networks.1,2 Global production of the epoxy thermosets such as printed circuit boards, packaging, coatings, and adhesives are about 2 000 000 t per year in recent years. The majority of them cannot be recycled due to their insoluble and infusible properties. As a result, the used epoxy thermosets are generally incinerated or landfilled like garbage or degraded to small molecules directly using strong acidic or basic agents.3,4 The separation and reuse of these small molecules are still big challenges. Indeed, these treatments for the used epoxy thermosets cause a huge material waste, energy consumption, and a great threat to the environment (e.g., production of dioxins during incineration). It is highly desired to develop a useful and controllable method for degrading the epoxy thermoset, which was predesigned to be stable in its service state, to soluble polymers. For such purpose, it would be useful to incorporate special chemical bonds, which can potentially unzip the 3-D network under external stimuli, into the epoxy thermoset. It would then be possible to transform the thermoset with infinite molecular weight (MW) to soluble high polymer (generally in the range © XXXX American Chemical Society

Received: September 18, 2014 Revised: December 14, 2014

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DOI: 10.1021/ma501934p Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Syntheses of 1, 2 and 3 Embedded with DA Bonds and the Proposed r-DA Reaction Induced by Ultrasound Irradiation

Figure 1. (A) DSC result of 2 under a N2 atmosphere (10 °C/min). (B) The powder of 2 swelled in DMSO for 4 h. (C) The solution obtained from (B) via sonication in DMSO for 75 min (ice bath). (D) The solid polymers from (C). (E) GPC curves of the soluble polymers obtained at different sonication times (entries 1−5 in Table 1). (F) The yield and Mn of the resultant soluble polymers as a function of sonication times.

reaction happens at 110−130 °C,11,16−27 releasing the furan and maleimide groups.16,24 Moreover, r-DA could be activated by force28 at low temperatures when a DA bond was placed at the center of a soluble polymer with high MW. In this case, MW kept unchanged when the polymer reached a MW threshold (Mlim), below which no further chain scission was observed.29−32 Since MW of the epoxy thermoset is infinite and nearly every site in the network could be considered as the midpoint of a chain, we envisioned that, if the epoxy thermoset was swelled well by a solvent, DA bonds embedded in the network could be broken via r-DA reaction under certain external force, leading to the transformation of the thermoset into the soluble polymer (Scheme 1). The existence of Mlim for r-DA reaction might ensure the achievement of high polymers

rather than small molecules. Herein, we shall show the proof-ofprinciple of this concept.

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RESULTS AND DISCUSSION For testing the aforementioned hypothesis, an epoxy−amine thermoset was designed and synthesized from the curing reaction of the bisepoxide (1) (FDB) containing two DA bonds in one molecule with diethylenetriamine (DETA). 1 was synthesized by [4 + 2] cycloaddition of furfuryl glycidyl ether (FGE) [1H (13C) NMR spectra are seen in Figure S1] with 4,4′-methylenebis(N-phenylmaleimide) in refluxed THF (66 °C) for 24 h.18 The obtained 1 had two DA bonds and two oxirane groups, which were confirmed by 1H (13C) NMR (Figure S2) and FT-IR spectra (Figure S3). B

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The effects of the sonication intensity and time on r-DA reaction in the swelled 2 were further examined (Table 1). The

The choice of the hardener for 1 and the corresponding cure procedure should strictly obey the rule that just one epoxy− amine network was formed with DA bonds between two crosslinked points because the production of the other network which resulted from the addition reaction of the generated secondary −OH group with epoxide induced by highly exothermic epoxy−amine addition reaction at high curing temperature would interpenetrate with the epoxy−amine network, and thus the resultant thermoset was difficult to degrade via sonication. Moreover, the heat-induced r-DA reaction could occur at above 110 °C in bulk. Therefore, a common hardener, DETA with five N−H bonds, was employed for curing with 1 for its moderate reactivity at low temperatures. 1 was then cured with the equivalent DETA at 25 °C for 4 days and postcured at 60 °C for 12 h under a N2 atmosphere. The long time and low temperature curing procedure were applied for equivalent reaction of the oxirane groups with N−H bonds, which was aimed to avoid the formation of another network. The oxirane groups of FDB were completely converted after curing reaction, which was proved by the comparison of FT-IR spectra of FDB/DETA thermoset (2) (Figure S4) with 1. The cross-linked structure of 2 was evident by the differential scanning calorimetric (DSC) and solubility test. The glass transition temperature (Tg) of 2 (Figure 1A) was estimated to be ca. 97 °C, which was much higher than that of the control (56 °C) which was the bisphenol A diglycidyl ether (DGEBA)/ DETA thermoset obtained under the same curing condition (Figure S5-A) because 2 had the rigid bismaleimide moieties. Moreover, an endothermic peak at 127 °C in the first heating trace could be attributed to r-DA reaction in 2,16 while the consecutive cooling and second heating processes exhibited nearly flat curves because DA reaction of the furan and maleimide moieties did not occur during DSC measurements.16 The solubility and swelling property of 2 in various solvents including tetrahydrofuran (THF), acetonitrile (ACN), dimethyl sulfoxide (DMSO), propylene carbonate (PC), N-methylpyrrolidone (NMP), and dimethylformamide (DMF) were examined at 25 °C (Figure S6). 2 was not dissolved by all the above solvents, but it was effectively swelled in DMSO, NMP, and DMF. THF, ACN, and PC presented poor swelling ability toward 2. NMP and DMF showed the swelling ratios of 157% and 175% to 2, respectively, while DMSO had the highest degree of swelling of 190% to 2 within 8 h. Therefore, DMSO was selected as the solvent for the controlled degradation of 2 under sonication. The ultrasound irradiation was carried out in the mode of 3 s on followed with 3 s off for the preset time. The crushed 2 (100 mg, the sizes of the granules were ca. 100 μm−5 mm) was put into DMSO (10.0 mL) in a 25 mL vial and swelled for at least 4 h under slow N2 bubbling (Figure 1B). The suspension was frozen to be solid in the ice bath because the melting point of DMSO is 18.5 °C. Once ultrasound irradiation was started, solid DMSO was melted immediately; the temperature of the suspension was raised and maintained at ca. 18−20 °C during sonication. With increasing the sonication times, the big granules became invisible gradually. Meanwhile, the color of the solution was changed from colorless to pale green and finally yellow green (Figure 1C). After sonication, the suspension was centrifuged. The supernatant was collected and dialyzed in methanol and then freeze-dried; a loose and pale yellow solid was obtained (Figure 1D).

Table 1. Soluble Polymers from the Sonochemical Reaction of the Epoxy Thermosets in the Selected Solventsa entry

solvent

int (%)

timeb (min)

yield of soluble polymers (%)c

Mn (kg/mol)/PDId

1 2 3 4 5 6 7 8 9 10e 11e 12e

DMSO DMSO DMSO DMSO DMSO DMSO DMSO NMP DMF DMSO NMP DMF

30 30 30 30 30 40 50 30 30 30 30 30

25 50 75 100 125 50 50 75 75 125 125 125

39.8 44.6 78.3 83.5 84.1 71.1 77.9 16.9 3.0 84.5 46.9 3.4

154.5/2.2 110.4/2.3 76.6/2.7 48.8/1.6 48.3/1.7 86.8/2.5 82.7/2.6 577.1/1.5 −/− 48.6/1.6 106.2/2.0 −/−

a

Conditions: 100.0 mg of 2 in 10.0 mL of solvent, ice bath; sonication mode: 3 s on and 3 s of f, and 10 min as one cycle (i.e., 5 min sonication on per cycle), the next cycle was then started after a stop time of 2 min. The maximum ultrasonic power of the instrument was 650 W; a 6 mm diameter amplitude transformer was immerged into the solvent in a 25 mL vial under a N2 atmosphere. bThe total time of the on mode of the sonicator. cCalculated from the residual solid after sonication, centrifugation, and completely dried process. dDetermined by gel permeation chromatography in DMF, 60 °C, PMMA standard. e The FHM/DETA thermoset (3).

increase of the sonication intensity from 30% to 50% led to the decrease of the number-average molecular weight (Mn) of the resultant soluble polymers from 110.4 to 82.7 kg/mol (entries 2, 6, and 7, the sonication time was 50 min, Table 1) as determined by gel permeation chromatography (GPC) (Figure S7). When the sonication intensity was set as 30%, with increasing the sonication times from 25 to 125 min, Mn decreased from 154.5 to 48.3 kg/mol (Figure 1E). We noticed that the decrease of Mn became very slow from 100 to 125 min, which could be considered as the Mlim (ca. 48.0−49.0 kg/mol) for the sonication degradation of 2 in DMSO (Figure 1F). Note that Mlim is closely dependent on the experimental conditions and technique used.29−32 The yield of the soluble polymers increased with increasing either the sonication intensity or the sonication times, as shown in Table 1. Note that the plot of the yield of the soluble polymer versus times presented an “S” type mode (Figure 1F), and a regime of high degradation rate could be identified for the sonication times from 50 to 75 min. Of importance, the sonication was easily to be regulated on demand. The sonication could be paused and restarted according to the requirements of the on and off of the external force. In other words, 2 could be degraded in a controllable manner under sonication at mild temperature. The r-DA reaction in the swelled 2 induced by sonication was clearly confirmed by 1H NMR spectrum (Figure 2) of the soluble polymer from entry 5 in Table 1. The 1H NMR spectra of FGE, FDB, and a heat-decomposed 2 (in DMSO, 130 °C, 30 min) are also shown in Figure 2 for comparison. Heat treatment of 2 in DMSO resulted in nearly complete disappearance of all DA bonds (6.62−6.51 and 5.20−5.33 ppm). Meanwhile, the furan ring (6.45−6.36 and 7.6−7.7 ppm) was released (curve 2) after heat treatment. In comparison, C

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Figure 2. 1H NMR spectra (400 MHz, d6-DMSO) of (1) the soluble polymer of entry 5 in Table 1, (2) heat-degraded product of 2 at 130 °C for 30 min, (3) FDB, and (4) FGE.

of 2 at mild temperature (ca. 18−20 °C). First, DMSO has a special dipole moment and is also a strong proton acceptor.36−38 DMSO could swell 2 via the formation of multihydrogen bonding between DMSO and hydroxyl groups (−OH) that was the strong proton donor in the network, which likely reduced the energy cost during the mixing of DMSO with the resultant soluble polymers. Such strong solvent−polymer interaction aided the extraction of a polymer chain from the swelled layer of the thermoset (Figure S11). Second, DMSO has a highly self-association structure when the temperature was close to its melting point (18.5 °C). Markarian et al. reported that the physical properties of DMSO had a sharp transition close to its melting point due to the formation of the bigger DMSO aggregates (or cluster), and the change of the temperature near the melting point caused the dramatic fluctuation concentration of the associates.39 In our system, because the suitable sonication temperature for 2 in DMSO was ca. 18−20 °C, the dramatic change of the self-association structure of DMSO, which was interpenetrated with the swelled 3-D network at the molecular levels, would cause a huge deformation of such swelled thermoset under sonication and thus activate the r-DA reaction effectively. Since no structural transition occurred when DMSO was heated from 20 to 60 °C,38−40 and the hydrogen bonding interaction between DMSO and water decreased with increasing the temperatures,41 we wondered if the degradation efficiency of 2 would decrease at a higher temperature or in both DMF and NMP which had no self-association structures in their liquid states because of their low melting points (DMF: −61 °C; NMP: −24.4 °C). To verify this idea, 2 was irradiated by ultrasound in DMSO without ice bath (ca. 40 °C) for 75 min; although the swelling ratio of 2 was 280% at 40 °C, the resultant solution was turbid (Figure S9−C) and only 9 wt % soluble polymer with a Mn of 219 kg/mol was obtained. When 2 was irradiated by ultrasound in DMF and NMP at ca. 10 °C (entries 8 and 9, Table 1), it was difficult to be degraded

ultrasound irradiation caused partial r-DA reaction in the swelled 2. There were 10.0 and 20.0 mol % DA bonds (curve 1) in the swelled 2 were broken via r-DA reaction after sonication for 75 and 125 min, respectively, according to 1H NMR spectra (the calculations are in Figure S8). The partial cleavage of DA bonds in the thermoset led to the production of the soluble polymers, which still contained considerable amounts of DA bonds. Therefore, the obtained soluble polymers could be postulated to be the statistical hyperbranched polymers. The partial degradation of the swelled 2 was solely caused by r-DA reaction rather than the degradation of epoxy−amine adducts in the thermoset (Scheme 1). It was confirmed by the sonication test for the swelled DGEBA/DETA thermoset (Tg = 56 °C, the degree of swelling was 156% in DMSO, 25 °C, 8 h) in DMSO under the same condition as that of entry 5 in Table 1. No soluble products were collected (Figure S9A,B). The weight loss of the DGEBA/DETA thermoset after sonication was ca. 3.0 wt %. Moreover, r-DA reaction in the swelled 2 was not caused by the local strongly exothermic heat, although the cavitation was formed and hot spots were produced under sonication.33,34 In a control experiment, we observed that r-DA reaction of FDB (1), which could be efficiently activated by heat,35 did not occur in d6-DMSO under the same sonication condition as entry 5 in Table 1 that was proved by 1H NMR spectra (Figure S10). The force-induced partial degradation of 2 to soluble polymers could be ascribed to the cavitation effect induced by ultrasound irradiation in DMSO.29,30 It is worthy to note that the sonochemical r-DA reaction of 2 was heterogeneous because 2 was cross-linked. The thermoset could only swell but not evenly disperse in DMSO. The sonication irradiation would be also helpful for the swelling of 2 in solution besides generating the mechanical force. Furthermore, the r-DA reaction may occur preferentially on the polymer chains that were pulled out by the solvation. DMSO was the effective solvent among of the above solvents for the partial degradation D

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MPa). G′ at 50 °C of 2 was 2600 MPa and higher than that of the recured 2 at the same temperature (2260 MPa), suggesting reduced storage modulus of the recured 2. Similar to the Tg values from DSC determination, both samples also showed nearly the same Tgs (85.9 and 85.5 °C for 2 and recured 2, respectively) from the peak position of tan δ. This sonochemical method could be applied to another epoxy thermoset, the FHM/DETA thermoset (3) containing DA bonds. The synthesis and characterization of FHM and 3 are given in the Supporting Information (Figures S12−S15, Tg = 15 °C). 3 was degraded to soluble polymers in DMSO under the same sonication condition (entry 10, Table 1) based on the same DA chemistry. The yields of the obtained soluble polymers in DMSO and NMP were 84.5% and 46.9%, respectively (entries 10 and 11, Table 1). Mn of the obtained soluble polymer in DMSO were 48.6 kg/mol and close to Mlim that was clearly lower than that (106.2 kg/mol) in NMP (entries 10 and 11 in Table 1), while 3 was hardly degraded in DMF (entry 12 in Table 1) under sonication. The partial degradation of 3 induced by sonication showed the similar results as that of 2. These obtained soluble polymers contain polar ether linkages, amino, carbonyl, and hydroxyl groups, which could be potentially used for scavenging the heavy metal ions or dyes in water.

although both solvents had good swelling abilities toward 2 and could easily dissolve the obtained polymers from sonication test in DMSO. Only 16.9 wt % of 2 was degraded to the soluble product (entry 8, Table 1) in NMP, while 2 was not degraded in DMF at all (entry 9, Table 1; Figure S9D,E) under sonication. Of interest, Mn of the soluble polymer obtained from the sonication-induced degradation of the thermoset in NMP was 577.1 kg/mol, which was higher than that (76.6 kg/ mol) obtained with DMSO as solvent under the same sonication conditions and the soluble polymers obtained after a 25 min sonication (154.5 kg/mol, entry 1, Table 1). This result suggested that the degradation rate of the soluble polymer in NMP was slower than that in DMSO. Such low efficiency of the mechanical activation in the control experiments might conversely prove the effective mechanical activation of r-DA reaction of 2 in DMSO at the temperatures close to the melting point of DMSO. As a result, the partial degradation of 2 to soluble polymers is a combination of the swelling and pulling-out effect caused by ultrasound irradiation in DMSO. The effective swelling of 2 is the prerequisite for converting the cross-linked network into soluble polymers, while the strong solvent−polymer interaction and highly self-association structure could facilitate the extraction of a polymer chain from the swelled layer of the thermoset (Figure S11) via the cavitation effect induced by ultrasound irradiation. Such sonochemical method represents a new effective route to transform epoxy thermosets into soluble polymers at mild temperature. The obtained soluble polymers (Figure 1D) could be easily reused. It was dissolved into DMF and poured into a mold. After evaporating DMF, the sample was cured at 70 °C for 2 days. A semitransparent flexible film was obtained (Figure 3,

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CONCLUSIONS In summary, we reported the first example of force-induced transformation of the epoxy thermoset into a soluble and reusable polymer via partial position-oriented cleavage of DA bonds in the thermoset in DMSO. This force-induced degradation protocol provides an unprecedented, useful, and efficient way to recycle the epoxy thermosets with dynamic covalent bonds like DA groups under mild temperature.

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EXPERIMENTAL SECTION

Materials. Furfuryl alcohol was purchased from Aladdin Reagent Ltd. 4,4′-Methylenebis(N-phenylmaleimide) (DPMBMI) and diethylenetriamine (DETA) were purchased from J&K Scientific Ltd. Epichlorohydrin (ECH) and solvents were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Diglycidyl ether of bisphenyl A (DGEBA, E-51, Brand KB-28) was granted kindly by Jiangsu Chemical Industry (Kingboard) Co., Ltd. (Wuxi, China). All solvents and the above reagents were used as received without further purification. Equipment and Measurements. Fourier transform infrared (FTIR) spectra were recorded by a Vector 22 FTIR spectrophotometer (KBr pellet). 1H nuclear magnetic resonance (1H NMR) spectra were obtained on a Bruker Advanced DMX 400-MHz spectrometer with tetramethylsilane (TMS) as internal standard. Ultrasound-induced degradation was carried on a sonifier cell disrupter (XO-650, Nanjing Xianou Instruments Manufacture Co. Ltd., Nanjing, China.). To obtain gel permeation chromatography (GPC) data, a PL-GPC 220 chromatograph (Polymer Laboratories Ltd.) equipped with an HP 1100 pump from Agilent Technologies. The GPC columns were eluted with DMF (0.05 M LiBr) at a rate of 1 mL/min at 60 °C. The injection volume was 100 μL. The molecular weight standard was PMMA. Differential scanning calorimetric (DSC) measurements were conducted on a TAQ200 instrument (New Castle, DE) with a ramping rate of 10 °C/min under a N2 atmosphere. The dynamic mechanical analysis (DMA) measurements were carried out on a Q800 analyzer (TA Instruments Corporation). The single cantilever mode was used, and the measurement was carried out from 0 to 140 °C with a heating rate of 3 °C/min and an oscillatory frequency of 1 Hz. Synthesis of Furfuryl Glycidyl Ether (FGE).18 51 g (0.55 mol) of epichlorohydrin (ECH) and 3.8 g of tetrabutylammonium bromide

Figure 3. DSC result of the recured sample (inserted chart) from the recycled epoxy polymer in DMF (cure condition: 70 °C/2 days).

inserted chart) and was not dissolved by DMF and DMSO. The resultant thermoset showed similar thermal behaviors (curve in Figure 3) with that of the pristine 2 (Figure 1A) and a Tg of 96 °C, which was higher than that (50 °C, Figure S5B) of the soluble polymers after sonication. Moreover, this film could be degraded to soluble polymers (yield: 51%) with a Mn of 103 kg/mol under ultrasound irradiation. Dynamic mechanical analysis (DMA) test was performed for obtaining the mechanical and thermal properties of the pristine 2 and recured 2 from the soluble polymers after sonication, as shown in Figure 4. The initial storage moduli (G′) at 0 °C of both pristine 2 and the recured 2 were nearly the same (ca. 3250 E

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Figure 4. Storage moduli (G′) and tan δ of the FDB/DETA thermoset (A) and the recured FDB/DETA thermoset (B) from soluble polymers as a function of temperature from dynamic mechanical analysis (DMA) scans. The glass fiber paper was used for strengthening 2 and recured 2, and the molding method was used for preparing the samples with a pressure of ca. 1−2 kg/cm2. The single cantilever mode was used, and the measurement was carried out from 0 to 140 °C at a heating rate of 3 °C/min and an oscillatory frequency of 1 Hz. (TBAB) were charged into a 250 mL three-necked round-bottom flask equipped with a magnetically stirred, a thermometer, and an inlet of dry nitrogen. When the solution was maintained at 25 °C with a water bath, 49 g (0.5 mol) of 2-furanemethanol was added dropwise in 30 min. The reaction mixture was stirred for 2 h at room temperature. After that, 80 mL of sodium hydroxide aqueous solution (50% (w/v)) was added dropwise within 1 h. The reaction proceeded for an additional 2 h at room temperature. Afterward, 100 mL of ethyl ether was added, and the organic layer was collected, washed with 50 mL waters for 3 times, and dried with anhydrous sodium sulfate, filtrated, and evaporated to get a light yellow liquid. The colorless liquid product was obtained after distillation of the crude product at 103− 105 °C in a vacuum (11 mmHg). 1H NMR (400 MHz, CDCl3): δ 7.44−7.39 (m, 1H), 6.38−6.31 (m, 2H), 4.53 (q, J = 12.8 Hz, 2H), 3.75 J = 11.5, 3.1 Hz, 1H), 3.44 (J = 11.5, 5.8 Hz, 1H), 3.16 (J = 5.8, 4.0, 2.9 Hz, 1H), 2.80 (J = 9.2, 4.4 Hz, 1H), 2.61 (J = 5.0, 2.7 Hz, 1H). 13 C NMR (100 MHz, CDCl3): δ 151.41, 142.89, 110.30, 109.58, 77.54, 77.28, 76.96, 70.57, 64.97, 50.68, 44.18. FIIR (KBr, cm−1): 3125, 3062, 3000, 1504, 1257, 1217, 1150, 1084, 1008, 916, 850, 751. Synthesis of 2,2′-(Methylenebis(4,1-phenylene))bis(4-((oxiran2-ylmethoxy)methyl)-3a,4,7,7a -tetrahydro-1H-4,7-epoxyisoindole1,3(2H)-dione) (FDB, 1).18 7.14 g (0.02 mol) of DPMBMI was dissolved in 50 mL of anhydrous THF. The solution was charged into a 100 mL three-necked round-bottom flask equipped with magnetic stirring and condenser and a thermometer. 6.16 g (0.04 mol) of FGE was then slowly dropped into the solution by a constant dropping funnel. Afterward, the solution was refluxed at about 66 °C for 24 h under a N2 atmosphere and then cooled down to room temperature. The reaction solution was poured into a large excess amount of diethyl ether. The precipitate was filtered and then dried under vacuum at room temperature. The obtained powder was dissolved into 5 mL of acetone and poured into 500 mL of diethyl ether again. Finally, the precipitate was filtered and dried under vacuum at room temperature. The precipitate was further purified by chromatography on a silica gel column with mixed solvent of CHCl3/CH3OH = 100/1 (Rf = 0.38) to obtain the yellow powder product. The exo and endo ratio of FDB was determined by 1H NMR as exo/endo = 7:1. Exo isomer 1H NMR (400 MHz, DMSO-d6): δ 7.39−7.32 (2H), 7.15 (2H), 6.57 (2H), 5.21 (1H), 4.23 (2H), 4.08−3.98 (1H), 3.85−3.75 (2H), 3.36−3.32 (1H), 3.19 (1H), 3.06 (1H), 2.74−2.68 (1H), 2.56−2.51 (1H). 13C NMR (100 MHz, CDCl3): 13C NMR (100 MHz, DMSO-d6): δ 176.29− 173.03, 140.80, 138.57−136.44, 128.93, 90.45−89.10, 80.63, 72.00, 68.34, 50.99−47.90, 43.85−37.67. Preparation of the FDB/DETA Thermoset (2). First, 0.06 g of curing agent DETA was dissolved into 1.0 mL of 1,2-dichloroethane. Then 1.0 g of FDB was dissolved in 2.0 mL of CH2Cl2. The two solutions were then mixed until a clear transparent mixture was formed. The mixture was degassed under vacuum for 3 h. Afterward, the mixture was cured at room temperature for 4 days and postcured at 60 °C for 12 h to get the cross-linked epoxy thermoset.

Sonication Test for 2. 100.0 mg of 2 was added into 10.0 mL of solvent in a 25 mL vial for at least 4 h, which was then cooled with an ice bath (0 °C) and subjected to pulsed ultrasound irradiation. The sonication test was carried on a sonicator with an 6 mm diameter amplitude transformer that was immerged into the solvent in a 25 mL vial under a N2 atmosphere. The maximum ultrasonic power was 650 W, and the intensity rate was set as 30% (195 W). The sonication mode: 3 s on and 3 s off, and 10 min as one cycle (i.e., 5 min sonication on per cycle); the next cycle was started after a stop time of 2 min. After the ultrasound degradation, the mixtures were centrifuged (3000 rpm, 20 min). The precipitates were dried and weighted to calculate the residual solid percentage. The supernatant were collected and loaded into the dialysis bags with molecular cutoff of MW3500 and placed in CH3OH for dialyzing 3 days at ca. 0 °C to replace the solvent with CH3OH. After that, the solutions in the dialysis bags were dried in vacuum oven and solid powders were obtained. For GPC measurement, 10.0 mg of obtained solid powders was first dissolved in 3.0 mL of DMF and filtered with 0.22 μm organic membranes; the obtained clear solutions were then subjected to GPC test. As a control experiment, 15.0 mg of cured sample and 1.0 mL of DMSO-d6 were loaded in a NMR tube. The NMR tube was placed in an oil bath with temperature of 130 °C for 30 min and then quenched to room temperature by ice water. The obtained heat-induced degradation product was directly characterized by 1H NMR spectroscopy. Preparation of Sample Bars for DMA Test. The glass fiber paper was used for strengthening 2 and the recured 2 because both were brittle and hard to remove bubbles by pouring method. The glass fiber paper was immersed into CH2Cl2 solution of FDB and DETA, and then the solvent was evaporated under vacuum for 3 h and cured at 25 °C for 4 days and 60 °C for 12 h under pressure of ca. 1−2 kg/cm2; the sample bar with 50 × 10 × 1.0 mm3 was obtained. For testing the mechanical properties of recured 2, the powder obtained by sonication degradation of 2 in DMSO were dissolved into DMF; the glass fiber paper was then immersed into the solution. Afterward, DMF was evaporated under vacuum for 3 days and then cured at 75 °C for 2 days under a pressure of ca. 1−2 kg/cm2, leading to the sample bar with 50 × 10 × 1.0 mm3.

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ASSOCIATED CONTENT

* Supporting Information S

Text, figures, and tables giving general experimental procedures and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax +86-571 87953732; e-mail [email protected] (X.Z.). F

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Macromolecules Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support by the National Science Foundation of the People’s Republic of China (No. 21274123 and 21474083).

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DOI: 10.1021/ma501934p Macromolecules XXXX, XXX, XXX−XXX