Journal of Applied Polymer Science

Transcription

1 Synthesis and Characterization of Bisphenol A Diphthalimide Bisbenzoxazine Monomers and the Properties of Their Polybenzoxazines Journal: Manuscript ID: APP Wiley - Manuscript type: Research Article Keywords: crosslinking, thermal properties, thermosets

2 Page 1 of 39 Synthesis and Characterization of Bisphenol A Diphthalimide Bisbenzoxazine Monomers and the Properties of Their Polybenzoxazines Chun Yan 1, Xinyu Fan 1, Juan Li 1, Shirley Zhiqi Shen 2 1 Ningbo Key Laboratory of Polymer Materials, Institute of Material Technology and Engineering, Chinese Academy of Science, Ningbo , P. R. China 2 CSIR Material Science and Engineering, P Box 56, Highett VIC 3190, Australia ABSTRACT: Three bisphenol A diphthalimide bisbenzoxazine monomers were successfully synthesized through a two-step procedure. First, a new bihydric phenol, bishydroxyphenylbisphenol A diphthalimide (HPBPADI), was prepared from bisphenol A dianhydride (BPADA) and 4-aminophenol. Then, different bisphenol A diphthalimide bisbenzoxazine monomers were synthesized from HPBPADI, paraformaldehyde, and various amines, such as aniline, allylamine, and 3-aminophenylacetylene. The chemical structures and compositions of the novel monomers were identified by Proton nuclear magnetic resonance ( 1 H NMR), Fourier transform infrared spectroscopy (FTIR), and elemental analysis. The curing behavior of the three monomers after various thermal treatments was investigated by using differential scanning calorimetry (DSC) and FTIR analysis. The polybenzoxazines from the three monomers have excellent high temperature properties demonstrated by dynamic mechanical analyses (DMA). The Corresponding author. Tel.: ; Fax: address: xinyu.fan@nimte.ac.cn (X. Fan) 1

3 Page 2 of 39 thermal stability of the three polybenzoxazines were also studied by thermogravimetric analyses (TGA), which revealed that the weight losses of these thermosets appeared to occur over 320 ºC, and the char yields at 800 ºC were up to 61%. Key words: benzoxazine; bisphenol A diphthalimide; crosslinking; thermal stability; thermosets INTRDUCTIN Polybenzoxazines, obtained by the ring-opening polymerization of benzoxazine monomers, have been investigated and developed recently as a new type of thermosetting phenolic resins. 1-2 They have not only the typical properties of traditional phenolic resins, such as the excellent thermal properties and flame retardance, 3 but also the unique characteristics, like near-zero shrinkage or expansion upon polymerization, 4-6 and low water absorption, 7 therefore, overcome the flaws of traditional phenolic resins. Benzoxazine monomers can be easily synthesized using phenols, formaldehyde, and primary amines either by employing solution or solventless methods. 8,9 Polybenzoxazines can be formed by thermally activated ring-opening of the benzoxazines without any catalysts, and no byproducts or volatiles are released. 2, The variation of raw materials allows flexible design in molecular level to synthesize benzoxazine monomers, which is important to meet specific requirements of various applications. However, there are still some disadvantages for polybenzoxazines derived from typical monofunctional and bifunctional benzoxazines, 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine (Bp) and bis-(3-phenyl-3,4-dihydro-2h- 1,3-benzoxazinyl)isopropane (Ba). For example, these 2

4 Page 3 of 39 polybenzoxazines are quite brittle and the polymerization temperature is relatively high. The glass translation temperatures of PBp and PBa are 161 and 185 ºC, 13,14 respectively, which is still too low for high temperature applications. Further enhancement of properties for the typical polybenzoxazine is therefore desired. Some approaches enable to improve the performance of polybenzoxazine for: (1) enhancing the toughness of polybenzoxazines through blending with elastomers; (2) reinforcing polybenzoxazines with inorganic materials or fibers ; and (3) improving the thermal stability of polybenzoxazines through blending with high performance polymers, 26,27 or introducing of other polymerizable groups, such as ethynyl or phenyl ethynyl 28-30, nitrile 31-34, propargyl groups 35 and allyl group 36-40, into the benzoxazine structure. These functional groups would form highly crosslinking network structures as well after cure. Thus, the glass transition temperature and the thermal decomposition temperature of polybenzoxazines would be improved significantly in comparison with typical polybenzoxazines. Polyimides are considered as high performance polymers containing imide moieties in the molecular backbone. They have unique temperature stability, high mechanical performance and excellent chemical resistance. By taking advantage of the molecular design flexibility, the benzoxazine monomers with imide moieties, named the imide-benzoxazines, were synthesized and the properties of their polybenzoxazines were investigated in literature. The imide moieties were introduced into benzoxazines to improve heat resistance. 41 A series of benzoxazine and benzylamine type intermediates were synthesized from hexamethylenetetramine (HMTA) and N-(hydroxyphenyl) 3

5 Page 4 of 39 succinimides, and their structures were investigated. 42 Maleimidobenzoxazines were synthesized from hydroxyphenylmaleimide (HPMI), formalin, and various amines. 13 The effect of HPMI on the curing behaviour and thermomechanical properties of rubber-modified polybenzoxazine were also studied. 43 The thermal and mechanical properties were enhanced through the polymerization of HPMI with a benzoxazine monomer or by incorporating HPMI into ATBN-modified PBa. A benzoxazine compound were prepared with a maleimide group, 3-phenyl-3, 4-dihydro-2H-6-(N-maleimido)-1, 3-benzoxazine (HPM-Ba), from N-(4-hydroxyphenyl) maleimide, formaldehyde, and aniline. 44 The completely cured polymer exhibited good thermal stability and flame retardancy. Maleimide and norbornene functionalized benzoxazines were synthesized 45 and the polybenzoxazines from these benzoxazine monomers had char yields above 55% and glass transition temperatures above 250 ºC. These polybenzoxazines had excellent thermomechanical properties and thermal stability, superior to those of traditional phenolics, epoxies or most of high-performance thermoplastics. In this paper, three bisphenol A diphthalimide bisbenzoxazine monomers were designed and expected to produce polymers with superior properties to the traditional polybenzoxazine. The designed monomers, containing other types of polymerizable groups, such as allyl and ethynyl groups, were expected to improve the thermal stability of polymers. The three bisphenol A diphthalimide bisbenzoxazine monomers were synthesized through a two-step procedure. First, a new bihydric phenol, bishydroxyphenylbisphenol A diphthalimide (HPBPADI), was prepared from bisphenol A dianhydride (BPADA) and 4-aminophenol. Then, different bisphenol A diphthalimide 4

6 Page 5 of 39 bisbenzoxazine monomers were synthesized from HPBPADI, paraformaldehyde, and three amines, aniline, allylamine, or 3-aminophenylacetylene. The structures of the novel monomers were identified by Proton nuclear magnetic resonance ( 1 H NMR), Fourier transform infrared spectroscopy (FTIR), and elemental analysis. The curing behavior of the new monomers after various thermal treatments was investigated by differential scanning calorimetry (DSC) and FTIR analysis. The thermal mechanical property and thermal stability of the novel polybenzoxazines were also investigated by dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA). EXPERIMENTAL Materials and characterization Bisphenol A dianhydride was obtained from Shanghai Research Institute of Synthetic Resins. 4-Aminophenol (98%), 3-aminophenylacetylene and allylamine were from Aladdin Reagent. Aniline (99%), N, N-dimethylformaide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMS), chloroform, toluene, N-methyl-2-pyrrolidone and paraformaldehyde (95%) from Sinopharm Chemical Reagent Company. All chemicals were used as received. The 1 H NMR spectra, used to identify the structures of the monomers, were recorded on a Bruker Avance Ⅲ (400MHz) instrument. All resonances were referenced with respect to the proton signal of tetramethylsilane (TMS). FTIR spectra were recorded at a resolution of 4 cm -1 with co-addition of 32 scans using a Thermo-Fisher Nicolet 6700 FTIR. All samples were prepared as KBr pellets. The quantitative element analysis of C, 5

7 Page 6 of 39 H, and N was carried out on a Horiba EMGA-620W oxygen/nitrogen analyzer. DSC was conducted on a Mettler Toledo DSC1 at a heating rate of 10 ºC/min over a temperature range of ºC under a nitrogen flowing atmosphere. DMA of polybenzoxazine films was performed on a Mettler Toledo Star e System DMA/SDTA861 e instrument. The film specimens, being about 10.5 mm 4 mm 0.04 mm, were tested in a tension mode over a temperatures range of ºC. The heating rate and dynamic frequency were 5 ºC/min and 1.0 Hz, respectively. TGA was carried out on a Mettler Toledo TGA/DSC1 with a heating rate of 10 ºC/min over a temperature range of ºC under nitrogen flow at a rate of 30 ml/min. Synthesis of three benzoxazine monomers Three bisphenol A diphthalimide bisbenzoxazine monomers were synthesized through a two-step procedure. Firstly, an intermediate bihydric phenol, bishydroxyphenylbisphenol A diphthalimide (HPBPADI), was prepared from bisphenol A dianhydride (BPADA) and 4-aminophenol. Secondly, the three bisphenol A diphthalimide bisbenzoxazine monomers were obtained through the reaction of HPBPADI, paraformaldehyde and three amines, aniline, allylamine, and 3-aminophenylacetylene.The three monomers, bis(3-phenyl-2, 4-dihydro-2H-1, 3-benzoxazinyl)-bisphenol A diphthalimide (BZ-HPBPADI), bis(3-allyl-2, 4-dihydro-2H-1, 3-benzoxazinyl)- bisphenol A diphthalimide (BZ-HPBPADI-al), and bis(3-(3-phenylacetylene)-2, 4- dihydro-2h-1, 3-benzoxazinyl)-bisphenol A diphthalimide (BZ-HPBPADI-apa), were produced as shown in Figure 1. 6

8 Page 7 of 39 (Figure 1) Synthesis of intermediate bihydric phenol (HPBPADI) In a 100 ml three-neck round bottom flask, 5.2 g (10 mmol) BPADA and 2.18 g (20 mmol) p-aminophenol were dissolved in 50 ml of glacial acetic acid by a magnetic stirrer at ambient temperature. The mixture was then stirred at 140 ºC for 24 hours (hrs) in an oil bath and a white precipitate was obtained. The precipitate was washed by glacial acetic acid and then dried in a vacuum chamber at 120 ºC for 24 hrs. The final product was white crystals of 6.17 g (88%), with a melting point of 272 ºC. HPBPADI can be dissolved in DMF, DMAc and DMS, but not in chloroform and toluene. The assignments of the 1 H NMR spectrum of HPBPADI (400 MHz, DMS-d 6, 298 K) are δ = 1.71 ppm (s, 6 H, 2 CH 3 ), ppm (20 H, Ar), and 9.73 ppm (s, 2 H, 2 H), respectively. Synthesis of bisphenol A diphthalimide bisbenzoxazine monomers Three bisphenol A diphthalimide bisbenzoxazine monomers were synthesized by the same method: HPBPADI (10 mmol), paraformaldehyde (40 mmol), and each amine of the three (20 mmol) were mixed in N, N-Dimethylformamide (DMF) (50 ml) and stirred at 105 ºC for 48 hrs. The products were precipitated into distil water and the precipitate was washed with ethanol for at least three times and then vacuum dried. The melting points of the three synthesized monomers tested by DSC are listed in TableⅠ. (Table Ⅰ) 7

9 Page 8 of 39 Preparation of polybenzoxazines The monomers were polymerized into polybenzoxazines films with thicknesses of 0.3 to 0.5 mm in a following procedure: (1) the monomers were dissolved in N-methyl-2-pyrrolidone (10 wt.%), (2) the solutions were cast on glass plates and kept at 70 ºC for 10 hrs for solvent evaporations, (3) the resultant films were cured at 150 ºC for 5 hrs, 180 ºC for 4 hrs, 200 ºC for 3 hrs, 220 ºC for 2 hrs, and 250 ºC for 2 hrs sequently. The films were then released from the glass plates in a water bath at ambient temperature and vacuum dried at 100 ºC for 24 hrs. RESULTS AND DISCUSSIN Characterization of the monomers Three benzoxazine monomers were found to be able to dissolve in many common organic solvents, such as acetone, chloroform, tetrahydrofuran, dioxane, and N, N-dimethylformamide. The chemical structures of the monomers were verified by FTIR, 1 H NMR and elemental analysis. Figure 2 depicts the FTIR spectra of the three monomers. The imide C= stretching absorptions appear at 1718 and 1774 cm -1 (symmetric and asymmetric) and the C-N stretching absorption at 1378 cm The characteristic absorptions for the benzoxazine ring appear at 1239 cm -1 (asymmetric stretching of C C), 1013 cm -1 (symmetric stretching of C C), 1340 cm -1 (wagging vibrations of CH 2 ), and 940 cm -1 (trisubstituted benzene ring), 11 while the characteristic absorptions for BZ-HPBPADI-al, assigned to allyl group, appear at 3068 cm -1 (stretching of =C H) and 1618 cm -1 (stretching of C=C) 36 and those for 8

10 Page 9 of 39 BZ-HPBPADI-apa, assigned to propargyl structure, at 3285 (stretching of H C ) and 2125 cm -1 (stretching of C C). 35 (Figure 2) Figure 3 shows the 1 H NMR spectra of the three monomers in CDCl 3. For the BZ-HPBPADI (Figure 3 (a)), the characteristic protons of the oxazine ring appear at 4.66 ( Ar CH 2 N ) and 5.37 ppm ( CH 2 N ), respectively. 2 The singlet at 1.76 ppm is assigned to the protons of CH 3. The multiplet at ppm is attributed to protons of the aromatic ring. For the BZ-HPBPADI-al (Figure 3 (b)), the characteristic protons of the oxazine ring appear as two singlets at 4.05 and 4.91 ppm, while the methyl protons appear as a singlet at 1.77 ppm, and the aromatic protons a multiplet at ppm. Two multiples at and ppm belong to the protons of =CH 2 and =CH in the allyl group and a doublet at ppm belongs to the protons of CH 2 of the allyl group. 36 For the BZ-HPBPADI-apa (Figure 3 (c)), the characteristic protons of the oxazine ring appear as two singlets at 4.69 and 5.38 ppm, while the methyl protons and the aromatic protons appear the same as a singlet at 1.77 ppm, and a multiplet at ppm, respectively. A singlet at 3.06 ppm belongs to the protons of C H. 35 In summary, all the assignments of peaks in the 1 H NMR spectra confirm that the structures of the designed monomers occur in the synthesized monomers. (Figure 3) The elemental analyses of the three monomers are shown in Table Ⅱ as a 9

11 Page 10 of 39 comparison of theoretically calculated values to experimental measured values. The measured values of C, H and N element contents of the monomers agree with the calculated values quite well. It confirms that the products are the designed monomers in terms of element compositions. (Table Ⅱ) Cure behavior of the monomers The cure behavior of the three monomers, BZ-HPBPADI, BZ-HPBPADI-al and BZ-HPBPADI-apa, was studied using DSC after a series of heat treatments at each temperature of 120, 160, 200 and 250 ºC for 2 hrs. BZ-HPBPADI showed an exotherm (Figure 4) with onset at 179 ºC and a maximum at 226 ºC corresponding to the ring-opening polymerization of benzoxazine. The cure enthalpy of BZ-HPBPADI was measured as 127 J/g (or 119 KJ/mol). And the exothermic peaks gradually decreased after each cure cycle and disappeared completely after 240 ºC/ 2 h treatment. The cure behavior was also monitored with FTIR spectra for BZ-HPBPADI after each cure treatment (Figure 5). FTIR spectra revealed that the characteristic absorption bands according to the benzoxazine structure at 940 and 1340 cm -1 gradually decreased, and disappeared after 240 ºC cure. It showed that the ring-opening reaction of benzoxazine was completed to form the cross-linked polybenzoxazine. (Figure 4) (Figure 5) 10

12 Page 11 of 39 For the system of BZ-HPBPADI-al, the DSC results in Figure 6 showed that the exothermic peak was observed with the onset at 131 ºC and maximum point at 226 ºC. The measured enthalpy of exotherm was as high as 148 J/g (or 128 KJ/mol), which was much higher than that of BZ-HPBPADI. The exotherm of BZ-HPBPADI-al is attributed to both the crosslinking of allyl group and the ring-opening polymerization of benzoxazine. However, the DSC curve of BZ-HPBPADI-al has the only one exothermic peak, which is different from those of benzoxazines containing oxazine ring and allyl group, showing two exothermic peak corresponding to the crosslinking of carbon-carbon double bond at about 210 ºC and the ring-opening polymerization of oxazine ring at about 250 ºC. 13, 36 It is believed that the appeared single exotherm in the DSC curve consists the two parts, occurring at similar time. The possible explanation might be that a small amount of phenolic hydroxyl groups of oligomers in the BZ-HPBPADI-al monomer on account of the difficulty of the purification decreased the temperature of oxzine ring-opening, therefore, both the ring-opening and allyl group crosslinking occurred almost simultaneously Similar to those results for BZ-HPBPADI, the FTIR spectra for BZ-HPBPADI-al showed that the absorptions peak of benzoxazine ring (1340 and 940 cm -1 ) decreased, and disappeared after completing heat treatment at 250 ºC for 2 hrs. Moreover, the characteristic absorption band of BZ-HPBPADI-al assigned to the unsaturated allyl group (3068 and 1618 cm -1 ) (in Figure 7) diminished gradually to disappeared, indicating the allyl groups were cross-linked after the heat treatments. (Figure 6) 11

13 Page 12 of 39 (Figure 7) Figure 8 shows the cure behavior of BZ-HPBPADI-apa after different cure treatment. BZ-HPBPADI-apa showed an exotherm starting at 159 ºC with the maximum point at 230 ºC. The exothermic enthalpy of monomer was measured as 240 J/g (or 236 KJ/mol) and was also much higher than that of BZ-HPBPADI or that of BZ-HPBPADI-al. The exotherm of BZ-HPBPADI-apa is attributed to both the ring-opening polymerization of benzoxazine and the crosslinking of ethynyl group. nly a single exotherm also suggests that the cure regimes of benzoxazine and ethynyl group occur simultaneously as reported previously The exothermic enthalpy decreased after each cure cycle and disappeared after 250 ºC/ 2 hrs cure, indicating the completion of curing. The FTIR spectra of BZ-HPBPADI-apa at different cure treatment are shown in Figure 9. The characteristic absorption bands at 3285 and 2125 cm -1 attributed to the ethynyl group disappeared obviously. In addition, similar to those for BZ-HPBPADI and BZ-HPBPADI-al, the FTIR spectra for BZ-HPBPADI-apa showed the disappearance of the absorptions assigned to benzoxazine ring. In summary, the three monomers can be cured completely after the designed heat treatment and the BZ-HPBPADI-apa monomer has the highest enthalpy of 240 J/g. The FTIR results confirm that the reactive groups in the three monomers, including oxazine ring, allyl and ethynyl group, have completely crosslinked to form the polymer after the set of heat treatments. (Figure 8) 12

14 Page 13 of 39 (Figure 9) Properties of the novel polybenzoxazines Dynamic mechanical analysis of the novel polybenzoxazines The viscoelastic properties of the new polybenzoxazines from the three monomers, PBZ-HPBPADI, PBZ-HPBPADI-al and PBZ-HPBPADI-apa, were investigated by DMA. Figure 10 and 11 show the temperature dependence of the storage modulus (E ), loss modulus (E ), and loss factor (tan δ) for the polybenzoxazines, respectively. The storage modulus at 50 ºC and T g defined from the peaks of E (E max ) or tan δ (tan δ max ), of the three polybenzoxazines are compared to those relevant polybenzoxazines reported, such as polybenzoxazine from bisphenol-a and aniline (PBa), 14 polybenzoxazine from bisphenol-a and allylamine (PB-ala), 36 polybenzoxazine from bisphenol-a and 3-aminophenylacetylene (PB-apa), 29 polybenzoxazine from maleimide and aniline (P(Mal-Bz)) 13 and polybenzoxazine from maleimide and allylamine (P(Mal-Bz-al)) 13 in Table 4. (Figure 10) (Figure 11) The E of PBZ-HPBPADI is 2.91 GPa at 50 ºC and decreases dramatically at about 205 ºC and its T g, defined from the peaks of E and tan δ, is 223 and 252 ºC, respectively. The E of PBZ-HPBPADI-al decreased more after 205 ºC than that of PBZ-HPBPADI, but that of PBZ-HPBPADI-apa retains much greater value at higher temperature. The 13

15 Page 14 of 39 E decrease of PBZ-HPBPADI-al may be due to the extra flexible aliphatic chain introduced in the additional polymerization of allyl group. The high E at high temperature of the PBZ-HPBPADI-apa may be due to the bicyclic chromene rings formed during the polymerization of the ethynyl group. Even though the E decreases, the T g of PBZ-HPBPADI-al is similar or slightly higher than those of PBZ-HPBPADI, while the T g of PBZ-HPBPADI-apa appears a lot higher than those of the other two polybenzoxazines. The increase of T g might be due to the increased crosslink density when additional crosslink sites were introduced 13. From the comparison in Table Ⅲ, PBZ-HPBPADI, PBa and P(Mal-Bz) were all prepared from the same aniline and different phenolic compounds. PBZ-HPBPADI have higher E and T g than PBa. It is because of the stiff imide ring in the main chain in the new polybenzoxazines. However, PBZ-HPBPADI have lower E and T g than P(Mal-Ba). It might be because of flexible aliphatic segments existed in the main chains of PBZ-HPBPADI. Though PBZ-HPBPADI-al, PB-ala and P(Mal-Bz-al) were all prepared from the same allylamine and different phenolic compounds, PBZ-HPBPADI-al have much lower T g than PBa and P(Mal-Bz-al). The reason may be because that BZ-HPBPADI-al monomer has higher molecular weight and longer molecular chains between two oxazine rings or allylamine groups than Ba and Mal-Bz-al monomers. Thus PBZ-HPBPADI-al has lower crosslink density than PBa-al and P(Mal-Bz-al), and the contribution of crosslink density to T g may be more than that of imide groups in the main chains. Higher crosslink density leads to higher T g. The similar trend to polybenzoxazines from the same 14

16 Page 15 of 39 3-aminophenylacetylene is found that PBZ-HPBPADI-apa has lower T g than PB-apa. In summary, more rigidity structure in polymer main chain leads to higher E and the possibly increased crosslink density of polybenzoxazines contributes to high T g. The PBZ-HPBPADI-apa has the highest E and T g values among the three novel polybenzoxazines. (Table Ⅲ) Thermal Stability of the Bisphenol A Diphthalimide Bisbenzoxazine Thermosets The thermal stability of the novel bisphenol A diphthalimide bisbenzoxazine thermosets was evaluated by TGA. The weight percentage versus temperature curves are shown in Figure 12. The results of 5% and 10% weight loss temperatures (T 5 and T 10 ) and char yields at 800 ºC in percentage are listed in Table Ⅳ also compared to the literature data of PBa, PB-ala, PB-apa, P(Mal-Bz) and P(Mal-Bz-al). (Figure 12) (Table Ⅳ) n one hand, the orders of T 5 and T 10 for the three polybenzoxazines are: PBZ-HPBPADI PBZ-HPBPADI-al < PBZ-HPBPADI-apa. The trend is similar to the data in the references 29 and 36, PB<PB-al<PB-apa. The initial thermal decomposition of polybenzoxazines is due to Mannich base cleavage (the cleavage of the C-N bond), leading to volatilization of aniline derivative fragments Since PBZ-HPBPADI-al or PBZ-HPBPADI-apa processes more crosslink points through 15

17 Page 16 of 39 the polymerization of allyl or ethynyl groups, the thermally labile Mannich bridge parts were anchored in the matrix, resulting in higher T 5 and T 10 compared to that of PBZ-HPBPADI, even though the increase for PBZ-HPBPADI-al is not significant. The ethynyl groups in PBZ-HPBPADI-apa polymerize to form the cyclic chromene structure with a higher thermal stability, resulting in much higher T 5 and T 10 than either PBZ-HPBPADI or PBZ-HPBPADI-al. The T 5 and T 10 values for PBZ-HPBPADI and PBZ-HPBPADI-al are higher those for PBa and PB-ala, respectively. The main reason may be that the main chains of PBZ-HPBPADI and PBZ-HPBPADI-al have high temperature resistant imide groups. However, PBZ-HPBPADI-apa with imide groups in main chain has lower T 5 and T 10 than PB-apa. The reason may be that the bicyclic chromene ring from ethynyl group has much higher thermal stability than imide group, and PB-apa posses more content of ethynyl group than that of PBZ-HPBPADI-apa. So the bicyclic chromene ring content of PB-apa is much more than that of PBZ-HPBPADI-apa, resulting in higher thermal stability of PB-apa. The T 5 and T 10 for PBZ-HPBPADI are higher than those of P(Mal-Bz) with imide groups, which indicate that the PBZ-HPBPADI has higher thermal stability than P(Mal-Bz) and also belongs to a novel high-temperature resistant material. However, the T 5 and T 10 for PBZ-HPBPADI-al are lower than those of P(Mal-Bz-al). The possible reason is that P(Mal-Bz-al) has higher crosslink density than PBZ-HPBPADI-al, leading to higher thermal stability. n the other hand, the char yields of the three polymers were found in following order: PBZ-HPBPADI-al < PBZ-HPBPADI < PBZ-HPBPADI-apa. It is also similar 16

18 Page 17 of 39 to the order of PB series in the literature: PB-al<PB<PB-apa. The char yield of PBZ-HPBPADI-al, being 41% is much lower than that of PBZ-HPBPADI, being 51%. It is mainly because of the nature of the aliphatic segments formed from the allyl polymerization, since aliphatic segments cause low thermal stability. 50 The char yield of PBZ-HPBPADI-apa, being 61%, is the highest among the three polybenzoxazines, again it is due to the higher thermal stability and stiffening effect of the cyclic chromene structure. The char yields of PBZ-HPBPADI and PBZ-HPBPADI-al are higher than those of PBa and PB-ala because the imide groups have higher char yield. However, the char yield of PBZ-HPBPADI-apa is lower than PB-apa. This is because that PB-apa has much more the cyclic chromene structure content than PBZ-HPBPADI-apa due to higher crosslink density of PB-apa. The char yield of PBZ-HPBPADI and PBZ-HPBPADI-al have lower char yield than P(Mal-Bz) and P(Mal-Bz-al) due to P(Mal-Bz) and P(Mal-Bz-al) with higher content of the imide group contributing to higher char yield. In summary, the three novel polybenzoxazines exhibit excellent thermal stabilities in terms of 5 and 10% weight loss temperatures, as high as over 360 and 405 ºC, as well as char yields at 800 ºC, being about 41 to 61%. The PBZ-HPBPADI-apa is proved to have the best thermal stability among the three polybenzoxazines. The 5 and 10% weight loss temperatures of the PBZ-HPBPADI-apa are 419 and 452 ºC, respectively, and the char yield at 800ºC is 61%. 17

19 Page 18 of 39 CNCLUSINS Three new benzoxazine monomers of bisphenol A diphthalimide bisbenzoxazine have been synthesized from hydroxyphenyl bisphenol A diphthalimide, paraformaldehyde and various amines through a two step process. The benzoxazine monomers have been crosslinked through the ring-opening polymerization. The exothermic enthalpy of BZ-HPBPADI-al or BZ-HPBPADI-apa is higher than that of BZ-HPBPADI because of additional exotherm of crosslinking of either allyl or ethynyl groups during the cure. The three novel polybenzoxazines have high storage modulus ( GPa) at 50 ºC, high T g (252 ~ 286 ºC) and high char yield (41 ~ 61%). The PBZ-HPBPADI-apa is proved to have the best dynamic mechanical property and thermal stability among the three polybenzoxazines since it has most rigid structures. The storage modulus at 50 ºC is 3.24 GPa and the T g is 286 ºC from maximum tan δ. The 5 and 10% weight loss temperatures of the PBZ-HPBPADI-apa are 419 and 452 ºC, respectively, and the char yield at 800 ºC is 61%. The novel polybenzoxazines have potential application as resin to process high performance composites for aerospace industry. ACKNWLEDGEMENTS The authors gratefully acknowledge the partial financial support from the Natural Science Foundation of Ningbo Government (grant number 2009A ). 18

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23 Page 22 of 39 Captions for Figures: Figure 1 Synthesis of the bisphenol A diphthalimide bisbenzoxazine monomers. Figure 2 FTIR spectra of the monomers. Figure 3 1 H NMR spectra of (a) BZ-HPBPADI, (b) BZ-HPBPADI-al, and (c) BZ-HPBPADI-apa. Figure 4 DSC cure of BZ-HPBPADI after each cure stage. Figure 5 FTIR spectra of BZ-HPBPADI: (a) monomer; (b) cured at 120 ºC for 2 hrs; (c) cured at 160 ºC for 2 hrs; (d) cured at 200 ºC for 2 hrs; (e) cured at 240 ºC for 2 hrs. Figure 6 DSC cure of BZ-HPBPADI-al after each cure stage. Figure 7 FTIR spectra of BZ-HPBPADI-al: (a) monomer; (b) cured at 120 ºC for 2 hrs; (c) cured at 160 ºC for 2 hrs; (d) cured at 200 ºC for 2 hrs; (e) cured at 240 ºC for 2 hrs. Figure 8 DSC cure of BZ-HPBPADI-apa after each cure stage. Figure 9 IR spectra of BZ-HPBPADI-apa: (a) monomer; (b) cured at 120 ºC for 2 hrs; (c) cured at 160 ºC for 2 hrs; (d) cured at 200 ºC for 2 hrs; (e) cured at 240 ºC for 2 hrs. Figure 10 Viscoelastic analyses of PBZ-HPBPADI, PBZ-HPBPADI-al, and PBZ-HPBPADI-apa. Figure 11 Temperature dependence of tanδ for PBZ-HPBPADI, PBZ-HPBPADI-al, and PBZ-HPBPADI-apa. Figure 12 TGA of PBZ-HPBPADI, PBZ-HPBPADI-al, and PBZ-HPBPADI-apa. 22

24 Page 23 of 39 Captions for Tables: TableⅠYield, appearance and melting point of the three synthesized monomers. Table Ⅱ The elemental analyses of three benzoxazine monomers. Table Ⅲ Summary of DMA analysis results Table Ⅳ Summary of TGA analysis results 23

25 Page 24 of 39 CH 3 C CH H 2 N H H CH 3 C CH 3 N N H + 2 R-NH HCH N CH 3 C CH 3 N N R BZ-HPBPADI N R R BZ-HPBPADI-al BZ-HPBPADI-apa Figure 1 Synthesis of the bisphenol A diphthalimide bisbenzoxazine monomers. 24

26 Page 25 of 39 BZ-HPBPADI-apa 2125 cm cm -1 BZ-HPBPADI-al 3068 cm -1 T / % BZ-HPBPADI 1618 cm cm cm cm cm cm cm cm v/ cm -1 Figure 2 FTIR spectra of the monomers. 25

27 Page 26 of ppm (a) ppm (b) ppm (c) Figure 3 1 H NMR spectra of (a) BZ-HPBPADI, (b) BZ-HPBPADI-al, and (c) BZ-HPBPADI-apa. 26

28 Page 27 of 39 Exo 240 C / 2 hrs cure 200 C / 2 hrs cure 160 C / 2 hrs cure 230 C 255 C 2 J/g 228 C 184 C 94 J/g Heat flow 226 C 181 C 125 J/g 120 C / 2 hrs cure 226 C Monomer 179 C 127 J/g Temperature / C Figure 4 DSC cure of BZ-HPBPADI after each cure stage. 27

29 Page 28 of 39 e d c T / % b a 1340 cm cm v / cm -1 Figure 5 FTIR spectra of BZ-HPBPADI: (a) monomer; (b) cured at 120 ºC for 2 hrs; (c) cured at 160 ºC for 2 hrs; (d) cured at 200 ºC for 2 hrs; (e) cured at 240 ºC for 2 hrs. 28

30 Page 29 of C / 2 hrs cure 200 C / 2 hrs cure 213 C 249 C 36 J/g Exo 160 C / 2 hrs cure 166 C 230 C 106 J/g Heat flow 120 C / 2 hrs cure 226 C 148 J/g Monomer 137 C 131 C 226 C 148 J/g Temperature / C Figure 6 DSC cure of BZ-HPBPADI-al after each cure stage. 29

31 Page 30 of 39 e d T / % c b a 3068 cm cm cm cm v / cm -1 Figure 7 FTIR spectra of BZ-HPBPADI-al: (a) monomer; (b) cured at 120 ºC for 2 hrs; (c) cured at 160 ºC for 2 hrs; (d) cured at 200 ºC for 2 hrs; (e) cured at 240 ºC for 2 hrs. 30

32 Page 31 of C / 2 hrs cure 261 C 200 C / 2 hrs cure 230 C 59 J/g Exo 237 C Heat flow 163 C 176 J/g 160 C / 2 hrs cure 230 C 239 J/g 163 C 120 C / 2 hrs cure Monomer 159 C 230 C 240 J/g Temperature / C Figure 8 DSC cure of BZ-HPBPADI-apa after each cure stage. 31

33 Page 32 of 39 e T / % d c b a 2125 cm cm cm cm v / cm -1 Figure 9 IR spectra of BZ-HPBPADI-apa: (a) monomer; (b) cured at 120 ºC for 2 hrs; (c) cured at 160 ºC for 2 hrs; (d) cured at 200 ºC for 2 hrs; (e) cured at 240 ºC for 2 hrs. 32

34 Page 33 of E' (MPa) E'' (MPa) PBZ-HPBPADI PBZ-HPBPADI-al PBZ-HPBPADI-apa Temperature ( C) 10 1 Figure 10 Viscoelastic analyses of PBZ-HPBPADI, PBZ-HPBPADI-al, and PBZ-HPBPADI-apa. 33

35 Page 34 of PBZ-HPBPADI PBZ-HPBPADI-al PBZ-HPBPADI-apa 0.4 tan δ Temperature ( C) Figure 11 Temperature dependence of tanδ for PBZ-HPBPADI, PBZ-HPBPADI-al, and PBZ-HPBPADI-apa. 34

36 Page 35 of PBZ-HPBPADI-apa 90 Weight Residue (%) PBZ-HPBPADI PBZ-HPBPADI-al Temperature ( C) Figure 12 TGA of PBZ-HPBPADI, PBZ-HPBPADI-al, and PBZ-HPBPADI-apa. 35

37 Page 36 of 39 TableⅠ Yield, appearance and melting point of the three synthesized monomers. Monomers Yield, % Appearance Melting point, ºC BZ-HPBPADI 80 yellow powder 130 BZ-HPBPADI-al 75 yellow powder 126 BZ-HPBPADI-apa 78 brown powder

38 Page 37 of 39 Table Ⅱ The elemental analyses of three benzoxazine monomers. Monomers Molecular formula Theoretical value Experimental value C, % H, % N, % C, % H, % N, % BZ-HPBPADI C 59 H 44 N BZ-HPBPADI-al C 53 H 44 N BZ-HPBPADI-apa C 63 H 44 N

39 Page 38 of 39 Table Ⅲ Summary of DMA analysis results Polymers [reference] E at 50ºC (GPa) T g (ºC) E max tan δ max PBa [14] PB-ala [36] PB-apa [29] P(Mal-Bz) [13] 3.00~ P(Mal-Bz-al) [13] PBZ-HPBPADI PBZ-HPBPADI-al PBZ-HPBPADI-apa

40 Page 39 of 39 Table Ⅳ Summary of TGA analysis results Polymers [reference] Weight-Loss Temperature (ºC) 5% 10% Char Yield at 800 ºC (%) PBa [36] PB-ala [36] PB-apa [29] P(Mal-Bz) [13] P(Mal-Bz-al) [13] PBZ-HPBPADI PBZ-HPBPADI-al PBZ-HPBPADI-apa