Dewetting and Nanoparticle Condensation Phenomena - American

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Stimuli Responsive Poly(1-[11-acryloylundecyl]-3-methylimidazolium bromide): Dewetting and Nanoparticle Condensation Phenomena Xiumin Ma,† Md. Ashaduzzaman,‡ Masashi Kunitake,‡ Rene Crombez,† John Texter,*,† Lisa Slater,§ and Thomas Mourey§ †

Coating Research Institute, School of Engineering Technology, Eastern Michigan University, Ypsilanti, Michigan 48197, United States New Frontier Science, Graduate School of Science and Technology, Kumamoto University, 39-1 Kurokami 2-chome, Kumamoto 860-8555, Japan § Corporate Research and Engineering, Eastman Kodak Company, Rochester, New York 14650-2136, United States ‡

bS Supporting Information ABSTRACT: A stimuli-responsive homopolymer poly(ILBr) is fabricated via a “two-phase” atom transfer radical polymerization (ATRP) process, where ILBr stands for the reactive ionic liquid surfactant, 1-[11-acryloylundecyl]-3-methyl-imidazolium bromide. An extraordinarily wide molecular weight distribution (PDI = 6.0) was obtained by introducing the initiator (4-bromomethyl methyl benzoate) in a heterogeneous two-phase process. The molecular weight distribution of poly(ILBr) was characterized by size-exclusion chromatography (SEC). The resulting homopolymer was found to be surface active and stimuli responsive. Poly(ILBr) films coated on quartz exhibit stimuli-responsive dewetting after ion exchange of Br
by PF6
. This dewetting phenomenon can be understood in chain segmental terms as a stimuliinduced structural relaxation and appears to be the first such reported stimuli-responsive polymeric dewetting. Titrating aqueous poly(ILBr) with aqueous bis(2-ethylhexyl)sulfosuccinate induces nanophase separation and results in the condensation of nanoparticles 30
60 nm in diameter.

’ INTRODUCTION Ionic liquids (ILs), well-known as “green” and “designer” solvents, are of growing interest for their applications in various fields, such as electrochemistry,1,2 organic synthesis,3,4 biochemistry,5,6 and chemical extraction and separation,7
9 due to their negligible vapor pressure and other properties, such as high thermal and chemical stability, wide liquid temperature ranges, electrochemical stability, high ionic conductivity, and low flammability. Moreover, ILs can be customized to meet specific demands, because their physicochemical properties of hydrophobicity, polarity, melting point, and solubility can be finely tuned by selecting appropriate combinations of anions and cations.10 In recent years, ILs have also found significant application in polymer science not only as reaction media11
15 but also as monomers for stimuli-responsive block copolymers,16 stimuliresponsive materials,17
20 and stimuli-responsive particles21,22 and as surfactants, water-immiscible liquid phases, oil-immiscible liquid phases, and polymeric additives.23
27 For instance, Taton et al.16 reported the self-assembly of IL-based block copolymers fabricated via sequential reversible addition
fragmentation chain transfer polymerization, which could be triggered by different stimuli including anion exchange of the polymeric ionic liquid block, or by the choice of a selective solvent of one block, or by chemical modification of hydrophilic poly(methacrylic acid) blocks into hydrophobic poly(methyl methacrylate) segments. Recent studies have also shown block copolymers r 2011 American Chemical Society

containing blocks derived from ionic liquids to have diverse and fascinating physical properties, including reversible nanoparticle formation across first-order phase boundaries with blocks fabricated via emulsion and dispersion free radical chain polymerization methods.21,22 It is well-known in colloid and surface science that mixtures of surfactants often are more effective than a pure compound, although the characterization of some surfactant mixtures can be technically difficult. Positive synergistic effects in surfactant mixtures are detailed by Rosen,28 and many practical applications such as emulsion stabilization have been detailed by Chattopadhyay et al.29 With respect to homopolymer adsorption, it is known that mixtures of molecular weights adsorb because of entropic effects.30 Janardhan et al.31 showed that, in the adsorption of polyurethane oligomers onto iron oxide pigments, the adsorption of higher molecular weight oligomers was favored initially, but at saturation the adsorption of lower molecular weights was favored. The nature of the mixture depended upon relative adsorption saturation as well as on the affinity of the solvent for the polymer. These trends are in agreement with the theoretical studies of Scheutjen and Fleer.32

Received: January 15, 2011 Revised: March 25, 2011 Published: April 28, 2011 7148

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Langmuir In this paper, a “two-phase” atom transfer radical polymerization (ATRP) method is used to produce homopolymeric poly(ILBr) with an extremely broad molecular weight distribution. The ILBr monomer itself is a surfactant with appreciable surface activity,33 and each oligomer is a polymeric surfactant.34 The resulting poly(ILBr) is characterized by various methods, and it is shown that poly(ILBr) exhibits both stimuli-responsive nanoparticle condensation and dewetting properties.

’ EXPERIMENTAL SECTION Materials. Acryloyl chloride (96%), 11-bromo-1-undecanol (98%), triethylamine (99.5%), 1-methylimidazole (99%), 2,6-di-tert-butyl-4methylphenol (minimum 99% GC, powder), 4-(bromomethyl methyl benzoate (98%; BMMB), potassium hexafluorphosphate (98%), cuprous bromide (99.99%; CuBr), 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA, 97%), and neutral aluminum oxide (activated, Brockmann I, standard grade, ∼150 mesh, 58 Å) were purchased from Aldrich. Tetrahydrofuran (THF, anhydrous, g99.9%, inhibitor free), dichloromethane (anhydrous, g99.8%), methyl sulfoxide-d6 (99.9 atom % D), deuterium oxide (D2O, 99.9 atom % D), and methanol-d4 (99.8 atom % D; contains 0.05% v/v TMS) were also obtained from Aldrich. Diethyl ether and Aerosol-OT (anhydrous; sodium bis[2-ethylhexyl]sulfosuccinate) were purchased from Fisher Scientific. Synthesis of 1-(11-Acryloyloxyundecyl)-3-methylimidazolium Bromide (ILBr). The intermediate compound, 11-bromoundecylacrylate, was prepared by the addition of acryloyl chloride to 11bromoundecanol in the presence of triethylamine. The reaction was conducted at room temperature under a nitrogen atmosphere with stirring for 2 days. ILBr was synthesized by stirring a small excess of 1-methylimidazole with the 11-bromoundecylacrylate intermediate for 2 days at 40 °C under a nitrogen atmosphere. Additional details are provided elsewhere.18,27 Synthesis of Poly(ILBr). Solubility of the initiator, BMMB, in the reaction solvent, methanol/water, was determined by optical turbidity assessments. The results are illustrated in Table 1. Methanol (4.2 mL) and water (3 mL) (7:5 v/v) were taken into a reaction flask and further purged with Ar for another 10 min. HMTETA (0.25 mmol, 57.6 mg, 68 μL) ligand and CuBr (0.2 mmol, 30 mg) catalyst were then charged in the flask successively. The solution appeared very light green in color after purging for 10 min. ILBr monomer (6.46 mmol, 2.50 g) was then added, and purging was continued for 5 min. Additional CuBr (0.1 mmol, 15 mg) was added, and the solution became bluish green. Finally, BMMB (0.2 mmol, 46 mg; a 6.4-fold excess for the solvent volume used) initiator was added and the reaction flask was then placed in a preheated oil bath at 40 °C with stirring, followed by degassing with a pump and backfilling with Ar (10 min). The emulsified reaction mixture was stirred for 48 h. After 48 h, the product reaction was cooled to room temperature and taken into a dialysis tube (SnakeSkin pleated dialysis tubing, MWCO 3500, length 25 cm). Product adhering to the reaction flask was removed by rinsing with deionized water and adding to the dialysis tubing contents. Dialysis was performed in a 4 L vessel containing a magnetic stirrer to accelerate the rate of dialysis against deionized water. Water was changed frequently during the first 12 h. Almost all CuBr/ HMTETA and monomer were removed from the system within this time. Dialysis was continued for another 36 h (changing water every

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Table 1. Solubility of BMMB in Various Methanol/Water Mixtures at Room Temperature (∼23°C) CH3OH/H2O (v/v) solubility (g/L)

5/5 0.22

4/3 0.5

7/5 0.95

12 h). A white polymer was obtained after lyophilization of the dialysis tube contents. The yield was ca. 60% w/w. Methods. The size-exclusion chromatography (SEC) system consisted of a Waters Corporation 2695 solvent delivery system, 2487 dual wavelength spectrophotometric detector, 410 differential refractive index (DRI) detector, a Precision Detectors PD2020 two-angle light scattering (LS) detector, and a Viscotek H502 differential viscometry (DV) detector. The DV and DRI were configured with a parallel split after the spectrophotometric and LS detectors. The column set consisted of three 8 mm  300 mm KF-806 L columns from Shodex, thermostatted at 40 °C, calibrated with narrow-molecular-weight distribution poly(methyl methacrylate) (PMMA) standards. The eluent was 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) containing 0.01 M tetraethylammonium nitrate, delivered at a nominal flow rate of 1.0 mL/min. The actual flow rate was determined from the retention volume of acetone added to the sample solvent as a flow marker. The specific refractive index increment dn/dc = 0.203 mL/g was obtained by integration of the DRI chromatogram. Differential scanning calorimetry (DSC) analysis was performed on a TA Instruments DSC Q2000 apparatus. Samples were encapsulated in Tzero aluminum pans, using Tzero standard aluminum lids (TA Instruments). All samples were analyzed by a heat/cool/heat cycle in which the sample was initially heated and held isothermally for 5 min at 150 °C, believed to be above the Tg in order to erase the previous thermal history of the given sample. The samples were then cooled to
85 °C and held isothermally for 5
10 min before heating again to the desired maximum temperature. All reported transitions were obtained from the second heating portion of the cycle. Thermogravimetric analyses (TGA) were performed on a TA Instruments TGA Q500 apparatus. Samples were placed in aluminum DSC pans (TA Instruments), which in turn were placed in platinum TGA pans. Samples were heated from room temperature to 568 at 20 °C/min. 1 H NMR structural analysis of IL-Br monomer and poly(ILBr) were performed on a JEOL 400 MHz NMR instrument. Samples were prepared by dissolving 15 mg of analyte in different solvents in a 5 mm O.D. NMR tube. 1H NMR spectra were obtained after averaging 8
32 scans for different samples. Scanning electron microscopy (SEM) analyses were performed on a Hitachi S-3400N scanning electron microscope. Energy-dispersive X-ray spectroscopy (EDAX; energy dispersive analysis by X-ray fluorescence) microanalysis was done with an IXRF 500 analyzer. The FE-SEM analysis was performed on a Philips XL 30 ESEM electron microscope. All of the samples were sputter coated with Au using a Denton Vacuum Desk IV cold sputter/etch unit to reduce charging effect by the electron beam. The sputter time was 90 s while keeping the sputter set point at 29%. Atomic force microscopy (AFM) images were taken using a Veeco Multi Mode scanning probe microscope (MMSPM) with a Nanoscope IIIa controller. The images were taken in tapping mode using a Veeco Multi 40a probe, driven at either 42 kHz or 272 kHz.

’ RESULTS AND DISCUSSION Poly(ILBr) Synthesis. After some initial attempts at ATRP of ILBr in aqueous dimethyl sulfoxide (DMSO) and in dimethylformamide (DMF), aqueous methanol at a volume ratio of 7:5 (water/methanol) was found to be effective with 4-bromomethyl methyl benzoate as initiator at 40 °C. The basic scheme is shown 7149

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Figure 1. Basic ILBr ATRP scheme.

Figure 2. 1H NMR of poly(ILBr) in CD3OD.

in Figure 1. While this appears to be the first ATRP synthesis of an IL bromide salt, we note that ATRP of a quaternary bromide, 2-(dimethylethylammonio)ethyl methacrylate bromide, was reported previously by Matyjaszewski et al.35 They explored the use of water and aqueous pyridine as reaction solvent with good results. They also concluded, along with Armes et al.,36 that the addition of halide salt retards the inactivation of catalyst. The polymerization of IL monomers that often are halide salts, therefore, should not be problematic. The most conventional biphasic ATRP is designed to separate reactants and products and to minimize the expense of separating the product polymer for the catalyst reaction mixture.37 Another application is where the38 catalyst system is maintained as a separate phase in order to maximize the ease of reusing the catalyst in a subsequent batch synthesis. In this study, the initiator, added at an excess of 6.4-fold the saturation level, constitutes a second (solid) phase. The excess initiator was expected to sustain for a good portion of the reaction time. In this system, both the monomer and the homopolymer are easily soluble in the reaction solvent. 1 H NMR Characterization. Poly(ILBr) was dissolved in CDCl3, DMSO, D2O, or CD3OD and examined by 1H NMR. The best spectra were obtained in CD3OD, and an example is illustrated in Figure 2. The structure of the polymer is shown as in inset in Figure 2 with alphabetical labels of resonance assignments. The proton signals of the initiator fragment composed of the methyl multiplet a (3H; δ = 3.45
3.75 ppm), ring protons b (2H; δ = 7.88 ppm) and c (2H; δ = 7.27 ppm), and methylene protons d (2H; δ = 2.63 ppm) fortuitously did not overlap with the polymerized oligomer peaks. The initiator methyl protons a are well separated from the imidazolium methyl protons p, as can be seen in the second inset in Figure 2. The backbone methine protons f

Figure 3. Differential weight fraction molecular weight distribution (primary y-axis) of poly(ILBr) (solid); viscosity-molecular weight data for linear narrow molecular weight distribution PMMA standards (symbols). Poly(ILBr) (dashed) are plotted on secondary y-axis.

(mH; δ = 2.2
2.4 ppm) and imidazolium methyl protons p (3mH; δ = 3.9
4.0 ppm) were distinguishable. The central 14 methylene protons i (28mH; δ = 1.2
1.4 ppm) of the undecyl linking group were also distinguishable. The imidazolium adjacent protons (
N
CHdCH
N
) q and r (2 mH; δ = 7.5
7.8 ppm) were identified between the phenyl ring proton resonances. The methylene protons adjacent the imidazolium ring k (2mH; δ = 3.9
4.1 ppm) were also distinguishable. The undecyl methylene protons g (2 mH; δ = 4.19
4.3) adjacent oxygen and the terminal methine proton f00 were upfield of the k methylene protons. The backbone methylene protons e and undecyl methylene protons h and j (6mH; δ = 2.45
1.95 ppm) were in overlapping peaks. The
N
CH
N
imidazolium protons l (mH) are easily ionized and exchanged with labile solvent deuterons. A detailed NMR study of the kinetics of polymerization of the homopolymer is given in the Supporting Information. The kinetics deviate significantly from first order, as is expected for the heterogeneous delivery of initiator. Extrapolation of the conversion plot (Supporting Information) to complete conversion suggests an overall degree of polymerization of about 60, corresponding to a Mn value of about 23 000 Da. However, little can be inferred about the polydispersity index (PDI) from these NMR data, except that it should be relatively large, in view of the heterogeneous delivery of initiator. SEC. Molecular weight averages measured by viscometry detection and universal calibration were Mn = 41 200 Da, Mw = 248 000 Da, Mz = 345 000 Da with polydispersity Mw/Mn = 6.02. The SEC method for measuring Mn can be insensitive to very-low-molecular weight species, resulting in a value that is 7150

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Figure 6. Surface tension of poly(ILBr) in water at 23 °C. Figure 4. TGA analysis of poly(ILBr) produced by ATRP with heterogeneous initiator delivery (—) in comparison with the ILBr monomer (
3
3 ) and poly(ILBr) made by bulk polymerization ( 3 3 3 ) and by solution polymerization in DMF (


) as reported by England.33

Figure 5. DSC of poly(ILBr) samples illustrating heat flow on heating for poly(ILBr) made by ATRP with heterogeneous delivery of initiator as reported here (—), for poly(ILBr) made by bulk polymerization (


), and poly(ILBr) made by solution polymerization ( 3 3 3 ) in DMF as reported by England.33.

underestimated compared to the value obtained from NMR. The weight average molecular weight measured by light scattering was 291 000 Da, which is in reasonable agreement with the viscometry detection result. The whole polymer weight-average intrinsic viscosity was [η] = 0.175 dL/g and a z-average rootmean square radius rgz equal to 49.2 nm was estimated from light scattering data. The molecular weight distribution is multimodal (Figure 3) with the higher modes being slightly greater than 2 and 4 multiples of the lowest mode. The polydispersity is much larger than generally obtained in a controlled ATRP process. The light-scattering molecular weight-size data were not suitable to assess conformation. The scaling exponent for intrinsic viscosity as a function of molecular weight (plotted on the secondary axis of Figure 3) is approximately 0.45, which is indicative of a

collapsed conformation such as a polymer near the theta state or a branched or other compact polymer architecture. Thermal Analysis. TGA results for poly(ILBr) are provided in Figure 4. The ILBr polymer is approximately 20 °C more stable thermally than the monomer. The decomposition ranges from 270 to 350 °C. This homopolymer appears only very slightly more thermally stable than ILBr polymerized in DMF at 60 °C via thermal initiation with Mn ∼ 9 kDa and Mw/Mn ∼ 3.27 We hesitate to rationalize these small differences, but we believe the lower stability of the bulk polymerized sample is a result of light branching from a 1,3-bis(acryoylundecyl)imidazloium bromide impurity33 caused by alkyl scrambling, which is consistent with the low slope of the conformation plot obtained by SEC (Figure 3 secondary y-axis data). DSC results for our poly(ILBr) are illustrated in Figure 5. This material appears to exhibit three Tg transitions at
40° to
30 °C, 0° to 15 °C, and at about 118° to 130 °C. At present, the molecular basis for these specific transitions is not known. These scans are highly reproducible, and two of the scans lie essentially on top of one another. A lightly cross-linked sample produced by thermally initiated bulk polymerization (in the ILBr melt) at 60 °C exhibited three Tg transitions at
25° to
30 °C, 5° to 10 °C, and at about 45° to 53 °C, but no apparent transition above 100 °C.33 A poly(ILBr) produced by thermally initiated solution polymerization in DMF at 60 °C exhibited three Tg transitions over
45° to
35 °C, over
15° to 0 °C, and over about 95° to 110 °C.33 While we believe each of these three transitions are glass transitions, their detailed origins are obscure and must await further spectroscopic characterization. Each of the samples appear to exhibit the lower two transitions, although the bulk polymerized sample, lightly cross-linked, has its lowest Tg (
25° to
30 °C) 20° higher than the other two samples (most likely due to the cross-linking). Also, this bulk polymerized sample has its highest Tg (45° to 53 °C) 50° to 70° lower than the other two samples (again, most likely due to the cross-linking). Surface Tension. Adsorption of our poly(ILBr) at the air
water interface was studied at 25 °C using the pendant drop method. Results are illustrated in Figure 6. This homopolymer is surface active at quite low concentration, although the ultimate surface tension lowering is to 43 dyn/cm. The steepest interval in the slope, from the Gibbs adsorption isotherm, corresponds approximately to an interfacial area density of 56 Å2/monomer. This is a quite reasonable value in view of the backbone constraint on all of the undecylimidazolium bromide chains; however, it does represent a fairly disordered system in comparison with close7151

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Figure 7. SEM images illustrating dewetting of poly(ILBr) at the quartz
water interface at 25 °C (a) after treatment with aqueous KPF6; (b) area taken at higher magnification from inside large dewetted region illustrated in frame (a).

Figure 8. Higher magnification SEM images of droplets within the large dewetting area of Figure 7a. Both images illustrate the occurrence of protuberances that appear to be coming out of the droplets. (a) Regions 1 and 2 in the left-hand image and (b) regions 1, 2, and 3 in the right-hand image were examined by EDAX analysis for elemental identification.

packed alkyl chains (18
20 Å2/chain), as might be expected from surface active segments tied to an acrylate backbone. Poly(ILBr) Dewetting. We have observed that 30
60 μm thick films derived from nanolatexes exhibit poration when contacted with a stimuli-responsive anion, such as hexafluorophosphate. We therefore examined thin films of poly(ILBr) adsorbed on quartz coverslips before and after soaking the films in 0.1 M KPF6. Films were prepared by drying 3% poly(ILBr) in water on a quartz coverslip and yielded a coverage of 1.95 mg/cm2. Before treatment, the films were smooth and featureless. Figure 7a illustrates a region after treatment with aqueous 0.1 M hexafluorophosphate with numerous areas of apparent dewetting. An expanded region within the large circular area in Figure 7a is shown in Figure 7b. Some droplets are in excess of 30 μm across and others are submicrometer in diameter. Higher magnification of selected regions (Figure 8) droplets with protuberances that appear whitish due to their apparent charging in the electron beam. Many of these protuberances appear cubic such as one shown in Figure 8b. Dewetting was confirmed by EDAX elemental analysis in regions between droplets and compared to elemental analyses in regions within droplets, as shown in Figure 8 and Table 2. The background area between two 20þ μm diameter droplets was analyzed by EDAX for atomic composition and the results are given in Table 2. The larger rectangular area “left-1” produced

Table 2. EDAX Elemental Analyses in Regions between Droplets and within Droplets regions of Figure 8

C (wt %)

P (wt %)

Si (wt %)

left-1

2.5

0.05

80.2

left-2 right-1

0.0 7.1

0.19 5.55

82.8 60.9

right-2

25.1

4.35

58.3

right-3

0.0

0.03

83.8

a strong Si signal from the quartz coverslip substrate and minor C and P background signals. Two very small droplets are visible in region left-1 but outside region left-2. The results for the smaller region left-2 gave no C signal and a slightly larger Si signal. These results, typical of many similar measurements made elsewhere in the sample, prove that dewetting has occurred. The region right3 also gave a very strong Si signal, a minor P signal, and no C signal, also supporting dewetting. The analysis of region right-2, centered on the apex of a 4 μm diameter droplet, shows a much diminished Si signal and large C and P signals, as would be expected for an amorphous droplet of poly(ILPF6). Bromide signals were negligible at all locations. Direct analysis of the protuberance, region right-2, gave even a higher P signal, slightly higher Si signal, and smaller C signal. The Si signal is easily detected through the droplet in region right-2. 7152

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Langmuir The reduced C signal in region right-1 can be understood in terms of a smaller portion of the droplet being subsumed by the electron beam. At present, we hypothesize that these protuberances are crystalline arrays of poly(ILPF6). The surface decorations caused us to initially think in terms of epitaxial salt precipitation, but

Figure 9. Optical micrograph (with dimensions of 330 μm  300 μm) illustrating dewetting of poly(ILBr) at the quartz
water interface at 25 °C after treatment with aqueous KPF6.

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these protuberances did not “wash away” when rinsed in water or when soaked in water. Additionally, various other SEM images give a clear view of such protuberances beginning as bulges, and with bulges combined with rounded protuberances not yet adopting a cubic habit. The poly(ILPF6) involved, we hypothesize, is composed of very short oligomers, more suitable to forming de facto crystalline regions. Figure 9 illustrates an optical micrograph of a dewetted region. This micrograph also illustrates the significant polydispersity in the droplets and also suggests the existence of grain boundaries of a type that must emanate from the unknown dynamics of the dewetting process. In Figure 9, the black dashed-dotted line and the black solid line identify “grain” regions wherein the droplets have nucleated in rows parallel with these lines. Other regions, such as in the lower right-hand corner, exhibit larger droplets with concomitantly fewer smaller droplets in between. Also evident in the lower right quadrant are doublets, some of which appear to have been just barely separated and others that appear to have been in the process of separating (undergoing fission), but were arrested in their present state prior to completely separating. AFM elevation and phase scans of an approximately 100 μm2 dewetted region are illustrated in Figure 10b and c. A control region is illustrated in Figure 10a for the poly(ILBr) film before treatment with stimui-responsive PF6
. The region depicted in Figure 10b and c was taken from the approximate center of the optical micrograph in Figure 9, around the “grain region” accentuated with the solid black line. The largest droplet illustrated in Figure 10 is ∼8 μm  10 μm in the plane and has an elevation of about 2 μm. From these dimensions, we

Figure 10. AFM images (each image area is 100 μm wide): (a) untreated control poly(ILBr) film (elevation image: 0
2 μm); (b) treated film (elevation image: 0
2 μm); (c) treated film (phase image: 0
30°); illustrating dewetting of poly(ILBr) upon transformation to poly(ILPF6) at the quartz
water interface at 25 °C after treatment with aqueous KPF6. 7153

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Langmuir estimate a contact angle a bit larger than 24°. An in-depth analysis of the contact angles and their droplet size dependence will be reported elsewhere. We believe the protuberance formation leads to distortion of what otherwise would be an amorphous polymer droplet. An introductory analysis of droplet cross sections for about 20 such droplets showed contact angles ranging over 14°
29° with most in the range of 20°
25°. These AFM images, both elevation and phase, very effectively illustrate that many of the larger droplets do not present a circular projection. The phase image in Figure 10c very distinctly distinguishes the protuberances, including those that are approximately equatorial or along the contact line with the substrate and those that have various azimuthal elevations. A dynamical AFM analysis may in the future allow more direct evidence to be obtained on the mechanism of shape formation. We believe this dewetting is not driven by the conventional interfacial energies between the substrate, air or water, and polymer. Our main rationale is that the stimuli-responsive poly(ILBr) is a higher energy material, being quite soluble in water. The quartz substrate has been estimated to have surface energies of 76 dyn/ cm40 and higher.41 The aqueous poly(ILBr) solution has a surface tension of about 50 dyn/cm, and so would be expected to wet the quartz substrate spontaneously. Further, the cationic nature of poly(ILBr) assures strong adsorption to hydroxyl and any anionic sites. Exchange of hexafluorophosphate for bromide, made facile in aqueous solution, results in a much more hydrophobic material that

Figure 11. Turbidity at 800 nm resulting from addition of AOT solution into a 0.5% (w/w poly(ILBr) solution.

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would be much more like a polyacrylate with surface energy in the range of 30
44 dyn/cm.42 If surface energy were all determining, we would expect the poly(ILPF6) to continue wetting the quartz substrate and to do so more tenaciously than the original wetting deposition solution of aqueous poly(ILBr). The small contact angles observed are consistent with a mechanism orthogonal to surface energy-based dewetting. This anion (PF6—) induced stimulus for dewetting can be understood in chain segmental terms as a stimuli-induced structural relaxation phenomenon, rather than an interfacial energy-induced dewetting. Upon contact with aqueous KPF6, ion exchange between bromide and hexafluorophosphate commences. The homopolymer chains become hydrophobic and undergo autocondensation and seek to shrink in one or more dimensions. This shrinking effect was demonstrated for the bulk state in gels of poly(ILBr-co-MMA) after contact with aqueous KPF617 and amounted to about 15% reduction in each linear dimension. Also, we reported recently43 that thin films produced by nanolatexes, after coalescence, also underwent pore formation upon contact with aqueous KPF6, but again the latexes were derived from poly(ILBr-co-MMA). In this system, it appears the stress from structural relaxation produces dewetting rather than poration. Such dewetting phenomena appear rare. Richardson et al.44 reported PMMA dewetting of spin-cast glassy films assisted by structural relaxation. Reiter et al.45 have asserted that residual stresses in thin polymer films cause rupture and dominate early stages of dewetting. It therefore appears the concept of stimuli-responsive-induced structural relaxation dewetting of polymer films is new. Nanoparticle Condensation. We also examined the effects of adding Aerosol-OT (AOT; NaOT; sodium bis(2-ethylhexyl)sulfosuccinate) to aqueous solutions of poly(ILBr). The structural relaxation accompanying poly(ILOT) condensation, in a solution or suspension state, may result in nanoparticle formation in ways similar to those reported by Tauer and co-workers.21,22 An aqueous solution of 0.5% poly(ILBr), 1.2 mL, was titrated with 1% NaOT, and the turbidity was measured by transmission absorbance at 800 nm. These measurements are illustrated in Figure 11. The 1:1 equivalence point between ILBr monomer and added NaOT corresponds to the addition of 688 μL of the NaOT solution. The onset of turbidity appears roughly at about 330 μL addition of titrant or just under 50% of the equivalence point. The average particle size obtained by photon correlation spectroscopy (PCS) for the sample at the equivalence point was about 210
260 nm. Since PCS is most sensitive to the largest particles present, the 210
260 nm values likely correspond to aggregates of smaller primary particles. Figures 12 and 13 illustrate FE-SEM micrographs of the resulting nanoparticles.

Figure 12. FE-SEM of poly(ILOT) nanoparticles produced by addition of 588 μL of AOT solution. The equivalence point is reached at 688 μL addition. 7154

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Figure 13. FE-SEM of poly(ILOT) nanoparticles produced by addition of 688 μL of NaOT solution.

The particles visible in Figure 13 obtained at the equivalence point, while aggregated, appear distinct. The particles obtained at 85% of the equivalence point and illustrated in Figure 12 appear to have film forming capability. While critical aggregation concentrations characterizing the nucleation of micelles along polyelectrolyte chains were discovered and characterized in the 1980s by a group from Kodak46
48 and subsequently nucleation and bridging flocculation of micelles was introduced by Spalla and Cabane,49 the interactions between polyelectrolytes and surfactants have been further studied at higher surfactant levels to nucleate nanoparticles. Our formation of nanoparticles by condensing poly(ILBr) with NaOT appears to be the first such formulation using a stimuliresponsive polymer. However, in the sense that traversing a firstorder phase boundary or particle precipitation is a stimulus response to a precipitating ion or sufficient change in some field variable, the papers we cite below report examples of stimuliresponsive systems. In our system here, more work is needed to improve the dispersion stability, but a variety of interesting metathesis reactions and effects appear possible as switches for dimensional change and cargo expulsion. Eisenberg et al.50 appear among the first to demonstrate microphase separation of polyelectrolyte
surfactant complexes to form nanoparticles, and it was recognized at this early stage that such particles may be invaluable in drug and chemical delivery processes. Th€unemann reviewed early work characterizing such particles,51 and drug delivery applications have been recently reviewed.52 Pispas and co-workers53 found nanoparticle formation upon complexation of NaOT (and SDS) with poly[3,5-bis(trimethylammoniummethylene-yl-iodide)-4-hydroxystyrene]. When particles were formed by mixing the polycation with NaOT above the critical micelle concentration (∼2 mM), larger particles (50
65 nm) were obtained and their size was explained by a mechanism based upon bridging of NaOT micelles by the polycation. They also found that the largest particle sizes were obtained at the equivalence point, when the number of complexed surfactants matched the number of cationic ammonium sites. In this system, the largest turbidity was obtained with a 21% excess amount of added NaOT, as shown in Figure 11. Particles at and below the equivalence point appear to be 20
50 nm in size. The FE-SEM images in Figure 13 suggest clusters of such primary particles compose larger particles in the range of 100
300 nm. This apparent aggregation of primary particles into clusters illustrates a limitation of the present method of producing nanoparticles. An improvement would consist of using a stabilizer, such as a neutral polymer (e.g., PEO), to

sterically stabilize the primary particles as demonstrated by Pojjak and Meszaros.54 The importance of cationic polyelectrolytes as important components in catanionic vesicle systems has been extensively investigated by Khan and co-workers in studies of polyelectrolyte interactions with catanionic systems.55
57 Catanionic systems are very well appreciated as giving rise to equilibrium vesicle formation.55 AOT (NaOT) readily forms vesicles by itself (and mild agitation)58,59 above its aqueous solubility limit (∼1.4% by weight60,61), and we would therefore expect poly(ILBr) to form gels and to exhibit other interactions with NaOT vesicles. Our experiments on particle formation were conducted in the polymer-rich domain, so that vesicles were not present. The titration of Figure 11 corresponds to a concentration of about 12.5 mM in cationic imidazolium group with a 22.5 mM NaOT titrant, and this equivalence point is reached after the onset of nanoparticle formation (onset of turbidity). The cationic polyelectrolyte
SDS interactions examined by Lindman et al.57 exhibited (polymer solution) one phase/two phase (precipitation) boundaries at 50% to 80% of SDS to polymer cation equivalence, depending on the polymer. Our results exhibit an onset of precipitation slightly below 50% of poly(ILBr) charge equivalence. In this system, we expect that the NaOT induces mixed micellization involving pendant imidazoluim groups along the poly(ILBr) backbone that subsequently condense to form microphase separated nanoparticles.

’ CONCLUSIONS In this work, we have developed a stimuli-responsive homopolymer, poly(ILBr), derived from a stimuli-responsive ionic liquid monomer, ILBr, via a “two phase” ATRP polymerization designed to produce large polydispersity in molecular weight. The molecular weight distribution of poly(ILBr) was measured by size-exclusion chromatography experiments and was found to have a PDI of about 6.0. The surface tension investigation of aqueous poly(ILBr) solutions show that this homopolymer is surface active at a quite low concentration and the interfacial area density is about 56 Å2 per monomer. Poly(ILBr) films deposited from water on quartz coverslips exhibit stimuli-responsive dewetting after ion exchange of bromide by hexafluorophosphate. This anion exchange induces a structural relaxation in the polymer chain that drives polymer dewetting. This appears to be the first stimuli-responsive-driven structural relaxation leading to polymer film dewetting to be reported. This relaxation process represents, we believe, a type of densification driven by an autocondensation of undecylimidazolium hexafluorophosphate 7155

dx.doi.org/10.1021/la200184c |Langmuir 2011, 27, 7148–7157

Langmuir ion pairs; but further work is needed to understand the mechanism. Introductory results on nanoparticles obtained by titrating aqueous poly(ILBr) with aqueous AOT (NaOT) are also reported. The precipitation from poly(ILBr)-rich solution appears to follow the nucleation of mixed micelles on the poly(ILBr). The polymer-rich solution phase
two phase boundary appears slightly below the mole fraction of 0.5 in NaOT relative to ILBr and NaOT.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional experimental details and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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