Temperature-Sensitive Phase Transition of Dendritic Polyethylene

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Temperature-Sensitive Phase Transition of Dendritic Polyethylene Amphiphiles with Core-Shell Architecture Revealed by a Rayleigh Scattering Technique Ling Zhang,* Jing Su, Wenzhi Zhang, Ming Ding, Xudong Chen,* and Qing Wu* PCFM Lab, School of Chemistry and Chemical Engineering, DSAPM Lab, OFCM Institute, Sun Yat-Sen (Zhongshan) University, Guangzhou 510275, China Received October 1, 2009. Revised Manuscript Received December 6, 2009 The phase-transition behavior of unimolecular dendritic polyethylene amphiphiles with core-shell architecture aqueous solutions was investigated by a Rayleigh scattering (RS) technique. Dendritic polyethylene (DPE)-poly(oligo(ethylenegylcol) methacrylate) (POEGMA) with a DPE hydrophobic core and a POEGMA hydrophilic shell was synthesized by the atom-transfer radical polymerization (ATRP) of OEGMA using DPE terminated by the bromine group as a macroinitiator. The fluorescence measurements implied that DPE-POEGMA molecules in aqueous solutions existed as the unimolecular micelles. To understand the phase-transition behavior of dendritic polyethylene amphiphilic unimolecular micelles in aqueous solutions, the temperature dependence of the RS spectra of DPE-POEGMA aqueous solutions under the heating-and-cooling cycle indicated that the heating and cooling processes were reversible but hysteresis existed. The phase transition of DPE-POEGMA aqueous solutions decelerated with increasing levels of PEGylation. DPE-POEGMA exhibited a lower phase-transition temperature in D2O than in water.

Introduction Dendritic polymeric amphiphiles with temperature-sensitive phase-transition behavior in aqueous solution have attracted a great deal of attention in past years because of their versatile applications, including catalysis, drug delivery, sensor design, and green chemistry.1 Unlike the linear polymeric systems, alternate topologies such as dendrimers and hyperbranched polymers provide a larger number of structural variables to affect their temperature-induced phase-transition behavior.2 Hyperbranched polyesters (Boltron-40) containing poly(N-isopropyl acrylamide) (PNIPAM) segments with double thermoresponsive coronas exhibit an interesting two-step chain collapse and double phase transitions.3 The introduction of isobutyramide (IBAM) groups to the chain ends of the poly(amidoamine) (PAMAM) or poly(propylenimine) (PPI) dendrimers could give them temperaturesensitive water solubility,4 contrary to the DAB-Am-32 PPI dendrimer having poly(N-isopropylacrylamide) arms.5 The biaryl-based neutral amphiphilic dendrons that carry switchable hydrophobic-hydrophilic segments are found to exhibit generation-dependent phase transitions.6 The phase-transition properties of hyperbranched polyethylenimines bearing terminal IBAM units have been compared with those of analogous dendrimers *Corresponding author. E-mail: [email protected] (L.Z.), cescxd@ mail.sysu.edu.cn (X.C.), [email protected] (Q.W.). (1) (a) Galaev, I. Y.; Mattiasson, B. Trends Biotechnol. 1999, 17, 335. (b) Lopez, V. C.; Raghavan, S. L.; Snowden, M. J. React. Funct. Polym. 2004, 58, 175. (c) Jeong, B.; Gutowska, A. Trends Biotechnol. 2002, 20, 305. (d) Yoshida, R.; Takahashi, T.; Yamaguchi, T.; Ichijo, H. Adv. Mater. 1997, 9, 175. (e) Yuk, S. H.; Cho, S. H.; Lee, S. H. Macromolecules 1997, 30, 6856. (2) (a) Saha, A.; Ramakrishnan, S. Macromolecules 2008, 41, 5658. (b) Haba, Y.; Kojima, C.; Harada, A.; Kono, K. Angew. Chem., Int. Ed. 2007, 46, 234. (3) (a) Xu, J.; Luo, S.; Shi, W.; Liu, S. Langmuir 2006, 22, 989. (b) Luo, S.; Xu, J.; Zhu, Z.; Wu, C.; Liu, S. J. Phys. Chem. B 2006, 110, 9132. (4) Haba, Y.; Harada, A.; Takagishi, T.; Kono, K. J. Am. Chem. Soc. 2004, 126, 12760. (5) Kimura, M.; Kato, M.; Muto, T.; Hanabusa, K.; Shirai, H. Macromolecules 2000, 33, 1117. (6) Aathimanikandan, S. V.; Savariar, E. N.; Thayumanavan, S. J. Am. Chem. Soc. 2005, 127, 14922.

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and linear systems.7 Investigations of poly(ethylene glycol) (PEG)modified hyperbranched polyethers reveal the existence of the lower critical solution temperature (LCST)8 and the effect of variation of the hydrophobicity in backbone-responsive hyperbranched polyethers on their LCSTs.9 Preparing dendrimers through stepwise synthesis offers precise structural control and uniformity, but the multistep synthesis involved in the preparation limits their general applicability. According to Guan et al.,10 the branching topology of ethylene homopolymers and copolymers can be systematically tuned in a single synthesis operation (controlling ethylene pressure) by using the chain-walking catalyst. On the basis of the unique chainwalking polymerization mechanism,10,11 obtained dendritic polyethylene possesses the complete carbon-carbon backbone construction and special characteristics in branching topology structure, being different from those prepared by typical condensation polymerization.12 The dendritic polyethylene amphiphiles are especially attractive because of their globular shape in aqueous solution with molecular dimensions in the nanometer range and their many potential applications, including controlled drug delivery and release, phase transfer, and molecular nanocarriers. Thus, it is necessary to study the phase-transition behavior of core-shell amphiphiles with dendritic polyethylene cores, but few attempts have been made. Considering that the investigation of this aspect would provide useful information about intramolecular (7) Liu, H.; Chen, Y.; Shen, Z. J. Polym. Sci., Polym. Chem. Ed. 2007, 45, 1177. (8) (a) Zhou, Y.; Yan, D.; Dong, W.; Tian, Y. J. Phys. Chem. B 2007, 111, 1262. (b) Yan, D.; Zhou, Y.; Hou, J. Science 2004, 65, 303. (c) Zhou, Y.; Yan, D. Angew. Chem., Int. Ed. 2004, 43, 4896. (d) Zhou, Y.; Yan, D. J. Am. Chem. Soc. 2005, 127, 10468. (9) Jia, Z.; Chen, H.; Zhu, X.; Yan, D. J. Am. Chem. Soc. 2006, 128, 8144. (10) (a) Guan, Z.; Cotts, P. M.; McCord, E. F.; McLain, S. J. Science 1999, 283, 2059. (b) Chen, G. H.; Huynh, D.; Felgner, P. L.; Guan, Z. B. J. Am. Chem. Soc. 2006, 128, 4298. (11) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414. (12) Hawker, C. J.; Frechet, J. M. J.; Grubbs, R. B.; Dao, J. J. Am. Chem. Soc. 1995, 117, 10763.

Published on Web 12/21/2009

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interactions of polymer chains and phase transitions, the phasetransition behavior of unimolecular dendritic polyethylene amphiphiles with core-shell architecture aqueous solutions was studied in this work. In this article, oligo(ethylenegylcol)methacrylate (OEGMA) was chosen as the comonomer to provide both water solubility and thermosensitivity to the unimolecular dendritic polyethylene amphiphiles. It has been reported recently that the introduction of a short oligo(ethylene glycol) chain in OEGMA to the polymer backbone can yield thermosensitive amphiphilic copolymers13 and their LCSTs can be tuned by varying the OEGMA/comonomer relative composition.14,15 Moreover, a hybrid core-shell micelle structure synthesized by the surface-initiated ATRPs of oligo(ethylenegylcol) methacrylate exhibited the LCST transition behavior.16 As an effective method, a Rayleigh scattering (RS) technique is used to investigate the changes in conformation, interaction, phase transition, aggregation, and assembly of biological and chemical species17-22 because of its high sensitivity, convenience in performance, simplicity, and rapidity. In previous work,19,20 we have applied RS to characterize the aggregation and extension of macromolecular chains during phase transition by monitoring the change in geometric size of particle from the RS intensity variances. In this work, we apply the RS technique to investigate the temperature-sensitive phase-transition behavior of core-shell dendritic polyethylene amphiphiles (DPE-POEGMA) in aqueous solutions in order to reveal the inherent nature of the phase transition of DPE-POEGMA in molecular terms. The effect of the level of PEG incorporation in the peripheral shell region on the phase-transition behavior is considered in this work to understand this unique molecular architecture further.

Experimental Section Materials. All manipulations involving air- and/or moisturesensitive compounds were carried out in an N2-filled drybox or using Schlenk techniques. The Pd-diimine catalyst was synthesized according to the literature.11 Polar comonomer 2-(2-bromoisobutyryloxy) ethyl acrylate (BIEA) was synthesized according to the literature.23 Oligo(ethylene glycol) methacrylate (OEGMA) (Aldrich, Mn = 300 calcd) was filtered through neutral alumina gel to remove inhibitors before use. Copper(I) chloride (>95%) was washed with glacial acetic acid to remove any soluble oxidized species and filtered and then was washed with ethanol and dried under vacuum. 2,20 -Bipyridine (bpy, >99%) and CuBr2 (>98%), obtained from Aldrich, were used without any purification. Other chemicals, including anhydrous dichloromethane, anisole, and tetrahydrofuran (THF), were refluxed for 24 h and distilled in a nitrogen atmosphere before use. (13) (a) Han, S.; Hagiwara, M.; Ishizone, T. Macromolecules 2003, 36, 8312. (b) Zhao, B.; Li, D. J.; Hua, F. J.; Green, D. R. Macromolecules 2005, 38, 9509. (c) Ishizone, T.; Seki, A.; Hagiwara, M.; Han, S. Macromolecules 2008, 41, 2963. (14) Ali, M. M.; St€over, H. D. H. Macromolecules 2004, 37, 5219. (15) (a) Lutz, J.-F.; Hoth, A. Macromolecules 2006, 39, 893. (b) Yamamoto, S. I.; Pietrasik, J.; Matyjaszewski, K. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 194. (16) Li, D.; Jones, G. L.; Dunlap, J. R.; Hua, F.; Zhao, B. Langmuir 2006, 22, 3344. (17) Pasternack, R. F.; Schaefer, K. F.; Hambright, P. Inorg. Chem. 1994, 33, 2062. (18) Huang, C. Z.; Li, K. A.; Tong, S. Y. Bull. Chem. Soc. Jpn. 1997, 70, 1843. (19) Li, Y. B.; Chen, X. D.; Zhang, M. Q.; Luo, W. A.; Yang, J.; Zhu, F. M. Macromolecules 2008, 41, 4873. (20) Zhang, W. Z.; Chen, X. D.; Luo, W. A.; Yang, J.; Zhang, M. Q.; Zhu, F. M. Macromolecules 2009, 42, 1720. (21) Zhang, Y. H. Physical Chemistry; Shanghai Jiao Tong University Press: Shanghai, 1988. (22) Lu, W.; Fernandez Band, B. S.; Yu, Y.; Li, Q. G.; Shang, J. C. Microchim. Acta 2007, 158, 29. (23) Matyjaszewski, K.; Gaynor, S. G.; Kulfan, A.; Podwika, M. Macromolecules 1997, 30, 5192.

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DPE-POEGMA Preparation. DPE-POEGMA was prepared by a tandem polymerization methodology, chain-walking polymerization (CWP) followed by atom-transfer radical polymerization (ATRP), as reported in the literature.10b The experimental details are described as follows. Synthesis of Dendritic Polyethylene Macroinitiators Terminated by a Bromine Group. A prescribed amount of BIEA solution (CM = 0.6) in dichloromethane was transferred into the reactor. After thermal equilibration (35 °C) for 10 min, 0.1 mmol of Pd-diimine catalyst dissolved in 10 mL of dichloromethane was added to the reactor and the ethylene pressure was controlled before starting the polymerization. After 48 h, the polymerization was terminated by venting the reactor and the solvent was subsequently evaporated to obtain the resultant oily polymer product. To remove catalyst residues, the polymer product was redissolved in petroleum ether and the solution was passed through a column packed with neutral alumina and silica gel until it became colorless. The polymer was finally precipitated out using methanol and was dried under vacuum. The microstructure of the products was characterized by 1H NMR spectroscopy, and all of the peaks in the 1H NMR spectra of the obtained dendritic polyethylene macroinitiators were assigned, in agreement with the previous report.24 Synthesis of DPE-POEGMA. Dendritic polyethylene macroinitiators was dissolved in degassed anisole and purged with dry nitrogen for 15 min. CuBr, CuBr2, bpy (molar ratio = 1:0.1:2), and OEGMA were dissolved in a 50 mL two-necked flask with degassed anisole and purged with dry nitrogen for 15 min before the above macroinitiator solution was added. The mixture was then subjected to three freeze-pump-thaw cycles to remove any residual oxygen. After the flask was charged with dry nitrogen, the reaction mixture was stirred at room temperature for the indicated time. After the reaction flask was opened to the air, methanol was added to dilute the mixture and then the resultant product was purified by dialysis against methanol (molecular weight cutoff (MWCO) of dialysis bag used: 14 000). Characterization. 1H NMR measurements were performed on a Varian INOVA 300 M spectrometer in CDCl3 or D2O. TMS and TSP (trimethylsiyl-3-propionic-d4 acid, Aldrich) were used as the internal reference, respectively. The molecular weights of the products were measured by SEC on a Waters Alliance GPC 2000 system with a multiangle laser-light scattering detector at 40 °C. THF was used as the solvent, and the flow rate was 1.0 mL/min. RS spectra were recorded on a Cary Eclipse fluorescence spectrophotometer (Varian, Inc.) equipped with a xenon flash lamp, a R928 photomultiplier detector, and dual monochromators. The spectrophotometer possesses a single-cell Peltier holder, which is an electronically controlled thermostatting single-cell accessory capable of controlling the temperature to within (0.1 °C. The variable-temperature measurements in the range of 0-100 °C at a heating rate of 0.5 °C min-1 were carried out by using this unit. RS spectra of the systems were recorded from 260 to 700 nm with synchronous scanning at λex = λem (i.e., Δλ = 0 nm).25 The theoretical background of the RS technique was described according to the literature.19,20 Prior to the experiments, DPE-POEGMA solutions were incubated for 1 day below the LCST for equilibration. The emission spectra of fluorescence were recorded at room temperature with a Shimadzu RF-5301 PC fluorescence spectrophotometer. UV-vis absorption spectra of DPE-POEGMA solutions were collected with a UV-3150 spectrophotometer (Shimadzu Corporation, Japan). The slit width was 1 nm during the measurements. Dynamic light scattering (DLS) measurements were performed with a Brookhaven Instruments BI-200SM goniometer (532 nm) and a BI-9000AT digital correlator. The concentration (24) Zhang, K. J.; Wang, J. L.; Subramanian, R.; Ye, Z. B.; Lu, J. M.; Yu, Q. Macromol. Rapid Commun. 2007, 28, 2185. (25) (a) Liu, S. P.; Luo, H. Q.; Li, N. B.; Liu, Z. F.; Zheng, W. X. Anal. Chem. 2001, 73, 3907. (b) Pasternack, R. F.; Bustamante, C.; Collings, P. J.; Giannetto, A.; Gibbs, E. J. J. Am. Chem. Soc. 1993, 115, 5393.

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Scheme 1. Synthesis of the Dendritic Polyethylene Amphiphiles (DPE-POEGMA)

of the sample in the aqueous solution was 1 mg/mL. Differential scanning calorimetry (DSC) analysis was conducted with a Perkin-Elmer DCS-7 system. The DSC curves were recorded at a heating rate of 10 °C min-1.

Results and Discussion Preparation and Characterization of DPE-POEGMA. A series of the dendritic polyethylene amphiphiles (DPE-POEGMA) were prepared by a tandem polymerization methodology, CWP followed by ATRP (Scheme 1). The branching topology of the polyethylene core was changed from dendrimer to hyperbranched architecture by varying the ethylene pressure (PE) of CWP conditions from 0.1 atm to 1 atm (i.e., from dendrimer polyethylene (namely, dPE) to hyperbranched polyethylene (hPE)),26 and the extent of PEGylation was varied by changing the ATRP time, which provided the possibility to investigate the effect of various molecular-structural parameters on the phase-transition behavior. By using the Brookhart palladium-R-diimine chain-walking catalyst, two dendritic macroinitiators bearing multiple initiation sites, including the bromine group for subsequent Cu(I)-mediated ATRP of OEGMA, were synthesized at the same comonomer (BIEA) feed concentration but at a variable ethylene pressure: dendrimer macroinitiator with a number-average molecular weight (Mn, measured by size exclusion chromatography using a multiangle light-scattering detector (SEC-MALS)) of 31 500 g/ mol, 4.5 mol % incorporated BIEA comonomer, 37 initiation sites, and hyperbranched macroinitiator with Mn =55 900 g/mol, 2.8 mol % incorporated BIEA, and 45 initiation sites as well as with branching numbers of ∼120 branches per 1000 carbons as determined by the 1H NMR spectra. A series of DPE-POEGMA with a broad range of molecular weights (from Mn = 99 100 to 473 000 g/mol) can be tailored through varying the ATRP time. The molecular weight of DPEPOEGMA and the number-average degree of polymerization (DP) of POEGMA increase with the ATRP time (Table 1). Despite the high hydrophobicity of dendritic polyethylene cores,27 the level of peripheral PEGylation makes DPE-POEGMA soluble in water at room temperature over wide ranges of composition and molecular weight. The good solubility of DPEPOEGMA in water directly proves that the placement of the PEG units on the peripheral shell region around the dendritic (26) (a) Guan, Z. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 3680. (b) Chen, G.; Ma, X. S.; Guan, Z. J. Am. Chem. Soc. 2003, 125, 6697. (c) Chen, G.; Guan, Z. J. Am. Chem. Soc. 2004, 126, 2662. (27) (a) Ye, Z.; Zhu, S. P. Macromolecules 2003, 36, 2194. (b) Pispas, S.; Hadjichristidis, N. Macromolecules 2003, 36, 8732.

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polyethylene core is very important to the hydrophilicity of DPEPOEGMA. Figure 1 shows the 1H NMR spectra of DPE-POEGMA in CDCl3 and D2O. In CDCl3, all of the characteristic signals of the dendritic polyethylene core and POEGMA shell can be clearly observed. The 1H NMR spectrum of DPE-POEGMA in CDCl3 reveals the presence of characteristic signals of POEGMA at δ = 4.07, 3.65, 3.54, 3.37, and 2.0-1.6 ppm. Three characteristic resonances in the range from 0.8 to 1.35 ppm in the 1H NMR spectrum are assigned to the methyl, methylene, and methine protons of the dendritic polyethylene core, respectively. It is interesting that DPE-POEGMA in D2O shows only characteristic peaks of the POEGMA shell and weak signals of the dendritic PE core, which is due to the insolubility of the dendritic polyethylene core in D2O. The exact incorporation level of the PEGylated comonomer is obtained by comparing the intensities of the peaks due to the methoxy protons (3.37 ppm) of the PEG units with those of all the methyl, methylene, and methine protons of dendritic polyethylene (∼0.8-1.35 ppm). The results are summarized in Table 1. The PEGylation level increases consistently with increasing ATRP time. Unimolecular Micelle Behavior in Water. To confirm the unimolecular micelle behavior of DPE-POEGMA in water, fluorescence measurements were performed by choosing coumarin 153 (C153) as a hydrophobic dye and a well-characterized solvatochromic fluorescence probe.28 C153 is insoluble and does not fluoresce in water, but once it is encapsulated inside micelles, its aqueous solutions exhibit significant fluorescence. The fluorescence intensities at the maximum emission intensity wavelength (534 nm) excited at 420 nm for C153 are plotted against the concentration of two kinds of DPE-POEGMA with dendrimer and hyperbranched polyethylene cores, respectively (Figure 2). It can be observed that the fluorescence intensities at 534 nm for C153 increase gradually with increasing concentration of DPEPOEGMA with dendrimer and hyperbranched polyethylene cores, being different from typical S-shaped curves formed by surfactants.29 The results reveal that there is no critical micelle concentrations (cmc) for DPE-POEGMA and that no association occurs between DPE-POEGMA molecules, which implies that DPE-POEGMA molecules in aqueous solutions exist as unimolecular micelles. Compared with the conventional block copolymer micelles, the dendritic polymeric unimolecular micelles possess much higher structural stability in aqueous solution because of the unique dendritic architecture and the covalent linkage between the hydrophobic and hydrophilic segments. To further understand the temperature-sensitive phase-transition behavior of the ultimate polymeric micelles, we study the phase-transition process of DPE-POEGMA aqueous solutions by using a Rayleigh scattering (RS) technique. Absorption and RS Spectra of DPE-POEGMA. Figure 3 displays the absorption spectra of DPE-POEGMA aqueous solutions at different concentrations (C = 0.4, 2, and 4 mg/mL). DPE-POEGMA exhibits two absorption bands at 203 and 305 nm and little absorbance in the range over 320 nm. Figure 4 shows the RS spectra of a DPE-POEGMA aqueous solution with varied concentrations recorded at 25 °C. The spectral characteristics of all of the solution systems are similar. The maximum scattering wavelength (λmax) of the solutions (C = 0.2, 0.4, 0.8, 1, 2, and 4 mg/mL) is located at 356 nm. By comparing Figures 3 and 4, we (28) Steege, K. E.; Wang, J. Z.; Uhrich, K. E.; Castner, W. E., Jr. Macromolecules 2007, 40, 3739. (29) Watkins, D. M.; Sayed-Sweet, Y.; Klimash, J. W.; Turro, N. J.; Tomalia, D. A. Langmuir 1997, 13, 3136.

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Zhang et al. Table 1. Molecular Parameters of Dendritic Polyethylene Amphiphiles

sample

tATRP (h)

BIEA content (mol %)a

PEGylation level (wt %)a

Mnb (kg/mol)

Mwb (kg/mol)

PDIb

DPb

46 61 73 44 59 70

31.5 55.9 99.1 154.3 293.6 169.7 271.9 473.0

54.1 94.8 178.4 281.6 502.0 290.5 498.6 805.1

1.7 1.7 1.8 1.8 1.7 1.7 1.8 1.7

225 409 874 379 720 1390

dPE-BIEA 4.5 hPE-BIEA 2.8 dPE-POEGMA1 24 dPE-POEGMA2 48 dPE-POEGMA3 72 hPE-POEGMA1 24 hPE-POEGMA2 48 hPE-POEGMA3 72 a Calculated from 1H NMR. b Data based on SEC-MALS.

Figure 1. 1H NMR spectra of DPE-POEGMA in different solvents: (a) CDCl3 and (b) D2O at 25 °C.

Figure 2. Fluorescence emission spectra of C153 (λex = 420 nm) with different concentrations of dPE-POEGMA2 in H2O solutions at 25 °C. (Inset) Fluorescence intensity at 534 nm for emission spectra of C153 as a function of the concentration of hPE-POEGMA3 (a) and dPE-POEGMA2 (b).

find that the maximum Rayleigh scattering intensities of DPEPOEGMA aqueous solutions at different concentrations all appear at 356 nm, which is located on the red side of the absorption band of the DPE-POEGMA solutions, indicating that the maximum scattering peaks become perceivable where the absorption decreases, as ascribed to Rayleigh scattering.22 Rayleigh scattering is able to characterize the phase-transition behavior in a polymer solution by a single parameter, the RS intensity (IRS), which can be considered to be an index of the degree of the change in macromolecular phase behavior. Figure 4 shows that the spectral peaks of DPE-POEGMA aqueous solutions exhibit no shift, indicating that the interactions between two adjacent unimolecular micelles are very weak and that these unimolecular micelles are uniformly distributed in such a concentration range. It can also be seen that IRS is enhanced with an increase in DPEA concentration because of the increase in the number of molecules of DPE-POEGMA per unit volume with increasing concentration. Phase Transition of DPE-POEGMA in Water. The maximum scattering wavelength (λmax) appears at 356 nm in the 5804 DOI: 10.1021/la903711e

Figure 3. Absorption spectra of DPE-POEGMA aqueous solutions at 25 °C at different concentrations (C = 0.4, 2.0, and 4.0 mg/mL).

Figure 4. RS spectra of DPE-POEGMA aqueous solutions at different concentrations (C = 0.2-4 mg/mL) at 25 °C.

temperature range of 30-55 °C, and little change in IRS is observed with increasing temperature, indicating that DPE-POEGMA unimolecular micelles do not dehydrate in such a temperature range. Considering the sensitivity of detection, the maximum scattering wavelength (356 nm) is selected for further work. A plot of the variation of the RS spectrum intensity at 356 nm (I356) as a function of temperature for dPE-POEGMA2 is presented in Figure 5. I356 increases sharply in a solution of dPEPOEGMA2 at a specific temperature during heating, suggesting the temperature-induced phase transition in the aqueous solution.30 At the same time, the dPE-POEGMA2 aqueous solution changes from a transparent liquid to a cloudy solution during the course of heating, being consistent with the RS observations in Figure 5. The stronger the RS intensity, the higher the extent of micelle aggregation. It is obvious that the dendritic polyethylene core is very hydrophobic and has no thermosensitivity. Thus, the phase-transition behavior of dPE-POEGMA2 is attributed to incorporating PEG units around a dendritic polyethylene core,9 (30) Swier, S.; Van Durme, K.; Van Mele, B. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 1824.

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Figure 5. Temperature dependence of I356 and the differential curve of I356 against temperature for dPE-POEGMA2 (C = 1 mg/mL).

Figure 6. Average hydrodynamic diameters of hPE-POEGMA2 in the temperature range of 25-70 °C as measured by dynamic light scattering.

Table 2. All Temperature-Sensitive Phase-Transition Parameters of DPE-POEGMA in H2O and D2O H2O sample dPE-POEGMA1 dPE-POEGMA2 dPE-POEGMA3 hPE-POEGMA1 hPE-POEGMA2 hPE-POEGMA3

D2O

LCST (°C)

Tmax (°C)

k

69.8 74.0

73.9 76.1 79.0 67.1 70.0 74.3

6.14 2.11 0.25 17.69 5.40 0.73

65.1 67.4

LCST (°C)

Tmax (°C)

k

64.7 65.5

70.0 73.0 75.6 66.1 68.9 73.1

7.38 2.26 0.33 6.35 2.40 1.51

64.3 66.9

indicating the temperature-sensitive hydrogen bonding ability of these moieties with the solvent, water. This suggests that the degree of hydration of the ethylene oxide chains decreases with increasing temperature and eventually the balance between the hydrophilicity and hydrophobicity of the individual polymer micelle as a whole is broken. The dehydration changes in DPEPOEGMA cause I356 to begin to increase gradually, which can be considered to be the signal for the onset of phase transition. On the basis of the RS spectrum intensity at 356 nm (I356) as a function of temperature in the DPE-POEGMA aqueous solutions, the LCST for the dPE-POEGMA2 solution is estimated at 74.0 °C from the intercept. Besides, the k factor, the slope of the straight line on the chart, is obtained. It is proportional to the rate of phase transition. The larger the value of k, the higher the rate of phase transition. Meanwhile, the temperature derivative of I356 of the DPE-POEGMA solution is also given in Figure 5. It is found that the phase transition is first accelerated and then decelerated with temperature. The temperature corresponding to the maximum rate of the phase transition (Tmax) is obtained at 76.1 °C. Table 2 lists all phase-transition parameters of DPE-POEGMA with different core-shell structural parameters in H2O solutions. It is clear that the increase in the level of PEGylation results in an increase in the phase-transition temperature; a similar result in the case of PEGylated hyperbranched polymers was also reported in the literature.2,8a It is concluded that the peripheral PEG segments of these DPE-POEGMAs affect the LCST, Tmax, and slope k. The higher the fraction of the PEG chain, the higher the desolvatation temperature of DPE-POEGMA and the lower the k value. The decrease in k reflects the deceleration of the phase transition with increasing level of PEGylation. For those DPEPOEGMAs with 72 h of ATRP time, all aqueous DPE-POEGMA micelle solutions exhibit weak LCST transitions with very low k values during heating, which makes it difficult to estimate the onset transition temperature (LCST). On the basis of the above results, the phase-transition behavior of DPE-POEGMA can effectively be modified by changing the level of PEGylation Langmuir 2010, 26(8), 5801–5807

Figure 7. 1H NMR spectra of hPE-POEGMA2 at various temperatures in D2O.

(i.e., the number of hydrogen bond sites along the chain increases with increasing level of PEGylation). The difference in the two kinds of dendritic topologies of polyethylene could be discriminated by the comparison of DSC data. The dendrimer polyethylene (dPE) and hyperbranched polyethylene (hPE) exhibit the glass transition at -70.6 and -68.8 °C and the weak melting endotherm at -39.4 and -33.8 °C, respectively. The low glass-transition temperatures Tg of the dendritic polyethylene chains ensures the high mobility of the chains in the micellar core.27 It demonstrates the higher mobility of the dPE chains than that of hPE, which has a better effect on the balance between the hydrophilicity and hydrophobicity of the individual polymer micelle. Moreover, it is noted that oligo(ethylene glycol) methacrylate(OEGMA) is a mixture of oligo(ethylene glycol) esters with a distribution of side-chain lengths, and it has been reported by St€ over et al.14 that POEGMA itself does not exhibit an LCST transition.16 However, incorporating POEGMA around a dendritic polyethylene core brings about a change in its temperature-sensitive phase-transition behavior for DPE-POEGMA systems. Thus, the phase-transition characteristics of DPEPOEGMA depend mostly on the hydrophilicity-hydrophobicity balance between the hydrophilic POEGMA shell and dendritic polyethylene core.6,9,31 The balance between hydrophilic and hydrophobic moieties in the molecular structure of the unimolecular (31) (a) Parrott, M. C.; Marchington, E. B.; Valliant, J. F.; Adronov, A. J. Am. Chem. Soc. 2005, 127, 12081. (b) Chen, H.; Jia, Z.; Yan, D.; Zhu, X. Macromol. Chem. Phys. 2007, 208, 1637. (c) Li, W.; Zhang, A.; Feldman, K.; Walde, P.; Schl€uter, A. D. Macromolecules 2008, 41, 3659.

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Figure 8. Schematic representation of the phase-transition process of the DPE-POEGMA aqueous solution during one heating-and-cooling cycle.

micelles is the key parameter influencing the LCST for DPEPOEGMA systems. To further confirm the temperature-sensitive properties of DPE-POEGMA, dynamic light scattering (DLS) was employed to provide the size of assembled structure at various temperatures (Figure 6). When the temperature was raised from 40 to 62 °C, the hydrodynamic diameter decreased from 59 to 32 nm, indicating the presence of the thermally induced collapse. Consistent with the observation in the RS analysis, a sharp increase in size was observed at temperatures above 60 °C, suggesting the aggregation of DPE-POEGMA at temperatures above the transition point. D2O Effect of the Phase Transition of DPE-POEGMA. To gain deeper insight into the effects of various structural parameters on their temperature-sensitive phase-transition behavior, DPE-POEGMA D2O solutions, compared to those in aqueous solutions, are examined by the RS technique. The temperature dependence of the RS spectrum intensity of the DPE-POEGMA D2O solution exhibits an S shape similar to that in aqueous solution (Figure 5). In all cases, the effect of various structural parameters on the phase-transition behavior in D2O solutions is similar to that observed in aqueous solutions, but the LCST and Tmax values of DPE-POEGMA are lower in D2O solutions than in aqueous solutions (Table 2) and the phasetransition behavior of DPE-POEGMA in D2O solutions is less sensitive to core-shell structural parameters than that in aqueous solutions. DPE-POEGMA in D2O solution exhibits a lower phasetransition temperature. Note that this result is in agreement with that of poly(2-alkyl-2-oxazoline).32 Light and heavy water are chemically identical, yet their physical properties differ significantly.33 The dissimilarities between H2O and D2O are believed to originate from differences in their intermolecular hydrogen bonds energies and from D2O being a more “structured” solvent than light water.34 In addition, this decrease in the transition temperature with D2O seems to parallel the relationship between the critical temperature of H2O and D2O.35 This significant effect corroborates the sensitivity to substituting D2O for H2O and may be taken as the added evidence of the existence of hydrogen bonding between DPE-POEGMA and water. The phase-transition behavior can also be monitored by temperature-variable 1H NMR measurement of DPE-POEGMA in D2O. Figure 7 shows the 1H NMR spectra of DPE-POEGMA in D2O during the heating process. The height of the water peak (32) (a) Chen, F. P.; Ames, A. E.; Taylor, L. D. Macromolecules 1990, 23, 4688. (b) Diab, C.; Akiyama, Y.; Kataoka, K.; Winnik, F. M. Macromolecules 2004, 37, 2556. (33) Nemethy, G.; Sheraga, H. A. J. Chem. Phys. 1964, 41, 680. (34) Ben-Naim, A. Hydrophobic Interactions; Plenum Press: New York, 1980. (35) Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water; Oxford University Press: New York, 1969.

5806 DOI: 10.1021/la903711e

are used as references. In the range of 35-55 °C, the peaks from the POEGMA shell are clearly observed, indicating the high mobility of the POEGMA shell in water. Additionally, the decrease in their heights also shows the presence of the dehydrated POEGMA shell over the temperature range of 35-55 °C. However, upon heating the polymer solution to over 60 °C, the peaks from the POEGMA shell become noticeably broadened and their heights decrease, suggesting that the degree of hydration of the ethylene oxide chains decreases with increasing temperature and that DPE-POEGMA micelles undergo a transition from a soluble state to an insoluble state above 60 °C, which is consistent with the observations via RS (Table 2) and DLS (Figure 6). Phase Transition of DPE-POEGMA during Cooling. In an effort to gain comprehensive knowledge of the phase transition, the solution of DPE-POEGMA is slowly heated and cooled to the desired temperature and RS spectra of the solution in the heating-and-cooling cycle are recorded. Although the heating and cooling processes are reversible, these two processes do not overlap and hysteresis exists. This hysteresis phenomenon stems from the existence of intramicelle and intermicelle interactions in the aggregated state and from the process of the gradual removal of these interactions during cooling.36 The micelle aggregates are reswelled, but they do not dissociate in the initial stage of cooling and the rate at which the micelles disengage themselves from aggregates would affect the dissolution rate in DPE-POEGMA systems.37-39 The phase-transition behavior is related to the temperature dependence of the interaction between hydrophilic PEG chains around the dendritic polyethylene core and water. DPE-POEGMA has hydrophilic PEG units in the peripheral shell region, which remains hydrated at low temperature and stably exists as unimolecular micelles in aqueous solution by forming strong hydrogen bonds with neighboring water molecules. With the temperature increasing, hydrophilic PEG segmemts on the peripheral shell begin to dehydrate, molecular chains in the shell exhibit thermally induced collapse, and the hydrodynamic diameter of DPE-POEGMA unimolecular micelles decreases gradually. As the temperature exceeds the LCST, such dendritic unimolecular micelles start to aggregate continually, leading to a sharp increase in aggregate size in accordance with the coalescence-induced mechanism of phase separation.40,41 However, the (36) Cheng, H.; Shen, L.; Wu, C. Macromolecules 2006, 39, 2325. (37) Brochard, F.; de Gennes, P. G. Physicochem. Hydrodyn. 1983, 4, 313. (38) Devotta, I.; Ambeskar, V. D.; Mandhare, A. B.; Mashelkar, R. A. Chem. Eng. Sci. 1994, 49, 645. (39) Devotta, I.; Premnath, V.; Badiger, M. V.; Rajmohanan, P. R.; Ganapathy, S.; Mashelkar, R. A. Macromolecules 1994, 27, 532. (40) Tanaka, H. Phys. Rev. Lett. 1994, 72, 1702. (41) Tanaka, H. J. Chem. Phys. 1997, 107, 3734.

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cooling process leads to the swelling and dissociation of globules. In the initial stage of cooling, the chain aggregates are swollen but do not begin to dissociate because of the formation of some intermicelle interactions in the collapsed state at higher temperature.36 As the solution temperature is decreased further, the aggregates begin to dissociate and gradually disappear, undergoing a reversible reswelling process upon cooling. Finally, the conformation of molecular chains returns to its original state below the LCST. Figure 8 presents a process for the temperaturesensitive phase transition of DPE-POEGMA micelles in a heating-and-cooling cycle.

Conclusions In this work, the temperature-sensitive phase-transition behavior in aqueous solutions of core-shell dendritic polyethylene amphiphiles (DPE-POEGMA) synthesized by CWP-ATRP tandem polymerization is investigated by the RS technique. The level of PEGylation on the peripheral shell plays an important role in the phase transition of DPE-POEGMA systems. The LCST of the

Langmuir 2010, 26(8), 5801–5807

Article

DPE-POEGMA solution increases with increasing level of PEGylation. During the heating-and-cooling process, the dendritic unimolecular micelles exhibit thermosensitive collapse or reswelling. The DLS and variable-temperature 1H NMR measurements of DPE-POEGMA are employed to provide further confirmation of their temperature-sensitive phase-transition behavior. The balance between hydrophilic and hydrophobic moieties in the molecular structure of unimolecular micelles is the key parameter influencing the LCST in DPE-POEGMA systems. A model was proposed to describe the phase-transition process of DPE-POEGMA micelles in an aqueous solution. The present work will shed new light on designing other types of temperature-sensitive core-shell polymer micelles by utilizing the dehydration of PEG segments in shells. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (grant nos. 20734004, 20804059, and 50673104) and the Natural Science Foundation of Guangdong Province (grant no. 8251027501000018).

DOI: 10.1021/la903711e

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