A simple method to introduce phosphoester groups into a polybenzoxazine backbone as multifunctional modifiers

Abstract

Phosphoester groups were conveniently introduced into benzoxazine monomers by chemical bonding through nucleophilic reaction of a benzoxazine ring with diethyl phosphite (DEP). The reaction process was monitored by phosphorus 31 nuclear magnetic resonance (³¹P-NMR). The characteristic peaks of ³¹P-NMR corresponding to DEP disappeared entirely when the reaction was run at 90 °C for 24 h, which implied the completion of the nucleophilic reaction. The products obtained then underwent direct ring-opening polymerization to produce DEP-modified polybenzoxazines. The introduction of DEP into polybenzoxazines led to a reduction of the onset of the polymerization temperature The adsorption ability for Cd(II) was improved because the phosphoester group provided an efficient chelating site for heavy metal ions.

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1. Introduction

As ideal alternatives to traditional phenol-formaldehyde (PF) thermosets, polybenzoxazines (PTBOZ) have attracted an increasing interest because of their superior properties. Use of polybenzoxazines overcomes some problems of using conventional PF thermosets and they exhibit excellent properties, such as high thermal stability, low water absorption, resistance to flames and chemicals, as well as limited volume shrinkage during the curing process.^1,2 Besides these advantages, the ring-opening polymerization of benzoxazine monomers is a thermally induced curing reaction which can be accomplished without any initiator or curing agent.^3–5 However, polybenzoxazines still have a few shortcomings and limitations, for example, unsatisfactory processability, brittleness and high polymerization temperatures, which hinder their practical applications.^6–8

The most important characteristic of polybenzoxazines is the variability of their structure and function which results from the molecular design flexibility of the monomer.^9–21 For example, to reduce brittleness and improve toughness of the polybenzoxazine, the concept of side-, main-, and end-chain polymeric benzoxazine precursors was suggested by various research groups, and these precursor polymers showed excellent film forming properties both before and after the curing reaction.^22–25 Introducing phosphoester groups into the polymer chain as an “inherent plasticizer” by chemical bonding could overcome the “transportation problem”.^26–29 Furthermore, difunctional benzoxazine monomers with second polymerizable sites which exhibited high thermal and mechanic stability, high resistance to solvents, moisture and chemicals had been designed and prepared, which could induce further crosslinking reactions to generate three-dimensional networks. These second curing groups included allyl,^13,30 acetylenyl,^31–34 propargyl,^35,36 and nitrile groups.³⁷In order to improve the curing process, active hydrogen containing groups such as carboxylic³⁸ and phenolic groups³⁹ were incorporated into the benzoxazine monomers, which could effectively decrease the curing temperature because of their catalytic effect on ring-opening polymerization. Recently, Gorodisher et al. reported that benzoxazines could undergo catalytic ring-opening reaction initiated by thiols at ambient conditions.⁴⁰ The process was called the “catalytic opening of the lateral benzoxazine rings by thiols” reaction (COLBERT) which proceeded via a two-step, acid catalyzed nucleophilic addition.⁴¹ Subsequently, Yagci et al.⁴² demonstrated that crosslinked but additionally curable, soft benzoxazine films could be prepared by simultaneously photo-induced thiol–ene and COLBERT reactions using difunctional thiol and diallyl benzoxazines. The polybenzoxazine precursors obtained still contained benzoxazine and allyl groups, which were readily available for the subsequent curing.

In this paper, we describe the preparation of phosphoester-modified polybenzoxazines based on the molecular design flexibility of the benzoxazine monomer. Phosphoester containing benzoxazine monomers were prepared firstly by a nucleophilic addition reaction of tri-functional benzoxazine (TBOZ) with a calculated amount of diethyl phosphite (DEP). Then DEP-modified polybenzoxazines were obtained by cationic ring-opening polymerization. The properties of the DEP-modified polybenzoxazine were comprehensively investigated by evaluating the influence of the introduction of DEP. Finally, the Cd(II) adsorption ability of this DEP-modified polybenzoxazine was studied because the phosphoester group could act as an efficient chelating site for many heavy metal ions.

2. Experimental

2.1. Chemicals

1,1,1-tris(4-hydroxyphenyl)ethane (98%) was purchased from TCI (Shanghai) Development Co., Ltd. Paraformaldehyde (95%), aniline (99%), DEP (99%), sodium hydroxide (99%) and cadmium sulfate (99%) were purchased from the Sinopharm Chemical Reagent Co., Ltd., China. 1,4-Dioxane (99%) was also purchased from Sinopharm and purified by distillation over calcium hydride (Sinopharm) and stored over molecular sieves. Standard stock solutions of Cd(II) were prepared by dissolving cadmium sulfate in deionized water.⁴³

Synthesis of the tri-functional benzoxazine monomer TBOZ. TBOZ was prepared according to a previously reported method⁴⁴ with a few modifications. The typical procedure was as follows: aniline (8.21 g, 0.09 mol) and paraformaldehyde (5.94 g, 0.198 mol) were mixed in a 250 mL three-necked flask and stirred in an ice bath for 1 h. Then 150 mL of toluene, and 1,1,1-tris(4-hydroxyphenyl)ethane (9.19 g, 0.03 mol) were added into the mixture with stirring. The mixture was gently heated to 110 °C and then refluxed for 10 h. Water produced during the reaction was separated out by azeotropic distillation with toluene. At the end of the reaction, toluene was removed by rotary evaporation and the residues were extracted with chloroform. The chloroform solution was successively washed with 0.5 M NaOH and distilled water, and then dried with anhydrous Na₂SO₄ over night. After filtration the filtrate was concentrated and dropped into petroleum ether with stirring. A white powder was collected and dried under vacuum at 50 °C for 48 h (Scheme 1).

Scheme 1 Synthesis of TBOZ and its nucleophilic reaction with DEP.

Yield: 91.6%. Melting point: 57–59 °C. ¹H-NMR (deuterated chloroform, trimethyl silane (TMS) ppm): 6.64–6.93 (24H, Ar–H), 4.52 (6H, Ar–CH₂–N), 5.34 (6H, O–CH₂–N), 2.04 (3H, –CH₃). ¹³C-NMR (deuterated dimethyl sulfoxide (DMSO-d₆), TMS, ppm): 152.4 (C–O), 148.3 (C–N), 141.7 (C–O), 129.6 (CH), 128.3 (CH), 126.9 (C), 120.8 (CH), 117.6 (CH), 115.9 (CH), 50.8 (Ar–CH₂–N), 78.9 (O–CH₂–N), 49.6 (C) [Fig. S1†].

Nucleophilic reaction of TBOZ with DEP. The example given is for the synthesis of (1/1)TBOZ/DEP ((1/1)TBOZ/DEP means molar ratio of TBOZ to DEP equals 1/1) TBOZ (3.29 g, 5 mmol) and DEP (0.64 mL, 5 mmol) were added into a 500 mL three-necked flask containing 200 mL of dried dioxane. The reaction mixture was gently heated to 90 °C and refluxed at this temperature for 24 h. The reaction was monitored by ³¹P-NMR and was stopped after the chemical shift corresponding to DEP disappeared. Then all the volatiles were removed under reduced pressure and the residual yellow precipitate was dried under vacuum at 40 °C for 48 h to obtain the desired product with a quantitative yield. The compound was used as obtained for the next studies without any further purification.

¹H-NMR (DMSO-d₆, TMS, ppm): 6.70–7.21 (Ar–H, m), 5.38 (O–CH₂–N, s), 4.53 (C–CH₂–N, s), 4.18 (N–CH₂–P, s), 3.75 (N–CH₂, s), 4.01–4.07 (O–CH₂, q), 1.21–1.24 (CH₃, t), 2.00 (C–CH₃, s), 9.36 (Ar-OH, s) [Fig. 2].

The synthetic procedures for (1/0.5)TBOZ/DEP and (1/2)TBOZ/DEP were the same as those for producing model compound II (Scheme 1).

Preparation of polybenzoxazines. Samples for dynamic mechanic analysis (DMA), impact resistance test, dielectric measurement and humidity absorption test were prepared as follows: TBOZ or DEP-modified TBOZ was added to a tinfoil mold and gently melted at 160 °C for 1 h and then at 180 °C for a further hour both times under vacuum. Later, the samples were heated stepwise and cured in a temperature-controlled oven at 180 °C for 8 h, followed by further heating at 200 °C for 8 h, then the sample was post-cured at 220 °C for 2 h and further cured at 240 °C for a further 2 h. Then, samples were slowly cooled down to room temperature to prevent cracking. Samples for batch adsorption of Cd(II) were prepared by a wet-milling process with the ratio of polybenzoxazines:alumina ball: ethyl alcohol equal to 2 : 50 : 10 for 8 h,⁴⁵ followed by sieving with an 80 mesh sieve.

2.2. Characterization

The structure of TBOZ and other compounds were verified by using solution state proton (¹H) and phosphorus (³¹P) nuclear magnetic resonance spectroscopy (NMR) using a Bruker AV400NMR spectrometer at a proton frequency of 400 MHz as well as the corresponding phosphorus frequency at room temperature. Chemical shifts were reported in ppm downfield from SiMe₄. Thermal transitions were monitored with a differential scanning calorimeter (DSC) using a NETZSCH Instruments Model 204 F1 with a scan rate of 10 °C min⁻¹ over a temperature range of 30–300 °C and nitrogen flow rate of 20 mL min⁻¹. Thermogravimetric analysis (TGA) was performed with a NETZSCH Instruments High Resolution STA 409 PC thermogravimetric analyzer that was purged with nitrogen at a flow rate of 70 mL min⁻¹. A heating rate of 20 °C min⁻¹ was used and a scanning range was from room temperature (RT) to 1000 °C. Infrared (IR) spectra were recorded using a Bruker VERTEX 70 Fourier transform infrared spectrometer (FT-IR) under ambient conditions. The potassium bromide disks were prepared by compressing the powder. Mechanic properties were measured using a PerkinElmer, Diamond DMA apparatus. Specimens (50 × 10 × 1.0 mm) were tested in a three point bending mode. The thermal transitions were studied in the range of 20–200 °C at a heating rate of 4 °C min⁻¹ and at a fixed frequency of 1 Hz.

2.3. Gel content

The gel fraction was determined by a standard extraction method⁴⁶ using chloroform. The process involved a continuous extraction with chloroform in a 500 mL round bottom flask for 72 h until constant weight obtained. After the extraction, the samples were dried and the gel content was calculated according to the formula (1), where m_t and m₀ represent the sample weights after and before extraction, respectively.

Impact resistance test. The impact strength of the cured specimens (80 mm × 10 mm × 4 mm) was measured with a Shenzhen Sans Testing Machine Co. Ltd, ZBC pendulum impact testing machine at 25 °C according to Chinese standard of GB/T 1043-1993 (Plastics Determination of charpy impact strength of rigid materials). The material's response (maximum stress or fracture) to the applied load was measured for five samples of each material group and the average values were recorded.⁴⁷

Dielectric measurements. Dielectric constant and dielectric loss were measured at room temperature in an air atmosphere using the two parallel plate modes at 125 Hz–18 MHz using an Agilent 4294A Precision Impedance Analyzer.¹⁴ A sample (about 20 mm × 10 mm × 2 mm) was placed between the two copper electrodes to form a parallel plate capacitor. Prior to each measurement, the sample was dried under vacuum at 100 °C for 3 h.

Humidity absorption. The cured samples were conditioned under vacuum at 90 °C for 20 h before being placed in air (75% and 33% relative humidity (RH)). All these experiments were conducted at room temperature. Then, the weight percentages of humidity absorption of the cured samples were calculated according to formula (2), where W_t and W₀ represent the sample weights after and before (dry sample) humidity absorption, respectively.⁴⁸

Batch adsorption experiments. A batchwise process was used to study the adsorption of Cd(II) from aqueous solutions onto the PTBOZ I (P(1/0.5)TBOZ/DEP), PTBOZ II (P(1/1)TBOZ/DEP) and PTBOZ III (P(1/2)TBOZ/DEP) adsorbents.⁴³ All the adsorption batch experiments were performed at 25 °C and the initial Cd(II) concentration was 100 mg L⁻¹. The mixtures were then centrifuged at 18000 rpm for 30 min, and the residual concentration of Cd(II) was determined using a Varian SpectrAA-55 atomic adsorption spectrometer. The effects of contact time and pH on the adsorption ability were determined. All the adsorption experiments were performed in triplicate, and the means were used in the data analysis. The amount of adsorbed Cd(II) q_t (mg g⁻¹) at any time t was calculated by using the general eqn (3):where C₀ was the initial concentration of Cd(II) (mg L⁻¹), C_t was the Cd(II) solution concentration at any time t (mg L⁻¹), V was the solution volume (L), and m was the adsorbent mass (g).

3. Results and discussion

3.1. Nucleophilic reaction of TBOZ with DEP

The NMR results of TBOZ are shown in Fig. S1.† The proton resonance signals at 4.52 and 5.34 were assigned to the methylene protons of Ar–CH₂–N (6H) and O–CH₂–N (6H) of the benzoxazine ring, respectively. The proton resonance signal at 2.04 was assigned to the methyl proton (3H). The integral area ratio of the methylene protons (6H) and the methyl protons (3H) combined with the ¹³C-NMR spectra of TBOZ indicated that the synthesis of TBOZ was successful.

The nucleophilic addition reaction of TBOZ with DEP was monitored using ³¹P-NMR and the results are presented in Fig. 1 and 2. From Fig. 1, it could be estimated that at a low temperature (70 °C), the addition reaction rate between TBOZ and DEP was slow. After 24 h, the unreacted DEP still existed, which was verified by the two peaks at 6.14 and 10.47 ppm which corresponded to DEP. The reaction rate was speeded up by increasing the reaction temperature. At 90 °C, the addition reaction was completed within 24 h. But further raising the temperature (110 °C) would cause some side reactions because some small peaks appeared (Fig. 1(C)) at about 0 ppm, which might be due to phosphite groups resulting from the hydrolysis of DEP at such conditions.

Fig. 1 ³¹P-NMR spectra of products from the reaction of TBOZ with DEP at (A) 70 °C; (B) 90 °C; (C) 101 °C for different time periods.

Fig. 2 ¹H-NMR spectrum of model compound (1/1)TBOZ/DEP.

It should be pointed out here that when TBOZ reacted with DEP at an equal molar ratio, compound II was not the only product. At these conditions, the product consisted of TBOZ, compounds II, III and IV, certainly compound II was the main product. In order to simplify the preparation procedure, we did not use any further purification of the final product and it was used directly for following studies. Thus, according to the molar ratio of TBOZ and DEP used in the reaction, the corresponding DEP-modified product was called (1/0.5)TBOZ/DEP, (1/1)TBOZ/DEP and (1/2)TBOZ/DEP. The synthetic procedures for (1/0.5)TBOZ/DEP and (1/2)TBOZ/DEP were the same as those used to synthesise model compound II.

The ¹H-NMR spectrum of (1/1)TBOZ/DEP is shown in Fig. 2. Peaks at 4.01–4.07 ppm (O–CH₂) and 1.21–1.24 ppm (-CH₃) verified the successful introduction of DEP to the TBOZ structure. Signals at 4.53 ppm (C–CH₂–N) and 5.38 ppm (O–CH₂–N) for the benzoxazine ring were also observed, confirming that there were still benzoxazine groups present, which could undergo ring-opening polymerization during curing. After nucleophilic reaction the phenolic groups appeared and the signal for the phenolic OH is located at 9.36 ppm. The rest of the signals in Fig. 2 were attributed to P–CH₂–N (4.18 ppm), N–CH₂–Ar (3.75 ppm), aryl (6.70–7.21 ppm), P–O–CH₂ (4.01–4.07 ppm), CH₃ (1.21–1.24) and Ar₃–C–CH₃ (2.00 ppm).

The IR spectra of all the cured samples are shown in Fig. 3. The wide peaks around 3418 cm⁻¹ were attributed to the phenolic groups resulting from ring-opening polymerization. The peaks at 1250 cm⁻¹, 1061 cm⁻¹ and 1380 cm⁻¹ were attributed to P=O, P–O–C and P–C absorption, respectively, these facts further proved that the DEP groups had been successfully introduced into the cured polymer after polymerization.

Fig. 3 FT-IR spectra of polybenzoxazines in the region between (A) 4200 and 2000 cm⁻¹; (B) 2100 and 500 cm⁻¹.

DSC studying the curing behavior and thermal properties of cured polymers. The polymerization behavior of TBOZ, (1/0.5)TBOZ/DEP, (1/1)TBOZ/DEP and (1/2)TBOZ/DEP was monitored by DSC as shown in Fig. 4(A) and Table 1. In order to see the melting endothermic transition (57.5 °C) more clearly, a separate DSC plot is shown in Fig. S2.† It is obvious from Fig. 4(A) that DEP-modification caused dramatic changes to the polymerization characters of TBOZ. It shows a sharp exothermic transition with an onset temperature around 215.8 °C and a peak maximum at 238.1 °C. After nucleophilic addition reaction with DEP, both the onset and peak temperatures of the resulting products ((1/0.5)TBOZ/DEP, (1/1)TBOZ/DEP and (1/2)TBOZ/DEP) shifted to lower temperatures (Please see Table 1 for details). These results were attributed to two factors. Firstly, it was known that the phenolic group could lower the polymerization temperature of the benzoxazine monomer because of its activated hydrogen.³⁸ From Scheme 1, it could be seen that after modification by DEP, a phenolic group appeared and that it would catalyze the curing reaction of newly obtained TBOZ derivatives. With the increase of the molar ratio of TBOZ : DEP, the number of phenolic groups increased simultaneously. For example, the onset and peak temperatures of (1/2)TBOZ/DEP were 98.1 °C and 195.3 °C, respectively. Secondly, Liu et al. proved that the phosphoester groups could also act as a thermally latent catalyst for the curing reaction of benzoxazine monomers.⁴⁹ As we expected, a higher DEP dosage would result in a lower polymerization temperature.

Fig. 4 DSC plots (A) of the four monomers ((a): TBOZ; (b): (1/0.5)TBOZ/DEP; (c): (1/1)TBOZ/DEP; (d): (1/2)TBOZ/DEP) and TGA curves (B) of the corresponding cured polymers.

Table 1 Thermal stability of polybenzoxazines modified with different contents of DEP

Samples	To^a (°C)	Tmax^b (°C)	T5%^c (°C)	T10%^d (°C)	Tmax^e (°C)	Yc^f (%)	LOI^g
a Onset temperature of exothermic peak.b Maximum of the polymerization exotherm.c The temperature for 5% weight loss.d The temperature for 10% weight loss.e Maximum weight loss temperature.f Char yields at 850 °C.g Calculated according to results in the literature.^50
TBOZ	229.7	250.3	315	343	393	44.0	35.1
(1/0.5)TBOZ/DEP	162.7	216.5	298	324	366	38.4	32.9
(1/1.0)TBOZ/DEP	116.8	206.8	276	296	294	31.3	30.0
(1/2.0)TBOZ/DEP	98.1	195.3	263	290	311	24.7	27.4

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In order to estimate the effect of DEP dosage on crosslinking density of the cured polymer, the gel contents of cured TBOZ, (1/\0.5)TBOZ/DEP, (1/1)TBOZ/DEP and (1/2)TBOZ/DEP were measured by solvent-extraction and the results are given in Fig. 5. After curing under the same conditions, the gel contents of PTBOZ, P(1/0.5)TBOZ/DEP, P(1/1)TBOZ/DEP and P(1/2)TBOZ/DEP were 99.79%, 99.63%, 98.35% and 97.06%, respectively. Though the gel content of the cured polymer decreases with the addition of DEP, all the samples showed a high gel content above 97%, this means that highly crosslinked polymers were obtained after the curing reaction. This fact was consistent with the proposal that the reaction product between TBOZ and DEP is a mixture of TBOZ and DEP-modified TBOZ. According to theoretical calculations, when TBOZ : DEP underwent a nucleophilic reaction at a molar ratio of 1 : 2, only compound III had one benzoxazine ring left. In this case, compound III would give a low gel content after the curing reaction. But the experimental data showed the opposite result. Thus, we presume that the reaction product was a mixture and the residual TBOZ and compound II could act as crosslinker during the curing reaction (see Scheme 2).

Fig. 5 Gel content of the polybenzoxazines ((1): PTBOZ; (2): P(1/0.5)TBOZ/DEP; (3): P(1/1)TBOZ/DEP; (4): P(1/2)TBOZ/DEP).

Scheme 2 Polymerization of TBOZ, (1/1)TBOZ/DEP and (1/2)TBOZ/DEP under heating.

The thermal stability of the cured polymer was studied by TGA and the results are shown in Fig. 4(B) and Table 1. Below 250 °C, all the cured polymer showed a similar thermal stability and weight loss could be neglected. The onset degradation temperature (T_5%) of the cured polymer decreases with addition of DEP, and for PTBOZ the onset temperature is 315 °C, for P(1/2)TBOZ/DEP which decreases down to 263 °C. T_10% and T_max showed similar trends. Between 300 °C and 500 °C, all the cured polymer showed a rapid thermal degradation. The char yields of the cured polymer reduced with the increase of the DEP dosage, the value for PTBOZ was 44.0% at 850 °C, whereas the value for P(1/2)TBOZ/DEP was reduced to 24.7%. It was obvious that the thermal stability of the polymer decreased with the introduction of the DEP groups into its structure. As is already known, except for the chemical structure, the crosslinking density is one of the most important factors in determining the thermal stability of a thermosetting polymer. Normally a higher crosslinking density of polymer results in higher thermal stability. For PTBOZ originating from a tri-functional monomer, it possesses the highest crosslinking density of the polymers studied, so it shows the best thermal stability. By introducing the DEP group into TBOZ, some of benzoxazine rings underwent a ring-opening reaction with DEP and the average functionality of the final products gradually declined, which would decrease the crosslinking density of the cured polymers. Meanwhile, DEP would undergo thermal degradation at a lower temperature range compared with polybenzoxazine itself. So this is another reason for the deterioration of the thermal stability of cured polymers. Thus, the limited oxygen index⁵⁰ (LOI) changed from 35.1 for PTBOZ to 27.4 for P(1/2)TBOZ/DEP. But all cured polymers showed good fire retardancy.

Mechanic properties of polybenzoxazine. Fig. 6 shows the typical DMA thermograms of all the cured samples. Though all the samples showed a high storage modulus at RT, the storage modulus decreased after the addition of DEP, for PTBOZ the value was 5.36 GPa, and this value gradually decreased to 5.29 GPa for P(1/0.5)TBOZ/DEP, 4.28 GPa for P(1/1)TBOZ/DEP and 2.69 GPa for P(1/2)TBOZ/DEP. The glass transition temperature was determined by the peak of tan  δ, it showed a similar trend as storage modulus. For example, the T_g of PTBOZ was 199 °C. With the introduction of DEP, the T_g of modified PTBOZ shifted down to 178 °C for P(1/0.5)TBOZ/DEP, 176 °C for P(1/1)TBOZ/DEP and 160 °C for P(1/2)TBOZ/DEP. The previous results proved that the introduction of DEP into benzoxazine monomers had dramatically altered the storage modulus and the T_g of the cured polybenzoxazines. The changes were attributed to the fact that the storage modulus and T_g of the cured polymers mainly depended on the crosslinking density of the cured polymers. By introduction of DEP through a nucleophilic reaction, the average functionality of the monomers decreased from three to one, which resulted in a low crosslinking density.

Fig. 6 Storage moduli (A) and tan  δ curves (B) of polybenzoxazines with different DEP contents.

Impact properties. The dependence of the impact strength of the cured polymers on DEP content is shown in Fig. 7. As expected, the impact strength at RT was appreciably enhanced by the introduction of DEP in to polybenzoxazines. The impact strength of neat PTBOZ was about 2.2 kJ m⁻², the value for P(1/2)TBOZ/DEP increased to 6.2 kJ m⁻². The increase in the impact strength of the DEP-modified PTBOZ was because of the fact that the decrease in crosslinking density of cured resins by the introduction of DEP would increase the flexibility of the thermosets. Meanwhile, the DEP groups suspended to the polymer network were easy to move and could act as an inherent plasticizer for the cured polybenzoxazines.

Fig. 7 Impact resistance of the polybenzoxazines – (1): PTBOZ; (2): P(1/0.5)TBOZ/DEP; (3): P(1/1)TBOZ/DEP; (4): P(1/2)TBOZ/DEP.

Dielectric analysis of polybenzoxazines. The dielectric property of polymers is one of the key characteristics for its application in microelectronic devices.¹⁴ The dependence of the dielectric constant and dielectric loss of all the samples in the frequency range of 125 Hz–18 MHz was studied at room temperature. Fig. 8 shows that both the dielectric constant (ε) and the dielectric loss (tan  δ) of the samples depended largely on the amount of DEP present. Because the dielectric constant and dielectric loss are directly related to the polarizability of materials, they could be increased by increasing the polarizability. Therefore, the dielectric constant and dielectric loss was strongly dependent on its chemical structure.¹⁰ In theory, pure PTBOZ is a uniformly crosslinked polymer and all its components are linked together by chemical bonds, so it shows the lowest polarizability. By introducing DEP into the polymer network, the DEP acted as polar pendant groups linked to the network by chemical bonds and thus increased the polarizability of the modified polymers. Furthermore, increasing the amount of DEP meant lowering the crosslinking density of the cured polymers, which would also result in the increase of polarizability for the cured polymers. The experimental results were consistent with the ones obtained from the theoretical analysis. The dielectric constant of PTBOZ at 125 Hz was about 5.14, for P(1/0.5)TBOZ/DEP, P(1/1.0)TBOZ/DEP and P(1/2.0)TBOZ/DEP were 5.21, 5.65 and 6.46, respectively. In the frequency range of 125 Hz–0.1 MHz, the dielectric constant for each sample showed a slight decrease. When the frequency of the applied field exceeded 1 MHz, the dielectric constants of all the samples declined rapidly because of the suppression of dipole orientation polarization. The dielectric loss of PTBOZ, P(1/0.5)TBOZ/DEP, P(1/1.0)TBOZ/DEP and P(1/2.0)TBOZ/DEP at 125 Hz were 0.027, 0.038, 0.039 and 0.062, respectively. These values showed a slight decrease in the frequency range of 125 Hz–0.1 MHz, and dropped remarkably when the frequency of the applied field increased beyond 1 MHz. The reason for such changes was the same as that of the dielectric constant.

Fig. 8 Frequency dependence of the dielectric constant and dielectric loss of DEP-modified polybenzoxazines.

Humidity absorption of the polybenzoxazines. Many results in various papers^18,51 showed that polybenzoxazines possessed low water uptake ability under humid environments. This advantage was because of the complete hydrogen-bond network formed between phenolic OH and nitrogen atoms, which hindered the water absorption and transportation. As shown in Fig. 9, all the polybenzoxazine samples exhibited low water uptake value (<1.0 wt%) at room temperature after nine days regardless of whether the RH value of the air was high or low. As expected the water uptake ability of the four samples changed regularly with the introduction of DEP. The final water absorption values of the four polybenzoxazines increased in the order of PTBOZ, P(1/0.5)TBOZ/DEP, P(1/1.0)TBOZ/DEP, P(1/2.0)TBOZ/DEP. The value of PTBOZ was about 0.46 wt%, which was very close to other reports,^18,51 and the value increased to about 0.93 wt% for P(1/2.0)TBOZ/DEP. The higher water uptake ability of DEP-modified PTBOZ was attributed to the polar phosphoester groups in the network and low crosslinking density.

Fig. 9 Humidity adsorption of the polybenzoxazines with different DEP contents at different RH at room temperature. (A) RH = 75%; (B) RH = 33%.

Absorption ability of polybenzoxazine for Cd(II) in water. Heavy metals being discharged into water can cause serious environmental damage because of its toxicity and non-biodegradability. Various methods have been proposed for this mandatory removal work, such as chemical precipitation, neutralization, membrane filtration, and adsorption. Among these techniques, adsorption by using chelating polymers has gained considerable attention because of its high efficiency, recyclability of the adsorbents, and ease of handling.^52–54 The chelating polymeric ligands are characterized by the reactive functional groups containing O, N, S, or P as donor atoms, and they are capable of forming a co-ordination complex with different metal ions. A few reports have proved that polybenzoxazine aerogel can act as a chelating polymer to remove metals from water.⁵⁵ The powder-like DEP-modified polybenzoxazines containing O, N and P atoms, could possibly act as a new chelating material to remove Cd(II) from waste water. As shown in Fig. 10, the equilibrium adsorption capacity (q_e) increased with the introduction of DEP because of the fact that the phosphoester group was an efficient chelating site for many metal ions. For all four samples, the most suitable pH value for Cd(II) adsorption was about 5. At low pH ranges, the H⁺ ion would compete for the chelating sites with Cd(II) and decrease the equilibrium adsorption capacity for Cd(II). At a high pH value range, the Cd(II) would precipitate from water in the form of Cd(OH)₂. The critical pH value for Cd(OH)₂ formation was about 8.1 calculated from its solubility product constant (K_sp = 5.27 × 10⁻¹⁵). Meanwhile, the adsorption rates of the four polymers were also studied at pH = 5. The q_e of neat PTBOZ was about 82 mg g⁻¹ after adsorption for 6 h, the value for P(1/2)TBOZ/DEP increased to over 100 mg g⁻¹ in the same absorption condition. The results showed that the introduction of DEP into polybenzoxazines by chemical bonding was an efficient and practical method to improve the adsorption ability of polybenzoxazines for Cd(II) and other heavy metal ions even in its powder form.

Fig. 10 Effect of time (A) and pH value (B) on the adsorption of Cd(II) from aqueous solution at 25 °C.

4. Conclusions

In this work, we have revealed a simple method to introduce phosphoester groups into the polybenzoxazine backbone by chemical bonding through a nucleophilic reaction of the benzoxazine ring with DEP. The novel products directly underwent ring-opening polymerization. Comprehensive research was carried out on the properties of the prepared DEP-modified polybenzoxazines. With the introduction of DEP, the onset of polymerization temperature greatly decreased and the weight loss at 850 °C increased. All the cured polymers showed excellent mechanic properties, higher impact strengths and high gel content after the addition of DEP. Furthermore, all the polymer powders showed a high absorption ability for Cd(II). We believe this characteristic could help researchers design and explore novel polybenzoxazine resins and apply them to a wide range of applications.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21274049) and Opening Project of Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, Jianghan University (no. JDGD-2013-06). All the authors give sincere thanks to the staff of the Analysis and Testing Center of HUST for carrying out the NMR and DMA testing.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05136j

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