Biomacromolecules 2004, 5, 1046-1051
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Morphology and Properties of Soy Protein Isolate Thermoplastics Reinforced with Chitin Whiskers Yongshang Lu, Lihui Weng, and Lina Zhang* Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China Received December 8, 2003; Revised Manuscript Received February 25, 2004
Environmentally friendly thermoplastic nanocomposites were successfully developed using a colloidal suspension of chitin whiskers as a filler to reinforce soy protein isolate (SPI) plastics. The chitin whiskers, having lengths of 500 ( 50 nm and diameters of 50 ( 10 nm on average, were prepared from commercial chitin by acid hydrolysis. The dependence of morphology and properties on the chitin whiskers content in the range from 0 to 30 wt % for the glycerol plasticized SPI nanocomposites was investigated by dynamic mechanical thermal analysis, scanning electron microscopy, swelling experiment, and tensile testing. The results indicate that the strong interactions between fillers and between the filler and SPI matrix play an important role in reinforcing the composites without interfering with their biodegradability. The SPI/chitin whisker nanocomposites at 43% relative humidity increased in both tensile strength and Young’s modulus from 3.3 MPa for the SPI sheet to 8.4 MPa and from 26 MPa for the SPI sheet to 158 MPa, respectively. Further, incorporating chitin whisker into the SPI matrix leads to an improvement in water resistance for the SPI based nanocomposites. Introduction Biodegradable materials have attracted much attention for sustainable development and environmental conservation. Among these materials, biodegradable plastic will be an important application in nonrecycled goods such as trash/ rubbish and compost bags, mulch films, and disposable diapers or nappies.1,2 Therefore, extensive studies have been made for the potential use of polymer materials derived from renewable resources, which can play a major role in helping alleviate these environmental concerns.3-8 Soy protein isolate (SPI), the major component of soybean,9 is readily available from renewable resources and agricultural processing byproducts.10 The utilization of SPI in the preparation of biodegradable materials, such as adhesives, plastics, and various binders, has received more attention in recent years.11 Plastics from SPI have very high strength and good biodegradability; however, they are also brittle and water sensitive,12-16 which limits their applications. Accordingly, the properties of the SPI have commonly been modified by physical, chemical, or enzymatic treatments. Such treatments mainly promote cross-linking within SPI or modify the side chains of SPI, for example, acetylation and esterification,17 denaturation,18,19 incorporating fillers,20 and blending with other polymers.21,22 Whiskers are very promising reinforcing materials for composites, because of their high stiffness and strength.23 Owing to their small diameter, whiskers are nearly free of internal defects, thereby yielding a strength close to the maximum theoretical value predicted by the theory of elasticity.24 The extent of their reinforcement has been found * To whom correspondence should be addressed. Phone: +86-2787219274. Fax: +86-27-87882661. E-mail:
[email protected].
to depend on such factors25-28 as the nature of the matrix, the generation of a strong fiber-matrix interface through physicochemical bonding, the aspect ratio, and dispersion of the whiskers in the matrix. Compared with inorganic whiskers, whiskers from renewable resources have advantages such as renewability, low cost, easy availability, good biocompatibility, and easy modification chemically and mechanically.29 Chitin, from shellfish, insects, and microorganisms, is the second most abundant structural biopolymer. Chitin has been known to form microfibrillar arrangements embedded in a protein matrix, and these microfibrils have diameters ranging from 2.5 to 2.8 nm.30 The low crystalline region can be dissolved away by acid hydrolysis, whereas the water-insoulable, highly crystalline region can be converted into a stable suspension by mechanical shearing.31 Chitin whiskers have been used as a new kind of nanofillers as a reinforcing phase in both synthetic polymeric matrixs32,33and natural ones.27,28 Utilizing natural fillers from renewable resources not only contributes to a healthy ecosystem but also makes them economically interesting for industrial application due to the high performance of the resulting composites. In the present study, we incorporate chitin nano-whiskers obtained from crab shells into SPI to improve the thermomechanical properties and to decrease water sensitivity of the SPI. The thermoplastic SPI/chitin whisker nanocomposites were processed by hot-press, and the effects of chitin whisker content on morphology, structure, and properties of the SPI/chitin whiskers composites were investigated in detail. Experimental Section Preparation of Chitin Whiskers. Chitin from crab shells was supplied by Yuhuan Sea Biochemical Co., Zhejiang,
10.1021/bm034516x CCC: $27.50 © 2004 American Chemical Society Published on Web 04/06/2004
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China. Suspensions of chitin whiskers were prepared according to Dufresne et al.27 Samples were boiled and stirred in a 5% KOH solution for 6 h to remove most of the protein. The resulting suspension was kept at room-temperature overnight being stirred, filtered, and washed several times with distilled water and then bleached with a NaClO2 aqueous solution of 17 g/L containing 0.3 M sodium acetate buffer for 6 h at 80 °C. The suspension was then kept in a 5% KOH solution for 48 h to remove residual protein. The suspension was centrifuged at 3600 rpm for 15 min. The protein-free chitin whisker suspension was prepared by hydrolyzing the purified chitin with 3 N HCl at 30 mL per 1 g of chitin boiling for 90 min while stirring. Then the suspension was diluted with distilled water and centrifuged (3600 rpm for 15 min), and the process was repeated three times. Next, the suspension was dialyzed in running water for 2 h and then overnight in distilled water until the pH reached 4. Then a further ultrasonic treatment was performed on the dispersion. Finally, the dispersion was stored at 6 °C after adding sodium azide to control bacterial growth. Processing of SPI/Chitin Whisker Nanocomposites. SPI of desired weight and various content of chitin were mixed and stirred to obtain a homogeneous dispersion. The dispersion was freeze-dried, and 30% glycerol was added. The resulting mixture was hot-pressed at 20 MPa for 10 min at 140 °C and then slowly cooled to room temperature. The SPI/chitin whisker nanocomposites (thickness about 0.4 mm) were coded as SPI-5, SPI-10, SPI-15, SPI-20, SPI-25, and SPI-30, corresponding to a chitin content of 5, 10, 15, 20, 25, and 30 wt %, respectively. The sheets prepared from pure SPI and glycerol-plasticized SPI were coded as SPI and GSPI, respectively. The sheets of SPI, GSPI, and nanocomposites were maintained for one week at a 0% relative humidity (RH) over P2O5 in a desiccator at room temperature before characterization. Characterization. Infrared spectroscopy of the chitin whiskers was performed with a Fourier transform infrared (FT-IR) spectrometer (1600, Perkin-Elmer Co., USA) at room temperature. Atomic force microscopy (AFM) was performed on a Digital 3100 IIIa microscope (USA). A droplet of a dilute whisker suspension was coated onto a flake of mica, and the water was evaporated at room temperature. The morphology of the nanocomposites was observed with a scanning electron microscope (S-570, Hitachi, Japan) at 10 kV. The specimens were frozen in liquid nitrogen, fractured, and then coated with gold. Dynamic mechanical thermal analysis (DMTA-V, Rheometric Scientific Co., USA) was performed at 1 Hz and a heating rate of 5 °C/min. The specimens were 10 mm × 10 mm (length × width). The kinetics of water absorption were determined. Rectangular specimens of 10 mm × 10 mm (length × width) and a thickness of about 0.3-0.5 mm were used to ensure one-dimensional molecular diffusion. The samples were conditioned at 20-25 °C in a desiccator containing a saturated salt solution of CuSO4‚5H2O to give a RH of 98%. After absorption, the samples were removed and weighed to an equilibrium value (W∞). The water uptake at equilibrium
Figure 1. AFM imaging of a dilute suspension of chitin whiskers.
(WU∞) of the samples was calculated as WU∞ (%) )
W∞ - W 0 × 100% W0
(1)
where W∞ and W0 are the equilibrium weight of the sample in 98% RH and the initial weight of the sample, respectively. The mean water uptake of each sample was calculated for various conditioning times (t). The mass of water absorbed at time t, (Wt - W0), can be expressed as follows:34 Wt - W0 W∞
∞
)
∑
n)0
8 (2n + 1)2π2
[
]
-D(2n + 1)2π2t
exp
4L2
(2)
where W∞ is the weight absorbed at equilibrium; 2L, the thickness of the film; and D, the diffusion coefficient. At short times, eq 2 can be written as Wt - W0 2 D 1/2 1/2 ) t W∞ Lπ
()
(3)
At (Wt - W0)/W∞ e 0.5, the error in using eq 3 instead of eq 2 to determine the diffusion coefficient is on the order of 0.1%.35 The tensile strength (σb) and elongation at break (b) of the samples were measured on a universal testing machine (CMT-6503, Shenzhen SANS Test Machine Co. Ltd., China) according to the ISO6239-1986 standard with an extension rate of 50 mm/min. Before testing, the films were allowed to rest for one week in 43% RH (saturated K2CO3 solution at room temperature). Each sample was tested at least four times. Results and Discussion Morphology and Structure of Nanocomposites. The AFM image of a dilute suspension of chitin whiskers is shown in Figure 1. The suspension consisted of individual
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Figure 2. FT-IR spectrum of the chitin whiskers.
chitin fragments having a spindle shape. These fragments have a broad distribution in length (L) ranging from 100 to 650 nm and diameter (D) ranging from 10 to 80 nm. The average of length and diameter were estimated to be 500 ( 50 nm and 50 ( 10 nm, respectively. Therefore, the average aspect ratio, L/D, is 10 ( 5, which is very close to dimensions of commercial chitin (16)27 and squid pen (15)32 but much lower than chitin from Riftia tubers (120).33 Figure 2 shows the FT-IR spectrum of chitin whiskers. The characteristic absorption bands of R-chitin were at 1662, 1625, and 1580 cm-1 in the carbonyl region. Absorption peaks at 1540 cm-1, assigned to the stretching vibration of protein,36 were absent. This is indicative of pure chitin. SEM images of the fractured surface of the GSPI sheet and nanocomposites of SPI-5, SPI-15, and SPI-30 are shown in Figure 3. The GSPI sheet exhibits a relatively uniform surface. The chitin whiskers, as particles, are easily identified. A relatively uniform distribution of the chitin whisker in the SPI matrix can be observed when the chitin content is lower than 15 wt %. However, as the chitin content increases, the resulting nanocomposites show agglomerates of whiskers. The diameter of whiskers determined by SEM is larger than identified by AFM shown in Figure 2, which resulted from a charge concentration effect due to the emergence of whiskers from the observed surface.37 This suggests that the adhesion between SPI and chitin whisker is strong. Figure 4 shows the temperature dependence of the storage modulus (E′) and loss factor (tan δ) of the pure SPI sheet and the GSPI sheet, respectively. The SPI sheet has significantly high storage moduli and thermal stability across the temperature range (-100 to +200 °C). This indicates a plastic characteristic of SPI.15 As the temperature is increased, a sharp decrease in storage modulus, assigned to thermal decomposition at about 225 °C,38 is observed. Compared with the pure SPI sheet, the GSPI sheet exhibits a significant decrease in storage modulus except at temperatures lower than -60 °C. Thus, glycerol is an antiplasticizer and stiffens
Figure 3. SEM image of GSPI sheet and SPI/chitin whiskers nanocomposites.
SPI at temperatures below the freezing temperature (-80 °C) of glycerol. The tan δ vs temperature curve reveals information concerning molecular or segment scale motions in polymers. Peaks located at 150 and 225 °C are observed in the pure SPI sheet, which are assigned to the glass transition temperature (Tg) of SPI39 and the thermal decomposition of SPI, respectively. No relaxation peak at subtemperature is observed. The relaxation peak, corresponding
Soy Protein Isolate Thermoplastics
Figure 4. Temperature dependence of storage modulus (E′) and loss peaks (tan δ) for pure SPI (b) and GSPI (O) sheets, respectively.
Figure 5. Temperature dependence of storage modulus (E′) for GSPI sheet and SPI/chitin whiskers nanocomposites.
to Tg of pure SPI, shifts to about -77 °C for the GSPI,15 implying more easy motion of SPI molecules due to plasticization. Both pure SPI and GSPI display one loss peak at about 75 °C in the tan δ vs T curves, from which no obvious changes in peak position are observed. These peaks could be attributed to the denaturation transition of the SPI.40 The temperature dependence of the storage modulus (E′) for GSPI and SPI/chitin nanocomposites is shown in Figure 5 and is similar to that of gluten.41 The drop in E′ of GSPI and the nanocomposites at Tg is not as obvious as in synthetic polymers (usually more than 3 orders of magnitude). At low temperature, the E′ of the nanocomposite does not vary significantly with respect to filler content and is about 109 Pa due to the fact that molecular motions are largely restricted to vibration and short-range rotational motions in the glassy state. This can be attributed to the synergistic effects of the reinforcement of chitin whisker and antiplasticization of glycerol in the composites. As the temperature increases, the nanocomposites show two decreases in storage modulus. Moreover, the trend of two-step decreases in E′ values becomes more obvious with increasing chitin content in the SPI matrix. With an increase of temperature, no obvious bump, assigned to increased interaction between SPI molecules resulting from the loss of water,39is observed in the E′-T curves, indicating that the plasticizing effect of water on SPI can be neglected in this DMTA experiment. The
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Figure 6. Temperature dependence of loss peaks (tan δ) for GSPI sheet and SPI/ chitin whiskers composites.
specific area of the chitin whisker fillers is very large; the interfacial interaction should result in a significant increase in storage modulus of the composites. Above Tg, a greater increase in E′ for composites is found with increasing chitin whisker content. For example, the E′ of the composite containing 30 wt % of chitin whiskers is 10 times higher than that of the GSPI at room temperature. Figure 6 shows the temperature dependence of tan δ for the GSPI sheet and the composites. All samples show two transitions over a temperature range of -80 to +100 °C. The transition occurs at lower temperature assigned to the Tg transition of SPI, which shifts from -77 to -22 °C as the chitin content increases from 0 to 30 wt %, indicating that chitin whiskers restrict molecular motions of SPI due to the strong interaction between the two polymers. All of the samples display a broad denaturation transition at about 75 °C, which hardly changes with whiskers content. Meanwhile, no significant broadening of the denaturation relaxation is observed either. The magnitude of the relaxation process, which is related to the modulus drop, decreases as the content of whiskers increases. This is ascribed to both the decrease of relaxing entities participating in the relaxation process and to the concomitant lowering of the modulus upon addition of whiskers.42 Water Resistance. The diffusion of water is strongly influenced by the microstructure of the polymers and plasticizers, which affects gas, water, and solute permeability.37 The water uptake up to equilibrium swelling of the GSPI sheet and the composites is plotted as a function of time in Figure 7. Two well-separated zones are observed at shorter times (zone I: t < 7 h) and longer times (zone II: t > 7 h), respectively. In zone I, a rapid increase in water uptake occurs, whereas in zone II, the absorption rate hardly changes, corresponding to equilibrium swelling. The water uptake at equilibrium for the GSPI sheet and the nanocomposites as a function of chitin whiskers content is plotted in Figure 8. The SPI sheet absorbs about 40% water, whereas the water uptake of the SPI/ chitin nanocomposites decreases with an increasing chitin whisker content, e.g., only about 23% water for SPI-30 composite, indicating an enhancement of water resistivity. Similar results have been reported in cellulose microfiber, or cellulose whisker, filled starch systems, in which the water uptake of
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Figure 7. Water uptake at equilibrium of GSPI sheet (b) and SPI/ chitin whiskers composites of SPI-5(O), SPI-10 (2), SPI-15 (4), SPI20 (0), SPI-25 (9) and SPI-30 (]) composite conditioned at 98 RH% as a function of time.
Figure 9. Young’s modulus (9), tensile strength (O) and elongation at break (4) as a function of chitin whiskers content for GSPI sheet and SPI/chitin whiskers nanocomposites.
Figure 8. Water uptake at equilibrium (b) and water diffusion coefficient (O) as a function of chitin whiskers content for nanocomposites conditioned at 98% RH.
Figure 10. Stress-strain curve of GSPI sheet and SPI/chitin whiskers nanocomposites conditioned at 43% RH.
the starch decreases from 62 to 40 wt % as the content of cellulose whiskers increases from 0 to 25 wt % due to the formation of a rigid cellulose network resulting from threedimensional hydrogen bonding between cellulose whiskers or microfibers during the films formation.37 (Wt - W0)/W∞ was plotted as a function of (t/L2)1/2 for all the composites when (Wt - W0)/W∞ e 0.5. The water diffusion coefficients (D) in the GSPI sheet and the composites were evaluated from the slope of the resulting plot. As shown in Figure 8, the GSPI sheet shows the highest D of about 2.56 × 10-10 cm2 s-1. However, with an increase of chitin whiskers in the SPI matrix from 0 to 30 wt %, the D of the nanocomposites decreases from 2.56 × 10-10 to 1.23 × 10-10 cm2 s-1, which can be ascribed to chitin whisker networks and strong interactions between whiskers and the SPI chains. For a three-dimensional network, the critical volume fraction (VRc) depends on the aspect ratio (L/D ) 10) of the chitin whiskers as VRc ) 0.7/(L/D).43 Substitution of the L/D value of chitin whiskers into that equation yields a value of VRc ) 7 vol %, namely, 10.5 wt %. The water diffusion coefficient decreases at a chitin content higher than 15 wt %, i.e., above the percolation threshold. This suggests that a relatively high filler content results in a dense three-
dimensional hydrogen-bonding network between fillers and between filler and molecular chains of SPI. Mechanical Properties. The tensile strength and Young’s modulus of the composites increase from 3.3 to 8.4 MPa and from 26.4 to 158 MPa with increasing chitin content from 0 to 20 wt %, whereas the elongation at break of the filled composites decreases from 205% to 29% (Figure 9). This indicates that incorporating chitin whisker into the SPI matrix results in strong interactions between whiskers and between whisker and matrix, which restricts the motion of the matrix. The composites containing more than 20 wt % chitin whiskers exhibit a decrease in mechanical properties, due to some phase separation as shown from SEM. The mechanical behavior of SPI/chitin whiskers nanocomposites is different to that of chitin whisker reinforced natural rubber nanocomposites28 which show relatively large changes in Young’s modulus (1.4-10.2 MPa) but the same elongation at break (176-179%) as the chitin whisker content increases from 2 to 20 wt %. All of the samples exhibit elastic nonlinear behavior of typical amorphous polymer at T > Tg. The stress continuously increases with increasing strain until break with no necking (Figure 10). This indicates some isotropy of the composites.28 GSPI and the composites filled with a lower content of chitin display brittle fracture, whereas
Soy Protein Isolate Thermoplastics
the nanocomposites containing more than 15 wt % of chitin exhibit tearing before break,42 indicating stronger interactions with more chitin. Therefore, incorporating chitin whiskers into the SPI matrix provides a novel modification of SPI. Conclusions A suspension of chitin whiskers having an average length of about 500 ( 50 nm and diameter around 50 ( 10 nm was prepared and used as a reinforcing agent for glycerol plasticized SPI to obtain nanocomposites by hot-pressing. Nanocomposites with lower whisker content exhibit a relatively uniform dispersion in the SPI matrix than those with higher chitin content. Compared with a glycerol plasticized SPI sheet, the chitin filled SPI composites increase in Young’s modulus and tensile strength from 26 to 158 MPa and 3.3 to 8.4 MPa with increasing chitin content from 0 to 20 wt %. As the chitin whiskers increase in the SPI matrix, the composites show greater water-resistance. The improvement in all of the properties of these novel SPI/chitin whisker nanocomposites may be ascribed to three-dimensional networks of intermolecular hydrogen bonding interactions between filler and filler and between filler and SPI matrix. Acknowledgment. This work was supported by the National Natural Science Foundation (59773026 and 59933070), as well as Major Grant of Science and Technology from Hubei Province and the Laboratory of Cellulose and Lignocellulosic Chemistry of the Chinese Academy of Sciences. References and Notes (1) Villar, M. A.; Thomas, E. L.; Armstrong, R. C. Polymer 1995, 36, 1869-1876. (2) Lu, Y.; Zhang, L. Ind. Eng. Chem. Res. 2002, 41, 1234-1241. (3) Li, F.; Larock, R. C. J. Appl. Polym. Sci. 2001, 80, 658-670. (4) Li, F.; Hanson, M.; Larock, R. C. Polymer 2001, 42, 1567-1579. (5) Lu, Y.; Zhang, L.; Zhang, X.; Zhou, Y. Polymer 2003, 44, 66896696. (6) Lu, Y.; Zhang, L. Polymer 2002, 34, 3979-3986. (7) Shin, J. H.; Kondo, T. Polymer 1998, 39, 6899-6904. (8) Mathew, A.; Dufresne, A. Biomacromolecules 2002, 3, 1101-1108. (9) Nielson, N. C. In New Protein Foods; Altschul, A. M., Wilcke, H. L, Eds.; Academic Press: New York, 1985; Vol. 5, pp 27-64.
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BM034516X
