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Nanoscale conformational ordering in polyanilines investigated by saxs and afm

Understanding the adsorption mechanisms in nanostructured polymer films has become crucial for their use in technological applications, since film properties vary considerably with the experimental conditions utilized for film fabrication. In this paper, we employ small-angle X-ray scattering (SAXS) to investigate solutions of polyanilines and correlate the chain conformations with morphological features of the nanostructured films obtained with atomic force microscopy (AFM). It is shown that aggregates formed already in solution affect the film morphology; in particular, at early stages of adsorption film morphology appears entirely governed by the chain conformation in solution and adsorption of aggregates. We also use SAXS data for modeling poly(o-ethoxyaniline) (POEA) particle shape through an ab initio procedure based on simulated annealing using the dummy atom model (DAM), which is then compared to the morphological features of POEA films fabricated with distinct pHs and doping acids. Interestingly, when the derivative POEA is doped with p-toluene sulfonic acid (TSA), the resulting films exhibit a fibrillar morphology—seen with atomic force microscopy and transmission electron microscopy—that is consistent with the cylindrical shape inferred from the SAXS data. This is in contrast with the globular morphology observed for POEA films doped with other acids.
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Transcripts - Nanoscale conformational ordering in polyanilines investigated by saxs and afm

  • 1. This article was published in an Elsevier journal. The attached copy is furnished to the author for non-commercial research and education use, including for instruction at the author’s institution, sharing with colleagues and providing to institution administration. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright
  • 2. Author's personal copy Journal of Colloid and Interface Science 316 (2007) 376–387 www.elsevier.com/locate/jcis Nanoscale conformational ordering in polyanilines investigated by SAXS and AFM Fabio L. Leite a,b,∗ , Mario de Oliveira Neto b , Leonardo G. Paterno c , Michel R.M. Ballestero b , Igor Polikarpov b , Yvonne P. Mascarenhas b , Paulo S.P. Herrmann a , Luiz H.C. Mattoso a , Osvaldo N. Oliveira Jr. b a National Nanotechnology Laboratory for Agriculture (LNNA), Embrapa Agricultural Instrumentation (EMBRAPA), Rua XV de Novembro, 1452, P.O. Box 741, CEP 13560-970, São Carlos-SP, Brazil b Instituto de Física de São Carlos, USP, P.O. Box 369, CEP 13560-970, São Carlos-SP, Brazil c Departamento de Engenharia de Sistemas Eletrônicos, EPUSP, 05508-900, São Paulo-SP, Brazil Received 16 July 2007; accepted 28 August 2007 Available online 4 September 2007 Abstract Understanding the adsorption mechanisms in nanostructured polymer films has become crucial for their use in technological applications, since film properties vary considerably with the experimental conditions utilized for film fabrication. In this paper, we employ small-angle X-ray scattering (SAXS) to investigate solutions of polyanilines and correlate the chain conformations with morphological features of the nanostructured films obtained with atomic force microscopy (AFM). It is shown that aggregates formed already in solution affect the film morphology; in particular, at early stages of adsorption film morphology appears entirely governed by the chain conformation in solution and adsorption of aggregates. We also use SAXS data for modeling poly(o-ethoxyaniline) (POEA) particle shape through an ab initio procedure based on simulated annealing using the dummy atom model (DAM), which is then compared to the morphological features of POEA films fabricated with distinct pHs and doping acids. Interestingly, when the derivative POEA is doped with p-toluene sulfonic acid (TSA), the resulting films exhibit a fibrillar morphology—seen with atomic force microscopy and transmission electron microscopy—that is consistent with the cylindrical shape inferred from the SAXS data. This is in contrast with the globular morphology observed for POEA films doped with other acids. © 2007 Elsevier Inc. All rights reserved. Keywords: Self-assembly; SAXS; AFM; TEM; Nanostructures; Thin films; PANI; Adsorption and conformation 1. Introduction The discovery and development of new semiconducting polymers have brought great promise for a number of techno- logical applications [1,2] and posed new challenges in terms of understanding fundamental properties of organic materials. A key feature of these polymers is the possibility of alter- ing their electrical and optical properties with small changes in composition or even in the experimental procedures to pro- duce the samples [3]. Indeed, the incorporation of a substituent group in a polymer such as polyaniline or polythiophene in- duces considerable changes in the physicochemical properties, * Corresponding author. Fax: +55 16 33725958. E-mail address: leite@cnpdia.embrapa.br (F.L. Leite). including solubility in organic solvents or in aqueous solu- tions. Samples are normally produced in the form of films, which then allows another avenue to pursue in obtaining spe- cific properties. If films are fabricated with techniques such as the Langmuir–Blodgett (LB) [4,5] or layer-by-layer (LbL) [6,7] methods, for instance, the properties may be controlled at the molecular level. One such example was demonstrated in LB films of polyaniline and a ruthenium complex [8], in which the intimate contact between the molecules of the two components led to electrical and electrochemical properties that differed completely from those obtained for cast or spin-coated films of the same materials. The reason for these differences was elu- cidated with Raman spectroscopy measurements, where PANI was found to exhibit enhanced oxidation induced by the inti- mate contact with the ruthenium complex [9]. 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.08.069
  • 3. Author's personal copy F.L. Leite et al. / Journal of Colloid and Interface Science 316 (2007) 376–387 377 The large ability to alter polymer properties is obviously advantageous to achieve a rich variety of features. In the lu- minescent, semiconducting polymers such as poly(p-phenyl- enevinylenes), the emission quantum efficiency, color and po- larization can be changed by mere introduction of functional groups in the polymer backbone [10]. In cases where con- ductivity is the property to be exploited, orders-of-magnitude changes are observed by varying the dopant concentration or pH [11–13]. There is a considerable disadvantage though in this ease with which polymer properties vary, particularly with regard to industrial applications. Reproducibility in the experi- mental results for any given type of sample or measurement is relatively poor, especially if one compares with the inorganic semiconductors. In the specific case of nanostructured films from polyanilines, several studies have shown that the adsorp- tion mechanisms and the film properties are entirely altered by a mere change in pH [14]. Parent polyaniline (PANI) and its derivatives have been widely used mainly due to their remark- able electrochemical, optical, electrical and mechanical prop- erties, and good environmental stability in the emeraldine base (EB) and emeraldine salt (ES) states [15,16]. These properties have been exploited in sensors [17,18] and other applications [19,20]. In order to identify the parameters that most affect polyani- line film properties, the morphology of LbL films has been studied in detail [21–23]. The choice of the LbL method to produce the films was based on the unique features offered by the method, which are basically the simplicity in the experi- mental procedures and ability to control film properties at the molecular level [24–26]. In the LbL films, polymer adsorption is in most cases driven by ionic interactions, with oppositely charged materials from aqueous solutions being deposited al- ternately on a solid substrate. Secondary interactions—e.g., hy- drogen bonding and van der Waals—may also be important for adsorption [27], as it has been shown for polyanilines, for which H-bonding contributes even when the molecules are pro- tonated. The layer thickness can be controlled by altering the interactions responsible for adsorption, which is carried out by changing materials and preparation conditions such as pH, ion dopant, ionic strength and concentration of the solutions. It was clear from the morphological studies mentioned above that film properties also depended strongly on the conforma- tion of the polymer in solution. Access to this type of infor- mation is not straightforward because the polyaniline solutions are polydisperse. Here we employ small angle X-ray scattering (SAXS) to investigate the properties of parent PANI, poly(o- methoxyaniline) (POMA) and poly(o-ethoxyaniline) (POEA) in solution at various pHs and doping acids. SAXS is useful to probe the material structure on a scale from 0.05 to 2000 Å [28, 29], as it may provide statistically meaningful measurements of total volumes, surface areas and scattering centers [30]. Knowl- edge about the external surface structure of the polymer can be obtained by measurements of the scattering intensity I(q) ver- sus the scattering vector modulus. We then study the influence from the conformation of polymers in solution on the properties of nanostructured films. Parameters such as roughness, aggre- gation, and fractal dimension were used to interpret the data. 2. Experimental details PANI, POMA, and POEA were chemically synthesized ac- cording to the methods described in Ref. [31] (Fig. 1). Solutions Fig. 1. Emeraldine base form of (a) PANI, (b) POMA, and (c) POEA.
  • 4. Author's personal copy 378 F.L. Leite et al. / Journal of Colloid and Interface Science 316 (2007) 376–387 were prepared by dissolving the dedoped polymers (EB) in wa- ter and dimethyl acetamide (DMAc) with a concentration of 3 g L−1. The solutions were left stirring overnight and then fil- tered obtaining a final concentration ≈2.5 gL−1. The polymers in these solutions were doped with HCl, with the pH adjusted at selected values between 1.5 and 10, as indicated. For the study of different dopants, POEA was doped with one of the follow- ing acids: p-toluenesulfonic acid (p-TSA), camphorsulfonic acid (CSA), sulfanilic acid (SAA), and hydrochloric (HCl) acid. The doping acids were all purchased from Sigma–Aldrich, and used as received. The SAXS experiments were conducted at the National Syn- chrotron Light Laboratory (LNLS), Campinas, Brazil, using a monochromatic X-ray beam (λ = 1.488 Å), which focuses the beam horizontally, and a one dimensional position-sensitive X- ray detector to record the scattering intensity. To perform the SAXS measurement, the aqueous solution was placed in a flat cell and sealed. The SAXS curves were normalized with respect to (i) the decreasing intensity of the coming synchrotron beam and (ii) the sample absorption. The SAXS intensity produced by the solvent (water and DMAc) was measured and subtracted from the total scattering intensity before the analysis. The scattered intensity was measured over the scattering vectors, q = (4π/λ)sinθ, where 2θ is the total scattering an- gle and λ is the wavelength generated from the rotating anode source which was monochromatized by a crystal monochrome- ter. In Guinier’s theory, the X-ray scattering intensity from the sample (I) depends on the number of particles per unit volume (Np), the electron density difference between particles and the medium ( ρ), volume of the particle (v), radius of gyration (Rg) and the scattered intensity of a single electron (Ie) [32]. (1)I(q) = Ie(q)Np( ρv)2 exp −R2 g 3 q2 . In a Guinier plot, lnI(q) vs q2(I(q) → I(0)), the slope of the linear region allows Rg to be obtained [33]. Since solutions containing particles of different sizes (polydisperse) show sev- eral linear regions in the Guinier plot, in order to generate low resolution models we considered only the SAXS curve from the last linear region. This region has a high minimum angle and therefore information on the large distances in the mole- cules is lost [34]. This was performed as an attempt to include the information due to the smallest particles in solution as if the system were monodisperse (three-dimensional models). In the absence of interference effects, a Fourier transform connects the normalized particle form factor (and hence I(q)) to the pair distance distribution function, p(r), the probability of finding a pair of small elements at a distance r within the entire volume of the scattering particle as [35] (2)p(r) = 1 2π2 ∞ 0 I(q)qr sin(qr)dq. This function provides information about the shape of the scattering particle as well as its maximum dimension, Dmax, accounted for a certain r value where p(r) goes to zero. More- over, the particle radius of gyration, Rg, value is given by [36] Table 1 Rg for the polyaniline and its derivates in solution at distinct pH values (HCl) Material pH Rg (Å) Max Min Region I—aggregates (q2 0.0016) POEA 1.5 Precipitate 3.0 112.9 51.9 5.0 60.8 37.6 10.0 77.2 45.9 POMA 1.5 Precipitate 3.0 77.1 56.8 5.0 47.9 31.1 10.0 65.7 42.9 PANI 2.5 Precipitate 3.0 87.4 59.7 5.0 94.4 48.6 10.0 52.8 41.2 Region II—smallest particles (q2 0.0016) RGuinier g RGNOM g POEA 3.0 39.2 ± 2.7 42.5 ± 0.6 5.0 31.5 ± 3.0 36.9 ± 0.8 10.0 28.8 ± 2.9 32.0 ± 0.8 POMA 3.0 39.1 ± 2.0 41.4 ± 0.6 5.0 29.8 ± 2.8 31.9 ± 0.3 10.0 26.2 ± 1.8 28.2 ± 0.3 PANI 3.0 33.3 ± 1.3 34.0 ± 0.3 5.0 30.3 ± 1.9 34.3 ± 0.4 10.0 31.2 ± 2.3 35.8 ± 1.4 (3)R2 g = Dmax 0 p(r)r2 dr 2 Dmax 0 p(r)dr . In this work, we make use of the GNOM program [37] to calculate p(r) and estimate the radii of gyration (RGNOM g ) (see Table 1). The smearing effect, caused by the length of the de- tector window, was corrected and the distance distribution func- tion p(r) was calculated. The dummy atom model (DAM) was generated with the program DAMMIN, which employs simu- lated annealing to obtain a model that minimizes the discrep- ancy between the theoretical and experimental curves [38]. For each structure, several models were tried, with no symmetry imposed, and an average DAM was calculated using the pro- gram DAMAVER [39]. The 3D-envelopes were visualized with VMD software [40]. The nanostructured films were prepared with POEA. At pH 3, POEA is not fully protonated, which leads to a chem- ical structure that is a mixture of emeraldine salt and base forms. For POEA, a few minutes are sufficient to form a stable and continuous layer [21]. Fractal dimension and film rough- ness were analyzed on films obtained with immersion times between 1 and 180 s at pH 10 (EB). To analyze the pH ef- fect on film morphology only the initial adsorption stage was considered, and therefore the immersion time was only 3 s, for solutions with pH varying from 3 to 10. The influence from the dopant was studied with films produced with POEA so- lutions at pH 3, immersion time of 180 s and 4 dopant acids (CSA, SAA, HCl and TSA). Substrates were prepared with optical glass slides (1 × 10 × 30 mm) previously cleaned in H2SO4/H2O2, 7:3 v/v solutions for 1 h, followed by extensive washing in ultra-pure water. The slides were then immersed into
  • 5. Author's personal copy F.L. Leite et al. / Journal of Colloid and Interface Science 316 (2007) 376–387 379 a H2O/H2O2/NH4OH 5:1:1 v/v solution for 40 min and again washed with large amounts of ultra pure water. The growth of the POEA layers was monitored by measuring the UV–vis ab- sorption spectrum with a UV–vis spectrophotometer Shimadzu UV-1601 PC. The POEA solution was also transferred onto a transmis- sion electron microscope (TEM) copper grid for TEM obser- vation on a Philips CM 200 operating at 200 kV. Atomic force microscopy (AFM) images were taken in a Topometrix micro- scope, model Discoverer TMX 2010, using silicon nitride tips (V shape) with spring constant of 0.06 N/m (nominal value). All images were obtained in the contact mode at a scan rate of 2 Hz. The root-mean-square roughness, Rms, and the surface fractal dimension were calculated using WSxM 4.0 software from Nanotec Electronica S.L. (Copyright © November 2003) and scanning probe image processor (SPIP) version 3.1.0.1 from Image Metrology A/S 2003. 3. Results and discussion 3.1. Polyanilines in solution investigated by SAXS Processing parameters are known to affect film morphology to a great extent, and there is evidence that some differences (a) (b) Fig. 2. lnI vs q2 curves for (a) POEA, (b) POMA, and (c) PANI in distinct pH values, which show two linear profiles, demarcated by the dashed line.
  • 6. Author's personal copy 380 F.L. Leite et al. / Journal of Colloid and Interface Science 316 (2007) 376–387 (c) Fig. 2. (continued) are already present in the solution used to produce the films. To elucidate this behavior, SAXS analysis was performed for polyanilines in various types of solution. Fig. 2 shows Guinier plots, lnI vs q2, for POMA, POEA, and PANI solutions at distinct pHs, namely 3.0, 5.0, and 10.0, where considerable heterogeneity is seen in all samples. With a Guinier plot one is able to calculate Rg since qRg should be around 1, i.e., qmaxRg 1.5. Two requirements to apply Guinier’s law are that the particles (scattering centers) should all have the same size (monodisperse) and be far from each other to avoid interpar- ticle scattering. Because neither of these conditions applies to the polymer samples, the analysis based on the Guinier region only gives an estimate of the scattering center sizes [41]. Two or three linear regions can be identified in Fig. 2, from which upper and bottom limits of aggregation radii may be estimated for the aggregated particle sizes. Table 1 shows the Rg values calculated with the procedures established in Ref. [42] where we used two regions of the curve, referred to as regions I (q2 0.0016) and II (q2 0.0016). A careful investigation of the Guinier plots reveals two linear regimes: I—a short linear profile at very low q values, and II— a wider linear profile. Since the q values probe the structural features at various length scales, the larger value of the slope of the linear region I (compared to II) indicates a larger size of the scatterers, i.e., chain aggregates [42]. In region I, Rg of the aggregates increases with decreasing pH, since aggregation is precluded by the charges incorporated upon adding acid to the solution. However, at low pHs ( 2.5), the solution becomes unstable and starts to precipitate. This explains why Rg of the structures in Table 1 increases as the pH was decreased from 5.0 to 3.0. Considering data for other pHs, we conclude that the best tradeoff between solubility and aggregation upon dedoping the polymers is attained at pH ≈ 5.0. The values of Rg in region II from Guinier analysis are comparable to those obtained from the shape factors (RGNOM). Therefore, we may infer that in di- lute solutions of conducting polymers, chain conformation can be estimated by SAXS. Fig. 3 shows the low-resolution particle shape for the polyanilines molecules in solution, determined from the exper- imental data using the ab initio procedure [43]. A less-packed, coiled structure is observed for pH 3, while at pH 10 blobs are formed, which are consistent with the radii of gyration in Ta- ble 1 (region II), and with the literature according to which a more extended structure is obtained by increasing the doping level [44]. The conformation of polyaniline molecules in solution also depends on the doping acid [45–47], which is illustrated here for POEA doped in pH 3.0 with four acids, namely HCl, CSA, SAA, and TSA. Fig. 4 shows low-resolution envelopes obtained with the same procedures as for those in Fig. 3. A less-packed, coiled conformation was observed for HCl and CSA, while for SAA and TSA the conformation is rod-like and cylindrical, respectively. Counterions of organic acids are strongly bound to the polymeric chain because they are less solvated. Conse- quently, the charges in the polymeric chains are more effec- tively screened and the polymer appears almost like a neutral polymer. In this situation the polymeric chains assume a rod- like conformation which results in aggregates of greater radii of gyration. On the other hand, ions easily solvated in water, as is the case of chloride, are less effective in screening the polyion charges, and therefore the polymeric chains assume a more extended coil-like conformation due to the intra and in- ter molecular electrostatic repulsion. The consequences for the film morphology of these distinct conformations will be dis- cussed in the next section. Another parameter that may be obtained from the SAXS data is the fractal dimension [48] of the molecular structures in so- lution. As I(q) follows a power law (Eq. (4)) [49], the fractal
  • 7. Author's personal copy F.L. Leite et al. / Journal of Colloid and Interface Science 316 (2007) 376–387 381 Fig. 3. Average DAM for (a) POEA, (b) POMA, and (c) PANI in (i) pH 10.0, (ii) pH 5.0, and (iii) pH 3.0 (HCl). dimension may be determined from the SAXS profile, analyz- ing the power-law profile (q > 0.01 Å −1 ) [42] shown in Fig. 5. In this region, the scattering profiles do not depend on the shape of the scattering units. (4)I(q) ∝ q−α , where I is the scattered intensity, q is the modulus of the scat- tering vector, and the exponent α is related to the fractal dimen- sion of the scattering particles. The slope of the linear region in logI(q) vs log(q) plot gives the exponent α, the dimensionality of the scattering object. Systems with fractal behavior are characterized by geomet- ric auto-similarity in a given region, i.e., the structure is inde- pendent of the observation scale size [50]. The fractal dimen- sion helps quantify properties such as mass (m) and surface area (A), since a fractal object varies with the radius of gyra- tion [51]. The slope (α) of logI(q) vs logq curves in the linear region was calculated, with linear regression analysis, leading to fractal dimensions (Df). For α between 1 and 3, Df is de- termined with Eq. (5) and the material is characterized as mass fractal (Dm f ) in a three-dimensional space. The so-called surface fractals (Ds f ) have α between 3 and 4, with the fractal dimension estimated with Eq. (6) [52]. Mass fractals are scattering centers whose mass increases proportionally to the volume while sur- face fractals are dense scattering centers associated with surface roughness. In Eq. (6), if Df = 2.0 we obtain the well-known Porod’s law I(q) ∝ q−4 for nonfractal structures with smooth interfaces [52]. (5)Dm f = |α|, (6)Ds f = −|α| + 6. The slopes in the linear region were calculated as −2.7, −2.4, and −2.6 for PANI-EB, POEA-EB, and POMA-EB (pH 10.0), respectively, corresponding to mass fractals. Values
  • 8. Author's personal copy 382 F.L. Leite et al. / Journal of Colloid and Interface Science 316 (2007) 376–387 of α for PANI, POEA, and POMA at pH 3.0 were −1.7, −1.3, and −1.4, respectively. Therefore, in solution all polyanilines exhibited mass fractals. As we shall see with the AFM data for nanostructured films, surface fractals are observed (see later Fig. 6). Typically, α = 2 for Gaussian chains, platelets or discs, or sheet-like lamellar objects; α = 1 for rigid rods, and α = 3 Fig. 4. Dummy residues model for POEA doped with (a) CSA (Rg = 28.8 Å), (b) HCl (Rg = 32.0 Å), (c) SAA (Rg = 36.4 Å), and (d) TSA (Rg = 36.9 Å) (pH 3.0). for tight compact structures. The values of α for the q-range used (0.01 < q < 0.1 Å) indicate that the polymers adopt a rela- tively more compact blob-like structure when undoped (pH 10), while in the doped state they exhibit Gaussian-like chain struc- tures. There are two aggregation regimes depending on the limiting factor for the aggregation: diffusion-limited cluster aggrega- tion (DLCA) and reaction-limited cluster aggregation (RLCA) [53,54]. For DLCA, aggregation is led by cluster diffusion with a fractal dimension Df ≈ 1.7–1.8. Particles under RLCA repel each other, growing more-compact aggregates with a limit frac- tal dimension of Df ≈ 2.0–2.1. A continuous transition between both regimes has also been reported, e.g., charged systems as a function of the range of the interactions [55,56]. Experiments have recently shown that aggregates grown in a 50/50 mix- ture of positive and negative particles show low-density frac- tal structures compared to those obtained from the universal regimes, Df ≈ 1.2–1.4 [57]. For our case, only PANI-ES is under the DLCA regime, the others showed low-density frac- tal structures i.e., the inner structure of the clusters was indeed fractal and the dimension characterizing it was lower than for DLCA [58]. In uncharged systems, POEA and POMA are un- der the RLCA regime, while for PANI the fractal dimension is higher than the typical values for diffusive aggregation. 3.2. Film morphology The influence from the state of aggregation, doping and con- formation in solution on the film properties has been mentioned in previous studies [59,60]. The question that then arises is whether a direct relationship can be established between the conformations in solution and in the solid film. The LbL tech- nique allows film preparation within short-time intervals, and therefore it is important to probe the kinetics of polymer ad- sorption to optimize the time of film fabrication. For POEA, a few minutes are sufficient to form a stable and continuous layer [21]. We observed the film morphology of POEA films Fig. 5. logI vs logq typical curve in the power-law region for POEA-ES (pH 3.0).
  • 9. Author's personal copy F.L. Leite et al. / Journal of Colloid and Interface Science 316 (2007) 376–387 383 Fig. 6. AFM images (10 × 10 µm) of POEA-EB (pH 10.0) for immersion times of (a) 1 s (Rs = 69 ± 25 Å; Df = 2.39), (b) 3 s (Rs = 74 ± 30 Å; Df = 2.25), (c) 5 s (Rs = 96 ± 30 Å; Df = 2.34), and (d) 180 s (Df = 2.23). Rt is 200 Å, calculated by SEM. at very early stages of adsorption with the same solutions used in SAXS experiments. The images are shown in Fig. 6, for ad- sorption times of 1, 3, 5, and 180 s. The size of the globules was determined from the images, but after correcting for the ef- fects from a tip having similar size as the particles [61–63]. The correction was made using a geometric relation: (7)Rt s ≈ L2 16Rt , where Rt s is the true radius of the particle, Rt is the tip radius, and L is the apparent size. The radii of the globules in the film imaged in Fig. 6 were 69 ± 25, 74 ± 30, and 86 ± 30 Å, for 1, 3, and 5 s, respectively. We did not calculate the size for 180 s because further aggrega- tion takes place and no comparison can be made with POEA conformation in solutions (see below). These radii are com- pared with the aggregate size in solutions. To estimate the latter, we assumed that the particles in solution are spherical, thus al- lowing Rs (radius of globules) to be calculated using [64]: (8)Rs = (5/3)1/2 Rg. The radii of gyration (GNOM) of the smallest particles for POEA-EB was 42.5 Å (Table 1), which leads to a Rs = 54 Å using Eq. (6), in agreement with the particle size in the AFM images for short adsorption times (ca. 1 s, Fig. 6a). For larger aggregates, i.e., 50 Rg 77 Å, the radius of the globules (Rs) obtained with Eq. (6) was 65 Rs 99 Å, consistent with the particle size in the AFM images for an adsorption time of 3 s (Fig. 6b). Therefore, at short immersion times (t 3 s) the smallest POEA aggregates (DAM) adsorb on the substrate, followed by further aggregation. In addition, within 3 s of ad- sorption nucleation appears to be complete, followed by growth of globules due to incorporation of additional polymer chains. For longer periods of adsorption, diffusion along the surface causes the film morphology to become flatter [21]; indeed, a less rough film is formed at the end of 180 s of immersion (Fig. 6d). As expected, the same applies to POMA and PANI, since the solutions are similar (emeraldine base). We also cal- culated the fractal dimension for the POEA films, according to the methods used in Ref. [21]. The values obtained varied from 2.23 to 2.39, and therefore all films presented surface fractals (Df > 2.2), i.e., rough with uniformly dense structures, in con- trast to those values for solutions which were characteristic of mass fractals. The effect from pH was analyzed by producing POEA films adsorbed with an immersion time of 3 s at several pHs, with the AFM images shown in Fig. 7. In general, the film morphology is globular, with globules ranging from 200 to 1000 Å in di-
  • 10. Author's personal copy 384 F.L. Leite et al. / Journal of Colloid and Interface Science 316 (2007) 376–387 Fig. 7. AFM images of POEA at 3 s on (a) pH 3.0, (b) pH 5.0, and (c) pH 10.0 (5 × 5 µm). ameter for pH 3 and from 600 to 2500 Å for pH 10. The larger diameter for the higher pH is due to aggregation in solution (see Table 1), which changes diffusion and the adsorption rates on the surface, leading to large irregularities in the films. At pH 3 (doped), the interactions between chains and with the surface are less significant, favoring formation of defects (small tun- nels) and less condensed packing of polymer chains with less aggregates (Fig. 7a). In this model, charged polyelectrolytes form thin films as the charged molecules adsorbed create a po- tential barrier that prevents more molecules from adsorbing, thus yielding flat, ultra-thin films [65]. The AFM images also exhibit wormlike structures represent- ing aggregates made of several blobs for pH 5.0, as illus- trated in Fig. 6a. This aggregation occurs on the sample sur- face, within very short times. Some of these structures could already exist in solution, as the aggregates depicted in Table 1 indicate. A possible mechanism for the formation of the struc- tures appearing in Fig. 7 is as follows. Chains collapse to form blobs due to hydrophobic interactions and form a compact core, which adsorb as a layer (Fig. 7b), whereas protonated polyani- line adopts an extended conformation due to the Coulomb re- pulsion and forms several tunnels (Fig. 7a). Thus, for pH 3.0, POEA structures are more extended than for higher pH, both on the surface and in solution. In summary, although details of conformation may be lost due to adsorption and solvent evaporation, features of the micro and nano-conformation are preserved on the films. As observed in the SAXS experiments, the chemical struc- ture of the dopant counter-ions has a strong influence on the properties of POEA solutions, and the same influence can be seen on the film fabrication process. In order to investigate such an effect, POEA films were produced with solutions contain- ing different doping acids. We could verify by UV–vis spec- troscopy, whose results are shown in Fig. 8, that the amount of POEA, taken as proportional to the absorption, depends on the acid used for film deposition. Although POEA is doped in all films as confirmed by the presence of the polaronic band at 700–800 nm, the amount of adsorbed POEA on glass increased in the following order: TSA > SAA > CSA > HCl, which is consistent with Paterno and Mattoso [66] who observed higher adsorption for TSA-doped POEA. The larger adsorption for POEA doped with the bulkiest anions may be explained by the low mobility and low solvation of these counter-ions, causing a higher screening effect of the charges in POEA chains. This
  • 11. Author's personal copy F.L. Leite et al. / Journal of Colloid and Interface Science 316 (2007) 376–387 385 Fig. 8. (a) UV–vis absorbance spectra of POEA films (1 layer of 3 min) doped with different acids in pH 3.0 (C = 0.6 gL−1). Fig. 9. (a) TEM micrograph of POEA doped with TSA and (b) AFM image of POEA in emeraldine salt state, doped with TSA. would lead to a more compact conformation of the polymer, thus resulting in a larger amount of material adsorbed [67–69]. The larger amount of deposited material for POEA doped with TSA and SAA (Fig. 8) in comparison with another or- ganic acid (CSA) can be explained by the large radii of gyration in solution. The differences observed on the amount of POEA adsorbed in films prepared with different acids are, analogously to the different radii of gyration, due to the different degrees of association between the charged polymer chains and the coun- terions from the doping acids. As mentioned before, the counte- rions of organic acids should be less solvated in water, interact- ing more strongly with the polymeric chains. The electrostatic repulsions between polymeric chains are therefore minimized, which allows the chains to assume a more compact conforma- tion and reach the substrate surface. Both effects are believed to increase the amount of POEA adsorbed. The opposite is found when the counterions are effectively dissolved in water and the polymeric chains are in a more extended conformation. The stiffness of the chains is caused by strong intra molecular electrostatic repulsion, which also hampers adsorption of poly- meric chains onto the substrate. Consequently, smaller amounts of POEA are expected to adsorb. The film morphology also depends on the doping acid, as illustrated in the TEM micrograph of Fig. 9. The films nor- mally display a globular morphology, with larger globules and higher roughness for those doped with inorganic acids, e.g., HCl. One exception is noted, though, for the film of POEA processed with TSA, for which a cylindrical morphology was observed (Fig. 9a). These well-defined cylinders may arise from the drying process in ultra-high vacuum (UHV) and treatment in an ultrasound bath. Electron diffraction analysis showed that POEA cylinders are amorphous, analogously to that observed for stretched amorphous polymers [70]. This cylindrical mor- phology is difficult to visualize with AFM because resolution is
  • 12. Author's personal copy 386 F.L. Leite et al. / Journal of Colloid and Interface Science 316 (2007) 376–387 Fig. 10. Schematic view of the cylindrical brush conformation where the cylin- ders contain POEA (TSA) chains. lost due to the tip radius (≈50 nm). Still, a quasi-fibrillar mor- phology was observed in Fig. 9b. The result in Fig. 9 should be expected on the basis of the SAXS measurements discussed in Section 3.1. The mor- phological structure at the surface can be attributed to the in- trinsic “comb-like” molecular architecture of the POEA-TSA system in solution, which forces the polymer chains to adopt the conformation of cylindrical brushes, similar to those of PANI(AMPSA)0.5 (Cnres) samples [71]. This conformation is caused by the steric overcrowding of toluenesulfonate anions which remain close to POEA cation radical segments due to their lower solubility in water. These features are depicted in a proposed model for the morphology, shown in Fig. 10, in which the distance between two neighboring layers corresponds to the distance between two neighboring POEA chains. Consid- ering (as an extreme case) that a fully extended POEA backbone with 120 monomers units has ∼1002 Å [72] for the chain con- tour length, we conclude that the objects appearing within the lamellae in the picture of Fig. 10 are indeed small molecular aggregates. Such features are similar to polyaniline nanotubes [73,74], whose electronic properties are size dependent. This opens the way for developing new materials for devices with tailored characteristics. Some structures shown in Fig. 9 have a diameter of a few nanometers, typical of nanotubes, but the diameter may reach 500 nm. The cylindrical structures found here are highly irregular in size as they were produced by drop- casting, rather than using controlled film fabrication processes such as the arc-evaporation method or chemical vapor deposi- tion (CVD) [75]. 4. Conclusions The combination of SAXS measurements of polyanilines in solutions and AFM imaging for adsorbed, nanostructured films allowed us to infer that film morphology for short adsorption times is governed by aggregation in solution. Indeed, estimates of the size of the globules in the films led to similar values to those of the size of the aggregates of the polymer molecules in solution. At higher adsorption times, film formation was ac- companied by further aggregation until eventually a less rough film was formed at ca. 180 s of immersion. Thus, the AFM images represent an “off print” of the solution conformation when molecules are adsorbed on the substrate. Significantly, the shape of the aggregates in solution depended on the pH, as ex- pected from the different degrees of doping. SAXS offers an effective tool for determining the fractal di- mension of aggregates of particles. The fractal dimension of particles formed by diffusion-limited process (DLCA) (i.e., fast aggregation) was in the range 1.4–1.8 for PANI-EB and its derivatives. At low pH, the fractal dimensions ranged from 1.9 and 2.7. The POEA and POMA values agree with findings in the literature, for which the fractal dimension is 2.1–2.2 for reaction-limited aggregation (RLCA). With the SAXS experi- ments we showed that the changes in conformation of polymer molecules in solution can be successfully reconstructed by an ab initio procedure. Also investigated was the influence of the doping acid for POEA. Interestingly, POEA doped with TSA exhibited cylin- drical aggregates in solution, which was then manifested as a cylindrical morphology in the adsorbed films studied by TEM. 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