Modification of Pea Starch and Dextrin Polymers with ... - MDPI

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Modification of Pea Starch and Dextrin Polymers with Isocyanate Functional Groups Reza Hosseinpourpia 1, * ID , Arantzazu Santamaria Echart 2 , Stergios Adamopoulos 1 , Nagore Gabilondo 2 ID and Arantxa Eceiza 2, * 1 2

*

Department of Forestry and Wood Technology, Linnaeus University, Lückligs Plats 1, 35195 Växjö, Sweden; [email protected] Materials + Technologies’ Group, Department of Chemical & Environmental Engineering, Polytechnic College of San Sebastian, University of the Basque Country UPV/EHU, Pza. Europa 1, 20018 Donostia-San Sebastián, Spain; [email protected] (A.S.E.); [email protected] (N.G.) Correspondence: [email protected] (R.H.); [email protected] (A.E.); Tel.: +46-470-708074 (R.H.); +34-943-017185 (A.E.)





Received: 5 July 2018; Accepted: 21 August 2018; Published: 23 August 2018

Abstract: Pea starch and dextrin polymers were modified through the unequal reactivity of isocyanate groups in isophorone diisocyanate (IPDI) monomer. The presence of both urethane and isocyanate functionalities in starch and dextrin after modification were confirmed by Fourier transform infrared spectroscopy (FTIR) and 13 C nuclear magnetic resonance (13 C NMR). The degree of substitution (DS) was calculated using elemental analysis data and showed higher DS values in modified dextrin than modified starch. The onsets of thermal degradation and temperatures at maximum mass losses were improved after modification of both starch and dextrin polymers compared to unmodified ones. Glass transition temperatures (Tg ) of modified starch and dextrin were lower than unmodified control ones, and this was more pronounced in modified dextrin at a high molar ratio. Dynamic water vapor sorption of starch and dextrin polymers indicated a slight reduction in moisture sorption of modified starch, but considerably lower moisture sorption in modified dextrin as compared to that of unmodified ones. Keywords: diisocyanate; DVS; pea starch; dextrin; functionalization; unequal reactivity

1. Introduction Starch, as the main reserve source and energy storage of plants, is widely available in agricultural products. This biopolymer has excellent applications in many industrial sectors such as food, textile, pharmaceutical, paper, and biofuel. Most of these applications rely on the colloidal properties of two structurally distinct α-D-glucan components of starch granules, amylose, and amylopectin [1]. Amylose is an essentially linear macromolecule with mostly α(1–4)-linked D-glucopyranosyl units and less than 0.5% of the glucose residues in α(1–6) linkages. Amylopectin is larger than amylose with α(1–4) linked chains and up to about 5% of the glucose residues in α(1–6) branch units [2]. Starch from different sources has different amylose/amylopectin ratios, and this, in turn, has a great effect on thermal behavior, mechanical strength and also accessibility of the starch granules to water molecule and other chemical reagents [3,4]. Over the last decades, starch has been modified chemically, physically, genetically, and enzymatically to enhance its functionalities and offer new applications. Chemical modification of starch has been carried out through cross-linking, acid hydrolysis, oxidation, etherification, esterification, cationization, and grafting of polymers [4–12]. These reactions partially or entirely

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destruct the hydrogen bonds of the starch and induce distinct changes in the properties of the ensuing polymers. Some examples of chemically modified starch are biodegradable packaging materials, thermoplastic starch (TPS), and thin films with improved mechanical and thermal properties. However, modification of starch polymers with isocyanate functional groups has received very little attention. Barikani and Mohammadi [5] grafted polycaprolactone terminated hexamethylene diisocyanate (HDI) onto the hydroxyl groups of corn starch polymer. Modified starch showed higher hydrophobicity than unmodified. Modification of starch nanoparticles with HDI also increased the hydrophobicity of thin films and enhanced their thermal stability and electrical conductivity [9]. Cross-linking of corn starch with a polypropyleneoxide toluene diisocyanate (PTD) oligomer at ambient temperature led to the production of amorphous elastomeric networks with reduced swelling capacity in different solvents due to the formation of polyurethane linkages [13]. The DSC curves showed an absence of glass transition temperature (Tg ) in unmodified starch but two defined Tg signals at −60 and 35 ◦ C were obtained, which were respectively attributed to the relaxation of the grafted oligopropylene oxide chains and possible glass transition of the starch macromolecules in the network. Isophorone diisocyante (IPDI) is an aliphatic isocyanate that contains primary and secondary isocyanate groups with unequal reactivity. Modification of cellulose nanocrystals with IPDI monomer facilitated the covalent bonding between the modified cellulose and polymer matrix, and brought a greater mechanical strength than that of unmodified cellulose nanocrystals [14]. Gómez-Fernández et al. [15] functionalized a kraft lignin with IPDI for producing polyurethane flexible foams. It was reported that the modification of lignin contributed to a higher chemical bonding of the lignin to the polyurethane matrix and also to the production of more soft and flexible foams than unmodified lignin containing foams. Reactivity of the secondary and primary isocyanate groups of the IPDI isomers increases by using a proper catalyst. Lewis acids such as dibutyltin dilaurate (DBTDL) preferentially enhance the reactivity of the cycloaliphatic (secondary) isocyanate group, while Lewis base like trimethylamine or 1,4-diazabicyclo[2.2.2]octane (DABCO) catalyzes the reaction of the primary isocyanate group [16–19]. Enhancement of the unequal reactivity of IPDI isocyanate groups by using DBTDL catalyst should make possible a reaction with the functional groups of natural polymers, like the hydroxyl groups of starch, through their higher reactive sites (e.g., –NCO groups connected to the rings). A pendant isocyanate group, which is sterically hindered by the neighboring methyl group, should then be available for reaction with additional monomers of interest. Therefore, this study aims at confirming the hypothesis by studying the highly functionalized pea starch and dextrin polymers with IPDI monomer and DBTDL reaction catalyst. Characterization of the modified starch and dextrin involved elemental analysis, Fourier transform infrared spectroscopy (FTIR), 13 C nuclear magnetic resonance (13 C NMR) in solid state, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). The effect of functional modification on water vapor sorption behavior of modified samples was assessed by using a dynamic vapor sorption (DVS) apparatus. 2. Materials and Methods 2.1. Materials Pea starch and pea dextrin were kindly provided by Emsland-Stärke GmbH (Wietzendorf, Germany). The functionalization of starch and dextrin polymers with isocyanate groups was carried out using isophorone diisocyanate, IPDI (>99.5%, DesmodurI® , kindly provided by Covestro, Leverkusen, Germany), with a NCO content of 37.8%. HPLC grade anhydrous dimethyl sulfoxide (DMSO) (>99.8%, Macron Fine Chemicals, Paris, KY, USA) was used as reaction medium and dibutyltin dilaurate DBTDL (>95%, Sigma Aldrich, Saint Louis, MO, USA) catalyzed the reaction of hydroxyl groups from glucoside unities of starch and dextrin with the secondary NCO groups of IPDI. HPLC grade toluene (>99.8%, Lab-Scan Analytical Sciences, Gliwice, Poland) and tetrahydrofuran (THF) (>99.8%, Macron Fine Chemicals) were used to wash the polymers and remove the unreacted IPDI after modification.

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2.2. Functionalization of Starch and Dextrin 1 g of vacuum oven dried-starch was dissolved in 70 mL of DMSO at 90 ◦ C by magnetic stirring for 120 min. The same concentration of dextrin was added to DMSO and dissolved at 40 ◦ C by magnetic stirring for 120 min. A separate 3-neck flask partially submerged in an oil bath containing IPDI was heated to 60 ◦ C under a flowing nitrogen atmosphere. The DBTDL (0.1% of the total weight) was then added to the IPDI and allowed to stir for 5 min. The dissolved starch was added dropwise to the stirring IPDI/DBTDL mixture with a separatory funnel over 45 min while maintaining the vigorous stirring in the flask. The reaction proceeded at 60 ◦ C for 24 h in a flask with N2 inlet. The reaction was run in excess of IPDI at a weight ratio of NCO:OH of 3:1 and 6:1. The reaction was halted by addition of THF. The DMSO and THF were then removed by 3 times centrifugation at 4500 rpm for 5 min replacing the supernatant by fresh THF after each centrifugation. The precipitates were then further washed with toluene and centrifuged at 4500 rpm for 10 min, and repeated 3 times, followed by replacing the used toluene with the fresh one after each centrifugation in order to remove the remaining unreacted IPDI. An identical procedure was applied to produce the modified dextrin. Finally, the modified starch at different ratios, namely MS 3.1 and MS 6.1, as well as the modified dextrin, called MD 3.1 and MD 6.1, were dried in a vacuum oven at 50 ◦ C for 24 h, and then the vacuum was kept at room temperature for the further 24 h. The samples were then stored in a tight bottle. 2.3. Characterization 2.3.1. Elemental Analysis The carbon, hydrogen, oxygen and nitrogen contents of unmodified and modified starch and dextrin were determined by elemental analysis using an EuroVector EA 3000 atomic absorption spectrometer (Pavia, Italy) heated at 980 ◦ C with a constant flow of helium. 2.3.2. Fourier Transform Infrared Spectroscopy The chemical structure of the unmodified and modified starch and dextrin as well as of the IPDI monomer was analyzed with FTIR (Nicolet-Nexus, Waltham, MA, USA) provided with a MKII Golden Gate accessory (Specac, Orpington, UK) with a diamond crystal at a nominal incidence angle of 45◦ and ZnSe lens. Spectra were recorded in attenuated reflection (ATR) mode and the evaluation was performed between 4000 and 400 cm−1 at room temperature, averaging 64 scans with resolution of 4 cm−1 . The samples were dried at 50 ◦ C for 24 h prior to the analysis. 2.3.3. Nuclear Magnetic Resonance Spectroscopy The insertion of isocyanate functional groups in starch and dextrin polymers were verified with solid-state 13 C NMR using a Bruker Avance III 400 MHz equipment (Billerica, MA, USA). The spectra were recorded using a decoupled sequence at 13 C frequency of 100.6338 MHz at 25 ◦ C. Samples were measured at a spinning rate of 10,000 Hz averaging 4096 scans with a recycling delay of 10 s. A time domain of 2 K was employed with a spectral width of 30 KHz. 2.3.4. Thermogravimetric Analysis Thermal stability of the modified starch and dextrin was analyzed using a Q500 TA equipment (New Castle, DE, USA). Around 5 mg of dried sample (24 h at 50 ◦ C) were heated from 50 to 650 ◦ C at a rate of 10 ◦ C·min−1 , under a flowing nitrogen atmosphere. 2.3.5. Differential Scanning Calorimetry The changes in glass transition temperature (Tg ) of starch and dextrin associated with functional modification with IPDI were performed using a DSC analyzer (Mettler Toledo DSC3+ equipment,

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Columbus, OH, USA), from −50 to 200 ◦ C at a heating rate of 30 ◦ C min−1 under a nitrogen flow of 10 mL·min−1 . Approximately 5 mg of oven-dried sample (at 50 ◦ C for 24 h) was used for each analysis. 2.3.6. Dynamic Vapor Sorption The vapor sorption behavior of the modified polymers was determined using a DVS apparatus (Q5000 SA, TA Instruments, New Castle, DE, USA) as reported previously [20–22]. Approximately 8 mg of oven-dried (at 50 ◦ C for 24 h) modified and control starch and dextrin granules were used for each measurement. The relative humidity (RH) increased from 0 to 90% in step sequences of 15% and then 5% from 90 to 95% RH. The instrument maintained a constant target RH until the mass change in the sample (dm/dt) was less than 0.01% per minute over a 10 min period. The target RH, actual RH, sample mass and running time were recorded every 30 s during the sorption run. The moisture content of the granules was calculated based on their equilibrium weight at each given RH step throughout the sorption run measured by the DVS device. 2.3.7. Scanning Electron Microscopy The morphology of the starch and dextrin granules before and after modification was characterized by SEM using a JEOL JSM-7000F equipment (Akishima, Japan) operating at 20 kV and a surrounding beam current between 0.01 and 0.1 nA, taking secondary electron images. Dried samples were placed over a double-sided carbon-based conductive tape and covered with a 20 nm Au in order to make the surface conductive. 3. Results and Discussion 3.1. Elemental Analysis and Degree of Substitution (DS) The results of elemental analysis and DS of the unmodified and modified starch and dextrin are given in Table 1. These results indicate a negligible nitrogen content of unmodified starch (S-control) and dextrin (D-control), but considerable amounts of nitrogen were found in modified samples. The content of nitrogen in MS 3.1 and MS 6.1 are 5.28% and 6.24%, respectively, and respective amount of nitrogen in MD 3.1 and MD 6.1 are 7.38% and 8.12%, respectively. The enhanced nitrogen contents of modified starch and dextrin suggested that IPDI was successfully substituted with the hydroxyl groups of polymers’ backbones. Table 1. Elemental composition of unmodified and modified starch and dextrin. C

H

O

N

DS

S-control MS 3.1 MS 6.1

38.60 45.58 46.97

6.53 7.45 7.58

52.94 26.96 27.92

0.08 5.28 6.24

1.11 1.54

D-control MD 3.1 MD 6.1

39.33 48.40 53.13

6.42 7.98 8.02

52.45 23.68 24.08

0.03 7.38 8.12

2.29 3.04

Although the molar ratio of IPDI in reaction medium of MS 6.1 was two times more than MS 3.1, the DS value of MS 3.1 (0.55) was slightly lower than that (0.77) of MS 6.1. This might be explained by the different reactivity of the three –OH groups in a glucose unit of the starch macromolecule. The primary C6 OH is more reactive and readily accessible than the secondary ones on C2 and C3 . The C2 OH is however more reactive than the C3 , mainly because the former is closer to the hemi-acetal and more acidic than the latter [23]. The hydroxyl groups in the dextrin polymer showed higher reactivity than those in the starch polymer, and thus the DS values of MD 3.1 and MD 6.1 were 1.15 and 1.52, respectively. This might be due to the higher solubility of dextrin in solvent than starch, and also to the low availability of starch’s hydroxyl groups due to the formation of gel-like product

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MD 6.1 were 1.15 and 1.52, respectively. This might be due to the higher solubility of dextrin in solvent starch, andreaction. also to the low availability of starch’s hydroxyl due to theof formation during thethan modification A more detailed description regardinggroups the calculation DS can be of gel-like product during the modification reaction. A more detailed description regarding the found in the Supplementary Materials. calculation of DS can be found in the Supplementary Materials. 3.2. Structural Characterization 3.2. Structural Characterization FTIR spectroscopy analysis was used to monitor changes in the structures of starch and dextrin FTIR spectroscopy analysis was to monitor changes in the structures of starch andspectra dextrin of polymers upon functionalization withused the IPDI monomer. Figure 1a,b illustrates the FTIR polymers upon functionalization with the IPDI monomer. Figure 1a,b illustrates the FTIR spectra of unmodified and modified starch and dextrin as well as of IPDI monomer. There were no differences unmodified and modified starch and dextrin as well as of IPDI monomer. There were no differences in the FTIR spectra of unmodified starch and dextrin, as they showed peaks at 3450 cm−1−1 , which is in the FTIR spectra of unmodified starch and dextrin, as they showed peaks at 3450 cm , which is 1 , which assigned to the stretching vibration of –OH group, and at 2960 cm− can be attributed to –CH assigned to the stretching vibration of –OH group, and at 2960 cm−1, which can be−attributed to –CH bond stretching vibration [24,25]. The absorption band between 1000 and 1200 cm 1 was characteristic bond stretching vibration [24,25]. The absorption band between 1000 and 1200 cm−1 was of the –CO stretching of the polysaccharide skeleton [9]. Obvious changes across all regions of the characteristic of the –CO stretching of the polysaccharide skeleton [9]. Obvious changes across all spectra were observed theobserved modifiedfor starch and dextrin. was an apparent in the regions of the spectrafor were the modified starchThere and dextrin. There wasdecrease an apparent −1 , which indicated that many of these functional –OH peak of modified starch and dextrin at 3450 cm −1 decrease in the –OH peak of modified starch and dextrin at 3450 cm , which indicated that many of −1 groups consumed. This consumed. together with appearance the isocyanate band at 2266 cm these were functional groups were This the together with theof appearance of the isocyanate band at , −1, which which related to the pendant –NCO groups, indicates successthe in success functionalization starch and 2266iscm is related to the pendant –NCO groups,the indicates in functionalization dextrin with IPDI [14,15,19]. After modification, the modified polymers were washed with toluene starch and dextrin with IPDI [14,15,19]. After modification, the modified polymers were washed three times. Thethree FTIR times. spectraThe of the third centrifuged product (see Figure S1) were with toluene FTIR spectra of the third centrifuged product (seealmost Figure comparable S1) were with the spectra of the neatthe toluene, thatwhich the –NCO peaks in the modified and almost comparable with spectrawhich of the indicates neat toluene, indicates that –NCO starch peaks in modified starch and dextrin be attributed to the attachment isocyanate into thepolymers. starch dextrin can be attributed to thecan attachment of isocyanate groups of into the starchgroups and dextrin −1 , and dextrin polymers. In addition, increase in the absorbance of –COcm groups at 1650–1750 cm−1, and of In and addition, increase in the absorbance of –CO groups at 1650–1750 bending vibration −1 − 1 bending vibrationvibration of –NH and stretching of C–N at cm confirmed thebonds presence of –NH and stretching of C–N at 1530vibration cm confirmed the1530 presence of urethane between urethane bonds between and starch and dextrin [14,15,26]. IPDI and starch and dextrinIPDI [14,15,26].

a Transmittance (a.u.)

IPDI

Transmittance (a.u.)

b

S-control MS 3.1 MS 6.1

IPDI

D-control

MD 3.1

MD 6.1

4000

3500

3000

2500

2000

1500

1000

-1

Wave number (cm ) Figure 1. Fourier transform infrared spectroscopy (FTIR) spectra of IPDI, S-control, MS 3.1 and MS 6.1 (a), and IPDI, D-control, MD 3.1 and MD 6.1 (b).

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Figure 1. Fourier transform infrared spectroscopy (FTIR) spectra of IPDI, S-control, MS 3.1 and MS 6.1 (a), and IPDI, D-control, MD 3.1 and MD 6.1 (b).

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The structural changes of starch and dextrin polymers after modification were analyzed by meansThe of solid NMR. Figure of2a,b illustrate almostpolymers identical after spectra for unmodified starch and structural changes starch and dextrin modification were analyzed by dextrin, although unmodified dextrin showed a more intense signalforatunmodified 61.8 ppm, which assigned means of solid NMR. Figure 2a,b illustrate almost identical spectra starch is and dextrin, toalthough C6 [27,28]. The overlapping signal ata around 68–78signal ppm is associated with to C2C ,C , and unmodified dextrin showed more intense at collectively 61.8 ppm, which is assigned 6 3[27,28]. CThe 5 [2,29]. Glycosidic carbons C1 and 68–78 C4 profiles most sensitive to chainwith conformations and overlapping signal at around ppm are is collectively associated C2 , C3 , and[30], C5 [2,29]. therefore thecarbons resonances that atare 81.4most andsensitive 103.2 ppm due to the amorphous domains of the Glycosidic C1 and C4appear profiles to are chain conformations [30], and therefore the Cresonances 4 and C1, respectively [2,31]. Modification of starch andto dextrin with IPDI domains caused apparent that appear at 81.4 and 103.2 ppm are due the amorphous of the Cchanges 4 and C1 , on NMR spectra, andModification the intensityof ofstarch signalsand was more pronounced at theapparent high NCO:OH ratio respectively [2,31]. dextrin with IPDI caused changes on (6:1). NMR Appearance of new peaks between 20 and 50 ppm corresponded to hydrocarbons that are present in spectra, and the intensity of signals was more pronounced at the high NCO:OH ratio (6:1). Appearance the isocyanate; in detail, thecorresponded signal at 45.2 to ppm is associatedthat with methylene in of structure new peaksofbetween 20 and 50 ppm hydrocarbons arethe present in thegroup structure the aliphatic IPDI ring, peaks at 26.9 are attributed to the tertiary carbons, signals of isocyanate; in detail, the signal at and 45.2 32.3 ppmppm is associated with the methylene group in and the aliphatic atIPDI 27.8ring, and 23.5 ppm are due theppm methyl groups [14]. appearance of new between peaks at 26.9 and to 32.3 are attributed to The the tertiary carbons, andpeaks signals at 27.8120 and and ppm correspond to urethane ppm) and (122.2–129.5) linkages in ppm the 23.5160 ppm are due to the methyl groups(155.2–158.0 [14]. The appearance of –NCO new peaks between 120 and 160 starch and dextrin polymers [14,15].ppm) The observation of distinct peaks forinthe urethane correspond to urethane (155.2–158.0 and –NCO (122.2–129.5) linkages theprimary starch and dextrin linkage can[14,15]. be attributed to the existence of excess which mayurethane cause some sidecan reactions with polymers The observation of distinct peaksIPDI, for the primary linkage be attributed the groups ofof starch and dextrin polymers. Thesome primary –NCOwith group at 123.6 ppm to OH the existence excess IPDI, which may cause sidefree reactions theappears OH groups of starch and secondary free –NCO group free is present 129.5appears ppm. The availability of the freesecondary primary and andthe dextrin polymers. The primary –NCO at group at 123.6 ppm and free secondary –NCO groups in the and dextrin polymers induce them –NCO as remarkable –NCO group is present at 129.5 ppm.starch The availability of free primary and secondary groups in bio-macromolecules forpolymers further reaction with as other polymerbio-macromolecules matrices. The NMRforresults the starch and dextrin induce them remarkable furthertogether reaction with the FTIR and elemental analysis data, confirmed that the starch and dextrin polymers with other polymer matrices. The NMR results together with the FTIR and elemental analysiswere data, highly modified withstarch acquisition of isocyanate and urethane thus theofhypothesis confirmed that the and dextrin polymers were highlyfunctionalities, modified with and acquisition isocyanate was andapproved. urethane functionalities, and thus the hypothesis was approved. a

S-control MS 3.1 MS 6.1

D-control MD 3.1 MD 6.1

hydrocarbons in IPDI

OH

OH

4

6 3

HO

5

b

Glucose unit (GU)

Glucose unit (GU)

O

hydrocarbons in IPDI

2

O

OH

IPDI

8

1

9

7

O

2 3

1 4

O

4

3

10 N C O 12

6 5

6

HO

C2,3,5 in GU

5

2

O

8

1

OH

7

O

2 3

IPDI

9

1 4

10 N C O 12

6 5

C2,3,5 in GU

n

n

N C O 11

N C O 11

C6 in GU

free -NCO

free -NCO

C6 in GU

-NHCOO

C11 in IPDI C12 in IPDI

200

C12 in IPDI C1 in GU

C1 in GU

C4 in GU

-NHCOO

150

C11 in IPDI

100

C4 in GU

50

0

200

150

 (ppm)

100

50

0

 (ppm)

13C nuclear magnetic resonance (1313 Figure NMR) spectra ofof S-control, MS Figure2.2. 13 C nuclear magnetic resonance (C C NMR) spectra S-control, MS3.1 3.1and andMS MS6.1 6.1(a) (a)and and D-control, MD 3.1 and MD 6.1 (b). D-control, MD 3.1 and MD 6.1 (b).

3.3. 3.3.Thermal ThermalBehavior Behavior Thermal behavior of unmodified starch andand dextrin as well as of the modified ones Thermaldegradation degradation behavior of unmodified starch dextrin as well as of the modified were by thermogravimetric (TG) (TG) and derivative thermogravimetric (DTG) analyses of ones examined were examined by thermogravimetric and derivative thermogravimetric (DTG) analyses samples that were dried previously in a vacuum oven at 50 °C for 24 h. Figure 3a,c shows the ◦ of samples that were dried previously in a vacuum oven at 50 C for 24 h. Figure 3a,c shows the temperature andmodified modifiedstarch starchand anddextrin dextrin lost their masses, Figure temperaturerange range that that unmodified unmodified and lost their masses, andand Figure 3b,d 3b,d demonstrates the peaks of DTG curves that are associated with the decomposition temperatures demonstrates the peaks of DTG curves that are associated with the decomposition temperatures of ofsamples samples [32]. There were considerable differences indegradation the degradation patterns of starch and [32]. There were considerable differences in the patterns of starch and dextrin dextrin after functionalization with IPDI. Modified and dextrin showed onset after functionalization with IPDI. Modified starch andstarch dextrin showed higher onsethigher temperature temperature of degradation (T onset) than the unmodified ones. The Tonset of S-control was ◦297.2 °C and of degradation (T ) than the unmodified ones. The T of S-control was 297.2 C and it was onset

onset

increased to 313.2 and 324.4 ◦ C in MS 3.1 and MS 6.1, respectively. D-control showed slightly lower Tonset (292.1 ◦ C) as compared with S-control. Modification of dextrin also increased the respective

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it was increased to 313.2 and 324.4 °C in MS 3.1 and MS 6.1, respectively. D-control showed slightly lower Tonset (292.1 °C) as compared with S-control. Modification of dextrin also increased the ◦ C319.8 respective Tonset MDMD 3.1 6.1 andtoMD 6.1 and to 318.9 and °C2). (Table 2). The DTG were curvesderived were derived T 3.1ofand 318.9 319.8 (Table The DTG curves by the onset of MD by theof slope of TG graphs 3b,d), and illustrate thetemperatures first temperatures at which the maximum slope TG graphs (Figure(Figure 3b,d), and illustrate the first at which the maximum mass ◦ mass ) ofpolymers the polymers occurred. These temperatures °Cstarch for starch loss (Tloss ) max1 of the occurred. These temperatures werewere 316.6316.6 and and 319.7319.7 C for and max1(T and dextrin, respectively, anddirectly were directly associated with their decomposition thermal decomposition [33]. As dextrin, respectively, and were associated with their thermal [33]. As indicated indicated Table 2, functionalization IPDI,shifted in contrast, shifted the Thigher max1 to temperatures slightly higher in Table 2, in functionalization with IPDI, inwith contrast, the Tmax1 to slightly at ◦ temperatures at ~330–338 which can bedecomposition attributed to the urethane groups into ~330–338 C, which can be°C, attributed to the of decomposition urethane groupsofinto the isocyanate and the isocyanate alcohol groupssecond [14,34],maximum and caused second maximum degradation alcohol groups and [14,34], and caused degradation temperatures (Tmax2 ) attemperatures ~413–434 ◦ C (T ) at ~413–434 °Cand for modified dextrin. These results are in accordance withcoworkers, Girouard formax2 modified starch dextrin. starch These and results are in accordance with Girouard and and who found one Tmax nanocrystals in cellulose nanocrystals after functionalization whocoworkers, found more than onemore Tmaxthan in cellulose after functionalization with IPDI with [14]. IPDI [14].& Thakore Valodkar[9]&also Thakore [9] analso reported an improved thermal of starch Valodkar reported improved thermal stability of starch stability nanoparticles after nanoparticles after1,4-hexamethylene modification with 1,4-hexamethylene diisocyanate (HMDI). modification with diisocyanate (HMDI). a

MS 6.1

MS 3.1

S-control

100

D-control

MD 6.1

MD 3.1

0

b

90 80

-2

DTG (% min-1)

Mass (%)

70 60 50 40 30 20

-4

-6

-8

10

c

0 100

-10 0

d

90 -2

80

-4

Mass (%)

70 60

-6

50

-8

40

-10

30 -12

20

-14

10

-16

0 100

200

300

400

500

600

100

200

300

400

500

600

Temperature (C)

Temperature (C)

Figure 3. Mass Mass loss loss and and first first derivative derivative (DTG) (DTG) of of S-control, S-control, MS MS 3.1 3.1 and and MS MS 6.1 6.1 (a,b) (a,b) and and D-control, D-control, MD 3.1 and MD 6.1 (c,d). ◦ C). Table of unmodified unmodified and and modified modified starch starch and and dextrin dextrin ((°C). Table 2. 2. Thermal Thermal degradation degradation properties properties of

Tonset S-control 297.2 S-control MS 3.1 297.2 313.2 MS 3.1 313.2 MS 6.1 324.4 MS 6.1 324.4 D-control 292.1 D-control 292.1 318.9 MDMD 3.1 3.1 318.9 319.8 MDMD 6.1 6.1 319.8 T onset

Tmax1 T max1 316.6 316.6 334.7 334.7 337.6 337.6 319.7 319.7 330.1 330.1 334.8 334.8

Tmax2 T max2 430.4 430.4 433.6433.6 415.1415.1 417.5417.5

The The glass glass transition transition temperature temperature (T (Tgg)) of of unmodified unmodified and and modified modified starch starch and and dextrin dextrin after after removing the first history of samples are shown in Figure 4a,b. The T g of unmodified starch and removing the first history of samples are shown in Figure 4a,b. The Tg of unmodified starch and dextrin ◦ C, 89.3 dextrin weretofound to be 113.2 °C, respectively. studies that quoted the moisture were found be 113.2 and 89.3and respectively. PreviousPrevious studies quoted the that moisture content

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Endo

Endo

has a considerable effect on shifting the Tg of the starch [35–37]. Thus, the lower moisture content of content has a considerable effect on shifting Tg of starch [35–37]. Thus, the lower moisture unmodified starch and dextrin in this study resulted to a higher Tto former The studies. respective g than content of unmodified starch and dextrin in this study resulted a higher Tg studies. than former ◦ TgThe of MS 3.1 and MS 6.1,MS however, 112.4 andshifted 105.7 to C. 112.4 Modification of dextrin caused further respective Tg of 3.1 andshifted MS 6.1,tohowever, and 105.7 °C. Modification of ◦ C and MD 6.1 showed a T of 44.1 ◦ C. The obvious decrease in T , as MD 3.1 presented a T of 63.1 g g g dextrin caused further decrease in Tg, as MD 3.1 presented a Tg of 63.1 °C and MD 6.1 showed a Tg of decrease Tg of modified polymers, mightmodified be attributed to the insertion 44.1 °C.inThe obvious decrease in Tg particularly of modified modified polymers,dextrin, particularly dextrin, might be of attributed IPDI in thetopolymers’ backbones creation of urethane linkages, which can increase the mobility the insertion of IPDIand in the polymers’ backbones and creation of urethane linkages, of which polymers due to lower amounts hydroxyl and thus reduce the hydrogen bonds [15]. can increase the mobility of of polymers duegroups, to lower amounts of hydroxyl groups, and thus In reduce addition, of bonds the pendant onofstarch and dextrin may hinder theonpackaging thepresence hydrogen [15]. Inisocyanate addition, groups presence the pendant isocyanate groups starch dextrin may hinderhindrance, the packaging polymer duethe to steric of and polymer due to steric andof thus decrease Tg . hindrance, and thus decrease the Tg.

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150

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Figure Differentialscanning scanningcalorimetry calorimetry (DSC) (DSC) thermographs MSMS 3.13.1 and MSMS 6.1 6.1 (a) (a) Figure 4. 4.Differential thermographsofofS-control, S-control, and and D-control, MD 3.1 and MD 6.1 (b). and D-control, MD 3.1 and MD 6.1 (b).

3.4. Water Vapor Sorption Behavior 3.4. Water Vapor Sorption Behavior Dynamic water vapor sorption analysis showed obvious changes in moisture adsorption Dynamic water vapor sorption analysis showed obvious changes in moisture adsorption behavior behavior of starch and dextrin as a result of IPDI modification (Figure 5a,b). Moisture content of of samples starch and dextrin as a result of IPDI modification (Figure 5a,b). Moisture content of samples increased with increasing the relative humidity (RH) from 0% to 95%. MS 3.1 and MS 6.1 increased with increasing the relative humidity (RH) 0% toacross 95%. all MSRH 3.1 and MSbut 6.1 demonstrated demonstrated a comparable sorption behavior as from S-control range, with lower a comparable sorption behavior as S-control across all RH range, but with lower moisture moisture content (MC). The upward bending of sorption curves at 75% RH might be due tocontent the (MC). The upward bendingregions of sorption at 75% RH might be in due to the relaxation of amorphous relaxation of amorphous of thecurves polymers, which resulted accommodation of more water regions of the polymers, resulted water molecules S-control molecules [20]. S-controlwhich exhibited a MCin ofaccommodation 21.6%, while MCof ofmore MS 3.1 and MS 6.1 were[20]. 19.7% and exhibited a MC of 21.6%, while MS 3.1inand MS 6.1content were 19.7% and 17.6%, respectively, at 95% 17.6%, respectively, at 95% RH.MC Theof increase moisture of D-control was more pronounced RH. The increase in moisture content of D-control was more pronounced in RHs of above 75%, and in RHs of above 75%, and it reached MC of 27% at 95% RH. This might be related to the high it reached MCstructure of 27% at RH. This might beeases related the vapor high amorphous structure of dextrin amorphous of 95% dextrin polymer which the to water accommodation. Modification of dextrin witheases IPDI,the however, decreased the moisture adsorption, as the with respective of polymer which water strongly vapor accommodation. Modification of dextrin IPDI,MCs however, MD 3.1 and MD 6.1 were 13.7% and 10.1% at 95% RH. Reduction in MC of modified starch and strongly decreased the moisture adsorption, as the respective MCs of MD 3.1 and MD 6.1 were 13.7% dextrin due to occupation hydroxyl groups byand cycloaliphatic urethane in the of and 10.1%can at be 95% RH. Reduction of in polar MC of modified starch dextrin can be due moieties to occupation polymer chains. Similar were reported by moieties Barikani & as well as by results Valodkar polar hydroxyl groups by results cycloaliphatic urethane inMohammadi the polymer [5] chains. Similar were & Thakore [9], who quoted that the hydrophilicity of starch polymer was decreased by urethane reported by Barikani & Mohammadi [5] as well as by Valodkar & Thakore [9], who quoted that the modification. of These results alongwas withdecreased the thermal provided further These evidence of thealong starchhydrophilicity starch polymer byanalysis urethane modification. results with and dextrin-urethane functionality and demonstrated the improved thermal stability and decreased the thermal analysis provided further evidence of the starch- and dextrin-urethane functionality and moisture adsorption behavior as a result of the IPDI modification. demonstrated the improved thermal stability and decreased moisture adsorption behavior as a result of the IPDI modification.

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60

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Relative Humidity (%) Figure 5. Moisture content (MC) of S-control, MS3.1 3.1and andMS MS6.1 6.1(a) (a) and and D-control, D-control, MD Figure 5. Moisture content (MC) of S-control, MS MD3.1 3.1and andMD MD 6.1 6.1 (b) for whole range of RH. (b) for whole range of RH.

3.5. Scanning Electron Microscopy (SEM)

3.5. Scanning Electron Microscopy (SEM)

SEM micrographs of controls and modified samples at the higher weight ratios (MS 6.1 and MD

SEM micrographs of Figure controls andThese modified samples the higher weight ratios (MS 6.1 and MD 6.1) are presented in 6a–d. images clearlyatshow an intense allocation of spherical 6.1) are presented inmodified Figure 6a–d. imagesgranules clearly (Figure show an intense allocation spherical particles particles on the starchThese and dextrin 6c,d). It is also obviousofthat the smooth surfaces of starch unmodified starch granules and dextrin granules to obvious entirely rough on the modified and dextrin (Figure 6c,d).turned It is also that thesurfaces smoothafter surfaces modification with IPDI. This was reported previously by Barikani & Mohammadi [5]. In addition, of unmodified starch and dextrin granules turned to entirely rough surfaces after modification with the reported dextrin granules was increased after&modification, while starch presented a IPDI.size Thisofwas previously by Barikani Mohammadi [5]. modified In addition, size of the dextrin more dense structure and intra-connection between granules. This might be due to the existence of granules was increased after modification, while modified starch presented a more dense structure and excess IPDI, which resulted in possible side reactions between modified starch and dextrin granules, intra-connection between granules. This might be due to the existence of excess IPDI, which resulted i.e., reaction between primary isocyanate groups with OH groups of starch and dextrin polymers. in possible sidemicrographs reactions between modified starch and granules, i.e., reaction primary The SEM provide the visual approval ondextrin successful modification of starchbetween and dextrin isocyanate groups with OH groups of starch and dextrin polymers. The SEM micrographs provide granules by IPDI. Further images from the modified samples MS 3.1 and MD 3.1 (lower mole ratios) the visual approval on S2. successful modification of starch and dextrin granules by IPDI. Further images are given in Figure from the modified samples MS 3.1 and MD 3.1 (lower mole ratios) are given in Figure S2.

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Figure Scanningelectron electronmicroscopy microscopy (SEM) (SEM) micrographs micrographs of (b), MSMS 6.16.1 (c),(c), Figure 6. 6. Scanning ofS-control S-control(a), (a),D-control D-control (b), MD 6.1 (d). MD 6.1 (d).

4. Conclusions 4. Conclusions Pea starch and dextrin were successfully modified with isophorone diisocyanate (IPDI) in order Pea starch and dextrin were successfully modified with isophorone diisocyanate (IPDI) in order to to reduce their hydrophilicity and improve their thermal stability. Assessment of degree of reduce their hydrophilicity and improve their thermal stability. Assessment of degree of substitution substitution (DS) from elemental analysis data indicated that most of the hydroxyl groups in starch (DS) from elemental analysis data indicated that most of13 the hydroxyl groups in starch and dextrin and dextrin polymers reacted with IPDI. FTIR and C NMR confirmed both isocyanate and polymers reacted with IPDI. FTIR and 13 C NMR confirmed both isocyanate and urethane functionalities urethane functionalities in starch and dextrin macromolecules after modification. This study in starch and dextrin macromolecules after modification. This study demonstrated remarkable demonstrated remarkable improvement in thermal stability of IPDI-modified starch and dextrin. In improvement thermalsorption stabilityofofstarch IPDI-modified starch dextrin. In by addition, the moisture addition, theinmoisture and dextrin was and greatly reduced IPDI modification. sorption of starch and dextrin was greatly reduced by IPDI modification. Functionalization Functionalization of starch and dextrin polymers with IPDI make them suitable for a wide range of of starch and dextrin with IPDI make them suitableasformatrices a widefor range of applications. applications. Thesepolymers sustainable materials can be employed composites or as These sustainable materials can be employed as matrices for composites or as reinforcement elements reinforcement elements in other matrices without any further functionalization. For example, such in polysaccharides other matrices without any further functionalization. Forpetroleum-based example, such polysaccharides would would constitute promising substitutes for counterparts, e.g., in constitute promising for petroleum-based polyurethane foamssubstitutes and wood adhesive applications. counterparts, e.g., in polyurethane foams and wood adhesive applications. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: FTIR Supplementary The following online Figure at http://www.mdpi.com/2073-4360/10/9/939/s1, spectra of neatMaterials: toluene and toluene fromare theavailable third washing, S2: SEM micrographs of MS 3.1 (a) and MD Figure S1: FTIR spectra of neat toluene and toluene from the third washing, Figure S2: SEM micrographs of 3.1 (b). MS 3.1 (a) and MD 3.1 (b). Author Contributions: R.H. prepared the functionalized starch and dextrin and characterized them. A.S.E. Author Contributions: the functionalized starch andindextrin and characterized them. participated in thermal R.H. study prepared and microscopy analysis. S.A. participated data processing. N.G. and A.E. A.S.E. participated in thermal study and microscopy analysis. S.A. participated in data processing. N.G. and A.E. contributed to the experimental design and data processing. R.H., A.S.E., S.A., N.G., and A.E. are participated in contributed to the experimental design and data processing. R.H., A.S.E., S.A., N.G., and A.E. are participated in preparing manuscript. preparing thethe manuscript.

Acknowledgments: Reza Hosseinpourpia and Stergios Adamopoulos thank VINNOVA, Swedish Governmental Agency for Innovation Systems (VINNMER Marie Curie Incoming project, grant No. 2015-04825).

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Arantzazu Santamaria-Echart and Arantxa Eceiza wish to acknowledge the financial support from the Basque Government in the frame of Grupos Consolidados (IT-776-13) and SGIker from the University of the Basque Country for their Technical support. Conflicts of Interest: The authors have declared no conflict of interests.

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