Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
pubs.acs.org/Macromolecules
Benzoxazine-Based Thermoset with Autonomous Self-Healing and Shape Recovery Mustafa Arslan,† Baris Kiskan,*,‡ and Yusuf Yagci*,‡ †
Faculty of Science and Letters, Department of Chemistry, Kirklareli University, 39000 Kirklareli, Turkey Faculty of Science and Letters, Department of Chemistry, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey
‡
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ABSTRACT: A novel approach is reported for self-healing of polybenzoxazine thermosets based on both supramolecular attractions and metal−ligand interactions. The relating smart material was synthesized by using bis(3aminopropyl)-terminated polydimethylsiloxane, formaldehyde, and bisphenol A. The films of the obtained main-chain polybenzoxazine precursor (Poly(SiBz)) containing 2% FeCl3 were prepared and cured at low temperatures (100− 120 °C). The structures of the precursors and final products were characterized by spectral analysis. The curing and thermal stability of the related materials were investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Self-healing efficiency was studied by stress−strain measurements. Potential shape recovery (SR) behavior was also demonstrated by preparing curled or spiral fixed shapes, and the transformation of temporary shapes to these fixed shapes was verified.
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INTRODUCTION Polybenzoxazines (PBz) as a contender to resol and novolac type phenolics gained much interest in polymer science due to their superior properties. These materials possess good thermal stability, high Tg, low water uptake, limited dimensional change during curing, good mechanical strength, and low dielectric constants and generally have char yields between 35% and 75% at 800 °C.1 As a result of such properties, these materials found applications in numerous areas, especially in aero-composites, blends, and electronics circuit boards in the past decade.2 The synthesis of polybenzoxazines can be performed by heating their corresponding 1,3-benzoxazine monomers in the range 160−250 °C. The substituents on the monomer and the purity have crucial impact on the polymerization temperature.3,4 For example, a monomer with 99% purity exhibits higher polymerization temperature than its 95% pure form.5 Moreover, the polymerization temperature can be reduced by several catalysts such as Lewis6−8 and organic acids,3,9 lithium compounds,10 thiols,11 toluenesulfonates,12 amines,13 and amine salts.14,15 These catalysts act as either cationic species or nucleophiles to initiate ring-opening polymerization (ROP) through cationic steps for ring cleavage and concomitant electrophilic aromatic substitution reaction (Scheme 1).16−18 Control of the structure of the materials and their properties has been a central goal of polymer and materials chemistry. Such objectives can readily be accomplished by taking advantage of huge molecular diversity of the corresponding 1,3-benzoxazine monomers. Any suitable primary amine, a phenol, and formaldehyde are the elements of classical benzoxazine synthesis, and there are numerous readily available phenols and amines that can tolerate the synthesis conditions. © XXXX American Chemical Society
Scheme 1. Ring-Opening Polymerization (ROP) of a Benzoxazine
Therefore, features of the polybenzoxazines can be arranged by selecting appropriate amines or phenols to obtain designed materials for specific applications.19−22 Although previous studies have mainly focused on the synthesis of composites and alternative products to classical phenolics, the design flexibility allows these materials to be used as smart materials by specifically chosen functional groups. Moreover, polybenzoxazines have unique polymeric structure consisting of phenolic −OH and tertiary amine repeat units that generate intra- and intermolecular hydrogen bonds and unusual chain conformations endowing supreme potency to fabricate smart materials.19 Thus, stimuli-responsive materials based on benzoxazines such as electrochemically activated coatings,23 electrochromic resins,24,25 shape memory,26−28 and self-healable polybenzoxazines29−34 were successfully designed and fabricated. In the past decade, among smart materials considerable attention has been directed toward to the self-healable polymers due to the possibility of prolonged service life. Apart from capsule-based systems,35 “stimuli-responsive” or Received: October 4, 2018 Revised: November 22, 2018
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DOI: 10.1021/acs.macromol.8b02137 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 2. Synthesis of Poly(Si-Bz) and DiBz-PolySi from Bis(3-aminopropyl)-Terminated Polydimethylsiloxane (PolySi)
“autonomous” healing routes were also found to be operative. As external stimulation, light, heat, ultrasound, or stress is required to trigger bond formations for repairing the damage. Mostly, chemical reactions with reversible nature were selected in these systems to obtain multihealable materials. In this manner, cycloadditions such as [2 + 2] and [4 + 2] are suitable for heat- and light-induced healings. Therefore, these reactions were extensively used, and many successive self-healable materials were produced.36 On the other hand, materials with autonomous healing property do not need external stimulation, and mending occurs spontaneously after damage takes place.37 In these systems generally supramolecular attractions are the main driving force, and these supramolecular materials generally associate with reversible interactions such as hydrogen bonding.38,39 These bonding regions behave as mobile sticking points throughout the material. In such manner, the hydrogen-bonding interactions of polybenzoxazines can be utilized to design a self-healing system.31 Additionally, polybenzoxazines can bind metal ions by N and O atoms and form coordination complexes.40 Although these metal ion−polymer attractions are generally strong and complexes are stable, a dynamic nature is also present for some of the metal−ligand bonds. These bonds are broken and re-form, which leads to unfolding and sliding of the dangling polymer chains and provides high mobility to chains, which thus contribute to the self-healing process. It seemed, therefore, appropriate to test the feasibility of hydrogen bonding and complex formation capacity of polybenzoxazines for fabrication of such smart materials. Thus, in the present work, we report a novel strategy to form polybenzoxazinebased thermoset as an efficient autonomous healing and shape recovery material.
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Molecular weights were determined by gel permeation chromatography (GPC). The measurements were performed on a TOSOH EcoSEC GPC system equipped with an autosampler system, a temperature controlled pump, a column oven, a refractive index (RI) detector, a purge and degasser unit, and a TSKgel superhZ2000, 4.6 mm i.d. × 15 cm × 2 cm column. Tetrahydrofuran was used as an eluent at flow rate of 1.0 mL min−1 at 40 °C. The refractive index detector was calibrated with polystyrene standards having narrow molecular weight distributions. Data were analyzed using Eco-SEC Analysis software. Uniaxial elongation measurements were performed on polymeric film samples. Measurements were performed using a PerkinElmer Pyris Diamond DMA (SII Nanotechnology Inc.) at 25 °C under 50 mN/min load speed. The tensile strength and percentage elongation at break were recorded. Synthesis of Main-Chain Poly(dimethylsiloxane−benzoxazine) (Poly(Si-Bz)). In a 250 mL round-bottomed flask, bis(3aminopropyl)-terminated polydimethylsiloxane (5 g, 2 mmol), bisphenol A (480 mg, 2.1 mmol), and paraformaldehyde (240 mg, 8 mmol) were dissolved with 75 mL of toluene/ethanol (2:1) and refluxed for 12 h. Thereafter, the reaction solution was concentrated by using a rotary evaporator, and polymer solution was added dropwise into 100 mL of methanol to afford an oily product. Then, the polymer was dried under vacuum at room temperature (RT) for 24 h. Synthesis of α,ω-Dibenzoxazine−Poly(dimethylsiloxane) (DiBz-PolySi). A mixture of bis(3-aminopropyl)-terminated polydimethylsiloxane (5 g, 2 mmol), phenol (376 mg, 4 mmol), and paraformaldehyde (240 mg, 8 mmol) was dissolved with 100 mL of CHCl3 in a 250 mL round-bottomed flask and refluxed for 24 h. Thereafter, the reaction solution was concentrated by evaporating three-fourths of the solvent under vacuum. Then, the raw material precipitated in 100 mL of cold methanol. Finally, the oily product was dried under vacuum at RT for 24 h. Film Preparations. To obtain polybenzoxazine films, Poly(Si-Bz) (2 g) was dissolved in 5 mL of THF, mixed with FeCl3·6H2O (9 mL, (90 mg in 20 mL of MeOH)), and charged into a Teflon mold. The solvent was evaporated at room temperature for 5 days. Then, the films were exposed to thermal curing at 100−120 °C for 1.5 h in an oven. After curing, black and opaque polybenzoxazine cross-linked soft films with smooth surface were obtained. A similar procedure was applied for DiBz-PolySi precursor and its mixture with Poly(Si-Bz).
EXPERIMENTAL SECTION
Materials. 4,4′-Isopropylidenediphenol (bisphenol A) (Aldrich, 97%), paraformaldehyde (Acros, 96%), phenol (Merck), bis(3aminopropyl)-terminated polydimethylsiloxane (Mn ∼ 2500 Da, Aldrich), iron(III) chloride hexahydrate (Aldrich, 97%), ethanol (≥99.5%, Aldrich), toluene (Carlo Erba, 99.5%), chloroform (Acros, 99+%), methanol (MeOH, Aldrich, 99%), and tetrahydrofuran (THF, Sigma-Aldrich, 99.9%) were used as received. Characterization. 1H NMR spectra were recorded using an Agilent VNMRS 500 MHz, and chemical shifts were recorded in ppm units using tetramethylsilane as an internal standard. FTIR spectra were recorded on a PerkinElmer FTIR Spectrum One spectrometer. Differential scanning calorimetry (DSC) was performed on PerkinElmer Diamond DSC from 30 to 320 °C with a heating rate of 10 °C/min under nitrogen flow. Thermal gravimetric analysis (TGA) was performed on PerkinElmer Diamond TA/TGA with a heating rate of 10 °C/min under nitrogen flow. UV−vis spectra were measured with a Shimadzu UV-1601 double-beam spectrometer.
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RESULTS AND DISCUSSION As stated, self-healable systems generally function by either reversible chemical bond formation or supramolecular attractions such as hydrogen bonding through entangled polymer chains. In these systems, polymers should have a degree of mobility for sufficient molecular collusion at the damaged zones to re-form bonds and anchor via chain entanglements. Therefore, molecular diffusions, segmental mobility of chains, and flow of molecules either with or without external stimulus are essential requirements for these systems.41 Accordingly, appropriate selection of the phenol and primary amine components in benzoxazine synthesis is crucial. B
DOI: 10.1021/acs.macromol.8b02137 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules In the synthesis, while bis(3-aminopropyl)-terminated polydimethylsiloxane (PolySi) was deliberately selected as the amine source to afford essential chain dynamics, phenol or bisphenol A was used as the phenol source to yield telechelic or main-chain polybenzoxazine precursors DiBz-PolySi or (Poly(Si-Bz), respectively (Scheme 2). Moreover, the toluene/ ethanol (2:1, v/v) solvent system was used for the synthesis of Poly(Si-Bz) to reduce the formation of triazine side products that generally cause gelation.42 The chemical structures of the Poly(Si-Bz) and DiBz-PolySi were confirmed by 1H NMR and FTIR spectral analyses. As seen from Figure 1 and Figure S1 where 1H NMR spectra of
Figure 2. FTIR spectra of bis(3-aminopropyl)-terminated polydimethylsiloxane (a), Poly(Si-Bz) (b), and DiBz-PolySi (c).
consequence of step-growth polymerization (Figure S3). The obtained Poly(Si-Bz) exhibited excellent film forming property, and solvent casting in a Teflon mold gave colorless, transparent and flexible films (Figure S4). It is well established that most of the features of polybenzoxazines mainly stem from the CH2−N−CH2 bridges and the hydrogen bonds between tertiary amino and phenolic −OH groups. Thus, this structure has the capability of binding metal ions over O and N atoms. For example, polybenzoxazine and Hg(II) ion coordination system was previously demonstrated.40 Similar metal−ligand interactions were used for the fabrication of self-healing materials, extensively.44−50 These materials combine the features of polymers and metal complexes, enabling the design of new smart materials with outstanding properties. For example, strong metal−ligand and weak binding sites were placed adjacently in a single ligand, 2,6-pyridinedicarboxamide, which resulted in highly dynamic metal−ligand interactions enabling efficient self-healing property.51 In this work, the versatility of metal−ligand interactions as a design concept was demonstrated and combined with supramolecular attractions of polybenzoxazine structure to obtain autonomous and reversible healing at room temperature. For this purpose, Poly(Si-Bz) was dissolved in THF and mixed with FeCl3·6H2O in methanol. FeCl3 salt was absorbed in Poly(Si-Bz) film, and swelling took place in 24 h. Then, the film was dried at room temperature for 5 days to obtain a brown-black film containing ∼2% w/w FeCl3·6H2O. As it is well-known and will be shown below, Lewis acids such as FeCl3 promote the ring-opening of benzoxazines, thus reducing the polymerization temperature drastically.52 The exotherm of ROP almost disappeared after treating Poly(SiBz) with FeCl3 for 24 h. Only a slight exotherm is observable around 100 °C (Figure S5). Although the FeCl3-treated films became insoluble in common solvents at room temperature, ring-opening polymerization successfully proceeded upon heating the film up to 100−120 °C for 1.5 h in an open air oven, yielding cured Poly(Si-Bz) (abbreviated as ΔPoly(SiBz), delta sign (Δ) designates the cured polymers). By this way, Fe(III) ligated polybenzoxazines were prepared for selfhealing applications. A similar procedure was also applied for DiBz-PolySi samples. Although DiBz-PolySi contained a large polysiloxane unit, the film quality of ΔDiBz-PolySi/FeCl3 was
Figure 1. 1H NMR spectrum of Poly(Si-Bz).
Poly(Si-Bz) and DiBz-PolySi were presented, the protons resonating at 4.82 and 4.87 ppm (O−CH2−N) and 3.94 and 3.99 ppm (Ar−CH2−N) are clear evidence for the formation of benzoxazine on polydimethylsiloxane. Moreover, the peak at 1.57 ppm (−CH3) discloses the bisphenol A moiety. The resonance signal belonging to the methyl group protons of the polydimethylsiloxane is observed at 0.08 ppm with a shoulder. The splitting of the resonance is caused by the silicon atom functionalized with a hydrogen atom adjacent to the CH3−Si− CH3 group.43 This indicates that some of the siloxane chains have a silicon atom with H atom substitution (CH3−Si−H). Accordingly, the signal of the Si−H proton is also observed at 3.76 ppm, revealing the defect structure of commercial bis(3aminopropyl)-terminated polydimethylsiloxane (also see Figure S2). FTIR spectra of the corresponding polymers also give evidence for the formation of the desired structures (Figure 2). The stretching vibrations of aromatic C−H (3024−3072 cm−1) and aromatic CC (1409−1604 cm−1), the out-ofplane aromatic C−H, and C−O−C bending at 937−939 cm−1 are detected for the precursors obtained. Moreover, broad and complex Si−O−Si vibration stretching bands at 1005−1083 cm−1 with overlapping bands reveal long siloxane chains. These results confirm successful synthesis of benzoxazine functional polydimethylsiloxane. Besides, further assessment of the FTIR spectra discloses a weak Si−H peak at 2060 cm−1. Gel permeation chromatography (GPC) analysis of Poly(SiBz) reveals successive condensation reactions resulting in formation of the polymeric precursor (Mn = 10874 g mol−1, PDI = 1.46). The band appearing at the longer retention times corresponds to the oligomeric structures as a typical C
DOI: 10.1021/acs.macromol.8b02137 Macromolecules XXXX, XXX, XXX−XXX
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also be regarded as the reason for the premature cross-linking at room temperature after addition of Fe(III) salt. It is quite probable that both ligation and oxidation reactions contribute to the observed deep color change. Benzoxazines have a broad ring-opening polymerization temperature range that generally lies between 150 and 260 °C depending on the functional groups. Figure 5 and Table 1
insufficient for further analysis because even under low mechanical stress, the films were crumbled easily. The reason for poor mechanical strength could be due to the low crosslinking degree since one polysiloxane macromonomer contains only two benzoxazine units at the end of the chain. Attempts to mix both polymers did not result in any improvement in the mechanical strength. Hence, ΔDiBz-PolySi/FeCl3 was discarded, and ΔPoly(Si-Bz)/FeCl3 films was determined to be the sole material with sufficient elasticity (Figure 3) for further self-healing studies.
Figure 3. Images of flexible ΔPoly(Si-Bz)/FeCl3 film.
As is known, Fe(III) generates strong chelates with phenols; thus, ferric chlorides are extensively used to test the presence of phenolic compounds in various samples.53 Also, iron chelation with amino compounds was extensively studied.54,55 Therefore, it can be anticipated that compounds containing both phenolic and amino groups would form strong chelates with Fe(III). 56 Strong complexation of iron(III) with polybenzoxazine structure through phenolic −OH and tertiary amines in Mannich bridges is strongly expected. Accordingly, the films were analyzed by XRD spectroscopy to detect the signals from remaining nonchelated FeCl3 salt. Figure 4 can be
Figure 5. DSC profiles of DiBz-PolySi (a), Poly(Si-Bz) (b), and ΔPoly(Si-Bz)/FeCl3 (c).
Table 1. DSCa Characteristics of Poly(Si-Bz) and DiBzPolySi under a N2 Environment polymer Poly(SiBz) DiBzPolySi
curing onset (°C)
curing endset (°C)
curing max (°C)
curing exotherm (J/g)
191
275
241
−17
249
301
278
−7
a
DSC analyses were performed under a N2 stream (20 mL/min) with a 10 °C/min heating rate.
show the DSC profiles of Poly(Si-Bz), DiBz-PolySi, and ΔPoly(Si-Bz)/FeCl3. Two of the structures are curable, and their curing temperatures are in accordance with telechelic or main-chain polybenzoxazine precursors. Poly(Si-Bz) showed an exotherm with an onset at 191 °C, an endset at 275 °C, and a curing maximum at 241 °C. However, a completely different curing profile was encountered for DiBz-PolySi. The corresponding exotherm has an onset at 249 °C, and a curing maximum emerges at 278 °C; these values are 58 and 37 °C higher than the curing exotherm of Poly(Si-Bz), respectively. This vast difference between these precursors can be explained by the dilution effect on curing. As known, collision of benzoxazine molecules is also important determining the ringopening temperature apart from structural characteristics. For example, in the case of two solutions of identical benzoxazine monomers in different concentrations, the diluted sample would exhibit a higher curing temperature. Benzoxazines as end-chains in DiBz-PolySi demonstrate this phenomenon quite clearly.32 On the other hand, the DSC profile of ΔPoly(Si-Bz)/ FeCl3 that was cured at 100−120 °C for 1 h is remarkably different, and the ring-opening exotherm of oxazine rings is not detectable even if benzoxazines are completely consumed. Remarkably, an unusual exotherm is visible starting from ca. 80 °C with a maximum at 89 °C probably due to the heat of
Figure 4. XRD pattern of Poly(Si-Bz) (a) and ΔPoly(Si-Bz)/FeCl3 (b).
considered as an evidence of the adequate ligation between Fe(III)−N−O atoms by the lack of crystalline segment or salt structure in ΔPoly(Si-Bz)/FeCl3. Apparently, both Poly(Si-Bz) and ΔPoly(Si-Bz)/FeCl3 are mainly amorphous with only slight differences. Another evidence of formation of Fe(III)− polybenzoxazine complex is the drastic color change of Poy(SiBz) from colorless to brown-black after mixing with FeCl3 and subsequent cross-linking. In this connection, it should be pointed out that color change could also stem from the oxidation of benzoxazine moieties, and dimerization may take place between two benzoxazine units.57 This possibility could D
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Macromolecules dynamic complexations58 with reorganization of opened oxazines. The endotherm observed between 210 and 250 °C corresponds to the removal of hydrates from FeCl3·6H2O.59 In general, self-healing systems require a dynamic nature based on diffusion and sliding of chains. Therefore, effective entanglement in such systems is important, and interdiffusion ability of the polymer chains on the cut edges is the key step to response to a damage for effective restoring. Hence, a designed system must have enough flexibility to re-establish chain entanglements at the damage zone. On the other hand, the mobile chains should attract each other by the reactive or inter- and intramolecular attraction sites present in the structure. For that reason, molecular chains must contain sort of supramolecular attractions or functional groups that could act as preferably, reversible binding sites. In our molecular design, PolySi was selected to grant the required flexibility to the self-healing material while ring-opened benzoxazine units act as supramolecular attraction sites. For the healing purpose, the design concept utilizes two distinct attractions: inter- and intramolecular hydrogen bonding between nitrogen atoms on the Mannich Bridge and phenolic hydroxyl groups of polybenzoxazines and metal−ligand interactions generated by the addition of FeCl3 into the system to form Fe(III)−O(phenolic)−N(tertiary amine) ligation. Accordingly, Poly(Si-Bz)/FeCl3 films were prepared and cured at low temperature such as 100−120 °C in order not to disturb the inner hydrogen-bonding interactions. To test the healing, the prepared film was beveled into two parts to maintain efficient contact on the cut edges. Then, the ends of the distal parts were pressed mildly between glass slides with a paper clip for 24 h at RT to avoid sample slippage. Autonomous self-healing with good mechanical property was observed after a given time, and the healed film was still flexible and could be folded as seen in the visual image (Figure 6).
Figure 7. Stress−strain (%) behavior of pristine specimen (a) and cut-healed specimen (b).
film to some extent with a healing efficiency of 61%. Moreover, a blank experiment was also conducted to understand the effect of metal−ligand interaction on healing ability. In this experiment, addition of FeCl3 was omitted, and pristine ΔPoly(Si-Bz) film was prepared by solvent casting and subsequent curing of Poly(Si-Bz). Consequently, it was observed that bare ΔPoly(Si-Bz) film exhibited a limited amount of healing with a low mechanical strength. The healed films could easily detach from the contact sites even by pulling away the film gently from the opposites edges by hand. Hence, a quantification via stress−strain test could not be performed due to the very weak mechanical strength. Those results reveal that the self-healing mechanism of the designed material acts by the cooperative effect of metal−ligand binding sites and hydrogen-bonding interactions that enables stretchability and a comparable autonomous self-healing. A similar healing mechanism was also reported previously.61 Moreover, ΔPoly(Si-Bz)/FeCl3 film was heated at 90−100 °C in water and bent to a curled or spiral shape, followed by rapid cooling to room temperature while maintaining external force to fix the shape. Then, samples were forced to a planar shape by hand, and deformation recovered easily to return the fixed curled or spiral shape in 5−7 s (Figure 8). These result confirm that apart from self-healing ability, ΔPoly(Si-Bz) exhibited a shape recovery (SR) property, which is another feature of smart materials. Despite the fact that SR can also be considered as shape memory (SM), we prefer to designate in
Figure 6. Images of cut-healed ΔPoly(Si-Bz)/FeCl3 film.
Healing of a polymer can be quantified by measuring the recovery of its properties, and polymers exhibit various properties that can be used for determining the extent of healing. In practice, measuring the recovery of all the features of a material is difficult. Thus, although sole mechanical measurements cannot represent the whole properties, recovery of fracture toughness and tensile strength are generally used to quantify the healing. Accordingly, the “healing efficiency” (η) of a self-healing system is expressed as the ratio of fracture toughness (the area of stress−strain curve) of healed (Khealed) and virgin (Kvirgin) specimens (eq 1).60 K η = healed × 100 K virgin (1) Therefore, stress−strain tests were performed to quantify the healing amount for ΔPoly(Si-Bz)/FeCl3 film. Figure 7 shows the curves of cut-healed and virgin specimens. The measurements clearly reveal the restoration of cut-healed cross-linked
Figure 8. Images of ΔPoly(Si-Bz)/FeCl3 film exhibiting shape recovery. E
DOI: 10.1021/acs.macromol.8b02137 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 3. Metal−Ligand Bond Breakage and Re-Forming at Certain Temperature Changes
this paper SR since the recovery mechanism can be controversial and may not fit with the usual SM mechanisms. As known, shape memory polymers (SMP) have great potential for the fabrication of medical apparatus, aerospace materials, and other possible smart devices.62,63 Although there are several kinds of stimuli transformations for SMPs, thermally triggered SM materials were reported extensively.64 In these systems a transition temperature (Ttrans) is present to fix the temporary shape. Generally, over Ttrans the original shape is recovered at a certain speed depending on the structure. Conventional examples bear in two phases as a “switch” that could soften or melt above Ttrans, and thermally stable “fixed joint” served to recover the shape. 64,65 Polybenzoxazines with SM ability based on epoxy and polyurethanes were reported previously.26,27 Moreover, pristine SM polybenzoxazines have been reported during the preparation of this paper. In the corresponding study, the SM property was obtained by admixing Lewis acids (AlCl3 and PCl5) with classical benzoxazine monomers to cure at low temperatures.28 The authors claimed that the SM ability is based on the reduction of the amount of strong −OH···N hydrogen bonding by Lewis acids; thus, −OH···π hydrogen bonding became prominent. Therefore, the microstructure of polybenzoxazines exhibited two phases to serve as switch and fixed points. It is also known that polybenzoxazines may contain N,O-acetal-type linkages, phenolic Mannich-type linkages, and arylamine Mannich-type linkages if aromatic amine was used in the synthesis. 66−68 The obtained polybenzoxazine mainly consisted of arylamine Mannich-type linkages, and this type of structure was asserted as the basis of SM of their samples. However, in our case, Poly(Si-Bz) lacks arylamines, and PolySi (bis(3-aminopropyl)-terminated polydimethylsiloxane) was used as amine source. Therefore, SR behavior of ΔPoly(Si-Bz)/FeCl3 may relate to the dynamic metal−ligand binding sites. The metal complex could exhibit high dynamism and break over Ttrans but re-form during rapid cooling and fix the shape at room temperature while the PolySi moiety acts as a soft segment and polybenzoxazine as a hard segment (Scheme 3). Therefore, it can be concluded that dynamic metal−ligand interactions and siloxane moiety together form the “switch”, and polybenzoxazine provides the “fix points” in this designed material. Thermal stability of the PolySi, ΔDiBz-PolySi, and ΔPoly(Si-Bz)/FeCl3 was studied by using thermogravimetric analysis (TGA). TGA traces and related thermogravimetric results are presented in Figure 9 and Table 2, respectively. Accordingly, ΔPoly(Si-Bz)/FeCl3 exhibited better thermal stability than ΔDiBz-PolySi and PolySi as reflected by T5%, T10%, Tc, and Tmax values; PolySi almost completely vaporizes after 645 °C,
Figure 9. TGA traces (1) of ΔPoly(Si-Bz)/FeCl3 (a), ΔDiBz-PolySi (b), and PolySi (c) and derivative TGA (2) of ΔPoly(Si-Bz)/FeCl3 (a′), ΔDiBz-PolySi (b′), and PolySi (c′).
Table 2. Thermal Properties of ΔPoly(Si-Bz)/FeCl3, ΔDiBz-PolySi, and PolySia sample
T5% (°C)
T10% (°C)
Tc (%)
Tmax (°C)
ΔPoly(Si-Bz)/FeCl3 ΔDiBz-PolySi PolySi
383 319 262
425 384 328
21 9 ∼0
572, 643 453, 588, 646 446, 583
a
T5%: the temperature for which the weight loss is 5%; T10%: the temperature for which the weight loss is 10%; Tc: the char yield at 800 °C; Tmax: the temperature for maximum weight loss extracted from derivative TGA graph (Figure 9-2).
and only 0.23% remains at 800 °C. These results are expected since ΔPoly(Si-Bz)/FeCl3 has a higher cross-linking degree and more rigid aromatic groups than ΔDiBz-PolySi. Moreover, previous studies showed that addition of various transition metals increased the thermal stability, especially a pronounced effect was observed on char yields with 10−12% increases when the amount of transition metal was 2 mol %.52 A similar positive effect was also observed for ΔPoly(Si-Bz)/FeCl3 obtained by Fe(III)-catalyzed ROP. Three reasons can be discussed to explain this phenomenon: (i) the high degree of cross-linking due to the catalyst, (ii) the chelation between Fe(III), oxygen, and nitrogen atoms that may retard the amine degradation, and (iii) Fe(III) oxidized the polybenzoxazine into a more stable intermediate structure. Therefore, fragmentation of phenolic moieties was delayed, increasing the char yield and overall thermal stability. F
DOI: 10.1021/acs.macromol.8b02137 Macromolecules XXXX, XXX, XXX−XXX
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CONCLUSION This work presents the design and synthesis of self-healable polybenzoxazine networks both by metal−ligand and supramolecular interactions. The Poly(Si-Bz)/FeCl3 films displayed excellent self-healing properties in the absence of any additives such as solvents or plasticizers and external stimuli to promote the healing process. Apart from self-healing ability, another feature of smart materials, the shape recovery (SR) property, was observed. Combination of these two important properties in one polymer generally requires complex synthesis and several steps. The described approach facilitated to achieve both properties through main-chain polybenzoxazine precursor synthesis methodology using simple and relatively cheap components as bis(3-aminopropyl)-terminated polydimethylsiloxane, bisphenol A, and formaldehyde. Moreover, this work demonstrates the capacity of benzoxazine chemistry to produce advanced materials by governing hydrogen bonding and affinity of polybenzoxazines against metal salts through design flexibility of this chemistry.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02137. 1 H NMR spectra, GPC chromatogram, DSC thermogram, and image of materials (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(B.K.) E-mail
[email protected]. *(Y.Y.) E-mail
[email protected]. ORCID
Baris Kiskan: 0000-0001-9476-2054 Yusuf Yagci: 0000-0001-6244-6786 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Istanbul Technical University Research Fund and Oguz Okay (Istanbul Technical University) for providing the infrastructure to measure mechanical properties.
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