Bio-based high performance thermosets: Stabilization and reinforcement of eugenol-based benzoxazine networks with BMI and CNT

Bio-based high performance thermosets: Stabilization and reinforcement of eugenol-based benzoxazine networks with BMI and CNT
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  Accepted Manuscript Bio-based high performance thermosets: stabilization and reinforcement of eu-genol-based benzoxazine networks with BMI and CNTLudovic Dumas, Leïla Bonnaud, Marjorie Olivier, Marc Poorteman, PhilippeDuboisPII:S0014-3057(14)00415-7DOI: 6638To appear in:  European Polymer Journal Received Date:6 October 2014Revised Date:19 November 2014Accepted Date:22 November 2014Please cite this article as: Dumas, L., Bonnaud, L., Olivier, M., Poorteman, M., Dubois, P., Bio-based highperformance thermosets: stabilization and reinforcement of eugenol-based benzoxazine networks with BMI andCNT,  European Polymer Journal  (2014), doi: is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customerswe are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, andreview of the resulting proof before it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.    Bio-based high performance thermosets: stabilization and reinforcement of eugenol-based benzoxazine networks with BMI and CNT Ludovic Dumas, a,b  Leïla Bonnaud, a  Marjorie Olivier, b  Marc Poorteman, b  and 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. Corresponding author *  Fax: +32 (0)65 37 3484; Tel: +32 (0)65 373480; E-mail:, Abstract This work presents the solventless synthesis, the reinforcement and the thermal stabilization of bio-based bis-benzoxazine resins derived from eugenol. The structure of this benzoxazine monomer was confirmed by 1 H NMR. The polymerization and degradation of the  precursor have been investigated and monitored by DSC and TGA and showed a limited crosslinking ability due to the steric hindrance of the eugenol based aromatic ring. A bis-maleimide (bMI) has been used to stabilize the benzoxazine moiety while it undergoes also a copolymerization through an ene -addition. Depending on its relative content, the bis-maleimide proved to limit the thermal degradation of the bis-benzoxazine monomer and allowed the formation of a dense crosslinked network with high thermomechanical stability. Furthermore, the properties of the network were significantly improved by the incorporation of a small amount of carbon nanotubes (CNTs) with an increase of the Tg as high as 60 °C. The combination of bMI and CNTs will allow the use of this new bio-based  benzoxazine resin as matrices for the preparation of sustainable high performance biocomposite materials. Keywords: Benzoxazine, bio-based, eugenol, nanocomposites, carbon nanotubes, bis-maleimide Introduction Benzoxazine resins represent a class of phenolic type thermosets that currently benefit some renewed interest as they allow for combining the advantages of both traditional epoxy and phenolic resins. First prepared by Holly and Cope in 1944 1 , Ning and Ishida demonstrated their real interest in the 90’s  by introducing a solventless synthesis process and by using benzoxazines as  precursors for new thermosetting materials offering an excellent  balance of materials properties. 2, 3  Their major advantages are a near-zero volume shrinkage during curing, low water absorption, a high char yield upon burning, a low coefficient of thermal expansion, easy thermal curing without the need of hardeners or catalysts, and, for some benzoxazines, the T g  may be higher than the curing temperature. 4-7 . Nevertheless, the main interesting characteristic of benzoxazines is their straight and easy synthesis  by a Mannich-like condensation of three compounds: a phenol, an amine and formaldehyde. As a consequence, the great versatility of monomer molecular design allows for readily tailoring a large range of properties and adding specific functionalities. 8  The number of newly synthesized monomers is therefore constantly increasing; however a particular interest for both academics and industrials is driven on those based on renewable organic materials as the development of environmentally friendly and sustainable polymers is one of the current challenges in polymer science. 9  Various bio-based benzoxazines have already been synthesized such as diphenolic acids, 10  cardanol, 11, 12  furfurylamine, 13-15  stearylamine and guaiacol. 14, 15  Very recently, eugenol (4-allyl-2-methoxyphenol) has also been used to prepare different bio-based bis-benzoxazines but the properties of these resins proved quite limited, with a glass transition temperature not higher than 140°C. 16  Eugenol is a main component (80 wt%) of clove essential oil, which is mainly produced in Indonesia. Clove oil is widely used in perfumes and foods, as flavoring agent and is also used in medicine as antioxidants and drugs, but also in preparation of gum and teeth. 17-19  Interestingly, eugenol is relatively cheap ( ca.  5 $.kg -1 ) making it an economically realistic feedstock for the synthesis of bio-based resins. 20  However, eugenol is an aromatic compound based on a trisubstitued  benzene on which the ortho  and  para  positions are occupied by the methoxy and the allyl groups respectively. The preferential site for the ring opening polymerization of benzoxazines is the ortho  position but the  para  one shows also an ability to react. 21-23  As these two positions are obstructed, the polymerisation of eugenol-based benzoxazines is theoretically not likely to occur. Despite a limited polymerizability, the eugenol part of a  benzoxazine molecule presents an advantage that can be leveraged: the allyl function. Indeed, eugenol has been successfully used in bis-maleimide (bMI) formulations to  produce high performance bio-based bis-maleimide resins. 24-26  Allyl functionalized benzoxazines have also been successfully copolymerized with a bMI to form homogeneous networks. 27, 28  Curing of this type of resin in stoichiometric amount proceeds through an ene -addition reaction of the allyl group to the maleimide one. Additionally, the use of maleimide presents    another serious advantage as the MI function may be used to react with carbon nanotubes (CNTs) through its dienophilic character. 29  CNTs are indeed one of the most promising nanofillers for polymer matrices due to their exceptional mechanical properties combined with high thermal and electrical conductivities. 30-32  CNTs have already been successfully dispersed into several benzoxazine matrices but the reinforcement of the material properties is closely related to the interactions of these nanofillers with the benzoxazine network. 33-39  Therefore, the use of bMI can allow the establishment of numerous covalent  bonds between CNTs and the thermoset network, globally enhancing the thermo-mechanical properties of the resulting nanocomposite. Herewith we propose, for the first time at least to the best of our knowledge, the introduction of a bis-maleimide to stabilize and reinforce the crosslinking ability of a new eugenol-based  benzoxazine. Furthermore, it will be shown that when the curing is performed in presence of CNTs the so produced nanohybrid materials do display exceptional thermo-mechanical performance. Experimental Materials The following chemicals were purchased from Aldrich and used without any further purification: 1,4-phenylenediamine (99%), Eugenol (99%), paraformaldéhyde (95%) and 1,1′ -(methylenedi-4,1-phenylene)bismaleimide (95%). Technical chloroform was  purchased from VWR and used as received. Multiwalled Carbon  Nanotubes (MWCNTs) were provided by Nanocyl (NC7000) and were used without any further treatment. According to the supplier, the MWCNTs have an average diameter of 9.5 nm, mean length of 1.5 µm, surface area ranging from 250 to 300 m 2 /g and purity of min. 90%. Characterization The 1 H NMR spectra were recorded with a NMR spectrometer (Bruker, 500 MHz), using deuterated dimethylsulfoxide (DMSO- d  6  ) 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 (70x12x3 mm 3 ) 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 µm 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 1 wt%. Spectra were recorded under vacuum from 500 to 4000 cm -1  with a wavenumber resolution of 4 cm -1 . 64 scans were collected for each sample. Preparation and characterization of the eugenol-based benzoxazine, E-pPDA The E-pPDA synthesis has been adapted from a procedure reported by Ishida et al. 3  Eugenol 61.97 g (3.69 10 -1  mol) and  paraformaldehyde in excess 25.72 g (8.14 10 -1  mol) 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. 1,4-phenylenediamine 20 g (1.85 10 -1  mol), 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. 40, 41   At this temperature, the triaza compound reacts quickly with eugenol and the gel is destroyed in a couple of min. The mixture was allowed to react for 30 min under continuous stirring. The crude reaction product was then dissolved in CHCl 3  (~150 mL) and washed several times with a solution of 2M NaOH until the aqueous layer was colorless. The organic solution was then rinsed with deionized water until it reached a pH of 7. The solvent was then evaporated on a large aluminium surface in a vacuum oven at 150 °C during 10 min. A crystallizing light yellow-orange resin was obtained (weight yield ~ 90 %). The E-pPDA monomer was characterized  by a T g  of ~10 °C, a melting enthalpy of 95 J.g -1  and a T m  of 150 °C and an apparent enthalpy of polymerization equal to 200 J.g -1 . Preparation of E-pPDA and bMI blends For each composition, 10 g of powdered E-pPDA were mixed at 160 °C with a corresponding amount of bMI in order to prepare  blends with the following E-pPDA:bMI molar proportions: 1:1; 1:0.8; 1:0.6; 1:0.4 and 1:0.2. The blend is manually mixed during five minutes at 160°C allowing the complete dissolution of bMI  powder into the melted E-pPDA. Vitrified resin precursors are obtained. Preparation of E-pPDA/bMI (1:1) nanocomposite with 0.5 wt% of CNTs The incorporation and dispersion of MWCNTs have been achieved by an ultrasonication step in solution (Branson S-50D equipment, 200 W, 20 kHz, 13 mm diameter ultrasonic-probe), for 5 min at r.t. A solvent (CHCl 3 ) is used to limit the risk of overheating during the sonication process. 10g of E-pPDA and 7.4 g of bMI were dissolved in 40 mL of CHCl 3  and 8.69 10 -2  g of MWCNTs was added to the solution. The mixture was subjected to sonication and it was then spread over a large surface. Finally, the solvent was completely removed at 140°C for 5min in a vacuum oven yielding. A dark vitrified resin  precursor was obtained. Curing procedure Each blend was introduced in a stainless steel 80x12x3 mm 3  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: 1h at 160 °C, 2h at 180 °C, 2h at 200 °C, and 30 min at 220 °C. Thereafter, the samples were allowed to slowly cool down to room temperature before their unmolding. Results and Discussion Synthesis and characterization of E-pPDA NONOOOOHONH 2 NH 2 OHH + 2+ 4 120 °C   Scheme 1 One pot synthesis of E-pPDA monomer. E-pPDA monomer was successfully prepared by a solventless synthesis from eugenol and 1,4-phenylene diamine with an easy and scalable procedure according to the reaction depicted in Scheme 1. The structure of the new benzoxazine was investigated  by 1 H NMR as shown in Figure 1. The attribution of peaks was  based on the analysis of 1 H NMR spectra of reagents and on similar benzoxazine derivatives. 16  The well-defined characteristic  peaks at 4.48 and 5.29 ppm corresponding to the Ph-CH 2 -N ( c ) and O-CH 2 -N ( b ) of oxazine ring, respectively, verify the formation of the benzoxazine ring. 42  Additional aromatic peaks labelled a , d  , e,  the presence of the singlet  f   at 3.67 ppm corresponding to the O-CH 3  and the doublet  g   at 3.23 ppm attributed to the methylene of the allyl function attest for the formation of the desired benzoxazine structure. The right matching of all integrals 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. Fig. 1 1 H NMR spectrum of E-pPDA in DMSO-d 6 . Water peak labeled *. The properties and reactivity of E-pPDA were further characterized by DSC. The DSC thermogram shown in Fig. 2. highlights, first, the purity of the monomer with the melting peak at 150°C. Secondly, the thermogram shows evidence for a reaction with the presence of a unique exothermic peak at ca. 250 °C. This exothermic peak could be attributed to the opening of the benzoxazine ring as it occurs at temperatures similar to those observed for similar monomers 16, 28  and as the allyl  polymerisation precedes the ring-opening of oxazine moieties when the allyl is attached to an aromatic structure. 43  The apparent value of the exothermic enthalpy of 200 J.g -1 , corresponding to a ΔH of 48.5 kJ per mole of benzoxazine ring is relatively low compared to the 73 kJ.mol -1 usually accepted for the reaction of a  benzoxazine ring. 44  This result indicates a poor polymerizability of the E-pPDA resin. However, when the profile of thermo-degradation (recorded by TGA) is superimposed to the DSC thermogram, it appears clearly that an important degradation/weight loss occurs within the curing temperature range. Indeed, as can be seen on Fig. 2, at  ca.  270 °C, a high weight loss of 30 % is recorded. This weight loss may arise from the eugenol part, since we have previously demonstrated an absence of thermal degradation of a benzoxazine monomer based on the  para -phenylene diamine during curing. 39  The enthalpy value of the exotherm is thus modified by the degradation and the evaporation of volatile species so as the polymerizability of the resin is likely affected by such degradation. It is worth pointing out that such a problem has not been discussed for other eugenol- based benzoxazines. 16   0 100 200 300 400-1,5-1,0-0,50,00,51,01,52,02,5       H  e  a   t   F   l  o  w   (   W   /  g   ) Temperature (°C) 20406080100 200 J.g -1             W  e   i  g   h   t   (   %   ) 95 J.g -1   Fig. 2  DSC and TGA profiles of E-pPDA monomer recorded under nitrogen upon the curing temperature range. Partial degradation taking place along with benzoxazine curing has been already reported in the literature however without mentioning that it can significantly affect the curing reaction of the resin in bulk. 45, 46  More problematic is the limited crosslinking degree of the so-cured E-pPDA as shown by its disaggregation and even solubilisation in chloroform. The 1 H NMR spectra of the cured sample (see SI) revealed the disappearance of the signals of the closed benzoxazine ring whereas a broad signal emerged around 3.7 ppm. Nevertheless, the quality of the spectra is insufficient to allow an accurate determination of the resulting cured structures. Therefore, if the benzoxazine rings can be opened with temperature, they cannot polymerize to form a crosslinked network due to the unfree ortho  and  para  positions. However, the polymerization of benzoxazines with occupied ortho  and  para  positions has yet been reported for an aniline- based benzoxazine, but in this case it alternatively proceeds on the  para -position of the aniline ring (see scheme 2). 43  In our case this position is no more available as we react a  para  phenylene diamine instead of aniline. In addition, the presence of the allyl function does not lead to further polymerization probably due to    the heavy stabilization of a formed radical by mesomeric effect as the allyl group is attached to an aromatic structure. 42  Furthermore the polymerization of E-pPDA did not proved possible with the aid of a free-radical initiator (data not shown). Post curing at higher temperature could not be performed due to the intensive degradation of the resin and the release of smelly by-products. In summary, E-pPDA can be readily synthesized offering a large content in bio-based carbon (80% with the use of bio-based  paraformaldehyde) but cannot be easily polymerized. NONOOO ortho para para  / amine   Scheme 2. Limitation of the availability of free ortho and para positions for the ring opening polymerization E-pPDA. E-pPDA and bMI blends. The presence of the inherent allyl function on the eugenol part of the E-pPDA benzoxazine, firstly seen as a disadvantage since it  prevents any polymerization from occurring, has been considered in a second time as the key-part for an alternative polymerization that could take place with bMI. The general reaction mechanism  proceeding through an ene -addition involving stoechiometric E- pPDA/bMI ratio (or with some defect of bMI) is depicted in Scheme 3. It is to note that only the reactions between E-pPDA and bMI are considered as no formaldehyde is present in the medium, thus the reaction between eugenol and formaldehyde could not possibly take place. NONO OONNOOOONNOOOOE-pPDA +ene-adduct n     Scheme 3. Representation of the reaction between E-pPDA and bMI in accordance with ref 25, 27   1,1′ -(Methylenedi-4,1-phenylene)bismaleimide was therefore incorporated into the E-pPDA molten resin in different stoichiometric ratios and the influence of bMI content was evaluated.  Effect of bMI concentration As aforementioned, the main disadvantage of the E-pPDA resin is its thermal degradation within the curing temperature range. Interestingly, in presence of 1,1′ -(methylenedi-4,1-phenylene)  bismaleimide, the monomer thermal degradation is severely circumscribed and an increase of the bMI content has a positive effect on the thermal stability of the blend as depicted in Fig. 3. As a result, the resin curing can occur in better conditions without  partial degradation within the curing temperature range. The influence of the bMI concentration on the polymerizability of the E-pPDA/bMI blend was studied by DSC. Firstly, as can be seen in Fig. 4, the monomer blends are homogeneous as only one T g  is observed around 50°C meaning an excellent miscibility of the two components. Secondly the presence of two exotherms at 185°C and 245°C suggests a curing taking place according to two different mechanisms. The evolution of the enthalpy of each peak in comparison with the proportion of bMI allows for identifying the first exotherm, which is due to the reaction between maleimide moieties and allyl functions while the second exotherm can be associated with the opening of the benzoxazine rings. These observations are in good accordance with the work of Kumar et al. 27  In addition, the homopolymerization of the used  bMI was also studied by a DSC scan. A large exothermic peak is observed with a T Peack of 215°C, but cannot be fitted with the exothermic peaks of the blend system. Moreover, in the presence of allyl functions, the homopolymerization of the maleimide group is not favoured as the allyl/MI occurs first. 47   10020030040020406080100    W  e   i  g   h   t   (   %   ) Temperature (°C) E-pPDA-bMI 1:1 E-pPDA-bMI 1:0,8 E-pPDA-bMI 1:0,6 E-pPDA-bMI 1:0,4 E-pPDA-bMI 1:0,2 EpPDA 0,00,20,40,60,81,0    D  e  r   i  v .   W  e   i  g   h   t   (   %   /   °   C   )   Fig. 3 Profiles of thermal degradation of E-pPDA-bMI blends determined  by TGA 050100150200250300-0,50,00,51,0  E-pPDA-bMI 1:1 E-pPDA-bMI 1:0,8 E-pPDA-bMI 1:0,6 E-pPDA-bMI 1:0,4 E-pPDA-bMI 1:0,2    H  e  a   t   F   l  o  w   (   W   /  g   ) Temperature (°C)   Fig. 4 Curing thermograms of E-pPDA-bMI blends with different contents in bMI A comparison of the evolution of enthalpy values as a function of the blend composition is detailed in Table 1.
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