Anomalous Isomeric Effect on the Properties of Bisphenol F-based

Aug 6, 2014 - Intrinsic self-initiating thermal ring-opening polymerization of 1,3-benzoxazines without the influence of impurities using very high pu...
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Anomalous Isomeric Effect on the Properties of Bisphenol F‑based Benzoxazines: Toward the Molecular Design for Higher Performance Jia Liu and Hatsuo Ishida* Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States ABSTRACT: A series of 2,2′-, 2,4′-, and 4,4′-substituted benzoxazine monomers based on isomeric bisphenol F and aniline has been synthesized in order to obtain a basic set of design rules toward making higher performance polybenzoxazines. All the monomer structures are confirmed by 1H and 13C NMR, and the purity is calculated by 1H NMR. The polymerization behavior is studied by differential scanning calorimetry (DSC) and in situ Fourier transform infrared spectroscopy (FT-IR), The rate of polymerization increases in the order 4,4′-, 2,4′-, and 2,2′-substituted monomer. After polymerization, the thermal stability and glass transition temperature (Tg) of each polymer are studied by thermogravimetric analysis (TGA) and temperature-modulated DSC. The polymer obtained from 2,2′-substituted monomer gives highest thermal stability as well as Tg, which is against many of such examples available in the literature. Discussions on understanding benzoxazine structure−property relationships have been provided.

1. INTRODUCTION Recently, the polybenzoxazine has been recognized as a new class of thermosetting phenolic resins with useful mechanical and thermal properties. It offers a number of attractive properties including near-zero shrinkage upon polymerization,1,2 high char yield,3−5 low dielectric constant,6 fast development of physical and mechanical properties at low conversion,7 and low water absorption.8 Furthermore, no catalyst is required to affect polymerization, and the crosslinked polymers are self-extinguishing and release no byproduct during polymerization. Owing to the very rich molecular design flexibility, over recent decades, intensive effort has been devoted to synthesis of new benzoxazine monomers.9−12 Few studies, if any, have examined the effects of ortho- and para-substitution on bisphenols used as starting material for polybenzoxazines. This is not surprising because for many years, bisphenol A (BPA) is the most commonly used starting material in benzoxazine synthesis. BPA comes from the reaction of phenol and acetone, overwhelmingly resulting in a 4,4′dihdroxylated structure. Although 4,4′-dihdroxylated BPA may produce benzoxazine that are good enough for commercial purposes, there have been no literature reports about other isomers. It may be possible to improve material properties through changes in the geometry of the bisphenol, that is, the typical p-hydroxy moiety could be changed to ortho or meta positioning. Owing to economic considerations, mixed isomers of bisphenol-F (BPF) are commercially available in large quantity and are becoming an attractive alternative to BPA, which is causing environmental concerns. Gu et al. reported 4,4′-bisphenol F-based benzoxazine13 and Ishida and coworkers synthesized, using a BPF isomer mixture, short chain benzoxazine oligomers which show excellent mechanical and physical properties, as well as ease of processing.14 Thus, understanding the structure and properties of benzoxazines derived from BPF isomer mixture is of interest. More © 2014 American Chemical Society

importantly, BPF, which is available in the form of pure 2,2′-, 2,4′- and 4,4′-substituted isomers, provides us with an opportunity to explore the influence of bisphenol stereoisomer on properties of benzoxazine monomers as well as polybenzoxazines. It is the purpose of this paper to study, systematically, the rate of oxazine polymerization, glass transition temperature (Tg) and other thermal properties of the subsequent polymers derived from the individual BPF isomeric components to provide us with a fundamental understanding of impact of changing bisphenol stereoisomeric structure on polybenzoxazine properties.

2. EXPERIMENTAL SECTION 2.1. Materials. 4,4′-Methylenediphenol (98%), paraformaldehyde (95%), aniline (99%), and 2,2′-methylenediphenol (98%) were purchased from Sigma-Aldrich Chemical Co.. Chloroform (99%), hexanes (a mixture of isomers), and ethyl acetate (99%) were obtained from the Fisher Scientific Company. 2,4′-Methylenediphenol (99%) were purchased from Tokyo Chemical Industry Company. Mixed isomers of bisphenol-F (a mixture of 16% 2-[(2-hydroxyphenyl)methyl]phenol (also known as 2,2′-bisphenol-F), 41% 2-[(4hydroxyphenyl)methyl]phenol (also known as 2,4′-bisphenol-F), and 43% 4-[(4-hydroxyphenyl)methyl]phenol (also known as 4,4′-bisphenol-F),14) were kindly supplied by Hexion Specialty Chemicals. All chemicals were used as received without further purification. 2.2. Preparation of BF-a Benzoxazine Monomer. The bisphenol-F isomer mixture and aniline based benzoxazine monomer (hereinafter abbreviated as BF-a isomer mixture), were prepared by the following procedures: Bisphenol-F isomers (20 mmol, 4.01 g), and aniline (40 mmol, 3.73 g) were dissolved in chloroform in a roundbottom flask which is equipped with a magnetic stirring bar. Paraformaldehyde (80 mmol, 2.4 g) was added to the solution followed by Received: June 23, 2014 Revised: July 31, 2014 Published: August 6, 2014 5682

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Scheme 1. Synthesis Route of BF-a Benzoxazine Monomers

heating the mixture at 80 °C in a preheated oil bath. By following the reaction by 1H NMR, it was found that the best conversion to benzoxazine structure was achieved after overnight reaction. The solution was cooled to room temperature and 1 N sodium hydroxide solution was used to perform a base wash to eliminate the unreacted phenol in the system.1 The powder was dried in a vacuum oven at 40 °C for 72 h (yield: 7.13g, 81%). The individual pure benzoxazine isomers bis(3-phenyl-3,4-dihydro2H-benzo[e][1,3]oxazin-6-yl)methane, 3-phenyl-8-((3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-yl)methyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine, bis(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-8yl)methane (hereinafter abbreviated as 4,4′-BF-a, 2,4′-BF-a, and 2,2′BF-a, respectively) were prepared according to Scheme 1 in a roundbottom flask as the following method: 1 molar ratio of bisphenol F and 2 molar ratio of aniline were dissolved in chloroform first and then 4 molar ratio of paraformaldehyde was added to the solution, followed by refluxing the mixture in an oil bath. The reaction was followed by thin layer chromatography (TLC) and proton nuclear magnetic resonance spectroscopy (1H NMR). After approximately 20 h, the solution became clear and yellow, and the best conversion to benzoxazine structure was found to be achieved. Column chromatography was used afterward to purify benzoxazine isomers with mixed solvents of hexane and ethyl acetate in 10:1 ratio. The purified samples were further recrystallized from 2-propornol/hexane mixtures (3:5). After recrystallization, needle like crystals were obtained for 4,4′- and 2,2′-substituted benzoxazines. 2,4′-Substituted benzoxazines were found to be extremely difficult to purify. It is possible that the steric reasons lead to poor molecular packing of this monomer. Repeated purification allowed all three isomers to have similar purities. The NMR end-group analysis indicates the purity of the three isomers, 4,4′-, 2,2′-, and 2,4′-isomer-based benzoxazines were 98.5%, 98.0% and 97.9%, respectively. 2.3. Polymerization of Polybenzoxazines. Solutions of 30% solid content of the monomers in DMF were prepared. Then, the solutions were cast over dichlorodimethylsilane-pretreated glass plates. The films were dried in an air circulating oven at 100 °C for 24 h to remove the solvent. The films as fixed on glass plates were polymerized stepwise starting from 120 °C for 1h each at 20 °C intervals until the final temperature is 40 °C above the Tg of the respective isomers to ensure near completion of the polymerization without detectable degradation, and then slowly cooled to room temperature. The films had brown color with thicknesses ranging from 0.1 to 0.8 mm, depending on the availability of the purified material. 2.4. Measurements. 1H and 13C NMR spectra were acquired in deuterated dimethyl sulfoxide on a Varian Oxford AS600 at a proton

frequency of 600 MHz and its corresponding carbon frequency. The average number of transients for 1H and 13C was 64 and 1024, respectively. A relaxation time of 10 s was used for the integrated intensity determination of 1H NMR spectra. The reactivity study involving these three purified benzoxazine monomers was accomplished by obtaining in situ Fourier transform infrared (FT-IR) spectra. A Bomem Michelson MB100 FTIR spectrometer, which was equipped with a deuterated triglycine sulfate (DTGS) detector and a dry air purge unit, was used. Coaddtion of 32 scans was recorded at a resolution of 4 cm−1. Transmission spectra were obtained by casting a thin benzoxazine monomer film on a KBr plate. The sample was then sandwiched between two KBr plates (a closed system) and inserted into a hot cell which was adapted to the FT-IR spectrometer and was adjusted to the desired temperature. The spectrometer was closed, and the experiment begun as soon as the hot cell temperature reached the desired temperature which took 2 min. The average time before and after the 32 scans was regarded the reaction time for that particular data point. The sample thickness was adjusted such that the Beer− Lambert law holds and quantitative spectral information on the bulk polymerization characteristics at each stage could be determined. TA Instruments DSC model Q2000 was used with a heating rate of 10 °C/min and a nitrogen flow rate of 50 mL/min for all tests of differential scanning calorimetric (DSC) study. The temperature modulated DSC technique was used to measure the glass transition temperature of polybenzoxazines derived from pure isomer benzoxazines and benzoxazine isomer mixtures. All samples were crimped in hermetic aluminum pans with lids. After benzoxazine monomers are fully polymerized in the DSC furnace, the measurements were performed at the same conditions as follows: (1) isothermal equilibration for 10 min at −50 °C, (2) setting the temperature amplitude of ±2 °C with period of 60 s, and (3) setting the temperature ramp rate at 2 °C/min to the specified maximum temperature, which was set to at least 50 °C above the glass transition temperature of the sample. The combination of underlying heating rate and period allowed at least six temperature modulation cycles across the width of the transition. Tg is defined as the middle point of the temperature interval when the baseline shifts upon heating. Thermogravimetric analyses (TGA) were performed on a TA Instruments Q500 TGA with a heating rate of 10 °C/min in a nitrogen atmosphere at a flow rate of 40 mL/min. Dynamic mechanical analyses (DMA) were done on a TA Instruments Q800 DMA, applying a controlled strain tension mode with an amplitude of 10 μm and a temperature ramp rate of 3 °C/min. 5683

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Table 1. Literature Examples of Isomeric Effect on Polymer Tg

ortho, like in the case of biphenylated cyanate esters.19 Table 1 also shows the Tg difference, ΔTg = Tg, p (para substitution) − Tg,o (ortho substitution). All the cases exhibit, as expected, positive Tg. 3.1. Spectral Assignment of Isomer Mixture Compounds. One goal of this study is to assign 1H NMR resonances of benzoxazine derived from commercially available bisphenol F isomer mixture as they are complex. Judging from

3. RESULTS AND DISCUSSION In an attempt to understand the effect of isomeric variation of bisphenol F on Tg, extensive literature survey has been performed at the early stage of this study to investigate the influence of isomeric effect among other well-known polymers, such as polyimides, polyesters, polyethers, polyurethanes, cyanate esters and others. As listed in Table 1,15−21 all the polymers investigated show that Tg drops in the order of para > 5684

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the NMR spectra obtained from the pure isomers, we should be able to assign the isomer benzoxazine components in the mixture. The chemical structures of benzoxazine monomers were confirmed by 1H NMR. Figure 1 shows the 1H NMR spectra of individual pure BF-a benzoxazine isomers and isomer mixture benzoxazine synthesized from the bisphenol F isomer mixtures.

Figure 2. 13C NMR spectra of BF-a isomers.

concentration of bisphenol F isomers in the starting mixture and the final product of benzoxazine determined by 1H NMR spectroscopy. As reported in the previous study,14 the isomeric ratio of starting bisphenol-F isomer mixtures can be determined by assigning the chemical shift location and integrating the peak intensity of the methylene protons in the Ar−CH2−Ar structure. Upon synthesizing benzoxazine isomer mixtures, these bisphenol-F isomer resonances shift from 3.70, 3.79, and 3.84 ppm to 3.75, 3.82, and 3.88 ppm for 4,4′-BF-a, 2,4′-BF-a, and 2,2′-BF-a, respectively.14 These resonances correspond well with the pure benzoxazine isomers as shown in Figure 3. On the basis of the integrated intensity, there is no substantial difference in concentration of each isomer before and after the oxazine ring formation within the experimental error as shown in Table 2. In situ FTIR analysis has been conducted to study the ringopening kinetics of the benzoxazine monomers as shown in Figures 4 and 5. The bands that were used to evaluate the polymerization were around 940 cm−1 which are assigned to the out-of-the-plane mode of the benzene to which oxazine ring is attached,23 and the band at 750 cm−1 was used as the internal standard which is from monosubstituted benzene ring. Conversion of the benzoxazine monomer to polybenzoxazines was measured by following the normalized decrease in the areas of the bands near 940 cm−1. Figure 5 displays the conversion curves of pure benzoxazine monomers at 160 °C. As reported by Dunkers,24 for the pure benzoxazine, the polymerization is catalyzed by phenols formed by ring-opening from trace impurities. The ring-opening and the Mannich bridge formation is a concerted reaction whereby one benzoxazine ring and one trisubstituted benzene ring should be consumed simultaneously.25 In other words, there is a one-to-one correspondence of oxazine ring-opening and trisubstituted benzene conversion. The infrared spectra in Figure 4 gives an indication of the species resulting from terminated ring-opening. By comparing the monomer concentration curves among these three isomer benzoxazines, we can see that at the desired temperature, the reactivity of the monomer decrease in the order of 2,2′-BF-a > 2,4′-BF-a > 4,4′BF-a. 3.3. Polymerization Behavior of BF-a Benzoxazine Monomers. The polymerization behavior of the benzoxazine monomers was studied by DSC. The DSC thermograms are shown in Figure 6 and the thermal properties are summarized in Table 3.

Figure 1. 1H NMR spectra of BF-a isomers.

Typically, benzoxazine monomers have two equal intensity singlet resonances in 1H NMR spectra due to the CH2s in the oxazine ring.22 Each characteristic CH2 resonance of oxazine ring appears as multiplets due to the isomeric nature of bisphenol-F mixture as starting material for benzoxazine synthesis. Three multiplets are observed at 5.53, 4.62, and 3.78 ppm for the monomer mixture which is assigned to −O− CH2−N−, Ar−CH2−N−, and Ar−CH2−Ar, respectively. In the case of 4,4′-BF-a, the characteristic resonances are observed as singlets at 5.32, 4.58, and 3.76 ppm corresponding to −O− CH2−N−, Ar−CH2−N−, and Ar−CH2−Ar, respectively. For 2,4′-BF-a, we observed two doublets at 5.39, 5.33 ppm and 4.65, 4.53 ppm for −O−CH2−N− and Ar−CH2−N−, respectively. The methylene group of 2,4′-BF-a locates at 3.84 ppm. For 2,2′-BF-a, the singlets at 5.36, 4.64, and 3.87 ppm correspond to −O−CH2−N−, −Ar−CH2−N− and −Ar−CH2−Ar, respectively. The monomer purity can be estimated by using the integrated peak area of methylene group from bisphenol F and from oxazine ring. The monomer purity for 4,4′-BF-a, 2,4′-BF-a, and 2,2′-BF-a was calculated to be 98.5%, 98.0%, and 97.9%, respectively. 13 C NMR analysis was performed to further confirm the structure as shown in Figure 2. The characteristic carbon resonances of the oxazine ring appear in the range of 49.36− 50.01 ppm for Ar−CH2−N− and 78.52−79.84 ppm for N− CH2−O−. The peaks in the range of 36.32−37.86 ppm are assigned to the bridging methylene carbons in bisphenol-F structure. 2,2′-Methylene carbons appear around 31 ppm, 2,4′methylene carbons around 36 ppm, and 4,4′-methylene carbons around 41 ppm.22 Resonances in the range of 154.20−155.83 ppm correspond to the C−N group in the benzoxazine ring. 3.2. Reactivity Difference between Bisphenol F Isomers. The mechanism of oxazine ring formation involves a Mannich base condensation reaction between phenol, amine, and paraformaldehyde. Since aniline is used in all the three benzoxazines synthesis, it would be interesting to compare if the steric hindrance difference has any effects on the 5685

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Figure 3. Expanded region of 1H NMR spectra.

Table 2. Isomer Composition Obtained from 1H NMR Measurement for the Bisphenol F Isomer Mixture and Benzoxazine Isomer Mixture Obtained Therefrom isomer

bisphenol F (%)

BF-a benzoxazine (%)

4,4′2,4′2,2′-

43 41 16

45 42 13

Figure 5. Conversion of BF-a isomers at 160 °C.

Figure 4. In situ FTIR spectra of 4,4′-BF-a at 160 °C.

The onset of the first exothermic peak for the BF-a benzoxazine mixtures is 78 °C lower than pure 4,4′-BF-a. This is because the benzoxazine synthesized from the isomer mixtures was the product that contained small amount of impurities (such as phenol and oligomers) that can initiate the benzoxazine polymerization, despite attempts to eliminate the raw materials via base wash. However, the base wash is not effective in eliminating the oligomeric compounds with the open phenolic structure that can act as the benzoxazine initiator. Owing to the nature of compound mixture, rigorous purification, such as recrystallization could not be performed whereas the individual isomers were purified by various purification methods. In addition to the existence of a small amount of initiating structure, the isomer mixture contains the isomers that polymerize at lower temperature than 4,4′-BF-a.

Figure 6. Nonisothermal DSC thermograms of BF-a isomers.

For 2,2′-BF-a and 4,4′-BF-a, the exothermic peak is very narrow and symmetry, however, for 2,4′ BF-a, a relatively broad exothermic peak is observed. This could be attributed to the contribution of both 2,2′-BF-a benzoxazine and 4,4′-BF-a benzoxazine components complicating the polymerization behavior. The idealized molecular structures of polybenzox5686

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Table 3. Thermal properties of benzoxazine monomers heat of polymerization

exotherm monomer

onset (°C)

max. (°C)

(J/g)

(kJ/mol)

4,4′-BF-a BZ 2,4′-BF-a BZ 2,2′-BF-a BZ BFs-a BZ mixtures

255 217 213 177

260 255 250 238

264 199 297 190

114.7 86.5 129 82.6

azines derived from the benzoxazine isomers studied are shown in Scheme 2. 3.4. Anomalous Glass Transition Temperature Trend Measured by TMDSC. In order to study the influence of the molecular architecture of benzoxazine monomers on glass transition temperature (Tg), we measured Tg from polymerized pure benzoxazine isomers as well as the isomer mixture using the temperature modulated DSC (TMDSC) technique. TMDSC offers advantage in that other non-Tg related thermal events can be eliminated from the reversible component where information from only Tg component can be extracted. Some components from melting/crystallization can also be detected by TMDSC in the reversing thermograms. However, the crosslinked polybenzoxazine is an amorphous polymer and thus complications from this phenomenon can be avoided. There is only a very limited amount of information in the literature on Tg determination of polybenzoxazine by temperature-modulated DSC (TMDSC).26 TMDSC thermograms of polymerized BF-a isomers are measured under nitrogen. Typically, a parasubstituted polymer tends to have higher glass transition temperature than the corresponding ortho substituted polymer due to the greater stiffness of the chain.27 It is of interest to verify if this also holds for benzoxazine chemistry as the main chain direction of the polymer changes dramatically in comparison to the monomer architecture. Figure 7 shows the reversible component of the TMDSC thermograms obtained from the polybenzoxazines derived from isomer mixture BF-a, 4,4′-BF-a, 2,4′-BF-a, and 2,2′-BF-a. Identical conditions were used to measure all the samples.

Figure 7. Temperature modulated DSC thermograms of poly(BF-a)s. Only the reversing component of the TMDSC measurements is shown here. The Tg is taken at the midpoint of the inflection.

The Tg for the cross-linked polybenzoxazines from the respective isomers are 148, 178, and 208 °C for 4,4′-BF-a, 2,4′BF-a, and 2,2′-BF-a-derived polybenzoxazines, respectively. It is interesting to note that the polybenzoxazine derived from pure 2,2′-BF-a monomer exhibits the highest Tg value among other isomer-derived polymers. The ΔTg for the polybenzoxazine isomers was a quite unexpected negative 60 °C. No other examples we found in the literature as summarized in Table 1 showed this negative difference, not to mention about the large difference in magnitude. The reason for this anomalous phenomenon is probably due to the higher cross-link density of the polymer derived from 2,2′-BF-a than 4,4′-BF-a as discussed later in the DMA section. On the basis of free volume model proposed by Fox et al.,28 it is reasonable for us to assume that polymerized 2,2′-BF-a chains give a “tighter” molecular packing, and hence lower free volume and higher Tg, while polymerized 4,4′-BF-a chains give a “looser” molecular packing, and hence higher free volume and lower Tg. Further detailed

Scheme 2. Structures of Poly(BF-a)s

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study is needed for molecular interpretation of this anomalous behavior. It is well-known that the Fox equation shown below can be used to predict the Tg of a miscible polymer blend or a random copolymer from the Tgs of their respective pure components.29 w w 1 = 1 + 2 Tg Tg1 Tg 2

Table 4. Thermal Properties of Bisphenol F−Aniline-Based Polybenzoxazines weight-loss temperature (oC)

Where wi is the weight fraction of the polymer 1 and 2. Although the Fox equation was originally proposed for twocomponent systems, Min et al. successfully applied this equation for a three-component system30 of plasticized isotactic poly(methyl methacrylate) and poly(vinyl chloride) blends, while Park et al. also applied this for a four-component system made of random acrylic quaternary copolymers31 using the following generalized equation.

1 = Tg

n

5%

10%

char yield at 800 oC (%)

4,4′-BF-a BZ 2,4′-BF-a BZ 2,2′-BF-a BZ BFs-a BZ mixtures

264 294 353 333

341 349 403 378

53 51 61 53

linkage and degradation of the Mannich base.32,33 As shown in poly(4,4′-BF-a), a shoulder, representing around 8% early degradation, was observed, implying the existence of higher concentration of branched or other chain ends than the polymers from other isomers. Other isomers show this effect much less. For the char yield, which is defined as the residual weight at 800 °C under N2 in this study, poly(4,4′-BF-a) has 52.7%, poly(2,4′-BF-a) 50.8%, poly(2,2′-BF-a) has 61.3%, and polybenzoxazine mixtures 53.4%. Poly(2,2′-BF-a) shows the highest thermal stability among the three isomers. The polybenzoxazine derived from the isomer mixture expectedly shows intermediate thermal stability. It should be noted that, despite using different polymerization temperatures for those three isomers due to the different ultimate Tg and, thus, using approximately 40 °C above the ultimate Tg for polymerization temperature, none of the polymerization temperatures span across the different degradation mechanisms thus far reported in the literature. This aspect is important when the observed results were attributed to isomeric structure related. In order to further understand the details of thermal degradation, derivative TGA thermograms are plotted in Figure 9.

⎛w ⎞ i ⎟ ⎟ T ⎝ gi ⎠

∑ ⎜⎜ i=1

type

Knowing that the bisphenol-F isomer mixture is dominated by the three isomers, we used this generalized Fox equation to predict the Tg of the polybenzoxazine derived from the isomer mixtures using the Tg of the polybenzoxazines derived from the pure isomer benzoxazines. The concentration used for the calculation was determined from the NMR analysis of the benzoxazine monomers derived from the monomer isomer mixtures. The calculated Tg of the polybenzoxazine derived from the mixed isomer is 168 °C whereas that of the observed by MDSC analysis is 169 °C. The excellent agreement between the calculated and measured values supports the reasonableness of the measured values and/or the approach taken. 3.5. Thermal Stability of Cross-Linked Polybenzoxazines. The thermal stability of the cross-linked polybenzoxazines derived from the three pure isomers as well as the isomer mixtures was determined and compared by thermogravimetric analysis (TGA). Figure 8 shows the TGA thermograms of

Figure 9. Derivative TGA thermograms of poly(BF-a)s.

There appear three stages of thermal degradation as reported elsewhere.32,33 The first stage of degradation occurs below 350 °C, which consists of minor, multiple degradation processes that are isomeric structure dependent. In the order of 4,4′-, 2,4′-, and 2,2′- isomers, this part of degradation temperature increases. A two-stage degradation is clearly seen in the derivative thermogram of poly(4,4′-BF-a) with overlapped peaks at 210 and 270 °C. These peaks shift to higher temperatures for poly(2,4′-BF-a) around 230 and 300 °C and for poly(2,2′-BF-a) around 275 and 350 °C. Owing to very

Figure 8. TGA thermograms of poly(BF-a)s.

polymer films under N2 atmosphere. The results are summarized in Table 4. For all the polymers studied, the 5% weight loss temperatures, Td5, varies from 264 to 353 °C. The thermal degradation of polybenzoxazines has been reported to show three main stages: evaporation of the amine from the chain ends and branches, evaporation of the amine from the main chain, and the simultaneous breakage of the phenolic 5688

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heavy overlap, detailed identification of the temperature is not possible. The second and third stage degradations take place at 400 and 500 °C. Interestingly, these two degradation stages are isomeric structure independent despite heavy dependence of the first stage degradation stated above. Previously, the low temperature degradation that occurs below 350 °C was proposed to occur due to the chain ends and branches.32,33 The second and third stage degradations were assigned to the amine evaporation and phenolic structure degradation form the main chain, respectively. Assuming the correctness of these assignments, it seems that poly(4,4′-BF-a) has many chain ends and branches, as already discussed in the previous section. A question remains whether the chain ends of poly(4,4′-BF-a) are intrinsically less stable than the others, or the structure of 4,4′isomer leads to lose network structure due to the effective termination of the propagating chains and, as a result, poor thermal stability. This is probably because the opened oxazine ring forms intramolecular 6-membered hydrogen bonding ring easier in the para-isomer than the ortho-isomer. It has been reported in the literature that this formation of intramolecular 6-membered hydrogen bonding ring interferes the propagation of the polybenzoxazine main chain, leading to branched structures.34,35 It is therefore suspected that polybenzoxazines from 2,2′-BF-a leads to more extensive networks with fewer dangling chain ends, leading to higher Tg. Another interesting observation is that the mode of the degradation of polybenzoxazine derived from isomer mixture is similar to the most stable poly(2,2′-BF-a), rather than showing the averaging behavior with the other isomers. This can be understood if we assume that 2,2′-structure has high reactivity and is an effective cross-linking agent that reduces the formation of loose chain ends that are likely formed for the other isomers. Regardless, considering that the majority of the reported bisphenol and diamine type benzoxazine monomers are based on the 4,4′- or para-configuration based, the engineering implication of the current study is quite significant as it is opposite of common wisdom reported in many polymers as shown in Table 1. 3.6. Dynamic Mechanical Analysis (DMA) of CrossLinked Polybenzoxazines. The viscoelastic properties of the cross-linked polybenzoxazines are studied by DMA. Figures 10 and 11 show the temperature dependence of the storage moduli (E′) and loss moduli (E″) for the cross-linked polybenzoxazine. The glass transition temperature (Tg) determined by the peak temperature of tan δ as shown in Figure 12 ranges from 161 to 218 °C. More rigorously, the peak position of E″ should be used. However, as is shown in Figure 11, the significant baseline shift makes precise determination of the peak position difficult. Thus, we used tan δ for the Tg determination by DMA. Consistent with the observation by MDSC, Tg increases in the order of poly(4,4′-BF-a), poly(2,4′-BF-a), poly(2,2′-BF-a). The difference between 4,4′- and 2,2′- is as large as 57 °C. On the other hand, the cross-link density which can be estimated by the E′ at which T ≫ Tg, increased in the order of poly(4,4′-BFa), poly(2,4′-BF-a), poly(2,2′-BF-a). Using the rubber elasticity theory, assuming the front factor to be 1, the cross-link densities of the three isometic polybenzoxazines are calculated to be 2.3 × 10−3, 6.1 × 10−3, and 45.5 × 10−3 mol/cm3, respectively. These results further confirm that the 2,2′structure is an effective cross-linking agent that reduces the

Figure 10. Temperature dependence of storage modulus of polybenzoxazines.

Figure 11. Temperature dependence of loss modulus of polybenzoxazines.

Figure 12. Temperature dependence of tan δ of polybenzoxazines.

formation of loose chain ends thus give higher cross-link density than other isomeric polybenzoxazines.

4. CONCLUSIONS Various bisphenol-F based benzoxazine isomers were successfully synthesized in this study from the respective pure bisphenol F isomers to understand the effect of isomeric variations on the NMR spectra as well as thermal properties, 5689

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such as the polymerization exotherm temperatures, monomer polymerization kinetics, glass transition temperature of crosslinked polybenzoxazines, and thermal degradation behavior under inert atmosphere. The generalized Fox equation has been utilized to predict Tg for isomeric benzoxazine mixtures. Unexpectedly, it was found that 2,2′-substituted benzoxazine has the highest Tg comparing to another two isomers, contrary to the majority of examples for similar isomeric effect reported in the literature for other type of polymers. Furthermore, the Tg difference between the 4,4′-substitueted benzoxazine and 2,2′substituted benzoxazine (Tg = Tg4,4′ − Tg2,2′) is as great as −60 °C. Aside from the opposite order of the property change, the literature examples of ΔTg are all around 20 °C. Meanwhile, polymerized 2,2′-substituted benzoxazine shows the highest thermal stability among the three isomer benzoxazines. The unique fact in bisphenol F based difunctional benzoxazine is important to draw our attention in the designing of high performance polybenzoxazine, additional studies addressing the universality of this concept for other benzoxazine isomers, and isomer structure and cross-link densities of the polybenzoxazines are in progress and will be reported elsewhere.

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AUTHOR INFORMATION

Corresponding Author

*(H.I.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.L. gratefully acknowledges the partial financial support of the China Scholarship Program. REFERENCES

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dx.doi.org/10.1021/ma501294y | Macromolecules 2014, 47, 5682−5690