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Hydrophobization of starch nanocrystals through esterification in green media
Industrial Crops and Products 59 (2014) 115–118
Contents lists available at ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Short communication
Hydrophobization of starch nanocrystals through esterification in green media Lili Ren a,b , Zhao Dong a , Man Jiang b , Jin Tong a , Jiang Zhou a,∗ a b
Key Laboratory of Bionic Engineering (Ministry of Education), College of Biological and Agricultural Engineering, Jilin University, Changchun 130022, China College of Chemistry, Jilin University, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 29 January 2014 Received in revised form 29 April 2014 Accepted 8 May 2014 Keywords: Starch nanocrystals Hydrophobization Esterification modification Alkenyl succinic anhydride Acetic anhydride
a b s t r a c t Starch nanocrystals (SNC) were successfully modified through esterification by using dodecenyl succinic anhydride (DDSA), 2-octen-1-ylsuccinic anhydride (OSA) and acetic anhydride (AA) in green media. The chemical modifications were confirmed using FT-IR. The esterification extent of the modified SNC was evaluated by degree of substitution (DS). The modifications imparted hydrophobic character to SNC so that the modified SNC could be dispersed in non-polar solvents such as chloroform, dichloromethane and toluene. It was found that the dispersibility of the modified SNC in non-polar solvents increased with increasing of DS and hydrophobicity of the group replacing the hydroxyl groups in starch. XRD analysis showed that the crystalline structure of SNC was preserved after the modifications. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Starch nanocrystals (SNC), isolated from starch granules by mild acid hydrolysis or enzymatic hydrolysis, have attracted significant interest over the past years (Le Corre et al., 2010; Lin et al., 2012). The unique characteristics, such as biodegradability, impressive mechanical properties, low density, and low permeability make SNC ideal candidate as reinforcement for fabrication of polymer nanocomposites to meet increasing demand for products made from renewable resources (Angellier et al., 2005a, 2006; García et al., 2009; Kristo and Biliaderis, 2007). However, the polar surface and hydrophilicity of SNC result in their poor dispersibility in non-polar solvents and poor compatibility with hydrophobic polymers, which limit processing of SNC reinforced nanocomposites and the function of mechanical reinforcement from SNC. Therefore, one of the main challenges with SNC application in nanocomposites is related to their homogeneous dispersion within polymeric matrix. Over the past decade, much effort has been devoted to modify SNC for nanocomposite applications. Modifications of SNC have been carried out via grafting polymers (Labet et al., 2007; Namazi and Dadkhah, 2008; Song et al., 2008; Thielemans et al., 2006) and reacting with various reagents (Namazi and Dadkhah, 2010;
∗ Corresponding author. Tel.: +86 431 85095760x414; fax: +86 431 85095760x888. E-mail address: [email protected] (J. Zhou). http://dx.doi.org/10.1016/j.indcrop.2014.05.014 0926-6690/© 2014 Elsevier B.V. All rights reserved.
Valodkar and Thakore, 2010). Esterification modifications of SNC with alkenyl succinic anhydrides (ASAs) and acetic anhydride (AA) were reported (Angellier et al., 2005b; Xu et al., 2010). However, most of these modifications used toxic and expensive solvents as reaction medium and required prolonged reaction time. The objective of this work was to modify SNC more environmentally friendly and efficiently through esterification using ASAs and AA in green media to improve dispersibility of SNC in non-polar solvents or hydrophobic polymers so that the application range of SNC as reinforcements in nanocomposites could be broadened. 2. Experimental Waxy maize starch was supplied by Changchun Jincheng Corn Development Co. Ltd., Da Cheng Group (China). Dodecenyl succinic anhydride (DDSA) and 2-octen-1-ylsuccinic anhydride (OSA) were purchased from Sigma-Aldrich (St. Louis, USA). Acetic anhydride (AA) and other chemicals were obtained from Beijing Beihua Fine Chemicals Co. Ltd. (China). All of these chemicals were used as received. One gram of SNC, prepared by acid hydrolysis of waxy maize starch according to the optimum condition determined by Angellier et al. (2004), was dispersed in 30 mL reaction medium (water, ethanol or water/ethanol mixture) using ultrasound and stirring for 30 min to obtain SNC suspension. After pH of the suspension was adjusted with NaOH aqueous solution (0.1 mol L−1 ), one gram of DDSA or OSA (diluted five times with ethanol, v/v) was added over 5 min. The reaction was allowed to last 1 h under stirring at
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Fig. 1. FT-IR spectra of unmodified SNC (a), DDSA modified SNC in water (pH 8.0–8.5) (b), in water/ethanol mixture (pH 8.0–8.5) (c) and in ethanol (pH 8.0–8.5) (d), OSA modified SNC in water/ethanol mixture (pH 8.5–9.0) (e).
Fig. 2. FTIR spectra of unmodified SNC (a), AA modified SNC (AA/SNC = 10/1, mL/g) at 30 ◦ C (b), AA modified SNC (AA/SNC = 30/1, mL/g) at 30 ◦ C (c), and AA modified SNC (AA/SNC = 30/1, mL/g) at 50 ◦ C (d).
35 ◦ C and selected pH maintained by adding NaOH aqueous solution. Upon completion of the reaction, pH was neutralized to 7 with HCl aqueous solution (0.1 mol L−1 ). For the AA modification, the reaction medium was 36 wt% acetic acid aqueous solution, and 10 mL mixture of H2 SO4 and acetic acid (10/90, v/v) was used as catalyst. After a certain amount of AA was added, the reaction was allowed to last 3 h under stirring at different temperatures. All the products were first washed by centrifugation thrice with 70% (v/v) ethanol aqueous solution (for the DDSA and OSA modifications) or water (for the AA modification), then thrice with ethanol to remove all traces of residual reagents and dried at room temperature. The modified SNCs were characterized with FT-IR (IRAffinity-1 spectrophotometer, Shimadzu, Japan). The specimen was prepared by grinding SNC sample together with KBr and then pressed into a disk. The resolution was 4 cm−1 and the total scans were 32. Degree of substitution (DS) refers to the average number of the hydroxyl groups substituted per D-anhydroglucose unit in starch, the maximum possible DS is 3.0. DS of the ASA (DDSA or OSA) modified SNCs was determined using the method of Jeon et al. (1999). DS of the AA modified SNC was determined by saponifying acetate groups (Miladinov and Hanna, 2001). Wettability test was used to assess surface polarity of SNCs. 5 mg of sample was mixed with two immiscible solvents (5 mL each) having different polarities and densities in a vial to observe the wettability of the sample with the solvents. Crystalline structure of SNCs was characterized by using a Rigaku D/max-2500 X-ray diffractometer (Rigaku Corporation, ˚ at 40 kV and Tokyo, Japan) with Cu-Ka radiation ( = 1.542 A) 250 mA. The X-ray diffraction patterns were recorded over the 2 range of 3–35◦ at a speed of 1◦ /min. Relative crystallinity was determined from the ratio of the areas of the diffraction peaks to the area of the whole diffraction pattern subtracted amorphous background patterns.
DS value was used to evaluate esterification extent of modified SNCs. As presented in Table 1, the modification with DDSA in the water/ethanol mixture (when pH > 8.0) yielded higher DS than that in water or ethanol, meaning there are synergistic interactions between water and ethanol which increase DS. It is known that SNC swell in water and water can increase mobility of starch molecules, while ethanol can dilute DDSA and help it diffuse into SNC. The water/ethanol mixture brought these factors together and therefore increased probability of the reactive moieties combining and resulted in higher esterification extent of SNC. Data in Table 1 also showed that DS of the DDSA modified SNC increased with increasing of pH in the reaction system and attained the maximum value when the pH ranged in 8.5–9.0. The results could be explained by the fact that higher pH values (>9.0) favor anhydride hydrolysis whereas lower pH values (<8.5) are not sufficiently to activate the hydroxyl groups in starch for nucleophilic attack of the anhydride moieties. Generally, DS and length of the alkenyl group determine the hydrophobicity of ASA modified SNC. In place of DDSA (C12 ), OSA (C8 ) was used to modify SNC. It was found that, reacted in the water/ethanol mixture with pH of 8.5–9.0 and temperature of 35 ◦ C for 1 h (weight ratio of OSA/SNC was 1), the DS of the OSA modified SNC was 0.034 which was higher than the DDSA modified one (0.021), indicating that modification of SNC with ASA having shorter chain of alkenyl group was more easily to yield a high DS. This is because, with decrease of the length of the alkenyl group, diffusion capability of ASA toward SNC increases and space baffle effect which can interrupt the reactive moieties combining decreases. Table 2 listed DS values of the AA modified SNCs with different amounts of AA at various reaction temperatures. The increase of AA amount from 10 to 30 (in proportion to SNC, mL/g) gave rise to a significant increase in DS. An increase of reaction temperature (below gelatinization temperature of starch) resulted in increased
3. Results and discussion Figs. 1 and 2 showed FT-IR spectra of the ASA and AA modified SNCs. Comparing with the spectrum of the unmodified SNC, there was a clear new peak which was located at 1732 cm−1 in the spectra of the ASA modified SNCs or located at 1722 cm−1 in the spectra of the AA modified SNCs. Since this peak is associated with carbonyl stretching from esters (Biswas et al., 2008) and the modified SNCs were thoroughly washed to remove the unbound anhydrides, this ester carbonyl bands peak is the evidence that esterification modification of SNC with ASA or AA occurred.
Table 1 DS of the modified SNCs using DDSA in different media with various pH at 35 ◦ C for 1 h. pH
7.5–8.0 8.0–8.5 8.5–9.0 9.0–9.5
Water
Ethanol
Water/ethanol (50/50, in volume)
0.004 0.013 0.017 0.015
0.005 0.012 0.016 0.014
0.005 0.017 0.021 0.019
L. Ren et al. / Industrial Crops and Products 59 (2014) 115–118
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Table 2 DS of the modified SNCs using different amounts of AA in 36 wt% acetic acid aqueous solution at various reaction temperatures for 3 h. Reaction temperature (◦ C)
AA/SNC (mL/g) 10/1
AA/SNC (mL/g) 30/1
30 40 50
0.041 0.078 0.196
0.083 0.169 0.558
DS. Comparing with the DS of the acetylated SNC prepared in the medium of glacial acetic acid and methanesulphonic acid for 5 h (Xu et al., 2010) and acetylated starch prepared by using AA and alkaline catalyst in aqueous medium (Wang and Wang, 2002), the preparation of the AA modified SNCs in this work was more efficiently. Wettability test is a simple and valuable method to qualitatively assess surface polarity of SNC (Angellier et al., 2005b; Namazi and Dadkhah, 2010). In this work, three solvent systems were used, namely, water/chloroform (d = 1.47 g cm−3 ), water/dichloromethane (d = 1.335 g cm−3 ) and toluene (d = 0.866 g cm−3 )/water. The polarities of water, chloroform, dichloromethane and toluene are 10.2, 4.4, 3.4 and 2.4, respectively. As showed in Fig. 3a, the unmodified SNC remained in water even after shaking the container; the DDSA modified one (DS = 0.021) could migrate into chloroform, dichloromethane and toluene (Fig. 3b–d) although more and more modified SNC stayed in water with decreasing of polarity of the organic solvent. In toluene/water system, more DDSA modified SNC stayed in water and more OSA modified SNC could migrate into toluene (see Fig. 3d and e), indicating the OSA modified SNC with DS of 0.034 has higher hydrophobicity than the DDSA modified one with DS of 0.021. As shown in Fig. 4, with increase of DS, more and more AA modified SNC migrated toward chloroform. Similar trend was also observed for DDSA modified SNC. This is because higher DS means more hydroxyl groups in starch molecules have been substituted by carbonyl group or alkenyl groups, which result in lower polar nature of the modified SNC. Comparing the dispersions of the DDSA modified SNC (DS = 0.021) and AA modified one (DS = 0.041) in water/chloroform system (Figs. 3b and 4a), it could be concluded that the hydrophobicity of the DDSA modified one is higher than the AA modified one even its DS is only half of the AA modified one. The reason is the hydrophobicity of the alkenyl group in DDSA is higher than that of the carbonyl group in AA. Dispersions of the DDSA modified SNC (DS = 0.021), OSA modified one (DS = 0.034) and AA modified one (DS = 0.558) in toluene/water system (Figs. 3d, e and 4d) indicated that the OSA modified one had higher hydrophobicity than the other two, suggesting that hydrophobicity of the modified SNC
Fig. 4. Dispersions of AA modified SNC in water/chloroform (DS = 0.041) (a), in water/chloroform (DS = 0.196) (b), in water/chloroform (DS = 0.558) (c) and in toluene/water (DS = 0.558) (d).
Fig. 5. X-ray diffraction patterns of unmodified SNC (a), DDSA modified SNC (DS = 0.021) (b), OSA modified SNC (DS = 0.034) (c), AA modified SNC (DS = 0.083) (d), AA modified SNC (DS = 0.558) (e).
depends on both DS and the nature of the groups replacing the hydroxyl groups. Therefore, affinity of esterification modified SNC for non-polar solvents or polymers could be regulated by controlling DS and choosing groups to substitute the hydroxyl groups. Fig. 5 showed the XRD patterns and relative crystallinity data of the unmodified SNC and some modified ones with different DS. The unmodified SNC showed a typical A-type crystalline structure and this crystalline structure was preserved after the modifications. Since the crystalline structure of SNC was not disrupted and hydrophobicity of SNC was significantly enhanced after the modifications, the modified SNCs can be used as reinforcements to prepare hydrophobic polymer based nanocomposites and those needing non-polar solvents for casting fabrication.
4. Conclusions
Fig. 3. Dispersions of unmodified SNC in water/chloroform (a), DDSA modified SNC (DS = 0.021) in water/chloroform (b), in water/dichloromethane (c) and in toluene/water (d), OSA modified SNC (DS = 0.034) in toluene/water (e).
SNC were modified through esterification with DDSA, OSA and AA. The hydrophobicity of the modified SNCs increased with increasing of DS and hydrophobicity of the group replacing the hydroxyl groups. The crystalline structure of SNC was preserved after the modifications. The presented work provides a route to improve dispersion of SNC in non-polar solvents or hydrophobic polymers and broaden application range of SNC as reinforcements in nanocomposites.
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Acknowledgements The authors are grateful to the National Natural Science Foundation of China (No. 51273083), Natural Science Foundation of Jilin Province of China (No. 20101538), Plan of Science and Technology Development of Jilin Province of China (No. 20110722) and “985 Project” of Jilin University for financial support. References Angellier, H., Choisnard, L., Molina-Boisseau, S., Dole, P., Dufresne, A., 2004. Optimization of the preparation of aqueous suspensions of waxy maize starch nanocrystals using a response surface methodology. Biomacromolecules 5, 1545–1551. Angellier, H., Molina-Boisseau, S., Dufresne, A., 2005a. Mechanical properties of waxy maize starch nanocrystal reinforced natural rubber. Macromolecules 38, 9161–9170. Angellier, H., Molina-Boisseau, S., Belgacem, M.N., Dufresne, A., 2005b. Surface chemical modification of waxy maize starch nanocrystals. Langmuir 21, 2425–2433. Angellier, H., Molina-Boisseau, S., Dole, P., Dufresne, A., 2006. Thermoplastic starch–waxy maize starch nanocrystals nanocomposites. Biomacromolecules 7, 531–539. Biswas, A., Shogren, R.L., Selling, G., Salch, J., Willett, J.L., Buchanan, C.M., 2008. Rapid and environmentally friendly preparation of starch esters. Carbohydr. Polym. 74, 137–141. García, N.L., Ribba, L., Dufresne, A., Aranguren, M., Goyanes, S., 2009. Physicomechanical properties of biodegradable starch nanocomposites. Macromol. Mater. Eng. 294, 169–177. Jeon, Y.S., Viswanathan, A., Gross, R.A., 1999. Studies of starch esterification: reactions with alkenyl-succinates in aqueous slurry systems. Starch/Starke 51, 90–93.
Kristo, E., Biliaderis, C.G., 2007. Physical properties of starch nanocrystal-reinforced pullulan films. Carbohydr. Polym. 68, 146–158. Labet, M., Thielemans, W., Dufresne, A., 2007. Polymer grafting onto starch nanocrystals. Biomacromolecules 8, 2916–2927. Le Corre, D., Bras, J., Dufresne, A., 2010. Starch nanoparticles. A review. Biomacromolecules 11, 1139–1153. Lin, N., Huang, J., Dufresne, A., 2012. Preparation, properties and applications of polysaccharide nanocrystals in advanced functional nanomaterials: a review. Nanoscale 4, 3274–3294. Miladinov, V.D., Hanna, M.A., 2001. Temperatures and ethanol effects on the properties of extruded modified starch. Ind. Crops Prod. 13, 21– 28. Namazi, H., Dadkhah, A., 2008. Surface modification of starch nanocrystals through ring-opening polymerization of -caprolactone and investigation of their microstructures. J. Appl. Polym. Sci. 110, 2405–2412. Namazi, H., Dadkhah, A., 2010. Convenient method for preparation of hydrophobically modified starch nanocrystals with using fatty acids. Carbohydr. Polym. 79, 731–737. Song, S.W., Wang, C., Pan, Z.L., Wang, X.F., 2008. Preparation and characterization of amphiphilic starch nanocrystals. J. Appl. Polym. Sci. 107, 418– 422. Thielemans, W., Belgacem, M.N., Dufresne, A., 2006. Starch nanocrystals with large chain surface modifications. Langmuir 22, 4804–4810. Valodkar, M., Thakore, S., 2010. Isocyanate crosslinked reactive starch nanoparticles for thermo-responsive conducting applications. Carbohydr. Res. 345 (16), 2354–2360. Wang, Y.J., Wang, L.F., 2002. Characterization of acetylated waxy maize starches prepared under catalysis by different alkali and alkaline-earth hydroxides. Starch/Starke 54, 25–30. Xu, Y., Ding, W.Q., Liu, J., Li, Y., Kennedy, J.F., Gu, Q., Shao, S.X., 2010. Preparation and characterization of organic-soluble acetylated starch nanocrystals. Carbohydr. Polym. 80, 1078–1084.
Contents lists available at ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Short communication
Hydrophobization of starch nanocrystals through esterification in green media Lili Ren a,b , Zhao Dong a , Man Jiang b , Jin Tong a , Jiang Zhou a,∗ a b
Key Laboratory of Bionic Engineering (Ministry of Education), College of Biological and Agricultural Engineering, Jilin University, Changchun 130022, China College of Chemistry, Jilin University, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 29 January 2014 Received in revised form 29 April 2014 Accepted 8 May 2014 Keywords: Starch nanocrystals Hydrophobization Esterification modification Alkenyl succinic anhydride Acetic anhydride
a b s t r a c t Starch nanocrystals (SNC) were successfully modified through esterification by using dodecenyl succinic anhydride (DDSA), 2-octen-1-ylsuccinic anhydride (OSA) and acetic anhydride (AA) in green media. The chemical modifications were confirmed using FT-IR. The esterification extent of the modified SNC was evaluated by degree of substitution (DS). The modifications imparted hydrophobic character to SNC so that the modified SNC could be dispersed in non-polar solvents such as chloroform, dichloromethane and toluene. It was found that the dispersibility of the modified SNC in non-polar solvents increased with increasing of DS and hydrophobicity of the group replacing the hydroxyl groups in starch. XRD analysis showed that the crystalline structure of SNC was preserved after the modifications. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Starch nanocrystals (SNC), isolated from starch granules by mild acid hydrolysis or enzymatic hydrolysis, have attracted significant interest over the past years (Le Corre et al., 2010; Lin et al., 2012). The unique characteristics, such as biodegradability, impressive mechanical properties, low density, and low permeability make SNC ideal candidate as reinforcement for fabrication of polymer nanocomposites to meet increasing demand for products made from renewable resources (Angellier et al., 2005a, 2006; García et al., 2009; Kristo and Biliaderis, 2007). However, the polar surface and hydrophilicity of SNC result in their poor dispersibility in non-polar solvents and poor compatibility with hydrophobic polymers, which limit processing of SNC reinforced nanocomposites and the function of mechanical reinforcement from SNC. Therefore, one of the main challenges with SNC application in nanocomposites is related to their homogeneous dispersion within polymeric matrix. Over the past decade, much effort has been devoted to modify SNC for nanocomposite applications. Modifications of SNC have been carried out via grafting polymers (Labet et al., 2007; Namazi and Dadkhah, 2008; Song et al., 2008; Thielemans et al., 2006) and reacting with various reagents (Namazi and Dadkhah, 2010;
∗ Corresponding author. Tel.: +86 431 85095760x414; fax: +86 431 85095760x888. E-mail address: [email protected] (J. Zhou). http://dx.doi.org/10.1016/j.indcrop.2014.05.014 0926-6690/© 2014 Elsevier B.V. All rights reserved.
Valodkar and Thakore, 2010). Esterification modifications of SNC with alkenyl succinic anhydrides (ASAs) and acetic anhydride (AA) were reported (Angellier et al., 2005b; Xu et al., 2010). However, most of these modifications used toxic and expensive solvents as reaction medium and required prolonged reaction time. The objective of this work was to modify SNC more environmentally friendly and efficiently through esterification using ASAs and AA in green media to improve dispersibility of SNC in non-polar solvents or hydrophobic polymers so that the application range of SNC as reinforcements in nanocomposites could be broadened. 2. Experimental Waxy maize starch was supplied by Changchun Jincheng Corn Development Co. Ltd., Da Cheng Group (China). Dodecenyl succinic anhydride (DDSA) and 2-octen-1-ylsuccinic anhydride (OSA) were purchased from Sigma-Aldrich (St. Louis, USA). Acetic anhydride (AA) and other chemicals were obtained from Beijing Beihua Fine Chemicals Co. Ltd. (China). All of these chemicals were used as received. One gram of SNC, prepared by acid hydrolysis of waxy maize starch according to the optimum condition determined by Angellier et al. (2004), was dispersed in 30 mL reaction medium (water, ethanol or water/ethanol mixture) using ultrasound and stirring for 30 min to obtain SNC suspension. After pH of the suspension was adjusted with NaOH aqueous solution (0.1 mol L−1 ), one gram of DDSA or OSA (diluted five times with ethanol, v/v) was added over 5 min. The reaction was allowed to last 1 h under stirring at
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Fig. 1. FT-IR spectra of unmodified SNC (a), DDSA modified SNC in water (pH 8.0–8.5) (b), in water/ethanol mixture (pH 8.0–8.5) (c) and in ethanol (pH 8.0–8.5) (d), OSA modified SNC in water/ethanol mixture (pH 8.5–9.0) (e).
Fig. 2. FTIR spectra of unmodified SNC (a), AA modified SNC (AA/SNC = 10/1, mL/g) at 30 ◦ C (b), AA modified SNC (AA/SNC = 30/1, mL/g) at 30 ◦ C (c), and AA modified SNC (AA/SNC = 30/1, mL/g) at 50 ◦ C (d).
35 ◦ C and selected pH maintained by adding NaOH aqueous solution. Upon completion of the reaction, pH was neutralized to 7 with HCl aqueous solution (0.1 mol L−1 ). For the AA modification, the reaction medium was 36 wt% acetic acid aqueous solution, and 10 mL mixture of H2 SO4 and acetic acid (10/90, v/v) was used as catalyst. After a certain amount of AA was added, the reaction was allowed to last 3 h under stirring at different temperatures. All the products were first washed by centrifugation thrice with 70% (v/v) ethanol aqueous solution (for the DDSA and OSA modifications) or water (for the AA modification), then thrice with ethanol to remove all traces of residual reagents and dried at room temperature. The modified SNCs were characterized with FT-IR (IRAffinity-1 spectrophotometer, Shimadzu, Japan). The specimen was prepared by grinding SNC sample together with KBr and then pressed into a disk. The resolution was 4 cm−1 and the total scans were 32. Degree of substitution (DS) refers to the average number of the hydroxyl groups substituted per D-anhydroglucose unit in starch, the maximum possible DS is 3.0. DS of the ASA (DDSA or OSA) modified SNCs was determined using the method of Jeon et al. (1999). DS of the AA modified SNC was determined by saponifying acetate groups (Miladinov and Hanna, 2001). Wettability test was used to assess surface polarity of SNCs. 5 mg of sample was mixed with two immiscible solvents (5 mL each) having different polarities and densities in a vial to observe the wettability of the sample with the solvents. Crystalline structure of SNCs was characterized by using a Rigaku D/max-2500 X-ray diffractometer (Rigaku Corporation, ˚ at 40 kV and Tokyo, Japan) with Cu-Ka radiation ( = 1.542 A) 250 mA. The X-ray diffraction patterns were recorded over the 2 range of 3–35◦ at a speed of 1◦ /min. Relative crystallinity was determined from the ratio of the areas of the diffraction peaks to the area of the whole diffraction pattern subtracted amorphous background patterns.
DS value was used to evaluate esterification extent of modified SNCs. As presented in Table 1, the modification with DDSA in the water/ethanol mixture (when pH > 8.0) yielded higher DS than that in water or ethanol, meaning there are synergistic interactions between water and ethanol which increase DS. It is known that SNC swell in water and water can increase mobility of starch molecules, while ethanol can dilute DDSA and help it diffuse into SNC. The water/ethanol mixture brought these factors together and therefore increased probability of the reactive moieties combining and resulted in higher esterification extent of SNC. Data in Table 1 also showed that DS of the DDSA modified SNC increased with increasing of pH in the reaction system and attained the maximum value when the pH ranged in 8.5–9.0. The results could be explained by the fact that higher pH values (>9.0) favor anhydride hydrolysis whereas lower pH values (<8.5) are not sufficiently to activate the hydroxyl groups in starch for nucleophilic attack of the anhydride moieties. Generally, DS and length of the alkenyl group determine the hydrophobicity of ASA modified SNC. In place of DDSA (C12 ), OSA (C8 ) was used to modify SNC. It was found that, reacted in the water/ethanol mixture with pH of 8.5–9.0 and temperature of 35 ◦ C for 1 h (weight ratio of OSA/SNC was 1), the DS of the OSA modified SNC was 0.034 which was higher than the DDSA modified one (0.021), indicating that modification of SNC with ASA having shorter chain of alkenyl group was more easily to yield a high DS. This is because, with decrease of the length of the alkenyl group, diffusion capability of ASA toward SNC increases and space baffle effect which can interrupt the reactive moieties combining decreases. Table 2 listed DS values of the AA modified SNCs with different amounts of AA at various reaction temperatures. The increase of AA amount from 10 to 30 (in proportion to SNC, mL/g) gave rise to a significant increase in DS. An increase of reaction temperature (below gelatinization temperature of starch) resulted in increased
3. Results and discussion Figs. 1 and 2 showed FT-IR spectra of the ASA and AA modified SNCs. Comparing with the spectrum of the unmodified SNC, there was a clear new peak which was located at 1732 cm−1 in the spectra of the ASA modified SNCs or located at 1722 cm−1 in the spectra of the AA modified SNCs. Since this peak is associated with carbonyl stretching from esters (Biswas et al., 2008) and the modified SNCs were thoroughly washed to remove the unbound anhydrides, this ester carbonyl bands peak is the evidence that esterification modification of SNC with ASA or AA occurred.
Table 1 DS of the modified SNCs using DDSA in different media with various pH at 35 ◦ C for 1 h. pH
7.5–8.0 8.0–8.5 8.5–9.0 9.0–9.5
Water
Ethanol
Water/ethanol (50/50, in volume)
0.004 0.013 0.017 0.015
0.005 0.012 0.016 0.014
0.005 0.017 0.021 0.019
L. Ren et al. / Industrial Crops and Products 59 (2014) 115–118
117
Table 2 DS of the modified SNCs using different amounts of AA in 36 wt% acetic acid aqueous solution at various reaction temperatures for 3 h. Reaction temperature (◦ C)
AA/SNC (mL/g) 10/1
AA/SNC (mL/g) 30/1
30 40 50
0.041 0.078 0.196
0.083 0.169 0.558
DS. Comparing with the DS of the acetylated SNC prepared in the medium of glacial acetic acid and methanesulphonic acid for 5 h (Xu et al., 2010) and acetylated starch prepared by using AA and alkaline catalyst in aqueous medium (Wang and Wang, 2002), the preparation of the AA modified SNCs in this work was more efficiently. Wettability test is a simple and valuable method to qualitatively assess surface polarity of SNC (Angellier et al., 2005b; Namazi and Dadkhah, 2010). In this work, three solvent systems were used, namely, water/chloroform (d = 1.47 g cm−3 ), water/dichloromethane (d = 1.335 g cm−3 ) and toluene (d = 0.866 g cm−3 )/water. The polarities of water, chloroform, dichloromethane and toluene are 10.2, 4.4, 3.4 and 2.4, respectively. As showed in Fig. 3a, the unmodified SNC remained in water even after shaking the container; the DDSA modified one (DS = 0.021) could migrate into chloroform, dichloromethane and toluene (Fig. 3b–d) although more and more modified SNC stayed in water with decreasing of polarity of the organic solvent. In toluene/water system, more DDSA modified SNC stayed in water and more OSA modified SNC could migrate into toluene (see Fig. 3d and e), indicating the OSA modified SNC with DS of 0.034 has higher hydrophobicity than the DDSA modified one with DS of 0.021. As shown in Fig. 4, with increase of DS, more and more AA modified SNC migrated toward chloroform. Similar trend was also observed for DDSA modified SNC. This is because higher DS means more hydroxyl groups in starch molecules have been substituted by carbonyl group or alkenyl groups, which result in lower polar nature of the modified SNC. Comparing the dispersions of the DDSA modified SNC (DS = 0.021) and AA modified one (DS = 0.041) in water/chloroform system (Figs. 3b and 4a), it could be concluded that the hydrophobicity of the DDSA modified one is higher than the AA modified one even its DS is only half of the AA modified one. The reason is the hydrophobicity of the alkenyl group in DDSA is higher than that of the carbonyl group in AA. Dispersions of the DDSA modified SNC (DS = 0.021), OSA modified one (DS = 0.034) and AA modified one (DS = 0.558) in toluene/water system (Figs. 3d, e and 4d) indicated that the OSA modified one had higher hydrophobicity than the other two, suggesting that hydrophobicity of the modified SNC
Fig. 4. Dispersions of AA modified SNC in water/chloroform (DS = 0.041) (a), in water/chloroform (DS = 0.196) (b), in water/chloroform (DS = 0.558) (c) and in toluene/water (DS = 0.558) (d).
Fig. 5. X-ray diffraction patterns of unmodified SNC (a), DDSA modified SNC (DS = 0.021) (b), OSA modified SNC (DS = 0.034) (c), AA modified SNC (DS = 0.083) (d), AA modified SNC (DS = 0.558) (e).
depends on both DS and the nature of the groups replacing the hydroxyl groups. Therefore, affinity of esterification modified SNC for non-polar solvents or polymers could be regulated by controlling DS and choosing groups to substitute the hydroxyl groups. Fig. 5 showed the XRD patterns and relative crystallinity data of the unmodified SNC and some modified ones with different DS. The unmodified SNC showed a typical A-type crystalline structure and this crystalline structure was preserved after the modifications. Since the crystalline structure of SNC was not disrupted and hydrophobicity of SNC was significantly enhanced after the modifications, the modified SNCs can be used as reinforcements to prepare hydrophobic polymer based nanocomposites and those needing non-polar solvents for casting fabrication.
4. Conclusions
Fig. 3. Dispersions of unmodified SNC in water/chloroform (a), DDSA modified SNC (DS = 0.021) in water/chloroform (b), in water/dichloromethane (c) and in toluene/water (d), OSA modified SNC (DS = 0.034) in toluene/water (e).
SNC were modified through esterification with DDSA, OSA and AA. The hydrophobicity of the modified SNCs increased with increasing of DS and hydrophobicity of the group replacing the hydroxyl groups. The crystalline structure of SNC was preserved after the modifications. The presented work provides a route to improve dispersion of SNC in non-polar solvents or hydrophobic polymers and broaden application range of SNC as reinforcements in nanocomposites.
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