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Green Synthesis, Thermal Analysis and Degradation Kinetics of Cross-Linked Potato Starch.

Byline: Azhar Abbas, Muhammad Ajaz Hussain, Muhammad Amin, Rizwan Nasir Paracha, Muhammad Ameer and Mazhar Hussain

Summary

Succinylation of starch was carried out with succinic anhydride homogeneously using N,N-dimethylacetamide (DMAc) at 70C for 24 h. Starch succinates (SS) were then cross-linked using 1,1'-carbonyldiimidazole (CDI). This green method for cross-linking of SS using CDI is being reported for the first time. The SS and cross-linked starch succinate (CLSS) were characterized by FTIR spectroscopy and thermal analysis. The CLSS was found thermally more stable than SS as degradation maxima (Tdm) of CLSS (311C) was 70C higher than Tdm of SS (241C) noted for first and major step of degradation. Additionally, initial (Tdi) and final (Tdf) degradation temperatures of CLSS were higher than SS which is indicative of extra thermal stability imparted after cross-linking of starch via succinylation. Thermal degradation kinetics were calculated using Friedman, Broido and Chang methods.

Energy of activation (Ea) was calculated for each step of degradation for SS and CLSS. Order of reaction (n) was calculated from Chang model and it was found that degradation in first step follows first order kinetics in SS and CLSS.

Keywords: Cross-linking, Degradation kinetics, Starch, Succinic anhydride, Thermal analysis.

Introduction

Starch and its cross-linked derivatives are widely used in various commercial applications like paper making, pharmaceutical and medicinal fields [1-4]. In order to improve thermal stability and mechanical properties, starch is modified by blending with other materials [5], esterification [6-8], oxidation and cross-linking [9-15].

The cross-linked starch has several desirable features, e.g., high temperature resistance, tensile texture, improved viscosity, high shear force, etc [16-20]. The cross-linking of starch is usually carried out using glutaraldehyde as cross-linking agent. But, glutaraldehyde gives aldehyde side products which are toxic hence not desirable. Alternatively, activated-dicarboxylic acids are being used as cross-linking agents for polysaccharide materials [21, 22].

Such cross-linked polysaccharides are used in pharmaceutical and food industries, ion separation filters and membranes, antibody and enzyme separation membranes [23]. One major drawback in dicarboxylic acid mediated cross-linking of polysaccharides is low DS due to less reactivity and necessary work up procedure. Therefore, need was felt to look for more efficient cross-linking methodology.

Herein, we present novel and green synthesis of cross-linked starch via succinylated intermediate using a green in situ carboxylic acid activating agent, i.e., CDI. The succinyl moiety also acts as a spacer arm and may enhance thermal stability of starch after cross-linking. We are also focused on thermogravimetric analysis and degradation kinetics to elucidate thermal behavior of SS and its cross-linked products, i.e., CLSS.

Experimental

Reagents and chemicals

Potato starch (Uni-Chem) was dried under vacuum at 50C overnight before use. 1,1'- carbonyldiimidazole (CDI), N,N-dimethylacetamide (DMAc) and DMF were obtained from Sigma- Aldrich and used as such. Lithium chloride was obtained from Guangdong Gaunghua, China. Succinic anhydride, other reagents and solvents (Fluka) were of analytical grade.

Measurements

IR Prestige-21 (Shimadzu, Japan) was used to acquire FTIR spectra. Thermal decomposition temperatures of the starch succinate (SS) and cross- linked starch succinate (CLSS) were analyzed using TA Instruments equipped with a thermo-balance (SDT Q600 USA). Temperature effects on samples were noted thermogravimetrically at heating rate of 10C/min under N2.

Dissolution of starch in DMAc/LiCl

Dried starch (1.0 g, 6.16 mmol) was added in DMAc (30 mL) and heated at 100C for 1 h. Lithium chloride (0.5 g) was added and reaction mixture was further heated at 135C for 45 min. the resultant starch solution was optically clear and used for chemical modifications.

Succinylation of starch using succinic anhydride

Succinic anhydride (3.70 g, 37.0 mmol) was added in parts to starch solution and reaction proceeded for 24 h at 70C. The reactions mixture was precipitated and subsequently washed thrice with 2-propanol. The SS obtained was dried under vacuum at 50C.

Yield: 1.27 g (70%); DS: 1.32; FTIR (KBr): 3444 (OH), 2947 (CH2), 1726 (CO Ester), 1458 (CH2) cm-1.

Cross-linking of starch-succinate using 1,1'- carbonyldiimidazole

The SS (1.0 g, 3.38 mmol) was dissolved in DMAc (25 mL). CDI (548 mg, 3.38 mmol) was added and reaction was preceded at 80C for 24 h. The reaction mixture was precipitated and washed thrice with 2-propanol. The product obtained as white powder was dried under vacuum at 50C.

Yield: 1.12 g (62%); DS: 1.30; FTIR (KBr): 3421 (weak signal, OH), 2951 (CH2), 1724 (CO Ester), 1453 (CH2) cm-1.

Degree of substitution (DS)

SS (100 mg) was dissolved in 1M aq.NaOH (50 mL) and stirred overnight. pH of reaction mixture was adjusted at 7 with help of HCl (0.01 M aq.). Afterwards, measured amount of 1M aq.NaOH was added. Extra amount of NaOH was back titrated against 0.1M HCl solution to restore pH at 7. DS of succinylation was then calculated using following formula;

Equation

where, n.NaOH is moles of NaOH added after saponification, M (Ru) is molar mass of repeating anhydroglucose unit, Ms is the mass of sample and Mr (RCO-) is molar mass of ester as a substituent.

Thermal analysis and degradation kinetics

DTG curves were drawn from TG curves. From TG and DTG curves, Tdi, Tdm and Tdf were assessed for comparison of thermal stability of each step in SS and CLSS. Friedman (eq. 1) [24], Chang (eq. 2) [25] and Broido (eq. 3) [26] models were used to calculate kinetic parameters.

Equation

where, in eq. 1-3, da/dt (rate of weight loss) was taken from DTG curve; n is the reaction order; Ea is the activation energy; R is the gas constant; Z is frequency factor of decomposition reaction; T is the absolute temperature recorded; 1-a is the weight of sample left at a certain temperature; y is (wt-w8)/(w0- w8); w8 is final weight; w0 is initial weight; wt is weight at a given time t.

Results and Discussion

Synthesis and characterization

Succinylation of starch was performed under homogenous reaction conditions. Starch was reacted with succinic anhydride [27, 28]. The starch succinates obtained were cross-linked using CDI. CDI first reacts with free COOH groups on succinyl moieties attached on starch to make their imidazolides [29-31]. Imidazolide groups generated in situ react with free OH of starch to yield cross- linked product. Additionally, CDI if remains unreacted towards COOH, it may cross-link starch hydroxyls in the form of ester which is an additional benefit of the reagent. The reaction scheme for the synthesis of starch succinate (SS) and cross-linked starch succinate (CLSS) is depicted in Fig. 1 and results are accumulated in Table-1.

Table-1: The conditions and results of starch (1g) with reagents to form esters of starch.

###Yield

Samples Molar ratio###DS###Solubility

###(g/ %)

###SS###1:6a###1.32 1.27/70 DMAc, DMF, DMSO

###CLSS###1:1b###1.30 1.12/62 DMAc, DMF, DMSO

The degree of substitution (DS) was found 1.32 for SS as calculated from acid-base titration after saponification. Both SS and CLSS were soluble in DMSO, DMAc and DMF. However, SS was partially soluble in water while CLSS was water insoluble even upon heating.

The SS and CLSS were characterized using FTIR spectroscopy. FTIR spectrum of SS showed a distinct ester peak at 1726 cm-1 indicating successful succinylation of starch. Other characteristic peaks of free OH were found at 3444 cm-1 as a broad signals and COC of polymer backbone centered at 1029 cm-1 while CH2 stretching appeared at 1458 cm-1. However, FTIR spectrum of CLSS showed still intact and distinct ester peak at 1724 cm-1 and all other characteristic peaks were comparable with SS as expected. Distinct ester peak along with all characteristic signals of starch indicated successful formation of SS and CLSS. An overlay FTIR spectrum of starch, SS and CLSS is shown in Fig. 2.

Thermal Analysis and Degradation Kinetics

Cross-linking is an important parameter to enhance heat resistant properties in biopolymeric materials. Such heat resistant biopolymers are used for film formation and coating applications. Therefore, it is important to study thermal properties of cross-linked starch derivatives. Thermal analysis and degradation kinetic of SS were studied before and after cross-linking. Thermal decomposition temperatures, Tdi, Tdm and Tdf were drawn from TG and DTG curves. Thermal analysis has revealed two step degradation profiles for SS and CLSS where 2nd step corresponds to furfural formation in polysaccharides [30] and is not being discussed here.

Therefore, first and major degradation step of SS and CLSS was compared for thermal properties. The overlay TG and differential TG (DTG) curves of SS and CLSS are shown in Figs. 3, 4, respectively.

Degradation started at 114C (Tdi) and completed at 388C (Tdf) with Tdm 241C for first step of thermal degradation of SS where total weight loss observed was 65%. Whereas, in CLSS, degradation started and ended later as Tdi and Tdf appeared at 176 and 407C, respectively. Similarly, Tdm (311C) of CLSS was 70C higher than SS (Tdm 241C) which is indicative of significant thermal stability imparted to SS after cross-linking. Thermal data of SS and CLSS is reported in Table-2.

Table-2: Results of thermogravimetric analysis for starch-succinate at10C/min.

###Tdi###Tdm###Tdf###Weight loss###Char yield

Sample Steps

###(C)###(C)###(C)###% at Tdf###Wt. (%)

SS

###I###114.27###241.26###387.78###64.55###4.81 at

###II###543.18###653.29###743.87###89.11###845.10C

###I###176.43###311.41###407.31###60.56

CLSS###II###552.06###639.08###728.77###83.80

###0.36 at

###916C

###III###764.29###846.88###893.05###98.30

Thermal kinetics was also calculated using three different kinetic models, i.e., Friedman, Broido and Chang. Friedman, Broido and Chang plots of SS and CLSS are shown in Figs. 5, 6 and 7, respectively.

The different kinetic parameters, e.g., order of reaction (n), activation energy (Ea) and frequency factor (Z) were drawn from thermal data. The Ea values for the first step degradation of SS as calculated from Friedman (28.42 kJ/mol) and Chang (29.47 kJ/mol) models were in good agreement. CLSS showed Ea values of 35.94 and 38.51 kJ/mol for first step degradation as calculated by Friedman and Chang models, respectively. The relatively higher Ea values for CLSS as compared to SS indicate higher thermal stability in the cross-linked product. Order of reaction (n) was also calculated from Chang model and found that degradation follows first order kinetics for SS and CLSS. Hence, it is confirmed from thermal analysis and degradation kinetics that CLSS has higher thermal stability than SS. The results of thermal kinetics of SS and CLSS are summarized in Table-3.

Table-3: Kinetics parameters of starch-succinate under nitrogen atmosphere.

###Sample###Step###Method###|r|###n###Ea (kJ/mol)###lnZ###H###S###G

###Friedman###0.999###-###28.42###6.06###24.14###222.31###138.44

###I###Broido###0.999###-###39.48###-###35.21###199.87###137.97

###Chang###0.999###1###29.47###6.39###25.20###219.26###137.93

###SS

###Friedman###0.999###-###138.47###17.32###130.77###130.20###251.36

###II###Broido###0.999###-###139.27###-###131.57###125.50###247.80

###Chang###0.999###1###137.16###16.17###129.46###130.28###250.12

###Friedman###0.999###-###35.94###6.84###31.09###217.06###157.88

###I###Broido###0.999###-###56.57###-###51.68###175.71###154.32

###Chang###0.997###1###38.51###7.53###33.66###210.75###156.76

###Friedman###0.994###-###75.95###8.14###68.36###211.15###260.96

###CLSS###II###Broido###0.999###-###82.16###-###75.25###205.93###263.09

###Chang###0.999###1###86.69###9.78###78.78###210.08###270.40

###Friedman###0.999###-###267.02###29.03###257.36###35.21###298.42

###III###Broido###0.999###-###269.13###-###259.43###33.07###297.99

###Chang###0.999###1###265.90###27.42###256.20###46.55###310.49

Conclusion

Succinylation of starch was achieved under homogeneous reaction conditions using succinic anhydride. Moreover, cross-linking of succinylated starch was achieved using CDI. Result of thermal studies and degradation kinetics has revealed that thermal properties of the starch succinate were significantly improved after cross-linking. Due to extra ordinary thermal stability imparted after cross- linking, CLSS could be a material of choice for surface coating applications.

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Author:Abbas, Azhar; Hussain, Muhammad Ajaz; Amin, Muhammad; Paracha, Rizwan Nasir; Ameer, Muhammad; Hussai
Publication:Journal of the Chemical Society of Pakistan
Article Type:Report
Date:Apr 30, 2015
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