Cross-Linking Highly Lubricious Phosphocholinated Polymer Brushes

Article pubs.acs.org/Macromolecules

Cross-Linking Highly Lubricious Phosphocholinated Polymer Brushes: Effect on Surface Interactions and Frictional Behavior Noa Iuster,† Odeya Tairy,† Michael J. Driver,‡ Steven P. Armes,§ and Jacob Klein*,† †

Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel Vertellus Biomaterials, Vertellus Specialties UK Ltd., Basingstoke, Hampshire RG25 2PH, U.K. § Department of Chemistry, University of Sheffield, Sheffield S3 7HF, U.K. ‡

ABSTRACT: Poly[2-(methacryloyloxy)ethylphosphorylcholine] (pMPC) brushes provide extremely low friction coefficients up to high compressions, but their use as boundary lubricating layers is limited by the challenge of surface grafting of the chains. Coating by thin layers of cross-linked pMPC hydrogels may provide an attractive alternative. Here we use a surface force balance (SFB) and other techniques to examine the effect of light cross-linking (0.1% cross-linker) on the surface interactions and frictional behavior of grafted-from pMPC brushes on mica, up to physiologically high contact pressures. Atomic force microscopy, X-ray photoelectron spectroscopy, and interferometric surface-excess measurements show little difference between the non-cross-linked (linear) pMPC brushes and the cross-linked brushes prepared under otherwise identical conditions. Normal force−distance profiles between the polymer-bearing surfaces, however, reveal a marked compaction of the unperturbed thickness of the cross-linked brushes relative to linear ones, attributed to the cross-linking which limits chain swelling. The crosslinked pMPC layers exhibit very low friction (friction coefficients of order 10−3−10−4, depending on sliding velocities), similar to the corresponding linear brushes and due to hydration lubrication by the highly hydrated phosphocholine monomer structure. Within the range of our parameters, however, there is a marked qualitative difference in the dependence of friction on sliding velocity vs. While for the linear brushes friction is only very weakly vs-dependent (over 3 orders of magnitude in vs), due to a selfregulating brush interpenetration, for the cross-linked brushes friction increases markedly with vs (∼vs1/2), an effect attributed to suppression of interpenetration by the cross-links.

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INTRODUCTION

Growing dense, stable, pMPC brushes as boundary lubricants, while useful in reducing friction and wear in aqueous and biological media,11,21,22 can be challenging, as they need to be attached to or grown from surfaces. An attractive alternative may be to coat surfaces with thin hydrogel layers that serve the same lubricating purpose. Studies on bulk pMPC hydrogels have examined the polymer either in the form of hydrogel, as a comonomer,23−25 or an additional matrix.26,27 As their monomers are highly hydrated the pMPC hydrogels swell considerably, function as anti-biofouling agents,28 and reduce friction.23,28 It is of interest to examine systematically lubrication by thin hydrogel layersof thickness of order 50−500 nmwhich are representative of the surfaces of bulk

Polymer brushes, which are long molecular chains grafted to (or from) a nonadsorbing substrate at one end, generally in a good solvent, are a commonly used means of modifying surface properties.1−5 Such brushes were initially exploited largely as steric stabilizers of colloidal dispersions6 (as the nonadsorbing brush chains do not form intersurface bridges), while later uses included the modification of wettability,7 adhesion,8 and biocompatibility.9 In particular, polymer brushes proved very efficient in reducing friction between surfaces.10−13 Charged or hydrated brushes, and especially the zwitterionic poly[2(methacryloyloxy)ethylphosphorylcholine] (pMPC) brushes, whose monomers comprise the highly hydrated phosphocholine group, are particularly efficient in reducing friction in aqueous media11−15 via the hydration lubrication mechanism.16−20 © XXXX American Chemical Society

Received: July 5, 2017 Revised: August 21, 2017

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DOI: 10.1021/acs.macromol.7b01423 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules hydrogels, whose lubricity29 is important in areas from biomedical devices to tissue engineering.30−32 One controlled way for creating thin hydrogel-like layers which may provide some insight into the surface lubricity of bulk hydrogels, pursued in the present investigation, is through cross-linking of polymer brushes in aqueous media. Cross-linking of brushes has earlier been examined as a method of tailoring surface properties such as chemical resistance,33 stimuli-responsive behavior,34 stiffness,35 elastic modulus, roughness, and swelling,36,37 and improved load bearing and wear resistance.38 Other studies have also examined the effect of brush cross-linking on their frictional behavior. Thus, Loveless et al.39 have grown PVP brushes which were later reversibly cross-linked, and their friction studied using friction force microscopy (FFM). Li et al.37 covalently crosslinked polyacrylamide brushes and examined their friction against a poly(dimethylsiloxane) (PDMS) surface, at pressures up to 0.04 MPa, finding the friction coefficient μ to increase with cross-linker concentration (from μ ≈ 0.01−0.1). This was attributed to decreased hydration and shorter chain-ends at the outer cross-linked brush surface. In a more recent study,38 the effect of cross-linking poly(N-hydroxyethylacrylamide) brushes was examined with respect to its lubrication and wear against a PDMS countersurface. The cross-linked brushes show a strong increase in friction (μ increasing from ca. 0.01 to ca. 1−2), attributed to surface roughening on cross-linking. Kobayashi et al.14 examined the effect of cross-linking on friction in a number of brush systems, including pMPC, sliding against a glass counter surface. They found an increase in friction coefficient on cross-linking of the pMPC brushes from μ ≈ 0.05 to 0.1− 0.2, at rather high pressures P (P ≈ 140 MPa). A recent molecular dynamics simulation of the effect of brush crosslinking on their lubricity found the friction coefficient to increase with cross-link density, attributed to shorter or fewer dangling (brush-like) chains at the surface, together with greater shear resistance of the cross-linked brushes.40 One limitation in measuring boundary friction (as between brushes or cross-linked brushes) is that a number of different pathways, in addition to the friction due to molecular processes at the actual slip plane between the surfaces, may contribute to the frictional dissipation. These include asperity contact/ adhesion or distortion of surface asperities (in the case of rougher surfaces), viscoelastic losses if the substrate surface is soft, or ploughing losses if a sharp tip (e.g., an AFM tip) is sliding past a surface.41 These may then obscure the detailed processes taking place at the brush (or cross-linked brush) surface itself. The use of a surface force balance (SFB), where the substrate for the surface-attached layers is a rigid, molecularly smooth mica surface, minimizes the effect of such additional dissipation pathways and facilitates insight into the molecular boundary friction due to the brush or cross-linked brush itself. This has the advantage of added insight through revealing precisely the absolute thickness of the interacting layers. Thus, we aim to create model brush layers on mica surfaces in water as well as similar but cross-linked layers. The latter form a thin, surface-anchored cross-linked (therefore hydrogel-resembling) film whose surface interactions and particularly lubrication properties can be studied in the SFB and compared with the properties of the non-cross-linked brushes. In this study we prepared both linear brushes and crosslinked brushes of pMPC, whose monomer is known for its biocompatibility,11 high hydration level,42 and, related to that

as noted earlierits remarkable lubrication properties.13 We first describe the synthesis and characterization of the brushes or cross-linked brushes, and then their surface interactions and frictional properties are carefully probed using the SFB up to physiological pressures (ca. 5 MPa). Insight into the similarities and differences between the brush-like and the hydrogel-like layers is provided by the normal force profiles, and in particular by the sliding-velocity-dependent frictional behavior of the layers, and is considered in terms of effects attributable to the cross-links.

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MATERIALS AND METHODS

Materials. The monomer 2-(methacryloyloxy)ethylphosphorylcholine (MPC) was obtained from Vertellus Biomaterials, Basingstoke, UK. The cationic macroinitiator (MI, see Scheme 1) was synthesized according to previously published protocols43−46 and was similar to that used earlier,13,47 except that its overall degree of polymerization was either 50 or 100 rather than 20 (with the same ratio of four anchoring N+-terminated monomers to each initiating Br−-terminated monomer). The monomer was used as received. For

Scheme 1. Upper: Schematic Representation of the Schlenk System Setup Used for the Synthesis of the PMPC Brushes and Cross-Linked Brushes (Tube A: EGDMA Cross-Linker Solution; Tube B: Monomer/Catalysts Solution; Tube C: Containing the SFB Lenses, Pre-Initiated with Macroinitiator and Pre-Vacuumed); Lower: Schematic Representation of the ATRP Process Starting with Macroinitiator Adsorption, Followed by ATRP of Either Linear Brushes (Left) or Cross-Linked Brushes (Right)

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DOI: 10.1021/acs.macromol.7b01423 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules atom transfer radical polymerization (ATRP)48,49 the catalysts Cu(I) Br (99.999%), Cu(II)Br2 (99.999%), 2,2′-bipyridine (bpy, 99%), and the cross-linker ethylene glycol dimethacrylate (EGDMA) were purchased from Sigma-Aldrich (Israel) and used as received. Water was purified by a Barnstead purification system and had resistivity ≥18.2 MΩ·cm and total organic carbon Fs,linear. This may be qualitatively attributed as follows: If we consider brushes and cross-linked brushes of similar grafting density and main-chain length, then, on strong compression in equilibrium at restno slidingthe linear brushes will be strongly interpenetrated, while the interpenetration of the cross-linked chains will be limited by the cross-linking. At the lowest shear rates the energy dissipation of the linear brushes, and thus Fs,linear, is therefore larger, since the interpenetrated linear-brush moieties sliding past each other are longer than the interpenetrated cross-linked moieties. As the sliding velocity increases, the self-regulating mechanism discussed above (eq 2) ensures that Fs,linear is largely unchanged, while Fs,x‑linked increases (eq 3). At the highest sliding velocities, Fs,x‑linked exceeds Fs,linear, which we attribute to a higher frictional dissipation due to the effect of the rubbing EDGMA cross-links themselves: the EDGMA units are not hydrated and thus do not benefit from the hydration lubrication mechanism, an effect which appears stronger at higher vs values. We remark that even at the highest velocities and compressions in this study, the friction coefficient of the cross-linked layers remains relatively low, at ca. μ ≈ 10−3, which we attribute to the predominance of the highly hydrated phosphocholine groups (and hydration lubrication) relative to the EDGMA groups at the slip interface. The fact that at high loads and very low sliding velocities the friction is lower for the cross-linked brushes relative to the noncross-linked ones, due to suppression of chain interpenetration by the cross-links, is suggestive and may point the way to improved, polymer-based boundary lubricants.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.K.). ORCID

Noa Iuster: 0000-0001-6978-2459 Steven P. Armes: 0000-0002-8289-6351 Present Address

O.T.: Pharmedica Ltd., P.O. Box 15060, Matam, Haifa 31905, Israel. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the European Research Council (Advanced Grant HydrationLube), the McCutchen Foundation, a research grant from Simon and Golde Picker, and the Israel Science Foundation for support of this work. The research described here has been made possible in part by the historic generosity of the Harold Perlman family.

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REFERENCES

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CONCLUSIONS Our study reveals subtle but significant differences between pMPC brush layers and the same brushes once they have been cross-linked to form what are effectively hydrogel-like layers. Structurally the hydrogel layer is denser, and thus thinner, than the precursor brush layer, an effect attributed to the trapping of more compact chain configurations arising from the crosslinking. More particularly, while the massive reduction in sliding boundary friction, arising from the highly hydrated nature of the phosphocholine monomer structure via the hydration lubrication mechanism, is largely retained in brush and hydrogel layers, the dynamic response is very different. This is attributed to the suppression of interpenetration by the opposing compressed hydrogel layers due to the cross-linking, relative to the brush layers, which eliminates the self-regulating I

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DOI: 10.1021/acs.macromol.7b01423 Macromolecules XXXX, XXX, XXX−XXX