Biofriendly ionic liquids for starch plasticization: A screening approach

Article abstract of DOI:10.1039/c6ra16573g

A series of cholinium cation-based bioionic liquids (BioILs) were synthetized with the aim of screening their performance as potential plasticizers of thermoplastic starch. To synthesize these BioILs, two easy and fast synthetic routes were selected: an ion exchange reaction, direct and economical, and a two-step acid-base reaction. Most of these BioILs allowed efficient plasticization of starch by film casting. The structure of the anion used significantly influences the thermo(hygro)mechanical and recrystallization behavior, making it possible to modulate the properties of the final material.

Full text of DOI:10.1039/c6ra16573g

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Biofriendly ionic liquids for starch plasticization:
a screening approach†
Cite this: RSC Adv., 2016, 6, 90331
a
G. Colomines, P. Decaen,bc D. Lourdinc and E. Leroyb
*
A series of cholinium cation-based bioionic liquids (BioILs) were synthetized with the aim of screening their
performance as potential plasticizers of thermoplastic starch. To synthesize these BioILs, two easy and fast
synthetic routes were selected: an ion exchange reaction, direct and economical, and a two-step acid–base
reaction. Most of these BioILs allowed efficient plasticization of starch by film casting. The structure of the
anion used significantly influences the thermo(hygro)mechanical and recrystallization behavior, making it
possible to modulate the properties of the final material.
Received 27th June 2016
Accepted 13th September 2016
DOI: 10.1039/c6ra16573g
www.rsc.org/advances
In recent years, ionic compounds such as Ionic Liquids
(ILs)9–11 or Deep Eutectic Solvents (DES)12–15 have been investi-
gated as new types of plasticizers for starch.9,16 Due to their
Introduction
Starch is one of the most abundant natural biopolymers
found in various crops worldwide.1 As a petroleum-based
polymer, it can be melt-processed as a thermoplastic mate-
rial using thermomechanical processes such as extrusion or
injection-molding.2 However, this requires the presence of
smaller molecules known as plasticizers, which play a double
role: rst, by favoring the destructuring of native starch,
strong ability to form hydrogen bonds, these compounds are
highly efficient both during the destructuration stage of native
starch and as glass transition depressors in the nal plasticized
starch material.9,17,18 In addition, it has been observed that the
presence of ILs or DES can hinder starch recrystallization
during storage,13,16 resulting in more stable thermomechanical
properties. Last but not least, it was shown that these new
plasticizers convey plasticized starch conductivity properties
due to their ionic nature. However, the range of conductivity
obtained is greatly inuenced by ionic liquid structure and
water content.9,11,19
Nevertheless, most of these scientic results have been ob-
tained for commercial ionic liquids based on imidazolium cations
that are currently too expensive and non-biodegradable (and toxic
in many cases; only EMIM acetate was shown to display relatively
low toxicity),20 impeding their use in large-scale applications.21–27
On the contrary, cholinium cation-based systems offer a prom-
ising alternative for cheap, environmentally-friendly ionic plasti-
cizers for starch. The use of cholinium chloride-based DESs has
already been studied.12,16,28,29 Depending on the hydrogen donor
counterpart used (glycerol, urea, etc.), various efficient plasticizers
can be designed.
However, an even higher versatility can be expected from the
design of cholinium-based ILs using different anions. The
objective of the present work is to propose a small-scale
screening approach that would make it possible to select the
most efficient combinations for starch plasticization. A series of
cholinium ILs was synthesized using naturally-derived counter
anions with structures similar to compounds formerly known
as starch plasticizers. For example, since citric acid and sodium
lactate are known as starch plasticizers,4 we studied cholinium
lactate and cholinium tricitrate. Other anion structures not
previously reported as starch plasticizers (furoate, salicylate,
a
granular semi-crystalline structure,3 and, second, by
reducing intermolecular interactions between starch chains
in the amorphous state. Water is the most efficient starch
plasticizer. Typically, an increase in moisture content of
1 wt% results in a decrease of 10 K of the glass transition of
the biopolymer.4 Unfortunately, water is a highly volatile
molecule. This results in a high sensitivity of the thermo-
mechanical properties of starch materials to relative humidity
(RH). In order to better control mechanical properties, various
non-volatile plasticizers have been studied.4–6 This results in
a wide range of mechanical properties that depend on plas-
ticizer type and content.7 Since the 1990s, polyols (in partic-
ular, glycerol) have emerged as the most widely used
compounds for starch plasticization. Nevertheless, they
present some severe drawbacks, the main one being their
inuence on plasticized starch recrystallization during
storage.8 This phenomenon results in a relatively fast stiff-
ening and embrittlement of plasticized starch objects.8
a
´
LUNAM Universite, IUT de Nantes, CNRS, GEPEA, UMR 6144, OPERP, 2 Avenue du
Professeur Jean ROUXEL, BP 539, 44475 Carquefou, France. E-mail: gael.
colomines@univ-nantes.fr
b
´
´
LUNAM Universite, CNRS, GEPEA, UMR 6144, CRTT, 37, Boulevard de l'Universite,
44606 St Nazaire Cedex, France
c
´
`
INRA Nantes-Angers, Unite Biopolymeres Interactions Assemblages (BIA-UR 1268),
´
`
Rue de la Geraudiere, BP 71627, 44316 Nantes Cedex 3, France
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra16573g
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etc.) were also selected for their low toxicity and/or biocompat- was evaporated under vacuum. The compound obtained was
ibility (cholinium salicylate has been recognized since 1960 as the ionic liquid.
a well-tolerated analgesic).30 The synthesis route of these bio-
Tricholinium citrate ([Chol][Cit]), cholinium furoate ([Chol]
friendly ionic liquids (BioILs) was kept as simple as possible, [Fur]) and cholinium lactate ([Chol][Lac]) were synthesized
focusing on starch plasticization studied by the characteriza- according to this method. Aer synthesis, ionic liquids were
tion of water-casted lms.
hermetically packaged and stored at 4 ꢁC without further
purication.
1H-NMR analysis. NMR spectra were recorded on a Bruker
Avance III 400 MHz NMR spectrometer. The chemical shis are
reported in parts per million, where s is a singlet, d a doublet, t
a triplet, and m a multiplet. Unless otherwise specied, all the
Experimental
Raw materials
Regular corn starch (amylopectin/amylose ratio: 70/30) was
purchased from Tate & Lyle (Meritena 100), with an initial
moisture content of 12%. Potassium acetate, furoic acid,
cholinium chloride ([Chol][Cl]), sodium saccharinate, anhy-
drous methanol and 1-butyl-3-methylimidazolium chloride
([Bmim][Cl]) were purchased from Sigma-Aldrich; Hydranal®
and Composite 5 from Fluka; citric acid, L-lactic acid and
sodium salicylate from Panreac; potassium hydroxide from
LabOnline; absolute ethanol from Alcogroup; silver nitrate from
Labosi; sodium chloride standard from Thermo Scientic; and
deuterium oxide from Euriso-top. They were all of analytical
grade and were used without further purication.
ꢁ
spectra were recorded at 30 C from D2O solutions.
The NMR spectra of [Chol][Lac] and [Chol][Cit] have already
been described.32
1
[Chol][Fur]: H NMR (400 MHz, D2O, TMS): d ¼ 3.13 (s, 9H,
–NCH3), 3.39 (m, 2H, –NCH2), 3.95 (m, 2H, –OCH2), 6.50 (m, 1H,
OC(COOꢀ)CHCHCH), 6.97 (m, 1H, OC(COOꢀ)CHCHCH), 7.55
(m, 1H, OC(COOꢀ)CHCHCH).
Differential scanning calorimetry (DSC). Differential scan-
ning calorimetry (DSC) analyses of the BioILs were performed
with a Mettler Toledo DSC 1 STARe System DSC apparatus
calibrated with indium. All samples were conditioned in 100 mL
sealed aluminum pans. The following temperature program
was used: precooling to ꢀ70 ꢁC, temperature ramp from ꢀ70 to
120 ꢁC at 1ꢀ01ꢁC minꢀ1, followed by cooling from 120 to ꢀ70 ꢁC at
Syntheses and characterization of ionic liquids
Synthesis by ion exchange. [Chol][Cl] (0.2 mol) and carbox-
ylate compound (0.2 mol of carboxylate equivalent) were sepa-
rately dissolved in absolute ethanol to obtain a 0.25 mol Lꢀ1
solution. Carboxylate solution was added to a stirred [Chol][Cl]
solution. A white precipitate of sodium chloride (or potassium
chloride) was formed and removed by ltration aer the solu-
tion was stirred for 1 h at room temperature. Ethanol was
evaporated under vacuum. The compound obtained was the
ionic liquid. The sodium chloride (or potassium chloride) ob-
tained was dried at 100 ꢁC for 24 h and then heated at 750 ꢁC to
eliminate organic compounds and to estimate the reaction yield
in another way. Cholinium acetate ([Chol][Ace]) and cholinium
salicylate ([Chol][Sal]) were synthesized according to this
method. Cholinium saccharinate ([Chol][Sac]) was synthesized
according to the Nockemann method31 but without further
purication. The NMR spectra of [Chol][Ace],32 [Chol][Sal]33 and
[Chol][Sac]31 have already been described. Our spectra were
made in D2O and present some differences in chemical shi
due to the nature of the deuterated solvent.
ꢁ
ꢁ
ꢁ
10 C min and, nally, heating from ꢀ70 to 120 C at 10 C
minꢀ1. The melting point (Tm) values were determined at the
onset of the peak during the second temperature ramp. In some
cases, no melting peak was observed, or a cold crystallization
was observed during the cooling step.
Thermogravimetric analysis (TGA). The thermal stability was
investigated by two types of experiments under a nitrogen
atmosphere:
(1) Anisothermal tests (temperature ramp from 30 ꢁC to
750 ꢁC at a heating rate of 10 ꢁC minꢀ1) were performed on
a LABSYS TGA/ATD (SETARAM). The mass loss curves were used
to evaluate the onset of thermal degradation and the content of
volatile compounds (% of volatile compounds, i.e., the residual
solvent from synthesis and water),ꢁwhich was dened as the
ꢁ
weight loss between 30 C and 100 C.
(2) In addition, the static thermal stability at 130 ꢁC (a
temperature typically used in starch extrusion) was estimated
using a TGA 2050 (TA Instruments). A temperature ramp from
room temperature to 130 ꢁC at 10 ꢁC minꢀ1 was applied, fol-
lowed by an isotherm at 130 ꢁC for 24 h. An initial mass loss was
observed corresponding to the release of volatile impurities
Synthesis using the acid–base method
On the one hand, cholinium hydroxide ([Chol][OH]) was (like in non-isothermal experiments), followed by a slow
prepared according to the ion exchange method. Potassium decrease of mass at 130 ꢁC. The slope of this second mass loss
hydroxide (0.1 mol) and [Chol][Cl] (0.1 mol) were dissolved in associated with the isothermal degradation of the ILs was
800 mL and in 200 mL absolute ethanol, respectively. Hydroxide measured in wt% per hour.
solution was added to a stirred [Chol][Cl] solution. The white
precipitate was removed by ltration aer stirring for 1 h at tained by potentiometric titration using an Ag-electrode
room temperature. (Orion®) and an Hg/Hg2SO4 reference electrode. Silver nitrate
On the other hand, carboxylic acid (0.1 mol of carboxylic acid (0.1 mol Lꢀ1) was used as titrant. Ionic liquid was dissolved and
equivalent) was dissolved in ethanol (50 mL). then diluted in deionized water until reaching an equivalence
Chloride titration. The amount of residual chloride was ob-
The carboxylic solution was added to the hydroxide solution. point at around 10 mL (estimated chloride concentration:
The solution was stirred at room temperature for 1 h. Ethanol approximately 0.01 mol Lꢀ1). The amount of potassium (or
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ꢁ
sodium) chloride in the ionic liquid was deduced in wt%. A was kept constant at 22 ꢃ 1 C and the relative humidity was
blank test (without ionic liquid) and a standard test (with also measured using a humidity sensor close to the sample.
a standard NaCl solution) were made to check the analysis.
Results and discussion
Synthesis and characterization of ionic liquid
Processing and characterization of thermoplastic starch lms
Film casting. Films were obtained by the casting method.4
Native starch was solubilized in a high-pressure stirred reactor
at 130 ꢁC for 20 minutes using 300 mL of ultrapure water, 12 g of
starch and 3.6 g of plasticizer (BioIL, glycerol, etc.) representing
30 wt% of plasticizer in starch (dry basis). The procedure was
performed under a nitrogen atmosphere to limit thermal
degradation. The solution was evenly spread on a Teon®-
coated hotplate maintained at 70 ꢁC until the lm no longer
adhered to the plate and edge curling occurred.
The transparent lm obtained had a mean thickness of
approximately 100 mm. In some cases, the lm was too brittle to
be removed and manipulated for further testing. Therefore, the
corresponding couples of IL/starch were considered to be “non
lm-forming”. For the lm-forming systems, the stable lms
obtained were equilibrated at 21 ꢁC and 57% relative humidity
(RH) (saturated sodium bromide solution) for at least 2 weeks
before being tested.
The synthesis of cholinium-based ionic liquids has already been
described by Fukaya et al.34 However, this process requires the
use of ion exchange resin made using several elutions and
a large amount of solvent. Our synthesis method is adapted
from the Nockemann method31 based on metathesis. The
equilibrium of ion solubility is used in this case. NaCl and KCl
are known to be very slightly soluble in ethanol,35 contrary to
cholinium chloride.36 When the alcoholic solution of the
carboxylic acid compound (or alcoholic potassium hydroxide)
and the cholinium chloride compound were mixed, KCl (or
NaCl), precipitate formation was observed. The partial dissoci-
ation of the ions in ethanol seems to be sufficient to bring
together the two cations and two anions and, therefore, the
establishment of four solubility equilibria (Scheme 1).
The low solubility of KCl (or NaCl) shis the equilibrium
until the total precipitation of KCl (or NaCl) and, thus, the
formation of the ionic liquid (or [Chol][OH]) in the supernatant.
However, the success of the synthesis depends on the solubility
of the starting carboxylate reagent. When its solubility is partial,
the reaction is oen unsuccessful. NMR analysis allows us to
check the success of the reaction (Table 1). Moreover, by-
product analysis (weighing, FTIR, calcination) conrms this.
It should be noted that many tests were performed and the
result is binary: either it works or it does not work at all, i.e., the
tripotassium citrate synthesis failed.
Water content in lms. Water content in lms was evaluated
by Karl Fischer analysis (KF analysis). Before analysis, the lms
were immersed in methanol under stirring in order to extract
water. Aerwards, the water content in the methanol was
determined by the Karl Fischer method.
X-ray diffraction. In order to determine the crystallinity of
the plasticized starch, diffraction diagrams were recorded using
a Bruker D8 X-ray diffractometer (Karlsruhe, Germany) equip-
ped with a GADDS detector. The following parameters were
used: supply voltage: 40 kV; current intensity: 40 mA; charac-
teristic radiation wavelength for a coordinated copper lamp:
0.1542 nm (Cu Ka1). Measurements were conducted aer 2 and
Chloride titration quanties the residual salt (Table 1). The
measured amount is around the same as in the Ma study,37
which shows that this amount of salt limits starch recrystalli-
zation, which is positive.
Ethanol is used because it is potentially bio-based, easy to
remove by evaporation (unlike water) and recyclable for many
syntheses without any noticeable difference. According to the
type of carboxylate, the carboxylate solution can be concen-
trated (i.e., potassium acetate) without modifying the yield.
ꢁ
8 weeks at 21 C and 57% relative humidity (RH).
Differential scanning calorimetry (DSC). Glass transition
temperatures of the cast lms were determined by differential
scanning calorimeter on a Q100 apparatus (TA Instruments,
New Castle, DE, USA). Stainless steel sealed pans were used in
order to avoid water loss during tests. A scan was then per-
formed from ꢀ60 to 100 ꢁC at 10 ꢁC minꢀ1. The glass transition
temperature (Tg) was evaluated from the thermogram dened as
the onset of the heat capacity increase.
Dynamic mechanical analysis under controlled relative
humidity. RH-DMA measurements were made on a Metravib
DMA 50 analyzer. The tensile mode was used at a frequency of 1
Hz, with a deformation amplitude of 0.01%, which is in the
range of linear viscoelasticity. Measurements were performed
on a piece of lm measuring approximately 20 ꢂ 10 mm and
with a thickness of about 100 mm. Simultaneously to the
dynamic solicitation, a static force of 0.5 N was applied in order
to maintain the lm under tension. The storage modulus was
measured under controlled atmosphere from approximately 5
to 95% relative humidity. The humidity was obtained by mixing
a dry air ow with a water-saturated air ow, thermostated by
a circulating water bath at 22 ꢁC. The measurement temperature Scheme 1 Synthesis of BioIL.
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Table 1 Composition of the synthesized BioILs
NMR 1H anion/cation
ratio (target)
Titration NaCl or
KCl residual (wt%)
TGA residual volatile
impurities (wt%)
Synthetic route
Ion exchange
BioIL
[Chol][Ace]
[Chol][Sac]
[Chol][Sal]
[Chol][Cit]
[Chol][Lac]
[Chol][Fur]
1.02 (1)
0.78 (1)
1.01 (1)
2.83 (3)
0.92 (1)
1.01 (1)
2.5
3.0
3.0
2.5
2.5
2.0
7
4
6
14
8
4
Acid–base reaction
Table 2 Thermal properties of the synthesized BioILs
TGA (non-isotherm at 10
K minꢀ1) degradation onset
temperature (ꢁC)
DSC Tm (ꢁC)
(cold crystallization)
TGA (isotherm at 130 ꢁC)
mass loss (wt%/per hour)
Synthetic route
Ion exchange
BioIL
[Chol][Ace]
[Chol][Sac]
[Chol][Sal]
[Chol][Cit]
[Chol][Lac]
[Chol][Fur]
51 (yes)
40 (no)
/(Liquid) (no)
/(Liquid) (no)
/(Liquid) (no)
/(Liquid) (no)
190
250
241
194
213
226
2.04
0.02
0.05
0.72
0.29
0.41
Acid–base reaction
When the ion exchange reaction fails or if the carboxylate isothermal tests show that the BioILs are not completely stable
compound is much more expensive than the carboxylic at 130 ꢁC. However, the mass loss per hour is lower than 1% for
compound, the acid–base method is used and requires two all the samples, except for [Chol][Ace] with a value of 2%.
synthesis steps (1st step: synthesis of [Chol][OH]; 2nd step:
The BioILs with an aromatic structure (saccharin and
synthesis of BioIL). The [Chol][OH] solution can't be concen- salicylate) display the highest thermal stability (Fig. 1). This is
trated otherwise it will undergo partial degradation (color not surprising since the salicylate group is known for its
evolution observed in the nal ionic liquid).
antioxidant effect.38 Our BioILs clearly appear to be less ther-
The volatile content shows that the evaporation step can be mally stable than imidazolium-based IL.39 However, their
improved because we have limited the time and temperature of thermal stability remains compatible with their use for
this evaporation step to avoid thermal degradation.
producing plasticized starch by extrusion at temperatures
Regarding the melting temperatures (Table 2), all the BioIL close to 130 ꢁC, since the typical residence times for that
synthesized meet the denition of an ionic liquid (Tm # 100 ꢁC). process are lower than 10 minutes. In the case of the less stable
Some of them even remain liquid despite low storage BioIL ([Chol][Ace]), this would result in a degradation/mass
temperatures.
loss of 0.3%.
Thermal stability was considered both in non-isothermal
conditions for a heating rate of 10 K minꢀ1 and in isothermal
conditions at 130 ꢁC (temperature typically used for starch
extrusion).
The onset temperatures of degradation for non-isothermal
tests range between 190 ꢁC and 290 ꢁC. Moreover, the
Properties of IL-plasticized starch lms
Casting tests were conducted to identify BioILs that could be
used as starch plasticizers (Table 3). Glycerol is used as a refer-
ence starch plasticizer. From a qualitative point of view, only
three BioILs ([Chol][Ace], [Chol][Cit] and [Chol][Lac]) make it
possible to obtain a comparable quality of lms such as
glycerol-based lm. Film quality is qualied as “good” if the
lm is homogeneous and easy to handle. For other BioILs, the
lms obtained, which were fragile and less homogeneous, are
qualied as “medium”. Finally, one BioIL did not make it
possible to obtain lms and so its lm-forming properties are
therefore qualied as “poor”.
At this time, it is difficult to qualify the inuence of the anion
structure on lm quality because not enough different struc-
tures were studied. Nevertheless, this approach could be easily
applied to a larger screening campaign.
Fig. 1 TGA results of [Chol][Ace]: (right) at 10 ꢁC minꢀ1; (left) isotherm
at 130 ꢁC; [Chol][Ace] (solid line); [Chol][Sal] (dotted line).
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Table 3 Properties of starch/BioIL films obtained by casting (equilibrium conditions: 21 ꢁC, 57% RH)
Compound: starch (77 wt%)/
BioIL (23 wt%)
Recrystallization
aer 8 weeks
Moisture content
(% wet basis)
Film-forming
Tg by DSC (ꢁC)
Starch/[Chol][Ace]
Starch/[Chol][Sac]
Starch/[Chol][Sal]
Starch/[Chol][Cit]
Starch/[Chol][Lac]
Starch/[Chol][Fur]
Starch/glycerol (reference)
Good
Poor
Medium
Good
Good
B-Type
None
None
None
None
None
B-Type
15.2 ꢃ 0.2
11.7 ꢃ 0.2
11.9 ꢃ 0.2
16.7 ꢃ 0.2
16.8 ꢃ 0.2
12.8 ꢃ 0.2
11.6 ꢃ 0.2
7 ꢃ 0.5
20 ꢃ 0.5
9 ꢃ 0.5
17 ꢃ 0.5
ꢀ6 ꢃ 0.5
2 ꢃ 0.5
Medium
Good
ꢀ5 ꢃ 0.5
from conditioning at a relative humidity of 57% and also
strongly depends on the type of ionic liquid. The gure clearly
shows that the type of ionic liquid is the rst-order parameter
controlling the decrease in Tg, i.e., the “efficiency of the
plasticizer”.
Evolution of crystallinity of starch/BioIL lm
X-ray diffraction analyses were made aer 2 and 8 weeks of
conditioning at 21 ꢁC and 57% relative humidity (RH) (saturated
sodium bromide solution).
These measurements highlight the disappearance of the
native crystal structure (A-type for corn starch) during the
solubilization of starch before the casting process. Further-
more, we did not observe V-type emergence for any of the
samples.40
However, depending on the BioIL used, B-type recrystalliza-
tion may occur. In Fig. 2, two cases can be observed: on one
hand, [Chol][Ace] seems to promote the B-type recrystallization
with the presence of a characteristic peak at 2q 17ꢁ aer only 2
weeks (behavior similar to that of starch/glycerol lms). On the
other hand, the use of [Chol][Lac], [Chol][Cit], [Chol][Fur],
[Chol][Ace] and [Chol][Sal] seems to preserve the amorphous
structure of plasticized starch lms as demonstrated by the
absence of a diffraction peak, at least for the investigated
storage time of 8 weeks. Nevertheless, slow recrystallization
cannot be excluded.
This conrms the strong interest in ionic liquids for the
limitation of starch recrystallization, which had already been
reported for imidazolium-based systems.13,16 However, the
inuence of the ionic liquid structure on this effect seems
difficult to predict.
Fig. 4 highlights the inuence of the BioIL anion on the Tg
and water content of the lm. However, glycerol seems to be the
most efficient Tg depressor compared to all of the BioILs.
Indeed, starch/glycerol lm has the lowest Tg and the lowest
water content. If we consider only these glass transition values,
the inuence of BioIL on starch thus seems to be similar to
a conventional plasticizer such as glycerol, but less efficient.
To compare our results with the previous study9 on [Bmim]
[Cl], we prepared an additional casted sample with this ionic
liquid, conrming the better plasticizing properties of [Bmim]
[Cl] compared to glycerol. This suggests that the Tg and water
uptake values of plasticized starch lms are mainly inuenced
by the type of cation. Nevertheless, the results obtained for the
present series of BioILs also show that the type of anion makes
it possible to modulate these properties.
Moreover, the evolution of the mechanical properties of the
starch/BioIL lms with relative humidity (Fig. 5) shows that
both the type of anion and cation greatly affect the thermo(hy-
dro)mechanical behavior of starch.
Glass transition temperature of starch/BioIL lm (DSC)
The glass transitions of lms containing different ionic liquids
were determined from the thermograms shown in Fig. 3 for
some of the examples. They are reported in Table 3 and plotted
in Fig. 4 as a function of water content. Water content results
Fig. 2 XRD diffractograms of plasticized starch-casted films using: (a)
[Chol][Ace] and (b) [Chol][Lac] as plasticizers; after 2 week conditioning Fig. 3 Examples of DSC thermogram of BioIL–plasticized starch films
(solid line) and after 8 week conditioning (dotted line).
(dotted lines indicated the glass transition onset).
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structures was investigated for starch plasticization applica-
tions. To synthesize these BioILs, two easy and fast synthetic
routes were selected: an ion exchange reaction, direct and
economical, and a two-step acid–base reaction.
We studied the starch plasticizing properties of six BioILs:
ve of them made it possible to obtain plasticized starch lms
by casting. In comparison to the glycerol/starch lm (reference),
the BioILs led to a stronger water absorption and a smaller
decrease in the glass transition temperature. The structure of
the anion, however, had a great inuence on thermo(hydro)
mechanical behavior, as well as the recrystallization of the
plasticized starch. Only [Chol][Ace] promoted the recrystalliza-
tion of the lm, while the other BioIL–plasticized starch
remained amorphous upon aging for the studied period of 8
weeks.
The screening approach of starch plasticization properties
described in the present work could be applied to other types of
newly synthesized ionic liquids available in small quantities.
In a future paper, we will study the extrusion of starch/BioIL
mixtures in more detail since they require larger quantities of
ionic liquid.
Fig. 4 Glass transition temperature of starch/BioIL film according to
moisture content. Squares (-) represent starch/BioIL mixtures with
good film-forming properties, triangles (:) represent medium film-
forming properties, and crosses (ꢂ) represent poor film-forming
properties. Starch/[Bmim][Cl] and starch/glycerol mixtures were used
as references (the precision of the measurements is ꢃ0.5 ꢁC for Tg
values, and ꢃ0.2% for moisture content).
Acknowledgements
We are grateful for the nancial support provided by GDR
BioMatPro and funded by INRA and the CNRS, and the LIM-
´
PONAN project funded by the “Region de Pays de la Loire”
Council. We thank O. Camatte, M. Oxeant and B. Pontoire for
their experimental contributions.
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