Specific anion effects on the hydration and tribological properties of zwitterionic phosphorylcholine-based brushes

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Contents lists available atScienceDirect

European Polymer Journal

journal homepage:www.elsevier.com/locate/europolj

Speci

ﬁc anion eﬀects on the hydration and tribological properties of

zwitterionic phosphorylcholine-based brushes

Yunlong Yu

a,b

, Yongchao Yao

a

, Simone van Lin

b

, Sissi de Beer

b,⁎

a_{National Engineering Research Center for Biomaterials, Sichuan University, 29 Wangjiang Road, Chengdu 610064, China}

b_{Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, the Netherlands}

A R T I C L E I N F O Keywords: Polymer brush Hofmeister series Friction Adhesion

Atomic force microscopy

A B S T R A C T

Polymer brushes are known to form highly efficient lubricating layers for a broad range of contacting surfaces in water. Here we focus on brushes of poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) polymers that are end-anchored to the substrate at a high density. We show that the friction coefficient, which is measured upon sliding atomic force microscopy colloid probes on PMPC brushes at different normal loads, decreases when salts are added (1 M sodiumfluoride (NaF), sodium chloride (NaCl), sodium iodide (NaI), cesium chloride (CsCl) and cesium iodide (CsI)). Interestingly, we observe that the reduction in the friction coefficient increases with in-creasing anionic size in the solution, while the size of the cation has no measureable effect. We relate the reduction in the friction coefficient to a decrease in the compliance of the brush. In pure water the brush is swollen and soft, resulting in a large contact area between colloids and brushes and, therefore, a relatively“high” friction coefficient. In salt solutions, the stiffness of the brush increases with increasing anionic size resulting in a lower contact area and, therefore, lower friction coefficient.

1. Introduction

In recent years, medical procedures are being advanced towards minimally invasive techniques where catheters and long needles are employed to reach the region of interest in the body. For a smooth manipulation without damage of the surrounding tissue, it is of critical importance to reduce friction for these tubular medical devices[1–3]. This is, however, challenging, because traditional oil-based lubricants are often toxic for humans and simple, biocompatible liquids, such as water, are easily squeezed out of the contact. Polymer brushes have been proposed as promising candidates for employment as low-friction surface coatings in biomedical applications such as artiﬁcial joints [4–6]. These brushes can be composed of biocompatible polymers and are able to absorb low-viscosity aqueous liquids and keep them from ﬂowing out of the contact such that it remains lubricated[7–9]even when pressures are high[10].

Polymer brushes are comprised of macromolecules that are attached with one end to a surface at a density that is high enough for the polymers to stretch away from the anchoring plane[11]. The degree of stretching of the polymers in brushes depends on the solvent quality: in poor solvents the brushes collapse to form a dense ﬁlm, while the polymers strongly stretch in good solvents. Besides being eﬀective lu-bricants, such brushes can also be employed as actuators [12]and

sensors [13]. In particular, zwitterionic polymers, which have both negative and positive charges within the same monomers, have recently been the subjects of many scientific studies[14–19]. For example, ex-tremely low friction has been observed between poly(2-methacryloy-loxyethyl phosphorylcholine) (PMPC) brushes in pure water[10], even for pressures up to 7.5 MPa. In PMPC, the repeat unit is composed of the phosphorylcholine group, which makes them very hydrophilic [20]. Moreover, the polymers are biocompatible [21,22]. This, combined with the excellent lubricity[10,23]and protein adsorption resistance [24]in water, makes them ideal candidates for application in human body, e.g. in replacement joints[25–27]or on the surface of needles and catheters. Since the composition of body fluids deviates from pure water, it is important to characterize how electrolytes affect the hy-dration and tribomechanical properties of zwitterionic polymers. Due to the specific cation-anion composition of the sidechains of these poly-mers, there are various phenomena that can occur depending on the salts in their aqueous solution[4,17,28–36]. For example, changes in the stretching following the (reversed) Hofmeister series, and ion-spe-cific effects can be anticipated.

In the 1880s, Hofmeister and co-workers[37]found there are ion-specific effects on the precipitation of proteins from blood and egg whites. These effects are still extensively studied[38–41]. Within the

Hofmeister series, anions are categorized as

https://doi.org/10.1016/j.eurpolymj.2019.01.013

Received 5 October 2018; Received in revised form 2 January 2019; Accepted 4 January 2019 ⁎_{Corresponding author.}

E-mail address:s.j.a.debeer@utwente.nl(S. de Beer).

European Polymer Journal 112 (2019) 222–227

Available online 06 January 2019

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CO32−˃ SO42−˃ H2PO4−˃ F−˃ Cl−˃ Br−≈ NO3−˃ I−˃ ClO4−˃

SC-N−. In light of the strength of ionic hydration, the ions on the left can be classified as kosmotropes and the ions on the right as chaotropes [42]. The strongly hydrated kosmotropes tend to immobilize water molecules to strengthen hydrophobic interaction resulting in pre-cipitation of the solutes. Whereas the weakly hydrated chaotropes in-crease water mobility and tend to inin-crease solubility of solutes in so-lution. Due to these specific ion effects, various salts can be applied to control polymer solubility [38,43] and, thus, friction [35,44] for polymer brushes in aqueous solutions.

The conformation of zwitterionic polymers in salt solutions depends on the segmental dipole orientation[45], on the charge density of the cationic and anionic groups [46]and on the distance between the charged groups[47,48]. For example, Kobayashi et al.[44]report that the hydration of poly[2-(methacryloyloxy)ethyl]-dimethyl(3-sulfo-propyl) ammonium hydroxide (PSBMA) brushes strongly increases for increasing salt concentrations. The same trend has also been found by Liu et al.[48]using quartz crystal microbalance (QCM) measurements to investigate the anion speciﬁcity on poly(sulfobetaine methacryla-mide) (PSBMAm) brushes. For another type of zwitterionic polymer [35]poly (3-(1-(4-vinylbenzyl)-1H-imidazol-3-ium-3-yl)propane-1-sul-fonate) (PVBIPS), it has been found that the speciﬁc ions in the solution alter the hydration and friction between brushes composed of these polymers in various ways, which can be tuned reversibly by changing the salts concentration.

For PMPC the salt-induced effects on the conformation of the polymers are still under debate[32,44]. Through different character-ization techniques, different results have been obtained. For example, Mahon et al.[32]uses size exclusion chromatography to investigate the effect of the type of salt and the concentration on the hydrodynamic radii of PMPC polymers. Their results show the presence of different cation types has little effect on the radius of gyration of the polymers, but the presence of different types of anions does decrease the hydro-dynamic volume. Moreover, they observe that monoatomic anions have a stronger effect than polyatomic ones. However, when dynamic light scattering is used to analyze the hydrodynamic radius of free PMPC and PMPC brushes immobilized on silica nanoparticles by Matsuda et al. [49], no effect of the salt concentration could be detected. Kobayashi et al. have reported similar results[44]using atomic force microscopy (AFM) imaging measurements. Recently, Zhang et al. [4]employed ellipsometry and QCM measurements to quantitatively characterize PMPC brushes in various salts solutions. Their results imply that the height of the brushes appears to increase upon increasing the ionic strength and that, when comparing different monovalent anions, the effect is the largest for the smallest anions.

Here, we show that the measured swelling of PMPC brushes depends on the AFM technique employed to probe the swelling. The reason for this is that both the swelling and the stiffness of the brushes changes with the salt concentrations and anionic radii. This can result in an apparent constant or even increase in the height when measuring under normal loads, while a reduction of the height is measured in force distance measurements. Building on these results, we show that the friction coefficient and adhesion between PMPC brushes and poly-styrene colloids correlates with the brush compressibility; a decrease in brush stiffness results in a decrease in the friction coefficient and ad-hesion.

2. Experimental section 2.1. Materials

Copper (I) bromide (CuBr, Aldrich, 98%) was puriﬁed twice by stirring in acetic acid andﬁltered. After washing with ethanol, the light gray powder was dried in a vacuum oven at room temperature over-night. Poly(glycidyl methacrylate) (PGMA, Mn= 1 × 104),

2-metha-cryloyloxyethyl phosphorylcholine (MPC, 97%), 2,2′-bipyridyl

(≥99%), triethylamine (TEA, ≥99%), α-bromoisobutyryl bromide (BiBB, 98%), copper (II) bromide (CuBr2, ≥99%), sodium ﬂuoride

(NaF, ≥99%), sodium chloride (NaCl, ≥99%), sodium iodide (NaI, ≥99.5%), cesium chloride (CsCl, ≥98%), cesium iodide (CsI, 99.9%), lithium iodide (LiI, 99.9%), D2O (99.9 atom% D), methyl ethyl ketone

(MEK) were purchased from Sigma-Aldrich, and used as received without any puriﬁcation. Methanol (absolute), sulfuric acid (95–97%), dimethylformamide (DMF), chloroform (AR), 2-proponal (AR) and to-luene (AR) were purchased from Biosolve. Ethanol, hydrogen peroxide (H2O2) was purchased from Merck. MilliQ water was made from a

MilliQ Advantage A 10 puriﬁcation system (Millipore, Billerica, Ma, USA).

2.2. Synthesis of PGMA-PMPC

The synthesis route and characterization (techniques) for the PGMA-PMPC brushes are explained in detail in Ref.[51]. In short,ﬁrst, the PGMA thinﬁlm was attached to Piranha cleaned substrates using dip coating in a concentration of 0.1% of PGMA in MEK solution, and dried in nitrogen atmosphere. Following a waiting period of 48 h, the samples were annealed for 30 min at 110 °C in an oven. With the in-tention to remove the non-chemically bonded macromolecules, we washed the substrates with chloroform and sonicated them for 2 min. By a one-step reaction with BiBB, the initiator was grafted on the substrates. Lastly, surface-initiated atom transfer radical polymeriza-tion (SI-ATRP) was conducted to graft PMPC brushes from the surfaces with a thickness in air of 20 ± 2 nm.

2.3. Atomic force microscopy measurements

AFM experiments were carried out on a Multimode 8 AFM (Bruker), with a NanoScope V controller, and a JV vertical engage scanner. The thickness of the PMPC brushes in air was measured using aluminum coated cantilevers, which were purchased from Olympus, with a re-sonance frequency around 70 kHz, and a force constant of 2 N/m. All the samples were gently scratched using tweezers to expose the silicon surface underneath. This allowed for measuring the relative height between silicon and brush layer. For the swelling ratio measurements in various salts solutions, a glass liquid cell (Bruker, San Barbara, CA) and silica (Cospheric Monodisperse Silica Microspheres – 2.0 g/cc) or polystyrene (PS, Thermo Fischer, 4000 Series Monosized Particles 4205A) colloid probes (∼5 μm diameter colloids mounted on TL-CONT-50, Nanosensors, Switzerland) were used. Both silica and PS colloids were glued on the tipless cantilevers using a microcontroller. The spring constant of the PS colloid probes was determined by thermal noise analysis, which gave values of 0.3 ± 0.03 N/m at room temperature. The deﬂection sensitivity of the cantilever was measured in each so-lution on the bare silicon surface.

To determine the swelling ratios two techniques were employed, which we call the imaging method and the force-separation method. For the imaging method, we employed AFM contact mode imaging near scratches in the brushes over a distance of 30μm with a scan rate of 15μm/s. We used a very small normal load (typically 1–1.5 nN) to minimize the deformation of the brushes. In experiments referred to as the“force separation method”, we ﬁrst set the set-point of the normal load for imaging to 2.5 nN. Under this load, the relative height between the bare silicon and the brushes was determined to get the swollen height of PMPC brushes (h0). Next, force-separation data was captured

on the brushes over a distance of 2μm using a maximum load of about 50 nN with a ramp speed of 1μm/s. The h0imaged at 2.5 nN is

em-ployed to convert the separation to‘distance from the silicon surface’ by shifting the separation observed for 2.5 nN to h0. After the

determina-tion of our zero, we determined the extra height (Δh), which is the diﬀerence between h0and the distance at which the force just starts to

deviate from zero. The maximum swelling height of brush was then calculated by: h = h0+Δh.

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Scheme 1shows a sketch of the system for the friction and adhesion measurements between PMPC brushes on the surface and a PS colloid on the AFM cantilever. With the movement of colloid in normal di-rection, force-separation curves and the adhesion force can be mea-sured. Additionally, via motion in the lateral direction, the friction re-sponse can be determined. After each measurement in salt solution, the substrate was rinsed with water and ethanol sequentially for three times, and dried with nitrogen before the next AFM measurement. The friction was measured by sliding the cantilever over the brushes in the lateral direction (with the scan angle: 90°) over a sliding distance of 5μm using a velocity of 10 μm/s. The torsional response of the canti-lever was calibrated using the noncontact method of Wagner et al.[52] resulting in a torsional conversion factor of Sθ= 3.5 × 10−8N/V.

Generally, three to four different positions on the surfaces for at least three different samples were measured. The friction coefficients were averaged from at least 50 force traces for each measured point. The force-distance curves were obtained by approaching and retracting the PS colloid to and from the PMPC brushes with a velocity of 2μm/s, under a maximum load of 50 ± 5 nN. We defined the adhesive force to be the minimum force in the retraction curve. The adhesive force was averaged from four different positions on the surfaces for at least three different samples (capturing at least 25 force curves per position). 3. Results and discussion

Fig. 1shows the swelling ratio of PMPC brushes in H2O, 1 M NaF,

NaCl and NaI solution normalized by the swelling ratio in H2O. Though

the salt concentration is known to affect the swelling of PMPC[4,32], we study only one concentration. The reason for this is that specific ion effects are not qualitatively altered for PMPC for different concentra-tions; these effects are only enhanced for higher concentrations. We employed two different AFM techniques to extract the swelling ratios: (1) by imaging under a small normal load (1.5 nN, top graph) and (2) via force-separation curves (bottom graph). The swelling ratio de-termined by the two different techniques appear to contradict. How-ever, as we will explain in detail below, our observations can be un-derstood considering that the stiffness of the brushes is altered by the presence of salts.

For the imaging method (Fig. 1top), the PMPC brushes appear to increase in swelling upon adding salts. Moreover, with increase of the radius of anions, the brushes also swell more. For the smallest anions (F−), there is a 2% increase of the swelling ratio. For Cl−, the brushes swell around 9–10% more. When I−_{is added in the solution, a 17%}

increase of the measured height is obtained. Interestingly, by varying the cations from sodium to cesium, there is almost no change (< 1%) in the swelling ratio, despite the relatively large diﬀerence in size (Na+

186 pm and Cs+265 pm). A reason for this might be that the anionic

groups on the polymer-sidechain are close to the backbone, thus it is diﬃcult for them to interact with the free cations in the solution. Si-milar eﬀects have been reported by Mahon et al.[32].

The swelling ratio for the PMPC brushes extracted from force dis-tance curves is shown in Fig. 1, the bottom graph. We observe the opposite trend compared to the imaging method upon adding salts. By adding 1 M NaF, the swelling decreases by 15%. For the Cl−ions, there is around 30% decrease in swelling, while for I−, the decrease is a bit larger (about 35%). According to the law of matching water affinity [53], chaotropic cations have a high binding affinity with chaotropic anions, while they have less binding affinity with kosmotropic anions. It was extracted from molecular dynamics simulations [40], that the ammonium group N+_(CH

3)3belongs to chaotropic ions. Therefore, the

ammonium group is more likely to combine with the chaotropic anion I−compared to the more kosmotropic F−and Cl−ions. This ionic in-teraction breaks the hydration and causes a slight collapse of brushes. Because of the stronger interaction, I−can induce a stronger collapse than the other two anions[50].

The question arises, why are the height data different when dif-ferent approaches are used to determine height variations? A possible answer is provided by the observations shown inFig. 2.Fig. 2shows force versus separation curves upon approach of the PS colloid towards the PMPC brush in pure water and in 1 M NaF, NaCl and NaI solutions. In pure water the force already deviates from zero at large distances (180–200 nm). However, the highly water-solvated brush is easily compressed by a small force. For a force of 2 nN, the separation de-creases to 106 nm. Upon adding salts, the force deviates from zero for much smaller separations (60–80 nm). It is known that the binding of ions will increase the rigidity of the phosphorylcholine groups[4,54]. Thus, under compression by a sufficiently large force, the apparent height of the salted brushes can be higher than the height for brushes in water. This might explain the apparent constant height found by Ko-bayashi et al.[44] who measured under compression. If one is not aware of the effect of the changing brush stiffness, different measure-ments (under different loads) can results in both an increase and

-+

-Scheme 1. Schematic representation of friction and adhesion measurement on PMPC brushes.

Fig. 1. Relative swelling ratio of PMPC brushes in H2O, and 1 M various salts solutions measured by AFM imaging (top) using a normal load of 50 nN over a scan size of 30μm at a scan rate of 0.5 Hz and force-distance (bottom). The numbers under the anions give the hydrodynamic radii in pm. The error bars denote the standard error of the mean with a 95% conﬁdence interval.

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decrease of the height with increasing anion size. Averaged over many measurements, this can appear as an effective constant height. The increase in stiffness has been reported by Zhang et al.[4]as well. We provide an important confirmation utilizing a complementary mea-surement technique.

Now the question arises‘What is the true brush height?’. The bru-shes measured under a constant normal load display artificial height changes induced by the changing brush stiffness. Therefore, the force-distance measurements should be employed to compare the heights for the different aqueous solutions. However, the exact height extracted from force-distance measurements can be sensitive to polydispersity. Therefore, we expect that the true height and swelling ratio is slightly lower than we present here.

Fig. 3(a) gives the adhesion between a PS colloid and a PMPC brush measured upon retraction of the colloid from the brush in pure water

and for various 1 M salt solutions. Before retraction, the brush is com-pressed under a load of 50 nN. We observe the highest adhesion (3 nN) in water and the lowest force in 1 M NaI solutions (1 nN). These results are consistent with the change in compressibility of the brushes: the water-filled brush can be compressed easily, such that the contact area is large and there can be a stronger interaction between the brush and the colloid (despite the lower polymer density in highly swollen bru-shes). The brushes become increasingly stiffer for increasing the size of the anions (F−< Cl−< I−) in the solutions, such that the contact area becomes smaller for solutions containing larger anions. This ex-plains the smaller adhesion measured in solutions of larger anions. An additional explanation could be that the change in the adhesion is due to an ion-induced change in electrostatic interactions between the ca-tions adsorbed on the charged PS[55]and the anions interacting with the ammonium groups in the PMPC. However, if this were to be true, the size of the cations should affect the adhesion, because ionic affi-nities depend on the anion and cation size[56]. This we do not observe. Moreover, we observe the same adhesive forces for silica colloids of the same radius (in 1 M NaCl adhesion is 1.79 ± 0.06 nN for PS and 1.72 ± 0.08 nN for SiO2), which are likely to have a different

surface charge compare to PS. Therefore, we can conclude that the speciﬁc ion induced change in electrostatic interactions between the colloid and the brush is small for our systems.

Besides observing different effects on the swelling ratio, different research groups also observed different effects of the addition of salts on the friction between PMPC brushes or for PMPC brushes in contact with colloids or AFM tips[4,10,44]. While some observed that friction in-creases[10], others found that the friction is not affected by the pre-sence of ions[44]and again others found that friction decreases[4]. Similarly, we measure different specific ion effects, depending on the applied normal load (Fig. 4).

Fig. 4shows the friction measured upon sliding a PS colloid over a PMPC brush using a sliding velocity is 5μm/s with a stroke length of 10μm for various normal loads between 50 and 200 nN. The results show that the friction increases linearly as a function of normal load, but that the exact form of the dependence strongly varies with the type of salt. Though there is no clear relation between the friction force and the size of the ions in the solutions, the friction coefficient does display a clear relationship. In the bottom graph ofFig. 3we show the friction coefficient determined after a linear fit to the data inFig. 4. We observe that, by adding salts in the aqueous solution, there is a decrease in the friction coefficient, which is also following the order of specific ion effects. In NaF, there is around 10% decrease compared to the friction coefficient measured in H2O. By changing the anion to Cl−and I−, the

decrease in friction coeﬃcient is around 15% and 20%, respectively. Fig. 2. Typical force versus separation curves (measured upon approach) for

various aqueous solutions: H2O, NaF, NaCl, NaI. The approach velocity of the PS colloid to the PMPC brushes is 2μm/s with a maximum load of 50 ± 5 nN. The inset gives a magniﬁcation of the NaF, NaCl, NaI curves.

Fig. 3. Adhesion force (a) and friction coeﬃcient (b) between the PMPC bru-shes and a PS colloid as a function of ion radius (pm) measured by AFM. The error bars denote the standard error of the mean with a 95% conﬁdence in-terval.

Fig. 4. Friction force between the PMPC brushes and PS colloids as a function of applied normal load measured at a velocity of 5μm/s, over a scan size of 10 μm. The error bars denote the standard error of the mean with a 95% conﬁdence interval.

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In agreement with the results of Zhang et al.,[4]wefind that the friction coefficient decreases upon adding salts. However, we observe the opposite effect of the anion size on the friction. While we find that friction decreases with increasing anion size, Zhang et al. found that friction decreases with decreasing anion size. The reason for this dis-crepancy is likely the difference in contact geometry. For our large colloids, friction is dominated by interfacial shear stresses due to ad-hesive interactions. Therefore, our friction strongly correlates with the adhesive interactions and decreases with increasing anion size due to increased brush-stiffness and, thereby, reduced contact area for larger anions. On the other hand, for the sharp tips employed in Zhang et al., friction is caused by plowing motion, which strongly depends on the polymer density in the brush. The presence of large anions results in denser brushes and, thus more entanglements and a higher effective viscosity[4].

Both adhesion and friction decrease by introducing ions in the so-lution. Moreover, both follow a decrease order as F−< Cl−< I−. For polymeric systems in contact with a chemically diﬀerent counter sur-face, friction and adhesion are often related[54,58–61]although some examples exist where friction and adhesion are caused by diﬀerent dissipation mechanisms [62–64]. Our results indicate that for PMPC brushes in contact with PS or silica colloids the adhesion and friction are closely related[65]. The reason for this is that friction is caused by interfacial shear stresses that arise due to adhesive interactions between the colloid and the brush.

4. Conclusion

In summary, we have shown that the swelling of PMPC brushes decreases, while the stiffness increases upon adding salts. The decrease of swelling and increase in stiffening is independent of the cation size, but is enhanced by increasing the size of the anions in the solution. We emphasize that an increase in height upon adding salt can be measured upon imaging the height under a positive normal load due to the in-crease in brush stiffening. Our measurements confirm the recently re-ported studies by Zhang et al.[4]using complementary measurements techniques and are, therefore, critical in resolving the debate on specific ion effects in the swelling of PMPC brushes. The change in brush stif-fening also affects the tribo-mechanical response of PMPC brushes. The friction coefficients are very low (< 0.0004) and decrease with the anionic size. This effect is opposite to the results reported by Zhang et al. [4]. We attribute this discrepancy to the difference in contact geometry: For colloids in contact with brushes (our systems), dissipa-tion is caused by interfacial shear stresses, which reduce in the presence of large anions due to the increased brush-stiffness resulting in smaller contact areas. For sharp tips in contact with brushes (Zhang’s systems), dissipation is dominated by plowing friction, which increases in the presence of large ions due to the increased effective viscosity of denser brushes.

Acknowledgment

We thank Hubert Gojzewski for help on the preparation of the colloid probes. This work was supported by the National Natural Science Foundation of China (Nos. 51703145), China Postdoctoral Science Foundation (2017M620426), the Postdoc Research Foundation of Sichuan University (2017SCU12009) and the MESA+ Institute for Nanotechnology of the University of Twente. SdB has been supported by the Foundation for Fundamental Research on Matter (FOM), which isﬁnancially supported by the Netherlands Organization for Scientiﬁc Research (NWO).

Appendix A. Supplementary material

Supplementary data to this article can be found online athttps:// doi.org/10.1016/j.eurpolymj.2019.01.013.

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