Phase Separation in Water-Ethanol Polymer Solutions Studied by NMR Methods

Transcription

1 WDS'07 Proceedings of Contributed Papers, Part III, , ISBN MATFYZPRESS Phase Separation in Water-Ethanol Polymer Solutions Studied by NMR Methods H. Kouřilová and L. Hanyková Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. We applied 1 H and 13 C NMR spectroscopy to investigate a phase transition and a cononsolvency effect of uncharged poly(n-isopropylmethacrylamide) (PIPMAm) and negatively charged PIPMAm copolymers in water/ethanol (D 2 O/EtOH) mixtures. For this purpose several PIPMAm solutions with various volume mixture compositions of D 2 O/EtOH (100/0-5/95 vol.-%) were prepared; sodium methacrylate was used as an ionic comonomer. Neat binary D 2 O/EtOH mixtures with the same compositions were studied for the comparison to be able to distinguish between effects caused by the phase transition and effects caused by the properties of binary D 2 O/EtOH mixtures. It is obvious from 1 H and 13 C spectra that with increasing temperature the concentration range of the phase separation broadens to lower values of ethanol content in the solution. We observed a difference in characteristics of the phase transition induced by a change of temperature or by a change of solvent composition. When the phase separation is induced by the solvent composition polymer groups at the ends of dangling chains are more mobile than those in backbone chains. After the temperature-induced phase transition PIPMAm groups behave in the same way. We confirmed this when investigating solvent dynamic properties by means of ethanol 13 C T 2. We found out that charge in chains prevents the polymer solution from the phase separation by strenghtening interactions between polymer and solvent. Introduction Some acrylamide-based polymers including poly(n-isopropylmethacrylamide) (PIPMAm) has very interesting behaviour in aqueous solutions, they exhibit so called lower critical solution temperature (LCST). They are soluble at low temperatures but at temperatures above LCST phase separation occurs [e.g., Fujishige et al., 1989; Netopilík et al., 1997; Idziak et al., 1999]. The process of phase separation is reversible. When lowering temperature under LCST, polymer dissolves again. The phase transition can also be induced at a constant temperature by addition of some polar solvent (for example ethanol) into water. Under LCST PIPMAm is soluble in pure water and in pure ethanol but is not soluble in their mixtures of certain compositions. This formation of nonsolvent by mixing of two solvents si called the cononsolvency effect [Winnik et al., 1990; Schild et al., 1991; Costa et al., 2002]. Both temperature-induced phase separation and cononsolvency effect are assumed to be a macroscopic manifestation of a coil-globule transition followed by aggregation as shown for acrylamide-based polymers in water by light scattering and small angle neutron scattering [Fujishige et al., 1989; Kubota et al., 1990; Zhu et al., 1999; Pleštil et al., 1987]. The transition is probably associated with competition between hydrogen bonding and hydrophobic interactions. Cononsolvency phenomenon is studied particularly in solutions of poly(n-isopropylacrylamide) (PI- PAAm); microcalorimetric measurements evidenced PIPAAm cononsolvency behaviour in mixtures of water and methanol, tetrahydrofuran, dioxane and other solvents [Schild et al., 1991], concluding that water-organic solvent complexes are preferred over PIPAAm-water hydrogen bonds. Cloud point measurements were applied to investigate PIPAAm in water-low molecular weight alcohols with respect to molecular structures of cononsolvents [Costa et al., 2002]. Semidilute solutions of PIPAAm in aqueous solution of tetrahydrofuran and n-butyl alcohol were measured by small angle X-ray scattering method [Shimizu et al., 2003] and the contribution of the segment-segment interactions to the entropy and the enthalpy were calculated. Although critical solution phenomena are mostly studied in solutions of PIPAAm, there are several studies on PIPMAm. LCST of PIPMAm in neat water is 315K [Netopilík et al., 1997; Starovoytova et al., 106

2 2004]. The presence of the α-methyl group in the PIPMAm monomer was found to increase the LCST [Kubota et al., 1990; Tiktopulo et al., 1995] compared with that of PIPAAm. Strong cononsolvency effect was observed for uncharged and negatively charged networks of PIPMAm in water/ethanol mixtures at room temperature [Alenichev et al., 2007]. Recently, we employed 1 H NMR spectroscopy for study of the temperature-induced phase transition of uncharged PIPMAm and negatively charged PIPMAm copolymers in water solutions in a broad range of polymer concentrations and ionic comonomer mole fractions [Starovoytova et al., 2003]. In the present paper we applied 1 H and 13 C NMR spectroscopy to investigate the cononsolvency effect of uncharged PIPMAm and negatively charged PIPMAm copolymers in water/ethanol (D 2 O/EtOH) mixtures. These water/ethanol (D 2 O/EtOH) mixtures contained various volumes of ethanol from 0 vol.- % to 95 vol.-%. We also studied neat binary mixtures of water and ethanol of the same compositions for comparison to be able to distinguish between effects caused by the phase transition and effects caused by properties of D 2 O/EtOH mixtures. Experimental Samples N-isopropylmethacrylamide (IPMAm, Fluka) and sodium methacrylate (MNa) were used to prepare uncharged and negatively charged PIPMAm/D 2 O/EtOH solutions of polymer concentrations c = 5 wt.- % and ionic comonomer mole fractions i = 0, 5 and 10 mole-%. 4,4 -azobis(4-cyanopentanoic acid) was used as initiator and polymerization was carried out in a D 2 O/EtOH mixture (6/94 vol.-%); the volume fraction of the sum of the monomers in the mixture was 0.2. After polymerization and subsequent drying of polymers to constant weight, the PIPMAm/D 2 O/EtOH solutions with polymer concentrations c = 5 wt.-% and volume fractions of ethanol in D 2 O/EtOH mixtures 0, 20, 30, 40, 50, 60, 80 and 95 vol.-% were prepared. All samples of PIPMAm/D 2 O/EtOH solutions in 5-mm NMR tubes were degassed and sealed under argon; sodium 2,2-dimethyl-2-silapentane-5-sulfonate (DSS) was used as an internal NMR standard. NMR Experiments High-resolution 1 H NMR spectra were recorded with a Bruker Avance 500 spectrometer operating at MHz. Typical measurements conditions were as follows: 16 scans, relaxation delay 5 s, spectral width 5 khz, acquisition time 1.64 s, π/2 pulse width 13.7 µs. High-resolution 13 C spectra were accumulated usually with 128 scans, relaxation delay 5 s, π/2 pulse width 13.9 µs, spectral width 27.8 khz and aquisition time 1.13 s under full proton decoupling. The integrated intensities were determined with the spectrometer integration software. Precision of this software is 3%, total accuracy of the integration is approximately 10%. The 13 C relaxation times T 2 of ethanol groups were measured using CPMG pulse sequence with a 1 H pulse added to remove cross-correlation between chemical shift anisotropy and dipole-dipole interactions [Kay et al., 1991]. The relaxation delay was set to 60 s which is adequately long to allow a complete recovery of 13 C magnetization. All obtained 13 C T 2 relaxation curves had a monoexponential character and the fitting process always enabled us to determine the single value of the respective relaxation time. In all types of experiment the temperature was maintained constant within 0.2 C using BVT 3000 temperature unit. Results and Discussion 1 H and 13 C NMR spectra Figure 1. Chemical structure of poly(n-isopropylmethacrylamide) 107

3 We present two 1 H high-resolution NMR spectra of uncharged PIPMAm/D 2 O/EtOH solutions (ionic comonomer mole fraction i=0 mole-%) (Fig. 2). These two solutions differ in ethanol fraction in D 2 O/EtOH mixture (20 and 80 vol.-% of ethanol). Both of them were measured at 328K under the same experimental conditions. The peak at 4.2 ppm is an overlapped signal of HDO/EtOH OH protons. Signals of PIPMAm methyl groups are overlapped by CH 3 signal of EtOH. Figure 2. 1 H high-resolution NMR spectra of uncharged PIPMAm/D 2 O/EtOH solutions containing 20 vol.-% (a) and 80 vol.-% (b) of ethanol with peak assignment measured at 328K. The spectrum of solution that contains 80 vol.-% of ethanol (Fig. 2b) corresponds to the homogeneous state; the sample is transparent (see Fig. 3). The other spectrum (Fig. 2a) corrensponds to a phase separated solution and the sample is turbid (see Fig. 3). Figure 3. Phase separated (on the left) and homogeneous (on the right) PIPMAm/D 2 O/EtOH solutions. The two spectra differ in the integrated intensity of PIPMAm peaks. In the phase separated solution (the sample containing 20 vol.-% of ethanol) polymer chains form compact globular structures. Their mobility is restricted so the corresponding NMR peaks are very broad and therefore are not visible in high-resolution NMR spectra. The integrated intensities of EtOH in Fig. 2 follow the ratio of molar ethanol fractions in corresponding solutions, so confirming that all ethanol molecules are directly detected in 1 H NMR spectra in both solutions. Such changes in shape and intensity of NMR lines have already been observed during temperatureinduced phase transition in uncharged PIPMAm/D 2 O solutions [Starovoytova et al., 2003]. We use a quantity p-factor for analyzing polymer spectral lines and thus finding ethanol concentration range where the phase separation occurs [Starovoytova et al., 2003; Spěváček et al., 2002]. p-factor is a fraction p p = 1 (I/I 0 ) (1) of phase separated polymer units. It is related to integral intensity of polymer peaks. In the definition (1) I 0 is integral intensity of a peak in a homogeneous solution (polymer is dissolved) and I is integral intensity of the same peak in a partially/totally phase separated solution. For measurements at 298K for I 0 we used values of integral intensities from spectrum of PIPMAm/D 2 O solution measured at 298K. For spectra that were obtained at higher temperatures we used the fact that integral intensities should decrease with absolute temperature as 1/T. 108

4 In the graph below (Fig. 4) you can see a dependence of p-factor of a 1 H CH peak of uncharged PIPMAm in PIPMAm/D 2 O/EtOH solutions at 298K, 313K and 328K. It follows from definition (1) that solutions showing p > 0 are partially (p < 1) or totally (p = 1) phase separated. For example at 328K for ethanol concentration from 0 vol.-% to 30 vol.-% p-factor p = 1. That means that all PIPMAm units are involved in globular structures. At 298K the concentration range of maximal value of p is around 40 vol.-% ethanol. For lower and higher ethanol concentrations the ratio of PIPMAm units with restricted mobility decreases and p-factor for solutions with the highest (95 vol.-%) and the lowest (0 vol.-%) ethanol fraction is zero. Similar cononsolvency effect was found for uncharged PIPMAm networks in water/etoh mixtures at room temperature - for solvent mixtures with EtOH fraction about 40 vol.-% a pronounced swelling minimum was observed [Alenichev et al., 2007]. Figure 4. Dependence of 1 H p-factor of CH peak of uncharged PIPMAm in PIPMAm/D 2 O/EtOH solutions on ethanol concentration at 298K, 313K and 328K. With increasing temperature the concentration range of phase separation expands to lower ethanol fraction values. At 298 K it is phase separated at approximately 40 vol.-% of ethanol, at 328K from 0 to 30 vol.-%. The higher temperature above LCST causes disruption of hydrogen bonds between polymer and solvent and hydrophobic polymer-polymer interactions are preferred. Such temperature-induced phase separation at temperatures around 315 K was observed in aqueous solutions of PIPMAm [Netopilík et al., 1997; Starovoytova et al., 2003]. The dependencies of CH and CH 2 1 H polymer p-factors exhibit minimum at the same ethanol concentration (at the same temperature). Other 1 H polymer peaks were overlapped with ethanol peaks. The p-factor dependencies of 1 H CH peak (the one in the dangling chain) on ethanol fraction are systematically shifted towards lower values of p-factor at all three experimental temperatures. This can mean that 1 H CH 2 (the one in the backbone chain) is less mobile than 1 H CH group in the dangling chain. Side chains probably do not participate in the phase separation as much as backbone chains. We measured 13 C spectra to verify this theory. When calculating 13 C p-factors we used the same method as in the case of 1 H p-factors. 13 C spectral lines of PIPMAm groups are well resolved except for the α-methyl line that is overlapped with CH 3 line of EtOH. From the dependence of 13 C p-factors of peaks of uncharged PIPMAm in PIPMAm/D 2 O/EtOH solutions on ethanol concentration at 313K (see Fig. 5) it follows that behaviour of each single polymer group depends on its position in PIPMAm. At 313K at the ethanol concentration range about vol.-% after phase separation the isopropyl CH 3 group is released from globules prior to other groups with increasing ethanol concentration. The p-factor of the CH 2 group in the backbone chain of PIPMAm is the highest in the whole ethanol concentration range. That means that this part of PIPMAm is more rigid. So there is a correspondence between 1 H and 13 C p-factors. We also investigated how ionization of PIPMAm affects the phase separation. We examined negatively charged PIPMAm/D 2 O/EtOH solutions with ionic comonomer mole fractions i = 5 and 10 mole-%. We again measured 1 H spectra, calculated 1 H p-factors and compared with p-factors of uncharged PIP- MAm - all of them at 298K. In the Fig. 6 there is the dependence of these 1 H p-factors of CH peak of uncharged and negatively charged PIPMAm. It is obvious that whereas solutions with uncharged PIPMAm clearly show phase transition depending on the ethanol content, behaviour of charged solutions is not influenced by solvent composition, e.g. p = 0 in the whole range of ethanol concentrations. The charge in polymer chains makes the polymer molecule more hydrophillic and it stabilizes hydrogen bonds between solvent molecules and polymer chains; that results in greater solubility and flexibility of the polymer. The charge prevents the solution 109

5 Figure 5. Dependence of 13 C p-factors of various peaks of uncharged PIPMAm in PIPMAm/D 2 O/EtOH solutions on ethanol concentration at 313K. Figure 6. Dependencies of 1 H p-factors of CH peak of uncharged and negatively charged (i = 5 and 10 mole-% of MNa) PIPMAm in PIPMAm/D 2 O/EtOH solutions on ethanol concentration at 298K. from the phase separation. Similar results were obtained from the study of temperature-induced phase transition in aqueous PIPMAm solutions [Starovoytova et al., 2003]. We also investigated dynamics of the solvent. We obtained useful results from measurements of 13 C transversal relaxation times T 2 of ethanol groups. The dependence of 13 C T 2 of ethanol CH 3 on ethanol concentration at two different temperatures is shown in the Fig. 7. We do not show dependence of 13 C T 2 of ethanol CH 2 because it is very similar. Figure 7. Dependence of 13 C transversal relaxation times T 2 of ethanol CH 3 on ethanol concentration at 298K (a) and 328K (b). In Fig. 7a the 13 C T 2 were measured at 298K. The dependencies are for 2 types of samples: PIPMAm/D 2 O/EtOH and neat solvent D 2 O/EtOH without PIPMAm. At 298K the concentration range of phase separation is about 40 vol.-% of ethanol. The T 2 are not affected by the phase separation; presence of polymer in the sample just shifts the dependence to lower values. The shallow minimum in 110

6 both dependencies is caused by water-ethanol properties. In that concentration range there is maximum of interactions between water and ethanol so their T 2 are shorter [Nakanishi et al., 1967; Coccia et al., 1975]. The dependencies in Fig. 7b were obtained at 328K. At this temperature the range of phase separation is from 0 to 40 vol.-% of ethanol. For sample containing PIPMAm a significant shortening of T 2 is observed - that is caused by the phase separation. After the phase separation part of the solvent molecules stays bound to polymer chains. Mobility of these bound solvent molecules is restricted so their T 2 is shorter. We measure average T 2 of all solvent molecules (bound and free) that is shorter because of these bound molecules [Hanyková et al., 2006]. Conclusion From polymer 1 H and 13 C NMR spectra in PIPMAm/D 2 O/EtOH solutions it follows that with increasing temperature the concentration range of phase separation broadens to lower values of ethanol fraction and that charge introduced into PIPMAm chains strenghtens interactions between polymer and solvent and thus prevents the solution from the phase separation. We found out that there is a difference between globules induced by change of solvent composition and globules induced by change of temperature. In the first case polymer groups behave differently, those at the ends of dangling chains are more mobile. In the latter case all groups behave in the same way. We found out that 13 C T 2 depend on the type of the phase separation. In the first case 13 C T 2 are not influenced by the phase transition, in the latter case 13 C T 2 are shorter with phase separated solutions. Acknowledgments. This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic (project MSM ). References Alenichev, I., Z. Sedláková, M. Ilavský, Polym. Bull., 58, , Coccia, A., P. L. Indovina, F. Podo, V. Viti, Chemical Physics, 7, 30-40, Costa, R. O. R., R. F. S. Freitas, Polymer, 43, , Farrar, T. C., E. D. Becker, Pulse and Fourier transform NMR, New York: Academic Press, p. 27, Fujishige, S., K. Kubota, I. Ando, J. Phys. Chem., 93, 3311, Hanyková, L., J. Labuta, J. Spěváček, Polymer, 47, , Idziak, I., D. Avoce, D. Lessard, D. Gravel, X. X. Zhu, Macromolecules, 32, 1260, Kay, L. E., L. K. Nicholson, F. Delaglio, A. Bax, D. A. Torchia, J. Magn. Reson., 97, , Kubota, K., S. Fujishige, I. Ando, J. Phys. Chem., 94, , Kubota, K., K. Hamano, N. Kuwahara, S. Fujishige, I. Ando, Polym. J., 22, 1051, Nakanishi K., N. Kato, M. Maruyama, The Journal of Physical Chemistry, 4, , Netopilík, M., M. Bohdanecký, V. Chytrý, K. Ulbrich, Macromol. Rapid. Commun., 18, 107, Pleštil, J., Y. M. Ostanevich, S. Borbely, J. Stejskal,M. Ilavský, Polym. Bull., 17, , Schild, H. G., M. Muthukumar, D. A. Tirrell, Macromolecules, 24, , Shimizu, S., T. Seyama, R. Sekine, K. Kurita, J. Appl. Cryst., 36, , Spěváček, J., L. Hanyková, L. Starovoytova, M. Ilavský, Acta Electrotechnica et Informatica, 3, 36-43, Starovoytova, L., J. Spěváček, L. Hanyková, M. Ilavský, Macromol. Symp., 203, , Starovoytova, L., J. Spěváček, L. Hanyková, M. Ilavský, Polymer, 45, , Tiktopulo, E. I., V. N. Uversky, V. B. Lushchik, S. I. Klenin, V. E. Bychkova, O. B. Ptitsyn, Macromolecules, 28, 7519, Winnik, F. M., H. Ringsdorf, J. Venzmer, Macromolecules, 23, , Zhu, P.W., D. H. Napper, Macromol. Chem. Phys. 200, ,