Name: preparation and friction force microscopy measurements of immiscible, opposing polymer brushes
Uploaded: 2014-12-24T14:00:00
Duration: 13 min 57 s
Description: Watch this Scientific Journal Video about Preparation and Friction Force Microscopy Measurements of Immiscible, Opposing Polymer Brushes at JoVE.com

We thank M. Hempenius and E. Benetti for fruitful discussions, Y. Yu for careful checking of the recipe, M. Vlot for the image design of Figure 1, C. Padberg and K. Smit for technical support. E.K. acknowledges the Netherlands Organization for Scientific Research (NWO, TOP Grant 700.56.322, Macromolecular Nanotechnology with Stimulus Responsive Polymers) for financial support. SdB has been supported by the Foundation for Fundamental research on Matter (FOM), which is financially supported by the Netherlands Organization for Scientific Research (NWO).

NOTE: Figure 2 illustrates the sample preparation procedure. Polymer brushes were grafted from silicon (Si) substrates (path (a)), from gold-coated substrates (100 nm gold evaporated on Si wafer having a 10 nm Cr adhesion layer, path (b)), and from gold colloidal AFM probes (6 &#956;m diameter, path (c)) by surface initiated atom-transfer radical polymerization (SI-ATRP)24. The AFM measurements were performed on a Multimode AFM with the low-noise head, vertical engage scanner and a liquid cell.
1. Sample Preparation
<ol>
	<li>Substrate preparation
	<ol>
		<li>Initiator deposition on silicon surfaces.
		<ol>
			<li>Clean the silicon substrates with chloroform, then piranha solution, subsequently rinse with purified water, ethanol and chloroform.</li>
			<li>Place the dry substrates into a desiccator around a vial containing 50 &#181;l (3-aminopropyl) triethoxysilane.</li>
			<li>Evacuate the desiccator with a rotary vane pump for 15 min and close it subsequently. Allow the vapor deposition to proceed O/N.</li>
			<li>Prepare a solution of 40 ml toluene (degased) and 40 &#181;l triethylamine in an Erlenmeyer flask.</li>
			<li>Place the substrates in the flask and drop-wise add 40 &#181;l 2-bromo-2-methylpropionyl. Keep the substrates in the solution, to avoid deposition of salt crystals formed in solution.</li>
			<li>Stir the solution for 4 hr.</li>
			<li>Rinse the substrates with toluene and ethanol. Next, place the substrates into the polymerization vials.</li>
		</ol>
		</li>
		<li>Initiator deposition on gold surfaces
		<ol>
			<li>Prepare a monolayer solution: dissolve 2-bromo-2-methyl-propionic acid 11-[11-(2-bromo-2-methyl-propionyloxy)-undecyldisulfanyl]-undecyl ester in 20 ml degassed chloroform (0.2 mM).</li>
			<li>Transfer ca. 1.5 ml monolayer solution into a small vial that was previously flushed with Argon.</li>
			<li>Clean the gold-coated substrates with chloroform and piranha solution. Subsequently rinse with purified water, ethanol and chloroform.</li>
			<li>Immerse the cleaned substrates into the monolayer-solution and close the flask. Store O/N in a dark place.</li>
			<li>Clean the gold colloidal probes with ethanol and chloroform by dipping them into the solvents.</li>
			<li>Immerse the colloidal probes into the vial containing the monolayer solution. Close firmly and keep it in a dark place O/N.</li>
			<li>Remove the substrates from the solution, wash with chloroform and ethanol. Next, transfer the initiator coated substrates into the polymerization flask.</li>
			<li>Remove the colloidal probes from the monolayer solution, wash with chloroform and ethanol. Next, transfer each probe separately into the polymerization vial.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Polymerization
	<ol>
		<li>SI-ATRP of PMMA
		<ol>
			<li>Purge the flask and vials, containing the initiator-covered substrates (both Si and gold) and the colloidal probes with Argon for 30 min.</li>
			<li>Dissolve 10 g methyl methacrylate (MMA) in the ATRP medium (10 ml methanol/water mixture with ratio 5:1) and degas the solution for 2 hr.</li>
			<li>Add 145 mg CuBr and 320 mg 2,2-bipyridine into a flask equipped with a magnetic stirring bar, and deoxygenize by 3 vacuum-Ar backfill cycles.</li>
			<li>Transfer the degassed monomer solution into the flask (containing the copper) and stir for another 15 min until a clear brown solution is observed.</li>
			<li>Attach a needle to a 2 ml syringe and flush the needle and the syringe with argon 2-3 times.</li>
			<li>Withdraw 1 ml polymerization solution with the syringe and inject the content into the small reaction vial, containing the colloid probe.</li>
			<li>Withdraw the remaining solution with the syringe and inject the content into the reaction vial, containing the flat substrates. Add enough solution to submerge each sample completely.</li>
			<li>Conduct the polymerization for 40 hr at RT.</li>
			<li>Remove the samples from the polymerization solution and wash with ethanol and chloroform in multiple cycles. The coloring of the sample indicates the presence of the brush.</li>
			<li>Finally, dry the substrates under a stream of nitrogen.</li>
			<li>Keep flat substrates in a nitrogen box. Store polymer brush modified probes in toluene.</li>
		</ol>
		</li>
		<li>SI-ATRP of PNIPAM10
		<ol>
			<li>Purge the flask and vials, containing the initiator-covered substrates (both Si and gold) and the colloid probes with argon for 30 min.</li>
			<li>Dissolve 5.6 g N-isopropylacrylamide and 320 &#181;l PMDETA in the ATRP medium (1.6 ml water and 18 ml methanol) and degas the solution for 2 hr.</li>
			<li>Add 76 mg CuBr into a flask equipped with a magnetic stirring bar, and deoxygenize by 3 vacuum-Ar backfill cycles.</li>
			<li>Transfer the degassed monomer solution into the flask that contains the copper. Stir for another 15 min until a clear green solution is observed.</li>
			<li>Attach a needle to a 2 ml syringe and flush the needle and the syringe with argon 2-3 times.</li>
			<li>Withdraw 1 ml polymerization solution with the syringe and inject the content into the small reaction vial, containing the colloid probe. Repeat for all probes.</li>
			<li>Withdraw the remaining solution with the syringe and inject the content into the reaction flask, containing the flat substrates. Add enough solution to submerge each sample completely.</li>
			<li>Conduct the polymerization for 2 hr at RT.</li>
			<li>Remove the samples from the polymerization solution and wash with ethanol and water through multiple cycles. The appearing color of the sample indicates the presence of the brush.</li>
			<li>Immerse the sample into 0.1 M EDTA solution and keep it in the solution O/N to remove all the copper. Wash with purified water and ethanol.</li>
			<li>Keep the flat substrates in a nitrogen box. Store the PNIPAM brush modified probes in purified water.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Brush characterization, Fourier Transform Infrared (Figure&#160;3) 
	NOTE: Use grazing-angle FTIR mode for polymer brushes on gold substrate and transmission FTIR mode for polymer brushes on Si substrate. Dry all samples carefully. Any water and solvent contamination can cause serious damage of the detector.&#160;
	<ol>
		<li>Start up the equipment according to the manual provided by the manufacturer.</li>
		<li>Set the following parameters for sample measurement: range between 900 and 3,700 cm-1 with a resolution of 4 cm-1 and average 32 scans in each measurement.&#160;</li>
		<li>Clean a blank substrate according to the protocol used for sample preparation, dry carefully and place it to the measurement chamber.</li>
		<li>Apply vacuum for 4 hr. Next, record a background scan.</li>
		<li>Remove the blank sample and place the dry polymer brush coated sample in the sample chamber.</li>
		<li>Apply vacuum in the sample chamber and record a sample-scan every 30 min for 5 hr.</li>
		<li>Identify all peaks in the spectra to confirm chemical composition of the polymer brush.&#160;(Only analyze the scans with a straight baseline.)</li>
	</ol>
	</li>
</ol>
2. AFM Measurement
<ol>
	<li>Scratch the PMMA brush covered substrate carefully with a needle, rinse with a good solvent to remove the excess of polymer and dry it.</li>
	<li>Mount the samples into the AFM instrument and mount the probe into the liquid cell.</li>
	<li>Align the laser on the end of the cantilever.</li>
	<li>With use of the camera, align the tip above a scratch.</li>
	<li>Before approaching and engaging the tip, set the scan size 0 nm. Next, engage the tip to the surface.</li>
	<li>Go to &#8216;ramp mode&#8217; and determine the deflection sensitivity via force distance curves. Calibrate the normal spring constant using the &#8216;thermal tune&#8217; as implemented in the software and torsional spring constant of the cantilever using the method of Wagner et al.25
	<ol>
		<li>Capture the torsional thermal noise of the cantilever in air for 2 sec using the high speed data capture in the software (6.25 MHz).</li>
		<li>Convert the thermal noise into the power spectral density (V2/Hz) using the Fourier Transform.</li>
		<li>Determine the fundamental resonance and the quality factor of the cantilever in air using the resonance peak in the power spectral density and the equation for a simple harmonic oscillator including baseline noise (eq. 2 of Ref. 25).</li>
		<li>Calculate the torsional spring constant using the inplane dimensions of the cantilever (length and width), the density and viscosity of the surrounding medium (air) and the quality factor and resonance frequency determined in step 2.6.3 using the method of Sader26. 
		NOTE: We used the tool provided on the website of John Sader: www.ampc.ms.unimelb.edu.au/afm/calibration.html.</li>
		<li>Calculate the torsional angle deflection sensitivity of the cantilever using eq. 6 and 7 of Ref. 25.</li>
		<li>Convert the torsional spring constant and deflection sensitivity into the lateral spring constant and deflection sensitivity using the size of the colloid, the thickness of the cantilever and eq. 8 of Ref. 25. 
		NOTE: the detector signal can now be converted into a force via: force [N] = lateral spring constant [N/m] * lateral deflection sensitivity [m/V] * detector signal [V].</li>
	</ol>
	</li>
	<li>Measure the brush height of the dry brush by imaging the brush at a scratch at the lowest possible deflection setpoint. Determine the brush height from the line-scans of the captured image.</li>
	<li>Solvate the brush in acetophenone by gently applying it to the surface with a syringe. 
	NOTE: The color of the sample changes as the solvent evaporates. This allows for following the drying process.</li>
	<li>Mount the sample to the AFM. Align the vertical laser signal to -1.0 V. Set the deflection setpoint to 0 V and engage the cantilever and surface. 
	NOTE: After bringing the cantilever in contact with the surface, the acetophenone moves into the brush on the colloid creating a capillary bridge between the tip and the surface.</li>
	<li>Set the scan size to 40 &#956;m, disable the slow scan axes and set image aspect ratio to 1:4. Record the height and the friction image channels (both trace and retrace).</li>
	<li>When the images are captured withdraw the cantilever.</li>
	<li>Apply a drop of water on the PNIPAM brush-covered surface, to solvate the PNIPAM brush.</li>
	<li>Lift the head and quickly replace the PMMA surface by the solvated PNIPAM surface to create the immiscible system. Be quick when exchanging the surfaces, to avoid evaporation of the acetophenone from the PMMA brush on the colloidal probe.</li>
	<li>Engage the tip and surface, and record images with same parameters as previously.</li>
</ol>

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The authors have nothing to disclose.

The presented results show that the friction, for immiscible systems of individually solvated brushes, is strongly reduced compared to traditional miscible systems of two of the same solvated brushes. The preferred absorbance of the different solvents in the two brushes prevents the brushes from interdigitating and consequently a major source of wear and dissipation in polymer brush friction is eliminated. The presented method is therefore fundamentally different from sliding dry hydrophilic on hydrophobic brushes, where the friction will be determined by brush-specific interactions27. In fact, upon shearing PMMA on PNIPAM (collapsed height 166 nm) without solvents, we found that the friction was 50% higher compared to dry PMMA on PMMA (collapsed height 236 m).
As already pointed out shortly in the notes of the &#8216;Protocol&#8217; section, there are a couple of crucial points that need to be kept in mind while performing these particular experiments: Firstly, acetophenone is a better solvent for PNIPAM than water. Thus, care should be taken that acetophenone does not enter the PNIPAM brush by wetting the PNIPAM brush with plenty of water. Since acetophenone and water do not mix, the acetophenone will now not enter the PNIPAM brush. That is why we did not immerse our system completely in acetophenone, but instead created an acetophenone capillary for the miscible system. Another reason for incomplete immersion is that complete immersion results in too strong hydrodynamics, such that we only measured the Stokes drag on the colloid and cantilever. Secondly, in AFM experiments the torsional and normal spring constants are coupled. Cantilevers with a low normal spring constant will also have a relatively low torsional spring constant and vice versa. This limits the lowest measurable friction coefficient to &#62;10-3. Thus, in order to measure the full friction reduction, the friction for the miscible system needs to be high. This is achieved by using long high-density brushes and a relatively high shear velocity of typically 100 m/sec. Moreover, the capillary between the brushes also increases the friction forces. We measured the lowest friction coefficient, for an immiscible system22, of &#956; = 0.003 under an estimated normal stress of 200 kPa. Using the same experimental conditions, we found that &#956; = 0.15 for the miscible system.
Note that the experiments were performed in a controlled laboratory environment and that surfaces used in industry are not as ideal as used in the presented experiments. Most surfaces have a non-uniform roughness distribution28 and thus many asperities of different shapes and sizes. During the collision of two brush-bearing asperities, the friction is composed out of different dissipation channels29. Next to steady-state dissipation mechanisms, such as interdigitation and solvent flow, there will be hysteretic effects in the shape30 due to the slow relaxation time of the polymers and solvent. Besides, capillaries are formed and broken. In the traditionally used miscible brush systems, transient interdigitation31 amplifies shape- and capillary hysteresis. With the immiscible system presented here, transient interdigitation is eliminated too. Moreover, capillary hysteresis can be circumvented by application of two immiscible solvents. Therefore, also for the more common rough surfaces, friction and wear will be reduced using immiscible brush systems22. The main source of friction that remains is brush deformation. Anchoring polyzwitterionic polymers, which are known for their intrinsic low friction32, onto one of the surfaces can minimize the latter. In such systems the osmotic pressure of the solvent is high resulting in little brush deformation under high normal loads.
The presented method of immiscible brush systems can be applied in almost any system where low friction is desirable. The method functions well under high pressures. However, care should be taken that the temperature is kept around RT. High temperatures damage the polymers, which will cause liquid-flow out of the contact and consequently high friction. Examples of potential application are: syringes, piston systems, axle bearings and hinges.

Perfect lubricants reduce friction and wear for solids in relative motion even when normal loads are high. To achieve this, the lubricant should remain in the contact during sliding and at rest. However, under a positive normal load, simple, low-viscosity liquids are quickly squeezed out of the contact area and even higher-viscosity oils are expelled eventually. Yet, biological contacts, e.g., in human joints, remain lubricated with low-viscosity fluids at all times. Nature realizes such efficient lubrication using sugar chains attached to solid surfaces1. The hydrophilic sugar chains keep an aqueous liquid in the contact provided that the normal pressure does not exceed the osmotic pressure of the solvent2. Therefore, a lot of effort has been directed towards mimicking biological lubricants by grafting polymers to solid surfaces forming so-called polymer brushes3-12.
When two opposing polymer brushes are brought into contact, segments of the polymer chains on one side can move into the brush chain segments at the opposite side. This effect is called interdigitation13. When the brushes are in relative sliding motion, interdigitation is the main source of wear14 and friction15-17. In fact, recently, friction-velocity relations for sliding polymer brushes have been derived18. These scaling laws are based on interdigitation and the consequent stretching and bending of the polymers upon sliding. The main characteristics agree with results of surface forces apparatus experiments19 and of molecular dynamics (MD) simulations20. In the latter the degree of overlap can be directly quantified. Moreover, it was shown that the overlap between polyelectrolyte brushes can be tuned by applying an electric field21. Thus, if interdigitation can be circumvented, friction and wear in these systems would be significantly reduced.
In a recent publication22 we have shown via MD simulations that two immiscible solvated polymer brush systems prevent overlap between the brushes. Moreover, upon sliding the brushes, we found a decrease of the friction force by two orders of magnitude compared to the traditional miscible brush systems, in excellent agreement with our atomic force microscopy (AFM) measurements. Here, we explain in detail how to set up the AFM experiments of Ref. 22. The basic principle is sketched in Figure&#160;1. On the two counter-surfaces, two different brushes, each solvated by their own preferred solvent, are needed. In this configuration each brush remains in its own solvent. Consequently, polymer segments from one brush do not penetrate into the other brush. Poly(methyl methacrylate) (PMMA) is grafted from an AFM colloidal probe and the brush is solvated by acetophenone. From the flat surface poly(N-isopropylacrylamide) (PNIPAM) is grafted and solvated in water. To compare the present system to traditional miscible systems, a second flat counter-surface bearing a PMMA brush solvated in acetophenone is made. The measured friction force upon sliding the immiscible system of PMMA on PNIPAM is approximately 1% of the friction for the miscible system of PMMA on PMMA. Note that the use of these particular brush systems is just one example. The presented method is generic and works due to the preferred absorbance of the solvents in the different brushes. Therefore, more types of brushes are expected to be applicable, as long as the chosen solvents de-mix in the two brushes. The effect is amplified by using two non-mixing solvents (like acetophenone and water) such that an extra slippery fluid-on-fluid-sliding interface is created22,23.

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			<td height="42" style="height:42px;width:241px;">Name of Material/ Equipment</td>
			<td style="width:108px;">Company</td>
			<td style="width:105px;">Catalog Number</td>
			<td style="width:520px;">Comments/Description</td>
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			<td style="width:105px;"></td>
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		<tr height="21">
			<td height="21" style="height:21px;">Methyl methacrylate</td>
			<td style="width:108px;">Sigma-Aldrich</td>
			<td>M55909&#160;</td>
			<td>Monomer for PMMA synthesis, cleaned by pressing through a basic alumina column</td>
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		<tr height="21">
			<td height="21" style="height:21px;">3-aminopropyl)triethoxysilane</td>
			<td style="width:108px;">Sigma-Aldrich</td>
			<td>440140</td>
			<td>vapor deposited silane monolayer</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:241px;">triethylamine</td>
			<td style="width:108px;">Sigma-Aldrich</td>
			<td>T0886</td>
			<td>Reagent for the ATRP initiator moiety coupling.</td>
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		<tr height="21">
			<td height="21" style="height:21px;">2-bromo-2-methylpropionyl</td>
			<td style="width:108px;">Sigma-Aldrich</td>
			<td>252271</td>
			<td>ATRP initiator moiety.</td>
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			<td height="21" style="height:21px;width:241px;">toluene</td>
			<td style="width:108px;">&#160;Biosolve</td>
			<td style="width:105px;">20150501</td>
			<td>Coupling medium for ATRP moiety</td>
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		<tr height="21">
			<td height="21" style="height:21px;">CuBr</td>
			<td style="width:108px;">Sigma-Aldrich</td>
			<td>212865</td>
			<td>ATRP catalyst.</td>
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		<tr height="21">
			<td height="21" style="height:21px;">2,2&#8242;-Bipyridyl</td>
			<td style="width:108px;">Sigma-Aldrich</td>
			<td>14453</td>
			<td>Cu complexing ligand for ATRP of MMA</td>
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		<tr height="21">
			<td height="21" style="height:21px;">N,N,N&#8242;,N&#8242;&#8242;,N&#8242;&#8242;-Pentamethyldiethylenetriamine</td>
			<td style="width:108px;">Sigma-Aldrich</td>
			<td>369497</td>
			<td>Cu complexing ligand for ATRP of NIPAM</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">acetic acid 98-100%</td>
			<td style="width:108px;">Merck</td>
			<td>8187551000</td>
			<td>For&#160; cleaning CuBr.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Sulfuric acid</td>
			<td style="width:108px;">Sigma-Aldrich</td>
			<td>320501</td>
			<td>For the preparation of Piranha solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:241px;">Hydrogen peroxide 33%</td>
			<td style="width:108px;">&#160;Merck</td>
			<td style="width:105px;">&#160;1.07210.1000</td>
			<td>For the preparation of Piranha solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Ethanol</td>
			<td style="width:108px;">&#160;Merck</td>
			<td style="width:105px;">1.00983.1000</td>
			<td>For cleaning substrates.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Basic aluminum oxide 60&#160;</td>
			<td style="width:108px;">&#160;Merck</td>
			<td style="width:105px;"></td>
			<td>For cleaning monomers.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Chloroform</td>
			<td style="width:108px;">&#160;Biosolve</td>
			<td style="width:105px;">3080501</td>
			<td>For monolayer deposition and substrate cleaning.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:241px;">Methanol</td>
			<td style="width:108px;">&#160;Biosolve</td>
			<td style="width:105px;">13680501</td>
			<td>For polymerization medium.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Acetophenone</td>
			<td style="width:108px;">Acros Organics</td>
			<td>&#160;102410010</td>
			<td>For AFM measurement environment.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">N-isopropyl acrylamide</td>
			<td style="width:108px;">Acros Organics</td>
			<td>412780250</td>
			<td>Monomer for PNIPAM synthesis, recrystallized from toluene/hexane</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Poly(ethylene glycol) methacrylate&#160;</td>
			<td style="width:108px;">Sigma-Aldrich</td>
			<td>409529</td>
			<td>Monomer for Si-POEGMA synthesis, cleaned by pressing through a basic alumina column.</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;">MilliQ water</td>
			<td style="width:108px;">MilliQ Advantage A 10 purification system&#160;</td>
			<td style="width:105px;"></td>
			<td style="width:520px;">ATRP medium, AFM measurement environment 
			and for substrate cleaning.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Silicon substrates&#160;</td>
			<td style="width:108px;"></td>
			<td style="width:105px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:241px;">Gold coated substrates</td>
			<td style="width:108px;"></td>
			<td style="width:105px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:241px;">AFM probe, CP-FM-Au,&#160;</td>
			<td style="width:108px;">SQube</td>
			<td style="width:105px;"></td>
			<td>AFM measurement</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:241px;"></td>
			<td style="width:108px;"></td>
			<td style="width:105px;"></td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:241px;">dithiodiundecane-11,1-diybis[2-bromo-2-methlpropanoate] (DTPR)</td>
			<td style="width:108px;"></td>
			<td style="width:105px;"></td>
			<td style="width:520px;">Initiator, for Si-ATRP on gold surfaces. 
			Synthesized according to Macromolecules, 2000, 33,597.</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:241px;"></td>
			<td style="width:108px;"></td>
			<td style="width:105px;"></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Atomic Frorce Microscope</td>
			<td colspan="2">Bruker Multimode V controller</td>
			<td></td>
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preparation and friction force microscopy measurements of immiscible, opposing polymer brushes

Solvated polymer brushes are well known to lubricate high-pressure contacts, because they can sustain a positive normal load while maintaining low friction at the interface. Nevertheless, these systems can be sensitive to wear due to interdigitation of the opposing brushes. In a recent publication, we have shown via molecular dynamics simulations and atomic force microscopy experiments, that using an immiscible polymer brush system terminating the substrate and the slider surfaces, respectively, can eliminate such interdigitation. As a consequence, wear in the contacts is reduced. Moreover, the friction force is two orders of magnitude lower compared to traditional miscible polymer brush systems. This newly proposed system therefore holds great potential for application in industry. Here, the methodology to construct an immiscible polymer brush system of two different brushes each solvated by their own preferred solvent is presented. The procedure how to graft poly(N-isopropylacrylamide) (PNIPAM) from a flat surface and poly(methyl methacrylate) (PMMA) from an atomic force microscopy (AFM) colloidal probe is described. PNIPAM is solvated in water and PMMA in acetophenone. Via friction force AFM measurements, it is shown that the friction for this system is indeed reduced by two orders of magnitude compared to the miscible system of PMMA on PMMA solvated in acetophenone.

Figure 4 shows representative AFM force traces upon sliding both the miscible and the immiscible polymer brush systems. The friction force F is normalized by the friction force at steady state sliding Fsym for the symmetric, miscible system. The swollen brush height in these experiments was 1,010 nm for PMMA and 532 nm for PNIPAM. The force traces are captured after following the procedure described in the Protocol section. In these experiments the surface was moved back and forth with a velocity v of 80 &#956;m/sec while applying a normal load of 30 nN. The difference in friction force for the miscible (left panel) and immiscible (right panel) brush systems can be clearly observed. The steady state friction force in the left panel is 90x higher than the steady state friction force in right panel. For the immiscible system the measured friction force is typically 0.5&#8211;2% of the friction force measured for the miscible system. Though the exact friction reduction depends on the grafting density, degree of polymerization, amount of solvent, and (weakly) on the normal load and sliding velocity, it is always around two orders of magnitude. If we increase the sliding velocity for the system described above by a factor 5 (to 400 &#956;m/sec), the friction reduction decreases by 2%. If we increase the normal load by a factor 10 (to 300 nN), the friction reduction decreases by 3%.
<img alt="Figure 1" src="/files/ftp_upload/52285/52285fig1highres.jpg" /> 
Figure 1. Schematic sketch of the setup. Left panel shows the miscible system, where the same polymers are grafted from the surface and the colloid. The brushes are solvated in a one-phase liquid. The right panel shows the immiscible system of two different polymer brushes. Each brush is solvated in its own preferred liquid. In traditional miscible systems the polymers of the opposite brushes overlap. For the immiscible system, opposite brushes do not interdigitate such that friction and wear during sliding is reduced.
<img alt="Figure 2" src="/files/ftp_upload/52285/52285fig2highres.jpg" width="700px" /> 
Figure 2. Schematic sketch of the sample preparation procedure as described in the Protocols section. From left to right shows the procedure of brush preparation via initiator deposition and surface initiated atom transfer radical polymerization (SI ATRP). Path (A) describes the brushes grafted from silicon surfaces, (B) brushes grafted from gold coated silicon surfaces and (C) brushes grafted from gold colloids on atomic force microscopy probes. <a href="https://www.jove.com/files/ftp_upload/52285/52285fig2highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/52285/52285fig3highres.jpg" /> 
Figure 3. FTIR spectra of the PMMA (blue) and PNIPAM (green) brushes on silicon (thick lines) and gold (thin lines). The data was taken from Suppl. Mat. of Ref. 22. PMMA wavenumbers (cm&#8722;1): 3,050&#8211;2,990 (CH stretching vibration), 1,730 C=O (double bond stretching vibration), 1,450 (CH3 and CH2 deformation vibration), 1,260&#8211;1,040 (C-O-C single bond stretching vibration), 880&#8211;960 (C-O-C single bond deformation vibration). At 1,730 cm&#8722;1 the characteristic stretching vibration peak of the C=O group is apparent. PNIPAM wavenumbers (cm-1): 3,289 (N-H symmetric and asymmetric stretching vibration), 3,078, 2,971, 2,933, 2,874 (asymmetric and symmetric C-H stretching vibration in -CH2-), 1,635 (C=O stretching vibration), 1,535 (amide II), 1,458 (C-H asymmetric bending deformations), 1,386 (C-H symmetric bending deformations), 1,366&#8211;1,170 (C-N asymmetric stretching vibrations). At 1,635 and 1,535 cm&#8722;1 the characteristic stretching vibration peaks of the amide group are apparent.
<img alt="Figure 4" src="/files/ftp_upload/52285/52285fig4highres.jpg" width="700px" /> 
Figure 4. Averaged, filtered and smoothed force traces upon sliding the miscible (left) and immiscible (right) systems (adjusted from Ref. 22). The surface is moved back and forth by 40 &#956;m at a scan-rate of 1 Hz and normal load of 30 nN.

Watch this Scientific Journal Video about Preparation and Friction Force Microscopy Measurements of Immiscible, Opposing Polymer Brushes at JoVE.com

Preparation and Friction Force Microscopy Measurements of Immiscible, Opposing Polymer Brushes

The methodology to perform friction force microscopy experiments for contacting brushes is presented: Two polymer brushes that are grafted from (a) substrates and (b) colloidal probes are slid to show that, by using two contacting immiscible brush systems, friction in sliding contacts is reduced compared to miscible brush systems.

Solvated polymer brushes are well known to lubricate high-pressure contacts, because they can sustain a positive normal load while maintaining low ...

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Engineering

Atomic Force Microscopy of Red-Light Photoreceptors Using PeakForce Quantitative Nanomechanical Property Mapping

Atomic force microscopy (AFM) uses a pyramidal tip attached to a cantilever to probe the force response of a surface. The deflections of the tip can be measured to ~10 pN by a laser and sectored detector, which can be converted to image topography. Amplitude modulation or &#8220;tapping mode&#8221; AFM involves the probe making intermittent contact with the surface while oscillating at its resonant frequency to produce an image. Used in conjunction with a fluid cell, tapping-mode AFM enables the imaging of biological macromolecules such as proteins in physiologically relevant conditions. Tapping-mode AFM requires manual tuning of the probe and frequent adjustments of a multitude of scanning parameters which can be challenging for inexperienced users. To obtain high-quality images, these adjustments are the most time consuming.
PeakForce Quantitative Nanomechanical Property Mapping (PF-QNM) produces an image by measuring a force response curve for every point of contact with the sample. With ScanAsyst software, PF-QNM can be automated. This software adjusts the set-point, drive frequency, scan rate, gains, and other important scanning parameters automatically for a given sample. Not only does this process protect both fragile probes and samples, it significantly reduces the time required to obtain high resolution images. PF-QNM is compatible for AFM imaging in fluid; therefore, it has extensive application for imaging biologically relevant materials.
The method presented in this paper describes the application of PF-QNM to obtain images of a bacterial red-light photoreceptor, RpBphP3 (P3), from photosynthetic R. palustris in its light-adapted state. Using this method, individual protein dimers of P3 and aggregates of dimers have been observed on a mica surface in the presence of an imaging buffer. With appropriate adjustments to surface and/or solution concentration, this method may be generally applied to other biologically relevant macromolecules and soft materials.

A method for investigating the structure of a protein photoreceptor using atomic force microscopy (AFM) is described in this paper. PeakForce Quantitative Nanomechanical Property Mapping (PF-QNM) reveals intact protein dimers on a mica surface.

Atomic force microscopy (AFM) uses a pyramidal tip attached to a cantilever to probe the force response of a surface. The deflections of the tip can be ...

Sub-nanometer Resolution Imaging with Amplitude-modulation Atomic Force Microscopy in Liquid

Atomic force microscopy (AFM) has become a well-established technique for nanoscale imaging of samples in air and in liquid. Recent studies have shown that when operated in amplitude-modulation (tapping) mode, atomic or molecular-level resolution images can be achieved over a wide range of soft and hard samples in liquid. In these situations, small oscillation amplitudes (SAM-AFM) enhance the resolution by exploiting the solvated liquid at the surface of the sample. Although the technique has been successfully applied across fields as diverse as materials science, biology and biophysics and surface chemistry, obtaining high-resolution images in liquid can still remain challenging for novice users. This is partly due to the large number of variables to control and optimize such as the choice of cantilever, the sample preparation, and the correct manipulation of the imaging parameters. Here, we present a protocol for achieving high-resolution images of hard and soft samples in fluid using SAM-AFM on a commercial instrument. Our goal is to provide a step-by-step practical guide to achieving high-resolution images, including the cleaning and preparation of the apparatus and the sample, the choice of cantilever and optimization of the imaging parameters. For each step, we explain the scientific rationale behind our choices to facilitate the adaptation of the methodology to every user's specific system.

We present a method for achieving sub-nanometer resolution images with amplitude-modulation (tapping mode) atomic force microscopy in liquid. The method is demonstrated on commercial atomic force microscopes. We explain the rationale behind our choices of parameters and suggest strategies for resolution optimization.

Atomic force microscopy (AFM) has become a well-established technique for nanoscale imaging of samples in air and in liquid. Recent studies have shown ...

Generating Lap Joints Via Friction Stir Spot Welding on DP780 Steel

Friction stir spot welding (FSSW), a derivative of friction stir welding (FSW), is a solid-state welding technique that was developed in 1991. An industry application was found in the automotive industry in 2003 for the aluminum alloy that was used in the rear doors of automobiles. Friction stir spot welding is mostly used in Al alloys to create lap joints. The benefits of friction stir spot welding include a nearly 80% melting temperature that lowers the thermal deformation welds without splashing compared to resistance spot welding. Friction stir spot welding includes 3 steps: plunging, stirring, and retraction. In the present study, other materials including high strength steel are also used in the friction stir welding method to create joints. DP780, whose traditional welding process involves the use of resistance spot welding, is one of several high strength steel materials used in the automotive industry. In this paper, DP780 was used for friction stir spot welding, and its microstructure and microhardness were measured. The microstructure data showed that there was a fusion zone with fine grain and a heat effect zone with island martensite. The microhardness results indicated that the center zone exhibited a greater degree of hardness compared with the base metal. All data indicated that the friction stir spot welding used in dual phase steel 780 can create a good lap joint. In the future, friction stir spot welding can be used in high-strength steel welding applied in industrial manufacturing processes.

Here, we present a friction stir spot welding (FSSW) protocol on dual phase 780 steel. A tool pin with high-speed rotation generates heat from friction to soften the material, and then, the pin plunges into 2 sheet joints to create the lap joint.

Friction stir spot welding (FSSW), a derivative of friction stir welding (FSW), is a solid-state welding technique that was developed in 1991. An industry ...

X-ray Beam Induced Current Measurements for Multi-Modal X-ray Microscopy of Solar Cells

X-ray beam induced current (XBIC) measurements allow mapping of&#160;the nanoscale performance of electronic devices such as solar cells. Ideally, XBIC is employed simultaneously with other techniques within a multi-modal X-ray microscopy approach. An example is given herein combining XBIC with X-ray fluorescence to enable point-by-point correlations of the electrical performance with chemical composition. For the highest signal-to-noise ratio in XBIC measurements, lock-in amplification plays a crucial role. By this approach, the X-ray beam is modulated by an optical chopper upstream of the sample. The modulated X-ray beam induced electrical signal is amplified and demodulated to the chopper frequency using a lock-in amplifier. By optimizing low-pass filter settings, modulation frequency, and amplification amplitudes, noise can efficiently be suppressed for the extraction of a clear XBIC signal. A similar setup can be used to measure the X-ray beam induced voltage (XBIV). Beyond standard XBIC/XBIV measurements, XBIC can be measured with bias light or bias voltage applied such that outdoor working conditions of solar cells can be reproduced during in-situ and operando measurements. Ultimately, the multi-modal and multi-dimensional evaluation of electronic devices at the nanoscale enables new insights into the complex dependencies between composition, structure, and performance, which is an important step towards solving the materials&#39; paradigm.

A setup for X-ray beam induced current measurements at synchrotron beamlines is described. It unveils the nanoscale performance of solar cells and extends the suite of techniques for multi-modal X-ray microscopy. From wiring to signal-to-noise optimization, it is shown how to perform state-of-the-art XBIC measurements at a hard X-ray microprobe.

X-ray beam induced current (XBIC) measurements allow mapping of&#160;the nanoscale performance of electronic devices such as solar cells. Ideally, XBIC is ...

Hydrogen Charging of Aluminum using Friction in Water

A new method of hydrogen charging of aluminum was developed by means of a friction in water (FW) procedure. This procedure can easily introduce high amounts of hydrogen into aluminum based on the chemical reaction between water and non-oxide coated aluminum.

In order to introduce high amounts of hydrogen in aluminum and aluminum alloys, a new method of hydrogen charging was developed, called the friction in water procedure.

A new method of hydrogen charging of aluminum was developed by means of a friction in water (FW) procedure. This procedure can easily introduce high ...

Co-localizing Kelvin Probe Force Microscopy with Other Microscopies and Spectroscopies: Selected Applications in Corrosion Characterization of Alloys

Kelvin probe force microscopy (KPFM), sometimes referred to as surface potential microscopy, is the nanoscale version of the venerable scanning Kelvin probe, both of which measure the Volta potential difference (VPD) between an oscillating probe tip and a sample surface by applying a nulling voltage equal in magnitude but opposite in sign to the tip-sample potential difference. By scanning a conductive KPFM probe over a sample surface, nanoscale variations in surface topography and potential can be mapped, identifying likely anodic and cathodic regions, as well as quantifying the inherent material driving force for galvanic corrosion.
Subsequent co-localization of KPFM Volta potential maps with advanced scanning electron microscopy (SEM) techniques, including back scattered electron (BSE) images, energy dispersive spectroscopy (EDS) elemental composition maps, and electron backscattered diffraction (EBSD) inverse pole figures can provide further insight into structure-property-performance relationships. Here, the results of several studies co-localizing KPFM with SEM on a wide variety of alloys of technological interest are presented, demonstrating the utility of combining these techniques at the nanoscale to elucidate corrosion initiation and propagation.
Important points to consider and potential pitfalls to avoid in such investigations are also highlighted: in particular, probe calibration and the potential confounding effects on the measured VPDs of the testing environment and sample surface, including ambient humidity (i.e., adsorbed water), surface reactions/oxidation, and polishing debris or other contaminants. Additionally, an example is provided of co-localizing a third technique, scanning confocal Raman microscopy, to demonstrate the general applicability and utility of the co-localization method to provide further structural insight beyond that afforded by electron microscopy-based techniques.

Kelvin probe force microscopy (KPFM) measures surface topography and differences in surface potential, while scanning electron microscopy (SEM) and associated spectroscopies can elucidate surface morphology, composition, crystallinity, and crystallographic orientation. Accordingly, the co-localization of SEM with KPFM can provide insight into the effects of nanoscale composition and surface structure on corrosion.

Kelvin probe force microscopy (KPFM), sometimes referred to as surface potential microscopy, is the nanoscale version of the venerable scanning Kelvin ...

Optimizing Magnetic Force Microscopy Resolution and Sensitivity to Visualize Nanoscale Magnetic Domains

Magnetic force microscopy (MFM) enables mapping local magnetic fields across a sample surface with nanoscale resolution. To perform MFM, an atomic force microscopy (AFM) probe whose tip has been magnetized vertically (i.e., perpendicular to the probe cantilever) is oscillated at a fixed height above the sample surface. The resultant shifts in the oscillation phase or frequency, which are proportional to the magnitude and sign of the vertical magnetic force gradient at each pixel location, are then tracked and mapped. Although the spatial resolution and sensitivity of the technique increases with decreasing lift height above the surface, this seemingly straightforward path to improved MFM images is complicated by considerations such as minimizing topographical artifacts due to shorter range van der Waals forces, increasing the oscillation amplitude to further improve sensitivity, and the presence of surface contaminants (in particular water due to humidity under ambient conditions). In addition, due to the orientation of the probe's magnetic dipole moment, MFM is intrinsically more sensitive to samples with an out-of-plane magnetization vector. Here, high-resolution topographical and magnetic phase images of single and bicomponent nanomagnet artificial spin-ice (ASI) arrays obtained in an inert (argon) atmosphere glovebox with &lt;0.1 ppm O2 and H2O are reported. Optimization of lift height and drive amplitude for high resolution and sensitivity while simultaneously avoiding the introduction of topographical artifacts is discussed, and detection of the stray magnetic fields emanating from either end of the nanoscale bar magnets (~250 nm long and &lt;100 nm wide) aligned in the plane of the ASI sample surface is shown. Likewise, using the example of a Ni-Mn-Ga magnetic shape memory alloy (MSMA), MFM is demonstrated in an inert atmosphere with magnetic phase sensitivity capable of resolving a series of adjacent magnetic domains each ~200 nm wide.

Magnetic force microscopy (MFM) employs a vertically magnetized atomic force microscopy probe to measure sample topography and local magnetic field strength with nanoscale resolution. Optimizing MFM spatial resolution and sensitivity requires balancing decreasing lift height against increasing drive (oscillation) amplitude, and benefits from operating in an inert atmosphere glovebox.

Magnetic force microscopy (MFM) enables mapping local magnetic fields across a sample surface with nanoscale resolution. To perform MFM, an atomic force ...

Atomic Force Microscopy Cantilever-Based Nanoindentation: Mechanical Property Measurements at the Nanoscale in Air and Fluid

An atomic force microscope (AFM) fundamentally measures the interaction between a nanoscale AFM probe tip and the sample surface. If the force applied by the probe tip and its contact area with the sample can be quantified, it is possible to determine the nanoscale mechanical properties (e.g., elastic or Young&#39;s modulus) of the surface being probed. A detailed procedure for performing quantitative AFM cantilever-based nanoindentation experiments is provided here, with representative examples of how the technique can be applied to determine the elastic moduli of a wide variety of sample types, ranging from kPa to GPa. These include live mesenchymal stem cells (MSCs) and nuclei in physiological buffer, resin-embedded dehydrated loblolly pine cross-sections, and Bakken shales of varying composition.
Additionally, AFM cantilever-based nanoindentation is used to probe the rupture strength (i.e., breakthrough force) of phospholipid bilayers. Important practical considerations such as method choice and development, probe selection and calibration, region of interest identification, sample heterogeneity, feature size and aspect ratio, tip wear, surface roughness, and data analysis and measurement statistics are discussed to aid proper implementation of the technique. Finally, co-localization of AFM-derived nanomechanical maps with electron microscopy techniques that provide additional information regarding elemental composition is demonstrated.

Quantifying the contact area and force applied by an atomic force microscope (AFM) probe tip to a sample surface enables nanoscale mechanical property determination. Best practices to implement AFM cantilever-based nanoindentation in air or fluid on soft and hard samples to measure elastic modulus or other nanomechanical properties are discussed.

An atomic force microscope (AFM) fundamentally measures the interaction between a nanoscale AFM probe tip and the sample surface. If the force applied by ...

Determination of the Friction Coefficients of Icy Pavements Under Different Amounts of Snowfall

Ice on road surfaces can lead to a significant decrease in the friction coefficient, thus endangering driving safety. However, there are still no studies that provide exact friction coefficient values for pavements covered in ice, which is detrimental to both road design and the selection of winter road maintenance measures. Therefore, this article presents an experimental method to determine the friction coefficient of icy road surfaces in the winter. A British portable tester (BPT), also known as a pendulum friction coefficient meter, was employed for the experiment. The experiment was divided into the following five steps: the preparation of the equipment, the calculation and analysis of the snowfall, equipment calibration, friction coefficient determination, and data analysis. The accuracy of the final experiment is directly affected by the equipment accuracy, which is described in detail. Moreover, this article suggests a method for calculating the ice thickness for corresponding amounts of snowfall. The results illustrate that even patchy ice formed by very light snowfall may lead to a significant decrease in the friction coefficient of the pavement, thus endangering driving safety. Additionally, the friction coefficient is at its peak when the ice thickness reaches 5 mm, meaning protection measures should be taken to avoid the formation of such ice.

Here, we present a method for determining the friction coefficient of pavements with different ice thicknesses indoors. The complete procedure includes the preparation of the equipment, the calculation and analysis of the snowfall, equipment calibration, friction coefficient determination, and data analysis.

Ice on road surfaces can lead to a significant decrease in the friction coefficient, thus endangering driving safety. However, there are still no studies ...

Parallel Active Cantilever Arrays in AFMS to Enable High-Throughput Inspections

Active Probe Atomic Force Microscopy with Quattro-Parallel Cantilever Arrays for High-Throughput Large-Scale Sample Inspection

An Atomic Force Microscope&#160;(AFM) is&#160;a powerful and versatile tool for nanoscale surface studies to capture 3D topography images of samples. However, due to their limited imaging throughput, AFMs have not been widely adopted for large-scale inspection purposes. Researchers have developed high-speed AFM systems to record dynamic process videos in chemical and biological reactions at tens of frames per second, at the cost of a small imaging area of up to several square micrometers. In contrast, inspecting large-scale nanofabricated structures, such as semiconductor wafers, requires nanoscale spatial resolution imaging of a static sample over hundreds of square centimeters with high productivity. Conventional AFMs use a single passive cantilever probe with an optical beam deflection system, which can only collect one pixel at a time during AFM imaging, resulting in low imaging throughput. This work utilizes an array of active cantilevers with embedded piezoresistive sensors and thermomechanical actuators, which allows simultaneous multi-cantilever operation in parallel operation for increased imaging throughput. When combined with large-range nano-positioners and proper control algorithms, each cantilever can be individually controlled to capture multiple AFM images. With data-driven post-processing algorithms, the images can be stitched together, and defect detection can be performed by comparing them to the desired geometry. This paper introduces principles of the custom AFM using the active cantilever arrays, followed by a discussion on practical experiment considerations for inspection applications. Selected example images of silicon calibration grating, highly-oriented pyrolytic graphite, and extreme ultraviolet lithography masks are captured using an array of four active cantilevers (&#34;Quattro&#34;) with a 125 &#181;m tip separation distance. With more engineering integration, this high-throughput, large-scale imaging tool can provide 3D metrological data for extreme ultraviolet (EUV) masks, chemical mechanical planarization (CMP) inspection, failure analysis, displays, thin-film step measurements, roughness measurement dies, and laser-engraved dry gas seal grooves.

Author Spotlight: Introduction to Active Probe Atomic Force Microscopy with Quattro-Parallel Cantilever Arrays

Large-scale sample inspection with nanoscale resolution has a wide range of applications, especially for nanofabricated semiconductor wafers. Atomic force microscopes can be a great tool for this purpose, but are limited by their imaging speed. This work utilizes parallel active cantilever arrays in AFMs to enable high-throughput and large-scale inspections.

An Atomic Force Microscope&#160;(AFM) is&#160;a powerful and versatile tool for nanoscale surface studies to capture 3D topography images of samples. ...