4534
Langmuir 2006, 22, 4534-4540
Effect of Block Length, Polydispersity, and Salt Concentration on PEO-PDEAMA Block Copolymer Structures in Dilute Solution Dave J. Adams,*,† Michael F. Butler,† and Anthony C. Weaver‡ UnileVer Corporate Research and Measurement Science, UnileVer R&D, Colworth, Sharnbrook, Bedfordshire, MK44 1LQ U.K. ReceiVed January 20, 2006. In Final Form: March 17, 2006 A series of poly(ethylene oxide)-block-poly(N,N-diethylaminoethyl methacrylate) (PEO-PDEAMA) block copolymers with relatively high polydispersity (1.36 < PDI < 1.96) have been prepared to determine the effect that polydispersity has on the self-assembly of amphiphilic block copolymers in dilute solution. Because monodisperse macroinitiators were used for the ATRP reactions, the polydispersity resides within the hydrophobic block. By adjusting the relative block lengths, spherical micelles, wormlike micelles, vesicles, or a precipitate is formed. Here, we show that relatively high polydispersity in the block copolymer does not preclude efficient self-assembly. We also discuss the effect of increasing the concentration of NaCl in the systems and show that this can result in a shift from one morphology to another. These shifts are reversible in some cases, but for PEO12-PDEAMA39, this method allows access to giant vesicles of between 500 nm and 1 µm in diameter.
1. Introduction Amphiphilic block copolymers can adopt several different morphologies in dilute aqueous solution. The size and shape of these morphologies are dictated by the molecular size, the composition, the architecture, and the concentration of the block copolymer.1,2 Many examples of different morphologies have been reported including spheres, rods, bilayers, and compound micelles.3 Another factor that potentially can affect the selfassembly of amphiphilic block copolymers is the polydispersity. (The polydispersity index (PDI) is calculated from Mw/Mn.) There are indications that the polydispersity can have a dramatic effect on the morphology of the final aggregates.4-7 Examples do exist of successful self-assembly of relatively polydisperse (PDI ≈ 1.6 to 1.7) block copolymers although this effect has not been well studied.8-10 Most studies in the literature on the effect of increased polydispersity have involved mixing two or more polymers, each of which has a low polydispersity, to increase the polydispersity artificially. For example, Eisenberg has mixed different poly(styrene)-block-poly(acrylic acid)s in varying amounts to create a mixture with a high polydispersity.4 This naturally creates a bi- or trimodal system that may not be a true reflection of monomodal but polydisperse systems. Also, the majority of the work has involved polydispersity in the watersoluble block as opposed to the hydrophobic block. One theoretical study on the effect of polydispersity in either the hydrophilic or hydrophobic block on the formation of vesicles was carried out * Corresponding author. E-mail:
[email protected]. † Unilever Corporate Research. ‡ Measurement Science. (1) Antionetti, M.; Forster, S. AdV. Mater. 2003, 15, 1323-1333. (2) Opsteen, J. A.; Cornelissen, J. J. L. M.; van Hest, J. C. M. Pure Appl. Chem. 2004, 76, 1309-1319. (3) Cameron, N. S.; Corbierre, M. K.; Eisenberg, A. Can J. Chem. 1999, 77, 1311-1326. (4) Terreau, O.; Luo, L.; Eisenberg, A. Langmuir 2003, 19, 5601-5607. (5) Terreau, O.; Bartels, C.; Eisenberg, A. Langmuir 2004, 20, 637. (6) Jiang, Y.; Chen, T.; Ye, F.; Liang, H.; Shi, A. Macromolecules 2005, 38, 6710-6717. (7) Jain, S.; Bates, F. S. Macromolecules 2004, 37, 1511-1523. (8) Hua, M.; Kaneko, T.; Liu, T.; Chen, M.; Akashi, M. Polym. J. 2005, 37, 59-64. (9) Nardin, C.; Hirt, T.; Leukel, J.; Meier, W. Langmuir 2000, 16, 1035-1041. (10) Sommerdijk, N. A. J. M.; Holder, S. J.; Hiorns, R. C.; Jones, R. G.; Nolter, R. J. M. Macromolecules 2000, 33, 8289.
by Jiang et al. using self-consistent-field theory.6 Their work suggests that an increase in polydispersity in the hydrophilic block leads to a segregation of polymers between the inner and outer monolayers. Those with shorter blocks tend to go to the inside of the vesicles and hence promote curvature, resulting in the formation of smaller vesicles. This effect has also been postulated by Eisenberg et al. to explain their experimental observations.4 However, increasing the polydispersity in the hydrophobic block leads to the formation of so-called quasivesicles that have less-well-defined cores. At very high polydispersities (PDI > 3), spherical micelles were formed. Here, we have used atom-transfer radical polymerization (ATRP) to prepare a series of poly(ethylene oxide)-block-poly(N,N-diethylaminoethyl methacrylate) (PEO-PDEAMA) copolymers. ATRP has been shown previously to give monodisperse block copolymers. (The PDI can be below 1.2 for polymers prepared by ATRP.11-16) However, by adjusting the reaction conditions, we have succeeded in increasing the polydispersities of the polymers such that 1.36 < PDI < 1.96. In all cases, monodisperse PEO-based macroinitiators were used such that the polydispersity in the samples resides mainly in the hydrophobic block. Here, we show that this level of polydipersity in the hydrophobic block does not preclude efficient self-assembly with spherical micelles, wormlike micelles, and vesicles being formed depending on the ratio of the hydrophobic to hydrophilic average block lengths. We also show that a shift from one morphology to another can be induced by increasing the concentration of NaCl in the system. 2. Experimental Section 2.1. Materials. N,N-Diethylaminoethyl methacrylate was obtained from Aldrich and passed through basic alumina before use. Copper(11) Vamvakaki, M.; Billingham, N. C.; Armes, S. P. Macromolecules 1999, 32, 2088-2090. (12) Vamakaki, M.; Papoutsakis, L.; Katsamanis, V.; Afchoudia, T.; Fragouli, P. G.; Iatrou, H.; Hadjichristidis, N.; Armes, S. P.; Sidorov, S.; Zhirov, D.; Kostylev, M.; Bronstein, L. M.; Anastasiadis, S. P. Faraday Discuss. 2005, 128, 129-147. (13) Lee, A. S.; Butun, V.; Vamakaki, M.; Armes, S. P.; Pople, J. A.; Gast, A. P. Macromolecules 2002, 35, 8540-8551. (14) Tan, J. F.; Ravi, P.; Too, H. P.; Haton, T. A.; Tam, K. C. Biomacromolecules 2005, 6, 498-506. (15) Dufresne, M.; Leroux, J. Pharm. Res. 2004, 21, 160-169. (16) Liu, S.; Weaver, J. V. M.; Tang, Y.; Billingham, N. C.; Armes, S. P.; Tribe, K. Macromolecules 2002, 35, 6121-6131.
10.1021/la060192x CCC: $33.50 © 2006 American Chemical Society Published on Web 04/15/2006
Effect of Polydispersity on PEO-PDEAMA Micelles
Langmuir, Vol. 22, No. 10, 2006 4535
Scheme 1. Effect of pH on PEO-PDEAMA Block Copolymers
(I) bromide (CuBr), 2,2′-bipyridine (bipy), 2-bromoisobutyryl bromide, triethylamine, toluene (anhydrous grade), diethyl ether, basic alumina, and monohydroxy-capped poly(ethylene oxides) with mean degrees of polymerization of 12, 16, 45, and 113 (designated mPEO12-OH, mPEO16-OH, mPEO45-OH, and mPEO113-OH, respectively) were purchased from Aldrich and used as received. 2-Butanone was purchased from Aldrich, distilled under nitrogen, and stored under nitrogen prior to use. 2.2. Polymer Synthesis. PEO macroinitiators were prepared by the methodology of Armes et al.16 The PEO macroinitiator (0.2 mmol), CuBr (0.005 g, 0.0035 mmol), and bipy (0.01 g, 0.065 mmol) were added to a three-necked roundbottomed flask. After being subjected to high vacuum for 30 min, the flask was filled with nitrogen, and 2-butanone (10 mL) was added. This solution was stirred for 5 min, and then N,Ndiethylaminoethyl methacrylate was added. The flask was heated to 80 °C for 8 h. After cooling to room temperature and opening to the air, the solution was passed through a basic alumina column, and the solvent was removed in vacuo. The crude product was then washed well with petroleum ether (60/80), dissolved in the minimum amount of THF, and again passed through an alumina column to remove any remaining copper species. The solvent was removed in vacuo. The crude product was washed with water and dried overnight at 40 °C in vacuo. 1H NMR (CDCl ): δ 3.99 (bs, COOCH ), 3.64 (s, OCH CH ), 3 2 2 2 3.37 (s, OCH3), 2.71 (bs, COOCH2CH2), 2.57 (bs, NCH2CH3), 1.89 (bs, CH2C(CH3)), 1.80 (bs, CH2C(CH3)), 1.03 (bs, NCH2CH3), 0.87 (bs, CCH3). IR 2967 (m), 2933 (w), 2803 (w), 1724 (s), 1449 (m), 1384 (w), 1266 (m), 1238 (m), 1146 (s), 1066 (m), 749 (w). 2.3. Preparation of Micellar Solutions. Solutions were prepared in three different ways: a. Aqueous Preparation. The PEO-PDEAMA block copolymers were dissolved in water at pH 2.0 at a concentration of 0.2 wt %. After filtering the solutions through a 200 nm filter (0.2 µm PVDF), the solution pH was adjusted to 12 by the addition of 1 M NaOH to induce self-assembly. Final polymer concentrations after this adjustment were not significantly changed. b. Aqueous Preparation in the Presence of NaCl. The same procedure was carried out as above except that water of the required NaCl molarity was used at pH 2 to dissolve the polymers. Alternatively, after altering the pH to 12, NaCl was then added to the solution to generate a solution of the required molarity. In each case, the samples were filtered through a 200 nm filter after dissolution of the NaCl. c. SolVent Preparation. The PEO-PDEAMA block copolymers were dissolved in an organic solvent at a concentration of 1.0 wt %. After filtering the solutions through a 200 nm filter, we slowly added prefiltered pH 12 water dropwise with gentle agitation between each addition to a final polymer concentration of 0.2 wt %. The solutions were then dialyzed against pH 12 water for 5 days. 2.4. Characterization. 1H NMR spectra were recorded using a Bruker AMX 400 MHz spectrometer. Infrared (IR) spectroscopy was carried out using a Biorad FTS 6000 FTIR spectrometer with a liquid-nitrogen-cooled MCT detector at 4 cm-1 resolution. Transmission electron microscopy (TEM) was carried out using a JEOL 1200 EX TEM with an ASID 10 scanning attachment. Samples (17) Won, Y.; Brannan, A. K.; Davis, H. T.; Bates, F. S. J. Phys. Chem. B 2002, 106, 3354-3364. (18) Giacomelli, C.; Le Men, L.; Borsali, R.; Lai-Kee-Him, J.; Brisson, A.; Armes, S. P.; Lewis, A. L. Biomacromolecules 2006, 7, 817-828.
were stained with methylamine tungstate. Dynamic light scattering (DLS) studies were carried out on a Malvern Nano-ZS zetasizer. The measurements were made at a scattering angle of θ ) 173° at 25 °C. The autocorrelation functions were analyzed with the CONTIN method. Molecular weight determinations were also carried out on the Nano-ZS zetasizer using a minimum of three concentrations for each sample. The block copolymers were characterized by GPC using THF (containing 2% triethylamine) as an eluent at a flow rate of 1.0 mL min-1 at 30 °C equipped with two 5 µm (30 cm) Mixed “C” columns, a WellChrom K-2301 refractive index detector operating at 950 ( 30 nm, a Precision detector PD 2020 light-scattering detector (at scattering angles of 90and 15°), and a BV400RT viscosity detector. Molecular weights of the polymers were determined by the tripledetection method using PL Cirrus Multi online software (version 2.0) supplied by Polymer Laboratories. A series of near-monodisperse linear PMMA standards (purchased from Polymer Labs) were used to construct the calibration curve.
3. Results 3.1. Preparation and Characterization of PEO-PDEAMA Block Copolymers. A series of PEO-PDEAMA block copolymers were prepared using ATRP. By using different PEO macroinitiators and different ratios of macroinitiator to N,Ndiethylaminoethyl methacrylate, block copolymers with differing ratios of hydrophilic to hydrophobic block lengths were prepared. Low polydispersities are usually obtained in protic media such as 2-propanol and methanol/water mixtures.12,14 Also, low polydispersities are encouraged when the polymerizations are carried out at high concentration. To encourage higher polydispersity, the reactions were carried out in dilute solution in 2-butanone and were allowed to run to completion. This led to final materials with a polydispersity of between 1.36 and 1.98. The GPC traces of these materials show broad monomodal traces (Supporting Information). This method encourages polydispersity only in the DEAMA block because monodisperse PEO-based macroinitiators were used. The PEO-PDEAMA block copolymers are molecularly soluble at low pH (where the tertiary amine groups are protonated) but phase separate at high pH where the methacrylic block becomes hydrophobic when deprotonated (Scheme 1). The molecular parameters of the block copolymers used in this study are shown in Table 1 along with data from the three literature examples of PEO-PDEAMA block copolymers. 3.2. Micellar Morphology. Each block copolymer was dissolved in pH 2 water at a concentration of 0.2 wt %. After complete dissolution, the solutions were filtered and adjusted to pH 12. The samples were then analyzed by DLS and TEM. Different morphologies are formed in different regions of the hydrophilic volume fraction (fEO) as exemplified by the images in Figure 1. The volume fractions were calculated from the stoichiometry using the amorphous density of PEO as reported elsewhere (FPEO ) 1.13 g/cm3).17 There is no reported value for the density of PDEAMA. Hence, that of PMMA was used instead (FPMMA ) 1.19 g/cm3). Although the densities of the lowermolecular-weight polymers used here will undoubtedly be lower
4536 Langmuir, Vol. 22, No. 10, 2006
Adams et al.
Figure 1. Representative TEM images from 0.2 wt % solutions of block copolymers 9 (spherical micelles), 10 (wormlike micelles), 1 (vesicles), and 3 (precipitate) at pH 12. The scale bars are 200 nm in each case, except for polymer 3 where it is 500 nm. Table 1. Molecular Parameters of PEO-PDEAMA Block Copolymers Prepared by ATRP entry
n, PEOna
m, DEAMAma
Mw/Mnb
%EOc
fEOd
1 2 3 4 5 6 7 8 9 10 ref 14 ref 15 ref 16
12 12 12 16 45 45 45 113 113 113 45 113 45
39 52 103 89 94 96 141 43 79 107 37 70 14
1.36 1.85 1.49 1.96 1.52 1.49 1.74 1.36 1.53 1.68 1.21 1.28 1.16
0.24 0.19 0.10 0.15 0.32 0.32 0.24 0.72 0.59 0.51 0.55 0.62 0.76
0.07 0.06 0.03 0.04 0.11 0.11 0.07 0.40 0.26 0.21 0.23 0.29 0.45
a Calculated from 1H NMR. b Measured by GPC. c Calculated from n/(n + m). d Volume fraction of the EO block calculated using FPEO ) 1.13 g/cm3 and FPDEAMA ) 1.19 g/cm3.
than these values (especially in the solvated state for PEO), it is anticipated that they will remain in a similar ratio to one another. From Figure 1, it can be seen that at a high fEO spherical micelles are formed. These are entirely consistent with the micelles imaged by Borsali et al. for micelles formed from poly(2-methacryloyloxyethylphosphorylcholine)-block-poly(2-(diisopropylamino)ethyl methacrylate).18 As fEO decreases, wormlike micelles are formed that coexist with spherical micelles. As fEO decreases further, bilayer structures (vesicles) are formed. At very low fEO, much larger structures are formed. The structures formed by PEO16-PDEAMA89 and PEO12-PDEAMA103 (the two examples with the lowest fEO values) are apparently not vesicles or other bilayer structures. Higher-resolution images show that these are nonspherical in nature and polydisperse (Figure 2). The structures from these two systems are similar. No internal structure can be seen from the TEM images, and it appears that this is precipitated
polymer. This change from one morphology to another as the fEO is decreased is consistent with reports on other block copolymers.17 It should be stressed that the TEM images seen here are consistent over several samples. Although the use of staining in TEM can potentially lead to artifacts, it should be stressed that the TEM images seen here are consistent over several samples with polymers with different fEO values giving different morphologies. The sizes and morphologies of the structures seen in the TEM images are also completely consistent with the DLS data (see below). We have found that the method of preparation can have a large effect on the final morphology adopted by PEO-PDEAMA block copolymers. This effect is exemplified here for PEO16PDEAMA89. Initially dissolving PEO16-PDEAMA89 in an organic solvent, adding pH 12 water, and then removing the solvent by dialysis results in very different structures being formed depending on the solvent used (Figure 3). It is also clear that the structures have already been fixed before dialysis because the TEM images and DLS data are very similar before and after dialysis. The trends are similar for all polymers studied here, whereby samples prepared initially in N,N-dimethylformamide (DMF) give the largest structures and those initially prepared in 1,4-dioxane give the smallest. In the case of PEO16-PDEAMA89, an order-of-magnitude difference in size is observed between the structures formed in these two solvents, and it is unclear as to why the different solvents lead to such different results. However, for the structures formed from PEO16-PDEAMA89 in DMF, the final diameters are almost identical to those formed by the pH adjustment method described above, and internal structure can be observed. Before dialysis, some of the structures burst open when analyzed by TEM, better showing the internal structure that is presumably due to the presence of solvent within the structure at this stage. This internal structure again demonstrates that these are not simply large vesicles. There are other
Effect of Polydispersity on PEO-PDEAMA Micelles
Langmuir, Vol. 22, No. 10, 2006 4537
Figure 2. High-resolution images of structures formed by PEO16-PDEAMA89 (4) and PEO12-PDEAMA103 (3). The scale bars are 200 nm.
Figure 3. TEM images of PEO16-PDEAMA89 prepared by quenching a solution in an organic solvent with pH 12 water: top images, before dialysis; bottom images, after dialysis. The scale bar in each case is 200 nm except for DMF before dialysis, 500 nm.
reports showing that the size of the aggregates formed by block copolymers can be affected by the solvents used to prepare the samples. Eisenberg et al. have rationalized such observations on the basis of polymer-solvent interactions (measured by the Hildebrand solubility parameter) and the dielectric constant.19 However, Vangeyte et al. have shown that when control of the aggregation size is kinetic no correlation is found with these solvent parameters.20 Similarly, here we observe no correlation with the Hildebrand solubility parameter, dielectric constant, or viscosity, suggesting that the differences observed between the structures formed from the different solvents are kinetic in origin. The TEM images agree well with the measurements from DLS. The sizes of the structures formed in dilute solution as measured by DLS are shown in Figure 4. The sizes have been plotted against the volume fraction of EO (fEO). Also included in Figure 4 is the data for the literature examples of more monodisperse (1.16 < PDI < 1.28) PEO-PDEAMA samples, all of which have been shown to adopt spherical micelle morphologies.13-15 All of the block copolymers give a single population as measured by DLS with the exception of PEO113-PDEAMA107 (polymer 10 as marked on Figure 4). Here, a bimodal distribution is observed with a population at 26 nm and a second population at approximately 800 nm. This bimodal distribution is explained (19) Yu, Y.; Zhang, L.; Eisenberg, A. Macromolecules 1998, 31, 1144-1154. (20) Vangeyte, P.; Gautier, S.; Jerome, R. Colloids Surf., A 2004, 242, 203211.
Figure 4. Diameters of structures formed on self-assembly of PEOPDEAMA block copolymers: b, this work; O, literature data.13-15 The errors bars refer to the variation in repeat preparations of the structures from fresh solutions of the polymers (minimum of three measurements in each case). The vertical lines represent boundaries between the different morphologies observed in this work: S, spherical micelles; C, wormlike micelles; B, bilayers; P, precipitate.
by the coexistence of spherical and wormlike micelles in this sample. In general, as the hydrophobic block to hydrophilic block ratio increases (and hence fEO decreases), the size of the structures formed as measured by DLS also increases. The size as measured by DLS and the structures observed by TEM are shown in Table 2.
4538 Langmuir, Vol. 22, No. 10, 2006
Adams et al.
Figure 5. Effect of added NaCl on the aggregate diameter of (a) PEO113-PDEAMA43, (b) PEO113-PDEAMA107, and (c) PEO12-PDEAMA39 (b, preadded NaCl; O, postadded NaCl) block copolymers.
Figure 6. TEM images of (a) PEO113-PDEAMA43 (left, scale bar 200 nm), (b) PEO113-PDEAMA107, (c) PEO12-PDEAMA39 after 3 h, and (d) PEO12-PDEAMA39 prepared by post-addition of 1.0 M NaCl after pH-induced self-assembly. Scale bars are 200 nm apart from d, 1 µm. Table 2. Diameters and Aggregation Numbers of the Structures Formed upon Self-Assembly of Each Block Copolymer in Dilute Solution polymer
diameter of structure/nma
structure observedb
1 2 3 4 5 6 7 8 9 10
36.3 ( 9.0 127.7 ( 14.9 131.5 ( 19.1 128.6 ( 23.9 49.3 ( 7.2 62.1 ( 6.5 33.6 ( 11.3 20.5 ( 0.9 23.2 ( 0.5 26.6 ( 0.2
vesicle vesicle precipitate precipitate vesicle vesicle vesicle micelle micelle wormlike micelles
a Measured by DLS (errors calculated from repeat measurements from at least three different fresh solutions). b Morphology as determined by TEM.
3.3. Effect of Salt on Self-Assembly. Because PDEAMA is charged at low pH and uncharged at high pH, there should be a pronounced effect on self-assembly in the presence of salt. The Rh value is expected to increase because the salt screens out the electrostatic repulsions between the PDEAMA blocks, allowing greater numbers of chains to aggregate to form the core as the pH is adjusted. It has previously been found for PEO45PDEAMA34 that including salt before aggregation does lead to a slight increase in the aggregation number and therefore in Rh.13 Here, the effect of NaCl on the self-assembly of the block copolymers prepared by the aqueous method was examined in two ways. First, the copolymers were dissolved in pH 2 water of differing NaCl concentrations, and then self-assembly was induced by adding NaOH as above. Second, NaCl was added after the self-assembly had already been induced. Example data for a system that, under the conditions described in the previous section, gives spherical micelles (PEO113-PDEAMA43), a system that gives wormlike micelles (PEO113-PDEAMA107), and a system that gives vesicles (PEO12-PDEAMA39) is shown in Figure 5. Here, for PEO113-PDEAMA43 we find a slight apparent
increase in micelle size with increasing NaCl concentration. For PEO113-PDEAMA107, we find a slow increase in micelle size with increasing NaCl concentration. For PEO12-PDEAMA39, the addition of salt up to 0.2 M NaCl has little effect. However, above 0.2 M, significant changes are observed with the size of the aggregates increasing rapidly with the molarity. At 2.0 M, the aggregates are over 10 times larger than at 0.2 M NaCl. In all cases, the polydispersity of the self-assembled system increases as the NaCl concentration is increased. It is to be expected that because the charges on the DEAMA block will have already been neutralized the postmicellization addition of NaCl should have little effect. However, there is an observable increase in size if 1.0 M NaCl is added to PEO113PDEAMA43 after micellization. The postmicellization addition of NaCl to solutions of PEO12-PDEAMA39 also causes significant increases in the sizes of the aggregates. This can be observed visually by the onset of turbidity a few minutes after the dissolution of the NaCl. TEM imaging of the samples prepared by the addition of 1.0 M NaCl after self-assembly showed the presence of nonspherical structures for PEO113-PDEAMA43, PEO113-PDEAMA107, and PEO12-PDEAMA39 (Figure 6). Wormlike micelles are observed in the PEO113-PDEAMA43 system, and these became more numerous over several days at the expense of the spherical aggregates. For PEO113-PDEAMA107, the fraction of wormlike structures is much greater. For PEO12-PDEAMA39, TEM images of samples that have been aged for approximately 3 h showed the presence of swollen vesicles. Images of samples aged longer showed the presence of a precipitate. The NaCl was then removed from these solutions by dialysis against pH 12 water for 5 days. For PEO113-PDEAMA43 and PEO113-PDEAMA107, micelles that were indistinguishable from the starting micelles were formed. However, for PEO12PDEAMA39, even after dialysis, the solution remained turbid. DLS shows the presence of large (>500 nm) species. The TEM images of this system show the presence of very large vesicles
Effect of Polydispersity on PEO-PDEAMA Micelles
Langmuir, Vol. 22, No. 10, 2006 4539
Figure 8. (a) Overlay of present data against volume fraction regions determined by Bates et al.17 for regions containing different morphologies: S, spheres; C, cylinders; B, bilayers; b, this work; O, literature data.14-16
Figure 7. TEM images of large vesicles formed by the dialysis of a PEO12-PDEAMA39 system with postaddition of 1 M NaCl salt. The scale bar represents 500 nm.
(Figure 7). These are polydisperse in size but between 500 nm and 1 µm in diameter.
4. Discussion The self-assembly of amphiphilic block copolymers has been widely studied, and a range of morphologies have been demonstrated. Several reports describe the behavior shown here, with increasing diameters observed as the volume of the hydrophobic block is increased.17,21,22 These literature results suggest that it is unlikely that all structures formed by these block copolymers are spherical. It is expected that a transition from spheres to cylinders to lamellae (bilayers) will occur as the relative length of the hydrophilic block decreases. Hence, in the PEO-PDEAMA systems presented here, the fact that spherical micelles, wormlike micelles, and vesicles are formed at different values of fEO is unsurprising. However, it is interesting that the vast majority of the work carried out on the self-assembly of amphiphilic block copolymers is done with polymers with a low polydispersity. Here, we have used block copolymers with a relatively high polydispersity in the hydrophobic block. This work therefore shows that a high polydispersity does not preclude efficient self-assembly into different morphologies that are dependent on fEO. This is highlighted by a comparison between PEO113-PDEAMA79 (entry 9, Table 1) and PEO113-PDEAMA70 (data from ref 14). Both form spherical micellar morphologies with diameters of approximately 20 nm (23 nm for PEO113PDEAMA79 and 17.3 nm for PEO113-PDEAMA70). Also, the aggregation numbers are very similar (82, as calculated from a Debye plot, and 85, respectively), despite the differences in polydispersity (1.53 as compared to 1.28). Another interesting observation using these PEO-PDEAMA block copolymers is the formation of what appears to be precipitated polymer at very low fEO. At very low fEO, the short PEO chains are no longer able to solubilize well-defined structures. This effect is not purely due to the high polydispersities used because PEO12-PDEAMA52 (which has a polydispersity of 1.85) still forms well-defined structures as shown by the TEM (21) Garnier, S.; Laschewsky, A. Macromolecules 2005, 38, 7580-7592. (22) Forster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. J. Chem. Phys. 1996, 104, 9956-9970.
images. PEO12-PDEAMA103 (with a lower polydispersity of 1.49) forms a precipitate (Figure 3). This precipitate morphology is possible because of the polyelectrolyte nature of PEOPDEAMA block copolymers. Because of the protonatable diethylamino groups on the DEAMA blocks, the polymers are soluble at low pH, and hence even polymers that are too hydrophobic to self-assemble into defined structures can still be solubilized at low pH. However, when the pH is raised, the polymers have no choice but to precipitate. The low concentration results in submicrometer particles being formed. This work shows that it is possible to prepare spherical micelles, wormlike micelles, and bilayer structures using relatively polydisperse block copolymers. Bates et al. have described similar morphologies and phase boundaries for monodisperse poly(ethylene oxide)-block-poly(butadiene) (PEO-PB) and poly(ethylene oxide)-block-poly(ethyl ethylene) (PEO-PEE) copolymers.17,23,24 However, the regions differ quite dramatically from those derived for these polymers in terms of the volumes of the hydrophobic block required to bring about each morphology (Figure 8). There are several possibilities for the differences shown in Figure 8. First, the density of PDEAMA is not accurately known, and that of PMMA has been used for the volume determination. However, this value is typical for a range of methacrylic polymers. Also, for the data to fall onto that predicted by comparison with the work of Bates et al., a density of approximately 4 would be required, which is clearly unrealistic. Second, the polydispersity of the polymers used in this study is higher than those of Bates et al., which are on the order of 1.09 < PDI < 1.22. However, the examples of PEO-PDEAMA in the literature have all been shown to give spherical micelles in solution. These also lie at values of fEO that would be predicted to give bilayers or wormlike micelles as can be seen in Figure 8, despite having polydispersities in the same range as for the polymers used by Bates et al. Third, the method of inducing self-assembly is also different. Bates et al. stir the block copolymers in water for 5 days before analysis. Here, the PEO-PDEAMA copolymers are self-assembled by pH adjustment. Hence, as self-assembly begins, there will be residual charge on the polymers that may affect the maximum number of chains that can come together, which may lead to an effect on aggregation number competing with the fEO. This would lead to there being fewer chains in each structure than would occur if the chains were charge neutral at the point of self(23) Jain, S.; Bates, F. S. Science 2003, 300, 460-464. (24) Jain, S.; Bates, F. S. Macromolecules 2004, 37, 1511-1523.
4540 Langmuir, Vol. 22, No. 10, 2006
Figure 9. (Left) comparison of fH and percentage hydrophobic block for data from Bates (b, PEO-PB) and this work (O, PEOPDEAMA); abbreviations are the same as for Figure 8. (Right) overlay of present data against percentage hydrophobic regions determined using the data of Bates et al. for regions containing different morphologies: b, this work; O, literature data.13-15
assembly. It is unclear, however, how this would lead to the differences highlighted in Figure 8. Fourth, Bates et al. have reported that increasing the size of the hydrophobic block while maintaining the same hydrophobic to hydrophilic balance shifts the composition window for wormlike micelles and perhaps vesicles and spherical micelles to higher values of the hydrophobic block volume (fH).23,24 DEAMA has a much greater volume than butadiene, so it would be expected that this effect might be magnified greatly. Qualitatively, this agrees with the results shown in Figure 8. However, the origin of this effect is unclear. Finally, the effect may simply arise from the way that the calculation is carried out. The molecular weights of the hydrophobic monomers differ dramatically, as do the densities. The result of this is that there is a nonlinear relationship between fEO and the percentage of the hydrophilic block by composition (NPEO/(NH + NPEO)), which is much more apparent for PDEAMA than PB (Figure 9). Figure 9 also demonstrates that the same hydrophobic block percentage for butadiene and DEAMA gives dramatically different fEO values. If the data for the PEO-PDEAMA polymers is overlaid with the morphological boundaries for PEO-PB using the EO monomer number percentage (n/(n + m)) as opposed to the fEO in both cases, then the data clearly falls much more successfully into the expected regions for each morphology. The implication of this is that the packing adopted by the hydrophobic block is similar in the cases of DEAMA and butadiene, with the morphology adopted is related to the length of the hydrophobic block as determined by the backbone. In each case, the backbone is two carbon atoms long, which is approximately the same for (CH2C(CH3)R in the case of DEAMA and for CH2CHR in the case of butadiene (R ) side chain). It is possible to induce systems to cross these morphological boundaries by the pre- or postmicellization addition of NaCl . In the PEO113-PDEAMA43 system, whereas spherical micelles are formed at low concentrations of NaCl, wormlike micelles are found at high concentrations of NaCl, and over time, these become both longer and more numerous at the expense of the spherical micelles. For PEO113-PDEAMA107, wormlike micelles are still formed, but these become more populous, becoming the dominant structure. For PEO12-PDEAMA39, the addition of NaCl initially causes the vesicles to become much larger in size, and then over time, the precipitated polymer is formed. These results can be explained by the fact that NaCl is known to dehydrate PEO chains, reducing the cloud point (i.e., salting out the PEO chains25-28). It is therefore expected that for the PEO-PDEAMA block copolymers the presence of NaCl will reduce the solubility of the PEO chains. As a result, it is to be expected that the effective hydrophilic volume of the block copolymer will be decreased as the hydrodynamic volume of the
Adams et al.
PEO chains decreases. The presence of wormlike micelles in the PEO113-PDEAMA43 systems demonstrates that the effective hydrophilic volume has indeed been decreased by the addition of NaCl. This shows that different morphologies can be accessed by the varying the solubility of the hydrophilic blocks by altering the pre- or post-self-assembly solvent quality. For PEO12PDEAMA39, the aggregates formed seem not to be well-defined structures, which implies that the much shorter PEO12 chains are no longer sufficient to solubilize the structures formed. When the salt is removed once again by dialysis, for PEO113PDEAMA43 and PEO113-PDEAMA107, solutions that are indistinguishable from the starting solutions are regenerated. However, for PEO12-PDEAMA39, large (500-nm-diameter) vesicles are formed. These are very different from the starting vesicles, which were 36 nm in diameter. This implies that there are far more block copolymer chains making up the final vesicles than in the starting vesicles. This occurs because the polymer precipitates and aggregates when the NaCl is first added. This provides a large reservoir of polymer. Hence, when the NaCl is removed by dialysis and the PEO chains become capable of solubilization once again, the final structures are by necessity much larger.
5. Conclusions A series of PEO-PDEAMA block copolymers of varying composition have been prepared. The polydispersities of the block copolymers are relatively high (1.36 < PDI < 1.96), with the polydispersity residing within the hydrophobic PDEAMA block. Nevertheless, by adjusting the relative block lengths, spherical micelles, wormlike micelles, and vesicles can be formed. Qualitatively, the same trend is observed as for other literature systems with micelles being formed at higher fEO, wormlike micelles being formed at intermediate fEO, and vesicles being formed at low fEO. At extremely low fEO, the PEO is unable to solubilize the structures, and irregular, nonspherical structures are formed. It is clear that the level of polydispersity in the hydrophobic block of a PEO-PDEAMA block copolymer does not preclude efficient self-assembly into micelles, wormlike micelles, or vesicles. It has been shown that using the percentage hydrophobic monomer allows a comparison between the literature data for predicting the morphology that will be adopted in dilute solution, whereas the volume of the hydrophobic block is a poor indicator when comparing to the literature. Finally, it has been shown that it is possible to induce these systems to cross morphological boundaries by the pre- or postmicellization addition of NaCl . These are reversible in some cases, but for PEO12PDEAMA39, this method allows access to giant vesicles of between 500 nm and 1 µm in diameter. Acknowledgment. We thank Sarah Adams and Jeroen Bongaerts for helpful discussions. We thank Steve Armes and Cong Duan Vo (University of Sheffield) for access to the GPC, Paul Sanderson and Dave Caswell for running the NMR spectra, and Andrzej Wolski for running the IR spectra. Supporting Information Available: Example GPC traces of block copolymers. This material is available free of charge via the Internet at http://pubs.acs.org. LA060192X (25) Ganguly, R.; Aswal, V. K.; Hassan, P. A.; Gopalakrishnan, I. K.; Yakhmi, J. V. J. Phys. Chem. B 2005, 109, 5653-5658. (26) Alexandridis, P.; Holzwarth, J. F. Langmuir 1997, 13, 6074-6082. (27) Armstrong, J. K.; Leharne, S. A.; Stuart, B. H.; Snowden, M. J.; Chowdhry, B. Z. Langmuir 2001, 17, 4482-4485. (28) Bailey, F. E.; Callard, R. W. J. Appl. Polym. Sci. 1959, 1, 56-62.
