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Influence of the Macromolecular Architecture on the Self-Assembly of Amphiphilic Copolymers Based on Poly(N,N-dimethylamino-2-ethyl methacrylate) and Poly(ε-caprolactone) F. Bougard,*,†,‡ C. Giacomelli,§ L. Mespouille,‡ R. Borsali,§ Ph. Dubois,‡ and R. Lazzaroni† Laboratory for Chemistry of NoVel Materials and Laboratory of Polymeric and Composite Materials, Center for InnoVation and Research in Materials and Polymers (CIRMAP), UniVersity of Mons-Hainaut, Materia NoVa, Place du Parc 20, B-7000 Mons, Belgium, and Centre de Recherche sur les Macromole´cules Ve´ge´tales (CERMAV, UPR 5301), BP53 38041, Grenoble Cedex 9, France ReceiVed March 11, 2008. ReVised Manuscript ReceiVed April 30, 2008 The self-assembly of amphiphilic copolymers consisting of poly(N,N-dimethylamino-2-ethyl methacrylate) (PDMAEMA) and poly(-caprolactone) (PCL) segments arranged in graft and linear diblock architectures was investigated in this work by means of dynamic light scattering (DLS) in aqueous solution and by atomic force microscopy (AFM) on thin deposits. The solid-state deposits of the micelles were generated by a “freeze-drying” technique that preserves the initial micelle morphology in solution. A comparison between the morphological properties of graft copolymers with corresponding diblock copolymers was established to demonstrate the effect of the copolymer architecture on the micelle structure and organization.
Introduction One important feature of amphiphilic copolymers is their capacity to self-assemble into micelles (and other ordered structures such as cylinders, vesicles, lamellae, etc.) spontaneously if dissolved in a so-called selective solvent (i.e., a solvent that is thermodynamically good for one block and poor for the other).1–11 In an aqueous medium, micelle formation originates from the minimization of the interactions between the solvent and the hydrophobic segments, which leads to a core-shell structure where the nonsoluble blocks form the core and the hydrophilic segments form the diffuse corona. Such compartmented nano-objects have been increasingly and successfully applied in many fields (drug delivery, cosmetics, fragrances, flavor-masking, pesticides, pollution remediation, colloid stabilization, stabilizers for organic reactions, and nanotemplates, etc.),12–14 as a result of their ability to incorporate, retain, and release poorly water-soluble, hydrophobic, and/or * Corresponding author. Tel: ++32 (0)65 37 34 77. Fax: ++32 (0)65 37 38 61. E-mail:
[email protected]. † Laboratory for Chemistry of Novel Materials, University of MonsHainaut. ‡ Laboratory of Polymeric and Composite Materials, University of MonsHainaut. § Centre de Recherche sur les Macromole´cules Ve´ge´tales (CERMAV, UPR 5301).
(1) Halperin, A. Macromolecules 1987, 20, 2943. (2) Nagarajan, R.; Ganesh, K. J. Chem. Phys. 1989, 90, 5843. (3) Gast, A. P.; Vinson, P. K.; Cogan-Farinas, K. A. Macromolecules 1993, 26, 1774. (4) Qin, A.; Tian, M.; Ramireddy, C.; Webber, S. E.; Munk, P.; Tuzar, Z. Macromolecules 1994, 27, 120. (5) Gao, Z.; Varshney, S. K.; Wong, S.; Eisenberg, A. Macromolecules 1994, 27, 7923. (6) Zhang, L.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168. (7) Zhang, L.; Yu, K.; Eisenberg, A. Science 1996, 272, 1777. (8) Torchilin, V. P. J. Controlled Release 2001, 73, 137. (9) Jain, S.; Bates, F. S. Science 2003, 300, 460. (10) Geng, Y.; Ahmed, F.; Bhasin, N.; Disher, D. E. J. Phys. Chem. B 2005, 109, 3772. (11) Giacomelli, C.; Le Men, L.; Barsali, R.; Lai-Kee-Him, J.; Brisson, A.; Armes, S. P.; Lewis, A. L. Biomacromolecules 2006, 7, 817. (12) Rapoport, N.; Pitt, W. G.; Sun, H. J. Controlled Release 2003, 91, 85. (13) Bromberg, L.; Temchenko, M.; Hatton, T. A. Langmuir 2003, 19, 8675. (14) Kabanov, A.; Batrakova, E. V.; Alakhov, V. Y. AdV. Drug DeliVery ReV. 2002, 54, 99.
highly toxic compounds, also minimizing degradation and waste. In solution, micellar structures are usually studied using light, X-rays, and neutron scattering and direct imaging (cryotransmission electron microscopy (Cryo-TEM)15 and atomic force microscopy (AFM))16 techniques. In the case of AFM imaging, micelles are usually prepared as thin deposits by casting from solution. The preparation of these thin deposits must be finely controlled to avoid micellar aggregation17 or destruction of the micelles during solvent drying so that it becomes possible to correlate the scattering analysis and the AFM characterization. As described by Lang et al., a freeze-drying process preserves the micelle morphology in the dry deposits.18 We investigate biocompatible amphiphilic copolymers combining poly(-caprolactone) (PCL) segments grafted along a poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) backbone.19 PCL is a hydrophobic polyester known for its biodegradability, its permeability, and the biocompatibility of its metabolites. PDMAEMA is a pH- and temperature-sensitive polymer with a pKa of 7.4 and a LCST between 32 and 46 °C, depending on the molecular weight.20 It is nontoxic in its nonquaternized form21 and water-soluble in its protonated form; it can be absorbed by endocytosis and can be used as a nonviral DNA vector. We focus on the self-assembly properties of these copolymers and on the morphological characterization of thin deposits prepared from the micellar solutions, in comparison with the results obtained for the corresponding linear diblock copolymers PDMAEMA-b-PCL.22 Because the micelle size and shape are strongly dependent on the copolymer molecular (15) He, Y.; Li, Z.; Simone, P.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 2745. (16) Ouarti, N.; Viville, P.; Lazzaroni, R.; Minatti, E.; Schappacher, M.; Deffieux, A.; Putaux, J. L.; Borsali, R. Langmuir 2005, 21, 9085. (17) Hou, X.; Sun, L.; Zou, B.; Wu, L. Mater. Lett. 2004, 58, 369. (18) Egger, H.; Nordskog, A.; Lang, P. Macromol. Symp. 2000, 162, 291. (19) Mespouille, L.; Dege´e, Ph.; Dubois, P. Eur. Polym. J. 2005, 41, 1187. (20) Bu¨tu¨n, V.; Armes, S. P.; Billingham, N. C. Polymer 2001, 42, 5993. (21) Yancheva, E.; Paneva, D.; Maximova, V.; Mespouille, L.; Dubois, P.; Manolova, N.; Rashkov, I. Biomacromolecules 2007, 8, 976. (22) Bougard, F.; Jeusette, M.; Mespouille, L.; Dubois, P.; Lazzaroni, R. Langmuir 2007, 23, 2339.
10.1021/la800765y CCC: $40.75 2008 American Chemical Society Published on Web 07/02/2008
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Scheme 1. Synthesis of Amphiphilic P(DMAEMA-g-CL) Graft Copolymers
structure (block length, block ratio, molecular weight distribution), the synthesis was carried out following a strategy that allows for efficient control of those parameters. PDMAEMA was produced via atom-transfer radical polymerization (ATRP), and PCL was synthesized by coordination-insertion ring-opening polymerization (ROP). Throughout the article, the graft copolymers will be referred to as Gxy, where xy is the PDMAEMA weight percentage (e.g., G80 contains 80 wt % PDMAEMA and 20 wt % PCL). Similarly, the diblock copolymers will be referred to as Bxy. The micelles were generated by the addition of water to polymers in THF solution,23,24 and the critical micelle concentrations (cmc’s) were measured by pyrene fluorescence.25 The micelles were then investigated both in the liquid medium by DLS and as thin deposits on a solid substrate by AFM. We have tested several methods for the preparation of thin micellar deposits, and we have found that freeze drying is the most efficient method for avoiding micellar aggregation or destruction. The influence of parameters such as the copolymer composition, the concentration, and the pH was also examined.
Experimental Section Materials. ε-Caprolactone (CL, Acros, 99%) was dried over calcium hydride for 48 h at room temperature and distilled under reduced pressure before use. Aluminum triisopropoxide (Al(OiPr)3, Acros, 98%) was distilled under vacuum, quenched in liquid nitrogen, rapidly dissolved in dry toluene, and stored under a nitrogen atmosphere. Ethyl 2-bromoisobutyrate (EBiB, Aldrich, 98%), 1,1,4,7,10,10-hexamethylenetetramine (HMTETA, Aldrich, 97%), and copper bromide (CuBr, Fluka, 98%) were used without further purification. N,N-Dimethylamino-2-ethyl methacrylate (DMAEMA, Aldrich, 98%) was passed through a column of basic alumina to remove the stabilizing agents and stored under a nitrogen atmosphere at -20 °C. Toluene (Labscan, 99%) was dried by refluxing over (23) Yu, K.; Hurd, A. J.; Eisenberg, A.; Brinker, C. J. Langmuir 2001, 17, 7963. (24) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (25) Wilhelm, M.; Zhao, C. L.; Wang, Y.; Xu, R.; Winnik, M. A. Macromolecules 1991, 24, 1033.
CaH2 and distilled before use. THF was passed through a column of basic alumina. Synthesis of Amphiphilic Copolymers. The poly(N,N-dimethylamino-2-ethyl methacrylate)-graft-poly(-caprolactone) graft copolymers P(DMAEMA-g-CL) were synthesized in a three-steps synthetic strategy as shown in Scheme 1: (i) ring-opening polymerization of CL, (ii) esterification reaction of hydroxyl end groups of PCL with a methacrylic acid derivative, and (iii) copolymerization of the resulting macromonomer with the DMAEMA comonomer by ATRP.26 The ROP27 of CL was initiated by Al(OiPr)3 to form PCLOH chains selectively. The second step was the quantitative conversion of PCL-OH into R-isopropyloxy,ω-methacrylate poly(caprolactone) (PCLMA) through an esterification reaction with methacrylic acid in the presence of N,N′-dicyclohexylcarbodiimide (DCCI), N,N-dimethylamino-4-pyridine (DMAP), and triethylamine. As the last step, the graft copolymer was synthesized by ATRPinduced copolymerization of the PCLMA macromonomer with the DMAEMA comonomer in THF at 60 °C using ethyl-2-bromoisobutyrate as the initiator and CuBr/HMTETA as the catalyst. In a typical procedure, the catalytic system along with a magnetic stirrer was first introduced into a three-way stopcock glass tube capped by a rubber septum. The tube was purged of oxygen by three repeated vacuum/nitrogen cycles. A round-bottomed flask was charged with the PCLMA macromonomer, THF, and DMAEMA, with nitrogen bubbling before transfer into the glass tube immersed in an oil bath at 60 °C. Finally, the ethyl 2-bromoisobutyrate initiator was added to the tube to start the polymerization. The reaction was stopped by cooling the glass tube down to room temperature. The polymer solution was diluted in excess THF and precipitated in cold heptane. The copper catalyst system was extracted by passing the copolymer in THF solution through a column of basic alumina; the purified copolymer solution was reprecipitated in cold ether, filtered, and dried under reduced pressure at 40 °C. A residual copper content lower than 1.0 ppm was reached, as measured by ICP-AES analyses of the purified DMAEMA-based copolymers. PDMAEMA-b-PCL diblock copolymers were synthesized as previously described.22 1H NMR Spectroscopy. The chemical structure and composition of the copolymers and the intermediate species were confirmed by (26) Matyjaszewski, K. Macromolecules 1998, 31, 4710. (27) Baran, J.; Duda, A.; Kowalski, A.; Szymanski, R.; Penczek, S. Macromolecules 1993, 123, 93.
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Table 1. Molecular Characteristics of the P(DMAEMA-g-CL) Graft Copolymers
copolymer G92 G86 G80 G50
Mn (NMR) PCLMA (g/mol)a
Mn (GPC) copolymer (g/mol)b
Mw/Mnb
Fw PDMAEMA (%)a
2150 2150 2150 2150
25 800 19 800 22 000 32 200
1.3 1.3 1.3 1.3
92 86 80 50
a Number-average molar mass (Mn) of PCL and weight fraction (Fw) of PDMAEMA determined by 1H NMR spectroscopy in CDCl3 (Supporting Information). b Apparent number-average molar mass (Mn) and polydispersity of P(DMAEMA-g-CL) copolymers estimated by GPC in THF (+ 2 wt % NEt3) using polystyrene standards.
1H
NMR spectroscopy. 1H NMR spectra were recorded on Bruker AMX-500 MHz spectrometers in CDCl3 (with 0.03% TMS as the reference). Gel Permeation Chromatography (GPC). GPC was employed to determine the molecular weight and the molecular weight distribution of the synthesized copolymers. GPC in THF at a flow rate of 1.0 mL/min was carried out using a 10 µm PLgel 50 mm × 7.5 mm precolumn and two 10 µm PLgel mixed-B 300 mm × 7.5 mm gradient columns on an apparatus equipped with a refractive index detector. A series of nearly monodisperse polystyrene (PS) standards was used for calibration. Thermal Analysis. Differential scanning calorimetry (DSC) measurements were carried out under nitrogen flow at a heating rate of 10 °C/min using a DSC Quest T.A. Instruments apparatus. The samples were prepared by freeze-drying micellar solutions. Preparation of Aqueous Micellar Solutions. P(DMAEMA-gCL) and PDMAEMA-b-PCL chains are expected to undergo selfassembly in the presence of water, leading to ordered structures composed of a hydrophobic compartment of collapsed PCL segments surrounded by a hydrophilic PDMAEMA corona. These assemblies were prepared by dissolving 5.0 mg of copolymer in 1.0 mL of THF (a good solvent for both constituents), followed by the slow addition (1 drop/10 s) of a given volume of Millipore water at pH 6.5 and 25 °C under vigorous stirring in order to reach a final copolymer concentration of 0.5 mg/mL. The residual THF was eliminated by evaporation under ambient temperature and reduced pressure during 24 h. UV adsorption measurements performed on a water-THF mixture with identical composition confirmed the complete removal of the organic solvent after 24 h under reduced pressure. The copolymer concentration was recalculated after evaporation. pH modification was carried out after the completion of water addition at pH 6.5 by adding a few drops of NaOH (0.1 M) or HCl (0.1 M) solution under the control of a pH-meter analyzer. Determination of Critical Micelle Concentration. The critical micelle concentration (cmc) was determined by using pyrene as a fluorescence probe. Pyrene was molecularly dissolved in the copolymer/THF mixture before the addition of water. Pyrene incorporation into the micelles takes place during micelle formation. The residual THF was removed following the procedure described above. The final concentration of pyrene in the micellar solutions was around 10-6 mol/L. The micelle concentration in these experiments varied from 10-4 to 3.0 mg/mL. The UV absorbance of the pyrene trapped in the micelle core was determined with a Varian Cary 50 UV/visible spectrophotometer at room temperature. Dynamic Light Scattering (DLS). The mean hydrodynamic diameter (Dh) and the size distribution of P(DMAEMA-g-CL) and PDMAEMA-b-PCL self-assembled structures were measured at a 90° scattering angle using a BI-160 Brookhaven Instruments Corporation DLS apparatus equipped with a HeNe laser (λ ) 633 nm). Solutions were filtered through 1.2 µm pore size Acrodisk filters and maintained at 25 °C throughout the experiments. The data were processed using multimodal size distribution (MSD) analysis based on non-negatively constrained least-squares fitting (NNLS). The experimental error in the measurements of micellar sizes by DLS remained within 10% around the average value obtained from >10 independent measurements.
DLS measurements were also carried out at different scattering angles using an ALV laser goniometer, which consists of a 22 mW HeNe linearly polarized laser operating at a wavelength of 632.8 nm and an ALV-5000/EPP multiple digital correlator with 125 ns of initial sampling time. The copolymer solutions were maintained at a constant temperature of 25.0 ( 0.1 °C in all experiments. Data were collected using the ALV correlator control software. The distribution of relaxation times, A(t), was obtained by using CONTIN analysis of the autocorrelation function, C(q, t).28,29 Atomic Force Microscopy (AFM). Thin deposits of micelles were prepared by applying 30 µL of a 0.5 mg/mL aqueous micellar solution onto freshly cleaved mica, and the deposited solution was freeze dried for 2 h. Mica is a negatively charged hydrophilic substrate at neutral pH (water contact angle of 90%), the weight fraction of PCLMA in the copolymer is close to the initial weight fraction in PCLMA. We have previously observed that the PCLMA macromonomer preferentially incorporates into the growing copolymer chains. This observation suggests the formation of graft copolymers with a palm-tree-like structure (especially for
the G50 copolymer) (i.e., with a higher number of polyester grafts close to the initiator residue extremity). cmc Measurements. It is well documented in the literature that the cmc, which is the copolymer concentration (Cp) below which only molecularly dissolved chains exist but above which both micelles and single chains are present simultaneously, is affected by the relative volume fraction of the core-forming block in a copolymer. In general, reducing the hydrophobic content results in an increase in the cmc. A micelle is thermodynamically stable with respect to dissociation provided that the copolymer concentration is above the cmc. If Cp < cmc, then micelles may still be kinetically stable and survive for a given period of time, which will depend on the characteristics of the core-forming block (size, glass-transition temperature, crystallinity, etc.). The cmc of P(DMAEMA-g-CL) graft copolymers in aqueous media was determined by steady-state fluorescence spectroscopy using pyrene as a probe. The fluorescence of pyrene is known
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Figure 3. DLS size distribution of 0.5 mg/mL graft copolymer micelles in water at pH 6.5 (left) and variation of the hydrodynamic diameter at the maximum in the population as a function of Fw PDMAEMA (right). The hydrodynamic diameter (Dh) at the maximum and the peak width at half-maximum (W) are presented in the inset. Table 2. Micelle Diameters Determined by DLS in Solution and by AFM on Freeze-Dried Micelles copolymer/architecture G50 G80 G86 G92 B50 B60 B70 B90
D h(DLS)a (nm)
Graft Copolymer Micelles 67 ( 10 52 ( 5 43 ( 5 19 ( 3 Block Copolymer Micelles 100 ( 20 53 ( 5 47 ( 5 30 ( 5
D(AFM)b (nm) 60 ( 9 40 ( 5 35 ( 5 20 ( 2 55 ( 3 45 ( 3 40 ( 3 25 ( 2
a Mean hydrodynamic diameter determined by DLS for 0.5 mg/mL micelle solutions at pH 6.5 and 25 °C. b Mean diameter measured by AFM of freeze-dried micelles on mica deposited from 0.5 mg/mL solutions at pH 6.5 and 25 °C.
to be sensitive to changes in the polarity of its microenvironment.30 The excitation spectra of pyrene in micellar solution exhibit a characteristic shift from 335 to 339 nm upon transition from a polar environment (water in this case) to an apolar environment (the hydrophobic micelle core). The cmc’s of the graft copolymers in aqueous media were determined from the intensity ratios of excitation bands at these two wavelengths (I339/I335) over a wide range of copolymer concentrations (Figure 1). At low concentrations, the intensity ratio remains basically constant. Beyond a given concentration, the absorbance to intensity ratio increase sharply, indicating the solubilization of pyrene molecules within the hydrophobic PCL-based micelle core. The cmc values were taken at the concentration at which the sharp increase in the I339/I335 ratio starts as illustrated in Figure 1, corresponding to the intersection of the two straight lines drawn through the experimental data. The cmc values estimated for the G92, G86, and G80 copolymers in pure water at pH 6.5 were found to be ∼0.1 mg/mL, in good agreement with measurements by tensiometry on similar copolymers systems.18 As expected, the cmc values decreased with PCL content; the G50 copolymer had a cmc of 0.05 mg/mL. Interestingly, the cmc values for graft copolymers were higher than for linear diblock copolymers (cmc ∼0.01 mg/mL) with similar composition. The self-organization of graft copolymer chains into spherical micelles implies the (30) Kalyanasundaran, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (31) Brown,W. Dynamic Light Scattering. The Method and Some Applications. Oxford University Press Inc.; New York, 1993.
curvature and looping of the PDMAEMA backbone in order to allow the PCL segments to assemble into the hydrophobic core. Such looping and bending of PDMAEMA segments is entropically unfavorable, which can explain the cmc increase from linear diblock to graft copolymers with similar compositions. Besides, the block architecture favors the hydrophobic interactions between nonpolar segments of adjacent chains in a regular micelle structure, as compared to graft architectures with a gradient distribution of (co)monomers. This obviously leads to a decrease in the cmc of the system. Dynamic Light Scattering (DLS). Figure 2 shows typical autocorrelation functions C(q, t) measured at different scattering angles for 0.5 mg/mL G50 aqueous micellar solutions. The inset illustrates the q2 dependence of the relaxation frequency Γ(Γ ) τ-1) calculated using CONTIN analysis. The linear behavior is characteristic of diffusive scattering particles,31 whose spherical morphology was evident by AFM. Essentially the same comments apply to the other samples. Therefore, the hydrodynamic diameter (Dh) of the particles could be correctly calculated using the Stokes-Einstein relation. The relaxation times A(t) at a scattering angle of 90° (left) and the variation of Dh values as a function of the PDMAEMA content (right) for 0.5 mg/mL solutions at pH 6.5 for P(DMAEMA-gCL) graft copolymers investigated in this work are shown in Figure 3. In general, fairly narrow distributions of relaxation times were obtained, with a single dominant mode corresponding to the diffusive motion of the spherical micelles in solution (see also AFM analysis hereafter) whose characteristic hydrodynamic diameters (Dh) varied from 20 to 80 nm depending on the copolymer composition; the higher the PCL content, the larger the micellar nanoparticles (Figure 3, right). Indeed, graft copolymers G86, G80, and G50, which present nearly the same PDMAEMA block length (i.e., 102-112 repeat units), developed micelles with decreasing interfacial curvature32 as the PCL weight fraction increased. However, morphology transitions in solution due to variations in the composition parameter30 did not take place for the systems investigated, and all of the samples produced spherical micelles upon self-assembly in water, as judged from direct imaging experiments that preserved the structures in solution (see below). It is also worth nothing that graft copolymers appear to form micelles smaller than those made from the (32) Zupancich, J. A.; Bates, F. S.; Hillmyer, M. A. Macromolecules 2006, 39, 4286. (33) Jain, S. J.; Bates, F. S. Science 2003, 300, 460.
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Figure 4. AFM height images of thin deposits prepared by drop-casting of 0.5 mg/mL solutions on mica: (a) G86 1 × 1 µm2; (vertical gray scale: 10 nm), (b) G50 1 × 1 µm2 (vertical gray scale: 30 nm). Scheme 2. Formation of Spherical Micelles and Aggregation into Irregular Objects or Merging into Cylindrical Assemblies
corresponding linear diblock copolymers, as observed for samples G50 (Dh ) 67 nm) and B50 (Dh ) 100 nm) in Figure 2 and Table 2. The decrease in micelle diameter in the graft architecture can be explained by the energy penalty in assembling graft copolymer chains. The constraint due to the bending and looping of the main backbone most probably decreases the aggregation number (not determined) and limits the hydration of the PDMAEMA corona. The broadening of the size distribution (slow relaxation) of micelles made from copolymers G80 and G50 (Figure 3, left) is most probably associated with the existence of secondary large aggregates in solution. Surely, this is a minor component of the system as long as the light-scattering intensity strongly depends on particle mass and size, implying that DLS reports an intensityaveraged size. We have also examined the effect of pH and temperature on the micelle size. As observed for the block copolymers, increasing the pH to basic values leads to a gradual decrease in the apparent hydrodynamic diameter of the micelles as a result of the deprotonation of the ammonium groups of the corona. As for the temperature dependence, we have observed the precipitation of copolymers above 35 °C at neutral pH as a result of the loss of solubility of PDMAEMA segments, which is consistent with the LCST behavior.
AFM on Thin Deposits The thin films obtained by drop casting the micellar solutions under ambient conditions on mica showed secondary aggregates (Figure 4). These objects are most probably formed during the solvent evaporation process. For G86 (Figure 4a), they seem to be formed by the irregular aggregation of smaller particles
(probably the micelles present in the liquid medium). In our opinion, there is no complete merging of individual micelles in this case (see scheme 2). In contrast, the G50 copolymer forms elongated structures (Figure 4b) with a diameter of 30 ( 5 nm. It can be noted that regardless of the PCL content the DSC curves of the P(DMAEMA-g-CL) graft copolymers do not exhibit the melting endotherm at 42 °C typical of the PCLMA graft (Supporting Information). This result indicates that the PCL segments do not crystallize in the solid state aggregates, in contrast to the diblock copolymer micelles of PDMAEMA-b-PCL studied previously.22 This might result from the palm-tree architecture and the steric hindrance of dangling PCL grafts. Therefore, the formation of wormlike micelles cannot be explained by the Vilgis-Halperin model.34 According to such a model, the PCL hydrophobic block would form crystals through adjacent folds within the core, and a sharp interface would divide the crystalline core from the solvent-swollen PDMAEMA corona. In the case of graft copolymers, the overall shape of the self-assembled structure depends on the interplay between the interfacial energy between the core and the solvent and stretching and looping of the PDMAEMA backbones within the corona. In the dilute regime, the copolymers with several grafted PCL segments selfassemble into flowerlike micelles by the association of the PCL chains into a hydrophobic core stabilized in water by loops of the hydrophilic PDMAEMA chains (Scheme 2). The entropic penalty associated with loop formation in the corona reduces the micelle stability.35,36 The transition from individual spherical micelles to long cylindrical assemblies is favored by the increase in the radius of curvature and thus the decrease in the PDMAEMA stretching in the corona when the spherical objects merge into cylindrical objects for the G50 copolymer (Scheme 2). Another relevant phenomenon is the loop-bridge transition: in the concentrated regime, the graft segments of a given copolymer chain can be incorporated into the cores of separate micelles, thus forming bridges between the micelles. Those bridges favor organized association of the micelles.37–40 Note that in the G86 copolymer there is on average only one PCL graft per chain. Therefore, no flowerlike configuration and no bridging are expected. As a consequence, the system does not tend to form cylindrical objects in the solid state. In contrast to the micelles prepared by drop casting, the thin deposits prepared by freeze-drying of the micellar solutions on mica (Figure 5) show individual spherical micelles for all graft
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Figure 5. AFM height images of thin deposits prepared by freeze drying 0.5 mg/mL micelle solutions on mica (a) G86, (b) G50, and (c) the B50 linear diblock copolymer.
copolymers studied. Very importantly, the micellar sizes determined by AFM using the freeze-drying technique are consistent with the DLS analysis. These results are indeed highly reproducible and confirm the presence of spherical micelles in the solutions. The comparison between images a and b shows the increase in micelle size when decreasing the amount of PDMAEMA in the copolymer. These results also highlight the interest in using the freeze-drying procedure for the preparation of the solid-state deposits on mica for AFM investigation that preserves the original morphology in solution. Finally, those observations confirm that the larger objects seen after slow solvent evaporation (Figure 4) are not present in the initial solution but are formed during the late stages of the drying process, as the micelle concentration increases. The data summarized in Table 2 indicate that (i) micelles of graft and diblock copolymers with similar compositions have very similar sizes in the solid state (compare G50 and B50) and (ii) micelles of graft and diblock copolymers can be quite different in size in solution. The micelles of the graft copolymers tend to be smaller, probably because the hydration of the PDMAEMA segments of the corona is not as efficient. (34) Vilgis, T.; Halperin, A. Macromolecules 1991, 24, 2090. (35) Semenov, A. N.; Joanny, J.-F.; Khokhlov, A. R. Macromolecules 1995, 28, 1066. (36) Borisov, O. V.; Halperin, A. Langmuir 1995, 11, 2911. (37) Xu, B.; Yekta, A.; Masoumi, Z.; Kanagalingam, S.; Winnik, M. A.; Zhang, K.; Macdonald, P. M.; Menchen, S. Langmuir 1997, 13, 2447.
Conclusions The influence of the macromolecular architecture on the selfassembly of amphiphilic copolymers (grafts and diblocks) based on poly(N,N-dimethylamino-2-ethyl methacrylate) and poly(caprolactone) was investigated both in aqueous solutions and in thin solid deposits. From the results herein reported on the micellar solutions, it was observed that the graft copolymer systems forms ordered nanostructures that are smaller than the corresponding block copolymers (Dh(grafts) < Dh(diblocks)) in self-assembly processes that usually begin at higher copolymer concentrations (cmc(grafts) > cmc(diblocks)). The morphology of solvent-free deposits on mica was examined by AFM, and an interesting deposition process that preserves the micelle morphology in solution was successfully applied in this study. The method consists of freeze drying a thin liquid layer deposited on a mica substrate, thus preventing the micelle aggregation phenomenon that usually takes place during water (solvent) evaporation. Indeed, in contrast to the micelles prepared by drop casting, the thin deposits prepared by freeze drying the micellar solutions on mica showed individual spherical micelles for all graft copolymers investigated. These results also highlight the interest in using such a freeze-drying procedure for the preparation of solid-state deposits on mica for AFM investigations that effectively preserve the original morphology in solution. (38) Alami, E.; Almgren, M.; Brown, W. Macromolecules 1996, 29, 2229.
Self-Assembly of Amphiphilic Copolymers
Acknowledgment. This work was partially supported by the Re´gion Wallonne and the European Commission in the framework of Phasing-out Hainaut: Materia Nova program, by the Belgian Federal Government Office of Science Policy (PAI 6/27) and by FNRS-FRFC. F.B. is grateful to F.R.I.A. for his Ph.D. grant.
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Supporting Information Available: 1H HMR spectra of PCLMA in CDCl3 and G86 and G50 graft copolymers in CDCl3. GPC traces of the PCLMA macromonomer and the G50 graft copolymer. DSC traces of G50 and B70 freeze-dried micelles. This material is available free of charge via the Internet at http://pubs.acs.org. LA800765Y
(39) Annable, T.; Buscall, R.; Ettelaie, R.; Wittelestone, D. J. Rheol. 1993, 37, 695.
(40) Nojima, R.; Sato, T.; Qiu, X.; Winnik, F. M. Macromolecules 2008, 41, 292.
