Article pubs.acs.org/IECR
Transparent Epoxy Acrylate Resin Nanocomposites Reinforced with Cellulose Nanocrystals Haifeng Pan,†,‡ Lei Song,*,† Liyan Ma,† and Yuan Hu*,†,‡ †
State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, People’s Republic of China ‡ USTC−CityU Joint Advanced Research Centre, Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, 166 Ren’ai Road Suzhou, Jiangsu 215123, People’s Republic of China ABSTRACT: A series of nanocomposites based on cellulose nanocrystals (CNCs) and epoxy acrylate resin (EA) were prepared using solution casting followed by UV curing. The microstructure of the CNCs and nanocomposites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The thermal behavior and dynamic mechanical properties of the nanocomposites were determined using dynamical mechanical analysis (DMA) and thermogravimetric analysis (TGA). The transparency of the nanocomposite films was examined by UV−vis transmission spectroscopy. TGA results showed that the thermal stability of EA was affected slightly after incorporation of CNCs. The results from DMA showed that the storage modulus of nanocomposites improved over the entire temperature range compared to pure EA derived from the reinforcing effect of the CNCs. The glass transition temperature of nanocomposites was increased with the increase of CNC loading levels. The ultraviolet−visible spectra of the films showed no obvious absorbance over a range of 400−800 nm, which revealed that the films were transparent.
1. INTRODUCTION Cellulose, as one of the most abundant biodegradable materials in nature, is a polysaccharide produced by plants and some animals (tunicates). Cellulose can form a crystalline structure via hydrogen bonds.1 However, cellulose is not a completely crystalline matrix. The part of paracrystalline or amorphous cellulose can be removed and broken into individual crystallites, namely cellulose nanocrystals (CNCs), through acid and/or enzymatic hydrolysis. Sulfuric acid treatment introduces sulfate groups to the surface of CNCs, which makes the aqueous CNC suspension become highly stable.2 CNCs display large aspect ratios, high specific surface area, high crystallinity, high specific strength, and high Young’s modulus. When CNCs are incorporated into a polymer matrix, a rigid CNC percolating network can be formed via hydrogen bonds and result in a reinforcement effect.3 CNCs have attracted considerable attention for exploring new applications and were found to be a good reinforcement agent for a wide variety of polymeric matrixes including thermoplastic matrixes such as poly(vinyl alcohol) (PVOH),4 polypropylene,5 PHBV,6 natural rubber,7 poly(lactic acid),8 cellulose acetate butyrate,9 waterborne polyurethane,10 poly(methyl methacrylate) (PMMA),11 and poly(ε-caprolactone)12 and thermosetting matrixes such as epoxy resins3,13 and phenolic resins.14 In general, the resulting composites exhibit improved mechanical properties due to the incorporation of CNCs. Recently, many optical transparent materials such as polycarbonate, poly(methyl methacrylate), polystyrene, and poly(vinyl chloride) have been widely used because of their excellent optical clarity and lower density. However, the wide application of these polymers is limited owing to relatively low strength. Some research interests were concentrated on the © 2012 American Chemical Society
formation of strong and transparent polymer composites using bacterial cellulose (BC) which can retain high transparency even in the case of high BC loading levels.15,16 BC nanofibers were acetylated to enhance the properties of optically transparent poly(lactic acid) composites.17 Optical transparencies of chitin nanofiber/(meth)acrylic resin composites were also studied in the case of high chitin nanofiber loadings.18 In addition, optically transparent epoxy19 and poly(vinyl alcohol)20 reinforced with cellulose nanofiber were also prepared. Preparation of CNCs from microcrystalline cellulose (MCC) made from cotton is a potential way to generate reinforcement nanomaterial for optical resins. Liu11 prepared poly(methyl methacrylate) (PMMA) composites with the reinforcement of CNCs manufactured from microcrystalline cellulose (MCC) using combined sulfuric acid hydrolysis and high-pressure homogenization techniques. The result showed that the PMMA/CNC nanocomposites had high optical transparency. UV-cured resins have found increasing applications in consumer electronics industries due to their rapid cure, solvent free characteristics, application versatility, low energy requirements, and low temperature operation. Previously, many kinds of nanofillers including graphite oxide nanoplatelets,21 carbon nanotubes,22 layered silicate clays,23 layered phosphate,24 and layered double hydroxide25 have been incorporated into UVcured resins to enhance the mechanical properties and thermal stability. Hydrophilic layered inorganic compounds are often modified with organic surfactants to improve their compatibility Received: Revised: Accepted: Published: 16326
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ratios were then put into the UV irradiation apparatus and irradiated for 20 s. The distance between samples and UV lamp was about 10 cm. Thicknesses of EA/CNC transparent films ranged from 200 to 400 μm. The pure EA and nanocomposites with CNCs of 2, 4, and 8 wt % were prepared and referred to as EA0, EA2, EA4, and EA8, respectively. 2.4. Characterization. Atomic force microscopy (AFM) observation of CNCs was performed on a DI Multimode V scanning probe microscope (Veeco). Prior to AFM analysis a drop of diluted CNC suspension was deposited on a mica surface and allowed to dry at room temperature. The scanning was conducted in tapping mode, and the height and phase images were collected. Nanoscope V software was used to measure the diameters of the CNCs from the height images. The CNCs’ lengths were determined via digital image analysis. Transmission electron microscopy (TEM) images of CNCs were obtained on a Jeol JEM-100SX transmission electron microscope with an acceleration voltage of 100 kV. A drop of CNC suspension was deposited on a Formvar and carbon coated copper grid; the water was naturally evaporated at ambient temperature. TEM images of CNCs were acquired without any sample staining. The morphologies of untreated MCC powder and the fracture surface of EA/CNC nanocomposites coated with a gold layer in advance were observed with scanning electron microscopy (SEM; AMRAY1000B, Beijing R&D Center of the Chinese Academy of Sciences). X-ray diffraction (XRD) measurements were employed to characterize CNCs and EA nanocomposites with a Japan Rigaku D=Max-Ra rotating anode X-ray diffractometer equipped with a Cu Kα tube and Ni filter (λ = 0.1542 nm). The scanning rate was 4 deg min−1, and the range was 3−60°. CNCs were analyzed as powders, and EA nanocomposites were analyzed as membranes. Dynamic mechanical analysis (DMA) was performed using a DMA Q800 instrument (TA Instruments Inc.) at a fixed frequency of 1 Hz and a temperature range from 30 to 170 °C at a linear heating rate of 5 °C min−1. The dynamic storage modulus was plotted. Thermogravimetric analysis (TGA) of the samples was examined on a TGA-Q5000 apparatus (TA Co.) from room temperature to 800 °C at a heating rate of 20 °C/min under a nitrogen atmosphere. The weight of all samples was kept within 3−5 mg in an open Al pan. The UV−vis analysis of the nanocomposite films was performed using a DUV-3700 UV−vis spectrometer (Sahimadzu). The transmission mode was used, and the wavelength range was from 400 to 800 nm.
with UV-curable resins and thus to achieve a uniform dispersion in the matrix. However, the optical transparency of these UV-cured nanocomposites was decreased significantly owing to the incorporation of those nanofillers, especially in the case of high loading levels. Epoxy acrylate resins (EAs) are extensively used as oligomers of UV-cured film because of their good chemical resistance and high performance-to-cost ratio. UV-cured epoxy acrylate resin (EA) is one of the important transparent commercial materials which have been utilized to make electronic components, optical lenses, and other optical devices. In this work, EA nanocomposites reinforced with CNCs were prepared. CNCs were obtained from acid hydrolysis of the MCC. CNCs were transferred from water to N,N-dimethylformamide (DMF) by a solvent exchange method, and then were introduced into EA oligomers. This method avoided drying and redispersion of CNC powder and therefore could reduce the probability of CNC agglomeration.6 Nanocomposite films were prepared using solution casting followed by UV curing. The thermal stability, dynamic mechanical properties, and optical transparency were investigated.
2. EXPERIMENTAL SECTION 2.1. Materials. EA, which is a bisphenol A epoxy acrylate with the unsaturation concentration of 3.73 mmol g−1 and a molar mass of 536 g mol−1, was supplied by Tianjin Tianjiao Co. MCC, commercially available as VIVAPUR, was supplied by Sinopharm Chemical Reagent Co., Ltd. Sulfuric acid (H2SO4) and N,N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. 2-Hydroxy-2methyl-1-phenyl-1-propanone (Darocur 1173), kindly supplied by Ciba Specialty Chemicals, was used as a photoinitiator. 2.2. Preparation of CNCs. First, CNCs were prepared by sulfuric acid hydrolysis according to the method described previously by Bondeson.26 MCC (1 g/8.75 mL of acid) was added in 64 wt % sulfuric acid solution and vigorously stirred at 45 °C for 1.5 h. Then the suspension was diluted to about 1/10 of its original acid concentration. The acid was removed by repeated centrifuge at 4800 rpm for 10 min for each run until the supernatant was turbid. The supernatant was collected and dialyzed (dialysis tube molecular weight cutoff 12 000−14 000) against deionized water for 4−5 days to remove the remaining acid and other chemicals. The supernatant was further neutralized with 1 wt % NaOH and sonicated for 30 min. 2.3. Preparation of EA/CNC Nanocomposites. The supernatant of CNCs was concentrated to 2% CNC concentration using a Rotavapor operating at 80 °C. Then various amounts of concentrated CNC suspension were added dropwise into 80 mL of DMF solution under continuous stirring and then sonicated for 30 min. Water was evaporated from the mixture using a Rotavapor operating at 80 °C under reduced pressure conditions. As a result, a CNC/DMF suspension could be obtained. A known amount of CNC/ DMF suspension was mixed with 2 g of EA resin (associated with 3% Darocur 1173, which was used as photoinitiator)/ DMF solution. The mixture was sonicated for 30 min until a homogeneous dispersion was obtained. The solution was cast on a Teflon Petri dish and evaporated for 48 h at 55 °C. The residual DMF content of the mixture was removed through vacuum drying at 65 °C. The ultraviolet light source used for irradiation was a lamp (80 W/cm; Lantian Co., Beijing, China) that emits light in the near-UV (characterized wavelength, 340−360 nm). The mixtures of EA with CNCs in different
3. RESULTS AND DISCUSSION 3.1. Morphologies of MCC, CNCs, and EA/CNC Nanocomposites. The morphology of raw MCC was observed by SEM (Figure 1). Large agglomerates with around 80 μm in length were observed. The reason is probably that the large sized MCC granules were composed of strong hydrogen bonding microfibrils.11 After acid hydrolysis treatments, individual CNCs were obtained. Figure 2 shows a TEM image of rodlike CNCs prepared by the acid hydrolysis of the MCC. Although the whiskers agglomerated on the copper grid after water evaporation,27 rodlike individual whiskers could still be clearly distinguished. The average length and width of CNCs were 200 ± 21 nm and 10 ± 2 nm, respectively. The average aspect ratio was calculated to be 20 ± 2. Figure 3 is the AFM 16327
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were caused by the crack forking due to the excess of energy associated with the relatively fast crack growth. 3.2. XRD Spectra of Neat EA and EA/CNC Nanocomposites. The XRD spectra of CNCs, pure EA, and EA/ CNC nanocomposites are shown in Figure 5. X-ray diffraction diagrams of CNCs showed three cellulose I characteristic peaks at 2θ = 14.7, 16.4, and 22.6°. The 2θ angle at 22.6° corresponded to the 200 reflection of cellulose I.28 Because of the low content of CNCs, no obvious peaks of cellulose I were visible in EA2. When the CNC content was above 4%, the peaks centered at 22.6° were clearly identified in EA4 and EA8 samples. These indicate that the original crystal structure of cellulose I is well preserved in the nanocomposites. 3.3. Dynamic Mechanical Analysis of EA/CNC Nanocomposites. The dynamic mechanical behavior of the EA/ CNC nanocomposites was investigated by dynamic mechanical analysis (DMA). Figure 6A shows the temperature dependence of the storage modulus of neat EA and EA-based nanocomposites. A relaxation transition corresponding to the glass transition temperature (Tg) of neat EA was detected at about 95 °C. The neat EA resin showed the typical behavior of an amorphous thermosetting resin. The storage modulus of neat EA matrix decreased with the increases of temperature, and it decreased significantly near the glass transmission temperature (95 °C). At the initial temperature (30 °C), the storage modulus of neat EA was 1.1 GPa. The EA/CNC nanocomposites displayed a behavior similar to that of neat EA. The storage modulus of nanocomposites increased by the incorporation of CNCs. From Table 1, when the CNC content was 8 wt %, the storage modulus of the nanocomposites was about 1.7 GPa (30 °C), a 54% increase compared to that of the pure EA. The more dramatic storage modulus reinforcement was observed above the Tg of neat EA. For example, at 120 °C, the storage modulus of nanocomposites with 8 wt % CNC loadings was 484 MPa, which was 10 times that of the neat EA resin (48 MPa). Figure 6B displays the evolution of the mechanical loss factor tan δ of the EA/CNC nanocomposites as a function of temperature and CNC content. The neat EA resin showed a peak at 95 °C, which correlated with the glass transition temperature of the polymer matrix. The incorporation of CNCs made the Tg of nanocomposites significantly shift to higher temperature. Moreover, the enhancement of Tg of EA/CNC nanocomposites increased with increases of CNC content. This could be related to the hydrogen bonds between the matrix and the surface of the cellulose. The presence of CNCs could hinder the polymer chain motion. Below the percolation threshold (3.5%),28 the storage modulus of the CNC-modified EA matrix increased slightly. The reason may be related to interactions between the EA matrix and the surface of CNCs. Hydrogen bond interactions between the hydroxyl groups on the surface of CNCs and polar sites of the EA chains, such as hydroxyl groups and carbonyl, existed. The incorporation of CNCs resulted in formation of a rigid cellulose network with the interaction between CNCs joined by hydrogen bonds for CNC content higher than the percolation threshold (3.5%). The percolating cellulose network which was facilitated by strong interactions between the CNCs made the reinforcement of mechanical properties of the polymer matrix.3 3.4. Thermal Stability of EA/CNC Nanocomposites. Figure 7 shows the TGA (Figure 7a) and DTG (Figure 7b) curves of neat EA, freeze-dried CNC powder, and EA/CNC nanocomposites under nitrogen atmosphere. Slight weight loss
Figure 1. SEM image of raw microcrystalline cellulose (MCC).
Figure 2. TEM image of CNCs.
images of CNCs. The height and phase images of CNCs are shown in Figure 3a,b. Figure 3c is the CNC dimension measurement of the height image. The diameter of CNCs could also be determined by AFM images. It was approximately 10 nm, which was similar to that of the TEM image. The morphologies of the EA/CNC nanocomposites were further characterized by SEM images. Figure 4 shows the fracture surfaces of nanocomposites with various CNC loading levels. Small white dots are visible. These dots were considered as transversal sections of CNCs embedded in the matrix phase. It could be observed that their concentration increases as a direct function of CNC content in the nanocomposites. The micrographs clearly showed the CNCs in EA resin which appeared to be generally well individualized and distributed in the EA matrix. A smooth and featureless surface was observed for the neat EA matrix. However, the fracture surface of CNCmodified EA matrix is rough. The reason may be that CNCs were likely to force a path change of the propagating crack and resulted in this change during fracture.14 The higher the whisker concentration the greater the density of crack deflection sites producing smaller and denser ripples and ridges. In addition, feather markings could be observed on the fractured surface of CNC-modified EA matrix. These features 16328
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Figure 3. AFM images of CNCs: (a) height image, (b) phase image, and (c) CNC dimension measurement of the height image.
Figure 4. SEM images of the fractured surface of EA/CNC nanocomposites.
of freeze-dried CNC powder below 100 °C could be attributed to the moisture evaporation. Two mainly pyrolysis processes
could be observed for the freeze-dried CNC powder. The first process took place between 160 and 300 °C, which 16329
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Table 1. Dynamic Storage Modulus (E′) and Glass Transition Temperature (Tg) of EA/CNC Nanocomposites with Different CNC Loadings E′ (MPa) sample
30 °C
95 °C
120 °C
Tg (°C)
EA0 EA2 EA4 EA8
1.1 1.2 1.8 1.7
147 298 541 938
48 77 179 484
95 112 116 137
Figure 5. XRD patterns for (a) EA/CNC nanocomposites with various CNC contents and (b) CNCs.
Figure 7. (a) TGA and (b) DTG curves of EA, freeze-dried CNC powder, and EA/CNC nanocomposites under nitrogen atmosphere.
corresponded to CNC depolymerization, dehydration, and decomposition of glycosyl units followed by the formation of a char. The second one was between 300 and 600 °C, which corresponded to the oxidation and breakdown of the char to lower molecular weight gaseous products. Thermostability of CNCs was compromised by sulfate groups owing to catalysis of surface sulfate groups. The above result was consistent with some literature reports.14 Table 2shows that the initial decomposition temperature (the temperature of 5% weight loss) of EA/CNC nanocomposites was slightly lower than that of neat EA resin because of the low decomposition temperature of CNCs. From DTG (Figure 7b) curves, it was also shown that the maximum decomposition temperature of nanocomposites was varied slightly compared to neat EA resin. As a result, the thermal stability of EA was affected slightly after incorporation of CNCs. 3.5. Appearance and Optical Transmittance of EA/ CNC Nanocomposites. The UV−vis transmittance spectra of the pure EA resin and EA/CNC nanocomposite sheets in the
Figure 6. Dynamic mechanical curves of neat EA and EA-based nanocomposites with various CNC contents.
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Table 2. Onset Temperaturea and Tmaxb of CNCs and EA/ CNC Composites with CNC Weight Contents of 0, 2, 4, and 8%
a
sample
onset temp (°C)
Tmax (°C)
char residue at 800 °C
CNCs EA0 EA2 EA4 EA8
183 206 210 212 196
194 440 442 445 440
27.4 10.5 8.5 6.0 9.9
nanosize effect of cellulose nanocrystals, the nanocomposites were optically transparent, even in the case of high loading levels.
4. CONCLUSIONS EA/CNC nanocomposites were successfully prepared by solution casting followed by UV curing. CNCs with an average length of 200 nm and an average diameter of 8−10 nm were obtained by acid hydrolysis. Then a solvent exchange method was used to transfer CNCs from water to DMF and then to introduce them into EA. The DMA data showed a marked increase in storage modulus from 1.1 MPa for pure EA to 1.7 GPa for the nanocomposite sheets containing 8 wt % CNC content at 35 °C due to the strong interactions (e.g., hydrogen bonding) between the CNCs and their relatively low aspect ratio. The transmittance of EA/CNC nanocomposites was reduced with the increase of the content of CNCs, but all nanocomposites still retained high transparency.
Temperature of 5% weight loss. bDegradation temperature.
visible wavelength range of 400−800 nm are shown in Figure 8. The regular transmittance of pure EA resin was high: 87.7% at
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AUTHOR INFORMATION
Corresponding Author
*Tel./fax: 86-551-3601664 (L.S.); 86-551-3601664 (Y.H.). Email:
[email protected] (L.S.);
[email protected] (Y.H.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the joint fund of NSFC and CAAC (61079015), the National Basic Research Program of China (973 Program) (2012CB719701), and the Fundamental Research Funds for the Central Universities (WK2320000007, WK2320000014).
Figure 8. UV−vis transmittance spectra of pure EA and EA/CNC nanocomposite sheets.
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600 nm. The transmittance of EA/CNCs nanocomposites was slightly lower than that of pure EA matrix. Even when the CNC content was 8 wt %, the transmittance of EA/CNC nanocomposites still remained 79.8% at 600 nm. In order to give an intuitive result, Figure 9 shows photographs of the EA/ CNC nanocomposite sheets with 0, 2, 4, and 8 wt % CNC content, respectively. The pattern and letters in the background can be clearly seen through the sheets, indicating that the nanocomposite sheets are transparent. These results reflect that the CNCs had good distribution in the EA matrix. In this paper, a solvent exchange method was used to transfer CNCs from water to N,N-dimethylformamide (DMF) and could reduce the probability of CNC agglomeration. The width of cellulose nanocrystals was between 10 and 20 nm. Because of the
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