Dynamics of Linear Poly (N-isopropylacrylamide) in Water around the

Article pubs.acs.org/JPCB

Dynamics of Linear Poly(N‑isopropylacrylamide) in Water around the Phase Transition Investigated by Dielectric Relaxation Spectroscopy Marieke Füllbrandt,†,‡ Elena Ermilova,§ Asad Asadujjaman,† Ralph Hölzel,§ Frank F. Bier,§ Regine von Klitzing,‡ and Andreas Schönhals*,† †

BAM Bundesanstalt für Materialforschung und -prüfung, Unter den Eichen 87, 12205 Berlin, Germany Stranski-Laboratorium für Physikalische und Theoretische Chemie/Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany § Fraunhofer-Institut für Biomedizinische Technik IBMT, Institutsteil Potsdam-Golm, Am Mühlenberg 13, 14476 Potsdam-Golm, Germany ‡

ABSTRACT: The molecular dynamics of linear poly(N-isopropylacrylamide) (pNIPAM) in aqueous media at temperatures below and above the lower critical solution temperature (LCST) are investigated using broadband dielectric relaxation spectroscopy in a frequency range from 10−1 to 1011 Hz. Below the LCST, two relaxation processes are observed in the megahertz and gigahertz region assigned to the reorientation of dipoles of the solvated polymer segments (pprocess) and water molecules (w-process), respectively. Both relaxation processes are analyzed using the Havriliak−Negami (HN) function, taking special attention to the w-process. Above the LCST, the dielectric spectra of the pNIPAM solutions resemble that of pure water, showing only the high frequency relaxation process of the water molecules with a more or less Debye-type behavior. The non-Debye behavior of the w-process below the LCST is mainly induced by the interactions between water and pNIPAM chains via hydrogen bonding. The relaxation time and strength of the w-process is studied with dependence on the concentration, temperature, and the polymer chain length (molecular weight). The information obtained is useful for a deeper understanding of the dehydration behavior at the phase transition. The suggestion of dehydration of the pNIPAM chains at the LCST is confirmed by calculating a dehydration number.

1. INTRODUCTION Poly(N-isopropylacrylamide) (pNIPAM) is a well-known thermosensitive polymer whose aqueous solutions exhibit a lower critical solution temperature (LCST) around 32 °C.1,2 At the LCST, pNIPAM undergoes a reversible coil-to-globule transition where polymer chains pass from a swollen coil conformation to a globular one. This transition phenomenon is related to the competition between two molecular effects: the hydrophobicity of the isopropyl groups as well as that of the backbone on one hand and the hydrogen bonding between the amide groups and the water molecules (solvent) on the other hand.3 From a more macroscopic point of view the phase transition is accompanied by a phase separation process leading to the formation of a kind of microsuspension above the LCST. The phase transition mechanism of pNIPAM is widely studied because of its importance in many biological aspects such as protein folding, DNA packing, and interchain complexation.4,5 Moreover, it is also related to the collapse of a gel network.6−8 Potential applications in biotechnology and medicine,9,10 including drug release and diagnostics, chemical separation, catalysis, surface modification, etc. have been suggested. There exist a variety of publications about pNIPAM in solution studied by different methods such as (light) scattering methods,11−13 infrared spectroscopy,14−16 differential scanning calorimetry,17−19 fluorescence microscopy,20 turbidimetry,18,21 and NMR.22−24 However, only a little research focuses on the © 2014 American Chemical Society

dynamics of concentrated solutions above and below the lower critical solution temperature. In that case polymer/water and polymer/polymer interactions will play a role. Broadband dielectric spectroscopy is a useful technique for investigating the dynamics of polymers and their solutions.25 Moreover, in contrast to other methods, a wide concentration range can be covered. Studies of concentrated pNIPAM solutions, i.e., above its overlapping concentration where different polymer coils start to interpenetrate, are possible. Dielectric spectroscopy is also a suitable tool to study biological systems,26,27 for example protein−water interactions28,29 and other aqueous mixtures.30−32 Ono et al.33,34 applied dielectric spectroscopy to study the hydration and dynamic behavior of NIPAM and its polymeric counterpart pNIPAM in aqueous solutions in a frequency range of 107 to 1011 Hz between 6 and 39 °C. The temperature dependence of the (de)hydration behavior of the monomer NIPAM and the polymer pNIPAM was investigated by analyzing the dynamics of water molecules hydrated to the solute. The molecular dynamics of pNIPAM in various solvents at one temperature below the LCST (25 °C) was studied by Nakano et al.35 They observed two relaxation processes that Received: February 6, 2014 Revised: March 3, 2014 Published: March 4, 2014 3750

dx.doi.org/10.1021/jp501325x | J. Phys. Chem. B 2014, 118, 3750−3759

The Journal of Physical Chemistry B

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were assigned to the fluctuations of solvated pNIPAM segments (around 106 Hz) and to the ones of solvent molecules that are influenced by the polymer segments (around 1010 Hz). However, a detailed study about the molecular (including the segmental) dynamics of pNIPAM with respect to the temperature and concentration dependence above and below the LCST is still missing. Recently, dielectric spectroscopy in the lower frequency regime (10−1 to 106 Hz) has been also employed to study the phase transition of linear pNIPAM and pNIPAM microgels in aqueous solution.36,37 There, the phase transition is deduced both by a change in temperature dependence of dc conductivity at the LCST and by the frequency dependence of the real part of the complex conductivity where the collapse of the pNIPAM system in water provokes interfacial polarization effects. Besides dielectric spectroscopy, neutron scattering has also been proven to be useful in investigating the dynamics of polymers.38−40 Especially, neutron spin echo studies41 are employed to investigate the diffusive motional processes on a spatial scale of chains or subchains for pNIPAM. But there are only few publications considering vibrations and molecular fluctuations on the length scales of group and segments42 by employing higher values of the scattering vector Q. In this study, broadband dielectric measurements were performed on aqueous solutions of linear pNIPAM with varying concentrations in the frequency range of 10−1 to 1.1 × 1011 Hz at temperatures from 15 to 50 °C. The relaxation time and strength obtained from the dielectric spectra are discussed with dependence on the concentration, the temperature and the chain length.

(Novocontrol, Hundsangen, Germany). The temperature is controlled by a Quatro Novocontrol cryo system with a temperature stability of 0.1 K. The polymer solutions are filled in a liquid sample cell supplied by Novocontrol (BDS 1309) with a diameter of 11 mm. The cell is made up of two gold plated electrodes with a Teflon cylinder as spacer in between. The electrode spacing of 5.5 mm is relatively wide to shift the effect of electrode polarization to lower frequencies. From 1 MHz to 1 GHz measurements were carried out by means of a coaxial reflectometer based on the Agilent E4991 analyzer. The samples were prepared in parallel plate geometry between two gold-plated electrodes with a diameter of 3 mm and a spacing of 1.5 mm. The capacitor was mounted as a part of the inner conductor of the transmission line. In brief, the impedance of the specimen is estimated from a complex reflection coefficient Γ*(ω) defined by the ratio of the complex voltages of the incident (U*Inc) and reflected (U*ref) waves: Γ* = Γx − iΓy =

1 iωZ*(ω)C0

Z* = Z 0

1 + Γ* 1 − Γ*

(2)

Z0 is the wave resistance of the coaxial line. To obtain correct values for Z*(ω), calibration procedures have to be carried out. First, the direction-dependent resistance of the line has to be measured employing an open, short, and 50 ohm (wave resistance) load calibration kit. The influence of the measuring cell is obtained by a second short and open calibration and is considered during the calculation of the sample impedance. The temperature of the sample was controlled by a Quatro temperature controller (Novocontrol) with nitrogen as a heating agent providing a temperature stability that was better than 0.1 K. To avoid any effects of water evaporation, a new drop of solution (from the same stock solution) was used at each temperature measured. In the frequency domain from 10 MHz to 110 GHz a vector network analyzer (VNA) Anritsu MS 4647A also in reflection mode was used. The VNA operates between 70 kHz and 70 GHz. To reach 110 GHz, the experimental system was extended with two microwave generators Anritsu MG37022, a broadband test set (Anritsu 3738A), a coupler (Anritsu WR10 66670-3), and a transmission-reflection module (Anritsu 3740A-EW). As a sensor for reflection measurements, an open-ended coaxial line prepared from a 1.19 mm precision 50 ohm semirigid coaxial cable (CA100FF, Kawashima Manufacturing Co., Ltd., Japan) with solid polytetrafluoroethylene (PTFE) was used. The sensor was connected to the output of the microwave coupler by a short (15 cm) and flexible coaxial cable (Gore, 3671 W1-50-3). With the use of W- type connectors TEM operation of the cables up to 112 GHz is guaranteed. The coaxial probe was immersed into the sample and the complex reflection coefficient Γ*(ω) = S11eiφ was measured. S11 and φ are the amplitude and the phase of the reflection coefficient, respectively. Solutions were placed in Eppendorf tubes with a volume of 1 mL and temperature controlled by a test tube incubator (Peqlab Thriller) with a temperature stability of 0.5 K. The system was calibrated by the standard one-port calibration procedures with the reference plane of the network analyzer set at the end of the 15 cm flexible coaxial cable based on W1 calibration kit (Model 3656B, Anritsu). The complex permittivity ε*(ω) was extracted from the measured reflection coefficients by43,44

2. EXPERIMENTAL SECTION Sample Preparation. Linear poly(N-isopropylacrylamide) (pNIPAM) with two different molecular weights were studied. The number-averaged molecular weight Mn and the polydispersity index (PDI) were obtained by gel permeation chromatography (GPC) and matrix-assisted laser desorption/ ionization time-of-flight mass spectroscopy (MALDI-TOF). The samples are named 1pNIPAM (Sigma Aldrich, Mn = 2.5− 3.0 kg/mol, PDI = 1.8) and 2pNIPAM (Polymer Source Inc., Mn = 100−150 kg/mol, PDI = 2.63) in the following. Aqueous pNIPAM solutions with different concentrations (5−20 wt %) were prepared by dissolving a defined amount of polymer in Milli-Q water (Millipore, USA). The pH value of the aqueous polymer was around 4 and 7.5 for 1pNIPAM and 2pNIPAM solutions, respectively. The difference in pH is probably due to the different polymer end groups and (radical) starter used in polymer synthesis. Dielectric Measurements. In general, the complex dielectric function ε*(ω) is derived by measuring the complex impedance Z*(ω) of the sample ε*(ω) = ε (́ ω) − iε (́ ω) =

U *ref U *Inc

(1)

where ε′ is the real part, ε″ is the loss part, ω is the angular frequency (f = 2πω), i = (−1)1/2, and C0 is the capacitance of the empty sample capacitor. The measurements were performed in a frequency range of 0.1 Hz to 110 GHz and in a temperature range of 15 to 50 °C (in steps of 5 K) combining three different measurement systems. In the frequency range from 0.1 Hz to 1 MHz, measurements are conducted using a high resolution ALPHA analyzer 3751



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Figure 1. Real ε′ part of the dielectric function of pure water (blue Δ) and an aqueous solutions of 1pNIPAM with 15 wt % polymer (green □) below the LCST at 25 °C (left panel) and above the LCST at 40 °C (right panel). The solid black line is a HN fit to the data according to eqs 4 and 5. For the pNIPAM solution the single contributions are calculated using eq 5 plus a conductivity contribution: dotted line = electrode polarization; dashed line = p-process; dot-dashed line: w-process.

ε*(ω) =

a1Γ*(ω) − a 2 a3 − Γ*(ω)

increase in ions due to water release during the phase transition. As a result, interfacial polarization effects become stronger. Similar findings were also reported by Gómez-Galván et al.50 For temperatures below the LCST, an additional process is observed between the high frequency process and electrode polarization indicated by a step-like increase of ε′ with decreasing frequency around 1 MHz. This relaxation process is assigned to the reorientation of dipoles of the solvated polymer segments (polymer process, p-process) whereas the process in the gigahertz region comes from the rotational fluctuations of water molecules in presence of pNIPAM chains (water process, w-process). These findings correspond to results found by Nakano et al.35 They studied pNIPAM only at 25 °C in different solvents and identified two relaxation processes in the same frequency range as discussed above. To analyze the data in a quantitative way, the model function of Havriliak−Negami51,52 (HN function) is employed.53 The whole relaxation function reads

(3)

where ai with i = 1−3 are complex coefficients estimated from reference measurements of ultrapure water with electrical conductivity lower than 0.1 μS/cm (dielectric parameter were taken from ref 45) for open and shorted coaxial sensors, respectively. Open was obtained by measurements in air with Γ = 1. For the short-circuit at the probe’s open end, freshly polished metal sheets of aluminum were used. Γ = −1 applies to the shortened end of the coaxial probe. Because of the temperature sensitivity of the semirigid coaxial cables filled with PTFE, all measurements including the reference once were performed after stabilization of the measured reflection coefficient at each temperature value.46 The data measured by the three setups agree with each other (Figures 1 and 2). As it is known, pNIPAM solutions show a phase separation at T > LCST leading to some kind of microsuspensions that are in fact inhomogeneous solutions. Therefore, measurements at higher temperatures were performed carefully and it was assured that in all cases the edge of the probe contacts the polymer phase (microsuspension) and not the pure water phase. Moreover, sample solutions were inspected by eyes to check for possible polymer sedimentation which was, however, not observed.

ε*(f ) = ε∞ +

σ0 Δε + EP ·f −m − i β ⎞γ ⎛ (2πf )s ε0 ⎛ f⎞ ⎜1 + ⎜i f ⎟ ⎟ ⎝ 0⎠ ⎠ ⎝ (4)

where f 0 is a characteristic frequency related to the frequency of maximal loss f P (relaxation rate), ε∞ describes the value of the real part ε′ for f ≫ f 0, and Δε is the dielectric strength. β and γ are fractional parameters (0 < β and 0 < γβ ≤ 1) characterizing the shape of the relaxation time spectra. The Debye function is recovered for β = γ = 1. A β-value smaller than one corresponds to a symmetrical broadening of the relaxation function compared to a Debye spectra whereas a γ-value smaller than 1 will lead to an asymmetrical broadening of the relaxation time spectra at the high frequency side. Electrode polarization effects are described by adding a term EP·f−m to the real part of the fitting function,25 where EP and m are constants (EP > 0, m > 1). Conduction effects (see below) are treated by adding a contribution (σ0/[(2πf)sε0]) to the dielectric loss where σ0 is related to the specific dc conductivity of the sample and ε0 is the dielectric permittivity of vacuum. The parameter s (0 < s < 1) describes for s = 1 ohmic and for s < 1 non-ohmic effects in the conductivity. For details see ref 25. For temperatures above the LCST, the data are well described by a single HN function (eq 4) as demonstrated in

3. RESULTS AND DISCUSSION Figure 1 compares the real part ε′ of the complex dielectric function versus frequency for an aqueous solution of 1pNIPAM with 15 wt % polymer with that of pure water. Pure water shows for both temperatures one relaxation process characterized by a step-like increase of ε′ with decreasing frequency around 19−20 GHz, as is known from the literature.47 The further increase of the real part ε′ of the complex dielectric function at lower frequencies ( f < 104 Hz) is due to electrode polarization effects48,49 caused by the accumulation of charges at the surface of the electrodes. For temperatures above the LCST, the behavior of the pNIPAM solution is similar to that of water, where only one relaxation process is observed. Due to a higher dc conductivity of the pNIPAM solution compared to water, electrode polarization is shifted to higher frequencies. Below the LCST, these effects start at f < 105 Hz whereas, above the LCST, the increase of ε′ is already observed at f < 106 Hz. That can be explained by the 3752



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Figure 2. Real ε′ and imaginary ε″ part of the complex dielectric function of pure water (a, b) and an aqueous solution of 1pNIPAM with 20 wt % polymer (c, d) at different temperatures (closed symbols, below LCST; open symbols, above LCST): 15 °C ( yellow ●), 20 °C (pink ■) 25 °C (green ▲), 30 °C (turquoise ▼), 35 °C (black □), 40 °C (red ○), 45 °C (dark yellow △), 50 °C (blue ◇). The insets in (b) and (d) show the shift of the relaxation peak with temperature on an enlarged frequency scale.

shown in Figure 2 at different temperatures. For water the observed behavior was as expected. At the temperature of 25 °C a relaxation process indicated by a peak in the dielectric loss is observed around 19−20 GHz, which shifts to higher frequencies with increasing temperature related to the rotational fluctuation of water molecules (inset Figure 2b).47,54,55 As known, its dielectric strength decreases with increasing temperature.25,56 Compared to water, for the aqueous polymer solutions at the same temperature the relaxation process broadens, and it is shifted to lower frequencies. At temperatures below the LCST, the temperature dependence of the relaxation and the dielectric strength in intensity is comparable to the behavior for water. But in contrast to pure water at the phase transition the intensity of the relaxation process starts to increase followed by a further decrease at even higher temperatures (see inset Figure 2d). Figure 3 depicts an example of fits of the HN function (eq 4) to the dielectric data of 1pNIPAM in water (c = 20 wt %) at 20 and 45 °C. As known from the literature, the dielectric spectra of pure water can be described by a Debye-type behavior (β = γ = 1).57 From the fit the characteristic relaxation frequency f 0, the dielectric strength Δε, and the shape parameters are obtained. The position of maximal loss f P is given by58

Figure 1, right panel. However, as is already obvious from the raw data below the LCST, besides electrode polarization and conductivity, the dielectric response of the polymer solutions has to be described by fitting two HN functions to the data in addition to conductivity and electrode polarization effect: ε*(f ) = ε∞ +

Δεp

γp ⎛ ⎛ ⎞ βp⎞ ⎜⎜1 + ⎜i f ⎟ ⎟⎟ ⎝ f0,p ⎠ ⎠ ⎝ σ0 + EP ·f −m − i (2πf )s ε0

+

Δεw γw ⎛ ⎛ ⎞ βw ⎞ ⎜⎜1 + ⎜i f ⎟ ⎟⎟ ⎝ f0,w ⎠ ⎠ ⎝ (5)

The subscripts p and w denote the p-process and w-process, respectively. Exemplary fits are shown in Figure 1 for a polymer solution at 25 °C (left panel) and 40 °C (right panel) using eqs 5 and 4, respectively. Note that Δεp ≪ Δεw. The analysis of the p-process delivers comparable results as discussed in ref 35. With regard to its frequency position it is located in the region where dielectric data from two measurement systems were combined together. Therefore, the data have a larger scattering, preventing a precise analysis of the p-process. Moreover, a detailed analysis of this process is given in ref 35. Hence here the focus is on the w-process observed in the gigahertz region, which is assigned to the rotational fluctuation of water molecules. This process is influenced by the presence of the polymer chains in solution and is discussed in the following in more detail. Typical dielectric spectra of the dielectric loss ε″ and the real part ε′ of the dielectric function versus frequency for the wprocess of a 1pNIPAM sample (c = 20 wt %) and water are

−1/ β 1/ β ⎡ βπ ⎤ ⎡ βγπ ⎤ fp = f0 ⎢sin ⎥ ⎥ ⎢sin ⎣ 2 + 2γ ⎦ ⎣ 2 + 2γ ⎦

(6)

Below the LCST, the dielectric spectra of the pNIPAM solution are well described by the HN function with β < 1 and γ < 1. Above the LCST, the spectra resemble that of water with an almost Debye-like behavior (β ≈ γ ≈ 1). Moreover, the p3753



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Figure 4. Relaxation rate f P over 1000/T for 1pNIPAM with c = 20 wt % (turquoise ⬢), 15 wt % (green ◆), 10 wt % (blue ●), 5 wt % (red ▲), and water (black ■). The gray zone marks the phase transition region for the polymer solutions. The solid and dashed lines are fits of the Arrhenius equation to the data for the pNIPAM solutions and water.

Figure 3. Dielectric loss ε″ as a function of frequency for 1pNIPAM with c = 20 wt % for 20 °C (■) and 45 °C (○). The lines are fits of one HN function to the data including a conductivity contribution: solid line, total fit; dashed line, HN; dotted line, conductivity contribution. The data for the two different temperatures are shifted at the y-scale for sake of clarity.

Table 1. Activation Energies EA for Water and the pNIPAM Solutions below and above the LCST Calculated from Arrhenius Fits Shown in Figure 4

process due to the molecular fluctuations of solvated pNIPAM segments seems to disappear. This behavior is explained by a dehydration process of the polymer at the LCST. Water is pressed out from the solvated coil, it collapses, and phase separation occurs. The free water molecules gained through this process contribute to the w-process whereas the fluctuations of the pNIPAM segments responsible for the p-process are hindered in the collapsed state and therefore no longer observed in the dielectric spectra within the experimental uncertainty. The rotational relaxation time of pure water at 25 °C was measured to be 8.03 ps in agreement with literature values (8.3 ps at 25 °C)59−61 corresponding to a relaxation rate of 19.8 GHz. As expected, the relaxation rates increase with increasing temperature. The temperature dependence of f P of water as well as that of pNIPAM in solution at temperatures below the LCST follows the Arrhenius equation indicating a thermal activated behavior: ⎛ −E ⎞ fP (T ) = f∞ exp⎜ A ⎟ ⎝ kBT ⎠

activation energy EA [kJ/mol] below LCST above LCST

water

1pNIPAM 5 wt %

1pNIPAM 10 wt %

1pNIPAM 15 wt %

1pNIPAM 20 wt %

18.3

19.8

22.2

21.9

23.9

19.4

an activation energy of 25 kJ/mol for a pNIPAM solution below the LCST.62 Probably the w-process for the polymer solutions reflects the averaged rotational relaxation mode of two kinds of water molecules: free molecules behaving like pure water and those which are interacting with the pNIPAM segments. The activation energy of the pNIPAM solutions is related to the energy necessary to break hydrogen bonds between hydrated water molecules and pNIPAM segments.33 Above the LCST, the activation energy of the polymer solutions decreases (EA = 19.4 kJ/mol) and approaches the EA value for pure water (18.3 kJ/mol). Figure 5 gives the concentration dependence of the relaxation rate for the w-process for two temperatures, below and above the LCST. In the latter case f P is independent of the

(7)

where EA is the activation energy, f∞ is the pre-exponential factor that corresponds to the relaxation rate at infinite temperature, which might relate to nonhindered rotation relaxation of water molecules, T is temperature, and kB is the Boltzmann constant. In Figure 4 the relaxation map is shown for aqueous 1pNIPAM solutions at different concentrations (5−20 wt %) and water. For water the fit yields an activation energy of EA = 18.3 kJ/mol, which compares well to literature values for pure water where EA ≈ 19 kJ/mol is given.59 For the pNIPAM solutions a discontinuity in the dependence of log f P versus 1/T in the phase transition region occurs. Therefore, the temperatures below and above the LCST were treated individually and separate Arrhenius fits were performed for the two different temperature regions. For temperatures above the LCST, the data points for all concentrations collapse more or less into one chart. These data can be described by a common fit yielding an activation energy similar to that for water. The corresponding activation energies EA are listed in Table 1. Below the LCST, EA increases with increasing concentration of pNIPAM. The values are higher than the activation energy for pure water. Ono et al.33 reported

Figure 5. Concentration dependence of the relaxation rate f P for the w-process at 25 °C (black ■) and 45 °C (black ▲). For comparison, literature values from ref 35 (red ○) are plotted at 25 °C. The solid lines are linear fits to the data; the dashed line represents the averaged value of all data at 45 °C. 3754



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Figure 6. Shape parameters β (left panel) and γ (right panel) versus 1000/T for aqueous 1pNIPAM solutions with c = 20 wt % (turquoise ⬢), 15 wt % (green ◆), 10 wt % (blue ●), 5 wt % (red ▲), and water (black ■). The gray zone marks the phase transition region for the polymer solutions. The dashed lines represent the averaged value of the pNIPAM data below the LCST. The dotted lines and the arrows in the gray zones are only guides to the eyes showing the general increase of β and γ in the phase transition regime.

polymer concentration and equal to that of pure water as it can be already seen from the raw data. For temperatures below the phase transition temperature the relaxation rate decreases with increasing polymer content of the solution. The rotational fluctuations of the water molecules are slowed down by the presence of the polymer segments. Figure 5 also compares the concentration dependence of the relaxation rates of the data obtained here with that given in ref 35. The absolute values as well as the concentration dependence of both data sets are almost the same. The decrease of the relaxation rate of the w-process with increasing pNIPAM concentration can be explained by an increasing amount of hydrogen bonds between the pNIPAM segments and the water molecules as suggested in ref 35. Further with increasing concentration of pNIPAM there will be less free volume available for the rotational fluctuations of water. For pure water it is well know that the relaxation process due to reorientation of the water molecules can be perfectly described by a Debye-process, this means β = γ = 1. This result was recovered by the investigations presented here (Figure 6). For the pNIPAM solution at temperatures below the LCST, the w-process broadens where both a symmetrical and asymmetrical broadening is observed (β < 1, γ < 1; Figure 6). This broadening is not due to the presence of the p-process. It was also found for aqueous solutions of small molecules.63 Feldman et al.63 explained this broadening by molecule (solved molecule)/matrix (water) interaction. A similar interpretation also accounts for the data presented here due to the increasing amount of hydrogen bonds between the pNIPAM segments and the water molecules. With increasing concentration of pNIPAM in the solution both the symmetrical and asymmetrical broadening increases (Figure 7), which can be explained by an increase of the interaction of the solved molecules with the water molecules. At the phase transition the behavior changes, the relaxation spectrum narrows and becomes symmetrically (Figure 6). For temperatures above the LCST, a nearly Debye-like process is recovered. A small symmetrical broadening remains that increases with increasing pNIPAM concentration in solution (inset Figure 7). The dielectric strength Δε versus inverse temperature is shown in Figure 8. Below the LCST, Δε decreases with increasing polymer concentration due to a decrease in the number density of free water molecules. A part of the water molecules are bound to the pNIPAM segments and thus no

Figure 7. Concentration dependence of the shape parameters β (black ■) and γ (red ▲) at 25 and 45 °C (inset). The dashed lines are linear fits to the data and only guides to the eyes.

Figure 8. Dielectric strength Δε versus 1000/T for aqueous 1pNIPAM solutions with c = 20 wt % (turquoise ⬢), 15 wt % (green ◆), 10 wt % (blue ●), 5 wt % (red ▲), and water (black ■). The gray zone marks the phase transition region for the polymer solutions. The straight and dashed lines are linear fits to the data for pNIPAM and water, respectively. The black dotted line and the arrow are only guides to the eyes showing the general increase of Δε in the phase transition regime.

longer contribute to the w-process. The number of bound water molecules increases with increasing pNIPAM content which results in a reduction of Δε. Below the LCST, Δε decreases with temperature (Δε ∼ 1/T), as expected. In the phase transition region (gray zone Figure 8) a discontinuous behavior of Δε versus 1/T is observed where Δε increases with 3755



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T approaching the value for pure water at T = 40 °C. During the phase transition the water molecules are released from the segments of the collapsing coil. Therefore, the number of free water molecules increases and with that also Δε. At higher temperatures (T > 40 °C) Δε decreases again with T and the curves for the pNIPAM solutions and water collapse more or less into one single curve. Due to experimental difficulties the values for the 20 wt % pNIPAM solution above 40 °C have a larger experimental error. For a better visualization the concentration dependence of the relaxation strengths Δε below the LCST (at 25 °C) and above the LCST (at 45 °C) are plotted in Figure 9.

water cb,water can be obtained using the Cavell equation66 which normalized to pure water results in67,68 cb,water =

Δεpolymer ρW Δεwater MW

(8)

where ρW is the density and MW the molar mass of pure water. A comparison of cb,water with the analytical water concentration ca,water and normalization to the polymer concentration cpNIPAM results in the hydration number NHyd:68 NHyd =

ca,water − cb,water M n NIPAM · 1pNIPAM c pNIPAM Mn

(9)

Figure 10 shows the hydration number per monomer unit NIPAM NHyd calculated from the dielectric strength values

Figure 9. Concentration dependence of the dielectric strength Δε for the w-process at 25 °C (black ■) compared to literature values from ref 35 (red ○) at 25 °C. The solid lines are linear fits to the data. The inset shows the concentration dependence of Δε at 45 °C (black ▲). The dashed line represents the average value of all data at 45 °C.

Figure 10. Temperature dependence of the hydration number NHyd per monomer unit NIPAM calculated from eq 9 for aqueous solutions of 1pNIPAM with c = 20 wt % (turquoise ⬢), 15 wt % (green ◆), 10 wt % (blue ●), and 5 wt % (red ▲). The solid line is a linear fit and guide to the eye only. The dashed line represents the average hydration number NHyd,av of all data between 15 and 30 °C. The scheme shows the monomer unit NIPAM hydrated by 5−6 water molecules (adapted from refs 33 and 34).

At temperatures above the LCST, Δε is independent of the polymer concentration (inset Figure 9) and approximately equals the Δε of pure water. This is a quite unforeseen result. It is expected from the current understanding of polymer solutions that the dielectric strength for the polymer containing systems should be lower than that of water as it is found for temperatures below the LCST. As mentioned above, attention was paid to measure the dielectric properties of the microsuspension and not these of the pure water. Therefore, it is a surprising result that for the suspension a dielectric strength close to that of water is observed because mixing rules should be applied.64 Until now that is difficult to understand. One possibility is that the collapsed pNIPAM globules still contain a lot of (free) water molecules as reported for single chain globules and pNIPAM microgel particles.11,65 These water molecules will give rise for a high permittivity of the pNIPAM globules. Nevertheless further experiments are necessary. At temperatures below the phase transition temperature Δε decreases with increasing polymer concentration for reasons as discussed above. The concentration dependence of the relaxation strength is compared to literature values (ref 35). The data show a more pronounced concentration dependence of Δε and hence deviate especially in the higher concentration regime from the data obtained here. That might be due to a different fitting procedure applied. For more details see ref 35. From the w-process the hydration number NHyd, which represents the number of water molecules that are affected by the polymer and therefore do not contribute to the free water relaxation can be calculated. The concentration of the bulk

shown in Figure 8 assuming a molecular weight Mn of 2500 and 113.16 g/mol for 1pNIPAM and NIPAM, respectively. NHyd was determined for different pNIPAM concentrations and temperatures below 30 °C and was evaluated to be between 4 and 6 irrespective of cpNIPAM and temperature. Above the LCST, NHyd decreases significantly and drops to zero. That means that about five water molecules are released to dehydrate one monomer unit NIPAM. However, below the LCST, a slight decrease of NHyd with increasing temperature could be observed (solid line (linear fit) in Figure 10) leading to the conclusion that the dehydration process already starts at temperatures below the phase transition as also reported by Kogure et al.69 for pNIPAM microgel particles. In literature a somewhat higher value is reported for NHyd. Ono et al.34 calculated a hydration number of 5−6 for the monomer NIPAM but a value of 11 for a monomer unit NIPAM (repeat unit of the polymer pNIPAM) using dielectric spectroscopy. The increased hydration number of the polymer compared to that of its monomer is explained by the additional water molecules that form hydrogen bonds to the water molecules directly hydrated to pNIPAM and to each other. That produces hydrogen bond bridges between the water molecules involved in hydration.34 It has to be mentioned that a different data analysis of the w-process was performed where a 3756



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Above the LCST, the pNIPAM microsuspensions and pure water showed similar characteristics of the w-process (dielectric strength and relaxation time) independent of polymer concentration and molecular weight of the pNIPAM. These unexpected results supported the idea of dehydration of the pNIPAM chains at the phase transition where water is pressed out and released from the solvated coil and a phase separation occurs. Due to the increase of free water molecules that contribute to the dielectric spectra of the pNIPAM solution, the dielectric characteristics of the w-process became water-like. Moreover, it was assumed that collapsed pNIPAM globules still contain a lot of water molecules, giving rise to a higher permittivity at T > LCST. However, as discussed in the contribution, it is an unexpected result that the dielectric strength of the polymer containing system at temperatures above the LCST is approximately equal to that of pure water. At temperatures below the LCST, the relaxation rate f P decreased with the increasing polymer content of the solution. It is explained by the slowing down of the rotational fluctuations of the water molecules in the presence of the polymer segments due to less free volume available. It was shown that there is almost no effect of the molecular weight on f P. Below the LCST, only a slight decrease of f P with increasing chain length was observed explained mainly by the hindrance of the translational or rotational motion of solvent molecules due to longer chains. At the LCST, the hydration number NHyd extracted from the dielectric strength dropped from about 5 to 0, meaning that around five water molecules are released to dehydrate one repeat unit of pNIPAM. The study has provided a better understanding of the chain dynamics and the dehydration behavior at the phase transition. Results showed that above the LCST pNIPAM chains dehydrate. However, the unexpected result that collapsed globules (microsuspension) dielectrically behaved like pure water is not fully understood yet and requires further measurements.

dielectric contribution of pure water was determined and subtracted from the total dielectric spectra to determine the contribution of the polymer. On this basis, the calculation of NHyd was carried out. A similar analysis procedure was applied to the dielectric data obtained here; however, no consistent results were obtained. To study the effect of the chain length on the dynamics of pNIPAM solutions, a second pNIPAM with a higher molecular weight was investigated (2pNIPAM, Mn = 100−150 kg/mol) at a concentration of 15 wt %. Figure 11 compares the relaxation

Figure 11. Relaxation rate f P and strength Δε (inset) of the w-process for 1pNIPAM c = 15 wt % (red ■), 2pNIPAM c = 15 wt % (black ●), and water (blue ▲) plotted over temperature.

rates obtained for pure water, 1pNIPAM and 2pNIPAM plotted over temperature. The temperature dependence of the relaxation rates for the polymer with longer chains (2pNIPAM) shows a similar trend as the rates of the pNIPAM with shorter chains (1pNIPAM). Relaxation rates are slightly lower below the LCST, which can be explained by the hindrance of the translational or rotational motion of solvent molecules due to longer chains. However, the effect is rather small. Above the LCST, the curves collapse into one single curve showing approximately the same behavior than water. The inset in Figure 11 shows the relaxation strength Δε versus temperature. As discussed, Δε is directly proportional to the number density of the water dipoles involved in the process, which is a concentration but not a molecular weight dependent parameter. This explains the similar behavior of Δε for the lower (1pNIPAM) and higher (2pNIPAM) molecular weight pNIPAM.

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AUTHOR INFORMATION

Corresponding Author

*A. Schönhals: tel, +49 30/8104-3384; fax, +49 30/8104-1617; e-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

4. CONCLUSIONS The molecular dynamics of linear poly(N-isopropylacrylamide) (pNIPAM) in aqueous media were studied using broadband dielectric relaxation spectroscopy in a frequency range from 10−1 to 1011 Hz. Below the lower critical solution temperature (LCST), two relaxation processes were observed. One due to the reorientation of dipoles on the solvated polymer segments (p-process) and a second one due to the rotation of water (solvent) molecules (w-process) molecules influenced by the presence of the polymer in solution. Above the LCST, surprisingly, the w-process approached Debye behavior (approximately similar to that of pure water). The p-process located in the megahertz region was no longer detectable. The relaxation time and strength of the w-process were studied with dependence on the concentration, temperature and polymer chain length (molecular weight).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Dr. J. Falkenhagen for the GPC analysis and the MALDI-TOF mass spectroscopy. Parts of this work were supported by the European Regional Development Fund and by the federal state of Brandenburg (projects TeraSens and 031IS2201A “Taschentuchlabor”).

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Dynamics of Linear Poly (N-isopropylacrylamide) in Water around the

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