Strong Stretching of Poly(Ethylene Glycol) Brushes mediated by Ionic Liquid Solvation

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1 Supplementary Information Strong Stretching of Poly(Ethylene Glycol) Brushes mediated by Ionic Liquid Solvation Mengwei Han, Rosa M. Espinosa-Marzal Dept. of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, IL-680 Urbana, USA. *Corresponding author s rosae@illinois.edu Experimental method The SFA is an instrument that directly measures forces occurring between two mica surfaces. A central element of this method is the optical measurement of the distance between the surfaces D based on multiple beam interferometry -2. The two surfaces are cylindrically curved (R 20mm) after being glued to cylindrical glass supports and facing each other such that the cylinder axes cross at an angle of 90. One surface is supported by a spring and is approached towards the other using an approach actuator, whose position is known over time. If there is a surface force, this spring deflects, and the surface force F results from the distance measurement with known spring constant k (2364±60 N/m was used in this work). The two transparent mica layers form an optical thin-film interferometer. The rear of each mica layer is coated with a semi-transparent silver mirror, in order to improve the reflectivity. White light is directed through the interferometer and analyzed in an imaging spectrograph. A number of improvements in accuracy, resolution, mechanical drift, thermal stability and imaging has been incorporated into the extended Surface Force Apparatus (esfa), which are all described in detail in the literature 3-4. Thus, the transmitted interference spectrum consists of fringes of equal chromatic order (FECO) that are analyzed by a numerical, fast-spectral correlation algorithm to determine both the distance D between the surfaces at the point of closest approach (PCA) with a precision of better than 30 pm, and the refractive index of the medium. It is important to note that at forces above ~0 mn/m, the flattening of the confined region leads to an overestimation of the normalized forced (F/R) since the radius is assumed to remain constant. The simplified disposition of the optical elements in the esfa minimizes human influence and allows the adjustment of the PCA to better than 2 µm laterally in an automated fashion. Thin mica sheets are prepared by manually cleaving ruby mica of optical quality Grade # (S&J Trading, Inc. NY, USA) in a class-00 laminar-flow cabinet. Uniformly thick (2 5.5 µm) mica sheets of size ~8 mm x 8 mm are cut using surgical scissors, to avoid possible contamination with nanoparticles 5. A 40 nm thick silver film is thermally evaporated onto mica sheets in vacuum (2 0-6 mbar). The silver-coated mica sheets are glued onto cylindrical lenses using resin glue (EPON 004F). The samples are then immediately inserted into the esfa, the fluid cell purged with dry N 2,

2 and the mica thickness determined by means of thin-film interferometry in mica-mica contact. After that, the liquid droplet is placed between the surfaces and the esfa cell is either continuously purged with dry N 2, or equilibrated with ambient air. Optimized geometry of the ILs 7.2 A 2.5 A 4.2 A 7.6 A 2.5 A 3.0A Figure S: Optimized geometry of [EMIM] +, and [TFSI] - calculated by Molecular Mechanics (force field =MMFF94s) with Avogadro software 6. Copolymer dissolution in [EMIM][TFSI] measured by Dynamic Light Scattering 0.3 mg PLL(20)-g[3.5]-PEG(5) copolymer was added into ml [EMIM][TFSI] that was pre-dried in vacuum for 2 days at 50. The solution was then stirred either in a dry nitrogen atmosphere for several days or equilibrated with ambient air at 40%RH before use. Dynamic light scattering (DLS) was performed to investigate the dissolution kinetics and before the solution was used for SFA measurements. As shown in Figure S2, the particle size of the copolymer decreased slowly over time to reach a final size of around 0 nm after 48 hrs. The same kinetics was observed in dry nitrogen atmosphere and at 40% RH. Diameter (nm) %RH (0.2 wt%) dry N Time (hr) Figure S2: Copolymer size in the IL as a function of time. The copolymer was dissolved under a dry nitrogen atmosphere (red circles) and in 40% relative humidity (blue diamonds). 2

3 Nuclear Magnetic Resonance (NMR) experiments To further understand the interplay between water and [EMIM][TFSI] in solvating the PEG chains, H -NMR tests were performed on a PEG(6kDa)-IL solution, dispersed in D 2O. Specifically, 50 µl of a saturated PEG-IL solution was pipetted into 0.7 ml D 2O (Cambridge Isotope Laboratories, Inc. Tewksbury, MA) and taken to a Carver-Bruker 500 CryoProbe NMR spectrometer (operating at a frequency of 500 MHz) in the NMR laboratory at the University of Illinois at Urbana-Champaign. - D H -NMR and 2-D Nuclear Overhauser Effect Spectroscopy (NOESY) were conducted on the sample. A reference sample was prepared by dissolving the same amount of PEG into D 2O. Figure S3 shows the peak of methylene protons on the PEG, measured with and without the presence of [EM- IM][TFSI]. An up-field shift of around 3-4 Hz is found when [EMIM][TFSI] is present; such shift persists after 24 hours and hour sonication. This shift may originate from the proximity-induced displacement of the electrons on H A of PEG, which suggests that the PEG chain may be solvated by [EMIM][TFSI], even when D 2O is present. 2-D NOESY confirmed this hypothesis. Here, a cross peak appeared for H A on the PEG chain and H C on the ethyl group in [EMIM][TFSI]. From these observations, it is inferred that the PEG chains are strongly associated to the [EMIM] + cations, even in excess D 2O, which implies that [EMIM][TFSI] outcompetes water in solvating PEG chains. PEG+[EMIM][TFSI] in D 2 O PEG in D 2 O A Figure S3: H -NMR spectrum of the PEG (6kDa)-[EMIM][TFSI] solution in D 2O (red) and the reference spectrum of PEG (6kDa) in D 2O (blue). The shift at δ = 3.69 ppm represents the methylene proton A on the PEG chains. 3

4 T~25 NOESY A C C A Figure S4: NOESY spectrum of the PEG(6kDa)-[EMIM][TFSI] solution in D 2O. A cross-peak (green circle) is found at the coordinate defined by the proton peaks of H c (ethyl group of [EMIM] + ) and H A (methylene group of PEG), indicating that the two protons are close in space (at a distance smaller than 5 Å), which is in agreement with X-ray and Molecular Dynamics simulations 7. 4

5 Calculation of electrostatic double layer forces We follow the method described in ref. 8 to determine the electrostatic double layer (EDL) forces. The refractive index (n) of the ILs was measured for films with thicknesses ranging from 20 nm to 00 nm with the esfa. We obtained n=.425±0.005 for [EMIM][TFSI]; the influence of the dissolved copolymer is not statistically significant. Relative permittivity of ε ~ 2.3 was assumed for the IL, according to the values given in ref. 9. These parameters lead to an approximate Hamaker constant of J. For similar surfaces, the EDL force is always repulsive, as shown in Figure S5. Fitting the DLVO equation to the measured surface forces at separations >0 nm gives an effective decay length κ!! for [EMIM][TFSI] of 6() nm, and a surface potential of ~-23 (2) mv at 25ºC, in good agreement with the results for [EMIM][TFSI] in ref. 8. The number density of dissociated ions, ρ, is obtained from: κ! = 2ρe!! εε! kt where ε! is the permitivitty in vacuum, k the Boltzmann constant, T the temperature (298.5 K), e c the elementary charge. This yields a dissociation degree of ~ 0.00± wt%, considering that the density of [EMIM][TFSI] is.58 g/cm 3. 5

6 0 a) F/R [mn/m] DL~6 nm IL = 6.6 (0.7) Å b) Count layer size [Å] Figure S5: a) Representative force-separation curves between mica surfaces in vacuum-dry [EM- IM][TFSI] while continuously purging the esfa cell with dry N 2 at 25ºC. Different symbols were used to represent different force-separation curves during approach. Approach and separation were conducted at constant speed of 0.5 nm/s. The line gives the fit to the DLVO theory with a Hamaker constant of J. The pull-off force in dry IL is 0.90 (.23) mn/m (not shown), 6

7 similar to those reported for other imidazolium-based ionic liquids with the same anion 0. The arrows point at film thickness transitions that occur when layers of ions are squeezed out from the confined film. b) Histogram of the thickness of the resolved layers during compression of [EM- IM][TFSI]. Calculation of grafting distance The area per PEG chain was determined according to A!"# = M!"# M!"#"$ /(N! M!"# M!"# ), where M!"# is the adsorbed dry mass per unit area, M!"#"$ is the number molecular weight of the copolymer, M!"# is the number molecular weight of only PEG in the copolymer, and N! is the Avogadro number. The number of chains per unit area is thus given by n!"# = /A!"#. We assume hexagonal packing of the PEG chains to calculate the grafting distance, which yields d! = /n!"# /(.5 3). For PLL(20)-g[3.5]-PEG(5), M!"# =95 kda and M!"#"$ =25 kda. For a dry mass of 268, 89 and 20 ng/cm 2, n!"# is equal to 0.29, 0.0 and 0.04 chains per nm 2 and the grafting distance d 0 is.5,.99 and 4.2 nm, respectively. Representative surface-forces between PEG-bearing mica surfaces in [EMIM][TFSI] in brush configuration 00 a) 2.5 b) F/R [mn/m] 0 F/R [mn/m] Figure S6: a) Representative surface forces between PEG-bearing mica surfaces in [EMIM][TFSI] measured while the esfa cell is continuously purged with dry N 2 at 25ºC, after dry adsorption. The red and blue symbols give loading and unloading curves, respectively. The two diagrams show the same surface forces with the Y-axis in a) log- and b) linear scale. The small adhesion between the two polymer-adsorbed films shown in b) is responsible for the observed hysteresis. Higher 7

8 grafting density reduces adhesion and hysteresis. The different symbols represent data from different force-separation curves. Water uptake kinetics determined by Fourier-Transform Infrared spectroscopy (FTIR) To determine the kinetics of water uptake by the polymer-[emim][tfsi] solution,.6 ml PLL-g- PEG solution was prepared in dryness, as described above, and separated evenly into 8 vials. The vials were then stored in a sealed chamber, in which the relative humidity was maintained at 44% by the vapor pressure of saturated K 2CO 3 solution. After 0, 0.5,,.5, 4.5, 24, and 35 hours, one vial was taken out and the solution was transferred onto a PerkinElmer Frontier FTIR spectrometer (Waltham, MA). An average of 2 scans were taken to generate a spectrum, with a resolution of 2cm -. Figure S7 shows the stacked spectra obtained over this period of time, showing a clear growth of the peak at 3634 cm -, which is indicative of an increase of the water content in the sample. The solution was saturated with water after 4.5 hrs of equilibration. Therefore, it was concluded that during the 5 hours exposure to ambient air in the esfa cell, the water content in the copolymer ionic liquid solution achieves equilibrium. The equilibrium water content was determined gravimetrically to be 0.23 wt% Transimisson [ %] wavenumber cm hr hr 4.5 hr 24 hr 35 hr Figure S7: FTIR spectra of PLL-g-PEG-[EMIM][TFSI] solutions stored at 44% RH for different periods of time. For simplicity, only the peak representing the vibrational mode of O-H stretching (attributed to water) is shown. 8

9 F/R [mn/m] a) Figure S8: a) Representative surface-forces between PEG-bearing mica surfaces in [EMIM][TFSI] measured while the esfa cell was continuous purged with dry N 2 at 25ºC after wet adsorption. The green and purple symbols give loading and unloading curves, respectively. The two diagrams show the same surface forces with the Y-axis in a) log- and b) linear scale. The low grafting density of the polymer increases adhesion and the adhesion between the two polymer-adsorbed films is responsible for the observed hysteresis. The different symbols represent data from different forceseparation curves. F/R [mn/m] b) Scaling theory for neutral brushes The scaling theory has been succesfully used to describe the surface forces between PEG brushes in systems similar to ours (i.e. PLL-g-PEG adsorbed to mica) but in aqueous buffer 2-3. The surface force according to the scaling theory is given by: Eq. (S) F R = 8π 35 2k!T H! d 7 D 2D!! 2H!!! + 5 D 2D! 2H!! 2 for D<2H, where D! is an offset that accounts for the thickness of the PLL backbone ( nm), d! the average grafting distance between PEG chains, and H the brush height. We note that d! is independently obtained from the measured refractive index. Fitting of the measured surface forces to eq. (S) gives a reasonable agreement, as shown in Figure S9. For the force-separation curves shown in Figure S9, we obtain H=40 nm and d! =8 nm. The fitting parameters across measurements on 7 different pairs of mica surfaces, brush length and grafting distance, vary in the range 30 to 45 nm and 6 to 0 nm, respectively. While the brush height cannot be larger than the contour length of PEG [5kDa] (L c~40 nm), the grafting distance was determined to be <4 nm based on the adsorbed dry mass, and hence, both fitting parameters have unrealistic values. We note that previous works have also shown good fits of the surface forces 9

10 between polyelectrolyte brushes to the scaling theory 4, and hence, the results in Figure S9 are not surprising. However, the unrealistic fitting parameters indicate that this theory does not adequately describe the surfaces forces between PEG-brushes in [EMIM][TFSI]. F/R [mn/m] a) scaling theory H=40 nm d 0 ~8 nm F/R [mn/m] 00 0 strong compression b) charged brush DL=6 nm H=29 nm 2-2 Nf/d ~0.7 nm Figure S9: Surface forces measured between PEG-brushes in [EMIM][TFSI] at 25ºC after dry adsorption. The lines give the fit to a) the scaling theory and b) the osmotic model for charged brushes. The different symbols represent data from different force-separation curves. Atomic Force Microscopy (AFM) imaging and force measurements A freshly cleaved piece of mica was mounted onto the air-tight flow cell of a JPK Nanowizard Ultra (JPK Instruments, Berlin, Germany) atomic force microscope. The cell was then purged with dry N 2 gas flow (99.999% purity) for 30 minutes before the PLL-g-PEG solution was introduced via an airtight tube. Before the first measurements, the system was purged with dry N 2 flow for 30 minutes more, and another 30 minutes were given to minimize mechanical and thermal drifts, during which the cell was maintained sealed. The first images were taken with a silicon sharp tip (spring constant=0.35 N/m, nominal radius ~0 nm) one hour after the solution was introduced, first in the quantitative-imaging (QI) mode 5 and then in contact mode. In the QI mode, force measurements are performed on every pixel of a 500 nm by 500 nm area. A single image is taken in ~ 5 hours. The force measurements are internally converted into topography and interfacial stiffness. Immediately after QI imaging, contact mode imaging was performed on the same area with a set point of 4 nn. The system was then purged with a dry N 2 flow for 24 hours longer before the same type of measurements (QI and contact mode imaging of the topography) were performed on different locations of the sample. All measurements were obtained immersed in the IL since removal of the IL would alter the polymer film. 0

11 a) b) 5 c) Force [nn] count Separation [nm] d) e) f) g) Figure S0: Bivariate histogram of the normal forces measured as a function of the separation with

12 an arbitrary zero at the hard wall, obtained after a) hour and b) 30 hours of adsorption of the copolymer on mica. While adsorption might have not been achieved equilibrium after 30 hours, the difference between the force curves in both diagrams is significant. The force-separation curves obtained after 30 hours (b) clearly showed longer-ranged repulsion than a); the steps in the force curves obtained in (a) are reminiscent to those measured in neat IL. The comparison of these results points at the gradual adsorption of the polymer with time. The force curves are highly reproducible during the imaging of the 500nmx500 nm area, indicating the homogeneity of the interface. The sharp tip radius is ~ 0 nm, and hence, mainly sensitive to the steric repulsion. In c) the forceseparations are shown in log-scale to visualize the onset of the repulsion, which is ~7 nm, while the maximum polymer film thickness measured with the esfa ranges from 5-30 nm depending on the grafting and charge density. Since the absolute separation in AFM force measurements is unknown, the measurements might underestimate the true onset of the repulsive force by a few nanometers. The individual force curves in b) correspond to the pixels on the QI images, which are shown in d) and e) (stiffness and topography, respectively). They were obtained after copolymer adsorption for 30 hours in [EMIM][TFSI]. The stiffness decreased from nn/µm to 6nN/µm after polymer adsorption, indicating that the interface became substantially softer. The image of the height in the QI mode shows features on the surface with an average thickness of ~ 0.8 nm ( nm at most), which corresponds mainly to the backbone of the bottle brush since the small tip squeezes the PEG chains during the approach to the surface. This is further supported by f), which shows a contact-mode image of the same area after QI imaging. In contact mode, the tip laterally slides on the surface, pushing away the side chains, and hence, mainly probing the adsorbed polymer backbone on the surface, as well. In contact mode, we cannot exclude that the adsorbed polymer film is modified with the sharp tip, though. Figure g) shows the mica surface immersed in [EMIM][TFSI] when imaged in contact mode, as reference. We thus conclude that polymer adsorption on mica, although slow, leads to a homogeneous polymer film on the surface, which is consistent with the brush conformation inferred from the SFA measurements. References. Israelachvili, J. N.; Tabor, D. The Measurement of Van Der Waals Dispersion Forces in the Range.5 to 30 nm. Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences 972, 33 (584), Tabor, D.; Winterton, R. H. S. The Direct Measurement of Normal and Retarded van der Waals Forces. Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences 969, 32 (5), Heuberger, M. The extended surface forces apparatus. Part I. Fast spectral correlation interferometry. Rev. Sci. Instrum. 200, 72 (3), Heuberger, M.; Vanicek, J.; Zäch, M. The extended surface forces apparatus. II. Precision temperature control. Rev. Sci. Instrum. 200, 72 (9),

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