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Physicomechanical and Thermal Properties of Jute-Nanofiber-Reinforced Biocopolyester Composites Kunal Das,† Dipa Ray,*,† Chitrita Banerjee,† N. R. Bandyopadhyay,‡ Saswata Sahoo,§ Amar K. Mohanty,§ and Manjusri Misra| Department of Polymer Science & Technology, UniVersity College of Science and Technology, UniVersity of Calcutta, 92 A.P. C Road, Kolkata 700009, India, School of Materials Science and Engineering, Bengal Engineering and Science UniVersity, Shibpur, Howrah 711103, India, and Bioproducts DiscoVery and DeVelopment Centre, Department of Plant Agriculture, Crop Science Building, and School of Engineering, Thornbrough Building, UniVersity of Guelph, Guelph N1G2W1, ON, Canada
Jute nanofibers (JNFs) were prepared by treating jute fibers with alkali and dimethyl sulfoxide (DMSO) and then applying acid hydrolysis and were characterized by transmission electron microscopy, scanning electron microscopy, atomic force microscopy, and X-ray diffraction. The JNFs exhibited both spherical an elliptical shapes, with diameter in the range of 50-120 nm. Biocopolyester matrix was reinforced with JNFs at three different loadings (5, 10, and 15 wt %), and the JNF-loaded biocomposites were characterized by X-ray diffraction, tensile testing, differential scanning calorimetry, and moisture uptake. The enhancement in properties was highest for 10 wt % JNF-loaded composites, indicating the most uniform dispersion in this material. Introduction Nature inspires us to develop new high-performance materials from its renewable resources. Natural fibers can be used as precursors for the preparation of crystalline micro-/nanoparticles having wide ranges of properties. Many researchers have prepared micro-/nanocrystalline cellulose particles from different agricultural sources by various physical and chemical treatments1-13 and used them as reinforcing fillers in polymer matrixes such as plasticized starch, poly(lactic acid), and poly(vinyl alcohol). The resulting materials were reported to exhibit significant improvements in characteristics such as mechanical, dynamic mechanical, thermal, barrier, and moisture absorption properties. Poly(vinyl alcohol) biocomposites reinforced with cellulose microfibrils isolated by high-intensity ultrasonication were prepared by Cheng et al.1 Ultrafine cellulosic nanofibers were prepared by enzymatic an hydrolysis route by Pa¨a¨kko¨ et al.3 Bondeson et al.5 detailed the optimization of nanofibril preparation by an acid hydrolysis route. Highly crystalline cellulose nanoparticles have also been prepared by some researchers through an enzymatic hydrolysis route.6,7 Tang et al.13 evaluated the changes in microcrystalline cellulose particles prepared from different sources by dilute acid hydrolysis. Choi and Simonsen14 prepared cellulose nanocrystals from cotton using 65% sulfuric acid followed by neutralization and sonication. The reinforcing potential of cellulose nanofibers in a starch-based thermoplastic polymer was investigated by Alemdar and Sain.15 Polymer nanocomposites with nanowhiskers isolated from microcrystalline cellulose (MCC) were prepared by Capadona et al.16 They observed a high reinforcing effect for the very first time by dint of nanowhisker fractionation and the template approach. Nakagaito et al.17 reported that the modulus, strength, and strain at fracture increased linearly with the microfibrillated cellulose (MFC) content. In the case of cellulose-based nanocomposites, a large reinforcing effect was * To whom correspondence should be addressed. Tel.: +91-0332350 1397. Fax: +91-033-2351 9755. E-mail: roy.dipa@ gmail.com. † University of Calcutta. ‡ Bengal Engineering and Science University. § Department of Plant Agriculture, University of Guelph. | School of Engineering, University of Guelph.
observed by Dalmas et al.18 Li et al.19 used staple cotton fibers for the preparation of cellulose nanocrystals. They first treated the cotton fibers with chemicals such as dimethyl sulfoxide (DMSO) and sodium hydroxide and then subjected the chemically treated fibers to acid hydrolysis. The main objective of such prior chemical treatment with sodium hydroxide and DMSO was to swell the cotton fibers so that the hydrolyst could diffuse into the fibers more easily for an effective break down from the macroscopic to the nanoscopic level. The cellulose nanofibers, when dried from aqueous suspensions, have a strong tendency to reagglomerate because of their high surface energy. They are therefore freeze-dried instead of oven-dried to minimize this reagglomeration.20 We report here the preparation of jute nanofibers (JNFs) by an acid hydrolysis route and their characterization by transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and nanoindentation. These jute nanofibers were incorporated as a filler in a biocopolyester matrix, and the resulting biocomposites were characterized by XRD, mechanical testing, differential scanning calorimetry (DSC), and water absorption. Experimental Section Materials. Jute felts were collected from the local market. The biocopolyester poly(tetramethylene adipate-co-terephthalate) (Eastar Bio GP copolyester, Eastman Company U.S.A. Limited) was used as the matrix material. DMSO and concentrated sulfuric acid (laboratory-grade, Merck) were used for the preparation of JNFs. Preparation of Jute Nanofibers. Jute felts (JFs) were cut to 3-4 cm in length, weighed, and finally shredded in a mixer. A weighed amount was soaked in 5 M NaOH solution and heated at 80 °C for about 2 h. The resultant mass was washed five to six times with demineralized water and was neutralized with 10% H2SO4 solution, after which it was dried in an oven at 70-80 °C for 24 h. The dried mass was then dipped in DMSO (liquor ratio 1:20), heated at 70 °C on a water bath, washed several times with distilled water, and then oven-dried at 70-80
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°C for 3 h.19 Finally, it was acid hydrolyzed with 47% H2SO4 solution to obtain JNFs following the standard procedure of Dong et al.21 The JNFs were then freeze-dried at -110 °C for 2 days. After the NaOH and DMSO treatments mentioned above, weight losses of 30% and 50%, respectively, were observed with respect to the weight of the starting material. After acid hydrolysis, the weight loss was 60% with respect to the weight of jute fibers taken initially. Thus, about 40% of the initial weight of the jute fibers was finally obtained as JNFs. Prior treatment with NaOH and DMSO removed the hemicellulose fraction, swelled the jute fibers, and facilitated breakdown of the material during acid hydrolysis. Acid hydrolysis of cellulose leads to hydrolytic cleavage of the glycosidic bond between two anhydroglucose units. Thus, the amorphous portion is dissolved by acid hydrolysis, leaving behind the crystalline regions. Acid hydrolysis followed by mechanical treatment results in the disintegration of the cellulose structure into microand nanocrystalline form.22 Preparation of Jute-Nanofiber-Reinforced Bionanocomposites. The prepared JNFs were incorporated into the biocopolyester (BCP) matrix by melt mixing in varying weight percentages ranging from 0% to 15%. Melt mixing was done in a Brabender 30/50 E apparatus. The resultant mass was then compression molded to form sheets. The biocomposites containing 0, 5, 10, and 15 wt % JNFs are denoted as BCP_JNF 0, BCP_JNF 5, BCP_JNF 10, and BCP_JNF 15, respectively. Characterization. The prepared JNFs were subjected to TEM analysis with a Technai G2 Spirit BioTWIN electron microscope (FEI) with a voltage range of 210-240 V and a frequency of 50-60 Hz operated with a thermo ionic tungsten electron gun at 80 kV. A drop of aqueous suspension of JNFs was poured onto a carbon-coated Cu grid (300 mesh) with a micropipet and dried prior to TEM examination. FE-SEM (model JEOL JEM6700F) observation of the JNFs was done by pouring a drop of aqueous nanofiber suspension onto a glass coverslip and coating it with platinum. The JNFs were characterized using a Veeco MultiMode scanning probe microscope with a Nanoscope IIIa controller. Images were collected using tapping mode with a phosphorus-doped silicon tip (model RTESP) at a nominal frequency of 312 kHz. A droplet of the aqueous nanofiber suspension was dried on a glass coverslip prior to AFM examination. The XRD analysis of the freeze-dried JNF powder was done with an X’Pert PRO model Rigaku MiniFlex instrument at a scanning rate of 4°/min with Cu KR radiation at 45 kV and 40 mA. Pellets of JNFs were prepared for nanoindentation tests. To make the pellets, a measured amount of JNF powder was compacted in a metal mold in a compression molding machine, as described in detail in our previous work.23 Nanoindentation was carried out in a CSM Instruments apparatus. Nanoindentation was done at eight different places in each pellet, and the reported values are the averages of eight results. The tensile properties of the bionanocomposite samples were investigated as in accordance with test method ASTM 638 with a crosshead speed of 5 mm/min, and the mean of at least five samples was reported for each set. Differential scanning calorimetry of the composite samples was done with a DSC Q200 instrunment at a heating rate of 5 °C/min in a nitrogen environment in the temperature range from -50 to 150 °C. The nitrogen flow rate during the test was 50 mL/min. The water uptake test was carried out by immersing the samples in water; each data point is the mean of three samples. Results and Discussion JNF Characterization. The jute nanofibers (JNFs) prepared by acid hydrolysis were examined by TEM to determine their
Figure 1. TEM image of the jute nanofibers.
Figure 2. FE-SEM images of the jute nanofibers.
size and shape (Figure 1). The nanofibers exhibited an elliptical shape with a diameter of 146 nm and a length of 950 nm. FESEM images of the jute nanofibers are shown in Figure 2. The particles were spherical as well as elliptical. Most of the particles were less than 100 nm in diameter, whereas some exhibited a size slightly higher than 100 nm. The AFM images of the JNFs shown in Figure 3a,b indicate an average diameter of 80 nm. The JNF dispersion was subjected to freeze-drying. The XRD patterns of the jute precursor and the JNFs are shown in Figure 4. The large difference in the XRD graphs is due to the removal of the amorphous portions of the jute fibers. Hemicelluloses make up part of the amorphous material in jute, and they affect the background shape of the powder diffraction pattern. It is evident from Figure 4 that the intensity of the broad peak due to the 101 and 101 planes of cellulose I that appeared in the range of 14.9-16.3° in the JFs diminished significantly in the JNFs. The sharp peak at 22.2° in the JFs was due to the 002 plane of cellulose I, and this peak also decreased in the JNFs. A very sharp peak appeared at 19° in the JNFs due to the 101 plane of cellulose II, but it was absent in the JFs. These results indicate that the cellulose I structures were decreased in the JNFs as a result of different chemical treatments followed by acid hydrolysis, and consequently, the cellulose II structures became prevalent.24,25 Similar results have been reported by other researchers as well.26 On the other hand, many sharp peaks appeared in the JNFs, which signifies that many new crystalline
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Figure 3. AFM images of the jute nanofibers.
regions were generated and that the amorphous portions were removed. The crystallite size was calculated using the Scherrer equation Lh,k,l ) Kλ/(β cos θ)
(1)
where K ) 0.94,27 based on the full width at half-maximum of the 101 and 002 reflections. The percent crystallinity was also calculated according to the formula28 crystallinity (%) )
(I002 - Iam) × 100% I002
(2)
The percent crystallinity and the crystallite sizes perpendicular to the 22.2° and 19° peaks are summarized in Table 1. It can be observed that the percent crystallinity with respect to the 002 plane (22.2°) decreased slightly in the JNFs and there was a slight increase in the crystallite size in comparison to that of jute fibers. The 101 plane of cellulose II became prominent at
Figure 4. XRD patterns of the jute fibers and jute nanofibers.
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Table 1. Properties of JNFs Obtained by XRD and Nanoindentation Study XRD analysis peak at 2θ ) 22.2°
peak at 2θ ) 19°
nanoindentation test
sample
crystallinity (%)
crystallite size (nm)
crystallinity (%)
crystallite size (nm)
modulus (GPa)
hardness (Vickers)
jute fibers JNFs
77 75
3.5 4.5
84
27.87
7.27 (std dev ) 2.64)
64.8 (std dev ) 39.42)
2θ ) 19° with large crystallites (27.8 nm) in the JNFs. The nanoindentation curve of a JNF pellet is shown in Figure 5. The modulus and hardness values of the JNFs, obtained by the nanoindentation technique, are summarized in Table 1. Nanoindentation was done at eight different places on the JNF pellet, and the mean of all results was reported. The JNFs showed a high modulus of 7.27 GPa and a Vickers hardness number of 64.8. Characterization of the Biocomposites. As jute fibers contain a large number of hydroxyl groups in their structures, they will have a good interaction with the biocopolyester matrix, which contains polar ester linkages in its repeat units. The probable chemical interaction of the polymer with the JNFs is shown in Figure 6. The XRD graphs of the biocomposites are shown in Figure 7. The addition of JNFs resulted in a decrease of the percent crystallinity from 59% in the unreinforced sample to 12% in BCP_JNF 5, 23% in BCP_JNF 10, and 30% in BCP_JNF 15. Thus, it was observed that addition of JNFs first led to a decrease in the degree of crystallinity at 5% loading, and then the percent crystallinity increased progressively with increasing JNF content. This initial decrease can be attributed to a lowering of the order in the arrangement of the chains in the presence of JNFs. Then, the progressive increase in crystallinity with increasing JNF content can be ascribed to the anchoring effect of cellulosic
nanoparticles, probably acting as nucleating agents for the biocopolyester matrix.
Figure 7. XRD graphs of the biocomposites.
Figure 5. Nanoindentation curve of a jute nanofiber pellet.
Figure 6. Chemical interaction between biocopolyester matrix and JNFs.
Figure 8. Water uptake behaviors of the unreinforced and reinforced biocomposite samples.
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Figure 9. (a) Tensile strength, (b) tensile modulus, and (c) breaking energy of the unreinforced and JNF-reinforced biocomposites.
The biocomposite samples were subjected to water immersion tests to investigate their water absorption properties. The increases in the weights of the samples due to immersion in water are shown in Figure 8. From Figure 8, one can see that the unloaded sample showed a negligibly small increase in weight after 24 h, followed by a loss in weight, which might be due to its dissolution in water. In contrast, incorporation of JNFs provided a stabililization effect to the matrix. This phenomenon can be ascribed to the presence of a threedimensional cellulosic network that strongly restricts the dissolution of the polymeric matrix in water. However, a swelling was observed up to 100 h, and then the increase in weight reached a constant value (Figure 8). The percent swelling was lowest in BCP_JNF 10 compared to the other samples, indicating a lower availability of free -OH groups and a higher fraction of polymer matrix entrapped within the cellulosic network and on the surface of the nanofibers. This could be due to the uniform dispersion of the JNFs in the matrix. In BCP_JNF 15, there was an increase in swelling compared to BCP_JNF 10, which might be due to overlapping of the jute nanofibers restricting the filler/matrix interfacial area and decreasing the entrapping matrix fraction due to a densification of the nanofiber network.
The tensile properties of the JNF-reinforced biocopolyester composites are shown in Figure 9a-c. It can be observed in Figure 9a that the tensile strength was increased by 20% and 19.5% in BCP_JNF 10 and BCP_ JNF 15, respectively, compared to that of the unreinforced sample (BCP_ JNF 0). However, a 19.6% decrease in tensile strength was observed in BCP_ JNF 5 compared to BCP_ JNF 0. At 5 wt % loading, the fillers might play the role of impurities, raising the stress concentration points and initiating fracture from these points. Figure 9b shows the change in tensile modulus with increasing JNF content in the composites. The tensile modulus was increased by 17%, 53%, and 77% for BCP_JNF 5, BCP_JNF 10, and BCP_JNF 15, respectively, compared to that of BCP_JNF 0. This increase can be attributed to the restricted Table 2. DSC Results for JNF-Reinforced Biocomposites Obtained from Cooling and Second Heating Curves cooling curve sample BCP_JNF BCP_JNF BCP_JNF BCP_JNF
0 5 10 15
second heating curve
Tc (°C)
Tg (°C)
Tm (°C)
∆Hm (J/g)
76 86 87 87
2.59 2.54 -1.32 3.08
115.8 118.6 119.5 120.2
10.7 8.6 6.6 6.9
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Figure 10. DSC curves of the biocomposites: (a) first heating curve, (b) cooling curve, and (c) second heating curve.
mobility of the polymer chains due to higher contact with the JNFs, which increased the rigidity of the composites. The variation of the breaking energy of the composites as a function of JNF content is shown in Figure 9c. The breaking energy value increased significantly in BCP_JNF 10 by 26%, whereas in BCP_JNF 15, the increase was only 7%. In contrast, in BCP_JNF 5, the breaking energy decreased by 38%. Such an increase in breaking energy in BCP_JNF 10 can be attributed to a strong filler/ matrix interaction throughout the matrix, which indicates a uniform
dispersion of the JNFs in this material, resulting in an overall improvement of properties. The biocomposite samples were subjected to heating, cooling, and second heating cycles in DSC analysis in the temperature range from -50 to 150 °C to determine the thermal transitions in the samples. The crystallization temperature (Tc), glass transition temperature (Tg), melting temperature (Tm), and melting endotherm (∆Hm) are summarized in Table 2 based on cooling and second heating cycles, which are shown in Figure
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development of green nanofillers that can be used to develop new materials holding a great deal of promise for the future. Acknowledgment D.R. is thankful to UGC (University Grants Commission), Government of India, for granting her a Major Research Project. K.D. is thankful to UGC for giving him a fellowship. M.M. and A.K.M. are thankful to the University of Guelph, The Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) 2009 Bioeconomy-Industrial Uses Research Program, for partial financial support. Literature Cited
Figure 11. Surface morphology of BCP_JNF 10 showing a uniform dispersion of JNFs.
10. Upon cooling, all of the samples exhibited crystallization showing an exothermic peak, but the incorporation of JNFs significantly influenced the crystallization temperature. The crystallization began at a higher temperature (86 °C) in all of the JNF-reinforced biocomposites compared to that in the unreinforced sample (76 °C), confirming the fact that the JNFs played the role of nucleating agent and facilitated crystallization. The Tg values of the biocomposite samples, as obtained from the second heating curve, were observed at 2.59, 2.54, -1.32, and 3.08 °C for BCP_JNF 0, BCP_JNF 5, BCP_JNF 10, and BCP_JNF 15, respectively. The lowest Tg value of BCP_JNF 10 suggests that the overall translational and rotational motion of the macromolecules in the amorphous region began at a lower temperature in BCP_JNF 10 compared to the other samples. This is also reflected in its higher breaking energy value. On the other hand, the melting point of the crystalline portion of BCP_JNF 10 shifted to a higher temperature (119.5 °C). The greater flexibility of the amorphous portions along with the higher rigidity of the crystalline fraction might be responsible for BCP_JNF 10 having the best mechanical properties. The decrease in the ∆Hm values of the JNF-reinforced biocomposites in comparison to that of the unreinforced sample indicates a lowering of crystallinity, which fully corroborates the XRD observations. The increase in tensile strength, tensile modulus, and breaking energy values; the decrease in percent swelling upon water immersion; and the shift of Tg and Tm indicate that the most uniform dispersion of the JNFs occurred in BCP_JNF 10, which resulted in an optimum balance of properties. This is also supported by Figure 11, which shows the uniform dispersion of JNFs in the matrix in BCP_JNF 10. Conclusions Spherical- and elliptical-shaped jute nanofibers were extracted from jute fibers by acid hydrolysis and were incorporated as a reinforcing filler in a biocopolymer matrix by the melt mixing method. TEM revealed that the JNFs had diameters in the range of 50-120 nm. XRD showed that the JNFs were much more crystalline than their precursor. The biocomposites exhibited improved mechanical properties. The 10 wt % JNF-loaded composite showed the highest enhancement in overall properties, indicating a strong filler/matrix interaction throughout the matrix. Thus, the conversion of jute fibers into JNFs can lead to the
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ReceiVed for reView October 14, 2009 Accepted January 28, 2010 IE9019984
