polymers

Article Modification of Pea and 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 Department of Forestry and Wood Technology, Linnaeus University, Lückligs Plats 1, 35195 Växjö, Sweden; [email protected] 2 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 13C nuclear magnetic resonance (13C 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 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

Polymers 2018, 10, 939; doi:10.3390/polym10090939 www.mdpi.com/journal/polymers Polymers 2018, 10, 939 2 of 12 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 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), 13C nuclear magnetic resonance (13C 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. Polymers 2018, 10, 939 3 of 12

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 13C NMR using a Bruker Avance III 400 MHz equipment (Billerica, MA, USA). The spectra were recorded using a decoupled sequence at 13C 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, Polymers 2018, 10, 939 4 of 12

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 Table1. 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 38.60 6.53 52.94 0.08 - MS 3.1 45.58 7.45 26.96 5.28 1.11 MS 6.1 46.97 7.58 27.92 6.24 1.54 D-control 39.33 6.42 52.45 0.03 - MD 3.1 48.40 7.98 23.68 7.38 2.29 MD 6.1 53.13 8.02 24.08 8.12 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 Polymers 2018, 10, x FOR PEER REVIEW 5 of 12 Polymers 2018, 10, 939 5 of 12

MD 6.1 were 1.15 and 1.52, respectively. This might be due to the higher solubility of dextrin in duringsolvent the than modification starch, and reaction. also to the A morelow availability detailed description of starch’s hydroxyl regarding groups the calculation due to the offormation DS can be foundof gel-like in the Supplementaryproduct during Materials.the modification reaction. A more detailed description regarding the 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 polymersFTIR upon spectroscopy functionalization analysis was with used the IPDIto monitor monomer. changes Figure in the1a,b structures illustrates of starch the FTIR and spectra dextrin of 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, which is in the FTIR spectra of unmodified starch and dextrin, as they showed peaks at 3450 cm−1, which is assigned to the stretching vibration of –OH group, and at 2960 cm−1, which 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 spectraregions were of the observed spectra for were the observed modified for starch the modi andfied dextrin. starch There and dextrin. was an apparentThere was decrease an apparent in the −1 –OHdecrease peak of in modified the –OH starchpeak of and modified dextrin starch at 3450 and cm dextrin, which at 3450 indicated cm−1, which that manyindicated of these that many functional of −1 groupsthese werefunctional consumed. groups were This consum togethered. with This thetogether appearance with the ofappear the isocyanateance of the isocyanate band at 2266 band cm at , which2266 iscm related−1, which to theis related pendant to the –NCO pendant groups, –NCO indicates groups the, indicates success the in success functionalization in functionalization starch and dextrinstarch with and IPDIdextrin [14 with,15,19 IPDI]. After [14,15,19]. modification, After mo thedification, modified the polymers modified were polymers washed were with washed toluene threewith times. toluene The three FTIR times. spectra The of theFTIR third spectra centrifuged of the third product centrifuged (see Figure product S1) were (see almostFigure comparableS1) were withalmost the spectracomparable of the with neat the toluene, spectra whichof the indicatesneat toluene, that which the –NCO indicates peaks that in the modified –NCO starchpeaks in and dextrinmodified can bestarch attributed and dextrin to the can attachment be attributed of isocyanate to the attachment groups of into isocyanate the starch groups and dextrin into the polymers. starch Inand addition, dextrin increase polymers. in theIn addition, absorbance increase of –CO in the groups absorbance at 1650–1750 of –CO cmgroups−1, and at 1650–1750 bending vibrationcm−1, and of –NHbending and stretching vibration vibration of –NH and of C–N stretching at 1530 vibration cm−1 confirmed of C–N at the 1530 presence cm−1 confirmed of urethane the bonds presence between of IPDIurethane and starch bonds and between dextrin IPDI [14 ,and15,26 starch]. and dextrin [14,15,26].

a IPDI

S-control

MS 3.1

MS 6.1 Transmittance (a.u.)

b IPDI

D-control

MD 3.1

MD 6.1 Transmittance (a.u.)

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). Polymers 2018, 10, x FOR PEER REVIEW 6 of 12

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). Polymers 2018, 10, 939 6 of 12

The structural changes of starch and dextrin polymers after modification were analyzed by meansThe of solid structural NMR. changes Figure of2a,b starch illustrate and dextrin almost polymersidentical afterspectra modification for unmodified were analyzedstarch and by dextrin,means ofalthough solid NMR. unmodified Figure2 a,bdextrin illustrate showed almost a more identical intense spectra signal for at unmodified 61.8 ppm, which starch andis assigned dextrin, toalthough C6 [27,28]. unmodified The overlapping dextrin showedsignal at a around more intense 68–78 signal ppm is at collectively 61.8 ppm, which associated is assigned with toC2 C, C6 3[,27 and,28 ]. CThe5 [2,29]. overlapping Glycosidic signal carbons at around C1 and 68–78C4 profiles ppm are is collectively most sensitive associated to chain with conformations C2,C3, and [30], C5 [ and2,29 ]. thereforeGlycosidic the carbons resonances C1 and that C 4appearprofiles at are81.4 most and sensitive103.2 ppm to are chain due conformations to the amorphous [30], anddomains therefore of the the Cresonances4 and C1, respectively that appear [2,31]. at 81.4 Modification and 103.2ppm of starch are due and todextrin the amorphous with IPDI domainscaused apparent of the C changes4 and C1 , onrespectively NMR spectra, [2,31 and]. Modification the intensity ofof starchsignals and was dextrin more pronounced with IPDI caused at the apparenthigh NCO:OH changes ratio on (6:1). NMR Appearancespectra, and of the new intensity peaks ofbetween signals 20 was and more 50 ppm pronounced corresponded at the high to hydrocarbons NCO:OH ratio that (6:1). are Appearancepresent in theof structure new peaks of between isocyanate; 20 andin detail, 50 ppm the corresponded signal at 45.2 toppm hydrocarbons is associated that with are the present methylene in the group structure in theof isocyanate;aliphatic IPDI in detail,ring, peaks the signal at 26.9 at and 45.2 32.3 ppm ppm is associated are attributed with theto the methylene tertiary carbons, group in and the aliphaticsignals atIPDI 27.8 ring,and 23.5 peaks ppm at 26.9are due and to 32.3 the ppm methyl are attributedgroups [14]. to The the tertiaryappearance carbons, of new and peaks signals between at 27.8 120 and and23.5 160 ppm ppm are duecorrespond to the methyl to urethane groups (155.2–158.0 [14]. The appearance ppm) and of –NCO new peaks (122.2–129.5) between 120linkages and 160 in ppmthe starchcorrespond and dextrin to urethane polymers (155.2–158.0 [14,15]. ppm)The observation and –NCO (122.2–129.5) of distinct peaks linkages for inthe the primary starch and urethane dextrin linkagepolymers can [ 14be, 15attributed]. The observation to the existence of distinct of exce peaksss IPDI, for the which primary may urethane cause some linkage side can reactions be attributed with theto OH the existencegroups of of starch excess and IPDI, dextrin which polymers. may cause The some primary side free reactions –NCO withgroup the appears OH groups at 123.6 of starchppm andand the dextrin secondary polymers. free –NCO The primary group freeis present –NCO at group 129.5 appears ppm. The at 123.6availability ppm and of thefree secondaryprimary and free secondary–NCO group –NCO is present groups at 129.5in the ppm. starch The availability and dextrin of free polymers primary andinduce secondary them –NCOas remarkable groups in bio-macromoleculesthe starch and dextrin for polymers further re induceaction themwith asother remarkable polymer bio-macromolecules matrices. The NMR for results further together reaction withwith the other FTIR polymer and elemental matrices. analysis The NMR data, results confirmed together that with the the starch FTIR and and dextrin elemental polymers analysis were data, highlyconfirmed modified that thewith starch acquisition and dextrin of isocyanate polymers and were urethane highly functionalities, modified with and acquisition thus the of hypothesis isocyanate wasand approved. urethane functionalities, and thus the hypothesis was approved.

a S-control D-control hydrocarbons in IPDI b MS 3.1 hydrocarbons in IPDI MD 3.1 MS 6.1 MD 6.1 Glucose unit (GU) Glucose unit (GU) OH OH 6 IPDI 6 IPDI O 4 5 O 89 O 4 5 O 89 2 2 10 3 1 10 3 1 7 7 O 1 O 1 OH N C O OH N C O HO 2 6 C2,3,5 in GU HO 2 6 12 12 C2,3,5 in GU 3 5 3 5 4 4 n n N C O N C O 11 11 C6 in GU

free -NCO free -NCO C6 in GU C11 in IPDI -NHCOO C11 in IPDI C12 in IPDI C12 in IPDI C1 in GU C1 in GU C4 in GU -NHCOO C4 in GU

200 150 100 50 0 200 150 100 50 0  (ppm)  (ppm) FigureFigure 2. 2. 1313C Cnuclear nuclear magnetic magnetic resonance resonance (13 (13C CNMR) NMR) spectra spectra of ofS-control, S-control, MS MS 3.1 3.1 and and MS MS 6.1 6.1 (a ()a )and and D-control,D-control, MD MD 3.1 3.1 and and MD MD 6.1 6.1 (b (b).).

3.3.3.3. Thermal Thermal Behavior Behavior ThermalThermal degradation degradation behavior behavior of unmodified of unmodified starch starch and anddext dextrinrin as well as wellas of the as ofmodified the modified ones wereones examined were examined by thermogravimetric by thermogravimetric (TG) (TG)and de andrivative derivative thermogravimetric thermogravimetric (DTG) (DTG) analyses analyses of samplesof samples that thatwere were dried dried previously previously in a in vacuum a vacuum oven oven at at50 50°C◦ Cfor for 24 24 h. h. Figure Figure 3a,c3a,c shows shows the the temperaturetemperature range range that that unmodified unmodified andand modifiedmodified starch starch and and dextrin dextrin lost lost their their masses, masses, and and Figure Figure3b,d 3b,ddemonstrates demonstrates the the peaks peaks of DTGof DTG curves curves that that are ar associatede associated with with the the decomposition decomposition temperatures temperatures of ofsamples samples [32 [32].]. There There were were considerable considerable differences differences in the in degradationthe degradation patterns patterns of starch of starch and dextrin and dextrinafter functionalization after functionalization with IPDI. with Modified IPDI. Modified starch and starch dextrin and showed dextrin higher showed onset higher temperature onset temperature of degradation (Tonset) than the unmodified ones. The Tonset of S-control was ◦297.2 °C and of degradation (Tonset) than the unmodified ones. The Tonset of S-control was 297.2 C and 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 respective Polymers 2018, 10, x FOR PEER REVIEW 7 of 12

Polymers 2018, 10, 939 7 of 12 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 ◦ Trespectiveonset of MD Tonset 3.1 of and MD MD 3.1 6.1and to MD 318.9 6.1 andto 318.9 319.8 andC 319.8 (Table °C2). (Table The DTG 2). The curves DTG werecurves derived were derived by the byslope the ofslope TG graphsof TG graphs (Figure (Figure3b,d), and3b,d), illustrate and illustrate the first the temperatures first temperatures at which at which the maximum the maximum mass ◦ massloss ( Tlossmax1 ()Tmax1 of the) of polymers the polymers occurred. occurred. These These temperatures temperatures were were 316.6 316.6 and and 319.7 319.7C for °C starchfor starch and anddextrin, dextrin, respectively, respectively, and were and directlywere directly associated asso withciated their with thermal their decompositionthermal decomposition [33]. As indicated [33]. As indicatedin Table2, functionalizationin Table 2, functionalization with IPDI, in with contrast, IPDI, shifted in contrast, the Tmax1 shiftedto slightly the T highermax1 to temperaturesslightly higher at ~330–338temperatures◦C, whichat ~330–338 can be °C, attributed which can to the be decompositionattributed to the of decomposition urethane groups of into urethane the isocyanate groups into and ◦ thealcohol isocyanate groups and [14,34 alcohol], and causedgroups second [14,34], maximum and caused degradation second maximum temperatures degradation (Tmax2) at temperatures ~413–434 C (forTmax2 modified) at ~413–434 starch °C and for modified dextrin. starch These and results dextrin. are inThese accordance results are with in accordance Girouard and with coworkers, Girouard andwho coworkers, found more who than found one moreTmax thanin cellulose one Tmax nanocrystalsin cellulose nanocrystals after functionalization after functionalization with IPDI with [14]. IPDIValodkar [14]. & ThakoreValodkar [ 9]&also Thakore reported [9] analso improved reported thermal an improved stability ofthermal starch stability nanoparticles of starch after nanoparticlesmodification withafter 1,4-hexamethylenemodification with 1,4-hexamethylene diisocyanate (HMDI). diisocyanate (HMDI).

S-control MS 3.1 MS 6.1 D-control MD 3.1 MD 6.1 100 a 0 b 90

80 -2 70 )

60 -1 -4 50

40 -6 Mass (%)

30 DTG (% min

20 -8

10

0 -10 100 c 0 d 90 -2 80 -4 70

60 -6

50 -8

40 Mass (%) -10 30 -12 20 -14 10

0 -16 100 200 300 400 500 600 100 200 300 400 500 600 Temperature (C) Temperature (C) Figure 3. MassMass loss loss and and first first derivative (DTG) (DTG) of of S-control, S-control, MS MS 3.1 3.1 and and MS MS 6.1 6.1 ( (aa,,bb)) and and D-control, D-control, MD 3.1 and MD 6.1 ( c,d).

TableTable 2. Thermal degradation properties of unmodifiedunmodified and modifiedmodified starch and dextrindextrin ((°C).◦C).

Tonset Tmax1 Tmax2 Tonset Tmax1 Tmax2 S-control 297.2 316.6 - S-controlMS 3.1 297.2 313.2 334.7 316.6 430.4 - MS 3.1 313.2 334.7 430.4 MSMS 6.1 6.1 324.4 324.4 337.6 337.6 433.6 433.6 D-control 292.1 319.7 - D-control 292.1 319.7 - MDMD 3.1 3.1 318.9 318.9 330.1 330.1 415.1 415.1

MDMD 6.1 6.1 319.8 319.8 334.8 334.8 417.5 417.5

The glass transition temperature (Tg) of unmodified and modified starch and dextrin after The glass transition temperature (Tg) of unmodified and modified starch and dextrin after removing the first history of samples are shown in Figure 4a,b. The Tg of unmodified starch and removing the first history of samples are shown in Figure4a,b. The Tg of unmodified starch and dextrin dextrinwere found were to found be 113.2 to be and 113.2 89.3 and◦C, 89.3 respectively. °C, respec Previoustively. Previous studies quotedstudies thatquoted the that moisture the moisture content

Polymers 2018, 10, 939 8 of 12

Polymers 2018, 10, x FOR PEER REVIEW 8 of 12 has a considerable effect on shifting the Tg of starch [35–37]. Thus, the lower moisture content of content has a considerable effect on shifting the Tg of starch [35–37]. Thus, the lower moisture unmodified starch and dextrin in this study resulted to a higher T than former studies. The respective content of unmodified starch and dextrin in this study resulted tog a higher Tg than former studies. T of MS 3.1 and MS 6.1, however, shifted to 112.4 and 105.7 ◦C. Modification of dextrin caused further gThe respective Tg of MS 3.1 and MS 6.1, however, shifted to 112.4 and 105.7 °C. Modification of ◦ ◦ decreasedextrin incausedTg, as further MD 3.1 decrease presented in T ag, Tasg MDof 63.1 3.1 presentedC and MD a T 6.1g of showed 63.1 °C and a Tg MDof44.1 6.1 showedC. The a obvious Tg of decrease44.1 °C. in TheTg ofobvious modified decrease polymers, in Tg particularlyof modified modifiedpolymers, dextrin, particular mightly modified be attributed dextrin, tothe might insertion be ofattributed IPDI in the to polymers’ the insertion backbones of IPDIand in the creation polymers’ of urethane backbones linkages, and creation which can of urethane increase thelinkages, mobility ofwhich polymers can increase due to lower the mobility amounts of ofpolymers hydroxyl due groups, to lower and amounts thus reduce of hydroxyl the hydrogen groups, and bonds thus [15 ]. Inreduce addition, the presence hydrogen of bonds the pendant [15]. In isocyanate addition, groupspresence on of starch the pendant and dextrin isocyanate may hinder groups the on packaging starch ofand polymer dextrin due may to sterichinder hindrance, the packaging and of thus polymer decrease due theto stericTg. hindrance, and thus decrease the Tg.

a b Endo Endo

D-control 113.2 S-control 89.3

MS 3.1 112.4 MD 3.1

63.1 105.7 MS 6.1 MD 6.1 44.1 Heat flow (a.u.) flow Heat (a.u.) flow Heat

0 50 100 150 200 0 50 100 150 200 Temperature (C) Temperature (C) FigureFigure 4. 4.Differential Differential scanning scanning calorimetrycalorimetry (DSC) thermographs of of S-control, S-control, MS MS 3.1 3.1 and and MS MS 6.1 6.1(a) (a) andand D-control, D-control, MD MD 3.1 3.1 andand MDMD 6.1 ( bb).).

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 starch and dextrin as a result of IPDI modification (Figure5a,b). Moisture content of samples samples increased with increasing the relative humidity (RH) from 0% to 95%. MS 3.1 and MS 6.1 increaseddemonstrated with increasing a comparable the relative sorption humidity behavior (RH) as fromS-control 0% toacross 95%. all MS RH 3.1 andrange, MS but 6.1 demonstratedwith lower a comparablemoisture content sorption (MC). behavior The upward as S-control bending across of sorption all RH curves range, at but 75% with RH lower might moisture be due to content the (MC).relaxation The upward of amorphous bending regions of sorption of the curves polymers, at 75% which RH mightresulted be in due accommodation to the relaxation of more of amorphous water regionsmolecules of the [20]. polymers, S-control which exhibited resulted a MC in of accommodation 21.6%, while MC ofof moreMS 3.1 water and MS molecules 6.1 were [20 19.7%]. S-control and exhibited17.6%, respectively, a MC of 21.6%, at 95% while RH. MC The ofincrease MS 3.1 in and mo MSisture 6.1 content were 19.7% of D-control and 17.6%, was more respectively, pronounced at 95% RH.in TheRHsincrease of above in 75%, moisture and it content reached of MC D-control of 27% was at 95% more RH. pronounced This might in be RHs related of above to the 75%, high and it reachedamorphous MC structure of 27% atof 95%dextrin RH. polymer This might which be ea relatedses the towater the vapor high amorphousaccommodation. structure Modification of dextrin polymerof dextrin which with eases IPDI, thehowever, water strongly vapor accommodation. decreased the moisture Modification adsorption, of dextrin as the withrespective IPDI, MCs however, of stronglyMD 3.1 decreased and MD 6.1 the were moisture 13.7% adsorption, and 10.1% as at the95% respective RH. Reduction MCs of in MD MC 3.1 of andmodified MD 6.1 starch were and 13.7% anddextrin 10.1% can at be 95% due RH. to occupation Reduction of in polar MCof hydroxyl modified groups starch by and cycloaliphatic dextrin can urethane be due moieties to occupation in the of polarpolymer hydroxyl chains. groups Similar by results cycloaliphatic were reported urethane by moietiesBarikani & in Mohammadi the polymer [5] chains. as well Similar as by resultsValodkar were reported& Thakore by Barikani[9], who "ed Mohammadi that the [hydrophilicity5] as well as by of Valodkarstarch polymer & Thakore was decreased [9], who quoted by urethane that the hydrophilicitymodification. ofThese starch results polymer along was with decreased the thermal by analysis urethane provided modification. further Theseevidence results of the along starch- with theand thermal dextrin-urethane analysis provided functionality further and evidence demonstrat of theed starch-the improved and dextrin-urethane thermal stability functionalityand decreased 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.

Polymers 2018, 10, 939 9 of 12 Polymers 2018, 10, x FOR PEER REVIEW 9 of 12

30 S-control a 25 MS 3.1 MS 6.1 20

15

10

5 Moisture content (%)

0

30 D-control b 25 MD 3.1 MD 6.1 20

15

10

5 Moisture content (%) content Moisture

0

0 20406080100

Relative Humidity (%) FigureFigure 5. Moisture 5. Moisture content content (MC) (MC) of S-control,of S-control, MS MS 3.1 3.1 and and MS MS6.1 6.1 ((aa)) andand D-control, MD MD 3.1 3.1 and and MD MD 6.1 (b) for6.1 whole (b) for range whole of range RH. 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 SEM6.1) are micrographs presented in of Figure controls 6a–d. and These modified images samples clearly at show the higheran intense weight allocation ratios (MSof spherical 6.1 and MD 6.1) areparticles presented on the in modified Figure6 a–d.starch These and dextrin images granules clearly show(Figure an 6c,d). intense It is allocationalso obvious of that spherical the smooth particles on thesurfaces modified of starchunmodified and dextrinstarch granulesand dextrin (Figure granules6c,d). turned It is also to obviousentirely thatrough the surfaces smooth after surfaces of unmodifiedmodification starch with andIPDI. dextrin This was granules reported turned previo tously entirely by Barikani rough & surfacesMohammadi after [5]. modification In addition, with IPDI.size This of was the reporteddextrin granules previously was increased by Barikani after & modification, Mohammadi while [5]. modified In addition, starch size presented of the dextrin a 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 possibleThe SEM side micrographs reactions between provide the modified visual approval starch and on dextrinsuccessful granules, modification i.e., reaction of starch between and dextrin primary isocyanategranules groups by IPDI. with Further OH groupsimages from of starch the modified and dextrin samples polymers. MS 3.1 and The MD SEM 3.1 (lower micrographs mole ratios) provide the visualare given approval in Figure on S2. successful modification of starch and dextrin granules by IPDI. Further images from the modified samples MS 3.1 and MD 3.1 (lower mole ratios) are given in Figure S2.

Polymers 2018, 10, 939 10 of 12 Polymers 2018, 10, x FOR PEER REVIEW 10 of 12

FigureFigure 6. 6.Scanning Scanning electron electron microscopy microscopy (SEM)(SEM) micrographs of of S-control S-control (a (),a ),D-control D-control (b (),b MS), MS 6.1 6.1 (c), ( c), MDMD 6.1 6.1 (d ).(d).

4. Conclusions4. Conclusions PeaPea starch starch and and dextrin dextrin were were successfully successfully modified modified with with isophorone isophorone diisocyanatediisocyanate (IPDI)(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 of the hydroxyl groups in starch and dextrin and dextrin polymers reacted with IPDI. FTIR and 13C NMR confirmed both isocyanate and polymers reacted with IPDI. FTIR and 13C 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 improvementaddition, the in moisture thermal sorption stability of of starch IPDI-modified and dextrin starch was andgreatly dextrin. reduced In by addition, IPDI modification. the moisture sorptionFunctionalization of starch andof starch dextrin and wasdextrin greatly polymers reduced with byIPDI IPDI make modification. them suitable Functionalizationfor a wide range of of starchapplications. and dextrin These polymers sustainable with ma IPDIterials make can be them employ suitableed as for matrices a wide for range composites of applications. or as Thesereinforcement sustainable elements materials in canother be matrices employed without as matrices any further for composites functionalization. or as reinforcement For example, elements such inpolysaccharides other matrices withoutwould constitute any further promising functionalization. substitutes for For petroleum-based example, such polysaccharides counterparts, e.g., would in constitutepolyurethane promising foams substitutesand wood adhesive for petroleum-based 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 Supplementaryspectra of neat Materials: toluene andThe toluene following fromare the available third washing, online Figure at http://www.mdpi.com/2073-4360/10/9/939/s1 S2: SEM micrographs of MS 3.1 (a) and MD , Figure3.1 (b). S1: FTIR spectra of neat toluene and toluene from the third washing, Figure S2: SEM micrographs of 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. Authorparticipated Contributions: in thermal R.H.study preparedand microscopy the functionalized analysis. S.A. participated starch and in dextrin data processing. and characterized N.G. and A.E. them. 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 preparingpreparing the the manuscript. manuscript.

Acknowledgments: Reza Hosseinpourpia and Stergios Adamopoulos thank VINNOVA, Swedish Governmental Agency for Innovation Systems (VINNMER Marie Curie Incoming project, grant No. 2015-04825). Polymers 2018, 10, 939 11 of 12

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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