J Polym Environ (2013) 21:359–365 DOI 10.1007/s10924-012-0473-y
ORIGINAL PAPER
Synthesis and Characterization of Biodegradable Starch-Polyacrylamide Graft Copolymers Using Starches with Different Microstructures Wei Zou • Xingxun Liu • Long Yu • Dongling Qiao • Ling Chen • Hongsheng Liu Nuozi Zhang
•
Published online: 7 June 2012 Springer Science+Business Media, LLC 2012
Abstract The effects of starch structures, in particular amylose content, on grafting reactions were investigated using thermal gravimetric analysis (TGA), nuclear magnetic resonance, X-ray diffraction (XRD). As a model system, corn starches with different amylose contents (0, 26, 50 and 80 %) were grafted onto acrylamide to produce superabsorbent polymers (SAPs). The weight loss measured by TGA at different temperature was used to analyze the grafting ratio in quantity. In general, the grafting ratio increased (about 10 %) with increasing starch amylose content, and graft chain segment lengths were much lower for the amylopectin-rich (waxy) starch. The high molecular weight and branched structure of the amylopectin reduced the mobility of the polymer chains and increased viscosity, which resulted in resistance to chain growth. The water absorption capability was increased with increasing amylose content for the starch-based SAPs. XRD detection showed that the crystalline structure of all starches was destroyed after grafting reactions. The thermal stability of the polyacrylamide grafted onto the starches increased by about 10 C, which could be explained by the strong bonding between the grafted polymer chains and the starch matrices.
W. Zou X. Liu L. Yu D. Qiao L. Chen H. Liu Centre for Polymers from Renewable Resources, ERCPSP, South China University of Technology, Guangzhou 510640, China L. Yu (&) N. Zhang CSIRO Materials Science and Engineering, Private Bag 33, Clayton, VIC 3169, Australia e-mail:
[email protected]
Keywords Starch Amylose Superabsorbent polymer Miscrostructure TGA
Introduction Superabsorbent polymers (SAPs) have been widely used for hygienic applications, in particular in disposable diapers and napkins, in which they capture secreted fluids such as urine and blood. In addition, agricultural grades of SAPs have been used in granular form to retain soil moisture in arid areas. Now, due to environmental concerns, biodegradable SAPs are attracting increasing attention [1, 2], particularly for agricultural applications. The biodegradable properties of SAPs rely on the starch backbone [1–4]. Of the various starches that have been used as base materials to develop biodegradable starch-based SAPs, potato [5–7], sago [8], cassava [9], and corn starch and its derivatives [10–20] are the most popular. One of the key characteristics of starches from different resources is their amylose/amylopectin content. Essentially, amylose has a linear structure of a-1,4 linked glucose units, which makes it behave in a similar manner to conventional synthetic polymers. The molecular weight of amylose is about 9106, which is 10 times higher than that of conventional synthetic polymers. Amylopectin, on the other hand, has a highly branched structure of short a-1,4 chains linked by a-1,6 bonds, and its molecular weight is about 9108, which is much higher than that of amylose. The high molecular weight and branched structure of amylopectin reduce the mobility of the polymer chains, and interfere with any tendency for them to become oriented closely enough to allow significant levels of hydrogen bonding.
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High amylose maize starch [21] and high amylopectin waxy cornstarch [22] have previously been used to produce starch-based SAPs. Comer and Jessop [23] compared two types of starch (high amylose and high amylopectin) and found no significant difference in the grafting efficiency of starch-g-poly (methyl methacrylate) prepared by emulsion photopolymerization. However, the differences in the grafting efficiency of high amylase and high amylopectiun starches with acrylamide have recently been observed [24]. The aim of this work is to investigate the effect of starch structures on chemical modifications, with a focus on the effects of amylose on the grafting reactions, microstructures and performance of the starch-based SAPs. Grafting acrylamide onto corn starches with different amylose contents to produce SAPs was used as a model system in this work. The weight loss measured by TGA at different temperature was used to analyze the grafting ratio in quantity. The thermal stability of the grafted groups and the starches after grafting were also investigated. Experimental Materials Commercially available corn starches (from Penford, Australia) with different amylose contents were used in this work (Table 1). All other chemicals used are also commercially available and were supplied by Huagong Chemicals (China) at the chemically pure grade. Preparation of Starch-Based SAPs The SAPs from different starches were prepared under the same conditions, as follows: •
•
5.00 g of starch in distilled water (80 mL) was firstly gelatinized at 121 C for 15 min. Then 0.50 g of ceric ammonium nitrate (CAN) was added into the gelatinized starch solution at 60 C to react for 10 min in a nitrogen atmosphere. A solution of acrylamide (AM) (15.00 g) containing N,N0 -methylene-bisacrylamide (N-MBA) (0.07 g) was added into the gelatinized starch at 60 C to react for
Table 1 Details of starches used in the experiments Starch
Amylose content (%)
The method has been described in detail previously [24–26]. The percentages of graft polymer, free polymer and soluble starch were determined through separation by washing using distilled water, and acid hydrolysis using HCl [1]. Characterization Thermal Gravimetric Analysis (TGA) A PerkinElmer Diamond TGA system was used to determine the grafting amount of the graft copolymer, based on the identified decomposition temperatures of the starch and grafted groups [1, 27, 28]. The thermal stability of the grafted groups and the graft starches were investigated by heating samples from 30 to 600 C at 10 C/min in a nitrogen atmosphere. Water release rates were also measured by TGA through heating swollen samples at the lower rate of 2 C/min. Nuclear Magnetic Resonance (NMR) A Varian NMR 300 system operating at a resonant frequency of 75 MHz for 13C was used to study the graft positions and graft ratios on specific carbons in the starches. Solution 13C-NMR spectra were measured at room temperature using a 10 mm solution probe-head via conventional methods. Tetramethylsilane was used as an internal reference to determine chemical shifts. Water Absorption Capacity Water absorption capacity of the polymer gels was measured by immersing samples in distilled water for 24 h at ambient temperature. Each swollen gel sample was passed through a 200-mesh screen to remove excess water, and then weighed.
Molecular weight* (Mn)
Moisture content (%)
X-ray Diffraction (XRD) A Bruker D8 diffractometer (operating at 40 kV, and 40 mA, with Cu Ka radiation monochromatized with a graphite sample monochromator, LynxExe array detector, scanning speed of 17.7 s per step, and scanning step 0.02) was used to study the crystalline structures of the starch granules and starch-based SAPs.
Waxy
0
20,787,000
12.9
Maize
26
13,000,000
12.7
Gelose 50 (G50)
50
5,115,000
12.3
Gelose 80 (G80)
80
673,000
12.2
*Average number molecular weight measured by GPC provided by Penford
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•
120 min. Then sodium hydroxide solution of NaOH (6.84 g) in distilled water (125 mL) was added to the flask at 90 C for 120 min. The modified starch was washed with distilled water to a pH of 7 then rewashed with ethanol. The sol fraction was removed during the water washing. The product was dried to a constant weight in an oven at 90 C.
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Results and Discussion TGA weight loss measurements were used to calculate grafting characteristics, which are expressed as the percentage of add-on and the percentage of grafting ratio, as reported previously [1]. In order to simplify the TGA curves, all samples were pre-dried at 105 C under vacuum conditions for 24 h so that weight loss due to moisture evaporation could be discounted. The percentage weight loss at the decomposition temperatures of a starch and polyacrylamide (PAM) grafted onto the starch can be determined in the TGA thermograms. Figure 1 presents the TGA thermograms obtained in this work for a typical sample of the dried waxy starch, as well as a waxy starchbased SAP (SAP-waxy), both before and after acid hydrolysis. The thermogravimetric (TG) curve for the native starch (Fig. 1a) shows that significant weight loss began at about 270 C. Correspondingly, there is a peak in the derivative thermogravimetric (DTG) curve at about 310 C due to starch decomposition. Lower decomposition temperatures were observed for the native starches with higher amylose content [29–31]. Figure 1b shows that two degradation steps occurred for the grafted starch, with the first peak P1 in the DTG curve at about 290 C being assigned to starch decomposition after graft modification. Correspondingly, D1 in the TG curve represents the weight loss of the starch backbone. It should be noted that the peaks of decomposition temperature for the modified starches were lower than those for the native starches (cf. Fig. 1a). A similar phenomenon has previously been observed [1, 32, 33], and can be explained by the destruction of the crystalline structure in the starch granules during gelatinization and modification [34, 35]. The second peak P2 at about 405 C is the thermogram of the acrylamide in the starch-g-PAM. The weight loss D2 represents the decomposition of the PAM grafted onto the starch. The thermogram for the acid-hydrolyzed SAP-waxy (Fig. 1c) also shows two decomposition stages. Apart from the acrylamide decomposition peak P21 in the DTG curve at about 395 C, a new peak P22 appears at about 230 C, which is attributed to the decomposition of the side groups and branches in the graft polymer [1]. Correspondingly, D21 and D22 represent the weight loss of P21 and P22 respectively. No second peak was detected for the starch-g-PAM (see Fig. 1b), since the PA chains grafted onto the starch were very short [20] and thus there were no side groups. The percentage weight loss detected by TG for starch and starch-g-PAM (Fig. 1b) can be converted to the percentage of starch and PAM grafted onto the starch respectively, which can then be used to calculate the grafting characteristics (expressed in terms of the percentages of add-on and grafting ratio). The weight in percentage of PAM in the graft polymer—or the so-called ‘‘percentage add-on’’ (AO)—was
Fig. 1 TGA thermograms of: a dried waxy starch, b SAP-waxy, and c acid-hydrolyzed SAP-waxy
computed from the weight difference between the grafted PAM and the total polymers. The grafting ratio (GR) reveals the degree of PAM grafted onto the starch backbone [1, 36]. AO and GR were calculated based on D1 and D2 respectively in Fig. 2b by: P ðweight loss at D2 Þ 100 P ð1Þ AOð%Þ ¼ ðweight loss at D1 and D2 Þ P ðweight loss at D2 Þ 100 P GRð%Þ ¼ ð2Þ ðweight loss at D1 Þ It can be seen in Fig. 2 that the TGA thermograms for the different starches and their corresponding starch-g-PAM
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Fig. 2 TGA thermograms of starches and their corresponding SAPs both before and after acid hydrolysis: a waxy, b maize, c G50, and d G80
both before and after acid hydrolysis exhibit similar patterns. The results listed in Table 2 show that the high amylopectin starch (SAP-waxy) recorded the lowest AO and GR values. The lower reaction efficiency could be because the double-helix crystalline structures formed by amylopectin’s short branched polymer chains are less flexible than the long chains in amylose. It should be noted, however, that the highest amylose starch (SAP-G80) did not record the highest AO and GR. This phenomenon could be due to the fact that the G80 has high and multiple gelatinization temperatures [37], so that it may not have been fully gelatinized during the first stage of the grafting reaction. This issue will be investigated in future work. The decomposition temperature and the percentage weight loss at different temperatures were also used to investigate the thermal stability of the starches and the groups grafted onto them. It has been noted that the thermal stability of PAM grafted onto starch (P2 in SAPs) was higher than that of the ungrafted polymer (P21 in acidhydrolyzed SAPs). For example, the decomposition temperatures of P2 and P21 were 405 and 395 C respectively for SAP-waxy (see Fig 1b, c). This could be explained by the strong bonding between the grafted polymer chains and the starch matrix in the graft polymer, and imidization [1]. There was no observable difference in the decomposition temperature of the PAM grafted onto the different starches. Graft positions and graft ratios on specific carbons in the starches was investigated by NMR. As the 13C-NMR
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spectra of PAM and starch-g-PAM have been widely reported previously [38–40], this work focused on the carbons in starches. The only observable difference in the peaks for the carbon atoms was a new peak detected at d = 61.8 ppm for the starch-based SAPs, which is close to the peak for C6 at d = 63.5 ppm (see Fig. 3). This result, which is similar to that previously reported [24, 41], indicates that the hydroxyl groups of C6 participated mainly in the graft reaction. The ratio of the peaks at d = 63.5 and 61.8 ppm were used to determine the ratio of C6-AM grafting. Table 2 shows the composition of the different SAPs, in which ‘‘Add-on (%)’’ represents the PAM segments in a sample, and ‘‘Reaction ratio of C6’’ represents the number ratio of C6 linked to the PAM. All SAP samples contained 70–80 wt% of PAM segments in the grafted starch. It has been noted that the sample based on waxy starch had 53.2 % of C6 involved in the grafting reactions, which was significantly higher than other starch-based SAPs (19–22 %). As grafting occurred mainly on C6, the length of the grafted PAM segments in each grafted starch was calculated based on the mole mass of glucose (160) and AM (71), and the results are also shown in Table 2. The results show that the segment length for the SAP-waxy was much shorter (3.0) than other starch-based SAPs (ranged from 8.2 to 9.5). This would indicate that the high molecular weight and branched structure of amylopectin reduced the mobility of the polymer chains, resulting in a higher viscosity and resistance to chain growth. The add-on
J Polym Environ (2013) 21:359–365 Table 2 Graft reactions, composition and properties of the starched-based SAPs
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Samples
Grafting reaction Add-on (%)
Grafting ratio (%)
Reaction ratio of C6 (%)
PAA segment length (no.)
Water viscosity (10 rad/s) (Pa.s)
Water absorption (%)
SAP-waxy
41.29
70.34
53.2
3.0
268
320 ± 57
SAP-maize
44.07
78.79
21.6
8.2
185
286 ± 36
SAP-G50
43.45
76.84
18.3
9.5
105
523 ± 44
SAP-G80
42.77
74.72
20.1
8.4
123
724 ± 68
Fig. 3 13C-NMR spectra of native starch (maize) and the starchbased SAPs
and graft ratio were also calculated by the gravimetric method [1, 24] based on the percentages of graft polymer, free polymer and soluble starch determined through separation by washing using distilled water, and acid hydrolysis using HCl. Table 2 also gives the results of water absorption capacity of the SAPs from different starches. It can be seen that the SAPs from amylase-rich starches generally had a higher capacity, which corresponds with the percentages of AO and GR. The high molecular weight and branched structure of amylopectin reduced the mobility of the polymer chains, and interfered with any tendency for them to become oriented closely enough to permit significant levels of hydrogen bonding. Furthermore, based on the NMR results, the longer grafted segments in high amylose
starch may have also contributed to the higher water absorption. An earlier study by Michaels [42] showed that PAM with longer chains was more effective in the flocculation of suspensions, and a similar phenomenon has also been observed for graft copolymers of PAM on starches [39, 43–45]. The rigid and branched structure of amylopectin hinders water movement in the gel, which results in lower water absorption. The shorter grafted chains of high amylopectin starch, as detected by NMR, also correspond with the observed water absorption behaviour. Figure 4 shows the water release rates during heating, as measured by TGA and, as expected, water evaporated during heating. As there was no significant difference in the release rates of the different SAPs, it would appear that the length of the grafted AM chains has not changed the nature of the bond between the AM and water. The XRD spectra for the different native starches and their corresponding SAPs are shown in Fig. 5. The spectra for the native waxy and maize starches (high amylopectin content) exhibited a typical A-type pattern, while those for the G50 and G80 starches (high amylose content) exhibited a strong B-type pattern. Overall, an increase in amylose content in the native starches led to a progressively weaker scanning intensity, except for the peaks observed at around 2h = 18, which is a characteristic of V-type starches. These results are similar to those previously reported [46–48]. In the spectra for the various SAPs, the weak but
Fig. 4 Water release rates of SAPs during heating at 2 C/min
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to chain growth. There was no significant difference in the water release rates of the different SAPs, which indicates that the length of grafted acrylamide chains did not change the nature of the bond between the acrylamide and water. XRD detection revealed that the crystalline structure of all starches were destroyed by the graft reactions. The thermal stability of the polyacrylamide grafted onto the starches was increased by about 10 C, which could be explained by the strong bonding between the grafted polymer chains and the starch matrices. Acknowledgments The authors from SCUT, China, would like to acknowledge the research funds NFSC (21174043, 31130042), RFDPHE (20110172110027) and FRFCU (2012ZZ0085). Thanks also to Dr S. Li (CSIRO) for contact angle measurements and analysis.
References
Fig. 5 XRD spectra of native starches and their corresponding SAPs
sharp peaks at around 2h = 30 could be residual PAM that was not totally removed by washing.
Conclusions Corn starches with different amylose/amylopectin contents (0, 26, 50 and 80 %) were grafted with acrylamide, for use as a model system to study the effects of starch microstructures on starch modification. The results showed that, in general, the grafting ratio was increased (about 10 %) with increasing amylose content, and the graft chain segment length was much lower for the SAPs based on the high amylopectin (waxy) starch. In generally, the water absorption capability of the starch-based SAPs increased with increasing amylose content, which corresponds with the graft ratio and relates to the segment length of the polyacrylamide grafted onto the starches. The high molecular weight and branched structure of amylopectin reduced the mobility of the polymer chains, resulting in higher viscosity and resistance
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1. Lanthong P, Nuisin R, Kiatkamjornwong S (2006) Carbohydr Polym 66:229 2. Zohuriaan MJ, Kabiri K (2008) Iran Polym J 17:451 3. Arvanitoyannis I (1999) J. Macromol Sci Rev Macromol Chem Phys C39:205 4. Psomiadou E, Arvanitoyannis I, Biliaderis CG, Ogawa H, Kawasaki N (1997) Carbohydr Polym 33:227 5. Zhang LM, Chen DQ (2001) Starch-Sta¨rke 53:311 6. Zhang LM, Hu ZH (2002) Starch-Sta¨rke 54:290 7. Shaikh MM, Lonikar SV (2009) J Appl Polym Sci 114:2893 8. Han TL, Kumar RN, Rozman HD, Noor MAM (2003) Carbohydr Polym 54:509 9. Nakason C, Wohmang T, Kaesaman A, Kiatkamjornwong S (2010) Carbohydr Polym 81:348 10. Cho CG, Lee K (2002) Carbohydr Polym 48:125 11. Song H, Zhang SF, Ma XC, Wang DZ, Yang JZ (2007) Carbohydr Polym 69:189 12. Fanta GF, Felker FC, Shogren RL (2004) Carbohydr Polym 56:77 13. Ma YHC, Manolache S, Sarmadi M, Denes FS (2004) StarchSta¨rke 56:47 14. Samaha SH, Nasr HE, Hebeish A (2005) J Polym Res 12:343 15. Xu SM, Wei J, Feng S, Wang JD, Li XS (2004) J Polym ResTaiwan 1(1):211 16. Xu S, Wang J, Wu R, Wang J (2006) Carbohydr Polym 66:55 17. Meshram MW, Patil VV, Mhaske ST, Thorat BN (2009) Carbohydr Polym 75:71 18. Mostafa KM, Morsy MS (2004) Starch-Sta¨rke 56:254 19. Kaith BS, Jindal R, Jana AK, Maiti M (2010) Bioresour Technol 101:6843 20. Salam A, Pawlak JJ, Venditti RA, Ei-tahlawy K (2010) Biomacromolecules 11:1453 21. Yamaguchi M, Watamoto H, Sakamoto M (1988) Carbohydr Polym 9:15 22. De Bruyn H, Sprong E, Gaborieau M, Roper JA, Gilbert RG (2007) J Polym Sci Pol Chem 44:4185 23. Comer CM, Jessop JLR (2008) Starch-Sta¨rke 60:335 24. Zou W, Yu L, Liu X, Qiao DL, Zhang X, Chen L, Zhang R (2012) Carbohydr Polym 87:1583 25. Meister JJ (1981) J Rheol 25:487 26. Wu J, Lin J, Zhou M, Wei C (2000) Macromol Rapid Commun 21:1032 27. McNeill IC, Sadeghi SMT (1990) Polym Degrad Stab 29:233
J Polym Environ (2013) 21:359–365 28. Athawale VD, Lele V (2000) Starch-Sta¨rke 52:205 29. Liu X, Yu L, Xie F, Li M, Chen L, Li X (2010) Starch-Sta¨rke 62:139 30. Liu X, Yu L, Liu H, Chen L, Li L (2009) Cereal Chem 86:383 31. Soares RMD, Lima AMF, Oliveira RVB, Pires ATN, Soldi V (2005) Polym Degrad Stab 90:449 32. Zhang X, Golding J, Burgar I (2002) Polymer 43:5791 33. Beninca C, Demiate IM, Lacerda LG, Carvalho Filho MAS, Ionashiro M, Schnitzler E (2008) Eclet Quı´m Sa˜o Paulo 33:13 34. Fares MM, Ei-faqeeh AS, Osman ME (2003) J Polym Res Taiwan 10:119 35. Athawale VD, Lele V (1998) Carbohydr Polym 35:21 36. Athawale VD, Rathi SC (1999) J Macromol Sci Rev Macromol Chem Phys C39:445 37. Liu H, Yu L, Xie W, Chen L (2006) Carbohydr Polym 65:357
365 38. Gurruchaga M, Goni I, Vazquez B, Valero M, Guzman GM (1992) Macromolecules 25:3009 39. Pal S, Nasim T, Patra A, Ghosh S, Panda AB (2010) Int J Biol Macromol 47:623 40. Sen G, Kumar R, Ghosh S, Pal S (2009) Carbohydr Polym 77:822 41. Zhang Q, Xu K, Wang P (2008) Fibers Polym 9:271 42. Michaels AS (1954) Ind Eng Chem 46:1485 43. Ghosh S, Sen G, Jha U, Pal S (2010) Bioresour Technol 101:9638 44. Rath SK, Singh RP (1997) J Appl Polym Sci 66:1721 45. Rath SK, Singh RP (1998) J Appl Polym Sci 70:1795 46. Douzals JP, Marechal PA, Coquille JC, Gervais P (1996) J Agric Food Chem 44:1403 47. Cheetham NWH, Tao LP (1998) Carbohydr Polym 36:277 48. Liu H, Yu L, Simon G, Dean K, Chen L (2009) Carbohydr Polym 77:662
123
