Structural Dependence of Salt-Responsive Polyzwitterionic Brushes

Subscriber access provided by University of Florida | Smathers Libraries

Article

Structural Dependence of Salt-Responsive Polyzwitterionic Brushes with Anti-polyelectrolyte Effect Shengwei Xiao, Yanxian Zhang, Mingxue Shen, Feng Chen, Ping Fan, Mingqiang Zhong, Baiping Ren, Jintao Yang, and Jie Zheng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03667 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 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.

Langmuir 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 27 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

Langmuir

Structural Dependence of Salt-Responsive Polyzwitterionic Brushes with Anti-polyelectrolyte Effect Shengwei Xiao†√, Yanxian Zhang¶√, Mingxue Shen‡, Feng Chen†, Ping Fan†, Mingqiang Zhong†, Baiping Ren¶, Jintao Yang†*, and Jie Zheng¶* †

College of Materials Science& Engineering

Zhejiang University of Technology, Hangzhou 310014, P. R. China ‡

School of Mechanical Engineering

Zhejiang University of Technology Hangzhou 310014, P. R. China ¶

Department of Chemical and Biomolecular Engineering The University of Akron, Akron, Ohio 44325, USA

√ These authors contributed equally to this work. *Corresponding Author: JY: [email protected] and JZ: [email protected]

Graphic Abstract

1

ACS Paragon Plus Environment

Langmuir 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

Page 2 of 27

ABSTRACT Some polyzwitterionic brushes exhibit strong “anti-polyelectrolyte effect” and ionic specificity that make them as versatile platforms to build smart surfaces for many applications. However, the structure-property relationship of zwitterionic polymer brushes still remains to be elucidated. Herein, we aim to study the structural-dependent relationship between different zwitterionic polymers and “anti-polyelectrolyte effect”. To this end, a series of polyzwitterionic brushes with different cationic moieties cationic moieties (e.g. imidazolium, ammonium, and pyridinium) in their monomeric units and with different carbon space lengths (e.g. CSLs=1, 3, and 4) between cation and anion were designed and synthesized to form polymer brushes via the surface-initiated atom transfer radical polymerization (SI-ATRP). All zwitterionic brushes were carefully characterized for their surface morphologies, compositions, wettability, and film thicknesses by atomic force microscopy (AFM), contact angle, and ellipsometer, respectively. Salt-responsive of all zwitterionic brushes on surface hydration and friction were further examined and compared both in water and in salt solutions with different salt concentrations and counterion types. Collective data showed that zwitterionic brushes with different cationic moieties and shorter CSLs in salt solution induced higher surface friction and lower surface hydration than those in water, exhibiting strong “anti-polyelectrolyte effect” salt-responsive behaviors. By tuning CSLs, cationic moieties, salt concentrations and types, surface wettability can be changed from highly hydrophobic surface (~60°) to highly hydrophilic surface (~9°), while interfacial frication can be changed from ultrahigh friction (µ ∼4.5) to superior lubrication (µ ∼ 10–3). This works important structural insights into how subtle structural changes in zwitterionic polymers can yield great changes in salt-responsive properties at interface, which could be used for the development of smart surfaces for different applications.

2


Page 3 of 27 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

Langmuir

1. INTRODUCTION Zwitterionic polymers, such as poly(carboxybetaine), poly(sulfobetaine methacrylate), and poly(2-(methacryloyloxy)-ethyl phosphorylcholine), contain the equal number of cationic and anionic groups on the same polymer chains to maintain an overall neutral state. Immobilization of zwitterionic polymers on different surfaces have demonstrated their ideal biocompatibility and antifouling capability for many biomedical and bioengineering applications such as bio-implants, biosensors, and immunoassays1-4. Zwitterionic polymer coatings, similar to hydrophilic polymer coatings, always possess high surface hydration. But, different from hydrophilic polymer coatings that achieve surface hydration via hydrogen bonding of polymers with water molecules, zwitterionic polymer coatings can bind water molecules even stronger via electrostatically induced hydration. More importantly, due to the presence of both positively and negatively charges, zwitterionic polymers displayed an unique salt-responsive behavior 5, in which polymer chains adopt the collapsed conformations in water, but the stretched conformations in salt solutions. Since this behavior in response to salt is completely opposite to that of polyelectrolyte, it is called as “anti-polyelectrolyte effect”. In addition, such salt-responsive behaviors of zwitterionic polymers highly depends on salt concentrations and ion types6-7, enabling many on/off applications for protein transport7-8, membranes separation9-11, self-cleaning surfaces12-14, controlled release15-16, and lubricant surfaces17-22. Due to the anti-polyelectrolyte effect, zwitterionic polymers have the less solubility in pure water than in salt solution. The poor solubility of zwitterionic polymers in pure water is attributed to the strong attractive electrostatic interactions (including charge-charge, charge-dipole, and dipole-dipole interactions) between zwitterionic polymer chains, which cause the collapse of polymer chains. In salt solution, the addition of counter ions will screen out such attractive

3


Langmuir 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

Page 4 of 27

electrostatic interactions, increasing the solubilization of zwitterionic polymers in salt solution. Moreover, since every zwitterionic group forms a dipole monument, zwitterionic chains adopt the stretching conformation that will further minimizes net dipole moments and electrostatic energy to increase their solubility23-24. From an intuitive viewpoint of structural design, the variations of cationic and anionic pairs (ammonium, imidazolium, and pyridinium corresponding to carboxylate, sulfonate, and phosphate)25-27 and carbon spacer lengths (CSLs)2, 28-30 between the pairs enable to not only design different zwitterionic polymers31-32, but also tune and optimize inter- and intra-molecular interactions among polymers, salts, and solvents. In principle, any change in these interactions will lead to different chain conformations and surface properties. However, when zwitterionic polymers are immobilized onto the surfaces to form polymer brushes, it still remains elusive how the bulk compositional and structural variations of zwitterionic polymers would affect their interfacial conformations and ion specificity via the anti-polyelectrolyte effect. In our previous study33-34, we developed zwitterionic polymer brushes formed by poly(3-(1-(4-vinylbenzyl)-1H-benzo[d]imidazol-3-ium-3-yl) propane-1-sulfonate) (polyVBIPS) via the surface-initiated atom transfer radical polymerization (SI-ATRP). Different from other zwitterionic polymers, we combined a hydrophobic vinylbenzyl group with zwitterionic imidazolium-sulfonate group to increase both salt-responsive sensitivity and anti-polyelectrolyte effect35. As a result, polyVBIPS brushes enable to achieve reversible surface properties between friction and lubrication and between protein adsorption and resistance33-34 in a wide range of salt concentrations (0-6.1 M) and salt types (SO42−, Cl−, NO3−, Br−, K+, Na+, Mg2+, and Ca2+). Motivated by our previous works33,34,35, herein we designed and synthesized a series of zwitterionic polymers including polyVBIPS, poly(3-(dimethyl(4-vinylbenzyl) ammonio) propyl sulfonate) (polyDVBAPS), and poly(4-(2-sulfoethyl)-1-(4-vinyl-benzyl) pyridinium betain

4


Page 5 of 27 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

Langmuir

(polySVBP)

with

different

cationic

moieties,

as

well

as

poly(3-(dimethyl(4-vinylbenzyl)ammonio)

methyl

sulfonate)

(polyDVBAMS)

(CSLs=1),

poly(3-(dimethyl(4-vinylbenzyl)ammonio)

propyl

sulfonate)

(polyDVBAPS)

(CSLs=3),

poly(3-(dimethyl(4-vinylbenzyl)ammonio) butyl sulfonate) (polyDVBABS) (CSLs=4) with different CSLs, and grafted them onto the silicon surface to form polymer brushes via the SI-ATRP method25, 27, 36. These zwitterionic polymers differ in zwitterionic pairs, i.e. different cationic groups (ammonium, imidazolium, and pyridinium) and CSLs (C1, C3, and C4), while still remaining the same anionic group of sulfonate and polymer backbones. Specifically, vinylbenzyl zwitterionic monomer contains one, three, and four methylene spacer groups between ammonium and sulfonate. We hypothesize that the combination of different cationic groups and CSLs between zwitterionic pairs would affect their interactions with themselves, nearby water molecules, and counter ions, and different CSLs would also change the hydrophilicity/hyrophobicity ratio and flexibility of the polymers. So, the ion- and CSLs-induced (i.e. compositional and structural-induced)

combination

effects

are

expected

to

alter

their

salt-responsive,

anti-polyelectrolyte properties on surface. To test our hypothesis, we systematically studied the surface hydration and surface friction/lubrication of different zwitterionic polymer brushes in water and in salt solutions with different salt concentrations and counterion types. As a result, strong dependence of salt-responsive surface friction/lubrication properties on intrinsic zwitterionic groups (cationic groups and CSLs) and extrinsic salt conditions (salt concentrations and ion types) was observed, and accordingly the salt-induced “anti-polyelectrolyte effect” of polymer brushes was also demonstrated. Very few work has been performed to investigate the salt-responsive “anti-polyelectrolyte effect” of zwitterionic polymers, thus this work provides a fundamental understanding of the relationship between the composition/structure of zwitterionic

5


Langmuir 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

Page 6 of 27

polymers and their salt-responsive “anti-polyelectrolyte effect”, which hopefully helps to design new smart polymer surfaces. 2. MATERIALS AND METHODS Materials. 4-vinlybenzyl chloride (90%), imidazole (99%), dimethylamine solution (40 wt.% in H2O), 1,3-propanesultone (98%), 4-Pyridineethanesulfoni cacid (99%), copper(I) bromide (CuBr) (98%), 2,2,2-Trifluoroethanol, and phosphate buffer saline (PBS, pH 7.4, 0.15M, 138mM NaCl, 2.7mM KCl), were purchased from Sigma-Aldrich (Shanghai), Co. and used as received. Tris[2-(dimethylamino)ethyl]amine (Me6TREN, 99%) was purchased from J&K Chemical Ltd. (Beijing, China). ATRP initiator for gold surface, ω-mercaptoundecyl bromoisobutyrate, was kindly provided by Professor Shaoyi Jiang at the University of Washington. The initiator for grafting polymer brushes on silica wafer and SiO2 prisms, 3-(2-bromoisobutyramido) propyl(trimethoxy)silane was purchased from Gelest, Inc.(Morrisville, PA). Water used in these experiments was purified by a Millipore water purification system with a maximum resistivity of 18.0 MΩ cm. All other reagents and solvents were commercially obtained at extra-pure grade and were used as received. SPR glass chips were coated with an adhesion-promoting chromium layer (2 nm in thickness) and a plasmon active gold layer (48 nm) by electron beam evaporation under vacuum. Synthesis of Zwitterionic Monomers. Three monomers with different cationic moieties in zwitterionic unit, VBIPS, DVBAPS, and SVBP were synthesized and purified using the previously method published25-27, 37 (Scheme 1a). The 1H-NMR spectrum (in D2O) was used to confirm structure of three monomers (Figure S1). 3-(1-(4-vinylbenzyl)-1H-benzo[d]imidazol-3-ium-3-yl) propane-1-sulfonate (VBIPS). Sodium hydrogen carbonate (10.5 g, 124.8 mmol) dissolved in 200 mL of a binary mixture of

6


Page 7 of 27 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

Langmuir

water/acetone (v/v = 1:1) was added into a 500 ml three-neck round-bottomed flask equipped with a reflux condenser, a thermometer, and a magnetic stirring bar. Imidazole (27.22 g, 0.398 mol) and hydroquinone (0.1 g, 0.908 mmol) were added, and the mixture was agitated for ~30 min under ambient temperature. 4-vinlybenzyl chloride (15.22 g, 99.6 mmol) was added to the flask in a drop-wise manner, and the solution was heated to 50 °C and maintained at this temperature for 20 h to complete the reaction. Upon reaction completion, the solution was cooled to room temperature and filtered to remove salt. Acetone in the filtrate was removed through distillation, and the remaining solution was diluted in 500 mL of ether and washed with 50 mL of deionized water for six times to remove unreacted imidazole. The product was back-extracted from the solution by adding 100 mL of 2 M aqueous hydrochloric acid solution. An opaque heterogeneous solution was obtained when 200 mL of 4 M aqueous sodium hydroxide solution was added to the hydrochloric acid solution. The product was recovered with 50 mL of ether for three times and dried with anhydrous magnesium sulfate. A transparent oily liquid sample was obtained when ether was distilled. The transparent oily liquid (8 g, 43.475 mmol) was dissolved in 180 mL of dry acetonitrile, and the solution was then added with hydroquinone (0.02 g, 0.182 mmol) and 1,3-propanesultone (5.312 g, 43.47 mmol). The reaction was carried out at 50 °C under stirring for 48 h. A white precipitate produced from the ring-opening reaction was recovered through filtration. The precipitate was then dried in a vacuum oven at 40 °C and stored at 2 °C to 4 °C. 3-(dimethyl (4-vinylbenzyl) ammonio) propyl sulfonate (DVBAPS). Potassium carbonate (55.3 g, 0.4 mol) dissolved in flask 200 ml of anhydrous ethanol equipped with a thermometer, a dropping funnel, and magnetic stirring. Then, a nitrogen inlet and 4-Vinlybenzyl chloride (30.5 g, 0.2 mol) and dimethylamine solution (20 ml, 0.9 g/ml) were added to the flask in a drop-wise manner, and the reaction mixture was heated to 50 °C and stirred for 24 h. Subsequently,

7


Langmuir 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

Page 8 of 27

evaporated the solvent and purified by column chromatography using petroleum ether as solvent, and

distilled

under

vacuum

to

get

a

light

yellow

transparent

oily

liquid

of

N,N-dimethylvinylbenzylamine. Then, N,N-dimethylvinylbenzylamine (3.22 g, 20 mmol) and 1,3-propanesultone (2.44 g, 20 mmol) were added and dissolved in 100 ml dry acetonitrile in 250 ml flask, the reaction was carried out at 50 °C under stirring for 48 h. A white precipitate produced was obtained and recovered through filtration. The precipitate was then dried in a vacuum oven at 40 °C and stored at 2 °C to 4 °C. 4-(2-Sulfoethyl)-1-(4-vinyl-benzyl) pyridinium betain (SVBP). 4-Pyridineethanesulfonicacid (18.72 g, 0.1 mol) and Sodium hydroxide (4.0 g, 0.1 mol) were dissolved in 150 ml of Formamid solutions with magnetic stirring under ambient temperature. Then, a nitrogen inlet and 4-Vinlybenzyl chloride (15.26 g, 0.1 mol) was added to the flask in a drop-wise manner, the reaction was carried out at ambient temperature and stirred for 72 h. The reaction solutions were precipitated in acetone and recovered through filtration, the crude products were recrystallized with anhydrous ethanol. Three monomers with varying CSLs, i.e., DVBAMS (CSLs=1), DVBAPS (CSLs=3), and DVBABS (CSLs=4) were synthesized and purified using previously published methods25,

38

(Scheme 1b). The 1H-NMR spectrum (in D2O) was used to confirm structure of three monomers (Figure S2). 3-(dimethyl

(4-vinylbenzyl)

ammonio)

methyl

sulfonate

(DVBAMS).

Sodium

hydoxymethanesulfonate (2.6 g, 20 mmol) was dissolved in 10 ml of water under 273.15 K, then 2.5 g of dimethylamine solutions (75 vol%) was added and kept reaction at ambient temperature for 72 h. Water and excess dimethylamine were removed to get dimethylamino methanesulfonate precipitate with light yellow colour. Dimethylamino methanesulfonate (3.22 g, 20 mmol) was

8


Page 9 of 27 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

Langmuir

dissolved in 50 ml ethanol (with 50wt% water), and dropped into 20 ml of ethnaol with amount of 4-vinlybenzyl chloride (2.82 ml, 20 mmol) slowly under 273.15 K. The reaction was heated to 45 °C and maintained 24 h. Light yellow precipitate was obtained after solvent removing, and washed with acetonitrile three times. 3-(dimethyl(4-vinylbenzyl)ammonio) butyl sulfonate (DVBABS). The synthesis of DVBABS is similar to that of DVBAPS. In brief, 4-Vinlybenzyl chloride (30.5 g, 0.2 mol) and dimethylamine solution (20 ml, 0.9 g/ml) were added to 200 ml of anhydrous ethanol under nitrogen, a light yellow transparent oily liquid of N,N-dimethylvinylbenzylamine was obtained after reaction with 50 °C and 24 h. Subsequently, N,N-dimethylvinylbenzylamine (3.22 g, 20 mmol) and 1,4-butanesultone (2.72 g, 20 mmol) were added and dissolved in 100 ml dry acetonitrile, the reaction was carried out at 50 °C under stirring for 48 h. A white precipitate produced was obtained and recovered through filtration. The precipitate was then dried in a vacuum oven at 40 °C and stored at 2 °C to 4 °C. Preparation of Polymer Brushes via SI-ATRP Method. Single crystal silicon wafers (20 mm × 20 mm) were placed into a fresh piranha solution (H2SO4: H2O2 = 3:1) at 120°C for ~0.5 h. Then repeatedly washed with deionized water and dried using a nitrogen stream. The wafer and prisms were subsequently treated with plasma (CORONA Lab. CTP-2000, Nanjing, China) for ~2 min to enhance hydrophilicity. The cleaned silica wafer and prisms were immediately immersed into 1 mM dehydrate toluene solution containing 3-(trimethoxysilypropyl)-2-bromo-2-methypropionate for 12 h at room temperature. The initiator-grafted surface was sequentially washed with toluene, ethanol, and water, then dried under a stream of nitrogen before use. The method of zwitterionic polymer brushes were synthesized through SI-ATRP published previously (Scheme 1c). Briefly, the monomer (1.96 mmol) and Me6TREN (0.14 mmol) were

9


Langmuir 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

Page 10 of 27

dissolved in water (2.5 mL), and the mixture was degassed by flowing a stream of nitrogen for 20 min. 2,2,2-Trifluoroethanol was also degassed in the same manner. CuBr (15.7 mg, 0.11 mmol) and the initiator-coated self-assembled monolayer (SAM) surface were placed in a reaction tube. The tube was immediately evacuated and back-filled with nitrogen for three times to remove oxygen. The degassed 2,2,2-trifluoroethanol (2.5 mL) and water solution containing the monomer and ligand were added to the reaction tube using a syringe under nitrogen protection. The tube was then subjected to two evacuation–nitrogen purging cycles and completely sealed for SI-ATRP polymerization at room temperature. Different film thicknesses of polymer brushes would be obtained by different reaction times. After the reaction, the solution was exposed to air to terminate the reaction. The silica wafer was removed, washed with 2,2,2-trifluoroethanol and saturated NaCl solution, and dried through nitrogen blowing.

Scheme 1. Synthesis procedures for (a) VBIPS, DVBAPS, and SVBP monomers and (b) DVBAMS, DVBAPS, and DVBABS monomers. (c) Schematic of the fabrication of polyzwitterionic brushes on a silica wafer using the SI-ATRP method. Polymer Thickness by Ellipsometry. The dry film thickness of polymer brushes were measured by Ellipsometric measurements performed on an α-SE ellipsometer (J.A. Woollam Co., Lincoln, 10


Page 11 of 27 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

Langmuir

NE) with a He-Ne laser (λ) 632.8 nm and a fixed angle of incidence of 70o. Bare silicon wafer was used to determine the thickness of SiO2 layer. Three different locations of each polymer brushes specimen were chose and took an average result. The in-situ film thickness of each polymer brushes were measured by a Variable Angle Spectroscopic Ellipsometer (J.A. Woollam Co., VASE M-2000) with the angle of incidence of 75o. The polymer surfaces were immersed by ~5 mL PBS solutions and ~5 mL salt solutions respectively, and measured each cycle in-situ film thickness for 10 min. Surface morphology and roughness by AFM. Surface morphology and roughness of polymer brushes were conducted on a Veeco multimode III AFM (Veeco Instruments Inc., USA) in tapping mode. A cantilever spring constant of 50 N/m was employed and driven at a vertical oscillation frequency of 170-180 kHz. All the measurements are carried out in ambient conditions. Contact angle measurement. Water or salt solution contact angle of polymer brushes surfaces were recorded with OCA 15EC Video-based Optical Contact Angle Measuring System (Eastern-Dataphy Instruments Co., Ltd., Beijing), equipped with a sessile drop shape analysis system and a video camera. A 4 µL droplet of water or salt solution was dropped on the dry surface by micro syringe. The data presented were averaged by five independent measurements on different positions. Friction coefficient measured through macroscopic friction test. The lubrication performance of three polyzwitterionic surfaces in different salt concentrations or counterion types was evaluated using the previously method published33, 39. Silicon wafer coated with zwitterionic polymer brushes were conducted on a Universal Micro-Tribometer (UMT-2, CETR), using an elastomeric poly-(dimethylsiloxane) (PDMS) hemisphere with a diameter of 6 mm as friction pin. The distance of a sliding cycle, sliding velocity, and loading of friction pin exerted on Silicon

11


Langmuir 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

Page 12 of 27

wafer surface were set as 10 mm, 2 × 10−3 m/s, and 0.5 N (Hertzian contact pressure = 0.23 MPa). Friction coefficient vs time plot was obtained and three independent measurements were carried on different positions. Each measurement included 20 sliding cycles, and 60 data points were used to obtain average value and standard deviation. 3. RESULTS AND DISCUSSION 3.1. Synthesis and characterization of polyzwitterionic brushes Polyzwitterionic brushes with different cations (i.e. polyVBIPS, polyDVBAPS, and polySVBP contain imidazolium, ammonium, and pyridinium, respectively) and CSLs (i.e. polyDVBAMS, polyDVBAPS, polyDVBABS contain one, three, and four methylene spacer groups between zwitterionic pairs) were prepared via the SI-ATRP method. To make a fair comparison of different polymer brushes, we first controlled the thickness of brushes by tuning polymerization time. Figure 1a shows the growth rate of polymer brushes as a function of SI-ATRP polymerization time for polyVBIPS, polyDVBAPS, and polySVBP. Clearly, all brushes showed a linear increase relationship between film thicknesses and SI-ATRP polymerization time. Specifically, polyVBIPS, polyDVBAPS, and polySVBP brushes can grow from 8.61 nm to 124.95 nm, from 15.22 nm to 130.08 nm, and from 8.91 nm to 65.45 nm in 48 hours, respectively. Different slopes indicate that different zwitterionic polymers have different surface activities and growth rates. The growth rates of film thickness for three polymer brushes were in a decrease order of 0.826 for polySVBP, 2.702 for polyVBIPS, and 3.474 for polyDVBAPS, respectively. To examine the CLSs effect on the growth of film thickness, we grew polyDVBAMS (CSLs=1), polyDVBAPS (CSLs =3), and polyDVBABS (CSLs=4) brushes at a constant SI-ATRP time of 24 h. It can be seen in Figure 1b that upon 24 h polymerization, the film thickness was 25.78 nm for polyDVBAMS brush, 68.21 nm for polyDVBAPS brush, and 86.35 nm for

12


Page 13 of 27 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

Langmuir

polyDVBABS brush, respectively. It appears that the longer CSLs enable the more active polymerization and the faster grow rate for achieving the longer brushes.

Figure 1. Film thickness of (a) polyVBIPS (imidazolium), polyDVBAPS (ammonium), and polySVBP (pyridinium) brushes with different cationic moieties as a function of SI-ATRP polymerization time and (b) polyDVBAMS (CSLs=1), polyDVBAPS (CSLs=3), and polyDVBABS (CSLs=4) brushes with different CSLs at a constant SI-ATRP time of 24 h. The microscopic morphology of six different polyzwitterionic brushes were imaged by the tapping-mode AFM, and surface roughness of the brushes was represented by root-mean-square (rms) value (Figure 2). Upon 24 h polymerization, polySBVP brushes (pyridinium) showed a relatively rough surface with a surface roughness of ~1.1 nm, while both polyVBIPS (imidazolium) and polyDVBAPS (ammonium) showed very smooth surfaces with surface roughness of 0.6-0.8 nm. As compared to ammonium and imidazolium cation types, pyridinium not only decreased the growth rate of polySVBP brush, but also slightly increased its surface roughness. In the case of CSL effect, polyDVBAMS brush with CSLs=1 (rms=1.7) showed a much higher surface roughness than polyDVBAPS brush with CSLs=3 (rms=0.4 nm) and polyDVBABS brush with CSLs=4 (rms=0.8 nm), indicating that the shorter CSLs lead to the higher surface roughness of polymer brushes. This phenomenon became more pronounced for polyDVBAMS brushes. For instance, many small clusters almost fully covered the whole surface of polyDVBABS brushes, 13


Langmuir 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

Page 14 of 27

and these clusters are likely aggregates of polymer chains caused by intra-/inter-chain attraction, which cause high surface roughness. Similar cluster-like aggregates were also observed for chemically-crosslinked brushes in hydrogel, where the cluster morphologies were caused by chain entanglement and interpenetration in hydrogel network. Taken together, both cation types and CSLs demonstrate their influence on the growth kinetics and surface morphologies of polyzwitterionic brushes.

Figure 2. Tapping-mode AFM images for (a) polyVBIPS (imidazolium) of 43.6 nm, (b) polySVBP (pyridinium) of 45.2 nm, and (c) polyDVBAPS (ammonium) of 46.3 nm with different cationic moieties and for (d) polyDVBAMS (CSLs=1) of 25.3 nm, (e) polyDVBAPS (CSLs=3) of 31.1 nm, and (f) polyDVBABS (CSLs=4) of 34.5 nm with different CSLs. 3.2. Salt-induced surface hydration of polyzwitterionic brushes Since anti-polyelectrolyte property of polyzwitterionic polymers is generally depended on the composition and structure of zwitterionic groups40-41, we examined six different polyzwitterionic brushes with different cationic moieties and CSLs for their salt-responsive 14


Page 15 of 27 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

Langmuir

anti-polyelectrolyte properties (surface hydration and surface friction) in both water and salt solutions. Since surface hydration of polymer brushes is critical for many interfacial properties such as antifouling capacity, friction force, and antibacterial property. Herein, we first evaluated the salt-responsive surface hydration of different cationic brushes using the contact angle test. All brushes were prepared by the SI-ATRP to achieve similar film thickness of ~45 nm. Figure 3 and Table S1 show the contact angles of water and salt solutions on the polyzwitterionic brushes with different cationic moieties. In Figure 3a, as a control, the contact angle of water was 62.8o on polyVBIPS brushes, 35.4o on polySVBP brushes, and 27.1o on polyDVBAPS brushes, respectively. PolyDVBAPS brushes had much smaller contact angle than polySVBP and polyVBIPS brushes, indicating the presence of ammonium groups in polyDVBAPS possess the higher hydration capacity than pyridinium groups in polySVBP and imidazolium groups in polyVBIPS. Differently, when switching pure water to salt solutions, all zwitterionic polymer brushes in salt solutions showed a large decrease of contact angles from 53.5o to 42.8o for polyVBIPS brushes, from 29.3o to 24.5o for polySVBP brushes, and from 24.6o to 14.8o for polyDVBAPS brushes, respectively. Such decrease in contact angle became more pronounced as salt concentrations increased from 0.05 M to 0.53 M NaCl. Further increase of NaCl to 6.1 M did not significantly change the contact angle of salt solution on all polymer brushes. Since 0.53 M NaCl is close to the salt concentration of sea water, these salt-responsive zwitterionic polymer brushes may have some potential for the applications related to marine environment. The smaller contact angles of polyzwitterionic brush in salt solution than that in water could be interpreted by the “anti-polyelectrolyte effect”. As shown in Figure 3a, in water solution the intra/inter chain associations between zwitterionic groups reduce polymer-water interaction, leading to a higher contact angle. However, in salt solution, since counterions could adsorb on and penetrate into

15


Langmuir 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

Page 16 of 27

polymer brushes, they will screen out electrostatic interactions between zwitterionic polymer chains and induce polymer chains to adopt the more extended conformations. Both effects in turn promote polymer-water interactions at the interface, leading to the smaller contact angles and the higher surface hydration. We further examined the effect of different anions (i.e. SO42−, Cl−, NO3−, and Br−) and cations (i.e. K+, Na+, Mg2+, and Ca2+) on the surface hydration of three polyVBIPS, polySVBP, and polyDVBAPS brushes. In these tests, all salt concentrations of different counterions were fixed at 1.0 M. In Figure 3b, anion tests showed that (i) in any salt solution with different anions, polyVBIPS brushes (~27°-55°) always exhibited higher contact angles than polySVBP brushes (~22°-50°) and polyDVBAPS brushes (~10°-40°). (ii) all three polyzwitterionic brushes in salt solution with SO42− (~40°-55°) had the higher contact angles of those in salt solutions with Cl− (~15°-40°), NO3− (~10°-27°), and Br− (~10°-27°). The decreased order of contact angles in SO42−>Cl−>NO3−≈Br− salt solutions is largely consistent with the hydration capability order of the Hofmeister effect (SO42−>Cl−> Br−>NO3−)42. In case of cation effects, Figure 3c shows the contact angles of polyVBIPS, polySVBP, and polyDVBAPS of ~30 nm in 1.0 M KCl, NaCl, MgCl2, and CaCl2 solutions. Different from the anion-induced large changes in contact angle for a given polymer brush, the same polymer brush displayed similar contact angle in different cation solutions, indicating that the surface wettability of these zwitterionic brushes are more sensitive to anions than to cations. Consistent with the anion effect, regardless in any salt solution containing different cations, three different brushes exhibited a decrease order of contact angles from polyVBIPS brushes (~40°-45°), polySVBP brushes (~25°-27°) to polyDVBAPS brushes (~17°). Furthermore, it appeared that monovalent cations (Na+ and K+) induced the higher contact angles on all three polymer brushes than divalent cations (Ca2+ and Mg2+), indicating that the divalent

16


Page 17 of 27 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

Langmuir

cations bind more strongly to sulfonate than monovalent cations43-45. Ammonium, pyridinium and imidazolium are well-known chaotropic cations with low hydration capability, while SO42− is a typical kosmotropic anion with strong hydration ability. Cl−, Br−, and NO3− anions are in between chaotropes and kosmotropes, and accordingly the hydration capacity of these three anions is in a decrease order of Cl−> Br−>NO3−. The Collins theory argues that only when a pair of ions has similar hydration capacity, they would strongly interact with each other to form a strong ionic bond. This implies that Br− and NO3− would have stronger interactions with ammonium, pyridinium, and imidazolium than other pairs of ions, as confirmed by the lower contact angles on all polymer brushes (Figure 3b-c). This phenomenon is likely attributed to the fact that the strong pairwise ionic interactions will promote ionic solvation, but weaken their interaction with polymer chains.

Figure 3. Contact angles of (a) NaCl solutions with different concentrations (0-6.1 M) (right: schematic of changes of polymer chains conformation before and after adding salt) and different 17


Langmuir 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

Page 18 of 27

salt solutions (1.0 M) containing (b) SO42−, Cl−, NO3−, or Br− and (c) K+, Na+, Mg2+, or Ca2+ on three polyzwitterionic brushes. 3.3. CSLs-induced surface hydration of polyzwitterionic brushes From a structural point of view, the change of CSLs from methylene to butylidene is expected to affect the hydrophobic:hydrophilic ratio, flexibility, and conformation of polymer chains. Herein, we examined the CSLs effect on the surface hydration of three polyDVBAMS (CSLs=1), polyDVBAPS (CSLs=3), and polyDVBABS (CSLs=4) brushes in both water and salt solutions of different concentrations and couterions. The three polymer brushes had similar film thickness of ~30 nm. The contact angles of water and salt solutions on these brushes are shown and summarized in Figure 4 and Table S2. As a control, polyDVBAMS (CSL=1), polyDVBAPS (CSL=3), and polyDVBABS (CSL=4) brushes in water displayed contact angles of ~45°, ~20°, and ~22°, respectively. When switching to NaCl solutions, the contact angles of NaCl on polyDVBAPS and polyDVBABS brushes decreased as salt concentrations, indicating the enhanced surface wettability. Different from polyDVBAPS and polyDVBABS brushes, polyDVBAMS brush showed constant or even higher contact angles almost independent of NaCl concentration. From the viewpoint of molecular structure, polyDVBAMS chains with short CSL=1 are more rigid and compact, leading to strong chain associations and high packing structure between them so that steric repulsion will prevent the adsorption and penetration of counterions on/into polymer chains. In the cases of polyDVBAPS and polyDVBABS chains with longer CSL (3 or 4), polymer chains become more flexible to interact with counter ions, leading to the decrease of polymer-polymer interactions but the increase of polymer-water interactions, thus the smaller contact angles and the higher surface hydration. Figure 4b-c show the contact angles of salt solutions with different counterions on the

18


Page 19 of 27 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

Langmuir

surfaces of zwitterionic polymers with different CSLs. First, regardless of counterion types, the contact angle generally decreased with the increase of CSLs, i.e. polyDVBAMS (CSLs=1) showed the high contact angles of ~45°-60°, polyDVBABS (CSLs=4) showed the low contact angles of ~9°-20°, and polyDVBAPS (CSLs=3) in between. In Figure 4b, the anion-induced contact angle changes in different polymer brushes showed a consistent anion-induced trend, i.e. contact angles decreased when polymer brushes were presented in the salt

solution containing

SO42−>Cl−>NO3−>Br−, indicating that salt solution with SO42− induces the lower surface hydration of polymer brushes almost independent of CSLs. Interestingly, salt solutions containing NO3- and Br- ions showed the reduced contact angles on polyDVBAMS, likely because their interactions with zwitterionic groups are strong enough to conquer inter/intra chain associations. In the case of cations (Figure 4c), the three polymer brushes exhibited a decrease transition of contact angle in salt solutions containing Ca2+ > Mg2+>Na+>K+. But, the change of cation types appears to have very little or no effect on the contact angle of polymer brushes. For zwitterionic polymer brushes, the origin of the difference in contact angles is believed to lie in the difference in ion-pairing between the quaternary ammonium groups (QA+) in the polymer and the two redox states of the counterions. Clearly, our data showed that the less hydrated, large, and highly polarizable species of SO42− interact very strongly with the QA+ groups, leading to hydrophobic-induced chain collapse due to the loss of water and conformational change of polymer chains46.

19


Langmuir 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

Page 20 of 27

Figure 4. Contact angles of (a) NaCl solutions with different concentrations (0-6.1 M) (right: schematic of changes in polymer chains conformation when switching water to salt solution) and different salt solutions (1.0 M) containing (b) SO42−, Cl−, NO3−, or Br− and (c) K+, Na+, Mg2+, or Ca2+ on the polyzwitterionic brushes with different CSLs=1, 3, and 4. 3.4. Salt-induced friction change of polyzwitterionic brushes Since all polymer brushes exhibited different extents of salt-induced surface hydration, it expects that interfacial friction should be related to surface hydration. Here, in situ friction tests were performed on different zwitterionic polymer brushes using elastomeric PDMS hemispheres of a diameter of 6 mm as a pin against the tested polymer surface. Figure 5 and Table S3 summarize the interfacial friction coefficients of polyVBIPS (imidazolium), polySVBP (pyridinium), and polyDVBAPS (ammonium) in water, in NaCl solutions of different concentrations (0.05-6.1 M), and in different salt solutions. As shown in Figure 5a, in water, all three polymer brushes showed high friction coefficients of 2.51-3.82. When these polymer brushes 20


Page 21 of 27 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

Langmuir

were immersed in 0.05 M NaCl solution, the friction coefficient was reduced to 2.23 for polyVBIPS brushes, 1.36 for polySVBP brushes, and 1.43 for polyDVBAPS brushes, respectively. Further increase of NaCl concentrations above 0.53 M led the interfacial friction to be signfidicantly reduced to a level of ~10-2. Increased salt concentrations promote polymer-water interactions at the interface, thus leading to the smaller contact angles, the higher surface hydration, and lower interfacial friction. Figure 5b shows the friction coefficients of polymer brushes in salt solutions containing different anions. It can be seen that friction coefficients in salt solutions containing SO42−, Cl−, NO3−, and Br− were ~3.6, ~0.05, ~0.03, and ~0.02 for polyVBIPS brushes, ~4.5, ~0.09, ~0.06, and ~0.05 for polySVBP brushes, and ~3.2, ~0.04, ~0.02, and ~0.01 for polyDVBAPS brushes, respectively, showing a monotonous decrease of interfacial friction in SO42−>Cl−>NO3−>Br− solutions, which follows the same trend with contact angles. In fact, both surface hydration and morphology are important factors for the friction coefficient of polymer brushes. AFM images showed that polySVBP brushes (~1.1 nm) possessed a relatively rougher surface than polyVBIPS brushes (~0.6 nm), which may explain the higher friction of poySVBP brush22, 39, 47. Along similar lines, the influence of different cations on friction coefficient of polyVBIPS, polySVBP, and polyDVBAPS brushes was also evaluated. In Figure 5c, friction coefficients of all three polymer brushes in different cation solutions were in an increased order of Na+≈K+

Structural Dependence of Salt-Responsive Polyzwitterionic Brushes

Recommend Documents