Eco-Friendly Sustainable Poly(benzoxazine-co-urethane) with Room-Temperature-Assisted Self-Healing Based on Supramolecular Interactions.

This work is an attempt to develop bio-based eco-friendly poly(benzoxazine-co-urethane) [poly(U-co-CDL-aee)] materials using cardanol-based benzoxazines (CDL) and hexamethylene diisocyanate (HMDI) to check their self-healing ability and thermal properties. CDL monomers were synthesized using cardanol, amino ethoxyethanol (aee) or 3-aminopropanol (3-ap), and paraformaldehyde through the Mannich reaction. Later, CDL-aee or CDL-3-ap monomers were copolymerized with a urethane precursor (HMDI), followed by ring-opening polymerization through thermal curing. The thermal properties of poly(U-co-CDL) were evaluated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The self-healing behavior of the bio-based poly(U-co-CDL) was checked by applying a mild external pressure. The results revealed that the developed poly(U-co-CDL) showed repeatable self-healing ability due to supramolecular hydrogen-bonding interactions. Further, the self-healing ability of poly(U-co-CDL) was studied using density functional theory (DFT). From the above results, the developed material with superior self-healing ability can be used in the form of self-healing coatings and composites for various applications with extended shelf-life and reliability.

Introduction

Self-healing methods are common to nature; for example, damaged or lost tissues and organs may be regained or regenerated in most of the living organisms. Inspired by this concept, scientists have attempted to develop synthetic self-healing polymeric materials in the last two decades in comparison with traditional polymeric materials. Due to their in-built capability to revamp physical damage triggered by both environmental and mechanical factors,[1−15] the self-healing abilities of polymeric materials not only extend their service life but also enhance the reliability of products in various applications; hence, this approach may surely be able to reduce the usage of available resources.[1,2] Therefore, many approaches are adopted for the development of self-healing materials. Generally, they are classified into autonomous self-healing and nonautonomous self-healing systems. Autonomous systems do not need external triggers for the self-healing ability, whereas nonautonomous systems utilize external triggers including temperature, light, pH, redox, etc.[1,2] In addition, self-healing materials can also be further classified into intrinsic self-healing and extrinsic self-healing systems. Extrinsic self-healing polymeric systems work in the presence of external healing agents like microcapsules or fibers, which heal the cracked or damaged portion at once. On the contrary, intrinsic self-healing materials undergo repeated multiple healings without any external healing agents.[1,2,8] These types of intrinsic self-healing polymeric materials are generally designed through dynamic reversible cross-linking by either covalent interactions, such as the Diels–Alder/retro-Diels–Alder reaction,[1,15] trans-esterification,[12] photodimerization,[13] acylhydrazone linkage,[14] disulfide linkage,[8,15] etc., or noncovalent supramolecular interactions, which include H-bonding,[14−16] π–π stacking,[16] metal-ion,[13] etc. Mostly, thermoplastic polymers have been utilized for self-healing applications. Generally, thermoset polymers are single-use polymers that cannot be used multiple times. If damaged, they have to be replaced with new polymeric materials.

Among the thermosets, polybenzoxazines are a new class of phenolic resins with good thermal and mechanical properties that allow overcoming several drawbacks of traditional phenolic resins. Also, this class has many added advantages such as a variety of molecular designs, low dielectric constant, high moisture resistance, high heat resistance, dimensional stability, good flame retardancy, excellent thermal stability, mechanical properties, and high residual char ratio.[12,13,17−23] Though polybenzoxazines have been utilized in various applications, the usage of polybenzoxazines in self-healing materials has barely been reported. Only limited studies have been published on self-healing. Yagci’s research group developed a self-healing material using poly(propyleneoxide)-based benzoxazine with varying weight ratios of acid-functionalized benzoxazine by supramolecular interactions.[24] The same group reported the self-healing behavior of polybenzoxazines based on both metal–ligand interactions and supramolecular attraction using a polydimethylsiloxane-based benzoxazine matrix.[25] The same research group reported that benzoxazine monomers can be used as self-healing agents for a polysulfone (PSU) matrix.[26] The self-healing property of polybenzoxazine by photo-induced coumarine dimerization[27] has also been reported, and reusable and self-healable polybenzoxazines have been developed by the inverse vulcanization approach.[15] Xiangdong Liu et al. developed self-healing polybenzoxazine using succinic anhydride and bisphenol-F benzoxazine through trans-esterification at a temperature of 140 °C in the presence of zinc acetate as a catalyst.[28] The reported polybenzoxazine materials with self-healing ability almost always involved multiple and nontrivial steps. To the best of our knowledge, to date, there has been no report on eco-friendly bio-based polybenzoxazines with self-healing or self-repairing ability in the literature. Here, an attempt has been made to develop eco-friendly bio-based polybenzoxazine-co-polyurethane matrices using cardanol-based benzoxazine derivatives (CDL) and hexamethylene diisocyanate (HMDI) and check their self-healing capability and thermal properties. The self-healing behavior of the bio-based poly(U-co-CDL) is checked by applying a mild external pressure without use of any external agents/trigger. The developed poly(U-co-CDL) shows good self-healing properties due to supramolecular action and inter- and intramolecular hydrogen-bonding interactions. Further, the self-healing ability of poly(U-co-CDL) is due to the supramolecular attraction and inter- and intramolecular hydrogen-bond interactions, which was revealed theoretically by DFT studies. The developed low-cost and environment-friendly self-healing poly(U-co-CDL) will be useful in various high-performance applications including coatings, automobile, printed circuit boards, etc. with a long shelf-life.

Results and Discussion

Structural Determination of the Composites

The structures of the synthesized renewable cardanol-based benzoxazine monomers CDL-aee and CDL-3-ap were confirmed with the help of 1H NMR shown in Figures and 2. From the NMR spectra of Figure , the peak that appeared at 4.01 ppm corresponds to Ph–CH2–N and the absorption peak at 4.93 ppm is related to the O–CH2–N protons of the oxazine ring. The appearance of peaks between 0.8 and 3.0 ppm corresponds to alkyl side-chain protons of cardanol. The peaks between 5.2 and 5.5 ppm represent the olefinic protons of cardanol. The peaks that appeared between 3.4 and 3.7 ppm represent protons of aminoethoxy ethanol. The peaks that appeared in the range of 6.6–6.8 ppm represent aromatic protons.

Figure 1

1H NMR spectrum of CDL-aee.

Figure 2

1H NMR spectrum of CDL-3-ap.

The 1H NMR spectrum of CDL-3-ap is shown in Figure . The peaks that appeared at 4.2 ppm and 5.3 ppm correspond to the protons of Ph–CH2–N and O–CH2–N, respectively. The alkyl side-chain protons of cardanol peaks appeared between 0.8 and 3.0 ppm. The triplet peak that appeared at 3.8 ppm corresponds to −C–CH2–O in the aminopropanol group. The aromatic protons appeared in the range between 6.6 and 6.8 ppm.

The Fourier transform infrared (FTIR) spectra of cardanol-based benzoxazines are presented in Figure . The appearance of a vibration band at 960 cm–1 corresponds to the oxazine ring, which confirms the formation of the benzoxazine ring.[19,29,30] In the oxazine ring, the C–H out-of-plane bending is observed at 728 cm–1. The vibration peaks around 1055–1118 cm–1 represent the symmetric stretching of C–O–C, and the peak at 1243 cm–1 represents the asymmetric stretching of C–O–C; also, the peak at 1355 cm–1 represents the C–N–C vibration. Further, the characteristic absorption peaks that appeared at around 2854 and 2923 cm–1 correspond to the asymmetric and symmetric stretching vibrations of CH2 of the oxazine ring as well as the alkyl side chain of cardanol. The peak at 3440 cm–1 shows the presence of the hydroxyl group in CDL.

Figure 3

FT-IR spectra of CDL-aee and CDL-3-ap monomers.

The successful formation of renewable cardanol-based polybenzoxazines and poly(U-co-CDL) was studied using FT-IR, and the results are shown in Figure . From Figure , the disappearance of the absorption band at 960 cm–1 and the appearance of a new peak at 1450 cm–1 are related to the formation of a tri- to tetra-substituted benzene ring, which confirms the occurrence of ring-opening polymerization of CDL.[31] The appearance of new peaks at 1254 and 1533 cm–1 represents the bending vibration of N–H and stretching vibration of C–N, respectively, which confirms the urethane linkage due to the introduction of HMDI into CDL. Further, the appearance of a peak at 1702 cm–1 shows the stretching vibration of C=O, and also the peak at 3332 cm–1 represents the NH stretching corresponding to the urethane linkage in CDL.

Figure 4

FT-IR spectra of poly(U-co-CDL-aee) and poly(U-co-CDL-3-ap).

Thermal Properties of Monomers and Composites

The curing properties of sustainable benzoxazines were studied with the DSC technique, and the curing thermogram is depicted in Figure . The DSC thermogram of sustainable benzoxazines was recorded at a heating rate of 10 °C/min under an inert atmosphere from 30 to 300 °C. From the DSC thermogram in Figure , the curing behavior of sustainable benzoxazines was observed from the single exothermic peak, which is attributed to the ring opening of the benzoxazine ring during thermal curing. Temperature peaks (exotherm maxima, Tp) were observed at 239 and 238 °C for CDL-aee and CDL-3-ap (monomers) benzoxazines, respectively (Table ). Generally, the curing of cardanol-based benzoxazine reveals the exothermic peak between 250 and 295 °C,[32] while in our work, the curing temperature of sustainable benzoxazines decreased to 238–239 °C. This may be due to the hydroxyl-terminated CDL that acts as a self-catalyst to decrease the curing temperature of the resulting cardanol benzoxazine monomers.

Figure 5

DSC thermogram of CDL-aee and CDL-3-ap.

Table 1

Thermal Properties of Cardanol-Based Benzoxazines of Poly(U-co-CDL-3-ap) Copolymers

	curing behavior
cardanol-based benzoxazines	Ti (°C)	Tp (°C)	Tf (°C)	5% weight loss (°C)	10% weight loss (°C)	Tmax (°C)	char yield % at 800 °C
CDL-aee	203	239	254	313	365	492	12
CDL-3-ap	223	238	253	256	343	489	7
poly(U-co-CDL-3-aee)				285	301	463	3
poly(U-co-CDL-3-ap)				256	274	467	4

Thermogravimetric analysis (TGA) provides valuable information with regard to the thermal stability of materials and the nature of degradation by measuring the weight loss at each instant. The thermal stability of cardanol-based polybenzoxazines was studied with the help of TGA, and the results are displayed in Figure a,b and Table . For cardanol-based polybenzoxazines with two different amines (CDL-aee and CDL-3-ap), the 5% weight loss temperature (T5) was noticed at 313 and 256 °C (Figure a) and the maximum degradation temperature (Tmax) was 492 and 489 °C, respectively. The char yield of CDL-aee- and CDL-3ap-based polybenzoxazines was obtained as 12 and 7%, respectively, at 800 °C. Further, the thermal stability of poly(U-co-CDL) copolymers was checked with TGA and is depicted in Figure . From the TGA curve, the maximum degradation temperature obtained for 1:1 weight percentage ratio of poly(U-co-CDL-3-aee) and poly(U-co-CDL-3-ap) was 463 and 467 °C (Figure b) and the char yield was 3 and 4%, respectively. The char yield percentages of the above blends decreased to below 5% (compared to the neat polybenzoxazine) due to the aliphatic polyurethane backbone in poly(U-co-CDL) copolymers.[1]

Figure 6

(a) TGA spectra of CDL-aee and CDL-3-ap and (b) poly(U-co-CDL-3-aee) and poly(U-co-CDL-3-ap) copolymers.

Self-Healing Behavior of Cured Samples

Self-healing in supramolecular polymers (Figure ) can be explained by the following basic steps: First, the undamaged materials consisting of polymer chains with attached inter- and intramolecular hydrogen bonds form a network,[33] and these bonds are able to connect and reconnect via a reversible “sticker-like” behavior. Thus, the strength of the polymeric materials is imparted by the “stickiness” of supramolecular hydrogen bonds, which is crucial for the formation of specific interactions between the bonds.[22] To justify the concept, the self-healing behavior of our polymers was tested on cured materials by applying a mild external pressure, and the test was recorded in the form of a video clipping (Video S1, supporting information). The prepared material film with 2 mm thickness and 2 cm length was taken and a cut was made in the middle to separate them into two equal halves. The equal parts were forced to join adjacently by applying a mild external pressure sideward to repair the damaged site. This process was repeated for 3–4 trials, and the material still showed good self-healing behavior.

Figure 7

Thermally cured poly(U-co-CDL-aee) of damaged and self-healed samples.

Thus, poly(U-co-CDL-aee) containing the urethane group motif shows complete healing (by applying a mild external pressure), probably caused by the delayed elasticity introduced by the supramolecular attraction; also, the inter- and intermolecular hydrogen bonding between the phenolic hydroxyl group and the urethane group present in the materials is responsible for self-healing. The O–H···O, N–H···N, and N–H···O types of hydrogen bonding were present in the material, and this was theoretically demonstrated using computational studies.

Computational Studies

The model calculation of polymeric chain and its interactions was examined using the state-of-the-art density functional theory (DFT) implemented in the Gaussian 09[34] suite of program. The monomer of poly(U-co-CDL-aee) was optimized at the B3LYP/6-31g(d,p)[35−38] level in the gas phase. The optimized structure was confirmed without any imaginary frequency through frequency calculations. Moreover, the monomers were duplicated and added in the Gaussian view to optimize the complex structure at the same level of theory. The optimized complex structure is shown in Figure . The hydrogen bond lengths are indicated by the dotted lines. Both the monomers interacted closely due to six strong hydrogen bonds, i.e., O–H···O, N–H···N, and N–H···O types of strong hydrogen bonds. These types of hydrogen bonds are reported in the literature as strong and responsible for the stability of the complexes.[39−42] The lengths of the hydrogen bonds are observed to be 1.8–2.0 nm. In the optimized complex, the center part of the polymer formed two hydrogen bonds, N–H···O type, which are observed to be strong as compared with the hydrogen bonds at both ends. Also, the complex formed a linear chain and elongated while forming a complex. It was experimentally observed that when a load was placed on the complex, applying a mild pressure, no cracks were formed due to the above-mentioned strong hydrogen bonds. Additionally, the interaction energy for the complex was calculated to be 29.99 kcal/mol. The strong interaction energy and the large number of strong hydrogen bonds in the complex are responsible for self-healing while testing the material with a mild external pressure.

Figure 8

Molecular structure of poly(U-co-CDL-3-aee) optimized at the B3LYP/6-31g(d,p) level in the gas phase.

Conclusions

In this work, a sustainable poly(U-co-CDL) copolymer was successfully developed using cardanol-based benzoxazines (CDL) and HMDI as a urethane precursor with autonomous self-healing ability. Cardanol-based benzoxazines were synthesized by the Mannich reaction using cardanol, AEE or 3AP, and paraformaldehyde. The structures of CDL monomers were confirmed by FT-IR and NMR. The developed CDL and poly(U-co-CDL) matrices were checked for their thermal stability. The self-healing behavior was demonstrated on cured films. The autonomous self-healing ability was due to the supramolecular attraction and inter/intramolecular hydrogen-bond interactions in the poly(U-co-CDL) matrices, which was revealed theoretically using DFT studies. The developed renewable, eco-friendly, and cost-effective poly(U-co-CDL) copolymer materials are useful for further expanding the use of benzoxazines in various high-performance applications with good reliability and long shelf-life.

Experimental Section

Materials

2-(2-Aminoethoxy) ethanol (aee), 3-aminopropanol (3-ap), and hexamethylene diisocyanate (HMDI) were procured from Sigma-Aldrich, India. Paraformaldehyde, 1,4-dioxane, ethyl acetate, anhydrous sodium sulfate (Na2SO4), ethanol, dichloromethane, and sodium hydroxide (NaOH) were purchased from Sisco Research Laboratories (SRL), India, and were used without further purification. Thanks are due to Satya Cashew Chemicals (P) Ltd., for providing cardanol as a complimentary sample.

Measurements

FT-IR spectra of benzoxazine and polybenzoxazine samples were obtained with an Agilent Cary 630 ATR spectrometer. About 10 mg of the sample was loaded in the ATR spectrometer, with a minimum of 16 scans collected for each sample at a resolution of ±4 cm–1. 1H NMR spectra were recorded with a Bruker 400 MHz using an 8000 Hz spectral width, a relaxation delay of 3.5 s, a pulse width of 45, 32 K data points, CDCl3 as a solvent, and tetramethylsilane (TMS) as an internal standard, with a minimum of 32 scans collected for each sample. Differential scanning calorimetry (DSC) measurements were carried out using a Hitachi DSC 7020. The instrument was calibrated with indium supplied by Hitachi. About 7–10 mg of the sample was used, and the thermograms were recorded from RT to 300 °C under N2 (60 mL/min) at a heating rate of 10 °C/min. Thermogravimetric analysis (TGA) was carried out with a Hitachi STA7000 series. The instrument was calibrated with calcium oxalate and aluminum supplied by Hitachi. About 10 mg of the samples was taken for each analysis under N2 (60 mL/min) and at a heating rate of 20 °C/min.

Synthesis of Cardanol Benzoxazine Monomers (CDL)

Synthesis of cardanol benzoxazine monomers (CDL) (Scheme ) was carried out by the following route: about 2 moles of paraformaldehyde were mixed with 1 mole of cardanol under vigorous stirring, 1 mole of different amine compounds (aee and 3-ap) dissolved in ethyl acetate was added, and then the temperature was increased to reflux. Stirring was continued at the same temperature for 48 h until the mixture became homogeneous. The product obtained was diluted with EA and filtered to remove unreacted materials and the organic layer was washed 3–4 times with 1 N NaOH, followed by distilled water. The organic layer was dried over anhydrous Na2SO4 and then the solvent was evaporated in a rotary evaporator and the hydroxyl-terminated cardanol benzoxazine monomers were stored at RT.

Scheme 1

Synthesis of Cardanol-Based Benzoxazine Monomers (CDL-aee and CDL-3-ap)

Development of Self-Healing Cardanol-Based Polybenzoxazine Matrices

Self-healing cardanol-based polybenzoxazine matrices (Scheme ) were prepared by the simple thermal curing ring-opening polymerization technique using cardanol benzoxazine (CDL-aee and CDL-3-ap) and HMDI. In a 50 mL round-bottom flask, CDL-aee and CDL-3-ap (1 mole) were dissolved in 10 mL of 1,4-dioxane, and then HMDI in 1,4-dioxane (1 mole) was added slowly under a N2 atmosphere at room temperature. Further, the reaction was stirred continuously for 6–8 h until the solution became viscous in nature. Then, the viscous solution was poured into a silane-treated glass plate and kept overnight at 45 °C for solvent evaporation. Then, a thermal curing cycle was performed at 60, 80, 100, 120, 140, and 160 °C for 1 h each and postcured at 180 °C for 2 h. Finally, the red brown poly(U-co-CDL-aee) and poly(U-co-CDL-3-ap) films were peeled off from the Petri dish and utilized for further characterizations.

Scheme 2

Schematic Representation of Ring-Opening Polymerization and Copolymerization of Poly(U-co-CDL-aee) Materials with Possible Self-Healing Ability by Supramolecular H-Bonding Interactions