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Article
Effects of Lateral Deformation by Thermoresponsive Polymer Brushes on the Measured Friction Forces Shivaprakash N. Ramakrishna, Marco Cirelli, Mohammad Divandari, and Edmondo Maria Benetti Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00217 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017
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Effects of Lateral Deformation by Thermoresponsive Polymer Brushes on the Measured Friction Forces 7
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Shivaprakash N. Ramakrishnaa,* Marco Cirellib, Mohammad Divandaria, Edmondo M. Benettia* 10 12
1 a
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Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich,
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Vladimir-Prelog-Weg 5/10, 8093 Zurich, Switzerland. 16 18
17 b
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Department of Materials Science and Technology of Polymers, MESA+ Institute for
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Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands 2
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* Corresponding Authors:
[email protected]; 28 30
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[email protected] 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment
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Abstract 5
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The nanotribological properties of hydrophilic polymer brushes are conveniently analyzed by 6 7
lateral force microscopy (LFM). However, the measurement of friction for highly swollen and 10
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relatively thick polymer brushes can be strongly affected by the tendency of the compliant brush 12
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to be laterally deformed by the shearing probe. This phenomenon induces a “tilting” in the 13 14
recorded friction loops, which is generated by the lateral bending and stretching of the grafts. In 17
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this study we highlight how the brush lateral deformation mainly affects the friction 19
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measurements of swollen PNIPAM brushes (below LCST) when relatively short scanning 20 2
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distances are applied. Under these conditions, the energy dissipation recorded by LFM is almost 24
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uniquely determined by stretching and bending of the compliant brush back and forth along the 25 26
scanning direction, and it is not correlated to dynamic friction between two sliding surfaces. In 27 29
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contrast, when the scanning distance applied during LFM is relevantly longer than the brush 31
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lateral deformation, sliding of the probe on the brush interface becomes dominant, and a correct 32 3
measurement of dynamic friction can be accomplished. By increasing the temperature above the 36
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LCST, the PNIPAM brushes undergo dehydration and assume a collapsed morphology, thereby 38
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hindering their lateral deformation by scanning probe. Hence, at 40°C in water the recorded 39 41
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friction loops do not show any tilting and LFM accurately describes the dynamic friction 43
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between the probe and the polymer surface. 4 45 46 47 48
KEYWORDS: atomic force microscopy; polymer brushes; friction; stimuli-responsive polymers; 51
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thin films; mechanical properties. 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment
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Introduction 5
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The lubricating properties of hydrophilic polymer brushes have been increasingly attracted the 6 7
attention of materials scientists1-3 and tribologists4-10 during the last two decades. Due to a 10
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combination of osmotic force and conformational entropy, densely grafted, swollen brushes can 12
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act as efficient boundary lubricants within aqueous media. This distinctive feature is revealed by 13 14
a markedly low coefficient of friction (COF) recorded on brush-coated surfaces by micro- and 17
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nano-tribometers,11-16 surface forces apparatus (SFA)17-19 and atomic force microscopy 19
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(AFM).7,20-22 The frictional dissipation mechanisms, as well as the effects of solvent quality on 20 2
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the nanotribological properties of polymer brushes were intensively studied both experimentally 24
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and by simulation.29-33 In particular, AFM-based techniques, such as lateral force
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microscopy (LFM), have been applied to study nanoscale friction generated between identical 27 29
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tribopairs, usually fabricated by grafting-to34 of surface reactive copolymers.25,35,36 Alternatively, 31
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LFM coupled to grafting-from methods, such as surface-initiated controlled radical 32 3
polymerizations (SI-CRP) were employed to investigate the nanotribological properties of 36
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asymmetric tribopairs, concentrating on the interaction between sharp or colloidal AFM probes 38
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and relatively thick brush layers. 37-40 39 41
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A particular interest was devoted to the nanotribological properties of stimuli-responsive 43
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polymer brushes, and especially poly(N-isopropylacrylamide) (PNIPAM)-based grafts, which 45
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across the lower critical solution temperature (LCST) of 30-33°C in water undergo a transition 46 48
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from swollen to dehydrated.41,42 AFM methods were applied for measuring friction and 50
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nanomechanical properties with nano-newton resolution between a well-defined asperity and 52
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PNIPAM brushes presenting different thicknesses and grafting densities.6,39,43-46 Although 53 5
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several studies helped dissecting the influence of these and other brush parameters on the 56 57 58 59 60 ACS Paragon Plus Environment
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nanotribological characteristics of PNIPAM brushes, a comprehensive and unambiguous 5
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understanding of the mechanics of interaction between the AFM probe and the brush surface 6 7
across its LCST is still missing. 10
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In this work, we demonstrate that the tendency of the PNIPAM grafts to be laterally 1 12
deformed by the shearing AFM probe, which varies over the temperature-induced 13 15
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transition, determines brush-probe interactions, while a throughout understanding of 17
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brush lateral deformability enables the correct measurement of friction by LFM. In 18 19
particular, we address these issues by analysing the frictional properties of PNIPAM 20 2
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brushes presenting different thicknesses below and above LCST. 24
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We recently demonstrated that the measurement of friction between polymer brushes and 25 27
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an AFM probe includes different contributions originating from the reciprocal sliding of 29
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the two surfaces, plus the lateral bending and stretching of the tethered polymer 31
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chains.46,47 The impact of this latter term on the measurement of friction can be 32 3
conveniently visualized through the analysis of the friction “loops” recorded using 36
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LFM.46,47 A tilted section of the loop is ascribed to the initial lateral deformation of the 37 38
swollen brush, while a following, flat friction force trace is produced by the steady sliding 39 41
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of the probe on the polymer interface. 43
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For PNIPAM brushes immersed in water below their LCST the lateral deformation by the 4 45
compliant brush affects the dissipation recorded via the friction loops, and thus the 46 48
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applied scanning distance must be accordingly tuned to obtain a reliable value of COF. 50
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In contrast, above LCST lateral deformability of the collapsed brush becomes irrelevant, 51 53
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and sliding of the probe on the brush interface uniquely determines the frictional 5
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dissipation. 56 57 58 59 60 ACS Paragon Plus Environment
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EXPERIMENTAL SECTION 6 7
Synthesis of PNIPAM brushes of different thicknesses 10
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PNIPAM brushes of two different thicknesses were synthesized by surface-initiated atom 1 12
radical polymerization (SI-ATRP) from initiator-functionalized silicon substrates (P/B 13 15
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, Si-mat, Germany) according to the previously reported procedure. 46 The catalyst 17
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system comprised CuBr (Sigma Aldrich), CuBr 2 (Sigma Aldrich) and N,N,N′,N′′,N′′18 19
pentamethyldiethylenetriamine (PMDETA) (Sigma Aldrich) in water/methanol mixtures. 20 2
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The chemical composition of the so-fabricated PNIPAM brushes was confirmed by 24
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Fourier-transform infrared spectroscopy (FTIR) using a spectrometer (BIO-RAD 25 27
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FTS575C FTIR equipped with a nitrogen-cooled cryogenic cadmium mercury telluride 29
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detector) with spectral resolution of 8 cm-1, applying 2048 scans for both the background 31
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spectra (performed on freshly cleaned silicon substrates) and the PNIPAM brush32 34
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modified samples. The surface-modification steps were also verified by contact angle 36
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(CA) measurements, using the sessile drop method. Both FTIR and CA characterizations 37 38
are reported in the Supporting Information (Figure S1 and Figure S2). 39 40 41 43
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Ellipsometry 45
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The dry and wet thicknesses of PNIPAM brushes were measured by variable angle spectroscopic 46 48
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ellipsometry (VASE) using a Woolam ellipsometer (J.A. Woolam Co. U.S.) equipped with a 50
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custom-built, liquid cell and a temperature controller. Ψ and Δ as a function of wavelength (27551 52
827 nm) were analyzed employing the package CompleteEASE (Woollam), using bulk dielectric 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment
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functions for silicon, silicon dioxide and water. The brush-underlying substrates were considered 5
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as silicon dioxide film on top of silicon substrate. 6 7
The analysis of the brush layers were performed using the Cauchy model: n = A + B·λ -2 + C·λ-4, 10
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where A, B and C represent the fitting parameters. We used C = 0 and assumed that the polymer 12
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films are fully transparent, i.e. the refractive index is a real quantity (the imaginary part is 13 14
neglected). In the case of dry PNIPAM brushes, a homogeneous layer with a refractive index 17
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expressed with a single Cauchy relation was considered. The films thickness d and the two 19
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Cauchy parameters A and B were used as fitting parameters. The dry thickness was obtained by 20 2
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performing the measurements at three different incident angles, namely 65°, 70° and 75°. 24
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For PNIPAM brushes immersed in water at 25 °C we considered a density gradient across the 25 26
film thickness employing a two-layers model to describe the optical response of the films (dense 27 29
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polymeric layer + graded polymeric layer) as it was previously described in detail. 39,46 For 31
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PNIPAM brushes immersed in water at 40°C a single-layer model was used.48 32 3 34 35 36 37 38 39 41
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Atomic Force Microscopy Measurements 43
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AFM measurements were carried out using a MFP-3D (Asylum Research, an Oxford 4 46
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Instruments company, Santa Barbara, CA, USA) equipped with a bio heater. Height micrographs 48
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of the brush surfaces were performed in milli-Q water below and above LCST of PNIPAM 49 50
(25°C and 40°C) using AC mode. Cantilevers (SNL-10, D triangular) from Bruker Corporation, 51 53
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having a normal spring constant of ~ 0.06 N·m-1 were used for the tapping mode imaging in 5
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liquid. 56 57 58 59 60 ACS Paragon Plus Environment
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Normal and lateral force measurements were carried out in milli-Q water at 25 and 40 °C, using 5
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a silica microsphere glued on a tip-less cantilever. The normal spring constant of the Au-coated, 6 7
cantilever (NSC-12, Mikromash, Bulgaria) was measured by thermal-noise method49 and the 10
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torsional spring constant was estimated according to the Sader’s method.50 Both the normal and 12
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the torsional spring constants of the cantilever were measured before attaching the colloidal 13 14
microsphere. A silica sphere of radius ~ 8 µm (EKA chemicals AB, Kromasil R) was glued with 17
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a two component Araldite glue to the end of the tip-less cantilever by means of a home-built 19
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micromanipulator.51 The colloidal probe was treated with UV/Ozone (BioForces Nanosciences) 20 2
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for 30 min just before the measurement. 24
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The adhesive properties of PNIPAM brushes, below and above LCST were examined by 25 26
acquiring the force vs distance (F-D) curves (40 curves) over 3 different areas. Gaussian function 27 29
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was used to calculate the distribution of the adhesion force over measured 120 F-D curves. 31
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Lateral-force calibration was done by using the “test-probe method” described by Cannara et 32 3
al.52 A freshly cleaned, smooth edge of the silicon wafer was used as a “wall” for measuring the 36
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lateral sensitivity. A test probe (cantilever glued with a silica colloidal sphere of diameter around 38
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40 μm) was moved laterally into contact with the wall. The lateral sensitivity of the photo 39 41
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detector was acquired by measuring the slope of the lateral deflection vs piezo displacement 43
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curve (a detailed procedure regarding the lateral force calibration is reported in the Supporting 45
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Information). Friction loops were recorded by lateral force microscopy (LFM) scanning the 46 48
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cantilever laterally over the brush surfaces. From the friction loops, the true friction values were 50
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obtained by averaging trace and retrace curves. Friction coefficient (COF) values were calculated 51 52
by the slope of friction vs applied load plots assuming Amontons’ law (F=µL). Where F is 53 5
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friction force, µ is the co-efficient of friction and L is the applied load. 56 57 58 59 60 ACS Paragon Plus Environment
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Results and Discussion 6 7
PNIPAM brushes were synthesized from initiator-functionalized silicon substrates by SI-ATRP, 10
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varying the polymerization time and the catalyst/monomer mixture in order to obtain the desired 12
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brush thickness.46 In particular, we aim to compare the frictional properties of relatively thin and 13 14
thick PNIPAM brushes. Thus, two types of PNIPAM brushes alternatively presenting 17
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ellipsometric dry thicknesses of 11 ± 1 and 417 ± 4 nm (Table 1) were synthesized (named thin 19
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PNIPAM and thick PNIPAM, respectively). Fourier transform infrared spectroscopy (FTIR) and 20 2
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water contact angle (CA) confirmed the successful formation of uniform PNIPAM films from 24
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initiator-functionalized silicon oxide substrates (data reported in the Supporting Information). 25 26
The dry thickness values recorded by VASE were additionally confirmed by AFM step-height 27 29
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measurements, imaging the height difference between the brush surface and areas on the 31
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substrates where the films were mechanically removed by means of a plastic tweezer (Figure 32 3
1).53 In addition, an average PNIPAM grafting density of 0.43 chains nm-2 was estimated by 36
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detaching the grafts via HF treatment, and subsequently analyzing the polymer sample by gel 38
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permeating chromatography (GPC) (see Supporting Information for details). 39 41
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The morphology of thin and thick PNIPAM immersed in water both below (25 °C) and above 43
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the LCST (40 °C) were subsequently studied by tapping mode AFM (Figure 2). Below LCST, 45
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PNIPAM brushes showed smooth topographies, characteristic of swollen brushes, and presented 46 48
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a root mean square (RMS) roughness of 0.1 ± 0.02 and 1.5 ± 0.5 nm, for thin PNIPAM and thick 50
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PNIPAM, respectively (RMS values were calculated over an area of 5x5 µm2). 51 52
Above the LCST, a marked increase of roughness was observed, due to the aggregation between 53 5
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PNIPAM grafts coupled to their vertical collapse. The formation of globular aggregates 56 57 58 59 60 ACS Paragon Plus Environment
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constituted by collapsed grafts was clearly observed for both thin and thick PNIPAM, and 5
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produced an increase of RMS values that reached 2 ± 0.3 and 8 ± 1 nm, respectively (calculated 6 7
over an area of 5x5 µm2) (Figure 2).54 9
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Figure 1. Tapping mode AFM micrographs (a,b) and corresponding section profiles (c,d) 36 37
recorded over the “scratched” brush and the silicon surface for thin PNIPAM and thick 40
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PNIPAM. The red lines highlighted in the micrographs in a) and b) indicate where the section 41 42
profiles reported in c) and d) were measured. The dotted lines in c) indicate the average values of 43 45
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thickness. The images were recorded under dry condition, and in Milli-Q water at 25° and 40°C. 47
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A cantilever (SNL-10, D triangular) from Bruker Corporation, presenting a normal spring 49
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constant of ~ 0.06 N m-1 was used for the tapping mode imaging in liquid. The scanning speed 50 52
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was kept at 1 Hz. The amplitude set point and the drive amplitude were set at 0.8 V and 0.3 V, 54
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respectively. 5 56 57 58 59 60 ACS Paragon Plus Environment
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The swelling properties of thin and thick PNIPAM in Milli-Q water were examined by VASE 5
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and AFM step-height measurements. The average thickness of PNIPAM brushes below LCST, 6 7
was measured by VASE as 47 ± 6 and 1033 ± 22 nm, for thin and thick PNIPAM, respectively 10
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(Table 1). When compared to the corresponding dry thickness values, swelling ratios of 3.2 and 12
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1.4 were calculated for the two polymer brush layers (calculated as the difference between 13 14
swollen thickness and dry thickness, divided by dry thickness). Above the LCST, the 17
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temperature-driven collapse produced a marked decrease of the ellipsometric thicknesses, which 19
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at 40 °C in water reached 13 ± 4 and 486 ± 2 nm for thin and thick PNIPAM, respectively. 20 2
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The thickness values obtained by VASE were subsequently confirmed by AFM step-height 24
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measurements. As shown in Figure 1 and Table 1, thin and thick PNIPAM brushes immersed in 25 26
water at 25°C showed height values of 33 ± 8 and 655 ± 15 nm, respectively. In contrast, at 40°C 27 29
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the height of the brushes dropped to 19 ± 10 and 440 ± 12 nm, due to the collapse of the 31
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PNIPAM grafts. The deviation of these height values from the swelling data obtained by VASE 32 3
was probably due to the compression of the brush by the AFM tip while scanning over the brush 36
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surfaces.53 This effect resulted more marked at 25°C in water, when the swollen and compliant 38
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brush can be compressed more easily by the AFM tip, originating an underestimate of the brush 39 41
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step-height. 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment
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Figure 2. Tapping-mode AFM micrographs of PNIPAM brushes immersed in water. (a) and (b) 31
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thin PNIPAM brushes in water at 25 and 40° C, respectively. (c) and (d) thick PNIPAM brushes 3
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in water at 25 and 40 °C, respectively. The same cantilever (SNL-10, D triangular) from Bruker 34 35
Corporation was used for recording all the micrographs. This presented a normal spring constant 38
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of ~ 0.06 N m-1. The scanning speed was kept in all cases at 1 Hz. The amplitude set point and 39 40
the drive amplitude were set at 0.7 V and 0.3 V, respectively. 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment
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Ellipsometry 8
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Dry (nm)
Water 25°C (nm)
Water 40°C (nm)
Dry (nm)
Water 25°C (nm)
Water 40°C (nm)
Thin PNIAPM
11±1
47±6
13±4
9±3
33±8
19±10
Thick PNIPAM
417±4
1033±22
486±2
390±6
655±15
440±12
Sample 9 10 1 12
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AFM
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Table 1. VASE and AFM thickness data for thin and thick PNIPAM brushes in dry condition 23
and in Milli-Q water at 25° and 40°C. 24 25 26 27 28 30
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Normal and Lateral AFM Force Measurements 31 32
The adhesive properties of thin and thick PNIPAM brushes, below and above LCST were 35
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examined by acquiring force vs distance (F-D) curves by colloidal probe AFM (CP-AFM). As 37
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shown in Figure 3a, at 25 °C in water both thin and thick PNIPAM brushes showed repulsive 38 40
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interactions during the approach of the colloidal probe, due to the repulsion against compression 42
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exerted by swollen and densely grafted brushes.54 Under these conditions, both thin and thick 4
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brushes displayed a slight hysteresis between the approaching and the retracting F-D profiles. 45 47
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This phenomenon was probably due to the stretching of the grafted chains up to several hundreds 49
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of nm from the grafting surface during the retraction of the colloidal probe, as recently observed 51
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for similar PNIAPM brushes by Yu et al..55 54
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Above the LCST, the dehydration of thin and thick PNIPAM brushes produced a marked 56
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steepening of the approaching F-D profiles (Figure 3b). In addition, the adhesive interactions 57 58 59 60 ACS Paragon Plus Environment
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recorded in the retracting F-D curve confirmed the presence of hydrophobic domains produced 5
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by the collapse of PNIAPM grafts above LCST.54 Remarkably, an order magnitude increase in 6 7
the adhesion force could be observed for thick PNIPAM grafts compared to thin ones. Namely, 10
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for thin PNIPAM brushes an average adhesion force of 0.8 nN was recorded, whereas thick 12
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PNIPAM brushes showed a much higher value of 6.3 nN, as shown in Figure 3c. The higher 13 14
adhesion force recorded on thick-PNIPAM brushes was due to the increased contact area 17
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between the AFM colloid and the compressed brush. In addition, the long-range interactions 19
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recorded in the retraction profiles of the F-D curves were presumably generated by the vertical 20 2
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stretching of long polymer grafts within thick brushes by the retracting probe. 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment
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Figure 3. F-D curves for thin and thick PNIPAM brushes at 25 °C (a) and 40 °C (b). The solid 37 39
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lines indicate the approaching curves, while the dashed lines indicate the retracting profiles. In c) 41
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the distributions of adhesion forces recorded at 40°C for thin (red) and thick PNIPAM (blue) 43
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brushes are reported. F-D curves were collected at a speed of 1 µm s-1 with a cantilever having 4 45
normal spring constant of 1.57 N m-1 and presenting a silica colloid of 8 µm radius. 47
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The nanotribological properties of the different PNIPAM brushes were subsequently investigated 51 53
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by LFM. 54 5 56 57 58 59 60 ACS Paragon Plus Environment
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In Figure 4a, typical friction loops recorded on thin and thick PNIPAM brushes at 25 °C in water 5
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are reported. In the swollen state PNIPAM brushes present tilted friction loops. As previously 6 7
described by us,47,56 the tilted sections of the friction traces originate from the deformation 10
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(lateral bending and stretching) of swollen and compliant brushes by the scanning probe (Xd in 12
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Figure 4b and 4c). When the shear stress exerted by the probe overcomes the spring force of the 13 14
deformed brush sliding finally occurs (Xs in Figure 4b and 4c), until scanning direction reversal 17
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and the consequent deformation and sliding towards the opposite direction. 19
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Comparing the friction loops recorded for thin and thick PNIPAM brushes, a much lower 20 2
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contribution of tilting was observed for the thinner brushes with respect to the thicker ones. 24
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Namely, the lateral piezo extension corresponding to the tilted portion of the friction trace, Xd (or 25 26
maximum lateral deformation) for thin PNIPAM brushes was estimated as 415 ± 30 nm, whereas 27 29
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thick PNIPAM showed a much higher Xd of 3010 ± 55 nm, as shown in Figure 4d. 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment
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Figure 4. (a) Friction loops recorded by LFM on thin and thick PNIPAM brushes immersed in 53
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water at 25 °C for the applied load of 10 nN. In (b) a typical friction loop recorded on thick 54 56
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PNIPAM brushes is reported; the tilted portion (Xd) and the sliding portion (Xs) of the loop are 57 58 59 60 ACS Paragon Plus Environment
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also highlighted. A schematic representing brush deformation and sliding during scanning is 5
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reported in (c). In (d) the lateral deformation of thin and thick PNIPAM brushes is plotted 6 8
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against the recorded lateral force. (e) Ff-L profiles for thin and thick PNIPAM brushes recorded 10
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in water at 25°C, the solid lines indicate the fit to the linear regression. The scanning distance 12
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and the scanning rate applied during CP-AFM friction measurements were 9 µm and 1 µm s-1, 13 15
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respectively. The radius of the colloid used was 9 µm. The normal and torsional spring constants 17
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of the cantilever used were 0.263 N m-1 and 5.52E-9 N m, respectively. 18 19 20 2
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Friction-vs-load (Ff-L) measurements further elucidated the frictional properties of thin and thick 24
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PNIPAM brushes. As reported in Figure 4e, at 25°C the recorded friction force increased nearly 25 26
linearly with the applied load. However, just a slight difference in COF was observed among thin 27 29
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and thick PNIPAM brushes (0.08 and 0.09 for thin and thick PNIPAM brushes, respectively).40 31
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It is noteworthy to mention that although no clear adhesion could be recorded between the 32 3
colloid and the swollen PNIPAM brushes, the Ff-L profiles measured at 25°C did not clearly 36
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follow Amontons’ law,40 i.e. the recorded friction force was not directly proportional to the 38
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applied load. We believe that the observed slight deviation was possibly due to the interaction 39 41
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between the AFM colloid and the PNIPAM grafts, which was recorded as a slight but visible 43
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hysteresis between the approaching and retracting F-D profiles (Figure 3a). This interaction and 45
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the consequent stretching of the chains affected the measured lateral force and resulted in a small 46 48
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variation of the slope of the Ff-L profiles. 50
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At 40°C, the friction loops recorded on thin and thick PNIPAM brushes displayed very similar 51 52
profiles, and no tilting effects were observed (Figure 5a). This result indicates that collapsed 53 5
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PNIPAM brushes could not be laterally deformed by the shearing probe, irrespective of their 56 57 58 59 60 ACS Paragon Plus Environment
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thickness. 5
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As expected, the Ff-L measurements for thin and thick PNIPAM brushes at 40 °C showed 6 7
relevantly higher friction compared to 25°C. However, as displayed in Figure 5b, thick PNIPAM 10
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brushes presented a steeper Ff-L profile if compared to the one recorded on thin PNIPAM 12
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analogues, with COF values of 0.9 and 0.5 for thick and thin PNIPAM brushes, respectively. The 13 14
higher friction and COF observed for thick PNIPAM brushes in the collapsed state was likely 17
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due to the high adhesion force and increased surface roughness recorded above LCST for 19
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relatively thick PNIPAM brushes with respect to thinner ones. 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 41
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Figure 5. (a) Friction loops recorded on thin (green trace) and thick PNIPAM brushes (black 43
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trace) recorded at 40 °C in Milli-Q water by applying a normal load of 10 nN. (b) Ff-L profiles 4 46
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recorded on thin (green trace) and thick PNIPAM brushes (black trace) at 40 °C in Milli-Q 48
47
water. The scanning distance and scanning rate applied during LFM were 9 µm and 1 µm·s-1, 49 50
respectively. The radius of the colloid used was 9 µm. The normal and torsional spring constants 51 53
52 54
of the cantilever used were 0.263 N·m-1 and 5.52E-9 N·m, respectively. 5 56 57 58 59 60 ACS Paragon Plus Environment
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Effect of Scanning Distance on Friction Measurements 5
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In order to investigate the effect of brush lateral deformation on the friction measurements, we 6 7
firstly analyzed the friction loops recorded on thick PNIPAM brushes at 25 °C, applying 10
9
8
scanning distances of 1, 5 and 9 µm, and maintaining a constant scanning speed of 1 µm s-1. As 12
1
displayed in Figure 6a, at the smallest scanning distance tested of 1 µm, the friction loop showed 13 14
a completely tilted shape, as the lateral deformation of the swollen brush dominates brush17
16
15
colloidal probe interaction, and sliding is never attained. Under these experimental conditions, 19
18
the dissipation recorded via the friction loop (green trace in Figure 6a) is solely due to the back 20 2
21
and forth bending and stretching of the swollen brushes. It is important to underling that at not 24
23
slipping the apparent friction force induced by the cantilever torsion was due to the resistance by 25 26
the PNIPAM grafts to bend and stretch along the lateral direction. An increase of the scanning 27 29
28
distance to 5 µm causes the brushes to be initially deformed and stretched until static friction is 31
30
overcome and sliding is finally gained (brown trace in Figure 6a). In this case, the dissipation 32 3
measured via the friction loop originates from a combination of both brush stretching and sliding 36
35
34
of the colloidal probe. A further increase of the scanning distance until 9 µm, which is a 38
37
considerably larger distance than the maximum lateral stretching of the grafted chains, produces 39 41
40
a friction loop presenting a tilted section after scanning direction reversal, followed by steady 43
42
sliding of the colloid over several µm of piezo displacement. Thus at relatively large scanning 45
4
distances, the frictional dissipation is dominated by the sliding of the colloid over the brush 46 48
47
surface (black trace in Figure 6a). 50
49
When a scanning distance of 5 µm is applied, a virtual increase in the recorded lateral force 51 52
towards direction reversal is observed. We believe this phenomenon was due to the piling-up of 53 5
54
the deformed grafts along the scanning direction, which caused an increase of the lateral force 56 57 58 59 60 ACS Paragon Plus Environment
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experienced by the AFM probe. Since the rate at which the deformed brushes are recovering 5
4
their equilibrium conformation is presumably lower compared to the applied scan rate, this 6 7
topographical change by the brush surface affected the friction measurements. As a confirmation 10
9
8
of this assumption, we laterally scanned the same brush with a sharp tip, and observed an 12
1
increase of the recorded height values on the edges of the Z-sensor micrographs (Figure 7), 13 14
indicating brush piling-up towards scanning direction reversal. 16
15 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54
Figure 6. (a) Friction loops recorded on thick PNIPAM brushes at 25°C in water applying 5 57
56
scanning distances of 1, 5 and 9 µm (individual curves are displayed separately in SI, Figure S6). 58 59 60 ACS Paragon Plus Environment
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(b) Ff-L profiles recorded on thick-PNIPAM brushes at 25 °C in water applying scanning 5
4
distances of 1, 5 and 9 µm. Friction loops and Ff-L profiles recorded on thick PNIPAM brushes 6 8
7
at 40 °C in water (c and d). The scanning velocity during the experiments was kept constant at 1 10
9
µm s-1. The normal and torsional spring constants of the cantilever used were 0.263 N m-1 and 1 12
5.52E-9 N m respectively. 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 37
36
Figure 7. AFM height micrograph in contact mode for thick PNIPAM brushes immersed in 38 40
39
Milli-Q water recorded at 25°C with a sharp tip (SNL-10, applied load of ~ 20 nN). 41 42 43 4
The comparison of the Ff-L plots recorded at the three different scanning distances 45 47
46
studied highlights how a variation of brush-colloidal probe interactions can lead to 49
48
different measurements of the COF. The lowest COF value of 0.03 was obtained in the 50 51
case of 1 µm of scanning distance. Across this relatively short scanning distance, the 52 54
53
measured friction force was solely due to the energy dissipation caused by the lateral 56
5
bending and stretching of the swollen brush (Figure 6a green trace). A marked increase in 57 58 59 60 ACS Paragon Plus Environment
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friction, with average COF of 0.08 was observed for Ff-L profiles at 5 µm of scanning 5
4
distance. In this particular case, the frictional energy dissipation recorded via the loops 6 8
7
derived from the combination of brush lateral deformation and sliding (Figure 6a brown 10
9
trace). In contrast, at the largest scanning distance of 9 µm the measured friction force is 1 12
mainly determined by sliding of the colloid, and an average COF of 0.1 was measured 13 15
14
(Figure 6b black trace). 17
16
The effect of brush lateral deformation on the measurement of friction becomes irrelevant at 40 18 19
°C, when PNIPAM brushes present a collapsed morphology. Under this condition, the friction 20 2
21
loops do not show any tilt, and the Ff-L plots display a similar profile irrespective of the scanning 24
23
distance applied. A constant COF of 0.9 is thus measured in all three cases, as indicated in Figure 25 26
6d. 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment
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37
Figure 8. (a) Friction loops recorded on thin PNIPAM brushes at 25°C in water applying 40
39
scanning distances of 0.5, 1.5 and 3 µm (individual curves are displayed separately in the 41 43
42
Supporting Information). (b) Ff-L plots obtained for thick PNIPAM brushes at 25 °C in water 45
4
applying 0.5, 1.5 and 3 µm of scanning distance. Friction loops and Ff-L profiles recorded on 46 47
thin PNIAPM at 40 °C are displayed in (c) and (d). The scanning velocity during the experiments 48 50
49
was kept constant at 1 µm s-1. The normal and torsional spring constants of the cantilever used 52
51
were 0.263 N m-1 and 5.52E-9 N m respectively. 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment
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Similar results are also obtained for thin PNIPAM brushes, although the occurrence of 5
4
sliding was observed at a value of scanning distance relevantly shorter with respect to the 6 8
7
more laterally deformable, thick PNIPAM brushes. As shown in Figure 8a, the transition 10
9
from a completely tilted loop to a combination of tilting and sliding was recorded at 1.5 1 12
µm of scanning distance, while at larger distances sliding of the colloidal probe 13 15
14
dominated the frictional properties of thin PNIPAM brushes. In this case, COF values of 17
16
0.04, 0.12 and 0.14 were obtained for 0.5, 1.5 and 5 µm of scanning distances, 18 19
respectively (Figure 8b). 20 2
21
Similar to the result observed for thick PNIPAM brushes, at 40°C the lateral deformation 24
23
of thin brushes was completely suppressed and no tilting was observed in the recorded 25 27
26
friction loops (Figure 8c). Hence, when recorded on the thin PNIPAM brushes above 29
28
LCST, the Ff-L profiles measured across different scanning distances overlapped each 31
30
other and indicated a constant COF of 0.5 (Figure 8d). 32 34
3
These results further confirmed that the contribution of brush lateral deformation to the 36
35
dissipation of frictional energy can strongly alter the friction and thus COF values when 37 38
the scanning distance applied during LFM is shorter or comparable to the maximum 39 41
40
lateral deformation of the measured brush (Xd). This is particularly valid for highly 43
42
swollen brushes, as in the case of PNIPAM immersed in water below its LCST, whereas 4 45
in a bad solvent, the shearing probe can hardly laterally deform and stretch the collapsed 46 48
47
and dehydrated brushes, and the frictional energy dissipation is mainly determined by the 50
49
sliding of the probe over the brush surface. 51 52 53 5
54
Conclusion 56 57 58 59 60 ACS Paragon Plus Environment
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In this report, we investigated the effect of the lateral deformation of a swollen brush by a 5
4
shearing AFM probe on the measurement of friction forces. Brush lateral deformation, 6 8
7
comprising bending and stretching of the grafts, can be quantified through the analysis of 10
9
the friction force loops recorded by LFM. The tendency of a brush to be laterally 1 12
deformed depends on brush parameters, such as the grafted-chain length (i.e. brush 13 15
14
thickness) and it is particularly relevant and interesting in the case of thermoresponsive 17
16
polymer brushes, being the swelling properties of such grafts precisely tunable by varying 18 19
the temperature of the medium. For both thin and thick PNIPAM brushes immersed in 20 2
21
water at 25 °C, the friction loops display a tilted behaviour, with thicker brushes that can 24
23
be laterally deformed more than thinner analogues. Thus, when relatively short scanning 25 27
26
distances are applied for recording the friction loops, brush deformation mainly 29
28
determines the frictional dissipation, while sliding of the colloid on the brush surface does 31
30
not occur. Under these conditions, a relatively low COF was measured, which is uniquely 32 34
3
originating from the bending and stretching back and forth of the swollen brushes. The 36
35
application of scanning distances relevantly longer than the brush lateral deformation 37 38
(expressed as Xd) produces friction loops displaying a tilted shape solely at scanning 39 41
40
direction reversal, while sliding of the colloid on the brushes dominates the frictional 43
42
dissipation recorded via the loops. In these cases, the slope of the Ff-L profiles as well as 4 45
the derived COF values increase. 46 48
47
In contrast, due to brush dehydration and collapse above LCST, PNIPAM brushes cannot 50
49
be laterally deformed at 40 °C in water. Tilting of the friction loops could not be observed 51 53
52
under these particular conditions and the Ff-L profiles showed a constant slope 5
54
irrespective of the scanning distance applied for recording the loops. 56 57 58 59 60 ACS Paragon Plus Environment
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Brush lateral deformation is thus proved to play a relevant effect on the measurement of 5
4
friction and COF, especially when the measured brush is immersed in a good solvent and 6 8
7
when relatively short scanning distances are applied during LFM. Our study demonstrates 10
9
that while measuring COF, brush thickness, swelling properties and applied scanning 1 12
distances are parameters to be carefully considered, as the simple recording of TMR 13 15
14
(Trace minus Retrace) values from the friction loops may yield a conceptually wrong 17
16
estimate of dynamic friction. 18 19 20 2
21
ASSOCIATED CONTENT 24
23
Supporting Information. This material is available free of charge via the Internet at 25 27
26
http://pubs.acs.org. 28 29 31
30
AUTHOR INFORMATION 32 34
3
Corresponding Author 36
35
* Corresponding Authors:
[email protected], 37 38
[email protected] 39 41
40
Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, 43
42
Vladimir-Prelog-Weg 5/10, 8093 Zurich, Switzerland. 4 45 46 48
47
Author Contributions 50
49
The manuscript was written through contributions of all authors. All authors have given approval 51 53
52
to the final version of the manuscript. 54 5 56 57 58 59 60 ACS Paragon Plus Environment
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ACKNOWLEDGMENT 5
4
This work was financially supported by the Swiss National Science Foundation (SNSF 6 7
“Ambizione” PZ00P2−790 148156). 9
8 10 12
1
References 13 14 15
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