Chavicol benzoxazine: Ultrahigh Tg biobased thermoset with tunable extended network

European Polymer Journal 81 (2016) 337–346

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Macromolecular Nanotechnology

Chavicol benzoxazine: Ultrahigh Tg biobased thermoset with tunable extended network Ludovic Dumas a,b,⇑, Leïla Bonnaud a, Marjorie Olivier b, Marc Poorteman b, Philippe Dubois a a Laboratory of Polymeric and Composite Materials, Center of Innovation and Research in Materials and Polymers (CIRMAP), Materia Nova Research Center & University of Mons, 23 Place du Parc, B-7000 Mons, Belgium b Department of Materials Science, Materials Engineering Research Center (CRIM), University of Mons, 23 Place du Parc, B-7000 Mons, Belgium

a r t i c l e

i n f o

Article history: Received 2 May 2016 Received in revised form 21 June 2016 Accepted 23 June 2016 Available online 25 June 2016 Keywords: Benzoxazine Bio-based Chavicol Allyl-benzoxazine Tunable network

a b s t r a c t A novel biobased benzoxazine monomer containing additional allyl functionality was synthesized using a solventless approach from the reaction of a natural occuring phenol: chavicol, para-phenylene diamine and formaldehyde. The chemical structure of this functionalized benzoxazine monomer was confirmed by 1H NMR and FTIR. Its polymerization was investigated and monitored by DSC showing two well defined exotherms allowing the selective ring-opening polymerization of benzoxazine functions and the preservation of the allyl functionality. The network crosslink density could be further increased via the controlled polymerization of allyl functionalities with a post-cure in order to adjust the thermo-mechanical properties. When both networks were polymerized, the thermoset presented an excellent thermo-mechanical stability with a Ta higher than 350 °C as measured by DMTA. This exceptional behavior for a potentially biobased benzoxazine resin will allow the preparation of sustainable high performance biocomposite materials. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Benzoxazine resins represent a relatively novel class of thermosets that currently benefit renewed growing interest as they allow for combining the advantages of both traditional epoxy and phenolic resins. The characteristic functional group of these resins consists in a heterocyclic six-membered oxazine ring fused to a benzene ring. First prepared by Holly and Cope [1], Ning and Ishida demonstrated their real interest for materials purposes in the 90s by introducing a solventless synthesis process and highlighting their excellent balance of material properties [2,3]. Their henceforth well-known main features are: (i) an easy thermal curing by ring-opening polymerization without the need of hardeners or catalysts, (ii) a limited shrinkage during curing, (iii) a high glass transition temperature, (iv) a low water absorption, (v) a high charring yield, (vi) a low coefficient of thermal expansion and (vii) low dielectric constants [4–7]. Nevertheless, the main interesting characteristic of benzoxazines is their easy and versatile synthesis by a Mannich-like condensation of three compounds: a phenol, an amine and formaldehyde offering thereby an extraordinary monomer design flexibility that allows for tailoring a large range of properties [8]. The number of newly synthesized monomers is therefore constantly increasing, in particular by including extra functionalities in order to provide additional specific properties. Furthermore the development of biobased and renewable organic materials is one of the hot current challenges in polymer science and is of particular interest ⇑ Corresponding author at: Laboratory of Polymeric and Composite Materials, Center of Innovation and Research in Materials and Polymers (CIRMAP), Materia Nova Research Center & University of Mons, 23 Place du Parc, B-7000 Mons, Belgium. E-mail address: [email protected] (L. Dumas). http://dx.doi.org/10.1016/j.eurpolymj.2016.06.018 0014-3057/Ó 2016 Elsevier Ltd. All rights reserved.

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for both academics and industrials [9]. Benzoxazine resins are by no means an exception and have recently become new players in the biobased polymers arena where researchers make an effort for replacing commercially available and inexpensive phenols and amines by natural products without significant loose of material properties. Various bio-based benzoxazines have already been synthesized from bio-based phenol such as diphenolic acid [10], cardanol [11–13], guaiacol [14,15], vanillin [16,17], eugenol [18–20], coumaric, ferulic and phloretic acids [21] or also resorcinol [22] while the most used amines were furfurylamine [14,23,24], and stearylamine [13,14]. Interestingly, a short review on biobased benzoxazine has been published very recently [25]. From these natural compounds, eugenol (4-allyl-2-methoxyphenol) appeared to present a high potential for green chemistry due to its realistic availability and low production cost [19], but benzoxazines synthesized with this trisubstituted aromatic ring were unable to homopolymerize correctly due to the occupied ortho and para positions [19,26]. Moreover, the thermal stability of eugenol-based benzoxazines was too low to allow the homopolymerization of the allyl function without addition of a comonomer or introduction of extra polymerizable sites [19,20]. In order to keep the interest to use an allyl phenol, eugenol can be replaced by another naturally occurring phenol, namely 4-allyl-phenol (chavicol), which to the best of our knowledge has never been studied to prepare benzoxazine resins. Interestingly, chavicol can be found in betel oil, bay oil or in sweet basil [27,28]. Other articles or patents describe also the possible enzymatic conversion of para-coumaryl/coniferyl alcohol esters, present in lignin, into chavicol but the processes are not yet easily scalable nor industrials [29,30]. Herewith we propose the synthesis of a potentially bio-based chavicol benzoxazine presenting the ability to be polymerized selectively through the ring opening reaction of benzoxazine rings and to further extend the network crosslinkability with the thermally activated reaction of the allyl functions. This selective crosslinking reaction allows an easy tuning of the network properties.

2. Experimental 2.1. Materials The following chemicals were purchased from Aldrich and used without any further purification: 1,4-phenylenediamine (99%), paraformaldehyde (95%). 4-allylphenol (95%) was purchased from Chembo Pharma and used as received. Ethanol was purchased from VWR.

2.2. Characterization The 1H NMR spectra were recorded with a NMR spectrometer (Bruker, 500 MHz), using deuterated dimethylsulfoxide (DMSO-d6) as solvent and the chemical shift was calibrated by setting the chemical shift of DMSO as 2.50 ppm. Calorimetric studies were carried out at a heating rate of 10 °C/min using a differential scanning calorimeter (DSC Q200 from TA Instruments) under nitrogen flow of 50 mL/min. An indium standard was used for calibration. Thermogravimetric analysis (TGA) was used to study the anaerobic thermal degradation of the precursor blends and cured systems. Approximately 10 mg of the sample was submitted to a temperature ramp from 25 to 1000 °C at a heating rate of 10 °C/min under a nitrogen flow of 60 mL/min. All TGA experiments were performed by using a TGA Q50 device from TA instruments. Thermo-mechanical properties were investigated using a dynamic mechanical thermal analysis (DMTA) apparatus (DMA 2980 Dynamical Mechanical Analyzer from TA Instruments). Specimens (70
12
3 mm3) were tested in a dual cantilever configuration with a dual cantilever length of 35 mm. The thermal transitions were studied in the temperature range of 25–370 °C at a heating rate of 3 °C/min and at a fixed frequency of 1 Hz. An amplitude of 18 lm was used corresponding to a strain of 0.043%. One representative sample was used for the measurements. Fourier Transform Infrared (FTIR) spectra were recorded in transmission mode using a Bruker IFS 66v/S spectrometer equipped with a vacuum apparatus. Precursors and crosslinked polymers were powdered and diluted into a KBr matrix with a weight concentration of about 0.5 wt%. Spectra were recorded under vacuum from 500 to 4000 cm1 with a wavenumber resolution of 4 cm1. 64 scans were collected for each sample. Preparation and characterization of the chavicol-based benzoxazine, C-pPDA. The C-pPDA synthesis has been adapted from a procedure reported by Ishida [3] chavicol 24.42 g (1.73
101 mol) and 12.02 g (3.80
101 mol) paraformaldehyde, 10% in excess, were introduced in a beaker at 50 °C. The mixture was stirred with a mechanical stirrer leading to the formation of a homogeneous white solution. 9.35 g (0.86
101 mol) 1,4-phenylenediamine, finely powdered, was then added into the beaker and immersed in an oil bath preheated at 120 °C. The addition of the diamine leads to the gelation of the mixture resulting from the condensation of the aromatic diamine and formaldehyde and the subsequent formation of a triazine network [31,32]. At this temperature, the triaza compound reacts quickly with chavicol and the gel is destroyed in a couple of minutes. The mixture was allowed to react for 25 min under continuous stirring. The crude reaction product was then dissolved in refluxing ethanol (600 mL). Then, the resin was allowed to precipitate upon cooling. The precipitate was collected whereas the filtrate was partially evaporated to allow a second precipitation of the residual soluble C-pPDA. Both precipitates were collected and rinsed with cold ethanol

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before drying at 70 °C. A crystallizing light yellow resin was obtained (31 g, weight yield  85%). The C-pPDA monomer was characterized by a Tg of 18 °C, a melting enthalpy of 65 Jg1 and a Tm of 112 °C. 2.3. Curing procedure of C-pPDA C-pPDA was introduced in a stainless steel 80
12
3 mm3 mould, melted, further degassed in a vacuum oven at 140 °C for 10 min, and then step-cured in an air-circulating oven according to the following cycle: 2 h at 180 °C, 2 h at 200 °C, and then 1 h at each of the following temperatures: 230 °C, 250 °C, 270 °C and 300 °C. Thereafter, the samples were allowed to slowly cool down to room temperature before their unmolding. 3. Results and discussion 3.1. Synthesis and characterization of C-pPDA C-pPDA monomer was successfully prepared by a solventless synthesis from chavicol and 1,4-phenylene diamine with an easy and scalable procedure without the use of toxic solvent according to the reaction depicted in Scheme 1. The structure of the new benzoxazine was investigated by 1H NMR as shown in Fig. 1. The attribution of peaks was based on the analysis of 1H NMR spectra of reagents and on similar benzoxazine derivatives [19,33]. The well-defined characteristic peaks at 4.51 and 5.3 ppm corresponding to the PhACH2AN (b) and OACH2AN (c) of oxazine ring, respectively, verify the formation of the benzoxazine ring [34]. Additional aromatic peaks labeled a, d, e, f, the presence of the doublet g at 3.23 ppm attributed to the methylene of the allyl function and the presence of peaks i and h at 5.0 and 5.9 ppm with respective relative intensities of 2 and 1 support the successful benzoxazine synthesis without loss of the allyl group of interest. However, the spectrum of the crude product (Fig. 1A) shows the presence of residual chavicol (signals labeled D, E, G, H, I and J) which cannot be consumed by a longer reaction time. Comparison of the integral values indicates a residue lower than 5% of the initial chavicol content. The presence of this residue is not critical for the resin curing but it would affect the polymerizability by catalyzing the ring-opening polymerization of benzoxazine ring or change also the network properties. A purification of the crude product has thus been carried out by a solubilisation/precipitation step in ethanol. This purification step does not require the use of toxic organic solvents or several washings with NaOH aqueous solutions as it is usual in benzoxazine synthesis area. As it can be seen on Fig. 1B, the right matching of all integrals, the disappearance of the characteristic peaks of residual chavicol and the absence of peaks related to oligomeric species clearly validate the investigated purification process. Moreover, in case of C-pPDA, the use of a solventless procedure instead of a solvent one not only allows to shorten significantly the reaction time (indeed, 25 min in bulk versus 74 h in toluene are needed to form a diamino allylphenol-based benzoxazine from ortho-allylphenol and 4,4-diaminodiphenylsulfone [35]) but it allows also to obtain the desired benzoxazine monomer with a high yield (85%) and minimize the risk of oligomerization. To complete the structure characterization, FTIR analysis was carried on the purified monomer (Fig. 2). The characteristic absorption peaks at 1226 cm1 and 1140 cm1 are assigned to the asymmetric and symmetric stretching vibrations of CAOAC of the oxazine ring respectively [36]. The asymmetric stretching vibrations of CANAC of the oxazine ring are observed at 1173 cm1 while the CAH out of plane deformation of the benzene ring attached to the heterocycle (mod 10a) is observed at 938 cm1 [36]. The important absorption bands at 1518, 1500 and 817 cm1 may be attributed to the CAC tangential vibration (mod 8a), CAC stretching vibration (mod 19b) and the CAH out of plane bending vibration (mod 11) of aromatic rings, respectively. The C@C stretching vibration at 1638 cm1 and the olefinic CAH out of plane bending vibration at 988 cm1 belong to the characteristic absorptions of the allyl group. The @CAH stretching vibrations of the allyl group and the aromatic ring appear at 3076 cm1, together with the peak at 2900 cm1 is the @CAH asymmetric stretching vibration of the allyl and aromatic rings [35,37]. The absorption peaks around 1362 cm1 may be attributed to the CH2 wagging of the benzoxazine ring and allyl function but their accurate attribution is not clearly elucidated. All these peaks confirm the presence of both the benzoxazine ring and allyl function in C-pPDA. 3.2. Curing behavior of the allyl functionalized benzoxazine The polymerizability and reactivity of C-pPDA were further characterized by DSC. The DSC thermogram shown in Fig. 3 highlights, first, the purity of the monomer with the melting peak appearing at 112 °C. Secondly, the thermogram shows evidence for a two stage reaction with the presence of two well defined exothermic peaks at ca. 240 °C and 340 °C. OH H H2 N

NH2

+ 4

O H

+ 2

120 °C

O

-4 H 2O

Scheme 1. One pot synthesis of C-pPDA monomer.

O N

N

C-pPDA

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(A)

(B)

0,0 3200 3000 2800

1600

1400

1000

817 cm

-1

-1

-1

744 cm

800

642 cm

-1

-1

988 cm

1173 cm

1200

910 cm

-1

938 cm

1226 cm -1

1362 cm

-1

-1

-1

-1

1638 cm

2900 cm

-1

0,5

3076 cm

Absorbance

-1

1,0

1500 cm

1520 cm

-1

Fig. 1. 1H NMR spectra of C-pPDA in DMSO-d6. (A) crude resin, (B) C-pPDA after purification. Water peak labeled ⁄.

600

Wavelength (cm-1) Fig. 2. FTIR spectrum of the C-pPDA monomer.

The first exothermic peak is attributed to the opening of the benzoxazine ring as it occurs at temperatures similar to those observed for analogous monomers [18,19] and as the allyl polymerization is known to follow the ring-opening of oxazine moieties when the allyl is attached to an aromatic structure [38]. The value of the exothermic enthalpy estimated by a rough deconvolution of the two exotherms is found to be about 335 Jg1, corresponding to a DH of 71 kJ per mole of benzoxazine ring which is in good accordance with the 73 kJ mol1

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100

1,0 80

0,5 0,0

60

-0,5 -1,0

Weight (%)

Heat Flow (W/g)

1,5

40

-1,5 -2,0

0

100

200

300

400

20

Fig. 3. DSC and TGA profiles of C-pPDA monomer recorded under nitrogen upon the curing temperature range.

Fig. 4. Samples of cured C-pPDA showing the ability of the resin to polymerize without formation of voids.

usually accepted for the heat reaction of a benzoxazine ring [39]. This result highlights the excellent polymerizability of the C-pPDA monomer contrary to the similar eugenol-based benzoxazine precursor (E-pPDA) bearing hindered methoxy groups in ortho position (relative to the hydroxyl of the used phenol) that prevented it from ring-opening polymerization (ROP) and promoted an irreversible thermal degradation with a weight loss, recorded at 400 °C, of 50% [19]. Instead, when chavicol is used, i.e. when the ortho position is free, the weight loss upon curing is very limited. In addition, the thermal stability of C-pPDA is found to be high enough to allow the second polymerization of allyl functions to take place at nearly 290 °C. This ability of C-pPDA to polymerize without the formation of defects or voids is illustrated in Fig. 4. More specifically, the broken sample presented in the picture aims at showing the smooth and fragile profile achieved after rupture stressing the successful preparation of a dense material. The exothermic enthalpy value of the second reaction is about 230 Jg1 corresponding to a value of 50 kJ per mol of allyl function. Unfortunately, it was not possible to compare this value to the existing literature on allyl functionalized benzoxazine as the exothermic peaks of both ROP and allyl addition polymerization are often superimposed [34,40–43], and sometimes the allyl groups were even found to alter the polymerization of benzoxazine group due to their position [35,38]. Nevertheless, a typical value of the polymerization enthalpy of allylic resin is known to be around 70 kJ per mol of allyl function when radical initiators are used [44]. This value may vary with the nature of initiators [45] while pure thermal polymerization is known also to lead to lower enthalpic values [44]. Additionally, allyl radicals present a lower reactivity and a lower tendency to initiate new polymer chains because of their ability to get stabilized by resonance [46]. This resonance is found to be exacerbated in case of an aromatic structure [34] which may lead to a lower degree of polymerization. Another possible explanation of this relatively low enthalpic value may also lie in the restricted mobility of the already formed benzoxazine network. Nevertheless, the presence of this exotherm stresses that the additional allyl functionality can readily polymerize with only a thermal activation. The degree of reaction can be considered as maximal in our case, as no residual exotherm can be found on a second DSC run. However, as the temperature needed to polymerize the whole sample appeared to be relatively high (400 °C), a step cure procedure with a maximal temperature of 300 °C was established to avoid degradation and the structural evolution of the network as well as the residual enthalpy were followed by infrared and DSC studies. Based on our past studies on diaminobased benzoxazine, the benzoxazine rings were first polymerized with a maximal cure temperature of 230 °C [33,47]. As it can be seen in Fig. 5. The exothermic peak associated to the ROP is entirely consumed indicating that all benzoxazine rings

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Heat Flow (W/g)

2,5 2,0 1,5 1,0 0,5 0,0 -0,5 0

50

100

150

200

250

300

350

400

Fig. 5. DSC of C-pPDA after each cure stage.

Table 1 Evolution of residual enthalpies after each cure stage.

Uncured Cured until Cured until Cured until Cured until

DHallyl J/g

% Allyl consumeda

Ta (DMTA)

335 0 0 0 0

230 180 140 83 12

0 22 39 64 95

– 200 222 297 353

% Allyl consumed = (DHallyl (Uncured)  DHallyl (Ti))/DHallyl (Uncured)
100.

-1

-1

988 cm

-1

1117cm

1173 cm

-1

910 cm

2,0

1270 cm

-1

1362 cm

-1

3076 cm

Absorbance

2,5

-1

1620 cm

-1

a

230 °C 250 °C 270 °C 300 °C

DHbenzo. J/g

1,5 1,0 0,5 0,0

3500

3000

1750

1500

1250

1000

750

Fig. 6. FTIR spectra of the step cured p(C-pPDA).

have reacted. In addition, a partial consumption of the second exotherm is also noticed with a 20% loss of the enthalpy associated to the allyl addition polymerization (Table 1). Each additional isotherm of 1 h at 250, 270 and 300 °C allows the pursuit of the polymerization of the allyl functions until their almost total consumption at 300 °C. The FTIR study (Fig. 6) corroborates the reaction of benzoxazine cycles after the cure step at 230 °C. The absorption bands at 938 and 1117 cm1 corresponding to the vibration of the CAH of the aromatic ring attached to the heterocycle (mod 10a and 18b respectively) disappeared while the main bands associated with the CANAC (1173 cm1) and CAOAC (1226 cm1) were severely decreased or modified into large and overlapped bands centered at 1270, 1250 and 1147 cm1 confirming the ring-opening reaction of benzoxazine and the formation of Mannich-type bridge structures. In addition, the absorption band located around 1620 cm1 has severely increased and the bands around 1500 cm1, characteristic of the substituted benzene ring, have shifted to lower wavenumbers witnessing a variation of the degree of substitution of the benzene ring [37,48]. The presence of the large absorption band at 3000–3500 cm1 is also an additional evidence of the benzoxazine polymerization as it produces phenol entities. Concerning the allyl function, its characteristic vibrational bands (988, 1362 and 3076 cm1) were not severely modified after the curing step at 230 °C. However, their intensities suffer a progressive decrease after each isotherm at higher temperatures and become negligible after the isotherm at 300 °C confirming the reaction of the allyl function and the subsequent formation of a secondary network as illustrated in Scheme 2.

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L. Dumas et al. / European Polymer Journal 81 (2016) 337–346 OH O

O N

OH N

N

Δ

n1

N

Δ

n1 m2

~ 300°C

~ 230°C N

N OH

n2

OH

m1

n2

Scheme 2. Illustration of the formation of the two embedded networks.

1000

0,8 0,6

100

0,4

Δ tan δ

Storage Modulus (MPa)

1,0

10 0,2 1 0

100

200

300

400

0,0

Fig. 7. Influence of the network extension on the thermo-mechanical properties. DMTA Dual cantilever, 1 Hz.

3.3. Properties of the network with different crosslinking degrees The thermo-mechanical properties of the crosslinked C-pPDA were further investigated by DMTA in dual cantilever mode. Fig. 7 highlights the impressive change of the thermo-mechanical behavior with different allyl reaction degrees obtained with the previously mentioned cure cycles. Firstly, the thermo-mechanical properties of the network formed only by the reaction of benzoxazine rings, i.e. cured at 230 °C, were found to be relatively high with a thermo-mechanical transition temperature Ta, observed at the maximal of the tan d peak, of 200 °C. This value is slightly lower than in the case of the same P-pPDA diamine benzoxazine which does not possess allyl groups (220 °C) [33,47]. This difference may account from the pendant unreacted allyl group, which increases, in the case of C-pPDA, the free volume of the network and reduces the crosslinking volume density. Secondly, by increasing the cure temperature, and the subsequent reaction of the allyl groups, the thermo-mechanical stability is significantly improved. Indeed, due to an increase of the crosslinking density, the drop of the storage modulus associated to the Tg is progressively shifted from 180 °C to 325 °C until the maximal allyl reaction degree is reached affording a Ta as high as 350 °C. This high value of Ta is quite exceptional for a biobased benzoxazine resin and in the same range of some of the best petrosourced allyl [34] or acetylene [49,50] functionalized bisbenzoxazines. It is also interesting to point here that the Tg is in this case higher than the polymerization temperature. Concerning the variation of the storage modulus, the elastic properties remain relatively identical in the glassy state, i.e. below the Tg. Indeed, in the case of benzoxazine, the crosslinking density is supposed to have little or no influence on stiffness in the glassy state as the rigidity in this region is mainly due to the strong intra- and inter-molecular hydrogen bonding [51,52]. In addition, the evolution of the storage modulus in the rubbery area, dependent on the crosslink density, is a little compromised as for the high cure temperature curves, there is no obvious plateau. However, the evolution of the tan d peak gives also some information about the network characteristics. The intensity of the peak decreases while it becomes wider when the curing temperature increased. These phenomena indicate that a lower number of polymer chains are involved in the thermo-mechanical transition for a given temperature while the network becomes more dense but less homogeneous [53]. These network heterogeneities, owing from the additional allyl polymerization, appear to be in good agreement with the well known difficulties to polymerize allyl functions due to the so-called ‘‘degradative chain transfer” [46]. The multiple chain terminations or heavy stabilizations by resonance limit the chain growth and introduce large length variation in the active polymer segment of the secondary network. Interestingly, by plotting the evolution of the Ta as a function of the percentage of consumed allyl functions, obtained from enthalpy measurements, a linear evolution can be found as evidenced in Fig. 8. Thus the thermomechanical properties are closely dependent on the extension of the network and can be easily modulated by adjusting this ‘‘extra crosslinking” with the post-cure treatment under a wide range of temperature, from 200 to 350 °C.

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400

Tα = 147,29 + %(a. c.)*2,19

R² = 0,977

T α °C

300

200

100

0 0

20

40

60

80

100

% (allyl consumed) Fig. 8. Evolution of the thermo-mechanical transition temperature as a function of the degree of allyl reaction.

1,5

100 80

1,0

Δ

60 40

0,5

20 0,0

0 0

200

400

600

800

1000

Fig. 9. Thermal properties of entirely cured p(C-pPDA), under N2 - 10 °C/min.

Another interesting feature for this benzoxazine is its high thermal stability. As evidenced by Fig. 9, the fully polymerized C-pPDA presents an excellent thermal stability up to 400 °C with a 5% weight loss temperature TD 5% of 413 °C. Such a high thermal stability is not common for allyl-functionalized benzoxazines where the TD 5%, obtained in the same conditions, are not higher than 350 °C [34,38,41,50]. Usually, the additional allyl function allow linking the weakest part of the benzoxazine molecule, i.e. the amino part, to the network thereby limiting its release at the initial stages of the degradation. In the case of p(C-pPDA), the amino part is already bound to two benzoxazine functions and is therefore well integrated into the benzoxazine network. Additional allyl crosslink points increase the overall crosslinking density and link all the molecule parts one to another reinforcing their thermal stability. Compared to a benzoxazine prepared with p-phenylene diamine and phenol, (the same molecule without allyl functions), the TD 5% was increased from 315 °C to 413 °C showing, without any doubt, the real benefit of the extra allyl-crosslink [47]. However, as the allyl additional network is aliphatic, there is no significant effect on the char yield which remains relatively high with a value of 40% at 1000 °C. This high charring lets expect improved fire properties with intrinsic values of limiting oxygen index higher than 33% according to the well-known Van Krevelen and Hoftyzer method [54,55]. 4. Conclusions A new multifunctional biobased benzoxazine was successfully synthesized starting from natural chavicol offering a high content of bio-based carbon (>75%). The thermal polymerization of C-pPDA proceeds in two separated steps: the ROP of the benzoxazine rings followed by the allyl addition polymerization. Indeed, the allyl function can be selectively preserved by an appropriate thermal curing and post-cured with different degrees of polymerization in order to readily modulate the network properties with Tg ranging from 200 °C to 350 °C. The evolution of the Tg was found to be in a linear correlation with the degree of allyl polymerization. These findings have highly significant implications for the design of new fully biobased polybenzoxazines with desirable properties. The thermal stability of this new biobased benzoxazine is also remarkable, close to 400 °C. The combination of an aromatic diamine and the biobased 4-allyl-phenol thus allows the preparation of high performances biobased materials with

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comparable or even improved properties when compared with the corresponding petroleum-based analogues. C-pPDA is thus very promising, especially in the frame of the increasingly demand from industries, which look forward to subscribing to sustainable approaches without sacrificing the material properties [56,57]. Acknowledgements The authors wish to thank the Wallonie and European Community for general support in the frame of the ‘‘Fonds de maturation: NANOBENZO project”, the FEDER 2014–2020 program: HYBRITIMESURF, MACOBIO, BIOMAT and BIORG-EL projects, the INTERREG V program (BIOCOMPAL project) and the ‘‘Programme d’Excellence FLYCOAT” and the Interuniversity Attraction Poles Program initiated by the Belgian Science Policy Office. References [1] F.W. Holly, A.C. Cope, Condensation products of aldehydes and ketones with o-aminobenzyl alcohol and o-hydroxybenzylamine, J. Am. Chem. Soc. 66 (11) (1944) 1875–1879. [2] X. Ning, H. 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