Direct Measurement of Colloidal Interactions between Polyaniline

Jan 8, 2014 - (1, 2) The main reason is the possibility of providing conductive composites .... of 60:40, 30:70 and 0:100, giving viscosities of ∼20...
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Direct Measurement of Colloidal Interactions between Polyaniline Surfaces in a UV-Curable Coating Formulation: The Effect of Surface Hydrophilicity/Hydrophobicity and Resin Composition Shadi Jafarzadeh,*,† Per M. Claesson,†,2 Jinshan Pan,† and Esben Thormann†,3 †

School of Chemical Science and Engineering, Department of Chemistry, Division of Surface and Corrosion Science, KTH Royal Institute of Technology, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden 2 Chemistry, Materials and Surfaces, SP Technical Research Institute of Sweden, P.O. Box 5607, SE-114 86 Stockholm, Sweden 3 Department of Chemistry, Technical University of Denmark, Kemitorvet 207, DK-2800 Kongens Lyngby, Denmark S Supporting Information *

ABSTRACT: The interactions between polyaniline particles and polyaniline surfaces in polyester acrylate resin mixed with 1,6-hexanediol diacrylate monomer have been investigated using contact angle measurements and the atomic force microscopy colloidal probe technique. Polyaniline with different characteristics (hydrophilic and hydrophobic) were synthesized directly on spherical polystyrene particles of 10 μm in diameter. Surface forces were measured between core/ shell structured polystyrene/polyaniline particles (and a pure polystyrene particle as reference) mounted on an atomic force microscope cantilever and a pressed pellet of either hydrophilic or hydrophobic polyaniline powders, in resins of various polymer:monomer ratios. A short-range purely repulsive interaction was observed between hydrophilic polyaniline (doped with phosphoric acid) surfaces in polyester acrylate resin. In contrast, interactions between hydrophobic polyaniline (doped with ndecyl phosphonic acid) were dominated by attractive forces, suggesting less compatibility and higher tendency for aggregation of these particles in liquid polyester acrylate compared to hydrophilic polyaniline. Both observations are in agreement with the conclusions from the interfacial energy studies performed by contact angle measurements.

1. INTRODUCTION Composites of conducting polymers, especially polyaniline (PANI), in a host polymer matrix have captured high research attention, and a wide range of applications have been suggested during the last decades.1,2 The main reason is the possibility of providing conductive composites where the limitations with weak processability and poor solubility of PANI can be eliminated by thoughtful design of the system’s components. Nanostructures of PANI are preferred to make homogenously dispersed composites with low percolation threshold.3 To improve the dispersion stability of such composites, compatibility of the components and the role of interaction between nanoparticles in the dispersed media should be considered.4 Repulsion between colloidal particles is preferable to provide well dispersed composites, which, in the case of good affinity between particles and the media, can be provided by the liquid adsorption onto the colloid surfaces. On the other hand, attractive forces between particles may lead to aggregation. For the specific case of making conductive composites, strong repulsion between the conductive constituents is not either favorable, as the dispersed PANI particles should be close enough to make electron conduction possible, providing a conductive network within the matrix above the percolation © XXXX American Chemical Society

threshold. Hence, there should be a balance between attractive and repulsive interactions when designing composites containing conductive additives. One of the conventional methods for investigating the free energy of the interparticle interaction and contact adhesion is surface tension and contact angle measurement.5 Here, the repulsive or adhesive nature of the interaction can be concluded from the calculated interfacial energy between two solid surfaces, or a solid and liquid interface. However, this technique does not give any direct information on the forces as a function of distance between two neighboring particles. The attempts on direct measurement of interaction forces between particles and surfaces in liquids have been reviewed extensively by Liang et al.6 The first direct measurements of surface forces was reported by Derjaguin et al. in 1956,7 where the forces were measured by an electrobalance, and the distance between two glass surfaces was detected using an optical technique. The first quantitative technique for measuring surface forces, named surface force apparatus (SFA), was Received: October 27, 2013 Revised: December 28, 2013

A

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reported by Israelachvili and Adams in 1978,8 and there are many reports on the interactions between two crossed mica cylinders (and a few other solid surfaces) in different liquids using the SFA. The main drawback of the SFA technique is the requirements for the substrate, which limits the technique to thin, molecularly smooth and semitransparent sheets. Reaching real equilibrium and reducing hydrodynamic effects could also be a problem in some cases due to the relatively large interacting areas, which is more pronounced when measuring forces in viscous polymer liquids.9 These limitations are surmounted by measuring surface forces using atomic force microscopy (AFM) between a micrometer-sized particle attached to the apex of a cantilever and a flat substrate. Colloidal probe AFM was first reported in 1991,10 and since then interactions between a wide range of surfaces in aqueous dispersions11 containing surfactants12 or polymer dispersants,13 and also in different solvents like pentanol14 and dodecane15 have been determined by this technique. However, just a few attempts have been focused on interactions in polymer melts from which the studies on silica and mica surfaces by Butt et al.9,16,17 can be mentioned. To our knowledge, there are no reports on direct measurements of surface forces between two polymer solid surfaces in viscous polymer liquids. In the present work, we investigate the compatibility of a hydrophilic PANI (doped with phosphoric acid) and a hydrophobic PANI (doped with n-decyl phosphonic acid) with a UV-curable resin based on polyester acrylate. The nature of the interactions between PANI particles is first determined by contact angle and interfacial energy measurements. Next, direct measurement of the interactions in polymer resin is performed by atomic force microscopy utilizing the colloidal probe technique. Here, the forces between core/shell structured polystyrene/polyaniline (and reference polystyrene) particles are measured versus distance to pressed pellets of PANI.

Conductivity of PANI was provided by the presence of phosphoric acid (PA) (Sigma-Aldrich, 99%) or n-decyl phosphonic acid (DecylPA) (PCI Synthesis, 99%) solutions as dopant, giving rise to PANIs with different characteristics in terms of hydrophilicity/ hydrophobicity. Conductivity of the synthesized polymers, measured by impedance spectroscopy on pressed pellets of PANI with 0.15 cm thickness and 1.28 cm diameter, was 9.7 × 10−2 Scm−1 for PANI-PA and 2.3 × 10−1 S cm−1 for PANI-DecylPA. Details of the conductivity measurements has been described elsewhere.3 The schematic structure of the conductive (emeraldine salt) state of PANI doped with PA and DecylPA are shown in Figure 1. Colloidal probes of spherical polystyrene (PS) microparticles, or spherical particles of PS coated with a shell of PANI, were used for the surface force measurements. PS was provided as monodisperse particles with 10 μm diameter in aqueous solution (G. Kisker GmbH, Germany). The core/shell structured PS/PANI-PA and PS/ PANI-DecylPA particles were prepared by direct synthesis of PANI (procedure described above) on PS microparticles acting as templates.19 This was done in order to prepare robust PANI surfaces with defined size shapes, which was not possible without the use of template particles. Pressed pellets of PANI-PA and PANI-DecylPA powders were used as the substrate in both contact angle and surface force measurements. Smooth and uniform films of polystyrene for contact angle measurements were made by melting PS particles on a silica substrate placed on a heating stage under a microscope. The topography and the homogeneity in terms of nanomechanical properties of the PANI pellets and of PS layer surfaces were characterized by PeakForce tapping mode AFM20,21 using a Multimode Nanoscope V (Bruker, USA) and a rectangular silicon cantilever of model DPE15 (Mikromasch, Estonia). The deflection sensitivity and spring constant were determined to be 30 nm/V and 33 N/m, respectively, by methods described elsewhere.22,23 Figure 2 displays AFM topography, elastic modulus, and deformation images of these three substrates taken in air with a scan size of 2 × 2 μm. The elastic modulus was calculated with the Derjaguin−Muller−Toporov (DMT) model,24 where the tip radius is required. This radius was determined to be 1.5 nm, by scanning a calibration sample with a known elastic modulus and adjusting the tip radius to achieve the correct bulk modulus using the DMT model. A force set point of 80 nN was applied in order to deform the sample surface by 2−3 nm, which is required to obtain quantitative material properties of the surface layer. Roughness analysis of the topography images shown in Figure 2 reveals an RMS roughness (Rq) value of 11.4, 5.6, and 2.5 nm for PANI-PA, PANI-DecylPA, and PS layers, respectively. The modulus image shows similar stiffness for PANI-PA pellets and PS layer, whereas PANI-DecylPA is significantly softer. 2.2. Contact Angle and Surface Tension Measurements. The surface properties of the materials to be used in surface force studies (PS, PANI-PA, and PANI-DecylPA) were examined in terms of water contact angle, surface energy components, and interfacial tension and wetting properties with the liquids in which interactions are of interest. Contact angles were assessed by the sessile drop method25 utilizing an OCA20 instrument (DataPhysics, Germany). The liquid droplet was illuminated while being dispensed on the PS or PANI surfaces, and the variation in droplet shape was captured by a high resolution CCD camera. The recorded series of images were examined afterward, and the contact angle was determined by employing the SCA20 software using the Ellipse fitting method. For the surface tension measurements, three liquids with known surface energy properties are needed to solve eq 1, which arises from the Good−Girifalco−Fowkes combining rule for interfacial Lifshitz− van der Waals interactions and the Young−Dupré equation considering acid−base interactions.26

2. EXPERIMENTAL SECTION 2.1. Materials. We have studied PANI−PANI interactions in the polymer matrix Ebecryl 584 from Cytec Surface Specialties, which is a chlorinated polyester acrylate (PEA) resin mixed with 40 wt % 1,6hexanediol diacrylate (HDDA) monomer. Figure 1 shows the schematic structures of HDDA monomer and polyester acrylate. PANI was synthesized by the so-called ‘rapid mixing’ method,3,18 which is chemical oxidative polymerization of aniline (Aldrich grade, 99%) with ammonium peroxodisulfate (Merck grade, 99%) as oxidant.

(1 + cos θ)γL = 2( γSLWγLLW +

γS+γL− +

γS−γL+ )

(1)

where θ is the contact angle between a droplet of a known liquid L and a solid S, γ is the surface energy, γLW is the Lifshitz−van der Waals (apolar) component of the surface energy, γ+ is the electron-acceptor

Figure 1. Schematic structures of HDDA monomer, PEA and PANI emeraldine salt with PA or DecylPA dopants. B

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Figure 2. (a) AFM topography, (b) DMT modulus, and (c) deformation images and line scan profiles (from the middle of each image) probed by PeakForce AFM for a pressed pellet of PANI-PA (first row), a pressed pellet of PANI-DecylPA (second row), and a PS layer (third row). C

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Figure 3. SEM images of the colloidal probes with glued particles of core/shell structured PS/PANI-PA (a) and PS/PANI-DecylPA (b), in comparison with the PS microparticles (c).

Table 1. Contact Angle (θ) of the Liquids Used for Surface Energy Component Calculations, HDDA Monomer and Original PEA Formulation (PEA:HDDA Ratio of 60:40) Droplets on PANI-PA and PANI-DecylPA Pressed Pellets, and a PS Layera liquids for surface energy component calculation water

diiodomethane

HDDA monomer

PEA formulation (PEA:HDDA ratio of 60:40)b

37 ± 3 78 ± 5 51 ± 6

45 ± 1 71 ± 5 45 ± 1

∼6 ∼58 18 ± 1

∼14 66 ± 2 ∼29

49 ± 2 94 ± 2 73 ± 1

PANI-PA pellet PANI-DecylPA pellet PS layer a

ethylene glycol c

c

c

All contact angle data are averages of at least three replicates. bData after 5 min of droplet relaxation on the surface. cThe same results as in ref 3.

surface energy parameter and γ¯ is the electron-donor surface energy parameter of the material. γ+ and γ¯ are nonadditive parameters of the Lewis acid−base (polar) component of the surface energy; γAB = 2(γ + γ −)1/2. The chosen contact angle liquids were diiodomethane, water and ethylene glycol. Information about the surface energy properties of these liquids can be found in the literature, and are reproduced in Table S1 in the Supporting Information. The interfacial energy of a solid material (material 1) immersed in liquid PEA resin (material 2) was calculated according to van Oss5 (eq 2).

and were found to be between 0.22 and 0.56 N m−1, respectively. Cantilevers with intermediate spring constants were chosen as a compromise between maximizing the sensitivity to allow detection of weak interactions, and minimizing the effect of drag forces on the cantilever body in the viscous media. The force versus separation measurements were conducted in three liquid mixtures of PEA polymer and HDDA monomer with PEA:HDDA weight ratios of 60:40, 30:70 and 0:100, giving viscosities of ∼2000 mPa·s (from Ebecryl 584 data sheet, Cytec Surface Specialties), 26.5 mPa·s (calculated from the viscosity blending index of components) and 10 mPa·s (from HDDA data sheet, Cytec Surface Specialties) at 25 °C, respectively. The liquid phase with 60 wt % PEA in HDDA is the original UV-curable formulation and the two others (30 wt % PEA in HDDA and pure HDDA monomer) were chosen for comparison and in order to study the influence of HDDA content in PEA resin as to correlate with the surface energies calculated. In order to lower the viscosity and make the movements of the cantilever being less affected by the high viscous media, all measurements were performed at 50 °C. The viscosity of the original formulation was measured by a Bohlin Gemini Rheometer 150, and showed a decrease from ∼2000 mPa·s at 25 °C to 230 mPa·s at 50 °C. The temperature was set by use of a thermal application controller attached to a Bioheater element (Bruker, USA) mounted under the sample. The temperature on the sample surface was calibrated by an external thermostat and controlled with an accuracy of ±1 °C. Force measurement were carried out at two different approach and retraction velocities of 80 and 400 nm/s, respectively, in order to detect possible hydrodynamic effects. Raw data representing photodetoctor output (related to cantilever deflection) versus piezo displacement were converted to force versus surface separation data by use of the cantilever spring constant and the deflection sensitivity as described in detail elsewhere.22 2.4. Scanning Electron Microscopy (SEM). Cured composite coatings of hydrophilic and hydrophobic PANI in a PEA matrix were analyzed by a high resolution FEG-SEM (SUPRA35, Carl Zeiss, Germany). Experiments were performed in In-Lens mode, and the data was collected using the SmartSEM software. The coatings were applied by utilizing a spin-coater and polished carbon steel substrates. They were cured under UV Fusion lamps of 1.5 W cm−2 intensity, giving a UV dose of 1.2 J cm−2, in the UV-A region. Surfaces of the coatings were gold-sputtered before SEM imaging by a BALZERS Sputter Coater SCD 050 for 60 s.

γ12 = γ12LW + γ12AB = ( γ1LW − −

γ2LW + )2 + 2( γ1+γ1− +

γ2+γ2− −

γ1+γ2−

γ1−γ2+ )

(2)

In order to calculate the interfacial energy between two different solid materials (1 and 2) immersed in liquid PEA resin (material 3) eq 326 can be used. AB γ132 = γ12LW − γ13LW − γ23LW + γ123

= ( γ1LW − − ( γ2LW − +

γ2LW )2 − ( γ1LW −

γ3LW )2

γ3LW )2 + 2[ γ3+ ( γ1− +

γ3− ( γ1+ +

γ2+ −

γ3+ ) −

γ2− −

γ1+γ2− −

γ3− )

γ1−γ2+ ] (3)

2.3. AFM force measurements. Force versus separation measurements between colloidal probes and planar substrates were performed by the colloidal probe technique10 with a Multimode Nanoscope III atomic force microscope equipped with a Picoforce controller and a fused quartz liquid cell (Bruker, USA). Pressed pellets of PANI-PA and PANI-DecylPA were used as the planar substrates. The colloidal probe cantilevers were prepared by gluing selected spherical particles of PS, PS/PANI-PA, or PS/PANI-DecylPA to tipless rectangular cantilevers (NSC12 Cr Au F-lever, from MikroMasch, Estonia). By using etched tungsten wires attached to an Eppendorf Micromanipulator 5171, a tiny amount of a two-component epoxy glue followed by the particle was placed on the end of the cantilever under a Nikon Optiphot 100S reflection microscope. Figure 3 shows some scanning electron microscope (SEM) images of the resulting colloidal probe cantilevers. The images were obtained by a PHILIPS SEM (Model XL30, FEI), in ‘MIX mode’ as a combination of 75% secondary electron and 25% backscattered electron. The spring constant of the cantilevers were calibrated before particle attachment by the Sader method, as described elsewhere,27

3. RESULTS AND DISCUSSION 3.1. Contact Angle and Surface Tension Measurements. The average contact angle values determined for water, ethylene glycol, diiodomethane, HDDA monomer, and original D

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Figure 4. Quantitative plots showing the change in contact angle together with the droplet volume and base diameter during the initial stages (plots in first row, with arrows pointing to the corresponding Y-axis) and snapshots illustrating the spreading behavior (rows 2−5) for liquid PEA resin droplet on PANI-PA (column to the left), on PANI-DecylPA (column in the middle) and on PS (column to the right).

the PS layer is in between those of PANI-PA and PANIDecylPA. Figure 4 shows some snapshots of a liquid PEA resin droplet on PANI-PA and PANI-DecylPA pressed pellets and PS layer, illustrating the change in wetting behavior during 40 min contact time. Quantitative data for the changes in contact angle together with the droplet volume and base diameter during the initial stages are plotted in the first row. The first decrease in PEA contact angle value on the PANI-PA surface together with the increase in droplet base diameter during the very first minutes demonstrates fast spreading of the liquid over the surface. Here, the decrease in contact angle continues by time until full-wetting, however, with a slow rate due to the high viscosity of the resin. Some decrease in the droplet volume is due to imbibition into the micropores of the PANI pellet, as no evaporation of the solvent-free polymer is expected. In the case of PANI-Decyl-PA, the droplet base diameter seems to be pinned from the beginning, and there is almost no change in the contact angle or droplet volume with time. This behavior confirms no imbibition and no further spreading of PEA liquid occurring on the PANI-DecylPA surface. It also confirms that

PEA formulation (PEA:HDDA ratio of 60:40) droplets on PANI-PA and PANI-DecylPA pressed pellets and a PS layer are presented in Table 1. A water contact angle of just below 50° is a common value for hydrophilic emeraldine salt PANI doped with pure phosphoric acid and is consistent with the values reported in the literature.28,29 In contrast, PANI doped with DecylPA, which is a long-chain phosphonic acid, is showing hydrophobic character with the water contact angle being over 90°. This illustrates how the wettability of PANI can easily be controlled by a proper choice of dopant. To our best knowledge a high water contact angle of PANI as a result of the dopant used has only been demonstrated once before.30 Very low contact angle values were obtained for both HDDA monomer and PEA resin formulation on hydrophilic PANI-PA substrate, indicating high wetting tendency of these liquids on PANI-PA. These liquids show less wettability on the hydrophobic PANI-DecylPA surface with the contact angle being around 58° for HDDA and around 66° for the PEA formulation. The contact angle for all the tested liquids on E

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3.2. AFM Force Measurements. The very different characteristic behavior of the two synthesized hydrophilic and hydrophobic PANIs (PANI-PA and PANI-DecylPA) in PEA resin, concluded from the surface tension studies discussed above, makes them interesting for direct surface force studies. Here, the nature of the interactions between different PANI surfaces in PEA resin is compared. The interaction between an uncoated PS probe and a PANI surface was also determined. Test of Reproducibility and Possible Hydrodynamic Effects. All measurements have been performed at two approach and retraction velocities of 80 and 400 nm/s, in order to detect possible hydrodynamic effects that would be observed in the form of an increasing repulsive force with decreasing surface separation and increasing measurement speed.32 However, negligible effect from hydrodynamic forces was seen for all measurements at the performed velocities. This is illustrated in Figure 5a, which shows that the force curves measured on approach for PANI-PA/PANI-PA across the PEA original formulation (PEA:HDDA ratio of 60:40) are the same at driving velocities of 80 and 400 nm/s. The remaining force curves presented in this work were all obtained at 400 nm/s. A total number of around 30 force curves have been collected at each condition, and the results showed good reproducibility with respect to the nature of interactions after the first couple of measurements. This is illustrated in Figure 5b, where approach and retraction force curves of eight repeated runs for PANI-DecylPA/PANI-DecylPA interaction in PEA original formulation (PEA:HDDA ratio of 60:40) are shown. However, for the PANI-coated probes a weak repulsive barrier was observed during approach in the first couple of force curves (see examples in Figure 5c). We attribute this feature to PANI nanoparticles loosely attached on the colloidal probe surface, which are removed when the two surfaces are approaching. Alternatively, or in addition, the apparent repulsive barrier can be an effect of small asperities which produce a high gradient force to which zero separation is assigned,33 but eventually break off the PANI-coated probe when hard contact is reached. However, after a couple of approach−retraction cycles reproducible force curves with no long-range repulsive barriers are achieved as shown in Figure 5b. Only these curves are considered in the following. Interaction Forces between PANI Particles in Resin. Representative force curves have been selected and are presented in Figure 6 for interactions between hydrophilic and hydrophobic PANIs in the three tested media. The surface forces measured between a PS colloidal probes and a PANI-PA substrate are also presented. PANI-PA/PANI-PA Interaction. Almost no attraction, neither as a jump-in during approach nor a contact adhesion during retraction, is observed between two hydrophilic PANI-PA surfaces in any of the three tested liquids. This is in agreement with the conclusions from interfacial energy measurements and suggests that PANI-PA has no or weak tendency to aggregate once dispersed in PEA resin. Prior to hard-wall contact, a short-ranged repulsion acting over a few nanometers seems to take place. Due to the high degree of surface roughness we are hesitant to draw any absolute conclusions about the nature of this repulsion. However, a possible origin could be related to the energy needed to remove the liquid layer in contact with the PANI-PA surface. Such solvent induced surface forces have previously been observed, especially in aqueous systems, in numerous cases ranging from smooth polar surfaces to molecular

no evaporation takes place. On the PS layer, a decrease in contact angle and increase in droplet base diameter is observed for the first minutes. However, equilibrium is reached earlier than on PANI-PA, and the surface never becomes fully wetted within the tested time frame. The known surface energy properties (data in Table S1) and measured contact angle values of water, ethylene glycol, and diiodomethane (data in Table 1) were utilized to calculate the surface energy components of the tested materials (PANI-PA, PANI-DecylPA, and PS) according to eq 1. These data are presented in Table 2, together with the interfacial energies Table 2. Surface Energy Components (mJ m−2) Calculated for the Tested Materialsa b

PANI-PA PANI-DecylPA PS cured PEA resinb

γ

γLW

γ+

γ‑

γ12

40.4 22.4 35.9 40.7

36.8 22.3 32.0 34.0

0.09 ∼0 0.3 0.8

36.9 5.94 12.5 13.7

−2.8 3.5 0.15 −

γ is the surface energy, γLW is the Lifshitz−van der Waals (apolar) component of the surface energy, γ+ and γ− are the electron-acceptor and the electron−donor surface energy parameters, and γ12 is the interfacial energy between PEA resin and the tested materials. bThe same results as in ref 3. a

between each tested material and PEA resin calculated according to eq 2. The surface energy components of the PEA resin used for interfacial energy calculations are taken from ref. 3. Negative interfacial energy between PEA and PANI-PA and positive interfacial energy between PEA and PANI-DecylPA indicates that the free energy of interaction (ΔG121 = −2γ12) between two PANI surfaces immersed in the PEA resin is positive and negative for PANI-PA and PANI-DecylPA, respectively. This is due to the much stronger Lewis base character of PANI-PA compared to PANI-DecylPA, knowing that the PEA resin has some Lewis acid character.3 This suggests repulsion between PANI-PA and attraction between PANI-DecylPA particles while dispersed in PEA resin.26 The theoretical adhesion force between PANI-DecylPA surfaces in PEA resin can be calculated by the contact mechanics model derived by Johnson et al.,31 known as the JKR model (eq 4).

FA = 3πγ12 (4) R where FA/R is the normalized force of adhesion (to the radius of the probe, R) and γ12 is the solid−liquid interfacial energy. The JKR model is based on the assumption that particle− surface interactions occur only in the area of contact, and that the particle is a perfect sphere with low surface roughness. The latter requirement is not fulfilled here as seen in Figure 3. The interfacial energy between PS and PANI-PA in liquid PEA has also been calculated since interaction between these two surfaces is being used as a control in this work. The reason for why PS is chosen is that it is used as a template for the PANI-coated colloidal probes prepared for surface force measurements. γ132 is calculated to be 1.56 mJ m−2 (according to eq 3), where materials 1, 2, and 3 are PANI-PA, PS, and liquid PEA, respectively. The sign and magnitude of the interfacial energy suggests an adhesive behavior which is slightly weaker than for the case of PANI-DecylPA particles in PEA resin. F

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systems.34,35 It is also known that if the interfacial interaction between the particles and liquid polymer is of attractive nature, an immobilized layer of the liquid forms around the particles, preventing them from aggregating.16 Formation of this layer can take several hours, but the rate of the process is affected by the conditions like temperature. In a study on the interactions between silicon nitride tips and silicon oxide substrates in liquid PDMS,16 it was revealed that annealing for 1 h at 50 °C, and thus changing the viscosity of the bulk material, has the same effect as waiting for more than 10 h at a lower temperature. Considering future applications of our system as a conductive composite, the important point is that no long-ranged repulsion is found. This suggests that the PANI-PA particles can come close enough to form conductive paths in PEA resin, but without a tendency to aggregate as concluded from the absence of attraction between the particles. PANI-DecylPA/PANI-DecylPA Interaction. A strong contact adhesion was observed between PANI-DecylPA surfaces in all three tested media, and the adhesion force is increased at high PEA content. This is illustrated in Figure 7, which shows the histograms for adhesion forces measured between PANIDecylPA colloidal particle and PANI-DecylPA substrate in the media with different PEA content. No large difference in the adhesion force histograms in the two media with PEA:HDDA ratios of 0:100 and 30:70 is observed, while the average magnitude of the adhesion force is 2.5 times larger in the original PEA formulation with PEA:HDDA ratio of 60:40 (mean and median values are shown for each histogram in Figure 7). This observation is qualitatively in agreement with the contact angle results, where a higher contact angle was observed for a PEA droplet than for a HDDA droplet on the PANI-DecylPA surface (see Table 1). Using the measured mean value of the adhesion force between PANI-DecylPA particles in PEA resin (Figure 7) and the calculated interfacial energy between PANI-DecylPA and PEA resin (data presented in Table 1), the apparent radius of the PANI-DecylPA-coated probe is, by the JKR model (eq 4), determined to be around 0.1 μm. This value is 50 times smaller than the radius of the PS particles used as the core of the probes. We suggest that this apparent discrepancy is an effect of the surface roughness of the PANI-DecylPA shell, which limits the contact area of the two surfaces and decreases the local contact radius. Consequently, the measured contact forces have a lower magnitude than the adhesion forces expected for smooth surfaces. The attractive interaction between hydrophobic PANI surfaces suggests a tendency of aggregation in a suspension of such particles in PEA resin. Figure 8 shows SEM images of 10 μm thick UV-cured composite coatings of PEA containing 3 wt % PANI-PA and 3 wt % PANI-DecylPA, applied on polished carbon steel substrate. No microscopic aggregates are seen on the surface of the coating containing hydrophilic PANI-PA (top view image in Figure 8a), and closely packed particles of several tens of nanometers are detected within the bulk of the coating by examining the wall-sides of a bore-hole using high resolution SEM (close-up image as inset in Figure 8a). On the other hand, consistent with predictions based on force measurements, poor dispersibility and microscopic defects in the coating with hydrophobic PANI-DecylPA is clearly seen in the SEM image in Figure 8b. Here, the inset shows close-up of one of the aggregates sticking out from the coating surface. In relation to the insert in Figure 8a it should be noted that the particle arrangements are not seen everywhere (since the total PANI-

Figure 5. (a) An example to illustrate the negligible effect of driving velocity during force measurements: Approach force curves for PANIPA/PANI-PA interaction in the media with PEA:HDDA ratio of 60:40, collected at driving velocities of 80 and 400 nm/s. (b) Illustration of reproducibility of consecutive force measurements: Approach (blue) and retraction (red) force curves for PANI-DecylPA/ PANI-DecylPA interaction in the media with PEA:HDDA ratio of 60:40. (c) Examples to illustrate the effect of loosely attached particles or weak thin asperities on PANI-coated colloidal probes during the first couple of force measurements: Approach force curves for PANIPA/PANI-PA interaction in HDDA. G

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Figure 6. Representative approach (blue) and retraction (red) force curves for PANI-PA/PANI-PA interaction (column to the left), PANIDecylPA/PANI-DecylPA interaction (column in the middle), and PS/PANI-PA interaction (column to the right) in the media with PEA:HDDA ratio of 0:100 (first row), 30:70 (second row) and 60:40 (third row). All force curves are collected at 50 °C.

PA concentration only is 3 wt %), but several such spots can be found in each bore hole. We believe that their existence is not directly related to the surface energies of PANI-PA and PEA but to limited mixing of the PANI particles in the high viscosity resin before curing. However, here the important message is that due to the right choice of components (in terms of surface energies) the particles are separated from each other, which means that no further aggregation of PANI-PA has taken place in the resin. Polystyrene/PANI-PA Interaction. Long-range repulsion upon approach and long-range attraction upon separation is observed in the force curves measured between a PS colloidal particle and a PANI-PA substrate. Both observations can be attributed to PS chains extending from the particle surface.36,37 The retraction force curves show step-like behavior and secondary adhesion events which is a characteristic indication of polymer bridging.38 This is especially pronounced in HDDA monomer, which could be either because of PS being more swelled in HDDA or because PEA shields the bridging interactions in the mixed resins. Based on the measured interfacial energies, the adhesion force between PS and PANI-PA was expected to have a magnitude between the adhesion forces measured for PANIPA/PANI-PA and PANI-DecylPA/PANI-DecylPA. This is, however, not the case and, instead, the PS/PANI-PA system shows even stronger adhesion than measured for PANIDecylPA/PANI-DecylPA in the media with PEA:HDDA ratios of 0:100 and 30:70 (see Figure 6). This could be due to a

Figure 7. Adhesion force histograms for PANI-DecylPA/PANIDecylPA interaction in the media with PEA:HDDA ratio of 0:100 (a), 30:70 (b), and 60:40 (c). Histograms are based on approximately 30 pull-off events in each case.

H

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Figure 8. Top view SEM images of 10 μm thick UV-cured composite coatings of PEA containing 3 wt % PANI-PA (a) and 3 wt % PANI-DecylPA (b). Insets in the images show closely packed PANI-PA particles of several tens of nanometers within the bulk and on the wall-sides of a bore-hole in the PEA/PANI-PA composite coating (a) and close-up of a PANI-DecylPA aggregate sticking out of the PEA/PANI-DecylPA composite surface.

than what can be deduced from surface energy calculations alone. In particular, steric interactions and polymer bridging was observed in the present study, and such features will affect the properties of dispersed particles.

geometry difference of the colloidal particles (see SEM images in Figure 3), as it is not well-defined for PANI-coated particles, and more contact area is available in the case of PS. It should, however, be noted that even in this case the measured adhesion forces are lower than expected based on the measured interfacial energies. This result shows that direct surface force measurements contains additional information from what can be derived from contact angle measurements. For instance, the estimation of adhesion forces based on the interfacial energies does not take steric repulsion and the repulsive solvation barrier from strongly attached PEA into account, both of which are of importance for the interaction between dispersed PANI particles in resin.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Surface energy properties (mJ m−2) of the chosen contact angle liquids reproduced from the literature. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

*Address: KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Division of Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden. Tel: +46-87906670; Fax: +46-8208284; E-mail address: [email protected].

4. CONCLUSIONS The interactions between hydrophilic PANI (doped with phosphoric acid) and hydrophobic PANI (doped with n-decyl phosphonic acid) particles in a UV-curable resin based on polyester acrylate have been investigated. One main observation is the strong adhesion force, measured by the AFM colloidal probe technique, between hydrophobic PANIDecylPA surfaces in PEA resin. This is in sharp contrast to the nonadhesive interaction between hydrophilic PANI-PA surfaces. These findings are in agreement with the conclusions from interfacial energy studies, where aggregation of PANIDecylPA and good dispersion stability of PANI-PA particles in liquid PEA resin were suggested. Both force measurements and interfacial energy measurements are in agreement with the SEM images that show well dispersed PANI-PA particles and large aggregates of PANI-DecylPA. A combination of the two techniques can be used as a powerful tool to select proper and compatible components while designing composites. In our particular case, the data demonstrate that the choice of dopant ions for PANI can be a useful tool to tune particle interactions in resin media. Our results also illustrate that the full interaction between polymer colloids in a polymer medium obtained by direct surface force measurements contains more detailed information

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by SSF: the Microstructure, Corrosion and Friction Control program. E.T. also acknowledges the Swedish Research Council (VR) for financial support. Per-Erik Sundell and Rodrigo Robinson are acknowledged for the help with SEM experiments.

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REFERENCES

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