Flexible Epoxy Resins Formed by Blending with the Diblock Copolymer PEO-b-PCL and Using a Hydrogen-Bonding Benzoxazine as the Curing Agent

In this study, we enhanced the toughness of epoxy resin by blending it with the diblock copolymer poly(ethylene oxide–b–ε-caprolactone) (PEO-b-PCL) with a benzoxazine monomer (PA-OH) as the thermal curing agent. After thermal curing, Fourier transform infrared spectroscopy revealed that intermolecular hydrogen bonding existed between the OH units of the epoxy–benzoxazine copolymer and the C–O–C (C=O) units of the PEO (PCL) segment. Differential scanning calorimetry and dynamic mechanical analysis revealed that the glass transition temperature and storage modulus of the epoxy–benzoxazine matrix decreased significantly upon increasing the concentration of PEO-b-PCL. The Kwei equation predicted a positive value of q, consistent with intermolecular hydrogen bonding in this epoxy–benzoxazine/PEO-b-PCL blend system. Scanning electron microscopy revealed a wormlike structure with a high aspect ratio for PEO-b-PCL as the dispersed phase in the epoxy–benzoxazine matrix; this structure was responsible for the improved toughness.

Polybenzoxazine derivatives are new thermosetting resins with excellent thermal and mechanical properties, low water absorption, and low surface free energies [33][34][35]. Similar to other thermosetting resins, benzoxazine resins also have the shortcoming of being brittle. Furthermore, the phenolic units produced during ring opening polymerization of benzoxazine monomers can react further with epoxy units at certain temperatures to form epoxy-benzoxazine copolymer matrices; it is not necessary to add any catalyst for epoxy thermal curing, because the phenolic units can perform this role for ring opening polymerization [36,37]. Strong intramolecular hydrogen bonding between the hydroxyl (OH) units and the nitrogen atoms strongly influences the properties of polybenzoxazines [30], but there is a drawback in that intermolecular hydrogen bonding can occur with other polymers after thermal curing. As a result, most epoxies and polybenzoxazines usually form immiscible or phase-separated blends with homopolymers (e.g., PCL or PEO [38,39]), and form miscible blends only with poly(N-vinylpyrrolidone) (PVP) as a result of strong hydrogen bonding [40].
To maintain strong intramolecular hydrogen bonding within a polybenzoxazine, we wondered whether incorporating another OH group into the benzoxazine monomer would allow the formation of an additional set of hydrogen bonding interactions with other homopolymers, and thereby solve the miscibility problem. Previously, we synthesized the monomer PA-OH from 4-hydroxybenzyl alcohol, CH2O, and aniline ( Figure 1). In this study, we prepared a flexible epoxy resin through blending with the diblock copolymer PEO-b-PCL and using this benzoxazine monomer as the curing agent ( Figure 2). The OH units of the epoxy and the polybenzoxazine could form hydrogen bonds with the ether units of the PEO segment or the C=O units of the PCL segment. Herein, we discuss the hydrogen bonding interactions, miscibility, mechanical properties, and phase behavior of these systems.

Flexible Epoxy Resin
The epoxy resin, PA-OH (curing agent), and PEO-b-PCL (flexibility agent) were dissolved in THF at the desired ratio. After the solution had become homogenous, the solvent was evaporated slowly and the residue was vacuum-dried overnight at room temperature. The epoxy-benzoxazine resins were cured using the following temperature profile: 110 °C for 3 h, 160 °C for 2 h, 180 °C for 2 h, 200 °C for 1 h, 220 °C for 1 h, and 240 °C for 0.5 h (heating rate: 1 °C min −1 ).

Characterization
1 H NMR spectra were measured using a Bruker AM 500 spectrometer (McKinley Scientific, Sparta, NJ, USA), with CDCl3 as the solvent and tetramethylsilane (TMS) as the external standard. The molecular weight and polydispersity of the PEO-b-PCL diblock copolymer were determined through gel permeation chromatography (GPC) using a Waters 510 high-performance liquid chromatography (HPLC) system and DMF as the eluent (flow rate: 0.6 mL min -1 ). The thermal behavior of the epoxy-benzoxazine resins was examined through differential scanning calorimetry (DSC) using a TA-Q20 instrument (TA Instrument, New Castle, DE, USA) operated over the temperature range from −90 to +240 °C under a N2 atmosphere (heating rate: 20 °C min -1 ). For nonisothermal crystallization experiments, the epoxy-benzoxazine resins with various amounts of

Flexible Epoxy Resin
The epoxy resin, PA-OH (curing agent), and PEO-b-PCL (flexibility agent) were dissolved in THF at the desired ratio. After the solution had become homogenous, the solvent was evaporated slowly and the residue was vacuum-dried overnight at room temperature. The epoxy-benzoxazine resins were cured using the following temperature profile: 110 • C for 3 h, 160 • C for 2 h, 180 • C for 2 h, 200 • C for 1 h, 220 • C for 1 h, and 240 • C for 0.5 h (heating rate: 1 • C min −1 ).

Characterization
1 H NMR spectra were measured using a Bruker AM 500 spectrometer (McKinley Scientific, Sparta, NJ, USA), with CDCl 3 as the solvent and tetramethylsilane (TMS) as the external standard. The molecular weight and polydispersity of the PEO-b-PCL diblock copolymer were determined through gel permeation chromatography (GPC) using a Waters 510 high-performance liquid chromatography (HPLC) system and DMF as the eluent (flow rate: 0.6 mL min -1 ). The thermal behavior of the epoxy-benzoxazine resins was examined through differential scanning calorimetry (DSC) using a TA-Q20 instrument (TA Instrument, New Castle, DE, USA) operated over the temperature range from −90 to +240 • C under a N 2 atmosphere (heating rate: 20 • C min -1 ). For nonisothermal crystallization experiments, the epoxy-benzoxazine resins with various amounts of PEO-b-PCL were first annealed at 240 • C for 5 min, and then the crystallization exotherm was obtained at temperatures down to -90 • C (cooling rate: 5 • C min -1 ). Fourier transform infrared (FTIR) spectra were measured using a Bruker Tensor-27 spectrophotometer (Billerica, MA, USA); 32 scans at a resolution of 4 cm -1 were collected using the KBr plate method. Field emission scanning electron microscopy (FE-SEM) was conducted using a JEOL JSM-7610F scanning electron microscope (JEOL, Tokyo, Japan); the samples were subjected to Pt sputtering for 2 min prior to measurement. The dynamic mechanical behavior of the samples was investigated using a PerkinElmer D8000 analyzer (PerkinElmer, Taipei, Taiwan); the cured samples were polished to approximately 30.0 × 13.0 × 3.0 mm 3 , and then the mechanical properties were determined at temperatures from -100 to +250 • C at a frequency of 1 Hz (heating rate: 2 • C min −1 ). PEO-b-PCL were first annealed at 240 °C for 5 min, and then the crystallization exotherm was obtained at temperatures down to -90 °C (cooling rate: 5 °C min -1 ). Fourier transform infrared (FTIR) spectra were measured using a Bruker Tensor-27 spectrophotometer (Billerica, MA, USA); 32 scans at a resolution of 4 cm -1 were collected using the KBr plate method. Field emission scanning electron microscopy (FE-SEM) was conducted using a JEOL JSM-7610F scanning electron microscope (JEOL, Tokyo, Japan); the samples were subjected to Pt sputtering for 2 min prior to measurement. The dynamic mechanical behavior of the samples was investigated using a PerkinElmer D8000 analyzer (PerkinElmer, Taipei, Taiwan); the cured samples were polished to approximately 30.0 × 13.0 × 3.0 mm 3 , and then the mechanical properties were determined at temperatures from -100 to +250 °C at a frequency of 1 Hz (heating rate: 2 °C min −1 ).

Analyses of Epoxy-Benzoxazine/PEO-b-PCL Mixtures.
DSC is generally applicable for determining the miscibility of polymer blend systems. Figure 4 displays the second heating scans of epoxy-benzoxazine/PEO-b-PCL blends of various compositions after thermal curing. For a 50:50 weight percentage blend of the epoxy and PA-OH (curing agent), the glass transition temperature was approximately 153 • C (Figure 4b). We used this composition to prepare blends featuring different amounts of PEO-b-PCL. The glass transition temperature of the epoxy-benzoxazine matrix decreased upon increasing the PEO-b-PCL composition, because of the lower glass transition temperature of the diblock copolymer.

Analyses of Epoxy-Benzoxazine/PEO-b-PCL Mixtures.
DSC is generally applicable for determining the miscibility of polymer blend systems. Figure 4 displays the second heating scans of epoxy-benzoxazine/PEO-b-PCL blends of various compositions after thermal curing. For a 50:50 weight percentage blend of the epoxy and PA-OH (curing agent), the glass transition temperature was approximately 153 °C (Figure 4b). We used this composition to prepare blends featuring different amounts of PEO-b-PCL. The glass transition temperature of the epoxy-benzoxazine matrix decreased upon increasing the PEO-b-PCL composition, because of the lower glass transition temperature of the diblock copolymer. Furthermore, the melting ( Figure 4a) and crystallization ( Figure 5) temperatures of the PCL and PEO block segments both decreased upon increasing the epoxy-benzoxazine concentration, due to hydrogen bonding interactions inducing thermodynamic and morphological effects. As a result, the melting temperatures for the PEO and PCL block segments were depressed in the presence of the epoxy-benzoxazine resin in this study. Because the glass transition temperatures of PCL and PEO are very close (Tg = −60 °C), the miscibility of PCL and PEO was difficult to determined based solely on the values of Tg. We observed, however, that the values of Tg of PEO-b-PCL shifted to higher temperatures upon increasing the epoxy-benzoxazine concentrations. In general, benzoxazine/PEO and benzoxazine/PCL blends usually form phase-separated and immiscible systems because of the absence of strong hydrogen bonding in these blend systems; in this study, however, we used the benzoxazine PA-OH to introduce additional OH groups into the epoxy matrix to form intermolecular hydrogen bonds with the C=O units of PCL and the ether units of PEO (Figure 2b). Furthermore, the melting ( Figure 4a) and crystallization ( Figure 5) temperatures of the PCL and PEO block segments both decreased upon increasing the epoxy-benzoxazine concentration, due to hydrogen bonding interactions inducing thermodynamic and morphological effects. As a result, the melting temperatures for the PEO and PCL block segments were depressed in the presence of the epoxy-benzoxazine resin in this study. Because the glass transition temperatures of PCL and PEO are very close (T g = −60 • C), the miscibility of PCL and PEO was difficult to determined based solely on the values of T g . We observed, however, that the values of T g of PEO-b-PCL shifted to higher temperatures upon increasing the epoxy-benzoxazine concentrations. In general, benzoxazine/PEO and benzoxazine/PCL blends usually form phase-separated and immiscible systems because of the absence of strong hydrogen bonding in these blend systems; in this study, however, we used the benzoxazine PA-OH to introduce additional OH groups into the epoxy matrix to form intermolecular hydrogen bonds with the C=O units of PCL and the ether units of PEO (Figure 2b).
We used dynamic mechanical analyzer (DMA) to determine the glass transition behavior and mechanical properties of the epoxy-benzoxazine/PEO-b-PCL blend systems after thermal curing ( Figure 6). The pure epoxy-benzoxazine thermosetting copolymer displayed a value of T g of approximately 160 • C, based on its tan δ-temperature curve. The values of T g determined using DMA were generally higher than those obtained through DSC analyses, as displayed in Figure 4. Nevertheless, similar to the results from the DSC analyses, the value of T g of the epoxy-benzoxazine matrix decreased upon increasing the content of PEO-b-PCL because of the lower glass transition temperature of the diblock copolymer. Because hydrogen bonding presumably existed in this system, we used the Kwei equation to predict the values of T g of the epoxy-benzoxazine copolymer blended with PEO-b-PCL [42]: where W 1 is the weight fraction of PEO-b-PCL, W 2 is the weight fraction of the epoxy-benzoxazine copolymer, T g1 is the glass transition temperature of PEO-b-PCL (both at -60 • C), T g2 is the glass transition temperature of the epoxy-benzoxazine copolymer (160 • C), and k and q are fitting constants. Figure 7 reveals that the values of T g decreased upon increasing the concentration of PEO-b-PCL, suggesting that the flexibility might indeed be enhanced through this approach. In addition, we determined the values of k and q to be 1 and 30, respectively, based on Figure 7. The positive value of q indicates that, after thermal curing, the intermolecular hydrogen bonding between the OH units of the epoxy-benzoxazine copolymer and the ether and C=O groups of the PEO-b-PCL diblock copolymer was stronger than the self-association hydrogen bonding of the OH units of the epoxy-benzoxazine copolymer. We used dynamic mechanical analyzer (DMA) to determine the glass transition behavior and mechanical properties of the epoxy-benzoxazine/PEO-b-PCL blend systems after thermal curing ( Figure 6). The pure epoxy-benzoxazine thermosetting copolymer displayed a value of Tg of approximately 160 °C, based on its tan δ-temperature curve. The values of Tg determined using DMA were generally higher than those obtained through DSC analyses, as displayed in Figure 4. Nevertheless, similar to the results from the DSC analyses, the value of Tg of the epoxy-benzoxazine matrix decreased upon increasing the content of PEO-b-PCL because of the lower glass transition temperature of the diblock copolymer. Because hydrogen bonding presumably existed in this system, we used the Kwei equation to predict the values of Tg of the epoxy-benzoxazine copolymer blended    Figure 8a presents the OH stretching signals of the epoxy-benzoxazine/PEO-b-PCL blend system after thermal curing; these signals were sensitive to the presence of intermolecular hydrogen bonding. The spectrum of the pure epoxy-benzoxazine copolymer featured two major peaks: one representing the free OH units (at 3560 cm -1 ), and the other, a broad band at 3420 cm -1 , representing the self-associated OH units. The intensity of the signal for the free OH units gradually decreased upon increasing the PEO-b-PCL concentration, while the broad band slightly shifted to 3435 cm -1 , suggesting that the OH units of the pure epoxy-benzoxazine copolymer interacted with both the C=O units of the PCL segment and the ether units of the PEO segment. Furthermore, the signal for the C=O units of the PCL segment was also sensitive to the intermolecular hydrogen bonding, as revealed in Figure 8b. The spectrum of the pure PCL segment featured two signals for the C=O groups: one corresponding to the amorphous conformation or free C=O units (1734 cm -1 ) and the other to the crystalline conformation (1724 cm -1 ) [43]. The intensity of the signal for the crystalline peak gradually decreased upon increasing the content of the epoxy-benzoxazine copolymer, eventually disappearing at 80 wt %-similar to our observations from the DSC analyses. A signal for the hydrogen-bonded C=O units of the PCL segment appeared near 1708 cm -1 at relatively higher concentrations of the epoxy-benzoxazine copolymer. Thus, a higher fraction of OH groups provided a higher number of hydrogen-bonded C=O units, as expected. Figure 8c displays the ether stretching bands of the PEO segments; the spectrum of the pure PEO-b-PCL exhibited this absorption at 1116 cm -1 , with this peak shifting to 1107 cm -1 upon increasing the concentration of the epoxy-benzoxazine copolymer [44,45], consistent with intermolecular hydrogen bonding occurring between the ether units of the PEO segment and the OH groups of the epoxy-benzoxazine copolymer after thermal curing (Figure 2b).
Polymers 2019, 11 FOR PEER REVIEW 8 After thermal curing, FTIR spectroscopy ( Figure 8) provided evidence for intermolecular hydrogen bonding between the OH units of the epoxy-benzoxazine copolymer and the ether or C=O groups of PEO-b-PCL. Figure 8a presents the OH stretching signals of the epoxy-benzoxazine/PEOb-PCL blend system after thermal curing; these signals were sensitive to the presence of intermolecular hydrogen bonding. The spectrum of the pure epoxy-benzoxazine copolymer featured two major peaks: one representing the free OH units (at 3560 cm -1 ), and the other, a broad band at 3420 cm -1 , representing the self-associated OH units. The intensity of the signal for the free OH units gradually decreased upon increasing the PEO-b-PCL concentration, while the broad band slightly shifted to 3435 cm -1 , suggesting that the OH units of the pure epoxy-benzoxazine copolymer interacted with both the C=O units of the PCL segment and the ether units of the PEO segment. Furthermore, the signal for the C=O units of the PCL segment was also sensitive to the intermolecular hydrogen bonding, as revealed in Figure 8b. The spectrum of the pure PCL segment featured two signals for the C=O groups: one corresponding to the amorphous conformation or free C=O units (1734 cm -1 ) and the other to the crystalline conformation (1724 cm -1 ) [43]. The intensity of the signal for the crystalline peak gradually decreased upon increasing the content of the epoxy-benzoxazine copolymer, eventually disappearing at 80 wt %-similar to our observations from the DSC analyses. A signal for the hydrogen-bonded C=O units of the PCL segment appeared near 1708 cm -1 at relatively higher concentrations of the epoxy-benzoxazine copolymer. Thus, a higher fraction of OH groups provided a higher number of hydrogen-bonded C=O units, as expected. Figure 8c displays the ether stretching bands of the PEO segments; the spectrum of the pure PEO-b-PCL exhibited this absorption at 1116 cm -1 , with this peak shifting to 1107 cm -1 upon increasing the concentration of the epoxy-benzoxazine copolymer [44,45], consistent with intermolecular hydrogen bonding occurring between the ether units of the PEO segment and the OH groups of the epoxy-benzoxazine copolymer after thermal curing (Figure 2b).

Mechanical Properties of Epoxy-Benzoxazine/PEO-b-PCL Mixtures
The storage moduli of the epoxy-benzoxazine/PEO-b-PCL blends after thermal curing indicated ( Figure 9) significantly improved toughness relative to that of PEO-b-PCL. The storage modulus deceased upon increasing concentration of PEO-b-PCL; it decreased significantly to 1.62 × 10 8 Pa at 25 • C when the content of PEO-b-PCL was 20 wt %-recall that, at this composition, a significant decrease occurred in the value of T g : from 160 • C for the pure epoxy-benzoxazine copolymer to 113 • C. Bates et al. proposed that the nature of the self-assembled nanostructures affected the mechanical properties of a thermoset resin blended with a block copolymer [19].

Mechanical Properties of Epoxy-Benzoxazine/PEO-b-PCL Mixtures
The storage moduli of the epoxy-benzoxazine/PEO-b-PCL blends after thermal curing indicated ( Figure 9) significantly improved toughness relative to that of PEO-b-PCL. The storage modulus deceased upon increasing concentration of PEO-b-PCL; it decreased significantly to 1.62  10 8 Pa at 25 °C when the content of PEO-b-PCL was 20 wt %-recall that, at this composition, a significant decrease occurred in the value of Tg: from 160 °C for the pure epoxy-benzoxazine copolymer to 113 °C. Bates et al. proposed that the nature of the self-assembled nanostructures affected the mechanical properties of a thermoset resin blended with a block copolymer [19]. Therefore, we used scanning electron microscopy (SEM) to investigate the morphologies and degrees of microphase separation in our epoxy-benzoxazine/PEO-b-PCL blends. We etched the thermosetting samples with NaOH to remove the PEO-b-PCL domains and maintain the epoxybenzoxazine matrix. The pure epoxy-benzoxazine copolymer possessed a smooth surface morphology, as expected in the absence of PEO-b-PCL (Figure 10a). Upon increasing the PEO-b-PCL concentration, several porous structures were evident, with dimensions of approximately 100-200 nm at 5 and 10 wt % of PEO-b-PCL within the epoxy-benzoxazine copolymer matrix (Figures 8b and  c). When the PEO-b-PCL content increased to 20 wt %, its domains changed from a dispersed phase to a semi-continuous phase, such that the PEO-b-PCL particles possessed an irregular wormlike structure (Figure 10d). Further increasing the PEO-b-PCL content to 30-50 wt % resulted in the epoxy-benzoxazine copolymer matrix changing from a continuous phase to a dispersed phase, with the particle size decreasing upon increasing the PEO-b-PCL concentration (Figures 8e-g). The toughening effect is known to depend significantly on the size and shape of the dispersed phase, and on the intermolecular interactions of the dispersed phase within the epoxy matrix; the wormlike micelle structure usually has a high aspect ratio, with an optimized length scale providing superior toughness over that of other self-assembled nanostructures, based on the toughness mechanism [19,20]. As a result, we conclude that the irregular wormlike structure of PEO-b-PCL dispersed in the epoxy-benzoxazine matrix (Figure 2c) was responsible for the improved toughness observed in our present DMA analyses. Figure 10h presents a flexible epoxy resin formed after blending with 30 wt Therefore, we used scanning electron microscopy (SEM) to investigate the morphologies and degrees of microphase separation in our epoxy-benzoxazine/PEO-b-PCL blends. We etched the thermosetting samples with NaOH to remove the PEO-b-PCL domains and maintain the epoxybenzoxazine matrix. The pure epoxy-benzoxazine copolymer possessed a smooth surface morphology, as expected in the absence of PEO-b-PCL (Figure 10a). Upon increasing the PEO-b-PCL concentration, several porous structures were evident, with dimensions of approximately 100-200 nm at 5 and 10 wt % of PEO-b-PCL within the epoxy-benzoxazine copolymer matrix (Figure 8b,c). When the PEO-b-PCL content increased to 20 wt %, its domains changed from a dispersed phase to a semi-continuous phase, such that the PEO-b-PCL particles possessed an irregular wormlike structure (Figure 10d). Further increasing the PEO-b-PCL content to 30-50 wt % resulted in the epoxy-benzoxazine copolymer matrix changing from a continuous phase to a dispersed phase, with the particle size decreasing upon increasing the PEO-b-PCL concentration (Figure 8e-g). The toughening effect is known to depend significantly on the size and shape of the dispersed phase, and on the intermolecular interactions of the dispersed phase within the epoxy matrix; the wormlike micelle structure usually has a high aspect ratio, with an optimized length scale providing superior toughness over that of other self-assembled nanostructures, based on the toughness mechanism [19,20]. As a result, we conclude that the irregular wormlike structure of PEO-b-PCL dispersed in the epoxy-benzoxazine matrix (Figure 2c) was responsible for the improved toughness observed in our present DMA analyses. Figure 10h presents a flexible epoxy resin formed after blending with 30 wt % of PEO-b-PCL; this thin film displayed bendability and recoverable properties-features that are difficult to observe for pure epoxy and polybenzoxazine resins prepared through the synthesis method.
Polymers 2019, 11 FOR PEER REVIEW 10 % of PEO-b-PCL; this thin film displayed bendability and recoverable properties-features that are difficult to observe for pure epoxy and polybenzoxazine resins prepared through the synthesis method.

Conclusions
We have used DSC, FTIR spectroscopy, DMA, and SEM to investigate the hydrogen bonding interactions, miscibility, and phase behavior of epoxy-benzoxazine/PEO-b-PCL blends. FTIR spectra revealed evidence for intermolecular hydrogen bonding of the ether units of the PEO segments and the C=O units of the PCL segments with the OH groups of the epoxy-benzoxazine copolymer after thermal curing; this behavior was confirmed through DMA analyses and predictions made using the Kwei equation. In addition, the glass transition temperature (DSC) and storage modulus (DMA) of the epoxy-benzoxazine matrix both decreased significantly upon increasing the PEO-b-PCL concentrations. SEM revealed that a wormlike structure, with a high aspect ratio, for the PEO-b-PCL block copolymer as the dispersed phase in the epoxy-benzoxazine matrix was responsible for the improved toughness of the blend.