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A new reactive polymethacrylate bearing pendant furfuryl groups: Synthesis, thermoreversible reactions, and self-healing
Accepted Manuscript A new reactive polymethacrylate bearing pendant furfuryl groups: Synthesis, thermoreversible reactions, and self-healing Sungmin Jung, Jiang Tian Liu, Sung Hwa Hong, Dhamodaran Arunbabu, Seung Man Noh, Jung Kwon Oh PII:
S0032-3861(16)31115-6
DOI:
10.1016/j.polymer.2016.12.029
Reference:
JPOL 19265
To appear in:
Polymer
Received Date: 4 November 2016 Revised Date:
9 December 2016
Accepted Date: 10 December 2016
Please cite this article as: Jung S, Liu JT, Hong SH, Arunbabu D, Noh SM, Oh JK, A new reactive polymethacrylate bearing pendant furfuryl groups: Synthesis, thermoreversible reactions, and selfhealing, Polymer (2017), doi: 10.1016/j.polymer.2016.12.029. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A new reactive polymethacrylate bearing pendant furfuryl groups: synthesis, thermoreversible reactions, and self-healing
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Sungmin Jung,a Jiang Tian Liu,a Sung Hwa Hong,a Dhamodaran Arunbabu,a Seung Man Noh,b Jung Kwon Oha*
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A new reactive polymethacrylate bearing pendant furfuryl groups: synthesis, thermoreversible reactions, and self-healing
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Sungmin Jung,a Jiang Tian Liu,a Sung Hwa Hong,a Dhamodaran Arunbabu,a Seung Man Noh,b Jung Kwon Oha* a
Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec, Canada H4B 1R6
b
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Corresponding author: J.K.Oh ([email protected])
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Research Center for Green Fine Chemicals, Korea Research Institute of Chemical Technology (KRICT), Ulsan 44412, Republic of Korea
Abstract
We report a new methacrylate copolymer having pendant furfuryl groups (PFu) reactive to maleimide-bearing compounds to form thermally-induced crosslinked networks exhibiting self-
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healability through thermoreversible Diels-Alder (DA)/Retro-DA reactions. The PFu is synthesized by a combination of a facile radical polymerization with post-modification methods through a basecatalyzed coupling reaction and a thermally-induced thiol-ene radical addition reaction. The resulting PFu is effective toward DA reaction with a maleimide-bearing crosslinker at moderate or elevated
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temperatures, yielding highly crosslinked films through DA linkages. Thermal analysis and spectroscopic studies along with NMR model study with small molecular weight precursors suggest the formation of DA-crosslinked networks at 50-100 °C and the occurrence of retro-DA reaction at
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>125 °C. The resulting DA-crosslinked network is dynamic, exhibiting self-healing through the occurrence of thermoreversible DA/retro-DA reactions, confirmed with sol-gel transition and optical microscopy.
Keywords: Post-modification, furan-containing polymethacrylates, crosslinked networks, Diels-Alder reaction, Retro-Diels-Alder reaction, self-healing
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Introduction Three-dimensionally crosslinked networks with covalent linkages providing high mechanical properties as well as great thermal and solvent resistance have been utilized as effective building blocks for the development of a variety of multifunctional polymeric materials useful in nanoscience
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and industrial fields.[1-3] Numerous methods have been explored to synthesize covalently-crosslinked networks. Conventional methods utilize either a chain-growth polymerization of vinyl monomers in the presence of multifunctional crosslinkers or a step-growth polymerization through polycondensation or polyaddition of polyfunctional monomers. These methods consequently yield polymeric materials
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crosslinked with permanent linkages, typically carbon-carbon bonds.[4-6] Recent advance is the development of new methods to synthesize reversible networks crosslinked with dynamic (labile)
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covalent linkages; the formed networks enable to self-heal or self-repair damaged cracks. Such selfhealability is a highly desired property of crosslinked materials because their built-in ability to repair physical damages effectively prevents catastrophic failure to extending their lifetime.[7-13] A variety of dynamic linkages have been explored, including disulfide,[14-18] hindered urea,[19, 20] alkoxyamine,[21, 22] diarylbibenzofuranone (a dimer of arylbenzofuranone),[23] boronic ester,[24-26] and etc.[27]
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Of particular interest is the covalent linkage formed through thermally-induced Diels-Alder (DA) reaction, a [4+2] cycloaddition of a diene and a dienophile (hereafter DA linkage). Promisingly, the formed DA linkage can be cleaved through retro-DA reaction at elevated temperatures to the corresponding diene and dienophile. Typically employing furan and maleimide moieties, this unique
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thermoreversible DA/retro-DA chemistry enables DA-crosslinked materials to attain self-healability. Several approaches have been explored for the synthesis of thermally-induced DA-crosslinked
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materials, which can be characteristic of furan-functionalized compounds or polymers.[28-30] Polycondensation or polyaddition of furan and maleimide-functionalized compounds were explored to synthesize various step-growth polymers[31-33] including polyurethanes[34-37] and epoxy resins[38] crosslinked with DA crosslinkages. Chain growth polymerization of monomers labeled with a pendant furfuryl group allows for the synthesis of relatively high molecular weight linear (co)polymers bearing pendant furan groups (PFu). These polymers include mainly polymethacrylates[39-42] as well as polycarbonate,[43] and epoxy oligomer.[44] DA-crosslinked networks can be formed from a reactive blend of the resulting PFu with maleimide-functionalized crosslinkers. The fabrication of DA-
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ACCEPTED MANUSCRIPT crosslinked networks based on polymethacrylates bearing both pendant furan and furan-protected maleimide groups is also reported.[45-47] Another approach involves post-modification of functional polymers with small furan-containing molecules. This approach could also allow for the synthesis of a variety of furan-bearing polymers
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with different chemical structures. Examples include polybutadiene,[48, 49] polyamides,[50] polyketones,[51, 52] and epoxy resins.[38, 53] Despite these advances, the exploration of postmodification to develop new furan-containing polymers, particularly polymethacrylates, is highly beneficial. This approach could circumvent the occurrence of premature crosslinking reactions because
furfuryl and maleimide groups during radical polymerization.
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of undesirable side reactions such as self-condensation of furfuryl groups or DA reactions between
Herein, we report the synthesis of a new methacrylate copolymer having pendant furfuryl groups
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(PFu) by a combination of a facile free-radical solution polymerization with post-modification methods through a base-catalyzed coupling reaction and a thermally-induced thiol-ene radical addition reaction. As illustrated in Scheme 2, a free radical polymerization of 2-hydroxyethyl methacrylate and 2ethylhexyl methacrylate yielded a POH copolymer. The pendant hydroxyl groups were converted to methacrylate esters by a facile esterification, thus forming PMA. Then, a click-type thiol-ene addition
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reaction of furfuryl mercaptan to the methacrylate groups allowed for the synthesis of PFu having pendant furfuryl groups. The ability of the resulting PFu toward DA crosslinking with different amounts of a bismaleimide (BM) as a model crosslinker was characterized with gel content measurements, thermal analysis, and spectroscopic studies of the formed DA-crosslinked networks. In
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addition, model NMR study for a reactive blend of small molecular weight furan and maleimide precursors were conducted to get an insight into the effect of temperature on the occurrence of DA/retro DA reactions. Further, the self-healability of the DA-crosslinked networks through
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thermoreversible DA/retro-DA reaction was examined with sol-gel transition in anisole and optical microscopy (Scheme 1).
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Scheme 1. Thermoreversible DA/retro-DA reactions for self-healing of dynamically-crosslinked networks prepared from a reactive mixture containing PFu having pendant furfuryl groups and a bismaleimide model crosslinker.
Experimental
Materials. 2,2´-Azobis(2-methybutyronitrile) (AMBN, >98%), n-butyl mercaptan (Bu-SH, 95%),
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triethylamine (Et3N, >99.9%), methacryloyl chloride (MA-Cl, >97%), furfuryl mercaptan (Fu-SH, 99%), and 1,1-(methylene di-4,1-phenylene)bismaleimide (BM) were purchased from Aldrich and used as received. 2-Ethylhexyl methacrylate (EHMA) and 2-hydroxyethyl methacrylate (HEMA) from
inhibitors.
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Aldrich were purified by passing them through a column filled with basic alumina to remove
Synthesis of POH. EHMA (15.3 g, 77 mmol), HEMA (10.0 g, 77 mmol), Bu-SH (0.5 wt% based
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on monomer mixtures, 150 µL), and anisole (34 mL) were mixed in a 50 mL Schlenk flask. The mixture was deoxygenated by purging under nitrogen for 40 min and placed in an oil bath preheated at 70 °C. The nitrogen-prepurged AMBN solution (0.15 g, 0.8 mmol) in anisole (0.5 mL) was injected into the Schlenk flask to initiate polymerization. Polymerization was stopped after 3 hrs by cooling the reaction vessel in an ice bath. For purification, as-synthesized polymer solutions were precipitated from cold diethyl ether and dried in vacuum oven at room temperature for 18 hrs. Synthesis of PMA. A mixture containing Et3N (3.5 mL, 25 mmol) and POH (1.0 g, 5 mmol of OH groups) dissolved in anhydrous tetrahydrofuran (THF, 150 mL) was purged with a nitrogen gas for 10 min in an ice-bath. After the dropwise addition of a clear solution of MA-Cl (2.4 mL, 25 mmol) in 4
ACCEPTED MANUSCRIPT THF (10 mL), the resulting mixture was stirred at room temperature overnight. The formed solids as by-products (Et3N-HCl adducts) were removed by vacuum filtration. Solvents were removed by a rotary evaporation, and residues were dissolved in dichloromethane (200 mL). The formed solution was washed with water (100 mL) and brine solution (50 mL) three times, and then dried over MgSO4.
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After being concentrated using a rotary evaporation, the solution was precipitated from hexane. The precipitates were isolated and dried in a vacuum oven at room temperature for 18 hrs.
Synthesis of PFu. PMA (0.6 g, 1.7 mmol of methacrylate group), AMBN (0.24 g, 1.3 mmol), and Fu-SH (1.34 mL, 13.3 mmol) were dissolved in a solvent mixture (10 mL) of anhydrous THF and
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DMF (1/3 v/v) in Schlenk flask. The resulting mixture was degassed by three freeze-pump-thaw cycles and stirred at 75 °C for 24 hrs. The reaction was stopped by cooling down to room temperature. After the removal of solvents using a rotary evaporation, the concentrated solution was precipitated from a
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solvent mixture (750 mL) of MeOH and water (3/2 v/v) twice. The precipitates were then dried in a vacuum oven at room temperature for 14 hrs.
Instrumentation and analyses. 1H-NMR spectra were recorded using a 500 MHz Varian spectrometer. DMSO-d6 multiplet at 2.5 ppm was selected as the reference standard. Monomer conversion was determined by gravimetry (120 °C/4 hr). Molecular weight and molecular weight
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distribution were determined by gel permeation chromatography (GPC) with a Viscotek VE1122 pump and a refractive index (RI) detector. Two PolyAnalytik columns (PAS-103L and 105L, designed to determine molecular weight up to 2,000,000 g/mol) were used with THF as eluent at 30 °C at a flow rate of 1 mL/min. Linear poly(methyl methacrylate) standards were used for calibration. Aliquots of
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polymer samples were dissolved in THF. The clear solutions were filtered using a 0.25 µm PTFE filter to remove any solvent-insoluble species. A drop of anisole was added as a flow rate marker. FT-IR spectra of all the samples were recorded on a Nicolet 6700 FT-IR spectrometer using an attenuated
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total reflectance (ATR) crystal. All spectra were recorded with 32 scans in a resolution of 4 cm-1 at room temperature in the range of 800-2000 cm-1. Differential scanning calorimetry (DSC). Thermal properties including glass transition temperature (Tg) of both polymers and DA-crosslinked films were measured with a TA Instruments DSC Q20 differential scanning calorimeter. Temperature range was set at -80-200 °C with heating and cooling cycles conducted at a rate of 10 °C/min (cycles : cool to -80 °C, heat up to 200 °C (1st run), cool to -80 °C, heat up to 200 °C (2nd run), and cool to 25 °C). The glass transition temperature (Tg) was determined from the 2nd heating run. 5
ACCEPTED MANUSCRIPT Thermogravimetric analysis (TGA). TGA measurements for the purified, dried polymers were carried out using a TA Instruments Q50 analyzer. Their pieces were placed onto a platinum pan, and the sample was heated from 25 to 800 °C at a heating rate of 20 °C/min under nitrogen flow condition. Preparation of crosslinked films through DA reactions. Reactive blends in anisole at 10 wt%
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was prepared at two different mole equivalent ratios of maleimide to furfuryl group (1/1 and 2/1). As an example to prepare a reactive blend at 1/1 mole equivalent ratio of maleimide/furfuryl group, PFu (28.7 mg, 0.064 mmol of furfuryl groups) was mixed with BM (11.5 mg, 0.032 mmol). Their aliquots were dropped on glass slides placed in oven preset at 60 °C for 12 hrs. The resultant crosslinked films
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were scraped using a knife and characterized for their thermal properties and gel contents.
Gel content measurements. Pieces of crosslinked films (approximately 23 mg, Wd) were mixed
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with THF (10 mL) for over 48 hrs. After THF was decanted using a pipette, the wet gels were dried in a vacuum oven for 12 hrs (Wex). Gel content was determined by Wex/Wd. Sol-gel transition. A mixture of PFu (100 mg, 0.18 mmol of furfuryl groups) with BM (31.5 mg, 0.09 mmol) in anisole (1.2 g) at 11 wt% in a vial was placed in an oil-bath preset at 60 °C for 12 hrs. The formed free-standing gel in the vial was taken out of the vial, broken to small pieces by spatula, and placed in fresh DMF (2 mL). The new samples are heated at 150 °C for 5 hrs.
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Optical microscopy. Self-healing of films cast on glass plates at 60 °C was observed by a microscope (Olympus BX51) with fluorescence filters (BP 460-490 excitation and BA520IF emission) coupled with a digital camera. Fresh cuts were made using a sharp blade on surfaces of the films. They
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were placed in oven preset at 150 °C for 5 hrs and then cooled down to room temperature.
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Results and Discussion
Synthesis and thermal properties of PFu having pendant furfuryl groups. Scheme 2 illustrates our approach utilizing a combination of free-radical solution polymerization with post-modification methods to synthesize a new methacrylate copolymer having pendant 2-ethylhexyl and furfuryl groups (PFu). Figure 1 and Figure S1 show the 1H-NMR spectra of POH, PMA, and PFu.
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O O
O O
m
n O
AMBN Bu-SH Anisole
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n O
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m O O
m
n O
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OH
O
O
O
S
POH
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HEMA PFu
Scheme 2. Synthetic route to PFu having pendant furfuryl groups by a combination of free-radical solution polymerization with post-modification methods.
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The first step is the synthesis of a methacrylate copolymer consisting of pendant hydroxyl groups (POH) by solution polymerization through a free-radical mechanism. A mixture of EHMA and HEMA
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at the mole ratio = 50/50 in feed was thermally initiated with AMBN (an azo-type initiator) at 70 °C in anisole. Bu-SH (0.5 wt% of monomer mixtures) was used as a chain transfer agent to reduce molecular weight. After 3 hrs, conversion was determined by gravimetry to be 30%. After being purified by precipitation from diethyl ether, the resulting POH was characterized for molecular weight as number average molecular weight, Mn = 29.6 kg/mol and molecular weight distribution as broad as Mw/Mn = 1.7, by GPC (Figure S2 for GPC trace). 1H-NMR of POH in Figure 1a shows the presence of
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methylene groups adjacent to ester groups in both EHMA units at 3.7 ppm (a) and HEMA units at 3.9 ppm (b), as well as methylene groups adjacent to OH groups in HEMA units at 3.6 ppm (c). Using the integrals of the peaks and their ratios (a, b, c), the HEMA units in the POH was determined to be 58 mol% (48 wt%), suggesting the synthesis of POH precursor having 58 mol% pendant OH groups.
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The second step is the coupling reaction of OH groups in POH with MA-Cl in the presence of Et3N in anhydrous THF at 0 °C to synthesize the corresponding methacrylate copolymer having
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pendant methacrylate groups (PMA). Excess MA-Cl and Et3N (5 mole equivalent to OH groups in POH) were used for higher coupling efficiency of pendant OH groups to the corresponding methacrylate groups. The product was purified by precipitation from hexane. 1H-NMR in Figure 1b shows the appearance of new peaks at 4.3 ppm (c′) corresponding to methylene protons adjacent to newly-formed ester groups due to methacrylation as well as at 5.6-6.1 ppm (d) corresponding to methacrylate protons. The spectrum also shows the complete disappearance of the peak corresponding methylene protons adjacent to hydroxyl groups (c, Figure 1a), suggesting the synthesis of PMA precursor having pendant methacrylate groups.
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ACCEPTED MANUSCRIPT The last step is the thermally-induced thiol-ene reaction of Fu-SH to the resulting PMA in the presence of AMBN as a radical initiator. Reaction conditions include [Fu-SH]o/[methacrylate groups in PMA]o = 8/1 at 75 °C. Note that the use of excess Fu-SH is required to minimize the probability to homopolymerization of pendant methacrylate groups of PMA through free radical mechanism. The
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product was purified by precipitation from a mixture of MeOH and water (3/2 v/v) to remove excess Fu-SH and AMBN species. GPC results suggest that its molecular weight, Mn = 35.3 kg/mol (Mw/Mn = 2.2), which is larger than that of POH, as a result of the modification of pendant methacrylate groups of PMA with furfuryl groups (Figure S2). 1H-NMR spectrum in Figure 1c shows the appearance of new peaks at 6.2-7.6 ppm (f) corresponding to furan ring protons and at 3.7 ppm (e) corresponding to
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methylene protons adjacent to sulfide and furan rings as well as the complete disappearance of the peaks corresponding to methacrylate protons (d in Figure 1b). In addition, FT-IR spectrum in Figure 3a
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shows the strong peak at 1728 cm-1 corresponding to ester carbonyl groups and the weak peak at 1502 cm-1 and 1025 cm-1 corresponding to furan rings in PFu. These results suggest the successful synthesis of PFu having 58 mol% pendant furfuryl groups.
water
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OH
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f
f
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Figure 1. 1H-NMR spectra of POH (a), PMA (b), and PFu (c) in DMSO-d6. 8
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The thermal properties of the copolymers were analyzed by measuring relative heat flow over temperatures ranging from -80 to 200 °C using DSC (Figure S3a). The DSC trace of POH shows one large glass transition at 49 °C, suggesting the random distribution of HEMA and EHMA units in POH
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chains. After the post-modification, the PFu had a glass transition at 30 °C, which is lower by19 °C than that of POH. The decrease in Tg is presumably attributed to the post-modification of pendant OH groups with bulkier groups. Further to DSC, the thermal properties of both POH and PFu were analyzed using TGA (Figure S3b). Both copolymers show the main weight loss at the similar
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temperature range of 300-450 °C.
A model study to investigate DA reaction. Prior to the investigation of DA-crosslinking reactions for a reactive blend containing PFu and BM, a model study was conducted for better
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understanding thermally-induced DA reaction (Figure 2a). FuMA and BM were selected as their low molecular weight precursors and mixed at 1/1 mole equivalent ratio of maleimide/furfuryl group in DMSO-d6 in NMR tubes. Aliquots of the mixtures were placed in an oil-bath preset at various temperatures ranging from room temperature to 120 °C and their 1H-NMR spectra were recorded in given times. As seen in Figure S4 as a typical example for 1H-NMR spectra recorded at 80 °C, the
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peaks representing FuMA and BM decreased, while the newly-appeared peaks representing the DA linkage increased. Particularly, the peak at 7.4 ppm (a) corresponding to an aromatic proton in FuMA was selected and the decrease in its integral ratio to the peak corresponding to water at 3.3 ppm was followed to calculate the conversion (or the extent of DA reaction). As seen in Figure 2b, the
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conversion increased over reaction time at all temperature ranges. Interestingly, conversion did not reach to completion due to reversible DA/retro-DA reactions. Importantly, the rate of DA reaction increased with an increasing temperature up to 80 °C; however, it decreased upon further increase in
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temperature, particularly at 120 °C. Such decrease of DA reaction rate is presumably due to the occurrence of retro-DA reaction, which competes with DA reaction. Promisingly, DA reaction occurred in a homogeneous solution at ambient temperature (at below 40 °C) to some extent. Note that small molecular weight FuMA, BM molecules, and the resulting FuMA-BM adducts were dissolved in an organic solvent. In crosslinked films based on PFu and BM, however, pendant furan and maleimide groups as well as the resultant DA linkages could have a limited mobility or diffusion through the networks for their exchange (or shuffling) to reform DA linkages upon the cleavage at elevated temperatures.
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100 °C 120 °C
0.4
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Figure 2. Scheme (a) and extent (b) of DA reaction of FuMA and BM at 1/1 mole equivalent ratio of maleimide/furfuryl group over reaction time at various temperature ranging from room temperature to 120 °C.
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Preparation and characterization of DA-crosslinked copolymer films. Scheme 3 shows the schematic illustration of thermally-induced DA reactions between pendant furfuryl groups of PFu and maleimide groups of BM, yielding DA-crosslinked copolymer networks. Given the results obtained from the above model NMR study, the temperatures of 60-80 °C could be suitable for DA reactions. In
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our most experiments, DA-crosslinked films were cast at 60 °C, a lower temperature.
Scheme 3. Illustration to prepare of DA-crosslinked networks through DA-crosslinking reactions of PFu and BM in reactive blends. 10
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Reactive mixtures consisting of PFu and BM at the different mole equivalent ratios of 1/1 and 2/1 of maleimide/furfuryl group were prepared in anisole. Aliquots of the mixtures were cast on glass plates at 60 °C for 12 hrs. The cast films were characterized for gel contents based on their solubility in an organic solvent. Note that gels here can be defined as insoluble species in the given solvent.
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Aliquots of the cast films were soaked in THF for 2 days and insoluble species were isolated and dried. The gel content was determined to be 89% for the 1/1 ratio film, which is caused by the occurrence of DA-crosslinking reactions between pendant furfuryl groups and maleimide groups in the reactive mixtures. However, the gel content for the 2/1 ratio film (more BM) was 75%; the lower gel content is
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presumably due to the more residues of unreacted BM molecules remaining in the film.
The DA-crosslinked films were further characterized by FT-IR and DSC analysis. For qualitative
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analysis of DA crosslinking reactions at 60 °C, Figure 3 compares the FT-IR spectra of DAcrosslinked films cast at 60 °C for 12 hrs (3d and 3e) with that of a physical blend of PFu and BM (3c) as well as those of individual PFu and BM (3a and 3b) as controls. First, the peak appeared at 1395 cm1
corresponds to –C-N-C- of maleimide groups. Upon annealing at 60 °C for 12 hrs, this peak
decreased in both 1/1 and 2/1 ratio films (3d and 3e). However, the peak was still detected in the 2/1ratio film, which is presumably due to residual maleimide groups remaining in the film. In addition,
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the peak at 1720-1728 cm-1 corresponds to ester carbonyl groups in PFu, BM, and their physical blend (3a, 3b, and 3c). Upon annealing, these peaks appeared to be decreased, but a broad peak newly appeared at 1705 cm-1 for the 1/1 ratio film and 1710 cm-1 at the 2/1 ratio film. The appearance of the peak at 1705-1710 cm-1 could be attributed to the formation of DA linkages in cast films, which is
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similar to the reported results for DA-crosslinked networks based on methacrylate copolymers having
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pendant lauryl, furfuryl, and a furan-protected maleimide groups.[45]
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220 a)
200 180
1502 cm-1
1728 cm-1
1025 cm-1
b)
%T
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c)
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120 100
d)
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1705 cm-1
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Figure 3. FT-IR spectra of PFu (a), BM (b), and a physical blend (1/1 mole ratio) cast at room temperature (c), and DA-crosslinked films cast at 60 °C from reactive blends at 1/1 (d) and 2/1 (e) mole equivalent ratio of maleimide/furfuryl group. Figure 4 shows DSC diagrams of the films cast at 60 °C, compared with the corresponding physical blend of PFu/BM (1/1 mole equivalent ratio of maleimide/furfuryl group). For both 1/1 and
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2/1 ratio films cast at 60 °C for 12 hrs, the 1st heating DSC diagrams exhibit a large endotherm with the valley at 125 °C, which indicates the occurrence of retro-DA reactions causing the cleavage of DA
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crosslinkages. Similar observation on different DA polymeric systems is reported.[47, 54] For a physical blend cast at room temperature, the 1st heating DSC diagram shows a large endotherm at 50100 °C, which is attributed to the occurrence of DA reactions between pendant furfuryl groups of PFu and maleimide groups of BM. As a consequence, its 2nd heating DSC diagram appeared to be similar to the 1st heating DSC diagram for the corresponding DA-crosslinked films cast at 60 °C. Unfortunately, both of the cast films appeared to be too brittle to be analyzed for viscoelastic and mechanical properties. Furthermore, the 2/1 ratio films (more BM) were more brittle than the 1/1 ratio
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Figure 4. DSC diagrams of DA-crosslinked films cast at 60 °C from reactive blends consisting of PFu and BM at 1/1 and 2/1 mole equivalent ratio of maleimide/furfuryl group, compared with a physical blend (1/1 mole equivalent ratio) cast at room temperature.
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Investigation of thermoreversible self-healing. The occurrence of retro-DA reaction was first visualized through sol-gel-sol transition in organic solution (Figure 5). The reactive mixture of PFu and BM at 1/1 mole equivalent ratio of maleimide/furfuryl group in anisole at 11 wt% was heated at 60
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°C (the same temperature for casting films) for 12 hrs, yielding a standing gel (a to b). The piece of the
gel was mixed with DMF (c). Upon heating at 150 °C for 5 hrs, the heterogeneous mixture became a sol solution (d), as a result of the occurrence of retro-DA reaction.
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Figure 5. For a PFu/BM reactive blend at 1/1 mole equivalent ratio of maleimide/furfuryl group, a sol solution dissolved in anisole (a), a gel formed at 60 °C for 12 hrs (b), a mixture of pieces of the gel with DMF (c), and a sol solution upon cooling down after being heated at 150 °C for 5 hrs (d). Next, thermally-induced self-healing on cuts through reversible DA/retro-DA reactions was examined by microscopy. Similar procedure was examined to cast DA-crosslinked films from reactive
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mixtures consisting of PFu and BM at the 1/1 and 2/1 mole equivalent ratios of maleimide/furfuryl group at 60 °C for 12 hrs. Several cuts with different widths and lengths were made on the surfaces of the cast films using a knife. The films were annealed at 150 °C for 5 hrs and then cooled down to room temperature. Figure 6 shows the microscope images before and after healing of the cuts. In the 1/1 ratio films, the healing appeared to be almost completed for small cuts. However, a trace of the original cuts remained for relatively large cuts. Similar healing behavior was observed in the 2/1 ratio films.
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length of the cuts.
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These results suggest that the self-healability for the systems (PFu and BM) relies on the width and
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Figure 6. Microscope images of DA-crosslinked films cast from a reactive blend of PFu and BM at mole equivalent ratio of maleimide/furfuryl group = 1/1 (a-b) and 2/1 (c) before and after being annealed at 150 °C for 5 hrs.
Conclusion
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A new PFu methacrylate copolymer having 58 mol% pendant furfuryl groups, whose Tg = 30 °C, was synthesized by a combination of a facile free-radical solution polymerization with postmodification methods through a base-catalyzed coupling reaction and a thermally-initiated thiol-ene radical addition reaction. A model NMR study with small molecular weight precursors exhibits
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temperature dependence on the extent of DA reaction; however, the extent of DA reaction did not reach completion, presumably because of DA reaction being competed with retro-DA reaction at elevated temperatures. Given the result, the reactive blends of PFu with a bismaleimide crosslinker in
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anisole were cast at 60 °C (a moderate temperature) to yield thermally-induced DA-crosslinked networks with gel contents >78% in THF. Their thermal analysis with DSC exhibiting endotherm suggest the occurence of DA reaction between furan and maleimide groups at 50-100 °C as well as the occurrence of retro-DA reaction at temperatures greater than 125 °C. Promisingly, the results from solgel transition and optical microscopy demonstrate the occurrence of thermoreversible DA/retro-DA reactions and self-healing in DA-crosslinked films.
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Acknowledgements Financial supports from Canada Research Chair (CRC) Award, Korea Research Institute of Chemical Technology (KRICT), and the Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Technology Innovation Program (No. 10067082) entitled “Development of scratch
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self-healable coatings and related process for automotive” are gratefully acknowledged. JKO is a Tier II CRC in Nanobioscience. Mr. Liu, a research project undergraduate student, thanks So Young An for her helpful discussions.
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A new reactive polymethacrylate bearing pendant furfuryl groups: synthesis, thermoreversible reactions, and self-healing
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Sungmin Jung,a Jiang Tian Liu,a Sung Hwa Hong,a Dhamodaran Arunbabu,a Seung Man Noh,b Jung Kwon Oha* Synthesis of a new methacrylate copolymer having pendant furfuryl groups (PFu)
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Utilization of a combination of FRP and most-modification methods
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Formation of dynamic crosslinked networks by thermally-induced Diels-Alder (DA) reaction
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Self-healability through thermoreversible DA/Retro-DA reactions
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A new reactive polymethacrylate bearing pendant furfuryl groups: synthesis, thermoreversible reactions, and self-healing Sungmin Jung,a Jiang Tian Liu,a Sung Hwa Hong,a Dhamodaran Arunbabu,a Seung Man Noh,b Jung Kwon Oha* a
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Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec, Canada H4B 1R6
b
Research Center for Green Fine Chemicals, Korea Research Institute of Chemical Technology (KRICT), Ulsan 44412, Republic of Korea
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Corresponding author: J.K.Oh ([email protected])
Figure S1. 1H-NMR spectra of POH (a), PMA (b), and PFu (c) in DMSO-d6.
water a
b
a
b
c´
c
Diethyl ether
OH
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d
Ratio of integral 2.70/4 = 0.675 (a and b) 1.86/2 = 0.93 (c) 0.93/(0.675+0.93) = 58%
DMSO
b
c
c´b
a
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a
d
a
b
f
c´ e
f
ff
e c´ba
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10 3
104
105
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POH PFu
106
10 7
Molecular weight (g/mol)
a)
Tg = 49.5 °C
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POH
0
Tg = 29.5 °C PFu
-1
-2 -50
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Heat flow (W/g)
1
0
50
100
Temperature (oC)
100 b)
80
Weight (%)
2
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Figure S3. DSC (a) and TGA (b) diagrams of POH and PFu.
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40 POH
PFu
20
150
200
0 0
100
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o Temperature ( C)
2
700
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Figure S4. Overlaid 1H-NMR spectra recorded at 80 °C for a reactive blend of FuMA and BM in DMSO-d6.
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S0032-3861(16)31115-6
DOI:
10.1016/j.polymer.2016.12.029
Reference:
JPOL 19265
To appear in:
Polymer
Received Date: 4 November 2016 Revised Date:
9 December 2016
Accepted Date: 10 December 2016
Please cite this article as: Jung S, Liu JT, Hong SH, Arunbabu D, Noh SM, Oh JK, A new reactive polymethacrylate bearing pendant furfuryl groups: Synthesis, thermoreversible reactions, and selfhealing, Polymer (2017), doi: 10.1016/j.polymer.2016.12.029. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A new reactive polymethacrylate bearing pendant furfuryl groups: synthesis, thermoreversible reactions, and self-healing
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Sungmin Jung,a Jiang Tian Liu,a Sung Hwa Hong,a Dhamodaran Arunbabu,a Seung Man Noh,b Jung Kwon Oha*
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A new reactive polymethacrylate bearing pendant furfuryl groups: synthesis, thermoreversible reactions, and self-healing
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Sungmin Jung,a Jiang Tian Liu,a Sung Hwa Hong,a Dhamodaran Arunbabu,a Seung Man Noh,b Jung Kwon Oha* a
Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec, Canada H4B 1R6
b
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Corresponding author: J.K.Oh ([email protected])
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Research Center for Green Fine Chemicals, Korea Research Institute of Chemical Technology (KRICT), Ulsan 44412, Republic of Korea
Abstract
We report a new methacrylate copolymer having pendant furfuryl groups (PFu) reactive to maleimide-bearing compounds to form thermally-induced crosslinked networks exhibiting self-
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healability through thermoreversible Diels-Alder (DA)/Retro-DA reactions. The PFu is synthesized by a combination of a facile radical polymerization with post-modification methods through a basecatalyzed coupling reaction and a thermally-induced thiol-ene radical addition reaction. The resulting PFu is effective toward DA reaction with a maleimide-bearing crosslinker at moderate or elevated
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temperatures, yielding highly crosslinked films through DA linkages. Thermal analysis and spectroscopic studies along with NMR model study with small molecular weight precursors suggest the formation of DA-crosslinked networks at 50-100 °C and the occurrence of retro-DA reaction at
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>125 °C. The resulting DA-crosslinked network is dynamic, exhibiting self-healing through the occurrence of thermoreversible DA/retro-DA reactions, confirmed with sol-gel transition and optical microscopy.
Keywords: Post-modification, furan-containing polymethacrylates, crosslinked networks, Diels-Alder reaction, Retro-Diels-Alder reaction, self-healing
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Introduction Three-dimensionally crosslinked networks with covalent linkages providing high mechanical properties as well as great thermal and solvent resistance have been utilized as effective building blocks for the development of a variety of multifunctional polymeric materials useful in nanoscience
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and industrial fields.[1-3] Numerous methods have been explored to synthesize covalently-crosslinked networks. Conventional methods utilize either a chain-growth polymerization of vinyl monomers in the presence of multifunctional crosslinkers or a step-growth polymerization through polycondensation or polyaddition of polyfunctional monomers. These methods consequently yield polymeric materials
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crosslinked with permanent linkages, typically carbon-carbon bonds.[4-6] Recent advance is the development of new methods to synthesize reversible networks crosslinked with dynamic (labile)
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covalent linkages; the formed networks enable to self-heal or self-repair damaged cracks. Such selfhealability is a highly desired property of crosslinked materials because their built-in ability to repair physical damages effectively prevents catastrophic failure to extending their lifetime.[7-13] A variety of dynamic linkages have been explored, including disulfide,[14-18] hindered urea,[19, 20] alkoxyamine,[21, 22] diarylbibenzofuranone (a dimer of arylbenzofuranone),[23] boronic ester,[24-26] and etc.[27]
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Of particular interest is the covalent linkage formed through thermally-induced Diels-Alder (DA) reaction, a [4+2] cycloaddition of a diene and a dienophile (hereafter DA linkage). Promisingly, the formed DA linkage can be cleaved through retro-DA reaction at elevated temperatures to the corresponding diene and dienophile. Typically employing furan and maleimide moieties, this unique
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thermoreversible DA/retro-DA chemistry enables DA-crosslinked materials to attain self-healability. Several approaches have been explored for the synthesis of thermally-induced DA-crosslinked
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materials, which can be characteristic of furan-functionalized compounds or polymers.[28-30] Polycondensation or polyaddition of furan and maleimide-functionalized compounds were explored to synthesize various step-growth polymers[31-33] including polyurethanes[34-37] and epoxy resins[38] crosslinked with DA crosslinkages. Chain growth polymerization of monomers labeled with a pendant furfuryl group allows for the synthesis of relatively high molecular weight linear (co)polymers bearing pendant furan groups (PFu). These polymers include mainly polymethacrylates[39-42] as well as polycarbonate,[43] and epoxy oligomer.[44] DA-crosslinked networks can be formed from a reactive blend of the resulting PFu with maleimide-functionalized crosslinkers. The fabrication of DA-
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ACCEPTED MANUSCRIPT crosslinked networks based on polymethacrylates bearing both pendant furan and furan-protected maleimide groups is also reported.[45-47] Another approach involves post-modification of functional polymers with small furan-containing molecules. This approach could also allow for the synthesis of a variety of furan-bearing polymers
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with different chemical structures. Examples include polybutadiene,[48, 49] polyamides,[50] polyketones,[51, 52] and epoxy resins.[38, 53] Despite these advances, the exploration of postmodification to develop new furan-containing polymers, particularly polymethacrylates, is highly beneficial. This approach could circumvent the occurrence of premature crosslinking reactions because
furfuryl and maleimide groups during radical polymerization.
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of undesirable side reactions such as self-condensation of furfuryl groups or DA reactions between
Herein, we report the synthesis of a new methacrylate copolymer having pendant furfuryl groups
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(PFu) by a combination of a facile free-radical solution polymerization with post-modification methods through a base-catalyzed coupling reaction and a thermally-induced thiol-ene radical addition reaction. As illustrated in Scheme 2, a free radical polymerization of 2-hydroxyethyl methacrylate and 2ethylhexyl methacrylate yielded a POH copolymer. The pendant hydroxyl groups were converted to methacrylate esters by a facile esterification, thus forming PMA. Then, a click-type thiol-ene addition
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reaction of furfuryl mercaptan to the methacrylate groups allowed for the synthesis of PFu having pendant furfuryl groups. The ability of the resulting PFu toward DA crosslinking with different amounts of a bismaleimide (BM) as a model crosslinker was characterized with gel content measurements, thermal analysis, and spectroscopic studies of the formed DA-crosslinked networks. In
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addition, model NMR study for a reactive blend of small molecular weight furan and maleimide precursors were conducted to get an insight into the effect of temperature on the occurrence of DA/retro DA reactions. Further, the self-healability of the DA-crosslinked networks through
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thermoreversible DA/retro-DA reaction was examined with sol-gel transition in anisole and optical microscopy (Scheme 1).
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Scheme 1. Thermoreversible DA/retro-DA reactions for self-healing of dynamically-crosslinked networks prepared from a reactive mixture containing PFu having pendant furfuryl groups and a bismaleimide model crosslinker.
Experimental
Materials. 2,2´-Azobis(2-methybutyronitrile) (AMBN, >98%), n-butyl mercaptan (Bu-SH, 95%),
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triethylamine (Et3N, >99.9%), methacryloyl chloride (MA-Cl, >97%), furfuryl mercaptan (Fu-SH, 99%), and 1,1-(methylene di-4,1-phenylene)bismaleimide (BM) were purchased from Aldrich and used as received. 2-Ethylhexyl methacrylate (EHMA) and 2-hydroxyethyl methacrylate (HEMA) from
inhibitors.
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Aldrich were purified by passing them through a column filled with basic alumina to remove
Synthesis of POH. EHMA (15.3 g, 77 mmol), HEMA (10.0 g, 77 mmol), Bu-SH (0.5 wt% based
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on monomer mixtures, 150 µL), and anisole (34 mL) were mixed in a 50 mL Schlenk flask. The mixture was deoxygenated by purging under nitrogen for 40 min and placed in an oil bath preheated at 70 °C. The nitrogen-prepurged AMBN solution (0.15 g, 0.8 mmol) in anisole (0.5 mL) was injected into the Schlenk flask to initiate polymerization. Polymerization was stopped after 3 hrs by cooling the reaction vessel in an ice bath. For purification, as-synthesized polymer solutions were precipitated from cold diethyl ether and dried in vacuum oven at room temperature for 18 hrs. Synthesis of PMA. A mixture containing Et3N (3.5 mL, 25 mmol) and POH (1.0 g, 5 mmol of OH groups) dissolved in anhydrous tetrahydrofuran (THF, 150 mL) was purged with a nitrogen gas for 10 min in an ice-bath. After the dropwise addition of a clear solution of MA-Cl (2.4 mL, 25 mmol) in 4
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After being concentrated using a rotary evaporation, the solution was precipitated from hexane. The precipitates were isolated and dried in a vacuum oven at room temperature for 18 hrs.
Synthesis of PFu. PMA (0.6 g, 1.7 mmol of methacrylate group), AMBN (0.24 g, 1.3 mmol), and Fu-SH (1.34 mL, 13.3 mmol) were dissolved in a solvent mixture (10 mL) of anhydrous THF and
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DMF (1/3 v/v) in Schlenk flask. The resulting mixture was degassed by three freeze-pump-thaw cycles and stirred at 75 °C for 24 hrs. The reaction was stopped by cooling down to room temperature. After the removal of solvents using a rotary evaporation, the concentrated solution was precipitated from a
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solvent mixture (750 mL) of MeOH and water (3/2 v/v) twice. The precipitates were then dried in a vacuum oven at room temperature for 14 hrs.
Instrumentation and analyses. 1H-NMR spectra were recorded using a 500 MHz Varian spectrometer. DMSO-d6 multiplet at 2.5 ppm was selected as the reference standard. Monomer conversion was determined by gravimetry (120 °C/4 hr). Molecular weight and molecular weight
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distribution were determined by gel permeation chromatography (GPC) with a Viscotek VE1122 pump and a refractive index (RI) detector. Two PolyAnalytik columns (PAS-103L and 105L, designed to determine molecular weight up to 2,000,000 g/mol) were used with THF as eluent at 30 °C at a flow rate of 1 mL/min. Linear poly(methyl methacrylate) standards were used for calibration. Aliquots of
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polymer samples were dissolved in THF. The clear solutions were filtered using a 0.25 µm PTFE filter to remove any solvent-insoluble species. A drop of anisole was added as a flow rate marker. FT-IR spectra of all the samples were recorded on a Nicolet 6700 FT-IR spectrometer using an attenuated
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total reflectance (ATR) crystal. All spectra were recorded with 32 scans in a resolution of 4 cm-1 at room temperature in the range of 800-2000 cm-1. Differential scanning calorimetry (DSC). Thermal properties including glass transition temperature (Tg) of both polymers and DA-crosslinked films were measured with a TA Instruments DSC Q20 differential scanning calorimeter. Temperature range was set at -80-200 °C with heating and cooling cycles conducted at a rate of 10 °C/min (cycles : cool to -80 °C, heat up to 200 °C (1st run), cool to -80 °C, heat up to 200 °C (2nd run), and cool to 25 °C). The glass transition temperature (Tg) was determined from the 2nd heating run. 5
ACCEPTED MANUSCRIPT Thermogravimetric analysis (TGA). TGA measurements for the purified, dried polymers were carried out using a TA Instruments Q50 analyzer. Their pieces were placed onto a platinum pan, and the sample was heated from 25 to 800 °C at a heating rate of 20 °C/min under nitrogen flow condition. Preparation of crosslinked films through DA reactions. Reactive blends in anisole at 10 wt%
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was prepared at two different mole equivalent ratios of maleimide to furfuryl group (1/1 and 2/1). As an example to prepare a reactive blend at 1/1 mole equivalent ratio of maleimide/furfuryl group, PFu (28.7 mg, 0.064 mmol of furfuryl groups) was mixed with BM (11.5 mg, 0.032 mmol). Their aliquots were dropped on glass slides placed in oven preset at 60 °C for 12 hrs. The resultant crosslinked films
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were scraped using a knife and characterized for their thermal properties and gel contents.
Gel content measurements. Pieces of crosslinked films (approximately 23 mg, Wd) were mixed
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with THF (10 mL) for over 48 hrs. After THF was decanted using a pipette, the wet gels were dried in a vacuum oven for 12 hrs (Wex). Gel content was determined by Wex/Wd. Sol-gel transition. A mixture of PFu (100 mg, 0.18 mmol of furfuryl groups) with BM (31.5 mg, 0.09 mmol) in anisole (1.2 g) at 11 wt% in a vial was placed in an oil-bath preset at 60 °C for 12 hrs. The formed free-standing gel in the vial was taken out of the vial, broken to small pieces by spatula, and placed in fresh DMF (2 mL). The new samples are heated at 150 °C for 5 hrs.
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Optical microscopy. Self-healing of films cast on glass plates at 60 °C was observed by a microscope (Olympus BX51) with fluorescence filters (BP 460-490 excitation and BA520IF emission) coupled with a digital camera. Fresh cuts were made using a sharp blade on surfaces of the films. They
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were placed in oven preset at 150 °C for 5 hrs and then cooled down to room temperature.
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Results and Discussion
Synthesis and thermal properties of PFu having pendant furfuryl groups. Scheme 2 illustrates our approach utilizing a combination of free-radical solution polymerization with post-modification methods to synthesize a new methacrylate copolymer having pendant 2-ethylhexyl and furfuryl groups (PFu). Figure 1 and Figure S1 show the 1H-NMR spectra of POH, PMA, and PFu.
6
ACCEPTED MANUSCRIPT O
O O
O O
m
n O
AMBN Bu-SH Anisole
O
O O
n O
Cl
Et3N/THF
O
O
m O O
m
n O
SH
O
O O
AMBN/DMF O O
OH
OH
O
O
O
S
POH
EHMA
PMA
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HEMA PFu
Scheme 2. Synthetic route to PFu having pendant furfuryl groups by a combination of free-radical solution polymerization with post-modification methods.
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The first step is the synthesis of a methacrylate copolymer consisting of pendant hydroxyl groups (POH) by solution polymerization through a free-radical mechanism. A mixture of EHMA and HEMA
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at the mole ratio = 50/50 in feed was thermally initiated with AMBN (an azo-type initiator) at 70 °C in anisole. Bu-SH (0.5 wt% of monomer mixtures) was used as a chain transfer agent to reduce molecular weight. After 3 hrs, conversion was determined by gravimetry to be 30%. After being purified by precipitation from diethyl ether, the resulting POH was characterized for molecular weight as number average molecular weight, Mn = 29.6 kg/mol and molecular weight distribution as broad as Mw/Mn = 1.7, by GPC (Figure S2 for GPC trace). 1H-NMR of POH in Figure 1a shows the presence of
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methylene groups adjacent to ester groups in both EHMA units at 3.7 ppm (a) and HEMA units at 3.9 ppm (b), as well as methylene groups adjacent to OH groups in HEMA units at 3.6 ppm (c). Using the integrals of the peaks and their ratios (a, b, c), the HEMA units in the POH was determined to be 58 mol% (48 wt%), suggesting the synthesis of POH precursor having 58 mol% pendant OH groups.
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The second step is the coupling reaction of OH groups in POH with MA-Cl in the presence of Et3N in anhydrous THF at 0 °C to synthesize the corresponding methacrylate copolymer having
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pendant methacrylate groups (PMA). Excess MA-Cl and Et3N (5 mole equivalent to OH groups in POH) were used for higher coupling efficiency of pendant OH groups to the corresponding methacrylate groups. The product was purified by precipitation from hexane. 1H-NMR in Figure 1b shows the appearance of new peaks at 4.3 ppm (c′) corresponding to methylene protons adjacent to newly-formed ester groups due to methacrylation as well as at 5.6-6.1 ppm (d) corresponding to methacrylate protons. The spectrum also shows the complete disappearance of the peak corresponding methylene protons adjacent to hydroxyl groups (c, Figure 1a), suggesting the synthesis of PMA precursor having pendant methacrylate groups.
7
ACCEPTED MANUSCRIPT The last step is the thermally-induced thiol-ene reaction of Fu-SH to the resulting PMA in the presence of AMBN as a radical initiator. Reaction conditions include [Fu-SH]o/[methacrylate groups in PMA]o = 8/1 at 75 °C. Note that the use of excess Fu-SH is required to minimize the probability to homopolymerization of pendant methacrylate groups of PMA through free radical mechanism. The
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product was purified by precipitation from a mixture of MeOH and water (3/2 v/v) to remove excess Fu-SH and AMBN species. GPC results suggest that its molecular weight, Mn = 35.3 kg/mol (Mw/Mn = 2.2), which is larger than that of POH, as a result of the modification of pendant methacrylate groups of PMA with furfuryl groups (Figure S2). 1H-NMR spectrum in Figure 1c shows the appearance of new peaks at 6.2-7.6 ppm (f) corresponding to furan ring protons and at 3.7 ppm (e) corresponding to
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methylene protons adjacent to sulfide and furan rings as well as the complete disappearance of the peaks corresponding to methacrylate protons (d in Figure 1b). In addition, FT-IR spectrum in Figure 3a
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shows the strong peak at 1728 cm-1 corresponding to ester carbonyl groups and the weak peak at 1502 cm-1 and 1025 cm-1 corresponding to furan rings in PFu. These results suggest the successful synthesis of PFu having 58 mol% pendant furfuryl groups.
water
DMSO
c
a
b
b
c
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OH
a
a)
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b
a c´
d
b)
a
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d
c´
b a
b
f
c´
e
f
f
c´b
e a
c)
9
8
7
6
5
4
3
2
1
0
Chemical shift (ppm)
Figure 1. 1H-NMR spectra of POH (a), PMA (b), and PFu (c) in DMSO-d6. 8
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The thermal properties of the copolymers were analyzed by measuring relative heat flow over temperatures ranging from -80 to 200 °C using DSC (Figure S3a). The DSC trace of POH shows one large glass transition at 49 °C, suggesting the random distribution of HEMA and EHMA units in POH
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chains. After the post-modification, the PFu had a glass transition at 30 °C, which is lower by19 °C than that of POH. The decrease in Tg is presumably attributed to the post-modification of pendant OH groups with bulkier groups. Further to DSC, the thermal properties of both POH and PFu were analyzed using TGA (Figure S3b). Both copolymers show the main weight loss at the similar
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temperature range of 300-450 °C.
A model study to investigate DA reaction. Prior to the investigation of DA-crosslinking reactions for a reactive blend containing PFu and BM, a model study was conducted for better
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understanding thermally-induced DA reaction (Figure 2a). FuMA and BM were selected as their low molecular weight precursors and mixed at 1/1 mole equivalent ratio of maleimide/furfuryl group in DMSO-d6 in NMR tubes. Aliquots of the mixtures were placed in an oil-bath preset at various temperatures ranging from room temperature to 120 °C and their 1H-NMR spectra were recorded in given times. As seen in Figure S4 as a typical example for 1H-NMR spectra recorded at 80 °C, the
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peaks representing FuMA and BM decreased, while the newly-appeared peaks representing the DA linkage increased. Particularly, the peak at 7.4 ppm (a) corresponding to an aromatic proton in FuMA was selected and the decrease in its integral ratio to the peak corresponding to water at 3.3 ppm was followed to calculate the conversion (or the extent of DA reaction). As seen in Figure 2b, the
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conversion increased over reaction time at all temperature ranges. Interestingly, conversion did not reach to completion due to reversible DA/retro-DA reactions. Importantly, the rate of DA reaction increased with an increasing temperature up to 80 °C; however, it decreased upon further increase in
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temperature, particularly at 120 °C. Such decrease of DA reaction rate is presumably due to the occurrence of retro-DA reaction, which competes with DA reaction. Promisingly, DA reaction occurred in a homogeneous solution at ambient temperature (at below 40 °C) to some extent. Note that small molecular weight FuMA, BM molecules, and the resulting FuMA-BM adducts were dissolved in an organic solvent. In crosslinked films based on PFu and BM, however, pendant furan and maleimide groups as well as the resultant DA linkages could have a limited mobility or diffusion through the networks for their exchange (or shuffling) to reform DA linkages upon the cleavage at elevated temperatures.
9
ACCEPTED MANUSCRIPT
b)
a)
0.8
80 °C
0.6
60 °C
Conversion
100 °C 120 °C
0.4
40 °C
RT
0.0 0
5
10
Time (hrs)
15
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0.2
20
25
SC
Figure 2. Scheme (a) and extent (b) of DA reaction of FuMA and BM at 1/1 mole equivalent ratio of maleimide/furfuryl group over reaction time at various temperature ranging from room temperature to 120 °C.
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Preparation and characterization of DA-crosslinked copolymer films. Scheme 3 shows the schematic illustration of thermally-induced DA reactions between pendant furfuryl groups of PFu and maleimide groups of BM, yielding DA-crosslinked copolymer networks. Given the results obtained from the above model NMR study, the temperatures of 60-80 °C could be suitable for DA reactions. In
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EP
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our most experiments, DA-crosslinked films were cast at 60 °C, a lower temperature.
Scheme 3. Illustration to prepare of DA-crosslinked networks through DA-crosslinking reactions of PFu and BM in reactive blends. 10
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Reactive mixtures consisting of PFu and BM at the different mole equivalent ratios of 1/1 and 2/1 of maleimide/furfuryl group were prepared in anisole. Aliquots of the mixtures were cast on glass plates at 60 °C for 12 hrs. The cast films were characterized for gel contents based on their solubility in an organic solvent. Note that gels here can be defined as insoluble species in the given solvent.
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Aliquots of the cast films were soaked in THF for 2 days and insoluble species were isolated and dried. The gel content was determined to be 89% for the 1/1 ratio film, which is caused by the occurrence of DA-crosslinking reactions between pendant furfuryl groups and maleimide groups in the reactive mixtures. However, the gel content for the 2/1 ratio film (more BM) was 75%; the lower gel content is
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presumably due to the more residues of unreacted BM molecules remaining in the film.
The DA-crosslinked films were further characterized by FT-IR and DSC analysis. For qualitative
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analysis of DA crosslinking reactions at 60 °C, Figure 3 compares the FT-IR spectra of DAcrosslinked films cast at 60 °C for 12 hrs (3d and 3e) with that of a physical blend of PFu and BM (3c) as well as those of individual PFu and BM (3a and 3b) as controls. First, the peak appeared at 1395 cm1
corresponds to –C-N-C- of maleimide groups. Upon annealing at 60 °C for 12 hrs, this peak
decreased in both 1/1 and 2/1 ratio films (3d and 3e). However, the peak was still detected in the 2/1ratio film, which is presumably due to residual maleimide groups remaining in the film. In addition,
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the peak at 1720-1728 cm-1 corresponds to ester carbonyl groups in PFu, BM, and their physical blend (3a, 3b, and 3c). Upon annealing, these peaks appeared to be decreased, but a broad peak newly appeared at 1705 cm-1 for the 1/1 ratio film and 1710 cm-1 at the 2/1 ratio film. The appearance of the peak at 1705-1710 cm-1 could be attributed to the formation of DA linkages in cast films, which is
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similar to the reported results for DA-crosslinked networks based on methacrylate copolymers having
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pendant lauryl, furfuryl, and a furan-protected maleimide groups.[45]
11
ACCEPTED MANUSCRIPT
220 a)
200 180
1502 cm-1
1728 cm-1
1025 cm-1
b)
%T
140
c)
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160 1720 cm-1
120 100
d)
1726 cm-1
1395 cm-1
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80 e)
40 1710 cm-1
20 2000
1800
1600
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1705 cm-1
60
1400
1200
1000
800
Wavenumber (cm-1)
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Figure 3. FT-IR spectra of PFu (a), BM (b), and a physical blend (1/1 mole ratio) cast at room temperature (c), and DA-crosslinked films cast at 60 °C from reactive blends at 1/1 (d) and 2/1 (e) mole equivalent ratio of maleimide/furfuryl group. Figure 4 shows DSC diagrams of the films cast at 60 °C, compared with the corresponding physical blend of PFu/BM (1/1 mole equivalent ratio of maleimide/furfuryl group). For both 1/1 and
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2/1 ratio films cast at 60 °C for 12 hrs, the 1st heating DSC diagrams exhibit a large endotherm with the valley at 125 °C, which indicates the occurrence of retro-DA reactions causing the cleavage of DA
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crosslinkages. Similar observation on different DA polymeric systems is reported.[47, 54] For a physical blend cast at room temperature, the 1st heating DSC diagram shows a large endotherm at 50100 °C, which is attributed to the occurrence of DA reactions between pendant furfuryl groups of PFu and maleimide groups of BM. As a consequence, its 2nd heating DSC diagram appeared to be similar to the 1st heating DSC diagram for the corresponding DA-crosslinked films cast at 60 °C. Unfortunately, both of the cast films appeared to be too brittle to be analyzed for viscoelastic and mechanical properties. Furthermore, the 2/1 ratio films (more BM) were more brittle than the 1/1 ratio
12
ACCEPTED MANUSCRIPT films with less BM. 1.0
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0.0
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-0.5
-1.0
-1.5
-2.0 -50
0
50
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Heat flow (W/g)
0.5
100
150
200
250
Temperature (oC)
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Figure 4. DSC diagrams of DA-crosslinked films cast at 60 °C from reactive blends consisting of PFu and BM at 1/1 and 2/1 mole equivalent ratio of maleimide/furfuryl group, compared with a physical blend (1/1 mole equivalent ratio) cast at room temperature.
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Investigation of thermoreversible self-healing. The occurrence of retro-DA reaction was first visualized through sol-gel-sol transition in organic solution (Figure 5). The reactive mixture of PFu and BM at 1/1 mole equivalent ratio of maleimide/furfuryl group in anisole at 11 wt% was heated at 60
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°C (the same temperature for casting films) for 12 hrs, yielding a standing gel (a to b). The piece of the
gel was mixed with DMF (c). Upon heating at 150 °C for 5 hrs, the heterogeneous mixture became a sol solution (d), as a result of the occurrence of retro-DA reaction.
13
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ACCEPTED MANUSCRIPT
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Figure 5. For a PFu/BM reactive blend at 1/1 mole equivalent ratio of maleimide/furfuryl group, a sol solution dissolved in anisole (a), a gel formed at 60 °C for 12 hrs (b), a mixture of pieces of the gel with DMF (c), and a sol solution upon cooling down after being heated at 150 °C for 5 hrs (d). Next, thermally-induced self-healing on cuts through reversible DA/retro-DA reactions was examined by microscopy. Similar procedure was examined to cast DA-crosslinked films from reactive
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mixtures consisting of PFu and BM at the 1/1 and 2/1 mole equivalent ratios of maleimide/furfuryl group at 60 °C for 12 hrs. Several cuts with different widths and lengths were made on the surfaces of the cast films using a knife. The films were annealed at 150 °C for 5 hrs and then cooled down to room temperature. Figure 6 shows the microscope images before and after healing of the cuts. In the 1/1 ratio films, the healing appeared to be almost completed for small cuts. However, a trace of the original cuts remained for relatively large cuts. Similar healing behavior was observed in the 2/1 ratio films.
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length of the cuts.
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These results suggest that the self-healability for the systems (PFu and BM) relies on the width and
14
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ACCEPTED MANUSCRIPT
Figure 6. Microscope images of DA-crosslinked films cast from a reactive blend of PFu and BM at mole equivalent ratio of maleimide/furfuryl group = 1/1 (a-b) and 2/1 (c) before and after being annealed at 150 °C for 5 hrs.
Conclusion
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A new PFu methacrylate copolymer having 58 mol% pendant furfuryl groups, whose Tg = 30 °C, was synthesized by a combination of a facile free-radical solution polymerization with postmodification methods through a base-catalyzed coupling reaction and a thermally-initiated thiol-ene radical addition reaction. A model NMR study with small molecular weight precursors exhibits
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temperature dependence on the extent of DA reaction; however, the extent of DA reaction did not reach completion, presumably because of DA reaction being competed with retro-DA reaction at elevated temperatures. Given the result, the reactive blends of PFu with a bismaleimide crosslinker in
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anisole were cast at 60 °C (a moderate temperature) to yield thermally-induced DA-crosslinked networks with gel contents >78% in THF. Their thermal analysis with DSC exhibiting endotherm suggest the occurence of DA reaction between furan and maleimide groups at 50-100 °C as well as the occurrence of retro-DA reaction at temperatures greater than 125 °C. Promisingly, the results from solgel transition and optical microscopy demonstrate the occurrence of thermoreversible DA/retro-DA reactions and self-healing in DA-crosslinked films.
15
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Acknowledgements Financial supports from Canada Research Chair (CRC) Award, Korea Research Institute of Chemical Technology (KRICT), and the Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Technology Innovation Program (No. 10067082) entitled “Development of scratch
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self-healable coatings and related process for automotive” are gratefully acknowledged. JKO is a Tier II CRC in Nanobioscience. Mr. Liu, a research project undergraduate student, thanks So Young An for her helpful discussions.
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A new reactive polymethacrylate bearing pendant furfuryl groups: synthesis, thermoreversible reactions, and self-healing
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Sungmin Jung,a Jiang Tian Liu,a Sung Hwa Hong,a Dhamodaran Arunbabu,a Seung Man Noh,b Jung Kwon Oha* Synthesis of a new methacrylate copolymer having pendant furfuryl groups (PFu)
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Utilization of a combination of FRP and most-modification methods
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Formation of dynamic crosslinked networks by thermally-induced Diels-Alder (DA) reaction
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Self-healability through thermoreversible DA/Retro-DA reactions
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A new reactive polymethacrylate bearing pendant furfuryl groups: synthesis, thermoreversible reactions, and self-healing Sungmin Jung,a Jiang Tian Liu,a Sung Hwa Hong,a Dhamodaran Arunbabu,a Seung Man Noh,b Jung Kwon Oha* a
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Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec, Canada H4B 1R6
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Research Center for Green Fine Chemicals, Korea Research Institute of Chemical Technology (KRICT), Ulsan 44412, Republic of Korea
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Corresponding author: J.K.Oh ([email protected])
Figure S1. 1H-NMR spectra of POH (a), PMA (b), and PFu (c) in DMSO-d6.
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Ratio of integral 2.70/4 = 0.675 (a and b) 1.86/2 = 0.93 (c) 0.93/(0.675+0.93) = 58%
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ACCEPTED MANUSCRIPT Figure S2. GPC traces for POH and PFu.
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POH PFu
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Molecular weight (g/mol)
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Figure S3. DSC (a) and TGA (b) diagrams of POH and PFu.
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Figure S4. Overlaid 1H-NMR spectra recorded at 80 °C for a reactive blend of FuMA and BM in DMSO-d6.
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