Synthesis of Self-Healing Bio-Based Tannic Acid-Based Methacrylates By Thermoreversible Diels-Alder Reaction.
Most polymeric materials are subjected to thermal, mechanical, as well as chemical degradation and destruction during their lifetime and thus affecting the longevity of its service life [1, 2]. Influenced by the self-healing phenomenon of biological features [3, 4], as new materials are designed, the quest for materials with self-healing properties is increasing, especially after mechanical deformation. As a result of researchers' extensive work, a variety of self-healing approaches have quickly been developed [5-7], including encapsulation [8, 9], hollow fibers , microvascular networks , and so forth. However, the dynamic bond, which has the ability to unlock the crosslinked network under external stimuli, can impart the properties of repeatable self-healing. The processes include ester exchange [12-14], disulfide exchange [15-17], and the Diels-Alder (DA) bond [18-26, 41] are generally used for reversible network formation.
Among all healing systems based on dynamic bonds to date, DA bonds are supposed to be one of the most reliable systems mainly because of its highly efficient reversibility and moderate sensitivity to temperature . The Wudl group successfully reported the self-healing of polymeric networks by using the thermoreversible nature of the DA reaction between the furfuryl and maleimide groups [19, 20]. The Wudl's approach was then followed by some researchers and they developed thermo-healable epoxy resin by modifying the epoxy monomer with the DA group. Liu and Hsieh  set epoxy compounds as precursors to simply synthesize thermally mendable crosslinked polymers. Subsequently, epoxy resin with furan groups was made by Tian et al. [22, 23] and reacted with bifunctional maleimide to form a crosslinked epoxy that can heal the cracks resulting from reverse DA (rDA) and DA reactions. The reaction was not limited to epoxy only and Bose et al. synthesized a one-component DA copolymer of methacrylates and pendant groups of both furan and maleimide moieties were incorporated into it . Schubert and coworkers studied a group of terpolymers consisting of a methacrylate backbone containing functional moieties for reversible crosslinking by DA reaction for its application as self-healing coatings . The use of DA cycloadditions (specifically those between various maleimide and furan derivatives) was accelerated by both the commercial availability and chemical comprehensibility of these functional groups [29-32]. Atom transfer radical polymerization (ATRP) is one of the most widely studied controlled living radical polymerization method to synthesize self-healing polymers via DA reaction . Through ATRP, DA groups can be easily introduced onto the polymer architectures and reversibility of the systems can be studied thereafter. The polymers prepared by ATRP consist of a living chain termini and this living part can easily be reacted with other reactive structures.
Another aspect to be worth noting is the use of bio-based starting materials in combination with polymers for various applications as it leads to low consumable energy synthesis and positive effects on the environment during processing while also improving the durability and self-healing capacity of these materials in the subsequent phase. However, till now petrochemical-derived acrylate monomers and oligomers has been used predominantly in many applications, which has a foreseeable limit due to the ever increasing cost and exhaustion. However, in the last few years, tannic acid (TA) and TA-based methacrylates in general has been used and studied by various researchers for its outstanding applications in coatings, adhesives as well as resin materials and electrolytes to name a few [34-38]. TA-based methacrylates thus can be a promising agent mainly because of its high functionality, better solubility, and low viscosity. Inspired by this, the present investigation reports the preparation of a DA self-healing polymer based on novel TA-based methacrylates. In this case, at first TA-based dendritic methacrylate (Mon-TAG) was prepared and then the furfuryl functionality was introduced via ATRP with furfuryl methacrylate (FMA). The performances of the synthesized polymers toward DA reaction with bismaleimide were studied. Bismaleimide was added during the last step to eliminate the possibility of copolymerization with the other components. This novel kind of TA-based methacrylates were characterized by Fourier transform infrared (FTIR), nuclear magnetic resonance ([sup.1]H NMR), and gel permeation chromatography (GPC). This investigation also reports the mechanical as well as the thermal properties of the crosslinked TA-based methacrylates. The self-healing property was analyzed by optical microscopy and scanning electron microscopy (SEM).
TA (Sigma Aldrich), triphenylphosphine (TPP) (>99%, Merck), 4-methoxyphenol (MEHQ) (99%, Sigma-Aldrich), anhydrous ethanol (Helix India), FMA (>95.0%, TCI Chemicals), ethyl 2-bromoisobutyrate (EBiB) (98%, Sigma-Aldrich), N,N,N',N",N"-pentamethyldiethylenetriamine (PMDETA) (99%, Sigma-Aldrich), 1,1'-(methylenedi-4, 1-phenylene)bismaleimide (BM) (95%, Sigma-Aldrich) were used as received. Solvents toluene (Emplura; [greater than or equal to]99.0%), dichloromethane (DCM; Emplura; [greater than or equal to]99.0%) were purified according to standard procedures. Glycidyl methacrylate (GMA) (97%, Sigma-Aldrich) was washed with NaOH prior to use.
Reactions and Synthesis
Synthesis of Tannic Acid-Based Methacrylates (Mon-TAG). Mon-TAG was synthesized as described by previously reported procedures [34, 38, 39]. A total of 17.00 g (0.01 mol) of TA, 0.36 g (1.5 wt%) of TPP, 0.03 g (0.1 wt%) of MEHQ, and 40 mL of anhydrous ethanol were placed into a 250 mL round-bottom flask quipped with a thermometer, a mechanical stirrer, a dropping funnel nitrogen inlet. Then, 7.11 g (0.05 mol) of GMA was added dropwise to the mixture. The reaction mixture was stirred at 95[degrees]C-100[degrees]C until the epoxy value approached to zero. A brown viscous liquid was obtained by evaporating the solvent. Yield: 95%.
Synthesis of Copolymer of TA-Based Methacrylate and FMA by ATRP (TAG-F-10, TAG-F-30, TAG-F-50). Synthesis of the copolymer was carried out using different ratios of 10%, 30%, and 50% of FMA with respect to Mon-TAG, respectively. In a typical experiment, CuBr (0.08 g), PMDETA (0.104 g), DMF (6 mL), MonTAG (3 g), and FMA were taken in a 100 mL three-necked, round-bottom flask. The flask was closed with a rubber septum in one neck, and another neck was equipped with a condenser. The polymerization was started by adding EBiB (0.039) and was carried out at 90[degrees] C under a nitrogen atmosphere. The copolymer was extracted by precipitation in ethanol. A conversion of ~79% was obtained after 12 h. A part of the polymer sample was analyzed by GPC to determine its molecular weight. Yield: 79%. The homopolymers of both MonTAG and Mon-FMA were also synthesized according to the above procedure.
Preparation of Thermally Reversible Crosslinked Polymer (DA Reaction) with BM (TAG-F-BM-10, TAG-F-BM-30, TAG-F-BM-50). Equimolar ratios of each of TAG-F-10, TAG-F-30, and TAG-F-50 with BM were dissolved in 4 mL of DCM ([CH.sub.2][C.sub.12]) and were stirred at room temperature for 35 h. The resultant crosslinked polymers TAG-F-BM-10, TAG-F-BM-30, and TAG-F-BM-50 were dried in a vacuum for 12 h.
rDA Reaction. Typically, 0.2 g of above synthesized DA polymers were placed in 1 mL of dimethylformamide and heated at 150[degrees]C for 9 h. The resulting product was completely soluble in the solvent. This reaction was characterized by FTIR analysis.
Self-Healing Demonstration. The self-healing tests were conducted on the test specimens according to the ASTM standard D5045-99. A loading rate of 10 mm/min was applied for the test. A razor blade was used to make cuts on the crosslinked polymer film (5 mm thick) so as to generate visible cracks. The samples were then thermally treated. Images of cracks on the surface of sample before and after healing were observed with an optical microscope and SEM.
CHARACTERIZATION AND METHODS
[sup.1]H NMR was measured by JEOL 400 MHz NMR instrument. The samples were dissolved in deuterated chloroform. FTIR spectra of the samples were recorded using a Nicolet Impact-410 IR spectrometer at room temperature in the range of 4,000-400 [cm.sup.-1]. Both the crosslinked and uncrosslinked polymer films were cast over KBr pellet and FTIR data were recorded. Molecular weights and the polydispersity index were measured by a GPC instrument equipped with a Waters Styragel column (HR series 3, 4E) with THF as eluent at a flow rate of 0.7 mL/min, and a Waters 2414 differential refractometer was used as detector. All the measurements by GPC were done relative to polystyrene standards. Thermogravimetric analyses (TGAs) of the polymers were done using a Shimadzu TA50 thermal analyzer. A preweighted amount of the samples was loaded in a platinum pan under nitrogen atmosphere and heated at a heating rate of 5[degrees]C/ min in the range of 30[degrees]C-600[degrees]C. Differential scanning calorimetry (DSC) of the samples was conducted on a DSC-60, Shimadzu analyzer over a temperature range of 0[degrees]-300[degrees]C at a heating (or cooling) rate of 10[degrees]C/min under a steady flow of ultrahigh-purity nitrogen purge. The DSC curve was plotted from the second heating scan. Optical microscopic images of the polymeric films were taken using Polarizing Microscope BA310 Pol. The surface morphology of the samples was determined by SEM (JEOL-JSM-6390LV) coupled with energy dispersive X-ray detector and the samples were sputter coated with platinum thickness of 200 [Angstrom]. The voltage and working distance was varied during the measurement. To evaluate self-healing ability of the materials, the method proposed by Jones et al. was employed . Healing efficiency, [H.sub.e] is defined as:
[H.sub.e] = [[sigma].sub.healed]/[[sigma].sub.virgin] * 100%
The testing was done according to ASTM standard D5045-99. Three samples were tested in each condition. A loading rate of 10 mm/min was applied for the test. The prenotched samples were first broken to failure, giving the fracture toughness, [[sigma].sub.virgin] and immediately clamped together and heated to a certain temperature to heal. Healed samples were tested again to measure the regained fracture toughness [[sigma].sub.healed]. All the testings were done using Universal Testing Machine (UTM, Zwick, Z010) at ambient temperature.
RESULTS AND DISCUSSION
Synthesis and Characterization of the TA-Based Methacrylates
The dendritic methacrylate, Mon-TAG was found to dissolve well in chloroform like Mon-FMA. However, large differences were observed between the homopolymerization of both MonTAG and Mon-FMA to form its corresponding hompolymers Hpol-FMA and H-pol-TAG. Due to the inhibitory effects of phenolic rings of Mon-TAG, Mon-FMA reacted much faster (90% conversion after almost 6 h) compared to Mon-TAG (65% conversion after 64 h). The copolymers obtained by ATRP from the reaction mixtures with Mon-FMA molar ratio of 10, 30 and 50% with respect to Mon-TAG mimic the behavior of H-pol-TAG. Copolymerization was not successful when the Mon-FMA content became too high (higher than the amount of Mon-TAG). The effect of CuBr on the phenolic rings of the tannins is negligible as the amount used is very low . The combination of higher concentration along with the higher reactivity of MonFMA causes a disproportional composition of the polymers that are being formed. DA reaction between BM and the furan functional groups of the copolymers having furan groups (TAG-F-10, TAG-F-30, and TAG-F-50) lead to a well-defined crosslinked polymer in which the main-chain segment is the methacrylate chain of Mon-TAG. The overall synthesis steps have been illustrated in Sch 1.
The [sup.1]H NMR spectra in Fig. 1 confirm the structure of the dendritic methacrylate Mon-TAG. The methaciylic double bond proton peaks appeared at 6.09 and 5.71 ppm which confirms the incorporation of GMA onto TA structure.
The [sup.1]H NMR spectra for TAG-F-10, TAG-F-30, and TAG-F50 in Fig. 2 show the introduction of furan ring onto the Mon-TAG structure at [delta] = 6.1, 7.3, and 7.6 ppm which are attributed to the hydrogen atoms of the furan ring. The specific contribution of the FMA moiety to the spectra of the co-polymers can be clearly discriminated by the increasing intensity of peaks at "a," "b," and "c" of TAG-F-10, TAG-F-30, and TAG-F-50, respectively. Additionally, the other protons "d," "e," "f," "g," "h," "k," "i," and "m" of the copolymers (specified in the spectra of TAG-F-50) can be observed clearly at [delta] = 5.34, 1.29, 1.91, 1.64, 2.02, 4.36, and 4.22 ppm and to a lesser extent in case of TAG-F-30 and TAG-F-10 respectively. The repeating unit ratio of the copolymers was determined using the indicative peak intensities of the [sup.1]H NMR spectra. The peak intensities of protons of the furan ring at [delta] = 6.1, 7.3, 7.6 ppm (a,b,c), proton of the -[CH.sub.2] group of the FMA block at [delta] = 1.91 ppm (f) as well as the peaks designated by 4.36 (i) and 1.64 ppm (g) of TAG block are compared and the results are summarized in Table 1. The results clearly suggest that phenolic rings of the tannins acts as inhibitor for the copolymerization and FMA reacts much faster comparatively. The homopolymers H-pol-TAG and H-pol-FMA are also synthesized for comparison purposes and their peaks in the [sup.1]H NMR spectra (Fig. S1) are found to be much broader compared to its corresponding monomers due to comparatively limited mobility within the chains and reduction in the freedom of side groups to rotate as commonly observed with polymers. The spectra also show the disappearance of peak at 6.09 and 5.71 ppm attributed to methacrylic double bond proton which were clearly observed in Mon-TAG. Similar results were also obtained from the spectra of H-pol-FMA.
The formation of the monomer Mon-TAG and its copolymers with Mon-FMA are also confirmed by FTIR (Fig. 3). The infrared spectra of Mon-TAG show the characteristic absorption bands for --[CH.sub.3] and --[CH.sub.2] (3,000-2,850 [cm.sup.-1]), C=0 (1,750 [cm.sup.-1]), and C=C (1,640 [cm.sup.-1]) due to the methacrylate moiety present in it. The infrared spectra of Mon-FMA are typical for acrylates  and show the characteristic absorption of C=0 stretch vibration (1,740 [cm.sup.-1]), the C=C stretch vibration (1,640 [cm.sup.-1]) and the =C--H bending vibration (812 [cm.sup.-1]). Upon copolymerization of Mon-TAG with Mon-FMA, the C=0 stretching vibration shifts to a higher wavenumber by 2-5 [cm.sup.-1] while the band around 1,640 [cm.sup.-1] disappears. Absorption specific to the furan rings only is found at 665 [cm.sup.-1]. This peak can be clearly observed in the spectra of Mon-FMA and decreases accordingly to its content from TAG-F-50, TAG-F-30, and TAG-F-10.
DA and rDA Reaction
In a controlled experiment when bismaleimide was added with H-pol-TAG, the resulting product H-pol-TAG-BM is not a DA adduct due to the absence of dienes. Thus, no changes in the FTIR spectra can be observed shown in Fig. 4. After addition of the same quantity of bismaleimide to the copolymer of the highest FMA content (TAG-F-50), the DA reaction can proceed to form a crosslinked polymer network (TAG-F-BM-50). The progress of the DA adduct formation is difficult to see in the TAG-F-BM-50 infrared spectrum and even less pronounced for the polymer networks with lower furan maleimide content (TAG-F-BM-30 and TAG-F-BM-10). However, when the DA reaction proceeds, the unsaturated maleimide ring is converted into the saturated succinimide ring which still absorbs at 665 [cm.sup.-1] with an additional absorption at 1,780 [cm.sup.-1] (marked by circle). Interestingly, the peak at 1,636 [cm.sup.-1] attributed to >C=C< which disappeared due to copolymerization further reappears because of the DA reaction between the furan functionalized copolymer TAG-F-50 and BM.
Solution Properties of the Crosslinked Polymers
Although the infrared spectra do not show much changes, the effect on the swelling behavior was large (Fig. 5). Before bismaleimide was added, all polymers (H-Pol-TAG, TAG-F-10, TAG-F-30, and TAG-F-50) dissolved readily in chloroform. The images in Fig. 5 show the solubility of Mon-TAG, homo-polymer H-pol-TAG, and copolymers TAG-F-10, TAG-F-30, and TAG-F-50 at room temperature. However, after addition of the bismaleimide, the reference material that could not be crosslinked (H-pol-TAG-BM) still dissolved rapidly. For the cases of TAG-F-BM-30 and TAG-F-BM-50, however, the materials became largely insoluble and remained as swollen precipitate at the bottom of the sample bottle due to the formation of crosslinked polymer networks.
The solution property of the crosslinked TAG-F-BM-50 adduct was also studied in toluene at 25[degrees]C as well as at higher temperature. Figure 6 indicates that when the crosslinked adduct is introduced into toluene at 25[degrees]C (Fig. 6a) it is insoluble and only swollen, indicating the crosslinking reaction between TAG-F-BM-50 and BM. When it was heated to 150[degrees]C, the crosslinked adduct becomes completely soluble after almost 9 h (Fig. 6b), indicating the cleavage of the furfuryl ring and BM.
The TGA measurements were performed to determine the thermal properties of the polymers after addition of bismaleimide shown in Fig. 7. The results indicated that all the polymers in nitrogen atmosphere are thermally stable below 250[degrees]C and rapidly decompose at 300[degrees]C-450[degrees]C. The degradation pattern (Curves c-e) in the polymers clearly shows that they have single as well as maximum degradation temperatures around 350[degrees]C-420[degrees]C. This clearly indicates that since the polymers are prepared by ATRP, thus side reaction or the bimolecular terminations are minimized. Interestingly, these curves also show that the crosslinked DA polymers have higher thermal stability as compared to their uncrosslinked counterparts (Fig. S2). The control experiment (Curve b) shows that although the polymer is not crosslinked due to absence of furan groups but the thermal degradation surely increases some fold as compared to its uncrosslinked H-pol-TAG (Curve a).
The GPC results of Mon-TAG and its copolymers with FMA (TAG-F-10, TAG-F-30, and TAG-F-50) are given in Table 1. The polydispersity of Mon-TAG was found to be around 1.81 as compared to the polydispersity of its copolymers with FMA which are found to be relatively lesser mainly because of their synthesis by ATRP which is a controlled radical polymerization technique. Also the molecular weight of the copolymers are observed to be increased as expected with increasing amount of FMA.
The synthesis of TAG-F-BM-50 crosslinking adduct by DA reaction was monitored by using FTIR shown in Fig. 8 at different time intervals of 0, 12, 24, and 35 h, respectively. The spectra show the time-dependent changes of the absorption peak at 1,012 [cm.sup.-1] attributed to the furan ring breathing of the furan ring of TAG-F-50. As expected initially, no crosslinking reaction occurs. Also after 12 and 24 h, complete crosslinking reaction is not observed as it is clearly seen that the peak for furan ring breathing is still observable although the intensity of the peak decreases. However, after 35 h, complete disappearance of the peak for furan ring breathing is observed. Thus, it can be said that the complete crosslinking reaction of TAG-F-50 with BM occurs after 32 h of reaction at room temperature.
Study of DA/rDA Reaction at Different Time Intervals
The DA/rDA reaction of the adduct TAG-F-BM-50 was also studied by FTIR at different time intervals of 2, 5, and 9 h shown in Fig. 9. The adduct was heated to 150[degrees]C in toluene. Initially, it shows a peak at about 1,636 [cm.sup.-1] attributed to >C=C< that appears to be due to the DA reaction between TAG-F-50 and BM (Fig. 9a). As can be seen in the figure, there is not much difference between Fig. 9a and b where the peak for >C=C< remains intact and which represent the FTIR spectra of the TAG-F-BM-50 adduct heated to 150[degrees]C at t = 0 and 2 h, respectively. However, in c and d (FTIR spectra at t = 5 and 9 h), it can be clearly observed that the peak at 1,636 [cm.sup.-1] slowly disappears. Thus, we can say that the cleavage of bonds between the furfuryl group and BM occurs after almost 9 h when treated to 150[degrees]C by rDA reaction, as shown in 1.
DSC Analysis of the Crosslinked DA Polymer
DSC was also performed to further confirm the formation of DA bond in TAG-F-BM-50 as illustrated in Fig. 10. The [T.sub.g] peak can be observed at 89[degrees]C, and when the temperature is increased, a broad endothermic peak corresponding to the rDA reaction can be observed at around 150[degrees]C. This clearly indicates that rDA reaction does takes place which leads to the decrosslinking of the DA adduct to its corresponding furan and maleimide moieties.
Mechanical Properties of the Thermoreversible Crosslinked Polymer
The stress-strain curves of the uncrosslinked as well as crosslinked adducts of the polymers are shown in Fig. 11. The stress versus strain curve clearly shows that with increasing content of FMA, the strength of the polymers clearly increases. However, it is also observed that its crosslinked counterparts impart much higher strength as compared to the uncrosslinked ones. The higher strength can be attributed to the fact that "intermonomer" linkages were formed by the DA cycloaddition reaction between furan group and BM. Due to lesser amount of furan groups in TAG-F-10, the strength of its crosslinked counterpart TAGF-BM-10 do not increase manifold as compared to TAGF-BM-30 and TAGF-BM-50. It should be noted that the introduction of TA segment enhanced not only the bond strengths but also the intermolecular and intramolecular interaction due to its aromatic nature and high polarity, which led to higher cohesive energy.
Self-Healing Characteristics of the Crosslinked Polymer
Initially, the self-healing property of TAGF-BM-50 was observed by optical microscopy as shown in Fig. 12. A razor blade cut was made on the film surface and immediately clamped together followed by heating up to 150[degrees]C. The sample was then cooled to room temperature and observed at various intervals of time (t = 0, 12, 24, and 35 h). It was observed that the whole crack could be healed within 35 h at 150[degrees]C.
The self-healing property of crosslinked polymer TAGF-BM-50 was also observed by SEM shown in Fig. 13. The images show the surface morphology of the crack created by the sharp razor blade before (Fig. 13a) and after healing (Fig. 13b, 35 h at 150[degrees]C). It was observed that complete recovery of the crack occurs. A controlled experiment was carried out using H-Pol-TAG with no furfuryl moiety content and the same procedure was applied as above. The images in Fig. 13cd clearly show that no self-healing occurs even after 2 weeks. The same procedure was applied and the temperature was raised to as high as 180[degrees]C but no self-healing was observed clearly indicating that softening of the sample is not the cause of self-healing but DA-rDA reaction is the prime cause.
The main concern of our study is to evaluate the self-healing efficiency of the self-healing TA-based methacrylates via DA reaction. The strength of the crosslinked polymer TAG-F-BM-50 before and after healing was studied at different time intervals. Three samples were tested in each case and the results are shown in Fig. 14a. The proposed mechanism for the self-healing process is like this, incorporation of BM into the polymer matrix and the subsequent crosslinking leads to increase in hardness and decrease in mobility. When the knife-cut sample is then heated to 150[degrees]C, it induces rDA reaction and decrosslinking of the network. This increases the chain mobility leading to reflow of the materials toward the crack site and when the temperature was then lowered to room temperature for almost 35 h; finally, full healing occurs due to reformation of the bonds via DA reaction.
The self-healing efficiency of the specimen was also determined and was found to be 89.04% as shown in Fig. 14b. The healing efficiency shows an exponential increase with time because although the cycloaddition reactions proceeds with increasing time but the DA reaction requires about 35 h to form a complete crosslinked adduct with maximum healing efficiency.
A series of furfuryl-functionalized methacrylates consisting largely of bio-based TA have been synthesized. Furfuryl methacrylate was used to introduce the furfuryl functionality by copolymerization through ATRP. Moieties have been included in the TA-based methacrylates to enable the formation of thermoreversible polymer networks through the DA reaction with a bismaleimide. The molecular stmcturc of the materials obtained has been verified by [sup.1]H NMR and FTIR spectroscopy. The crosslinking reaction was analyzed by means of solvent exposure tests and its thermo-reversible nature has been confirmed by FTIR and mechanical properties. When the polymer was heated to 150[degrees]C, it was soluble and the reason is attributed to the cleavage of the DA adduct by rDA reaction. The self-healing property of the crosslinked adduct was studied by SEM analysis, and complete healing was observed after 35 h. The SEM, mechanical, and solution studies confirmed the thermoreversibility and selfhealing character of the crosslinked polymer.
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Junali Handique, Joly Gogoi, Jayashree Nath, Swapan Kumar Dolui
Department of Chemical Sciences, Tezpur University, Napaam, Tezpur, Assam, 784028, India
Additional Supporting Information may be found in the online version of this article.
Correspondence to: S. K. Dolui; e-mail: swapankumardoluilO@gmail.com DOI 10.1002/pen.25267
Published online in Wiley Online Library (wileyonlinelibrary.com).
Caption: Sch 1. Preparation of thermally remendable crosslinked TA-based methacrylates by DA reaction. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 1. [sup.1]H NMR spectra of Mon-TAG. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 2. [sup.1]H NMR spectra of furan functionalized uncrosslinked TA-based methacrylates (Mon-FMA is included for comparison purposes only). [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 3. FTIR spectra of monomer Mon-TAG and uncrosslinked tannic acid-based methacrylates, prior to addition of thermoreversible crosslinker. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 4. FTIR-spectra of the homopolymer and copolymer networks obtained after the addition of bismaleimide, H-pol-TAG (a), H-pol-TAG-BM (b), TAGF-BM-10 (c), TAG-F-BM-30 (d), and TAG-F-BM-50 (e). [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 5. Photographs of Mon-TAG (a), H-pol-TAG (b), TAG-F-10 (c), TAG-F-30 (d), TAG-F-50 (e) in chloroform, H-pol-TAG-BM (f), TAG-F-BM-30 (g), and TAG-F-BM-50 (h) in chloroform. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 6. Photographs of observation of the thermally reversible crosslinking behavior of TAGF-BM-50 polymer film at (a) 25[degrees]C and (b) 150[degrees]C in toluene. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 7. TGA analysis of (a) H-pol-TAG, (b) H-pol-TAG-BM, (c) TAG-F-10, (d) TAG-F-BM-30, and (e) TAG-F-BM-50. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 8. FTIR spectra of the DA reaction of polymer TAG-F-BM-50 at different time intervals. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 9. FTIR spectra of the rDA reaction of the crosslinked polymer TAG-F-BM-50 heated to 150[degrees]C at different time intervals: (a) t = 0 h; (b) t = 2 h; (c) t = 5 h; and (d) t = 9 h.
Caption: FIG. 10. DSC curve of TAG-F-BM-50 DA adduct.
Caption: FIG. 11. Stress-strain curves of uncrosslinked and crosslinked furan functionalized TA-based methacrylates. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 12. Optical microscope images of TAGF-BM-50 treated at 150[degrees]C followed by cooling to room temperature. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 13. Self-healing study of crosslinked polymer TAG-F-BM-50. SEM micrographs of the (a) razor cut crosslinked polymer, (b) thermally self-healed sample of crosslinked polymer TAGF-BM-50 after 35 h, (c) razor cut polymer film h-pol-TAG, and (d) thermally treated h-Pol-TAG polymer film after 2 weeks.
Caption: FIG. 14. UTM results of the crosslinked product TAG-F-BM-50 at various intervals of time (a) and variation of self-healing efficiency of the TAG-F-BM-50 with respect to time (b). [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. GPC results. Samples [M.sub.n] [M.sub.w] [M.sub.z] [M.sub.z+1] Mon-TA-G 4,203 7,019 8,116 8,725 TAG-F-10 14,373 17,105 20,272 21,339 TAG-F-30 16,247 16,899 22,083 24,533 TAG-F-50 18,925 22,899 25,789 Samples Polydispersity [Mon-TA-G]: [FMA] ratio in the copolymer Mon-TA-G 1.81 100:0 TAG-F-10 1.63 65:32 TAG-F-30 1.55 49:42 TAG-F-50 1.59 24:70