Bioinspired Adhesion Polymers: Wear Resistance of Adsorption

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Bioinspired Adhesion Polymers - Wear Resistance of Adsorption Layers Illia Dobryden, Medeina Steponaviciute, Vaidas Klimkevicius, Ricardas Makuska, Andra Dédinaité, Xiaoyan Liu, Robert William Corkery, and Per Martin Claesson Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01818 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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Bioinspired Adhesion Polymers - Wear Resistance of Adsorption Layers Illia Dobryden1,*, Medeina Steponavičiūtė2, Vaidas Klimkevičius2, Ričardas Makuška2, Andra Dėdinaitė1,4, Xiaoyan Liu3, Robert W. Corkery1,*, and Per Martin Claesson1,4 1,*

KTH Royal Institute of Technology, School of Engineering Sciences in Chemistry, Biotechnology and Health, Department of Chemistry, Division of Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden. 2

Institute of Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania

3 School

of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062,

China 4

RISE Research Institutes of Sweden, Division of Bioscience and Materials, SE-114 86 Stockholm, Sweden. * Communicating authors email: [email protected] and [email protected] Abstract Mussel adhesive polymers owe their ability to strongly bind to a large variety of surfaces under water due to their high content of 3,4-dihydroxy-l-phenylalanine (DOPA) groups and high positive charge. In this work we use a set of statistical copolymers that contain medium length poly(ethylene oxide) side chains that are anchored to the surface in three different ways: by means of i) electrostatic forces, ii) catechol groups (as in DOPA), and iii) the combination of electrostatic forces and catechol groups. A nanotribological scanning probe method was utilized to evaluate the wear resistance of the formed layers as a function of normal load. It was found that the combined measurement of surface topography and stiffness provided accurate assessment of the wear resistance of such thin layers. In particular, surface stiffness maps allowed to identify initiation of wear before a clear topographical wear scar was developed. Our data demonstrate that the molecular and abrasive wear resistance on silica surfaces depends on the anchoring mode and follows the order: catechol groups combined with electrostatic forces > catechol groups alone > electrostatic forces alone. The devised methodology should be generally applicable for evaluating wear resistance or “robustness” of thin adsorbed layers on a variety of surfaces. Key-words: Biomimetic polymers, Catechol polymers, Initiation of wear, Abrasive wear, Molecular wear, Surface stiffness, Nanotribology

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Introduction Polymer layers of a few nm to a few tens of nm thickness are commonly used for controlling surface properties. The aims may vary from providing steric stabilization to a dispersion, achieving a non-fouling surface layer, promote adhesion or lubrication. Such polymer layers can be formed by either chemical grafting methods or by means of a simple adsorption process. The grafting methods have the advantage of providing strong anchoring to the surface via chemical bonds, whereas the disadvantage is that such layers do not provide any self-healing properties if worn in a particular area of the surface. In contrast, adsorption layers are often less robust, but, when the adsorbing polymer remains present in solution, readily heal when damaged. In many applications a robust and wear resistant polymer layer is needed, and it would of course be useful to combine the advantages of grafting and adsorption methods to achieve a strong anchoring using an adsorption method. As so often is the case, humans can find inspiration of the evolutionary structures found in nature. For instance, mussels have developed protein-based glues that bind to a wide range of surfaces efficiently under water [1]. The main constituents of these glues are known as mussel adhesive proteins (MAP), and they are characterized by a high positive charge and large content of the amino acid 3,4dihydroxy-l-phenylalanine (DOPA). The DOPA groups facilitate strong anchoring to surfaces via cooperative hydrogen bonding and metal chelating capability, and the DOPA group can also participate in redox reactions [2]. The high positive charge of the MAPs also contribute by providing an electrostatic surface affinity and fast adsorption kinetics [3]. Not surprisingly, layers containing MAPs have shown good protective properties. For instance, when MAPs are combined with ceria nanoparticles excellent corrosion protection performance is achieved [4]. In recent years, there has also been significant interest in biomimetic adhesives inspired by the MAPs [5-8]. A range of applications requires a coating that counteracts adsorption of contaminants or proteins. There are many strategies for achieving this, ranging from the use of superhydrophobic surfaces [9] to superhydrophilic ones [10]. In mammals the large glycoproteins of the mucin family provide the hydrophilic and protective coating on all internal surfaces [11]. In technical applications a commonly utilized alternative is poly(ethylene oxide), PEO, also known as poly(ethylene glycol), PEG. The non-fouling properties of PEO [12] have been suggested to arise from its extensive hydration [13] as well as the low refractive index and high flexibility of the PEO chains that results in predominance of protein – PEO steric forces over van der Waals forces [14]. In this work we utilize polymers that have anchoring groups inspired by the MAPs, i.e. there are cationic charges and/or catechol groups present. The polymers also contain mediumlength PEO chains to facilitate high hydration of the surface layer. Polymer structures with catechol and PEO groups have also been reported previously [15, 16], and they may show promise in non-fouling applications for surfaces in contact with bodily fluids or on surfaces of e.g. solar cells. However, a prerequisite is a strong anchoring to the surface to give the adsorption layer sufficient robustness and wear resistance. The adsorption strength and wear resistance cannot be judged by simply measuring the adsorbed amount, as realized by considering adsorption of polyelectrolytes to oppositely charged surfaces. In this case it is often found that the adsorbed amount of a low charge density polyelectrolyte is higher than that of a more highly charged one [17, 18], whereas the electrostatic affinity is expected to increase with increasing polyelectrolyte charge density. This is well established

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experimentally and understood theoretically [19]. Thus, if we would like to understand how the wear resistance of an adsorbed polymer layer is affected by the polymer structure we cannot rely on measurements of the adsorbed amount but need to find another method. For this purpose scanning probe microscopy has been shown to be promising [20]. In this work, we utilize nanoscale wear measurements as a function of load to compare the robustness of three polymer layers and thereby learn the importance of electrostatic forces and catechol mediated forces. All three polymers contain 19 units long PEO chains that are attached to the surface by means of i) cationic groups, ii) catechol groups, and iii) a combination of cationic and catechol groups. We also analyze how the stick-slip frictional events are affected by the anchoring mode of the adsorbed polymer layer. Methods and Materials The structure of the three copolymers used in this study is provided in Figure 1, and key characteristics are listed in Table 1. Statistical copolymers were synthesized by RAFT copolymerization of poly(ethylene glycol) methyl ether methacrylate (molecular weight 950 Da, PEO19MEMA) with [2-(methacryloyloxy) ethyl] trimethylammonium chloride (METAC), dopamine methacrylamide with acetonide protected hydroxyl groups (DOPMAA) or 2-(dimethylamino)ethyl methacrylate (DMAEMA) according to the procedure described before [21]. In a second step, the protected hydroxyl groups in DOPMAA copolymers were deprotected by trifluoroacetic acid, and the copolymers containing DMAEMA units were quaternized by 2-chloro-3′,4′-dihydroxyacetophenone giving copolymers containing units of PEO19MEMA and [2-(methacryloyloxy)ethyl] 3′,4′-dihydroxyacetophenone dimethylammonium chloride (MEDAPDAC) (Figure 1). Molecular weight Mn and dispersity Ð of the copolymers were determined by size exclusion chromatography, SEC, with triple detection, and their composition was assessed by 1H NMR spectroscopy. 

 y

x O

O O

 O

O N

 y

x O

O



 y

x O

O NH

O

Cl

O N

O

Cl

OH

O

19

OH

O 19

O 19

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OH OH p(PEO19MEMA-METAC)

p(PEO19MEMA-DOPMAA)

p(PEO19MEMA-MEDAPDAC)

Figure 1. The structures of the copolymers used in this investigation. The monomeric units have a statistical distribution along the polymer chain. Chemical names and the abbreviations we use in this paper are: i) p(PEO19MEMA-METAC), PEO-cationic, ii) p(PEO19MEMADOPMA), PEO-catechol and iii) p(PEO19MEMA-MEDAPDAC), PEO-cationic-catechol. Table 1. Key characteristics of the copolymers used in this investigation Copolymer Chemical name

Copolymer composition,

Abbreviation

PEO19MEMA, mol.%

3


Mn, kDa

Ð

DP*

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p(PEO19MEMAMETAC) p(PEO19MEMADOPMAA)

PEO-cationic

46

62.5

1.18

103

PEO-catechol

56

55.4

1.19

88

46

61.9

1.13

99

p(PEO19MEMA- PEO-cationicMEDAPDAC)

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catechol

*Degree of polymerization Adsorption was evaluated by means of a Q-sense E4 microbalance (Biolin Scientific, Gothenburg, Sweden). This technique allows monitoring of adsorption processes by measurements of changes in frequency and energy dissipation of a quartz sensor. Here we utilized sensors with silica coating (QSX-303, Biolin Scientific). All parts of the QCM-D cell were cleaned using 2% Deconex solution (Borer Chemie AG, Switzerland) followed by sonication for half an hour and extensive rinsing with MilliQ water and ethanol. The QSX303 sensors were cleaned using 2% Hellmanex solution (Hellma GmbH) for half an hour without sonication, followed by rinsing with MilliQ water, ethanol and drying in a filtered nitrogen jet. All cleaning and assembly operations were carried out in a laminar flow cabinet. All solutions were adjusted to pH 4 as monitored by using pH-indicator strips (pH 2.0-9.0) MQuant (Merck). The Sauerbrey model [22], valid for thin and rigid layers, was used for evaluating the mass oscillating with the crystal (polymer and associated solvent), , and calculated as: (1) where is the overtone number, is the frequency change and for the sensor used, and for these sensors it is 0.177 mg m-2 Hz-1.

is a characteristic constant

Nanoscale wear experiments were performed with a JPK NanoWizard 3 Atomic Force Microscope (JPK Instruments AG, Berlin, Germany) using diamond-like-carbon coated probes (All-In-One-DLC, Budget Sensors) with measured spring constants of 7.0-7.1 N/m. The nominal tip end radius was 10 nm. The adsorption of the polymer layers on 22×22 mm cleaned silicon substrates was carried out in an AFM liquid cell using a polymer concentration of 100 ppm. After allowing 10 min for the adsorption process, the cell was rinsed with MilliQ water of pH 4 to remove all polymers that remained in the bulk solution. The rinsing was executed to prevent self-healing of the layer by adsorption from solution when worn by the sliding AFM tip. The wear measurements were carried out in contact mode at different loads up to 450 nN for only one wear cycle. The sliding tip speed was about 4.5 µm/s. We also present some data for three times repeated wear measurements in the supplementary information. The wear measurements were conducted on areas of about 1 x 1 µm2. One dedicated surface area was used for each load, and the load was increased step-wise at the border of each area. These measurements result in lateral force images (measured in mV), which reflect the friction force at different loads. Changes in topography and stiffness of the adsorbed layer due to the combined action of normal and lateral forces experienced under the influence of the AFM tip 4


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were recorded in aqueous solution using the Quantitative Imaging mode (QI mode) at the setpoint force of 25 nN. Here the worn area and a surrounding unworn area were imaged at the same time. It was found that the surface stiffness, evaluated from the measured force curve as the slope of the approach curve in the repulsive contact region, was clearly affected by molecular wear of the polymer layer. Stick-slip phenomena were investigated by analyzing the lateral force images by employing the Fast Fourier Transform (FFT) method. The power spectra (2D) of the AFM lateral force images were generated with the built-in FFT java script in ImageJ (version 1.51s). ImageJ employs a Fast Hartley Transform algorithm as an intermediate in generating the FFT power spectrum. To obtain 1D FFTs the intensity values in 2D data sets were radially integrated. The integration angle was restricted to a finite arc chosen to maximize the signal to noise ratio. Peaks in the 1D FFT spectra were fitted with Gaussians (after background subtraction, see the supplementary information, Figure S1) using three fit parameters for each peak (peak height, peak position, peak width) over the k-space from kmin to 0.12 nm-1. Tables including the fit parameters are provided in the supplementary information, Tables S1. Reciprocals of the peak positions returned the average stick-slip length in nm. Results and Discussions Adsorption The QCM-D data are summarized in Table 2 and show the changes in resonance frequency and dissipation values together with the calculated Sauerbrey mass and thickness for the three polymers used in this investigation. The thickness was calculated by assuming a density, , of the layer of 1 g/mL. (The exact density of the layer is not known and the calculated thickness scales with the density as 1/). The data were collected at a polymer concentration of 100 ppm. All measurements were carried out at pH 4 to avoid cross-linking reactions involving catechol groups. The data is presented for the 7th overtone. The raw frequency shift and dissipation plots for the measured systems are shown in the supplementary information, Figures S2-S4. These figures also show that the polymers are essentially irreversibly adsorbed with respect to dilution, which is typical for polyelectrolytes on oppositely charged surfaces and polymers showing a high affinity adsorption isotherm [23]. Table 2. Change in resonance frequency, ∆f, dissipation value, ∆D, Sauerbrey mass, QCM-D, and thickness, t, as calculated using data from the 7th overtone. Polymer -∆f (Hz) ∆D x 106 t (nm) QCM-D (mg/m2) PEO-cationic 12.7±0.2 0.6±0.1 2.3±0.1 2.3 PEO-catechol 21.5±0.2 1.0±0.1 3.8±0.1 3.8 PEO-cationic12.4±0.2 0.7±0.1 2.2±0.1 2.2 catechol The Sauerbrey mass, which is includes the mass of the polymer and the water associated with the layer, for the case of PEO-catechol, was found to be larger than for the two polymers containing cationic groups. The main reason for this is that once the surface charge density has been compensated by the charges of the polymer, then electrostatic forces counteract further adsorption. Thus, electrostatic forces appear to limit the adsorption for PEO-cationic and PEO-cationic-catechol. This limiting factor does not exist for PEO-catechol, but here adsorption is instead limited by excluded volume effects between adsorbed chains [24]. The 5


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dissipation change was in all cases low, ≤ 1x10-6, which validates the use of the Sauerbrey method for evaluation of the wet mass and also demonstrates that the formed layer is rather thin and stiff. Nanowear Prior to polymer adsorption, a bare silica surface was exposed to the combined action of load and shear by the AFM tip to assess its eventual wear under our experimental conditions. The resulting topographical and lateral force images are provided in Figure 2. The silica surface used is smooth with RMS roughness of 0.22 nm over a 2x2 µm2 area (Figure 2a). The lateral force image obtained during wear measurements is shown in Figure 2b. The photodetector signal increases as the load is increased due to the increasing friction. At each load the lateral force is relatively constant, but some variations due to stick-slip are observed [25]. This feature will be discussed in detail later in the text. The topography was measured again after the wear measurement, and neither the topography image nor the surface stiffness image can distinguish the worn area from the rest of the surface. Thus, we conclude that the silica surface is not measurably affected by the wear measurement at these applied loads.

Figure 2. Silica in aqueous solution without any adsorbed polymer layer. Panel a: Topography prior to wear measurements. Panel b: Lateral photodetector signal during wear measurements. The normal force in the different areas from bottom to top was 50, 100, 150, 200 and 300 nN. Panel c: Topography after wear measurements. Panel d: Surface stiffness map after wear measurements. The worn area is located at the center of the image in panels c and d. The wear measurements were conducted over an area of about 1 x 1 µm2 located at the centre of panel 2c and 2d, as also indicated by the blue square in panel 2c.

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Adsorption of the PEO-cationic polymer does not significantly affect the topography of the silica surface as viewed over a 2x2 µm2 surface area (Figure 3a), as expected for such a thin and stiff layer (Table 2). The lateral force image (Figure 3b) is also similar to that observed for bare silica, with an increase in lateral force with increasing load. More importantly, when inspecting the topography after the wear measurements, one can distinguish the worn area from the unworn one (Figure 3c), where a small pile-up of material is observed at the left and right edges of the worn area. The thickness of the pile-up region is ˂ 1 nm relative to the worn area and this is exemplified by one line scan shown in Figure 3e taken at the highest load applied (450 nN). This signifies that some polymers have been dragged along the surface by the lateral motion of the tip, and abrasive wear has occurred at normal loads higher than 25 nN. Removal of the polymer from the worn area also results in an increase in surface stiffness, which is clearly distinguished in Figure 3d. Thus, removal of some polymers enhances the contribution of the underlying silica surface to the measured surface stiffness. We conclude that electrostatic anchoring of the PEO-cationic polymer is not sufficiently strong to avoid molecular abrasive wear at the normal forces employed, and, as expected, the wear increases with the applied load.

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Figure 3. Silica with an adsorbed layer of the PEO-cationic polymer. Panel a: Topography prior to wear measurements. Panel b: Lateral photodetector signal during wear measurements. The normal force in the different areas from bottom to top was 25, 75, 150, 300 and 450 nN. Panel c: Topography after wear measurements. Panel d: Surface stiffness map after wear measurements. Panel e: A height profile over the border of the worn and unworn area (red line in panel c) at a load of 450 nN. The worn area is located at the center of the image in panels c and d. Adsorption was carried out from a 100 ppm solution, and AFM measurements were carried out in water after rinsing away the polymer from bulk solution at pH 4. The wear measurements were conducted over an area of about 1 x 1 µm2 located at the centre of panel 3c and 3d. The data shown in Figure 4 is for the adsorbed PEO-catechol polymer layer. In this case anchoring does not occur due to electrostatic polymer-surface interactions, but rather due to interactions between the OH-groups on the silica surface and the catechol groups on the polymer. Again, a smooth layer following the underlying features of the silica surface is observed (Figure 4a). It was found that the PEO-catechol polymer layer was more wear resistant than that formed by the PEO-cationic polymer. It is difficult to obtain any clear 8


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evidence for neither abrasive wear nor pile up in the topography image (Figures 4c) recorded after the wear measurement. However, there may be a hint of it at the highest loads applied. That some wear indeed has occurred is more clearly seen in the surface stiffness image (Figure 4d) where a slightly higher stiffness is found in the worn area at the highest loads. It appears as individual molecules have been removed while the topography image still suggests a homogeneous layer. We will refer to this situation as “molecular wear” rather than abrasive wear even though there is no clear boarder between the two. We also note a slight blurring of the topography image outside the worn area, which likely is caused by some polymers being attached to the tip during the wear measurements.

Figure 4. Silica with an adsorbed layer of the PEO-catechol polymer. Panel a: Topography prior to wear measurements. Panel b: Lateral forces during wear measurements. The normal force in the different areas from bottom to top was 15, 150, 300 and 450 nN. Panel c: Topography after wear measurements. Panel d: Surface stiffness map after wear measurements. The worn area is located close to the middle of the image in panels c and d. Adsorption was carried out from a 100 ppm solution at pH 4, and AFM measurements were carried out in water at pH 4 after rinsing away the polymer from bulk solution. The wear measurements were conducted over an area of about 1 x 1 µm2 located at the centre of panel 4c and 4d.

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The PEO-cationic-catechol polymer that is attached to the surface due to both electrostatic and catechol anchoring groups also forms a smooth surface layer (Figure 5a), hardly distinguishable from the bare silica surface. The wear measurements (up to 450 nN, Figure 5b) did not cause any detectable change in the surface morphology or evidence for any pile up at the boarder of the worn area (Figure 5c). Just as for the adsorbed PEO-catechol polymer, some surface rearrangement can possibly be observed, but neither the topography image nor the surface stiffness image provide any clear evidence of molecular wear. However, as shown below an analysis of the surface stiffness image provides evidence for limited molecular wear.

Figure 5. Silica with an adsorbed layer of the PEO-cationic-catechol polymer. Panel a: Topography prior to wear measurements. Panel b: Lateral forces during wear measurements. The normal force in the different areas from bottom to top was 15, 150, 300 and 450 nN. Panel c: Topography after wear measurements. Panel d: Surface stiffness map after wear measurements. The worn area is located at the center of the image in panels c and d. Adsorption was carried out from a 100 ppm solution at pH 4, and AFM measurements were carried out in water at pH 4 after rinsing away the polymer from bulk solution. The wear measurements were conducted over an area of about 1 x 1 µm2 located at the centre of panel 5c and 5d. In order to compare the change in surface stiffness, S, due to wear we have calculated the average stiffness in the worn area and the unworn area and express the relative change as:

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(2) The results are shown in Figure 6. When a cationic anchoring group is used the surface stiffness increases by 27% ± 2.1% due to the wear under a load of 450 nN, though as evident from Figure 3, the cationic polymer is worn already at loads above 25 nN. The increase in stiffness value is just 8.3% ± 0.9 when instead catechol groups anchor the polymer to the surface, and when both electrostatic and catechol groups contribute to the anchoring, the surface stiffness increases only by 3.3% ± 1.3%at loads of 450 nN. The high wear resistance for the adsorbed catechol polymers was confirmed by measurements where the same surface area was worn three times, see Figure S5. The repeated wear measurement resulted in a slight increase in surface stiffness as shown in Figure 6 (open circles). (Note that repeated measurements are not relevant for the PEO-cationic polymer due to the high wear at the initial scan).

Figure 6. The relative change in surface stiffness, Srel, in the worn area compared to that of the undisturbed layer, evaluated from the stiffness maps shown in Figures 3-5. The normal force was 450 nN. Unfilled symbols represent data where the same surface area was worn three times at a load of 450 nN. The x-axis is just used to distinguish the different polymer layers. On the macroscopic and microscopic length scales it is common to distinguish between adhesive and abrasive wear. When the contact zone deforms plastically, the wear, Q (unit m2) have been described by the equation [26]: (3) where V is the worn volume, L the sliding length, Ar the real contact area and ki is a dimensionless constant that depends on the material combinations and the wear mechanism. For adhesive wear, relevant for materials with similar hardness, the constant can be termed kadh, and the resulting equation is known as the Archard equation [27]. For abrasive wear, where one material (the tip in our case) is much harder than the other material (the adsorbed

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layer) the equation with ki = kabr is known as the Rabinowicz equation [26, 28] and in this situation the hard tip indents the soft material. For atomistic wear at the nanoscale, material is removed atom by atom as illustrated in simulations [29]. For atomistic wear it has been argued that the abrasive process due to lateral forces is more important than the adhesive process due to normal forces, and an equation somewhat similar to Eq. 3 has been proposed [30]. Thus, even though the wear mechanisms and the corresponding constants are very different, the guiding equations are conceptually similar for different wear mechanisms and on different length scales. We note that for conditions at which atomic wear predominates it is very difficult to distinguish the wear volume in topographical images. In our case we investigate wear of an adsorbed layer, and here we either remove individual molecules from the worn area, conceptually similar to atomistic wear, or abrade the adsorbed layer. Just like for atomistic wear we find that it is difficult to notice molecular wear from topography images (Figures 4 and 5), whereas abrasive wear involving a larger fraction of the adsorbed molecules (Figure 3) is readily observed. To assess molecular wear, i.e. the initiation of the wear process, we instead inspect surface stiffness images, where molecular wear of the soft adsorbed layer results in a larger contribution from the stiffer substrate to the measured surface stiffness. With this approach we could clearly demonstrate that polymer structures containing catechol groups are more wear resistant on silica surfaces in aqueous media than polymers that are held to the surface by electrostatic forces. Stick-slip Sliding motion is in most cases not smooth, but rather characterized by stick and slip behavior [31]. The concept of stick-slip means that at some occasions the relative motion of the surfaces slows down or stops (stick), but once sufficient lateral energy has been built up they suddenly move (slip), and then the process repeats. The distance moved between two stick events is referred to as the “stick-slip distance”. Stick-slip is often attributed to atomistic [32], nanoscale [33], microscale [34], [35] roughness on the sliding surfaces. Stick-slip phenomena are also often associated with wear, and for polymer surfaces it has been shown that the characteristic stick-slip length can be directly linked to plastic deformations induced by the sliding motion [25]. In this work we do not observe any plastic deformation, but stick-slip is evident from the variation in the lateral force during sliding, as summarized in Figure 7.

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Figure 7. Lateral force images recorded during wear measurements in contact mode with grey scale image contrast (optimized for maximum contrast in each case). The lateral force images correspond to bare silica (a), silica with an adsorbed layer of PEO-cationic (b), PEO-catechol (c), and PEO-cationic-catechol (d). All data were recorded in aqueous solution at pH 4, and lighter colors represent higher lateral forces. A visual observation of these images makes it evident that there are larger variations in lateral force at higher loads, which means that the stick-slip motion becomes more pronounced. There is a hint of a regularity in these images, but image analysis is needed to evaluate it. To this end the images in Figure 7 were used to generate the FFT maps as described in the methods section, and these are reported in Figure 8.

Figure 8. The 2D FFT maps of the lateral force variation for bare silica (a), and silica carrying an adsorbed layer of PEO-cationic (b), PEO-cationic-catechol (c) and PEO-catechol (d) evaluated from the region exposed to the highest load reported for each system as shown in Figure 7. The degree of correlation decreases from red to blue. The FFT maps demonstrate that there is an order in the stick-slip patterns (i.e. in the intensity variations shown in Figure 7). The correlation pattern observed on bare silica and silica carrying an adsorbed layer of PEO-cationic show similarities in that two loobs of high intensity are observed, and these loobs are centred at a small angle away from the scan direction. The pattern observed for the catechol containing polymers differes in that a second high intensity arc region at larger wavenumbers in Fourier space, i.e. shorter distances in real space, are observed. This means that instead of having one characteristic stick-slip length we now have two.

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(a)

(b)

(c)

(d)

Figure 9. The 1D FFT plot for the lateral force evaluated from the 2D images shown in Figure 8. The data are for bare silica (a), and silica carrying an adsorbed layer of PEOcationic (b), PEO-catechol (c) and PEO-cationic-catechol (d). To further quantify these differences a radial 2D intergration was carried out as detailed in the Methods section. This results in a 1D FFT plot of intensity as a function of wavenumber, and such plots are shown in Figure 9. We note that one broad peak is observed for bare silica. The fact that the peak is broad means that the there is a large variation in the stick-slip length, as can be expected for a surface with random topographical features. A similar peak is also observed for the three polymer coated surfaces, but the peak is more narrow meaning that the characteristic stick-slip length is better defined. A second peak is also clearly visible for the two catechol containing polymers and hinted at for the PEO-cationic polymer.

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Figure 10. The characteristic stick-slip length as a function of load evaluated from the peak position in the 1D FFT correlation function shown in Figure 9. The data are for bare silica (a), and silica carrying an adsorbed layer of PEO-cationic (b), PEO-catechol (c) and PEOcationic-catechol (d). The peak position and half width at half maximum (the error bar) of the two peaks are shown in Figure 10. Both peak positions move slightly to larger distances as the load is increased. This is as expected since an increased normal load makes it more difficult to climb over surface features. As a result the cantilever stores more energy by bending before slip occurs, and as this energy is released during the slip period the surface can move further, i.e. the stick-slip length is increased. We note that the stick-slip length typically increases with load and also depends on the sliding speed (not investigated in this study), as discussed in several reports, see e.g. [36, 37]. Not only surface features but also adhesive interactions between the tip and the surface can result in a stick event [38]. We propose that this is the origin of the second stick-slip length observed for the polymers containing catechol groups. When a polymer interacts with the laterally moving tip it will be stretched. If the binding to the underlying surface is of smaller or similar strength as the applied lateral force, the polymer will be dragged along the surface and the adsorbed layer will be worn. This process dominates for the PEO-cationic layer (Figure 3), and as a result the stick-slip pattern is similar to that observed for silica (Figures 8 and 9) and a second peak is not clearly distinguished. In contrast, when the binding strength of the polymer to the surface is larger than the lateral force applied, the polymer will be stretched but not move along the surface. Instead, the sliding motion will come to a halt (stick) until the energy stored in the cantilever due to lateral bending exceeds the tip-polymer adhesion. When this occurs a slip event will occur. For this 15


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mechanism to be plausible the stick-slip length corresponding to the second peak should be less than the contour length of the polymer. This is indeed the case, as we find a stick-slip length of 15-18 nm, whereas the extended length of the polymer is just above 40 nm. Thus, the finding of a second characteristic stick-slip length for silica carrying adsorbed layers of the catechol containing polymers is consistent with their high surface affinity that counteracts molecular and abrasive wear (Figures 4 and 5). Conclusions As pointed out by Colaço [26], it is particularly difficult to understand wear on the length scale that is too large for atomistic modelling and too small for conventional experimental studies. The most promising approach for nanoscale wear studies is offered by AFM-based methodologies. However, even with such approaches the initiation of wear is difficult to assess for such thin layers as of interest in this work. Here the evaluation based on wearinduced changes in surface topography that is useful for polymeric samples [25] was found to be inadequate. Instead, more accurate evaluation of the molecular wear resistance could be achieved by comparing the surface stiffness of worn and unworn areas of the adsorbed layer. In particular, it allowed us to identify the initiation of wear before a topographical wear scar had developed. Since the surface stiffness of the worn and unworn areas is evaluated at the same time with the same tip, where is no ambiguity (for any given sample) in this comparison due to variations in tip radius or cantilever stiffness. Our data also show the appearance of a second characteristic stick-slip length for silica carrying adsorbed layers of the catechol containing polymers. We propose that it arises from stretching of firmly adsorbed polymer molecules, and this suggestion is consistent with the high wear resistance observed for these layers. We used the methodology outlined above to investigate the wear resistance of a series of synthetic mimics of mussel adhesive proteins. Key features of mussel adhesive proteins are their high content of 3,4-dihydroxy-l-phenylalanine, which contains a catechol group, and their high positive charge [2]. These features allow them to bind to most surfaces under water [1]. Here we utilized a set of statistical copolymers containing poly(ethylene oxide) side chains that were anchored to negatively charged silica surfaces with electrostatic forces, or via catechol groups, or with both. We demonstrated, for the first time, that the presence of catechol groups in the polymer structure resulted in a higher underwater wear resistance of the adsorbed layers than anchoring the polymers by only electrostatic forces. The combination of cationic and catechol anchoring groups, as in the mussel adhesive proteins, provided the most wear resistant polymer layer. These findings are important for designing thin wear resistant polymer layers in a number of applications, including non-fouling surfaces, primer layers and lubricating layers. Further studies are needed in order to elucidate how the details of the polymer structure affects the wear resistance, for instance, if it is an advantage to utilize a block copolymer structure rather than a statistical copolymer structure as in this work. Acknowledgements PC acknowledges financial support by the Swedish Research Council, VR, contract number [2015-05080]

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Graphical abstract

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Bioinspired Adhesion Polymers: Wear Resistance of Adsorption

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