Cohesion Mechanisms of Polystyrene-Based Thin Polymer Films

Sep 9, 2013 - The influence of film preparation conditions and film architecture on polymer cohesion mechanisms is probed by comparing spin-coated PS ...
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Cohesion Mechanisms of Polystyrene-Based Thin Polymer Films Bizan N. Balzer,† Markus Gallei,‡ Katrin Sondergeld,‡ Markus Schindler,§ Peter Müller-Buschbaum,§ Matthias Rehahn,‡ and Thorsten Hugel*,† †

IMETUM and Physik-Department, Technische Universität München, Boltzmannstr. 11, 85748 Garching, Germany Ernst-Berl Institute for Chemical Engineering and Macromolecular Science, Technische Universität Darmstadt, Petersenstraße 22, 64287 Darmstadt, Germany § Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München, James-Franck-Str. 1, 85748 Garching, Germany ‡

S Supporting Information *

ABSTRACT: The cohesion mechanisms of end-functionalized high molar mass polystyrene with very low polydispersity (PS, Mn = 1.26 × 106 g mol−1, PDI = 1.06) and polylysine (PLL, 150−130 × 103 g mol−1) on silicon (Si) supported thin PS films are investigated by desorbing single polymers covalently bound to an atomic force microscope (AFM) cantilever tip. The influence of film preparation conditions and film architecture on polymer cohesion mechanisms is probed by comparing spin-coated PS films (scPS) with a thickness range of 6−52 nm and covalently surface-attached PS films (saPS) with a thickness of 15−83 nm. Annealed scPS prevents cohesion of further PS polymers unless the scPS partly dewets. In all other cases, two different cohesion mechanisms are observed: first, a previously described equilibrium desorption similar to hydrophobic solid substrate desorption, represented by a plateau of constant force in the force−extension curve, and second, a nonequilibrium mechanism with nonlinear force−extension behavior. The second requires a geometrical interlock between the tip bound single molecule and the PS film. Remarkably, this mechanism is observed below the glass transition temperature of PS films and is promoted by good solvent conditions. These findings contrast many bulk measurements assuming a glassy state of the complete polymer film, but they are consistent with fluid like boundary layers having a high mobility. Our results further underline the decisive influence of polymer film conformation and mobility close to its solvent exposed boundary layer for the cohesion of polymer coatings.

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layer.8 The molecular mechanisms largely lack an experimental verification. In the present investigation, we use atomic force microscopy (AFM)-based single molecule force spectroscopy (SMFS). The AFM has increasingly been used not only as imaging device but also for the detection of molecular interactions with piconewton (pN) resolution. This technique uses a single polymer covalently attached to a passivized AFM cantilever tip via a flexible linker and enables the detection of structural and mechanical properties of polymers.10 Previously, adhesion properties of single polymers on solid substrates in aqueous environment,11 ion specific effects,12 cosolute influence on polymers tension,13 polymer conformation close to solid substrates,14 and temperature dependence on desorption behavior of singe polymers could be elucidated.15 We extend this technique to probe the interaction of tip attached single polymers with thin polymer films. In other words, we

INTRODUCTION

Polymer films on solid surfaces are of great interest for surface modification and composite material preparation.1−6 In industrial products polymer coatings are often used to tune the adhesion properties. The stability of such coatings is crucial but lacks basic understanding on the molecular level. The role of the interfacial layers is essential for the stability of these films and their adhesion to substrates. Studying the mechanistic basis of these interfacial interactions is of utmost importance for their further development. Different theories state that adhesion can be of chemical, mechanical, or diffusive origin.7,8 While the first mechanism assumes electrostatic interactions or covalent bonds, the second relies on a mechanical interlock model that refers to asperities of solid substrates. The third theory describes how the interdiffusion of polymers of coated substrates across the interface determines their joint strength. The diffusion kinetics are typically described by the reptation model.9 Failure of adhesive joints is usually assumed to be of cohesive manner, either located in the adhesive, the adherent, or some boundary © XXXX American Chemical Society

Received: June 7, 2013 Revised: July 30, 2013

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least five times at different positions of the sample. The errors correspond to the standard deviation. The AFM measurements were performed with a MFP3D-SA (Asylum Research). They were carried out at RT in a closed fluid cell and in ultrapure H2O (Biochrom AG, Germany) or in dry chloroform (CHCl3, >99.9%, Sigma-Aldrich). The AFM cantilever tip is pressed onto the film (with an indentation force of 100−1000 pN). After a variable dwell time the AFM cantilever tip is retracted with the polymer successively desorbing (deadsorbing) from the film. During indentation of the functionalized tip, the polymer interacted with the film. The tip was then retracted with a constant velocity of 1 μm s−1. Force−extension traces were derived from the deflection-piezopath signal as described elsewhere.10,30 The spring constant (with typical values of 10−50 pN nm−1) of each cantilever was determined after the experiment by integrating over the power spectral density from 2.5 Hz to the local minimum between the first and the second resonance peak and by applying the equipartition theorem.31,32 Each measurement was done with a sampling rate of 5 kHz and contains at least 100 force− extension curves gathered from at least three different spots. The obtained velocity-independent plateaus represent an equilibrium desorption process.11 Evaluation was done with the program Igor Pro (Wavemetrics) and self-programmed procedures. The desorption force Fplateau and the detachment length zdet of plateaus were evaluated by fitting a sigmoidal function to the final part of the plateau. In case of multiplateaus33 only the last plateau, representing the desorption of a single polymer from the substrate, was taken for evaluation. For AFM imaging a MFP3D-SA AFM (Asylum Research) was used with AC 240 TS cantilevers (Olympus, Japan) having a force constant of about 1.8 N m−1 and a resonance frequency of 70 kHz. Typically, a scan rate of 0.5 Hz was used for intermittent-contact mode imaging. Substrate Preparation. Spin-Coated Polystyrene Films (scPS). The polystyrene films were prepared on Si wafers which were cleaned in an acidic bath consisting of deionized H2O (54 mL), H2O2 (84 mL), and H2SO4 (198 mL) for 15 min at 80 °C.19 Then the substrates were rinsed with deionized H2O and dried in a dry nitrogen flow. Polystyrene (PS, from Polymer Standard Service, Mainz, Germany; Mn = 6.7 kg mol−1, Mw = 7.0 kg mol−1, PDI = 1.04) was dissolved in toluene (Roth, Germany). These solutions were spin-coated at a velocity of 2500 rpm for 30 s. In order to avoid peeling of the polymer film from the substrate under H2O, the samples were annealed in vacuum at 90 °C for 45 min. The thickness values determined via XRR were 5.6, 10.0, 20.7, and 50.5 nm for concentrations of PS in toluene of 1, 2, 4, and 10 g L−1, respectively. Synthesis of 4-(11′-Triethoxysilylundecanyl)diphenylethylene (TEOS-DPE). In dry toluene (20 mL) 4-(10′-undecenyl)-DPE (4.41 g, 13.3 mmol, 1 equiv) and triethoxysilane (TEOS) (3.67 mL, 19.9 mmol, 1.5 equiv) were charged with PtO2 (15 mg, 6.64 × 10−5 mol). The dispersion was stirred for 18 h at 55 °C. To separate the catalyst, the reaction mixture was filtrated twice using Celite. Solvent and the excess of TEOS were removed in vacuo. The product was quantitatively obtained as yellow oil. 1H NMR and 13C NMR spectra are given in Figures S1a and S1b of the Supporting Information. 1 H NMR (300 MHz, CDCl3): δ = 0.53−0.57 (m, 1H, H11′), 1.13− 1.40 (m, 20 H, H3′−9′,10′,11′)), 1.49−1.59 (m, 2H, H2′), 1.87−1.98 (m, 1H, H10′), 2.52 (t, 2H, H1′), 3.73 (q, 4H, OCH2), 5.33 (d, 2H, H14), 7.06 (m, 2H, H3,5), 7.17 (m, 2H, H2,6), 7.21−7.27 (m, 5H, H8−12) ppm. 13 C NMR (75 MHz, CDCl3): δ = 10.62 (C11′), 18.56 (−CH3), 23.01 (C10′), 29.48, 29.61, 29.82, 29.70, 29.81, 29.89 (C3′−8′), 31.69 (C2′), 33.43 (C9′), 35.91 (C1′), 58.49 (OCH2), 113.82 (C14), 127.80 (C10), 128.30 (C3,5,8,12), 128.37 (C9,11), 128.52 (C2,6), 138.92 (C1), 141.95 (C7), 142.82 (C4), 150.15 (C13) ppm. Synthesis of 4-(11′-Trichlorosilylundecanyl)diphenylethylene (TCS-DPE). In a flask 4-(10′-undecenyl)-DPE (2 g, 6.9 mmol, 1 equiv) was dissolved in dry toluene (10 mL). Under an atmosphere of nitrogen platinum(0)−1,3-divinyl-1,1,3,3-tetramethyldisiloxane (Karstedt Catalyst, 0.3 mL, 2 wt % in xylene) was added, and the reaction vessel was cooled with liquid nitrogen. Trichlorosilane (TCS, 1.05 mL, 10.4 mmol, 1.5 equiv) was added dropwise, and the solution was stirred for 12 h at RT. The solvent and the excess of TCS were

determine the desorption force of a single polymer after deposition onto a substrate. These spatially resolved measurements allow us to understand cohesion mechanisms in thin polymer films boundary layers. As a model system we investigate the molecular cohesion of single polymers such as polystyrene (PS) and polylysine (PLL) to boundary layers of supported thin PS films where we do not expect ionic bonds for polymer−polymer interaction.16 Therefore, styrene is polymerized via anionic polymerization to obtain narrowly distributed high molar mass polymers. By taking advantage of this method, a suitable end group for AFM tip modification has been successfully introduced. Spin-coated PS (scPS), resembling thin homogeneous physisorbed polymer films,17−19 and covalently surface-attached PS (saPS) chains on silicon (Si), enabling us to measure under different solvent conditions, are compared.20−23 Surface-attached PS chains have been prepared by using surface-initiated anionic polymerization posterior to the substrate functionalization with two different diphenylene ethylene-based moieties. PS films have been frequently used as model systems for fundamental investigations on polymer film stability and film mobility.5,24−27

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EXPERIMENTAL SECTION

Reagents. All solvents and reagents were purchased from Alfa Aesar, Sigma-Aldrich, Fisher Scientific, ABCR, and Merck and used as received unless otherwise stated. Tetrahydrofuran (THF), toluene, and cyclohexane (CH) were distilled from sodium/benzophenone under reduced pressure (cryo-transfer) prior to the addition of 1,1diphenylethylene and n-butyllithium (n-BuLi) followed by a second cryo-transfer. Styrene, divinylbenzene (DVB), and propylene sulfide were purified by 3-fold distillation over CaH2. Prior to use, the monomers were freshly distilled from these solutions. Deuterated solvents were purchased from Deutero GmbH (Germany). 4Bromodiphenylethylene and 4-(10′-undecenyl)diphenylethylene were synthesized by a similar protocol reported by Advincula et al.23 and Zhou et al.28 All syntheses were carried out under an atmosphere of nitrogen using a Schlenk technique or a glovebox equipped with a Coldwell apparatus. Poly(tetrafluorethylene), PTFE, samples of 1.5 mm thickness were purchased from GM GmbH (Germany). Poly-Llysine hydrobromide (PLL, 150−130 kg mol−1) was purchased from Sigma-Aldrich (Germany). Instrumentation. NMR spectra were recorded on a Bruker ARX 300 NMR spectrometer working at 300 MHz (1H NMR) and 75 MHz (13C NMR). NMR chemical shifts are given relative to tetramethylsilane; the signal assignment was carried out according to the numbering of protons and carbons as specified in the corresponding schemes. Standard SEC was performed with THF as the mobile phase (flow rate 1 mL min−1) on a SDV column set from PSS (SDV 1000, SDV 100000, SDV 1000000) at 30 °C. Calibration was carried out using PS standards (from Polymer Standard Service, Mainz, Germany). For the film thickness determination of the scPSs X-ray reflectivity (XRR) was performed on a D8 Discovery (Bruker, Germany) and evaluated by using the Parratt algorithm to fit the measured data. Dry film thickness was measured using an imaging ellipsometer (nanofilm EP3) equipped with an incidence beam wavelength of λ = 658 nm. One zone angle-of-incidence (AOI) variation scans were routinely performed between AOIs of 60°−86°. For analysis the obtained angles Δ and Ψ were fitted with the ellipsometric analysis EP4 software. For contact-angle detection, we used a home-built goniometer equipped with a charge-coupled device (CCD) camera. Both angles of the drop were recorded and determined with the drop analysis plugin29 for Java-based freeware ImageJ. For determination of the angles, a polynomial was fit to the edge of the droplet. Then, a tangent was fit to this polynomial where the sample and the edge met to determine the enclosed angle. Static H2O contact angles (using a H2O volume of 1.5 μL) were taken at room temperature (RT, 25 °C) at B

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Figure 1. AFM cantilever tip functionalization with PS: (a) scheme of PS−PSS synthesis; (b) scheme of the coupling of PS−PSS to the linker system bound to a Si3N4 AFM cantilever tip. stirring for 3 h at 0 °C by using the Coldwell apparatus. After that time degassed MeOH was added to terminate the reaction. The polymer was precipitated in MeOH, filtered, and dried in vacuo (Figure 1a). The polymer was stored in a fridge at −15 °C under an argon atmosphere. SEC measurement vs PS standard revealed Mn = 1.26 × 106 g mol−1, Mw = 1.34 × 106 g mol−1, and PDI = 1.06. AFM Cantilever Tip Functionalization. The preparation of the AFM cantilever tips and attachment of single polymers as molecular sensor were performed with Si nitride cantilevers (MLCT from Bruker AXS). Covalent attachment to the AFM cantilever tip via a flexible poly(ethylene glycol) (PEG) linker made long measurements over a period of many hours with one and the same AFM cantilever on the different substrates possible. The linker covalently binds one end of a single probe polymer to the tip shifting the polymer away from the tip apex. This separates the adhesion-related polymer−substrate interaction from the undesired contributions such as unspecific interaction of the substrate with the tip material itself. Functionalization with PS−PPS. Chemical and oxygen plasma treatments (Femto, Diener Electronic, Germany) were used for cleaning and activation of the AFM cantilevers. Amino-functionalized tip surfaces for the covalent coupling were obtained by using Vectabond reagent (Axxora, Germany) on the activated Si nitride tips for 15 min. The aminated cantilevers were immersed for 45 min in a PEG (1:1500 mixture of the heterobifunctional linker malhex-NHPEG-O-C3H6-CONHS ester 5 kg mol−1, PDI = 1.03, and the monofunctional CH3O-PEG-NHS ester 5 kg mol−1, PDI = 1.03, RappPolymere, Germany) solution in dry CHCl3 (>99.9%, Sigma-Aldrich) with 5 vol % triethylamine (Sigma-Aldrich) in order to provide coupling sites and a passivation. Alternatively, short linkers, such as heterobifunctional NHS-PEG4-maleimide (0.5 kg mol−1, Thermo Scientific) and monofunctional CH3O-PEG4-NHS (0.3 kg mol−1, Thermo Scientific) were applied. Afterward, a PS−PPS solution (maximum 10 mg mL−1 in CHCl3) was used for the coupling of the polymer. After overnight incubation the functionalized cantilevers were rinsed and stored in CHCl3 until use (Figure 1b). Functionalization with PLL. The preparation followed the same protocol as for PS−PSS. Instead of heterobifunctional linker malhexNH-PEG−O-C3H6-CONHS ester 5 kg mol−1 homobifunctional linker PEG-(NHCO-C2H4-CONHS)2 ester 6 kg mol−1, PDI = 1.07 was used in order to provide NHS active esters at both ends for those amineterminated homo-poly(amino acid)s. After PEG incubation a PLL solution (maximum 10 mg mL−1 in 50 mM borate buffer, pH = 8.0) was used for the coupling of the polymer for 90 min followed by

removed in vacuo. The product was quantitatively obtained as green oil. The 1H NMR spectrum of aliquots indicated complete consumption of 4-(10′-undecenyl)-DPE. 1H NMR is given in Figure S1c of the Supporting Information. 1 H NMR (300 MHz, CDCl3): δ = 0.75−1.67 (m, 18H, H3′−11′), 1.85 (m, 2H, H2′), 2.30−2.51 (m, 2H, H1′), 5.25−5.29 (m, 2H, H14), 6.67−7.18 ppm (m, 9H, H2,3,5,6,8−12) ppm. Preparation of Si Wafers and Initiator Immobilization. For the immobilization of TEOS-DPE on Si wafers, 1 × 1 cm2 pieces were cut and cleaned by Soxhlet extraction using dry toluene. For the immobilization of TCS-DPE, 1.5 × 6.5 cm2 pieces of Si wafers were cut, and every wafer was scratched into 1.5 × 1.5 cm2 pieces for subsequent breaking after polymerization steps. The wafers were cleaned by Soxhlet extraction using dry toluene followed by treatment with Caro’s acid for 45 min and rinsing with distilled H2O prior to the immobilization of either TEOS-DPE or TCS-DPE. Wafers were dried in vacuo. Under an atmosphere of nitrogen cleaned wafers were immersed in a 2 × 10−3 M solution of either TEOS-DPE or TCS-DPE in dry toluene charged with dry triethylamine (0.1 mL). The reaction mixture was stirred for 12 h at RT. Then, the wafers were cleaned by using Soxhlet extraction with dry toluene for 24 h. Surface-Initiated Anionic Polymerization (SI-AP) of Styrene. In a glovebox both TCS-DPE and TEOS-DPE initiator functionalized Si wafers were immersed in dry CH containing a large excess of n-BuLi (1.6 M) for at least 2 h to initiate the DPE-containing precursors. Wafers were taken off the vessel and cleaned very carefully by rinsing with dry CH. All wafers were transferred to another vessel containing dry CH and styrene (for TEOS-DPE 3 mL of CH and 0.5 mL of styrene were used; for TCS-DPE 30 mL of CH and 2 mL of styrene were used) and stirred for 12 h at RT. The polymerization was terminated by adding methanol (MeOH). Wafers with immobilized TCS-DPE were carefully broken at previously scratched positions, and all wafers were cleaned by using Soxhlet extraction with toluene. Molecular Force Sensor Preparation. Synthesis of Poly(styrene-b-propylene sulfide), PS−PPS. PS−PPS was synthesized as discussed in ref 34. In short, neat styrene (3.6 g) was dissolved in dry CH (150 mL) in an ampule equipped with stirring bar. sec-BuLi (22 μL, 0.13 M in n-hexane, 2.86 μmol) solution was added quickly via syringe at RT. The mixture was stirred at RT for 48 h to ensure complete conversion, and an aliquot (0.1 mL) was taken for SEC measurements. Freshly distilled propylene sulfide (110 mg, 1.48 mmol) was added, and the solution was stirred for 30 min prior to the addition of precooled THF (50 mL, −30 °C) followed by further C

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Figure 2. PS desorption on PTFE, Si, and annealed scPS in H2O: (a) Types of force−extension curves: type 1 represents equilibrium plateaus of constant force, type 3 constitutes a nonlinear force−extension relation representing nonequilibrium events, and type 2 is a mixture of both. (b) Plateau desorption force Fplateau, (c) detachment length zdet, and (d) probability of the various curve types for the desorption of PS from PTFE and Si coated with annealed scPS. The dwell time on the substrate is 1 s, and the pulling velocity is 1 μm s−1. Errors correspond to the standard deviation. *No plateau events are observed. rinsing in Tris buffer (10 mM, pH = 8.4). The molecular force sensors were stored in H2O until use.

shows nonequilibrium stretching and rupture events (Figure 2a, lower curve). Those resemble the force−extension curves for the rupture of single covalent bonds43 or unfolding of secondary structures14,44 and can be fitted using a worm-like chain model.45−47 In addition to these two types of SMFS force−extension curves, more complex curves including type 1 and type 3 motifs exist, which we term type 2. We observe that PS in H2O on the 6 nm thick annealed scPS shows a similar plateau desorption force Fplateau as the hydrophobic PTFE substrate (Figure 2b), caused by the hydrophobic interaction.11,48,49 The detachment lengths zdet obtained were in a comparable range to those substrates but higher than on bare Si (Figure 2c). Many plateaus (type 1) on PTFE and on the 6 nm annealed scPS could be observed, while on Si most of the curves are type 3 (Figure 2d). All annealed samples of films thicker than 10 nm do not show any single polymer cohesion events. This on first sight surprising behavior can be explained by evaluating AFM images of the scPS (Figure 3 and Supporting Information Figure S2). Although annealed, the thin scPS (6 nm) still exhibits dewetting (Figure 3a,b), and only thicker annealed scPS remain stable during many long and repeated measurements (Figure 3c). Figure 3d shows the distribution of the adhesion peak maximum of several scPS. The data on the 6 nm thick annealed scPS reveal two peaks. One peak results from the closed PS film, and the other from the dewetted film areas, exposing the Si substrate. By contrast, all measurements on thicker annealed scPS only show one peak, indicating a stable polymer film during the time of the experiment. This difference in stability of the scPS can be explained by the strong dependence of the characteristic dewetting time τ on the film thickness h (for nucleation and growth τ ∼ h3 and for spinodal dewetting τ ∼ h5).50−52 Irrespective of the underlying dewetting mechanism, the thinnest film is prone to immediate

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RESULTS A. Single Polymer Desorption from Spin-Coated Polystyrene (scPS). In order to determine the polystyrene− polystyrene (PS−PS) cohesion, the desorption of PS has been monitored on annealed scPS with thickness values determined by XRR in air of 6, 10, 21, and 52 nm (Supporting Information, Table S1). The root-mean square (RMS) surface roughness of these films obtained by intermittent-contact AFM measurements is around 0.5 nm (see Table S1). Obviously, the absence of a covalent attachment of the PS chains at the Si substrate makes them prone to dewetting35−39 when immersed into polar solvents such as H2O. Annealing the scPS at 90 °C for 45 min, which is expected to be above the glass transition temperature of such thin films,40 results in an increased film stability in H2O within the time frame of our SMFS experiments. As probe polymer, which is covalently attached to the AFM cantilever tip, we use a polystyrene-b-poly(propylene sulfide), PS−PSS. H2O was used as a surrounding medium in the experiment to minimize the repulsive PS−PS interactions to receive fundamental mechanistic insights. H2O constitutes the limiting case of a poor solvent, also referred to as nonsolvent.41 In general, in aqueous environment two simple typical types of force−extension curves and thus cohesion mechanisms can be distinguished. The first type (denoted as type 1) is characterized by plateaus of constant force Fplateau with the force dropping to zero as soon as the polymer detaches (Figure 2a, upper curve). In the present experiment, the detached polymer shows a length of about 160 nm corresponding to about 67 kg mol−1. This process occurs close to equilibrium conditions and is a characteristic of high in-plane mobility.11,42 Another typical force−extension motif (denoted as type 3) D

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an AFM imaging and indentation approach,61,62 the PS grafting densities can be estimated to 0.03 nm−2 (TEOS-DPE) and 0.06 nm−2 (TCS-DPE). The grafted chain length is 238 (TEOSDPE) and 2241 (TCS-DPE) corresponding to a mass of 25 kg mol−1 and 233 kg mol−1, respectively (Figures S3, S4 and Tables S2, S3). The TCS-DPE substrate (thickness in air of 83 nm) is more hydrophobic than TEOS-DPE (15 nm) as macroscopically measured by the static contact angle of H2O with a value of 116° and 91°, respectively (Table S2). PS cannot be dissolved in H2O, resulting in a compact and collapsed saPS structure.63 A higher grafting density was determined for TCS-DPE leading to lower surface tension with H2O, which is reflected by increasing static contact angle. Still the grafting density determination by AFM characterizes local film properties while contact angle measurements average over large areas. Furthermore Advincula et al.23 found that contact angles are sensitive to the topmost surface composition, which they assume to be mainly due to varying surface morphology and grafting density. Our desorption experiments on saPS show similar results for molecular force sensors for PS and PLL in H2O, respectively (Figure 5). Hence, the cohesion mechanism is independent from the hydrophobicity of the force sensor molecules used. TCS-DPE reveals a higher plateau desorption force Fplateau (type 1 and 2) and a higher probability of plateau occurrence than TEOS-DPE (Figure 5a,c). Still a significant amount of force−extension curves show a nonlinear force behavior (Figure 5c). The equilibrium desorption plateau values are around 50−60 pN on TEOS-DPE and 70−80 pN on TCSDPE following the trend of decreasing surface tension. In case of TEOS-DPE in CHCl3 (where saPS was more extended due to self-repulsion of the PS chains) the Fplateau values are around 10 pN (Figure 5a). This solvent related decrease in Fplateau is similar to that of desorption on hydrophobic solid substrates.48,49 Additionally, the probability for type 1 traces is strongly reduced (Figure 5c) while the detachment length distribution is similar to those observed in H2O. The detachments lengths zdet were around 200−400 nm (84−168 kg mol−1) with a quite broad detachment length distribution independent of the solvent used (Figure 5b and Figure S5). The force peaks for type 2 and 3 events are below 1 nN and therefore well below the forces to break covalent bonds like the

Figure 3. Dewetting behavior of scPS: (a) AFM intermittent-contact mode imaging in H2O and (b) cross section of 6 nm thick layer shows about 6 nm deep holes revealing dewetting. (c) Thicker layers remain stable. (d) Adhesion peak maximum of SMFS measurements with PS force sensors in H2O. The partly dewetted 6 nm thick sample shows two distinct peaks.

dewetting, resulting in a high mobility53,54 in thin scPS.19 The hole structures (Figure 3a) constitute a nonequilibrium state showing rims with high roughness. With ongoing dewetting the rim shape reaches a smooth shape.38 B. Single Polymer Desorption from Covalently Surface Attached Polystyrene (saPS). In order to prevent dewetting and enable measurements in different types of solvents, surface-initiated anionic polymerization is applied as technique of choice to produce well-defined surface-attached PS (saPS) on Si substrates (without annealing).23,28,55−58 Two different types of diphenylethylene-based initiators were synthesized in a similar manner as reported by Advincula et al. 23 In our studies we chose immobilized 4-(11′triethoxysilylundecanyl)diphenylethylene (TEOS-DPE) and 4(11′-trichlorosilylundecanyl)diphenylethylene (TCS-DPE) as initiator precursors for surface-initiated anionic polymerization of styrene (Figure 4a,b). As a result, saPS with different thicknesses ranging between 15 nm (TEOS-DPE) and 83 nm (TCS-DPE) as determined by intermittent-contact mode AFM imaging in air. The height of grafted polymer chains scaling with grafting density and polymer chain mass has been extensively studied based on theories by Alexander,59 de Gennes,60 and Milner et al.21 Using

Figure 4. Preparation of saPS: (a) DPE-based initiator precursors for (b) immobilization on Si wafers for surface-initiated anionic polymerization of styrene. E

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dewetted scPS but to a lesser extent on saPS in H2O. This is caused by a high in-plane mobility42 or by a zipper-like detachment65 of the several 100 nm long (100 nm are about 42 kg mol−1) PS chain attached to the AFM tip.66 In other words, the constant plateau force during desorption in the type 1 curves shows that the PS−PS friction is undetectably low in the observed velocity regime.34 This type of PS−PS interaction constitutes cohesion with low internal friction. Such events can happen even below the glass transition temperature, likely because the glassy saPS exposes a rather fluid boundary layer to the liquid environment (see discussion below). The second cohesion mechanism leads to nonequilibrium events (type 3) resulting in stretching and rupture events. This curve type goes hand in hand with low in-plane polymer mobility. Electrostatic interaction cannot explain this behavior in the case of PS−PS cohesion as there are no charges present. In addition, a tip with attached hydrophilic PLL shows similar results (Figure 5 and Figure S5). In order to explain the latter findings which cannot be due to electrostatics, hydrophobic interaction, and covalent bonds, a geometrical interlock is introduced. Here, the polymer feels a constraint when it is pulled in the z-direction. The relaxation times are much slower than the time scale of our experiment. Upon desorption, the polymer is stretched up to a point where the conformation of neighboring polymers is altered to release the tip bound polymer. Type 2 and 3 curves are observed on dewetted scPS but more frequently on saPS. The higher occurrence of type 2 and 3 curves on saPS is consistent with the larger chain length of 25 kg mol−1 and 233 kg mol−1 compared to scPS with 7 kg mol−1. The long saPS chains favor the formation of entanglements and exhibit much higher relaxation times. Type 2 and 3 curves are much more pronounced in a good solvent such as CHCl3,67 and their amount further increases with longer dwell times and higher indentation force (Figure S7). The forces during rupture, on the other hand, are found to be similar (Figure 6). A good solvent should expose more

Figure 5. PS desorption behavior on saPS: (a) plateau desorption force Fplateau; (b) detachment length zdet; (c) probability of curve type occurrence for PS in H2O, PLL in H2O, and PS in CHCl3 for TEOSDPE and TCS-DPE. The dwell time on substrate is 1 s, and the pulling velocity is 1 μm s−1. For the plateau forces the relative error is given. Absolute errors are around 10%, caused by the uncertainty in cantilever force constant calibration.12 For detachment length data the errors correspond to the standard deviation. *No plateau events are observed.

C−C bond.43,64 Still, the work done to desorb the polymers, which is given by the area under the force−extension curve Edes, is around 103 kBT (Figure 6 and Figure S6) which is remarkably high and furnished proof for a rather strong interaction both in H2O and in CHCl3.

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DISCUSSION Desorbing the PS from PS film boundary layers shows two different cohesion mechanisms. The first results in plateau-like force−extension curves (type 1), as mainly observed on

Figure 6. Solvent dependence of nonequilibrium events: Overlay of about 100 force−extension curves of type 2 and 3. PS on (a) TEOS-DPE in H2O, (b) TEOS-DPE in CHCl3, (c) TCS-DPE in H2O, and (d) TCS-DPE in CHCl3. Zoom-ins show fits for selected curves using the WLC model. F

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In summary, a geometrical interlock model (summarized in Figure 7) is required by several observations: First, we obtain

polymer tails and make them accessible for the AFM cantilever tip attached polymer and lead to an increased ability to form entanglements. In all saPS experiments the detachment lengths of PS and PLL (200−400 nm) are considerably shorter than on hydrophobic solid substrates like PTFE, i.e., only parts of the polymer interact with the PS film (Figure S5). We assume that the AFM cantilever tip pushes the single polymer being in a coiled conformation into the film entering the boundary area. During this process a part of the PS force sensor polymer interlocks with the PS film, while some part of the force sensor molecule tail might stay coiled on top of the PS film. Upon retraction of the AFM tip, the polymer buried into the PS film is pulled off, leading to type 3 traces. At the end, the tail part detaches all in one (because it is not interlocked), explaining the shorter average detachment length zdet. The broad distributions result both from the coiled state of the polymer on the AFM cantilever tip prior to PS film contact and from differences in local PS film conformation in the boundary layer. Both variation of indentation force Findent and dwell time tdwell do not show any significant change in desorption energy Edes (Figure S6). This can be explained by a fast (