Influence of Hydrophobic Anion on Solution Properties of PDMAEMA

Mar 5, 2014 - not only decreases the cloud point of PDMAEMA but also triggers an upper critical solution temperature (UCST) type behavior in acidic pH...
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Influence of Hydrophobic Anion on Solution Properties of PDMAEMA Erno Karjalainen, Vladimir Aseyev, and Heikki Tenhu* Laboratory of Polymer Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, 00014 Helsinki, Finland S Supporting Information *

ABSTRACT: The effect of bis(trifluoromethane)sulfonimide, NTf2, anion on solution properties of the thermoresponsive poly(2-(dimethylamino)ethyl methacrylate), PDMAEMA, has been studied. Nonstoichiometric amounts of LiNTf2 were added to aqueous solutions of PDMAEMA, with or without a buffer in the pH range 6−10. Since PDMAEMA is a weak polybase, the interaction between PDMAEMA and NTf2 can be manipulated by the concentration of the anion and also by varying the degree of charging of PDMAEMA with pH. PDMAEMA has a well-known LCST behavior which can be modulated by the counterion. It was observed that the hydrophobic NTf2 anion not only decreases the cloud point of PDMAEMA but also triggers an upper critical solution temperature (UCST) type behavior in acidic pH. In a higher pH regime, NTf2 makes the cloud point increase because the anion turns PDMAEMA to a stronger base, presumably by effectively shielding the charges.

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form micelles upon interaction with persulfate anions.18 The formed aggregates could be dissolved either by heating or by the reduction of the anions. Similarly, Hu et al. studied multiblock copolymers in which the other block was poly(4vinylpyridine) (4PVP) and the other one a random copolymer of N-isopropylacrylamide, NIPAM, and dimethylacrylamide.19 They found that in the presence of divalent 2,6-naphthalenedisulfonate anion the block copolymers showed both LCST and UCST type behaviors at low pH, where 4PVP is in its protonated form. The common approach in all of these studies is that the change in the transition temperature or even the introduction of UCST behavior to a LCST system may be realized by the addition of ions with an overall charge of two or more. Bis(trifluoromethane)sulfonimide (NTf2) is a monovalent anion well-known from the world of low molar mass ionic liquids (ILs). NTf2 turns several organic cations practically insoluble in water, owing to the fact the anion is not capable of forming strong hydrogen bonds.20−25 Water is usually moderately soluble in ionic liquids with an NTf2 anion, but not vice versa.21,24 Also, NTf2 as a counterion often decreases viscosities and melting points of ILs, even in some cases turning them completely amorphous with low glass transition temperatures (T g ).20,23,25−27 These experimental findings are attributed to the inability of the anion to form strong cation−anion hydrogen bonds, the weak cation−anion interaction, and the high charge delocalization resulting from exceptionally short nitrogen−sulfur bonds.28,29 Apart from low molar mass ILs, NTf2 has also been used in polymeric ionic liquids (PILs).30−40 Often the anion is

INTRODUCTION Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) is a weak polyelectrolyte, which in water shows a lower critical solution temperature (LCST) type behavior.1 As the polymer is a weak base its cloud point (Tc) increases with lowering pH, i.e., with an increasing degree of charging.1−5 The Tc shows some variation with molecular mass, especially in basic solutions.1,6 The reported values for the apparent pKa of the conjugate acid of PDMAEMA vary from 6.2 to 7.8, the variation reflecting the condition dependence of the pKa of polyelectrolytes.1,4,7−10 One of the much studied applications of PDMAEMA and its copolymers with various architectures is gene transfection.8,10−13 Plamper et al. studied the effect of multivalent hexacyanocobaltate(III) on buffered solutions of both linear and star-shaped PDMAEMA.14 They found that PDMAEMA with the multivalent counterion develops an upper critical solution temperature (UCST) type of behavior in buffers with pH of 5−8 and relatively high ionic strength. The UCST type Tc was found to be tunable by light due to a change from trivalent hexacyanocobaltate(III) to divalent anions with illumination. The investigation was further extended to miktoarm stars of poly(ethylene oxide), PEO, and PDMAEMA.15 The miktoarm stars formed aggregates both at high and low temperatures and, similar to the previous case, the system responded to UV-light. Even vesicles were formed in some cases. Miktoarm stars with quaternized PDMAEMA were later studied with added hexacyanoferrate II and III.16 The aggregation could now be controlled electrochemically, and vesicles were formed also in this case. Plamper has also studied the solution behavior of quaternized star-shaped PDMAEMA in the presence of hexacyanocobaltate(III), which causes photoreversible precipitation of the polymer.17 Somewhat similar systems have been studied by Jia et al. with block copolymers of PEO and the protonated form of poly(2-vinylpyridine), which © 2014 American Chemical Society

Received: January 9, 2014 Revised: February 26, 2014 Published: March 5, 2014 2103

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Measurements. Size Exclusion Chromatography. Size-exclusion chromatography (SEC) was conducted using a Waters 515 HPCL pump, Waters Stryragel columns, and a Waters 2410 refractive index (RI) detector. The eluent was THF containing 1% of TBAB, and the system was calibrated using poly(methyl methacrylate) standards. Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI). The MALDI spectrum of the synthesized polymer was recorded using a Bruker MicroFlex-instrument. Solutions of IAA, PDMAEMA, and NaTFA were prepared in methanol with concentrations 40, 10, and 10 mg/mL, respectively. The sample was prepared by mixing 20 μL of IAA solution, 5 μL of PDMAEMA solution, and 1 μL of NaTFA solution. 0.5 μL of the mixed solution was injected to the sample plate and allowed to dry. Proton Nuclear Magnetic Resonance (1H NMR). 1H NMR spectra were recorded with a 300 MHz Varian Unity INOVA spectrometer for determining the conversion of the polymerization and with a Bruker Avance III 500 spectrometer for verifying the purity of the synthesized polymer. Dynamic Light Scattering (DLS). DLS measurements were conducted using a Brookhaven Instruments BI-200SM goniometer, a BI-9000AT digital correlator, and a Coherent Sapphire 488-100 CDRH laser operating at wavelength of 488 nm. Temperature was maintained at 20 °C in all of the DLS measurements with a Lauda RC 6 CP thermostated bath. The scattering angle was 90° in every case. All the stock solutions for light scattering measurements were prepared by dissolving a needed amount of polymer in water and diluting the solution to a concentration 1 mg/mL (6.36 mM of repeating units) in a volumetric flask. In the case of LiCl-containing samples a known amount of LiCl was dissolved in a small volume of water, prior to the final dilution. All the solutions were allowed to equilibrate with shaking at least overnight. The light scattering cuvettes were washed with membrane-filtered water and methanol prior to measurements. The actual samples were prepared by measuring 1000 μL of the aforementioned stock solution to a light scattering cuvette with a micropipet. Then the LiNTf2 was added as a 0.1 M solution while vigorously stirring the cuvette in a vortex stirrer. For the measurement of the intensity of scattered light as a function of pH, the pH was set to above 10 with 0.1 M NaOH. The HCl additions were done similarly to the LiNTf2 addition; the pH was recorded at this point, and the scattering intensity was measured right after that. pH was cycled with varying amounts of HCl or NaOH, and the experiments were conducted as above. pH Measurements. The pH of the solutions was measured with Radiometer Copenhagen PHM 210-pH meter. The buffers used in calibration were a phosphate buffer with pH 7 (Aldrich), a citrate buffer with pH 4 (Fluka), and a borate buffer with pH 10 (Aldrich). Two of the aforementioned buffers were used to calibrate the electrode. The other was pH 7, and the other was either pH 4 or pH 10, depending on the target pH of the solution. The exception of this rule was the pKa determination, where the electrode was calibrated with pH 4 and 10. pH Titrations. The titration with LiNT2 was conducted using a similar solution as in the DLS measurements. 5000 μL of the solution with PDMAEMA concentration 1 mg/mL was titrated with additions of 7.95 μL of 0.1 M LiNTf2, which corresponds to 2.5% of the repeating units. The solution was vigorously stirred while adding the LiNTf2, and the pH was recorded. The pKa of PDMAEMA was determined by titration of 1 mg/mL solution with 0.1 M HCl. The pKa was taken to be the pH at degree of neutralization 0.5. In both titrations, after each addition the stirring was ceased, and the pH was allowed to stabilize before it was recorded. Transmittance Measurements. Transmittance as a function of temperature was measured with a JASCO J-815 CD spectrometer equipped with a PTC-423S/15 Peltier-type temperature control system. The transmittances of the samples were monitored at wavelength of 600 nm. The measurements were conducted in 10 mm cuvettes. The sample cuvettes were degassed in vacuum prior to measurements. The sample holder was heated or cooled at the rate of 1 °C/min, and the sample temperature was monitored. Prior to the

introduced to modify the solubility of the polycation, turning the polymer water-insoluble and soluble in organic solvents.31,32,38−40 NTf2 has also been shown to improve the thermal stability and lower the Tg of a strong polycation compared to traditional counteranions like halides.30,33,34,36,40 Also, NTf2 can be used to modify the morphologies of PILcontaining copolymers, both in solution and in bulk.33,35,38,39,41 Surprisingly, it seems that only Hunley et al. have combined protonated PDMAEMA and NTf2 in a study where different counterions were used.42 They studied the influence of the anion to thermal decomposition, Tg, solubility, and electrospinning properties of PDMAEMA. They noticed that protonated PDMAEMA can be turned water-insoluble in the presence of NTf2. However, they did not study the influence of the counterion on thermoresponsive or polyelectrolyte properties of PDMAEMA. In this context it needs to be added that Moreno et al. have observed the neutralization of PDMAEMA with octanoic or oleic acid, to produce a water-insoluble polymer.43 As is shortly described above, PDMAEMA as a weak polycation as well as polycations with NTf2 counterion show interesting solubility/solution properties. The purpose of the present study is first to shed light on the effect of NTf2 on solution properties of PDMAEMA. Second, it will be shown that polymer behavior similar to that observed earlier in the presence of multivalent ions can also be realized with a monovalent anion. The system under study is very flexible, since the cationic content of PDMAEMA is easily varied with pH. The focus is mostly on thermoresponsive properties of PDMAEMA with nonstoichiometric amounts of NTf2 in solutions with varying pH. Polyelectrolyte properties of such a system are discussed as well.

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EXPERIMENTAL SECTION

Materials. Azobiscyanopentanoic acid (ACPA) (Fluka, 98%) was recrystallized from methanol. 2-(Dimethylamino)ethyl methacrylate (DMAEMA) (Acros Organics, 99%) was passed through aluminum oxide and distilled. (4-Cyanopentanoic acid)-4-dithiobenzoate (CPA) was synthesized following a literature procedure.44,45 The water used for salt solutions and buffers was distilled, and for experiments in pure water the water was purified with the ELGA purelab ultrapurification system. Acetone (VWR, HPLC grade), hexane (Aldrich, HPLC grade), lithium bis(trifluoromethane)sulfonimide (LiNTf2) (Aldrich, 99%), HCl solutions (FF-Chemicals), NaOH solutions (FF-Chemicals), phosphoric acid (85%, Merck), tetrahydrofuran (THF), tetrabutylammonium bromide (TBAB), trans-3-indoleacrylic acid (IAA) (Aldrich, 99%), sodium trifluoroacetate (NaTFA) (Aldrich, 98%), and borax (Fluka, 99.5%) were used as received. The buffer solutions with pH 6−8 were prepared by neutralization of phosphoric acid with NaOH, and buffers with pH 9−10 were prepared from borax and HCl. All the buffers had ionic strengths of the order of 20−40 mM. Synthesis of Poly(2-(dimethylaminoethyl) methacrylate) (PDMAEMA). PDMAEMA was synthesized with the reversible addition−fragmentation chain transfer (RAFT) polymerization method.46−48 In a flask, 0.0754 g (0.270 mmol) of CPA and 0.0076 g (0.0271 mmol) of ACPA were dissolved in 8.4845 g (53.0 mmol) of DMAEMA. The mixture was deoxygenated by five freeze−thaw cycles and allowed to react at 100 °C under a nitrogen atmosphere. After 22 h, the polymerization was stopped by immersing the flask in liquid nitrogen. At this point, a sample for determining the conversion of the reaction was taken. The polymer was purified by three precipitations from acetone to hexane. The product was further purified by dialysis against water for 6 days, changing the water twice during the process. Finally, the polymer was isolated by freeze-drying. 2104

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Scheme 1. Suggested Effect of Acid and LiNTf2 on PDMAEMA

measurement, the sample was allowed to equilibrate at the starting temperature (5 or 90 °C) for 30 min. The transmittance of the pure solvent at 20 °C was set to be 100%, and the volume of the sample was always 2400 μL. The cloud point (Tc) was defined to be the onset determined by intersection of two tangents. See Figure S1a for details. The buffered stock solutions were prepared by dissolving a known amount of PDMAEMA in the buffer to an approximate concentration 20 mg/mL. Then the pH of the solution was set to match that of the buffer by addition of 1 M phosphoric acid, and the solution was diluted with the buffer in a volumetric flask to the concentration 10 mg/mL. The acid addition was needed only in phosphate buffers with pH range of 6−8. In higher pH values, PDMAEMA is so weak base that the acid addition was not needed. The samples in water were made by directly dissolving PDMAEMA to a small volume of water and diluting the solution to the concentration 10 mg/mL. The stock solutions were always kept shaking overnight or more, after which their pH was checked. The samples were prepared using the 10 mg/mL stock solutions of the polymer. First, varying amounts of LiNT2 as 0.1 or 1 M solution were added to the vials. In order to not to affect the buffer capacity, the amount of LiNTf2 solution was always less than 1.3% of the final sample volume of 3 mL. Next, the solvent, buffer or water, was added in such a fashion that the sum of all additions was 2700 μL. Finally, 300 μL of 10 mg/mL PDMAEMA solution was added with vigorous stirring. Thus, the final concentration of PDMAEMA, assuming no volume change, was 1 mg/mL. The pH of the solution was measured at this point. In the cases where pH was varied, acid or base was added prior to the addition of PDMAEMA, keeping the sum of additions at that point in a constant value of 2700 μL. In a vast majority of measurements with varying pH, the added solution was 0.1 M HCl. Microcalorimetry. Differential scanning calorimetry (micro-DSC) measurements on solutions were conducted with a MicroCal VP-DSC microcalorimeter. The samples were prepared in exactly the same way as in transmittance measurements. The samples were degassed at 5 °C prior to measurements. The heating was done exactly as in the transmittance measurements. The temperature of maximum heat

capacity (Tmax) and the enthalpy change associated with the phase transition were determined from the thermograms (see Figure S1b).

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RESULTS AND DISCUSSION The main hypothesis behind this study is presented in Scheme 1, illustrated for one repeating unit of PDMAEMA. The hypothesis is that the protonated PDMAEMA, which as such is more hydrophilic than its nonprotonated counterpart, turns water-insoluble when NTf2 is added in the solution. Three polymeric species should exist in equilibrium in a pH range where PDMAEMA is partially protonated and when NTf2 is present in substoichiometric concentrations compared to DMAEMA repeating units. The molecular mass distribution of the synthesized PDMAEMA was analyzed with SEC and MALDI. The results are summarized in Table 1. The conversion of the polymerTable 1. Molecular Mass of PDMAEMA

a

method

Mn (g/mol)

Mw (g/mol)

PDI

theoreticala SEC MALDI

24 900 24 100 33 800

32 300 35 300

1.34 1.04

[DMAEMA]/[CPA] × conversion × M(DMAEMA) + M(CPA).

ization was 79.2%. The values of Mn listed in Table 1 are in a good agreement with the theoretical value. It is important to note that all the experiments were conducted using the very same batch of PDMAEMA. The apparent pKa of the conjugate acid of PDMAEMA used in this study was determined by titration with 0.1 M HCl (Figure 1). The pKa was taken to be the pH at half 2105

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between the protonated form of PDMAEMA and NTf2 anion exists, which turns the polymer insoluble at 20 °C. Figure 2 also illustrates the high sensitivity of protonated PDMAEMA to the presence of LiNTf2. The onset of the increase in turbidity keeps essentially unchanged after the addition of LiCl, although the LiCl concentration is approximately 15 times higher than that of LiNTf2. The reversibility of the effect shown in Figure 2 was studied by cycling the pH in pure water, adding either 0.1 M NaOH or 0.1 M HCl to a similar PDMAEMA/LiNTf2 solution as used in the previous experiment at 20 °C (see Figure 3a). The added volume of acid or base was each time slightly varied. In this way, a set of pH−intensity data points were collected, which could then be plotted as in Figure 3b. Comparison of Figures 2 and 3b shows that almost identical curves were produced using two different titration methods. On the basis of the two graphs of Figure 3, one can conclude that the effect of PDMAEMA charging is in the present system reversible. Thus, the solubility of PDMAEMA with a constant amount of LiNTf2 is seen to be strongly affected by the degree of PDMAEMA charging, i.e. pH. Figures 2 and 3 also show that the steep increase of intensity of scattered light occurs in the pH range 8−9, regardless whether pH is decreased monotonously (Figure 2) or cycled back and forth (Figure 3b). No special precautions were taken to prevent introduction of dust to the samples during the experiments described in Figures 2 and 3. However, the possible, or even probable, effect of dust to the scattering intensity is not significant since it is possible to decrease the scattering intensity by increasing pH (Figure 3). As Figure 3a shows, pH of an aqueous PDMAEMA solution increases upon the addition of LiNTf2. This point was further studied by slowly adding 0.1 M LiNTf2 solution to aqueous PDMAEMA (1 mg/mL), while monitoring the pH (see Figure 4). The initial additions of LiNTf2 increase the pH rapidly, but after the addition exceeds 50% relative to the DMAEMA repeating units, the increase slows down and the value keeps constant at 9.25 ± 0.03. The effect of LiNTf2 on pH may be rationalized by assuming that a protonated DMAEMA unit forms a tight ion pair with the NTf2 anion. The effective shielding of charge increases the ability of the neighboring repeating units to protonate, and thus their basicity increases. The DMAEMA units do not turn into strong bases as it does not protonate completely, however, but the increase of pH stops well before every base unit has been saturated with LiNTf2. A similar effect has been reported previously with a different counterion.14 The increase of pH is similar also if the solution is acidified before the addition of LiNTf2. It should be noted that the concentration of LiNTf2 during the course of the titrations discussed above is very low; a full saturation of DMAEMA units is reached at 6.36 mM. The effect of increased ionic strength should thus be minimal. The increase of pH does not arise from the NTf2 anion itself. This was verified by measuring the pH of a 20 mM solution of LiNTf2, which was found to be 7.8. This is actually a surprisingly high value, since the conjugate acid of NTf2 is known to be a relatively strong one.54−56 Since PDMAEMA is a thermoresponsive polymer, it is vital to study its solution behavior as a function of temperature. This was done with two different methods, measuring transmittance as a function of temperature and by microcalorimetry. The effect of LiNTf2 on the cloud point, Tc, of PDMAEMA in pure water is shown in Figure 5. Also, the pH of the solutions was rechecked. The sample preparation was now

Figure 1. pH of 1 mg/mL solution of PDMAEMA as a function of the relative amount of added HCl.

neutralization, which was calculated from the equivalent point. The value for the apparent pKa determined in this way was 6.1, which is in a good agreement with the value 6.22 obtained by Plamper et al.1 The method gives an apparent pKa rather than a “true” pKa because the dissociation constant for a weak polyacid is known to depend on the degree of ionization and concentration; also, the activities of different species are not known.49−53 The effect of pH on a PDMAEMA solution with concentration 1 mg/mL, containing an equimolar amount of LiNTf2 (i.e., the same concentration as that of the repeating DMAEMA units), was studied by means of light scattering (Figure 2) at 20 °C, starting the titration from the alkaline end. The PDMAEMA solutions were studied both in water and in 0.1 M LiCl. In both cases, the intensity of scattered light, i.e. turbidity, starts to increase below pH of approximately 9. At pH higher than 9, PDMAEMA is practically completely deprotonated. The finding supports the hypothesis that an interaction

Figure 2. Intensity of scattered light as a function of pH from a PDMAEMA solution in 0.1 M LiCl (■) and in pure water (●) at 20 °C. Both samples contain equal concentrations of LiNTf2 and DMAEMA repeating units. 2106

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Figure 3. pH cycling experiment at 20 °C. (a) Intensity of scattered light upon addition of varying amounts of acid (red line) or base (blue line) to a 1 mg/mL solution of PDMAEMA, after addition of an equimolar amount of LiNTf2 (black line). pH of the solutions after each addition is indicated next to the data point. (b) Intensity of scattered light as a function of pH, after the addition of LiNTf2 (■). The original PDMAEMA solution before the addition of LiNTf2 (▲) is shown for comparison.

Figure 4. pH of 1 mg/mL aqueous solution of PDMAEMA as a function of LiNTf2 concentration. Concentration is given as mol % of DMAEMA units.

Figure 5. Tc (■) and pH (▲) of 1 mg/mL solution of PDMAEMA as a function of LiNTf2 concentration. Concentration is given as mol % of DMAEMA units. All the solutions have been prepared separately.

different from the case where pH was monitored as a function of LiNTf2 concentration (Figure 4). In the present case, PDMAEMA was added to a solution of LINTf2 (see Experimental Section). The cloud point decreases fast with increasing LiNTf2 concentration, but at a certain salt concentration the value reaches a minimum and starts to slowly increase. The final Tc is close to that in pure aqueous polymer solution. Evidently with low LiNTf2 concentrations the formation of insoluble ion pair is the factor mostly affecting the cloud point. However, as the LiNTf2 concentration increases, the increased basicity of the polymer and thus the degree of charging begins to dominate, this leading to an increase of Tc. The pH values in Figure 5 are close to those in Figure 4 though slightly higher, an observation which supports the hypothesis that the presence of LiNTf2 increases the basicity of PDMAEMA.

Buffered polymer solutions were studied with varying amounts of LiNTf2 in a pH range 6−10. For reference, the values of Tc, Tmax, and ΔH for PDMAEMA solutions without any added LiNTf2 are shown in Figure S2. The values are close to those reported by others, though the enthalpy of phase transition has been shown to vary significantly.1−3 Also solutions buffered to pH 6 were studied, but they show no thermal transition without LiNTf2. Figure S2 is an important reference when the discussion on the effect of added LiNTf2 in solutions with varying pH proceeds. Addition of LiNTf2 to a solution of PDMAEMA in a buffer with pH 6 has a remarkable effect, as can be seen from the transmittance data in Figure 6. The solution shows a clear UCST behavior, turning turbid upon cooling. Formation of hydrophobic ion pairs makes the polymer insoluble at low temperatures. Heating of such a solution above the critical temperature disturbs the interactions between repeating units and counterions. The liberation of counterions during the 2107

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Figure 6. (a) Transmittance as a function of sample temperature upon cooling 1 mg/mL solutions of PDMAEMA buffered to pH 6 with various amounts of LiNTf2. Heating rate 1 °C/min. The percentages next to the curves represent the relative amounts of LiNTf2 given as mol % of DMAEMA units. (b) Dependence of Tc on the amount of LiNTf2.

dissolution makes the process entropically favorable.57 The fact that the critical temperature may be tuned in a wide temperature range by the salt content points to the dynamic nature of the ion pairs. The increase of Tc appears to be linear in respect of the amount of LiNTf2 (Figure 6b). Owing to the fact that the samples need to be degassed at low temperature to avoid formation of gas bubbles, it was not possible to determine the enthalpy associated with the phase transition in this case. The concentration of the sample would most probably change during prolonged periods below Tc due to precipitation. At pH 7, the behavior is more as what could be expected for PDMAEMA. The system shows LCST behavior, and the Tc decreases with the addition of LiNTf2 (Figure 7). NTf2 is seen to decrease the water solubility of the DMAEMA units. Even 10 mol % of LiNTf2, which translates to a concentration 0.636 mM, is enough to produce an observable effect. The shape of the transmittance versus temperature curves changes essentially

when the amount of LiNTf2 (relative to the number of DMAEMA units) exceeds 40%. It seems possible that at high LiNTf2 concentrations the counterions bind to the polymer in a cooperative manner, this leading to block-like structures, as is the case in polyelectrolyte−surfactant systems.58 These structures lead to the buildup of aggregates which strongly scatter light. The process is observed as the lowering of transmittance below Tc with higher concetrations of LiNTf2. This assumption is supported by the facts that the transmittances are low, and Tc becomes practically independent of the LiNTf2 concentration. The cloud points of solutions with pH 7 and 8 (Figure 8a) decrease monotonously with increasing LiNTf2 concentration, and soon the polymer turns insoluble at the measured temperature range. No apparent associate formation, as observed with higher concentrations at pH 7 (Figure 7), can be observed at pH 8, however. Evidently, certain competition between the counterions exists since the effect of LiNTf2 does not level off to any constant value. Alternatively, the increasing basicity of PDMAEMA may again play a role. Either way, insoluble ion pairs clearly build up. As has been shown already, PDMAEMA with a stoichiometric amount of LiNTf2 starts to dissolve above the pH range 9.1−9.3 (Figures 2 and 3). This is also the value the pH reaches upon the addition of LiNTf2 (Figure 4). In this pH range Tc starts to increase in water (Figure 5). Just slightly below this value, at pH 9, Tc decreases much slower than in pH 7 and 8 (Figure 8a), and the system tolerates much more LiNTf2 without the polymer turning completely insoluble. This may be understood in terms of competition between increased basicity (which increases Tc) and the formation of insoluble ion pairs (which decreases Tc). In the end the ion pair formation, however, is the dominant factor, and the polymer precipitates. On the other hand, in pH 10 the increased basicity starts to dominate and the cloud point increases with LiNTf2. Importantly, the pH dependence of Tmax (Figure 8b) is similar to that of Tc. Tmax is determined by measuring the heat capacity, while Tc is based on optical transmittance. ΔH of the transition at pH 7−9 (Figure 8c) follows similar trends as Tc and Tmax. The enthalpy values indicate that PDMAEMA gets less hydrated with the introduction of LiNTf2, or the stability of the hydration layer decreases. This is in

Figure 7. Transmittance as a function of sample temperature for cooling 1 mg/mL solutions of PDMAEMA buffered to pH 7, containing various amounts of LiNTf2. Heating rate 1 °C/min. The percentages next to the curves represent the relative amounts of LiNTf2 given as mol % of DMAEMA units. 2108

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Figure 8. Tc (a), Tmax (b), and ΔH (c) for buffered solutions with pH 7 (■), pH 8 (●), pH 9 (▲), and pH 10 (▼). Lines are to guide the eye.

accordance with Scheme 1, where hydrophobic ion pairs exist in equilibrium with hydrated and dissociated ions. When more NTf2 ions are added, the equilibrium shifts toward the hydrophobic ion pairs and ΔH decreases. In pH 10, the changes in ΔH are small. The initial addition of LiNTf2 slightly increases the enthalpy change, but the system soon reaches equilibrium with further increasing the salt concentration. So far, the studies have been centered on altering the amount of LiNTf2 while maintaining a constant pH (Figure 8). Finally, two cases with constant LiNTf2 concentrations were taken to closer examination. The first case comprises solutions with 30 mol % of LiNTf2 of DMAEMA units. In the preceding experiments this amount of salt was seen to considerably affect the transition temperature, but the polymer was soluble at some temperature. In the second case, the amount of salt was 40 mol % since this was the lowest amount where UCST type behavior was observed. The 30 mol % series was studied in buffered solutions with varying pH (Figure 9). Tc values are lower compared to the ones obtained without LiNTf2 (Figure S2) when pH is below 9.2. Above the pH range 9.2−9.4, Tc is higher than without the salt. This is in line with previous experiments. The effect of LiNTf2 is at its highest at pH 8. At this point PDMAEMA is charged enough for LiNTf2 to have a significant effect, but

Figure 9. Tc as a function of pH in buffered solutions for 1 mg/mL solution of PDMAEMA with 30 mol % of LiNTf2 of DMAEMA units. The solid line is to guide the eye. The dashed line marks the Tc at pH 9 and pH 10 without LiNTf2 for comparison.

when pH increases, the lower charging eases the effect of the NTf2 ion. 2109

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hydrophobe content is high enough, as the insolubility of the polymer. The effect seems to be at its highest at about pH 8. In basic solutions with pH above approximately 9.2, the polymer character changes dramatically. In this area of pH, the main factor affecting the solubility and phase separation starts to be the increase in the basicity of PDMAEMA. Contrary to solutions with lower pH, the Tc now starts to increase with additions of NTf2. This reflects the increased charging and can be achieved not only in buffers but also in solutions in pure water. In short, the interaction between PDMAEMA and NTf2 has been shown to be a versatile way to influence the solution properties of the polymer.

Two separate regions of solubility exist when the solutions contain 40 mol % LiNTf2 (Figure 10). Here the sample was

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ASSOCIATED CONTENT

S Supporting Information *

Definitions used in the study, results for PDMAEMA without LiNTf2, a representative graph of transmittance as a function of temperature, and Tc as a function of pH in unbuffered solutions. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 10. Tc as a function of pH in buffered solutions for 1 mg/mL solution of PDMAEMA with 30 mol % of LiNTf2 to DMAEMA units. The solid lines are to guide the eye. The dashed line marks the Tc at pH 9 and pH 10 without LiNTf2.

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

defined to be in the two-phase region also if the transmittance was below 80% at every temperature. Again, the Tc grows higher than in buffered PDMAEMA without any LiNTf2 between pH 9.1 and 9.3. The LCST type phase separation was always monitored while heating the sample and the UCST behavior during cooling (see Figure S3). In unbuffered solutions with varying pH and constant LiNTf2 (Figures S4 and S5), the LCST behavior is similar to that in the buffered solutions, but no UCST behavior could be detected. This is in line with the results of Plamper et al.14 To the end, it is worth noting that the point of highest pH was remeasured in both cases, but no change of Tc was observed. Thus, the hydrolysis of the polymer is not significant under the experimental conditions and time scale.

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ACKNOWLEDGMENTS

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REFERENCES

Financial support from the Finnish Funding Agency for Technology and Innovation TEKES (grant 40207/09), Academy of Finland (grant 264990), and ERA.Net RUS (Project SILICAMPS) is gratefully acknowledged.

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CONCLUSIONS In a series of experiments, it has been shown that introduction of NTf2 to aqueous solutions of PDMAEMA has a tremendous effect in both the polyelectrolyte and thermoresponsive properties of the polymer. The anion forms hydrophobic ion pairs with the DMAEMA units and also turns the polyelectrolyte more basic. High enough number of protonated units in the chain is needed for this effect to take place. Contrary to usual behavior of polyelectrolytes, the introduction of charges to PDMAEMA makes the polymer actually less soluble in water when NTf2 is present. In acidic and mildly basic solutions the dominant factor affecting the solutions is the formation of hydrophobic ion pairs, even to the extent that the well-known LCST polymer shows a UCST-type behavior, provided that the degree of charging is high enough. The UCST behavior can, however, only be observed in buffered solutions. In less acidic conditions the effect of NTf2 is similar to that of copolymerizing hydrophobic comonomers to the PDMAEMA chain. This is seen as a decrease of the cloud point and eventually, when the 2110

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