New Method To Predict the Thermal Degradation Behavior ofPolybenzoxazines from Empirical Data Using Structure PropertyRelationshipsIan Hamerton,*,† Scott Thompson,† Brendan J. Howlin,† and Corinne A. Stone‡

†Department of Chemistry, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, Surrey, GU2 7XH, U.K.‡Dstl, Porton Down, Salisbury, SP4 0JQ, U.K.

*S Supporting Information

ABSTRACT: The degradation behavior of five polybenzox-azines is studied and the effect of selected experimentalparameters (particle size, heating rate, and atmosphere) on thenature of the degradation pathway is examined. The particlesize within the samples (systematically varied in four discretesize ranges: <106, 106−150, 150−250, >250 μm) influencesthe progress of the early stage in the degradation mechanism(the cleavage of the bridging groups) such that the smallerparticles are less stable, but the latter stages of the degradationmechanism remain largely unaffected. In contrast, the changein heating rate (5, 10, 15, 20 K min−1) of thethermogravimetric analysis has little effect on the first step inthe degradation mechanism, but has a strong influence on the progress of the ring breakdown mechanism. Molecular simulationis shown to reproduce the thermo-mechanical behavior of the polybenzoxazine of bisphenol A/aniline very well, with the nuancesof the glass transition and degradation onset temperatures simulated very closely (e.g., within 10 °C of the degradationexperiment at a mass loss of 5 wt %). Quantitative structure property relationships are shown to predict the experimental charyields for all the polybenzoxazines studied within the data set, with the calculated values for the polymers based solely on thevolume and surface area of the monomer structures.

■ INTRODUCTION

It has been reported1 that approximately 20% of the 1153fatalities on U.S. transport airlines between 1981 and 1990 werecaused by fire, with the vast majority resulting in postcrash fireaccidents; 40% of the passengers who survive the impact of anaircraft accident subsequently die in these postcrash fires.2 Thedevelopment of structural materials with improved fireresistance relative to commodity plastics is key to retard thefire, increase the time available for passengers to escape theaircraft interior, and thus reduce the loss of life. The EC hasrestricted the use of brominated diphenyl oxide flameretardants because highly toxic and potentially carcinogenicbrominated furans and dioxins may form during combustion.3

The World Health Organisation (WHO) and the USEnvironmental Protection Agency (EPA) also recommendexposure limit and risk assessment of dioxins and similarcompounds.4,5 It is essential that new (halogen-free) flameretardant systems are developed to meet the constantlychanging demand of new regulations, standards, and testmethods. This is often achieved by introducing highly aromaticor heteroaromatic materials such as thermoset polymercomposites that form intumescent chars during the combustionprocess, with the polymer swelling and becoming porous toprotect the underlying structure.6 Thermoset polymers have an

established history in civil aviation, in applications involvingdecorative panels, secondary composite structures andadhesivesaround 90% of the interior furnishings of a typicalcivil airliner will contain thermoset composites.7

Polybenzoxazines (PBZs) form a comparatively new familyof thermosetting resins that are being explored8 as potentialhigher performance replacements for phenolic or epoxy resinsand they occupy a niche intermediate between high glasstransition temperature (Tg), tetrafunctional epoxy resins,cyanate esters, and bis(maleimide)s.9 While they are notcurrently widely used in civil aviation, PBZs potentially offer thebest properties from conventional phenolics (in particular thecombination of high thermal stability and flame resistance), andmight be able to be used in place of phenolics in a number ofapplications. The synthetic route employed (in which bothpolyphenol and amine might be varied) offers the potential toyield polymers with improved toughness over conventionalphenolics. PBZs are formed through step growth ring-openingpolyaddition from bis(benzoxazine) monomers (Figure 1),which are in turn the products of the Mannich reaction

Received: July 10, 2013Revised: September 5, 2013

Article

pubs.acs.org/Macromolecules

© XXXX American Chemical Society A dx.doi.org/10.1021/ma401444z | Macromolecules XXXX, XXX, XXX−XXX

between a bis(phenol), formaldehyde, and a primary amine.10

Unlike many other commercial thermosetting resins, whichevolve condensation products such as water or ammonia,benzoxazine monomers react relatively cleanly to form apolymer with few reaction byproducts,11 although the exactmechanism of the polymerization reaction to form a networkhas not been fully elucidated, a recent report has focused on thedevelopment of phenolic and phenoxy network structures as afunction of the catalyst used.12 Recent work in our group hasalso examined the influence of additives on the nature of thepolymerization mechanism, the formation of the polymernetwork structure and the resulting final properties.13

We are particularly interested in rationalizing the effects ofthermal conditioning on the development of char structure incured PBZs. The ability to predict the degree to which charformation occurs and the thermal degradation onset, based onmonomer structure, would assist with the design of polymerswith specific thermal characteristics. If this approach is morewidely applicable then this could be an important tool in thedesign of flame retardant polymers. In the present context, thenature of the bisphenol has been varied to examine the effect ofthe chemical structure on the manner in which the cross-linkedpolymer undergoes degradation.

■ EXPERIMENTAL SECTIONMaterials. 2,2-Bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)-

propane (BA-a, Figure 2), bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)methane (BF-a, mixture of isomers), bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)-2-benzofuran-1(3H)-one (BP-a, mixtureof containing ca. 30 wt % BF-a), 3,3′-bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine) sulfide (BT-a), and bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)tricyclo[5.2.1.0]decane (BD-a, mixture of isomers) wereall characterized fully using 1H NMR, Raman spectroscopy, andelemental analysis and were used as received without furtherpurification (unless otherwise stated). In the interests of brevity thecharacterization data for the monomers have been deposited asSupporting Information. It is well-known that monomer purity (inparticular the oligomer content) can have a significant effect on bothcure mechanism and kinetics. 1H and 13C NMR spectroscopic analyseswere performed in parallel on the monomers (e.g., using DEPT-135,HSQC, and HMBC pulse sequences) and the NMR spectroscopicdata are deposited as Supporting Information.All monomers (Figure 2), except BP-a, were degassed at

approximately 90 °C for 1 h using a vacuum oven to reduce voidformation during the subsequent curing process. BP-a was found to bevery difficult to control under degassing conditions (excessive voidformation) and so was simply melted and held at 120 °C over the

same time period that the other samples were degassed. Followingdegassing, all samples were placed in an air circulating oven at 120 °Cand the following cure schedule was applied: heating 2 K min−1 to 180°C (isothermal 2 h) followed by heating at 2 K min−1 to 200 °C(isothermal 2 h).

Characterization and Measurements. Differential scanningcalorimetry (DSC) was undertaken using a TA Instruments Q1000running TA Q Series Advantage software on samples (5.0 ± 0.5 mg)placed in hermetically sealed aluminum pans. Experiments wereconducted at a heating rate of 10 K min−1 from −10 to +400 °C(heat/cool/heat) under flowing nitrogen (50 cm3 min−1). In order togauge the reactivity of the monomer in the bulk, dynamic DSCanalysis was performed on all of the systems. Thermogravimetricanalysis (TGA) was performed using a TA Q500 on milled, curedresin samples (6.5 ± 0.5 mg) in a platinum crucible and heating from20−800 °C at 10 K min−1 in static air and nitrogen (40 cm3 min−1).Prior to analysis cured PBZ samples were separated, using three sievesof specified mesh size, into four discrete size ranges <106, 106−150,150−250, >250 μm. Dynamic mechanical thermal analysis (DMTA)(in single cantilever mode at a frequency of 1 Hz) was carried out oncured neat resin samples (3 mm × 5 mm × 17 mm) using a TA Q800in static air from −50 to +260 °C at 2 K min−1 at 0.1% strain.

Molecular Simulation. The molecular modeling program AccelrysMaterials Studio versions 5.5 and 6.014 were utilized within this workand all the modeling work was carried out using an in house PC. Thepotential energies for all models throughout this work were calculatedusing the condensed-phase optimized molecular potential for atomisticsimulation studies (COMPASS),15 a force field specifically designedfor polymer calculations. The benzoxazine monomers were drawnatom by atom in the polymer builder module, six copies of the

Figure 1. Schematic showing polymerization of bis(benzoxazine)s through ring-opening and cross-linking.

Figure 2. Structures of the monomers examined in this study.

Macromolecules Article

dx.doi.org/10.1021/ma401444z | Macromolecules XXXX, XXX, XXX−XXXB

monomers were made and monomers that were in close proximitywere manually linked to form the polymer by opening the oxazinering, energy minimization was performed at each stage. After thisprocedure one oxazine ring was left effectively uncured. Theamorphous cell module was used to replicate the polymer in threedimensions with the target density for the cell set to 1.2 g cm−3,

consistent with the density found for the polymer (1.195 g cm−3) inthe literature.16 A super cell of two cells was used to link the polymerin three dimensions and this super cell was replicated 2 × 2 × 2 inspace to make a cubic structure containing 3136 atoms. Oxazine ringsin close proximity in this superstructure were then linked and theresulting network copied to produce a double cube.

Figure 3. TGA data for of PBA-a in nitrogen as a function of heating rate.

Figure 4. TGA data for PBA-a in nitrogen as a function of particle size.

Macromolecules Article

dx.doi.org/10.1021/ma401444z | Macromolecules XXXX, XXX, XXX−XXXC

Molecular dynamics (MD) simulation was performed under theNPT ensemble at 300 °C (573 K) for 10 ps and the resulting modelwas subjected to temperature ramped MD simulations using thetemperature cycle option in the amorphous cell protocols. A collectionof MD simulations was run over different temperatures, withdecrements of 10 K from the starting temperature. The startingtemperature was set at 300 °C (573 K) and a total of 31 MDsimulations were performed, ranging between 300 °C (573 K) and 0°C (273 K). At each temperature stage a 125 ps MD simulation wascreated. The first 25 ps of each simulation were used to equilibrate thesystem and the subsequent 100 ps simulation was used to record theresults. The NPT ensemble (25 °C, 298 K, 0.0001 GPa) with a timestep of 1 fs was utilized with the Anderson thermostat in combinationwith the Parinello Barostat.17 COMPASS was used with the atomicvan der Waals summation, a cutoff at 9.50 Å, a spline width of 1.00 Å,and a buffer width of 0.50 Å. The Tg is a second order phase change,which shows a change in thermal expansion coefficient when thetemperature and volume of a polymer are plotted.18 The gradientchange in the plot locates the Tg and a further transition locates the Td.Generation of QSPR Models. The molecular operating environ-

ment (MOE) software (Chemical Computing Group, Cambridge,U.K.) was used for QSPR modeling. MOE was used to generatemodels to calculate char yield (Yc) for various bis(benzoxazine)monomers. The general procedure used can be summarized as follows:the variable of interest (e.g., char yield) was fitted to a range ofindependent variables (descriptors) within the database to generate a

preliminary QSPR model. The process for selection of appropriatedescriptors was broadly based on trial and error, with the criteria that asuitable QSPR model should incorporate as few descriptors as possibleand display a correlation coefficient value (R2) greater than 0.99. Thedescriptors were “pruned” by removing those in which poorcorrelation was found (a discussion of this process follows withinthe paper) in order to select the optimum set. The most significant orinfluential descriptors or sets of descriptors were identified for the charyield.

■ RESULTS AND DISCUSSION

Dependence of Thermal Stability on ExperimentalParameters. All monomers (see Experimental Section) arereferred to by designations indicating the components used intheir preparation. Thus, BA-a is formed from bisphenol A (BA)and aniline (a); the cured polybenzoxazine is prefixed with a P(e.g., PBA-a). Initially, a series of parameters (experimentalheating rate, atmosphere and particle size) were varied tooptimize the analytical methodology. Thus, as expected theheating rate was observed to have an effect on the position ofthe peak maximum in the derivative data for PBA-a (Figure 3)and with increasing heating rate the peak maximum is raised toa higher temperature regime.

Table 1. Mass Loss Temperatures of the Polybenzoxazines Measured in Nitrogen and (in Air) Obtained Using TGAa

temperature (°C) at which mass loss (%) recorded

sample 5% 10% 20% 30% 40% 50% 60% Yc (%)

PBA-a 260 (295) 313 (371) 348 (450) 368 (499) 389 (524) 417 (542) 456 (558) 27.07 (1.64)PBF-a 254 (269) 309 (377) 381 (473) 428 (504) 480 (531) 562 (554) N/A (572) 45.48 (1.42)PBT-a 289 (323) 315 (390) 369 (479) 451 (511) 535 (532) N/A (549) N/A (566) 52.63 (1.65)PBP-a 285 (317) 351 (406) 437 (469) 472 (498) 516 (520) N/A (542) N/A (564) 52.50 (1.42)PBD-a 237 (238) 310 (365) 378 (415) 398 (420) 412 (423) 426 (426) 440 (428) 22.06 (1.17)

aNB, char yield (Yc) measured at 800 °C. Italicized entries were recorded in air.

Figure 5. TGA data for polybenzoxazines in nitrogen atmosphere.

Macromolecules Article

dx.doi.org/10.1021/ma401444z | Macromolecules XXXX, XXX, XXX−XXXD

The char yield is very slightly elevated as the heating rateincreases, but it seems to have less influence on the nature ofthe initial degradation (between 200 and 300 °C), which isapparently invariant with increasing heating rate. Low andIshida19,20 identified aniline as a major degradation componentin the initial stages of the thermal decomposition of PBA-a,which was postulated to arise from the cleavage of the Mannichbase (Figure 4) as evidenced by the presence of an infrared-active band at 1735 cm−1, consistent with a carbonyl group, thatbecame negligible at 390 °C, arising from a secondary amide. Inthe same study, it was proposed that the cured polybenzox-azines contain terminal Schiff base and secondary amides asdefect structures.In contrast, the maximum derivative peak in the TGA data at

388 °C has also been assigned6 to the phenolic cleavage; thelatter appears much more sensitive to heating rate. Low andIshida6 proposed a degradation mechanism wherein thenitrogen atom of the Mannich base is hydrogen bondedresulting in a stable six-membered ring.21 This degradationscheme involves two routes: one producing aniline and theother a stable conjugated Schiff base. Primarily, cleavage of theC−N bond outside of the six-membered ring is more likely tooccur1 as it is less energetically stable than that inside. The C−N bond is more susceptible to cleavage than the C−C bond ofthe Mannich base as it has a lower dissociation energy than thelatter.22

The effect of particle size on the degradation mechanism,through thermal lag, is more pronounced. In this study thePBA-a was milled to a variety of particle sizes from below 106μm to greater than 250 μm before being analyzed at a singleheating rate (10 K min−1) under nitrogen. The major effect ofchanging particle size is to influence the temperature stability:

the smallest particles display lower thermal stability with thefirst thermal event occurring at a lower temperature than thelarger particles (with the largest sizes, > 250 μm, occurringbetween 50 and 75 °C higher). On the other hand, theinfluence on the thermal events occurring at higher temper-atures is less marked. The char yield (measured at 800 °C) isalso influenced by particle size (Figure 4), with the largestparticles yielding a value some 3%−5% greater than thesmallest.

Thermal and Thermo-Oxidative Stability of Poly-benzoxazines. The nature of the polymer backbone has adramatic effect on the thermal stability and char yield of thepolymers when analyzed under air and nitrogen. For instance,the char yields measured at 800 °C in nitrogen for both PBA-aand PBD-a are less than 30%, in contrast with PBT-a and PBP-awhich show char yields of around 54%, suggesting thatsuperficially the phenolphthalein (benzofuran-1(3H)-one) andsulfur linkages contribute more strongly to char formation thantheir counterparts. PBF-a shows intermediate stability with achar yield of 45% (Table 1).The TGA profiles of the polybenzoxazines in the current

study (Figure 5) show significant differences in threetemperature regimes (i.e., covering initial degradation, max-imum mass loss, and char yield).DSC analysis of the five monomers was carried out to

determine the degree of polymerization (from a comparison ofthe polymerization enthalpy) achieved during the curingprocess from the first heating cycle. The exothermsrepresentative of ring-opening for all five monomers (Figure6 shows the first cycle of a heat/cool/heat experiment for allmonomers) fall within a similar range of 220−242 °C (Table2).

Figure 6. DSC data for benzoxazine monomers in nitrogen atmosphere (heating rate 10 K min−1).

Macromolecules Article

dx.doi.org/10.1021/ma401444z | Macromolecules XXXX, XXX, XXX−XXXE

Interestingly the two monomers that underwent a meltingprocess both have particularly broad exotherms, whereas theother monomers display much more pronounced curves whilecuring. A single exothermic peak is observed for each monomerand demonstrates that the curing results from a single chemicalprocess as a first approximation, but an overall process formedby two or more simultaneous or very close chemical reactionscannot be ruled out.23 The lowest energy was recorded for theBD-a sample at 87.7 kJ/mol monomer (Table 2), while thevalues for BA-a, BP-a, and BT-a are essentially the same withinthe limits of the technique; the BF-a monomer yields a valuethat lies between the other monomers studied. As each of themonomers are difunctional this confirms that four of thosetested have undergone polymerization to a similar degree; theBD-a producing a lower degree of cure (presumably because ofthe bulkiness/rigidity of the bridging group). Consequently, itis possible to make a comparison of the polybenzoxazines of thefour similar monomers on the basis of cross-link density.Thermomechanical Behavior of the Polybenzoxa-

zines. BD-a displayed the poorest thermal stability of theseries studied and was eliminated from the more detailed study.In order to determine whether the data reflect the nature of themonomer backbone, it is important to eliminate the effect ofthe cross-link density. Thus, dynamic mechanical thermalanalysis (DMTA) experiments were used to generate thermo-mechanical data from which the cross-link density could becalculated. Typical DMTA plots are shown in Figure 7 for (a)PBA-a and (b) PBP-a and the data for the series are presentedin Table 3. Although it is customary to report the Tg as the peakmaximum in the loss modulus, the storage modulus and tan δdata are also given for comparison (the latter consistentlypresents a somewhat inflated value as is commonly seen inother polymer systems in the literature).The breadth of the tan δ peaks also indicate differences in the

damping behavior of the polybenzoxazines and the peak widthsrepresent the temperature ranges over which the glasstransition temperatures occur. Thus, the broadest tan δ peak(PBA-a) can be attributed to more heterogeneous networkscontaining both highly- and less densely cross-linked regions.24

This, in turn, results in a broad distribution of molecularmobilities or relaxation times. The cross-link density (ν) foreach polybenzoxazine was calculated from the DMTA datausing eq 1:

ν φ= G / RTe e (1)

Here φ is taken as unity, Ge is the storage modulus strictly froma sample at equilibrium, but is taken at Te, where Te = (Tg + 50

K). This equation25 is technically most appropriate for lightlycross-linked materials, so it should only be used as acomparison between similar materials.From this analysis, it is apparent that perhaps unsurprisingly,

similar cross-link densities are calculated for the PBA-a andPBF-a polybenzoxazines (given the similarity in the bridginggroups) with PBT-a yielding the highest value, Table 4. Thismight be influenced by the rotational freedom of the thio etherallowing perhaps the unreacted benzoxazine ring more latitudeto move within the network to meet unreacted monomers. Thevalue calculated for PBF-a (intermediate between PBA-a andPBF-a) is less easy to explain, but might relate to the bulk of thebridging group inhibiting the polymerization reaction andlimiting the cross-link density. It is known26 that while highglass transition temperatures can achieved for this polybenzox-azine, a carefully designed progressive cure schedule is requiredto achieve its full potential. In a previous study, Howlin et al.reported27 the comparative flexibility of the (analogous) etherand isopropylidene groups, showing that the simulated energybarrier to rotation was 6 kcal/mol for the isopropylidenelinkage and 4 kcal/mol for the ether linkage; the phenyl-phthalein bridge should confer the greatest rigidity/greatestbarrier to rotation, given the bulkiness of the group, coupledwith the polar nature of the substituted γ-butyrolactone ring.Comparison of the nitrogen and air TGA thermograms

demonstrates that the type of atmosphere has a considerableeffect on the degradation processes that are occurring. Notunexpectedly, under an atmosphere of air (Figure 8) all thepolybenzoxazines showed near complete degradation at 800 °C,but the initial stages of the degradation also show the influenceof the bridging group.Thus, the presence of oxygen yields additional reaction

pathways alongside those arising solely from thermaldegradation. Interestingly, the initial mass loss is morepronounced in a nitrogen atmosphere for all of the polymersstudied. Hemvichian and Ishida28 used evolved gas analysis(EGA) to reveal a complex thermal degradation pathwayinvolving polymer degradation and recombination of radicalspecies to yield a range of primary decomposition products(substituted benzenes, anilines, phenols, and Mannich bases),secondary products (biphenyl compounds, benzofurans,isoquinolines, and phenathridines) and ultimately char.Examination of the derivative plots of the mass loss curves is

very informative in visualizing these degradation processes. Forexample, the derivative data for PBA-a in air and nitrogen(Figure 9) atmospheres display significantly different profiles,particularly in the proportion of the data collected between 450and 700 °C. There are at least three processes visible, but theirrelative proportions differ in air or nitrogen. In nitrogen, thefirst process (between 200 and 350 °C) is slightly pronouncedwhen compared with the air atmosphere; the second process(between 300 and 450 °C) is also more prominent.The effect of atmosphere is most marked in the case of the

third process (between 450 and 700 °C), wherein the thermalevent is significantly delayed in air and also accounts for thegreatest rate of mass loss in the thermogram. Low and Ishidaobserved similar TGA profiles in their study,20 and found theinitial mass loss rates under an oxidative environment to belower than those observed under a nitrogen environment. Theyattributed these differences to possible mass gains arising fromoxidation processes. Results presented here suggest that similarprocesses might be associated with higher temperature thermalevents. Work continues in this program to examine the

Table 2. DSC Data of First Heat Cycle for BenzoxazineMonomersa

ΔHpd

sample Tmb (°C) Tmax

c (°C) J/g (kJ/mol) kJ/mol, Bz

BA-a 36 242 311 (128.8) 64.4BF-a − 241 298 (115.0) 57.5BP-a 70 230 263 (132.6) 66.3BT-a − 221 323 (130.5) 65.3BD-a 57 234 174 (87.7) 43.9

aN.B., all samples heated under nitrogen from 20 to 300 °C at 10 Kmin−1. bTm = melting temperature (measured from minimum inendotherm). cTmax = temperature of polymerization peak maximum.dΔHp = polymerization enthalpy (expressed as J/g or kJ/mol ofmonomer and kJ/mol of benzoxazine groups).

Macromolecules Article

dx.doi.org/10.1021/ma401444z | Macromolecules XXXX, XXX, XXX−XXXF

products of these degradation pathways involving pyrolysis−gaschromatography−mass spectrometry and the technique ofTGA hyphenated to residual gas analysis. Low and Ishida20

attributed the second loss in mass in a thermo-oxidativeenvironment (amounting to around 20% at 400 °C) to the lossof carbon dioxide; the final stage was attributed to thebreakdown of the carbonaceous char. Carbon dioxide has beenfound via hyphenated analysis using TGA-FTIR20 to be themajor degradation product of PBA-a above 500 °C, which

would possibly contribute to the large final stage observed inair.

Prediction of Thermal Degradation Using Simulationand Mathematical Techniques. It has already beendemonstrated29 for thermosetting polymers that the thermaldegradation can be well modeled by monitoring closely changesin the simulated cell volume (or bulk density). The simulatedstructure of the PBA-a network (using Materials Studio andcomprising 6272 atoms) is depicted in Figure 10.

Figure 7. DMTA plots for (a) PBA-a and (b) PBP-a.

Macromolecules Article

dx.doi.org/10.1021/ma401444z | Macromolecules XXXX, XXX, XXX−XXXG

From the plot of temperature vs density (Figure 11) the Tdand Tg of the polymers can be determined from the raw data bynoting the change in the cell density (the black bars are thestandard deviations of the values at each point). The shadedregion in the plot shows the range of Tg determinedexperimentally (from the fall in the storage modulus data

using DMTA) and the simulation Tg values fall well within thisrange. PBA-a starts to degrade from around 200 to 250 °C(TGA data), corresponding to the degradation of the bridginggroups and loss of aniline, and this is also observed in thesimulation with a small decrease in density prior to acatastrophic drop from 250 °C onward. This demonstratesthat MD techniques yield realistic predictions of the range overwhich the onset of thermal degradation occurs for the curedpolybenzoxazines, but the technique does not allow the highertemperature processes (notably char yield formation) to beexamined, since the force field used does not allow covalentbonds to break. In some applications (e.g., the production of aflame retardant polymer), the ability to predict the formation ofchar would be of great interest and benefit, but in order to doso it was necessary to use another approach.In a previous study,30 we reported the use of quantitative

structure property relationships (QSPR) to predict variousphysical data in polymers, including the onset of polymerizationand glass transition temperature. Using a similar approach, anequation to predict char yield was derived from the thermaldata of the five monomers presented here (Table 6). TheQSPR model data were fitted to the TGA data obtainedexperimentally in this work and the QSPR model presented ineq 2 was applied to reproduce char yields precisely for each ofthe polybenzoxazines (measured at 800 °C in nitrogen) and thedata are presented in Table 6.

− + +

−

341.65 0.07(vsurfHB2) 16.75(aheavy) 3.41(vol)

5.05(VSA) (2)

Key: VSA = van der Waals surface area, vsurf_HB2 = H bondcapacity at −0.5, a_heavy = number of heavy atoms, vol = vander Waals volume.

Table 3. Determination of Glass Transition Temperature(Tg) for Four of the Polybenzoxazines Using DifferentParameters from DMTA Dataa

Tg (°C) measured by DMTA

samplefrom storage

modulus (midpoint)from loss modulus(peak maximum)

from tan δ (peakmaximum)

PBA-a 172 173 190PBF-a 165 166 184PBP-a 202 204 227PBT-a 197 199 213

aN.B., all samples (3 mm × 5 mm × 17 mm) were analyzed in singlecantilever mode at a frequency of 1 Hz in static air from −50 to +260°C at 2 K min−1 at 0.1% strain.

Table 4. Cross-Link Densities for Four of the CuredPolybenzoxazines Determined from DMTA Dataa

sample Teb (K) Te

c (°C) Ged (MPa) 10‑3νe (mol cm‑3)

PBA-a 496 223 21.3 5.2PBF-a 489 216 14.7 3.6PBP-a 527 254 19.1 4.3PBT-a 522 249 67.5 15.6

aN.B., all samples (3 mm ×5 mm ×17 mm) were originally analyzed insingle cantilever mode at a frequency of 1 Hz in static air from −50 to+260 °C at 2 K min−1 at 0.1% strain. bTe = equilibrium temperature(Tg + 50 K). cTe = equilibrium temperature (Tg + 50 K). dGe = storagemodulus at equilibrium temperature, Te.

eν = cross link density.

Figure 8. TGA data for polybenzoxazines in air atmosphere.

Macromolecules Article

dx.doi.org/10.1021/ma401444z | Macromolecules XXXX, XXX, XXX−XXXH

Figure 9. TGA data of PBA-a in air and nitrogen with derivative data.

Figure 10. Model of cured, pure PBA-a equilibrated at 300 °C (N.B., carbon atoms = gray, nitrogen atoms = blue, and oxygen atoms = red).

Macromolecules Article

dx.doi.org/10.1021/ma401444z | Macromolecules XXXX, XXX, XXX−XXXI

Given that the effect of HB2 on the observed char yield isnegligible (the relative importance of the descriptors in eq 1are: 0.01 vsurf_HB2, 0.29 a_heavy, 0.70 vol, 1.00 VSA), it maybe excluded from the equation to yield eq 3.

+ + −341.65 16.75(aheavy) 3.41(vol) 5.05(VSA) (3)

Note the set of five monomers produced linear equations (witha high degree of fit, R2 = 1.00), making it possible to predictchar yield with a high degree of confidence. Note, the first valuein the eq (341.65) is a constant that is not accounted for by theparameters and is standard in the QSPR method.The parameters that have the most importance in the QSPR

are the van der Waals surface area of the monomer and thevolume of the monomer. Of these, the relative importance ofthe van der Waals surface area is 1.0 and that of the volume 0.7.However, these parameters are negatively correlated in thatvolume has a factor of +3.41 and surface area −5.05. Hence,having a monomer with a large volume helps to generate a largechar yield, but this is moderated by the van der Waals surfacearea. In our data the monomers with the largest volumes giveboth the second largest and the smallest char yields as they have

different van der Waals surface areas. Therefore, from amolecular design point of view, in order to generate a newmonomer that would give a good char yield when tested, it isnecessary to develop a monomer that has a large volume and asmall van der Waals surface area. In other words keeping theexposed surface small contributes to increasing the char yield.This makes excellent physical sense as producing a good charyield requires that combustion is less efficient. These data onlyapply to the five monomers studied so far and work continuesto verify whether this conclusion holds true for a larger data setof literature data.

■ CONCLUSIONS

The degradation data presented here indicate that the sameprocesses are occurring both in air and nitrogen atmospheresbut that the relative proportion of each process changes withatmosphere. Thus, the second stage is dominant in nitrogenand the third is dominant in air. The observed char yield differswith respect to atmosphere (e.g., 20−30 wt % for PBA-a innitrogen versus <5 wt % in air), particle size and heating rate,with char yield increasing as a function of particle size/diminishing surface area (for a given monomer). The level ofchar formed is increased, albeit to a lesser extent, as heating rateincreases. The cross-link densities of the polybenzoxazines werefound to be similar ((3.6−5.2) × 10−3 mol cm−3), with theexception of the thio ether (PBT-a), which displayed more thana 3-fold increase in cross-link density over the other polymers(15.6 × 10−3 mol cm−3), without displaying a markedly higherdegree of conversion. This suggests that the polymerizationmechanism is subtly different: leading to a similar evolution ofheat (i.e., numbers of benzoxazines undergoing ring-opening)in the other monomers, but a more open network through theformation of e.g. ether linkages.Molecular simulation (particularly using molecular dynamics

techniques) can be used to excellent effect to reproduce (andpredict) Tg and Td with a good degree of accuracy (e.g., for

Figure 11. Plot of simulated density versus temperature for PBA-a (N.B. the shaded area shows the empirically determined range for Tg forcomparison; the black bars show the standard deviation in the simulation experiments).

Table 6. Char Yields Predicted from a QSPR Model for theFive Selected Polybenzoxazinesa

polymervan der Waals

Surface Area (Å2)volume(Å3)

predicted charyield (%)

observed charyield (%)

PBA-a 463 438 27.07 27.07PBF-a 497 473 45.48 45.48PBP-a 464 442 52.50 52.50PBT-a 554 533 52.63 52.63PBD-a 582 564 22.06 22.06

N.B. Char yield measured at 800°C in nitrogen. The error inthe observed char yields (based on replicate measurements) is±0.01%.aa.

Macromolecules Article

dx.doi.org/10.1021/ma401444z | Macromolecules XXXX, XXX, XXX−XXXJ

PBA-a the measured Tg is 173 °C from the loss modulus dataand 182 °C using MD simulation). The QSPR modelingapproach has shown the importance of the volume and van derWaals surface area of the monomer in allowing the char yield inthe cured polymer to be predicted precisely for the data set.Only three parameters (van der Waals surface area, van derWaals volume, and the number of heavy atoms) are required todescribe the char yield. This approach has indicated theimportance of the surface area of the monomer in generating alarge char yield for a given cross-link density or degree ofpolymerization. This important finding offers the chance todesign polymers specifically to undergo thermal degradation ata desired temperature and to produce a specific char yield,based on the monomer structure. If this approach is morewidely applicable then this could be an important tool in thedesign of flame retardant polymers.

■ ASSOCIATED CONTENT*S Supporting InformationSpectroscopic and characterization data associated with thecharacterization of the monomers. This material is available freeof charge via the Internet at http://pubs.acs.org/.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: (I.H.) i.hamerton@surrey.ac.uk.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank the Ministry of Defence for funding this work andsupporting ST in the form of research contract (Dstlx-10000065719) and Huntsman Advanced Materials (Basel) forsupplying the monomers.

■ REFERENCES(1) Lyon, R. E. Fire Resistant Materials: Research Overview DOT/FAA/AR-97/99; FAA: Washington, DC, 1997.(2) Special Study: U.S. Air Carrier Accidents Involving Fire, 1965−1974and Factors Affecting the Statistics, National Transportation SafetyBoard: Washington, DC, Report NTSB-AAS-77-1, February 17, 1977.(3) Lu, S.-Y.; Hamerton, I. Prog. Polym. Sci. 2002, 27, 1661−1712.(4) Van Esch, G. J.Environmental health criteria 218flameretardants: tris(2- butoxyethyl)phosphate, tris(2-ethylhexyl)-phosphateand tetrakis-(hydroxymethyl)phosphonium salts; WHO: Geneva, 2000.(5) EPA/600; U.S. Environmental Protection Agency: Washington,DC, 1994; Vols. 1−3.(6) Alger, M. S. M. High-temperature and fire resistant polymers,Chapter 3. In Specialty Polymers; Dyson, R. W., Ed.; Blackie Academicand Professional: Glasgow, 1998; pp 189−227.(7) Hamerton, I.; Mooring, L. The use of thermosets in aerospaceapplications, Chapter 7. In Thermosets Structure, Properties andApplications, Guo, Q., Ed.; Woodhead Publishing:: Cambridge, U.K.,2012; pp 189−227.(8) Handbook of Polybenzoxazine Resins; Ishida, H., Agag, T., Eds.;Elsevier: New York, 2011, and references cited therein.(9) Chemistry and Technology of Cyanate Ester Resins; Hamerton, I.,Ed.; Blackie A&P: Glasgow, 1994.(10) Ishida, H.; Ohba, O. Polymer 2005, 46, 5588−5595.(11) Sudo, A.; Sudo, R.; Nakayama, H.; Arima, K.; Endo, T.Macromolecules 2008, 41, 9030−9034.(12) Liu, C.; Shen, D.; Sebastian, R. M.; Marquet, J.; Schonfield, R.Polymer 2013, 54, 2873−2878.(13) Hamerton, I.; McNamara, L. T.; Howlin, B. J.; Smith, P. A.;Cross, P.; Ward, S. Macromolecules 2013, 46, 5117−5132.

(14) Accelrys Materials Studio, Versions v 5.5.0.0 (2010) and v 6.0.0.0(2012). Accelrys Inc.: San Diego, CA, 2010 and 2012.(15) Sun, H.; Mumby, S. J.; Maple, J. R.; Hagler, A. T. J. Am. Chem.Soc. 1994, 116, 2978−2987.(16) Rimdusit, S.; Tiptipakorn, S.; Jubsilp, C.; Takeichi, T. React.Funct. Polym. 2013, 73 (2), 369−380.(17) Berendsen, H.; Postma, J.; van Gunsteren, W.; Dinolaand, A.;Haak, J. J. Chem. Phys. 1984, 81, 3684−3690.(18) Leach, A. Molecular Modelling - Principles and Applications, 2nded.; Pearson Education Limited: Harlow, U.K., 2001.(19) Low, H. Y.; Ishida, H. J. Polym. Sci., Phys. Ed. 1998, 36, 1935−1946.(20) Low, H. Y.; Ishida, H. Polymer 1999, 40, 4365−4376.(21) Dunkers, J. P.; Zarate, A.; Ishida, H. J. Phys. Chem. 1996, 100,13514.(22) Pauling, L. Chemical bonds. In The Nature of the Chemical Bond:Cornell University Press: New York, 1960; p 85.(23) Ishida, H.; Rodriguez, Y. Polymer 1995, 36, 3151−3158.(24) Kannurpatti, A. R.; Anseth, J. W.; Bowman, C. N. Polymer 1998,39, 2507−2513.(25) Allen, D. J.; Ishida, H. J. Appl. Polym. Sci. 2006, 101, 2798−2809.(26) Spitzer, M. Huntsman Advanced Materials, Basel, Switzerland,personal communication, 2010.(27) Hamerton, I.; Heald, C. R.; Howlin, B. J. Macromol. Theor.Simul. 1996, 5, 305−320.(28) Hemvichian, K.; Ishida, H. Polymer 2002, 43, 4391−4402.(29) Crawford, A. O.; Howlin, B. J.; Cavalli, G.; Hamerton, I. PLoSOne 2012, 7 (9), e44487.(30) Mhlanga, P.; Wan Hassan, W. A.; Hamerton, I.; Howlin, B. J.PLoS One 2013, 8 (1), e53367.

Macromolecules Article

dx.doi.org/10.1021/ma401444z | Macromolecules XXXX, XXX, XXX−XXXK