Crucial Role of Covalent Surface Functionalization of Clay

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Crucial role of covalent surface functionalization of clay nanofillers on improvement of the mechanical properties of bio-epoxy resin Van Son Vo, Samia Mahouche-Chergui, Vu-Hieu Nguyen, Salah Naili, and Benjamin Carbonnier ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.9b02088 • Publication Date (Web): 19 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019

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ACS Sustainable Chemistry & Engineering

Crucial Role of Covalent Surface Functionalization of Clay Nanofillers on Improvement of the Mechanical Properties of Bio-epoxy Resin V-S. Voa,b, S. Mahouche-Cherguia,*, V-H. Nguyenb, S. Nailib, B. Carbonniera,* a)

Université Paris-Est, ICMPE (UMR 7182), CNRS, UPEC, 2-8 Rue Henri Dunant, 94320 Thiais, F- 94320 THIAIS France b) Université Paris-Est, Laboratoire Modélisation et Simulation Multi-Echelle, MSME UMR 8208 CNRS, 61 avenue du Général de Gaulle, Créteil Cedex 94010, France *

Corresponding Authors

[email protected] [email protected]

ABSTRACT: Although the unique mechanical strength and adhesion properties of epoxy resins, they still suffer from poor toughness and brittleness inducing poor resistance to cracks. Herein, we report an efficient way of synthesis of bio-epoxy resin nanocomposites filled with highly exfoliated epoxy-grafted montmorillonite. The filled resin network was produced by covalent incorporation of a binary nanocomposite (MMT-PGMA) synthesized via in situ photo-induced polymerization of glycidyl methacrylate, into a Bio-epoxy resin matrix to design a ternary nanocomposite (MMT-PGMA/Bio-epoxy) and this in the presence of a green polyamine used as curing agent. The materials structure and morphology were characterized by FTIR, TGA, XRD, SEM and TEM which show the key role of the MMT surface modification on its interfacial adhesion with the epoxy resin. The results showed that the clay interlayer d-spacing increases from 1.23 nm to more than 2.2 nm upon grafting of the polymer. The homogeneous solvent-free dispersion of hybrid clay nanofillers, via sonication process, enhanced remarkably the bio-epoxy resin glass transition temperature (Tg) by 26.5°C. This can be rationalized by both the nanofillers fine dispersion and the chemical surface reactivity ensuring strong interfacial adhesion with the matrix. KEYWORDS: Bio-epoxy, clay nanofillers, photochemistry, solvent-free dispersion, ternary nanocomposites, mechanical performances. 1 ACS Paragon Plus Environment

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INTRODUCTION Along with the great development of the polymer-based materials industry increasing the sensitiveness about eco-conception has become a key challenge nowadays. The use of polymeric materials must move towards renewable resources in order to cope with the depletion of nonrenewable resources but also to meet the criteria of sustainable development. Indeed, new bio-inspired materials allow not only replacement of fossil fuels by limiting the use of petroleum-based polymers but also limitation of the generation of harmful substances to the environment. This is particularly accentuated for composite materials since great amounts of petrochemical-derived polymers are used in this area.1-3 Among the sustainable different polymers originating from biomass, bio-based thermosetting epoxy resins have attracted a particular increasing attention in last few years as an alternative to the diglycidyl ether of Bisphenol A due their compatibility with the environment as well as rapid cure time, facile processing, chemical resistance, and adhesion properties.4-8 However, this class of polymers suffers from a high flammability, a low glass transition temperature, a low fracture toughness, and poor elongation at break which limit their widespread application. Liu et al.

9

and Tan et al.

10

prepared epoxidized soybean oil based

composite films using triethylenetetramine (TETA) and methylhexahydropthalic anhydride (MHFA) as curing agent, respectively. The glass transition temperature (Tg) and the storage modulus determined by dynamic mechanical analysis are, respectively, of 11.8 °C and 590 MPa for the TETA-cured system, and 62°C and 400 MPa, for the MHFA/epoxy film. Nevertheless, the poor mechanical properties restrict their application. In the work of Lu et al.11, biobased composites have been prepared via cationic polymerization of conjugated soybean oil (CSOY) or conjugated LoSatSoy oil (CLS) with styrene (ST) and divinylbenzene (DVB). The storage modulus (< 1GPa) and Tg ( 56%). Moreover, it can be cured under ambient conditions using a stoichiometric resin/hardener ratio. It is also to note that on the one hand, the hardener contains more than 55% of "green" carbon and, on the other hand, the epoxy-clay nanocomposites were processed under environmental friendly conditions (solventless) ensuring a green label to the final nanocomposites.

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EXPERIMENTAL SECTION Chemicals Natural sodium montmorillonite (MMT-Na) was supplied by Southern Clay Products (ionexchange capacity = 92 mequiv/100g, d001= 1.17 nm, surface area 750m2/g). Silver nitrate (AgNO3), (trimethoxysilyl) propyl methacrylate, MAPS), glycidyl methacrylate (GMA), 2,2dimethoxy-2-phenylacetophenone (DMPA), were purchased from Aldrich and used as received. The hydrochloric acid 37% (HCl), ethanol (EtOH), acetonitrile (ACN), and chloroform, solvents were purchased from Alfa-Aesar and used without further purification. Ultrapure water was prepared using a Milli-Q plus purification system. In this study a commercial bio-based epoxy resin containing 56% of carbon from bio-renewable sources “greenpoxy 56”, and a polyamine curing agent (SD GP 505 V2) containing around 58% of “green” carbon were used. Both were purchased from Sicomin composites, France.

Characterization and Instruments Fourier Transform Infrared spectroscopy (FTIR) measurements were conducted on a Bruker TENSOR 27 FTIR spectrometer in the ATR mode in the wavenumber range of 400-4000 cm-1 at a resolution of 4 cm-1. For comparison purposes, all spectra were normalized to the area under the Si-O vibration band at about 1000 cm-1. Thermogravimetric analyses (TGA) were performed on a SetaramSetsys Evolution 16 apparatus in the temperature range of 20-800°C at a heating rate of 10°C/min under argon flow rate of 20 mL/min. X-ray diffraction measurements were carried out on a Bruker D8 Advance diffractometer using Cu Kα source ( =1.54 Å) and 2

scans were obtained from 4 to 10°. The basal distance of the silicate layer (d001) was

calculated using the Bragg's equation (n =2.d001.sin ) where n=1 and

is the diffraction

angle. Transmission electron microscopy (TEM) analyses were performed on a FEI Tecnai F20 microscope operating at an accelerating voltage of 200 kV. The modified clay powders were dispersed in ethanol under ultrasonication, and the suspension was dropped onto the Formvar carbon film-coated copper grid. In the case of the ternary nanocomposite films, the samples were prepared using an ultramicrotome equipped with a diamond knife to obtain ultrathin specimens. Dynamic mechanical measurments (DMA) were performed on a TA Q800 Dynamic apparatus in the tension mode with a constant frequency of 1 Hz, and a heating rate of 5°C/min in the range of 0-145°C. The nanocomposite samples were cut into rectangular specimens of 20 x 5 x 1 mm3.


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Activation of clay (MMT-OH) 2 g of MMT-Na were dispersed in 100 mL of 0.05 M HCl under continuous stirring at 60°C for 3 h. The clay powder was then collected by centrifugation and washed several times with ultrapure water until total removal of chlorine ions (as tested by AgCl formation using AgNO3), and finally dried in a vacuum oven at 60 °C for 24 h. The activated clay was abbreviated as MMT-OH. Silanization of clay (MMT-MAPS) 1 g of MMT-OH was first dispersed in 100 mL of ultrapure water under continuous stirring at 60 °C for 1 h. The mixture was sonicated for 5 min in order to obtain a stable suspension and then kept under nitrogen flow for 15 min, before adding the desired amount of MAPS. The suspension was then continuously stirred under nitrogen at 80 °C for 24 h. To optimize the silanization reaction on the activated clay, three amounts of MAPS (0.5, 0.75 and 1mL) were used and the obtained silanized clays were abbreviated MMT-MAPS-21mM, MMT-MAPS31.5mM, and MMT-MAPS-42mM, respectively. Subsequently, the powder was collected by centrifugation and washed abundantly with water, ethanol, followed by Soxhlet extraction with EtOH to remove the physisorbed silanes. Finally, the obtained methacrylate-functionalized clays were dried in a vacuum oven at 60 °C for 24 h. In situ photo-induced polymerization of glycidyl methacrylate in presence of methacrylate-functionalized clay (MMT-PGMA) 100 mg of MMT-MAPS-42mM were dispersed in 10 mL of ACN and continuously stirred under nitrogen flow for 15 min, followed by sonication for 5 min. Then, GMA monomer and DMPA photo-initiator (1wt.% in respect to the monomer) were added to the previously prepared suspension. The obtained mixture was continuously stirred and degassed under nitrogen flow for 15 min before UV irradiation at 365 nm in a spectrolinker XL-1500 UV. The polymerization reaction was performed under continuous stirring. The polymerization time was optimized to produce exfoliated nanocomposite. For this, three polymerization reaction times were investigated, i.e. 1, 2 and 3 h, and the obtained polymer-grafted clays were abbreviated as MMT-PGMA-1h, MMT-PGMA-2h and MMT-PGMA-3h. After the desired polymerization time, the powder was collected by centrifugation and washed abundantly with ACN, ethanol, and then followed by Soxhlet extraction with chloroform. Finally, the obtained nanofillers were dried in a vacuum oven at 60 °C for 24 h.



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Fabrication of MMT-PGMA/Bio-epoxy resin nanocomposites The ternary nanocomposites MMT-PGMA/Epoxy were prepared following two steps: Firstly, the MMT-PGMA nanofillers were dispersed in the green epoxy resin noted here bio-epoxy under ultrasonication at 80°C for 1 h. Then, the MMT-PGMA/Epoxy mixture was cooled down to room temperature, and the polyamine hardener (SD Green Pox 505 V2) was added to the MMT-PGMA/Epoxy mixture at the epoxy/hardener of 42/40 weight ratio. The mixture was stirred for 5 min, then poured into silicon mold and degassed under vacuum to remove trapped air bubbles and finally cured 24 h at 60 °C and then 48 h at room temperature. Ternary nanocomposites were prepared at varying amount of nanofiller, from 1 to 15 wt.% of the total weight of resin and hardener. Using the same procedure, MMT-Na/epoxy nanocomposite was prepared and used as reference using 2 wt.% of MMT-Na.

RESULTS AND DISCUSSION Fabrication of bio-epoxy filled with MMT/PGMA The procedure for the MMT-PGMA/Epoxy ternary nanocomposite consists of two parts: firstly, synthesis of MMT-PGMA binary nanocomposites used as nanofillers, and secondly, the preparation of the ternary nanocomposite by incorporation of the MMT-PGMA nanofillers in a bio-epoxy resin via a covalent anchoring (see Scheme 1). The synthesis of MMT-PGMA is done following three-steps: (i) activation of the montmorillonite clay surface using chlorhydric acid in order to generate hydroxyl groups in between the clay layers (abbreviated as MMT-OH, step (i) in Figure 1), (ii) covalent attachment of 3-(trimethoxysilyl)propyl methacrylate at the interlayer clay surface through silanization reaction with the internal silanol groups leading to methacrylate-functionalized clay (abbreviated as MMT-MAPS, step (ii) in Figure 1). This step aims to convert the clay surface from hydrophilic to hydrophobic in order to ensure its compatibility with the GMA monomer, to separate the clay layers from each other facilitating the penetration of GMA monomer, and to functionalize the clay layers surface with methacrylate groups in order to ensure the covalent bonding of the epoxy resin onto the functionalized clay surface. (iii) in situ photo-induced polymerization of GMA in the presence of the methacrylate-functionalized clay, leading to the growth of poly(glycidyl methacrylate) chains inside the clay layers and tethered covalently to their surface (abbreviated as MMTPGMA, step (iii) in Scheme 1). Covalent grafting of polymer ensures more separation of the clay layers which facilitates the insertion of resin matrix chains in the step of fabrication of the ternary nanocomposites. The choice of PGMA polymer is related to the presence of the reactive


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epoxy groups able to ensure strong interactions with the epoxy matrix through the polyamine hardener playing also the role of a bridge between clay and matrix.

Scheme 1. Preparation procedure of the MMT-PGMA/Epoxy ternary nanocomposites. GMA: glycidyl methacrylate.

The MMT-PGMA reactive nanofillers were dispersed in a bio-epoxy resin for 1 h at 80 °C using an ultraturrax before adding the polyamine hardener. The mixture was then transferred into a silicon mold and cured at 60°C for 24h. Optimization of the experimental preparation and dispersion parameters of nanofillers as well as ternary nanocomposites is crucial to ensure fine and homogeneous dispersion of the clay nanofillers in the epoxy resin matrix and thus, to produce performant final epoxy-clay nanocomposites. For this, we were interested in both the effect of the amount of silane coupling agent used for clay surface functionalization and the GMA polymerization time on the structure of the nanofillers. Then, we have shown the great influence of the method of dispersion of the nanofillers in the epoxy matrix on the morphology of MMT-PGMA/Epoxy ternary nanocomposite. Ultraturrax instrument was adopted for mixing the nanofillers with the epoxy matrix under heating as these conditions facilitate dispersion of the nanofillers in the viscous epoxy resin in the absence of a solvent; details are given in the supporting information (Figure S 1). Characterization of methacrylate-functionalized clay layers After reaction between 3-methacryloxypropyl trimethoxysilane and the activated clay (step (ii) in Scheme 1, the structure of the resulting MMT-MAPS was characterized by infra-red spectroscopy as shown Figure 1a. As it can be observed, the spectrum of MMT-OH shows a large band centered at 3625 cm-1 and a sharp peak at 1635 cm-1, corresponding to the silanol groups and the deformation band of adsorbed water, respectively.30 Previous studies highlight three possible clay-sites to graft 7 ACS Paragon Plus Environment


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silane coupling agents (a) “broken” layer edges, (b) external surface, or (c) in between platelets. This depending both on the silane structure and the solvent surface energy. 31-32 As our aim was to exfoliate the clay layers, we have selected the optimal conditions for the grafting silane inside the clay platelets, i.e. trifunctional silane and water as silanization solvent. After the silanization reaction, FTIR spectrum displays a decrease of the intensity of the silanol band, indicating consumption of this group and thus transformation to Si-O-Si groups. Two bands appeared at 2945 and 2890 cm-1, attributed to the asymmetric and symmetric stretching of -CH2 groups in the alkyl chains of MAPS, respectively. 33-34 Moreover, the new characteristic peaks detected at 1726 and 1634 cm-1 are ascribed to C=O and C=C groups, respectively. 34-35

The effect of the initial MAPS silane concentration (21, 31.5, and 42 mM) has been investigated and FTIR results show obvious increase of the C=O stretching mode peak intensity with increasing silane concentration, indicating that the degree of silane grafting increased correspondingly. These FTIR results evidenced the successful silanization reaction. 36 TGA analysis was further employed to quantify the grafting of organic amount on the clay layers. One can note from Figure 1b, a clear decrease of both the physically adsorbed water amount at 100°C and the onset temperature attributed to the reduced thermal stability after silanization reaction. This revealed conversion of clay surface from hydrophilic character to hydrophobic or organophilic one. The total weight loss showed for activated clay MMT-OH corresponding to the evaporation of physorbed water at about 100°C and the irreversible dehydroxylation of clay at 639°C was found to be of about 14 wt.%.

33

After silanization

reaction, the weight loss increased to approximately 19 wt.% for MMT-MAPS-21mM, 28 wt.% for MMT-MAPS-31.5mM and 32 wt.% for MMT-MAPS-42mM, which confirm the FTIR results. To gather information about the structure of the clay layers, XRD measurements were

conducted on clay before and after silanization reaction and the interlayer d-spacing was calculated according to Bragg equation (Figure 1c). An intensive diffraction peak is detected at 2

=7.2° for MMT-OH, reflecting an interlayer

spacing of about 1.23 nm. After silanization reaction, the diffraction peak continuously shifted towards lower 2

angles, decreased in intensity and became broader with increasing of initial

silane concentration, which implies a significant insertion of the silane coupling agent moities into the clay interlayer. The methacrylate-functionalized clays show an interlayer spacing of about 1.48, 1.52 and 1.64 nm for MMT-MAPS prepared using 21, 31.5 and 42 mM of (trimethoxysilyl) propyl methacrylate, respectively. Moreover, broadening of the XRD peak measured for MMT-MAPS-42 mM evidences the loss of parallel organization of clay layers 8 ACS Paragon Plus Environment

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and their separation. Thereby, MMT-MAPS-42mM was retained for the preparation of the MMT-PGMA binary nanocomposites which would allow for a facile penetration of GMA monomer molecules inside the clay layers providing efficient growth of clay-grafted PGMA chains.

MMT-OH MMT-MAPS 21 mM MMT-MAPS 31.5 mM MMT-MAPS 42 mM

(c) 10000

d001=1.23

d001=1.48

8000

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


d001=1.52 d001=1.64

6000 4000 2000 0

4

5

6

7

8

9

10

2(deg.)

Figure 1. (a) FTIR spectra, (b) TGA curves, and (c) XRD patterns of activated clay (MMT-OH) and silanized clay prepared from activated clay at different silane concentration 21, 31.5 and 42 mM.

Characterization of MMT-PGMA binary nanocomposites The FTIR results (Figure 2a) of polymer-grafted clay layers prepared from MMT-MAPS42mM, exhibited obvious changes with regard to the MMT-MAPS, confirming the success of the PGMA chains growing inside the clay interlayers. The effect of the polymerization reaction time (varied from 1 to 3 h) on the grafted amount of PGMA on the clay layers is demonstrated through the increasing intensity of the polymer characteristic signals. First, appearance of new peaks at 904, 845 and 757 cm-1, related to the stretching of epoxide groups, and strong decrease (MMT-PGMA-1h and MMT-PGMA-2h) or complete disappearance (MMT-PGMA-3h) of the peak of the C=C bonds at 1634 cm-1, and finally increase in the intensity of the 9 ACS Paragon Plus Environment


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carbonyl stretching vibrations peak at 1726 cm-1 are clear proofs for the growing PGMA chain on the silicate layers. The TGA analysis (Figure 2 b) provided information about the PGMA grafted amount and confirmed the FTIR results, showing that the grafted amount increased with polymerization time. The calculated mass loading of PGMA on the clay layers was determined after subtraction of the MAPS grafted amounts, and it is about 7, 48 and 59 wt.%, for polymerization times of 1, 2, and 3h, respectively. From these TGA results, the PGMA grafting degree was estimated using the equation 1 (eq 1): GD (%) = RMAPS-RPGMA-nh)/(RMAPS-RP)

(eq 1)

Where, n is the polymerization time (1, 2 or 3h), RMAPS (68 wt.%), RPGMA-1h, (61 wt.%), RPGMA2h

(20 wt.%), and RPGMA-3h (9 wt.%) represent the residual weight percentages. While Rp

corresponds to PGMA pyrolysis which is assumed to be complete, so Rp = 0, and thus polymer grafting degree was about 10.3%, 70.6%, and 86.8% for 1, 2 and 3h of polymerization time. XRD analysis was used to characterize the structure of the MMT-PGMA binary nanocomposites (Figure 2c). XRD pattern of MMT-MAPS-42mM is shown again in the Figure 6 for comparison purposes. MMT-PGMA-1h pattern shows a shift of the diffraction peak toward lower angles (2θ = 4.7°), indicating an increase in the interlayer spacing from 1.64 nm (MMT-MAPS-42mM) to 1.88 nm for the MMT-PGMA-1h. Both MMT-PGMA-2h and MMTPGMA-3h do not show any visible diffraction peak, indicating disordered structure with high interlayer spacing. As the absence of diffraction peak alone is not sufficient to predict fully exfoliated structure of the clay layers, TEM analysis is used to provide clear and unambiguous evidence.


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5500

(c)

5000

MMT-MAPS-42mM MMT-PGMA-1h MMT-PGMA-2h MMT-PGMA-3h

4500

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4000 3500 3000 2500 2000 1500 1000 4

5

6

7

8

9

10

2(deg.)

Figure 2. a) FTIR spectra, (b) TGA curves, and (c) XRD patterns of silanized clay MMT-MAPS-42mM, and polymer-grafted clay prepared with different polymerization times 1, 2 and 3 h.

As can be seen in the top of the Figure 3, SEM micrographs show an obvious change in the montmorillonite surface morphology after polymer grafting. The MMT-Na exhibits agglomerates of sharp flakes, while the MMT-PGMA shows much smoother surface due to the surface coverage with amorphous polymer chains. TEM images (bottom of the Figure 3) confirmed the suggested morphological structures based on XRD characterization, revealing an intercalated morphology for the MMT-PGMA-1h and an exfoliated one for MMTPGMA-2h and MMT-PGMA-3h. The clay layers in MMT-PGMA-1h are still parallel with an increased interlayer spacing from about 1.64 nm before polymerization reaction (as calculated from XRD) to 1.9 nm after 1 h of polymerization time (as shown by TEM which is in excellent agreement with the 1.88 nm value determined by XRD results). TEM image of MMT-PGMA-2h showed that most of the clay platelets are highly separated from each other, giving increased interlayer spacing and random structure. In the case of MMT-PGMA3h nanocomposite, fully delamination and random distribution of the clay layers are observed. The TEM observations are in well agreement with the XRD results, and both of them showed that the polymerization time affects significantly the MMT-PGMA binary nanocomposite structure. Taking in account the different obtained results and the preparation conditions, MMT-PGMA-2h has been selected to be incorporated in the epoxy resin as nanofiller. As it showed random structure and rich-epoxy groups at the surface of clay, MMT-PGMA-2h could ensure facile and homogeneous dispersion in epoxy matrix.



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Figure 3. (Top images) SEM micrographs of pristine clay (MMT-Na) and poly(glycidyl methacrylate)grafted clay binary nanocomposites (MMT-PGMA-2h). (bottom images) TEM micrographs of MMTPGMA binary nanocomposites at polymerization times of 1h (MMT-PGMA-1h), 2h (MMT-PGMA-2h) and 3h (MMT-PGMA-3h).

Structural characterization of the epoxy-rich clay/bio-resin ternary nanocomposite In order to study the reactivity of the curing system, FTIR analysis was performed to follow the conversion of the epoxy groups from the resin via reaction with the polyamine hardener at three different reaction times, T0 corresponding to the resin/hardener mixture just after mixing, T1 for 40 minutes at 60 ° C and T2 mixing overnight at 60 °C. It is to note that the mixture was prepared using epoxy/hardener system of 42/40 weight ratio. Figure S2 (SI) shows that the characteristic signals of epoxide groups detected at 758 and 913 cm-1 decreased from T0 to T1, to completely disappear in the case of T2. The change in the chemical structure was accompanied by appearance of a large band centered at 3364 cm1

, ascribed to hydroxyl groups coming from the epoxides ring opening with the amine

functions from the hardener. Figure S3 (SI) shows the XRD patterns of MMT-Na and MMT-PGMA-2h before and after their incorporation in the epoxy resin. The ternary nanocomposite prepared using pristine clay MMT-Na as nanofiller revealed a diffraction peak at the same 2

position as pure

pristine clay, indicating that the epoxy resin chains did not intercalate into the clay interlayer and therefore all clay platelets remained completely stacked. This is explained by the incompatibility between clay and epoxy resin which leads to very poor interfacial interactions. However, no peak was detected in the XRD pattern of the epoxy-functionalized clay nanofillers filled resin, indicating a uniform dispersion of the functionalized clay 12 ACS Paragon Plus Environment

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platelets in the epoxy resin matrix, which most possibly results from both pre-exfolliated structure of the PGMA-modified nanofillers as shown by Figure 3 and strong interfacial adhesion between these nanofillers and the matrix through covalent attachment. Indeed, the rich-epoxy groups at the surface of clay ensure chemical bonding with the matrix via bioamine hardener. It is well-established that the unique use of XRD data is not enough for assessing precisely the distribution of the clay layers embedded in epoxy resin and deducing their exfoliated morphologies because of different parameters (e.g. clay dilution). Indeed, the absence of peak is not a proof of the exfoliation of the clay nanofillers but only indicates that clay layers do not exhibit short range order as typically observed for clay lamellae. To study the morphology of the nanocomposite without ambiguity, TEM observation is are required. Another possible solution is the use of small-angle X-ray scattering (SAXS) which may detect a peak not visible in XRD pattern as it can also measure d-spacing values of about 3 nm and above. Herein, XRD findings were supported by TEM observations (Figure 4) which gave clear morphological structure of the nanoscale dispersion state of the clay platelets in the ternary nanocomposites prepared by incorporation of 2 wt.% of MMT-Na pristine clay, and different amounts of MMT-PGMA-2h (2, 5, and 15 wt.%). Incorporation of the unmodified clay in the epoxy resin matrix leads to high degree of agglomerated morphology consisting of stacked layers in the matrix. This is assigned to both the incompatibility of the hydrophilic pristine clay with the organic epoxy resin matrix and its small interlayer spacing preventing the polymer chains to access into the interlayer. TEM images of the MMT-PGMA-2h/Epoxy resin ternary nanocomposites prepared with 2 and 5 wt.% show that the clay platelets are predominantly thin and randomly dispersed in the matrix. Several individual platelets were detected for 5 wt.% loading, otherwise more layers are assembled to form tactoids randomly distributed in the matrix in the case of nanocomposites containing 15 wt.% of nanofillers. The pre-exfoliated structure of the epoxy-rich clay and the presence of epoxycompatible groups at the clay surface as well as the reactivity of these epoxy functions towards the amine-curing agents resulted in homogeneous distribution of clay nanofillers in bio-epoxy resin.



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Figure 4. TEM images of MMT-Na/Bio-epoxy with 2 wt% and MMT-PGMA-2h/Bio-epoxy nanocomposite with 2, 5 and 15 wt% of MMT-PGMA-2h nanofillers

Dynamic mechanical properties of the ternary nanocomposite The mechanical reinforcing effect of the amount of MMT-PGMA-2h binary nanocomposite on the epoxy resin was investigated using dynamic mechanical analysis (DMA) by measuring the storage modulus and determination of loss factor (tan

as a function of

temperature in the range of 0 to 145°C is discussed and rationalized. For this, unfilled and filled resin films with MMT-Na and MMT-PGMA-2h at different contents have been prepared. From Figure 5, it should be noted that incorporation of pristine clay did not induce any variation of the storage modulus and the tang

of the resin. This can be explained by

the stacked structure of the clay layers in the matrix as shown by XRD and TEM analyses, leading to limit the interfacial interactions, thus resulting in non-improvement of the resin properties. Whereas, MMT-PGMA-2h filled epoxy resin nanocomposites show a higher storage modulus than the pure epoxy resin throughout the whole temperature range and this behavior is displayed for all nanofillers content. Storage modulus of the ternary nanocomposites, for different nanofillers content are summarized in table 1 at three temperatures (below, close and above the glass transition temperature, Tg). It can be concluded that even at temperatures below glass transition temperature, covalent incorporation of MMT-PGMA-2h nanofillers into the bio-epoxy resin enhanced significantly the storage modulus. For example at 25°C, incorporation of 2 wt.% and 10 wt.% of MMTPGMA-2h in the resin increased the storage modulus from 1984 to 2403 and 3725 MPa, 14 ACS Paragon Plus Environment

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which corresponds to 21 % and 88 % increase, respectively. In can be noted that above Tg, greater improvement of the mechanical performances of the epoxy nanocomposite was observed for all nanofillers contents. Amounts of 2 wt.% and 10 wt.%, for examples, induced an increase of the storage modulus of about 54 % and 489 %, respectively. The improvement in resin nanocomposites storage modulus observed in presence of MMTPGMA-2h nanofillers can be attributed to the homogeneous and fine distribution of the nanofillers within the epoxy resin matrix as demonstrated by both TEM and XRD results. Due to the nanofillers delaminated morphology, individual nanofillers platelets are highly confined in the crosslinked network leading to a significant reduction of the matrix chains movement and therefore increase the stiffness of the crosslinked clay-filled epoxy resin. In addition to their morphology, nanofillers are multifunctional as they are densely decorated with epoxy groups as demonstrated by FTIR and TGA results, which permit chemical reaction during synthesis of the nanocomposite between the polyamine hardener and both nanofillers and epoxy resin via epoxide opening with nucleophilic amine groups. This chemical crosslinking generates strong interfacial adhesion between nanofillers and epoxy resin matrix, which plays a key role in the enhancement of the mechanical properties of the resin. 12, 37-38 As can be seen from Figure 5, obvious continuous increase of the glass transition temperature with a simultaneous decrease in peak intensity were shown with increasing of the nanofillers content in the ternary nanocomposites comparing to the pure bio-epoxy matrix. This is attributed to increasing of loading of the highly dispersed platelets resulting in more confinement of the molecular polymer chains which could restrict more their mobility. The bio-epoxy resin shows a peak at 85 °C, corresponding to its transition from the glass to the rubber state (Tg). Incorporation of 10 wt.% of nanofillers in the resin shifts Tg to 112°C which corresponds to 26.5 °C increase.39-40 However, the reinforcement effect as well as Tg values were slightly decreased when the amount of incorporated nanofillers is increased to 15 wt.% in comparison with 10 wt.% loading while remaining significantly higher than that of the neat bio-epoxy network. This can be explained by the lower fine dispersion level of the nanofillers because of presence of some tactoids which led to decreased interaction between nanofillers and epoxy resin matrix as shown by TEM images (Figure 4). Interestingly, contrary to most of the literature results where incorporation of high nanofiller contents (> 5wt.%) yield deterioration of the dynamic mechanical properties of epoxy resins (e.g. Tg and storage modulus), in our system even at 10 and 15wt.% of PGMA-functionalized 15 ACS Paragon Plus Environment


Page 16 of 23

nanofiller contents, Tg and storage modulus are still significantly improved over the neat epoxy.41-44 This beneficial effect can be explained by the quality of dispersion/distribution of the nanofillers even at such loadings as a result of the chemical functionalization of the clay layers surface with epoxy-rich polymer. Indeed, the presence of these reactive polymer chains ensure both covalent binding between the nanofillers and the bio-epoxy matrix and the delamination of the clay layers induce an increase in the basal spacing which improve remarkably their level of dispersion in the matrix. As increasing the amount of functionalized nanofillers induces high degree of strong bonding and hence crosslinking causing significant changes in the dynamic of polymer chains.45-46 These relevant results demonstrated the beneficial synergistic effects of nanofillers dispersion and covalent nanoscale interfacial bonding between nanofillers and the bio-epoxy in the mechanical reinforcement of the ternary nanocomposite.37, 39-40

Figure 5. Storage modulus and loss factor (tan as function of temperature of neat bio-epoxy resin and MMT-PGMA-2h/Bio-epoxy nanocomposite at different nanofillers content. The MMT-Na/ Bio-epoxy nanocomposite at 2 wt.% is represented by the dash line. The inset shows Tg as function of MMT-PGMA2h content.

MMTPGMA-2h (wt.%) 0

3°C

90°C

140°C

2076

34

14

2

2535

111

20

5

3095

584

30

7

3620

1275

51

10

3886

1884

69

15

3688

1883

46

Storage modulus E’ (GPa)

Table 1. Storage modulus and glass-transition temperature (Tg) of neat bio-epoxy resin and MMT-PGMA2h/Bio-epoxy nanocomposites at different nanofillers content.


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Fracture mechanism of the crosslinked bio-epoxy resin To better understand and explain the effect of the incorporation of nanofillers in the reinforcement of the mechanical performance, especially the fracture toughness of the bioepoxy resin networks which plays a crucial role for applications, the microstructures of tensile fracture surface of the pure epoxy resin, the pristine clay physically mixed with the epoxy resin (MMT-Na/epoxy resin) and epoxy resin filled with PGMA functionalized-clay were investigated by SEM. As it can be observed from Figure 6, the pure epoxy resin exhibits a smooth and flat fracture surface with occasional linear cracks. It can be seen that the lines are parallel to the direction of crack propagation. This behavior is usually associated to brittle failure in cured epoxy materials.37, 47-48 Nevertheless, the fracture surface changes significantly after incorporation of clay nanoparticles into the resin, it becomes clearly rougher. It addition, the continuous cracks direction seen in the pure epoxy resin was lost in both epoxy resin filled with MMTNa and MMT-PGMA-2h due to the limited propagation of the cracks induced by the presence of the clay platelets.49 In the case of MMT-Na/epoxy resin nanocomposite, due to aggregated clay layers and their poor interfacial interactions with the epoxy resin, separation between the two constituents and appearance of voids around the clay tactoids were observed. These voids act as stress concentration areas which generated some new fracture surfaces in the nanocomposite.37, 50 Bio-epoxy resin filled with MMT-PGMA-2h nanofillers showed a much more uniform distribution and displayed smoother fractured surface than that of the MMT-Na, indicating the effective role of the layered nanofillers in improving fracture toughness of the thermosetting polymer. Furthermore, there is no void around the clay particles, clearly explained by the strong adhesion between the functionalized clay nanofillers and the epoxy resin matrix. This further demonstrated the remarkable effect of the surface functionalization of the clay layers with epoxy groups in the quality of their dispersion in the epoxy resin matrix due to their covalent bonding through hardener amino groups via epoxides ring opening reactions.



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Figure 6: Microstructure of tensile fracture surface of pure bio-epoxy film and its nanocomposites

CONCLUSIONS Ternary nanocomposites-based on bio-epoxy resin filled with epoxy functionalized-clay layers were prepared using an ultra-turrax disperser at moderate temperature of 80°C without using solvent for a facile dispersion of the MMT-PGMA-2h nanofillers in a bio-epoxy resin in the presence of polyamine hardener, followed by a thermal curing at 60°C for 24h and then 24h at room temperature. The nanofillers were prepared via a green polymerization technique, i.e. photopolymerization. The novel experimental procedure proposed here for the synthesis of MMT-PGMA nanofillers permits control of the grafting amount of PGMA coating by the polymerization time, and thus control of the interlayer spacing. FTIR and TGA analyses confirmed that the poly(glycidyl methacrylate) chains are densely grafted to clay layers surface, while both XRD and TEM results indicated a highly exfoliated morphology for the clay layers in the MMT-PGMA at polymerization times of 2 and 3h. As the MMT-PGMA-2h nanofillers incorporated in the epoxy resin are pre-exfoliated, significant improvement in clay layers dispersion homogeneity in the epoxy resin have been shown. In addition to the initial random morphological structure of the nanofillers, their surface functionality plays crucial role. Indeed, the poly amine hardener is able to react with epoxy groups from both PGMA grafts and epoxy resin via epoxide ring opening reaction, playing a role of interfacial covalent bridge between the resin and the clay surface. This generated strong interactions between clay and epoxy resin result in improvement of the storage modulus and increasing of glass temperature transition in the MMT-PGMA filled epoxy resin even at high nanofillers loadings (10 and 15wt.%). Furthermore, the presence of functionalized-clay nanofillers makes the fracture surface smoother as revealed by SEM analysis. 18 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information Figure S1: Schematic illustration of dispersion of neat clay and epoxy-rich clay nanofillers (MMT-PGMA) into the bio-epoxy resin matrix. Figure S2: FTIR spectra of resin epoxyhardener system at 60 °C and at different crosslinking times T0 (0 minute), T1 (40 minutes) and T2 (12h). Figure S3: XRD patterns of (a) MMT-Na, (b) MMT-Na/bio-epoxy nanocomposite 2 wt.%, (c) MMT-PGMA-2h and (d) MMT-PGMA-2h/Bio-epoxy nanocomposite 5 wt.%.

AUTHOR INFORMATION Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work has benefited from a French government grant managed by ANR within the frame of the national program of Investments for the Future ANR-11-LABX-022-01 (LabEx MMCD project).

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MMT‐PGMA (5 wt. %)/Epoxy

Strong interfacial interactions OH R1

R3

OH

N

R2

R1: PGMA; R2: bio epoxy resin; R3: bio curing agent

MMT‐PGMA/Bio epoxy Montmorillonite MMT

O O

Si O

O

Bio‐based epoxy resin (GreenPoxy 56)

CH3

O

44 % petrobased C

n CH3

O

C

O

56 % biobbioased C

bio polyamine (curing agent)

O

MMT‐PGMA ACS Paragon Plus Environment

Crucial Role of Covalent Surface Functionalization of Clay

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