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Structural effects of diamines on synthesis, polymerization, and properties of benzoxazines based on o-allylphenol
Polymer 57 (2015) 29e38
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Structural effects of diamines on synthesis, polymerization, and properties of benzoxazines based on o-allylphenol Yanfang Liu*, Zhanzhan Hao, Shufang Lv, Jinbai Huang, Chunyan Liao, Mingtao Run Key Laboratory of Analytical Science and Technology of Hebei Province, College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 July 2014 Received in revised form 1 December 2014 Accepted 2 December 2014 Available online 12 December 2014
Two allyl-containing bifunctional benzoxazines (oAP-hda and oAP-dds) were synthesized from o-allylphenol, formaldehyde, and two diamines [1,6-hexamethylenediamine (HDA) and 4,40 -diaminodiphenyl sulfone (DDS)]. The chemical structures of oAP-hda and oAP-dds were confirmed by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The reactivity of HDA is higher than that of DDS in the synthetic reaction of benzoxazines, and the polymerization temperature of oAP-hda is lower than that of oAP-dds. The structure changes from benzoxazines to their corresponding polybenzoxazines [Poly(oAP-hda) and Poly(oAP-dds)] were studied by FTIR and solid state 13 C NMR. The dynamic mechanical properties and thermal stabilities of Poly(oAP-hda) and Poly(oAP-dds) were studied by dynamic mechanical analysis and thermogravimetry, respectively. The storage moduli in the glassy state are 1.44 and 3.18 GPa for Poly(oAP-hda) and Poly(oAP-dds), respectively, and the glass transition temperatures are 87 and 193 C, respectively. The thermal stability of Poly(oAP-dds) is much higher than that of Poly(oAP-hda). © 2014 Elsevier Ltd. All rights reserved.
Keywords: Benzoxazine Electronic effect Sulfone
1. Introduction Benzoxazines are a new class of thermosetting resins that can be polymerized into polybenzoxazines with excellent physical properties [1,2]. Thus, they are attracting much attention from both academia and industry. Owing to the molecular design flexibility, benzoxazines with different structures have been synthesized from phenols, amines, and formaldehyde [3e9]. Naturally, the structures of phenols and amines are essential not only to affect the ringforming reaction and thermally activated ring-opening polymerization of benzoxazines but also to govern physical properties of the resultant polybenzoxazines. In practice, several structural factors from phenols and amines are frequently considered, such as steric and electronic effects from substituent groups on the reactivity in both synthetic reaction and polymerization of benzoxazines, and the effect of conformational flexibility or rigidity of substituent groups on physical properties of polybenzoxazines. Many researchers explored the structural effects of various substituents on synthesis, polymerization, and properties of a series of benzoxazines based on monofunctional amines.
* Corresponding author. E-mail address: [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.polymer.2014.12.005 0032-3861/© 2014 Elsevier Ltd. All rights reserved.
Ishida and Sanders indicated that electron-donating alkyl substituent groups at one or both meta positions on arylamine rings facilitate ring-opening polymerization of monofunctional alkylsubstituted aromatic amine-based benzoxazines [10]. Ronda and coworkers reported that an increase of the polymerization temperature takes place when the electron-withdrawing character of the groups in the para-position of the phenyl substituent increases for monofunctional benzoxazine monomers [11]. Endo et al. disclosed that the allyl group in allylamine can stabilize the zwitterionic intermediates and promote ring-opening polymerization of benzoxazines [12], though bulkier substituents on the nitrogen atom correspond to slower polymerization of benzoxazines [12,13]. Gu et al. revealed that the electron-withdrawing bridging groups in bisphenols inhibit the synthetic reaction and promote the thermally activated polymerization of bis-benzoxazines [14]. Besides, the substituting groups on phenolic units can make a notable impact on the molecular structure parameters of benzoxazines and on the ring-opening reaction mechanism, and the ring-opening reactions of benzoxazines with electron-withdrawing substituting groups proceed easier to some extent than those of benzoxazines with electron-donating ones [15]. Ishida and Liu synthesized a series of 2,20 -, 2,40 -, and 4,40 -substituted benzoxazine monomers based on isomeric bisphenol F and aniline, and they found an
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anomalous phenomenon that the rate of polymerization increases in the order 4,40 -, 2,40 -, and 2,20 -substituted monomer and the polymer obtained from 2,20 -substituted monomer gives highest thermal stability as well as glass transition temperature (Tg) [16]. Xin et al. reported that the surface energies of polybenzoxazines decrease with the increase of alkyl chain length and both the chain length and bulkiness of the alkyl group of N-substituents have an effect on the hydrogen bonding network of polybenzoxazines [17]. Ishida et al. found that the intensity of the hydrogen bonding increases with the increased amine substituent size and the nature of hydrogen bonding is very similar if the amines have the similar basicity [18]. Moreover, the onset temperature of weight loss is lower and the progression of weight loss is more rapid for the longer chain amine polybenzoxazine [19], and various phenols substantially affect the char yield of a series of polybenzoxazines based on aniline under an inert environment and have a significant influence on the degradation behavior of polybenzoxazines below 600 C under an oxidative environment [20]. In addition, different phenolic substituents at various points on the phenolic units can have critical influences on the degree to which the photooxidation reaction occurs and can impact upon the general photooxidation mechanism [21], and the structures of the amines in polybenzoxazines have a significant impact on the photooxidation process [22]. As for diamine based benzoxazines, Ishida and Allen reported that an electron-donating methyl group attached to a phenol ring has the effect of either stabilizing the benzoxazine rings against thermal activation, or reducing the effectiveness of the phenolic eOH groups of open rings as initiators or catalysts [23]. They found that the melting temperatures of linear aliphatic diamine-based series of benzoxazines decrease as a function of diamine chain length [23,24], but increase by adding a methyl substituent to phenol in the order of meta > ortho > para > unsubstituted monomers [23], while the thermally activated polymerization temperatures of the benzoxazines increase with the rising of diamine chain length [23,24], or by adding a methyl substituent to phenol [23]. The Tgs of the corresponding polybenzoxazines decrease with the rising of diamine chain length [23,25], or by adding a methyl substituent to phenol in the order of ortho < para < meta < unsubstituted polymers [23], and the density, room temperature modulus, crosslink density, yield stress, and char yield of aliphatic amine-based polybenzoxazines decrease as a function of aliphatic diamine chain length [25]. Moreover, the yield strain and thermal degradation temperature increase as a function of aliphatic diamine chain length, and methyl substitution at the ortho, para, or meta position of these benzoxazines seems to adversely affect both the onset of degradation and the char yield [23]. Therefore, aliphatic amine-based polybenzoxazines are much more flexible than the bisphenol-type polybenzoxazines [25]. Lin et al. synthesized a series of benzoxazines based on various aromatic diamines and compared the properties of the resultant polybenzoxazines [26e28], and they found that the melting temperatures and polymerization temperatures of aromatic diamine-based benzoxazines are affected by the bridging group between two amine groups, so are the Tgs and the thermal degradation temperatures of the corresponding polybenzoxazines [26]. Gu and Ling incorporated 2-(6-oxido-6H-dibenzo[c,e][1,2]oxaphosphorin-6-yl) 1,4-benzenediol into an aromatic diamine based benzoxazine and found that the polymerization temperature of the monomer and the thermal degradation temperature of the polymer decrease while the Tg and mechanical properties decrease [29]. Yan et al. reported that introducing rigid group to the backbone of benzoxazines hinders the chain mobility and inhibits the polymerization reaction [30]. These studies establish a foundation for understanding the mechanism for synthesis and polymerization of
benzoxazines and properties of polybenzoxazines. However, less attention has been paid to the structural effects of diamines on synthetic reaction, polymerization, and properties of benzoxazines based on allylphenol. Recently, we synthesized two allyl-containing benzoxazines (oAP-ddm and oAP-dde) from o-allylphenol, formaldehyde, and two aromatic diamines [4,40 -diaminodiphenyl methane (DDM) and 4,40 -diaminodiphenyl ether (DDE)] [31,32]. In the synthesis of oAPddm and oAP-dde, the reaction takes a relatively long time, implying the reactivity of o-allylphenol/aromatic diamine system is relatively low. When oAP-ddm and oAP-dde are thermally polymerized, the onset and peak temperatures in their differential scanning calorimetry (DSC) curves are relatively high, indicating that the ring-opening polymerization for oAP-ddm and oAP-dde is not easy. Nevertheless, the chain flexibilities in the corresponding polybenzoxazines are relatively high due to the existence of unreacted allyl groups. In this study, to evaluate the structural effects of diamines on synthetic reaction, polymerization, and properties of benzoxazines based on o-allylphenol, two new allyl-containing bifunctional benzoxazines, 1,6-bis(8-allyl-2H-benzo[e][1,3]oxazin-3(4H)-yl) hexane (oAP-hda) and bis(4-(8-allyl-2H-benzo[e][1,3]oxazin3(4H)-yl)phenyl)sulfone (oAP-dds), were synthesized from oallylphenol, formaldehyde, and two diamines [1,6hexamethylenediamine (HDA) and 4,40 -diaminodiphenyl sulfone (DDS)]. The chemical structures of oAP-hda and oAP-dds were confirmed by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The thermally activated polymerization behaviors of oAP-hda and oAP-dds were investigated by FTIR, solid-state 13C NMR, and DSC. In addition, the dynamic mechanical properties and thermal stabilities of the corresponding polybenzaoxazines [Poly(oAP-hda) and Poly(oAP-dds)] were determined. 2. Experimental 2.1. Materials o-Allylphenol was supplied by Shandong Laizhou Hualu Accumulator Co., Ltd., China. Formaldehyde (37% aqueous), 1,6hexamethylenediamine (HDA), toluene, and chloroform were obtained from Tianjin Chemical Reagent Co., Ltd., China. 4,40 -Diaminodiphenyl sulfone (DDS) was offered by Shanghai Jingchun Chemical Reagent Co., Ltd., China. All chemicals were used as received. 2.2. Synthesis of oAP-hda In a 100 mL three-necked round bottom flask equipped with a mechanical stirrer, a thermometer, and a reflux condenser, 3.3 mL of HDA, 8 mL of formaldehyde, and 40 mL of toluene were added. The mixture was stirred for 30 min in an ice bath before adding 6.6 mL of o-allylphenol. Then the temperature was raised gradually up to reflux and kept stirring for 24 h. Thereafter, the solvent was removed by distillation under reduced pressure, and the residue was dissolved in 30 mL of chloroform. The chloroform solution was washed several times with 1 mol/L NaOH aqueous solution and deionized water, respectively. Subsequently, the solution was washed several times with 0.1 mol/L HCl aqueous solution and deionized water, respectively, followed by evaporation of the chloroform under reduced pressure. The product was dried at 70 C in a vacuum oven for 24 h, and finally a garnet brown transparent high viscous liquid was obtained. The yield was approximately 83%. FTIR (KBr, cm1): 3074 (aromatic and allylic ]CeH), 2976, 2932, 2855 (CeH), 1637 (allylic C]C), 1593 (aromatic C]C), 1464, 1334
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(CH2), 1225 (CeOeC), 1137 (CeNeC), 995 (allylic CeH), 972, 927 (out-of-plane mode of the benzene ring to which an oxazine ring is attached), 817, 750 (aromatic CeH). 1H NMR (CDCl3, ppm): 7.01e7.03 (d, 2H), 6.83e6.87 (m, 4H), 6.00e6.06 (m, 2H), 5.07e5.10 (m, 4H), 4.91 (s, 4H), 4.01 (d, 4H), 3.37e3.38 (d, 4H), 2.74e2.77 (t, 4H), 1.61 (s, 4H), 1.40 (s, 4H). 13C NMR (CDCl3, ppm): 151.90, 136.87, 127.88, 127.48, 119.94, 119.83 (C aromatic); 115.35, 82.56, 51.38, 50.43, 33.65, 28.11, 27.17 (C aliphatic). 2.3. Synthesis of oAP-dds In a 100 mL three-necked round bottom flask equipped with a mechanical stirrer, a thermometer, and a reflux condenser, 6.2 g of DDS, 10 mL of formaldehyde, and 50 mL of toluene were added. The mixture was stirred for 1 h in an ice bath before adding 6.6 mL of oallylphenol with a pressure-equalizing dropping funnel. Then, the temperature was raised gradually up to reflux and kept stirring for 24 h. Thereafter, the flask was equipped with a DeaneStark trap, and the water from the reaction was collected in the trap. After the reaction was continued for another 50 h, the solvent was removed by distillation under reduced pressure, and the residue was dissolved in 30 mL of chloroform. The chloroform solution was washed several times with 1 mol/L NaOH aqueous solution and deionized water, respectively. Subsequently, the solution was washed several times with 0.1 mol/L HCl aqueous solution and deionized water, respectively, followed by evaporation of the chloroform under reduced pressure. The product was dried at 80 C in a vacuum oven for 24 h, and finally a yellow powder was obtained. The yield was approximately 50%. FTIR (KBr, cm1): 3074 (aromatic and allylic ] CeH), 3007 (aromatic CeH), 2923, 2853 (CeH), 1637 (allylic C]C), 1592, 1506 (aromatic C]C), 1464 (CH2), 1296 (SO2), 1253 (CH2), 1223 (CeOeC), 1147, 1110 (SO2), 1074 (CeS), 991 (allylic CeH), 954 (out-of-plane mode of the benzene ring to which an oxazine ring is attached), 916 (allylic CeH), 823, 750 (aromatic CeH), 731, 645 (CeS), 553 (SO2). 1H NMR (CDCl3, ppm): 7.81e7.79 (d, 4H), 7.06e7.07 (d, 2H), 7.04 (s, 2H), 7.03 (s, 2H), 6.91e6.97 (m, 4H), 5.99e6.06 (m, 2H), 5.35 (s, 4H), 5.11e5.14 (m, 4H), 4.63 (s, 4H), 3.40e3.41 (d, 4H). 13C NMR (CDCl3, ppm): 151.94, 151.54, 136.57, 132.71, 129.10, 128.46, 128.42, 124.98, 121.06, 120.22, 115.97 (C aromatic); 115.88, 77.26, 49.50, 33.67 (C aliphatic). 2.4. Preparation of polybenzoxazines First, oAP-hda or oAP-dds was put into a steel mold, and the mold was put into a vacuum oven. Then, for oAP-hda, the vacuum oven was step-heated to 100, 120, 140, 160, and 180 C and hold at each temperature for 1 h, thereafter, hold at 200 C for 6 h; whereas for oAP-dds, the vacuum oven was step-heated to 100, 120, 140, 160, 180, and 200 C and held at each temperature for 1 h, and then at 230 C for 6 h.
13 C spectra were externally referenced to the carbon signal of solid adamantane (38.48 ppm relative to TMS). The FTIR spectra were obtained with a Nicolet 380 FTIR spectrometer at a resolution of 4 cm1. The benzoxazine sample was dissolved in chloroform, and the solution was coated on a KBr disk to form a thin uniform film. When the solvent was completely evaporated at 50 C in a vacuum oven, the disk was scanned by the FTIR spectrometer. Thereafter, the disk was heated isothermally in a static air oven. During the polymerization reaction, the disk was removed periodically for measurement. The dynamic polymerization reactions for oAP-hda and oAP-dds were monitored by a PerkinElmer Diamond differential scanning calorimeter operating in nitrogen, and the samples were scanned at a heating rate of 10 C/min. A PerkineElmer DMA-8000 dynamic mechanical analyzer was used to determine the dynamic storage modulus (E0 ) and loss factor (tan d) using three-point bending mode. Measurements were performed on rectangular specimens with dimensions of 50.0 mm 9.0 mm 2.6 mm by heating from 25 to 250 C, with a heating rate of 2 C/min and a frequency of 1 Hz. Thermogravimetric analysis was performed on a Netzsch TG 209 F3 thermogravimeter, and polybenzoxazine fine powder samples of approximately 10 mg were heated to 800 C at a heating rate of 10 C/min in a nitrogen (or air) flow of 100 mL/min.
3. Results and discussion 3.1. Structural effects of diamines on synthesis of benzoxazines oAP-hda and oAP-dds were synthesized from o-allylphenol, formaldehyde, HDA, and DDS via a solution method [31,32], and the synthesis mechanism for oAP-hda and oAP-dds is shown in Scheme 1. In synthesis for o-allylphenol based benzoxazines, the reaction is thought complete when the solution color changes from colorless to reddish-brown, and it takes less time (around 24 h) for the reaction of o-allylphenol/HDA/formaldehyde than for those of both oallylphenol/DDM/formaldehyde and o-allylphenol/DDE/formaldehyde. However, the solution color does not change in a relatively long time (48 h) under the same reflux temperature for o-allylphenol/DDS/formaldehyde, indicating that the reactivity of DDS is obviously lower than those of HDA, DDM, and DDE. To overcome the low reactivity of DDS and enhance the reaction rate of o-allylphenol/DDS/formaldehyde system, the water from the reaction is distilled after the reaction temperature is raised up to reflux for a period of time (such as 24 h). The same distillation procedure was
2.5. Measurements Both proton (1H) and carbon (13C) NMR spectra were recorded using a Bruker Avance III 600 NMR spectrometer at a proton frequency of 600 MHz and the corresponding carbon frequency. Deuterated chloroform (CDCl3) was used as the solvent and tetramethylsilane (TMS) was used as an internal standard. Solid-state NMR experiments were performed at room temperature (25 C) on a Bruker Avance III 400 NMR spectrometer operating at a 13C resonance frequency of 100.568 MHz. The samples were analyzed under cross-polarization/magic-angle spinning (CP/MAS) conditions using 4-mm zirconia rotors at a spinning frequency of 5 kHz. A 90 pulse width of 4 ms was employed, and the CP HartmanneHahn contact time was set at 3.0 ms. The chemical shifts of
31
Scheme 1. Mechanism of synthesis of oAP-hda and oAP-dds.
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used in the synthetic reaction for oAP-ddm, oAP-dde, and oAP-hda could also effectively enhance the reaction rate and shorten the reaction time. It is certain that the reactivity difference between HDA and DDS is due to the difference in the steric and electronic effects exerted by the substituent groups on nitrogen atom. On one hand, the mobility of the bulky molecule of DDS is relatively lower than that of HDA at the same temperature, which results in a lower molecule collision chance in o-allylphenol/DDS/formaldehyde system than in o-allylphenol/HDA/formaldehyde system. On the other hand, the electronic effect of N-substituent plays an important role on the reactivity (nucleophilicity and basicity) of diamines. An amine group can react with formaldehyde as a nucleophile by attacking the electron-deficient carbonyl carbon to form a carbinolamine intermediate (Scheme 2) [33], and the reaction rate depends on the nucleophilicity of the amine group. The basicities of aromatic amines are lower than those of aliphatic ones, and the sulfone group greatly decreases the electron density on nitrogen atom in DDS both by induction and resonance [34e36]. Therefore, DDS is a weaker nucleophile than HDA and the oxazine ring formation in oallylphenol/DDS/formaldehyde system is inhibited in comparison with the reaction of o-allylphenol/HDA/formaldehyde system. For aromatic diamines, the nucleophilicity of DDS is lower than those of DDM and DDE due to the strong electron-withdrawing effect of the sulfone group. In addition, an insoluble white color gel is frequently formed in the synthetic reaction of diamine based benzoxazines [7,27], which is due to the formation of a triazine network intermediate [33]. The formation of the triazine network often appears in the synthetic reaction for bifunctional benzoxazines based on o-allylphenol (Scheme 3) [33], which hinders the reaction proceeding, especially in the synthetic reaction of oAP-dds, whereas the formation of the insoluble gel is not obviously observed in the synthetic reaction of oAP-hda. Thus, the synthetic reaction of oAP-hda proceeds easier than that of oAP-dds.
3.2. Characterization of oAP-hda and oAP-dds The chemical structures of oAP-hda and oAP-dds were confirmed by 1H and 13C NMR and FTIR. Fig. 1a shows the 1H NMR spectrum of oAP-hda. The resonances at 4.91 and 4.01 ppm correspond to the methylene protons (H10 and H11) of OeCH2eN and AreCH2eN of the oxazine ring, respectively. The chemical shifts (ppm) at 6.00e6.06 (2H, H2), 5.07e5.10 (4H, H1), and 3.37e3.38 (4H, H3) correspond to the aliphatic protons in the allyl group, and 2.74e2.77 (4H, H12), 1.61 (2H, H13), and 1.40 (4H, H14) are assigned to the protons of the methylene groups between the two oxazine rings. The peaks at 7.01e7.03 (2H, H5), 6.85e6.87 (2H, H7), and 6.83e6.84 (2H, H6) ppm are assigned to the aromatic protons. The ratio of the corresponding integration areas of the eleven protons (H1, H2, H3, H5, H6, H7, H10, H11, H12, H13, and H14) was determined roughly to be
2:1:2:1:1:1:2:2:2:2:2, which corresponds well with the theoretical proton ratio based on the chemical structure. Fig. 1b show the corresponding 13C NMR spectrum of oAP-hda, the resonances at 82.56 and 50.43 ppm correspond to the methylene carbons (C10 and C11) of OeCH2eN and AreCH2eN of the oxazine ring, respectively. Other chemical shifts (ppm) are assigned to the resonances of the carbons: 27.17 (C13), 28.11 (C14), 33.65 (C3), 51.38 (C12), 115.35 (C1), 119.83 (C6), 119.94 (C8), 125.49 (C7), 127.48 (C5), 127.88 (C4), 136.87 (C2), 151.90 (C9). Fig. 2a shows the 1H NMR spectrum of oAP-dds. The resonances at 5.35 and 4.63 ppm correspond to the methylene protons (H10 and H11) of OeCH2eN and AreCH2eN of the oxazine ring, respectively. The chemical shifts (ppm) at 3.40 and 3.41 (4H, H3), 5.11 and 5.14 (4H, H1), and 5.99e6.06 (2H, H2) correspond to the aliphatic protons in the allyl group. The peaks at 6.91e6.97 (4H, H13), 7.03 (2H, H6), 7.04 (2H, H7), 7.06 and 7.07 (2H, H5), and 7.79e7.81 (4H, H14) ppm are assigned to the aromatic protons. The ratio of the corresponding integration areas of the ten protons (H1, H2, H3, H5, H6, H7, H10, H11, H13, and H14) was determined roughly to be 2:1:2:1:1:1:2:2:2:2, which corresponds well with the theoretical proton ratio based on the chemical structure. In the corresponding 13C NMR spectrum of oAP-dds in Fig. 2b, the resonances at 77.26 and 49.50 ppm correspond to the methylene carbons (C10 and C11) of OeCH2eN and AreCH2eN of the oxazine ring, respectively. Other chemical shifts (ppm) are assigned to the resonances of the carbons: 33.67 (C3), 115.88 (C1), 115.97 (C13), 120.22 (C6), 121.06 (C8), 124.98 (C7), 128.42 (C5), 128.46 (C4), 129.10 (C14), 132.71 (C15), 136.57 (C2), 151.54 (C12), 151.94 (C9). The FTIR spectrum of oAP-hda is shown in Fig. 3a. The characteristic absorption at 1137 cm1 is assigned to the asymmetric stretching vibration of CeNeC of the oxazine ring [37]. The asymmetric and symmetric stretching vibrations of CeOeC of the oxazine ring are observed at 1225 and 1076 cm1, respectively [37]. The absorptions at 972 and 927 cm1 are the out-of-plane mode of the benzene ring to which an oxazine ring is attached. The C]C stretching vibration at 1637 cm1 and the olefinic CeH out of plane bending vibration at 995 cm1 belong to the characteristic absorptions of the allyl group. The ]CeH stretching vibrations of the allyl group and the aromatic ring appear at 3074 cm1, together with the peaks at 3040 and 3002 cm1 are the ]CeH stretching vibrations of the aromatic ring. The CeH asymmetric stretching vibrations of CH2 are at 2976 and 2932 cm1, whereas the CeH symmetric stretching vibration of CH2 is at 2855 cm1. The absorption peaks at bands of 1464, 1375, and 1334 cm1 are due to CH2 bending, wagging, and twisting vibrations, respectively. The absorption at 1593 cm1 is associated with the C]C stretching vibrations of the aromatic ring. The bands at 817, 772, 750, and 625 cm1 are assigned to the CeH out-of-plane bending vibrations of the aromatic ring. The FTIR spectrum of oAP-dds is shown in Fig. 4a. The absorption peak at 1223 cm1 is assigned to the asymmetric stretching vibration CeOeC of the oxazine ring [37], and the absorption at 954 cm1 is the out-of-plane mode of the benzene ring to which an
Scheme 2. Mechanism of formation of a carbinolamine intermediate in synthesis of benzoxazines.
Y. Liu et al. / Polymer 57 (2015) 29e38
33
Scheme 3. Mechanism of formation of a triazine network intermediate in synthesis of benzoxazines.
oxazine ring is attached. The C]C stretching vibration at 1637 cm1 and the olefinic CeH out of plane bending vibrations at 991 and 916 cm1 belong to the characteristic absorptions of the allyl group. The ]CeH stretching vibrations of the allyl group and the aromatic ring appear at 3074 cm1, together with the peak at 3007 cm1 is the ]CeH stretching vibration of the aromatic ring. The CeH asymmetric stretching vibrations of CH2 are at 2953 and 2923 cm1, whereas the CeH symmetric stretching vibration of CH2 is at 2853 cm1. The absorption peaks at bands of 1464, 1401, 1384, and 1253 cm1 are due to CH2 bending, scissoring, wagging, and twisting vibrations of the oxazine ring, respectively. The absorptions at 1592 and 1506 cm1 are associated with the C]C stretching vibrations of the aromatic ring. The bands at 823 and 750 cm1 are assigned to the CeH out-of-plane bending vibrations
Fig. 1. 1H and
13
C NMR spectra of oAP-hda.
of the aromatic ring. The peak at 1296 cm1 is assigned to the asymmetric stretching vibration of SO2, whereas the peaks at 1147 and 1110 cm1 are due to the symmetric stretching vibrations of SO2. The absorptions at 1074, 731, and 645 cm1 are due to the stretching vibrations of aromatic CeS, whereas the peak at 553 cm1 belongs to the in-plane bending vibration of SO2. 3.3. Structure changes in polymerization Fig. 3aee shows the FTIR spectra of oAP-hda and the polymerized products obtained at 200 C for various times. The intensities of the absorptions at 927, 1076, 1137, and 1225 cm1 decreased gradually with the polymerization proceeding, indicating that the
Fig. 2. 1H and
13
C NMR spectra of oAP-dds.
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Fig. 3. FTIR spectra of (a) oAP-hda and (b)e(e) polymerization products obtained at 200 C in air for various times.
Fig. 4. FTIR spectra of (a) oAP-dds and (b)e(e) polymerization products obtained at 230 C in air for various times.
oxazine ring-opening reaction occurred. Correspondingly, the absorption of phenolic eOH appears at 3375 cm1 and the intensity increases with increasing time. At the same time, the absorption intensity of the olefinic CeH out of plane bending vibration at 995 cm1 decreases with the reaction proceeding, implying that the radical polymerization occurred for allyl groups. In addition, the absorption intensity of the peak at 1676 cm1 increases with increasing time, due to the carbonyl absorption resulted from the oxidation of the allylic C]C double bonds. Fig. 4aee shows the FTIR spectra of oAP-dds and the polymerized products obtained at 230 C for various times. The decrease in the intensities of the peaks at 954 and 1223 cm1 indicates that the oxazine ring-opening reaction occurred, whereas the decrease in the intensities of the peaks at 916, 991, and 1637 cm1 indicates that the radical polymerization occurred for allyl groups. The intensities of two new peaks increase with increasing time, one peak at 3375 cm1 is ascribed to the absorption of the phenolic eOH, whereas the other peak at 1680 cm1 is attributed to the carbonyl groups resulted from the oxidation of the allylic C]C double bonds. Based on the cationic ring-opening mechanism of benzoxazine polymerization and the structure characteristics of oAP-hda and oAP-dds, the structures of the polymerized products for oAP-hda and oAP-dds can be described in Scheme 4 [31,32]. To better understand the structure changes from benzoxazine monomers to polybenzoxazines, Fig. 5 shows the solid-state 13C CP/ MAS NMR spectra of oAP-hda and oAP-dds monomers and their corresponding polybenzoxazines. From benzoxazine monomers to polybenzoxazines, the significant changes in the solid-state 13C NMR spectra are the intensity decrease or disappearance of the resonance peaks of the carbons of oxazine rings and the unsaturated carbons of allyl groups. In the solid-state 13C NMR spectrum for oAP-hda shown in Fig. 5a, the peaks at 84.58 and 47.75 ppm correspond to the resonances of the methylene carbons (C10 and C11) of OeCH2eN and AreCH2eN of the oxazine ring, respectively, whereas the resonances of C1, C2, and C3 of the allyl group are at 116.08, 137.65, and 34.23 ppm, respectively. Correspondingly, in the solid-state 13C NMR spectrum for Poly(oAP-hda) shown in Fig. 5b, the resonance peaks of the methylene carbons (C10 and C11) of OeCH2eN and AreCH2eN are overlapped at 51.99e59.14 ppm, together with the resonance peak of C12, whereas the resonance peaks of C1, C2, and C3 in Poly(oAP-hda) are overlapped at ~34.40 ppm [31,32]. In addition, the resonance peak at 21.80 ppm is assigned to the terminal methyl carbons [31,32], overlapping with the peak of the terminal C1.
Scheme 4. Structure changes from benzoxazines to polybenzoxazines.
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Fig. 5. Solid-state
35
13
C NMR spectra of (a) oAP-hda, (b) Poly(oAP-hda), (c) oAP-dds, and (d) Poly(oAP-dds).
In the solid-state 13C NMR spectrum for oAP-dds shown in Fig. 5c, the peaks at 76.61 and 46.35 ppm correspond to the resonances of the methylene carbons (C10 and C11) of OeCH2eN and AreCH2eN of the oxazine ring, respectively, whereas the resonances of C1, C2, and C3 of the allyl group are at 112.10, 137.06, and 33.36 ppm, respectively. Correspondingly, in the solid-state 13C NMR spectrum for Poly(oAP-dds) shown in Fig. 5d, the resonance peaks of the methylene carbons (C10 and C11) of OeCH2eN and AreCH2eN are overlapped at ~45.90 ppm, whereas the resonance peaks of C1, C2, and C3 in Poly(oAP-dds) are overlapped at ~33.65 ppm [31,32]. In addition, the resonance peak at 20.86 ppm is assigned to the terminal methyl carbons [31,32], overlapping with the peak of the terminal C1. Moreover, from monomers to polymers, the chemical shifts of aromatic carbons and aliphatic saturated carbons does not change, but most of the chemical shifts of these carbons become hard to be distinguished because the corresponding peaks are highly overlapped. In addition, comparing the solid-state 13C NMR spectra of Poly(oAP-hda) and Poly(oAP-dds) with those of polybenzoxazines based on bisphenol-A/methylamine and bisphenol-A/aniline reported by Koenig and coworkers [38], the common characteristic can be noted that the peaks of the methylene carbons of oxazine rings disappear or almost disappear and the chemical shifts of the methylene carbons of Mannich bridge structures of polybenzoxazines locate near the characteristic chemical shift of the methylene carbon of AreCH2eN of oxazine rings.
3.4. Polymerization behavior The non-isothermal DSC curves of oAP-hda and oAP-dds are shown in Fig. 6. Besides an endothermic peak corresponding to the melting process appeared at 98 C in the DSC curve of oAP-dds, each of the two DSC curves shows an exothermic peak
Fig. 6. DSC curves of oAP-hda and oAP-dds.
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corresponding to both the oxazine ring-opening polymerization and the radical polymerization of allyl groups. The onset temperatures of the exothermic peaks are at 185 and 223 C for oAP-hda and oAP-dds, respectively, with respective maxima at 238 and 281 C. The difference in onset and peak temperatures between the DSC curves of oAP-hda and oAP-dds is due to the difference in structure of the substituent groups on nitrogen atom. In view of the bulkiness of the substituent groups, sterically less hindered alkyl group on the nitrogen atom allows faster polymerization of benzoxazine monomers [12] and the bulkier substituent on the nitrogen atom corresponds to the slower polymerization [13]. Therefore, the polymerization of oAP-hda with a flexible methylene chain is faster than that of oAP-dds with rigid sulfone-bridged benzene rings. On the other hand, the linear methylene groups can enhance the stability of zwitterionic intermediates and facilitate ring opening at lower temperatures, whereas the electronwithdrawing sulfone-bridged benzene rings will destabilize the intermediates [12,39]. Generally, the ring-opening of benzoxazines would form two corresponding zwitterionic intermediates (with aminomethyl and imminium cations, respectively), and the two intermediates could react with benzoxazine monomers and thus the increase of the concentrations of the intermediates would achieve faster polymerization [39]. The methylene chain than the sulfonebridged benzene rings could have better effect on stability of the zwitterionic species, which would lead to the polymerization of oAP-hda at lower temperature than oAP-dds. 3.5. Dynamic mechanical properties Fig. 7 shows the curves of the storage modulus (E0 ) and loss factor (tan d) for Poly(oAP-hda) and Poly(oAP-dds), and the analysis
results on the curves are summarized in Table 1. The E0 of Poly(oAPdds) is much higher than that of Poly(oAP-hda) in the glassy region, indicating a higher stiffness of Poly(oAP-dds) over Poly(oAP-hda), which may be due to the difference in rigidity between sulfonebridged benzene rings and linear methylene chain. On the other hand, a higher crosslinking density often leads to a high E0 . With an increase in temperature, E0 decreases, and a sharp decline in E0 can be observed in the glass transition region, due to the increase of chain mobility at higher temperatures. In the rubbery region, no significant change in E0 can be seen. The difference in modulus between the glassy and rubbery states is smaller for Poly(oAP-hda) than for Poly(oAP-dds) due to the high modulus of Poly(oAP-dds) in the glassy region. According to rubber elasticity theory, the plateau of the elastic modulus in the rubbery state is associated with the crosslinking density of the materials [40], and an empirical equation (1) was proposed by Nielsen and Landel to describe the approximate relationship [41], which is reported to better describe the elastic properties of dense networks [42e44].
log
0 Ee ¼ 7:0 þ 293ðrx Þ 3
(1)
where E0 e (dyne/cm2) is an equilibrium elastic modulus in rubbery plateau, rx (mol/cm3) is the crosslinking density which is the mole number of network chains per unit volume of the polymers. Based on equation (1), the crosslinking densities of Poly(oAP-hda) and Poly(oAP-dds) can be calculated from the plateau of the storage shear modulus in the rubbery state (G0 e), which equals E0 e/3, and E0 e in the rubbery region is determined by taking the value at the inflection point of the plateau. The calculated results are summarized in Table 1. It is evident that the crosslinking density of Poly(oAP-dds) is much higher than that of Poly(oAP-hda). The degree of interchain crosslinking depends on the electron density of the carbon (C6) on para position (with respect to the ether of the benzoxazine ring), because a high electron density facilitates the carbon undergoing electrophilic attack of carbocations formed in the oxazine ringopening reaction. The chemical shifts of hydrogen on C6 for oAPhda and oAP-dds show that the electron density of C6 in oAP-dds is higher than that in oAP-hda (Figs. 1a and 2a). Thus the reactivity index for electrophilic substitution at C6 of oAP-dds is higher than that of oAP-hda. As a result, the obtained crosslinking density is higher for Poly(oAP-dds) than for Poly(oAP-hda). On the other hand, the crosslinking density of Poly(oAP-hda) is lower than the values of polybenzoxazines based on phenol/HDA, o-cresol/HDA, and p-cresol/HDA reported by Ishida and Allen [23], indicating that the flexibility of o-allylphenol based polybenzoxazines is higher than those of polybenzoxazines based on phenol, o-cresol, and pcresol due to the bulk unreacted allyl group in o-allylphenol based polybenzoxazines. Corresponding to the decrease in E0 , an a-relaxation peak appears in each of the two tan d curves, and the a-relaxation peak temperatures taken as the Tg values, together with peak heights and peak widths at half-height are listed in Table 1. The Tg of Poly(oAPhda) is much lower than that of Poly(oAP-dds), whereas the height
Table 1 Analysis of the DMA curves of Poly(oAP-hda) and Poly(oAP-dds). Width of Polybenzoxazines E' (25 C) Tg rx (103) Height of tan d peaks (GPa) ( C) tan d peaks (arbitrary units) (half ht.) ( C)
Fig. 7. Dynamic mechanical curves of Poly(oAP-hda) and Poly(oAP-dds).
Poly(oAP-hda) Poly(oAP-dds)
1.44 3.18
87 1.33 193 1.05
26 37
2.12 3.08
Y. Liu et al. / Polymer 57 (2015) 29e38
of the tan d peak of Poly(oAP-hda) is higher than that of Poly(oAPdds), implying that the flexibilities of the molecular chains of Poly(oAP-hda) are higher than those of Poly(oAP-dds), because the flexibility of the long methylene chain is higher than that of the sulfone-bridged benzene rings. On the other hand, the crosslinking densities of polybenzoxazine networks also affect the flexibilities of the molecular chains. Moreover, the peak width at half-height of the tan d curve of Poly(oAP-dds) is broader than that of Poly(oAPhda), indicating the segmental mobility in Poly(oAP-dds) is lower than that in Poly(oAP-hda). Therefore, Poly(oAP-dds) has more number of kinetic units adequately to move than Poly(oAP-hda), which results in a broader distribution of structures. Consequently, the glass transition of Poly(oAP-dds) takes place in a relatively broad temperature range. Comparing the dynamic mechanical properties of Poly(oAPhda) and Poly(oAP-dds) with those of Poly(oAP-ddm) and Poly(oAP-dde) [31,32], it can be seen that the moduli in the glassy state and Tgs of Poly(oAP-ddm) and Poly(oAP-dde) are higher than those of Poly(oAP-hda) but lower than those of Poly(oAP-dds), due to the difference in structural effects (including the flexibility of the substituent groups on nitrogen atom and the crosslinking density of the networks). In addition, it is noted that the modulus in the glassy state and Tg of Poly(oAP-dds) are lower than those of phenol-DDS based polybenzoxazine [7], which indicates that the crosslinking density of the former might be lower than that of the latter, due to a low degree of polymerization formed through the less favorable paraposition. On the other hand, the unreacted allylic C]C double bonds in Poly(oAP-dds) might also be contributed to the relatively low E0 and Tg.
37
Poly(oAP-dds), the weigh loss temperature is higher in air than in nitrogen for a given weight loss below 604 C for Poly(oAP-hda) and 549 C for Poly(oAP-dds), and a weight increase is obviously seen for Poly(oAP-dds) before weight loss in air. In a previous report, the oxidation of the unreacted allylic C]C double bonds in Poly(oAPddm) was confirmed by element analysis on the thermal degradation residues [31]. Alternately, the weight increase phenomenon in thermal degradation in dynamic air atmosphere reflects the existence of unreacted allylic C]C double bonds in polybenzoxazines based on o-allylphenol. It is reasonable that the weight increase in dynamic air for Poly(oAP-dds) is much obvious than those in static air for Poly(oAP-ddm) and Poly(oAP-dde) [31,32], because more oxygen is supplied from air flow. But no weight increase for Poly(oAP-hda) can be seen, which is due to the low thermal stability of Poly(oAP-hda). The acceleration effect of oxygen on thermo-oxidation degradation of Poly(oAP-hda) and Poly(oAP-dds) can be clearly seen in high temperature range. The char yields of Poly(oAP-hda) and Poly(oAP-dds) are 22.3% and 40.0% at 800 C in nitrogen, respectively, whereas the char yields of Poly(oAP-hda) and Poly(oAP-dds) are close to 0 at 700 C in air. 4. Conclusions Two allyl-containing benzoxazines were synthesized based on o-allylphenol. The reactivity of HDA is higher than that of DDS in the synthetic reaction of benzoxazines, and the polymerization temperature of oAP-hda is lower than that of oAP-dds. The storage moduli in the glassy state are 1.44 and 3.18 GPa for Poly(oAP-hda) and Poly(oAP-dds), respectively, and the glass transition temperatures are 87 and 193 C, respectively. The thermal stability of Poly(oAP-dds) is much higher than that of Poly(oAP-hda).
3.6. Thermal stability Acknowledgments Fig. 8 shows the TG curves of Poly(oAP-hda) and Poly(oAP-dds) in nitrogen and air. Obviously, the thermal stability of Poly(oAPdds) is much higher than that of Poly(oAP-hda), due to the higher crosslinking density and the higher aromatic ring content in Poly(oAP-dds) over Poly(oAP-hda). The 5% weight loss temperatures for Poly(oAP-hda) and Poly(oAP-dds) are 242 and 370 C in nitrogen, respectively, which are lower than their corresponding weight loss temperatures (251 and 403 C) in air, respectively. However, these results do not mean more stable in air than in nitrogen for Poly(oAP-hda) and Poly(oAP-dds). In fact, due to the oxidation of the unreacted allylic C]C double bonds in Poly(oAP-hda) and
This work was financially supported by the Natural Science Foundation of Hebei Province (B2013201107). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
Fig. 8. TG curves of Poly(oAP-hda) and Poly(oAP-dds).
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Structural effects of diamines on synthesis, polymerization, and properties of benzoxazines based on o-allylphenol Yanfang Liu*, Zhanzhan Hao, Shufang Lv, Jinbai Huang, Chunyan Liao, Mingtao Run Key Laboratory of Analytical Science and Technology of Hebei Province, College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 July 2014 Received in revised form 1 December 2014 Accepted 2 December 2014 Available online 12 December 2014
Two allyl-containing bifunctional benzoxazines (oAP-hda and oAP-dds) were synthesized from o-allylphenol, formaldehyde, and two diamines [1,6-hexamethylenediamine (HDA) and 4,40 -diaminodiphenyl sulfone (DDS)]. The chemical structures of oAP-hda and oAP-dds were confirmed by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The reactivity of HDA is higher than that of DDS in the synthetic reaction of benzoxazines, and the polymerization temperature of oAP-hda is lower than that of oAP-dds. The structure changes from benzoxazines to their corresponding polybenzoxazines [Poly(oAP-hda) and Poly(oAP-dds)] were studied by FTIR and solid state 13 C NMR. The dynamic mechanical properties and thermal stabilities of Poly(oAP-hda) and Poly(oAP-dds) were studied by dynamic mechanical analysis and thermogravimetry, respectively. The storage moduli in the glassy state are 1.44 and 3.18 GPa for Poly(oAP-hda) and Poly(oAP-dds), respectively, and the glass transition temperatures are 87 and 193 C, respectively. The thermal stability of Poly(oAP-dds) is much higher than that of Poly(oAP-hda). © 2014 Elsevier Ltd. All rights reserved.
Keywords: Benzoxazine Electronic effect Sulfone
1. Introduction Benzoxazines are a new class of thermosetting resins that can be polymerized into polybenzoxazines with excellent physical properties [1,2]. Thus, they are attracting much attention from both academia and industry. Owing to the molecular design flexibility, benzoxazines with different structures have been synthesized from phenols, amines, and formaldehyde [3e9]. Naturally, the structures of phenols and amines are essential not only to affect the ringforming reaction and thermally activated ring-opening polymerization of benzoxazines but also to govern physical properties of the resultant polybenzoxazines. In practice, several structural factors from phenols and amines are frequently considered, such as steric and electronic effects from substituent groups on the reactivity in both synthetic reaction and polymerization of benzoxazines, and the effect of conformational flexibility or rigidity of substituent groups on physical properties of polybenzoxazines. Many researchers explored the structural effects of various substituents on synthesis, polymerization, and properties of a series of benzoxazines based on monofunctional amines.
* Corresponding author. E-mail address: [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.polymer.2014.12.005 0032-3861/© 2014 Elsevier Ltd. All rights reserved.
Ishida and Sanders indicated that electron-donating alkyl substituent groups at one or both meta positions on arylamine rings facilitate ring-opening polymerization of monofunctional alkylsubstituted aromatic amine-based benzoxazines [10]. Ronda and coworkers reported that an increase of the polymerization temperature takes place when the electron-withdrawing character of the groups in the para-position of the phenyl substituent increases for monofunctional benzoxazine monomers [11]. Endo et al. disclosed that the allyl group in allylamine can stabilize the zwitterionic intermediates and promote ring-opening polymerization of benzoxazines [12], though bulkier substituents on the nitrogen atom correspond to slower polymerization of benzoxazines [12,13]. Gu et al. revealed that the electron-withdrawing bridging groups in bisphenols inhibit the synthetic reaction and promote the thermally activated polymerization of bis-benzoxazines [14]. Besides, the substituting groups on phenolic units can make a notable impact on the molecular structure parameters of benzoxazines and on the ring-opening reaction mechanism, and the ring-opening reactions of benzoxazines with electron-withdrawing substituting groups proceed easier to some extent than those of benzoxazines with electron-donating ones [15]. Ishida and Liu synthesized a series of 2,20 -, 2,40 -, and 4,40 -substituted benzoxazine monomers based on isomeric bisphenol F and aniline, and they found an
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anomalous phenomenon that the rate of polymerization increases in the order 4,40 -, 2,40 -, and 2,20 -substituted monomer and the polymer obtained from 2,20 -substituted monomer gives highest thermal stability as well as glass transition temperature (Tg) [16]. Xin et al. reported that the surface energies of polybenzoxazines decrease with the increase of alkyl chain length and both the chain length and bulkiness of the alkyl group of N-substituents have an effect on the hydrogen bonding network of polybenzoxazines [17]. Ishida et al. found that the intensity of the hydrogen bonding increases with the increased amine substituent size and the nature of hydrogen bonding is very similar if the amines have the similar basicity [18]. Moreover, the onset temperature of weight loss is lower and the progression of weight loss is more rapid for the longer chain amine polybenzoxazine [19], and various phenols substantially affect the char yield of a series of polybenzoxazines based on aniline under an inert environment and have a significant influence on the degradation behavior of polybenzoxazines below 600 C under an oxidative environment [20]. In addition, different phenolic substituents at various points on the phenolic units can have critical influences on the degree to which the photooxidation reaction occurs and can impact upon the general photooxidation mechanism [21], and the structures of the amines in polybenzoxazines have a significant impact on the photooxidation process [22]. As for diamine based benzoxazines, Ishida and Allen reported that an electron-donating methyl group attached to a phenol ring has the effect of either stabilizing the benzoxazine rings against thermal activation, or reducing the effectiveness of the phenolic eOH groups of open rings as initiators or catalysts [23]. They found that the melting temperatures of linear aliphatic diamine-based series of benzoxazines decrease as a function of diamine chain length [23,24], but increase by adding a methyl substituent to phenol in the order of meta > ortho > para > unsubstituted monomers [23], while the thermally activated polymerization temperatures of the benzoxazines increase with the rising of diamine chain length [23,24], or by adding a methyl substituent to phenol [23]. The Tgs of the corresponding polybenzoxazines decrease with the rising of diamine chain length [23,25], or by adding a methyl substituent to phenol in the order of ortho < para < meta < unsubstituted polymers [23], and the density, room temperature modulus, crosslink density, yield stress, and char yield of aliphatic amine-based polybenzoxazines decrease as a function of aliphatic diamine chain length [25]. Moreover, the yield strain and thermal degradation temperature increase as a function of aliphatic diamine chain length, and methyl substitution at the ortho, para, or meta position of these benzoxazines seems to adversely affect both the onset of degradation and the char yield [23]. Therefore, aliphatic amine-based polybenzoxazines are much more flexible than the bisphenol-type polybenzoxazines [25]. Lin et al. synthesized a series of benzoxazines based on various aromatic diamines and compared the properties of the resultant polybenzoxazines [26e28], and they found that the melting temperatures and polymerization temperatures of aromatic diamine-based benzoxazines are affected by the bridging group between two amine groups, so are the Tgs and the thermal degradation temperatures of the corresponding polybenzoxazines [26]. Gu and Ling incorporated 2-(6-oxido-6H-dibenzo[c,e][1,2]oxaphosphorin-6-yl) 1,4-benzenediol into an aromatic diamine based benzoxazine and found that the polymerization temperature of the monomer and the thermal degradation temperature of the polymer decrease while the Tg and mechanical properties decrease [29]. Yan et al. reported that introducing rigid group to the backbone of benzoxazines hinders the chain mobility and inhibits the polymerization reaction [30]. These studies establish a foundation for understanding the mechanism for synthesis and polymerization of
benzoxazines and properties of polybenzoxazines. However, less attention has been paid to the structural effects of diamines on synthetic reaction, polymerization, and properties of benzoxazines based on allylphenol. Recently, we synthesized two allyl-containing benzoxazines (oAP-ddm and oAP-dde) from o-allylphenol, formaldehyde, and two aromatic diamines [4,40 -diaminodiphenyl methane (DDM) and 4,40 -diaminodiphenyl ether (DDE)] [31,32]. In the synthesis of oAPddm and oAP-dde, the reaction takes a relatively long time, implying the reactivity of o-allylphenol/aromatic diamine system is relatively low. When oAP-ddm and oAP-dde are thermally polymerized, the onset and peak temperatures in their differential scanning calorimetry (DSC) curves are relatively high, indicating that the ring-opening polymerization for oAP-ddm and oAP-dde is not easy. Nevertheless, the chain flexibilities in the corresponding polybenzoxazines are relatively high due to the existence of unreacted allyl groups. In this study, to evaluate the structural effects of diamines on synthetic reaction, polymerization, and properties of benzoxazines based on o-allylphenol, two new allyl-containing bifunctional benzoxazines, 1,6-bis(8-allyl-2H-benzo[e][1,3]oxazin-3(4H)-yl) hexane (oAP-hda) and bis(4-(8-allyl-2H-benzo[e][1,3]oxazin3(4H)-yl)phenyl)sulfone (oAP-dds), were synthesized from oallylphenol, formaldehyde, and two diamines [1,6hexamethylenediamine (HDA) and 4,40 -diaminodiphenyl sulfone (DDS)]. The chemical structures of oAP-hda and oAP-dds were confirmed by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The thermally activated polymerization behaviors of oAP-hda and oAP-dds were investigated by FTIR, solid-state 13C NMR, and DSC. In addition, the dynamic mechanical properties and thermal stabilities of the corresponding polybenzaoxazines [Poly(oAP-hda) and Poly(oAP-dds)] were determined. 2. Experimental 2.1. Materials o-Allylphenol was supplied by Shandong Laizhou Hualu Accumulator Co., Ltd., China. Formaldehyde (37% aqueous), 1,6hexamethylenediamine (HDA), toluene, and chloroform were obtained from Tianjin Chemical Reagent Co., Ltd., China. 4,40 -Diaminodiphenyl sulfone (DDS) was offered by Shanghai Jingchun Chemical Reagent Co., Ltd., China. All chemicals were used as received. 2.2. Synthesis of oAP-hda In a 100 mL three-necked round bottom flask equipped with a mechanical stirrer, a thermometer, and a reflux condenser, 3.3 mL of HDA, 8 mL of formaldehyde, and 40 mL of toluene were added. The mixture was stirred for 30 min in an ice bath before adding 6.6 mL of o-allylphenol. Then the temperature was raised gradually up to reflux and kept stirring for 24 h. Thereafter, the solvent was removed by distillation under reduced pressure, and the residue was dissolved in 30 mL of chloroform. The chloroform solution was washed several times with 1 mol/L NaOH aqueous solution and deionized water, respectively. Subsequently, the solution was washed several times with 0.1 mol/L HCl aqueous solution and deionized water, respectively, followed by evaporation of the chloroform under reduced pressure. The product was dried at 70 C in a vacuum oven for 24 h, and finally a garnet brown transparent high viscous liquid was obtained. The yield was approximately 83%. FTIR (KBr, cm1): 3074 (aromatic and allylic ]CeH), 2976, 2932, 2855 (CeH), 1637 (allylic C]C), 1593 (aromatic C]C), 1464, 1334
Y. Liu et al. / Polymer 57 (2015) 29e38
(CH2), 1225 (CeOeC), 1137 (CeNeC), 995 (allylic CeH), 972, 927 (out-of-plane mode of the benzene ring to which an oxazine ring is attached), 817, 750 (aromatic CeH). 1H NMR (CDCl3, ppm): 7.01e7.03 (d, 2H), 6.83e6.87 (m, 4H), 6.00e6.06 (m, 2H), 5.07e5.10 (m, 4H), 4.91 (s, 4H), 4.01 (d, 4H), 3.37e3.38 (d, 4H), 2.74e2.77 (t, 4H), 1.61 (s, 4H), 1.40 (s, 4H). 13C NMR (CDCl3, ppm): 151.90, 136.87, 127.88, 127.48, 119.94, 119.83 (C aromatic); 115.35, 82.56, 51.38, 50.43, 33.65, 28.11, 27.17 (C aliphatic). 2.3. Synthesis of oAP-dds In a 100 mL three-necked round bottom flask equipped with a mechanical stirrer, a thermometer, and a reflux condenser, 6.2 g of DDS, 10 mL of formaldehyde, and 50 mL of toluene were added. The mixture was stirred for 1 h in an ice bath before adding 6.6 mL of oallylphenol with a pressure-equalizing dropping funnel. Then, the temperature was raised gradually up to reflux and kept stirring for 24 h. Thereafter, the flask was equipped with a DeaneStark trap, and the water from the reaction was collected in the trap. After the reaction was continued for another 50 h, the solvent was removed by distillation under reduced pressure, and the residue was dissolved in 30 mL of chloroform. The chloroform solution was washed several times with 1 mol/L NaOH aqueous solution and deionized water, respectively. Subsequently, the solution was washed several times with 0.1 mol/L HCl aqueous solution and deionized water, respectively, followed by evaporation of the chloroform under reduced pressure. The product was dried at 80 C in a vacuum oven for 24 h, and finally a yellow powder was obtained. The yield was approximately 50%. FTIR (KBr, cm1): 3074 (aromatic and allylic ] CeH), 3007 (aromatic CeH), 2923, 2853 (CeH), 1637 (allylic C]C), 1592, 1506 (aromatic C]C), 1464 (CH2), 1296 (SO2), 1253 (CH2), 1223 (CeOeC), 1147, 1110 (SO2), 1074 (CeS), 991 (allylic CeH), 954 (out-of-plane mode of the benzene ring to which an oxazine ring is attached), 916 (allylic CeH), 823, 750 (aromatic CeH), 731, 645 (CeS), 553 (SO2). 1H NMR (CDCl3, ppm): 7.81e7.79 (d, 4H), 7.06e7.07 (d, 2H), 7.04 (s, 2H), 7.03 (s, 2H), 6.91e6.97 (m, 4H), 5.99e6.06 (m, 2H), 5.35 (s, 4H), 5.11e5.14 (m, 4H), 4.63 (s, 4H), 3.40e3.41 (d, 4H). 13C NMR (CDCl3, ppm): 151.94, 151.54, 136.57, 132.71, 129.10, 128.46, 128.42, 124.98, 121.06, 120.22, 115.97 (C aromatic); 115.88, 77.26, 49.50, 33.67 (C aliphatic). 2.4. Preparation of polybenzoxazines First, oAP-hda or oAP-dds was put into a steel mold, and the mold was put into a vacuum oven. Then, for oAP-hda, the vacuum oven was step-heated to 100, 120, 140, 160, and 180 C and hold at each temperature for 1 h, thereafter, hold at 200 C for 6 h; whereas for oAP-dds, the vacuum oven was step-heated to 100, 120, 140, 160, 180, and 200 C and held at each temperature for 1 h, and then at 230 C for 6 h.
13 C spectra were externally referenced to the carbon signal of solid adamantane (38.48 ppm relative to TMS). The FTIR spectra were obtained with a Nicolet 380 FTIR spectrometer at a resolution of 4 cm1. The benzoxazine sample was dissolved in chloroform, and the solution was coated on a KBr disk to form a thin uniform film. When the solvent was completely evaporated at 50 C in a vacuum oven, the disk was scanned by the FTIR spectrometer. Thereafter, the disk was heated isothermally in a static air oven. During the polymerization reaction, the disk was removed periodically for measurement. The dynamic polymerization reactions for oAP-hda and oAP-dds were monitored by a PerkinElmer Diamond differential scanning calorimeter operating in nitrogen, and the samples were scanned at a heating rate of 10 C/min. A PerkineElmer DMA-8000 dynamic mechanical analyzer was used to determine the dynamic storage modulus (E0 ) and loss factor (tan d) using three-point bending mode. Measurements were performed on rectangular specimens with dimensions of 50.0 mm 9.0 mm 2.6 mm by heating from 25 to 250 C, with a heating rate of 2 C/min and a frequency of 1 Hz. Thermogravimetric analysis was performed on a Netzsch TG 209 F3 thermogravimeter, and polybenzoxazine fine powder samples of approximately 10 mg were heated to 800 C at a heating rate of 10 C/min in a nitrogen (or air) flow of 100 mL/min.
3. Results and discussion 3.1. Structural effects of diamines on synthesis of benzoxazines oAP-hda and oAP-dds were synthesized from o-allylphenol, formaldehyde, HDA, and DDS via a solution method [31,32], and the synthesis mechanism for oAP-hda and oAP-dds is shown in Scheme 1. In synthesis for o-allylphenol based benzoxazines, the reaction is thought complete when the solution color changes from colorless to reddish-brown, and it takes less time (around 24 h) for the reaction of o-allylphenol/HDA/formaldehyde than for those of both oallylphenol/DDM/formaldehyde and o-allylphenol/DDE/formaldehyde. However, the solution color does not change in a relatively long time (48 h) under the same reflux temperature for o-allylphenol/DDS/formaldehyde, indicating that the reactivity of DDS is obviously lower than those of HDA, DDM, and DDE. To overcome the low reactivity of DDS and enhance the reaction rate of o-allylphenol/DDS/formaldehyde system, the water from the reaction is distilled after the reaction temperature is raised up to reflux for a period of time (such as 24 h). The same distillation procedure was
2.5. Measurements Both proton (1H) and carbon (13C) NMR spectra were recorded using a Bruker Avance III 600 NMR spectrometer at a proton frequency of 600 MHz and the corresponding carbon frequency. Deuterated chloroform (CDCl3) was used as the solvent and tetramethylsilane (TMS) was used as an internal standard. Solid-state NMR experiments were performed at room temperature (25 C) on a Bruker Avance III 400 NMR spectrometer operating at a 13C resonance frequency of 100.568 MHz. The samples were analyzed under cross-polarization/magic-angle spinning (CP/MAS) conditions using 4-mm zirconia rotors at a spinning frequency of 5 kHz. A 90 pulse width of 4 ms was employed, and the CP HartmanneHahn contact time was set at 3.0 ms. The chemical shifts of
31
Scheme 1. Mechanism of synthesis of oAP-hda and oAP-dds.
32
Y. Liu et al. / Polymer 57 (2015) 29e38
used in the synthetic reaction for oAP-ddm, oAP-dde, and oAP-hda could also effectively enhance the reaction rate and shorten the reaction time. It is certain that the reactivity difference between HDA and DDS is due to the difference in the steric and electronic effects exerted by the substituent groups on nitrogen atom. On one hand, the mobility of the bulky molecule of DDS is relatively lower than that of HDA at the same temperature, which results in a lower molecule collision chance in o-allylphenol/DDS/formaldehyde system than in o-allylphenol/HDA/formaldehyde system. On the other hand, the electronic effect of N-substituent plays an important role on the reactivity (nucleophilicity and basicity) of diamines. An amine group can react with formaldehyde as a nucleophile by attacking the electron-deficient carbonyl carbon to form a carbinolamine intermediate (Scheme 2) [33], and the reaction rate depends on the nucleophilicity of the amine group. The basicities of aromatic amines are lower than those of aliphatic ones, and the sulfone group greatly decreases the electron density on nitrogen atom in DDS both by induction and resonance [34e36]. Therefore, DDS is a weaker nucleophile than HDA and the oxazine ring formation in oallylphenol/DDS/formaldehyde system is inhibited in comparison with the reaction of o-allylphenol/HDA/formaldehyde system. For aromatic diamines, the nucleophilicity of DDS is lower than those of DDM and DDE due to the strong electron-withdrawing effect of the sulfone group. In addition, an insoluble white color gel is frequently formed in the synthetic reaction of diamine based benzoxazines [7,27], which is due to the formation of a triazine network intermediate [33]. The formation of the triazine network often appears in the synthetic reaction for bifunctional benzoxazines based on o-allylphenol (Scheme 3) [33], which hinders the reaction proceeding, especially in the synthetic reaction of oAP-dds, whereas the formation of the insoluble gel is not obviously observed in the synthetic reaction of oAP-hda. Thus, the synthetic reaction of oAP-hda proceeds easier than that of oAP-dds.
3.2. Characterization of oAP-hda and oAP-dds The chemical structures of oAP-hda and oAP-dds were confirmed by 1H and 13C NMR and FTIR. Fig. 1a shows the 1H NMR spectrum of oAP-hda. The resonances at 4.91 and 4.01 ppm correspond to the methylene protons (H10 and H11) of OeCH2eN and AreCH2eN of the oxazine ring, respectively. The chemical shifts (ppm) at 6.00e6.06 (2H, H2), 5.07e5.10 (4H, H1), and 3.37e3.38 (4H, H3) correspond to the aliphatic protons in the allyl group, and 2.74e2.77 (4H, H12), 1.61 (2H, H13), and 1.40 (4H, H14) are assigned to the protons of the methylene groups between the two oxazine rings. The peaks at 7.01e7.03 (2H, H5), 6.85e6.87 (2H, H7), and 6.83e6.84 (2H, H6) ppm are assigned to the aromatic protons. The ratio of the corresponding integration areas of the eleven protons (H1, H2, H3, H5, H6, H7, H10, H11, H12, H13, and H14) was determined roughly to be
2:1:2:1:1:1:2:2:2:2:2, which corresponds well with the theoretical proton ratio based on the chemical structure. Fig. 1b show the corresponding 13C NMR spectrum of oAP-hda, the resonances at 82.56 and 50.43 ppm correspond to the methylene carbons (C10 and C11) of OeCH2eN and AreCH2eN of the oxazine ring, respectively. Other chemical shifts (ppm) are assigned to the resonances of the carbons: 27.17 (C13), 28.11 (C14), 33.65 (C3), 51.38 (C12), 115.35 (C1), 119.83 (C6), 119.94 (C8), 125.49 (C7), 127.48 (C5), 127.88 (C4), 136.87 (C2), 151.90 (C9). Fig. 2a shows the 1H NMR spectrum of oAP-dds. The resonances at 5.35 and 4.63 ppm correspond to the methylene protons (H10 and H11) of OeCH2eN and AreCH2eN of the oxazine ring, respectively. The chemical shifts (ppm) at 3.40 and 3.41 (4H, H3), 5.11 and 5.14 (4H, H1), and 5.99e6.06 (2H, H2) correspond to the aliphatic protons in the allyl group. The peaks at 6.91e6.97 (4H, H13), 7.03 (2H, H6), 7.04 (2H, H7), 7.06 and 7.07 (2H, H5), and 7.79e7.81 (4H, H14) ppm are assigned to the aromatic protons. The ratio of the corresponding integration areas of the ten protons (H1, H2, H3, H5, H6, H7, H10, H11, H13, and H14) was determined roughly to be 2:1:2:1:1:1:2:2:2:2, which corresponds well with the theoretical proton ratio based on the chemical structure. In the corresponding 13C NMR spectrum of oAP-dds in Fig. 2b, the resonances at 77.26 and 49.50 ppm correspond to the methylene carbons (C10 and C11) of OeCH2eN and AreCH2eN of the oxazine ring, respectively. Other chemical shifts (ppm) are assigned to the resonances of the carbons: 33.67 (C3), 115.88 (C1), 115.97 (C13), 120.22 (C6), 121.06 (C8), 124.98 (C7), 128.42 (C5), 128.46 (C4), 129.10 (C14), 132.71 (C15), 136.57 (C2), 151.54 (C12), 151.94 (C9). The FTIR spectrum of oAP-hda is shown in Fig. 3a. The characteristic absorption at 1137 cm1 is assigned to the asymmetric stretching vibration of CeNeC of the oxazine ring [37]. The asymmetric and symmetric stretching vibrations of CeOeC of the oxazine ring are observed at 1225 and 1076 cm1, respectively [37]. The absorptions at 972 and 927 cm1 are the out-of-plane mode of the benzene ring to which an oxazine ring is attached. The C]C stretching vibration at 1637 cm1 and the olefinic CeH out of plane bending vibration at 995 cm1 belong to the characteristic absorptions of the allyl group. The ]CeH stretching vibrations of the allyl group and the aromatic ring appear at 3074 cm1, together with the peaks at 3040 and 3002 cm1 are the ]CeH stretching vibrations of the aromatic ring. The CeH asymmetric stretching vibrations of CH2 are at 2976 and 2932 cm1, whereas the CeH symmetric stretching vibration of CH2 is at 2855 cm1. The absorption peaks at bands of 1464, 1375, and 1334 cm1 are due to CH2 bending, wagging, and twisting vibrations, respectively. The absorption at 1593 cm1 is associated with the C]C stretching vibrations of the aromatic ring. The bands at 817, 772, 750, and 625 cm1 are assigned to the CeH out-of-plane bending vibrations of the aromatic ring. The FTIR spectrum of oAP-dds is shown in Fig. 4a. The absorption peak at 1223 cm1 is assigned to the asymmetric stretching vibration CeOeC of the oxazine ring [37], and the absorption at 954 cm1 is the out-of-plane mode of the benzene ring to which an
Scheme 2. Mechanism of formation of a carbinolamine intermediate in synthesis of benzoxazines.
Y. Liu et al. / Polymer 57 (2015) 29e38
33
Scheme 3. Mechanism of formation of a triazine network intermediate in synthesis of benzoxazines.
oxazine ring is attached. The C]C stretching vibration at 1637 cm1 and the olefinic CeH out of plane bending vibrations at 991 and 916 cm1 belong to the characteristic absorptions of the allyl group. The ]CeH stretching vibrations of the allyl group and the aromatic ring appear at 3074 cm1, together with the peak at 3007 cm1 is the ]CeH stretching vibration of the aromatic ring. The CeH asymmetric stretching vibrations of CH2 are at 2953 and 2923 cm1, whereas the CeH symmetric stretching vibration of CH2 is at 2853 cm1. The absorption peaks at bands of 1464, 1401, 1384, and 1253 cm1 are due to CH2 bending, scissoring, wagging, and twisting vibrations of the oxazine ring, respectively. The absorptions at 1592 and 1506 cm1 are associated with the C]C stretching vibrations of the aromatic ring. The bands at 823 and 750 cm1 are assigned to the CeH out-of-plane bending vibrations
Fig. 1. 1H and
13
C NMR spectra of oAP-hda.
of the aromatic ring. The peak at 1296 cm1 is assigned to the asymmetric stretching vibration of SO2, whereas the peaks at 1147 and 1110 cm1 are due to the symmetric stretching vibrations of SO2. The absorptions at 1074, 731, and 645 cm1 are due to the stretching vibrations of aromatic CeS, whereas the peak at 553 cm1 belongs to the in-plane bending vibration of SO2. 3.3. Structure changes in polymerization Fig. 3aee shows the FTIR spectra of oAP-hda and the polymerized products obtained at 200 C for various times. The intensities of the absorptions at 927, 1076, 1137, and 1225 cm1 decreased gradually with the polymerization proceeding, indicating that the
Fig. 2. 1H and
13
C NMR spectra of oAP-dds.
34
Y. Liu et al. / Polymer 57 (2015) 29e38
Fig. 3. FTIR spectra of (a) oAP-hda and (b)e(e) polymerization products obtained at 200 C in air for various times.
Fig. 4. FTIR spectra of (a) oAP-dds and (b)e(e) polymerization products obtained at 230 C in air for various times.
oxazine ring-opening reaction occurred. Correspondingly, the absorption of phenolic eOH appears at 3375 cm1 and the intensity increases with increasing time. At the same time, the absorption intensity of the olefinic CeH out of plane bending vibration at 995 cm1 decreases with the reaction proceeding, implying that the radical polymerization occurred for allyl groups. In addition, the absorption intensity of the peak at 1676 cm1 increases with increasing time, due to the carbonyl absorption resulted from the oxidation of the allylic C]C double bonds. Fig. 4aee shows the FTIR spectra of oAP-dds and the polymerized products obtained at 230 C for various times. The decrease in the intensities of the peaks at 954 and 1223 cm1 indicates that the oxazine ring-opening reaction occurred, whereas the decrease in the intensities of the peaks at 916, 991, and 1637 cm1 indicates that the radical polymerization occurred for allyl groups. The intensities of two new peaks increase with increasing time, one peak at 3375 cm1 is ascribed to the absorption of the phenolic eOH, whereas the other peak at 1680 cm1 is attributed to the carbonyl groups resulted from the oxidation of the allylic C]C double bonds. Based on the cationic ring-opening mechanism of benzoxazine polymerization and the structure characteristics of oAP-hda and oAP-dds, the structures of the polymerized products for oAP-hda and oAP-dds can be described in Scheme 4 [31,32]. To better understand the structure changes from benzoxazine monomers to polybenzoxazines, Fig. 5 shows the solid-state 13C CP/ MAS NMR spectra of oAP-hda and oAP-dds monomers and their corresponding polybenzoxazines. From benzoxazine monomers to polybenzoxazines, the significant changes in the solid-state 13C NMR spectra are the intensity decrease or disappearance of the resonance peaks of the carbons of oxazine rings and the unsaturated carbons of allyl groups. In the solid-state 13C NMR spectrum for oAP-hda shown in Fig. 5a, the peaks at 84.58 and 47.75 ppm correspond to the resonances of the methylene carbons (C10 and C11) of OeCH2eN and AreCH2eN of the oxazine ring, respectively, whereas the resonances of C1, C2, and C3 of the allyl group are at 116.08, 137.65, and 34.23 ppm, respectively. Correspondingly, in the solid-state 13C NMR spectrum for Poly(oAP-hda) shown in Fig. 5b, the resonance peaks of the methylene carbons (C10 and C11) of OeCH2eN and AreCH2eN are overlapped at 51.99e59.14 ppm, together with the resonance peak of C12, whereas the resonance peaks of C1, C2, and C3 in Poly(oAP-hda) are overlapped at ~34.40 ppm [31,32]. In addition, the resonance peak at 21.80 ppm is assigned to the terminal methyl carbons [31,32], overlapping with the peak of the terminal C1.
Scheme 4. Structure changes from benzoxazines to polybenzoxazines.
Y. Liu et al. / Polymer 57 (2015) 29e38
Fig. 5. Solid-state
35
13
C NMR spectra of (a) oAP-hda, (b) Poly(oAP-hda), (c) oAP-dds, and (d) Poly(oAP-dds).
In the solid-state 13C NMR spectrum for oAP-dds shown in Fig. 5c, the peaks at 76.61 and 46.35 ppm correspond to the resonances of the methylene carbons (C10 and C11) of OeCH2eN and AreCH2eN of the oxazine ring, respectively, whereas the resonances of C1, C2, and C3 of the allyl group are at 112.10, 137.06, and 33.36 ppm, respectively. Correspondingly, in the solid-state 13C NMR spectrum for Poly(oAP-dds) shown in Fig. 5d, the resonance peaks of the methylene carbons (C10 and C11) of OeCH2eN and AreCH2eN are overlapped at ~45.90 ppm, whereas the resonance peaks of C1, C2, and C3 in Poly(oAP-dds) are overlapped at ~33.65 ppm [31,32]. In addition, the resonance peak at 20.86 ppm is assigned to the terminal methyl carbons [31,32], overlapping with the peak of the terminal C1. Moreover, from monomers to polymers, the chemical shifts of aromatic carbons and aliphatic saturated carbons does not change, but most of the chemical shifts of these carbons become hard to be distinguished because the corresponding peaks are highly overlapped. In addition, comparing the solid-state 13C NMR spectra of Poly(oAP-hda) and Poly(oAP-dds) with those of polybenzoxazines based on bisphenol-A/methylamine and bisphenol-A/aniline reported by Koenig and coworkers [38], the common characteristic can be noted that the peaks of the methylene carbons of oxazine rings disappear or almost disappear and the chemical shifts of the methylene carbons of Mannich bridge structures of polybenzoxazines locate near the characteristic chemical shift of the methylene carbon of AreCH2eN of oxazine rings.
3.4. Polymerization behavior The non-isothermal DSC curves of oAP-hda and oAP-dds are shown in Fig. 6. Besides an endothermic peak corresponding to the melting process appeared at 98 C in the DSC curve of oAP-dds, each of the two DSC curves shows an exothermic peak
Fig. 6. DSC curves of oAP-hda and oAP-dds.
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corresponding to both the oxazine ring-opening polymerization and the radical polymerization of allyl groups. The onset temperatures of the exothermic peaks are at 185 and 223 C for oAP-hda and oAP-dds, respectively, with respective maxima at 238 and 281 C. The difference in onset and peak temperatures between the DSC curves of oAP-hda and oAP-dds is due to the difference in structure of the substituent groups on nitrogen atom. In view of the bulkiness of the substituent groups, sterically less hindered alkyl group on the nitrogen atom allows faster polymerization of benzoxazine monomers [12] and the bulkier substituent on the nitrogen atom corresponds to the slower polymerization [13]. Therefore, the polymerization of oAP-hda with a flexible methylene chain is faster than that of oAP-dds with rigid sulfone-bridged benzene rings. On the other hand, the linear methylene groups can enhance the stability of zwitterionic intermediates and facilitate ring opening at lower temperatures, whereas the electronwithdrawing sulfone-bridged benzene rings will destabilize the intermediates [12,39]. Generally, the ring-opening of benzoxazines would form two corresponding zwitterionic intermediates (with aminomethyl and imminium cations, respectively), and the two intermediates could react with benzoxazine monomers and thus the increase of the concentrations of the intermediates would achieve faster polymerization [39]. The methylene chain than the sulfonebridged benzene rings could have better effect on stability of the zwitterionic species, which would lead to the polymerization of oAP-hda at lower temperature than oAP-dds. 3.5. Dynamic mechanical properties Fig. 7 shows the curves of the storage modulus (E0 ) and loss factor (tan d) for Poly(oAP-hda) and Poly(oAP-dds), and the analysis
results on the curves are summarized in Table 1. The E0 of Poly(oAPdds) is much higher than that of Poly(oAP-hda) in the glassy region, indicating a higher stiffness of Poly(oAP-dds) over Poly(oAP-hda), which may be due to the difference in rigidity between sulfonebridged benzene rings and linear methylene chain. On the other hand, a higher crosslinking density often leads to a high E0 . With an increase in temperature, E0 decreases, and a sharp decline in E0 can be observed in the glass transition region, due to the increase of chain mobility at higher temperatures. In the rubbery region, no significant change in E0 can be seen. The difference in modulus between the glassy and rubbery states is smaller for Poly(oAP-hda) than for Poly(oAP-dds) due to the high modulus of Poly(oAP-dds) in the glassy region. According to rubber elasticity theory, the plateau of the elastic modulus in the rubbery state is associated with the crosslinking density of the materials [40], and an empirical equation (1) was proposed by Nielsen and Landel to describe the approximate relationship [41], which is reported to better describe the elastic properties of dense networks [42e44].
log
0 Ee ¼ 7:0 þ 293ðrx Þ 3
(1)
where E0 e (dyne/cm2) is an equilibrium elastic modulus in rubbery plateau, rx (mol/cm3) is the crosslinking density which is the mole number of network chains per unit volume of the polymers. Based on equation (1), the crosslinking densities of Poly(oAP-hda) and Poly(oAP-dds) can be calculated from the plateau of the storage shear modulus in the rubbery state (G0 e), which equals E0 e/3, and E0 e in the rubbery region is determined by taking the value at the inflection point of the plateau. The calculated results are summarized in Table 1. It is evident that the crosslinking density of Poly(oAP-dds) is much higher than that of Poly(oAP-hda). The degree of interchain crosslinking depends on the electron density of the carbon (C6) on para position (with respect to the ether of the benzoxazine ring), because a high electron density facilitates the carbon undergoing electrophilic attack of carbocations formed in the oxazine ringopening reaction. The chemical shifts of hydrogen on C6 for oAPhda and oAP-dds show that the electron density of C6 in oAP-dds is higher than that in oAP-hda (Figs. 1a and 2a). Thus the reactivity index for electrophilic substitution at C6 of oAP-dds is higher than that of oAP-hda. As a result, the obtained crosslinking density is higher for Poly(oAP-dds) than for Poly(oAP-hda). On the other hand, the crosslinking density of Poly(oAP-hda) is lower than the values of polybenzoxazines based on phenol/HDA, o-cresol/HDA, and p-cresol/HDA reported by Ishida and Allen [23], indicating that the flexibility of o-allylphenol based polybenzoxazines is higher than those of polybenzoxazines based on phenol, o-cresol, and pcresol due to the bulk unreacted allyl group in o-allylphenol based polybenzoxazines. Corresponding to the decrease in E0 , an a-relaxation peak appears in each of the two tan d curves, and the a-relaxation peak temperatures taken as the Tg values, together with peak heights and peak widths at half-height are listed in Table 1. The Tg of Poly(oAPhda) is much lower than that of Poly(oAP-dds), whereas the height
Table 1 Analysis of the DMA curves of Poly(oAP-hda) and Poly(oAP-dds). Width of Polybenzoxazines E' (25 C) Tg rx (103) Height of tan d peaks (GPa) ( C) tan d peaks (arbitrary units) (half ht.) ( C)
Fig. 7. Dynamic mechanical curves of Poly(oAP-hda) and Poly(oAP-dds).
Poly(oAP-hda) Poly(oAP-dds)
1.44 3.18
87 1.33 193 1.05
26 37
2.12 3.08
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of the tan d peak of Poly(oAP-hda) is higher than that of Poly(oAPdds), implying that the flexibilities of the molecular chains of Poly(oAP-hda) are higher than those of Poly(oAP-dds), because the flexibility of the long methylene chain is higher than that of the sulfone-bridged benzene rings. On the other hand, the crosslinking densities of polybenzoxazine networks also affect the flexibilities of the molecular chains. Moreover, the peak width at half-height of the tan d curve of Poly(oAP-dds) is broader than that of Poly(oAPhda), indicating the segmental mobility in Poly(oAP-dds) is lower than that in Poly(oAP-hda). Therefore, Poly(oAP-dds) has more number of kinetic units adequately to move than Poly(oAP-hda), which results in a broader distribution of structures. Consequently, the glass transition of Poly(oAP-dds) takes place in a relatively broad temperature range. Comparing the dynamic mechanical properties of Poly(oAPhda) and Poly(oAP-dds) with those of Poly(oAP-ddm) and Poly(oAP-dde) [31,32], it can be seen that the moduli in the glassy state and Tgs of Poly(oAP-ddm) and Poly(oAP-dde) are higher than those of Poly(oAP-hda) but lower than those of Poly(oAP-dds), due to the difference in structural effects (including the flexibility of the substituent groups on nitrogen atom and the crosslinking density of the networks). In addition, it is noted that the modulus in the glassy state and Tg of Poly(oAP-dds) are lower than those of phenol-DDS based polybenzoxazine [7], which indicates that the crosslinking density of the former might be lower than that of the latter, due to a low degree of polymerization formed through the less favorable paraposition. On the other hand, the unreacted allylic C]C double bonds in Poly(oAP-dds) might also be contributed to the relatively low E0 and Tg.
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Poly(oAP-dds), the weigh loss temperature is higher in air than in nitrogen for a given weight loss below 604 C for Poly(oAP-hda) and 549 C for Poly(oAP-dds), and a weight increase is obviously seen for Poly(oAP-dds) before weight loss in air. In a previous report, the oxidation of the unreacted allylic C]C double bonds in Poly(oAPddm) was confirmed by element analysis on the thermal degradation residues [31]. Alternately, the weight increase phenomenon in thermal degradation in dynamic air atmosphere reflects the existence of unreacted allylic C]C double bonds in polybenzoxazines based on o-allylphenol. It is reasonable that the weight increase in dynamic air for Poly(oAP-dds) is much obvious than those in static air for Poly(oAP-ddm) and Poly(oAP-dde) [31,32], because more oxygen is supplied from air flow. But no weight increase for Poly(oAP-hda) can be seen, which is due to the low thermal stability of Poly(oAP-hda). The acceleration effect of oxygen on thermo-oxidation degradation of Poly(oAP-hda) and Poly(oAP-dds) can be clearly seen in high temperature range. The char yields of Poly(oAP-hda) and Poly(oAP-dds) are 22.3% and 40.0% at 800 C in nitrogen, respectively, whereas the char yields of Poly(oAP-hda) and Poly(oAP-dds) are close to 0 at 700 C in air. 4. Conclusions Two allyl-containing benzoxazines were synthesized based on o-allylphenol. The reactivity of HDA is higher than that of DDS in the synthetic reaction of benzoxazines, and the polymerization temperature of oAP-hda is lower than that of oAP-dds. The storage moduli in the glassy state are 1.44 and 3.18 GPa for Poly(oAP-hda) and Poly(oAP-dds), respectively, and the glass transition temperatures are 87 and 193 C, respectively. The thermal stability of Poly(oAP-dds) is much higher than that of Poly(oAP-hda).
3.6. Thermal stability Acknowledgments Fig. 8 shows the TG curves of Poly(oAP-hda) and Poly(oAP-dds) in nitrogen and air. Obviously, the thermal stability of Poly(oAPdds) is much higher than that of Poly(oAP-hda), due to the higher crosslinking density and the higher aromatic ring content in Poly(oAP-dds) over Poly(oAP-hda). The 5% weight loss temperatures for Poly(oAP-hda) and Poly(oAP-dds) are 242 and 370 C in nitrogen, respectively, which are lower than their corresponding weight loss temperatures (251 and 403 C) in air, respectively. However, these results do not mean more stable in air than in nitrogen for Poly(oAP-hda) and Poly(oAP-dds). In fact, due to the oxidation of the unreacted allylic C]C double bonds in Poly(oAP-hda) and
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Fig. 8. TG curves of Poly(oAP-hda) and Poly(oAP-dds).
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