Straightforward route to design bio-renewable networks based on

Research Article pubs.acs.org/journal/ascecg

Straightforward Route To Design Biorenewable Networks Based on Terpenes and Sunflower Oil Carine Mangeon,†,‡ France Thevenieau,† Estelle Renard,† and Valérie Langlois*,† †

Institut de Chimie et des Matériaux de Paris-Est, UMR 7182, CNRS-UPEC, 2-8 rue Henri Dunant, 94320 Thiais, France Avril, 11 rue Monceau, 75008 Paris, France

‡

S Supporting Information *

ABSTRACT: By using a combination of bio-based monomers, sunflower oil (S) and monoterperne, the β-myrcene (M), or a sesquiterpene, the β-farnesene (F), a series of novel networks SxM100−x and SxF100−x with different compositions were prepared by a green chemistry route, without any solvent or catalyst, using the Diels−Alder reaction at 100 °C to obtain branched materials. The presence of oxygen during this process is responsible of the formation of oxidation products that are characterized by Fourier transform infrared (FTIR) and Raman spectroscopies, a swell as X-ray photoelectron spectroscopy (XPS) analysis. Materials prepared with 100% terpenes (PM100 and PF100) are very hard and brittle, whereas materials prepared from vegetable oil (PS100) are tearable. The properties of SxM100−x and SxF100−x are closely linked to the composition and, in particular, to the content of terpenes. It is observed that, by increasing the proportion of terpenes from 0 to 80 wt %, there was a marked increase in the glass-transition temperature (Tg) values, from 13.8 °C to 58.2 °C for S20F80 networks and up to 92.3 °C for S20M80 networks. Although the S20M80 network presents higher tensile modulus (97 MPa) and tensile strength (8.8 MPa), the S50M50 network is the more promising material, combining high resistance and good flexibility. These resulting materials exhibited remarkable swelling capacity when they were immersed in eugenol, which is an interesting antibacterial compound. The release profile indicated a Fickian diffusion model with the release of 90% of eugenol in 30 days, providing interesting antibacterial properties to the materials. KEYWORDS: Renewable networks, Terpenes, Sunflower oil, Mechanical properties

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INTRODUCTION During the last decades, the development of bio-based molecules to replace petroleum-based resources has been constantly increasing. This global effort from the scientific community is in perfect agreement with the industrial and social demands of sustainable management for reducing our reliance on fossil resources and decreasing the environmental impacts of the petroleum chemistry. In this context, natural vegetable oils have been intensively studied for the synthesis of polymers, because of their renewable character. Vegetable oils can be used to yield different functionalized derivatives such as polyesters, polyurethanes, and polyamides.1 Nevertheless, because of the flexibility of their long alkyl chains in their aliphatic structure, polymers based on vegetable oils generally exhibit limited thermomechanical properties. The objective was here to prepare networks by using vegetable oils with terpenes to increase the mechanical properties of the materials. Terpenes are natural hydrocarbons, usually obtained from plants by steam distillation, extraction, or enfleurage,2 with worldwide production of ∼109 tons per year.2,3 The terpenoids constitute a wide family of compounds4 composed of 5-carbon isoprene (2-methyl-1,3-butadiene) units and are successively assembled into terpenes. The resulting © 2017 American Chemical Society

terpenoids are classified by the number of isoprene units contained within the structure; for example, the β-myrcene is a monoterpenoid with two isoprene units and β-farnesene is a sesquiterpenoid with 3 isoprene units (see Table 1). The term terpenoid refers to a compound biologically derived from isoprene units and terpene denotes an alkene.5 The largest-scale industrial production of a terpenoid from its native producer is the natural poly(isoprene) from the hevea tree. Terpenoids are generally used as solvents and diluting agents for dyes and varnishes, and ingredients in fragrances, pharmaceutical compounds, foods, and cosmetics.6 Even though terpenes are involved in numerous catalytic chemical processes, only a few of them have been the subjects of studies related to their use as polymeric units.7−13 Among the different terpenes, β-myrcene (7-methyl-3-methylene-octa-1,6-diene) is an interesting natural molecule,2,14,15 because of the presence of three highly reactive bonds, including one conjugated double bond and its low oral and dermal toxicity.16,17 Myrcene is a versatile starting raw material for flavors, fragrances, cosmetics, vitamins, and Received: March 29, 2017 Revised: June 23, 2017 Published: July 11, 2017 6707

DOI: 10.1021/acssuschemeng.7b00959 ACS Sustainable Chem. Eng. 2017, 5, 6707−6715

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Structures of the Reactive Molecules

Farnesene is also a natural product used as an alarm pheromone emitted by aphids,30 a natural coating of apples,31 or as a component of essential oils. This substance has many uses as a feedstock molecule for the production of cosmetic oils,32 surfactants,33 lubricants34 and fuels,33 and polymers.33,35 Copolymers derived from β-farnesene and vinyl monomers were prepared by polymerizing β-farnesene in the presence of a catalyst suitable for polymerizing olefins such as ethylene, styrene, or isoprene.36 In this context, here, we propose an efficient and straightforward route based on the Diels−Alder reaction in the presence of oxygen to prepare renewable networks without using any solvent or catalyst. β-myrcene or β-farnesene and sunflower oil were used as they were received without any chemical modification. This work describes the synthesis and the characterization of a new class of networks with different contents of β-myrcene or β-farnesene and sunflower oil. The structure, the morphology and the mechanical properties of these networks have been studied by 1H nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), dynamic mechanical analysis (DMA), and Fourier transform

pharmaceuticals. Diels−Alder reactions have been used by combining myrcene and the unsaturated compounds such as dienophiles as (meth)acrolein, (meth)acrylate and maleic anhydrid in the presence of catalyst, under microwave irradiation or ionic liquids.18−22 As the other 3-dienes, such as butadiene and isoprene, myrcene was also used as monomer to synthesized elastomeric bio-based materials. It can undergo different ways of polymerization using a combination of metathesis and cationic polymerization,23 coordination polymerization from lanthanide-based catalysts,24 cationic lutetiumbased coordination complexes,25 Ziegler-type catalysts,26 or by using a redox-initiated emulsion polymerization.27 Recently, a novel monomer was prepared from a Diels−Alder reaction, glycidylation, and epoxy ring-opening esterification of myrcene using maleic anhydride, epichlorhydrin, and an acrylic monomer. It was further polymerized with tung oil acrylicacid-polymerized glycidyl methacrylate28 or acrylated epoxidized soybean oil.29 A series of copolymers was prepared by mixing with acrylated epoxidized soybean oil under UV irradiation by using a photoinitiator. 6708


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ACS Sustainable Chemistry & Engineering

were treated by means of the “Avantage” software provided by ThermoFisher. The C 1s and O 1s envelopes were analyzed and peakfitted using Gaussian line shapes. The binding energy scale was fixed by assigning a binding energy of 284.8 eV to the −CH− carbon (1 s) peak. Thermal and decomposition characteristics of the materials were determined by thermogravimetric analysis (TGA), conducted on a Setaram Setsys Evolution 16 apparatus, in the temperature range of 20−800 °C with a heating rate of 10 °C/min under a flow of air at 20 mL/min. Mechanical properties were carried out on an Instrom Model 5965 universal testing machine, at crosshead displacement rate of 2 mm/min at room temperature (23 °C ± 2 °C). Dynamical, mechanical thermal analyses (DMTA) were performed with a Model DMA Q800 system from Texas Instruments, at a frequency of 1 Hz in the tensile configuration.

infrared (FTIR) and Raman spectroscopies. The release profile of eugenol, which is known for its antioxidant and antibacterial properties, is studied to test the ability of these new networks to release bioactive molecules.

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EXPERIMENTAL SECTION

β-Myrcene, β-farnesene, limonene, and geraniol were purchased from Sigma−Aldrich and used without further purification. β-farnesene is commercially available only as a mixture with its positional isomer αfarnesene. Eugenol was obtained from Alpha Aesar and was used as received. Sunflower oil was kindly provided by Lesieur (Coudekerque, France) and used as supplied. Sunflower oil contains palmitic acid, stearic acid, oleic acid, and linoleic acid. Synthesis of a Network Based on Sunflower Oil and Myrcene (SxM100−x) or Sunflower Oil and Farnesene (SxF100−x). Initially, sunflower oil and myrcene were mixed uniformly with different weight ratios, respectively, at ambient temperature with a magnetic stirrer to give a homogeneous solution. Thus, a network obtained from a mixture of 0.5 g of sunflower oil and 0.5 g of myrcene was noted as S50M50. After stirring, the mixture was poured into a silicon mold (5 cm × 2.5 cm) to form a film with constant thickness and the solution was cured in an oven for different curing times at 100 °C. Farnesene and sunflower oil networks were prepared under the same experimental conditions as described previously. Networks obtained from farnesene and sunflower oil were noted as SxF100−x; thus, a network obtained from a mixture of 0.5 g of sunflower oil and 0.5 g of farnesene was S50F50. Swelling Measurements of S50M50 or S50F50 Networks. Swelling measurements were performed as follows. A quantity of ∼0.15 g of network mixture was immersed into 5 mL of pure eugenol at room temperature, under moderate stirring. The samples were allowed to swell during different times and they were removed from the solution. The films then were dried under a paper sheet and the samples were immediately weighed. A total of three replicates were measured and the degree of swelling (q) was calculated according to the following equation:

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RESULTS AND DISCUSSION Preparation of Networks Based on Sunflower Oil and β-Myrcene (SxM100−x) or Sunflower Oil and Farnesene (SxF100−x). β-Myrcene and geraniol were separately heated at 100 °C without any solvent and catalyst in the objective to evaluate the behavior of the different unsaturated groups during the thermal treatment (see Table 1). After heating the geraniol for 24 h in the presence of oxygen, FT-IR analysis showed the presence of peaks at 3500, 1700, and 1100 cm−1, characteristic of oxidation products (see Figure S1 in the Supporting Information) confirming that oxidation reactions occurred. The material based on geraniol is very soft and tearable, whereas the material based of β-myrcene is very rigid and brittle. This difference is explained by the chemical nature of these two terpenes. The geraniol has nonconjugated double bonds, whereas the β-myrcene contains three double bonds, including conjugated double bonds that are sensitive to Diels− Alder reactions and also oxidation reactions. In order to investigate the effect of time on the polymyrcene PM100 structure, the reaction was studied at different times, from 0 h up to 140 h. Figure 1 represents the Raman spectra of the monomer, β-myrcene (M100) and polymyrcene (PM100). The −CH2 and −CH3 asymmetric stretching bands appear as a broad band at 2915 cm−1. The chemical structure of PM100 was confirmed with the disappearance of the band at 3011 cm−1, assigned to the aliphatic C−H stretching mode of C1, C3, and C4 (spectrum not shown). The Raman bands at 1635 and

⎛ M − Mi ⎞ q=⎜ s ⎟ × 100 ⎝ Mi ⎠ where Mi is the initial mass and Ms is the mass of the material after immersion in eugenol. Release of Eugenol in Aqueous Medium. S50M50 films (2 cm × 2.5 cm) containing 30% of eugenol were introduced into a glass vial containing 10 mL of distilled water and stirred at 25 °C. At predetermined times intervals, the films were withdrawn and introduced into a new distilled water solution. The total amount of eugenol released from the films was quantified with a calibration curve plotted by examining the absorbance versus concentration obtained by ultraviolet−visible light (UV-vis) using a Cary 60 UV-vis spectrophotometer from Agilent Technologies from 200 nm to 800 nm. The absorbance of eugenol was determined at 282 nm. The experiment was carried out in triplicate. Instrumentation. 1H NMR spectra were recorded on a Bruker Avance II NMR spectrometer, working at 400 MHz. The solvent peak (chloroform, 7.26 ppm) was used as reference. The FTIR spectra were obtained on a Tensor 27 Bruker apparatus (Digi Tect DLATGS detector, 32 scans, 4 cm−1) in the range of 500−4000 cm−1. Raman spectroscopy investigation was performed using a LabRAM HR spectrometer from Horiba Jobin Yvon (Longjumeau, France) equipped with a laser emitting at 633 nm. X-ray photoelectron (XPS) measurements were made with a Thermo Scientific K-Alpha apparatus. Survey scans were done using a monochromatic Mg Kα Xray source (12 keV, 2 mA) with a spot diameter of 25 mm2 operated in a low power mode (24 W). A pass energy of 10 eV was used for the detailed XPS scans. XPS spectra were obtained with an energy step of 0.05 eV with a dwell time of 200 ms. Data acquisitions have mainly been focused on the C 1s, O 1s core level lines. The spectra obtained

Figure 1. Raman spectra of β-myrcene (M100) and polymyrcene (PM100) after 140 h of reaction at 100 °C. 6709


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Figure 2. Conversion of double bonds of myrcene during the reaction at 100 °C, as determined by Raman spectroscopy.

Figure 3. XPS spectra of PM100 after 48 h of thermal treatment at 100 °C in the presence of oxygen.

1670 cm−1 are respectively characteristic of the conjugated C1C2 and C3C4, and the isolated C7C8 unsaturations of M100.27 After 3 h of reaction at 100 °C, a new band appeared at 1656 cm−1, which is characteristic of the presence of the C2′C3′ that corresponds to the appearance of the unsaturation in the hexatomic ring37 formed via Diels−Alder reaction. At the same time, we observed the decrease of the C1C2 and C3C4 bands. After 140 h of reaction in the presence of oxygen, a large band at 1740 cm−1, characteristic of oxidation products, was observed, whereas it was not present during the reaction without oxygen. Figure 2 represents the conversion of the double bonds during the first 8 h of reaction. The conversion was calculated by the ratio of the band intensity of unsaturated groups and the band at 1433 cm−1 due to the −CH2 bending vibration of C5−C6 centers.27 There are, at the same time, a rapid decrease of the proportion of the C1C2 and C3C4 conjugated double bonds and an increase of the C2′C3′ peak at 1656 cm−1, attesting to the presence of a branched structure (see Figure S2 in the Supporting Information). The Diels−Alder reaction occurs during the first 5 h. At this stage, the PM100 looks like a viscous gel. The characteristic band at 1670 cm−1 seems to be constant over time, because the initial C7C8 and C7′C8′ double bonds that were obtained after the Diels−Alder reaction possess the same local environment and are not differentiated by Raman spectrophotometry. The oxidation reactions then proceed and

explain the formation of the cross-linked network that can be removed from its support after 48 h. To investigate the presence of oxidized products at the surface, XPS analysis were performed. The deconvolution of C 1s band (Figure 3) reveals the presence of peaks at 285.9, 287.3, and 288.8 eV characteristic of C−OH, CO, and OC−O, respectively. Deconvolution of O 1s spectrum also demonstrated the presence of two oxygen group contributions, with binding energies of 531.8 and 533.4 eV, characteristic of CO and C− O/H−O, respectively. The O/C ratio is 0.39, which attests to the presence of oxidized products, because there is no oxygen in the initial myrcene. The acyclic structure with three double bonds for myrcene (Table 1) provide it a high reactivity with oxygen in the atmosphere.38,39 The oxidation proceeds through several routes and yields a wide variety of products.40 The initial step involves abstraction of a hydrogen atom by OH radicals to the carbon atom of the double bonds. Although abstraction can proceed through several sites, the sites with methyl substituents are the most susceptible to free radical cleavage.39,41 Recent studies42,43 emphasized the importance of hydroperoxy alkyl radical chemistry in the oxidation of polyunsaturated hydrocarbons and can explain here the importance of the oxidation products during the formation of the PM100 networks. Oxidation reactions are considered as secondary reactions, compared to Diels−Alder reaction, but they are nonetheless indispensable to obtain cohesion of the 6710


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Figure 4. Visual aspects of PM100, S50M50, PF100, S50F50, and PS100 films.

film. Without oxygen, only a viscous gel is obtained. Although the structure of the network is not well-defined, because of both the formation of the cycle by Diels−Alder reactions and oxidation reactions, the materials present an interesting aspect. To increase the flexibility of these materials, networks were prepared with different proportions of vegetable oils, from 20% to 80% in weight. On one hand, the sunflower oil contains ∼4.5 unsaturated groups that are able to enhance the density of cross-linking by Diels−Alder reaction with the terpenes; on the other hand, they possess long alkyl chains that are able to increase the flexibility of the system. The spectrum of S50M50 clearly presents the different unsaturated groups of myrcene and sunflower oil with distinct Raman shifts (see Figure S3 in the Supporting Information). The bands at 1655 and 1740 cm−1 correspond to the double bonds and ester groups of sunflower oil,44−46 whereas the bands at 1635 and 1670 cm−1 are characteristic of myrcene.27 As a consequence, the intensities of these bands have been changed during the reaction and a dramatic decrease is observed in the presence of oxygen showing the importance of the formation of oxidized products. We can estimate the importance of the presence of oxygen during the formation of the network and the formation of oxidized products via FTIR (see Figure S4 in the Supporting Information). The bands at 891, 1594, and 3093 cm−1, which are characteristic of the double bonds, decrease during the reaction in presence (or absence) of oxygen. Broad and intense bands appear after 3 h of reaction, at 3400 and 1194 cm−1. These bands are characteristics of the products of oxidation that do not appear when the reaction is performed in the absence of oxygen. Although the presence of oxygen leads to secondary reactions other than the Diels−Alder reaction, it turns out that the films obtained present a very smooth, homogeneous aspect and an interesting flexible behavior, as it was observed in the case of the S50M50 and S50F50 films. The aspect of these materials depends on the chemical compositions (Figure 4) and the presence of the two components, vegetable oil and terpene are essential to obtain interesting films, because the polyterpenes PM100 and PF100 are brittle and the PS100 has a tearable aspect. Reactions have also been realized using limonene and sunflower oil, giving S50L50, and using eraniol and sunflower oil, giving S50G50. In this context, Diels−Alder reactions may not occur with limonene and geraniol, because they do not have two conjugated unsaturated bonds in their structure. Consequently, the S50L50 and S50G50 remain soft and as tearable as PS100, which attests to the fact that the Diels− Alder reaction is important to obtain stronger and flexible materials. Since the networks were not soluble in solvents, the molar masses could not be determined.

Rheological Studies and Mechanical Properties of the Synthesized Network SxM100−x. The thermal stabilities of sunflower oil PS100, myrcene PM100, and the network S50M50 have been studied by TGA (Figure 5). The S50M50 exhibited

Figure 5. TGA curves of PS100, PM100, and S50M50.

three steps of degradation behavior, with the first step occurring between 200 °C and 375 °C, followed by a second step occurring from 375 °C to 450 °C, and the third step from 450 °C to 550 °C. The thermal initial decomposition (5% weight loss) occurs at >300 °C. The dynamic mechanical characterizations of the cured networks were performed by means of DMA analysis. The DMA thermograms for the cured systems containing from 0% to 80 wt % of myrcene are reported in Figure 6 The storage modulus (E′) is correlated to the molecular packing density in the glassy state, due to better packing of the chain fragments. The highest value of E′ at 25 °C is 1000 MPa, which is obtained with 80 wt % myrcene. This value dramatically decreases (3 MPa) when the sunflower oil content is ∼80 wt %. This is related to the free movements of the polymer chains in networks that contain more vegetable oil and are, therefore, less cross-linked. The tan δ curve shows a maximum that is attributed to be the glass-transition temperature (Tg) of the cured networks. Each of the networks has a single Tg value, which demonstrates that all systems are compatible and homogeneous, except the S20M80 network, which seems less defined. The data clearly evidence a decrease of the Tg value of the cured network with the increase the amount of sunflower oil in the formulation. The highest value, 92.3 °C, is obtained 6711


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Figure 7. Variation of Tg values with sunflower oil contents in the SxM100−x and in the SxF100−x networks.

Table 2. Thermal and Mechanical Properties of SxM100−x Networks PS100 S80M20 S60M40 S50M50 S20M80

for the S20M80 network, whereas a decrease of Tg is observed for the S80M20 network, which reaches 19.8 °C. This flexibilization behavior must be attributable to the introduction of vegetable oil units into the networks. Nevertheless, the curves show a wide region of Tg values for all materials. This behavior is typical of thermoset materials derived from vegetable oils, because of the plasticizing effect of the fatty acid chains. The variation of Tg is not linear for all the compositions and the effect is particularly important when the content of vegetable oil is between 20% and 60% (see Figure 7). According to the rubber elasticity theory, eq 1 was employed to describe the relationship between the average molar mass of the segment between cross-linking points (Mc) and the crosslinking density (νe, given in mol cm−3) with the storage modulus (E′) of thermosets at the rubbery plateau: 3dRT d = E′ νe

E′(Tg+30°C) (MPa)

νe (mol m−3)

Mc (g mol−1)

13.8 19.8 30.5 46.7 92.3

0.37 0.61 0.91 0.96 1.45

46.8 75.7 109.4 110.1 147.1

21141 13068 9048 8994 6732

segment between cross-linking points (Mc), because the resulting network is only formed by cross-linking reactions in the presence of oxygen. It is clear that the densities of crosslinking increased with the contents of myrcene, from 46.8 mol m−3 up to 147.1 mol m−3. Theoretically, myrcene has three reactive double bonds able to react by Diels−Alder reactions and oxidation reactions. Although the molar masses between cross-linking points decreased from 21 141 g mol−1 for S80M20 to 6732 g mol−1 for S20M80, these values are always superior to the molar masses of myrcene and sunflower oil. Consequently, we can suppose that all the double bonds are not engaged in these reactions, but the resulting networks have the capacity to form films easily, except for the PM100 polymer. This material is too brittle to form a homogeneous film and to measure the mechanical properties by DMA. This increase in cross-linking is simultaneously correlated with a decrease in the Mc and an increase in Tg. The Tg value reaches 92.3 °C when the content of myrcene is ∼80%. As a result, the myrcene unit is a shorter unit than fatty acids, decreasing the distance between the crosslinks. In contrast, fatty acid units contained more flexible aliphatic chains and a greater amount of free volume. Figure 8 shows the tensile properties of the different networks. Although the mechanical performance of the S20M80 network with high tensile modulus (97 MPa) and tensile strength (8.8 MPa) are very interesting, this material showed an important brittle character. Consequently, the S50M50 network has been chosen for further analysis, because of its remarkable flexibility. By comparison, the properties of our materials are totally different than those obtained by UV curing of modified myrcene and tung oil due to the lower cross-linking density obtained by our process, compared to the UV photoinitiated polymerization.28,29 The networks obtained here are surprisingly homogeneous and flexible and present an important breaking elongation (reaching 108%), whereas it did not exceed 11% in the systems that were obtained by UV curing. The biodegradation tests showed a 12% mass loss over a

Figure 6. Thermomechanical behavior of PS100, S80M20, S60M40, S50M50, and S20M80 determined by DMA.

Mc =

Tg (°C)

(1)

where E′ is the storage modulus (MPa) at Tg + 30 °C, d the density of network (d = 0.99 g cm−3), R the gas constant (R = 8.314 J mol−1 K−1), and T the temperature (K). The crosslinking densities, νe (which represent the average number of cross-links per unit volume), were calculated for different concentrations of myrcene (see Table 2). The E′ value at Tg + 30 °C was taken to be sure that the networks were in the rubbery state. The PS100 material, which was based only on vegetable oil, presents the higher average molar mass of the 6712


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Figure 9. Swelling rate of S50M50 and S50F50 films in eugenol.

inside the S50F50. Moreover, the S50F50 presents a Tg value of 22 °C near room temperature; therefore, both the rate of penetration of active molecules and their diffusion will be difficult to control in the case of biomedical and environmental applications. In the case of S50M50, Tg = 47 °C, which is significantly higher than the room temperature. The release rate of the active ingredients is closely related to the Tg value of the polymer matrix54 and, therefore, S50M50 seems best-suited for potential applications. Almost 90% of eugenol was released within 30 days from the S50M50 films (see Figure S5 in the Supporting Information) corresponding to a concentration of 6.2 mg/mL that is superior to the minimum inhibitory concentration (MIC) of eugenol against both E. coli (gram− bacteria) and S. aureus (gram+ bacteria).55,56 These data showed the interesting antibacterial properties of these networks and support their possible use as active coatings. To study the release kinetics of the systems, several mathematical models have been developed.57 One of the most commonly used is the Korsmeyer−Peppas semiempirical model, which described drug release from a polymeric system (see eq 2).58,59

Figure 8. Mechanical properties of the different networks S20M80, S30M70, S40M60, S50M50, and S60M40.

period of one year. The material is not totally biodegradable, because of the presence of cross-linkings. Swelling Behavior of S50M50 and S50F50 Films in the Presence of Eugenol. According to Flory swelling theory, swelling behavior is affected by three factors: rubber elasticity, affinity to the solution, and cross-linking density.47,48 In this respect, the swelling behavior of S50M50 and S50F50 networks were measured. Since the networks are based on hydrophobic moieties, efficiency of swelling in organic compounds is expected. The essential oils constitute an important source of free monoaromatic compounds having a large spectrum of chemical structures. Among them, eugenol extracted from the clove oil is a very interesting phenolic compound and is readily available. The biological properties of eugenol, because of the presence of the phenol group, have been used for a long time in dentistry49−52 to prepare dental resins or to improve the antibacterial activity of bio-based polymers.53 The incorporation of eugenol in the S50M50 and S50F50 films at room temperature was tested to further prepare novel bio-based antibacterial materials by contact or as a eugenol delivery system. The rate of eugenol diffusion inside the films was dependent on the nature of the terpene (Figure 9). The swelling rate increased very quickly in the case of S50F50 films 120 wt % in 2 hwhereas it only reaches 43% in 5 h in the case of S50M50 films. This difference can be explained by the structure of the two networks. As the molar masses between cross-linking points decreased from 17 074 g mol−1 for S50F50 to 8994 g mol−1 for S50M50, the diffusion of eugenol is easier

Mt = Kt n M∞

(2)

Mt/M∞ is the fraction of drug released at time t, k the release rate constant, and n the diffusion exponent that is dependent on the release mechanism. For a thin film geometry delivery system, n < 0.5 suggests a Fickian diffusion. In this case, the molecule release rate is time-dependent; 0.5 < n < 1.0 supports an anomalous non-Fickian transport, where the drug release rate is time-dependent but other factors such as polymer solute transport can occur. For n = 1.0, the release mechanism is represented by a case II, which indicates that the release rate is time-independent. Finally, when n > 1, the release mechanism is super case II transport, suggesting that the release rate is time-dependent (tn−1).57,60,61 In our study, the release exponents n were determined to be 0.18. Consequently, the mathematical modeling of the molecule release data suggested a Fickian diffusion mechanism.

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CONCLUSION Networks based on myrcene or farnesene and sunflower oil were successfully engineered according to an environmentally sustainable green chemistry method without any catalyst and solvent. The preparation of the networks requires two reactions: Diels−Alder reaction between the myrcene or 6713


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farnesene and sunflower oil, which result in an n-armed star formation, and oxidation reactions in the presence of oxygen, which will generate a network structure, as was demonstrated by FTIR and Raman spectroscopy and XPS analysis. The obtained materials are homogeneous and flexible, and this method allows one to obtain a large variety of networks having different glass-transition temperature (Tg) values, from 19.8 °C to 92.3 °C for the S20M80 network and to 58 °C for the S20F80 material. The materials also demonstrate remarkable swelling ability in eugenol. Moreover, investigations showed that ∼90% of the loaded eugenol is released in 30 days and the release is governed by Fickian diffusion. This network is a very promising material for coating application with antibacterial activity. To reinforce the mechanical properties, further studies are continuing, concerning the incorporation of lignin, which is also an attractive feedstock that can give greater value to these bio-based materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00959. FT-IR spectra of geraniol after 24 h of reaction (Figure S1); schematic representation of the preparation of PM100 network (Figure S2); Raman spectra of initial S50M50, S50M50 after 140 h of reaction at 100 °C in the presence (and absence) of O2, initial S50F50, and S50F50 after 140 h of reaction at 100 °C in the presence (and absence) of O2 (Figure S3); FT-IR spectra of initial S50M50 and S50M50 after 68 h of reaction at 100 °C in the presence (and absence) of O2 (Figure S4); and total release of eugenol from the S50M50 in water at 25 °C (Figure S5) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 33 (0)1 49 78 12 17. E-mail: [email protected]. ORCID

Estelle Renard: 0000-0003-0452-7507 Valérie Langlois: 0000-0002-4465-7827 Notes

The authors declare no competing financial interest.

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REFERENCES

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Straightforward route to design bio-renewable networks based on

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