Enhanced Thermal Property and Flame Retardancy via Intramolecular

Article pubs.acs.org/Macromolecules

Cite This: Macromolecules XXXX, XXX, XXX−XXX

Enhanced Thermal Property and Flame Retardancy via Intramolecular 5‑Membered Ring Hydrogen Bond-Forming Amide Functional Benzoxazine Resins Jia Liu,† Natallia Safronava,‡ Richard E. Lyon,§ Joao Maia,† and Hatsuo Ishida*,† †

Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States Technology and Management International, LLC, Toms River, New Jersey 08753, United States § Fire Safety Branch ANG-E21, Federal Aviation Administration, Building 277, William J. Hughes Technical Center, Atlantic City International Airport, Atlantic City, New Jersey 08405, United States

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‡

ABSTRACT: Polybenzoxazines containing an ortho-amide group in the backbone with a significant degree of aromaticity have been synthesized and characterized in this study. This class of materials makes use of smart chemistry to be able to readily process the monomer and later transform from polybenzoxazine into polybenzoxazole. All the benzoxazine monomers studied contain amide groups that form 5membered intramolecular hydrogen bonds with the oxazine rings. Systematic manipulation of the diamine structure has allowed the structure−property relationship of polybenzoxazines to be examined. Polymerization behavior of monomers is studied by differential scanning calorimetry (DSC) and in situ Fourier transform infrared spectroscopy (FT-IR). Thermal stability and flammability are characterized by thermogravimetric analysis (TGA) and microscale combustion calorimetry (MCC), respectively. The experimental results are fitted to the major thermal decomposition processes contributing to the specific heat release rate histories Q′(T). This family of materials show one of the lowest heat release capacity values of all polymers indicating the excellent flame retardancy.

1. INTRODUCTION Despite the appealing features such as high strength-to-weight potential polymer composites, the relatively low thermal stability limits their applicability for certain applications where flame retardency is of particular concern.1 Development of fire safe polymers with high performance for applications such as in the electronic and aerospace industries has been the subject of strong interest of many advanced material researchers. This requires materials exhibiting high glass transition temperature, high thermal stability, low toxicity, and high flame retardancy.2 Among various high temperature polymers, polybenzoxazine is a newly developed thermosetting resin. Extensive research in the polybenzoxazine area has been reported in the past decade due to their fascinating properties, for example, no release of volatiles during polymerization, no added initiator and/or catalyst required, near-zero shrinkage during polymerization,3,4 excellent thermal stability,5−7 self-extinguishing,8 and, above all, very rich molecular design flexibility. Improvement of flame retardancy of polybenzoxaiznes using phosphorus and bromine atoms has actively been studied.9−27 However, it is well-known that many halogen-containing flame-retardants lead to toxicity to both humans and the environment, and some phosphorus compounds are also a suspect for some health issues.28,29 Therefore, development of halogen and/or phosphorus-free flame retardant materials is of great interest. © XXXX American Chemical Society

A benzoxazine containing amide group at the ortho position with respect to the phenolic oxygen atom was first reported by Agag et al. as an attractive class of precursor that has potential to form benzoxazole resin upon heating,30 resulting in very high temperature stability. More recently, detailed study of the role of hydrogen bonding on the acceleration of the monomer synthesis rate, improved solubility in a common organic solvent, reduction of the polymerization temperature, and increased thermal degradation temperature31−34 have been reported. All these are associated with the stable interaction via 5-membered or 6-membered intramolecular hydrogen bonding of an amide proton with the oxazine ring.35 Detailed modes of interaction have been studied by an NMR spectroscopic method termed two-dimensional (2D) 1H−1H nuclear Overhauser effect spectroscopy (NOESY).36 In the present study, the thermal stability and flammability of the amide containing polybenzoxazins are investigated. Furthermore, the correlation between monomer backbone structure and polymer properties is discussed. Received: September 21, 2018 Revised: November 15, 2018

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DOI: 10.1021/acs.macromol.8b02033 Macromolecules XXXX, XXX, XXX−XXX

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NH),13C NMR (DMSO-d6), ppm δ = 49.71 (Ar−C−N, oxazine), 81.07 (O−C−N, oxazine), 165.60 (s, Ar−CO−NH−). (yield: 81%) 4,4-Oxidianiline (ODA) Based Benzoxazine. White powder. 1H NMR (DMSO-d6), ppm: δ = 4.62 (s, Ar−CH2−N, oxazine), 5.45 (s, O−CH2−N, oxazine), 6.81−7.94 (24H, Ar). 9.74 (s, NH), 13C NMR (DMSO-d6), ppm δ = 49.94 (Ar−C−N, oxazine), 80.68 (O−C−N, oxazine), 165.61 (s, Ar−CO−NH−) (yield: 78%). 3,3′-Diaminodiphenyl Sulphone (DDS) Based Benzoxazine. Orange powder. 1H NMR (DMSO-d6), ppm: δ = 4.76 (s, Ar− CH2−N, oxazine), 5.57 (s, O−CH2−N, oxazine), 6.91−8.01 (24H, Ar), 9.39 (s,NH). 13C NMR (DMSO-d6), ppm δ = 49.01 (Ar−C−N, oxazine), 79.22 (O−C−N, oxazine), 165.64 (s, Ar−CO−NH−) (yield: 74%). 4-Amino-N-(4-aminophenyl)benzamide (DABA) Based Benzoxazine. Yellow powder. 1H NMR (DMSO-d6), ppm: δ = 4.61 (s, Ar− CH2−N, oxazine), 6.01 (s, O−CH2−N, oxazine), 6.94−8.03 (24H, Ar), 9.15 (s, NH), 13C NMR (DMSO-d6), ppm δ = 49.60 (Ar−C−N, oxazine), 80.63 (O−C−N, oxazine), 165.65 (s, Ar−CO−NH−) (yield: 70%). 4,4′-Methylenebis(2,6-dimethylaniline) Based Benzoxazine. Pink fine powder. 1H NMR (DMSO-d6), ppm: δ = 2.12 (s, CH3), 3.96 (s, Ar−CH2−Ar), 4.61 (s, Ar−CH2−N, oxazine), 6.16 (s, O−CH2−N, oxazine), 6.64−8.17 (20H, Ar), 9.37 (s, NH), 13C NMR (DMSO-d6), ppm δ = 19.48 (s, CH3), 49.16 (s, Ar−CH2−Ar), 67.04 (Ar−C−N, oxazine), 82.02 (O−C−N, oxazine), 165.56 (s, Ar−CO−NH−). (yield: 76%) 4,4′-(Biphenyl-4,4′-diylbis(oxy))dianiline (BAPB) Based Benzoxazine. Light red powder.1H NMR (DMSO-d6), ppm: δ = 4.54 (s, Ar− CH2−N, oxazine), 6.23 (s, O−CH2−N, oxazine), 6.73−8.32 (32H, Ar), 9.25 (s, NH). 13C NMR (DMSO-d6), ppm δ = 49.86 (Ar−C−N, oxazine), 80.58 (O−C−N, oxazine), 165.61 (s, Ar−CO−NH−) (yield: 71%). Polymerization of Polybenzoxazines. DMF was used to dissolve benozxazine monomers to make a 30% solid content solution. The solution was cast over a dichlorodimethylsilane-pretreated glass plate to prepare a thin film. The film was dried in an air-circulating oven at 100 °C for 24 h to remove the solvent completely, which is followed by stepwise heating at 120, 140, 160, and 200 for 2 h each and 225 °C for 1 h, and then slowly cooled to room temperature. The films have dark brown color with thickness ranging from 0.1 to 0.3 mm.

2. EXPERIMENTAL SECTION 2.1. Materials. 2-Aminophenol, 4-chlorobenzene, benzoyl chloride, isophthalic chloride, sodium borohydride, trifluoroacetic anhydride, tetrachlorophthalic anhydride, and paraformaldehyde, 2,4dimethylphenol, 4,4′-diaminodiphenylmethane (DDM), 4,4′-oxidianiline (ODA), and aniline were purchased from Sigma-Aldrich. All these compounds are of reagent grade and used as received except for aniline. Aniline was purified by distillation before use. Chloroform, hexane, 2-propanol, methanol, xylenes, N,N′-dimethylacetamide (DMAc) and sodium sulfate were purchased from Fisher Scientific and used as received. 4,4′-(Biphenyl-4,4′-diylbis(oxy))dianiline (BAPB) was purchased from Wakayama Seika Kogyo. 1,4-Phenylenediamine, 3,3′-dianmiodiphenyl sulphone (DDS), 1,3-phenylenediamine (1,3-PDA), 1,4-phenylenediamine (1,4-PDA), and 4-amino-N(4-aminophenyl)benzamide (DABA) were purchased from Tokyo Chemical Industry (TCI). 2.2. Measurements. Proton and 13C nuclear magnetic resonance (1H and 13C NMR) spectra were acquired on a Varian Oxford AS600 at a proton frequency of 600 MHz and its corresponding carbon frequency of 150.9 MHz. The average number of transients for 1H and 13C MNR measurements were 64 and 1024, respectively. A relaxation time of 10 s was used for the integrated intensity determination of1 H NMR spectra. Fourier transform infrared (FTIR) spectra were obtained using a Bomem Michelson MB100 FTIR spectrometer, which was equipped with a deuterated triglycine sulfate (DTGS) detector and a dry air purge unit. Co-addition of 32 scans was recorded at a resolution of 4 cm−1. Transmission spectra were obtained by casting a thin film on a KBr plate for partially polymerized samples. A TA Instruments DSC model 2920 was used with a heating rate of 10 °C/min and a nitrogen flow rate of 60 mL/ min for all tests of differential scanning calorimetric (DSC) study. All samples were sealed in hermetic aluminum pans with lids. Thermogravimetric analysis (TGA) was performed on a TA Instrument Q500 TGA with a heating rate of 10 °C/min in a nitrogen atmosphere at a flow rate of 40 mL/min. Microscale combustion calorimetry (MCC) experiments were conducted using the equipment that was developed and built by researchers at the FAA.37 Tests were run under standard operating conditions with flow rates of 80 mL/min N2, 20 mL/min O2 and a heating rate of 1 °C/s and combustor temperature set to be 900 °C according to ASTM D7309 Method A.38 Synthesis of N-(2-Hydroxyphenyl)benzamide. 2-Aminophenol (10.838 g, 99.3 mmol) was first dissolved in 100 mL of DMAc, followed by cooling the solution to 0 °C and dropwise adding benzoyl chloride (13.96 g, 99.3 mmol) into the reaction mixture with vigorous stirring. After maintaining 0 °C for 4 h, the reaction continued at room temperature for overnight to achieve the maximum yield. The structure was confirmed by 1H and 13C NMR. Yield 18 g, 85%, light pink crystal, mp 175 °C. 1H NMR (DMSO-d6), ppm δ = 5.35 (s, OH), 6.77−7.95 (9H, Ar). 9.15 (s, NH), 13C NMR (DMSO- d6), ppm δ = 125.8 (−C−NH−),148.4 (HC−OH, s), 164.7 (Ar−CO−, s). Preparation of Benzoxazine Monomers. The aromatic diamine, N-(2-hydroxyphenyl)benzamide and paraformaldehyde were added to a round-bottom flask in stoichiometric amounts (1:2:4). The reactants were mixed with 1,4-dioxane and further refluxed for 5 h. After cooling, the reaction mixture was dropwise added into 100 mL of cold water while stirring to give powder-like precipitates. The product was further purified by washing with 1 N NaOH solution and deionized water to remove unreacted materials, afterward drying in a vacuum oven at 40 °C to obtain a fine powder. 1,3-Phenylenediamine (1,3-PDA) Based Benzoxazine. White powder. 1H NMR (DMSO-d6), ppm: δ = 4.57 (s, Ar−CH2−N, oxazine), 5.41 (s, O−CH2−N, oxazine), 6.84−8.13 (20 H, Ar). 9.32 (s, NH), 13C NMR (DMSO-d6), ppm δ = 67.04 (Ar−C−N, oxazine), 81.07 (O−C−N, oxazine), 165.59 (s, Ar−CO−NH−) (yield: 84%). 1,4-Phenylenediamine (1,4-PDA) Based Benzoxazine. White powder. 1H NMR (DMSO-d6), ppm: 4.58 (s, Ar−CH2−N, oxazine), 5.42 (s, O−CH2−N, oxazine), 6.81−8.22 (20 H, Ar). δ = 9.30 (s,

3. RESULTS AND DISCUSSION 3.1. Synthesis of Amide Containing Benzoxazine. Scheme 1 represents the generalized scheme for the syntheses of amide functional benzoxazine monomer. N-(2Scheme 1. Generalized Scheme for Benzoxazine Monomer Synthesisa

a X = O, CH2, 3,3′-SO2, O-ph-ph-O, NHCO, O-ph-O. The diamine structure uses an expression for isomers, indicating the use of m- and p-forms of isomers.

B


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Table 1. Molecular Structure of the Synthesized Monomers and Their Corresponding Abbreviated Names, and the Diamines Used To Synthesize the Corresponding Benzoxazines along with Abbreviated Names

Hydroxyphenyl)benzamide (abbreviated as HPB) is first synthesized by solution polycondensation reaction of oaminophenol with benzoyl chloride in an amide solvent

using triethylamine as an acid scavenger. Then a series of aromatic diamines was used to make a family of amide containing benzoxazines. C


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Macromolecules Table 1 lists the structure of diamines used in this study, which have been grouped as three categories based on differences in benzene backbone. Group 1 seeks to explore the effect of isomeric substitution, group 2 emphasizes the anchoring effect of a two benzene ring linkage, and group 3 is the more branched type structure and main chain type structure. Table 1 further lists the molecular structures of benzoxazine monomers synthesized using these diamines along with the abbreviated name of the monomer. The abbreviation followed the widely used convention by abbreviating the phenolic component in capital letters, amine components in small letters, and these two abbreviated names are connected by a hyphen to indicate the benzoxazine monomer. 3.2. Polymerization Behavior of Benzoxazine Monomers. The polymerization behavior of amide containing benzoxazine monomer was studied by nonisothermal DSC. As shown in Figure 1, upon heating, an endothermic peak was observed around 145−150 °C followed by an exothermal peak around 175−210 °C. The endothermic peak indicates either the melting of benzoxazine monomer or the evaporation of solvent residue contained in the benzoxazine monomer, whereas the exothermic peak relates to the polymerization reaction of benzoxazine. These thermograms show somewhat broader peaks than typical benzoxazine monomers as well as lower onset temperature. This indicates that the polymerization occurs at lower temperatures and in a more complex way. To further understand the polymerization behavior, in situ FTIR spectra were obtained by heating a benzoxazine monomer at various temperatures for 1 h. HPB-14pda was used as an example. As shown in Figure 2(a), the presence of the cyclic ether structure in the benzoxazine ring is supported by the absorbance peaks at 1228 and 1035 cm−1 that correspond to the C−O−C asymmetric and symmetric stretching modes, respectively.39 The characteristic mode of the oxazine ring is located at 949 cm−1.40 The two bands around 750 and 690 cm−1 indicate a monosubstituted benzene ring, which corresponds to the terminal benzene ring that is attached to a carbonyl group. Two bands at 1650 and 1600 cm−1, which correspond to the in-plane carbon−carbon stretching of the trisubstituted benzene ring, are also observed.39 Notice that the amide stretch located at 1609 cm−1 splits into multiple peaks upon heating. When the temperature is above 225 °C, the newly appeared bands at 1616 cm−1 (CN stretch) and 1006 cm −1 (C−O stretch) indicate the formation of the oxazole ring.41,42 This matches well with the decreased NH absorption at 3421 cm−1 upon heating as shown in Figure 2(b) since the NH group must be consumed for the benzoxazole formation according to the proposed mechanism. 3.3. Thermal Property of Polybenzoxazines. The thermal stability of polybenzoxazines is determined by thermogravimetric analysis (TGA) as shown in Figure 3. For all the polymers studied, the 5% weight loss temperatures, Td5, vary from 243 to 292 °C. Compared to the polybenzoxazine based on DDM with phenol, which has Td5 of 380 °C, these values are relatively low. This is because of the formation of water upon the oxazole ring closure reaction that takes place in the temperature range of 250 to 300 °C. Structural transformation from hydroxyimide into the benzoxazole structure has been actively studied43−48 though such reaction was sometimes questioned.49 More recent study using solid state 13C NMR analysis supports benzoxazole ring formation.30

Figure 1. DSC thermogram of benzoxazine monomers. (a): phenylenediamine isomers; (b) phenylene ethers; (c): aliphatic bridges between phenylene groups. D


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Figure 2. Representative in situ FTIR spectra of a benzoxazine monomer (HPB-14pda) upon heating. (a) 1800−600 cm−1 region, and (b) 3600−2600 cm−1 region.

Transformation to an oxazole structure is possible whether from hydroxyl imide43 or ortho-amide.30 Overall, the thermograms clearly show two stages in the case of amide containing polybenzoxazine, which include cyclization and degradation of the material. The temperature range of the cyclizaion matches well with the FT-IR data as discussed in the previous results. For the char yield at 800 °C under N2, this series of polybenzoxazines range from 42% to 51%, which is much higher than that of polybenzoxazines based on neat difunctional benzoxazine monomer, such as P-ddm (Char yield is around 41% at 800 °C under nitrogen atomosphere) even considering the loss of water during the transformation to an oxazole ring. These char yields in Table 2 can be compared with the corresponding polybenzoxazines made from phenol rather than HPB. For example, the polymer derived from HPB-oda exhibited the char yield at 45% whereas the polybenzoxazine that is from the monomer based on phenol and 4,4′-

Figure 3. TGA thermograms of o-amide-containing polybenzoxazines.

diaminodiphenylether (dds) showed the char yield at 48%.50 Another example is HPB-13pda which showed the char yield of 52% as compared with the polymer based on phenol and E


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specimen by oxygen consumption calorimetry. The MCC thermal combustion properties are the maximum (peak) specific heat release rate, Q′p, heat of combustion of the pyrolysis gases Hc (J/g-gas), the pyrolysis residue ϕ (gresidue/g-sample), the thermal decomposition temperature Tp (°C) at Q′p, and the heat of combustion of the pyrolysis gases per unit mass of sample, Q∞ (J/g-sample) = (1 − ϕ)Hc. The pyrolysis residue is the mass fraction of sample remaining after anaerobic thermal decomposition as per ASTM D7309, Method A, obtained by weighing the sample before and after the test. The heat release capacity of the solid, ηc (J/g-K) is an additional thermal combustion property that is one of the best single predictors of the flammability of a material. Heat release capacity is a material property that is rooted in the chemical structure of the polymer and can be calculated from additive molar group contributions for simple materials.52 Milligram size samples were tested at a constant heating rate of 1 K/s over the temperature range of 150−900 °C. If the material exhibits multiple HRR peaks, the heat release capacity is calculated as if each peak represents a separate component, i = 1, 2, 3, ..., n having mass mi and heat release capacity ηi = (Q’i /mi)/β. In this case, the heat release capacity of the ncomponent mixture having ∑mi = m0 is simply the massfraction-weighted average of the heat release capacities of the components, which is the sum of the individual peak heights after deconvolution (peak fitting) to remove peak overlap. Because the multiple Q′i peaks often overlap, they must first be separated and resolved using a peak-fitting procedure. The

Table 2. Thermal Properties of Polybenzoxazines Weight-Loss Temperature Abbreviation of the benzoxazine monomer used

5%

10%

Char yield at 800 °C

HPB-13pda HPB-14pda HPB-oda HPB-dds HPB-daba HPB-44m26dma HPB-bapb

286 243 248 290 292 253 265

312 296 288 319 323 295 320

52 46 45 44 48 40 41

1,3-phenylenediamine whose char yield was 53%.51 It can be seen that, despite loss of weight by about 10% at relatively low temperature due to the evaporation of water produced during the structural transformation into benzoxazole, the char yield for two benzoxazines, one with phenol and another with HPB for the same diamine are nearly identical. It is interesting to notice that despite a significant difference in the diamine structures used, the thermal stability, in particular char yield, is rather similar for the same o-amide-functional phenol used. The formed char is not powdery but is an amorphous carbon with somewhat porous structure. 3.4. Flammability Testing of Polybenzoxazines. Microscale combustion calorimetry (MCC) is typically a 15 min, constant heating rate pyrolysis test in which the evolved gases are combusted in excess oxygen and the specific heat release rate Q′(W/g) is determined for a milligram-sized Table 3. MCC Test Results for Polybenzoxazinesa

ηc: heat release capacity; Q′: specific heat release rate; Q∞: heat of combustion of the pyrolysis gases per unit mass of sample; Hc: heat of combustion of the pyrolysis gases; Tp: thermal decomposition temperature.

a

F


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Macromolecules minimum number of peaks that provide an error of less than 5% in the composite curve maximum should be used to obtain ηc. Triplicate tests of each sample were conducted using the nitrogen flow rate at 80 cm3/min as a purge gas according to ASTM D 7309 Method A, and the average value was reported. Heat release capacity was calculated by the peak summation method.37,53 The parametric results of these tests are given in Table 3. An important property for flammability characteristics, according to Walters and Lyon,53 is the heat release capacity for which molar group contribution can be used to calculate,54 thus allowing theoretical comparison of flammability. According to their calculations, polybenzoxazines can be inherently good flame-retardant materials as molar group contribution of hydroxyl groups, tertiary amine, and benzene groups are high. Such a molar contribution approach for heat release capacity has very recently been successfully applied to several benzoxazines.55 Their results are summarized in Table 4 for comparison. It can be seen that the o-amide-based benzoxazines show consistently low heat release capacity regardless of the diamine structure.

P=

Phenol/pphenylenediamine Bisphenol A/aniline Chavicol/pphenylenediamine Hydroquinone/ furfurylamine Resorcinol/ fufurylamine Bisphenol A/ aminophenylacetylene Bisphenol A/ propargylamine

Char yield (%)

Measured heat release capacity (J/g·K)

Calculated heat release capacity (J/g·K)

60

88

109

30 48

247 173

280 189

65

30

46

65

40

46

60

135

146

40

222

229

n

Global Interaction Term = ≡P−

∑ ∑ ϕijPij (1)

∑ ϕiPi + 2∑ ∑ i=1 j=i+1

λijxijPij

∑ xiPi = ηcmeas − ηccal

(4)

As seen in Table 5, the heat release capacities measured in the MCC tests are considerably lower than the ηc calculated assuming simple molar additivity, i.e., using only the first term on the right-hand side of eq 3. We might interpret this to mean that the chemical moieties of the polybenzoxazines do not act independently during thermal decomposition. Instead, the pairwise interactions of the polybenzoxazine moieties are so strong to significantly reduce ηc by participating in a complex sequence of thermal decomposition reactions that extend the charring process over a broad temperature range so that the rate of production of fuel gases at any single temperature, Q′(T), is reduced. This complex charring process involving numerous pairwise and higher-order interactions between the benzoxazine moieties is evidenced in the large and negative global interaction terms in Table 5. Figure 4 illustrates this point by comparing polybenzoxazine derived from HPB-13pda to polyphenylsulfone (PPSU), which has no oxazine ring in the molecule and decomposes to gases and char in a single step with Q∞ = 12.8 kJ/g and ϕ = 0.42 (42%).37,56 Although PPSU decomposes at a higher temperature than the polybenzoxazine, both of these polymers have similar char yields and heats of combustion. However, the polybenzoxazine decomposes over a much broader temperature range (about 2.5 times that of PPSU) because of the complex decomposition process associated with interactions of the chemical moieties and the specific heat release rate Q′(T) is correspondingly reduced. From the flame retardation point of view, this broader thermogram is preferred over the narrow and intense peak of PPSU since it minimizes the excessive heat generation per unit time. Thus, it reduces the generation of fuel by accelerated decomposition of the yet to be burned solid. It is highly likely that this broad decomposition scheme of the polybenzoxazines studies relates to the structural transformation of the o-amide group into benzoxazoles releasing water molecules and changing into a further thermally stable group during high temperature degradation. This could explain why the calculated and actual measurement exhibits such high discrepancy toward more favorable flame retardancy in the current study. Although only one representative example is shown in Figure 4, the thermograms of all the polybenzoxazoles derived from o-amide-based polybenzoxazines studied in the current paper show these complex multiple-peak thermograms. This helps broaden the temperature range of the thermogram while reducing the maximum heat release rate,

The order of the subscripts i and j is interchangeable, and using contracted notation, Φij = Φi and Pii = Pi P=

∑ ∑ i=1 j=i+1

n

i=1 j=1

(3)

The pairwise interaction terms λij can produce positive or negative deviations from additivity. Empirical additive molar group contributions for polymers that decompose in a single step [50] were used to calculate the heat release capacity, P = ηc, for the polybenzoxazines using the first term on the right-hand side of eq 3, and the results are given in Table 5 along with the mean values of ηc. Also shown in Table 5 are the global interaction terms for the polybenzoxazines computed as the difference between the measured heat release capacity ηcmeas and the calculated heat release capacity ηccal according to eq 3,

One approach to calculating the contribution of a chemical moiety in a polymer to an intrinsic property is to assume that groups i and j contribute an amount Pij to a property P according to a weighted sum over all of the pairwise interactions between groups.52 P=

λijxijPij

i=1 j=i+1

Table 4. Thermal Properties of Benzoxazines without Ortho-Functional Phenolic Components (ref 56) Benzoxazines derived from

∑ xiPi+∑ ∑

ϕijPij (2)

If the weighting factors are defined to be the geometric mean of the mole, mass or volume fractions of the components xi multiplied by an interaction parameter λij that describes the strength of the pairwise interaction between groups i and j, then Φij = λij (xixj)1/2. Furthermore, if Pij is the arithmetic mean of the properties, Pij = (Pi + Pj)/2 and λij = 1, G


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Macromolecules Table 5. Measured ηc of Polybenzoxazines and Calculated ηc Using Additive Molar Group Contributions

calorimetry (MCC), respectively. This family of materials showed very low heat release capacity values as well as high thermal stability, indicating the excellent flame retardancy without added flame retardants. The measured heat release capacities are considerably lower than that calculated value, indicating the chemical moieties of the polybenzoxazines chains do not act independently during thermal decomposition. Instead, the pairwise interactions of the polybenzoxazine moieties are strong enough to significantly reduce heat release capacities by participating in a complex sequence of thermal decomposition reactions that extend the charring process over a broad temperature range. The unexpectedly lower heat release capacity values of o-amide-functional benzoxazines in comparison to the predicted values using the molar contribution approach of heat release capacity that was successfully applied to ordinary polybenzoxazines are consistent with the expected formation of benzoxazole structure upon heating the o-amide-functional polybenzoxazines.

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Figure 4. Comparison of specific heat release rate Q′(T) of polybenzoxazole derived from HPB-13pda (the thermogram with double peaks) and polyphenylsulfone (PPSU) (the thermogram with a single peak).

AUTHOR INFORMATION

Corresponding Author

*E-mail: (H.Ishida) [email protected]. ORCID

Joao Maia: 0000-0001-6285-566X Hatsuo Ishida: 0000-0002-2590-2700

both of which are desirable characteristics of flame-retardant polymers.

Notes

The authors declare no competing financial interest.

4. CONCLUSIONS A series of o-amide-containing difunctional benzoxazine monomers were synthesized and characterized in this study. The thermal stability and flammability were characterized by thermogravimetric analysis (TGA) and microscale combustion

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REFERENCES

(1) Elias, H. G. In An Introduction to Polymer Science; Wiley, New York, 1997; pp 27−31.

H


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Macromolecules (2) Cullis, C. F.; Hirschler, M. M. The Combustion of Organic Polymers; Oxford University Press: Oxford, UK, 1981; pp420−424. (3) Ishida, H.; Allen, D. J. Physical and mechanical characterization of near-zero shrinkage polybenzoxazines. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 1019−1030. (4) Ishida, H.; Low, H. Y. A study on the volumetric expansion of benzoxazine-based phenolic resin. Macromolecules 1997, 30, 1099− 1106. (5) Low, H. Y.; Ishida, H. Structural effects of phenols on the thermal and thermo- oxidative degradation of polybenzoxazines. Polymer 1999, 40, 4365−4376. (6) Low, H. Y.; Ishida, H. Mechanistic study on the thermal decomposition of polybenzoxazines: Effects of Aliphatic Amines. J. Polym. Sci., Part B: Polym. Phys. 1998, 36, 1935−1946. (7) Laobuthee, A.; Chirachanchai, S.; Ishida, H.; Tashiro, K. Asymetric mono-oxzine: an inevitable product from manich reaction of benzoxazine dimers. J. Am. Chem. Soc. 2001, 123, 9947−9955. (8) Walters, R. N.; Lyon, R. E. Molar group contributions to polymer flammability. J. Appl. Polym. Sci. 2003, 87, 548−563. (9) Choi, S. W.; Ohba, S.; Brunovska, Z.; Hemvichian, K.; Ishida, H. Synthesis, characterization and thermal degradation of functional benzoxazine monomers and polymers containing phenylphosphineoxide. Polym. Degrad. Stab. 2006, 91, 1166−1178. (10) Lin, C. H.; Cai, S. X.; Leu, T. S.; Hwang, T. Y.; Lee, H. H. Synthesis and properties of flame-retardant benzoxazine by three approaches. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 3454−3468. (11) Chang, S. L.; Lin, C. H.; Lin, H. T.; Huang, H. J.; Hwang, K. Y.; Tu, A. P. Development of an aromatic triamine-based flame-retardant benzoxazine and its high-performance copolybenzoxazines. Eur. Polym. J. 2009, 45, 680−689. (12) Sponton, M.; Lligadas, G.; Ronda, J. C.; Galia, M.; Cadiz, V. Development of a DOPO-containing benzoxazine and its highperformance flame retardant copolybenzoxazines. Polym. Degrad. Stab. 2009, 94, 1693−1699. (13) Li, S.; Huang, W.; Liu, X.; Yu, X.; Xiao, W. Synthesis, characterization, and polymerization of brominated benzoxazine monomers and thermal stability/flame retardance of the polymers generated. Polym. Adv. Technol. 2010, 21, 229−234. (14) Espinosa, M. A.; Galia, M.; Cadiz, V. Novel phosphorinated flame retardant thermosets: epoxy-benzoxazine-novolac systems. Polymer 2004, 45, 6103−6109. (15) Espinosa, M. A.; Galia, M.; Cadiz, V. Novel phosphorinated flame retardant thermosets: epoxy-benzoxazine-novolac systems. Polymer 2004, 45, 6103−6109. (16) Espinosa, M. A.; Cadiz, V.; Galia, M. Development of novel flame retardant thermosets based on benzoxazine-phenolic resin and a glycidyl phosphinate. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 279−289. (17) Lin, C. H.; Cai, S. X.; Leu, T. S.; Hwang, T. Y.; Lee, H. H. Synthesis and properties of flame-retardant benzoxazines by three approaches. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 3454−3468. (18) Sponton, M.; Ronda, J. C.; Galia, M.; Gadiz, V. Studies on thermal and flame retardant behaviour of mixtures of bis(maminophenyl)methylphosphine oxide based benzoxazine and glycidylether or benzoxazine of bisphenol A. Polym. Degrad. Stab. 2008, 93, 2158−2165. (19) Hwang, H. J.; Lin, C. Y.; Wang, C. S. Flame retardancy and dielectric properties of dicyclopentadiene-based benzoxazine cured with a phosphorus-containing phenolic resin. J. Appl. Polym. Sci. 2008, 110, 2413−2423. (20) Sponton, M.; Ronda, J. C.; Galia, M.; Cadiz, V. Cone calorimetry studies of benzoxazine-epoxy systems flame retarded by chemically bonded phosphorus or silicon. Polym. Degrad. Stab. 2009, 94, 102−106. (21) Sponton, M.; Ronda, J. C.; Galia, M.; Cadiz, V. Development of flame retardant phosphorus- and silicon-containing polybenzoxazines. Polym. Degrad. Stab. 2009, 94, 145−150. (22) Sponton, M.; Lligadas, G.; Ronda, J. C.; Galia, M.; Cadiz, V. Development of a DOPO-containing benzoxazine and its high-

performance flame retardant copolybenzoxazines,. Polym. Degrad. Stab. 2009, 94, 1693−1699. (23) Ling, H.; Gu, Y. Improving the flame retardancy of polybenzoxazines with a reactive phosphorus-containing compound. J. Macromol. Sci., Part B: Phys. 2011, 50, 2393−2404. (24) Chang, H. C.; Lin, C. H.; Lin, H. T.; Dai, S. H. A. Deprotection-free preparation of propargyl ether-containing phosphorinated benzoxazine and the structure-property relationship of the resulting thermosets. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 1008−1017. (25) Zhang, T. T.; Men, W. W.; Liu, Y.; Lu, Z. J. Synthesis and characterization of polybenzoxazine containing phosphorus. Chin. J. Polym. Sci. 2012, 30, 250−257. (26) Chang, H. C.; Lin, H. T.; Lin, C. H. Benzoxazine-based phosphorinated bisphenols and their application in preparing flameretardant, low dielectric cyanate ester thermosets. Polym. Chem. 2012, 3, 970−978. (27) Su, H. X.; Liu, Z. G. The structure and thermal properties of novel DOPO-containing 1,3-benzoxazines. J. Therm. Anal. Calorim. 2013, 114, 1207−1215. (28) Betts, K. S. New Thinking on Flame Retardants. Environ. Health Perspect. 2008, 116, A210−A213. (29) Araki, A.; Saito, I.; Kanazawa, A.; Morimoto, K.; Nakayama, K.; Shibata, E.; Tanaka, M.; Takigawa, T.; Yoshimura, T.; Chikara, H.; Saijo, Y.; Kishi, R. Phosphorus flame retardants in indoor dust and their relation to asthma and allergies of inhabitants,. Indoor Air 2014, 24, 3−15. (30) Agag, T.; Liu, J.; Graf, R.; Spiess, H. W.; Ishida, H. Benzoxazole resin: A novel class of thermoset polymer via smart benzoxazine resin. Macromolecules 2012, 45, 8991−8997. (31) Zhang, K.; Ishida, H. Thermally stable polybenzoxazines via ortho-norbornene functional benzoxazine monomers: Unique advantages in monomer synthesis, processing and polymer properties. Polymer 2015, 66, 240−148. (32) Zhang, K.; Ishida, H. An anomalous trade-off effect on the properties of smart ortho-functional benzoxazines. Polym. Chem. 2015, 6, 2541−2550. (33) Zhang, K.; Ishida, H. Smart synthesis of high-performance thermosets based on ortho-amide-imide functional benzoxazines. Front. Mater. 2015, 2, Art. 5, 1−9. (34) Zhang, K.; Han, L.; Froimowicz, P.; Ishida, H. A Smart Latent Catalyst-Containing Ortho-Trifluoroacetamide Functional Benzoxazine: Precursor for Low Temperature Formation of Very High Performance Polybenzoxazole with Low Dielectric Constant and High Thermal Stability. Macromolecules 2017, 50, 6552−6560. (35) Han, L.; Zhang, K.; Ishida, H.; Froimowicz, P. Study of the Effects of Intramolecular and Intermolecular Hydrogen-Bonding Systems on the Polymerization of Amide-Containing Benzoxazines. Macromol. Chem. Phys. 2017, 218, 1600562. (36) Froimowicz, P.; Zhang, K.; Ishida, H. Intramolecular hydrogenbonding in benzoxazines: When structural design becomes functional,. Chem. - Eur. J. 2016, 22, 2691−2707. (37) Lyon, R. E.; Walters, R. N.; Safronava, N.; Stoliarov, S. I. Principles and Practice of Microscale Combustion Calorimetry, Federal Aviation Administration Final Report, DOT/FAA/TC-12/53, April 2013. (38) ASTM Standard D7309, Test method for determining flammability characteristics of plastics and other solid materials using microscale combustion calorimetry; American Society for Testing and Materials, West Conshohocken, PA, 2013. (39) Dunkers, J.; Ishida, H. Vibrational assignments of 3-alkyl-2,4dihydro-1,3-benzoxazines in the fingerprint region. Spectrochim. Acta, Part A 1995, 51A, 1061−1074. (40) Han, L.; Iguchi, D.; Heyl, T.; Sedwick, V. M.; Arza, C. R.; Ohashi, S.; Lacks, D. J.; Ishida, H.; Gil, P. J. Phys. Chem. A 2017, 121, 6269−6282. (41) Hsiao, S. H.; Huang, Y. H. A new class of aromatic polybenzoxazoles containing ortho-phenylenedioxy groups. Eur. Polym. J. 2004, 40, 1127−1135. I


Article

Macromolecules (42) Yang, G.; Matsuzono, S.; Koyama, E.; Tokuhisa, H.; Hiratani, K. A new synthetic route to benzoxazole polymer via tandem Claisen rearrangement. Macromolecules 2001, 34, 6545−6547. (43) Tullos, G. L.; Powers, J. M.; Jeskey, S. J.; Mathias, L. J. Thermal conversion of hydroxyl-containing imides to benzoxazoles: polymer and model compound study. Macromolecules 1999, 11, 3598−3612. (44) Okabe, T.; Morikawa, A. Synthesis of polyimides from 3,3′dihydroxybenzidine and conversion to polybenzoxazoles. High Perform. Polym. 2008, 20, 53−66. (45) Park, H. B.; Jung, C. H.; Lee, Y. M.; Hill, A. J.; Pas, S. J.; Mudie, S. T.; Wagner, E. V.; Freeman, B. D.; Cookson, D. J. Polymers with cavities tunes for fast selective transport of small molecules and ions. Science 2007, 318, 254−258. (46) Schab-Balcerzak, E.; Jikei, M.; Kakimoto, M. Thermal rearrangement of poly(o-hydroxyimide)s synthesized from 4,6diaminoresorcinol dihydrochloride. Polym. J. 2003, 35, 208−212. (47) Guzman-Lucero, D.; Likhatchev, D. Imide-to-benzoxazole rearrangement in ortho substituted poly(4,4’-diphenylene pyromellitimide)s. Polym. Bull. 2002, 48, 261−269. (48) Chen, B. K.; Tsai, Y. J.; Tsay, S. Y. 2,6-Diamino-4-phenylphenol (DAPP) copolymerized polyimides: synthesis and characterization. Polym. Int. 2006, 55, 93−100. (49) Hodgkin, J. H.; Dao, B. N. Thermal conversion of hydroxylcontaining polyimides to polybenzoxazoles. Does this reaction really occur? Eur. Polym. J. 2009, 45, 3081−3092. (50) Liu, Y. F.; Li, Z. H.; Zhang, J.; Zhang, H. L.; Fan, H. Y.; Run, M. T. Polymerization behavior and thermal properties of benzoxazine based on 4,4’diaminodiphenyl ether. J. Therm. Anal. Calorim. 2013, 111, 1523−1530. (51) Ren, S. T.; Yang, X.; Zhao, X. J.; Zhang, Y.; Huang, W. An mphenylenediamine-based benzoxazine with favorable processability and its high-performance thermoset. J. Appl. Polym. Sci. 2016, 133, 43368. (52) Lyon, R. E.; Safronava, N.; Quintiere, J. G.; Stoliarov, S. I.; Walters, R. N.; Crowley, S. Material properties and fire test results. Fire Mater. 2014, 38, 264−278. (53) Lyon, R. E.; Takemori, M. T.; Safronava, N.; Stoliarov, S. I.; Walters, R. N. A molecular basis for polymer flammability. Polymer 2009, 50, 2608−2617. (54) Walters, R. N.; Lyon, R. E. Molar group contributions to polymer flammability. J. Appl. Polym. Sci. 2003, 87, 548−563. (55) Sonnier, R.; Otazaghine, B.; Dumazert, L.; Menard, R.; Viretto, A.; Dumas, L.; Bonnaud, L.; Dubois, P.; Safronava, N.; Walters, R.; Lyon, R. Prediction of thermosets flammability using a model based on group contributions. Polymer 2017, 127, 203−213. (56) Karlsson, L. E.; Jannasch, P. Polysulfone ionomers for protonconducting fuel cell membranes: 2. Sulfophenylated polysulfones and polyphenylsulfones. Electrochim. Acta 2005, 50, 1939−1946.

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