Highly Unsaturated Microcrystalline Cellulose and Its Cross-Linked

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Highly Unsaturated Microcrystalline Cellulose and its Crosslinked Soybean Oil-based Thermoset Composites Tingting Chen, and Wendi Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05968 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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ACS Sustainable Chemistry & Engineering

Highly Unsaturated Microcrystalline Cellulose and its Crosslinked Soybean Oil-based Thermoset Composites

Tingting Chen and Wendi Liu*

College of Transportation and Civil Engineering, Fujian Agriculture and Forestry University, 63 Xiyuangong Road, Shangjie, Minhou, Fuzhou, Fujian 350108, P. R. China *Corresponding author E-mail: [email protected]; Phone: 86-591-83719095; Fax: 86-591-83768536.

ABSTRACT: Microcrystalline cellulose (MCC) was highly decorated with unsaturation functionality through esterification with methacrylic anhydride (MAA) with the assistance of ultrasonic treatment. The degree of unsaturation on the MCC surface was tailored by adjusting the mass ratio of MAA to MCC during the modification process. The unsaturated-MCC was characterized by FTIR, XRD, XPS, NMR, and contact angle measurements. The degrees of substitution and unsaturation of the unsaturated-MCC were quantitatively determined by NMR and XPS analyses to reveal the grafting efficiency of MAA on MCC surface. The unsaturated-MCC was further used as a reactive reinforcing agent for crosslinking with acrylated epoxidized soybean oil (AESO) to manufacture fully biobased

MCC/AESO

composites.

The

crosslinking

mechanism

between

the

unsaturated-MCC and AESO was discussed through the curing kinetic behavior of the MCC/AESO systems based on Kissinger’s theory. The unsaturation functionalization of MCC resulted in significantly improved flexural strength and modulus, water resistance, storage modulus, glass transition temperature, thermal stability, and interfacial adhesion of the resulting unsaturated-MCC/AESO composites. 1

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KEYWORDS: Microcrystalline cellulose; Unsaturation functionality; Acrylated epoxidized soybean oil; Crosslinking agent; Biocomposites

INTRODUCTION Vegetable oil, consisting of triglycerides, has been utilized for a number of years in the application of coatings, inks, lubricants, and composites due to its availability, relatively low cost, and environmental sustainability.1 Triglyceride is a kind of esters that are derived from glycerol with three saturated or unsaturated fatty acids. The ester linkages and unsaturated sites in the triglycerides provide the routes for chemical modification to formulate various vegetable oil-based polymers including unsaturated polyester, polyurethane, and epoxies.2 Acrylated epoxidized soybean oil (AESO) is a commercially available soybean oil derivative with reactive acrylate functionality such that it is able to polymerize via a free-radical reaction to generate thermoset AESO.3 However, the self-polymerized AESO resin behaves like a crosslinked rubber with poor mechanical properties due to the flexible fatty acid chains of triglycerides. Co-polymerization with petroleum-based monomers including typical styrene could significantly increase the crosslinking degree and thus achieve optimized properties of the obtained AESO resins. For maintaining the sustainability of the AESO resins, numerous unsaturated monomers have been synthesized from renewable resources such as eugenol,4 vanillyl alcohol,5 rosin acids,6,7 isosorbide,8 and furan9. Incorporation of fillers to develop biocomposites is another effective strategy for increasing the mechanical strength of the AESO resins. Contributed by their virtues such as high specific strength and modulus, renewability, and low costs, lignocellulosic fibers including dissolving pulps,10 regenerated cellulose,11 jute,12 ramie,13 and hemp fibers14 showed their great ability and prospect to reinforce AESO resins. However, the most intractable issue for the developed composites is the poor interfacial adhesion originated from the highly hydrophilic

2


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lignocellulosic fibers in contrast to the hydrophobic AESO resins. Cellulose

is

highly

abundant

renewable

materials

on

earth.

Cellulose

is

a polysaccharide composed of linear chains of repeating D-glucopyranose linked by β-1,4-glucosidic bonds. Intrigued by its unique characteristics such as high aspect ratio, high strength and modulus, and 100% renewability, cellulose is attracting a tremendous degree of scientific interests in pharmaceuticals, papermaking, food processing, composites, etc.15-17 The intriguing capability of cellulose to reinforce polymer matrices endows the obtained composites with high renewability as well as significantly enhanced physical, mechanical, and thermal properties.18 But the presence of abundant hydroxyl (–OH) groups on the surface of cellulose results in pronounced hydrophilicity and aggregation of cellulose. Numerous chemical modifications for cellulose have been developed to broaden its application scope, which involves the utilization of surface hydroxyl chemistry of cellulose to introduce new functionality onto the cellulose.19 However, most functionalization only occurs on the surface of cellulose because of the heterogeneous reaction between the cellulose and chemical reagents. The functionalized cellulose with a high degree of substitution is not easy to achieve. In addition to improving the compatibility of cellulose-reinforced composites, the highly functionalized cellulose might functions as a reactive component for polymer matrices and thus affect the curing and crosslinking degree of the matrices. It was reported that cellulose nanocrystals (CNCs) could be used as a green crosslinking agent for epoxy resin when a high loading (75%) of CNCs was used.20 As a result of its low nucleophilicity, the ability of CNCs –OH groups to induce the ring-opening of epoxides is not sufficient to obtain high performance nanocomposites. Hence, amine functionalization of CNCs via amino-silanes gave a high ring-opening efficiency of epoxies and generated a CNC-epoxy nanocomposite with improved thermomechanical properties.21 Nanocellulose was modified with unsaturated

3



fatty acid to produce crosslinkable nanocellulose for being used as reactive reinforcement in unsaturated polyester resins, which significantly increased the properties of the obtained nanocomposites.22 Cellulose/epoxidized soybean oil (ESO) composites with improved mechanical properties were prepared from ESO and dialdehyde cellulose as reactive reinforcing fillers.23 The current work focused on the unsaturation functionalization of microcrystalline cellulose (MCC) and its use as a reinforcing and crosslinking agent in AESO resins. The MCC was highly decorated with unsaturation functionality by reacting with methacrylic anhydride (MAA) in the presence of ultrasonic assistance. The functionalized MCC carrying different contents of methacrylic bonds was obtained by changing the concentration of MAA during the modification. The surface chemistry of the unsaturated-MCC was fully characterized along with the quantification of MAA substitution and degree of unsaturation. The effects of the unsaturated-MCC on the crosslinking and curing behavior of AESO resins and the properties of the resulting unsaturated-MCC crosslinked AESO composites were investigated in details through discussion of curing kinetics of MCC/AESO systems as well as evaluation of flexural properties, dynamic mechanical properties, water resistance, and thermal stability of the composites.

EXPERIMENTAL SECTION Materials MCC with an average particle size of 50 µm was purchased from Thermo Fisher Scientific. AESO with an average molecular weight of ~1200 g/mol (inhibitor: 3500–4500 ppm monomethyl ether hydroquinone), methacrylic anhydride (MAA, 94%, inhibitor: 2,000 ppm topanol A), 4-(dimethylamino)pyridine (DMAP, 99%), N,N-dimethylformamide (DMF, 99.8%), tert-butyl peroxybenzoate (TBPB, 98%), sodium bicarbonate (NaHCO3), and

4


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acetone were obtained from Sigma-Aldrich.

Preparation of unsaturated-MCC MCC (10 g) and DMAP (0.5 g, 5 wt% based on MCC) were suspended in DMF (150 mL) in a 250-mL three-necked flask. Then, MAA (5 g) was poured into the mixture; the mass ratio of MAA to MCC was 0.5:1. The flask was placed in an ultrasonic water bath (80 W) and then was magnetically stirred (500 rpm) at 60°C for 2 h. After being cooled to room temperature (RT), the mixture was filtered to separate the modified MCC from the solution. The obtained MCC was sequentially washed with acetone, saturated NaHCO3 and water, and then oven-dried under vacuum at 80°C for 24 h to generate the unsaturated-MCC (designated as 0.5MCC). Using the similar procedure, the mass ratios of MAA to MCC of 1:1 and 1.5:1 were also used for the reaction to obtain the MCC with different unsaturation level, i.e., 1.0MCC and 1.5MCC.

Manufacture of unsaturated-MCC/AESO composites In our previous work24, an optimized weight ratio (3:7) of MCC to AESO was used for the preparation of unsaturated-MCC/AESO composites. Pure-MCC or unsaturated-MCC (3 g) and AESO (7 g) were continuously dispersed in acetone (50 mL) in a 100-mL flask with an ultrasonic water bath (80 W, 10 min). The obtained mixture was mechanically stirred at 80°C to evaporate acetone and form a MCC-dispersed AESO solution. After being cooled to RT, the solution was added with 2 wt% TBPB (0.2 g) with a stirring of 500 rpm for 2 min. The resulting solution was poured into silicon molds (80 × 10 × 3 mm3) to perform the crosslinking reaction of unsaturated-MCC with AESO. The process was continuously conducted in an oven at 120°C for 2 h and 160°C for 4 h, respectively, to prepare pure-MCC/AESO and unsaturated-MCC/AESO composite samples.

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Characterization The surface chemistry of the unsaturated-MCC was analyzed by attenuated total refraction Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and nuclear magnetic resonance (NMR). FTIR spectra were scanned on a PerkinElmer Spectrum One spectrometer (PerkinElmer, USA) from 4000–650 cm-1 at a resolution of 4 cm-1. XPS analysis was performed on an ESCALAB 250 XPS (Thermo Fisher Scientific, USA) under a vacuum of 2e-10 mBar at a spot size of 500 μm with Al Kα source. A low-resolution survey was run from 1200 to 0 eV with a pass energy of 100 eV; the atomic high-resolution spectra were collected with a pass energy of 30 eV and an increment of 0.05 eV. Solid-state

13C

NMR analysis was conducted on an Avance III 500 NMR spectrometer

(Bruker, USA), equipped with a 4-mm double-resonance CP-MAS probe, at a contact time of 2 ms and at a magic angle spinning rate of 5 kHz. The crystalline structure of MCC was analyzed with a D8 Advance diffractometer (Bruker, USA) fitted with a Lynxeye XE high-resolution energy dispersive 1-D detector. The X-ray unit was operated at 40 kV and 40 mA using a Ni-filtered Cu-Kα radiation of 0.1542 nm. Angular scanning was conducted from 5 to 40° at 0.2°/s. The crystallinity index of MCC was calculated as the empirical method proposed by Segal et al.25 for native cellulose: CrI=[(I002-IAmorph)/I002]×100

(1)

where, CrI, I002, and IAmorph are the crystallinity index, the maximum intensity of (002) lattice diffraction, and the diffraction intensity at 18° of 2θ degree, respectively. The surface wettability of MCC was evaluated by pressing the MCC powders at 15 MPa for 1 min to form pellets for contact angle measurements. The tests were conducted on a First Ten Angstroms Analyzer System (FTA 1000, USA). A drop with 2 µL of distilled water was placed on the MCC surface using a micro-syringe. The contact angles were recorded in 1 and

6


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30 s after the probe liquid was applied. At least 5 measurements were performed for each sample. The curing process between the unsaturated-MCC and AESO was monitored by differential scanning calorimetry (DSC) analyses. The tests were performed on a STA 449 F3 Jupiter Simultaneous Thermal Analyzer (NETZSCH, Germany) with a N2 flow (30 mL/min) from 25 to 250°C at different heating rates (5, 10, 15, and 20 °C/min). The flexural properties of the unsaturated-MCC crosslinked AESO composites were evaluated on a CMT6104 universal testing machine (MTS Systems, USA) in accordance with ASTM D 790-10. Five rectangular specimens (80 × 10 × 3 mm3) were tested for each composite at a crosshead rate of 10 mm/min. Water absorption measurements were performed using rectangle specimens (80 × 10 × 3 mm3) based on ASTM D570-10 in distilled water at RT and boiling water. The specimens were placed in an oven at 50°C for 3 h before cooling down to RT in sealed plastic bags. The dry weight (wd) of the samples was obtained with a balance. Then, the samples were soaked in distilled water at RT for 24 h, and subsequently they were removed from water, wiped with tissue papers, and weighed to determine the wet weight (ww). Similarly, the samples were soaked in a boiling distilled water bath for 2 h. Afterward, the specimens were removed to distilled water at RT for 15 min before obtaining the ww. The water absorption rate of the specimens was calculated as (ww‒wd)/wd × 100%. Three replicates were tested for each composite. Dynamic mechanical analysis (DMA) were carried out on a DMA 242 (Netzsch, Germany) under single cantilever bending mode with a strain of 0.1%. Rectangular specimens (55 × 10 × 3 mm3) were used for the tests from ‒50 to 150 °C at a heating rate of 5 °C/min and a frequency of 1Hz. Thermogravimetric (TG) analysis were performed on a DTG-60 TG instrument (Shimadzu, Japan) at a heating rate of 10 °C/min from RT to 600 °C with a N2 flow of 50 mL/min. Scanning electron microscopy (SEM) images were obtained from a Zeiss Supra 35 VP field

7



emission SEM at an operation voltage of 10 kV. Samples were coated with elemental gold film (8‒10 nm) before testing.

RESULTS AND DISCUSSION Unsaturation functionalization of MCC

Scheme 1. Proposed reaction mechanism between MCC and MAA with ultrasonic assistance. MCC contains abundant –OH groups on the surface and thus easily agglomerates via hydrogen bonds. The modification of MCC with MAA aims to substitute the hydrophilic –OH groups with hydrophobic moieties while introducing the unsaturation functionality onto MCC surface. The esterification between the anhydride groups of MAA and the –OH groups of MCC could occur and generate the MAA-grafted MCC and methacrylic acids as byproducts with the catalysis of DMAP (Scheme 1). This reaction could be greatly accelerated by ultrasonic assistance because, 1) ultrasound is effective in improving the reactivity of –OH groups in cellulose by breaking hydrogen bonds;26 and 2) acoustic cavitation could improve the dispersion of MCC in organic solvents and disturb the phase boundary between cellulose and MAA.27 The reaction between MCC and MAA can be verified by the FTIR spectra of pure-MCC and unsaturated-MCC (Fig. 1A): 1) The unsaturated-MCC presented a strong peak at 1722 cm-1 assigned to the C=O bonds (stretching vibration) resulted from the ester linkages between MAA and MCC; 2) There were two other peaks at 1637 cm-1 (stretching vibration of C=C bonds) and 811 cm-1 (rocking vibration of C=CH2) appearing in the unsaturated sample due to the grafting of methacrylic

8


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groups from MAA; 3) The peak at 1104 cm-1 indicating the C–O bonds (stretching vibration) of secondary alcohols from glucoses became stronger because of the overlap of the stretching vibration of ester C–O–C bonds. As shown in the magnified FTIR spectra (Fig. 1B), the unsaturated-MCC showed two slight bands at 1454 and 1405 cm-1 due to the C–H bending vibration of –CH3 and =CH2 bonds, respectively;28 the bands at 1334 and 1200 cm-1 were associated with the –COH in-plane bending of secondary (C2 or C3) and primary (C6) alcohols from glucose units,29,30 which were diminishing as the increased MAA concentration. This indicates that both two types of –OH groups on MCC could react with MAA, which is in line with the previous work on the modification of nanocellulose.31

Fig. 1 ATR-FTIR spectra of pure-MCC and unsaturated-MCC at 4000–650 cm-1 (A); magnified spectra at 1500–1150 cm-1 (B). The esterification between MCC with MAA can be further elucidated by

9


13C

NMR


spectra (Fig. 2AB). A slight signal at 167.5 ppm indicating the carbons (C7) of ester groups was observed in the unsaturated-MCC. The emerged characteristic signals at 135.8 and 18.0 ppm were attributed to the unsaturated carbons (C8, 9) and the carbons (C10) only connected with carbon and hydrogen from the introduced methacrylic moieties. Generally, the carbons C2, 3, 5 of cellulose I showed a cluster of resonances at 70–80 ppm in the NMR spectrum; the C3 was assigned at 74.6 ppm and the signals at 72.1 and 71.6 ppm were resulted from the overlap of C2 and C5.32 With respect to those of C2 and C5 signals, the intensity of C3 signal was decreasing with the increase in MAA/MCC mass ratio. This confirms that the –OH group on C3 (secondary alcohol) would react with MAA. The change in the intensity of C2 signal was difficult to determine since the signal was overlapped by C5 signal that keeps unchanged during the reaction. The two signals at 65.0 and 62.3 ppm of C6 were attributed to the crystalline (C6’) and amorphous (C6’’) cellulose, respectively.33 The peak height of C6’’ signal was reduced after MAA grafting, which indicates that the –OH groups of C6 in amorphous zone is more reactive than those in crystalline region because the former is more accessible than the later. The FITR spectra of the unsaturated-MCC showed two strong peaks at 1722 cm-1 (C=O) and 811 cm-1 (C=CH2) that were becoming stronger with the increase of MAA concentration (Fig. 1A), which indicates the increased degree of substitution (DS) and degree of unsaturation (DU) on the MCC surface. Similar results were observed from the increase in the C7, C8, 9, and C10 signals of NMR spectra (Fig. 2A). To quantitatively compare the effect of MAA/MCC mass ratio on the efficiency of esterification between MCC and MAA, the DS and DU of the unsaturated-MCC were determined from the NMR data.34 The DS was calculated by comparing the integral of the terminal carbon (C10, –CH3) peak to half of the integral of carbons in MCC because only half of the carbons have –OH groups for substitution (Eq. 2).35,36

10


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DS = Integral (C10)/[Integral (CMCC)/2] × 100%

(2)

The DSs of 0.5MCC, 1.0MCC and 1.5MCC were calculated to be 2.38, 4.50 and 6.94, respectively (Fig. 2C), which were much higher than that (2.18) of the modified CNC with acid anhydride using iodine as a catalyst for epoxy nanocomposites.37 This demonstrates that ultrasonic assistance greatly contributes to increasing the grafting efficiency of MAA on MCC. This was further verified by the different DU of the unsaturated-MCC, which was determined from the integral of the introduced unsaturated carbons (C8, 9) divided by the integral of the total carbons (Eq. 3). DU = Integral (C8, 9)/Integral (Cunsaturated-MCC) × 100%

(3)

As shown in Fig. 2C, the DUs of the unsaturated-MCC ranging from 0.66 to 1.39 was a function of MAA/MCC mass ratio, which would provide the MCC with different crosslinking efficiency for AESO resins.

Fig. 2 NMR spectra of pure-MCC and unsaturated-MCC (A and B); Plots of calculated DS and DU of unsaturated-MCC from NMR data vs. MAA/MCC mass ratio (C). 11



XRD patterns of pure-MCC and unsaturated-MCC were shown in Fig. 3. In addition to the common peaks with the control, no additional crystallographic plane appeared on the unsaturated samples, which indicates that there was no significant change in the crystalline structure of the MCC after esterification. The calculated crystallinity index of 0.5MCC and 1.0MCC were comparable with that of pure-MCC; however, further increase MAA/MCC mass ratio to 1.5 slightly decreased the crystallinity index of MCC from 76.4 % (pure-MCC) to 73.5 %. Both types of ‒OH groups (i.e., primary and secondary alcohols) from cellulose could be the positions for the esterification with MAA. However, the reaction only occurs in the amorphous cellulose when the MAA/MCC mass ratio was lower than 1 because the crystallinity index of the MCC is not significantly affected. This also indicates that the ultrasonic treatment does not affect the crystallinity index of the MCC. However, with the aid of ultrasonic activation, the increased MAA concentration might lead to MAA molecules penetrating into the crystalline region of cellulose, which would destroy the crystalline structure of cellulose and thus decrease the crystallinity index of the MCC.

Fig. 3 XRD patterns of pure-MCC and unsaturated-MCC.

Surface chemical components and hydrophobicity of unsaturated-MCC The esterification between MCC and MAA was processed in a solid-liquid two-phase 12


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system and hence the reaction usually occurs on the MCC surface because the MAA is not easy to penetrate inside cellulose. The elemental distributions of the MCC were determined by the low-resolution XPS measurements (Table 1). The MAA grafting resulted in lower O/C ratio of the unsaturated-MCC due to the much lower O/C ratio (0.25) of the grafted moieties (CH2=C(CH3)C=O) compared to pure-MCC; the O/C ratio of the modified samples decreased from 0.60 to 0.48 with the increased grafting efficiency (i.e., DS). Similar to the calculation method of lignin content on the surface of pulps,38,39 the surface coverage percentage of MAA on MCC was determined from O/C ratio using Eq. 4. ΦMAA = [O/Cunsaturated-MCC – O/CMCC]/[O/CMAA – O/CMCC] × 100%

(4)

where O/CMCC and O/Cunsaturated-MCC are the O/C ratios of MCC before and after grafting, respectively; O/CMAA is the O/C ratio (0.25) of the grated moieties. The calculated content of MAA coverage on MCC of the unsaturated-MCC ranged from 20.5 to 47.7 % (Table 1), which were greatly higher than the DS values determined by NMR. This is likely because the MAA-MCC reaction mainly occurs on the MCC surface, and XPS only collects the chemical information from surface 5–10 nm region.40 The functional groups of the MCC surface were further revealed by high-resolution C1s spectra (Fig. 4). Pure-MCC showed three characteristic functional groups of cellulose, i.e., C1 (C–C), C2 (C–O), and C3 (O–C–O)41,42. Excluding the three common peaks with pure-MCC, a new C4 peak assigned to the O–C=O linkages of ester was seen in the unsaturated-MCC. Meanwhile, the C1 content greatly increased from 11.2 to 40.6% (1.5MCC) due to the grafting of C=C bonds and –CH3 groups from MAA. The C2 fraction was dramatically reduced from 72.9 to 36.8% (1.5MCC) due to the consumption of –OH groups by MAA grafting, which is responsible for the improved hydrophobicity of the unsaturated-MCC.

13



Table 1 The element concentrations, O/C ratios and calculated surface MAA coverage of pure-MCC and unsaturated-MCC obtained by XPS. Sample pure-MCC 0.5MCC 1.0MCC 1.5MCC

Element concentration (%) O/C ratio C O 59.2 40.8 0.69 62.4 37.6 0.60 63.6 36.4 0.57 67.5 32.5 0.48

Surface coverage of MAA (%) 20.5 27.7 47.7

Fig. 4 XPS C1s spectra of pure-MCC and unsaturated-MCC and their hydrophobicity 14


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measured by contact angle measurements. The surface hydrophobicity of the MCC was examined by contact angle tests (Fig. 4). The contact angle of pure-MCC after dropping of water for 1 s was measured to be 28.73°; however, the drop was breakdown after 30 s. Similar phenomenon was seen in 0.5MCC, although its contact angle after 1s was much higher than that of pure-MCC. The contact angles of 1.0MCC and 1.5MCC were 69.53° and 84.23° after 1 s and decreased to 48.49° and 78.20° after 30 s, respectively. These results indicate that the MAA grafting changes the surface characteristics of the MCC from highly hydrophilic to more hydrophobic, and the hydrophobicity of the unsaturated-MCC are closely related to the grafting efficiency of MAA. As mentioned above, there were four different C components in the unsaturated-MCC surface, which are mainly associated with the physical interaction of the MCC with other materials such as organic solvents. It is reported that C1 is recognized as representative non-polar groups, while C2, C3 and C4 are typical polar groups in organics.43 Therefore, the increased C1 component on the MCC surface leads to the reduced hydrophilicity and the increased contact angle of the MCC with water.

Crosslinking of AESO with unsaturated-MCC The crosslinking reaction between MCC and AESO initiated by TBPB was monitored by DSC scans at a heating rate of 10 °C/min (Fig. 5). The curing process of pure-MCC/AESO blend exhibited an intense peak with a shoulder at higher temperature resulting from the free radical polymerization of AESO molecules. The shoulder is attributed to the post-curing process caused by the improved molecular mobility and accelerated decomposition of initiator with the increased temperature.8 The DSC curves of unsaturated-MCC/AESO system had similar shape with that of pure-MCC/AESO system, where there were not significant difference in the peak temperatures (Tp) (Supporting Information, SI1). However, the 15



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shoulder at high temperature (Ts) became more obvious for the unsaturated-MCC/AESO with the increased DU of the unsaturated-MCC. This indicates that the methacrylic groups on the unsaturated-MCC would crosslink with the acrylate groups of AESO via free-radical polymerization at high temperature, hence accelerating the crosslinking of AESO resins.

Fig. 5 DSC curves of pure-MCC/AESO and unsaturated-MCC/AESO systems at a heating rate of 10 °C/min. Table 2 Calculated curing kinetic parameters from DSC results based on Kissinger’s theory. Systems pure-MCC/AESO 0.5MCC/AESO 1.0MCC/AESO 1.5MCC/AESO

Temperature position

E (kJ/mol)

lnA

Tp Ts Tp Ts Tp Ts Tp Ts

65.8 91.7 66.6 91.3 71.0 105.3 65.8 104.7

12.1 17.5 12.3 17.3 13.8 21.2 12.1 21.0

The activation energy (E) of the crosslinking reaction, reflecting the needed energy for the reaction to proceed, was determined from the nonisothermal curing kinetic analysis according to Kissinger’s theory.44 The detailed calculation was provided in the Supporting Information. As shown in Table 2, the E values of pure-MCC/AESO at Tp and Ts were 65.8 and 91.7 kJ/mol, respectively, which were much lower than those (83.4 and 124.6 kJ/mol) of neat AESO due to the presence of MCC.8 The E values of 0.5MCC/AESO at the two 16


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different temperatures were comparable with those of pure-MCC/AESO. With the increased DU of the unsaturated-MCC, no significant increase was observed in the E values of 1.0MCC/AESO and 1.5MCC/AESO at Tp, while their E values at Ts were much higher than those of 0.5MCC/AESO and pure-MCC/AESO. This confirms that the co-polymerization between the acrylate of AESO and the methacrylate of MCC would proceed at high temperature due to the low molecular mobility of the solid MCC. A similar case was seen in the pre-exponential factor (A) of the curing systems that represents the collision frequency of molecules in the reaction (Table 2).45 Increments in the A values at Ts were observed in 1.0MCC/AESO and 1.5MCC/AESO with respect to 0.5MCC/AESO and pure-MCC/AESO. This indicates that more free-radicals were generated due to the incorporation of methacrylates on the MCC, hence increasing the collision frequency of the reaction system.

Properties of unsaturated-MCC crosslinked AESO composites

Fig. 6 SEM images for (a) pure-MCC, (b) 1.0MCC, (c) pure-MCC/AESO, and (d) 1.0MCC/AESO composites and proposed interfacial bonding mechanism between unsaturated-MCC and AESO. As shown in Fig. 6ab, both SEM images of pure-MCC and 1.0MCC presented

17



well-separated macrofibrils with a small part of agglomeration. No significant difference in the morphology of MCC was observed between pure-MCC and 1.0MCC. The length and diameter of the macrofibrils were unable to be statistically determined. However, the size distribution of the MCC could be obtained with a particle size distribution analyzer (Supporting Information, SI2). The average size of pure-MCC, 0.5MCC, 1.0MCC and 1.5MCC were 47.9, 47.6, 49.8 and 49.5 µm, respectively (Fig. S2), which indicates that MAA grafting did not significantly change the fiber size of the MCC. The grafting of MCC with C=C bonds provides the opportunity for the crosslinking between the unsaturated-MCC and AESO, which would significantly increase the crosslinking density of the AESO resins and the fiber-matrix interfacial bonding of the resulting unsaturated-MCC/AESO composites. As shown in Fig. 6c, most fibers were pulled out and a large amount of voids were generated on the surface of pure-MCC/AESO composites, indicating an inferior interfacial adhesion between pure-MCC and AESO matrix. By contrast, for the unsaturated-MCC/AESO composites, the fibers were mostly fractured at the root and the fiber-matrix interface was indistinguishable, showing a superior interfacial adhesion (Fig. 6d). This is likely due to the formation of an interfacial layer from the crosslinking between the unsaturated-MCC and AESO.

Fig. 7 Flexural properties of pure-MCC/AESO and unsaturated-MCC/AESO composites. Significant increments in the flexural strength and flexural modulus of the obtained 18


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MCC/AESO composites were achieved after the unsaturation functionalization of MCC (Fig. 7). The flexural strengths of the composites from 0.5MCC, 1.0MCC and 1.5MCC greatly increased by 17.5%, 40.2% and 67.2%, respectively, when compared to that of pure-MCC/AESO composite. The unsaturated-MCC/AESO composites achieved 32.4%, 51.1% and 77.0% higher flexural modulus than the pure-MCC/AESO composite, depending on the DU of the unsaturated-MCC. The improved crosslinking density of AESO resins and fiber-matrix interfacial adhesion of the composites explain the greatly increased mechanical strength and modulus of the composites. However, the flexural strain of the composites significantly reduced when using the unsaturated-MCC to replace pure-MCC as reinforcing agent, while the DU of the unsaturated-MCC did not significantly affect the flexural strain of the composites. As for the pure-MCC/AESO composites, the AESO matrix consists of a large amount of flexible fatty acid chains that is not sufficiently crosslinked, which would effectively disperse instantaneous loading and avoid stress concentration during the loading process for the composites.46,47 But the toughening effect is eliminated for the unsaturated-MCC/AESO composites thanks to the enhanced crosslinking degree of the AESO matrix.

Fig. 8 Water absorption of pure-MCC/AESO and unsaturated-MCC/AESO composites. The water absorption rates (Wms) of the composites at RT for 24 h and boiling water 19



(100°C) for 2 h were given in Fig. 8. The Wms at 100°C for all the composites were much higher than those at RT, which is due to the increased water diffusivity inside the composites at high temperature. The water absorption of MCC/AESO composites is mainly resulted from the highly hydrophilic characteristics of MCC due to its abundant –OH groups. It is observed that the Wms of the unsaturated-MCC/AESO composites at both environments were greatly lower than those of the pure-MCC/AESO composite; the increase in the DU of the unsaturated-MCC resulted in continuous decrease in the Wms of the composites. The reduced hydrophilicity of the unsaturated-MCC is the main reason for the decreased Wms of the composites. The crosslinking between the unsaturated-MCC and AESO contributes to an enhanced fiber-matrix interfacial bonding of the composites, which also effectively prevent the entry of water molecules into the composites through interface gaps.

Fig. 9 DMA curves of pure-MCC/AESO and unsaturated-MCC/AESO composites. 20


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The storage modulus (E’) and tan δ of the composites vs. temperature were given in Fig. 9. The MCC/AESO composites had much higher E’s at all the measured temperature than neat AESO thermoset due to the reinforcing effect of MCC. The E’s of the unsaturated-MCC/AESO composites were higher than that of the pure-MCC/AESO composite; the increases were more significant in rubbery region. This confirms the improved interfacial adhesion and crosslinking degree of the composite because of the crosslinking between the unsaturated-MCC and AESO. The E’s at initial stage (‒50°C) of 0.5MCC/AESO, 1.0MCC/AESO and 1.0MCC/AESO composites were 1.33, 1.42 and 1.50 GPa, respectively, which demonstrates that the increased DU of the unsaturated-MCC contributes to increased stiffness of the composites. Meanwhile, a significant increase in the glass transition temperature (Tg) (the peak temperature of the tan δ curve) from 49.1 to 60.5°C was obtained after the incorporation of MCC into AESO resin. The Tgs of MCC/AESO composites increased to 61.3 and 63.5°C, respectively, when using 0.5MCC and 1.0MCC to replace pure-MCC as reinforcing agents for AESO resin, indicating an improved crosslinking density of the composites. Further increase in the DU of the unsaturated-MCC decreased the Tg of 1.5MCC/AESO composite when compared to 1.0MCC/AESO, which might be related to the excess level of unsaturation functionality on the MCC surface that are not reacted with AESO.

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Fig. 10 Thermal decomposition behavior of (a, b) MCC and (c, d) MCC/AESO composites. Table 3 Characteristic weight loss temperatures of MCC and MCC/AESO composites. MCC pure-MCC 0.5MCC 1.0MCC 1.5MCC

Characteristic weight loss temperature (°C) T10 Tmax 312.1 334.7 286.5 339.8 261.7 343.5 263.8 347.1

Composites pure-MCC/AESO 0.5MCC/AESO 1.0MCC/AESO 1.5MCC/AESO

Characteristic weight loss temperature (°C) T10 Tmax1 Tmax2 305.9 325.2 386.9 327.7 348.9 384.1 321.1 351.5 389.0 331.4 353.8 387.8

The thermal degradability of MCC and their AESO composites was measured by TG analysis (Fig. 10 and Table 3). Both pure-MCC and unsaturated-MCC presented a fast degradation behavior with a maximum weight loss temperature (Tmax) due to the thermal decomposition of β-1,4-glucosidic linkages in cellulose. Significant increases in the Tmax of the unsaturated-MCC with respect to pure-MCC were observed due to the reduced number of –OH groups on the MCC. However, the introduced methacrylic groups contain C=C bonds that is very sensitive to heat and hence is easy to decompose at low temperatures, which resulted in dramatically decreased initial weight loss temperatures (T10) of the 22


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unsaturated-MCC. As for the composites from the unsaturated-MCC and AESO, there were two Tmaxs presented on the curve, which corresponds to the degradation of MCC (Tmax1) and AESO (Tmax2), respectively. In contrast with the results of MCC, the unsaturated-MCC/AESO composites had much higher T10s and Tmax1s than the pure-MCC/AESO composite, which is probably due to the elimination of C=C bonds on the unsaturated-MCC after curing with AESO. The Tmax2s of the composites is attributed to the decomposition of AESO matrix and thus did not have significant change after the replacement of pure-MCC with the unsaturated-MCC.

CONCLUSIONS MCC with a high degree of unsaturation functionality was successfully manufactured via esterification with MAA with assistance of ultrasonic treatment, which was confirmed by FTIR, XPS and NMR results. The degree of substitution of MCC with MAA and the unsaturation level of the unsaturated-MCC were tailored by MAA/MCC mass ratio. NMR results indicated that the substitution and unsaturation degrees of 1.5MCC reached 6.94 and 1.39, respectively. However, unsustainable chemicals such as MAA and DMF were necessary to obtain MCC with a high degree of substitution. Therefore, a green unsaturated monomer, i.e., itaconic anhydride, has great potential to replace MAA and solvent-free reaction will be considered in the future. MAA grafting significantly changed the surface chemical components of the MCC, which resulted in improved hydrophobicity due to the introduction of nonpolar functional groups. The unsaturated-MCC showed its potential application in crosslinking with AESO, which significantly increased the crosslinking degree of AESO resins and interfacial adhesion of the resulting unsaturated-MCC/AESO composites. Curing kinetic analysis revealed that the crosslinking between unsaturated-MCC and AESO tended to occur at high temperature

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with an increased activation energy. The use of unsaturated-MCC as reactive reinforcing agent for AESO resins induced the MCC/AESO composites with significantly improved flexural strength and modulus, storage modulus, Tg, thermal stability, and water resistance.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Calculation

on

the

nonisothermal

curing

kinetics

of

pure-MCC/AESO

and

unsaturated-MCC/AESO systems; Size distributions of pure-MCC and unsaturated-MCC.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (W.L.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors are grateful for the financial supports from the National Natural Science Foundation of China (Grant No. 31800486) and the China Postdoctoral Science Foundation (Grant No. 2017M622039).

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TOG graph:

Synopsis: An efficient and facile method was successfully developed to highly decorate microcrystalline cellulose (MCC) with unsaturation functionality for the formulation of MCC-crosslinked soybean oil-based thermoset composites.

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Highly Unsaturated Microcrystalline Cellulose and Its Cross-Linked

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