Article pubs.acs.org/IECR
Preparation and Characterization of Plasticized Starch/Carbon BlackOxide Nanocomposites Dayan Qian,† Peter R. Chang,‡ Pengwu Zheng,§ and Xiaofei Ma†,* †
Chemistry Department, School of Science, Tianjin University, Tianjin 300072, People's Republic of China Bioproducts and Bioprocesses National Science Program, Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, Saskatchewan, S7N 0 × 2, Canada § School of Pharmacy, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013, People's Republic of China ‡
ABSTRACT: The carbon black-oxide (CBO) particles were prepared by oxidizing carbon black (CB) in a modified Hummer’s method to improve the dispersion of CBO in water. The carboxylic acid, epoxide groups, and hydroxyl groups were formed in the resulting golden CBO particles, which in the size of about 30−50 nm were smaller than CB particles. And the dispersion of CBO particles was so good in distilled water that CBO particles in aqueous solution followed the Lambert−Beer’s law well. The nanocomposites were also prepared using CBO or CB particles as the fillers in glycerol-plasticized starch (GPS) matrix by the casting process. CBO fillers had good dispersion in GPS matrix and exhibited an obvious reinforcing effect. Both tensile strength and Youngs modulous of GPS/CBO composites were higher than GPS/CB composites. In views of the values of water vapor permeability (WVP), GPS/CBO composites exhibited better water resistance than GPS/CB composites. 90−99% carbon atoms.15 The interfacial interaction is important to improve the properties of polymer, but raw hydrophobic CB cannot be dispersed well in the hydrophilic PS matrix.16 In this study, CB particles were oxidized by the modified Hummer’s method, which was often used to prepare GO. The obtained carbon black-oxide (CBO) particles were explored as a promising filler to prepare glycerol-plasticized starch (GPS)/CBO composites. The various functional groups including hydroxyl, epoxide, and carbonyl groups in CBO particles would form the hydrogen-bond interaction with starch in GPS matrix, which could improve the dispersion of CBO in the matrix and the properties of the composites. In addition, CBO particles could also be used as the filler for other natural polysaccharide (guar gum, agar, alginate, and chitosan) matrices.
1. INTRODUCTION Starch is a renewable carbohydrate polymer procurable at low cost from a great variety of crops. It has been investigated for potential applications in agriculture, medicine, and packaging.1 Native granular starch can be processed into plasticized starch (PS). However, PS is usually sensitive to moisture and shows lower tensile strength and thermal stability compared to conventional polymers.2 Nanofillers are often introduced into the PS matrix to overcome the above-mentioned drawbacks. Many natural polysaccharides such as cellulose,3 starch,1,4 chitosan,5 chitin,6 and their derivatives,7 have been used to produce nanofillers to composite with PS matrix. Much effort has been focused on mineral nanofillers8,9 and metal oxide nanoparticles.10 Recently, carbon materials also have been used as filler to prepare PSbased composites. Carbon materials have a strong van der Waals force, resulting in self-aggregation, which interrupts a good dispersion of carbon materials in the PS matrix. Therefore, surface modification is required to improve the interfacial interaction between carbon fillers and GPS film. Fama et al.11 wrapped the carbon nanotubes with a starchiodine complex to obtain very well-dispersed nanotubes in the PS matrix and strong adhesion between the phases. The carboxylate multiwalled carbon nanotubes were used to prepare homogeneous PS-based nanocomposites.12 Compared to graphene, graphene oxide (GO) is hydrophilic and can form strong physical interactions with GPS. And PS/GO composite films with different loading levels of GO were prepared by the casting method.13 Carbon black (CB) is an important industrial pigment which has been widely used for coatings, plastics, and ink, due to its excellent properties such as low price, low pollution, and high color saturation.14 And CB is much cheaper than CNT and graphene. However, CB particles are strongly hydrophobic and tend to aggregate in water because the surface of CB contains © 2012 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. Corpren CB3000 was purchased from SPC, Sweden. This grade of CB had a DBP value of 380 cm3/100 g, an iodine adsorption of 1000 mg/g, as provided by the manufacturer. Potato starch was supplied by Manitoba Starch Products (Manitoba, Canada). Glycerol, H2SO4, NaNO3, KMnO4, H2O2 and HCl were of analytical grade from Tianjin Chemical Reagent Factory (Tianjin, China). 2.2. Preparation of the CB-Oxide (CBO) Particles. The CBO particles were prepared by oxidizing CB in a modified Hummer’s method.17 The concentrated H2SO4 (46 mL) and NaNO3 (1 g) were added into a flask, followed by the addition of CB (2 g) at below 5 °C. The solid KMnO4 (6 g) was gradually added under stirring at below 20 °C, and stirred for 1 Received: Revised: Accepted: Published: 7941
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h. The temperature was increased and kept at 35 °C for 2 h. 92 mL distilled water was then added, and the temperature was increased to 98 °C for another 15 min. Excess distilled water (150 mL) was added to the mixture, and then 30% H2O2 (5 mL) was added. The mixture was immediately centrifuged and the resulting CBO particles were washed three times with 5% aqueous HCl to remove metal ions and then washed with distilled water to remove the acid. The CBO particles were dried in air and dispersed into water. 2.3. Preparation of GPS/CB and GPS/CBO Composites. CB and CBO particles were respectively dispersed into the solution of distilled water (100 mL) and glycerol (1.5 g) using ultrasonication for 10 min. 5 g starch was added. The filler loading level (0, 1, 2, 3, or 4 wt %) was based on starch. The mixture was heated at 90 °C for 0.5 h for the plasticization of starch with constant stirring. The mixture was cast into a membrane on a dish. The formed solid-like membranes were placed in an air-circulating oven at 50 °C until they were dry (about 6 h). The composite films of GPS/CB and GPS/CBO composites were preconditioned in a climate chamber at 25 °C and 50% RH (relative humidity) for at least 48 h prior to the testing. 2.4. Fourier Transform Infrared (FTIR). FTIR spectra of CB and CBO particles were obtained on a BIO-RAD FTS3000 IR Spectrum Scanner. 2.5. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were obtained on a Perkin-Elmer 5600 (U.S.) using monochromatic microfocused Al K Alpha X-rays. Survey scans using a 200 eV pass energy were initially taken to identify all components. The atomic percentages (atom %) of the different elements were calculated from the survey spectra by considering the integrated areas of the main XPS peaks of the elements that were found. 2.6. Scanning Electron Microscopy (SEM). CB, CBO particles, and the fracture surfaces of GPS/CB and GPS/CBO composites were examined using a Nanosem 430 Scanning Electron Microscope. CB and CBO particles were respectively dispersed into water using ultrasonication for 10 min. The suspension drops were drawn on a glass flake, dried to remove water, and then vacuum coated with gold for SEM. The composites were cooled in liquid nitrogen, and then broken. The fracture faces were vacuum coated with gold for SEM. 2.7. UV−visible Spectra. The UV−visible (UV−vis) spectra of the aqueous solution with different concentrations of CB and CBO particles were recorded from 200 to 800 nm using a UV−vis spectrophotometer model U-1800, Hitachi Company. 2.8. Mechanical Testing. The Testometric AX M350− 10KN Materials Testing Machine was operated at a crosshead speed of 50 mm/min for tensile testing (ISO 1184−1983 standard). The data was averaged over 6−8 specimens. 2.9. Thermogravimetric (TG) Analysis. Thermal properties of the composites were measured with a STA409PC thermal analyzer, NETZSCH, Germany. The weights of samples were about 15 mg, and heated from room temperature to 500 °C at a heating rate of 15 °C/min in a nitrogen atmosphere. 2.10. Water Vapor Permeability (WVP). WVP tests were carried out by ASTM method E96 (1996) with some modifications.18 The films were cut into circles, sealed over with melted paraffin, and stored in a desiccator at 25 °C. RH 0% was maintained using anhydrous calcium chloride in the cell. Each cell was placed in a desiccator containing saturated
sodium chloride to provide a constant RH 75%. Water vapor transport was determined by the weight gain of the permeation cell. Changes in the weight of the cell were recorded as a function of time. Slopes were calculated by linear regression (weight change vs time) and correlation coefficients for all reported data were >0.99. The water vapor transmission rate (WVTR) was defined as the slope (g/s) divided by the transfer area (m2). After the permeation tests, the film thickness was measured and WVP (g Pa−1s−1m−1) was calculated as follows: WVP =
WVTR ·x P(R1 − R 2)
(1)
where P is the saturation vapor pressure of water (Pa) at the test temperature (25 °C), R1 is the RH in the desiccator, R2, the RH in the permeation cell, and x is the film thickness (m). Under these conditions, the driving force [P(R1 − R2)] is 1753.55 Pa.
3. RESULTS AND DISCUSSION 3.1. Characterization of CBO. FT-IR spectra of CB and CBO particles are shown in Figure 1(a). Compared to CB, CBO showed the collection of absorption bands corresponding to carboxylic acid, epoxide groups, and hydroxyl groups. A
Figure 1. (a) FTIR spectra of CB and CBO particles. (b) XPS spectrum in the C 1s region of CBO particles. 7942
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broad band centered at 3430 cm−1 was attributed to the hydrogen-bonded hydroxyl groups. The peak at around 1736 cm−1 was ascribed to CO stretching vibrations from carbonyl and carboxylic groups. The peak at 1632 cm−1 was related to CC stretching vibrations.19 The bands at 1300−1000 cm−1 were assigned to C−O stretching vibrations, and the out-ofplane deformation of O−H appeared at 880 cm−1.20 The oxidation degree of CBO was confirmed by XPS. The survey spectra for CBO yielded C/O atomic ratios of 1.94. The oxidation degree of CBO was a little higher than that of graphitic oxide (C/O atomic ratios between 2.1 and 2.9) from the Hummer’s method.17 Curve-fitting of the carbon region (C 1s) of CBO in Figure 1(b) could be decomposed into four peaks with binding energies of 287.9 (CO), 286.3 (C−O), 289.3 (O−CO), and 284.6 eV (C−C), respectively.21 As shown in Figure 2(a), CB nanoparticles exhibited an approximately spherical morphology with a size range of 50−
100 nm. Figure 2(b) revealed that CBO nanoparticles were composed of even smaller subunits with a dimension of 30−50 nm. The oxidized processing decreased the size of CB particles. Figure 3(a,b) shows the UV−vis spectra of different concentrations of CB or CBO in distilled water. There was
Figure 3. UV−visible spectra of CB (a) and CBO (b) particles in water at the increasing concentrations from the bottom to top, the inset (b) is Lambert-Beer’s plot for the absorption peak of CBO at 230 nm.
no obvious absorption of CB particles in the 200−800 nm regions, while CBO exhibited the absorption at 230 nm. The straight line passing through the origin described the linear relationship of the observed absorption peak intensity and the CBO concentrations (Figure 3(b), inset). It indicated that the optical behavior from the aggregation of any multimolecular species was not displayed in the concentration range tested.22 CBO particles in aqueous solution followed the Lambert− Beer’s law well, because the carboxylic acid, epoxide groups and hydroxyl groups improved the hydrophilicity and the dispersion of CBO in distilled water. The dispersions of CB and CBO in distilled water are displayed in Figure 4. It was found that the black CB turned into golden CBO particles when CB particles were oxidized. It is similar to the graphene. The CB and CBO particles were
Figure 2. SEM micrograph of CB (a) and CBO (b) particles. 7943
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3.2. Morphology of the Composites. As shown in Figure 5(a), no residual granular structure of starch was observed in the continuous GPS phase. At the high temperature, water and glycerol were known to physically break up the granules of starch and disrupt intermolecular and intramolecular hydrogen bonds and make the native starch plastic. The distributions of CB or CBO particles in the GPS matrix were shown in Figure 5(b−e). Since CB had the poor dispersion in water, the obvious aggregation of CB particles formed in the GPS matrix at the low loading (only 2 wt %) in Figure 5(b), and more CB aggregation was observed at the loading of 4 wt % CB for GPS/ CB composites in Figure 5(c). However, CBO particles could be evenly dispersed in GPS matrix at low loading (2 wt %), revealed by Figure 5(d). The good dispersion was attributed to the strong hydrogen-bond interaction between carboxylic acid, epoxide groups, and hydroxyl groups of CBO particles and starch of GPS matrix. At the higher contents (4 wt %) of CBO particles, as shown in Figure 5(e), the aggregation appeared in GPS/CBO composites, but the dispersion was still better than that of GPS/CB composites. 3.3. Mechanical Properties. Figure 6 revealed the effect of the filler (CB or CBO) contents on the mechanical properties of the composites. As the filler in the GPS matrix, CBO had an obvious reinforcing effect. With the increasing of the filler contents, the tensile strength and Youngs modulous of the composites increased. When the CBO contents varied from 0 to 3 wt %, the tensile strength increased from 7.6 to 13.5 MPa, and Youngs modulous improved from 143 to 240 MPa. The improvement was related to the good interfacial interaction between the GPS matrix and the CBO filler containing carboxylic acid, epoxide groups and hydroxyl groups. CBO particles could act as physical cross-linking points of starch molecules, which results in the increased tensile strength and Youngs Modulous. However, when GPS matrix was loaded with more CBO filler (4 wt %), the tensile strength of the composites decreased to 12.7 MPa. The agglomeration of CBO, as shown in Figure 5(e), actually reduced the effective cross-linking points and the interaction between CBO and GPS matrix.16 The GPS/CB composites exhibited similar dependence of tensile strength on the CB contents, however, the maximal tensile strength (12.5 MPa) appeared at the lower loading (2 wt %) of CB particles. It was related to the poor dispersion of CB in GPS matrix, as shown in Figure 5(b). And the tensile strengths and Youngs Modulous of GPS/CB composites were lower than those of GPS/CBO composites. Since the well-dispersed fillers could constrain the surrounding polymers, decrease the mobility of matrix chains, and further result in lower elongation at break, both CB and CBO particles reduced the elongation at break of the composites with the increasing of CB and CBO contents. And the GPS/CBO composites also had the better elongation at break than GPS/ CB composites. 3.4. Thermal Stability of the Composites. TG and DTG curves of GPS/CB and GPS/CBO composites were shown in Figure 7. The mass loss of GPS before the onset temperature was related to the volatilization of both water and glycerol plasticizer. As revealed by the DTG curves, the degradation of GPS took place at about 313 °C, i.e., the temperature at maximum rate of mass loss. GPS/CB composites containing 2 and 4 wt % CB particles degraded at higher 316 °C and lower 308 °C, respectively, while GPS/CBO composites containing 2 and 4 wt % CBO particles degraded at 319 and 318 °C,
Figure 4. Photographs of the dispersions of CB (a and b) and CBO (c and d) in water after 0 h (a and c), 1 h (b), and 24 h (d).
respectively added into distilled water and shaken on a rotary shaker at 150 rpm for 10 min. The stability of the dispersion was studied with the storage time. At the beginning, there was no obvious precipitation for CB (Figure4(a)) and CBO (Figure 4(c)). After the suspension was stored for 1 h, the CB particles were completely precipitated from the solution and the solution was clear (as shown in Figure 4(b)). In contrast, no obvious CBO precipitation was observed and the solution was not transparent in Figure.4 d, when the suspension was stored for 24 h. Over the storage period of one month, no CBO particles precipitated from the solution and the stability was maintained. It was probably dependent on the hydrophilic groups from the CB oxidation23 and the smaller size of CBO. This good stability in water would contribute to form the good dispersion of CBO particles in starch pasting, furthermore in GPS matrix during the casting process. 7944
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Figure 5. SEM micrograph of the fragile fractured surface for GPS/CB or GPS/CBO composites. (a) GPS, (b) 2 wt % CB, (c) 2 wt % CBO, (d) 4 wt % CB, and (e) 4 wt % CBO.
could decrease the effective filler contents, and the improvement of thermal stability was restrained. 3.5. WVP of the Composites. Since hydrophilic starchbased materials are sensitive to water vapor, the composite films are expected to avoid or at least to decrease moisture transfer. Water vapor permeability (WVP) was often used to research
respectively. GPS/CBO composites exhibited the better thermal stability than both GPS and GPS/CB composites. This could be ascribed to the better interaction between CBO and GPS.24 When more filler particles were added, either GPS/ CBO composites or GPS/CB composites showed the lower thermal stability. The aggregation (as shown in Figure 5(e)) 7945
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Figure 6. The effect of CB or CBO contents on tensile strength/ Youngs Modulous (a) and elongation at break (b) of the composites. Figure 7. The effect of CB or CBO contents on the thermal stability of the GPS/CB (a) or GPS/CBO (b) composites.
the moisture transport through the composite films. As shown in Figure 8, water vapor easily went through GPS film with the highest WVP values of 5.68 × 10−10 g m−1 s−1 Pa−1. With the increasing of CBO contents, WVP values of GPS/CBO composites obviously decreased, and then gradually changed. When CBO contents were more than 3 wt %, WVP values basically keep unchanged at about 2.88 × 10−10 g m−1 s−1 Pa−1. With the increasing of CB contents, WVP values of GPS/CB composites decrease gradually to 4.5 × 10−10 g m−1 s−1 Pa−1, but increased at 2 wt % content of CB particles. The CB or CBO particles probably introduced a tortuous path for water molecule to pass through.25 Since CBO particles could disperse well in the matrix, there were few paths for water molecule to pass through. And superfluous aggregations of CB or CBO particles actually decreased the effective contents of CB or CBO particles and make WVP of the composites decrease less or even increase a little. GPS/CBO composites have a better water vapor barrier than those of GPS/CB composites because the good dispersion of CBO in GPS matrix (as shown in Figure 5) leads to the reduction of permeability.
4. CONCLUSIONS In the present study, a simple and effective method was used to prepare CBO particles. GPS/CBO composites were obtained by incorporating CBO fillers into plasticized starch matrix. The carboxylic acid, epoxide groups, and hydroxyl groups from the oxidization of CB particles played an important role on the
Figure 8. The effect of CB or CBO contents on water vapor permeability of the composites.
dispersion of CBO particles in water, which could contribute to form the good dispersion of CBO in starch pasting, 7946
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furthermore in GPS matrix by the casting process. The introduction of CBO improved mechanical properties, thermal stability, and water vapor resistance of GPS. The GPS/CBO composites exhibited the better dispersion of the filler, mechanical properties, thermal stability, and water vapor resistance than GPS/CB composites. The improvement in these properties could be related to the good interaction between CBO filler and GPS matrix.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 22 27406144. Fax: +86 22 27403475. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We appreciate the financial support from the National Natural Science Foundation of China (No. 51162011). REFERENCES
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