Langmuir 2001, 17, 8085-8091
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Association Properties of Diblock Copolymers of Propylene Oxide and Ethylene Oxide in Aqueous Solution. The Effect of P and E Block Lengths Antonis Kelarakis Physical Chemistry Laboratory, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, 157 71 Athens, Greece
Zhuo Yang,* Evangelia Pousia, S. Keith Nixon, Colin Price, and Colin Booth Department of Chemistry, University of Manchester, Manchester M13 9PL, U.K.
Ian W. Hamley, Valeria Castelletto, and Johan Fundin School of Chemistry, University of Leeds, Leeds LS2 9JT, U.K. Received August 9, 2001. In Final Form: October 22, 2001 Three diblock copolymers of propylene oxide and ethylene oxide were prepared by sequential oxyanionic polymerization; P94E316, P60E348, and P43E312. Light scattering and dye solubilization were used to confirm micellization in dilute aqueous solution and to determine the temperature dependence of the critical micelle concentration (cmc), and eluent gel permeation chromatography was used to determine the extent of micellization. Comparison with results for the series of copolymers EmPn (m ) 98-104, n ) 37-73, published previously) shows that values of the cmc in molar units depend almost entirely on P-block length, i.e., that the effect of E-block length on critical micelle concentration is insignificant in the range m ) 100-300. Light scattering was also used to determine micellar association numbers (Nw) and radii. Present and previous values of Nw fit well to a scaling law for block lengths (P and E) similar to that proposed by Nagarajan and Ganesh for EmPn copolymers. Gel boundaries were determined by a tube-inversion method, and critical gel concentrations were found to coincide with values predicted by using micellar parameters determined by static light scattering.
1. Introduction There are many reports of the micellization, micelle properties, and gelation of triblock EPE copolymers of ethylene oxide and propylene oxide in aqueous solution, as summarized in recent reviews.1,2 We use E to represent an oxyethylene unit, OCH2CH2, and P to represent an oxypropylene unit, OCH2CH(CH3). These copolymers are available from commercial sources, and have allowed the effects of block length to be studied.1-4 However, there are drawbacks to using EPE copolymers. Apart from the fact that the reproducibility of results using commercial EPE copolymers cannot by guaranteed,5 it is also the case, as summarized recently by Linse,6 that theoreticians have focused attention on the properties of copolymers with diblock architecture, an exception being Linse himself. Previously we have examined the micellization and micelle properties of a set of copolymers prepared with different block architectures (EP, EPE, and PEP) but similar overall composition and chain length.7 While that study provided evidence of the effect of block (1) Chu, B.; Zhou, Z.-K. In Nonionic Surfactants: Polyoxyalkylene Block Copolymers; Surfactant Science Series Vol. 60; Nace, V. M., Ed.; Marcel Dekker: New York, 1996; Chapter 3. (2) Alexandridis, P.; Hatton, T. A. Colloids Surf., A 1995, 96, 1. (3) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. J. Am. Oil. Chem. Soc. 1995, 72, 823. (4) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (5) Yu, G.-E.; Altinok, H.; Nixon, S. K.; Booth, C.; Alexandridis, P.; Hatton, T. A. Eur. Polym. J. 1997, 33, 673. (6) Linse, P. In Amphiphilic Block Copolymers. Self-Assembly and Applications; Alexandridis, P., Lindman, B., Eds.; Elsevier: Amsterdam, 2000; Chapter 2.
architecture on the cmc (EP , EPE < PEP) and association number (EP > EPE > PEP), similar to that summarized recently for a range of block copoly(oxyalkylene)s,8 more detailed information for diblock copolymers was obtained from a systematic study of the effect of P-block length within a series of EmPn copolymers (m ) 98-104, n ) 37-73, where the subscripts denote block lengths in repeat units).9 In addition to confirming the expected linear dependence of log(cmc) on P-block length (n),9 results from light scattering were used to show that the mass-average association number (Nw) scaled directly as the P-block length.8 The work described in this paper extends this aspect of our work to diblock copolymers with E blocks of 300 or more units, thus enabling a first investigation of the scaling relationship between association number and E-block length for diblock EP copolymers. Specifically, the copolymers synthesized for this work were P94E316, P60E348, and P43E312. Some results for copolymer P94E316 have been described already in connection with a study of the solubilization of a liquid crystal by its micellar solutions.10 Because of differences in the rate of initiation of the second block,11 diblock EP copolymers with short E blocks are more uniform in chain length and composition than PE copolymers, where the (7) Altinok, H.; Yu, G.-E.; Nixon, S. K.; Gorry, P. A.; Attwood, D.; Booth, C. Langmuir 1997, 13, 5837. (8) Booth, C.; Attwood, D. Macromol. Rapid Commun. 2000, 21, 501. (9) Altinok, H.; Nixon, S. K.; Gorry, P. A.; Attwood, D.; Booth, C: Kelarakis, A.; Havredaki, V. Colloids Surf., B 1999, 16, 73. (10) Z. Yang, E. Pousia, F. Heatley, C. Price, C. Booth, V. Castelletto and I. W. Hamley Langmuir, 2001, 17, 2106. (11) Nace, V. M.; Whitmarsh, R. H.; Edens, M. W. J. Am. Oil Chem. Soc. 1994, 71, 77.
10.1021/la015528b CCC: $20.00 © 2001 American Chemical Society Published on Web 11/27/2001
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Table 1. Molecular Characteristics of the PnEm Copolymersa NMR
GPC
copolymer
wt % E
Mn/g mol-1
Mw/Mn
Mw/g mol-1
P94E316 P60E348 P43E312
72 81 83
1940 18800 16200
1.07 1.06 1.04
20700 19900 16900
a M calculated from M and M /M . Estimated uncertainties: w n w n Mn, (2%; Mw/Mn, (0.02; Mw, (3%.
order of polymerization is reversed. Additionally, as polymerized and neutralized, the P blocks in EP copolymers terminate with an OH group, whereas those in PE copolymers terminate (in our case) with a methoxy group. However, effects caused by the difference in polymerization sequence should be marginal when the E and P blocks are long and so should be unimportant in a comparison of results from the two series under present consideration. 2. Experimental Section 2.1. Preparation and Characterization. The copolymers were prepared by sequential anionic polymerization of propylene oxide followed by ethylene oxide. The methods used in preparation and characterization followed closely those described previously.7 Vacuum line and ampule techniques were used to eliminate moisture. The monofunctional initiator 1-methoxy-2-propanol was activated by reaction with potassium metal (mole ratio OH/K ≈ 9). The precursor poly(oxypropylene)s and the final copolymers were sampled and characterized by gel permeation chromatography (GPC, N,N-dimethylacetamide eluent at 60 °C) calibrated with poly(oxyethylene) standards in order to define the width of their chain length distributions, i.e., Mw/Mn, where Mw and Mn are the mass-average and number-average molar mass, respectively. A small correction to Mw/Mn was made to account for instrumental spreading in the system. Small shoulders on the low elution volume side of the GPC peak of the copolymers indicated some 1-2 wt % impurity, assigned as triblock copolymer. Analysis of the 13C NMR spectra was based on the assignments of Heatley et al.12 Comparison of integrals from end group and backbone carbons gave absolute values of molar mass (Mn) and overall composition (mol % E units). Small fractions (approximately 0.3 mol %) of unsaturated ends originating from the transfer reaction in the anionic polymerization of propylene oxide were included in the calculations. The formulas of the copolymers were obtained accurately from Mn of the precursor poly(oxypropylene) and the overall composition. Results are set out in Table 1. 2.2. Light Scattering from Micellar Solutions. Copolymer solutions were clarified by filtering through Millipore Millex filters (Triton free, 0.22 µm) directly into the cleaned scattering cell. Static light scattering (SLS) intensities were measured by means of a Brookhaven BI 200S instrument using vertically polarized incident light of wavelength λ ) 488 nm supplied by an argon-ion laser operated at 500 mW or less. The intensity scale was calibrated against benzene. Dynamic light scattering (DLS) measurements were made under similar conditions, using a Brookhaven BI 9000 AT digital correlator to acquire data. Experiment duration was in the range 5-20 min, and each experiment was repeated two or more times. Scattered light intensity was measured at angle θ ) 90° to the incident beam. More detailed descriptions of the methods employed can be found elsewhere.7,9,13 The correlation functions from DLS were analyzed by the constrained regularized CONTIN method14 to obtain distributions of decay rates (Γ), hence distributions of apparent mutual diffusion coefficient (Dapp ) Γ/q2, q ) (4πn/λ) sin(θ/2), n ) refractive index of the solvent) and ultimately of apparent hydrodynamic radius (rh,app, radius of the hydrodynamically (12) Heatley, F.; Luo, Y.-Z.; Ding, J.-F.; Mobbs, R. H.; Booth, C. Macromolecules 1988, 21, 2713. (13) Deng, N.-J.; Luo, Y.-Z.; Tanodekaew, S.; Bingham, N.; Attwood, D.; Booth, C. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1085. (14) Provencher, S. W. Makromol. Chem. 1979, 180, 201.
equivalent hard sphere corresponding to Dapp) via the StokesEinstein equation
rh,app ) kT/(6πηDapp)
(1)
where k is the Boltzmann constant and η is the viscosity of the solvent at temperature T. In section 3.1 the data are presented as normalized intensity-fraction distributions of log(rh,app). Average values of Γ, delivered by the CONTIN program by integration over the intensity distributions, were similarly converted to intensity-average values of rh,app. The basis for analysis of SLS data was the Debye equation
K*c/(I - Is) ) 1/Mw + 2A2c + ...
(2)
where I is intensity of light scattering from solution relative to that from benzene, Is is the corresponding quantity for the solvent, c is the concentration (in g dm-3), Mw is the mass-average molar mass of the solute, A2 is the second virial coefficient (higher coefficients being neglected in eq 2), and K* is the appropriate optical constant. Values of the specific refractive index increment, dn/dc, its temperature increment, and other quantities necessary for the calculations have been given previously.9 Values of dn/dc are very similar for poly(oxyethylene) and poly(oxypropylene), making dn/dc insensitive to exact composition of the copolymers and making correction for refractive index difference within the copolymer unnecessary. 2.3. Critical Micelle Temperature by Light Scattering. Static light scattering was used to detect the onset of micellization in solutions of known copolymer concentration. Scattering intensities relative to benzene were measured at intervals as the temperature was raised slowly, 0.1-1.0 °C min-1. The critical micelle temperature (cmt) was defined as that at which the scattering curve left the baseline established at low temperatures. 2.4. Critical Micelle Temperature by Dye Solubilization. This method depended on the change in spectrum caused by solubilization of the dye DPH (1,6-diphenyl-1,3,5-hexatriene) in the hydrophobic micellar cores. The procedure followed closely that described by Alexandridis et al.15 Absorption at 356 nm was determined for copolymer solutions at several temperatures by means of a Cary 1E UV-vis spectrometer with temperature controller. The DPH concentration in the copolymer solution was 0.008 mmol dm-3. As described previously,5,10 the cmt was defined as the temperature at which the absorption curve left the baseline established at low temperatures. 2.5. Extent of Micellization by Elution Gel Permeation Chromatography. Elution gel permeation chromatography (EGPC) was used to determine the extent of micellization (mass fraction) of a copolymer in solution at a given concentration and temperature. The method has been described elsewhere,16 and has been applied previously to EP copolymers.9 In EGPC, the eluent is an aqueous solution of the block copolymer and the micelle-molecule equilibrium in the eluent is probed by injecting a solution of different concentration. It is necessary that the probe does not significantly disturb the equilibrium, but this condition is not difficult to achieve.16 The EGPC system comprised two columns, each 30 cm long packed with TSKgel-PW (G4000 and G3000). Detection was by differential refractometry. The system was calibrated with poly(oxyethylene) standards covering the molar mass range from 103 to 106 g mol-1 and was found to have satisfactory resolution in the appropriate elution volume range. Eluent, prepared by dissolving copolymer in distilled water followed by filtration, was pumped at 0.5 cm3 min-1. Probe solutions, prepared by dissolving the same copolymer in the eluent, were injected via a 0.1 mm3 loop. The column temperature was controlled (to (0.5 °C) by means of a constant-temperature oven (ICI Instruments, model TC1900), allowing several hours at any temperature for thermal equilibration. Mass fractions of molecules and micelles were obtained from peak areas. Additional information from EGPC was the temperature at which the micelle peak was first detected, i.e., the cmt. (15) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (16) Wang, Q.-G.; Yu, G.-E.; Deng, Y.; Price, C.; Booth, C. Eur. Polym. J. 1993, 29, 665.
Association Properties of Diblock Copolymers
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Figure 1. Determination of critical micelle temperature for aqueous solutions of copolymer P43E312: (a) temperature dependence of light scattering intensity for solutions of the concentrations indicated; (b) temperature dependence of absorbance (356 nm) of DPH solubilized in solutions of the concentrations indicated. For both plots the ordinate scales and zeros are arbitrarily chosen for clarity of presentation. 2.6. Gel Boundary by Tube Inversion. Samples of solution (0.5 g) were enclosed in small tubes (internal diameter ca.10 mm) and observed while slowly heating the tube in a water bath within the range 0-100 °C. The heating/cooling rate was 0.5 deg min-1 or less. The change from a mobile to an immobile system (or vice versa) was determined by inverting the tube. The method served to define a mobile-gel transition temperature to (1 °C. In comparable systems, this simple method of detecting the gel boundary, which is sensitive to the yield stress of the gel, has been shown give the same results as rheometry and differential scanning calorimetry.17
3. Results and Discussion Copolymer solutions with concentrations up to 30 wt % were observed over the temperature range 0-100 °C and were found to remain optically clear. 3.1. Critical Micelle Conditions and Enthalpy of Micellization. Figure 1 shows examples of the experimental results obtained by two of the methods employed to investigate the onset of micellization. The critical micelle temperatures obtained are collected in Table 2. The results are also plotted as log(c) against 1/T in Figure 2. Agreement between the two methods is excellent. Also included in Figure 2 is a data point for a critical micelle concentration (cmc) of copolymer P94E316 at 20 °C obtained previously from measurements of surface tension,10 and data points for cmts of copolymers P60E348 and P43E312 obtained from the EGPC measurements reported in section 3.3. Given closed association (see sections 3.2 and 3.3 for confirmation) the slopes of these plots can be used in the usual way8 to obtain values of the apparent standard enthalpy of micellization from
∆micHoapp ) R d ln(c)/d(1/T)
(3)
True values of ∆micHo can be obtained in this way if the association number of the micelles is high and if any
temperature dependence of Nw has an insignificant effect in eq 3. For the present case, the first requirement is satisfied for copolymers P94E316 and P60E348 but not for copolymer P43E312, and the second may not be satisfied for any of the copolymers (see section 3.4). However, the consequences of similar departures from ideal have been explored for EP copolymers,7 and the errors in the present case should be small. The values of ∆micHoapp listed in Table 2 lie in the range 150-345 kJ mol-1 and amount to 3.53.7 kJ mol-1 per P unit transferred to the micelle core under standard conditions. 3.2. Gel Boundary. Gel boundaries obtained by tube inversion are shown in Figure 3. The lower gel boundary is attributed to micellar dissociation in the better solvent at low temperature and consequent decrease in the effective hard-sphere volume fraction of the micelles to a value below that for packing.13,18 As seen in Figure 3, copolymer P43E312 in solution at 20 °C does not gel even when c ) 25 wt %, which directly characterizes it as a poor micellizer at that temperature, and limits investigation of micellar properties to solutions at significantly higher temperatures. Critical gel concentrations (cgcs) at a given temperature can be read off this diagram. For example, for solutions at 50 °C we find values of 11.5 wt % (P94E316), 6.8 wt % (P60E348), and 7.4 wt % (P43E312). These values are discussed in section 3.7. 3.3. Extent of Micellization. EGPC was used to confirm the extent of micellization of copolymers P94E316 and P43E312 in dilute aqueous solution. Examples of the results obtained are shown in Figure 4. Extents of micellization, calculated from the areas of the micelles peak at ca. 12 cm3 and the molecules peak at ca. 16 cm3, are listed in Table 3. The results for copolymers P60E348 and P43E312 also served to locate the cmts of the solutions as follows: P60E348, 5 g dm-3, 25 °C; P43E312, 10 g dm-3, 32 °C; P43E312, 20 g dm-3, 28 °C. 3.4. Concentrations of Solutions for Light Scattering. The concentrations used in the light scattering experiments were adjusted in view of the results presented in sections 3.2 and 3.3. For measured properties to be characteristic of block copolymer micelles, and not micelle-molecule mixtures, it is necessary that the copolymers are effectively fully micellized over the concentration range investigated. However, the increase in solution viscosity and the clustering of micelles as concentration is increased and the gel region is approached creates a problem for extrapolation of data to infinite dilution. Experience with other diblock copolymers with long E blocks19 has shown that acceptable results are obtained for a well-micellized system at a given temperature if the concentration does not exceed approximately one-half the critical gel concentration. The upper limit for SLS is higher than that for DLS, as equilibrium theory can be used to guide the analysis of SLS data. Taking the EGPC and gel-boundary results into account, there is a good concentration range for investigation of micelle properties for solutions of copolymer P94E316 suitable for both DLS and SLS measurements and a useful concentration range for copolymer P60E348 allowing satisfactory analysis of SLS data and approximate analysis of DLS data. However the concentration range available for solutions of copolymer P43E312 is very narrow, allowing only approximate analysis of SLS data. In practice the lower concentration limits (17) Li, H.; Yu., G.-E.; Price, C.; Booth, C.; Hecht, E.; Hoffmann, H. Macromolecules 1997, 30, 1347. (18) Bedells, A. D.; Arafeh, R. M.; Yang, Z.; Attwood, D.; Padget, J. C.; Price, C.; Booth, C. J. Chem. Soc., Faraday Trans. 1993, 89, 1243. (19) Kelarakis, A.; Havredaki, V.; Viras, K.; Mingvanish, W.; Heatley, F.; Booth, C.; Mai, S.-M. J. Phys. Chem. B 2001, 105, 7384.
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Table 2. Critical Micellization Temperatures for PnEm Copolymers in Aqueous Solutiona copolymer P94E316
10.0 20.0
18 16.5
P60E348
5.8 14.9 17.6 20.0 27.4 39.7 49.7 5.6 11.9 28.8 44.6
24 22 22 21 21 20 17 32 30 26 24
P43E312
a
light scattering c/g dm-3 cmt/°C
dye solubilization c/g dm-3 cmt/°C 1 20 50 5.0 10.0 20.0
22 16 14 22 21 19
50.0
17
15 20 30 50
26 24 22 18
∆micHoapp/ kJ mol-1
∆micHoapp/ n/kJ mol-1
345 ( 25
3.7
215 ( 35
3.6
160 ( 20
3.7
Estimated uncertainty in cmt: (1 °C.
Figure 2. log(concentration) versus reciprocal temperature for aqueous solutions of the PmEn copolymers indicated. The data points denote (b) cmt by light scattering, (O) cmt by dye solubilization, (9) cmc by surface tension (ref 10), and (0) cmt by EGPC (see section 3.3).
Figure 4. EGPC curves for PnEm copolymers in aqueous solution at the concentrations and temperatures indicated. Table 3. Extent of Micellization from EGPC
Figure 3. Gel boundaries determined by tube inversion for aqueous solutions of PmEn copolymers: (9) P94E316; (b) P60E348; (O) P43E312.
were set at approximately 5 g dm-3 (P94E316), 15 g dm-3 (P60E348), and 30 g dm-3 (P43E312), and the upper limit was set at 40 g dm-3 in all cases. Solution temperatures were 35-40 and 50 °C, plus 25 °C for solutions of copolymer P94E316 only. 3.5. Micelle Size Distribution by DLS. Examples of intensity fraction distributions of log(rh,app) obtained for aqueous solutions of the copolymers are shown in Figure 5. The single-peaked distributions indicate closed micellar association under the conditions used, confirming the results from EGPC. The peak positions were insensitive to temperature (range 25-50 °C for P94E316, 35-50 °C otherwise), as expected for micelles of this type.1,8,20 Average values of 1/rh,app obtained as a function of concentration are plotted in Figure 6. The quantity 1/rh,app relates directly to the apparent diffusion coefficient (20) Attwood, D.; Collett, J. H.; Tait, C. J. Int. J. Pharm. 1985, 26, 25.
copolymer
c/g dm-3
T/°C
wt % micelles
P94E316
5
P60E348
5
P43E312
10
27 38 49 25 39 49 25 32 39 49 28 35 39 49
50 85 ∼100 2 75 ∼100 0
