Characterization of esterified cassava starch with long alkyl side chains and different substitution degrees

Accepted Manuscript Title: Characterization of esterified cassava starch with long alkyl side chains and different substitution degrees Author: Sim´on E. Barrios Giuseppe Giammanco Jes´us M. Contreras Estrella Laredo Francisco L´opez-Carrasquero PII: DOI: Reference:

S0141-8130(13)00262-6 http://dx.doi.org/doi:10.1016/j.ijbiomac.2013.04.079 BIOMAC 3728

To appear in:

International Journal of Biological Macromolecules

Received date: Revised date: Accepted date:

8-3-2013 24-4-2013 27-4-2013

Please cite this article as: S.E. Barrios, G. Giammanco, J.M. Contreras, E. Laredo, F. L´opez-Carrasquero, Characterization of esterified cassava starch with long alkyl side chains and different substitution degrees, International Journal of Biological Macromolecules (2013), http://dx.doi.org/10.1016/j.ijbiomac.2013.04.079 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

 

Characterization of esterified cassava starch with long alkyl side chains and different substitution degrees Simón E. Barrios1, Giuseppe Giammanco1, Jesús M. Contreras1, Estrella Laredo2, Francisco López-

Grupo de Polímeros ULA, Departamento de Química, Facultad de Ciencias, Universidad de los

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Andes, Mérida 5101A, Venezuela.

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Carrasquero1*

Grupo FIMAC, Departamento de Física, Universidad Simón Bolívar, Aptdo. 89000, Caracas 1080,

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Venezuela.

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Abstract

The present work describes the characterization and thermal properties of hydrophobic starch obtained

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by the esterification of cassava starch with acyl imidazoles, acid chlorides and methyl ester derivatives

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of fatty acids with n-alkyl chains with 12 to 22 carbon atoms, in order to compare the dependence of their properties as a function of the length of the side chain and the methodology used for their

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synthesis. The n-acyl starches presented degrees of substitution (DS) between 0.06 and 1.2. Most of the derivatives obtained with acyl imidazoles were found to be stable at temperatures up to 300ºC, whereas those synthesized with acid chlorides or methyl ester decomposed below. Finally, when the n-acyl starches were substituted with n-alkyl side chains of 16 or more carbon atoms, they were capable to crystallize in separate paraffinic phases independent of the starch backbone. Key words: Cassava starch, esterified starch, acyl imidazoles, acid chlorides, methyl n-alkyl esters, side chain crystallization *Corresponding author Telephone: 58 274 2401381 Fax: 58 24012 86 E-mail address: [email protected] (F. López-Carrasquero)

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1. Introduction Starch is an important natural resource which can be obtained from many renewable sources such as corn, potato, rice, wheat, peas or cassava, at low cost. Its field of application is very broad from food, paper and pharmaceutical to plastic industries [1-4]. Nevertheless, some of the physical characteristics of the native

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starch (NS) limit some of its possible applications [5-7]. Thus, the chemical modification of starch, as well

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as other polysaccharides, can be considered as an interesting alternative to expand these biopolymers applications and allow the development of new green products from sources other than oil, such as these

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biodegradable polymers. Depending on the nature of the modifying agent or chemical modification, hydrophilic or hydrophobic starches can be synthesized increasing the potential industrial applications of

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starch [8, 9]. It has also been suggested that long-chain esters of starch may find application as substitutes

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for oil-based plastic materials especially in the packaging industries [10, 11]. Esterification has been carried out by a variety of pathways [12-18]. Etherification reactions have been

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also widely explored [19-22] and among them, the carboxymethylation reactions have been frequently

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employed [9, 23-24]. The chemical modification of starch, cellulose and others related materials by the insertion of alkyl chains to obtain hydrophobic materials has recently attracted the interest of many

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researchersas it is thought that they may find application as substitutes for oil-based plastic materials especially in the packaging industry [10-25]. For example, the preparation of long-chain esters of starch using fatty acid chlorides or methyl esters in absence of solvent have been reported [10, 26]. More recently, fatty acid chlorides from 8 to 18 carbon atoms were also used to esterify cellulose in LiCl/DMAc. The esterified cellulose is organized in a layered structure where the cellulosic backbones are arranged in a plane with the side chains being fully extended in a direction perpendicular to the backbone. The side chains with more than 12 carbon atoms are able to crystallize into an hexagonal lattice [27]. This behavior is similar to that observed in comb-like polymers derived from natural occurring monomers, such as esters of aspartic [28], glutamic [29] or itaconic acids [30]. A study of the synthesis of fatty acid potato starch esters, using supercritical carbon dioxide as solvent, varying the pressure, temperature and 2 

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the basic catalyst was recently reported and the starches were obtained with degree of substitution (DS) values ranging from 0.01 to 0.3 [11]. The esterification of amylose enriched starch was also carried out using carboxylic acid imidazoles with n-alkyl side chain of 8, 12 and 16 carbon atoms, obtaining starch esters with DS in the range of 1.55-2.0 [25]. Synthesis of complexes of long alkyl chain quaternary

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ammonium salts with carboxymethyl starch [31] or with pectinic and alginic acid [32], have also been

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reported. In both cases the alkyl side chains are able to crystallize when they reach a minimum length. To the best of our knowledge the detailed crystallization study in the case of long side chains attached to

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starch by esterification has not been performed.

In this work we describe the esterification of cassava starch with derivatives of fatty acid with 12, 14, 16,

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18 and 22 carbon atoms with the aim to determine if the n-alkyl side chain attached to the starch back

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bone were able to crystallize and how the length and the DS influence this behavior. The effect on the side chain length in the thermal stability is also explored. These starches esterified with long alkyl side chain,

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especially those with side chains with 22 carbon atoms, could act as compatibilizers in mixtures of starch

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with other polymeric materials. For this purpose, the esterification of the cassava starch was carried out using three different types of reagents, i.e. acyl imidazoles, acid chlorides and methyl esters derivatives of

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fatty acids with the same number of carbon atoms. The chemical structure of the esterified starches was characterized by FTIR, the thermal stability and the degradation process by Thermogravimetric Analysis (TGA) and Thermal Volatilization Analysis (TVA). The insertion of long side chains with 18 or 22 carbon atoms, with different degrees of substitution also allowed us to study the crystallization of the side chains attached to the starch back bone by Differential Scanning Calorimetry (DSC) and Wide Angle X-Ray Scattering (WAXS).

2. Materials and methods 2.1. Materials

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Cassava starch with an amylose content of about 17%, estimated according to the procedure of McGrance, Cornell, and Rix [33], was kindly supplied by Agroindustriales Mandioca S.A., Venezuela, and was dried in a oven al 100ºC for two hours before being used. Fatty acids were purchased from Aldrich Chemicals and used as received. Other chemicals and reagents used here were of analytical grade or more and were

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employed without further purification, except for the thionyl chloride which was distilled immediately

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before used. The acid chlorides were prepared by chlorination of the fatty acids with thionyl chloride. Acyl imidazoles were synthetized using the method previously reported [25]. Methyl esters were prepared

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by methylation of fatty acids with diazomethane using dry ether as solvent. After reactions were completed, solvent was removed by evaporation and the purity of the esters was confirmed by thin layer

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chromatography (TLC).

2.2 Starch modification

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Starch modification was carried out by three different methods. In the first one the cassava starch was

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esterified by acyl imidazole series with n-alkyl chains with 12 to 22 carbon atoms. The reaction was carried out by dropwise addition  of the acyl imidazole dissolved in dimethyl sulfoxide (DMSO) to a

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dispersion of the NS and dissolved in DMSO (1g / 20mL) with a little amount of sodium methoxide (5%) previously heated at 90ºC. For the acyl imidazoles with chains 18 and 22 carbon atoms long, the solution must be kept at least at 60ºC due to their low solubility in cold DMSO. The addition took about an hour. When the addition was done speedily the starch tends to “aggregate” from the mixture, the modification was superficial and the esterified starches were obtained with low DS. After the addition, the mixture was allowed to react during 3 hours at 90ºC. Then, the esterified starch was precipitated in methanol and filtered in vacuum. The solid was allowed to dry in vacuum overnight, washed with acetone for 20 minutes, filtered and dried in vacuum. Starch was also esterified directly using acyl chlorides in similar way to that previously reported by Aburto, et al. [10, 26]. The third method was carried out through a transesterification of methyl esters with 4 

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n-alkyl chains of 12, 14, 16, 18 and 22 carbon atoms with the starch. In a typical procedure a solution 5% w/w of NS in DMSO at 100ºC was treated with few drops of methanolic solution of potassium methoxyde (KOMe) at 10 torr to maintain the reflux of the solvent and to allow the generated MeOH to distill. Then a solution of the methyl ester in DMSO was added and it was allowed to react for 6 hours. Afterward, the

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hot mixtures was poured in ethanol and shaken for a few minutes, filtered and washed with ethanol,

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acetone or dichloromethane, and finally filtered and dried in vacuum during one night.

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2.3 Chemical characterization

Infrared spectra were recorded on a Perkin-Elmer 2000 instrument from KBr discs samples or films

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prepared by casting. All the spectra were performed with 32 scans and spectral resolution of ±4.

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NMR spectra were obtained with a Bruker Avance DRX 400 spectrometer operating at 400 MHz at room temperature. NMR spectra were recorded (16 scans) in DMSO-d6.

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The DS is generally defined as the molar ratio of alkyl substituents to anhydroglucose units present in the

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starch molecules. In this work the degrees of substitution (DS) were estimated gravimetrically and by

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saponification. The gravimetric method consists on measuring the difference in weight of the purified esterified products with respect to the native starch. The weight difference is due to the acyl group attached to the starch. This method allows an estimate of the minimum DS by using the equation (1):

DS= (162 x nRCO)/(wES- (MWRCO x nRCO))

(1)

Where, nRCO : number of moles of attached RCO chains 162: molecular weight of the anhydroglucose unit wES: weight of the esterified starch 5 

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MWRCO: molecular weight of the RCO moiety Saponification was carried out by heating at 70ºC the esterified starch in a 75% ethanol/water solution with a previously standardized 20mL of KOH (0.5 M). After the heating time (3 hours for starches with

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12, 14 and 16 carbon atoms and 24 hours for those with 18 and 22) the unreached KOH was back titrated. The DS was calculated by the equation (1), but in this case nRCO was obtained from the unreacted KOH.

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In the case of some samples with low DS which were soluble in DMSO-d6, the DS were also determined

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by 1H NMR by measuring and comparing the areas of the aliphatic protons with those corresponding to

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the anhydroglucose units [11].

2.4 Thermal behavior

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Calorimetric measurements were performed with a Perkin-Elmer DSC-7 calibrated with indium. Samples

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2.5 Thermal stability

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temperatures ranging from –40 to 120ºC.

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of about 5 mg were heated or cooled at rates of 10ºC min-1 under an ultra-pure nitrogen atmosphere for

Thermogravimetric analyses, TGA, were carried out on a Perkin-Elmer TGA-7 thermobalance with similar conditions as those used in DSC runs but within a temperature range from 25 to 500ºC. Thermal volatilization analyses, TVA, were performed in a vacuum line, heating samples of about 100 mg from room temperature to 390ºC during 30 minutes under a moderate primary vacuum. Gases or noncondensing fractions were collected in a gas cell equipped with KBr windows at liquid nitrogen temperature and volatile fractions condensing at room temperature were collected in the so-called cold ring. The different fractions were then analyzed by FTIR spectroscopy and in some cases, when needed, by 1H NMR.

2.6. Structural characterizations 6 

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Wide angle X-rays scattering (WAXS) experiments were performed in a Panalytical automatic

θ−θ horizontal axis diffractometer using Cu Kα-Ni filtered radiation. The sample temperature varied from -20 to 75°C in an Anton Paar TTK 450 with the scattering angle, 2θ, varying from 3 to 50°.

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3. Results and discussion

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3.1 Synthesis and characterization of esterified starches

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The NS was esterified by reaction between the starch and acyl imidazoles, acid chlorides or methyl esters with n-alkyl chains of 12, 14, 16, 18 and 22 carbon atoms using the previously described methods; the

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reactions are shown schematically in Figure 1.

The nomenclature used for these esterified starches (ES) is ESIC-n-X, ESCC-n, y ESMC-n where C-n is

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the carbon number of the n-alkyl side chain, the letters I, C and M are related to the method employed in

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represents the experiment number.

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the esterification: I correspond to the acyl imidazoles, C to the acid chlorides and M to methyl esters. X

The DS are strongly influenced by the esterification method used in each case. As can be observed in

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Tables 1 and 2, most of the esterified starches obtained with highest DS were synthesized by using the acyl imidazoles.

Although all the starch modifications with acyl imidazoles were performed under the same conditions, the obtained DS for this series vary from 0.1 up to values near 1. As it was mentioned above, the DS values are dependent on the rate of addition of the acyl imidazoles solution to the reaction vessel. For example, in the case of the sample ESIC-14-2, due to a very fast addition of the acyl imidazole the DS obtained was 0.1 the lowest of this series as may be appreciated in Table 1. The DS estimated gravimetrically or by saponification show comparable results in most of the cases. However, for ESIC-22 samples the saponification method was not satisfactory due the poor solubility of these samples. This was also observed for the same derivatives synthesized by the other two methods. 7 

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Most of the ES obtained with highest DS, synthesized using acyl imidazoles, were insoluble in dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), chloroform, acetone, alcohols and water. However, they significantly swell in chloroform and THF but not in polar solvents as water, DMSO, acetone or methanol. This behavior is clearly different to that shown by NS which has no affinity for these organic solvents and

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shows a certain solubility in water specially after having been heated in a basic medium. This fact is a

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clear evidence that the modification was indeed carried out and also shows the hydrophobic character of these esterified starches.

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However, all derivatives obtained by using acid chlorides, as well as some derivatives obtained with low DS as ESIC-14-2, ESMC-14 and ESMC-16, were soluble in DMSO. This solubility behavior is in

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agreement with the literature where starch derivatives with low DS are reported to be soluble in this

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solvent [11].

FTIR spectra of all modified starches were similar, as may be expected for a series where the only

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difference is the alkyl side chain length. The spectra of the C-22 derivatives of each series are showed as

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an example in Figure 2, together with the NS spectrum used for comparison. The spectra of the derivatives show many of the characteristic bands of the starch. For ESIC-22-2 and

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ESMC-22 it can be observed a decrease of the O-H stretching band centered at about 3600 cm-1, a significant increase in the intensity of the stretching signal of C-H that appears near 2928 cm-1 and a new band at 2850-2855 cm-1, both corresponding to the stretching of the methyl and methylenes of the ester alkyl chain. These changes are less evident in the ESCC-22 due to the low DS of this sample. The spectra of all derivatives also exhibit a signal about 1720 cm-1 due to the stretching of the carbonyl group of the ester and another signal of medium to low intensity at 720 cm-1 characteristic of the long aliphatic chains. The band that appears at 1640 cm-1, which is associated with moisture present in the starch, is not observed in ESIC-22-2, it is very weak for ESMC-22 but it becomes significant for ESCC-22. These results are consistent with the values of the DS previously discussed. On the other hand, no signals due to the fatty acid or any other reagents are observed in the spectra, suggesting that the samples are fairly pure. 8 

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In the case of ESCC-n derivatives, in addition to being obtained with low DS and in smaller amount than for the other series, they also suffered degradation or side reaction during synthesis as may be evidenced by NMR. The 1H NMR spectra in DMSO of ESIC-14-2, ESMC-14 and ESCC-14 are compared in Figure 3. The spectra of ESIC-14-2 and ESMC-14 are quite similar and most signals were assigned according to

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previous reports in the literature [11, 34-35]. The signals corresponding to the anhydroglucose units

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appear between 5.7 and 3.5 ppm. The peaks at 4.6, 5.2 and 5.6 ppm (labeled with asterisks) correspond to the hydroxyl groups of the unsubstituted starch units; the peak located at 5.1 ppm to the anomeric proton

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and the signal centered at 3.6 ppm is due to the protons 2 to 6. The two weak signals appearing behind the anomeric proton near 5.0 pm are due to the protons located at position 6’. Protons located in position 6 on

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the anhydroglucose unit, can suffer a shifting from about 0.5 to 1.5 ppm to lower field when they are

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acylated. Its low intensity also corroborates the low degree of substitution of the samples. Finally, the shoulders that appear at low and high field of the signals at 5.6 and 4.6 ppm respectively (best viewed in

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ESMC-14 spectrum) are due to hydroxyl groups of acylated units. The methylene and methyl protons of

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the side chains of the acylated starch unit appear between 2.3 and 0.9 ppm and their assignations are indicated in Figure 3. The areas ratio between the protons of anhydroglucose unit and the side chains

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protons allow us to estimate the DS of these samples. While the 1H NMR spectra of ESIC-14-2, ESMC-14 are similar, the spectrum of ESCC-14 shows significant differences, the most relevant being a change of the hydroxyl protons signals of the anhydroglucose ring into a broad pattern centered at approximately 3.5 ppm which couldn’t be assigned. The changes in the spectrum might be due to side reactions involving, among others, exchange reactions between chlorine and hydrogen atoms of the anhydroglucose ring because of the high reactivity of the acid chlorides. These exchange reactions can also be favored by the presence of the formiate which is a good leaving group. The spectroscopic results together with the solubility of the samples confirm beyond all doubt that the modified esters were obtained. 9 

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3.2 Thermal properties The thermal stability of the polymers was evaluated by TGA. In Figure 4 TGA traces of the whole ESIC-

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n series are shown together with that corresponding to NS. As it may be appreciated, the degradation process takes place in one step and all the modified esters begin to lose weight at temperatures near

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300°C. The total mass loss in the process is of about 90%. The maximum decomposition temperatures are

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observed between 365 and 332°C and these values tend to diminish with the increase of the side chain length, but in all cases the decomposition temperature corresponding to the maximum of the DTGA curve

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is higher than the observed one for NS (330ºC) as may be seen in Table 1.

All the esterified starches synthesized using acid chlorides and methyl esters begin to lose weight at lower

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temperatures than those observed for the ESIC-n series and their decomposition temperatures are lower than those of NS (see Table 2). As previously discussed, the ESCC-n suffered a prior degradation process

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or side reactions during the esterification process and these facts probably reduce their thermal stability.

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On the other hand, although the ESMC-n samples do not show degradation evidence by NMR, their

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synthesis was carried out at 100ºC for a long time in a strong basic media, and it is well known that in these conditions the starch undergoes chain breakages with the consequent reduction of the molecular weight and hence of their thermal stability [36-37]. In Figure 4, it can be observed that the NS loses approximately 10% of its total weight in a process that begins at a temperature below 100°C and which is due to the loss of moisture present in the sample. This loss is not observed for the other samples, thus confirming the hydrophobic nature of the esterified starches. In order to elucidate how the thermal degradation occurs for the modified starch, a preliminary analysis by TVA was carried out with representative members of all the series. Samples were heated to a temperature near 300ºC to collect and examine by FTIR the volatiles produced during the degradation process.

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Almost all bands of the FTIR spectra of the volatile fraction condensed in the cold ring at room temperature, correspond basically to those of the fatty acid, regardless of the DS, side chain length or the series, clearly indicating that during the degradation process the cleavage of the ester takes place with the consequent release of the side chain as a fatty acid. In the fraction condensed at liquid nitrogen

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temperature, water, CO and CO2 were detected. The water probably is formed by a similar process to that

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present in the thermal degradation of starch, in which the condensation of hydroxyl groups of the anhydroglucose ring takes place with the subsequent dehydration [38-39]. Additionally, the presence of

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CO2 and CO is confirmed by the presence of the bands that appear at 2359 cm-1 and at 2170 and 2120 cmrespectively in the FTIR spectra of the fraction recovered in the gas cell. These products are mainly

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formed by decarboxylation of the carboxylic group of the ester.

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Fatty acids formation, is not completely clear, it could imply an alpha-beta elimination with the consequent generation of a double bond in the substituted anhydroglucose units, or an hydrolysis of the

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ester group by the adsorbed or produced water during the heating or a combination of both processes. In

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the first case, no double bounds were observed by FTIR in the residue of the degraded samples, which additionally had not given a positive test with Br2/CCl4 solution. These results might suggest that the

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process of alpha-beta elimination does not occur, although the low concentration of double bonds could also make it difficult its detection. In our opinion, hydrolysis of the ester moiety seems a more reasonable way for fatty acid formation. The heating at temperatures around 300°C causes the starch dehydration [40] and then the liberated water would be responsible for the process of hydrolysis. In order to determine if the n-alkyl chains of the ester inserted to the starch were able to crystallize a DSC study was carried out. In Figure 5 are shown the DSC runs for a representative member of the series of ESIC-n. Thermograms correspond to the second heating in order to have samples with the same thermal history.

It can be observed that the 11 

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thermograms of ESIC-12 and 14 show no significant transition between -30 to 100ºC. However, ESIC-16, 18 and 22 display an endothermic transition in which the enthalpy ΔH and melting

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temperature (Tm) rise when the side chain length increases. These transitions are attributed to

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the melting of the n-alkyl side chains and as it is illustrated in the inset of Figure 5, these

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transitions are reversible on cooling and reheating. This shows that after reaching a certain size,

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the n-alkyl side chains are able to crystallize in paraffinic phases, as is a common feature in the comb like polymers. In the case of ESIC-16 the transition is too weak suggesting that the

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paraffinic phase must be poorly crystallized. In Table 1 and in the foot note of Table 2, ΔH and Tm

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the DSC thermograms.

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are reported for all the samples that showed thermal transitions associated with the melting process in

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Although in this study only few points were available, a plot of ΔH vs DS suggest that ΔH value increases in a more or less linear way with the rise of the DS as may be appreciated in Figure 6 for ESIC-18 y ESC-22 derivatives. The minimum DS values which allow crystallization of the side chains in each case may be estimated by the intersection value with the DS axis after a linear extrapolation. Values for ESIC-18 and ESIC-22 are 0.4 and 0.14, respectively. Due to the lack of available samples of ESIC-16 with lower DS it was not possible to do a similar estimate for this derivative. Nevertheless, the melting enthalpy for ESC-16 with a DS = 1 is significantly lower than those observed for ESC-18 and 22 with lower DS values. It is also noticeable that, with the exception of ESMC-22, none of the

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ESMC-n or ESCC-n derivatives with side n-alkylic chain with 16 or more carbon atoms were able to crystallize; this fact can be attributed to the low DS that in the cases of ESMC-18 and ESCC-18 and 22 did not reach the estimated minimum values.

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In order to identify the crystalline phase that was formed and which fusion was detected by DSC, WAXS

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experiments in a variable temperature cell were performed. The diffractograms of ESIC-18 and 22 were recorded at various temperatures and they are showed in Figure 7 (a) and (b). At room temperature, which

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is below the melting temperature determined by DSC, the mean interplanar spacing is at 4.14 Å,

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characteristic of a crystalline paraffinic phase formed by the n-alkyl side chain of the ester moiety and it appears as a very sharp reflection. At higher temperatures this sharp reflection is replaced by a broad halo

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centered at 4.6 Å caused by the amorphous material indicating the fusion of the paraffinic crystalline structure. In Figure 7 it can be observed for both samples that the crystallinity is almost totally recovered

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after cooling at temperatures below the melting ones obtained by DSC. Only a somewhat smaller width is

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observed for the ESIC-22 indicating that the sample kept at room temperature has more perfect lamellae than after a short anealing at 10ºC. When comparing the recrystallized ESIC-18 and ESIC-22 it can to be

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noted that the former gives narrower Bragg peaks indicating that the lamellae perfection is higher even though the alkyl chains are shorter than in ESIC-22 but it has a higher DS (see Table 1). These results agree with the crystallinity values reported in Table 1 after DSC experiments. The diffractogram of ESMC-18 (DS=0.1) appears as a broad halo centered 4.6 Å at all temperatures, indicating the absence of crystallization, which is in agreement with DSC results. In similar WAXS experiments performed on ESIC-16 the existence of crystallization was not as evident. At 20°C a broad reflection is recorded at 4.44 Å which shifts to 4.55 Å at 70 ºC. When the sample is now cooled to -20 ºC the peak becomes somewhat narrower and its center corresponds to a distance of 4.26 Å. These variations are indicative of the existence of very imperfect paraffinic crystals in agreement with the weak DSC trace. The existence of more imperfect, thinner and less abundant crystals than in the samples 13 

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with longer n-alkyl side chains is thus demonstrated (see Table 1). Finally, the shorter lateral chain derivatives were 100% amorphous regardless their DS, as showed on their DSC trace and WAXS spectra.  

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Conclusions The three methods employed here allowed the obtention of esterified starches. However, the DS varies

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with the chosen method. The best results were obtained when acyl imidazoles were used as the modifying

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agent. With acid chlorides and methyl esters as modifying agents the products suffered degradation or side reactions and in both cases the DS are significantly lower than those obtained with the first method. All

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the modified starches obtained with high DS were insoluble in water or organic solvents but swell in chloroform and THF. All the modified starches begin to lose weight near 300ºC and the thermal stability

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of ESIC-n is slightly greater than ESMC-n and ESCC-n. During the thermal degradation the cleavage of the ester takes place with the consequent release of the side chain as a fatty acid. When the aliphatic side

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chains of the esters contain 16 or more carbon atoms they are able to crystallize in paraffinic phases

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independent of the carbohydrate backbone whose perfection increased with the number of carbon atoms in

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the side chains and the degree of substitution, as usually found in comb-like polymers.

Acknowledgements

This work has been supported by the Consejo de Desarrollo Científico Humanístico y Tecnológico de la Universidad de Los Andes, Mérida (Venezuela) (CDCHT-ULA) through the grants C-1744-08-A and C1745-11-08-ED and FONACIT (grant G2005-000776). The authors also thank Dr. Alí Bahsas of the Laboratorio de Resonancia Magnética Nuclear, Departamento de Química, Facultad de Ciencias, Universidad de Los Andes (Mérida, Venezuela) for performing the NMR experiments and helping with the interpretation of the spectra.  

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behaviours, Carbohydrate Polymers 56 (2004) 111–122. [7] D. Briassoulis, An overview on the mechanical behavior of biodegradable agricultural films, Journal of Polymers and the Environment 12 (2004) 65–81. [8] S. Jaspreet, K. Lovedeep, O.J. Mc Carthy, Factors influencing the physico-chemical, morphological, thermal and rheological properties of some chemically modified starches for food applications-A review, Food Hydrocolloids 21 (2007) 1-22. [9] T. Heinze, A. Koschella, Carboxymethyl ethers of cellulose and starch – A review, Macromolecular Symposia 223 (2005) 13-39. [10] J. Aburto, I. Alric, E. Borredon, Preparation of long-chain esters of starch using fatty acid chlorides in absence of an organic solvent, Starch/Stärke 51 (1999) 302–307. 15 

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[11] H. Muljana, S. van der Knoop, D. Keijzer, F. Picchioni, L.P.B.M. Janssen, H.J. Heeres, Synthesis of Fatty Acid Esters in Supercritical Carbon Dioxide, Carbohydrate Polymers 82 (2010) 346–354. [12] J.M. Fang, P.A. Fowler, J. Tomkinson, J. Hill, The preparation and characterization of a series of

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chemically modified potato starches, Carbohydrate Polymers 47 (2002) 245–252. [13] J. Kapusniak, P. Siemion, Thermal reactions of starch with long-chain unsaturated fatty acids. Part 2.

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Linoleic acid, Journal of Food Engineering 78 (2007) 323–332.

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[14] A.D. Sagar, E.W. Merrill, Properties of fatty-acid esters of starch, Journal of Applied Polymer Science 58 (2003) 1647–1656.

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[15] R. Bhosale, R. Singhal, Effect of octenylsuccinylation on physicochemical and functional properties

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of waxy maize and amaranth starches, Carbohydrate Polymers 66 (2006) 521–527. [16] H. Chi, K. Xu, D. Xue, C. Song, W. Zang, P. Wang, Synthesis of dodecenyl succinic anhydride

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(DDSA) corn starch, Food Research International 40 (2007) 232–238.

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[17] A.N. Jyothi, K.N. Rajasekharan, S.N. Moorthy, J. Sreekumar, Microwave assisted synthesis and

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characterization of succinate derivatives of Cassava (Manihot Esculenta Crantz) starch, Starch/Stärke 57 (2005) 319–324.

[18] I. Rivero, V. Balsamo, A.J. Müller, Microwave-assisted modification of starch for compatibilizing LLDPE/starch blends, Carbohydrate Polymers, 75 (2009) 343–350. [19] F. Bien, B. Wiege, S. Warwel, Hydrophobic modification of starch by alkali-catalyzed addition of 1,2-epoxyalkanes, Starch/Stärke 53 (2001) 555–559. [20] U. Funke, M. Lindhauer, Effect of reaction conditions and alkyl chain lengths on the properties of hydroxyalkyl starch ethers, Starch/Stärke 53 (2001) 547–554.

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[21] T. Heinze, V. Haack, S. Rensing, Starch derivatives of high degree of functionalization. 7. Preparation of cationic 2-hydroxypropyltrimethylammonium chloride starches, Starch/Stärke 56 (2004) 288–296.

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[22] S. Pal, D. Mal, R.P. Singh, Cationic starch: An effective flocculating agent, Carbohydrate Polymers 59 (2005) 417–423.

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[23] W. Lazik, T. Heinze, K. Pfeiffer, G. Albrecht, P. Mischnick, Starch derivatives of a high degree of

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functionalization. VI. Multistep carboxymethylation, Journal of Applied Polymer Science 86 (2002) 743– 752.

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[24] Z.P. Stojanovic, K. Jeremic, S. Jovanovic, M.D. Lechner, A comparison of some methods for determination of the degree of substitution of carboxymethyl starch, Starch/Stärke 57 (2005) 79–83.

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[25] U. Newman, B. Wiege, S. Warwel, Synthesis of Hydrofobic Starch Esters by Reaction of Starch with

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Various Carboxylic Acid Imidazolides, Starch/Stärke 54 (2002) 449–453.

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[26] J. Aburto, I. Alric, E. Borredon, Organic solvent-free transesterification of various starches with

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[28] F. López-Carrasquero, S. Montserrat, A. Martínez de Ilarduya, S. Muñoz-Guerra, Structure and thermal properties of new comblike polyamides: Helical poly (β-L-aspartate)s containing linear alkyl side chains, Macromolecules 28 (1995) 5535–5546. [29] M. Morillo, A. Martínez de Ilarduya, S. Muñoz-Guerra, Comblike alkyl esters of biosynthetic poly (γ-glutamic acid). 1. Synthesis and characterization, Macromolecules 34 (2001) 7868–7875.

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[36] S. Barrios, J. Contreras, F. Lopez-Carrasquero, A.J. Müller, Chemical modification of cassava starch by carboxymethylation reactions using sodium monochloro acetate as modifying agent, Revista de la Facultad de Ingeniería U.C.V., (2012). Acepted for publication. [37] S. Mollega, S. Barrios, J.L. Feijoo, J. Contreras, A.J. Muller, F. López-Carrasquero, Modificación química de almidón de yuca nativo Mediante la reacción de carboximetilación en medio acuoso, Revista de la Facultad de Ingeniería U.C.V., 26 (2011) 77-88.

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resolution solid-state NMR spectroscopy, Polymer 43 (2002) 5791–5796.

13

C high-

ip t

[39] X. Zhang, J. Golding, I. Burgar, Thermal decomposition chemistry of starch studied by

cr

[40] X. Liu, L. Yu, F. Xie, M. Li, L. Chen, X. Li , Kinetics and mechanism of thermal decomposition of

Ac ce p

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d

M

an

us

cornstarches with different amylose/ amylopectin ratios. Starch/Stärke 2010, 62, 139–146.

19 

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Table 1. Results obtained in starch esterification with acyl imidazoles(a).

(g)

ES (g)

DS

(d)

(e)

DS

Td (f)

ΔH2(h)

Tm(h)

(ºC)

(J/g)

(°C)

365

-

-

cr

NS

(c)

1.92

0.79

0.94

ESIC-12-2

1.00

2.01

0.87

0.86

-

-

-

ESIC-14-1

1.00

2.57

1.20

-

351

-

-

ESIC-14-2(g)

1.01

1.21

0.15

0.10

-

-

-

ESIC-14-3

1.00

2.32

1.00

0.94

-

-

-

ESIC-16-1

1.00

2.53

1.03

1.13

343

10.7

-14.5

ESIC-16-2

0.50

1.26

1.02

-

-

10.1

-14.1

ESIC-18-1

1.02

1.93

0.54

0.68

-

7.2

-9.0

ESIC-18-2

1.00

2.32

0.80

0.85

347

27.4

14.7

ESIC-18-3

1.00

1.66

0.66

0.72

-

14.9

-5.5

ESIC-22-1

0.50

0.89

0.51

-

332

21.1

33.0

0.50

1.18

0.68

-

-

32.6

36.0

te

Ac ce p

ESIC-22-2

an

us

1.01

d

ESIC-12-1

M

Derivative

(b)

ip t

TABLES

(a) In DMSO at 90ºC during 3 h using NaOCH3 as base and with a relation acyl imidazol/NS 2:1 (mol:mol) (b) Native starch (NS) used in the synthesis. (c) Amount product obtained after the purification. (d) Degree of substitution estimated gravimetrically (see text). (e) Degree of substitution estimated by saponification. (f) Decomposition temperature corresponding to the maximum of the DTGA curve. Td for NS 330ºC. (g) Soluble in DMSO, DS 0.12 (determined by NMR). (h) Measured by DSC.

20 

Page 20   of 29

 

 

 

 

Table 2. Results obtained in starch esterification with acid chlorides and methyl esters. NS(c) (g)

ES(d) (g)

DS(e)

DS(f)

ESCC-12

1.00

1.09

-

0.13

315

ESCC-14

1.00

1.01

-

0.62

322

ESCC-16

1.01

0.98

-

ESCC-18

1.01

0.83

-

ESCC-22

1.00

0.96

-

0.50

0.65

0.50

0.60

ESMC-16

0.50

0.59

ESMC-18

1.03

1.15

ESMC-22(i)

1.00

1.55

Derivative

0.36

312

-

310

0.21

0.23

308

0.15

0.20

309

0.13

0.06

309

0.08

0.10

315

0.33

-

307

an M

d

ESMC-14

(h)

cr

314

us

0.20

Methyl esters(b) ESMC-12

ip t

Acid chlorides(a)

Td (g) (ºC)

Ac ce p

te

(a) Carried out in absence of solvent under N2 stream. HCOOH is initially added and maintained at 25°C for 4 min and then the acyl chloride is added and the reaction is carried on at 55°C for 2h. (b) In DMSO at 100°C and 10mmHg 100ºC during 6 h and using CH3OK. (c) Native starch (NS) used in the synthesis. (d) Esterified starch after purification. (e) Degree of substitution obtained gravimetrically (see text). (f) Degree of substitution obtained by saponification. (g) Decomposition temperature corresponding to the maximum of the DTGA curve. Td for NS 330ºC. (h) DS 0.04 (determined by NMR). (i) Melting transition of the n-alkyl side chain measured by DSC ΔH = 12.4 J/g and Tm = 22.4ºC.

21 

Page 21   of 29

 

 

 

 

Figure captions

Figure 1. Synthesis of esterified starches using acyl imidazoles, acid chlorides and methyl esters. Although in the

ip t

Figure the substitution is represented only on the OH in position 6, it also can occur on hydroxyl groups located at

cr

positions 2 or 3.

us

Figure 2. FTIR spectra of a representative member of each series compared with the native starch (NS).

an

Figure 3. 1H NMR spectra in DMSO-d6 of some esterified starch with side chains of 14 carbons and low DS.

M

Figure 4. TGA traces for the series of starch modified with the acyl imidazoles compared with the native starch.

Figure 5. (A) DSC heating scans at 10°C/min for series of starch modified with the acyl imidazoles (B) Insert

te

d

ESIC-18-2 (a) first heating, (b) cooling, (c) second heating.

Figure 6. Variation of the melting enthalpy vs. the degree of substitution. (●) ESIC-18 and (○) ESC-22.

1.54184 Å).

Ac ce p

Figure 7. WAXS of the ESIC-18 (a) and ESIC-22 (b) at room temperature, after melting and recrystallization. (λ = 

22 

Page 22   of 29

O R

N

N

Method 1:

ip t

[NaOCH3] / DMSO

1)

H

HO H

H

O

Method 2:

O

H

H

OH

H

2)

HO O

25°C / 5 min

O

n R

Cl

55°C / 2 h

HO

us

OH H

cr

O

H

R

O H

H

O H

HO O

n

an

R = C 11H23, C 13H27, C 15H31, C 17H35, C 21H43

O

Method 3:

M

R

O

CH3

Figure 1

Ac ce p

te

d

[KOCH3] / DMSO

(Barrios et al., Characterization of esterified cassava starch with long alkyl side chains and different

substitution degrees).

Page 23 of 29

ip t cr

% Transmittance

NS

%T

us

ESIC-22-2

an

ESMC-22

4000,0

3600

3200

2800

2400

d

M

ESCC-22

2000

1800

1600

1400

1200

1000

800

600

600

400,0

400

te

4000 3600 3200 2800 2400 2000 1800cm-11600 1400 1200 1000 800

Ac ce p

Wavenumber (cm-1)

Figure 2

(Barrios et al., Characterization of esterified cassava starch with long alkyl side chains and different

substitution degrees).

Page 24 of 29

8

O

10

7

OH

H

HO 3

13

15

18 17

20 19

H 6’ O

O H2

1 H

4

5

HO

H 3

HO O

H

11

16

O H2 1

H

HO O

H

ip t

5

9

14

m

n

cr

4

H 6

12

H10-19

H1

us

H2-6 *

**

H8

an

ESIC-14-2 water

H20

DMSO-d6

M

H6’

H9

ESCC-14 5

4

3

2

1

[ppm]

0

Ac ce p

6

te

d

ESMC-14

Figure 3

(Barrios et al., Characterization of esterified cassava starch with long alkyl side chains and different

substitution degrees).

Page 25 of 29

ip t

110 100

cr

90

us

70 60

40

30 20 10 25 50

100

150

an

ESIC-12-1 ESIC-14-4 ESIC-16-1 ESIC-18-2 ESIC-22-2 NS

50

M

Weight (%)

80

200

250

300

350

400

450

500

Ac ce p

te

d

Temperature (ºC)

Figure 4

(Barrios et al., Characterization of esterified cassava starch with long alkyl side chains and different

substitution degrees).

Page 26 of 29

ip t

ESIC-18-2

cr

(a)

(c)

-20

0

20

40

60

80

100

an

Temperature (ºC)

us

(b)

5 mW

Heat flow (Endo)

(B)

ESIC-22-2

E) AlmC22

ESIC-18-2

D) AlmC18

ESIC-16-1

C) AlmC16

ESIC-14-1 B) AlmC14 ESIC-12-1

A) AlmC12

2 mW

Ac ce p

te

d

Heat flow (Endo)

M

(A)

-20

Figure 5

0

20

40

60

80

100

120

Temperature (ºC)

(Barrios et al., Characterization of esterified cassava starch with long alkyl side chains and different

substitution degrees).

Page 27 of 29

ip t

35 30

cr

20 15

us

H (J/g)

25

5 0

0.2 0,2

0.3 0,3

0.4 0,4

0.5 0,5

an

10

0.6 0,6

0.7 0,7

0.8 0,8

0.9 0,9

Ac ce p

te

d

M

Gravimetricdegree degreeof ofsubstitution substitution (DS) (DS) Gravimetric

Figure 6

(Barrios et al., Characterization of esterified cassava starch with long alkyl side chains and different

substitution degrees).

Page 28 of 29

ip t cr us an M d te Ac ce p

Figure 7

(Barrios et al., Characterization of esterified cassava starch with long alkyl side chains and different

substitution degrees).

Page 29 of 29