Comparative Investigation of the Thermoresponsive Behavior of Two

Article pubs.acs.org/JPCB

Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX

Comparative Investigation of the Thermoresponsive Behavior of Two Diblock Copolymers Comprising PNIPAM and PMDEGA Blocks Dionysia Aravopoulou,† Konstantinos Kyriakos,‡ Anna Miasnikova,§ André Laschewsky,§,∥ Christine M. Papadakis,‡ and Apostolos Kyritsis*,† †

Physics Department, National Technical University of Athens, Iroon Polytechneiou 9, Zografou Campus, Athens 15780, Greece Physik-Department, Fachgebiet Physik weicher Materie, Technische Universität München, James-Franck-Str. 1, 85748 Garching, Germany § Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany ∥ Fraunhofer Institut für Angewandte Polymerforschung, Geiselbergstr. 69, 14476 Potsdam-Golm, Germany ‡

ABSTRACT: The thermoresponsive behavior of two diblock copolymers PSb-PNIPAM and PS-b-PMDEGA, which both comprise a hydrophobic polystyrene (PS) block but different thermoresponsive blocks, also differing in length, poly(N-isopropylacrylamide) (PNIPAM) and poly(methoxy diethylene glycol acrylate) (PMDEGA), respectively, was comparatively investigated in a wide temperature range. Concentrated aqueous solutions containing 25 wt % polymer were studied by small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and broadband dielectric spectroscopy (BDS). DSC measurements show that, during the demixing phase transition, the hydration number per oligo(ethylene glycol) side chain in the PS-b-PMDEGA solution decreases rather gradually, even up to 20 °C above the onset of the transition, i.e., the cloud point (CP). In contrast, the PS-bPNIPAM solution exhibits an abrupt, stepwise dehydration behavior at its CP, indicated by the sharp, narrow endothermic peak. BDS measurements suggest that the organization of the expelled water during the phase transition and the subsequent evolution of the micellar aggregates are different for the two copolymers. In the PS-b-PMDEGA solution, the long-range charge transport process changes significantly at its CP and strong interfacial polarization processes appear, probably due to charge accumulation at the interfaces between the micellar aggregates and the aqueous medium. On the contrary, in the PS-b-PNIPAM solution, the phase transition has only a marginal effect on the long-range conduction process and is accompanied by a reduction in the high-frequency (1 MHz) dielectric permittivity, ε′. The latter effect is attributed to the reduced polarization strength of local chain modes due to an enhancement of intra- and interchain hydrogen bonds (HBs) in the polymer-rich phase during the water detaching process. Surprisingly, our BDS measurements indicate that prior to both the demixing and remixing processes the local chain mobility increases temporally. Our dielectric studies suggest that for PS-b-PNIPAM the water detaching process initiates a few degrees below CP and that the local chain mobility and intra- and/or interchain HBs of the PNIPAM blocks may control its thermoresponsive behavior. Dielectric “jump” experiments show that the kinetics of micellar aggregation in the PS-b-PMDEGA solution is slower than that in the PS-b-PNIPAM solution and is independent of the target temperature within the two-phase region. From the experimental point of view, it is shown that the dielectric susceptibility, especially, the dielectric permittivity, ε′, is a well-suited probe for monitoring both the reversible changes in the molecular dipolar bond polarizability and the long-range interfacial polarization at the phase transition.

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heating.9 The onset of the demixing phase transition may, among others, be identified by the single endothermic behavior in differential scanning calorimetry (DSC) or the increased turbidity,10,11 and the demixing temperature is usually called the cloud point (CP). The thermoresponsive behavior is controlled by the hydrogen bonds (HBs) among water molecules and the hydrophilic moieties of the macromolecules, whereas the so-

INTRODUCTION

Thermoresponsive polymers constitute a class of stimuliresponsive polymers that react strongly to a small change of temperature.1−4 In particular, the polymers that exhibit the socalled lower critical solution temperature, LCST, behavior have been in the focus over the last decades due to numerous possible applications, e.g., in tissue engineering, drug delivery, surface engineering, etc.2,4−8 In a certain temperature range, these macromolecules undergo a reversible coil-to-globule transition, whereas their aqueous solution undergoes a phase separation into a polymer-rich and a polymer-poor phase upon © XXXX American Chemical Society

Received: September 28, 2017 Revised: February 8, 2018 Published: February 8, 2018 A

DOI: 10.1021/acs.jpcb.7b09647 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B called hydrophobic effect seems crucial as well. This latter effect may be considered as a complex result of the tendency of hydrophobic molecules to minimize the interface with water molecules and of the consecutive entropy change of water molecules around the hydrophobic group. Thus, this phase separation results from the delicate balance between the entropy and the enthalpy of mixing.12−17 Block and graft copolymer structures featuring thermoresponsive and hydrophobic blocks can self-assemble in water or other solvents because of opposite interactions of the blocks. A great variety of architectures has been obtained..2,8,18−20 One of the most studied thermoresponsive polymers is poly(Nisopropylacrylamide) (PNIPAM), which exhibits a CP around 32 °C in aqueous media.3,10,21,22 The phase transition of PNIPAM solutions, which is accompanied by the formation of inter- and/or intrachain HBs above the CP, spans over a very narrow temperature range, and in a wide range, the CP has only a weak dependence on the degree of polymerization and the concentration of the solutions. The sharpness of the transition is usually assigned to a highly cooperative dehydration of PNIPAM.23−25 Moreover, the rather weak dependence26 of this phase transition on the degree of polymerization and concentration in a wide range indicates that entropic contributions of the polymer chains play a minor role in the phase transition, implying, thus, that other parameters must control the phase transition of PNIPAM in aqueous media. It was suggested that the strength and the structure of the HB network among water molecules that hydrate the hydrophobic isopropyl groups in the PNIPAM side group, the so-called water cage molecules,27,28 govern the phase behavior of PNIPAM.14,29 Additionally, it is well established that, in parallel with the HB network of hydration water molecules, the intraand/or intermolecular interactions among the side chains of PINPAM play an important role for its phase behavior.13,30,31 PNIPAM, homo- and copolymer systems have been studied by various methods, such as small-angle neutron scattering (SANS),32−34 dynamic light scattering,35,36 nuclear magnetic resonance,37−39 infrared30,40,41 and Raman31,42 spectroscopy, differential scanning calorimetry,29,43−46 dielectric spectroscopy (which will be discussed in the following paragraph), and solvated in water or other solvents, together with theoretical approaches like simulation studies.17,47−51 Recently, an alternative type of thermoresponsive copolymers based on poly(diethylene glycol) (acrylate)s blocks was introduced.20,52 They belong to the class of comb polymers composed of a carbon−carbon backbone and oligo(ethylene glycol) side chains,53 which undergo the demixing phase transition through thermally induced dehydration but without formation of inter- and/or intrachain hydrogen bonds in the polymer-rich phase. Within this family of polymers, poly(methoxy diethylene glycol acrylate) (PMDEGA) has the shortest side chain to achieve water solubility12,20 and thus is structurally the most alike to PNIPAM. Furthermore, the CP of aqueous solutions of PMDEGA homopolymers having intermediate to high molar mass is around 35−45 °C, i.e., similar to the CP of PNIPAM. Yet, as PMDEGA shows LCST behavior of type I (Flory−Huggins type) whereas PNIPAM follows type II behavior,22,43 characteristic differences exist between solutions of block copolymers featuring PNIPAM or PMDEGA blocks, concerning the self-organization, the width and hysteresis of the transition, and the switching kinetics.33,54−57

Broadband dielectric spectroscopy (BDS) is a noninvasive technique to study the structure and dynamics of polymers, including their solutions, which has been increasingly used to study smart gels.58,59 BDS covers a broad frequency range, from 10−2 to 1011 Hz; thus, processes of different time and length scales can be studied. For aqueous solutions of thermoresponsive polymers, their demixing transition and the subsequent assembly process were monitored, usually, in the highfrequency domain (1 MHz to 50 GHz).58−67 In this frequency range, the investigated underlying molecular mechanisms include the polarization of either condensed counterions or freely mobile ions, the polarization of the ionic atmosphere, long-range solvent-ordering effects and, at higher frequencies, the orientational polarization of the water molecules themselves, possibly modified by the interaction with the solute. Recent measurements in the terahertz (THz) frequency range provided new insight into the dynamics of individual water molecules, related to the hydrogen bond interactions they are involved in, either with the polymer chain or with other water molecules.16 Moreover, dielectric measurements at lower frequencies, in the range of 10−2−106 Hz, were also performed on solutions of thermoresponsive polymers, mainly based on PNIPAM.66−72 Such measurements allow for the study of the evolution of micellar aggregates, of changes in microgel structures, and of their charge accumulation capacity during the demixing phase transition, on the basis, mainly, of monitoring interfacial polarization processes that are activated because of the long-range transport of ions and their accumulation at interfaces between the aggregates and the aqueous medium. In the present work, we investigate, in a comparative way, two amphiphilic diblock copolymers featuring both a short polystyrene (PS) block, which is hydrophobic, and different thermoresponsive blocks, namely, the well-studied PNIPAM or PMDEGA, respectively. The demixing phase transition in concentrated solutions of 25 wt % polymer is investigated by small-angle X-ray scattering (SAXS), DSC, and BDS. The observed characteristics of the thermoresponsive behavior are discussed in terms of the structure of the micelles and their aggregation as well as the reorganization of water molecules and polymer chains during the demixing/remixing process. The main goal of this work is to study the differences in the thermoresponsive behavior of these two copolymers. Also, we demonstrate that dielectric susceptibility, in particular, its real part, dielectric permittivity ε′, is a well-suited probe for investigating the thermoresponsive behavior of polymers.

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EXPERIMENTAL SECTION Materials. Concentrated aqueous solutions of two thermoresponsive diblock copolymers, namely, diblock copolymers PS62-b-PNIPAM115 and PS11-b-PMDEGA331 (Scheme 1), were investigated at a polymer concentration of 25 wt % (i.e., water fraction hw = 0.75). Details on their synthesis and their chemical characteristics were reported elsewhere.52,57 Turbidimetry measurements revealed CP values of 31.6 °C for a similar copolymer PS50-b-PNIPAM16033 and 40.0 °C for PS11-bPMDEGA33152 solutions, both with water concentrations slightly lower than those for the solutions under study here. For the remainder of the article, we will refer to the 25 wt % polymer solutions of the PS62-b-PNIPAM115 and PS11-bPMDEGA331 copolymers simply as PS-b-PNIPAM and PS-bPMDEGA solutions, respectively. B


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The Journal of Physical Chemistry B

micellar shell75 and gives the scaling factor, IOZ, and the characteristic correlation length of the concentration fluctuations, ξ. Ibkg was adapted for each curve. All model fits were carried out using NIST SANS package 7.04b in the Igor Pro 6.12 environment.76 For T = 42 °C, i.e., above the CP, the Guinier form factor was additionally used to model the forward scattering from large aggregates.

Scheme 1. Chemical Structures of the Two Diblock Copolymers under Study: (a) PS62-b-PNIPAM115 and (b) PS11-b-PMDEGA331

⎛ q 2r 2 ⎞ g ⎟ PGuinier(q) = G(0) exp⎜⎜ − 3 ⎟⎠ ⎝

where rg is the radius of gyration of the aggregates and G(0) is a scaling factor. Differential Scanning Calorimetry (DSC). The thermal properties of the solutions were investigated using a TA Q200 series DSC instrument with Tzero functionality. Nitrogen was used as a purge gas (50 mL/min). Empty furnace and sapphire runs were previously performed to calibrate the capacitance and resistance values of the Tzero heat flow circuit. Then, indium was used for temperature and enthalpy calibration. Samples of similar size, 8−11 mg, were used to avoid side effects of the sample size on the baseline signal. Hermetic aluminum pans were used while cooling and heating rates were fixed at 1, 3, and 10 °C/min. The usual protocol of the measurements consisted of heating from 10 up to 80 °C and subsequently cooling down to 0 °C. Broadband Dielectric Spectroscopy (BDS). For BDS measurements, the complex dielectric function (also known as dielectric permittivity) was determined in dependence on frequency, f (10−1−106 Hz), at constant temperature (10−60 °C in steps of 10 or 3 °C, isothermal measurements), controlled to better than 0.1 °C, using a Novocontrol Alpha Analyzer in combination with a Novocontrol Quatro Cryosystem. Additional isochronal measurements were performed by changing the temperature with a constant rate and monitoring the dielectric response at four preselected frequencies (1, 10, 100, and 1000 kHz). A self-made cell for liquids was used. The cell had the geometry of planar capacitor and consisted of a Teflon cylinder and two fine polished brass electrodes that are fixed to the Teflon cylinder on top and bottom of the cell. A two-part, regular body hydrophilic vinyl polysiloxane impression material was used as sealer of the Teflon cylinder around the electrodes of the capacitor. The distance between the electrodes was 3 mm with the round electrodes having a diameter of 13 mm. The applied voltage was 0.1 V (root mean square (rms) value); thus, an alternating electric field with a rms strength of 3.3 × 10−3 V/cm was applied. To characterize the bulk conductivity, we used the complex conductivity, σac*, formalism.77 In the admittance presentation, the real part of the alternating current (ac) conductivity, σ′ac, is calculated from the imaginary part of dielectric function, ε″, using

Methods. Small-Angle X-ray Scattering (SAXS). SAXS experiments were performed using a GANESHA 300XL SAXS instrument (SAXSLAB ApS, Copenhagen, Denmark) with a GENIX 3D microfocus X-ray source (wavelength λ = 1.54 Å) and point collimation. The entire beam path was in vacuum. A two-dimensional Pilatus 300 K detector was installed at the sample−detector distances (SDDs) of 406.2 and 1056.2 mm, while the counting times were 3600 and 7200 s, respectively. A pin diode was used to measure the sample transmission. The sample was mounted in a capillary of 1.2 mm diameter. A heating run between 24 and 42 °C was carried out in steps of 2 °C with a waiting time of 15 min after temperature change, to ensure thermal equilibrium. All images were corrected for cosmic background and parasitic scattering. The background from a water-filled capillary was subtracted. Diblock copolymer PS11-b-PMDEGA331 was dissolved in demineralized water at a water fraction hw = 0.75. It was placed on a shaker for a few days and stored in the fridge prior to measurements. The scattered SAXS intensity, I(q), was modeled using the following expression I(q) = IoPmic(q)SHS(q) + SOZ(q) + Ibkg

(2)

(1)

where Io is a scaling factor, Pmic(q) denotes the form factor of the micelles, SHS(q) is the hard-sphere structure factor describing their correlation, SOZ(q) is the Ornstein−Zernike structure factor of the swollen PMDEGA blocks, and Ibkg is the background. For Pmic(q), the form factor of spherical core−shell micelles with a polydisperse core was used, along with a uniform shell thickness, revealing the micellar radius, rmic, which is the sum of the core radius, rcore, and the shell thickness, t: rmic = rcore + t, whereas the polydispersity of the core radius was taken into account using a Schulz−Zimm distribution of rcore.73 Because PS is not thermoresponsive, rcore is supposed to be temperature-independent. The correlation between the micelles was modeled by the hard-sphere structure factor, SHS(q), using the Percus−Yevick approximation,74 which allows us to obtain the hard-sphere radius, rHS, which, in our case, is equivalent to half the distance between the centers of two micelles, and parameter ηHS that denotes the fraction of the micelles that are correlated. The Ornstein−Zernike (OZ) structure factor, SOZ(q), accounts for the concentration fluctuations in the

σ ′ac (ω) = ωε0ε″(ω)

(3)

where ε0 is the permittivity in vacuum and ω is the angular frequency of the alternating electric field.

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RESULTS AND DISCUSSION Structural Characterization. To shed light on the structure and correlation of the micelles formed by the PS-bPMDEGA solution, temperature-resolved SAXS was used. C


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The Journal of Physical Chemistry B Representative SAXS curves at 32−42 °C, i.e., below and above the CP from turbidimetry, are given in Figure 1. At higher temperatures, the polymer precipitated.

Figure 1. Representative SAXS curves of PS-b-PMDEGA solution in dependence on temperature. Black squares: 32 °C, blue triangles: 36 °C, pink triangles: 38 °C, orange triangles: 40 °C, and dark yellow circles: 42 °C. The lines are fits; see the text. Only every second point is presented for clarity.

At 32 °C, a maximum is observed at q ∼ 0.2 nm−1, which is attributed to the correlation between the positions of the micelles. Moreover, at ∼0.5−2.0 nm−1, a shoulder is present, and above, a smooth decay. With increasing temperature, the correlation peak becomes weaker and, at the same time, the forward scattering below 0.1 nm−1 increases, implying the formation of larger aggregates. At 42 °C, the maximum is not discernible any longer because of the increased forward scattering. The model given in eq 1 was fitted, and the fits obtained are good in the entire q-range (Figure 1). The resulting fitting parameters are given in Figure 2. During fitting, the core radius, rcore, was fixed at values of 2.3−2.6 nm. The micellar radius, rmic, is 8.3 ± 0.5 nm in the range of 32−40 °C. The value decreases slightly (7.3 ± 0.5 nm) at 42 °C, i.e., above CP. The correlation length of concentration fluctuations, ξ, increases from 1.0 ± 0.04 nm at 31 °C to 1.15 ± 0.06 nm at 42 °C. Thus, at the level of the single micelle, no discontinuous changes are observed. The hard-sphere radius, rHS, is ca. 11 nm at 32−38 °C, i.e. only slightly larger than rmic, i.e., the micelles are quite close to each other, as expected from the relatively low water content. At 40 and 42 °C, rHS decreases to 10.1 ± 0.4 nm, i.e. the average distance between the micelles slightly decreases at the CP. The volume fraction, ηHS, is rather constant 0.12 ± 0.05 at 32−36 °C and then decreases to 0.05 ± 0.01 at 40.0 °C, implying that the correlation between micelles gradually weakens as the CP is approached. At 42.0 °C, ηHS = 0.17 ± 0.06; this higher value shows that the correlation is reinstalled and even better than far below the CP. At 42 °C, forward scattering is observed (Figure 1), which was modeled using an additional Guinier form factor (eq 2). This way, the average radius of gyration of the aggregates, rg, was obtained as 11.4 ± 0.2 nm. We note that the overall behavior and the values are consistent with our previous results on a P(S-d8)11-bPMDEGA505 diblock copolymer (the PS block was fully deuterated) with the same degree of polymerization of the PS block.72

Figure 2. Parameters resulting from model fitting to the SAXS curves of the PS-b-PMDEGA solution. (a) Core radius, rcore (black squares), and micellar radius, rmic (red circles). (b) Hard-sphere radius, rHS (green triangles, left axis), and volume fraction, ηHS (dark yellow circles, right axis). (c) Correlation length of the fluctuations, ξ. The gray dashed line indicates the cloud point from DSC.

Regarding the PS-b-PNIPAM solution, previous synchrotron SAXS measurements78 and small-angle neutron scattering (SANS) measurements33 on concentrated solutions (30 wt % polymer) of the similar PS50-b-PNIPAM160 diblock copolymer revealed thermoresponsive behavior different from that of the PS-b-PMDEGA solution. PS-b-PNIPAM forms core−shell micelles with a peculiar behavior around the CP: At temperatures up to 26 °C, the PNIPAM shell is swollen, however, less than expected for a good or a θ solvent. At 27−28 °C, just below the CP, the micellar shell starts to collapse and the micelles form already small aggregates. Between 28 and 31 °C, the number of aggregates increases. Above 31 °C, the micellar shells are fully collapsed, with the micellar core−shell structure being preserved. The distance between the micelles is minimal, and their liquidlike correlation becomes stronger again. The aggregates grow as temperature is increased. Obviously, the experiments indicate that for PS-b-PNIPAM solutions, the phase separation consists of discrete stages, the first stage taking place below the CP where aggregation of the individual micelles occurs. On the contrary, the results of the D


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The Journal of Physical Chemistry B present study on PS-b-PMDEGA solutions indicate a smooth demixing transition around the CP, where individual micelles start to form aggregates only above the CP. These are consistent with the smooth behavior observed in PS-bPMDEGA-b-PS triblock copolymer solutions.55 Thermal Behavior. Figure 3 presents DSC thermograms obtained on the two solutions focused on the temperature

Figure 3. DSC heating () and cooling (- - -) thermograms obtained on PS-b-PNIPAM (red) and PS-b-PMDEGA (blue) solutions. The heating/cooling rate was 3 °C/min. Figure 4. Heating (a) and cooling (b) thermograms obtained on both copolymers at three different heating/cooling rates: 1 °C/min (black), 3 °C/min (red), and 10 °C/min (blue).

range of the phase transition. The applied heating/cooling rate was 3 °C/min. It is obvious that the response of the two systems is significantly different. For the PS-b-PNIPAM solution, the peaks are more symmetric and narrower in both heating and cooling thermograms. During heating, the signal drops below the baseline after the completion of the endothermic peak, reaching a plateau value. This effect, indicating that heat capacity decreases slightly at temperatures above CP, is considered as the onset of partial vitrification because of the dissociation of water molecules and polymer units.79,80 On the contrary, for the PS-b-PMDEGA solution, the signal meets the (extended) baseline after the transition. Figure 4 presents DSC heating (Figure 4a) and cooling (Figure 4b) curves for both copolymers measured at three different heating/cooling rates. The parameters resulting from the analysis of the thermograms are compiled in Table 1. The calorimetrically obtained demixing temperature of the thermoresponsive polymers is often defined as the maximum of the endothermic peak, TDSC, or the onset of the DSC transition endotherm, Ton, i.e., the temperature at the intersection of the baseline and the leading edge of the endotherm. We note both in Table 1. It is noteworthy here that in the case of PS-b-PNIPAM solution our DSC measurements point to TDSC values (27−28 °C) that are lower than the CP estimated by turbidimetry (31.6 °C);33,78 however, they are within the temperature range where the structure of the micelles starts to change.33,78 In contrast, for the PS-bPMDEGA solution, the TDSC values (41−44 °C) are higher than the CP estimated by turbidimetry (40.0 °C). The breadth, ΔTbreadth, of the transition, which can be defined as ΔTbreadth = Tend − Ton, where Tend denotes the temperature where the thermogram after the endothermic/exothermic peak merges again with the baseline or reaches the plateau value, is found to be about 11 °C for the PS-b-PNIPAM solution and 22 °C for the PS-b-PMDEGA solution, at the heating rate of 3 °C/min.

To quantify the hysteresis between heating and cooling, we may calculate the difference ΔThyst = TDSC(heating) − TDSC(cooling). According to the data listed in Table 1, the hysteresis is equal to 1 °C for both solutions at the heating/cooling rate of 1 °C. For higher rates, the hysteresis increases, the effect being more pronounced for the PMDEGA copolymer, and can be attributed to the high rates that may broaden the metastable regions of the transition.15 This observation agrees with the findings of Füllbrandt et al. on linear PNIPAM70 and with similar studies on PNIPAM and PMDEGA copolymer solutions.57 Regarding the characteristic TDSC temperatures in Figure 4, we observe that the change of the (constant) heating/cooling rate results in a shift of TDSC(heating) to higher and of TDSC(cooling) to lower values for the demixing and the remixing process, respectively (Table 1). These results are consistent with findings on PNIPAM reported in the literature.45,70 Taking into account that the estimated enthalpies of the phase transition, ΔH (Table 1) exhibit also a heating rate dependence, we may conclude that the recorded values correspond to enthalpy changes in nonequilibrium.81 It is known that such changes of the corresponding ΔH values imply changes in the level of water dissociation from the hydrophilic chains.82 Therefore, the trend of the estimated ΔH values, shown in Table 1, i.e., their increase with increasing rate, may suggest that with increasing heating/cooling rate, the association/ dissociation process is related to higher energy barriers, the effect being more pronounced in the case of the PS-b-PNIPAM solution. For concentrated solutions, like in this study, we would expect that interchain associations mainly control the enthalpy changes during the phase transition, especially at relatively low heating rates.81 The remarkable increase of ΔH at E


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Table 1. Thermoresponsive Transition-Related Parameters Obtained from DSC Studies on PS-b-PNIPAM and PS-b-PMDEGA Solutions thermoresponsive behavior heating

1 °C/min 3 °C/min 10 °C/min

sample

TDSC (°C) (±0.1 °C)

Ton (°C) (±0.5 °C)

PS62-b-PNIPAM115 PS11-b-PMDEGA331 PS62-b-PNIPAM115 PS11-b-PMDEGA331 PS62-b-PNIPAM115 PS11-b-PMDEGA331

27.1 41.1 28.0 44.4 28.1 45.1

25.4 38.3 23.1 35.4 25.0 38.9

cooling ΔH (J/g) (±0.1 J/g)

TDSC (°C) (±0.1 °C)

Ton (°C) (±0.5 °C)

ΔH (J/g) (±0.1 J/g)

3.9 2.5 4.2 2.6 4.7 3.5

26.1 42.0 25.3 40.1 24.1 39.3

28.0 51.9 29.0 51.4 26.0 52.7

3.7 2.3 4.3 2.5 4.3 4.6

the high heating rate of 10 °C/min may imply that intrachain contraction (of higher enthalpy) also takes place. On the other hand, it cannot be excluded that the number of water molecules involved in the transition, i.e., the degree of completion of the dehydration process, may be dependent on the heating/cooling rate. In summary, DSC measurements show that during the demixing process the hydration number per oligo(ethylene oxide) side chain in PMDEGA blocks may decrease gradually and that the whole process is completed at about 20 °C above the CP. On the contrary, the sharp, narrow endothermic peaks recorded on the PS-b-PNIPAM solution point to an abrupt, stepwise dehydration behavior of PNIPAM at CP, which has been suggested to be of cooperative character.1,23−25 Dielectric Behavior. In Figure 5, we present the temperature dependence of the real part of dielectric permittivity, ε′, and ac conductivity, σ′ac, measured isochronally at f = 1 MHz on the PS-b-PNIPAM (Figure 5a) and PS-b-PMDEGA (Figure 5b) copolymer solutions, during heating/cooling with a rate of 3 °C/min. For both solutions, ε′(T) decreases initially with increasing temperature, which is expected for aqueous solutions,83 and reflects the dominance of dipolar orientational polarization processes at frequencies higher than those covered by us. In the same temperature range, the conductivity, σ′ac, increases steadily for both solutions. Considering the PS-b-PNIPAM solution, a steep increase of ε′ is recorded at a critical temperature TcH = 24.6 °C, i.e., far below CP (Figure 5a). However, still below CP, the dielectric permittivity values start to decrease again with increasing temperature, exhibiting thus a peak in the ε′(T) curve and reaching finally saturation values lower than the initial one, at temperatures as high as 30 °C above CP. This ε′(T) behavior is reversible, as seen from the values recorded during cooling shown in Figure 5a. On returning to the initial temperature, we observe that the value of ε′ remains slightly lower than the initial value. This result implies that, within the time scale of the experiment, the orientational polarization in the solution is not fully recovered. Considering now the PS-b-PMDEGA solution (Figure 5b), at temperatures higher than a critical temperature, TcH = 41 °C (slightly above CP), ε′ increases rapidly with temperature, reaching a plateau at temperatures 30 °C above CP. During cooling, no hysteresis with respect to the heating curve is observed. Regarding the σ′ac(T) curves shown in Figure 5, we observe that the temperature dependence of the conductivity around the CP is different in the two solutions. For the PS-b-PNIPAM solution (Figure 5a), crossing the CP seems to have no serious effects on the temperature dependence of the conductivity (measured at 1 MHz), which continues to increase with temperature, however with a reduced slope. On the contrary,

Figure 5. Real part of dielectric permittivity, ε′, and ac conductivity, σ′ac, measured at 1 MHz, as a function of temperature for PS-bPNIPAM (a) and PS-b-PMDEGA (b) copolymers. The dotted lines indicate the cloud point temperature as measured by turbidimetry (CP) and by DSC (TDSC). The critical temperature, TcH, where ε′ suddenly changes during heating, is indicated for both solutions.

for the PS-b-PMDEGA solution (Figure 5b), the demixing transition leads to a decrease of the conductivity. By a further increase of temperature, the conductivity values increase again, presenting a similar trend as that of the PS-b-PNIPAM solution. The effects are fully reversible during cooling, and excellent agreement between the data of conductivity measured initially and finally at T = 0 °C is observed. Figure 6 shows the real part of dielectric permittivity, ε′, of the PS-b-PNIPAM (Figure 6a) and PS-b-PMDEGA (Figure 6b) solutions, measured isothermally during heating in the temperature region around CP. Only the high-frequency part of the spectra is shown here. Below TDSC of each solution (27 °C for the PS-b-PNIPAM solution and 41 °C for the PS-b-PMDEGA solution in the F


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T = 44 °C, the measured real part of dielectric permittivity increases in the whole frequency range (Figure 6b). The spectra reveal the appearance of a new polarization process at temperatures above CP, in the frequency range of 0.1−1 MHz, i.e., having a time scale of 10−5 s. The isothermal plots of the real part of the ac conductivity, σ′ac, versus frequency, shown in Figure 7, provide additional

Figure 6. Real part of the dielectric permittivity, ε′, measured at the temperatures indicated in the graphs, as a function of frequency for (a) PS-b-PNIPAM and (b) PS-b-PMDEGA solutions. The dashed lines mark the responses recorded at temperatures above the TDSC of each solution.

quasi-static case), all spectra exhibit a plateau at high frequencies, whereas a steep increase of the ε′ values is observed at frequencies lower than ∼104 Hz. The constant values at high frequencies correspond to dipolar polarization processes relaxing at frequencies higher than 1 MHz due to the reorientation of water molecules with possible contributions from fast local chain polarization processes. The steep increase of ε′ at lower frequencies is attributed to ionic polarization effects usually observed in aqueous solutions.59,83 The highfrequency dielectric permittivity, ε′, decreases with increasing temperature, as expected for orientational polarization processes. For both solutions, the dielectric response changes significantly, when crossing TDSC, however in different ways. Indeed, for the PS-b-PNIPAM solution (Figure 6a), at T = 29 °C, ε′ increases in the low-frequency range of the spectrum and a polarization step is discerned at the high-frequency range, slightly below 1 MHz. Upon further increase of temperature, at T = 32 °C, i.e., above the CP, this polarization step disappears and the values on the low-frequency side of the spectrum increase further, whereas the values in the high-frequency range of the spectrum decrease. The decrease of ε′ at 1 MHz indicates that the total dielectric strength of the polarization processes, activated at frequencies above 1 MHz, decreases at temperatures above the CP. On the contrary, for the PS-bPMDEGA solutions, above the CP and when crossing TDSC, at

Figure 7. Real part of ac conductivity, σ′ac, vs frequency measured at the same temperatures as in Figure 6 for the PS-b-PNIPAM (a) and PS-b-PMDEGA (b) solutions. The dashed lines mark the responses recorded at temperatures above the TDSC of each solution. The arrows show the evolution of the spectra with increasing temperature.

information on the charge mobility and the polarization processes activated in the solutions under study. Starting with the PS-b-PNIPAM solution, the curves exhibit the conductivity plateau related to the bulk direct current (dc) conductivity of the solution at frequencies higher than about 1 kHz (Figure 7a). We note here that the absolute values of the dc conductivity, σ′dc, of the neat solvent (high-purity water) at 20 and 50 °C are around 15 and 19 μS/cm, respectively. Obviously, the dc conductivity of the PS-b-PNIPAM solution is about 10 times higher compared to that of neat solvent. The drop in the σ′ values at low frequencies is attributed to the electrode polarization effect. The dc conductivity increases with increasing temperature; however, during the increase of temperature from 26 °C (below TDSC) to 29 °C (above TDSC), the conductivity does not increase further, implying that all of the thermal energy provided to solution is consumed by the water reorientation/detaching process. By further increase of the temperature, an additional enhancement of the dc G


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The Journal of Physical Chemistry B conductivity is recorded (for T > CP (=31.6 °C)). The spectra obtained on the PS-b-PMDEGA solution feature the same trend in the temperature dependence of dc conductivity only at low temperatures, below TDSC (Figure 7b). During the temperature rise from 41 to 44 °C (above the CP (=40.0 °C) and around TDSC), the values of σ′ in the kilohertz region decrease. Careful inspection of the graph leads to the conclusion that the conductivity values at the frequency of 1 MHz continuously increase with temperature; however, a strong polarization process activated at lower frequencies results in the decrease of the measured real part of conductivity at the kilohertz region. It is important to note here that the dielectric behavior of both solutions is thermally reversible. To extract additional information from the dielectric data, which suffer from a large conductivity contribution, we transform the values of the real part of dielectric permittivity, ε′, to the dielectric loss values, ε″der, using a derivative method84 ε″der (f ) = −

π ∂ε′(f ) ≈ ε″rel 2 ∂ ln f

range, the spectra show that changes in the polarization process occur already at T = 26 °C (below TDSC). At T = 29 °C, the activation of a polarization process is evident, having a maximum in the frequency range above 1 MHz (i.e., outside our frequency window), which disappears at higher temperatures. For T > 32 °C (above CP), the spectra show no evidence for a polarization process. Again, for the PS-bPMDEGA solution (Figure 8b), the spectra are different from those of PS-b-PNIPAM for temperatures above the CP. We observe that the dielectric behavior changes dramatically at T = 44 °C and the activation of a polarization process with a peak frequency above 1 MHz (at T = 44 °C) is implied, which shifts to lower frequencies with increasing temperature, entering into our frequency window at T = 53 °C. This strong polarization process is also evident in the isothermal spectra of σ′ac for the PS-b-PMDEGA solution (Figure 7b). As can be seen, the σ′ac( f) spectra for T > 41 °C exhibit a dc plateau at frequencies below the frequency range of this polarization step, indicating, thus, that this process may be of interfacial polarization character (Maxwell−Wagner−Sillars type of polarization).77 Probably, the micellar aggregates formed right at the CP provide the medium for motion and accumulation of charge carriers detached from the polymers. In aqueous solutions of colloidal particles, similar polarization processes are usually related to ionic polarization processes.59,83,86 When aggregation of micelles occurs above the CP of thermoresponsive polymers, ionic polarization originates usually in one of the two following mechanisms: (i) the free charge carriers move across the size of the aggregates being trapped at their inner surfaces, or (ii) a double-layer polarization process occurs at the well-defined interface between the micellar aggregates and the aqueous medium. In the latter case, the length scale of the charge motions equals the so-called debye length of this double-layer polarization.77 Interestingly, for both processes, the time scale of the polarization process exhibits the same length scale dependence, given by eq 558,66,83

(4)

where ε″rel represents the ohmic-conduction-free dielectric loss. The corresponding ε″rel values, evaluated by applying eq 4 to the data from Figure 6, are presented in Figure 8. For both solutions, high loss values are observed in the lowfrequency range of the spectra, indicating the complex nonohmic character of the electrode−solution interface.85 For the PS-b-PNIPAM solution (Figure 8a), in the high-frequency

τ≈

L2 D

(5)

where length L refers either to the dimensions of the micellar aggregates or to the debye length of the interfacial double-layer polarization. Diffusion coefficient D refers to the mobility of the charge carriers. Because, in the present work, the frequencies where this polarization process is observed in the PS-b-PMDEGA solution are rather low as compared to those in the literature,59−61,63,64,66,67 we discuss our results on the basis of the first kind of polarization processes, i.e., we assume that water molecules expelled from the hydration shells of PMDEGA chains become trapped within the micellar aggregates and contribute to enhanced protonic charge mobility. To quantify the length scale of the aggregates where charges are trapped, we use the results of Schwartz theory about a polarization process that may be activated in colloidal solution and can be treated according to the Maxwell−Wagner equation for conducting spherical particles of radius α in a dielectric medium.87 The time scale of this process is determined by ionic diffusion in the inner surface of the micelles and is given by Figure 8. Dielectric loss, ε″rel, data evaluated by the derivative method from the data presented in Figure 6 for PS-b-PNIPAM (a) and PS-bPMDEGA (b) solutions. Temperatures are indicated in the graphs.

τ= H

eα 2 2μkT

(6) DOI: 10.1021/acs.jpcb.7b09647 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B where μ is the surface mobility of the charge carriers in unit electric field. Taking into consideration that the diffusion D kT coefficient, D, is given by Einstein relation μ = e , we get for

the reduction of the strength of the m-process due to the demixing process that occurs in the PNIPAM/water system. Our BDS results imply, thus, that dielectric permittivity ε′ (at 1 MHz) is a sensitive probe of the m-process in PS-b-PNIPAM solutions. Consequently, we may suggest that the sudden increase of ε′ at temperatures just below the CP (Figure 5a) is also related to the local PNIPAM chain polarization process (m-process). In agreement with the proposed water detaching mechanism, this enhancement of the local chain polarization process indicates that enhanced local chain motions occur at the first stages of the demixing transition.13 This finding may suggest that just prior to (or more accurately, at the first stages of) the transition, water reorganization takes place, resulting in a temporal increase of the local chain mobility, which in terms of BDS spectroscopy results in the temporal increase of the strength of the m-process. However, in parallel with this water reorganization process, the solution enters into the metastable phase of the transition and some of the amide (−NH) and carbonyl (−CO) groups prefer to form intramolecular HBs. The establishment of these intra- and/or interchain HBs together with the existing local chain mobility alters dramatically the conformations of the hydrophobic isopropyl groups and consequently the “cagelike” rigid HB network of the water molecules around them.14,16 The recorded gradual decrease of the dielectric strength of the m-process with increasing temperature above the CP reflects a gradual decrease of local chain mobility, implying, thus, that intra- and/or interchain HBs gradually increase, leading probably to a decrease in the hydrophobic surface that is exposed to water. We note here that this interpretation is supported by smallangle neutron scattering (SANS) measurements on concentrated solutions (70 wt % water) of a similar diblock copolymer PS50−PNIPAM160, which, as discussed earlier, showed that the diblock copolymers form core−shell micelles that show a peculiar aggregation behavior at temperatures just below the CP.33 Aggregation Kinetics. To shed more light on the kinetics of the micellar aggregation process above the CP, we performed “jump” experiments where, starting from equilibrium at an initial temperature Tin, far below TDSC, we increase the temperature of the solution as fast as possible up to a target temperature Teq > TDSC, where the solution is allowed to equilibrate for a certain time (20 min). Then, a “jump” in opposite direction, i.e., from Teq to Tin, is carried out, and the solution is left to equilibrate again at the initial temperature. During the whole procedure, the dielectric response is recorded continuously at 4 preselected frequencies (1, 10, 100, and 1000 kHz). Changing Teq and, thus, the distance Teq − TDSC within the spinodal decomposition range of demixing transition15,22 allows us to study the evolution of the dielectric behavior of the solution during the equilibration process at two different temperatures within the two-phase region. In addition, during the “jump” experiments to and from Teq, valuable information on both demixing and remixing processes is collected. For the PS-b-PNIPAM copolymers, we equilibrated the solution at Tin = 15 °C and then forced the temperature of the measuring cell to “jump” to Teq = 30 and 33 °C in a subsequent experiment (i.e., 2 and 5 °C above TDSC). We may note here that at Teq = 30 °C the solution may be in the metastable region of the transition,15,22 whereas at Teq = 33 °C, the solution is in the two-phase region. Because of the large volume of the measuring cell, the whole procedure of the “jump” takes more than 500 s (300 s for change of the temperature and more than

the time scale of the polarization process τ=

α2 2D

(7)

Equation 7 describes the relationship between the time scale of the polarization process and the distance that may be covered by the charge carriers. Denoting this length scale L and expressing the time scale in terms of the frequency of the loss peak in susceptibility measurements, f max (τ = 1/2πf max), eq 7 takes the form L2 =

D πfmax

(8)

Assuming that the charges stem from the hydration shell from which they were expelled, we use the value that was recently estimated for the self-diffusion of water molecules in a PS-bPNIPAM solution: D ∼ 10−5 cm2/s.34 Using f max = 106 Hz (corresponding to the time scale of the process at T ∼ 50 °C), we get L ∼ 17 nm, whereas for f max = 1.6 × 105 Hz (T = 62 °C), L ∼ 44 nm is obtained. Thus, the BDS results suggest that, immediately above the CP, aggregates are formed with a length scale smaller than 17 nm and that these aggregates increase in size with increasing temperature, reaching dimensions about 44 nm at the saturation (T = 62 °C). On comparing with the finding from SAXS, that 2rg of the aggregates right above the CP is about 22.8 nm, we conclude that the BDS and SAXS methods provide the same order of magnitude regarding the size of the aggregates. Regarding the PS-b-PNIPAM solution, our BDS results suggest that the micellar aggregation process is different from the one in the PS-b-PMDEGA solution, at least with respect to their polarization capability: no aggregates are formed that can trap water molecules and can lead to stable charge accumulation within length scales of 20−100 nm (Figure 8a). On the other hand, the most striking result of our BDS study is that, during the water detaching process, an enhanced polarization in the megahertz region appears only just below the CP (Figures 5a, 6a, and 8a), whereas at temperatures above the CP, the high-frequency polarizability decreases significantly (Δε′ ∼ [69.3 (at 26 °C) − 64.3 (32 °C)] ∼ 5 at 1 MHz; Figure 6a). Seeking for interpretation of these findings, we consider the relaxation processes that have been reported to contribute to the dielectric response of PNIPAM solutions. Dielectric investigations on PNIPAM homopolymer solutions showed that, at frequencies higher than 1 MHz, two relaxation processes contribute to the dielectric spectra: the relaxation due to reorientation of solvent molecules (for water molecules, this relaxation has a peak frequency of about 10 GHz at 25 °C, the so-called w-process) and a relaxation due to local chain motion of PNIPAM (that has been found to be affected by hydrogen bonding with water molecules, the so-called mprocess).65,88 Taking into account recent dielectric measurements in the THz frequency range showing that the strength of the w-process is practically not affected by the coil-to-globule transition of PNIPAM chains16 and recent QENS studies on PNIPAM solutions, which suggest that the majority of the detached water molecules do not behave as bulk water,34 we may assign the observed decrease of ε′ at high frequencies to I


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Figure 10 shows the ε′ values recorded at f = 1 MHz for the PS-b-PMDEGA solution by performing similar “jump” experi-

200 s for its stabilization); therefore, the procedure seems like changing the temperature at a heating/cooling rate of ∼3 °C/ min. The temperature of the measuring cell and the ε′ values measured at 1 MHz continuously are plotted together with a common time axis in Figure 9. We note that in Figure 9 we

Figure 10. Real part of dielectric permittivity, ε′, measured in dependence on time for the PS-b-PMDEGA solution in the jump experiments: (a) from T = 20 to 45 °C and returning back and (b) from T = 20 to 50 °C and returning back.

Figure 9. Real part of dielectric permittivity, ε′, measured in dependence on time on the PS-b-PNIPAM solution in the jump experiments: (a) from T = 15 to 30 °C and returning back and (b) from T = 15 to 33 °C and returning back.

ments. Figure 10a,b shows the data recorded during the temperature sequence 25 → 45 → 25 °C (Teq − TDSC = 1 °C) and 25 → 50 → 25°C (Teq − TDSC = 6 °C), respectively. In Figure 10, we observe that, initially, the dielectric permittivity, ε′, starts to decrease immediately when temperature increases. At TcH = 40.6 °C, this trend is stopped and ε′ starts to increase with temperature up to Teq. The values of ε′ exhibit a strong oscillation toward its equilibrium value ε′eq, probably due to the temperature oscillation toward its stabilization at Teq. Like in the PS-b-PNIPAM solution, the equilibrium values ε′eq at 1 MHz increase with increasing Teq. Furthermore, at Teq and although the temperature was already stable, ε′ shows a tendency to decrease slightly, indicating a higher time for equilibration (>800 s) than for the PS-bPNIPAM solution (∼400 s). During cooling, the ε′ values start to decrease immediately after the initiation of the cooling step, reaching a minimum at Tmin = 41.5 °C for Teq = 30 °C and Tmin = 39.4 °C for Teq = 33 °C. Upon further decreasing the temperature, the ε′ values start to increase with a constant rate, which implies that at T min the solution becomes a homogeneous mixture, i.e., the remixing transition is completed. During cooling, the entire transition takes place in the 44.5−41.5 and 49.8−39.4 °C ranges for Teq = 45 and 50 °C, respectively.

show only the data recorded at 1 MHz; however, crossing TcH (indicated in Figure 5), the dielectric response changes dramatically at all of the four preselected frequencies (not shown here). Figure 9 shows that in both experiments when the temperature starts to increase, ε′ immediately starts to decrease from its initial value. Suddenly, at TcH = 25.0 °C, ε′ increases and, within a rise of temperature of about 4 °C, ε′ reaches a maximum before its further rapid reduction. At Teq, during temperature stabilization and solution equilibration, ε′ values show an oscillation around the equilibrium value, ε′eq. We observe that at Teq = 33 °C (Figure 9b) ε′ has a significantly lower value (ε′eq = 60.4) than the saturation value of 66.8 recorded at Teq = 30 °C in the first experiment (Figure 9a). During cooling, it is more clear the appearance of the temporal maximum in the recorded permittivity values during the increase of ε′ toward a higher value at T = 15.0 °C. In both experiments, the time scale for the equilibration at Teq, i.e., the time interval for the stabilization of ε′ at the saturation value after temperature stabilization, is approximately 400 s. During cooling, the whole transition takes place in the range of 29.0− 22.8 and 32.5−22.8 °C, for Teq = 30 and 33 °C, respectively. J


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The Journal of Physical Chemistry B On comparing the results from the “jump” experiments in both solutions (Figures 9 and 10), we can make the following comments: (a) The experiments confirm the different high-frequency polarization behaviors of PS-b-PNIPAM and PS-bPMDEGA solutions during both the demixing and remixing transitions. Indeed, for the former solution, the dielectric permittivity, ε′, measured at 1 MHz decreases at temperatures above CP and TDSC, whereas a remarkable increase of ε′ when crossing CP/TDSC is recorded for the latter. (b) Δε′ = |ε′in − ε′eq| depends on ΔT = Teq − TDSC for both solutions. (c) The equilibration of the PS-b-PNIPAM solution at temperatures 2 and 5 °C above TDSC is faster than the one of the PS-b-PMDEGA solution at 1 and 6 °C above TDSC, and the equilibration time is independent of the difference Teq − TDSC. (d) In the jump experiments, information on the initiation and the completion of the demixing/remixing process is obtained. For the PS-b-PMDEGA solution, we observe a difference in the corresponding critical temperatures. Indeed, we find that the demixing process starts at 40.6 °C during heating (for both experiments), whereas the remixing process is completed during cooling at 41.5 °C (for the shallow jump within the two-phase region, i.e., Teq = 45 °C) and at 39.4 °C (for the deeper jump with Teq = 50 °C). For the PS-b-PNIPAM solution, the corresponding values are as follows: the demixing process starts at 25.0 °C during heating (for both experiments), whereas the remixing process is completed at 22.8 °C during cooling (again for both experiments). The difference of 2.2 °C of the two characteristic temperatures may reflect the hysteresis effect that is observed in the thermoresponsive behavior of the PS-bPNIPAM solution in the DSC experiments. (e) For the PS-b-PNIPAM solution, the experiments confirm the temporal polarization enhancement at 1 MHz, just prior to the demixing process, and reveal its existence also prior to the remixing process during cooling. The maximum value of ε′ (at 1 MHz) is recorded at 29.2 °C during heating (in both experiments) and at 27.1 and 26.7 °C during cooling in the two subsequent experiments. Again, the difference of 2.1−2.5 °C between the critical temperatures during heating and cooling may be considered as an indicator of the hysteresis effect.

Significantly different thermal and dielectric behaviors of PSb-PNIPAM and PS-b-PMDEGA solutions during the phase transition is found. The endothermic peak in calorimetry measurements is broader for the PS-b-PMDEGA solution (the breadth of the transition being double in the PS-b-PMDEGA solution) and highly asymmetric. Dielectric measurements suggest that the organization of the expelled water during the macromolecular water detaching process and the subsequent evolution of micellar aggregates may be different. Our results suggest that in the PS-b-PMDEGA solution the water detaching process triggers the rearrangement of the whole PMDEGA blocks, leading to the formation of micellar aggregates, where a large part of expelled water molecules are trapped and charge accumulation may take place. We estimate the size of the aggregates on the order of 17−44 nm, depending on the temperature within the two-phase region. These values are consistent with the value found in SAXS measurements (22.8 nm). On the other hand, our measurements reveal that in the PS-b-PNIPAM solution the demixing transition in the PS-bPNIPAM solution is associated with a restriction of the local chain mobility of PNIPAM and, consequently, with a reduction in the strength of the corresponding polarization process (mprocess). This structural change during the phase transition is supported also by the DSC measurements on PNIPAM where a remarkable decrease of the heat capacity is detected during the transition to the two-phase region (partial vitrification effect). Jump experiments confirm that the change of dielectric permittivity in the two solutions by crossing the calorimetric CP has opposite trends: for PS-b-PNIPAM, the polarizability at the megahertz range decreases, whereas it is strongly enhanced for PS-b-PMDEGA. The degree of these changes depends mainly on the temperature within the two-phase region. The kinetics of micellar aggregation, for temperatures slightly above CP, is slower in the PS-b-PMDEGA solutions. Our BDS measurements reveal that the dielectric permittivity, ε′, measured at 1 MHz is a very sensitive probe of the local chain polarization process of PNIPAM (m-process). They highlight the significant role of the local PNIPAM chain mobility during the demixing/remixing transition. More specifically, we found that PNIPAM local chain mobility increases temporally just below CP during heating and just above CP during cooling. This finding is a strong indication that the unique thermoresponsive behavior of PNIPAM blocks may be closely related to their ability to form intra- and interchain HBs that affect the local chain mobility and, consequently, the hydrophobic hydration of the whole macromolecule.

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CONCLUSIONS We investigate in a comparative way the thermoresponsive behavior of two diblock copolymers featuring both a short hydrophobic polystyrene block and two different long thermoresponsive blocks, PNIPAM and PMDEGA, respectively. The demixing phase transition in concentrated aqueous solutions of 25 wt % polymer is investigated by SAXS, DSC, and BDS. Our study shows that the dielectric susceptibility is very sensitive to morphological changes occurring in the solutions during the demixing transition and to the assembly process within the two-phase high-temperature region. Especially, its real part, the dielectric permittivity ε′, proves to be a well-suited probe for monitoring both the reversible changes in molecular dipolar bond polarizabilities and the longrange ionic interfacial polarization during the phase transition.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

André Laschewsky: 0000-0003-2443-886X Christine M. Papadakis: 0000-0002-7098-3458 Apostolos Kyritsis: 0000-0001-5893-7849 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Deutscher Akademischer Austauschdienst for the travel support within the program “Hochschulpartnerschaften mit Griechenland” (ResComp). K


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K.K., A.L., and C.M.P. thank Deutsche Forschungsgemeinschaft for funding within the DFG priority program SPP1259 “Intelligente Hydrogele” (Pa771/4 and AL611/7).

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