Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST
Letter
Strong Stretching of Poly(Ethylene Glycol) Brushes Mediated by Ionic Liquid Solvation Mengwei Han, and Rosa M. Espinosa-Marzal J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01451 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on August 1, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry Letters
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-61801 Urbana, USA.
*Corresponding author’s email:
[email protected] KEYWORDS: Surface forces apparatus, polymer brush, ionic liquids, solvation, nanoconfinement, solid-like behavior. ABSTRACT We have measured forces between mica surfaces coated with a poly(ethylene glycol) (PEG) brush
solvated
by
a
vacuum
dry
ionic
liquid,
1-ethyl-3-methyl
imidazolium
bis(trifluoromethylsulfonyl)imide, with a surface forces apparatus. At high grafting density, the solvation mediated by the ionic liquid causes the brush to stretch twice as much as in water. Modeling of the steric repulsion indicates that PEG behaves as a polyelectrolyte; the hydrogen bonding between ethylene glycol and the imidazolium cation seems to effectively charge the polymer brush, which justifies the strong stretching. Importantly, under strong polymer compression, solvation layers are squeezed out at a higher rate than for the neat ionic liquid. We propose that the thermal fluctuations of the PEG chains, larger in the brush than in the mushroom configuration, maintain the fluidity of the ionic liquid under strong compression, in contrast to the solid-like squeezing-out behavior of the neat ionic liquid. This is the first experimental study of the behavior of a polymer brush solvated by an ionic liquid under nanoconfinement.
ACS Paragon Plus Environment
The Journal of Physical Chemistry Letters
TOC GRAPHICS
-
+
-
-
+
-
+
+ +
+
_
+
+
_
+
+
_
_ _
+
+
_
+
+
+
_
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
TOC: SFA measurements show that a poly(ethylene glycol) brush stretches in an ionic liquid twice as much as in water and responds to compression as effectively charged, while polymer fluctuations maintain the fluidity of the ionic liquid at high compression.
MAIN TEXT It is well-known that the fluid-like cushioning layer of well hydrated polymer brushes is protein-resistant1, reduces friction2, and can serve to stabilize colloidal suspensions3-4. The underlying key mechanism relies on a repulsive force of osmotic and steric origin that efficiently separates the two approaching surfaces, even under an applied pressure5. Necessary for this repulsion is a combination of high grafting density and excellent polymer solvation, which causes the chains to stretch vertically away from the surface to avoid overlap with their neighbor chains while maximizing their interactions with the solvent6-7. The polymer thus achieves a height that is greater than its unperturbed diameter in the bulk solution, despite the involved loss of conformational entropy on stretching. Polymer brushes are thus systems of tremendous
2 ACS Paragon Plus Environment
Page 2 of 23
Page 3 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry Letters
technological importance in the fields of lubrication 8, colloidal science 9, and biofouling 10, due to their ability to influence surface forces. Understanding the influence of solvent quality on the steric repulsion between polymer brushes has been the subject of many studies11-14. Although room temperature ionic liquids have been recognized as a new generation of solvents15, research on the molecular interactions between polymers and ionic liquids is still in its infancy
16-18
, with even less attention given to study
surface-adsorbed polymer films19. Originating from their ionic and amphiphilic molecular structure, Coulombic, van der Waals, solvophobic, π- π, and steric interactions as well as hydrogen bonding play pivotal roles in determining polymer-ionic liquid interactions20. Ionic liquids (IL) thus open up an unprecedented possibility of incorporating a variety of interactions in a single room-temperature solvent and the capability to tune them by design, which earns them the reputation as designer solvents21. Here, we present results obtained with one imidazolium-based IL: 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide (abbreviated as [EMIM][TFSI], Iolitec, 99% purity) was dried in vacuo at 50ºC (see molecular structure in Figure S1). The copolymer poly(L-lysine)-graft-poly(ethylene glycol) –abbreviated as PLL-g-PEG– with molecular weights of the PLL backbone and of the PEG chains of 20 kDa and 5 kDa, respectively, and a grafting ratio of 3.5, was purchased from SuSoS AG (Dübendorf, Switzerland). The contour length of PEG (5 kDa) is ~39.5 nm, estimated for a polymerization degree of N= 113 and monomer size a= 0.35 nm. In aqueous solutions, PLL-g-PEG was shown to adsorb on negatively charged surfaces, including mica, via the positively charged PLL backbone, while the PEG chains form brush-like structures22-23. For PLL(20 kDa)-g[3.5]-PEG(5 kDa), the grafting density of PEG chains was determined to be ~0.29±0.02 chains per nm2, which corresponds to a grafting
3 ACS Paragon Plus Environment
The Journal of Physical Chemistry Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
distance between the PEG chains d0 of 1.2±0.10 nm24, and the brush stretched to a height of 15 nm in water25-26. Solutions of the copolymer in vacuum-dry [EMIM][TFSI] were prepared at the concentration of 0.3 mg/ml under a dry N2-atmosphere. The dissolution of the copolymer in [EMIM][TFSI] was studied by Dynamic Light Scattering (Figure S2). Importantly, 2D-NMR showed the spatial proximity of PEG with the imidazolium cation (Figures S3-S4), which is consistent with the hydrogen bonding between the imidazolium cation and the hydroxyl groups of PEG shown in MD simulations27. Moreover, NMR confirmed that the interaction between PEG and the imidazolium cation is still present in the presence of deuterated water, thereby indicating the preferential solvation of PEG by [EMIM][TFSI] compared to water. An extended surface forces apparatus (eSFA)28-29 was used to measure the surface forces between the adsorbed copolymers on opposite mica surfaces. A transmitted white-light interference spectrum was analyzed by fast-spectral-correlation interferometry to determine the gap distance and the refractive index simultaneously. Mica surfaces were prepared according to the usual protocol (see SI for experimental details). A 200 µl-droplet of vacuum-dried IL either neat (as reference) or with the dissolved copolymer was introduced between the mica surfaces using a syringe while continuous purging the eSFA cell with dry N2 to keep the mica surface as dry as possible. The copolymer was allowed to adsorb for 1 day in a dry N2-atmosphere, which we refer to as “dry” adsorption, before force measurements started. Force-distance curves were measured by approaching/separating the surfaces at a constant speed of 0.5 nm/s. During force measurements, the temperature was maintained constant at 25ºC and the solution was kept dry by continuously purging the eSFA cell with dry N2 gas.
4 ACS Paragon Plus Environment
Page 4 of 23
Page 5 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry Letters
Figure S5 shows characteristic force–separation curves between mica surfaces across vacuumdry [EMIM][TFSI]. It consists of an electrical double layer (EDL) force with a decay length of 6(1) nm, corresponding to an effective ion dissociation ~0.010(0.004) % (see details of calculation in SI). At D
