pubs.acs.org/Langmuir © 2009 American Chemical Society
Hierarchical Micellar Structures from Amphiphilic Invertible Polyesters: 1 H NMR Spectroscopic Study Ananiy Kohut and Andriy Voronov* Deptartment of Coatings & Polymeric Materials, North Dakota State University, Fargo, North Dakota Received February 26, 2009. Revised Manuscript Received March 16, 2009 The environment-dependent behavior of invertible polyesters has been studied by 1H NMR spectroscopy. In dilute toluene solutions, the micelle exterior is made up of the lipophilic fragments, and the interior consists of the hydrophilic constituents. The polyester inverts the structure in an aqueous medium to form micelles with a hydrophobic inner part and a hydrophilic outer part. Increasing polyester concentration leads to the formation of hierarchical structures both in toluene and in an aqueous medium as a result of the aggregation of unimolecular micelles and the formation of hydrophilic and lipophilic domains. On the contrary, no unimolecular micelles or micellar aggregation has been observed in acetone or chloroform.
Recently, we described invertible amphiphilic polyesters prepared by a polycondensation reaction from poly(ethylene glycol) (hydrophilic constituent) and aliphatic dicarboxylic acids (hydrophobic constituent) alternately distributed along the polymeric backbone.1 Their macromolecules form micellar architectures in solvents differing by polarity and demonstrate a unique switching behavior in polar and nonpolar environments with changing polarity.2-8 It is an assumption that when the polarity of an environment changes from polar to nonpolar the poly(ethylene glycol) constituents collapse and form the interior of the micelle and the aliphatic dicarboxylic acid fragments switch their location to the outer side of the micelle and vice versa. These changes occur by changing the polarity from nonpolar to polar. The preparation of smart adaptive micellar structures from such amphiphilic polyesters is a novel approach based on the manipulation of micelle formation and following the self-arrangement of the micelles into aggregates by changing the polymer concentration and environmental polarity. The resulting invertible complex hierarchical nanostructures with controlled size and morphology can be functional both in a polar and nonpolar environment and might be functional in a broad range of chemical and biochemical applications. They can be used as nanoreactors for the synthesis of smart nanoparticles having a protective shell made from both the hydrophilic and hydrophobic polymeric fragments and thus are very stable both in polar and nonpolar environments.2,3,6-8 The ability to encapsulate substances, some of which are insoluble, makes the smart polymeric nanostructures promising for the pharmaceutical industry, agriculture, and cosmetics.4,5 *Corresponding author. Tel +1 701 2319563. Fax +1 701 2318439. Email:
[email protected]. (1) Voronov, A.; Kohut, A.; Peukert, W.; Voronov, S.; Gevus, O.; Tokarev, V. Langmuir 2006, 22, 1946–1948. (2) Kohut, A.; Voronov, A.; Samaryk, V.; Peukert, W. Macromol. Rapid Commun. 2007, 28, 1410–1414. (3) Voronov, A.; Kohut, A.; Peukert, W. Langmuir 2007, 23, 360–363. (4) Voronov, A.; Vasylyev, S.; Kohut, A.; Peukert, W. J. Colloid Interface Sci. 2008, 323, 379–385. (5) Martinez Tomalino, L.; Voronov, A.; Kohut, A.; Peukert, W. J. Phys. Chem. B 2008, 112, 6338–6343. (6) Voronov, A.; Kohut, A.; Peukert, W.; Ranjan, S.; Voronov, S.; Tokarev, V.; Gevus, O. Langmuir 2006, 22, 6498–6506. (7) Kohut, A.; Voronov, A.; Peukert, W. Langmuir 2007, 23, 504–508. (8) Voronov, A.; Kohut, A.; Vasylyev, S.; Peukert, W. Langmuir 2008, 24, 12587–12594.
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To identify factors that are important for the formation of micellar structures from invertible polyesters and their selfassembly, we perfomed an experimental NMR spectroscopic study of polyester structural changes by changing the solvent polarity over a wide concentration range. Two main questions are targeted in this study: (i) How does the invertible polyester micelle behave in changing medium polarity? (ii) Will the invertible polyester micelle be capable of self-assembly and form secondary structures (aggregates) both in a polar and a nonpolar medium with increasing polymer concentration? In the past two decades, 1H NMR spectroscopy has been proven to be a very powerful technique for studying macromolecular conformation when macromolecules comprise the micelles in a wide range of temperatures and concentrations. The formation of surface-active micelles has been extensively studied for both polymeric and low-molecular-weight surfactants.9-27 In our work, the 1H NMR technique has been used for the systematic study of polyester composition, solvent polarity, and polyester concentration effect on the structural properties of the amphiphilic polyester macromolecules. (9) Nakashima, K.; Bahadur, P. Adv. Colloid Interface Sci. 2006, 123, 75–96. (10) Kositza, M. J.; Bohne, C.; Alexandridis, P.; Hatton, T. A.; Holzwarth, J. F. Macromolecules 1999, 32, 5539–5551. (11) Guo, Q.; Thomann, R.; Gronski, W.; Thurn-Albrecht, T. Macromolecules 2002, 35, 3133–3144. (12) Malka, K.; Schlick, S. Macromolecules 1997, 30, 456–465. (13) Yang, L.; Alexandridis, P. Langmuir 2000, 16, 4819–4829. (14) Booth, C.; Attwood, D.; Price, C. Phys. Chem. Chem. Phys. 2006, 8, 3612– 3622. (15) Su, Y.; Wang, J.; Liu, H. Z. J. Phys. Chem. B 2002, 106, 11823–11828. (16) Sommer, C.; Pedersen, J. S.; Stein, P. C. J. Phys. Chem. B 2004, 108, 6242– 6249. (17) Nixon, S. K.; Hvidt, S.; Booth, C. J. Colloid Interface Sci. 2004, 280, 219– 223. :: (18) Soderman, O.; Stilbs, P. Prog. NMR Spectrosc. 1994, 26, 445–482. (19) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145– 4159. (20) Zhang, Z.; Khan, A. Macromolecules 1995, 28, 3807–3812. (21) Cau, F.; Lacelle, S. Macromolecules 1996, 29, 170–178. (22) Navaggioli, T.; Tsao, B.; Alexandridis, P.; Hatton, T. A. Langmuir 1995, 11, 119–126. (23) Steinbeck, C. A.; Hedin, N.; Chmelka, B. F. Langmuir 2004, 20, 10399– 10412. (24) Fleischer, G. J. Phys. Chem. 1993, 97, 517–521. (25) Ma, J. H.; Guo, C.; Tang, Y. L.; Wang, J.; Zheng, L. L.; Liang, X. F.; Chen, S.; Liu, H. Z. J. Colloid Interface Sci. 2006, 299, 953–961. (26) Lovino, A.; Mesa, C. L.; Capitani, D.; Segre, A. L. Colloid Polym. Sci. 2003, 281, 1136–1141. (27) Ma, J. H.; Guo, C.; Tang, Y. L.; Liu, H. Z. Langmuir 2007, 23, 9596–9605.
Published on Web 3/23/2009
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Figure 1. 1H NMR spectra of 1% w/v solutions of amphiphilic polyesters S6 (left) and D3 (right) in solvents differing by polarity.
Two polyesters have been investigated in detail with 1H NMR spectroscopy. Polyester S6 based on decanedioic (sebacic) acid and PEG with a molecular weight of 600 g/mol has been chosen as a more hydrophilic polymer. That synthesized from dodecanedioic acid and PEG-300 polyester (D3) has been taken as more hydrophobic. The weight-average molecular weights of S6 and D3 are 6400 and 5500 g/mol, respectively, as determined by gel permeation chromatography. Langmuir 2009, 25(8), 4356–4360
The general chemical formula of the polyesters is depicted in Figure 1. Samples for 1H NMR spectroscopy were prepared by dissolving an appropriate amount of polyester in a corresponding deuterated solvent under gentle agitation. The solutions were left for at least 16 h to equilibrate at 25 °C before measuring. All NMR spectra were recorded at 500 MHz on a Varian VXR-500 NMR spectrometer. DOI: 10.1021/la900700u
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solvent acetone-d6
D3
S6
toluene-d8
D3
concentration, % 0.1 1.0 10 30 0.1 1.0 10 30 0.1 1.0 10 30 0.1 1.0 10 30
a
b
c
d
e
f
2.318 2.317 2.315 2.309 2.312 2.312 2.310 2.303 2.155 2.153 2.136 2.117 2.167 2.169 2.155 2.130
1.609 1.609 1.605 1.595 1.604 1.605 1.602 1.592 1.553 1.550 1.525 1.483 1.582 1.581 1.588 1.511
1.335 1.339 1.333 1.325 1.321 1.321 1.319 1.309 1.142 1.141 1.122 1.099 1.190 1.189 1.170 1.136
4.178 4.178 4.176 4.169 4.175 4.176 4.173 4.166 4.155 4.152 4.134 4.107 4.145 4.148 4.133 4.103
3.672 3.672 3.670 3.662 3.670 3.670 3.668 3.660
3.590 3.590 3.588 3.581 3.589 3.589 3.587 3.581
Effect of Solvent Polarity H NMR spectra of 1% w/v S6 and D3 solutions have been recorded in D2O, acetone-d6, CDCl3, and toluene-d8. Figure 1 shows the local expanded spectra of each peak region. In acetoned6 and CDCl3, all groups show a distinct hyperfine structure indicating that amphiphilic polyesters S6 and D3 are dissolved as unimers, the macromolecules are expanded, and the segments of the chains can move freely.19 However, when D2O has been used as a solvent, the 1H NMR spectra of S6 and D3 changed considerably. The signal width corresponding to methylene groups localized in the internal part of the hydrophobic fragment (peak c in Figure 1) increases as compared with those in acetone-d6 and CDCl3. The broadening of the signal indicates that the methylene groups avoiding contact with an aqueous medium aggregate and form a micellar core, thus reducing the mobility of the protons in a hydrophobic polyester fragment. Interestingly, each -CH2- group in the R and β positions in relation to the carbonyl groups in the dicarboxylic acid moieties (Figure 1, protons a and b, respectively) shows two different signals in D2O. The significant upfield shifts of a part of protons a (signals at 2.15 ppm for both S6 and D3) and b (signals at 1.53 ppm for both S6 and D3) imply that these protons are apparently in a nonpolar micellar core. The chemical shift is known to be sensitive to the chemical nature of the related protons, and transferring part of protons a and b to the nonpolar microenvironment induces the shift toward lower ppm values as a result of the change in magnetic susceptibility of the protons.28 The sudden 1H downfield shifts experienced by retaining protons a and b indicate that they are transferred to a polar aqueous medium. Because the interaction with water enhances the deshielding effect of the C-H protons, it results in the appearance of the peaks at 2.41 and 1.61 ppm for S6 (a and b, respectively) and at 2.34 and 1.60 ppm for D3 (a, and b, respectively). Thus, protons a and b are located partially in a nonpolar micellar core and partially in a polar aqueous medium. The reason for this is a strong -I- effect of the carbonyl groups resulting in enhanced polarizability of the C-H bonds in the R and β positions. At the same time, the peaks that are due to the poly(ethylene glycol) fragments of polyester S6 (protons d-f) move considerably downfield, indicating that the PEG units are at the micellar outer surface and hence in contact with water. The peaks remain 1
(28) Kim, B. J.; Im, S. S.; Oh, S. G. Langmuir 2001, 17, 565–566.
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3.499 3.498 3.480 3.456 3.475 3.474 3.454 3.441
sharp, showing that PEG fragments are long enough to move freely. In turn, the PEG fragment length in D3 is much lower in comparison with those in S6 (on average, 6.4 ethylene oxide links per PEG fragment in D3 and 13.2 links in S6). Here, the moving downfield signals attributed to the PEG protons of D3 broaden considerably. The PEG fragments in D3 are rather short, and their motions are limited. When toluene-d8 has been used as a solvent for S6 and D3, the chemical shifts of all protons moved upfield (Figure 1). It is well known that in aromatic solvents the high-field shifts arise from the magnetic anisotropy of the solvent molecules. When the H atom of the polyester is bonded to the center of the π-electron cloud of a toluene molecule, the ring current effect leads to an upfield shift.29 So-called aromatic solvent-induced shifts have been observed in this case. Broadening of the peaks corresponding to the PEG fragments (protons e and f) in S6 and D3 in toluene indicates the formation of the microenvironment restricting the mobility of the fragments due to their assembly. It is our assumption that polar PEG fragments form the core (interior part) of the amphiphilic polymeric architectures in nonpolar toluene. Remarkably, the broadening of the signal attributed to the PEG protons of D3 is much more prominent as compared with that of S6 (Figure 1), showing that the mobility of the shorter PEG fragments in D3 is restricted. Interestingly, in comparison with the spectra taken in acetone-d6 and CDCl3, the chemical shifts of hydrophobic group protons a-c of polyester D3 remain unchanged in terms of the width in the toluene medium, which shows that the hydrophobic groups of D3 move freely in toluene and form the outer part of the polyester micelle. Unlike that for D3, the signal of protons c of S6 experiences a slight broadening in toluene. In our opinion, the reason is the shorter length of the hydrophobic units (8 methylene groups in S6 instead of 10 -CH2- groups in D3). Apparently, although the hydrophobic units remain at the outer surface of the S6 micelles in toluene, they are too short to move freely. Similar behavior of the hydrophobic and hydrophilic fragments has been observed in aqueous and toluene media for two other polyesters developed recently in our laboratory1 (S3 based on sebacic acid and PEG300 and D6 made from dodecanedioc acid and PEG-600) (Figure 1S in Supporting Information).
(29) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 3rd ed.; Wiley: Weinheim, Germany, 2003; p 383.
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Figure 2. 1H NMR spectra of the solutions of amphiphilic polyesters S6 (left) and D3 (right) in D2O at different concentrations.
Effect of Polyester Concentration To study the concentration effect, the 1H NMR spectra of S6 and D3 have been taken in various deuterated solvents (acetone, toluene, and water) over a wide range of polyester concentration. In acetone-d6 solutions, the peaks of all protons of S6 and D3 remain almost unchanged in the range of 0.1-10% w/v (Figure 2S in Supporting Information and Table 1). Remarkably, the hyperfine structure of all groups does not disappear even at the highest investigated concentration showing that the amphiphilic chains can move freely (Figure 2S, see Supporting Information). Evidently, macromolecules are dissolved as unimers in a 30% w/v acetone solution. The slight upfield shifts are attributed Langmuir 2009, 25(8), 4356–4360
to the decreasing medium polarity as a result of the dilution of acetone by the polyesters. No changes in the chemical shift and peak width have been observed for 0.1-1% solutions of S6 and D3 in toluene (Table 1, Figure 3S in Supporting Information). However, further increases in the concentration to 10% result in upfield shifts becoming more essential for the 30% solutions (∼0.04-0.05 ppm for protons a and c-f and 0.07 ppm for protons b) (Table 1). As we described above, at low concentration, microphase separation occurs, and the polyesters form unimolecular micelles with an interior consisting of the polar PEG fragments and an exterior made up of the lipophilic -(CH2)n- moieties. As the polyester concentration DOI: 10.1021/la900700u
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increases, the formation of the hierarchical structures takes place because of the aggregation of single unimolecular micelles. The formed structures are obviously of lower polarity, which causes the high-field shifts of the proton signals in 1H NMR spectra. Conspicuously, a peak of PEG fragment protons e and f of polyester D3 narrows slightly with increasing polymer concentration. It is concluded that there are hydrophilic domains formed by the PEG fragments in the polyester aggregates. The motion of the PEG fragments within the larger domain is less limited than within the smaller core of a unimolecular micelle, which leads to narrowing of the PEG proton signal in the spectrum of polyester D3. At low concentration in aqueous solution, S6 and D3 form micelles with a hydrophobic inner part built by dicarboxylic acid moieties and with an outer part made up of the hydrated PEG fragments. The highest investigated concentrations of S6 in D2O are 10 and 5% for D3 (Figure 2) because the latter is more hydrophobic and it does not form a 10% solution in water. It can be concluded from the data presented in Figure 2 that the polyester micelles aggregate with increasing concentration to form structures containing hydrophilic and lipophilic domains. Increasing polyester concentration in water leads to a broadening of the signals attributed to the PEG protons d-f (drastically for the polyester D3 with the shorter PEG fragments) indicating that the motions of the PEG fragments became limited due to their close packing in the hydrophilic domain. Disappearing of the hyperfine structures of the methylene groups a and b located in the area of the PEG fragments supports the idea that the mobility of the polar units decreases with increasing polyester concentration. A slight shift of the signal in d-f toward lower ppm values implies that the polarity within the hydrophilic domain is lower as compared with those in the outer part of polyester micelles.
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The signals of hydrophobic protons c experience a slight narrowing that shows inessential enhancement in the mobility of the -(CH2)n- groups when they are forming a hydrophobic domain in water. Interestingly, the c signals practically do not shift with increasing polyester concentration in D2O, indicating that in terms of polarity there is no essential difference between the micelle inner part and the hydrophobic domain of the hierarchical structures. In summary, the environment-dependent formation of amphiphilic polyester micelles (unimers) has been confirmed by the 1 H NMR spectroscopic study. In dilute toluene solutions, the micelle exterior is made up of lipophilic -(CH2)n- moieties, and the interior consists of the polar PEG fragments. In turn, the polyester inverts the structure in the aqueous medium to form micelles with a hydrophobic inner part and with an outer part made up of hydrated PEG fragments. Increasing polyester concentration leads to the formation of hierarchical structures both in toluene and the aqueous medium as a result of the aggregation of single unimolecular micelles and the formation of hydrophilic and lipophilic domains. On the contrary, the studied polyesters have shown a distinct hyperfine structure in acetone-d6 and CDCl3 solutions, indicating that both solvents are good environments for the amphiphilic polyesters. No unimolecular micelle formation or micellar aggregation has been observed in acetone or chloroform. Acknowledgment. We thank Dr. J. Bagu (Department of Chemistry, North Dakota State University) for help with NMR spectroscopy measurements. Supporting Information Available: 1H NMR spectra of the amphiphilic polyesters. This material is available free of charge via the Internet at http://pubs.acs.org.
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