WDS'07 Proceedings of Contributed Papers, Part III, 106–111, 2007.
ISBN 978-80-7378-025-8 © MATFYZPRESS
Phase Separation in Water-Ethanol Polymer Solutions
Studied by NMR Methods
H. Kouřilová and L. Hanyková
Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic.
Abstract. We applied 1 H and 13 C NMR spectroscopy to investigate a phase transition and a cononsolvency effect of uncharged poly(N-isopropylmethacrylamide)
(PIPMAm) and negatively charged PIPMAm copolymers in water/ethanol
(D2 O/EtOH) mixtures. For this purpose several PIPMAm solutions with various volume mixture compositions of D2 O/EtOH (100/0 - 5/95 vol.-%) were
prepared; sodium methacrylate was used as an ionic comonomer. Neat binary
D2 O/EtOH mixtures with the same compositions were studied for the comparison
to be able to distinguish between effects caused by the phase transition and effects
caused by the properties of binary D2 O/EtOH mixtures.
It is obvious from 1 H and 13 C spectra that with increasing temperature the concentration range of the phase separation broadens to lower values of ethanol content in
the solution.
We observed a difference in characteristics of the phase transition induced by a
change of temperature or by a change of solvent composition. When the phase
separation is induced by the solvent composition polymer groups at the ends
of dangling chains are more mobile than those in backbone chains. After the
temperature-induced phase transition PIPMAm groups behave in the same way.
We confirmed this when investigating solvent dynamic properties by means of
ethanol 13 C T2 .
We found out that charge in chains prevents the polymer solution from the phase
separation by strenghtening interactions between polymer and solvent.
Introduction
Some acrylamide-based polymers including poly(N-isopropylmethacrylamide) (PIPMAm) has very
interesting behaviour in aqueous solutions, they exhibit so called lower critical solution temperature
(LCST). They are soluble at low temperatures but at temperatures above LCST phase separation occurs
[e.g., Fujishige et al., 1989; Netopilı́k et al., 1997; Idziak et al., 1999]. The process of phase separation is
reversible. When lowering temperature under LCST, polymer dissolves again.
The phase transition can also be induced at a constant temperature by addition of some polar
solvent (for example ethanol) into water. Under LCST PIPMAm is soluble in pure water and in pure
ethanol but is not soluble in their mixtures of certain compositions. This formation of ”nonsolvent” by
mixing of two solvents si called the cononsolvency effect [Winnik et al., 1990; Schild et al., 1991; Costa
et al., 2002].
Both temperature-induced phase separation and cononsolvency effect are assumed to be a macroscopic manifestation of a coil-globule transition followed by aggregation as shown for acrylamide-based
polymers in water by light scattering and small angle neutron scattering [Fujishige et al., 1989; Kubota
et al., 1990; Zhu et al., 1999; Pleštil et al., 1987]. The transition is probably associated with competition
between hydrogen bonding and hydrophobic interactions.
Cononsolvency phenomenon is studied particularly in solutions of poly(N-isopropylacrylamide) (PIPAAm); microcalorimetric measurements evidenced PIPAAm cononsolvency behaviour in mixtures of
water and methanol, tetrahydrofuran, dioxane and other solvents [Schild et al., 1991], concluding that
water-organic solvent complexes are preferred over PIPAAm-water hydrogen bonds. Cloud point measurements were applied to investigate PIPAAm in water-low molecular weight alcohols with respect to
molecular structures of cononsolvents [Costa et al., 2002]. Semidilute solutions of PIPAAm in aqueous
solution of tetrahydrofuran and n-butyl alcohol were measured by small angle X-ray scattering method
[Shimizu et al., 2003] and the contribution of the segment-segment interactions to the entropy and the
enthalpy were calculated.
Although critical solution phenomena are mostly studied in solutions of PIPAAm, there are several
studies on PIPMAm. LCST of PIPMAm in neat water is 315K [Netopilı́k et al., 1997; Starovoytova et al.,
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KOUŘILOVÁ AND HANYKOVÁ: PHASE SEPARATION IN WATER-ETHANOL POLYMER...
2004]. The presence of the α-methyl group in the PIPMAm monomer was found to increase the LCST
[Kubota et al., 1990; Tiktopulo et al., 1995] compared with that of PIPAAm. Strong cononsolvency effect
was observed for uncharged and negatively charged networks of PIPMAm in water/ethanol mixtures
at room temperature [Alenichev et al., 2007]. Recently, we employed 1 H NMR spectroscopy for study
of the temperature-induced phase transition of uncharged PIPMAm and negatively charged PIPMAm
copolymers in water solutions in a broad range of polymer concentrations and ionic comonomer mole
fractions [Starovoytova et al., 2003].
In the present paper we applied 1 H and 13 C NMR spectroscopy to investigate the cononsolvency effect of uncharged PIPMAm and negatively charged PIPMAm copolymers in water/ethanol (D2 O/EtOH)
mixtures. These water/ethanol (D2 O/EtOH) mixtures contained various volumes of ethanol from 0 vol.% to 95 vol.-%. We also studied neat binary mixtures of water and ethanol of the same compositions for
comparison to be able to distinguish between effects caused by the phase transition and effects caused
by properties of D2 O/EtOH mixtures.
Experimental
Samples
N-isopropylmethacrylamide (IPMAm, Fluka) and sodium methacrylate (MNa) were used to prepare
uncharged and negatively charged PIPMAm/D2 O/EtOH solutions of polymer concentrations c = 5 wt.% and ionic comonomer mole fractions i = 0, 5 and 10 mole-%. 4,4’-azobis(4-cyanopentanoic acid) was
used as initiator and polymerization was carried out in a D2 O/EtOH mixture (6/94 vol.-%); the volume
fraction of the sum of the monomers in the mixture was 0.2. After polymerization and subsequent drying
of polymers to constant weight, the PIPMAm/D2 O/EtOH solutions with polymer concentrations c = 5
wt.-% and volume fractions of ethanol in D2 O/EtOH mixtures 0, 20, 30, 40, 50, 60, 80 and 95 vol.-%
were prepared. All samples of PIPMAm/D2 O/EtOH solutions in 5-mm NMR tubes were degassed and
sealed under argon; sodium 2,2-dimethyl-2-silapentane-5-sulfonate (DSS) was used as an internal NMR
standard.
NMR Experiments
High-resolution 1 H NMR spectra were recorded with a Bruker Avance 500 spectrometer operating
at 500.1 MHz. Typical measurements conditions were as follows: 16 scans, relaxation delay 5 s, spectral
width 5 kHz, acquisition time 1.64 s, π/2 pulse width 13.7 µs. High-resolution 13 C spectra were accumulated usually with 128 scans, relaxation delay 5 s, π/2 pulse width 13.9 µs, spectral width 27.8 kHz
and aquisition time 1.13 s under full proton decoupling. The integrated intensities were determined with
the spectrometer integration software. Precision of this software is 3%, total accuracy of the integration
is approximately 10%.
The 13 C relaxation times T2 of ethanol groups were measured using CPMG pulse sequence with a 1 H
pulse added to remove cross-correlation between chemical shift anisotropy and dipole-dipole interactions
[Kay et al., 1991]. The relaxation delay was set to 60 s which is adequately long to allow a complete
recovery of 13 C magnetization. All obtained 13 C T2 relaxation curves had a monoexponential character
and the fitting process always enabled us to determine the single value of the respective relaxation time.
In all types of experiment the temperature was maintained constant within 0.2◦ C using BVT 3000
temperature unit.
Results and Discussion
1
H and
13
C NMR spectra
Figure 1. Chemical structure of poly(N-isopropylmethacrylamide)
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We present two 1 H high-resolution NMR spectra of uncharged PIPMAm/D2 O/EtOH solutions
(ionic comonomer mole fraction i=0 mole-%) (Fig. 2). These two solutions differ in ethanol fraction
in D2 O/EtOH mixture (20 and 80 vol.-% of ethanol). Both of them were measured at 328K under the
same experimental conditions. The peak at 4.2 ppm is an overlapped signal of HDO/EtOH OH protons.
Signals of PIPMAm methyl groups are overlapped by CH3 signal of EtOH.
Figure 2. 1 H high-resolution NMR spectra of uncharged PIPMAm/D2 O/EtOH solutions containing
20 vol.-% (a) and 80 vol.-% (b) of ethanol with peak assignment measured at 328K.
The spectrum of solution that contains 80 vol.-% of ethanol (Fig. 2b) corresponds to the homogeneous
state; the sample is transparent (see Fig. 3). The other spectrum (Fig. 2a) corrensponds to a phase
separated solution and the sample is turbid (see Fig. 3).
Figure 3. Phase separated (on the left) and homogeneous (on the right) PIPMAm/D2 O/EtOH solutions.
The two spectra differ in the integrated intensity of PIPMAm peaks. In the phase separated solution
(the sample containing 20 vol.-% of ethanol) polymer chains form compact globular structures. Their
mobility is restricted so the corresponding NMR peaks are very broad and therefore are not visible in
high-resolution NMR spectra.
The integrated intensities of EtOH in Fig. 2 follow the ratio of molar ethanol fractions in corresponding solutions, so confirming that all ethanol molecules are directly detected in 1 H NMR spectra in
both solutions.
Such changes in shape and intensity of NMR lines have already been observed during temperatureinduced phase transition in uncharged PIPMAm/D2 O solutions [Starovoytova et al., 2003].
We use a quantity p-factor for analyzing polymer spectral lines and thus finding ethanol concentration range where the phase separation occurs [Starovoytova et al., 2003; Spěváček et al., 2002]. p-factor
is a fraction p
p = 1 − (I/I0 )
(1)
of phase separated polymer units. It is related to integral intensity of polymer peaks. In the
definition (1) I0 is integral intensity of a peak in a homogeneous solution (polymer is dissolved) and I is
integral intensity of the same peak in a partially/totally phase separated solution. For measurements at
298K for I0 we used values of integral intensities from spectrum of PIPMAm/D2 O solution measured at
298K. For spectra that were obtained at higher temperatures we used the fact that integral intensities
should decrease with absolute temperature as 1/T .
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In the graph below (Fig. 4) you can see a dependence of p-factor of a 1 H CH peak of uncharged
PIPMAm in PIPMAm/D2 O/EtOH solutions at 298K, 313K and 328K. It follows from definition (1) that
solutions showing p > 0 are partially (p < 1) or totally (p = 1) phase separated. For example at 328K
for ethanol concentration from 0 vol.-% to 30 vol.-% p-factor p = 1. That means that all PIPMAm units
are involved in globular structures. At 298K the concentration range of maximal value of p is around 40
vol.-% ethanol. For lower and higher ethanol concentrations the ratio of PIPMAm units with restricted
mobility decreases and p-factor for solutions with the highest (95 vol.-%) and the lowest (0 vol.-%)
ethanol fraction is zero. Similar cononsolvency effect was found for uncharged PIPMAm networks in
water/EtOH mixtures at room temperature - for solvent mixtures with EtOH fraction about 40 vol.-%
a pronounced swelling minimum was observed [Alenichev et al., 2007].
Figure 4. Dependence of 1 H p-factor of CH peak of uncharged PIPMAm in PIPMAm/D2 O/EtOH
solutions on ethanol concentration at 298K, 313K and 328K.
With increasing temperature the concentration range of phase separation expands to lower ethanol
fraction values. At 298 K it is phase separated at approximately 40 vol.-% of ethanol, at 328K from 0 to 30
vol.-%. The higher temperature above LCST causes disruption of hydrogen bonds between polymer and
solvent and hydrophobic polymer-polymer interactions are preferred. Such temperature-induced phase
separation at temperatures around 315 K was observed in aqueous solutions of PIPMAm [Netopilı́k et
al., 1997; Starovoytova et al., 2003].
The dependencies of CH and CH2 1 H polymer p-factors exhibit minimum at the same ethanol concentration (at the same temperature). Other 1 H polymer peaks were overlapped with ethanol peaks.
The p-factor dependencies of 1 H CH peak (the one in the dangling chain) on ethanol fraction are systematically shifted towards lower values of p-factor at all three experimental temperatures. This can
mean that 1 H CH2 (the one in the backbone chain) is less mobile than 1 H CH group in the dangling
chain. Side chains probably do not participate in the phase separation as much as backbone chains. We
measured 13 C spectra to verify this theory.
When calculating 13 C p-factors we used the same method as in the case of 1 H p-factors. 13 C spectral
lines of PIPMAm groups are well resolved except for the α-methyl line that is overlapped with CH3 line of
EtOH. From the dependence of 13 C p-factors of peaks of uncharged PIPMAm in PIPMAm/D2 O/EtOH
solutions on ethanol concentration at 313K (see Fig. 5) it follows that behaviour of each single polymer
group depends on its position in PIPMAm. At 313K at the ethanol concentration range about 20 - 30
vol.-% after phase separation the isopropyl CH3 group is released from globules prior to other groups
with increasing ethanol concentration. The p-factor of the CH2 group in the backbone chain of PIPMAm
is the highest in the whole ethanol concentration range. That means that this part of PIPMAm is more
rigid. So there is a correspondence between 1 H and 13 C p-factors.
We also investigated how ionization of PIPMAm affects the phase separation. We examined negatively charged PIPMAm/D2 O/EtOH solutions with ionic comonomer mole fractions i = 5 and 10 mole-%.
We again measured 1 H spectra, calculated 1 H p-factors and compared with p-factors of uncharged PIPMAm - all of them at 298K. In the Fig. 6 there is the dependence of these 1 H p-factors of CH peak of
uncharged and negatively charged PIPMAm.
It is obvious that whereas solutions with uncharged PIPMAm clearly show phase transition depending on the ethanol content, behaviour of charged solutions is not influenced by solvent composition, e.g.
p∼
= 0 in the whole range of ethanol concentrations. The charge in polymer chains makes the polymer
molecule more hydrophillic and it stabilizes hydrogen bonds between solvent molecules and polymer
chains; that results in greater solubility and flexibility of the polymer. The charge prevents the solution
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KOUŘILOVÁ AND HANYKOVÁ: PHASE SEPARATION IN WATER-ETHANOL POLYMER...
Figure 5. Dependence of 13 C p-factors of various peaks of uncharged PIPMAm in PIPMAm/D2 O/EtOH
solutions on ethanol concentration at 313K.
Figure 6. Dependencies of 1 H p-factors of CH peak of uncharged and negatively charged (i = 5 and
10 mole-% of MNa) PIPMAm in PIPMAm/D2 O/EtOH solutions on ethanol concentration at 298K.
from the phase separation. Similar results were obtained from the study of temperature-induced phase
transition in aqueous PIPMAm solutions [Starovoytova et al., 2003].
We also investigated dynamics of the solvent. We obtained useful results from measurements of 13 C
transversal relaxation times T2 of ethanol groups. The dependence of 13 C T2 of ethanol CH3 on ethanol
concentration at two different temperatures is shown in the Fig. 7. We do not show dependence of 13 C
T2 of ethanol CH2 because it is very similar.
Figure 7. Dependence of 13 C transversal relaxation times T2 of ethanol CH3 on ethanol concentration
at 298K (a) and 328K (b).
In Fig. 7a the 13 C T2 were measured at 298K. The dependencies are for 2 types of samples:
PIPMAm/D2 O/EtOH and neat solvent D2 O/EtOH without PIPMAm. At 298K the concentration
range of phase separation is about 40 vol.-% of ethanol. The T2 are not affected by the phase separation;
presence of polymer in the sample just shifts the dependence to lower values. The shallow minimum in
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KOUŘILOVÁ AND HANYKOVÁ: PHASE SEPARATION IN WATER-ETHANOL POLYMER...
both dependencies is caused by water-ethanol properties. In that concentration range there is maximum
of interactions between water and ethanol so their T2 are shorter [Nakanishi et al., 1967; Coccia et al.,
1975].
The dependencies in Fig. 7b were obtained at 328K. At this temperature the range of phase separation is from 0 to 40 vol.-% of ethanol. For sample containing PIPMAm a significant shortening of
T2 is observed - that is caused by the phase separation. After the phase separation part of the solvent
molecules stays bound to polymer chains. Mobility of these bound solvent molecules is restricted so their
T2 is shorter. We measure average T2 of all solvent molecules (bound and free) that is shorter because
of these bound molecules [Hanyková et al., 2006].
Conclusion
From polymer 1 H and 13 C NMR spectra in PIPMAm/D2 O/EtOH solutions it follows that with
increasing temperature the concentration range of phase separation broadens to lower values of ethanol
fraction and that charge introduced into PIPMAm chains strenghtens interactions between polymer
and solvent and thus prevents the solution from the phase separation. We found out that there is a
difference between globules induced by change of solvent composition and globules induced by change of
temperature. In the first case polymer groups behave differently, those at the ends of dangling chains are
more mobile. In the latter case all groups behave in the same way. We found out that 13 C T2 depend
on the type of the phase separation. In the first case 13 C T2 are not influenced by the phase transition,
in the latter case 13 C T2 are shorter with phase separated solutions.
Acknowledgments. This work was supported by the Ministry of Education, Youth and Sports of the
Czech Republic (project MSM0021620835).
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