molecules
Communication
A New Determination Method of the Solubility
Parameter of Polymer Based on AIE
Shan Jiang 1 , Tian Ya Huang 1 , Ke Min Wang 1 , Ben Zhong Tang 2 and Qiang Yu 1, *
1
2
*
School of Materials Science and Engineering, Changzhou University, Changzhou 213164, China;
[email protected] (S.J.); [email protected] (T.Y.H.); [email protected] (K.M.W.)
Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon,
Hong Kong, China; [email protected]
Correspondence: [email protected]; Tel.: +86-1391-5023-269
Academic Editor: Youhong Tang
Received: 29 November 2016; Accepted: 21 December 2016; Published: 30 December 2016
Abstract: An accurate method of the fluorescence probe approach based on an aggregation-induced
emission (AIE) molecule (tetraphenylethylene) for measuring the solubility parameter of the polymer
is reported. This method is distinctive in that the approach can make the polymer chain conformation
in solution be related to the fluorescence intensity. Since the solubility parameter of the polymer is
also closely linked to its chain conformation in solution, the solubility parameter can be determined
by the fluorescence intensity. The range of the solubility parameter of polymethyl methacrylate
(PMMA) tested by this method was from 9.00 cal1/2 cm−3/2 to 10.00 cal1/2 cm−3/2 . The results are
more accurate than those obtained from the traditional turbidimetric titration method, ranging
from 8.60 cal1/2 cm−3/2 to 12.15 cal1/2 cm−3/2 . According to the photoluminescence (PL) intensities
spectra, the solubility parameters of PMMA and polyvinyl acetate (PVAc) are 9.19 cal1/2 cm−3/2 and
9.85 cal1/2 cm−3/2 , respectively.
Keywords: aggregation-induced emission; conformation transition; luminescence; solubility parameter
1. Introduction
Luminescent materials are rapidly being developed and have aroused great academic
interest [1–3]. Generally, a luminescent substance in the state of aggregation would produce
two different kinds of photoluminescence (PL) effects: aggregation-induced emission (AIE) and
aggregation-caused quenching (ACQ) [4–6]. When the AIE molecules aggregate in concentrated
solutions or on surfaces of solid materials, they become highly luminescent. Thus, the AIE phenomenon
can be applied in some special fields [7,8].
The solubility parameter is defined as the square root of the total energy of vaporization of the
liquid [9]. Since polymers would decompose before the heat of the vaporization could be measured,
the solubility parameter of polymer is measured indirectly, and it can be estimated from experimental
testing methods such as turbidimetric titration [10,11], viscometry [12], the swelling method [13], etc.
The interactions of polymer chains and solvent molecules are most conveniently described by the
Flory-Huggins (F-H) model [14,15], and the interaction parameters χ and the second virial coefficient
A2 are closely related to the conformations of polymer chains in solution [16]. When a polymer
dissolves in a good solvent, the polymer chains are in stretched conformations, A2 > 0, χ < 1/2; with a
bad solvent added, the polymer chains are in entangled conformations, A2 < 0, χ > 1/2. On the basis of
the Hildebrand approximation [17], polymer chains are in stretched conformations when the solubility
parameters of the solvent and polymer are similar. When choosing a good solvent for the polymer,
a mixed solvent is often adopted rather than a single solvent. The solubility parameter of the mixed
solvent δmix can be adjusted to an accurate value by Equation (1), as follows:
Molecules 2017, 22, 54; doi:10.3390/molecules22010054
www.mdpi.com/journal/molecules
Molecules 2017, 22, 54
Molecules 2017, 22, 54
2 of 6
δmix = δ1 φ1 + δ2 φ2
2 of 6
(1)
mix =types
δ1φ1 +of
δ2φ
2
where δ1 , δ2 are the solubility parameters of δtwo
pure
solvents; φ1 , φ2 are the volume(1)fractions
of two
types
pure
In addition
considering
parameters of the
where
δ1, of
δ2 are
thesolvents.
solubility parameters
of to
two
types of purewhether
solvents; the
φ1, φsolubility
2 are the volume fractions
solvent
and
polymer
are
similar,
some
physical
parameters
such
as
the
dispersion
force,
polarity
of two types of pure solvents. In addition to considering whether the solubility parameters
of theforce,
and hydrogen
bond
should
takensome
into account
choosing
solvent for
polymers.
solvent and
polymer
are be
similar,
physical when
parameters
such aasgood
the dispersion
force,
polarity
force,
hydrogen
bond shouldprobe
be taken
into on
account
when choosing
a good solvent for
polymers.
In
ourand
work,
a luminescence
based
4-Acryloyl
tetraphenylethylene
(TPE-a)
[18] was
In into
our the
work,
a luminescence
probe
based on 4-Acryloyl tetraphenylethylene
(TPE-a) [18]by
was
introduced
polymers
polymethyl
methacrylate(PMMA)
and polyvinyl acetate(PVAc)
radical
introduced
into
the
polymers
polymethyl
methacrylate(PMMA)
and
polyvinyl
acetate(PVAc)
by
radical
copolymerizations, giving birth to AIE polymers P(MMA-co-TPE-a) (PMT) and P(VAc-co-TPE-a) (PVT)
copolymerizations, giving birth to AIE polymers P(MMA-co-TPE-a) (PMT) and P(VAc-co-TPE-a)
(Scheme
1). The solubility parameter of the solvent, in which the polymer chains are in fully stretched
(PVT) (Scheme 1). The solubility parameter of the solvent, in which the polymer chains are in fully
conformations, can serve as the solubility parameter of the polymer itself. The state of the polymer
stretched conformations, can serve as the solubility parameter of the polymer itself. The state of the
chains
was observed
by luminescent
intensity
inina aseries
ofsolvents
solvents
with
different
polymer
chains wascontinuously
observed continuously
by luminescent
intensity
series of
with
different
solubility
parameters.
solubility parameters.
Scheme
1. Synthesis
aggregation-induced emission
polymers
PMT
andand
PVT.PVT.
Scheme
1. Synthesis
ofofaggregation-induced
emission(AIE)
(AIE)
polymers
PMT
2. Results and Discussion
2. Results and Discussion
The PL spectra of PMT in THF-water mixtures with different water contents are shown in Figure 1.
The
PLa spectra
of PMT
in THF-water
with
different
water contents
are peak
shown
in Figure 1.
When
large amount
of water
(50% vol.)mixtures
was added
to the
THF solution,
an emission
emerged
Whenata465
large
of water
(50% vol.)
was added to
the THF
solution,
anpolymer
emission
emerged
nm,amount
demonstrating
a typical
AIE phenomenon.
Despite
being
weak, the
is peak
somewhat
in the THF solution.
TheAIE
emission
became stronger
progressively
withthe
increasing
theisamount
at 465fluorescent
nm, demonstrating
a typical
phenomenon.
Despite
being weak,
polymer
somewhat
of waterin
in the
the mixed
solvent. During
process,became
the solubility
of theprogressively
PMT got worse with
and the
polymer the
fluorescent
THF solution.
The the
emission
stronger
increasing
chains
changed
from
stretched
conformations
to
curled
conformations.
At
90%
vol.
water
content,
amount of water in the mixed solvent. During the process, the solubility of the PMT got worsethe
and the
PL intensity is clearly higher than that of the 50% vol. water content solution, while the emission
polymer
chains changed from stretched conformations to curled conformations. At 90% vol. water
maximum is still located at 465 nm.
content, the PL intensity is clearly higher than that of the 50% vol. water content solution, while the
Different kinds of solvents were chosen to dissolve PMT. Although all these solutions were
emission
maximum is still located at 465 nm.
transparent, the PL spectra presented big differences. From the PL spectra (Figure 2), when PMT was
Different
kinds
of solvents (DCM),
were chosen
to dissolve
PMT.
these
dissolved in
dichloromethane
the PL intensity
reached
theAlthough
minimum all
at 465
nm.solutions
Therefore, were
transparent,
the PL spectra
presented
From the PL spectra (Figure 2), when PMT was
DCM is considered
as a good
solventbig
fordifferences.
PMT.
dissolvedThen,
in dichloromethane
(DCM),
the PL intensity
minimum
at 465giving
nm. Therefore,
DCM was mixed with
cyclohexane
and ethyl reached
alcohol inthe
varying
proportions,
birth
to
a
series
of
solvents
with
different
solubility
parameters
(Figure
3a).
It
was
found
that
when
the
δ
DCM is considered as a good solvent for PMT.
1/2cm−3/2 or above 10.00 cal1/2cm−3/2, the PL intensity and the relative
of
mixed
solvent
was
below
9.00
cal
Then, DCM was mixed with cyclohexane and ethyl alcohol in varying proportions, giving birth
fluorescent
quantum
yielddifferent
(ln(I/I0)) (Figure
3b) parameters
changed monotonically,
indicating
that thethat
polymer
to a series
of solvents
with
solubility
(Figure 3a).
It was found
when the
chains tend to shrink. Additionally, when
the−δ3/2
was below 8.50 cal1/2cm−3/2
or above
12.00 cal1/2cm−3/2, the
1/2
1/2
−
3/2
δ of mixed solvent was below 9.00 cal cm
or above 10.00 cal cm
, the PL intensity and
polymer precipitated thoroughly. From the range of 9.00 cal1/2cm−3/2 to 10.00 cal1/2cm−3/2, the PL intensity
the relative fluorescent quantum yield (ln(I/I0 )) (Figure 3b) changed monotonically, indicating that
and ln(I/I0) decreased at first, and then they increased, indicating the minimum intensity
must be in
the polymer
chains tend to shrink. Additionally, when the δ was below 8.50 cal1/2 cm−3/2 or above
this range. Based on the analysis, the intersection point of the trendline on both sides of 9.50 cal1/2cm−3/2
−3/2 the polymer precipitated thoroughly. From the range of 9.00 cal1/2 cm−3/2 to
12.00refers
cal1/2tocm
1/2cm−3/2 (Figure 3b), which can be considered as the solubility parameter of PMT.
δ = 9.19, cal
1/2
−
10.00 cal cm 3/2 , the PL intensity and ln(I/I0 ) decreased at first, and then they increased, indicating
Molecules 2017, 22, 54
3 of 6
the minimum intensity must be in this range. Based on the analysis, the intersection point of the
Molecules
22,22,
54 54 of 9.50 cal1/2 cm−3/2 refers to δ = 9.19 cal1/2 cm−3/2 (Figure 3b), which
6
trendline
on2017,
both
sides
be
Molecules
2017,
3 of36ofcan
considered
as 2017,
the 22,
solubility
parameter of PMT. Furthermore, since the molar content of TPE
Molecules
54
3 of 6 units is
Furthermore,
since the
only 0.47%,
0.47%,which
whichisisextremely
extremely
low,
impact
Furthermore,
themolar
molarcontent
contentof
ofTPE
TPE units
units is only
low,
itsits
impact
only 0.47%,
which issince
extremely
low,
its impact
on the solubility
parameter
could be
neglected
and the
on
solubility
parameter
could
beneglected
neglected
theonly
solubility
ofofPMT
and
PMMA
areare
on Furthermore,
thethe
solubility
parameter
could
be
and is
solubility
parameters
PMT
and
PMMA
since
the molar
content
of TPE units
0.47%, parameters
which
is extremely
low,
its
impact
solubility
parameters
of
PMT
and
PMMA
are
the
same.
the same.
theon
same.
the solubility parameter could be neglected and the solubility parameters of PMT and PMMA are
the same.
Figure 1. Photoluminescence (PL) spectra of PMT in THF-water mixtures with different water contents
Figure 1. Photoluminescence (PL)
spectra of PMT in THF-water mixtures with different water contents
−1).
(λex =1.310
nm, [P] = 1 mg·mL(PL)
Figure
Photoluminescence
THF-watermixtures
mixtureswith
withdifferent
different
water
contents
Figure
1. Photoluminescence
(PL)spectra
spectraof
of PMT
PMT in
in THF-water
water
contents
(λex = 310 nm, [P] = 1 mg·mL−−11−1).
(λex(λ=ex310
nm,
[P][P]
= 1= mg·mL
= 310
nm,
1 mg·mL ).).
Figure 2. PL spectra of PMT in different solvents (λex = 310 nm, [P] = 1 mg·mL−1).
Figure 2. PL spectra of PMT in different solvents (λex = 310 nm, [P] = 1 mg·mL−1).
Figure
2. PL spectra of PMT in different solvents (λex==310
310nm,
nm,
[P] = 1 mg·mL−1 ).
Figure 2. PL spectra of PMT in different solvents (λex
[P] = 1 mg·mL−1).
(a)
(b)
Figure 3. (a) PL spectra(a)
of PMT in mixed solvents with different solubility (b)
parameters (δ = 8.50~12.00
−3/2, λex = 310 nm, [P] = 1 mg·mL−1); (b) Plots of ln(I/I0) of PL spectra (a).
cal1/2cm3.
Figure
(a) PL spectra
(δ = 8.50~12.00
(a)of PMT in mixed solvents with different solubility parameters
(b)
1/2
−3/2
cal cm , λex = 310 nm, [P] = 1 mg·mL−1); (b) Plots of ln(I/I0) of PL spectra (a).
Figure 3. (a) PL spectra of PMT in mixed solvents with different solubility parameters (δ = 8.50~12.00
Figure 3.
(a) PL spectra of PMT in mixed solvents with different solubility parameters (δ = 8.50~12.00
cal1/2cm−3/2, λex = 310 nm, [P] = 1 mg·mL−1); (b) Plots of ln(I/I0) of PL spectra (a).
cal1/2 cm−3/2 , λex = 310 nm, [P] = 1 mg·mL−1 ); (b) Plots of ln(I/I0 ) of PL spectra (a).
Molecules 2017, 22, 54
4 of 6
For
comparing
Molecules
2017, 22, 54 the fluorescence probe technique with the turbidimetric titration method,
4 of which
6
is now the most commonly used method for determining the solubility parameter of a polymer, the
For comparing
the fluorescence
technique the
withluminousness
the turbidimetric
method,
is the
turbidimetric
titration was
carried outprobe
by measuring
of titration
the solutions
towhich
estimate
now
the
most
commonly
used
method
for
determining
the
solubility
parameter
of
a
polymer,
the
cloud point. The solubility parameter of PMT obtained by the turbidimetric titration method ranged
titration
carried1/2
measuring the luminousness of the solutions to estimate
−3/2 towas
from turbidimetric
8.60 cal1/2 cm
12.15
cal out
cmby−3/2
, which is much wider than the range (9.00 cal1/2the
cm−3/2
cloud point.
The
solubility
parameter
of
PMT
obtained
by the turbidimetric titration method ranged from
1/2
−
3/2
to 10.00 cal 1/2 cm
) determined by our fluorescence method above.
8.60 cal cm−3/2 to 12.15 cal1/2cm−3/2, which is much wider than the range (9.00 cal1/2cm−3/2 to 10.00 cal1/2cm−3/2)
To
further
explore
the application of the fluorescence method, the solubility parameter of PVT was
determined by our fluorescence method above.
also determined.
n-hexane,
DCM and
ethyl
alcohol were
used
preparing
mixed
To furtherThe
explore
the application
of the
fluorescence
method,
thefor
solubility
parameter
of solvents
PVT was with
different
solubility parameters
to measure
solubility
parameter
PVT (Figure
When
the δ of
also determined.
The n-hexane,
DCM andthe
ethyl
alcohol were
used forof
preparing
mixed4a).
solvents
with
1/2 cm−3/2 , the PL intensity of solution reached the relative minimum
the mixed
solvent
was
10.00
cal
different solubility parameters to measure the solubility parameter of PVT (Figure 4a). When the δ of
1/2 cm
−3/2 or at
1/2cm−3/2, the PL intensity of solution reached the relative
thenm.
mixed
solvent
was 10.00
minimum
at 463
The
intensity
andcal
ln(I/I
above
0 ) changed monotonically below 9.50 cal
1/2
−3/2
1/2cm−3/2.
1/2
−
3/2
1/2
−
3/2
1/2
−
3/2
The intensity
and
ln(I/I
0) changed
monotonically
9.50 cal
10.50
10.50463
calnm.cm
. From
the
range
of 9.50
cal cm below
to 10.50
calcm cmor above
, the
PLcal
intensity
and
−3/2 to 10.50 cal1/2cm−3/2, the PL intensity and ln(I/I0) decreased at first,
the rangeatoffirst,
9.50 and
cal1/2cm
ln(I/IFrom
)
decreased
then
they
increased,
indicating
the
minimum
intensity
must
be
in
this
0
and then they increased, indicating the minimum intensity must be in this range. Figure 4b shows
1/2
−
range. Figure 4b shows that the intersection point of the trendline1/2on −3/2
both sides of 10.00 cal cm 3/2
that the intersection
point
of the trendline on both sides of 10.00 cal cm refers to δ = 9.85 cal1/2cm−3/2,
1/2
−
3/2
refers to δ = 9.85 cal cm
, which can be considered as the solubility parameter of PVT. Since the
which can be considered as the solubility parameter of PVT. Since the molar content of TPE units is only
molar0.37%,
content
of TPE units is only 0.37%, which is extremely low, the solubility parameter of PVT
which is extremely low, the solubility parameter of PVT could be identical to the solubility
couldparameter
be identical
to the solubility parameter of PVAc.
of PVAc.
(a)
(b)
Figure 4. (a) PL spectra of PVT in mixed solvents with different solubility parameters (δ = 8.50~12.00
Figure 4.
(a) PL spectra of PVT in mixed solvents with different solubility parameters (δ = 8.50~12.00
cal1/2cm−3/2, λex = 310 nm, [P] = 1 mg·mL−1); (b) Plots of ln(I/I0) of PL spectra (a).
cal1/2 cm−3/2 , λex = 310 nm, [P] = 1 mg·mL−1 ); (b) Plots of ln(I/I0 ) of PL spectra (a).
3. Materials and Methods
3. Materials and Methods
3.1. General
3.1. General
Tetrahydrofuran (THF), methyl methacrylate (MMA), vinyl acetate (VAc), and triethylamine (Et3N)
were freshly purified,
respectively.
(AIBN)
purified
by recrystallization.
Tetrahydrofuran
(THF),
methyl Azobisisobutyronitrile
methacrylate (MMA),
vinylwas
acetate
(VAc),
and triethylamine
4-hydroxylbenzophenone,
benzophenone,
and acryloyl
chloride were purchased(AIBN)
from Aladdin
(Et3 N)
were freshly purified,
respectively.
Azobisisobutyronitrile
wasIndustrial
purified by
Co.,
(Fengxian,
Shanghai,
China)
and
used
as
received
without
further
purification.
Number-average
recrystallization. 4-hydroxylbenzophenone, benzophenone, and acryloyl chloride were purchased
and polydispersity of polymers by GPC (Waters Co., Milford, MA, USA) and THF as
frommolecular
Aladdinweight
Industrial
Co., (Fengxian, Shanghai, China) and used as received without further
the eluent. NMR spectra were measured on AVANCE III 400 MHz (Bruker Co., Fällanden, Switzerland).
purification. Number-average molecular weight and polydispersity of polymers by GPC (Waters Co.,
PL spectra were measured on a PerkinElmer LS45 spectrometer (Fremont, CA, USA).
Milford, MA, USA) and THF as the eluent. NMR spectra were measured on AVANCE III 400 MHz
(Bruker
Fällanden,
Switzerland).
PL spectra were measured on a PerkinElmer LS45 spectrometer
3.2. Co.,
Synthesis
of 4-Hydroxyl
Tetraphenylethylene
(Fremont, CA, USA).
Into a 500 mL two-necked round-bottom flask were added 4-hydroxylbenzophenone (1.9 g, 10
mmol), benzophenone (2.2 g, 12 mmol) and Zn powder (2.9 g, 44 mmol). The flask was evacuated under
3.2. Synthesis of 4-Hydroxyl Tetraphenylethylene
vacuum and flushed with dry N2 three times. Then 80 mL fresh dried THF was added. After all the
solidsawere
dissolved,round-bottom
the solution wasflask
cooledwere
to 0 °Cadded
and TiCl
4 (2.4 mL, 22 mmol) was slowly
Into
500completely
mL two-necked
4-hydroxylbenzophenone
(1.9 g,
injected
into
the
flask.
After
refluxing
at
70
°C
overnight,
the
mixture
was
cooled
to
room
temperature
10 mmol), benzophenone (2.2 g, 12 mmol) and Zn powder (2.9 g, 44 mmol). The flask was evacuated
under vacuum and flushed with dry N2 three times. Then 80 mL fresh dried THF was added. After all
Molecules 2017, 22, 54
5 of 6
the solids were completely dissolved, the solution was cooled to 0 ◦ C and TiCl4 (2.4 mL, 22 mmol)
was slowly injected into the flask. After refluxing at 70 ◦ C overnight, the mixture was cooled to room
temperature and 80 mL dilute hydrochloric acid (1 mol·L−1 ) was added to it, which was extracted with
dichloromethane (3 × 80 mL). The collected organic layer was dried over anhydrous magnesium sulfate.
After solvent evaporated, the crude product was purified by a silica gel column chromatography using
petroleum ether (60–90 ◦ C)/ethyl acetate (40:1) as eluent. White crystal; yield 58% (2.4 g). 1 H-NMR
(400 MHz, CDCl3 , δ): 7.16–7.09 (m, 8H), 7.07–7.01 (m, 9H), 6.91 (d, 1H), 6.58 (d, 1H), 4.78 (s, 1H).
3.3. Synthesis of 4-Acryloyl Tetraphenylethylene
Into a 250 mL two-necked round-bottom flask were added 4-Hydroxyl Tetraphenylethylene
(TPE-OH) (1.0 g, 3 mmol), trimethylamine (2 mL) and fresh dried THF (30 mL). After all the solids
were completely dissolved, acryloyl chloride (0.3 mL, 3.6 mmol) was injected into the flask dropwise
over 20 min. After stirring at 0 ◦ C for 2 h, the reaction mixture was concentrated under vacuum.
The residue was dissolved in ethyl acetate and the resulting solution was washed with water and
brine, dried with Na2 SO4 and concentrated. The crude product was purified by a silica gel column
chromatography using petroleum ether (60–90 ◦ C)/ethyl acetate (100:1) as eluent. White crystal; yield
62% (0.83 g). 1 H-NMR (400 MHz, CDCl3 , δ):7.14–7.07 (m, 9H), 7.05–6.99 (m, 8H), 6.88 (d, 1H), 6.55
(d, 1H), 6.31–6.24 (q, 1H), 5.99 (d, 1H), 5.97 (d, 1H).
3.4. Synthesis of PMT and PVT
The polymerization reaction was carried out by radical polymerization under N2 and equipped
with a condenser and magnetic stirring. Into 500 mL three-necked round-bottom flask were added
TPE-a (0.3 g, 0.7 mmol), MMA (15 g, 150 mmol), AIBN (0.1 g, 0.6 mmol) and DMF (80 mL). The reaction
mixture was stirred on an oil bath at 70 ◦ C under nitrogen for 16 h. The solution was then cooled
to room temperature, concentrated under vacuum and added dropwise to 500 mL of methanol
under stirring to precipitate the polymer and meanwhile remove the unreacted reactants and solvent.
The polymer was filtered by Buchner funnel, washed with methanol several times, and dried in
vacuum overnight at 40 ◦ C to a constant weight. White solid; yield 86% (13.2 g). Mn : 16500, PDI: 1.78
(GPC); 1 H-NMR (400 MHz, CDCl3 , δ): 7.13–6.99 (multi-peak of TPE), molar ratio of TPE: 0.47%.
The PVT was synthesized using the same synthesis methods of PMT. White solid; yield 82%
(12.6 g). Mn : 12800, PDI: 1.68 (GPC); 1 H-NMR (400 MHz, CDCl3 , δ): 7.13–6.98 (multi-peak of TPE),
molar ratio of TPE: 0.37%.
4. Conclusions
We have demonstrated a luminescence probe approach based on the AIE molecule to measure
the solubility parameters of PMMA and PVAc. The solubility parameters of PMMA and PVAc are
9.19 cal1/2 cm−3/2 and 9.85 cal1/2 cm−3/2 , respectively. The measured parameter of PMMA ranges
from 9.00 cal1/2 cm−3/2 to 10.00 cal1/2 cm−3/2 , while the solubility parameter of PMMA measured
by the turbidimetric titration ranges from 8.6 cal1/2 cm−3/2 to 12.15 cal1/2 cm−3/2 . All these results
demonstrated that the values obtained from our method should be more accurate than those obtained
by the turbidimetric titration method, and the luminescence probe approach can also be applied to
other kinds of polymers.
Acknowledgments: The research was supported by the National Science Foundation of China (21304011).
Author Contributions: K.M.W., B.Z.T. and Q.Y. performed the research, analyzed the data and discussed the
results; S.J. and T.Y.H. designed and performed the research, and did the data analysis, manuscript writing and
revision. All authors read and approved the final manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
Molecules 2017, 22, 54
6 of 6
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