Phase Transformations in Aqueous Low Molar Mass Poly(vinyl methyl

Jan 24, 2007 - Supported by theoretical predictions based on the Wertheim Lattice Thermodynamic Perturbation Theory, modulated temperature differentia...
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J. Phys. Chem. B 2007, 111, 1288-1295

Phase Transformations in Aqueous Low Molar Mass Poly(vinyl methyl ether) Solutions: Theoretical Prediction and Experimental Validation of the Peculiar Solvent Melting Line, Bimodal LCST, and (Adjacent) UCST Miscibility Gaps Kurt Van Durme,† Guy Van Assche,† Erik Nies,*,‡,§ and Bruno Van Mele*,† Department of Physical Chemistry and Polymer Science, Vrije UniVersiteit Brussel, Belgium, Polymer Research DiVision, Department of Chemistry, Katholieke UniVersiteit LeuVen, Belgium, and Laboratory of Polymer Technology, EindhoVen UniVersity of Technology, The Netherlands ReceiVed: May 30, 2006; In Final Form: December 18, 2006

Supported by theoretical predictions based on the Wertheim Lattice Thermodynamic Perturbation Theory, modulated temperature differential scanning calorimetry (MTDSC) was used to further the knowledge of the phase behavior of aqueous poly(vinyl methyl ether) (PVME) solutions. Using a narrowly dispersed low molar mass PVME, we determined the following phase boundaries: (i) a bimodal lower critical solution temperature (LCST) miscibility gap at physiological temperature (around 37 °C), (ii) an upper critical solution temperature (UCST) two-phase area at sub-zero temperatures and high polymer concentration, and (iii) the melting line of the solvent across the entire concentration range, showing a peculiar stepwise decrease with composition. The location of the glass transition region and its influence on the crystallization/melting behavior of the solvent is discussed.

Introduction Aqueous polymer systems often display lower critical solution temperature (LCST) miscibility behavior, which implies that the polymer dissolves in cold water and phase separates during heating. Depending on the specific LCST phase behavior of the polymer, the cross-linked analogues may show an orderof-magnitude shrinking in hydrogel size upon raising the temperature.1-6 This makes such materials suitable for numerous applications, including actuators,7,8 artificial muscles,9 dewatering membranes,10 drug delivery systems,11-14 thermoresponsive surfaces,15-17 light modulation systems,18 and molecular recognition agents.19,20 Aqueous polymer systems can be experimentally characterized by different types (i.e., I, II, and III) of LCST phase behavior.1,2,21 Type I represents the traditional Flory-Huggins (FH) behavior, for which the liquid-liquid (L-L) critical composition shifts to lower polymer concentrations with increasing polymer molar mass. Type II demixing behavior stands for liquid-liquid demixing with a critical composition that hardly changes with the polymer chain length and that remains nonzero, even for infinite molar mass. Finally, Type III denotes bimodal L-L demixing behavior with two stable critical points, one at low polymer concentration (type I) and a second one at high polymer concentration (type II). The stable critical condition in the dilute (or semidilute) regime changes with polymer molar mass as expected from the Flory-Huggins theory, and * To whom correspondence should be addressed. Address: Vrije Universiteit Brussel, Faculty of Engineering, Department of Physical Chemistry and Polymer Science, Pleinlaan 2, B-1050 Brussels, Belgium (B.V.M.); Eindhoven University of Technology, Laboratory of Polymer Technology, P.O. Box 513, 5600MB Eindhoven, The Netherlands (E.N.). Fax: +32-(0)2-6293278 (B.V.M.); +31-(0)40-2474954 (E.N.). Phone: +32-(0)2-6293288 (B.V.M.); +31-(0)40-2474954 (E.N.). E-mail: bvmele@ vub.ac.be (B.V.M.); [email protected] (E.N.). † Department of Physical Chemistry and Polymer Science. ‡ Polymer Research Division. § Laboratory of Polymer Technology.

the second stable critical point at high polymer concentration is nearly molar mass independent, yielding a nonzero limiting critical concentration. These different types of phase behavior can be described using the Flory-Huggins lattice theory amended with an extended composition and temperaturedependent FH interaction function.1,22 Nonetheless, this phenomenological classification cannot provide molecular insight regarding the observed phase behavior. Conversely, Nies et al. recently mapped the Thermodynamic Perturbation Theory of Wertheim for saturation interactions (e.g., hydrogen bonding)23-25 on the lattice model and demonstrated that the atypical liquid-liquid and solid-liquid phase behavior of aqueous poly(vinyl methyl ether) (PVME) solutions could be predicted.26 That is to say, using the Wertheim Lattice Thermodynamic Perturbation Theory (Wertheim-LTPT), we predicted the well-known bimodal LCST phase behavior2,3,27 together with two adjacent narrow upper critical solution temperature (UCST) miscibility gaps at low temperature. Moreover, the predicted solid-liquid equilibrium of the solvent displays an unusual change with composition, showing a pronounced drop in melting temperature (to ca. -10 °C) at intermediate composition. This was experimentally confirmed using Fourier transform infrared spectroscopy (FTIR).26 The peculiar concentration dependence of the melting line of water provided a new explanation for the shape of the double melting endotherm observed in (MT)DSC, rather than being linked to the melting of bound and free water, respectively.28,29 Furthermore, these novel theoretical predictions and experimental findings allow us to explain the inhibited crystallization at high PVME concentration, in contrast with an interpretation based on an intermolecular complex previously suggested in lit erature.28-31 That is, at high polymer concentration, the actual supercooling becomes smaller than could be anticipated for a conventional course of the melting curve. Hence, before attaining the required supercooling (at the experimental time scale), the temperature reaches the glass transition (Tg) region,

10.1021/jp063322j CCC: $37.00 © 2007 American Chemical Society Published on Web 01/24/2007

Phase Transformations in PVME/Water

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which dramatically slows down the nucleation and succeeding crystallization. In order to verify this reasoning, we decided to study a low molar mass PVME of narrow polydispersity, for which the glass transition region is expected to shift to lower temperature in comparison with the previously discussed commercially available PVME. Hence, the temperature gap between the upper limit of Tg and the melting curve will enlarge, which might reveal (i) the direct crystallization/melting of water at high PVME concentration and (ii) the UCST miscibility gap at sub-zero temperatures. Throughout this work, the phase behavior of the PVME/water system will be investigated by means of MTDSC and the obtained results will be discussed in relation to the aforementioned predictions of the Wertheim-LTPT. Experimental Section Materials. Poly(vinyl methyl ether) (PVME) has been synthesized by living cationic polymerization as described elsewhere.32 The obtained polymer (PVME4050) has a number average molar mass, Mn ) 4.05 kg mol-1 (determined by NMR, Bruker AM 300) with a polydispersity index, Mw/Mn ) 1.10 (determined by size exclusion chromatography, SEC, Waters).33 The polymer was dried at 40 °C under vacuum for several days, until the water mass fraction was less than 0.002 as determined by thermogravimetric analysis (TA Instruments 2950 TGA) at 120 °C. The glass transition temperature of this dried PVME4050 sample is ca. -30.5 °C. Sample Preparation. A range of compositions was prepared by adding the appropriate amount of water to dried PVME4050 directly in Mettler aluminum pans that subsequently were hermetically sealed. The inserted sample mass typically varied between 0.5 and 3 mg. Mixture compositions are expressed using polymer mass fractions, wPVME. The sample pans were stored at 4 °C for at least 3 weeks to obtain homogeneous mixtures. Modulated Temperature Differential Scanning Calorimetry (MTDSC). A first series of MTDSC measurements was performed on a TA Instruments 2920 DSC with the MDSC option and a refrigerated cooling system; a second series of MTDSC experiments was performed on a TA Instruments Q1000 (T-zero DSC-technology) with a liquid nitrogen cooling system. Helium was used as a purge gas (25 mL min-1). Indium and cyclohexane were used for temperature calibration. Indium was also used for enthalpy calibration. Standard modulation conditions are an amplitude of 0.5 °C with a period of 60 s. Heat capacity calibration was performed in standard modulation conditions with water, using the heat capacity difference between two temperatures, one above and one below the melting temperature. In this way, the most accurate measurements of heat capacity changes and “excess” contributions, cpexcess, were obtained. Data are expressed as specific heat capacities (or changes) in J g-1 K-1. Non-isothermal experiments were performed at an underlying heating/cooling rate of 0.2 or 1 °C min-1. Results and Discussion Theoretical Phase Behavior for a Model Polymer Solution according to the Wertheim Lattice Thermodynamic Perturbation Theory. From the theoretical point of view, it is of interest to explore the complicated phase behavior of systems involving hydrogen-bonding using a theoretical approach that incorporates the formation of specific interactions. In this respect, we apply the seminal work of Wertheim regarding saturation interactions, whereby the Wertheim theory is mapped

Figure 1. Theoretical reduced temperature-composition (T*-φ1) phase diagram for model polymer solutions with different values of the polymer chain length s1 () 500, 100, 50, 10): LCST spinodals (solid lines) and critical conditions (9, s1 ) 500; 0, s1 ) 100; b, s1 ) 50; o, / s1 ) 10); melting temperature Tm,0 of the solvent versus composition (dashed line, s1 ) 500; dash-dotted line, s1 ) 100; dash-dot-dotted line, s1 ) 50; dotted line, s1 ) 10). Parameter values used are summarized in Table 1.

onto the lattice model. In short, a binary mixture of a solvent (component 0) and a monodisperse polymer (component 1) are considered, causing the lattice positions to be occupied either by a solvent molecule or by a segment of a polymer molecule, occupying s1 consecutive lattice sites. Each solvent molecule carries a specific site A, and each polymer segment carries a specific site B. Nearest neighbor molecules and segments interact by dispersive interactions with an interaction energy, -ij (i,j ) 0, 1), depending on the pair of units (solvent molecules or polymer segments) involved. Furthermore, the specific sites A on molecules of component 0 interact with interaction strength -AA 00 if they are properly oriented relative to each other, and specific sites A and B interact with interaction BA strength -AB 01 ) -10 . No specific interaction exists between BB sites B; i.e., 11 ) 0. The proper orientation of the specific sites A and B on segments i and j, respectively, is quantified by the parameters KAB ij , the ratio of the nearest neighbor positions with the proper orientation to all possible orientations. Finally, to obtain the melting line of the solvent, the (reduced) 0 (*) equilibrium melting temperature Tm,0 and the (reduced) 0 (*) melting enthalpy ∆Hm,0 are calculated. More details about the theory and the equations needed to calculate the phase behavior (spinodals, binodals, critical conditions, and melting line of the solvent) are presented elsewhere.26 Figure 1 shows the reduced temperature-composition T*-φ1 phase diagram of model polymer solutions in which the polymer chain length (molar mass) is varied. The phase boundaries, i.e., the high-temperature (T* > 0.265) bimodal LCST spinodal, the adjacent UCST spinodals at low temperature (T* < 0.125), and the concentration dependence of the solvent melting line, together with all critical conditions are calculated using the parameter values summarized in Table 1. The most important chain length dependence is noticed for the low composition lobe of the bimodal LCST miscibility gap: the bimodality becomes less pronounced with decreasing chain length as indicated by the dashed arrow in Figure 1. At sufficiently small chain lengths, the bimodality even disappears, turning the type III phase behavior into an apparent type II phase behavior. These large variations with chain length are dominated by the combinatorial translational entropy of mixing. The high concentration lobe of the bimodal LCST spinodal is less affected by changes in the polymer molar mass, except for very small

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TABLE 1: Parameter Values in the Wertheim Lattice Thermodynamic Perturbation Theory Used to Calculate the Phase Behavior Depicted in Figure 1a parameter s1 z 01/k 00/k ) 11/k AA 00 /k BA AB 01 /k ) 10 /k

value

parameter

value

500, 100, 50, 10 6 -0.1 0 1.5 2.8

KAA 00 KBB 11 BA KAB 01 ) K10 0 ∆Hm,0* 0 Tm,0 *

6 6 495 1.500 0.200

a s1 the number of lattice sites taken by the polymer (component 1); z the lattice coordination number; -ij(i,j)0,1) the dispersive interaction energy between segments of component i and j; -AB ij the specific interaction energy between specific sites A and B on segments of components i and j, respectively; KAB ij the ratio of the nearest neighbor positions with the proper orientation to all possible orientations for specific sites A and B on segments of component i and j, respectively; 0 * 0 * ∆Hm,0 and Tm,0 the reduced enthalpy of melting and the reduced equilibrium melting temperature of the solvent, respectively.

chain lengths for which the bimodality has disappeared (Figure 1). The observed changes in the bimodal LCST phase behavior according to the Wertheim-LTPT are in agreement with the variations expected from the semi-phenomenological analysis of the type III demixing behavior as well as with the experimentally observed molar mass dependence of some polydisperse polymers.2 Recently, the molar mass dependence of the bimodal LCST phase behavior was systematically studied for well-defined PVME samples with narrow polydispersity.33 These experiments confirm the theoretical predictions of the chain length dependence presented here, i.e., a shift of the low concentration lobe. However, the change from bimodal LCST phase behavior to an apparent type II phase behavior is not observed experimentally. In fact, for the lowest molar mass studied, a bimodal LCST miscibility gap seems to be retained. Moreover, the entire LCST miscibility gap shifts to lower temperatures, resulting in a reduced solubility instead of the improved miscibility predicted by theory. This observation is most likely related to the influence of end-groups, gaining importance with decreasing molar mass. In the Wertheim-LTPT, as presented here, the end-groups are assumed to be identical to the middle segments, even though they are expected to have different interactions with the surrounding solvent molecules. The (LCST) miscibility gap may consequently shift to either lower or higher temperatures depending on the specific interactions of these end-groups with the other interacting units of the binary mixture. The integration of end-group effects in the Wertheim-LTPT will be the subject of future work. Nevertheless, the end-group effect has already been addressed using the classical Flory-Huggins theory,34 illustrating that the change from homogeneous to heterogeneous and vice versa can indeed vary both with temperature and/or composition. The predicted influence of the polymer chain length on the UCST miscibility gaps at low temperatures (Figure 1, T* < 0.125) is insignificant. In fact, the UCST two-phase area at high polymer concentration is virtually independent of molar mass, whereas the UCST in the (semi)-dilute composition range displays a small shift to lower temperature as the polymer chain length decreases. The solvent melting line is also hardly influenced by the polymer chain length: a decrease in polymer molar mass causes the drop in melting temperature at intermediate compositions (Figure 1, 0.40 e wPVME e 0.70) to be slightly more pronounced only.

The theoretical predictions of the Wertheim-LTPT will now be evaluated by studying aqueous PVME solutions spanning the entire composition range. To minimize anomalies, a low molar mass PVME (PVME4050) of narrow polydispersity is used (Mw/Mn ) 1.10). For the low molar mass PVME, the Tgcomposition curve will shift to lower temperatures. This might reveal the direct crystallization/melting of water (even at high PVME4050 concentrations) in combination with the UCST miscibility gap(s) at sub-zero temperatures. These results will be compared to the commercially available and previously investigated aqueous PVME system (Mw ) 20 kg mol-1, Mw/ Mn ∼ 2.5).26 Bimodal LCST Demixing of Aqueous PVME4050 Solutions. Upon heating, PVME/water mixtures undergo LCST-type phase separation, which is accompanied by large compositional changes and a significant endothermic heat effect. Conventional DSC reveals a complex thermal behavior with two overlapping endothermic peaks, reflecting the shape of the bimodal demixing curve.2,27,30,35 A broad, increasing shoulder that appears as the sample is heated through a two-phase region precedes the large peak that originates from extensive compositional changes upon passing the temperature at which three liquid phases coexist. Using MTDSC, the total demixing enthalpy is separated into two endothermic contributions (depending on the modulation conditions used).27 The largest part is usually found in the apparent heat capacity signal (and as such in the reversing heat flow), whereas the non-reversing heat flow contains the remainder. The heat capacity signal is termed apparent, cpapp, because it contains a so-called “excess” contribution, cpexcess, in addition to the baseline specific heat capacity, cpbase (see eq 1). The excess contribution originates from fast demixing/ remixing processes at the polymer/water interphase of coexisting phases. These fast demixing/remixing processes occur during each modulation cycle and thus contribute to the amplitude of the modulated heat flow and to the heat capacity signal derived from it. This phenomenon has been elaborated previously.6,27,33,36,37 Note that cpapp is temperature- and time-dependent and therefore changes during the phase transformation (e.g., due to the phase morphology development).

cpapp ) cpbase + cpexcess

(1)

Figure 2 illustrates the evolution of both cpapp (thick) and the non-reversing heat flow (thin) upon heating an aqueous PVME4050 solution with wPVME ) 0.27. The initial deviation of cpapp with respect to the extrapolated experimental baseline specific heat capacity, cpbase (dashed line), is considered as the start of phase separation (Figure 2, O). This was previously discussed in more detail for commercially available PVME.27 Instead of using the extrapolated experimental heat capacity as an estimate for the baseline inside the phase separation region, cpbase can be calculated in a more quantitative manner by considering the absolute heat capacity values of both water and PVME.38 Such a detailed analysis is beyond the scope of this paper. The evolution of the apparent specific heat capacity resembles that of the total heat flow from conventional DSC, reflecting the shape of the bimodal LCST demixing curve. In contrast, only a single, sharp transition is observed in the non-reversing heat flow, occurring just before the maximum of the large peak in cpapp (Figure 2, indicated by the vertical dotted line). The onset temperature of the heat effect in the non-reversing heat flow is independent of the polymer concentration and can therefore be used to determine the invariant three-phase equilibrium, demonstrating the added value of MTDSC in comparison with conventional DSC.27

Phase Transformations in PVME/Water

Figure 2. cpapp (thick) and non-reversing heat flow (thin) during nonisothermal demixing for a wPVME ) 0.27 mixture: demixing temperature (O), temperature at the three-phase equilibrium (vertical dotted line). Extrapolated experimental baseline specific heat capacity cpbase is indicated (dashed line).

Figure 3. Demixing enthalpy per gram of solution (a) or per gram of PVME (b) as a function of concentration.

Integration of the total heat effect from MTDSC yields the demixing enthalpy. The composition dependence of the demixing enthalpy (per gram solution) is nearly symmetrical, with a maximum at wPVME ≈ 0.50 (Figure 3a), in agreement with literature.35 Normalizing the demixing enthalpy to the amount of PVME reveals two distinct regions (Figure 3b). At higher PVME concentrations (wPVME g 0.30), the endothermic heat effect increases linearly with increasing water content, indicating the continuous increase of the number of water molecules surrounding the polymer chains.39 At lower PVME concentrations (wPVME < 0.30), the normalized demixing enthalpy levels off, indicating that the available water molecules exceed the amount needed to fully hydrate the polymer chains. Aqueous PVME4050 Solutions at Sub-zero Temperatures. The low-temperature properties of aqueous (high molar mass) PVME solutions have attracted quite some attention during the

J. Phys. Chem. B, Vol. 111, No. 6, 2007 1291 last couple of years, triggered by the inhibited crystallization of water at high polymer concentration (wPVME g 0.60). This observation was attributed to the formation of a stable intermolecular complex consisting of ca. two water molecules per polymer repeating unit,28-31 rather than being attributed to the vitrification of the homogeneous sample prior to crystallization, a phenomenon observed for aqueous solutions of poly(Nisopropyl acrylamide) (PNIPAM)4,37 and of poly(N-vinyl caprolactam) (PVCL).5 Both recent theoretical predictions (by the Wertheim-LTPT) and experimental studies emanating from these predictions elucidated that the melting curve of ice shows an unusual drop starting at intermediate compositions (0.50 e wPVME e 0.70). This causes the actual supercooling at high polymer concentration to become smaller than what could be anticipated for a conventional course of the melting curve.26 The vicinity of the glass transition region therefore causes the solution to vitrify upon cooling, slowing down the rate of nucleation and subsequent crystallization. According to this kinetic argumentation, the crystallization of water is expected to occur over the entire composition range if the Tg-composition curve is sufficiently lowered by using low molar mass PVME (in comparison with the commercially available PVME). The phase behavior at sub-zero temperatures was studied by cooling homogeneous PVME4050/water solutions to -100 °C (at 1 °C min-1) for compositions spanning the entire concentration range. Figure 4a shows the total heat flow upon cooling a wPVME ) 0.27 mixture, demonstrating that in this particular case a supercooling of ca. 20 °C is needed to induce crystallization of water (), onset crystallization exotherm). The observed crystallization process causes phase separation between icecrystals and a PVME-rich solution, which will inevitably vitrify upon further cooling. In the subsequent heating, the devitrification of the glassy PVME-rich phase (Figure 4b, ×) is followed by the melting of ice (Figure 4b, b, endset melting endotherm). Note that the observed glass transition temperature at ca. -37 °C (Figure 4b, ×) corresponds to the Tg of a homogeneous solution with wPVME ≈ 0.98, which indicates that nearly all the water was able to crystallize upon cooling. Similar observations hold for solutions with wPVME e 0.64, as illustrated in Figure 4b. Note that the melting endotherm consists of two overlapping peaks, in accordance with observations made for the commercially available polymer. This was previously attributed to the existence of bound and free water that melt at different temperatures.28,29 In this work, it will be illustrated that the shape of the melting endotherm basically reflects the change with composition of the solid-liquid equilibrium. Even though upon cooling crystallization (on the experimental time scale) occurs only for mixtures with wPVME e 0.64, upon heating, cold crystallization and subsequent melting could be observed up to wPVME e 0.94 (Figure 5), after the devitrification of the initially vitrified homogeneous solution (Figure 5, 9, Tg). As a result, the melting curve of ice could be obtained over the entire composition range, showing an atypical stepwise decrease to ca. -13 °C at 0.40 e wPVME e 0.70 (Figure 6, b), in agreement with theoretical predictions. This observation confirms that the inhibited crystallization in the commercial PVME/ water system (discussed earlier) was kinetically driven rather than being associated with the formation of a stable intermolecular complex. Evidence for the peculiar evolution of the melting line of the solvent is also found in the shape of the double melting endotherm (Figure 4b), which can be related to the temperature derivative of the melting curve.

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Figure 6. Change in composition with temperature (continuous line) for a wPVME ) 0.30 mixture, obtained by partial integration of the melting endotherm of the same mixture (dashed line) after cooling the solution to -100 °C. Experimentally determined melting temperatures (endset) for different sample compositions upon heating (b).

Figure 4. Total heat flow during (a) non-isothermal cooling for a wPVME ) 0.27 mixture, crystallization temperature ()), and (b) non-isothermal heating for solutions of different compositions PVME/water, Tg PVMErich phase (×), and endset melting endotherm (b). Curves are shifted vertically for clarity.

Figure 5. Total heat flow during non-isothermal heating for a wPVME ) 0.70 mixture, Tg homogeneous solution (9), cold crystallization exotherm (indicated by arrow), and endset melting endotherm (b).

Indeed, the course of the solid-liquid line can (partly) be reconstructed from the shape of the double melting peak observed for one specific composition. It is assumed that the

liquid water phase formed at any temperature during the melting process quickly remixes with the liquid PVME-rich phase to make the composition of the PVME/water mixture in accordance with the thermodynamic solid-liquid equilibrium. The heat of fusion needed up to each temperature in the melting region (i.e., the partial integrated heat flow up to that particular temperature) will determine the amount of liquid water formed and as such the new composition of the liquid phase in equilibrium with the remaining ice. To calculate the heat of fusion at each temperature, the heat capacity values of water in both the liquid and the solid state are needed. The resulting calculated change of the liquid-phase composition with temperature is shown in Figure 6 for a PVME4050/water mixture of initial composition wPVME ) 0.30 (continuous line). Note that the compositional change at low temperature starts from wPVME ≈ 0.98 after (slowly) cooling to -100 °C (independent of the initial sample composition). The agreement between the melting curve calculated from the shape of one particular melting endotherm and the experimental endset temperatures of the melting endotherm using samples of different compositions is quite reasonable, although not perfect. Especially at high PVME concentrations, deviations are noticed (e1.5 °C), which might indicate that the assumption made about the quasi-instantaneous remixing of freshly molten ice with the liquid PVME-rich phase only holds approximately. However, the error made must be quite small because the shape of the LCST demixing endotherm (in capp p ) does not depend on the sample thermal history, independent of the sample composition (Figure 7), illustrating that the remixing process catches up throughout the melting of ice. UCST Demixing of Aqueous PVME4050 Solutions. The novel theoretical developments based on the Wertheim Lattice Thermodynamic Perturbation Theory also predict the existence of two narrow, adjacent UCSTs at low temperatures. A previous study on the commercially available PVME mixed with D2O revealed a remarkable increase in the width of the solution’s Tg for certain concentrated compositions,26 suggesting inhomogeneities in the samples studied, and thus possibly indicating the existence of a UCST-miscibility gap. However, it was impossible to disregard the effect of the broad molar mass

Phase Transformations in PVME/Water

Figure 7. cpapp during non-isothermal heating for a wPVME ) 0.86 mixture, either starting from 10 °C (thin line) or starting from -100 °C (thick line).

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Figure 9. Width of the glass transition region (∆Tg) as a function of concentration of PVME/water.

Figure 8. cpapp during non-isothermal cooling for solutions of different compositions PMVE/water: Tg homogeneous solution (9). The arrow indicates the plasticizing effect of water. Curves are shifted vertically for clarity.

distribution of the commercially available polymer. For that reason, a more elaborate study was performed in this work on the low molar mass PVME4050 having a narrow polydispersity. On the basis of the theoretical predictions, it is anticipated that the UCST miscibility gaps are influenced little by the lower molar mass (Figure 1), making them experimentally more accessible because the Tg-composition curve shifts to lower temperatures for the low molar mass PVME. Figure 8 shows the evolution upon cooling of several highly concentrated aqueous PVME4050 solutions, i.e., mixtures in which crystallization of water solely occurs upon heating (see explanation in Figure 5). Hence, the Tg of the homogeneous solution can be measured, which, as expected, lowers with increasing water content due to the plasticizing effect of the solvent (Figure 8, indicated by the arrow). In addition, the width of the glass transition region (∆Tg) can be evaluated, as indicated in Figure 8 for a solution with wPVME ) 0.98. A significant increase of ∆Tg is observed for mixtures with 0.78 e wPVME e 0.94 (Figure 9). For these compositions, the glass transition region clearly consists of two distinct Tg values (Figure 8, thick

Figure 10. Temperature-derivative of cpapp during non-isothermal cooling for solutions of different compositions of PVME/water. Tg of the coexisting phases is taken as the maximum in dcpapp dT-1 (/). Dashed lines are a guide to the eye. Curves are shifted vertically for clarity.

curves), indicating that a UCST-type phase separation occurred prior to vitrification. The two Tg values are easily visualized using the derivative of cpapp with temperature, allowing one to determine the glass transition temperature of the (co)-existing PVME-rich phase(s) (Figure 10, /). Even though we have established the location of the UCST at high polymer concentration, the temperature at which demixing sets in remains unknown, because no heat effect upon cooling was detected, in contrast with the LCST demixing upon heating (Figure 2). This is quite reasonable, as the heat effects associated with compositional variations of the coexisting phases in such a narrow UCST miscibility gap are expected to be much smaller than the heat effects observed during the LCST phase separation. However, earlier work illustrated that quasiisothermal phase separation in binary polymer mixtures often

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Figure 11. cpapp during stepwise quasi-isothermal cooling for a wPVME ) 0.88 mixture at indicated temperatures.

introduces time-dependencies in the apparent specific heat capacity signal, which are attributed to morphological changes or interphase development within the sample toward an equilibrium situation.6,27,36,37 Therefore, we examined the UCST demixing at low temperature in a similar manner, i.e., by performing stepwise quasi-isothermal cooling experiments. These are termed quasi-isothermal because the underlying temperature is kept constant although the instantaneous temperature varies due to the imposed modulation of (0.5 °C. Heat capacities measured in stepwise quasi-isothermal experiments upon cooling show no time-dependency down to a certain temperature (e.g., -15.0 °C for a wPVME ) 0.88 mixture, Figure 11) and thus coincide with the non-isothermal cooling curve (not shown). However, at -17.5 °C, cpapp increases with time, reflecting certain (morphological) changes in the sample, which cannot be attributed to crystallization of water or to a (partial) vitrification of the polymer rich phase, because both transformations are expected to invoke a decrease in the apparent specific heat capacity signal.40,41 Hence, the observed time-dependency is most likely associated with the temperature-induced UCST-type phase separation upon cooling (such as the creation of an interfacial contact surface). The initial temperature at which cpapp starts to increase with time is considered as the demixing temperature of the mixture studied. Thus, for each highly concentrated solution that displayed a double Tg upon cooling (Figure 10), the demixing temperature can be found, allowing the UCST demixing curve to be constructed (see next section). Note that at temperatures near -25.0 °C cpapp starts decreasing with time due to vitrification of the (co)-existing polymer rich phase(s). State Diagram of PVME4050/Water. The thermal transitions discussed so far were collected to construct a state diagram (Figure 12). The bimodal LCST demixing curve (O) that consists of two lower two-phase areas separated from one upper twophase area by a three-liquid-phases coexistence line at invariant temperature is in agreement with the commercially available polymer described in literature.2,3,22,27 This bimodal LCST miscibility gap of the PVME4050/water system studied was predicted by the Wertheim-LTPT provided that the solvent-polymer interaction strength is (somewhat) larger than the solvent-solvent interaction strength.26 Additionally, and closely related to the existence of bimodal LCST phase behavior, two adjacent UCSTs at low temperature were predicted. The UCST at high PVME4050 concentration is experimentally confirmed using MDSC for compositions of 0.78

Figure 12. State diagram of PVME/water determined with MTDSC: crystallization curve ()) and UCST demixing curve (0) upon cooling; Tg PVME-rich phase (×), melting curve (b), LCST demixing curve (O), and Tg-composition curve (homogeneous (9), see also Figure 8; coexisting phases within UCST area (/), see also Figure 10; width I) upon cooling.

e wPVME e 0.94 through the detection of two Tg values, indicative for the preceding phase separation (Figure 12, /). The associated demixing temperatures (Figure 12, 0) could be determined through well-defined stepwise cooling experiments. The intersection of the UCST-type demixing curve with the Tgcomposition curve (9, width I) causes the Tg values of both coexisting phases (Figure 12, /) to remain nearly constant. The melting curve of ice (b) is also included. Its shape is again confirming the theoretically predicted peculiar step for compositions between 0.40 e wPVME e 0.70. Unlike for the commercially available PVME/water system wherein crystallization of water was completely inhibited at wPVME g 0.60, the solidliquid equilibrium could now be evaluated for compositions spanning the entire concentration range. This different behavior stems from the lower molar mass of PVME4050, which gives rise to an adequate temperature gap between the upper limit of Tg and the solvent melting curve, facilitating nucleation and subsequent crystallization. Conclusions Modulated temperature differential scanning calorimetry (MTDSC) was successfully applied to determine the phase behavior of aqueous PVME solutions, confirming the liquidliquid and solid-liquid phase boundaries of a model polymer solution predicted by the Wertheim Lattice Thermodynamic Perturbation Theory. The bimodal LCST phase behavior of the low molar mass PVME4050/water system studied was established and is in agreement with the one for commercially available polymers. Furthermore, the melting curve of water, which shows an unusual step with composition, was directly determined from the melting endotherm observed and indirectly calculated through partial integration, assuming immediate remixing during melting. This peculiar shape of the melting line demonstrates that the actual supercooling becomes smaller than could be anticipated for a conventional course of the melting curve. This

Phase Transformations in PVME/Water effect is responsible for the arrested crystallization in the commercially available PVME/water system because the homogeneous solution vitrifies before crystallization occurs. However, such phenomena were not observed in this work due to the low molar mass of the PVME used, which shifts the Tgcomposition curve to lower temperatures enabling crystallization prior to vitrification. The double melting endotherm observed in MTDSC is probably not related to the melting at different temperatures of bound and free water, it simply reflects the atypical shape of the melting line. The sensitivity of MTDSC for measuring broad, overlapping glass transitions and its ability to perform quasi-isothermal measurements allowed for the experimental confirmation of the predicted UCST-type miscibility gap at low temperature and high polymer concentration. Work is in progress to experimentally detect the adjacent UCSTmiscibility gap at low temperature and low polymer concentration. These findings from MTDSC confirm all non-trivial predictions made by the Wertheim-LTPT, in particular illustrating some atypical properties at low-temperature, which are not associated to intermolecular complexes as was previously assumed. They also confirm that the theory captures the essential features of the phase behavior of systems involving saturation interactions. Acknowledgment. Kurt Van Durme thanks the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT) for a PhD-grant. The work was supported by the bilateral (international) scientific and technological cooperation of the Ministry of the Flemish Community and the Ministry of Science and Technology of the People Republic of China (BIL01/06). The authors thank the Fund for Scientific Research Flanders (FWO) for financial support. References and Notes (1) Sˇ olc, K.; Dusˇek, K.; Koningsveld, R.; Berghmans, H. Collect. Czech. Chem. Commun. 1995, 60, 1661-1688. (2) Scha¨fer-Soenen, H.; Moerkerke, R.; Berghmans, H.; Koningsveld, R.; Dusˇek, K.; Sˇ olc, K. Macromolecules 1997, 30, 410-416. (3) Moerkerke, R.; Meeussen, F.; Koningsveld, R.; Berghmans, H. Macromolecules 1998, 31, 2223-2229. (4) Afroze, F.; Nies, E.; Berghmans, H. J. Mol. Struct. 2000, 554, 5568. (5) Meeussen, F.; Nies, E.; Verbrugghe, S.; Goethals, E.; Du Prez, F.; Berghmans, H. Polymer 2000, 41, 8597-8602. (6) Van Durme, K.; Loos, W.; Du Prez, F. E.; Van Mele, B. Polymer 2005, 46, 9851-9862. (7) Osada, Y.; Kishi, R.; Hasebe, M. J. Polym. Sci., Part C: Polym. Lett. 1987, 25, 481-485. (8) Shinohara, S.; Tajima, N.; Yanagisawa, K. J. Intell. Mater Syst. Struct. 1996, 7, 254-259.

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