Counterion-Induced UCST for Polycations - ACS Publications

Article pubs.acs.org/Macromolecules

Counterion-Induced UCST for Polycations Erno Karjalainen, Vladimir Aseyev, and Heikki Tenhu* Laboratory of Polymer Chemistry, Department of Chemistry, University of Helsinki, PB 55, 00014 Helsinki, Finland S Supporting Information *

ABSTRACT: A method to promote upper critical solution temperature (UCST) type of behavior for polycations is introduced. This relies on in situ introduction of a hydrophobic anion to an aqueous solution of a polycation in the presence of sufficient ionic strength. This was studied using two polycations: poly(2methacryloyloxyethyltrimethylammonium iodide) and poly(3-methyl-1-(4vinylbenzyl)imidazolium chloride). The solution behavior of the polymers was investigated in the presence of bis(trifluoromethane)sulfonamide (NTf2) and trifluoromethanesulfonate (OTf), adjusting the ionic strength with sodium chloride. All the four studied cation−anion pairs undergo an UCST type phase separation. The phase separation was reversible and only very weakly dependent on polymer concentration in the studied range.

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INTRODUCTION Thermoresponsive polymers in water have been much studied, but most of the studies have been centered on polymers with lower critical solution temperature (LCST).1−4 Studies on aqueous polymers with upper critical solution temperature (UCST) are fewer by far.3 Nonionic homopolymers with UCST in liquid water under atmospheric pressure often contain primary amide groups as, e.g., poly(N-acryloylglycinamide) (poly(NAGA)), poly(N-acryloylasparaginamide) (poly(NAAAM)), and poly(methacrylamide) (PMAAm).5−9 Some polyelectrolytes are known to show UCST behavior. Examples of those are polymers with zwitterionic groups, as polysulfobetaines.10−13 Cloud points (Tc) of these are usually strongly dependent on concentration and added electrolytes; they often experience increased solubility in higher ionic strengths.10−12 It is also possible to induce a UCST type of Tc to a weak polycation in a suitable pH range by the introduction of a multivalent counterion.14−17 Flory and Osterheld showed already in 1954 that partially neutralized poly(acrylic acid) undergoes phase separation at the UCST in a solution with high ionic strength.18 There are cases where copolymers show UCST, even if the respective homopolymers do not.4 Seuring and Agarwal synthesized copolymers of acrylamide and acrylonitrile and observed a tunable Tc in aqueous systems.9 Noh et al. sulfopropylated branched polyethylene imine and obtained copolymers with cloud points adjustable with pH and ionic strength.19 Shimada et al. reported ureuido-modified derivatives of poly(allylamine) and poly(L-ornithine) with tunable Tc.20 In some cases the cloud point was also affected by pH. The residual amines in poly(allylamine)-co-poly(allylurea) were later acetylated or succinylated, this creating a wide variety of different UCST polymers with the capability of capturing proteins.21 Meiswinkel and Ritter observed the copolymers of N-vinylimidazole and 1-vinyl-2-(hydroxymethyl)imidazole to show UCST behavior which can be modulated not only by the copolymer composition but also with pH and ionic strength.22 © 2014 American Chemical Society

They further developed the polymer by introducing an adamantly modified monomer, thus creating a system where the cloud point could be manipulated also by host−guest interactions with randomly methylated β-cyclodextrin.23 Other examples of polymers capable of forming cyclodextrin-based supramolecular assemblies that exhibit “pseudo UCST” have been reported as well.24−26 The solubility of polycations, some of which are called polymerized ionic liquids (PILs), can be manipulated with the choice of counterion.27−29 In particular, bis(trifluoromethane)sulfonimide (NTf2) and trifluoromethanesulfonate (OTf) are able to turn many polycations insoluble in water but soluble in organic solvents.28−35 NTf2 is often used to turn low molecular mass ionic liquids (ILs) poorly soluble in water.36−40 OTf is different; ILs with OTf as counterion may be completely miscible with water, whereas PILs are typically insoluble.28−31,37,38,41 Several examples of PILs having an LCST in aqueous solution have also been reported.42−46 Strong polycations (i.e., polycations with degree of charging independent of pH) can form associates in water upon introduction of suitable counterions.33−35,47−50 Yoshimitsu et al. prepared PILs by substituting poly(vinyl ethers) with imidazolyl. When the original chloride counterions were exchanged to tetrafluoroborates, aqueous solutions showed UCST type phase separation. The cloud points were strongly affected by polymer concentration and added salts.35 The Tc was below 40 °C in every case, however. The effect of partial counterion exchange was not studied. It seems possible to create a UCST polymer by in situ manipulating the relative numbers of hydrophilic and hydrophobic units in a polycation by ion exchange. The purpose of this report is to explore this possibility, which could greatly increase the number of available aqueous UCST systems. The Received: September 17, 2014 Revised: October 15, 2014 Published: October 28, 2014 7581

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precipitations to cold methanol, using acetone as good solvent, and dried in vacuum. The yield was 2.3542 g (67%). Poly(3-methyl-1-(4-vinylbenzyl)imidazolium chloride) (PIL-1). PClMeSt 2 (2.0118 g, 13.2 mmol of repeating units) was dissolved to 25 mL (313 mmol) of 1-methylimidazole containing 5 mL of methanol. The solution was purged with nitrogen flow for 45 min and allowed to react at 70 °C for 14 h. The polymer was precipitated to acetone and dialyzed against water for 5 days, changing the water twice in the process. The product was isolated by freeze-drying with yield of 2.761 g (89%). Instrumentation. Size Exclusion Chromatography (SEC). SEC was conducted using a Waters 515 HPCL pump, Waters Stryragel columns. and a Waters 2410 refractive index (RI) detector. Chromatography was performed with poly(methyl methacrylate standards) in THF containing 1% of TBAB for PDMAEMA and with polystyrene standards in pure THF for PClMeSt. Proton Nuclear Magnetic Resonance (1HNMR). 1HNMR spectra were recorded with a 300 MHz Varian Unity INOVA-spectrometer or with a Bruker Avance III 500 spectrometer at room temperature. Transmittance Measurements. Transmittance as a function of temperature was measured with JASCO J-815 CD spectrometer equipped with PTC-423S/15 Peltier-type temperature control system. The transmittances of the samples were monitored at wavelength of 600 nm. The sample cuvettes were degassed in vacuum at 5 °C prior to measurements. The sample holder was cooled at the rate of 1 °C/ min from 95 to 5 °C, and the sample temperature was monitored. Prior to measuring, the sample was allowed to equilibrate at the starting temperature of 95 °C for 30 min. In the case of POMTAI− KNfO system, the starting temperature was 90 °C and equilibration time 10 min. In the case of the reversibility measurements, the sample was cooled from 50 to 15 °C with initial stabilization of 10 min and heated back without any stabilization period. The transmittance of pure water at 20 °C was set to be 100%, and the volume of the sample was always 2800 μL. Tc was determined by onset defined by intersection of two tangents (see Figure S1 for details). The polymers were always allowed to dissolve to the mother solution for at least overnight prior to any sample preparation. The samples were prepared by first mixing salt solutions, always starting with LiNTf2 or LiOTf, with suitable concentrations and adding water to set the final concentration (taking into account also the volume of polymer solution to be added), assuming no volume change. Polymer solution was then added with vigorous stirring. As an example, a solution with 150 mM NaCl, 100 mM LiOTf, and 1 mg/mL PIL-1 was done by mixing 300 μL of 1 M LiOTf, 450 μL of 1 M NaCl, and 1950 μL of water. To this solution, 300 μL of 10 mg/mL solution of PIL-1 was then added with stirring. Prior to starting of the measurement itself it was verified that no macroscopic precipitate could be observed, and the sample was swirled until the refractive index was visually constant through the whole solution.

approach is conceptually similar to the case where the UCST is tuned by copolymerization of acrylamide and acrylonitrile.9 The choice of counterions allows easy postpolymerization variation of the cloud point.

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EXPERIMENTAL SECTION

Materials. (4-Cyanopentanoic acid)-4-dithiobenzoate (CPA) was synthesized as described in the literature.51,52 2-(Dimethylamino)ethyl methacrylate (DMAEMA) (Acros Organics, 99%) was passed through aluminum oxide followed by distillation. Dimethylformamide (DMF) (VWR, HPLC grade) and p-chloromethylstyrene (ClMeSt) (Aldrich, 90%) were purified by vacuum distillation. Azobis(isobutyronitrile) (AIBN) (Fluka, 98%) was recrystallized from methanol. Water used to prepare polymer solutions, salt solutions, and samples was purified with ELGA purelab ultrapurification system to conductivity of 0.05− 0.07 μS/cm, and a fresh batch was taken every day. Water used in dialyses was distilled. Acetone (Sigma-Aldrich, HPLC grade), n-hexane (LiChrosolv, HPLC grade), acetonitrile (VWR, HPLC grade), iodomethane (Fluka, 99%), 2-cyano-2-propyldodecyl trithiocarbonate (Aldrich, 97%), 1-methylimidazole (Sigma-Aldrich, 99%), methanol (VWR, HPLC-grade), tetrahydrofuran (THF) (Sigma-Aldrich, HPLCgrade), tetrabutylammonium bromide (TBAB) (Fluka, 99%), lithium bis(trifluoromethane)sulfonimide (LiNTf2) (Aldrich, 99%), lithium trifluoromethanesulfonate (LiOTf) (Aldrich, 96%), potassium trifluoromethanesulfonate (KOTf) (Aldrich, 98%), potassium nonafluorobutanesulfonate (KNfO) (Aldrich, 98%), and sodium chloride (Fisher Scientific UK, 99.9%) were used as received. Syntheses. Both studied polymers were synthesized with the reversible addition−fragmentation chain transfer (RAFT) method.53−55 Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA). In a flask, 66.4 mg (0.238 mmol) of CPA and 3.9 mg (0.0238 mmol) of AIBN were dissolved to 7.4696 g (47.5 mmol) of DMAEMA. The mixture was bubbled with nitrogen for an hour and immersed to an oil bath, which was preheated to 70 °C. The polymerization was allowed to proceed for 14 h, after which the reaction was stopped by sudden cooling with liquid nitrogen and exposure to the atmosphere. A conversion sample was taken at this point. The polymer was purified with three consecutive precipitations from acetone to cold hexane. The polymer was collected with acetonitrile, which was then evaporated to dryness. The polymer was dissolved to water and isolated by freezedrying. Poly(2-(methacryloyloxy)ethyltrimethylammonium iodide) (PMOTAI). The synthesis was adapted from Plamper et al.50 To 150 mL of acetone, 3.0928 g (19.7 mmol of repeating units) of PDMAEMA was dissolved in. With vigorous stirring, 3.8 mL (61.0 mmol) of iodomethane was added. The system gelated within minutes, but the gel was broken by the stirrer. Dispersion formed, and the reaction was allowed to proceed in the dark overnight; the polymer was filtered from the dispersion. The polymer was further purified by dialysis against water for 2 days, changing the water twice in process. Dry polymer was obtained by freeze-drying. The yield was 4.0563 g (69%). Poly(4-chloromethylstyrene) (PClMeSt 1). A flask was charged with 0.1036 g (0.300 mmol) of 2-cyano-2-propyldodecyl trithiocarbonate, 9.8 mg (0.0597 mmol) of AIBN, and 9.1479 g (59.9 mmol) of ClMeSt. The mixture was deoxygenated with five freeze−thaw cycles and filled with nitrogen. The reaction was allowed to proceed at 120 °C for 38 h. The reaction was stopped with liquid nitrogen, and conversion sample was taken. The polymer was purified by three precipitations from acetone to cold methanol. The polymer was dried in vacuo. End-Group Modification of PClMeSt 1 (PClMeSt 2). The method for end-group modification was adapted from Perrier et al.56 A flask was charged with 3.5148 g (0.229 mmol of end groups) of PClMeSt 1, 0.5578 g (3.30 mmol) of AIBN, and 10 mL of DMF. The solution was bubbled with nitrogen for 45 min and allowed to react at 80 °C overnight. Pressure was periodically released from the system with nitrogen flow during first 2 h. The product was purified by three

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RESULTS AND DISCUSSION Noncharged precursors of the polymers were first synthesized in order to be able to characterize them with SEC (Figure S2). These precursors were then derivatized to yield polycations. PMOTAI was synthesized with a reaction between PDMAEMA and iodomethane in a quantitative fashion (Figures S3 and S4). The hydrophobic end group of PClMeSt 1 was removed with AIBN (Figure S5). The resulting PClMeSt 2 was then reacted with 1-methylimidazole to yield PIL-1. This reaction also proceeded to completion, which was confirmed by the disappearance of the chloromethyl signal (“c” in Figure S5) and by comparing the integral of the methylene bridge (“d” in Figure S6) to the integrals of the aromatic region (“a−c” in Figure S6). The aromatic end groups of PMOTAI were not manipulated, since the hydrophobic contribution of a single phenyl ring should not be as great as that of a long alkyl tail. The derivatization of the polymers as well as the counterions is 7582

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Scheme 1. Structures of the Polycations and Counterions

upon addition of LiNTf2.33,34 Particle formation was also observed by Guo et al. when the counterions of the polymers were partially pre-exchanged from chloride to PF6.47 The temperature dependence was not studied in either case. PMOTAI and LiNTf2. When NaCl, in addition to LiNTf2, is introduced to the PMOTAI solutions, the stability of the solutions changes considerably (Figure 1). A certain amount of NaCl is required for the observation of a cloud point. It has been previously observed that within a certain pH range and concentration of LiNTf2 weakly cationic PDMAEMA can undergo a UCST type phase separation in a buffer.57 No UCST was observed in unbuffered solutions in otherwise similar conditions. This is in an agreement with observations by Plamper et al., who studied PDMAEMA with multivalent counterions.14 As may be seen in Figure 1, a threshold amount of LiNTf2 is required before Tc can be observed. The trend observed in Figure 1 is clear. In every case, Tc initially decreases with increasing NaCl concentration but then reaches a constant value. The plateau value varies with the amount of LiNTf2. The observed increase in solubility with increasing ionic strength (“salting in”) is opposite to what might be expected for a polyelectrolyte. It is even possible to turn the solution stable within the whole temperature range by adding sodium chloride, assuming the LiNTf2 content is low enough (20% in Figure 1). A high enough ionic strength is needed to screen the intra- and intermolecular electrostatic interactions and to increase the flexibility of the polymer chains. The screening of the charges also turns the thermal transition sharper (see Figure 2).

illustrated in Scheme 1, and the molecular weights of the polymers are listed in Table 1. Table 1. Synthesized Polymers polymer PDMAEMA PMOTAI PClMeSt 1 PClMeSt 2 PIL-1 a c

Mn (g/mol) b

27400 51900c 15400b 16400b 25100c

conv (%)

Mn(theor)a (g/mol)

b

76.0

24200

1.24b 1.32b

66.7

20700

Mw/Mn 1.07

[M]/[CTA]·conv·M(M) + M(CTA). Calculated from the precursor polymer.

b

Measured with SEC.

The effect of LiNTf2 and LiOTf on the solution behavior of PMOTAI and PIL-1 was studied by adding the salts in the polymer solutions and adjusting the ionic strength with sodium chloride. Earlier, the effect of LiNTf2 on aqueous PDMAEMA had been studied in a similar manner.57 The concentrations of LiNTf2 are reported as mol % of the repeating units of the polycations. In most cases the polymer concentration was 1 mg/mL, and then 100% corresponds to 3.34 mM and 4.26 mM LiNTf2 for PMOTAI and PIL-1, respectively. The addition of LiNTf2 to aqueous PMOTAI or PIL-1 decreases the transmittance of the solutions, i.e., leads to a certain degree of aggregation (Figure S7). The aggregates, however, did not show clear temperature dependence in their transmittance. Vijayakrishna et al. have reported that block copolymers with a polycation block form associates in water 7583

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Figure 1. Tc of 1 mg/mL solution of PMOTAI with 20% (■), 30% (●), 40% (▲), 50% (▼), and 60% (◆) of LiNTf2 relative to the repeating units as a function of NaCl concentration. The absolute concentrations are given in parentheses after each relative concentration. The solutions with 20% LiNTf2 showed no Tc with NaCl concentration above 100 mM. Samples with 10% LiNTf2 showed no Tc, regardless of NaCl concentration.

Figure 3. Tc as a function of polymer concentration at 100 mM NaCl either with 1.67 mM LiNTf2 (■) or with MOTAI/NTf2-unit −molar ratio of 50% (▲). At 1 mg/mL, 1.67 mM corresponds to 50%. In the 50% series (▲), also polymer concentration of 0.25 mg/mL was measured, but no Tc was observed.

PIL-1 and LiNTf2. When LiNTf2 is added to aqueous PIL-1, the phase separation temperature changes similarly to what was observed for PMOTAI (Figure 4). This polymer is able to

Figure 2. Transmittance as a function of temperature for 1 mg/mL PMOTAI solutions with constant LiNTf2 concentration of 50% (1.67 mM) and varying NaCl concentrations, shown as mM left of each curve.

Figure 4. Tc of 1 mg/mL solution of PIL-1 with various relative amounts (concentrations) of LiNTf2 to repeating units as a function of NaCl concentration. 10% showed no Tc.

Chloride and NTf2 ions compete on binding to the cationic polymers and equilibrium is reached at high NaCl concentrations. The affinity of NTf2 to the polymer is very high; looking, e.g., solutions with 30% LiNTf2 content, one can see that even a 1000-fold NaCl concentration does not make the Tc disappear. The finding is in line with observations of Vijayakrishna et al., that in order to change NTf2 to bromide in a polycation required a large excess of LiBr.33 Figure 1 deserves further clarification. Either the factor determining Tc is the ratio between PMOTAI and NTf2 or the concentration of NTf2. This was studied using a 1 mg/mL polymer solution with 50% (1.67 mM) of LiNTf2 at 100 mM NaCl as a starting point. The polymer concentration was varied either keeping the concentration of LiNTf2 constant at 1.67 mM or keeping the ratio between NTf2 ions and MOTAI repeating units constant at 50% (Figure 3). With polymer concentration of 1 mg/mL 1.67 mM corresponds to 50%; i.e., the data point is the same. Clearly, both factors have an effect but that of LiNTf2 concentration is much stronger.

tolerate much less LiNTf2, both in absolute and in relative units compared to PMOTAI, before turning completely insoluble over the studied temperature range. When detected, Tc appears at rather high temperatures. 61.5 °C was the lowest observed Tc (the corresponding temperature was 22.6 °C for PMOTAI). The observations can be rationalized by the hydrophobicity of the phenyl containing side group in PIL-1, where in addition the charge is delocalized over the imidazolium ring. However, the colloidal stability of the system at temperatures below Tc is much better compared to PMOTAI. The effect of NaCl on Tc is less pronounced than in the case of PMOTAI, but still, it is possible to “salt in” PIL-1 (cf. 30% and 40% in Figure 4). The transitions occur in a very wide temperature region (see Figure 5). Comparing Figures 2 and 5, one comes to conclusion NTf2 ions bind much stronger to PIL-1 than to PMOTAI. 7584

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those with LiNTf2. Tc decreases with addition of NaCl and may even disappear. In most cases, an equilibrium value of Tc is reached at high salt concentrations. At least in the low range, Tc is independent of polymer concentration (Figure 7) in solutions with a given concentration of LiOTf. This is true whether or not the total amount of added salts is kept constant.

Figure 5. Transmittance as a function of temperature for 1 mg/mL PIL-1 solution with constant LiNTf2 concentration of 30% (1.28 mM) with varying NaCl concentrations (indicated next to each curve).

PIL-1 and LiOTf. OTf is another anion often used in studies on ionic liquids, and it is of interest to compare the effect of OTf to that of NTf2. It was immediately observed that much more of OTf is needed than NTf 2 to turn PIL-1 thermoresponsive. Tc became observable in concentrations between 10 mM (235%) and 20 mM (469%). Owing to the high LiOTf concentration, Tc was observable even without the addition of sodium chloride. This makes the system very flexible since the Tc can be adjusted to practically any liquid water temperature by adding NaCl. The transitions are also very sharp (Figure S8). The amounts of LiOTf needed to induce Tc are very high, and correspondingly, when discussing the effect of NaCl the Tc will be plotted as a function of total salt concentration (Figure 6). The dashed red line in Figure 6 indicates the samples where

Figure 7. Tc as a function of PIL-1 concentration in various concentrations of LiOTf, given as mM above the lines. The measurements have been conducted either with LiOTf as the only salt (■) or keeping the total salt concentration constant at 100 mM with added NaCl (▲).

The data in Figures 3 and 7 are in contrast with the results of Yoshimitsu et al., who studied an imidazolium containing poly(vinyl ether) with BF4− as a counterion. They observed Tc to be strongly dependent on polymer concentration.35 However, in their case the hydrophobic anion was a part of polymer structure, which means that increasing polymer concentration also increases the concentration of the hydrophobic counterion. The present case is different because the hydrophobic anion is added in varying amounts to a readymade polymer solution. This enables changing the concentrations polymer and the hydrophobic ion independently of each other. Comparisons. With LiOTf one can induce Tc even in solutions of hydrophilic PMOTAI. Very high LiOTf concentrations are needed, 200 mM or more (Figure 8). The cloud point increases with increasing concentration of LiOTf, then reaches a maximum, and starts to decrease again. The maximum is around 500 mM of LiOTf. With this high ionic strength the solutions differ drastically from those discussed in the previous cases, and reasonable comparisons have to be done with caution. However, it is interesting to note that the maximum in Figure 8 appears at approximately the same salt concentration where the reaches a plateau value in Figures 1, 4, and 6. The origin of the decrease of Tc with increasing LiOTf concentration is still open for discussion. To ascertain that this is not a phenomenon related to the added cation, the cloud points were also measured in the presence of KOTf. As can be seen in Figure 8 the dependence is qualitatively the same. The reversibility of the system was studied with cycling the temperatures of 1 mg/mL solutions of PMOTAI (with 40% LiNTf2 and 750 mM NaCl) and PIL-1 (with 35 mM LiOTf at a

Figure 6. Tc of 1 mg/mL aqueous PIL-1 with various concentrations of LiOTf (shown next to the lines as mM) as a function of total concentration of added salts. The dashed red line connects points with LiOTf as the only salt.

only LiOTf was added to the solutions. Tc changes steeply with LiOTf concentration. In previous cases it was concluded that high enough ionic strength and the presence of a hydrophobic counterion are needed for the transition to be possible. In the present case, however, LiOTf concentration is so high that both conditions are met. Otherwise, the observations are similar to 7585

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As one further example, cloud points for a PMOTAI− nonafluorobutanesulfonate (NFO) are shown in the Supporting Information (Figure S9).

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CONCLUSIONS In situ introduction of a hydrophobic anion to an aqueous solution of a polycation induces UCST type of behavior. The phenomenon was illustrated by using two different polycations, PMOTAI and PIL-1, together with two different anions, NTf2 and OTf. Certain ionic strength together with sufficient concentration of the hydrophobic anion is needed for the observation of UCST. Adjustment of the ionic strength is needed to screen the intra- and intermolecular electrostatic interactions. The hydrophobic anion and the added smaller anion (chloride in this case) compete of the binding sites of the polymer. As a consequence, the solubility of the polyelectrolyte increases with increasing sodium chloride concentration when the concentration of the hydrophobic ion is kept constant. High concentrations of LiOTf were needed to induce the phase transition, which then could be observed even without added NaCl. PMOTAI is less sensitive to hydrophobic counterions compared to PIL-1 because of its high hydrophilicity. To observe the UCST, NTf2 was needed in much smaller amounts than OTf, which is in line with results on small molecular ionic liquids.37,41 In a low concentration range, the polymer concentration had little or no effect on the cloud point when the concentrations of other species were kept constant. The phase separation was reversible, and the cloud points kept constant over five heating−cooling cycles. In short, a method to induce a UCST behavior to polycations in water has been introduced.

Figure 8. Tc of 1 mg/mL PMOTAI solution as a function concentration of OTf ions, with either LiOTf (■) or KOTf (▲).

total salt concentration of 100 mM) (see Figure 9). The cooling cycle was always done first. These particular samples

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ASSOCIATED CONTENT

* Supporting Information S

Definition of Tc, NMR spectra, SEC chromatograms, transmittance as a function of temperature for several systems, and the Tc of PMOTAI in 100 mM NaCl as a function of KNfO concentration. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. Tc on cooling (▼) and heating (▲) cycles for 1 mg/mL solution of PMOTAI at 750 mM NaCl with 40% (1.33 mM) of LiNTf2 (black) and PIL-1 with 35 mM LiOTf at total salt concentration of 100 mM (red).

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were chosen because in both cases the transition was relatively sharp and occurred in temperatures far from the extremes of the studied range. The phase separation is reversible in both cases. Much less hysteresis is observed in the LiOTf-PIL-1 solution than in the PMOTAI-LiNTf2. The difference in the degree of hysteresis owes to the different hydrophilicities of the polymers and to their different affinities toward hydrophobic anions. It has also been observed during the course of the studies that polycations respond much slower to NTf2 than to OTf. This is due to either steric hindrance in NTf2 or the fact that NTf2 is not a very polarized ion because of the presence of many electronegative atoms.58 Only few polymers show a UCST-type Tc in aqueous solutions.3 Although the method used in the present study to promote or even induce the cloud point is illustrated with only four polycation−small molecular anion pairs, it should be applicable to a great variety of such systems. This leads to a nearly unlimited amount of polymers with UCST behavior in water.

AUTHOR INFORMATION

Corresponding Author

*E-mail heikki.tenhu@helsinki.fi (H.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Academy of Finland (grant number 264990) and ERA.Net RUS (Project SILICAMPS) is gratefully acknowledged.

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REFERENCES

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dx.doi.org/10.1021/ma501924r | Macromolecules 2014, 47, 7581−7587