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Langmuir 2006, 22, 1946-1948
Invertible Architectures from Amphiphilic Polyesters Andriy Voronov,*,† Ananiy Kohut,†,‡ Wolfgang Peukert,† Stanislav Voronov,‡ Orest Gevus,‡ and Viktor Tokarev‡ Institute of Particle Technology, UniVersity of Erlangen-Nuremberg, Cauerstrasse 4, 91058 Erlangen, Germany, and LViV Polytechnic National UniVersity, Bandera str. 12, 79013 LViV, Ukraine ReceiVed August 16, 2005. In Final Form: December 21, 2005 We synthesized and characterized novel amphiphilic polyesters with both hydrophilic and hydrophobic functionalities. The polyesters are soluble in organic and aqueous media and reveal the formation of inverse architectures whose behavior could be correlated to their chemical structure. We foresee that the amphiphilic properties of the polyesters reported here are obviously the basis of new architectures both in solution and on the solid surfaces, which could be used in a broad range of applications. The described synthesis of the copolymers is very simple and is based on commercially available products. That makes this approach attractive in various uses.
Introduction Responses and conformation to local changes in pH, temperature, and solvent quality are well known for flexible polymer chains.1-4 Surface modification with polymers that provides possibilities for controlling and changing surface compositions, thus allowing for properties that are in demand, is significant for practical applications such as sensing and biocompatibility.5,6 Intensive efforts have been made to prepare, characterize, and understand the structure and properties of adaptive layers attached to material surfaces.7,8 The multistep synthetic concept for manufacturing polymer films still remains challenging with several factors to be considered. Amphiphilic block copolymers have recently attracted great interest, for example, in biomedical applications.9 In previous research, poly(sebacic anhydride-co-ethylene glycol) copolymers have been synthesized using sebacic anhydride prepolymer and commercial poly(ethylene glycol) via melt condensation.10,11 A number of papers have provided information on the synthesis of these polymers and applications for their controlled release.12-14 The main advantage of the degradable polymers synthesized before is that these materials as well as the degradation products are biocompatible. However, no responsive behavior * Corresponding author. E-mail:
[email protected]. Tel: +49 9131 8529 418. Fax: +49 9131 8529 402. † University of Erlangen-Nuremberg. ‡ Lviv Polytechnic National University. (1) Ionov, L.; Minko, S.; Stamm, M.; Gohy, J.-F.; Jerome, R.; Scholl, A. J. Am. Chem. Soc. 2003, 125, 8302. Luzinov, I.; Minko, S.; Tsukruk, V. V. Prog. Polym. Sci. 2004, 29, 635. Minko, S.; Luzinov, I.; Luchnikov, V.; Mu¨ller, M.; Patil, S.; Stamm, M. Macromolecules 2003, 36, 7268. (2) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163. Jagur-Grodzinski, J. React. Funct. Polym. 1999, 39, 99. Gilcreest V. P.; Carroll W. M.; Rochev Y. A.; Blute I.; Dawson K. A.; Gorelov A. V. Langmuir 2004, 20, 10138. (3) Stimuli-ResponsiVe Water Soluble and Amphiphilic Polymers; McCormick, C. L., Ed.; ACS Symposium Series No. 780; American Chemical Society: Washington, DC, 1999. (4) Russel T. P. Science 2002, 297, 964. (5) Galaev I.; Mattiassom, B. Trends Biotechnol. 1999, 17, 335. (6) Jones, D. M.; Smith, R. R.; Huck, W. T. S.; Alexander, C. AdV. Mater. 2002, 14, 1130. (7) Halperin, A.; Tirrell, M.; Lodge T. P. AdV. Polym. Sci. 1992, 100, 31-71. (8) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677-710. (9) Kumar, N.; Ravikumar, M. N. V.; Domb, A. J. AdV. Drug DeliVery ReV. 2001, 53, 23. (10) Chan, C.-K.; Chu, I.-M. Biomaterials 2003, 24, 47. (11) Jiang, H. I.; Zhu, K. J. Polym. Int. 1999, 48, 47. (12) Park, Y. H.; Cho, C. G. J. Appl. Polym. Sci. 2001, 79, 2067. (13) Deschamps, A. A.; Claase, M. B.; Sleijster, M. W. J.; De Bruijn, J. D.; Grijpma, D. W.; Feijen, J. J. Controlled Release 2002, 78, 175. (14) Nagata, M.; Kiyotsukuri, T.; Takeuchi, S.; Tutsumi, N.; Sakaj, W. Polym. Int. 1997, 42, 33.
in micelles or in thin films (being attached to the surface) has been reported. We describe here a new class of amphiphilic polyesters forming stimuli-responsive architectures. They offer adaptive properties, do not require a complex multistep synthesis, and are accessible on a larger scale. Polymers with such properties can find use in applications such as stimuli-responsive release.15-18 Our letter describes the development and characterization of invertible polyesters based on poly(ethylene glycol) (PEG) of various molecular weights and aliphatic dicarboxylic acids. The obtained copolymers have been synthesized via polycondensation in solution. We believe that the possibility of conformational switching for polyester architectures can be reached with a controllable presence of oxygen atoms in the main backbone of the polymer. Our structural hypothesis suggests that the hydrophilic PEG and the hydrophobic aliphatic acid residues can change the micelle architecture in solvents of different polarities. We have investigated several copolymer compositions that differ in (1) the molecular weight and (2) the hydrophilic-lipophilic balance of the monomer units. Herein we show that the change on the surface of the polymer assembly is a result of changes in molecular conformations within monomer units in solvents of different polarity. Until now, very little attention has been paid to the organization of the environment-dependent switching of a polymer assembly with a hydrophilic compartment to an inverted assembly with a lipophilic compartment.19,20 Experimental Section Polyesters 5-7 (Figure 1) have been synthesized by the polycondensation of poly(ethylene glycols) 1, 2 (molecular weight 300 and 600 g/mol) with aliphatic dicarboxylic acids 3, and 4 (sebacic or dodecanedioic acid). A three-necked flask fitted with a nitrogen inlet, a thermometer, and a Dean-Stark trap with a backflow condenser was charged with 1.01 mol of poly(ethylene glycol), 1.0 mol of dicarboxylic (sebacic or dodecanedioic) acid, and toluene. A 1:3 (w/v) monomer mixture/toluene solution was used. The DeanStark trap was filled with toluene. The reactive mixture was agitated (15) Kataoka, K.; Harada, A.; Nagasaki, Y. AdV. Drug DeliVery ReV. 2001, 47, 113. (16) Gerweck, L. E.; Seetharaman, K. Cancer Res. 1996, 56, 1194. (17) Liu, S.; Billingham, N. C.; Armes, S. P. Angew. Chem., Int. Ed. 2001, 40, 2328. (18) Liu, F.; Eisenberg, A. J. J. Am. Chem. Soc. 2003, 125, 15059. (19) Rodriguez-Hernandez, J.; Lecommandoux, S. J. Am. Chem. Soc. 2005, 127, 2026. (20) Basu, S.; Vutukari, D.; Shyamroy, S.; Sandanaraj, B. S.; Thayumanavan, S. J. Am. Chem. Soc. 2004, 126, 9890.
10.1021/la052225z CCC: $33.50 © 2006 American Chemical Society Published on Web 01/27/2006
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Langmuir, Vol. 22, No. 5, 2006 1947 Polymers 5-7 were dried under vacuum at 60 °C for at least 24 h. All the polyesters are viscous colorless or yellowish transparent substances. We confirmed the structure of synthesized polyesters 5-7 by FTIR spectroscopy (Figures 1-3 in Supporting Information) and determined the molecular weight, Mw, by dynamic light scattering (DLS). The composition of the polyesters was proved by the intensive doublet of absorption bands in the range of 1000-1270 cm-1 resulting from valence oscillations of the C-O bond. The presence of carbonyl groups is displayed as a broken intensive absorption band at 1730 cm-1. The spectra show a doublet of absorption bands in the range of 2800-3000 cm-1 (valence oscillations) and absorption bands at 1450 cm-1 (deformation oscillations) and 730 cm-1 (pendulum oscillations) that are characteristic of -(CH2)n- groups. The intrinsic [η] and specific ηsp viscosities of substances 5-7 in acetone and toluene solutions have been measured by Ubbelohde viscosimeter. The surface tension σ of aqueous solutions of polyesters has been investigated using the Wilhelmy approach. Critical micelle concentration cmc for every polyester was found from the inflection points on the surface tension isotherms. The experimental data are presented in Table 1. For the contact angle measurements, a drop of bidistilled water or diiodomethane (CH2I2) was placed on the surface of silicon wafers or mica. Thin films of polystyrene (PS) and polyvinylpyridine (PVP) were deposited onto polished silicon wafers by spin-coating from chloroform (3 mg/mL) and dried. The polyester films were spincoated (from acetone solution, 3 mg/mL, on PS film and from toluene solution, 3 mg/mL on PVP film) at the top of virgin mica as well as on PS and PVP. An advanced contact angle (Θ) was measured immediately after the drop was settled. Measurements were done with a Contact Angle System OCA 15 plus instrument with a precision of (0.1°. An average value of the contact angles measured from the left and right sides of at least three drops has been accepted.
Figure 1. Polycondensation reaction of the synthesis of amphiphilic polyesters.
Figure 2. UV-vis spectroscopy of malachite green solubilized in toluene by polyesters 5-7 (1 g/100 mL).
Results and Discussion The polycondensation of dicarboxylic acids with diols is known to be an equilibrium process; therefore, the water evolved during the reaction must be continuously removed from the reactive mixture to shift the equilibrium toward the desired polyester formation. Hence, the acylation of poly(ethylene glycols) by dicarboxylic acids has been carried out in toluene as an azeotrope former; a Dean-Stark trap has been used to remove water. The reaction has been performed in the presence of catalytic amounts of sulfuric acid (1-4 mol %). A small excess of poly(ethylene glycol) has been used to synthesize polyesters with terminal hydroxyl groups. A 1:1.01 molar ratio of PEG/dicarboxylic acid was used. The progress of the reaction was monitored by the analysis of the acid value of the reactive mixture and the amount of water released during polycondensation. The value of hydrodynamic radius Rh in nonpolar toluene was systematically higher than those in polar acetone (Table 1). It could be explained that according to the solubility parameters toluene is a better solvent for hydrocarbon blocks as compared to acetone. To investigate host-guest properties of polyesters 5-7, two dyessSudan red B and malachite greenswere selected for extraction studies in water and toluene polymer solutions, respectively. It is well known that Sudan red B remains insoluble in water and also that malachite green is insoluble in toluene.
Figure 3. UV-vis spectroscopy of Sudan red B solubilized in water by various concentrations of polyester 7 (1-0.46, 2-0.92, and 3-1.89 mmol/mL). and heated to dissolve the dicarboxylic acid completely. Then 1 mL of 75% sulfuric acid was added to catalyze the polycondensation reaction. The acid value of the reactive mixture was determined. The mixture was refluxed under nitrogen for about 20 h. When the acid value decreased to 3.6-4.1 mg of KOH/g, the reactive mixture was cooled to room temperature. To neutralize H2SO4, 1.75 mL of a NaOH solution (40%) was added, and the mixture was stirred at room temperature for 12 h. Sodium sulfate that formed was separated by centrifugation. Toluene was removed under reduced pressure.
Table 1. Properties and Characteristics of Synthesized Polyesters ηsp (1% solution)
Rh of the macromolecule, nm
polyester sample
Mw, kDa
[η] at 25°C, cm3/g
cmc, mmol/l
σ at the cmc, mN/m
in toluene
in acetone
in toluene
in acetone
5 6 7
22.6 21.1 9.2
14 26 13
1.3 × 10-4 1.4 × 10-4 2.6 × 10-4
45.0 48.9 47.5
0.25 0.35 0.14
0.19 0.26 0.12
9.0 ( 0.6 6.9 ( 0.7 7.4 ( 0.6
2.8 ( 0.2 3.3 ( 0.1 2.0 ( 0.1
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Figure 4. Photographs of Sudan red B and malachite green samples, respectively, in water and toluene before and after the addition of amphiphilic polyesters.
In the presence of polyesters 5-7, the dyes were immediately extracted into the solvent phase, which was clearly visible from changes in the solutions’ colors to red and green, respectively. Figures 2 and 3 show the observed color changes for both dye solutions via UV/vis spectra. The data revealed that upon addition of polyester 7 to Sudan Red B in water the adsorption of light at 514 nm increases with increasing polyester concentration. Photographs in Figure 4 show the observed color changes for both dye solutions. This behavior can be explained by the solubilization of the dye molecules by the hydrocarbon groups of polyester and hence the extraction of Sudan red B into aqueous polymer solution. Increasing polymer concentration in water leads to an increase of the total number of architectural units that are able to extract the hydrophobic dye (Figure 3, inset). A similar experiment was performed with malachite green, extracted into toluene solutions of polyesters 5-7. The UV/vis spectra results are summarized in Figure 3, demonstrating the highest loading capacity of polyester 7. Because the longer hydrophilic poly(ethylene glycol) part of polyester 7 is solved in toluene, it can obviously encapsulate more hydrophilic dye molecules than polyesters 5 and 6. Therefore, the incorporation of PEG of various lengths into the polyester macromolecule also allows for the adjustment of solubilization activity in nonpolar media. To confirm the invertibility and adaptive properties of polyester 7, the following experiments were done. The polyester thin films were spin-coated at the top of virgin mica as well as on polystyrene (PS) and polyvinylpyridine (PVP) thin films. PS and PVP were deposited onto silicon wafers by spin-coating from solution and dried. We measured water and diiodomethane contact angles at the top of a polyester film deposited on PS, PVP, and a freshly cleaved mica surface. In the next step, we exposed the polyestercoated wafers and mica for few seconds to water or diiodomethane and measured the contact angles again. The contact angle data and the calculated polar and dispersive constituents of surface energy21 are presented in a Table 2. The polar and dispersive energy constituents of the polyester film remained nearly constant for the wetting experiments performed on the different substrates. The data clearly show that the top layer of the polyester film switches upon exposure to either a polar or nonpolar environment. When we measured water contact angles, poly(ethylene glycol) fragments dominated at the top of (21) Van Krevelen, D. W. Properties of Polymers: Correlations with Chemical Structure; Elsevier: London, 1972; p 412.
Table 2. Advanced Contact Angles of Wetting the Modified Silicon Wafers with Water and Diiodomethane no.
sample type
1 2 3 4 5 6 7
PS film PVP film PE7 on PS PE7 on PS PE7 on PVP PE7 on PVP PE7 on mica
contact angle contact angle γps , γds , with water (θ°) with CH2I2 (θ°) mJ/m2 mJ/m2 90.0 60.5 17.3 11.1a 18.4 20.5a 10.0
11.2 19.2 9.0 13.4b 15.1 16.9b 9.1
50.9 40.1 35.1 34.1 34.0 33.7 34.6
0.2 12.3 37.2 39.5 37.5 37.0 39.3
a The samples were immersed for a few seconds in diidomethane and then dried. b The samples were immersed for a few seconds in water and then dried.
the film. When diiodomethane was used as a wetting liquid, nonpolar aliphatic units of polyester came to the top, and the surface switched its energetic state. The switchability on the order of a few seconds has been found for the polyester deposited on a PS film as well as on more polar PVP and at the hydrophilic mica surface.
Conclusions To summarize, we synthesized a new range of amphiphilic polyesters with both hydrophilic and hydrophobic functionalities alternately distributed in the polymer backbone. These polyesters are soluble in aqueous and organic media, where they reveal inverse behavior that could be correlated to their chemical structure. Amphiphilic properties of polyesters reported here are obviously the basis of new architectures, both in solution and on solid surfaces, and could be used in a broad range of applications. Coatings of switchable properties depending on the effect of the environment may be obtained on nanoparticulate surfaces (fillers, pigments, and semiconductors such as SiO2, TiO2, and ZnO). Tailoring particle interactions is a promising way to develop new and functional products in the form of thin films, coatings, and biomedical applications. Acknowledgment. A.K. thanks the German Academic Exchange Service (DAAD) for financial support during his stay at the Friedrich-Alexander University of Erlangen-Nuremberg. Supporting Information Available: FTIR spectra of the synthesized polyesters.This material is available free of charge via the Internet at http://pubs.acs.org. LA052225Z
