Deep eutectic solvents as simultaneous plasticizing and crosslinking agents for starch

Accepted Manuscript Deep eutectic solvents as simultaneous plasticizing and crosslinking agents for starch

Magdalena Zdanowicz, Roman Jędrzejewski, Ryszard Pilawka PII: DOI: Reference:

S0141-8130(18)36838-7 https://doi.org/10.1016/j.ijbiomac.2019.02.103 BIOMAC 11763

To appear in:

International Journal of Biological Macromolecules

Received date: Revised date: Accepted date:

9 December 2018 29 January 2019 16 February 2019

Please cite this article as: M. Zdanowicz, R. Jędrzejewski and R. Pilawka, Deep eutectic solvents as simultaneous plasticizing and crosslinking agents for starch, International Journal of Biological Macromolecules, https://doi.org/10.1016/j.ijbiomac.2019.02.103

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ACCEPTED MANUSCRIPT Deep eutectic solvents as simultaneous plasticizing and crosslinking agents for starch Magdalena Zdanowicza*, Roman Jędrzejewskib, Ryszard Pilawkaa,c, a

West Pomeranian University of Technology, Szczecin, Faculty of Chemical Technology and

Engineering, Polymer Institute, Ul. Pulaskiego 10, 70-322 Szczecin, Poland. b

West Pomeranian University of Technology, Szczecin, Faculty of Mechanical Engineering

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and Mechatronics, Institute of Materials Engineering, Al. Piastow 10, 70-310 Szczecin,

New Era Materials, Sp. z o.o., ul. Komandosow 1/7 32-085 Modlniczka, Poland

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c

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Poland.

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* Corresponding author e-mail address: [email protected], West Pomeranian University of Technology, Szczecin, Faculty of Chemical Technology and Engineering,

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Polymer Institute, Ul. Pulaskiego 10, 70-322 Szczecin, Poland. Abbreviations: CBit-choline bitartrate, CCit-choline citrate, CitA-citric acid, CLac-choline

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lactate, CMal-choline malate, EB-elongation at break, G-glycerol, PS- potato starch, TPS-

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thermoplastic starch, TS-tensile strength, YM- Young’s modulus ABSTRACT

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The aim of this work was to prepare deep eutectic solvents (DES) made with choline salts with α-hydroxylate anions and glycerol and apply them as starch plasticizers. Additionally,

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crosslinking potential of polyfunctional anions from the salts was investigated. Starch/DES premixtures were thermally analyzed (DSC, TGA). Thermoplastic starch (TPS)/DES films were prepared via thermocompression molding. Influence of choline salt to glycerol molar ratio, type of anion and compression parameters on mechanical, thermal and sorption properties as well as structural morphology (XRD, FTIR analysis) was studied. DES containing citrate anion exhibited parallel crosslinking and plasticizing ability of polysaccharide matrix as applied techniques confirmed. The higher choline citrate (CCit) 1

ACCEPTED MANUSCRIPT content in DES the higher tensile strength, more amorphous structure and lower sorption degree to CCit:G 1:6 molar ratio. XRD revealed that TPS/DES films did not retrograde even after one year of storage. The best compression parameters for studied systems were: 140 °C, 12 tons, 10 min.

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Keywords: thermoplastic starch; deep eutectic solvents; crosslinking

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1. Introduction

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Starch is one of the most abundant polysaccharides consists of two polymeric fractions: amylose and amylopectin, widely used in food, pharmaceuticals, paper processing and

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packaging applications [1]. Strong interactions between starch chains and glass temperature (Tg) near their degradation indispose starch processing as a thermoplastic material [2].

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Introduction of plasticizer into polymer creates more flexible materials with lower Tg, facilitating starch processing e.g. via thermocompression. Plasticizers are polar, low

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molecular weight molecules which are capable of disrupting intermolecular bonds between

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chains and of forming new ones with -OH groups on the glucosidic units (AGU) of polysaccharide chains, leading to increased chain movements with a more flexible material

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with lower Tg as a result. Thus, starch in the presence of plasticizer can be easily processed and can form thermoplastic derivatives. The most common plasticizers for starch are water

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and glycerol, but other polyols [3,4], amines [5-8], carboxylic acids or their salts [5,9], sugars [10] and ionic liquids [11] can be also applied. However, glycerol tends to migrate in starch materials, leading to starch retrogradation after longer time of storage. Other plasticizers, like some amines, are toxic; ionic liquids are expensive and carboxylic acids lead to changes in molecular weight of polysaccharide chains [12,13].

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ACCEPTED MANUSCRIPT Recently, a novel group of media named deep eutectic solvents - DES, being a cheap and more environmentally friendly alternative for molecular ionic liquids, have been tested as starch plasticizing agents [14]. DES are mixtures of two or more components (containing hydrogen bond donor and acceptor) where their melting temperature (Tm) is much lower than the Tm of the individual components [15]. So far, mainly choline chloride (CC) based DES

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with urea [16-19] glycerol (G) [17] carboxylic acids and imidazole [20,21] have been tested

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as starch plasticizers. Nonetheless, CC is highly hygroscopic and can increase sensitivity to moisture of starch materials. Thus, in the presented study their derivatives like choline

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dihydrogen citrate salt (choline citrate – CCit) were applied.

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Carboxylic acids can act not only as crosslinking agents but also as plasticizers [9,22].

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Although they crosslink starch materials by forming ester bonds between their carboxyl groups and hydroxyl groups onto AGU, they can also cause chain scission by acidic hydrolysis [13]. Therefore, addition of carboxylic acid to starch, especially during

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thermocompression molding and extrusion where shear stress and high temperature occur, should be strictly controlled. Most of work related to TPS crosslinked with carboxylic acids described materials obtained via casting method [23-28], with only few papers presenting

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TPS films with carboxylic acids prepared via thermocompression molding methods, including

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extrusion [22,29-31].

The aim of this work was preparation of starch-based films plasticized with deep eutectic mixtures consisting of choline salts with polyfunctional carboxyl anions (citrate, bitartrate) and glycerol via simple and convenient thermocompression molding. So far, such DES have been used as plasticizers for TPS films obtained only from casting method [32,33]. Moreover, prepared DES have reactive functional groups that can simultaneously act as plasticizer and crosslinking agent. Influence of choline salt to G molar ratio as well as processing parameters 3

ACCEPTED MANUSCRIPT on morphological and mechanical properties was investigated. Additionally, cholinum salts (ionic liquids) with different α-hydroxylate anions were synthesized and used in DES preparation to study the amount of functional group influence on TPS final properties. Starch/DES premixtures of DES were characterized by DSC and TGA. Mechanical tests, dynamic mechanical thermal analysis, characterization of molecular structure by FTIR and

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XRD as well as behavior in water (sorption and solubility degree, perspiration tests) were

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performed. Some obtained results were compared with TPS plasticized with G or DES CC:G

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1:2.

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2. Materials

Native potato starch (16.5 wt% moisture) was supplied by Zetpezet S.A. (Poland), choline

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dihydrogencitrate salt - CCit (≥ 98%), L-malic acid (≥ 98%) and citric acid – CitA (99%) were purchased from Sigma-Aldrich choline chloride – CC, and choline bitartrate – CBit (≥

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98%) from Alfa Aesar, Glycerol - G (pure) from Eurochem and lactic acid (pure) from POCH

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(Gliwice, Poland). 2.2 Methods

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2.2.1. Preparation of deep eutectic solvents and cholinium ionic liquids

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Deep eutectic solvents were prepared by stirring the components at 80 °C until homogenous pellucid liquid was obtianed. Then the mixture was placed in a glass vial and kept in a vacum chamber (105 °C, 250 bar, 30 min) to remove moisture from DES. Viscosity and transition temperatures values of DES are presented in Supplementary Information - SI, Table S1, Fig. S1. Degradation temperature values of CCit-based DES were presented in previous work [32]. Cholinium IL was prepared according to work [34]. Choline chloride was dissolved in ethanol and then KOH metanol solution was added to exchange Cl- onto OH-. The disspersion was 4

ACCEPTED MANUSCRIPT stirred for 1 h with a magnetic stirrer, filtrated to remove KCl, then α-hydroxycarboxylic acid was stepwise added and the mixture was stirred for 1 h. Ethanol was evaporated by distillation to obtain a homogenous medium viscous IL. Moisture content determination (using RADWAG MAX 60NH) indicated that ILs had some water content: choline lactate (CLac) – 2.6%, choline malate - CMal – 1.6% and CCit (obtained in laboratory) 1.7%, so their

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acronyms will be CXh.

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2.2.2. Preparation of TPS/DES films

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Starch and DES (35 parts of DES per 100 parts of dry starch) were placed in a mortar and

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ground to obtain a homogenous paste, then the composition was placed in PP vial, sealed and stored for 24 h at ambient conditions. The premixtures were thermocompressed (standard

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parameters: 140 °C, 12 tons for 10 min; influence of parameters on films properties was investigated, thus they were changed for certain compositions) cooled down to 85 °C under

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pressure and then placed in a climate chamber (25 °C, RH 50%) for 7 days before testing.

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2.2.3. Differential scanning calorimetry (DSC) Phase transitions of DES and starch/DES premixtures were investigated applying DSC

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technique (DSC Q100 TA Instruments). Parameters of DES characterization are presented in

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SI. Starch/DES premixtures (before thermocompression) after 24 h storage were analyzed in aluminium hermetic pans (ca. 10 mg of sample) with nitrogen as cooling agent with heating rate of 5 °C/min. Standard run at the temperature range from 20 to 250 °C for starch/DES premixtures was applied. 2.2.4. Rheological behavior

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ACCEPTED MANUSCRIPT Starch/DES premixtures were stored for 24 h before measurements. The rheological behavior of compositions was investigated using a stress rheometer ARES (Anton Paar MCR 502) equipped with a parallel plate system. Temperature range was 30-200 °C, heating rate 5 °C/min, frequency 1 Hz.

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2.2.5. Thermogravimetric analysis (TGA) Thermal stability of DES, starch/DES premixtures and thermocompressed films was

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investigated using TGA (Q500, TA Instruments). Tests were performed on platinum pans

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under 25 mL/min air flow, in the temperature range of 40–700 °C at a heating rate 10 °C/min.

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2.2.6. Mechanical tests

Mechanical tests for TPS/DES films were performed using Instron 5982 (load cell 1 kN). The

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films (thickness 0.5-0.6) were cut into 10 mm wide, 100 mm long stripes. The initial grip separation was 50 mm and the cross head speed was set to 10 mm/min. Ten replicated

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samples for each system were tested and the mechanical parameters (tensile strength - TS,

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elongation at break - EB and Young’s modulus - YM) were calculated with Bluehill 3 software.

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2.2.7. X-ray diffractiometry (XRD)

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Crystallinity of TPS/DES films was analyzed by X-ray diffraction (X’pert Pro, PANalytical, operated at the CuK(alfa) wavelength 1,54 Å). XRD analysis of the samples was repeated after 12 months of storage in ambient conditions. 2.2.8. DMA measurements Dynamic mechanical thermal analysis (DMA Q800 TA Instruments) was used to measure tan δ of transition temperatures Tα and Tβ of TPS/DES films. The measurements were carried out 6

ACCEPTED MANUSCRIPT in a dual cantilever mode at frequency of 1 Hz, heating rate 3 °C/min, and temperature range from -80 to 140 °C. 2.2.11. Behavior in water Water sorption and solubility degrees were determined by immersing of dried (60 °C,

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overnight before tests) samples (20 x 20 mm) in distilled water for 24 h. Wet samples were placed on a paper towel for a few minutes do remove water excess and, in the case of sorption

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degree, they were weighted, for solubility degree determination samples were dried before

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weighting. Sorption degree (S%) and solubility degree (R%) were calculated according to the following equations: SD = (ms-md/md)*100%; R% = (md-mr/md)*100%, respectively, where

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md is mass od dried sample, ms – mass of swollen sample, mr – mass of remaining dried

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sample. 2.2.10. Resistivity measurements

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Electrical surface and bulk resistivity of the films were measured using a 6517A electrometer

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with electrode a Keithley 8009 set (Keithley Instruments, Inc.).

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2.2.9. FTIR-ATR analysis

FTIR analysis was performed using Nexus (Thermo-Nicolet) technique equipped with ATR.

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For each sample 32 scans were taken from 4000–400 cm-1. 3. Results and discussion 3.1. Differential scanning calorimetry (DSC) Transition temperature values and exemplar DSC curves of DES are presented in Table S1 and Fig S1. The higher content of choline salt in the mixture the higher its Tg value.

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ACCEPTED MANUSCRIPT Moreover, CCit-based mixtures exhibited glass transition peaks at higher temperatures than analogues with CBit. Figure 1 shows DSC curves for starch/DES premixtures as a function of CCit to G molar ratio in eutectic mixture. Starch plasticization including e.g. swelling of starch granules is an

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endothermic process. It can be observed that the higher CCit content the lower ∆H of plasticization. It can be concluded that CCit presence decreases heat requirements of the

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transformation and facilitates thermocompression. At ca. 230 °C some slightly intense

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exothermic peaks appeared. This change is assigned to starch carbonization in the presence of carboxylic group, in which, firstly, some esteryfication reaction can occur with release of CO2

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and polysaccharide chain scission. Similar peaks were observed in other studies [35], where

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starch with different moisture content was investigated. 3.2. Rheological behavior

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Changes of starch/DES premixtures viscosity in temperature function were investigated using

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ARES rheometer. Rapid drop of viscosity ending up at 110 °C for PS with CCit:G 1:12, 1:2 and G and at ca. 85 °C for CCit:G 1:8 and 1:6. For the premixtures with CCit:G DES, there

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was viscosity increase at higher temperatures, and the maximum of the values increased with CCit content on the system (1:8 –3610 Pa·s/106 °C; 1:6 – 4480 Pa·s /129 °C; 1:2-6860

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Pa·s/178 °C). However, only slight increase of this parameter was observed for starch with G and plateau for CCit:G 1:12. Increase of viscosity can be related to starch swelling occurring at lower temperatures with the presence of CCit:G and crosslinking reaction between starch chains and CCit from spherical plasticizer. 3.3. Thermogravimetric analysis (TGA)

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ACCEPTED MANUSCRIPT Native potato starch, its premixture with Ccit:G 1:8 and thermocompressed TPS were thermogravimetrically analyzed. As we can see from Figure 3A, granular native starch exhibits rapid weight loss to ca. 100 °C which is related to moisture evaporation, next drastic decrease at ca. 260 °C is caused by carbohydrate polymer degradation. Introduction of CCitbased DES decreases degradation temperature to ca. 210 °C. Comparing Stach/DES

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premixture and its thermoplasticized form, it can be noticed that moisture evaporation from

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the material is more gradual, suggesting better water molecules bonding and trapping them

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into the film structure so they act as a co-plasticizer. In Fig. 3B presenting DTA a small peak with maximum at 165 °C for the premixture is observed. It can be an effect of reaction

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between CCit and starch and some CO2 release. For TPS/DES there is a barely visible peak with maximum at 188 °C which is related to degradation of chemically unreacted CCit from

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DES (see SI, Fig.S2). Moreover, TPS/DES has a new peak at 291 °C, that is not present in other samples, which can indicate a fraction with substituted OH groups (by esterification

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reaction). As both TGA and DTA curves show, at high temperature around 500 °C TPS/DES

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has the highest resistance to thermal degradation (carbonisation). 3.4. Mechanical properties of TPS/DES films obtained via thermocompression

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Figure 4 presents mechanical properties i.e. tensile strength, elongation at break and Young’s

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modulus for TPS plasticized with DES in dependence on CCit:G or CBit:G molar ratio in the plasticizer. Mechanical tests of TPS/G ad TPS/CCG were also prepared for comparison purposes. As can be observed from the data, when CCit amount increased in TPS, a slightly increase of TS and YM, but decrease of EB values occured. This behavior could have been caused by partial crosslinking which forms bridges between polysaccharide chains and limits their mobility. For CCitG 1:12 and 1:10 TS is comparable to G and EB is even higher, which can be related to summarized size of the plasticizer (quite a big size of CCit molecule + 9

ACCEPTED MANUSCRIPT glycerol) and insufficient amount of functional groups to form crosslinking structure. A similar correlation between EB and size of plasticizer was observed in another study [36], where branched plasticizers where used or carboxylic acid was added into a system in low concentration [22,29]. However, for citric acid, TS increased for 0.75% CA content in TPS [22] further increase of acid addition caused decrease of TS values [22,29]. For TPS/CBit:G

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films this trend was not so pronounced. TS values are higher than for TPS with G and Ccit:G,

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although EB is almost the same for different CBit content. Due to the fact that CBit has only

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one functional group able to react, it can not crossling the matrix. Moreover, systems with molar ratio of 1:6 and higher tended to DES recrystallization, even after about a week of

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conditioning, which confirms their unbonded form in the TPS matrix. Plasticizers with higher molecular weight like glycerol diacetate/triacetate compositions were

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shown to be inefficient as plasticizers [9] whereas phase separation has been observed for starch and starch blends plasticized with polyethylene glycols [5,37]. Here, plasticizers

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possess functional groups that can react with AGU leading to internal plasticization and

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decrease of phase separation phenomenon (more in DMA section). Additionally, starch was also thermoplasticized with other DES based on cholinium salts and

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a mixture of citric acid and G (CitA:G 1:8) at the same molar ratios to compare with CCit:G.

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Moreover, starch was plasticized with CCit and G added as individual components (CCit+G 1:8) Results are shown in Table 1. Laboratory obtained choline derivatives with αhydroxyacidic anions have some water content, thus are liquids at RT. Moreover, their eutectic mixtures with G exhibit slightly lower apparent viscosity in comparison with DES made with commercial choline salts. According to literature [14] water presence in DES decrease its viscosity. TPS plasticized with such DES have higher EB values than anhydrous DES. On the one hand, water acts as starch plasticizer, on the other, their small molecules are 10

ACCEPTED MANUSCRIPT more mobile, ad therefore bound to facilitate TPS elongation. Lower TS and high EB of TPS/Clach:G 1:8 can be caused by smaler lactate anions which are not able to crosslink starch chains. TS increase and ε decrease with increasing of carboxylic groups in choline-derived anions. Despite the fact that cholinium IL were applied as starch plasticizer [34,38], their mixtures with G effectively acts as plasticizers and forms flexible transparent films.

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Furthermore, they cost less than IL and to the best of our knowledge have not been described

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to date.

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One selected DES was also chose to plasticization corn starch. A comparison of stres-strain curves for TPS from two types of starches is presented in SI, Figure S3. Corn starch formed

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film with lower TS but higher ε compared to potato starch.

Mechanical properties of TPS/CCit:G 1:8 films prepared at different thermocompression

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parameters are presented in SI, Table S2. Lower temperature or pressure led to films with lower TS or EB, caused by some unplasticized remainings (films were slightly opaque, see

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discussion for XRD results below), in turn, higher temperature (150 °C) and longer time of

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compression cause sample degradation. 3.5. XRD analysis

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XRD diffractometry was used to investigate morphological structure of TPS/DES films. For

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comparison TPS/G was also analyzed. The pattern for granular potato starch exhibits characteristic peaks at 5.4, 17.1, 19.5, 22.5, 24.0 and 26.0° forming by B-type semi-crystalline structure (SI, Fig. S4). After thermocompression with plasticizer presence these peaks got flattened, indicating a more amorphous structure as Fig 5A presents. XRD patterns for TPS with conventional plasticizer – glycerol and DES with different amounts of CCit were compared. Generally, the higher content of CCit the more amorphous the sample is. For TPS/G and TPS/CCit:G 1:12 there are small peaks at 17.1° and 19.5° identified as the 11

ACCEPTED MANUSCRIPT remianing B-type semicrystalline structure formed by amylopectin [38] and unplasticized core of granule. DES size is much greater than glycerol, moreover, it has more spherical complex structure that can easily disrupt external and internal bonds between starch chains. Similar results, where TPS structure was more amorphous for branched plasticizer than for conventional one were obtained in work [36]. Starch plasticized with choline ionic liquid [34]

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or in citric acid presence [22] also has more amorphous structure than that modified only with

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e.g. glycerol. Data presented here are correlated to mechanical test results, where TPS

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plasticized with DES with lower content of CCit had higher ε values than TPS/G, caused by greater size of plasticizer and more amorphous polysaccharide chains with higher mobility.

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Fig 5B shows TPS/CCit:G 1:8 films after storage. As XRD pattern revealed there is no difference between the patterns, that indicates lack of starch retrogradation even after one year

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storage period.

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XRD diffractograms of TPS/CCit:G 1:8 thermocompressed at lower temperature, pressure

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and shorter time are presented in SI, Fig. S5. It can be fund that there are some peaks with low intensity at 17 and 19° suggesting presence of remains of B-type semicrystalline structure of

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starch granule core.

Fig. S6 shows XRD patterns of TPS/CBit:G films, where evidence of plasticizer

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recrystallization in polysacharide matrix can be observed. 3.6. DMA analysis of TPS/DES films DSC for TPS/DES films was performed for analysed samples, however, due to moisture and plasticizer presence, it was not able to observe Tg (SI, Fig. S7), thus instead DMA results are demonstrated. Figure 6 presents tan delta curves for TPS plasticized with two CCit:G DES, as well as G and CCG. Influence of functional group presence on some structural changes as

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ACCEPTED MANUSCRIPT temperature function was investigated and compared. DMA for TPS/DES and TPS/G materials revealed two relaxation regions: β-relaxation (Tβ) placed near -50 °C and αrelaxation (Tα) at range between 30-50 °C. TPS/CCit:G 1:4 is an exception where only Tα peak was observed. β-relaxation came from plasticizer rich phase and TPS plasticized with CC:G and G have much higher peak intensity than that of films with CCit presence. Much

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lower intensity for TPS/CCit:G 1:8 and lack of Tβ for TPS/CCit:G 1:4 can be caused by

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chemical reaction between DES component and polysaccharide chains increasing starchplasticizer homogeneity and limiting their mobility a well as by lower content in DES. Type

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of plasticizer influenced also on β-relaxation related to starch-rich phase. Tα values are 34.9,

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53.3, 36.2, 62.8 °C for TPS plasticized with: CC:G, G, CCit:G 1:8 and CCit:G 1:4, respectively.

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Influence of processing parameters on starch-plasticizer interactions was presented in SI, Fig. S8. α-relaxation peak was much more intense when lower pressure or shorter time of

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compression were applied. The results indicated that mutual interactions between starch and

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plasticizer are weaker and the system required higher pressure and longer time to achieve high plasticizing efficiency. These data corresponds with XRD, where a more amorphous sample

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(highly plasticized) was obtained at 140 °C/12 tons/10 min.

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3.7. Behavior of TPS/DES films in water and moisture Table 2 presents sorption and solubility degrees of TPS plasticized with CCit-based DES, CCG and G. Generally, solubility degrees do not much differ and are placed in the range of 22-23%, except for TPS films with higher content of CCit. For the sample with the highest CCit concentration determination of sorption degree was not possibile due to its defragmentation in water. It can be related to high content of citrate groups which lead to polysaccharide chain scissions during thermocompression. These results corresponds with 13

ACCEPTED MANUSCRIPT those obtained from mechanical tests (the higher CCit the higher TS). Sorption degrees for TPS/CCit:G films are much higher than for TPS/G ( ̴65%) and are placed in the range of 89.7-119.6%. Much higher sorption properties of TPS/CCitG films in comparison with TPS/G are caused by presence of highly hydrophilic carboxylic groups, but also by slightly crosslinked starch matrix with spherical large molecules of plasticizer. Sorption degree

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decreased with increase of CCit content, but increased for CCit:G 1:4 and for CCit:G 1:2

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defragmented, as mentioned above.

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Films were placed in climate chamber at 70% to study if perspiration would occur. Only for TPS/CCG sample plasticizer perspirated on sample surface (perspirated droplets were

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collected and analyzed by FTIR, data not shown). Choline chloride is highly hygroscopic so its presence can increase sensitivity of a material to moisture. However, replecemant of Cl

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with citrate anion that additionally can partially react with the matrix hinders migration of

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CCit-based plasticizer on the TPS surface.

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3.8. Electrical resistivity of TPS/DES films Results of surface and bulk electrical resistivity measurements for TPS/DES films are presented in Table 3. Generally, while comparing TPS samples with CCit presence there is no

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significant difference between the results. Resistivity slightly increased with increase of CCit

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content but the differences are negligible. Lower values were observed for TPS plasticized with CC:G which was caused by more mobile small chloride anion from CC. This sample exhibits conductive properties, while the rest of analyzed samples exhibit dissipative activity. 3.9. FTIR-ATR spectroscopy FTIR-ATR spectroscopy was applied to investigate changes in molecular structure of starch/DES films. Figure 7 shows FTIR-ATR spectra for TPS/DES (CCit:G 1:8), DES and unmodified potato starch. DES have bands assigned to carboxyl groups from the citrate anion 14

ACCEPTED MANUSCRIPT at 1718 cm-1. For TPS/DES these bands are shifted to higher wavenumber (1725 cm-1), which may indicate an esterification reaction between carboxyl groups from the salt and the polysaccharide. Moreover, the peak at 3273 cm-1 of the broad band in the wavenumber range of 3600-3000 cm-1 assigned to -OH groups from glucosidic units was shifted to 3285 cm-1 indicating formation of hydrogen bonds with DES (3295 cm-1). Similar results were obtained

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in another study where CCit-based DES were used to crosslink TPS films obtained via casting

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method and where scheme of crosslinking reaction was presented [32]. In Fig. S9 the intensity

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of the peak related to ester bond formed by CCit is increased with increase of CCit content in DES.

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4. Conclusions

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In this work deep eutectic solvents were prepared and characterized. The higher amount of CCit in the mixture, the higher its viscosity and glassdegradation transition temperature . As

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DSC results revealed, the higher CCit amount in the plasticizing system the lower ∆H of

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starch/DES premixtures. The type of DESmixture, concentration of choline salt and thermocompression parameters influenced the mechanical, sorption and morphological

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properties of the final products. Tensile strength increased while elongation at break decreased when CCit content in DES increased. Increased CCit content in TPS film also

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resulted in its lower sorption degree. This indicates that higher amount of citrate anion in the system led to higher degree of crosslinking. Partial crosslinking reaction between polysaccharide and DES component was confirmed also by FTIR spectra and DMA. X-ray diffractometry revealed that TPS/CCit:G films did not retrograde even after 12 months of storage. The best compression parameters for the studied systems were: 140 °C, 12 tons, 10 min.

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ACCEPTED MANUSCRIPT Reactive plasticizer is advantageous over conventional plasticizer (e.g. glycerol) owing to the fact that as covalently bonded compounds do not migrate or evaporate from polymeric matrix. Such materials can be applied for example in safe food packaging or agricultural industry. Acknowledgements

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This work was supported by National Science Centre, Poland [SONATA 9 grant number: 2015/17/D/ST8/01290]. Authors thank Dr Szymon Kugler for electrical resistivity

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measurements and Dr Honoata Mąka for rheometric analysis.

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ACCEPTED MANUSCRIPT Table 1 Mechanical properties of TPS films with different plasticizers. Parameters of thermocompression: 140 °C/12 tons/10 min. Tensile strength Elongation at break Young’s modulus (MPa) (%) (MPa) 4.3 (±0.18) 82.6 (±3.3) 133 (±24) 5.5 (±0.22) 59.0 (±3.4) 185 (±15) 5.7 (±0.03) 54.5 (±1.2) 212 (±30) 5.8 (±0.07) 61.2 (±4.9) 220 (±28) 5.2 (±0.09) 36.8 (±4.7) 194 (±24) degraded during thermocompression

Plasticizer

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CLac:Gh 1:8 CMal:Gh 1:8 CCit:G 1:8 CCit:Gh 1:8 CCit+G 1:8 CitA:G 1:8

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ACCEPTED MANUSCRIPT Table 2 Sorption and solubility degrees in distilled water and perspiration test results at 25 °C and RH 70%. Solubility degree (%) 22.0 (±3.6) 22.9 (±3.7) 22.9 (±1.7) 23.4 (±1.3) 22.3 (±1.9) 24.0 (±1.6) 26.5 (±2.3) 23.0 (±1.6)

Perspiration no no no no no no no yes

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TPS/G TPS/CCit:G 1:12 TPS/CCit:G 1:10 TPS/CCit:G 1:8 TPS/CCit:G 1:6 TPS/CCit:G 1:4 TPS/CCit:G 1:2 TPS/CC:G 1:2

Water sorption degree (%) 65.1 (±4.2) 106.0 (±8.2) 103.9 (±14.3) 102.2 (±5.0) 89.7 (±7.8) 119.6 (±6.2) defragmented 80.4 (±3.4)

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ACCEPTED MANUSCRIPT Table 3 Electrical (surface and bulk) resistivity of TPS/DES films at ambient conditions. Resistivity Surface (Ω) Bulk (Ω·m) TPS/CCit:G 1:12 7·107 5·105 TPS/CCit:G 1:10 7·107 6·105 TPS/CCit:G 1:8 8·107 5·105 7 TPS/CCit:G 1:6 8·10 5·105 TPS/CCit:G 1:4 1·108 6·105 8 TPS/CCit:G 1:2 2·10 1·106 TPS/CC:G 1:2 1·106 3·104

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ACCEPTED MANUSCRIPT Figure captions Fig. 1. DSC curves for potato starch/DES premixtures. Fig. 2. Viscosity changes (rotational rheometry measurements) of starch/DES premixtures as temperature function. Fig. 3. TGA (A) and DTA (B) curves for native potato starch (PS), premixture of PS and DES

Fig. 4. Mechanical tests results for TPS/DES and TPS/G films.

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and TPS/DES.

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Fig. 5. XRD diffractograms for TPS plasticized with DES with different CCit:G molar ratio (A) and TPS/CCit:G 1:8 (B) after storage at ambient conditions.

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Fig. 6. Tan delta curves for TPS plasticized with G, CC:G and selected DES with CCit.

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Fig. 7. FTIR-ATR spectra for native potato starch, DES CCit:G 1:8 and TPS plasticized with

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7