Methanol Mixtures

Article pubs.acs.org/JPCB

Solvent Dynamics in Solutions of PNIPAM in Water/Methanol MixturesA Quasi-Elastic Neutron Scattering Study Konstantinos Kyriakos,† Martine Philipp,† Luca Silvi,‡ Wiebke Lohstroh,‡ Winfried Petry,†,‡ Peter Müller-Buschbaum,† and Christine M. Papadakis*,† †

Fachgebiet weicher Materie/Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München, James-Franck-Strasse 1, 85748 Garching, Germany ‡ Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstrasse 1, 85748 Garching, Germany S Supporting Information *

ABSTRACT: The solvent dynamics of concentrated solutions of poly(N-isopropylacrylamide) (PNIPAM, 25 wt %) in water/methanol mixtures (85:15 v/v) are measured with the aim of shedding light onto the cononsolvency effect. Quasi-elastic neutron scattering (QENS) with contrast variation has been carried out at temperatures below and above the cloud point by using in the first set of experiments the mixture H2O:d-MeOD (d-MeOD denotes fully deuterated methanol) as a solvent and in the second set of experiments the mixture D2O:MeOH (MeOH denotes methanol). As a reference, bulk H2O, bulk MeOH and the mixtures H2O:d-MeOD and D2O:MeOH (both 85:15 v/v) have been investigated as well. In the PNIPAM solution in H2O:d-MeOD, two water populations are identified, namely strongly and less strongly arrested water. At the cloud point, the former is partially released from PNIPAM. The diffusion coefficient of the latter one is similar to the one in the water/methanol mixture, and its residence time decreases at the cloud point. The PNIPAM solution in D2O:MeOH reveals similar dynamics to the one in H2O:d-MeOD which may reflect that the dynamics of MeOH near the PNIPAM chain is similar to the one of H2O. The similarity may, however, partially be due to H/D exchange between D2O and MeOH. In both PNIPAM solutions, the mean-square displacement of the PNIPAM chain decreases gradually above the cloud point.

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INTRODUCTION The thermoresponsive behavior of poly(N-isopropylacrylamide) (PNIPAM) has attracted strong interest over the last decades.1−3 The proximity of its lower critical solution temperature to the biologic window designates this polymer as a perfect candidate for a vast range of applications.4−7 Whereas the polymers are hydrated and water-soluble below the cloud point, Tcp, phase separation occurs above, along with a partial release of the associated water molecules and a collapse of the polymer chain.8−12 In spite of the large number of studies, certain aspects of its hydration mechanism are still under vivid debate.13−15 In particular, the mechanisms that govern the solvation of the PNIPAM chain in a mixed solvent of water and any short chain alcohol (e.g., methanol)termed cononsolvencyis far from being well understood.16 Albeit, at room temperature, both water and methanol are good solvents for the polymer, phase separation is encountered in a certain composition range of the solvent mixture.17−26 Different models were put forward to explain the molecular origin of cononsolvency.27−37 For instance, Zhang and Wu27 highlighted the importance of the water−methanol interactions. Also known as kosmotropic ef fect,38,39 these interactions induce the formation of small hydration clusters around the methanol molecules. As a consequence, the water network is strengthened, and the mobility of the water molecules is © XXXX American Chemical Society

severely reduced. According to this approach, the solvent− solvent interactions are more preferable than those between polymer and solvent, and thus the polymer chain phase separates.27,31 Recent studies focused on the impact of the addition of the cosolvent on the energetic state of water:36 Addition of cosolvent molecules reduces the enthalpy of bulk water, which, in turn, affects the strength of the hydrophobic hydration of certain parts of the polymer chain. Tanaka et al. postulated that the cooperative character of the solvation mechanism of PNIPAM is at the origin of cononsolvency.28,29,33 This model predicts the formation of sequences of the two species along the polymer chain, giving rise to a “pearl-necklace” conformation. Computer simulations showed that methanol binds strongly to PNIPAM, mainly with its hydroxyl group.34 The importance of this mechanism was confirmed recently.37 Therefore, the methyl groups of methanol are oriented toward the solvent, and the polymer plus solvent shell appears hydrophobic. Using FT-IR, it was shown that the carbonyl group is of importance.26 Recently, Mukherji et al. suggested that the preferential adsorption of the alcohol molecules causes bridging between neighboring polymer segments and thereby leads to phase separation.33 In Received: February 3, 2016 Revised: April 11, 2016

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DOI: 10.1021/acs.jpcb.6b01200 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

shaken and stored in the fridge (at ∼4 °C) for several days in turns, until clear solutions were obtained. Then, additional polymer was added to achieve the final concentration of 25 wt %. Again, these solutions were shaken and stored in the fridge for several days. This preparation route was chosen to avoid phase separation at room temperature. The final solutions were left to equilibrate in the fridge for 1 week prior to the QENS measurements. The deuterated solvents (Deutero GmbH) were both 99.9%. In all cases, the vials were hermetically closed to avoid the evaporation of methanol. The cloud points were determined by optical inspection using a thermobath and were found at Tcp = 26.0 ± 0.2 °C and Tcp = 25.5 ± 0.2 °C for the solutions in H2O:d-MeOD and D2O:MeOH, respectively. The slight difference of the values is probably caused by the different compositions of the partially deuterated solvent mixtures.53,54 Quasi-Elastic Neutron Scattering (QENS). QENS experiments were performed at the time-of-flight direct spectrometer TOF-TOF at MLZ, Garching, Germany.55,56 The experiment focuses on the proton dynamics of H2O and MeOH. By selecting a wavelength of 6 Å and a frequency of the mainframe chopper at 14 000 rpm, an energy resolution of 0.048 meV was achieved. The detector bank consists of approximately 1000 He3 tubes, covering an angular range from −15° to 140°. Energy transfers in the range −1.4 to 10.0 meV and momentum transfers 0.4 to 2.0 Å−1 were probed. In the temperature resolved experiments, a thermostat with stability better than 0.5 K was used. The 25 wt % PNIPAM solutions in the solvent mixtures H2O:d-MeOD (85:15 v/v) and D2O:MeOH (85:15 v/v) were investigated in the temperature range 21 to 34 °C and 21 °C to 32 °C, respectively. The two pure solvents and the two solvent mixtures were measured at 21 and 34 °C. After each temperature change, the equilibration time was 30 min. Acquisition times were 4 h at each temperature. The samples were inserted in cylindrical aluminum cells, and special precautions were taken to avoid the evaporation of the highly volatile methanol during the measurements. The slit width of the cells was 0.1 mm in all cases, except for the solvent mixture D2O:MeOH 85:15 v/v, where a slit of 0.2 mm width was used to keep the acquisition time of the sample containing only 15 vol % of protonated material (MeOH) at 4 h. For all samples, the respective empty sample cells and a sample of vanadium powder were measured, and the corresponding corrections were applied to the data. Analysis of the QENS Data. Whereas the incoherent scattering length density values of the nondeuterated species H2O and MeOH are 20.69 × 10−6 Å−2 and 18.43 × 10−6 Å−2, respectively, i.e., relatively high, they take much lower values, namely 3.30 × 10−6 Å−2 and 2.93 × 10−6 Å−2 for the deuterated species D2O and d-MeOD. Thus, the signal is dominated by the protonated solvent, whereas the signal of the deuterated solvent is significantly weaker. The QENS spectra were analyzed by assuming that the vibrational, translational, and rotational degrees of freedom of the solvent molecules are decoupled. Thus, the experimentally measured signal was modeled by a sum of an elastic component, theoretically given by a δ-function and approximated here by a Gaussian function, and the necessary number of Lorentzian functions (up to three) having amplitudes Ai and widths Γi to adequately describe the experimental data. The model function reads as follows:

addition to homopolymers, the cononsolvency effect was observed in chemically cross-linked gels16,40 and amphiphilic block copolymers,41−43 and for a number of cosolvents, e.g., ethanol, acetone, and isopropanol.22,44,45 However, the picture is still far from being clear, and information on the molecular mechanism of the solvation process will be beneficial for gaining a better understanding. Quasi-elastic neutron scattering (QENS) has frequently been employed to study the water dynamics in the presence of synthetic polymers46 or biologic molecules.47 With respect to PNIPAM, QENS studies addressed the hydration mechanism of homopolymers12,48 and of microgels.49,50 Our recent study revealed insights into the hydration of PNIPAM homopolymers in aqueous solution around Tcp.12 The most important findings were that, even several Kelvin above Tcp, PNIPAM is hydrated; moreover, the temperature range over which the phase transition evolves, is quite large (on the order of 4 K). Especially the former finding is in agreement with results from previous spectroscopic studies8,9 and from measurements on the mechanical properties of the PNIPAM-rich phases above Tcp.11,51 In the present work, we focus on the diffusion dynamics in concentrated solutions of PNIPAM in water/methanol mixtures, aiming to elucidate the molecular interactions giving rise to cononsolvency. The polymer was dissolved in a mixture of D2O and (protonated) methanol or in a mixture of H2O and fully deuterated methanol (d-MeOD). This way, we take advantage of the possibility of neutron scattering that allows the distinction between nondeuterated and deuterated species. The composition of the H2O:d-MeOD and D2O:MeOH mixtures was chosen at 85:15 v/v (water:methanol). The polymer concentration was chosen at 25 wt %, as in our previous experiments of PNIPAM in pure water.12 At this high polymer concentration, the majority of the solvent molecules are expected to be associated with the polymer chain. The dynamics of the latter is about 2 orders of magnitude slower than that of the solvent molecules52 and thus well-separated from the solvent dynamics. The proton diffusion dynamics of the polymer is deduced from the reduction of the intensity of the elastic line. The temperature range was chosen to cover the cloud point, Tcp. The paper is structured as follows: First, we describe the sample preparation and the protocol of the QENS measurements and the analysis of the QENS data in the Experimental Section. In the following Results section, we describe the results from the H2O:d-MeOD system, i.e., the water dynamics in the pure state, in the H2O:d-MeOD mixture, and in the solution of PNIPAM in H2O:d-MeOD. Then, we turn to the D2O:MeOH system, studying the MeOH dynamics in the pure state, in the D2O:MeOH mixture and in the solution of PNIPAM in D2O:MeOH. In each case, the mean-square displacement of the PNIPAM segments is analyzed alongside the diffusion coefficients and residence times of the solvents. Finally, we compile and discuss the results and compare them with theoretical models for cononsolvency.

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EXPERIMENTAL SECTION Sample Preparation. Poly(N-isopropylacrylamide) (PNIPAM) with a molar mass Mn = 20−25 kg/mol from SigmaAldrich (Germany) was used without further purification. The polymer was dissolved in mixtures of H2O:d-MeOD and D2O:MeOH, both at a methanol volume fraction of 15% as follows: First, polymer solutions of 10 wt % were prepared,

Sm(q , ω) = Ael (q)δ(ω) +

∑ Ai(q) i

B

Γi 1 π Γ i2(q) + ω 2

(1)


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line in Figure 1, Γ1 in Figure 2a) is assigned to the long-range diffusion of water molecules, whereas the broad one (dashdotted red line in Figure 1), corresponding to fast dynamics, is assumed to be a superposition of various local motions of the water molecule.57 If the latter process was entirely due to rotational dynamics, then the difference Γ2-Γ1 would be qindependent,58 which is not observed (Figure 2b). The deconvolution of these local motions is not possible with the information at hand; thus, the fast dynamic process will not be discussed further. Returning to the long-range diffusion of water molecules, it is best described by the isotropic jump diffusion model.50,57,59 According to this model, each H2O molecule is part of a tetrahedron, and the diffusion proceeds via jumps between different tetrahedrons. The corresponding width is given by the following:

Sm(q,ω) was convoluted with the instrumental resolution R(q,ω) to give the measured spectrum: S(q , ω) = R(q , ω) ⊗ Sm(q , ω) + bkg

(2)

where bkg denotes a constant background. The relative peak areas of the components are defined as Ai /∑i Ai .

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RESULTS Dynamics of Water in the Bulk, in the H2O:d-MeOD Mixture and in the Solution of PNIPAM in H2O:d-MeOD. In this part, we describe the water dynamics in the pure state and in the H2O:d-MeOD mixture as a reference for the results from the solution of PNIPAM in H2O:d-MeOD which are discussed afterward. The mean-square displacement of the PNIPAM segments is analyzed alongside the diffusion coefficients and residence times of the solvents. Bulk H2O. A QENS spectrum of bulk H2O at 21 °C is given in Figure 1 at the selected q-value of 1.1 Å−1. Two Lorentzians

Γ(q) =

Dq2 1 + Dτq2

(3)

where D is the diffusion coefficient, and τ is the residence time, i.e., the characteristic time that the molecule stays at a position before jumping to the next one. The fits of eq 3 to the experimental data are presented in Figure 2a as well. At 21 and 34 °C, D is found to be (1.9 ± 0.2) × 1011 Å2s−1 and (2.8 ± 0.2) × 1011 Å2s−1, respectively. Both values are in excellent agreement with the values reported in literature by various techniques.57,60−63 τ is ∼0.13−0.14 ps at both temperatures and thus also lies in the expected range.57 H2O:d-MeOD Mixture. We now turn to the diffusion of H2O in the 85:15 v/v H2O:d-MeOD mixture. Again, two Lorentzians are used to describe the QENS spectra (Figure 3); one

Figure 1. QENS spectrum S(q,ω) versus energy transfer ΔE of pure H2O at q = 1.1 Å−1 and 21 °C. Open black squares, experimental data; solid red line, full fit; dashed green line, long-range diffusion of H2O molecules; and dash-dotted red line, superposition of local motions of the H2O molecule.

(eq 1) are required to fit the data. The widths of the two contributions are presented in Figure 2. The narrow component, corresponding to slow dynamics (dashed green

Figure 3. QENS spectrum S(q,ω) versus energy transfer ΔE of the mixture H2O:d-MeOD at q = 1.1 Å−1 and 21 °C. Open black squares, experimental data; solid red line, full fit; dashed green line, long-range diffusion of H2O molecules; and dash-dotted red line, superposition of local motions of the H2O molecule.

accounting for the long-range diffusion and one for a superposition of local motions. The q-dependence of the width Γ of the former process is well described by the isotropic jump diffusion model (eq 3). The resulting diffusion coefficients are D = (1.5 ± 0.2) × 1011 Å2s−1 and (1.9 ± 0.2) × 1011 Å2s−1 at 21 and 34 °C, respectively (Figure 4b), i.e., in good agreement with previous studies63,64 and reduced compared to the ones in bulk water. This reduction reflects the structure-forming influence of methanol on the structure of water, termed as kosmotropic ef fect.38,65,66 The residence time of the water molecules is τ = 0.68 and 0.35 ps at 21 and 34 °C, respectively. These values are significantly higher than the ones found in pure water.

Figure 2. Results from pure H2O. (a) q-dependence of the width of the Lorentzian describing the long-range diffusion of the H2O molecules, Γ1. The solid red lines are the fits of eq 3. (b) Difference Γ2−Γ1 of the two Lorentzians describing the local motions of H2O. Solid symbols: 21 °C, open symbols: 34 °C. C


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Figure 4. Results from the H2O:d-MeOD mixture. (a) q-dependence of the width of the Lorentzian describing the long-range diffusion of the H2O molecules at 21 °C (solid black squares) and at 34 °C (open black squares). The solid red lines are fits of eq 3. (b) Resulting diffusion coefficients D of bulk H2O (solid black circles) and of H2O in the H2O:d-MeOD mixture (open black circles) in dependence on temperature. Figure 5. (a) QENS spectrum S(q,ω) versus energy transfer ΔE of the 25 wt % solution of PNIPAM in 85:15 v/v H2O:d-MeOD at q = 1.1 Å−1 and 21 °C. Open black squares: experimental data, solid red line: full fit, dashed green line: long-range diffusion of H2O molecules with strongly arrested dynamics, dash-dotted blue line: long-range diffusion of H2O molecules with arrested dynamics, dash-double-dotted red line: superposition of local motions of the H2O molecule, and magenta solid line: elastic component. (b) q-dependence of the width of the Lorentzian describing the water population with strongly arrested dynamics and (c) of the one with arrested dynamics at the temperatures given in the graphs. The solid lines are fits of eq 3.

We note that the effect of the H/D exchange of the hydroxyl deuteron of d-MeOD with H2O on the scattering cross section is negligible,67 thus the observed dynamic processes are nearly entirely due to H2O. The same holds for the PNIPAM solution in H2O:d-MeOD discussed below. Solution of PNIPAM in H2O:d-MeOD. Figure 5a shows a representative QENS spectrum for the 25 wt % solution of PNIPAM in 85:15 v/v H2O:d-MeOD at 21 °C. Three contributions, i.e., three Lorentzian functions, are required for the adequate description of these data. As before, the fastest process giving rise to the broadest Lorentzian (dash-doubledotted red line in Figure 5a) is ascribed to a superposition of local motions of the water molecules. The other two contributions account for the long-range diffusion of two different water species. The q-dependence of their widths is presented in Figure 5b and c, respectively. Qualitatively, the two species have very different diffusional behavior. The narrow contribution (dashed green line in Figure 5a) exhibits relatively low values of Γ, which do not change significantly with temperature (Figure 5b) and are attributed to strongly arrested water dynamics. The second population, Γ2(q), giving rise to the broader decay (dash-dotted blue line in Figure 5a), is faster and alters its behavior with temperature (Figure 5c). The diffusion coefficients of these two water species arising from the analysis of the q dependence of Γ1 and Γ2 within the isotropic jump diffusion model (eq 3) are given in Figure 6a together with the values in bulk water and in the H2O:d-MeOD mixture. The two populations exhibit significantly different values of D. The comparison with the dynamics of the water molecules in bulk water and in the H2O:d-MeOD mixture reveals that the origin of the slowest population with strongly arrested dynamics can be safely assigned to water−polymer interactions, since it has not been observed in the absence of polymer. The diffusion coefficient of this population is D = (0.4 ± 0.2) × 1011 Å2s−1 for all temperatures, thus almost 4 times smaller than the one of bulk water and 3 times smaller than the

one of water in H2O:d-MeOD. Interestingly, the signal of this contribution persists even 8 K above Tcp (26 °C). As a consequence, water resides in the PNIPAM-rich aggregates even deep inside the demixed phase. This observation is in agreement with previous studies.8,9,12 The diffusion coefficients of the faster water population are found to be in the same range as the ones obtained for the water molecules in H2O:d-MeOD without polymer (Figure 6a). This is a strong argument for assigning this population to water molecules that interact solely with methanol molecules via the kosmotropic effect. One can, of course, not exclude interactions of water with the polymer−previous studies have reported D values of water molecules with restricted dynamics due to interaction with the polymer which are in the same range as in purely aqueous solutions.12,48 Hence, interactions with the polymer may be at the origin of this restricted diffusional behavior of water, such as the participation in hydration layers of higher order or the formation of cages hydrating the hydrophobic isopropyl groups. The behavior of the residence times τ of the two water populations is in accordance with the picture described in the previous paragraph (Figure 6b). The population with strongly arrested dynamics has a value τ = 2.90 ± 0.76 ps at 21 °C and decreases slightly above Tcp to a lower value, i.e., τ = 2.40 ± 0.17 ps, which is essentially independent of temperature. Thus, D


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Figure 6. Results from the 25 wt % solution of PNIPAM in H2O:dMeOD. Temperature dependence of (a) the diffusion coefficient D, (b) the residence time τ and (c) the relative peak amplitudes A of the two Lorentzians for water molecules with strongly arrested dynamics (half-filled blue symbols) and with arrested dynamics (dotted red symbols). In (a) and (b), the respective values of bulk water (solid black symbols) and of water in H2O:d-MeOD without polymer (open black symbols) are given as well. The dashed vertical line marks Tcp.

Figure 7. (a) q-dependence of the intensity of the elastic component of the 25 wt % solution of PNIPAM in H2O:d-MeOD at the temperatures given in the graph. The solid lines are fits of eq 4. (b) Temperature dependence of the mean-square displacement ⟨u2⟩ of the vibrational mode of the PNIPAM chain. The red solid line is a guide to the eye. The dashed black line marks Tcp.

transition between the swollen and the collapsed state spans over several Kelvin. The absolute values are in the expected range.12 We conclude that water shows complex dynamics in the ternary system PNIPAM:H2O:d-MeOD, and two arrested populations are present. The strongly arrested water has a low diffusion coefficient and a high residence time, both nearly unaffected by the phase transition. Only the amplitude of this process decreases above Tcp, i.e., strongly arrested water is released by the contracting PNIPAM chain. The diffusion coefficients of the faster water population are higher and similar to the one in H2O:d-MeOD without polymer. Its residence time decreases above Tcp, and its amplitude increases at the expense of the one of the strongly bound water. The meansquare displacement of the PNIPAM monomers decreases above Tcp. Dynamics of Methanol in the Bulk, in the D2O:MeOH Mixture and in the Solution of PNIPAM in D2O:MeOH. To obtain a comprehensive picture of the dynamics, we investigate the solvent dynamics in D2O:MeOH. Under these conditions, one might expect that the signal is dominated by MeOH; however, due to H/D exchange of the hydroxyl proton of MeOH with H2O, a certain fraction of H2O transforms into HDO, and a non-negligible fraction of the signal (∼25%) is due to HDO. The dynamics of methanol are investigated in the bulk, in the D2O:MeOH mixture and in the concentrated solution of PNIPAM in D2O:MeOH. Bulk MeOH. The QENS spectrum of bulk methanol (Figure S1 in the Supporting Information, SI) is described by a sum of two Lorentzians. The broad Lorentzian is assigned to a superposition of local motions of the methanol molecule and the narrow one to the long-range diffusion. We focus again on

these water molecules reside for quite a long time at a position before diffusing to the next one. The residence time of the faster population is significantly lower. It decreases at Tcp from τ = 0.8 ± 0.2 ps at 21 °C to τ = 0.17 ± 0.08 ps at 34 °C. Whereas the latter value is comparable to the one in bulk water and in H2O:d-MeOD, the lower value of D at the same temperature reveals that the diffusive behavior of this population of water is slowed down (Figure 6a) compared to bulk water. The behavior of the relative peak amplitudes Ai of the two Lorentzians as a function of temperature is presented in Figure 6c. The number of molecules with severely arrested dynamics decreases above Tcp, in favor of the number of those with faster dynamics. This observation is in agreement with the expectation that the collapse of the PNIPAM chain leads to a release of a fraction of those water molecules in direct interaction, since the contraction of the chain at Tcp is expected to reduce the number of the available H-bonding sites. Interestingly, this transition does not occur abruptly at Tcp, but spans over several Kelvin, in good agreement with our previous studies.12 From the q-dependence of the elastic intensity (Figure 7a), it is possible to extract the mean square displacement ⟨u2⟩ of the PNIPAM monomers in dependence on temperature. At this, it is assumed that the vibrational mode of the chain can be approximated by a harmonic oscillator. In that case, the elastic intensity can be expressed as follows:12 2

Iel(q) = I0e−⟨u ⟩q

2

/3

(4)

where I0 is a constant. The resulting ⟨u ⟩ values (Figure 7b) decrease strongly above 28 °C, which reflects the collapse of the polymer chain above Tcp. Similar to the peak area A, the 2

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The Journal of Physical Chemistry B the latter. Analogous to the diffusion of water molecules, the diffusion of methanol follows the isotropic jump diffusion model. Analyzing the q-dependence of the width Γ (Figure S2 in the SI) by applying eq 3 gives diffusion coefficients D = (2.7 ± 0.2) × 1011 Å2s−1 and (3.3 ± 0.3) × 1011 Å2s−1 at 21 and 34 °C, respectively. These values are close to the ones reported in literature,68,69 although higher70−72 and lower values73 have been reported as well. A possible reason for this discrepancy is the sensitivity of the results to the selected energy resolution. In any case, the values presented here will be used as reference in the next steps of our analysis. The values of τ are 0.039 ± 0.005 ps and 0.059 ± 0.008 ps at 21 and 34 °C, respectively. The comparison of the diffusion dynamics of bulk water and bulk methanol reveals that the latter diffuses faster, which seems contradictory in view of their molar volumes. This result implies that, rather than the molar volume, the coordination number is the decisive factor for molecular diffusion−the water molecules form a more complex, three-dimensional network than the methanol molecules, which form a simpler, linear one.74 D2O:MeOH Mixture. In the D2O:MeOH mixture, slower processes than in bulk MeOH are observed (Figures S3−S5). We find values of D = (1.2 ± 0.2) × 1011 Å2s−1 and (1.7 ± 0.2) × 1011 Å2s−1 at 21 and 34 °C, respectively, thus significantly lower than the values in bulk MeOH. The absolute values are compatible with the ones reported in the literature.75,76 As expected from complex formation, the residence time τ of MeOH in D2O:MeOH is increased compared to bulk MeOH: Values τ = 0.35 ± 0.05 ps and 0.40 ± 0.08 ps are found at 21 and 34 °C, respectively. PNIPAM Solution in D2O:MeOH. Figure 8a shows a representative QENS spectrum of the 25 wt % PNIPAM solution in 85:15 v/v D2O:MeOH at 21 °C. The analysis of the experimental data requires the use of three Lorentzians. The broadest Lorentzian is attributed, as before, to local motions of the solvent molecules and will not be discussed further. The other two contributions are assigned to the long-range diffusion of two different populations of solvent molecules with distinctive molecular mobilities. The q-dependences of their widths Γ1 and Γ2 are analyzed using eq 3 (Figures 8b and c), and the resulting diffusion coefficients D for the two populations, the residence times τ, and the peak areas A are given in dependence on temperature in Figure 9. Starting from the diffusion coefficient D (Figure 9a), it is seen that the slowest population of solvent features diffusion coefficients, D = (0.3 ± 0.1) × 1011 Å2s−1, throughout the entire temperature range, which are strongly reduced compared to the ones in D2O:MeOH and which are not affected by the cloud point. Thus, this (strongly arrested) solvent population interacts strongly with the PNIPAM chain. The value of D is very similar to the one of H2O in the concentrated solution of PNIPAM in H2O:d-MeOD (Figure 6a). This indicates that a strongly associated methanol fraction exists. However, one cannot exclude that the signal is, at least partially due to a fraction of HDO which has been formed by H/D exchange. The contributions of MeOH and HDO cannot be separated, e.g., by adding another component to the model function (eq 1), which would result in unstable fits. The dynamics of the faster solvent population features values of D, which increase slightly with temperature from (1.7 ± 0.1) × 1011 Å2s−1 at 21 °C to (2.2 ± 0.2) × 1011 Å2s−1 at 32 °C (Figure 9a). These values are slightly higher compared to the ones found for D2O/ MeOH. We suggest that this solvent population may be located

Figure 8. (a) QENS spectrum S(q,ω) versus energy transfer ΔE of the 25 wt % solution of PNIPAM in 85:15 v/v D2O:MeOH at 21 °C and q = 1.1 Å−1. Open black squares, experimental data; solid red line, full fit; dashed green line, long-range diffusion of solvent molecules with strongly arrested dynamics; dash-dotted blue line, long-range diffusion of solvent molecules with arrested dynamics; dash-double-dotted red line:, superposition of local motions of the solvent molecule; and magenta solid line, elastic component. (b) The q-dependence of the width of the Lorentzian describing the solvent population with strongly arrested dynamics and (c) the one with arrested dynamics for representative temperatures. The solid lines are fits of eq 3.

in hydration layers of higher order or at solvation sites of the chain. It may be considered as being loosely bound to PNIPAM. Thus, it seems that also loosely bound methanol is present in the solution. The residence time τ of the strongly arrested solvent population is τ = 3.2 ± 0.3 ps, independent of temperature (Figure 9b); this value is a factor of 8−9 higher than the one in D2O:MeOH, which supports the picture of MeOH being directly bound to PNIPAM. The faster, loosely bound solvent population has τ = 0.25 ± 0.11 ps i.e. similar to the value in D2O:MeOH, it is thus highly mobile. Since both the diffusion coefficients and the residence times observed in the PNIPAM solutions in D2O:MeOH are similar to the ones in the PNIPAM solutions in H2O:d-MeOD (Figure 6a,b), we conclude that the H/D exchange in D2O:MeOH hampers an unambiguous assignment of the dynamic process in the PNIPAM solutions in D2O:MeOH to MeOH. The relative intensity of the strongly bound solvent fraction and the fact that no additional processes are found support a very similar dynamics of water and methanol near the PNIPAM chain. The number of strongly bound solvent molecules, as reflected in the relative amplitude of the contribution (Figure 9c), decreases above Tcp, in favor of the loosely bound population. This transition spans over several Kelvin, as observed for H2O in the solution of PNIPAM in H2O:dMeOD. F


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Figure 9. Results from the 25 wt % solution of PNIPAM in D 2 O:MeOH. Temperature dependence of the (a) diffusion coefficients D, (b) the residence times τ, and (c) the relative peak amplitudes A of the respective Lorentzians for solvent molecules with strongly arrested dynamics (half-filled blue symbols) and with arrested dynamics (dotted red symbols). In parts (a) and (b), the values of bulk MeOH (solid black symbols) and of D2O:MeOH at 85:15 v/v (open black symbols) are given as well.

Figure 10. (a) The q-dependence of the intensity of the elastic component at the temperatures given in the graph for the 25 wt % solution of PNIPAM in 85:15 v/v D2O:MeOH. The solid lines are fits of eq 4. (b) Temperature dependence of the mean-square displacement ⟨u2⟩ of the vibrational mode of the PNIPAM chain. The red solid line is a guide to the eye. The dashed black line marks Tcp.

In analogy with this previous study, the PNIPAM concentration has been chosen to be as high as 25 wt % to ensure that most solvent molecules are in the vicinity of PNIPAM. In the solution of PNIPAM in pure water, studied below and above its cloud point, three types of water were identified:12 strongly associated water (which was detected indirectly), loosely associated water and almost freely diffusing water. The dynamics of both strongly and loosely associated water are both described by jump diffusion. In the present study, strongly associated water is identified as well, which exhibits severely reduced diffusion dynamics. At the cloud point, a fraction of this population is liberated, whereas the rest remains attached to the PNIPAM chain, even far above Tcp. A second water population is detected as well, namely loosely associated water, exhibiting dynamics between the one of freely diffusing and strongly associated water. Moreover, its diffusion coefficient is similar to the one in H2O:d-MeOD. We postulate that this population consists of water molecules that participate in hydration layers of higher order. In the solution of PNIPAM in pure water, the diffusion coefficient of the loosely associated water increases slightly above the cloud point and its residence time decreases 4 K above the cloud point.12 Its fraction is nearly constant over the entire temperature range. The diffusion coefficient of the faster water population in the solution of PNIPAM in water/ methanol is similar to the one in the solution of PNIPAM in pure water, and again, the residence time on the PNIPAM chain is reduced. The fraction of this population increases slightly above the cloud point. The measurements on the water/methanol mixture where the methanol signal is expected to dominate reveal that, indeed, the methanol dynamics is slowed down compared to the one in

The dynamics of the PNIPAM chain, as evident from the intensity of the elastic line is presented in Figure 10. The mean square displacement ⟨u2⟩ of the chain decreases above Tcp, due to the collapse. Again, the transition from the swollen to the collapsed state occurs over several Kelvin. The values are very similar to the ones of PNIPAM in H2O:d-MeOD (Figure 7b), i.e., the chain dynamics are unaffected by the difference in solvent deuteration, as expected.

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DISCUSSION The cononsolvency phenomenon displayed by PNIPAM in water/methanol mixtures has been discussed vividly in recent years. A number of mechanisms have been put forward which relate to the interaction between the solvents and PNIPAM as well as to the interactions of the solvents among each other. The binding of the solvents to PNIPAM is a key aspect of the cononsolvency phenomenon: modification of the hydrophobic hydration by methanol,36 cooperative binding of water and methanol to PNIPAM in sequences,28,29,33 and bridging of PNIPAM chains by methanol have been discussed.34 A separate characterization of the diffusional behavior of the two solvents in a solution of PNIPAM in a water/methanol mixture may further the understanding. QENS allows us to determine the diffusion coefficient and the residence time of the solvents as well as the mean-square displacement of the PNIPAM chain. Contrast variation enables the selective investigation of the dynamics of water in the solution of PNIPAM in water/ methanol. Moreover, it is instructive to compare the results with those from the pure solvents, the water/methanol mixture and with the ones from our previous study on a concentrated solution of PNIPAM in water.12 G


The Journal of Physical Chemistry B pure methanol, which confirms the complex formation which was discussed as one of the reasons for cononsolvency.27 In the presence of PNIPAM, two populations of solvent are identified: one with strongly arrested dynamics, i.e., strongly bound to PNIPAM, and one with faster dynamics. For both species, both the diffusion coefficients and the residence times are similar to the ones of the strongly and loosely associated water species discussed above. We suppose that the association behavior of methanol and PNIPAM is very similar to the one of water and that methanol is present in all hydration shells,34,40 although the observed dynamics is, at least partially, due to HDO which has formed by H/D exchange. The presence of methanol close to PNIPAM, directly bound or in the hydration shells, may explain our recent findings on the kinetics of aggregation of micelles formed by PS-bPNIPAM (PS stands for polystyrene) in D2O or in D2O:dMeOD after heating through the cloud point.43 We attributed the faster aggregation of the micelles in D2O:d-MeOD to the perturbation of the hydration shell by methanol. This is consistent with the present findings. The mean-square displacement of the PNIPAM chain in both contrast conditions behaves differently from the one in the PNIPAM solution in pure water:12 Whereas it decreases abruptly in the solution of PNIPAM in pure water, it decreases more gradually in the solution of PNIPAM in water/methanol; the chain collapse in water/methanol thus seems to proceed more smoothly than in pure water. This finding may be in accordance with the finding in computer simulations that methanol forms bridges between different segments along the chain, resulting in its gradual collapse.34

ACKNOWLEDGMENTS

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REFERENCES

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CONCLUSIONS We conclude that the solvent dynamics in concentrated PNIPAM solutions in water/methanol mixtures are complex. Both methanol and water show similar slow dynamics, possibly both forming hydrogen bonds with the amide group of PNIPAM. This population is affected by the cloud point, in contrast to the less strongly bound populations. We postulate that models which attribute the cononsolvency effect only to water/methanol interactions or only to solvent/polymer interactions do not fully describe the situation, but rather both aspects play a role. QENS together with contrast variation is a powerful tool to detect and characterize the local diffusional processes of different populations of solvent and is complementary to spectroscopic techniques highlighting the dynamics of moieties belonging to the PNIPAM chain. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b01200.

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We gratefully acknowledge fruitful discussions with A. Kyritsis. This work was supported by the DFG priority program SPP1259 “Intelligente Hydrogele” (Pa771/4, Mu1487/8). This work is based upon experiments performed at the TOFTOF instrument operated by Technische Universität München at the Heinz Maier-Leibnitz Zentrum (MLZ), Garching, Germany.

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Article

QENS results of pure MeOH and of the D2O:MeOH mixture (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +49 089 289 12447. Fax: +49 089 289 12473. E-mail: [email protected] (C.M.P.). Notes

The authors declare no competing financial interest. H


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