Subsurface Imaging of Functionalized and Polymer-Grafted Graphene

Article pubs.acs.org/Macromolecules

Subsurface Imaging of Functionalized and Polymer-Grafted Graphene Oxide Martin Dehnert,*,† Eike-Christian Spitzner,† Fabian Beckert,‡ Christian Friedrich,‡ and Robert Magerle*,† †

Fakultät für Naturwissenschaften, Technische Universität Chemnitz, D-09107 Chemnitz, Germany Freiburger Materialforschungszentrum, Albert-Ludwigs-Universität, D-79098 Freiburg, Germany

‡

S Supporting Information *

ABSTRACT: We investigate the surface and subsurface morphology of stearylamine-modified graphene oxide sheets and polystyrene-grafted functionalized graphene oxide sheets through atomic force microscopy (AFM) operated in multi-set point intermittent contact (MUSIC) mode. This allows a depth-resolved mapping of the nanomechanical properties of the top surface layer of the functionalized graphene oxide sheets. On the surface of stearylamine-functionalized graphene oxide sheets, we can distinguish areas of hydrophilic graphene oxide from hydrophobic areas functionalized with stearylamine. We find that every sheet of graphene oxide is functionalized with stearylamine on both sides of the sheet. The exposure of polystyrene-grafted functionalized graphene oxide to chloroform vapor during the AFM measurement causes a selective swelling and a softening of the polystyrene envelope. The depth-resolved mapping of the tip−sample interaction allows the shape of the folded and wrinkled graphene oxide sheets within the polystyrene envelope to be imaged; furthermore, it allows the thickness of the swollen polystyrene envelope to be measured. This yields the swelling degree, the grafting density, and the average chain conformation of the grafted polystyrene chains, which we find to be in the brush regime. Our work demonstrates a versatile methodology for imaging and characterizing functionalized and polymer-grafted two-dimensional materials on the nanometer scale.

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INTRODUCTION The functionalization of graphene oxide (GO) allows its use as a nanofiller in GO−polymer composites with enhanced material properties,1−5 for example, an increased elastic modulus6 and increased thermal conductivity.7 Generally, a fine dispersion of GO particles within the polymer matrix is necessary to improve the properties of the resulting nanocomposite material. To this end, the graphene agglomerates must be exfoliated and the GO surface must be chemically tailored to compatibilize the GO sheets with the polymer matrix. Furthermore, the functionalization of graphene and GO is of great interest for organic electronics and for sensor applications.8 Different approaches for functionalizing the GO surface have been reported, such as noncovalent8−10 and covalent11−13 modification with small molecules, DNA aptamers,14 peptides,15,16 and polymers.17,18 Moreover, the decoration of GO sheets with metal nanoparticles19,20 and star polymer micells21 as well as polymer grafting2,22−24 has been shown. Beckert et al. demonstrated the grafting of graphene with polystyrene and styrene copolymers by means of thermal selfinitiated free radical “grafting-from” and “grafting-to” polymerization in the presence of organophilic stearylamine-functionalized GO (Stearyl-GO).23 Morphological and rheological investigations revealed that the in-situ formation of polystyrene-grafted Stearyl-GO (PS-g-Stearyl-GO) accounts for much © XXXX American Chemical Society

more uniform graphene dispersion in polystyrene melts than in corresponding graphene/polystyrene solution blends. Furthermore, the resulting PS-g-Stearyl-GO/polystyrene nanocomposites displayed an enhanced elasticity and a markedly improved electrical conductivity.23 The graphene functionalization scheme demonstrated by Beckert et al.23 proceeds in two steps (Figure 1i). Since GO is extremely hydrophilic and is only readily dispersed in water and highly polar organic solvents, the first step involves functionalizing GO with stearylamine to render the resulting Stearyl-GO sheets organophilic.23 In a second step, polystyrene is grafted onto the Stearyl-GO sheets via a self-initiated free radical grafting-from polymerization. The resulting PS-g-Stearyl-GO particles are dispersed in the styrene homopolymer byproduct, which is favorable for blending with commercially available polystyrene (PS) to obtain PS/PS-gStearyl-GO composites.25 The covalent attachment of PS chains to the Stearyl-GO sheets greatly improves the dispersibility of PS-g-Stearyl-GO in polar and nonpolar organic media, including PS melts.23,25 To develop and understand GO functionalization schemes, it is essential to characterize and quantify the efficiency of the GO exfoliation and the individual surface functionalization steps. An Received: July 14, 2016 Revised: September 5, 2016

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DOI: 10.1021/acs.macromol.6b01519 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. IC-mode AFM height images of (a) a GO sheet deposited on a Si substrate, (b) Stearyl-GO, (c) PS-g-Stearyl-GO at ambient conditions, and (d) PS-g-Stearyl-GO swollen in CHCl3 vapor with p/ps = 60% vapor pressure. (e−h) Corresponding IC-mode AFM phase images. (i) Diagram of the functionalization of GO sheets to Stearyl-GO and PS-g-Stearyl-GO. The areas enclosed in black perimeters are investigated with MUSICmode AFM.

efficient functionalization reaction requires a fine dispersion of GO sheets. Whether GO sheets are present as individual monolayers or as stacks of multiple sheets in the suspension during the functionalization reaction is also of particular interest. After the reaction, one would like to know the homogeneity and the thickness of the functionalization layer. To this end, high-resolution imaging is essential, since the surface chemistry of GO sheets is not uniform. For example, heavily oxidized ∼50−100 nm large domains were revealed on the GO surface via electrostatic force microscopy (EFM),26 and the spatial heterogeneous distribution of the surface potential on GO sheets was imaged with Kelvin probe force microscopy (KPFM).27 A functionalization with polymer chains can be achieved either by physisorption onto the surface of GO sheets or by covalently binding polymer molecules via a grafting-to reaction or a grafting-from reaction. In the latter case, a GO sheet has to be functionalized with initiators for the polymerization reaction. After the sheet has been functionalized, similar questions arise as those after the functionalization with smaller molecules: What is the grafting density? How thick and how homogeneous is the polymer layer? Do the GO sheets retain their shape, and do polymer-functionalized GO sheets agglomerate in the suspension? From the polymer point of view, the molecular weight,22,28,29 the chain conformation (brush, mushroom, or Gaussian coil),30,31 and the mechanical properties of the grafted polymer layer are of great interest.1,4 The properties of individual GO sheets,32−34 the morphology of GO surfaces,33,35 and the morphology of polymer−GO composites21,36−42 can be studied with atomic force microscopy (AFM), scanning tunneling microscopy (STM), scanning electron microscopy (SEM), and related techniques. AFM is particularly suited for measuring geometrical quantities, such as the shape and the thickness of GO sheets as well as their surface roughness.33,34,43,44 In addition, the local nanomechanical properties of individual GO sheets can be determined with AFM-based techniques.32,45,46

Here we use multi-set point intermittent contact (MUSIC) mode AFM47 for imaging and characterizing the nanomechanical surface properties of individual stearylaminefunctionalized GO (Stearyl-GO) and polystyrene-grafted Stearyl-GO (PS-g-Stearyl-GO) particles prepared as described in refs 22 and 23. MUSIC-mode AFM is based on pointwise measurements of the amplitude and the phase as a function of the tip−sample distance to generate so-called amplitude− phase−distance (APD) curves.47 From the APD data, the position of the unperturbed (true) surface can be determined, which allows for measuring the shape of soft polymeric surfaces without tip-indentation artifacts.48 Furthermore, the tip indentation can be measured, which is an indicator of the local compliance of the material. MUSIC-mode AFM allows height and phase images that correspond to different amplitude set points to be reconstructed from one APD data set. The resulting height and phase images are free of feedback-loop artifacts, which is advantageous for imaging soft polymer surfaces49−51 and nanoscaled objects with height corrugations, such as self-assembled nanofibrils47 and hydrated collagen fibrils.52 Moreover, the amplitude and phase signal can be deconvoluted to discriminate between conservative and dissipative contributions to the tip−sample interaction.53−55 One variant of MUSIC-mode AFM is the reconstruction of three-dimensional (3D) depth profiles of the tip−sample interaction, where the measured tip indention is used as the depth coordinate.49,50 Here, we expose the polystyrene-grafted Stearyl-GO to chloroform vapor during the AFM measurement which causes a softening of the PS envelope of PS-g-Stearyl-GO particles and allows the shape of the stiff graphene oxide sheet embedded within the soft PS envelope to be imaged.

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RESULTS AND DISCUSSION Figure 1 shows intermittent contact (IC)-mode AFM height images and phase images of individual GO, Stearyl-GO, and PS-g-Stearyl-GO particles deposited on a Si substrate with a B


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Macromolecules native oxide layer. Figures 1b and 1f show the effect of the stearylamine functionalization, which leads to an inhomogeneous coverage of the GO particle. We use MUSIC-mode AFM to investigate the local nanomechanical properties of the Stearyl-GO particles in detail. This allows us to discriminate and to identify areas with different amounts of stearylamine coverage. After the polystyrene (PS) was grafted on the StearylGO particles, the thickness of the PS-g-Stearyl-GO particles increased to about 10 nm (Figure 1c). Imaging in air does not allow the glassy and therefore stiff PS to be distinguished from the stiff GO sheet in the AFM phase image (Figure 1g). This is in line with the measured glass transition temperature of the PS/PS-g-Stearyl-GO composite of 105 °C.23 Exposing the polystyrene-grafted Stearyl-GO to chloroform vapor during the AFM measurement causes a selective swelling and a softening of the polystyrene envelope. As a result, the mechanical contrast between the PS envelope, the embedded GO sheet, and the Si substrate increases dramatically, and the contrast in the AFM phase image improves significantly (Figure 1h). However, the AFM height image is affected by the tip indentation into the soft PS envelope. To overcome this problem, we use MUSIC-mode AFM to determine the unperturbed shape of the swollen PS-g-Stearyl-GO particles and to reconstruct 3D depth profiles of the particles’ nanomechanical properties. This allows us to determine the swelling degree, the grafting density, and the chain conformation of the polystyrene layer on the individual PS-gStearyl-GO particles. The remaining text describes the successive functionalization steps and their effect on the particles’ morphology and nanomechanical properties. Folding and Stacking of GO Sheets. We first address the efficiency of the GO exfoliation. Figures 1a and 1e show the ICmode AFM height image and phase image of a single GO sheet deposited on a Si substrate. Large-area AFM height images (see Supporting Information Figure S1) show numerous single-layer and multilayer GO particles with a morphology similar to the particle shown in Figure 1a. The GO sheet partly folds back on itself, displays wrinkles, and is crumpled in the lower half of the image. Such a morphology of GO sheets deposited on a substrate has been described in the literature.33,34,43,44 The flat areas of the GO sheet appear darker than the surrounding Si substrate in the IC-mode phase image (Figure 1e). Differences in the phase shift reflect local differences in the energy dissipated from the oscillating AFM tip.54,56 We examined the GO sheet shown in Figure 1a with MUSICmode AFM to obtain detailed information about the tip− sample interaction mechanism. The data (not shown here) yield the width w of the attractive regime,52 which is 14.1 ± 0.8 nm on the GO surface and 12.6 ± 0.9 nm on the substrate. The width of the attractive regime is mainly determined by the formation of a water bridge between the tip and the specimen,52,57 and it is much larger than the thickness of the water layer adsorbed on hydrophilic substrates at ambient conditions (0.2−0.3 nm on SiO2).58−60 The difference in tip indentation between the GO surface and the SiO2 surface is less than 0.1 nm. This shows that the phase contrast (1°) is not caused by differences in mechanical properties but by differences in the hydrophilicity of the two materials. GO is extremely hydrophilic,61,62 and SiO2 is less hydrophilic than GO.63 Figure 2a shows the histogram of height values within a 12.5 × 12.5 μm2 large AFM height image (Figure S1b) containing about 100 GO particles of different size and thickness. The

Figure 2. (a) Histogram of values of an IC-mode AFM height image of GO deposited on a Si substrate. (b) Position of the peaks in the height histogram shown in (a). (c) Diagram of stacked GO sheets (blue) deposited on a Si substrate (black). The space between the GO sheets is shown in gray. (d−f) Same as (a−c) for Stearyl-GO sheets.

largest peak corresponds to the substrate, and its position is set to z = 0 nm. The other peaks correspond to GO particles consisting of multiple GO sheets. The peak positions were determined by fitting a Gaussian to each peak in the histogram (Figure 2b). The first layer has a height of 1.2 nm relative to the substrate. The height difference between all the other layers is 0.79 ± 0.01 nm, as determined by a linear regression of the peak positions. This corresponds to the thickness of a single GO sheet within a stack of multiple GO sheets. A diagram of this arrangement is shown in Figure 2c. The thickness values correspond well to the reported spacing between GO layers of 0.65−0.75 nm44 and the thickness of the first monolayer of GO on SiO2 of 0.9−1.3 nm.64 Our own XRD measurements yield a distance of 0.75 nm65 between the GO layers. For determining the average thickness of the stearylamine functionalization, we will use 0.79 nm for the thickness of a single GO sheet in the following. Surface Morphology of Stearyl-Functionalized GO Sheets. An IC-mode AFM height image of a stearylaminefunctionalized GO sheet is shown in Figure 1b together with the corresponding AFM phase image (Figure 1f). Both images indicate an inhomogeneous stearylamine coverage on the GO sheet, which we will discuss in more detail below. The average thickness of the stearylamine layer can be obtained from the histogram of the height values (Figure 2d) within a large-area AFM height image depicting multiple Stearyl-GO particles (Figure S2b). The histogram shows a series of peaks with a distance of 2.05 ± 0.01 nm, as determined by a linear regression to fit the peak positions. A diagram of a stack of multiple Stearyl-GO layers is shown in Figure 2f. The appearance of distinct and equally spaced peaks in the height histogram indicates that all GO sheets have been functionalized on both sides. This suggests that the GO is fully exfoliated and is present as a dispersion of GO monolayers in N-methyl-2pyrrolidone during the stearylamine functionalization. An alternative explanation is that the stearylamine is intercalated into stacks of multiple GO sheets. As a result, each GO sheet is functionalized with stearylamine, with an average thickness of 0.6 nm on each side. We obtain this value by subtracting the thickness of an individual GO sheet (0.79 nm) from the average distance between Stearyl-GO sheets (2.05 nm) and dividing the result by 2. The average thickness of the stearylamine coverage C


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marked with a white dashed line. On the Stearyl-GO sheet, three different types of areas can be distinguished which differ in height relative to the substrate: (1) the nonfunctionalized GO areas, (2) GO functionalized with a monolayer of stearylamine, and (3) stearylamine islands on top of the Stearyl-GO sheet. A cross section is depicted in a diagram in Figure 3e, with the AFM tip partially indenting into the stearylamine layers. Each area type is characterized by a characteristic set of parameters in the different MUSIC-mode AFM data channels (Figure 3 and Table 1). We manually

is smaller than the thickness of a dense stearylamine monolayer on mica.66,67 Our AFM images suggest that this smaller value is due to an inhomogeneous stearylamine coverage. We note that the average thickness 0.6 nm is obtained for the Stearyl-GO sheets shown in Figure S2b. XRD measurements of a StearylGO powder sample yield a distance of 1.67 nm between the functionalized GO sheets,65 which confirms the increase in the distance between GO sheets due to the stearylamine functionalization. An interesting detail is that Stearyl-GO sheets do not show wrinkles and back-folds as the GO sheets do. We attribute this to an increased bending stiffness of Stearyl-GO sheets compared to the not functionalized GO sheets. A similar stiffening effect has been observed for GO sheets decorated with star-polymer micelles.21 We now turn to the morphology of the stearylamine coverage on the Stearyl-GO sheets. The area marked with a black perimeter in Figure 1f was investigated with MUSICmode AFM, which allows a detailed and quantitative interpretation of the tip−sample interaction. Figure 3a shows the map of the undisturbed height z0, which is the position where the tip first starts to interact with the sample as measurements are taken for an amplitude−phase−distance approach curve. The outer edge of the Stearyl-GO sheet is

Table 1. Averages and Standard Deviations of the MUSICMode AFM Parameters Measured on Stearyl-GO area 1a

area 2a

area 3a

Sia

assignment

GO surface

stearylaminefunctionalized GO surface

stearylamine island

substrate

z0 [nm] z̃Bc [nm] Edisc [eV] w [nm]

2.30 ± 0.74 0.15 ± 0.20 219 ± 23 3.99 ± 0.44

2.67 ± 0.78 0.40 ± 0.24 203 ± 23 3.32 ± 0.57

4.87 ± 0.94 2.02 ± 0.59 293 ± 43 3.03 ± 0.90

0.00b ± 0.59 0.13 ± 0.17 222 ± 14 3.73 ± 0.30

a

Areas marked in Figure 3a−d and Figure S3. bReference height set to zero. cAt a set point of A/A0 = 0.80.

marked each area type (Figure S3), and for each area type, we determined the average and the standard deviation of the height z0 relative to the substrate, the tip indentation z̃B, the energy dissipated during one oscillation cycle Edis, and the width w of the attractive regime (Table 1). The histogram of the z̃B values measured at a set point of A/A0 = 0.80 is shown in Figure S4. Each of the four different area types causes a distinct peak in the histogram. Nonfunctionalized GO areas (1) are characterized by a large width w of the attractive regime, which corresponds well to the hydrophilic character of the bare GO surface.68 The tip indentation z̃B = 0.15 ± 0.20 nm is small and similar to the one on the Si substrate, which corroborates the assignment of area 1 as a nonfunctionalized GO surface. Functionalized GO areas (2) display a 25 nm larger tip indentation than the nonfunctionalized GO areas. The difference in height is only 0.37 nm, which is smaller than the thickness of a dense stearylamine monolayer on mica (1.6 nm).66,69 This could have two reasons. First, the stearylamine functionalization might not be densely packed, and/or the height measurement might be disturbed due to noise in the z0 data. The width of the attractive regime is 0.67 nm, which is smaller than on the nonfunctionalized GO areas. We attribute this to the more hydrophobic character of functionalized GO. The third area type on the GO sheets (marked as 3) are islands of multiple stearylamine layers. Their most distinct properties are the large height z0, the large tip indentation z̃B, and the large dissipated energy Edis. The height difference relative to the nonfunctionalized GO surface corresponds to two or three stearylamine layers, depending on the thickness assumed for one monolayer. The width of the attractive regime for the islands is similar to that of functionalized GO. This indicates that the surfaces of the two areas are chemically similar, suggesting the presence of a stack of three stearylamine layers. Finally, we note that the small globular structures visible on the Si substrate next to the Stearyl-GO sheet are most likely small stearylamine islands, since they display similar MUSIC-

Figure 3. (a−d) MUSIC-mode images of Stearyl-GO deposited on a Si substrate. (a) Unperturbed height image z0. Tip indentation z̃B (b) and a map of Edis (c) for A/A0 = 0.80, representing the dissipative, repulsive tip−sample interactions. The attractive regime’s width w (d) represents the attractive forces acting on the AFM tip. The maps of z̃B and w were filtered with a 1 pixel wide median filter. (e) Diagram of the AFM tip indenting into different areas on the Stearyl-GO sheet: (1) nonfunctionalized area, (2) stearylamine-functionalized area, and (3) stearylamine islands on the Stearyl-GO sheet. A blue line indicates the GO sheet. Black lines with red ends represent stearylamine molecules. The AFM tip is shown with a radius of 8 nm. D


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Figure 4. (a) Unperturbed height image z0 of PS-g-Stearyl-GO deposited on a Si substrate, imaged at p/ps = 0% CHCl3 vapor pressure. (b) MUSICmode AFM phase image at A/A0 = 0.95. (c) MUSIC-mode AFM phase image at A/A0 = 0.50. (d) Depth profile of kTS, reconstructed from MUSICmode AFM data along the dashed line shown in (a−c). (e−h) Same as (a−d) for p/ps = 30% CHCl3 vapor pressure. (i−l) Same as (a−d) for p/ps = 60% CHCl3 vapor pressure.

Figure 5. (a) Average tip indentation ⟨z̃0⟩ as a function of the amplitude set point A/A0 on bright and dark areas in the phase image of PS-g-StearylGO as well as the Si substrate in Figure 4. Each curve is the average of 200 different positions. Each error bar indicates the standard deviation. Data are shown for different CHCl3 vapor pressures p/ps. (b) Tip indentation ⟨z̃0⟩ as a function of the amplitude set point A/A0 on PS deposited on a Si substrate for different vapor pressures of CHCl3. Each curve is the average of eight APD curves. Each error bar indicates the standard deviation. The raw data are taken from ref 70.

next to the phase images. At ambient conditions with p/ps = 0% and at a set point of A/A0 = 0.95 (Figure 4b), only small differences are visible in the phase shift between the PS-gStearyl-GO particle and the Si substrate. At a set point of A/A0 = 0.50, the contrast between the PS and the GO increases on the particle surface. As the chloroform vapor pressure increases, the phase contrast on the particle surface as well as the phase contrast between the particle and the Si substrate increases dramatically (Figure 4f,g,j,k). In particular, the internal structure of the PS-gStearyl-GO particle becomes clearly visible. We interpret the bright areas as the stiff and wrinkled GO sheet embedded within its envelope of PS grafted on the GO sheet. As the chloroform vapor pressure increases, the particle’s height and the tip indentation into the particle increase. Both are due to the swelling and the softening of the particle’s PS envelope. The maps of amplitude−phase−distance (APD) curves from data measured with MUSIC-mode AFM can be deconvoluted to discriminate between conservative and dissipative contributions to the tip−sample interaction.53−55 The conservative contribution to the total tip−sample interaction is expressed by an additional tip−sample spring constant kTS, which characterizes the elastic response of the specimen. The tip indentation z̃0 can also be determined from the APD curves and can be used as a depth coordinate for reconstructing 3D depth profiles of the kTS parameter, characterizing the tip−sample interaction49

mode AFM parameters as the stearylamine islands (area 3) on the Stearyl-GO sheet. Probably they stem from excess free stearylamine from the dispersion. Subsurface Imaging of Polystyrene-Grafted StearylGO. PS-g-Stearyl-GO sheets display a much larger thickness than Stearyl-GO sheets as well as an irregular surface morphology. In the conventional AFM phase images, the glassy and therefore stiff PS is difficult to distinguish from the stiff GO (Figure 1c). We therefore exposed the specimen to chloroform vapor, which causes a selective swelling and softening of the PS grafted on the GO sheet. In this way, the mechanical contrast between the swollen PS and the GO increases dramatically, as seen in the conventional AFM phase image (Figure 1h). To investigate in more detail, we imaged the PS-g-Stearyl-GO particle with MUSIC-mode AFM at increasing chloroform vapor concentrations ranging from p/ps = 0% to 60% (Figure 4). Figure 4 shows maps of the unperturbed height z0 and MUSIC-mode phase images at the set points A/A0 = 0.95 and 0.50. The small positional variations of the imaged areas during the swelling experiment are due to the drift of the AFM setup, which causes an apparent distortion of the particle’s outline. Despite this drift, the dashed horizontal lines in the z0 and in the phase images mark the same cross section for the particle during the swelling experiment. The depth profiles of kTS reconstructed along these dashed lines are shown in Figure 4 E


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Macromolecules (Figure 4d,h,l). The solid black line is the z0 profile, marking the onset of attractive interaction during APD-approach-curve measurements. The swelling and the softening of the PS envelope are clearly visible in the kTS depth profiles shown in Figure 4d,h,l. The color code is chosen to distinguish the attractive regime (negative kTS values, blue) from the repulsive regime (positive kTS values, red) of the tip−sample interaction. The attractive regime’s width w is due to attractive van der Waals forces and capillary forces between the tip and the specimen.52 With increasing solvent vapor pressure, the width w increases on the PS-g-Stearyl-GO particle as well as on the Si substrate. In the kTS depth profiles, a white stripe marks the position of zB, where kTS = 0 N/m (balance of forces). The repulsive regime of the tip−sample interaction is shown in different shades of red. Its width also increases with increasing chloroform vapor pressure due to the increasing tip indention into the PS-g-Stearyl-GO particle. The average tip indentation ⟨z̃0⟩ measured on the dark (PS) and bright (GO) areas as determined from the MUSIC-mode AFM phase images is shown in Figure 5 as a function of the set point A/A0. The average indentation ⟨z̃0⟩ on the Si substrate is plotted as a reference. On the Si substrate, the average tip indentation ⟨z̃0⟩ is constant for set points A/A0 < 0.7, as observed earlier.48 We note that z̃0 includes the width of the attractive regime w.48 Therefore, an apparent average indentation ⟨z̃0⟩ = 3.2 nm is observed on the Si substrate. On the PS-g-Stearyl-GO particle, a similar behavior is observed, however, with an average tip indentation that is 1.5 nm higher. This is also observed on a pure PS surface.48 On both types of areas (PS and GO), the same average tip indentation is observed, since the entire particle is covered with a PS layer. As vapor pressure increases, the average tip indentation ⟨z̃0⟩ into the PS-g-Stearyl-GO particle increases up to 15 nm at A/A0 = 0.5 for p/ps = 60%. In addition, larger lateral variations of z̃0 are observed, as can be seen from the increasing standard deviations. These lateral heterogeneities are in line with the increased contrast in the corresponding MUSIC-mode AFM phase images. On PS areas, the average tip indentation ⟨z̃0⟩ is up to 3.5 nm larger than on GO areas. The behavior of the average tip indentation ⟨z̃0⟩ is similar to that observed on a 33 nm thick PS film deposited on a Si substrate (Figure 5b). Wrinkling of Embedded GO Sheets. The softening of the PS envelope makes the shape of the embedded GO sheet and its wrinkling visible. A detail is shown from the IC-mode height and phase images generated from measurements taken at a solvent vapor pressure of p/ps = 60% (Figure 6a,b). As discussed above, the tip pushes through the PS top layer, and the conventional IC-mode AFM height image corresponds to the shape of the embedded GO sheet. Figures 6a and 6b show wrinkles in the GO sheet. Similar wrinkles are also seen on other PS-g-Stearyl-GO particles (not shown here). From the height profile along the black line, we determine an undulation periodicity of 70 ± 10 nm, with the peaks of the high areas and the valleys of the low areas differing in height by 2.5 ± 0.5 nm. Figure 6b shows lower phase values in the valleys than on the ridges, where higher phase values are observed. The valley (ridge) areas are marked as PS (GO) areas in Figure 5 and display different tip indentations. Therefore, one explanation for the lower phase values in the valleys could be that the PS layer is thicker in the valleys than on the ridges. Another explanation could be that stronger attractive forces act on the AFM tip in the valleys due to the concave surface curvature.71 Figure 6c shows an IC-mode AFM height image of the same

Figure 6. (a) IC-mode AFM height image and (b) the corresponding phase image of PS-g-Stearyl-GO swollen in CHCl3 vapor with p/ps = 60% vapor pressure (details of Figure 1d,h). (c) IC-mode AFM height image after swelling treatment with CHCl3 vapor, taken at p/ps = 0%. The white line indicates the outline of the height image of the same particle before swelling in CHCl3 vapor. (d) Diagram of the GO sheet embedded in the PS matrix with dimensions taken from (a).

spot measured at p/ps = 0% after the swelling treatment and drying. The white line indicates the particle’s outline before swelling occurs in chloroform vapor. The initial area covered by the particle is 1.51 μm2 (as determined from the entire height image shown in Figure 1c). After the swelling procedure, the particle’s area on the substrate is reduced to 1.47 μm2, which is a relative reduction of 2.5%. In the swollen state, the PS envelope is soft and contracts in order to minimize the total surface area. This is accompanied by a local retraction of the particle’s outline, as indicated by the arrows in Figure 6c. During swelling, the particle becomes 1 nm thicker, indicating volume conservation. The area on the substrate which was previously covered by the particle is smooth, and no residual PS is visible. This indicates that the entire PS envelope slips on the substrate, and the particle is laterally compressed. This compression is likely a reason for the wrinkle formation in the GO sheets. A summary of the measured geometrical data is provided in Figure 6d, which shows a schematic cross section of the PS-g-Stearyl-GO particle along the black line shown in Figure 6a−c. At this position, the particle is 10.2 ± 1.4 nm thick. The thickness of the embedded Stearyl-GO sheet is 2.05 nm, as determined from the height histogram in Figure 2b. Swelling Behavior of the PS Envelope. The position of the cross section shown in Figure 4 was chosen to include the Si substrate on both sides of the PS-g-Stearyl-GO particle. This allows us to use the Si substrate as a reference for accurate measurements of the particle’s height and the particle’s swelling behavior. Despite the heterogeneous morphology of the PS-gStearyl-GO particle, the depth profiles shown in Figure 4 indicate an approximately uniform swelling behavior for the PS envelope. This leads us to conclude that the surface is completely covered with grafted PS. To determine the relative swelling, we determined the area a of the cross section between the unperturbed height z0 and the F


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Figure 8. (a) Mean distance D of PS chains grafted on GO as a function of the layer thickness h for different molecular weights Mw and (b) reduced grafting density Σ as a function of the layer thickness h for different assumed Kuhn lengths lK of PS. (c) Diagram of the conformation of two PS chains with Mw = 3 and 30 kg/mol grafted on a surface and confined in a 4.2 nm thick film.

Figure 7. Relative swelling ratios a/a0 and d/d0 of PS-g-Stearyl-GO as a function of the CHCl3 vapor pressures p/ps (red line). The error bars indicate the standard deviation of d/d0. The data for the relative swelling of a pure PS film are from Knoll et al.72 (gray line). The inset shows the height profiles z0 taken at p/ps = 0% (black), 30% (dark gray), and 60% (light gray). The data are from Figure 4d,h,l.

shorter chains, we assume Mw = 3 kg/mol, and for calculating the grafting density σ, we therefore assume molecular weights Mw in the range of 3−110 kg/mol. The resulting average distance between the grafting points D is plotted in Figure 8a as a function of the layer thickness h for the aforementioned values of molecular weights Mw. To determine the reduced grafting density Σ, we calculate the radius of gyration RG = √NlK/√6, where N = Mw/CMs is the number of Kuhn segments, lK is the length of a Kuhn segment, and Ms = 0.104 kg/mol is the molecular weight of a styrene monomer.74 In the calculation of Σ, we assume C = 5, 7, and 9 monomers per Kuhn segment,75 which leads to lK = 1.25, 1.75, and 2.25 nm, and we plot the corresponding curves in Figure 8b. The grafting density σ and the radius of gyration RG both depend on the molecular weight Mw; however, the reduced grafting density Σ = (πρNAlK2/6CMs)h is independent of the molecular weight Mw and depends only on the thickness h of the grafted polymer layer, which we can measure accurately with AFM. The truebrush regime is characterized by Σ > 5, and the mushroom regime is characterized by Σ ≈ 1.30 Between these two regimes, the polymer chains are in a mushroom-to-brush transition. We measure an average height z0 = 10.5 nm for the PS-gStearyl-GO particle shown in Figure 1c (for p/ps = 0%). From this, we obtain an average thickness h = 4.2 nm for the grafted PS layer, which leads to a reduced grafting density Σ between 4.18 (for C = 5) and 7.52 (for C = 9). We therefore conclude that the grafted PS chains are in the brush regime. Other works have shown that in the brush regime at higher values of Σ the polymer chains swell primarily in the direction normal to the substrate.76,77 This could be an explanation for the larger swelling ratio of the grafted PS layer compared to that of a pure PS film. From Figure 8a, we can also estimate the mean distance between the grafting points D. For a brush height of h = 4.2 nm, D ranges between 1.06, 3.4, and 6.4 nm, depending on which molecular weight is assumed (Mw = 3, 30, and 110 kg/ mol). These distances are much larger than the mean distance between stearylamine molecules in a densely packed monolayer (0.47 nm).69 It is interesting to compare the contour length of the grafted polymers and their gyration radius with the thickness of the polymer layer and the mean distance between grafting points. Since the grafted PS is polydisperse, we consider a chain with Mw = 3 and 30 kg/mol. The

measure of the relative polymer concentration in the swollen film. For comparison, we determined the particle’s average height d on the cross section’s left-hand side, as shown in the inset of Figure 7. The ratio d/d0 is also shown in Figure 7, where d0 is the height for p/ps = 0%. The ratio d0/d is a measure of the local polymer concentration in the swollen film. Both parameters characterizing the swelling increase in the same way with increasing solvent vapor pressure. In contrast, a pure PS film (with Mw = 520 kg/mol) swells less than the grafted PS layer on Stearyl-GO as the data from Knoll et al.72 indicate (Figure 7, gray dots). This difference shows that the swelling ratio depends on the detailed conformation of the PS chains in the thin film. Grafting Density and Chain Conformation in the PS Envelope. The swelling behavior of grafted polymer films allows the grafting density and the average chain conformation in the film to be determined.30 One distinguishes the brush conformation, where all polymer chains are stretched, from the mushroom regime, where the grafted polymer chains are compressed toward the substrate. The grafting density σ = (hρNA)/Mw, where h is the brush thickness, ρ = 1.05 g/cm3 is the bulk density of PS,73 NA is Avogadro’s number, and Mw is the weight-averaged molecular weight. The average distance between grafting points D = 1/√σ is plotted in Figure 8a for different values of Mw along with the reduced grafting density Σ = σπRG2, where RG is the gyration radius (Figure 8b).30 From the image of the unperturbed height, we can determine the average height ⟨z0⟩ of a PS-g-Stearyl-GO sheet relative to the substrate. The Stearyl-GO sheet has a thickness of 2.05 nm, and we assume that it is located in the middle of the particle, with an approximately homogeneous coverage of grafted PS chains on both sides. Therefore, the mean thickness of the grafted polymer layer is h = (⟨z0⟩ − 2.05 nm)/2. The molecular weight is only approximately known. The upper limit is the molecular weight of unattached PS that forms simultaneously during the grafting reaction. The unattached PS chains have been separated from the PS/PS-g-Stearyl-GO composite, and their molecular weight was measured to be 110 kg/mol.65 For a similar system to the one studied in this work, Beckert et al. determined Mw = 30 kg/mol for PS chains that were detached from the PS-g-Stearyl-GO sheet after the grafting reaction and found a polydispersity of Mw/Mn = 5.0.22 This shows that shorter chains are also present in the grafted PS layer. For these G


Article

Macromolecules corresponding contour lengths lC are 7.2 and 72 nm; the gyration radii are 1.6 and 5.2 nm (assuming C = 9 monomers per Kuhn segment). A diagram of the conformations of these polymer chains confined in a 4.2 nm thick polymer layer is shown in Figure 8c. From this, we conclude that the short chains are more stretched than the longer chains.

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CONCLUSION In summary, we imaged the surface and subsurface morphology of stearylamine-functionalized and polystyrene-grafted graphene oxide particles and determined their swelling behavior in chloroform vapor. All GO sheets were exfoliated and functionalized with stearylamine on both sides. The stearylamine functionalization forms, on average, 0.6 nm thick layers on GO sheets. The surface of GO is not completely covered with stearylamine. In addition to monolayers of stearylamine, triple layers of stearylamine as well as nonfunctionalized areas are also observed on Stearyl-GO. The attachment of stearylamine causes the bending stiffness of Stearyl-GO to increase, as indicated by the absence of wrinkles in the Stearyl-GO sheets compared to bare GO sheets. PS grafted on GO forms an approximately 4 nm thick layer. Swelling treatment with chloroform vapor softens the PS envelope and allows us to use MUSIC-mode AFM to image the embedded GO sheets, which are wrinkled and partly folded. From the thickness of the PS layer and its swelling behavior in chloroform vapor, we can draw conclusions about the average chain conformation of the grafted PS chains, which are found to be in the brush regime.

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Figure 9. (a) Phase φ and amplitude A measured on PS-g-Stearyl-GO swollen in CHCl3 vapor with p/ps = 60% vapor pressure as a function of the tip−sample distance d (approach curves). The thin solid lines indicate the amplitude on a stiff reference surface. (b) Additional tip− sample spring constant kTS, tip indentation z̃0 (black), and tip indentation z̃B (red) as a function of the tip−sample distance d.

interact with the surface.48 Technically, z0 is the position where the phase starts to deviate from the phase of the free oscillation. As the cantilever further approches the surface, the cantilever oscillation is damped as the tip interacts with the surface at the inflection point of the oscillation. On a compliant surface, the tip indents into the surface.48,80 For a given tip−sample distance d, the tip indentation is the additional distance that the tip has to approach the surface in order to reach the same damped amplitude A as on a stiff reference surface that does not allow for any indentation.81 This is a purely geometric interpretation of the tip−sample contact geometry, and it does not require any model assumptions. However, the value of the tip indentation depends on the definition of the unperturbed surface, as Spitzner et al.52 have shown. The position z0 is often defined by the first appearance of an attractive interaction as the tip approaches the surface.48,49,81,82 In the case of strong attractive tip−sample interactions (van der Waals forces, capillary forces), this choice can lead to astonishingly large values for the tip indentation z̃0. In these cases, an alternative definition of the sample surface is the position zB where a balance of attractive and repulsive forces is observed in the phase−distance curves.52 At zB, the transition from the attractive to the repulsive regime of the tip−sample interaction occurs. The width of the attractive regime is w = z0 − zB. It is mainly determined by the formation of a water bridge between the tip and the specimen and the range of adhesive forces acting between the tip and the specimen.52,57 The measured amplitude and phase values can be deconvoluted to discriminate between conservative and dissipative contributions to the tip−sample interaction. The theory is described in refs 53−55. Briefly, the influence of the tip−sample interaction on the forced oscillation of the AFM cantilever is modeled using first-order perturbation theory.55 Conservative and dissipative contributions to the total tip−sample interaction force are expressed by an additional tip−sample spring constant kTS and the effective damping parameter αeff/m. Within the attractive regime, kTS is negative (Figure 9b). The energy Edis dissipated during one tip oscillation is determined from the measured A and φ values according to refs 53 and 54. 3D Depth Profiling.49 The tip indentation z̃0 measured as a function of the tip−sample distance d (Figure 9b) can be used as a depth coordinate relative to the unperturbed surface z0 for plotting a depth profile of quantities that can be determined from the measured amplitude and phase values, for example, the conservative contribution to the tip−sample interaction kTS.55 The result is a 3D depth profile of the tip−sample spring constant kTS.

MATERIALS AND METHODS

Sample Preparation. We investigated PS-g-Stearyl-GO, StearylGO, and GO synthesized as in ref 23. GO was dispersed in water and Stearyl-GO and PS-g-Stearyl-GO in toluene with a concentration of 2 mg/mL. 60 μL of the dispersion was spin-coated at 3000 rpm on a 1 × 1 cm2 large Si substrate with a native SiO2 layer. The concentration was chosen to result in isolated particles deposited on the substrate. Prior to spin-casting, the dispersion was treated in an ultrasonic bath for 15 min; the substrates were cleaned in a 1:1 solution of acetone/ toluene and were mounted on a heating plate (150 °C), and the residual contamination was removed with a jet of carbon dioxide.78 Swelling in Chloroform Vapor. We used a custom-built setup similar to the one described by Knoll et al.72 for in-situ AFM imaging during swelling treatment with chloroform vapor. A stream of dry nitrogen gas and a stream of nitrogen gas saturated with chloroform (with vapor pressure ps) was mixed for controlling the relative vapor pressure of chloroform p/ps.72 The total gas flow was set to 15 sccm. For imaging at ambient conditions, the N2 flow was turned off. AFM Imaging. For AFM measurements, a NanoWizard and a NanoWizard II instrument (JPK Instruments AG, Berlin, Germany) were used with silicon AFM cantilevers (Pointprobe NCH, NanoWorld AG, Neuâchtel, Switzerland). The tip radius was taken to be 8 nm, as specified by the manufacturer. The spring constants of the cantilevers were between 13 and 26 N/m (determined as in ref 79) with Q factors ranging from 290 to 480 and resonance frequencies between 260 and 310 kHz. The free amplitude A0 was set to 60 nm and was damped to either 30 or 15 nm at the end of the approach curve. To reconstruct MUSIC-mode images, we measured data for arrays of either 50 × 50 or 70 × 70 APD curves. Data Analysis. The protocols for measuring the amplitude− phase−distance (APD) curves and MUSIC-mode AFM are described in refs 47 and 52. The data analysis is illustrated in Figure 9a. The amplitude A and the phase φ of the oscillating cantilever are recorded as a function of the tip−sample distance d on a typically 50 × 50 large array of positions on the surface. From each APD curve we determine the position of the unperturbed surface z0 where the tip starts to H


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Macromolecules

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01519.

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Figures S1−S4 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.D.). *E-mail: [email protected] (R.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank A. Süsselbeck for sample preparation and for help with the AFM measurements, C. Riesch for implementing the software used for analyzing the MUSIC-mode AFM data, M. Neumann, M. Fröbe, M. Uhlig, D. Voigt, and M. Zerson for discussions, and S. McGee for proofreading the manuscript. The acquisition of the AFM was funded by the Volkswagen Foundation and the Deutsche Forschungsgemeinschaft.

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