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Lyotropic and Thermotropic Phase Transitions in Films of Ionene-Alkyl Sulfate Complexes Quanwei Yu,† Jens Fro¨mmel,† Thomas Wolff,*,† Miroslav Stepanek,‡ and Karel Procha´zka‡ Institut fu¨ r Physikalische Chemie und Elektrochemie, Technische Universita¨ t Dresden, D-01062 Dresden, Germany, and Department of Physical and Macromolecular Chemistry, Charles University in Prague, Albertov 2030, CZ-12840 Prague 2, Czech Republic Received April 6, 2005. In Final Form: May 20, 2005 Five [X,Y]-ionenes [(CH2)XN+(CH3)2(CH2)YN+(CH3)2]nBr-2n were prepared (X ) 3, 5; Y ) 10, 12, 22). Using new preparation methods, dry, salt-free complexes with three n-alkyl sulfates (octyl, decyl, and dodecyl) were obtained. The ionenes and their complexes were characterized in methanol solution by light scattering, viscometry, and fluorescent probe studies. The solid materials were investigated by means of DSC, TG, and ATR-IR. Transparent films were formed from stoichiometric [3,10], [3,12], and [5,10]-ionene complexes with decyl and dodecyl sulfate. In the films, mesogenic phase transitions could be induced: dry films were optically isotropic; when exposed to elevated humidity, the films slowly became optically anisotropic because of a lyotropic transition to a hexagonal mesogenic phase. The relative humidity, at which the hexagonal phase developed, was distinct for each complex. The anisotropic phases were converted to isotropic in a thermotropic transition under controlled relative humidity at specific clearing temperatures, which were higher for dodecyl complexes than for decyl complexes. This thermotropic isotropic-anisotropic transition could be cycled several times, but partial hydrolysis of the alkyl sulfates reduced the reproducibility of transition points.
1. Introduction 1
Polyelectrolyte-surfactant complexes have gained interest in areas as different as electroluminescent displays,2 biosynthetic3 and nanostructured4 materials, wastewater treatment,5 and antigraffiti coatings.6 While in contact with aqueous salt solutions mesomorphic phases of the complexes can be induced by varying the ionic strength of the solution,7 most publications on solid stoichiometric complexes claim the formation of mesomorphous phases throughout, until it turned out that carefully dried samples can be optically isotropic.8 We recently published a note on the observation of both lyotropic and thermotropic transitions between an optically isotropic and an anisotropic hexagonal phase in the [3,12]-ionene dodecyl sulfate stoichometric complex, the lyotropic transition being induced by water vapor through the gas phase.9 In the present study, this complex is compared with related ones formed from [3,10], [3,22], and [5,10]-ionenes and octyl, decyl, and dodecyl sulfate * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +49-351-46333633. Fax: +49-351-46333391. † Technische Universita ¨ t Dresden. ‡ Charles University in Prague. (1) For recent reviews, see (a) Thu¨nemann, A. F. Prog. Polym. Sci. 2002, 27, 1473-1572. (b) Thu¨nemann, A. F.; Mu¨ller, M.; Dautzenberg, H.; Joanny, J. F. O.; Lowne, H. Adv. Polym. Sci. 2004, 166, 113-171. (2) Thu¨nemann, A. F. Adv. Mater. 1999, 11, 127-130. (3) (a) Perez-Camero, G.; Garcia-Alvarez, M.; de Ilarduya A. M.; Fernandez, C.; Campos, L.; Munoz-Guerra, S. Biomacromolecules 2004, 5, 144-152. (b) Mu¨ller, M.; Rieser, T.; Dubin, P. L.; Lunkwitz, K. Macromol. Rapid Commun. 2001, 226, 390-395. (4) Liao, X. M.; Higgins, D. A. Langmuir 2001, 17, 6051-6055. (5) Guo, W.; Uchiyama, H.; Tucker, E. E.; Christian, S. D.; Scamehorn, J. F. Colloids Surf., A 1997, 123, 695-703. (6) Thu¨nemann, A. F.; Lieske, A.; Paulke, B. R. Adv. Mater. 1999, 11, 321-324. (7) Leonard, M. J.; Strey, H. H. Macromolecules 2003, 36, 95499558. (8) Klaussner, B.; Fro¨mmel, J.; Wolff, T. Des. Monomers Polym. 1999, 2, 53-59. (9) Yu, Q.; Fro¨mmel, J.; Wolff, T.; Procha´zka, K. Colloid Polym. Sci. 2004, 282, 1039-1043.
Scheme 1. Structural Formula of Stoichiometric [X,Y]-Ionene Alkyl Sulfate Complexes
(cf. Scheme 1). Modified methods9 had to be used to prepare the dry, salt-free complexes. We will show that stable transparent films are formed in 5 of 12 complexes that exhibit lyotropic transitions at distinct relative humidities and thermotropic transitions at distinct temperatures (under controlled humidity). Both the transition temperature and humidity can be determined from changes in the optical anisotropy of the films, and the transition humidity is also reflected by the enthalpy and temperature of a certain DSC peak. In addition, the properties of the ionenes and their complexes in methanol solutions were studied by viscometry, and complex formation in solution was followed by fluorescent probe investigations using 1,8-anilinonaphthalene sulfonate (ANS), whose sensitivity to the polarity of the surroundings is well known.10 Relations between solution and film properties will be discussed. 2. Experimental Section 2.1. Preparations. 2.1.1. Ionenes. The synthesis of ionenes widely followed literature procedures11 (i.e., the chlorides of R,ω-alkyldiacids were prepared, reacted to the corresponding (10) (a) Kosower, E. M. Acc. Chem. Res. 1982, 15, 259. (b) Kosower, E. M.; Dodiuk, H.; Kanety, H. J. Am. Chem. Soc. 1978, 100, 4179. (c) Chakraborti, K.; Ware, W. R. J. Chem. Phys. 1971, 55, 5494. (11) (a) Nieves, V. E. R.; Ribaldo, E. J.; Baroud, R.; Quina, F. H. J. Polym. Sci., Polym. Lett. Ed. 1982, 20, 433-437. (b) Soldi, V.; Magalhaes, N. d. M.; Quina, F. H. J. Am. Chem. Soc. 1988, 110, 5127-5143. (c) Rembaum, A.; Baumgartner, W.; Eisenberg, A. Polym. Lett. 1968, 6, 159-171. (d) Noguchi, H.; Rembaum, A. Macromolecules 1972, 5, 253260. (e) Rembaum, A.; Noguchi, H. Macromolecules 1972, 5, 262-269.
10.1021/la050907g CCC: $30.25 © 2005 American Chemical Society Published on Web 06/21/2005
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Table 1. Refraction Index Increment, dn/dc, Second Virial Coefficient, A2, and Apparent Hydrodynamic Radii, rapp H , for Ionenes and Some Alkyl Sulfate Complexes in Methanola ionene, complex [3,10]-ionene [3,12]-ionene [3,12]-ionene dodecyl sulfate complex [5,10]-ionene [5,10]-ionene dodecyl sulfate complex [3,22]-ionene
A2 (10-6 Mw (103 rapp dn/dc (10-4 H (nm) dm3 g-1) mol dm3 g-2) g mol-1)b at 10 g/dm3 1.95 1.95 1.5
4.43 0.08 0.88
1.3 1.9 6.0
1.8 2.6
1.95 1.5
1.05 1.63
2.1 6.7
2.0 3.8
1.95
-5.0
8.4
a
Without added salt. b Molecular weight evaluated from SLS data by the standard Zimm technique. Table 2. Apparent Properties of Films of Stoichiometric Complexes of X,Y-Ionenes and Alkyl Sulfatesa [3,22]-ionene [3,12]-ionene [5,10]-ionene [3,10]-ionene
dodecyl sulfate
decyl sulfate
octyl sulfate
bad good good good
bad bad good good
bad bad bad bad
a Good film: clear, transparent, flat, and optically isotropic; bad film: cloudy and undulated.
dimethylamides, reduced to the amines, and quaternized by R,ω-dibromoalkanes; see Supporting Information). The ionenes showed the published physical and spectroscopic properties.11 2.1.2. Ionene Alkyl Sulfate Stoichiometric Complexes. Dry complexes were prepared by the reaction of the silver salts of the alkyl sulfates and the ionene bromides in methanol solution under electrochemical control (see Supporting Information), as previously described for the [3,12]-ionene dodecyl sulfate complex.9 Molecular weights and other light-scattering data of the ionenes and of their complexes are given in Table 1. 2.1.3. Film Formation. After the preparation in methanol solution and slow evaporation of the solvent at low pressure and low temperature (e35 °C), the salt-free stoichiometric complexes were obtained. [3,10]-Ionene octyl sulfate, [5,10]-ionene octyl sulfate, [3,12]-ionene decyl sulfate, [3,12]-ionene octyl sulfate, [3,22]-ionene dodecyl sulfate, and [3,22]-ionene decyl sulfate stoichiometric complexes are powders or did not form clear films on a quartz plate placed at the bottom of the reaction beaker. However, the [3,12]-ionene dodecyl sulfate, [5,10]-ionene dodecyl sulfate, [5,10]-ionene decyl sulfate, [3,10]-ionene dodecyl sulfate, and [3,10]-ionene decyl sulfate stoichiometric complexes formed suitable films. Finally, the films were dried in a desiccator over phosphorus pentoxide for 2 days at normal pressure and for another 3 days in vacuum. 2.1.4. Controlled Humidity. Distinct relative humidities (100% corresponds to an activity of water vapor of a(H2O) ) 1) were adjusted by confining the respective sample and a dish with a suitable aqueous salt solution in an appropriate chamber (e.g.,
a desiccator) for several hours until the sample weight was constant. The salt concentrations providing the desired relative humidity can be found in handbooks12 (see Supporting Information). 2.2. Apparatus. For attenuated total reflection(ATR)-IR spectra, a Thermo-Nicolet AVATR instrument, fitted with a model 360 FT-IR detector, was available. Textures of mesogenic samples were obtained via polarizing microscopy using a Jenapol polarizing microscope with a Mettler FP80Ht central processor. The sample was held in the Mettler FP80Ht hot stage. During the course of the experiment, the sample could be heated or cooled by the control of the Mettler FP80Ht central processor, which also allowed us to adjust and control the heating and cooling speed of the sample. This was used to determine phase-transition temperatures THI (Table 3). For tests of repeated phase transitions (Figure 10), the apparatus described elsewhere9 was used (cf. Supporting Information), which allows the detection of anisotropy under controlled temperature. The sample is placed between two perpendicularly crossed filters for linearly polarized light, and the detector voltage UD for He-Ne laser light passing through filters and samples is measured, which is set equal to zero in the case of isotropic samples (Uisotropic). Transition temperatures THI were determined from Uisotropic and its standard deviation (σU) in that the mean temperature value for the interval +0.4σU > UD - Uisotropic > -0.4σU is taken as THI. This explains small deviations of Figure 10c here from Figure 7c in the preceding note.9 NMR spectra were taken on Bruker models DRX-500 and Ac-200 at 500 and 200 Hz, respectively. The light-scattering setup (from ALV GmbH, Langen, Germany), which was used for both static and quasielastic light scattering (QELS), consists of a 632.8 nm, 22 mW He-Ne laser light source and detector optics coupled via a monomodal fiber to an ALV/CSG-8 goniometer. An ALV/high QE APD detector was connected to an ALV-5000/EPP multiple τ digital correlator. The cuvettes were sealed after filtration through 0.22 µm Millipore filters and immersed in a large-diameter thermostated bath containing toluene placed at the axis of a goniometer. Apparent hydrodynamic radii of ionene and its complexes, rapp H , were measured at 90° (the evaluation of the data is described elsewhere13), and the error of the rapp H determination was (0.1 nm. Static light scattering was performed with the same apparatus by measuring the total scattering intensity and comparing it with that from toluene. The data were treated by the cumulant technique. Refractive index increments have been measured with a Brice-Phoenix differential refractometer. Problems with filtration (i.e., scattering from dust particles) sometimes appeared during the measurement as a peak in the time dependence of the scattered intensity. If that happened during the time range of a measurement (30 s), then the measurement was rejected and repeated. Both static and dynamic LS data in Table 1 are averages from three measurements qualified in this way. Thermogravimetric analyses were performed using a Netzsch TG/DTA STA 409 instrument, which is connected to a mass spectrometer. The low-molecular-weight gas products in pyrolysis and steam gasification were measured by a Balzers MID at the same time. During the experiment, argon gas was fed into the thermobalance reactor.
Table 3. Transition Humidities, aIH(H2O), Slope of Plots of Water Taken up by Complexes vs Relative Humidity, dw/da, Number of Water Molecules per Ion Pair at Transition Humidity, n(H2O)/n(Ion Pairs), Hexagonal-Iostropic Transition Temperatures, THI, and Temperature of the Emission Peak in DSC Curves at the Transition Humidity, Θ, for Five Stoichiometric Ionene Alkyl Sulfate Complexes
ionene complex
transition humidity aIH(H2O)
[3,12]-ionene dodecyl sulfate [5,10]-ionene decyl sulfate [3,10]-ionene decyl sulfate [5,10]-ionene dodecyl sulfate [3,10]-ionene dodecyl sulfate
0.90 0.69 0.65 0.62 0.58
dw/da
n(H2O)/n(ion pairs) at transition humidity
transition temperaturea THI/°C
temperature of the endothermic DSC peakb ΘHI/°C
0.46 0.245 0.215 0.212 0.178
4 2.4 2 2 1.5
104 76 86 105 131
102 62 80 97 154
a Samples prepared at relative humidities slightly above the ones given in the transition humidity column. b Peak maximum measured at the transition humidity.
Phase Transitions in Ionene-Alkyl Sulfate Complexes Differential scanning calorimetry (DSC) curves were taken on a Setaram DSC 121 instrument. For low temperatures, liquid nitrogen was used to control the temperature. Steady-state fluorescence spectra (i.e., corrected excitation and emission spectra and steady-state anisotropy) were recorded at 23 ( 0.1 °C with a SPEX Fluorolog 3 fluorescence spectrometer in a 1 cm quartz cuvette closed with a Teflon stopper. Excitation of fluorescence emission spectra was at 370 nm, and the concentration of 8-anilino-1-naphthalenesulfonic acid (ANS, Sigma >99%) was 3 µmol dm-3. The solvent was methanol (Fluka, >99.5%, for luminescence). Viscosities of ionenes were measured at 25 ( 0.01 °C using Ubbelohde viscometers. No correction for shear dependence was applied because only low-molecular-weight ionenes were investigated. 2.3. Samples. Mesogenic samples for polarized microscopy were prepared as follows: a film of the complex was cast from methanol solution on a glass plate; the film was dried in a desiccator over phosphorus pentoxide for 2 days at normal pressure. The dry complex was then placed in a desiccator together with a cup of aqueous salt solution at the desired water activity for 24 h. Grease (Glisseal) was cast on the four side edges on another glass plate, and then the two glass plates were put together tightly to keep the sample and absorbed water between them. This method prevents water taken up by the complex from escaping when the complex is heated on the hot stage. For differential scanning calorimetry (DSC), ca. 0.1 g of the predried stoichiometric complex was placed into a suitable stainless steel crucible. It was dried in a desiccator over phosphorus pentoxide for 2 days at normal pressure and then placed in a desiccator together with a cup of aqueous salt solution at the desired water activity for 24 h. The swollen sample then was sealed tightly to prevent water from escaping when the complex was heated during the DSC process. To follow the swelling of ionene-alkyl sulfate stoichiometric complexes as a function of controlled relative humidity (Figure 9), a film of the stoichiometric complex was cast from methanol on the inner wall of an open polyethylene screw cap bottle. The sample and bottle were dried over phosphorus pentoxide until constancy of weight was achieved. A high complex concentration in methanol ensured a high viscosity of the solution so that the film at the wall remained stable during the drying process. The weighed bottle with the dry film was placed inside a desiccator together with a cup of aqueous salt solution at the desired water activity for 24 h and weighed again. The bottle was tightly capped while weighing it in order to rule out the uptake of moisture from ambient air (or the loss of water).
3. Results 3.1. Thermal Stability of Ionenes and Ionene Surfactant Complexes. [3,10]-, [3,12]-, [3,22]-, and [5,10]-ionene were prepared. The ionenes were characterized by 1H NMR and IR spectra (see Supporting Information) as well as by static and dynamic light scattering (DLS), differential scanning calorimetry (DSC), and thermogravimetry (TG, with mass spectroscopic detection of the fragments). DSC as well as TG revealed the stability of both the ionenes and their stoichiometric complexes with alkyl sulfates in the temperature range relevant for this study (i.e., only volatile compounds such as methanol or water (left from the preparation) were detected by the mass spectra of TG fragments up to ca. 150 °C, which corresponded to mass losses of a few percent at most). According endothermic peaks appeared in DSC curves that vanished in second runs, peaks around 3400 cm-1 (12) (a) CRC Handbook of Chemistry and Physics, 75th ed.; Lide, D. L., Ed.; CRC Press: Boca Raton, FL, 1994; pp 15-25. (b) Gmelins Handbuch der Anorganischen Chemie; Verlag Chemie: Weinheim, Germany, 1957; Vol. 28 (Ca), Part B, p 488 ff. (c) Gmelins Handbuch der Anorganischen Chemie; 8th ed.; Verlag Chemie: Weinheim, Germany, 1973; Vol. 21 (Na), 7th Supplement, p 10 ff. (13) Stepanek, M.; Procha´zka, K.; Tuzar, Z.; Brown, W. Langmuir 2001, 17, 4240-4244.
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(O-H) in the ATR-IR spectra lost intensity upon heating. The ATR-IR spectra differed only slightly for the ionenes investigated, as expected for compounds composed of identical functional groups. At higher temperatures, the ionenes and their complexes started to decompose (the [5,10]-ionene being the most labile one), as indicated by TG fragments with masses of 2, 15 (CH3+), and 29 (CH3N+). The ionenes thus followed the thermal degradation patterns observed previously in related systems.14 3.2. Solution Properties. Viscometry. According to textbooks, the viscosity η of dilute solutions of uncharged polymers can be represented as a linear function of concentration
η - η0 ) [η] + k[η]2c′ η0c′
(1)
η0 is the solvent viscosity, [η] is the intrinsic viscosity, c′ is the mass concentration, and k is the Huggins constant. According to Fuoss,15 the reduced viscosity, ηsp/c′ (with ηsp ) (η - η0)/η0), of solutions containing a pure, highly charged, flexible, linear polyelectrolyte can be represented by the empirical relationship
ηsp ) c′
A 1+B
x
c′ cΘ
+D
(2)
where A, B, and D are characteristic constants of each sample, with A being identical to [η]. Typical curves are reproduced in Figure 1, and the Fouss parameters were A ) 8921 cm3 g-1, B ) 464 (102 cm3/g)1/2, and D ) 15.26 cm3 g-1 for [3,22]-ionene and A ) 2971 cm3 g-1, B ) 128 (102 cm3/g)1/2, and D ) 21.01 cm3 g-1 for [3,12]-ionene, respectively. The intrinsic viscosity [η] (identical to coefficient A in the Fuoss equation) can be taken as a relative measure of molecular weight:
[η] ) KMxw
(3)
The [η] value for [3,22]-ionene exceeds that for [3,12]ionene by a factor of 3. Because we do not have sufficient data to determine exponent x in eq 3 (cf. ref 16), we can state only qualitative agreement with light-scattering data, which indicate a factor of 4 between the molecular weights of the two ionenes (Experimental Section). Light Scattering. Results from static light scattering (SLS) were used to determine molecular weights Mw and second virial coefficients A2. These data can be found in section 2. Dynamic light scattering experiments can be exploited to determine apparent hydrodynamic radii rapp H .
Figure 1. Reduced viscosity as a function of the mass concentration of [3,22]-ionene in the presence (4) and absence (2) of NaBr, which was added at concentrations chosen to keep the total ion concentration constant at 3.34 × 10-3 mol dm-3. All data points were taken at 25 °C.
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Figure 2. Apparent hydrodynamic radius, rapp H , of [3,12]ionene as a function of the molar concentration of sodium dodecyl -3 sulfate (cSDS). Ionene concentration 10 g dm .
Yu et al.
Figure 4. Emission maximum wavelengths (3, 4) and maximum intensities (1, 2) of the ANS fluorescence in methanol as a function of the sodium decyl sulfate concentration. Curves 2 and 3 are in the presence and curves 1 and 4 are in the absence of [5,10]-ionene. Lines are to guide the eye only.
Figure 3. Emission maximum wavelengths (3, 4) and maximum intensities (1, 2) of the ANS fluorescence in methanol as a function of the sodium dodecyl sulfate concentration at 22 ( 1 °C. Curves 2 and 3 are in the presence and curves 1 and 4 are in the absence of [5,10]-ionene. Lines are to guide the eye only.
At 10 g/dm3, rH values (without added salt) of 1.8 nm for [3,12]-ionene (Mw ) 1900 g/mol from SLS), 2.6 nm for the corresponding dodecyl sulfate complex (Mw ) 6000 g/mol), 2.0 nm for [5,10]-ionene (Mw ) 2100 g/mol), and 3.8 nm for the corresponding dodecyl complex (Mw ) 6700 g/mol) were found. The ratios of the various hydrodynamic radii nicely reflect the molecular weights of the noncomplexed ionenes, whereas the [5,10] complex appears to be less coiled in solution than the [3,12] complex. Attempts to find the stoichiometric concentration of [3,12]-ionene (present at 10 g/dm3) and added SDS from a change in the slope of a plot of rapp H vs c(SDS) failed. The slope changed below 20 mmol dm-3, whereas the stoichiometric point should be at 31.5 mmol dm-3. This is a consequence of the presence of NaBr salt that is set free upon addition of SDS to the ionene solution containg Brcounterions. Fluorescent Probe Study. 1-Anilinonaphthalene-8-sulfonic acid (ANS) was used as a probe in methanol solutions of the ionenes. The results are displayed in Figures 3 and 4. In each Figure, the absorption maximum and the fluorescence intensity of ANS are depicted as functions of the concentration of either sodium dodecyl sulfate or sodium decyl sulfate in the presence and absence of a constant amount of [5,10]-ionene, respectively. In the absence of ionene, the maximum does not change much in the dodecyl sulfate cases (curve 4 in Figure 3). In the presence of ionene, one notes a gradual decrease of the maximum (curve 3 in Figure 3) until a leveling off is observed at an SDS concentration corresponding to the equivalence point of ionene cationic groups and dodecyl sulfate anions (31.5 mmol dm-3). The intensities increase continuously upon addition of SDS. For [3,10]- and [3,12]ionene very similar results were obtained; see Supporting Information. Somewhat different features were observed when sodium decyl sulfate was added to the ionenes in methanol,
Figure 5. Fan texture of the stoichiometric complex of 3,10ionene and decyl sulfate in the anisotropic state on a polarizing microscope indicating a hexagonal mesogenic structure. Magnification 100×; sample prepared at a(H2O) ) 0.75.
Figure 6. Swelling (mass fraction w of water) of [5,10]-ionene decyl sulfate stoichiometric complexes and number of water molecules per ion pair unit (n(H2O)) as a function of the relative humidity (a(H2O)) to which the dry sample was exposed at room temperature.
as shown in Figure 4 (numbering of curves as in Figure 3): in the absence of ionene, the addition of sodium decyl sulfate leads to a less polar environment of ANS; in the presence of [5,10]-ionene, the leveling off at the stoichiometric point (at 30.53 mmol/L sodium decyl sulfate) is not observed. [3,10]-Ionene behaves analogously; see Supporting Information. 3.3. Films. 3.3.1. Film Formation. A general recipe for preparing polyelectrolyte-surfactant complexes can be found in the literature:17 a white solid precipitates from (14) Berwig, E.; Severgnini, V. L. S.; Soldi, M. S.; Bianco, G.; Pinheiro, E. A.; Pires, A. T. N.; Soldi, V. Polym. Degrad. Stab. 2003, 79, 93-98. (15) Fuoss, R. M. J. Polym. Sci. 1948, 3, 603. (16) Schultes, K.; Wolf, B. A.; Meyer, W. H.; Wegner, G. Macromol. Chem. Phys. 1995, 196, 1005-1016. (17) Antonietti, M.; Conrad, J.; Thu¨nemann, A. Macromolecules 1994, 27, 6007.
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Figure 7. (a, b) Birefringence indicating the hexagonal phase (measured as detector voltage UD, see Supporting Information) under the cyclic variation of temperature T for [3,12]-ionene dodecyl sulfate. (c, d) Drift of transition temperatures THI during experiments a and b (1) obtained upon heating and (2) obtained upon cooling. (a, c) At 91.4% relative humidity; (c, d) at 93% relative humidity; a and b were modified from ref 9.
Figure 8. (a) Birefringence at 55.9% relative humidity indicating the hexagonal phase (measured as detector voltage UD, see Supporting Information) under the cyclic variation of temperature T for [5,10]-ionene dodecyl sulfate. (b) Drift of transition temperatures THI during the experiments in a (1) obtained upon heating and (2) obtained upon cooling.
an aqueous solution of polyelectrolyte and surfactant, which coagulates and can be separated from the aqueous phase. This crude complex, however, contains large amounts of excess surfactant, salt, and water. For purification, this mixture is redissolved in alcohol, and water is added until a phase separation of a complex-rich gel phase and a salt- and surfactant-rich water phase takes place. After removal of the water phase, this phase separation is repeated until the water phase is free of halogenide ions. It is, however, difficult to prepare water free complexes by this method. Following our previously published8 synthesis of salt- and water-free complexes of acrylic acid and an alkyltrimethylammonium surfactant via an acid-base reaction, ionene hydroxide as a necessary component would be unstable at room temperature. Therefore, stoichiometric complexes of ionene and alkyl sulfate were prepared via the reaction of the bromide of the ionene and the silver salt of the alkyl sulfate in methanol under precipitation of the silver halide with
potentiometric stoichiometry control.9 After removing the solvent (methanol containing some water formed in the reaction), films of the stoichiometric complexes were formed, which were further dried in a desiccator (Experimental Section). Films produced from [3,22]-ionene dodecyl sulfate, [3,22]-ionene decyl sulfate, [3,22]-ionene octyl sulfate, [3,12]-ionene decyl sulfate, octyl sulfate, [5,10]-ionene octyl sulfate, and [5,10]-ionene octyl sulfate stoichiometric complexes turned out to be cloudy and to form undulated surfaces. These complexes were not investigated further. Dry films produced from [3,12]-ionene dodecyl sulfate, [5,10]-ionene dodecyl sulfate, [5,10]-ionene decyl sulfate, [3,10]-ionene dodecyl sulfate, and [3,10]-ionene decyl sulfate, however, were clear, transparent, optically isotropic, and flat (i.e., they showed the desired properties). Table 2 displays qualitative film properties of stoichiometric complexes. An inspection of the Table reveals that the optical properties of the films become better when the
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Figure 10. ATR-IR spectra of the [3,12]-ionene dodecyl sulfate complex as prepared and after several experiments at R(H2O) ) 0.91 and at temperatures between 50 and 110 °C.
Figure 9. DSC curves for the stoichiometric complexes of (a) [3,10]-ionene and dodecyl sulfate and (b) [5,10]-ionene and decyl sulfate at two relative humidities, one slightly below and one at the transition humidity.
longer CH2 segments of the ionene are shortened and when the hydrocarbon tail of the alkyl sulfate is longer. An influence of the molecular weight of the ionenes on the optical quality of the films cannot be detected. 3.3.2. Mesogenic Transitions. The clear, transparent films were investigated with respect to their optical isotropy by the use of two crossed filters for linearly polarized light, between which the sample film is placed under controlled humidity and temperature. Light can pass through this arrangement only when the films shows optical anisotropy. Dry films at low relative humidity were optically isotropic at room temperature and up to 150 °C. At elevated relative humidity, all films at room temperature became optically anisotropic at a transition humidity depending on the specific complex. Once the anisotropic phase is reached, the anisotropy signal increases with further increases in humidity. When the anisotropic films were heated (at constant relative humidity), the films changed to optically isotropic at distinct clearing temperatures. Table 3 displays transition humidities, aIH(H2O), and transition temperatures, THI (clearing points), for five different complexes. The latter were determined on a polarizing microscope for samples prepared at humidities above but close to the transition humidities. Textures appearing on the polarizing microscope upon cooling cleared samples indicated hexagonal mesogenic structures in all cases investigated here. An example is given in Figure 5. In all five complexes, the mass of the sample increases linearly with the relative humidity to which the sample is exposed. The slopes of plots of increasing mass fractions w versus relative humidity a(H2O) are given in the dw/ da(H2O) column of Table 3. An example is depicted in Figure 6. From the masses at the transition humidity, the minimum number of water molecules per ionic group of the complex, which are needed to form the hexagonal phase, can be calculated. Values are given in Table 3. Repeated Phase Transition. A fair reproducibility of phase-transition temperatures THI was reported for the [3,12]-ionene dodecyl sulfate complex in cyclic experiments at 91.4% relative humidity9 (Figure 7a and b). When the same experiments were performed at 93% relative humidity, higher transition temperatures were found, and
Figure 11. Temperature θ (9) of the maximum of the DSC emission peak and enthalpy (2) of this peak as a function of controlled relative humidity for the [3,10]-ionene decyl sulfate (a) and for the [5,10]-ionene dodecyl sulfate complex (b). Lines are to guide the eye only.
the sample readily degenerated (Figure 7c and d), probably via chemical decomposition (see below). Note that the transition temperature rises from 50 to 55 to >80 °C upon changing the relative humidity from 91.4 to 93% (i.e., there is a strong dependence of THI on humidity, which makes obtaining reproducible THI data extremely difficult). For [5,10]-ionene dodecyl sulfate, cyclic phase changes had poor reproducibility even at very low humidity, as shown in Figure 8. After 18 h, there is practically no return to the anisotropic phase upon cooling. Thermal Decomposition of the Complexes in the Presence of Humidity. Although dry samples of the complexes have proven to be stable up to at least 150 °C, the observed drift of the transition temperature (Figures 7 and 8) indicates the decomposition of complexes when they are exposed to higher relative humidity. This is in keeping with the observation of endothermic and exothermic peaks in DSC curves, indicating chemical reactions, that appear only in samples prepared under certain humidities (and not in dry samples). Examples of such curves are depicted in Figure 9, in which the endothermic peak (upward) is shown at various temperatures depending on the humidity offered. A broader exothermic peak (around 120 °C in Figure 9b) is also observed in most of the complexes at elevated humidity but can be shifted to higher temperatures. To prove the chemical decomposition, we analyzed a sample after it was used for thermal phase-transition
Phase Transitions in Ionene-Alkyl Sulfate Complexes
experiments at elevated temperature and humidity. Among others, new 1H NMR peaks at 1.39 and 4.31 ppm as well as IR peaks at 851 and 1048 cm-1 clearly indicate the formation of dodecanol due to the hydrolysis of dodecyl sulfate. In Figure 10, the ATR-IR spectra of a freshly prepared and a partially decomposed complex of [3,12]ionene and dodecyl sulfate are shown. Temperature Dependence of the Endothermic DSC Peak. The temperature θ at which the endothermic DSC peak (Figure 9) occurs strongly depends on the relative humidity present. Interestingly, this temperature passes through a minimum at approximately the transition humidity. The same is due to the enthalpy determined by the endothermic peak. Two examples of the temperature dependence are given in Figure 11. The minimum values for the temperature θHI are included in Table 3. 4. Discussion 4.1. Solution Behavior. Viscosities. From an inspection of Figures 1 and 2, it follows that normal (textbooklike) polyelectrolyte viscosity behavior was observed for ionenes in methanol (i.e., with increasing concentration of the ionene, a decrease in the reduced viscosity occurs as the intermolecular repulsion decreases because of charge shielding by the counterions). In the presence of sufficient NaBr, however, the electrostatic interactions are always lower. Such behavior was discussed in terms of some mobile electrolyte diffusing into the polyion coil.18 Fluorescent Probe Study. It is known19,20 that the fluorescence of ANS is sensitive to the polarity of the immediate surrounding of the ANS probe molecule. When the probe enters a region of changing polarity, the wavelength at which a maximum of the emission spectrum occurs will shift21 (i.e., the maximum varies from 515 nm in H2O to 473 nm in methanol to 460 nm in octanol and eventually to 430 nm in hexane). The fluorescence intensity decreases with increasing polarity. The results for increasing sodium dodecyl sulfate concentration in methanol suggest that in the absence of ionene there is generally no marked effect of SDS, which indicates that SDS micelles are not formed in methanol under the conditions investigated. Solutions of the ionenes in methanol provide an environment that is more polar than methanol. This can be interpreted in terms of ANS anions bound to the cationic units of the ionenes. This situation is gradually changed while SDS is added; the dodecyl sulfate anions of SDS obviously bind more strongly to the cationic units than the ANS ions, which consequently are replaced and find themselves in a methanol environment. Therefore, the maximum emission wavelength decreases until the stoichiometric point is reached and levels off thereafter at approximately the methanol value, 473 nm (Figure 3). These experiments clearly monitor and prove the ionene-surfactant complex formation. Stepwise or strictly cooperative complex formation as discussed for aqueous solutions22 cannot be derived from our experiments. For sodium decyl sulfate, differing features were observed. The fact that the stoichiometric point is not indicated suggests that decyl sulfate binds less strongly to the ionenes so that the complexes are not completely formed in methanol solution. Nevertheless, because good films can be prepared from [3,10]- and [5,10]-ionene and (18) Kogej, K.; Skerjanc, J. Acta Chim. Slov. 1999, 46, 481-492. (19) Buschmann, H.; Wolff, T. J. Photochem. Photobiol., A 1999, 121, 99-103. (20) Nomula, S.; Cooper, S. L. Macromolecules 2000, 33, 6402-6406. (21) Mistiniene, E.; Luksa, V.; Sereikaite, J.; Naktinis, V. Bioconjugate Chem. 2003, 14, 1243-1252.
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decyl sulfate (Table 2), the complexation in solution is not a necessary prerequisite for the formation of films. However, the decrease in the fluorescence maximum below the methanol value (473 nm) indicates that ANS induces the formation of micelles of sodium decyl sulfate, which shield ANS from methanol (Figure 4). 4.2. Films. Hygroscopy. It is known that ammonium salts are generally hygroscopic. Consequently, the presence of water in solid ionenes appears to be a general feature, even in crystallized samples,23 unless special efforts are taken to prepare water-free samples. The slopes of the plots (Figure 6) of water taken up by ionene alkyl sulfate complexes as a function of relative humidity, dw/ da (Table 3), may be used to quantify the hygroscopy of the samples. If linear plots such as the one in Figure 6 are extrapolated to w ) 0, then the a(H2O) axis is reached at between 0.2 and 0.25 for all samples investigated. Thus, there is no direct proportionality but an induction period at low a(H2O) after which the linear increase of w starts. A strong influence of humidity on the properties of ionene alkyl sulfate complexes is established: upon inspection of Table 3, a nice correlation of transition humidities aIH(H2O) and the number of water molecules per ionic pair at aIH(H2O) can be observed. The latter strongly corresponds to the fraction of water that is taken up by the respective complex below, at, and above the transition humidity (dw/da column in Table 3). Transitions. The water molecules taken up by the complexes can be expected to assemble around the ionic pairs of the ionenes. Obviously, the presence of these water molecules reduces the rigidity of directions in the ionic bonds and imparts the flexibility needed to form the mesogenic phase. The THI clearing points were determined as the temperatures at which the samples on a polarizing microscope become anisotropic upon cooling after heating them to above the transition temperature. Because (i) slight variations in humidity due to a possible loss of water during this process as well as (ii) the chemical decomposition of alkyl sulfates (at elevated humidity and temperature) affect transition temperatures, these values can vary significantly. Nevertheless, the trend of increasing transition temperatures with the length of the alkyl sulfate can be noted. Less obviously, the transition temperatures decrease with increases in the average length of the alkyl segments of the ionenes. Another way to determine transition (clearing) temperatures arises from the humidity dependence of exothermic DSC peaks (Figure 11). Without further experiments, such as small-angle X-ray scattering, we cannot decide whether the optically isotropic phases in dry samples and in humid samples at high temperature are related (i.e., cubic phases as discussed in contact with solution7). DSC Peaks. The examples in Figure 9 illustrate that at elevated humidity there is an endothermic process or reaction that occurs in all complexes investigated. This process is eased close to the transition humidity (i.e., in the two-phase region of the binary system of complex and water, both the temperature of the maximum of the peak and its enthalpy pass through a minimum; Figure 11). The peak does not appear in dry samples. It cannot be attributed to the isotropic-hexagonal phase transition itself because the peak is also found at high humidities when the sample is already hexagonal at room temper(22) (a) Chen, L.; Yu, S.; Kagami, Y.; Osada, Y. Macromolecules 1998, 31, 787-794. (b) Zheng, X.; Cao, W. Eur. Polym. J. 2001, 37, 22592269. (c) Zheng, X.; Cao, W. Polym. Int. 2001, 50, 484-486. (23) Soldi, V.; Erismann, N. M.; Quina, F. H. J. Am. Chem. Soc. 1988, 110, 5137-5143.
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ature. A possible interpretation of this endothermic peak arises from the analysis of samples that were treated at controlled humidity for considerable times. In these samples, alkyl alcohol, which is a product of the hydrolysis of alkyl sulfate was detected. Therefore, the endothermic peaks may be attributed to the reaction enthalpy of this hydrolysis. The temperature at which these peaks occur is specific for the complex and depends on the relative humidity and on whether the curve was measured in the isotropic or mesogenic phase. The reaction enthalpies calculated from the peaks range from 150 to 600 J/mol of alkyl alcohol. Because the process does not take place in free water (where the solvation of free ions such as HSO4have to be taken into account) and because the entropy change in our systems is not known, the reaction can be endothermic in our systems. The hydrolysis then is less endothermic close to the transition humidity because there can be some exothermic hydration of free ions in the twophase region. The effect can be used to determine transition humidities (cf. θΗΙ column in Table 3). The broader exothermic peak at temperatures above 100 °C (Figure 9b and ref 9) may be due to another chemical decomposition reaction of the ionene taking place in the presence of water at high temperatures (such as Hofmann elimination). Possible Application of the Observed Effects. Applications of the systems described here are restricted by their chemical instability described above. Nevertheless, one might think of exploiting the sensitivity for humidity because the intensity of the anisotropy signal varies with the humidity surrounding the film. A humidity sensor working at 50% relative humidity or above (corresponding to the transition humidities of our systems) is not demanded because commercial hygrometers can be used. Polyelectrolyte surfactant complexes exhibiting lower transition humidities might require a weak acid or base as a complex component (e.g., a poly(acrylic acid) complex8). Future studies will investigate this point. 5. Conclusions Although all of the ionenes investigated show typical polyelectrolyte behavior in methanol solution, ionene alkyl sulfate complexes in solution form with dodecyl sulfate but not with decyl or octyl sulfate. Because clear,
Yu et al. Scheme 2. Transitions in Ionene Alkyl Sulfate Stoichiometric Complex Films
transparent films can be obtained from a variety of ionene alkyl sulfate complexes, the formation of complexes in solution is not a precondition for the formation of good films. Among the complexes investigated, the optical quality of the films rises with longer alkyl chains of the sulfate and upon shortening the longer CH2 segment of the ionene. The results establish phase transitions according to Scheme 2 in all of the film-forming ionene alkyl sulfate complexes investigated: when prepared water-free films are optically isotropic, upon exposure to sufficient humidity the films convert to optically anisotropic. The anisotropic phases exhibit hexagonal structures that melt at specific clearing points THI that do not depend very strongly on the kind of ionene backbone but are higher for dodecyl than for decyl complexes. THI strongly depends on the humidity present. The materials are thus sensitive to humidity and temperature. Although the ionenes and their dry alkyl sulfate complexes prove stable up to 150 °C, at elevated humidity a degradation of the complexes occurs after several hours; this is partly due to the hydrolysis of the alkyl sulfate moieties. Acknowledgment. Q.Y. thanks the Deutsche Forschungsgemeinschaft for a grant from the European Graduate College “Advanced Polymeric Materials”. Thanks are due to Mrs. Ch. Meissner, A. Go¨pfert, M. Ro¨sler, and H. Dallmann for performing light scattering, DSC, ATRIR, and TG measurements, respectively. We thank Mr. M. So¨hnel and Mr. M. Gestrich for excellent mechanical work. Supporting Information Available: Preparation of the chlorides of R,ω-alkyldiacids. Description of the apparatus used for tests of repeated phase transitions. 1H NMR and IR spectra of the ionenes. This material is available free of charge via the Internet at http://pubs.acs.org. LA050907G
