Indentation modulus and hardness of polyaniline thin films by atomic force microscopy

Synthetic Metals 161 (2011) 7–12

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Indentation modulus and hardness of polyaniline thin films by atomic force microscopy D. Passeri a,∗ , A. Alippi a , A. Bettucci a , M. Rossi a , E. Tamburri b , M.L. Terranova b a b

Dipartimento di Scienze di Base e Applicate per l’Ingegneria, Sapienza Università di Roma, Via A. Scarpa 16, 00161 Roma, Italy Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma “Tor Vergata”, Via della Ricerca Scientifica, 00133 Roma, Italy

a r t i c l e

i n f o

Article history: Received 19 February 2010 Received in revised form 20 July 2010 Accepted 25 October 2010 Available online 3 December 2010 Keywords: Atomic force microscopy Elastic modulus Hardness Thin film

a b s t r a c t Polyaniline (PANI) thin films with different thicknesses have been deposited on indium tin oxide (ITO) coated glass substrates by electrochemical polymerization of the aniline monomer in H2 SO4 aqueous solution. By using the tip of an atomic force microscopy (AFM) apparatus as an indenter, cantilever deflection versus sample vertical displacement curves have been acquired and analyzed for evaluating the contact stiffness, by using an approach analogous to that developed for standard depth sensing indentation (DSI) tests. After the calibration performed using a set of polymeric reference materials, indentation modulus and hardness of PANI films have been deduced as a function of the reached maximum penetration depth. By using a model originally proposed for the analysis of standard DSI measurements, indentation modulus and hardness values of only PANI are finally deduced from the corresponding apparent values measured for the film-substrate systems, although they have to be considered as semi-quantitative estimations, since the roughness of the films does not allow a certain determination of the local thickness in correspondence of the probed points. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Polyaniline (PANI) – with its derivatives – has attracted great interest as a material for protective coating [1,2], sensor [3], and actuator applications [4–7]. To this aim, it is necessary to characterize its mechanical properties such as Young’s modulus, Poisson ratio, and hardness. In particular, as for other polymers [8,9], PANI mechanical properties have been found to be strongly depend on the doping characteristics, on structure of the sample (e.g., film or fiber), on the oxidation state, on the sample being stretched or as-realized, and on the measurement being performed in dry environment or in electrolyte solutions [10–21]. Consequently, it is necessary to characterize the mechanical properties of the effective particular realized samples. As far as thin films on stiff substrates samples are concerned, diffused reliable techniques for the mechanical characterization are represented by indentation, either quasi-static, i.e., depth sensing indentation (DSI), or dynamic, i.e., dynamic mechanical analysis (DMA). Nevertheless, in case of ultrathin films, the use of such techniques is limited by the effect of

∗ Corresponding author at: Dipartimento di Scienze di Base e Applicate per l’Ingegneria, Sapienza Università di Roma, Via A. Scarpa 16, 00161 Roma, Italy. Tel.: +39 06 49766591; fax: +39 06 49766932. E-mail address: [email protected] (D. Passeri). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.10.027

the mechanical properties of the substrate itself. Moreover, they do not ensure an adequate lateral resolution in case of micro- and nano-structures, due to the dimensions of the diamond indenter. To overcome such limitations, we recently proposed an approach which enables the characterization of elastic modulus and hardness of viscoelastic films, where the tip of an atomic force microscopy (AFM) apparatus is used as an indenter and the control of the vertical motion of the sample is used for acquiring the cantilever deflection versus the sample height curves, both in the approaching and in the retracting direction, to be converted into force versus indentation curves analogous to those obtained in standard DSI tests [22]. In the present work, we describe the characterization of indentation modulus and hardness of PANI thin films with different thicknesses, deposited on indium tin oxide (ITO) coated glass substrates by electrochemical polymerization, performed by AFM based nanoindentation technique by imposing different values of normal load applied on the samples surfaces by the deflection of the AFM cantilever, thus reaching different values of penetration depth. In the following, firstly the technique is briefly summarized which is illustrated in more details elsewhere [22]; then the investigated samples and the experimental apparatus are described; finally, the indentation modulus and hardness of PANI are obtained by analyzing the apparent measured values of indentation modulus and hardness as a function of the penetration depth.

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D. Passeri et al. / Synthetic Metals 161 (2011) 7–12

2. Technique

in the BC portion the time t can be obtained as

Mechanical properties of the investigated sample can be evaluated by analyzing the force F versus indentation depth h curves, obtained using the AFM tip as an indenter. This requires a post-experiment data processing analogous to that developed for standard DSI measurements [23,24]. In DSI tests, F(h) curves are collected by directly monitoring h as a function of F which is increased up to a maximum value (loading phase), then kept constant (holding phase), and finally decreased down to its initial value (unloading phase). Differently from DSI, in AFM based nanoindentation the extension z of the piezoelectric actuator along the vertical axis (i.e., the z-direction displacement of the sample) is controlled. Therefore, the cantilever deflection value d is acquired as a function of z. Then d(z) curve is converted into the corresponding F(h) one [23,24]. In the following, the data processing procedure is briefly summarized, which is reported in more details elsewhere [22–24]. An example of d(z) curve is shown in Fig. 1, where each value of d is obtained as the average of 50 measurements. In the approaching phase, i.e., while z is increasing with constant velocity z up to a selected maximum value (zC ), d remains equal to zero until it is reached the contact point A, the corresponding value of z being assumed as the origin of the z-axis. The exact determination of A is a relevant issue in the technique, since its uncertainty may be significative in case of compliant materials (such as the polymeric samples involved in the present work) and is a source of experimental error [25]. In the present study, it has been determined as the point where d is observed to increase from zero above the noise level [22]. After the contact is reached, any increase z of z produces an increase d of d. For an ideally stiff material d/z = 1, while for a compliant one d/z < 1 since the sample surface is indented by the AFM tip. For each value of d, the corresponding value of indentation depth h is obtained as [22,26]

t=

h = z − d.

(1)

For obtaining an holding phase in our technique, a limiting value dlim (point B) is imposed. After dlim is reached, the corresponding value of z is maintained constant [22]. The subsequent variation of z corresponding to the portions BC and CD is virtual, being introduced by our software, and allow us to analyze the viscoelastic behavior of the sample. The abscissa values in the portion BC and CD do not represent the real extension of the piezoelectric actuator. They are the z values that would have been reached if the piezoactuator had not been stopped but had continued extending. Being z constant,

z − zB . z

(2)

The decreasing of d is thus obtained as a function of time and is produced by the increase of h due to viscoelastic relaxation [22]. When the imposed maximum value of zC is ‘virtually’ reached, the piezoactuator is ‘virtually’ retracted (actually, zB ) is still maintained, which corresponds to the portion CD of the curve. Here, the time t is deduced by the abscissa z as t=

zC − z 2zC − zB − z zC − zB + = . z z z

(3)

Eqs. (2) and (3) allow to reconstruct the complete d(t) curve during the holding phase [22]. Nevertheless, as shown below, such explicit conversion is not strictly required thus avoiding the exact determination of z . The holding phase ends when z = zB = zD during the ‘virtual’ retraction of the piezoactuator. After point D is reached, z is actually decreased and thus d is reduced (unloading phase). The retracting d(z) curve does not trace the approaching one due to plastic effects. These are present during loading while are assumed to be absent during unloading [27]. In the final portion of the unloading curve, negative values of d are observed due to the presence of adhesion and capillary effects. Finally, the tip-sample contact is broken (point E) and the cantilever returns in its equilibrium position with zero deflection. The d(z) curve has then to be converted into the F(h) one. To this aim, both for the loading and the unloading curve, h is obtained by Eq. (1). The corresponding applied load is calculated as F = kc d, where kc is the cantilever spring constant. For evaluating M and H of the sample, a number of parameters are required. They should be deduced from the F(h) curve, but may be also evaluated directly by the portion of the unloading d(z) curve in correspondence of the point D, and immediately below and above it. The load and the penetration depth at the beginning of the unloading phase (namely, Fmax and hmax ) are evaluated as Fmax = kc dD ,

(4)

and hmax = zD − dD .

(5)

From the slope of the onset of the unloading d(z) curve (namely, Sdz ), the slope of the onset of the unloading F(h) curve (namely, the contact stiffness Smeas ) may be calculated by Smeas = kc

Sdz . 1 − Sdz

(6)

For a viscoelastic material, the effective elastic contact stiffness Se can be obtained as [28] 1 = Se



1 Smeas

−

h˙ h F˙ u



1 1 − F˙ h /F˙ u

,

(7)

where h˙ h and F˙ h are the variation rate of the penetration depth and of the load at the end of the holding and F˙ u is the variation rate of the load at the onset of unloading phase. Eq. (7) can be rewritten as [22] 1 = Se

Fig. 1. Example of cantilever deflection d as a function of the sample displacement z curve during the loading and unloading phase (continuous and dashed line, respectively). In the inset, a detail of the curve is reported evidencing both the end of the holding and the onset of unloading phases, where the slopes of the d(z) curve at the end of the holding and at the beginning of the unloading (d˙ h and d˙ u , respectively) are also indicated.



1 Smeas

d˙ + h kc d˙ u



1 1 − d˙ h /d˙ u

,

(8)

where d˙ h and d˙ u are the slope of the d(z) curve at the end of the holding and that at the beginning of the unloading, respectively (both shown in the inset of Fig. 1). It has to be explicitly pointed out that, while the symbols h˙ h , F˙ h , and F˙ u in Eq. (7) refer to the derivative on time, the symbols d˙ h and d˙ u in Eq. (8) refer to the derivative on the variable z. Being constant, z does not affect Se

D. Passeri et al. / Synthetic Metals 161 (2011) 7–12

and thus its explicit evaluation is not strictly required [22]. The penetration contact depth hc is then calculated by hc = hmax − ε

Fmax , Se

(9)

where ε is a parameter dependent on the shape of the tip. In case of parabolical tips, the value ε = 0.75 has to be assumed. An ancillary parameter f can be introduced which is defined, for a parabolical tip, as [22] f =

S

e .

(10)

hc

The parameter f is proportional to the reduced Young’s modulus E* which is defined by E∗ =

1 M

+

1 Mt

−1 ,

(11)

where M and Mt are the indentation moduli of the sample and of the tip, respectively. The indentation modulus quantifies the elastic response of a material during indentation: for a generic isotropic sample, the indentation modulus M is related to the Young’s modulus E and to the Poisson ratio  by the simple equation M=

E , 1 − 2

3.2. Experimental apparatus

(13)

Analogously, a second ancillary parameter g can be defined as [22] g=

Fmax , hc

(14)

which is proportional to the sample indentation hardness H by the relation [22] g = cgH H,

these experimental conditions a totally oxidized state of the polymer (the pernigraniline form) is reasonably realized. Three PANI samples (namely, sample A, B, and C, respectively) were obtained by adopting the different values of deposition time  d indicated in Table 1, in order to produce films with different thickness tf . The value of tf of each PANI film has been determined by the AFM imaging of an area of each sample where the polymer had been removed by a gentle scratch. The average values of tf for the three samples are reported in Table 1. Standard AFM morphological characterization of the PANI films has shown that the maximum variation of the samples height is a significative fraction of the thickness and consequently tf has to regarded to only as a average estimation of the local thickness of the samples. In order to perform the calibration of the technique, low-density polyethylene (LDPE), polymethylpentene (TPX), and polycarbonate (PC) thick samples (with thickness varying in the range 1–2 mm) have been used as reference materials, whose mechanical properties have been characterized by standard DSI tests, as reported elsewhere [22]. After correcting for viscoelasticty, for LDPE, TPX, and PC indentation modulus has been evaluated as 0.27 ± 0.01 GPa, 1.5 ± 0.2 GPa, and 3.5 ± 0.4 GPa, respectively, while hardness has been evaluated as 18 ± 2 MPa, 110 ± 20 MPa, and 250 ± 30 MPa, respectively [22].

(12)

while, for an anisotropic sample, M has to be calculated from the cij elements of the elastic tensor [29]. If indentation measurements are performed on compliant materials like polymers by a stiff tip, Mt  M so that E* ≈ M. Consequently, a proportionality relation exists between f and M that can be express by introducing the proportionality coefficient cfM as [22] f = cfM M.

9

(15)

where a proportionality coefficient cgH has been introduced [22]. If the geometry of the is completely known, the parameters cfM and cgH can be determined. Actually, the shape of the AFM apex is generally only roughly known so that its independent characterization would be required. Alternatively, a phenomenological calibration can be performed by measuring f and g on a set of reference samples with known M and H: cfM and cgH are evaluated by using Eqs. (13) and (15). After the calibration is performed, cfM and cgH allow one to evaluated M and H for an unknown sample on which f and g have been measured. 3. Experimental 3.1. Samples description PANI samples were prepared by electrochemical polymerization of 0.1 M aniline monomer in 0.1 M H2 SO4 aqueous solution. The electrosynthesis was performed at room temperature in an one compartment three-electrode electrochemical cell by means of a PalmSens instrument. ITO coated glasses and a Pt coil were used as working and counter electrodes, respectively. The reference was an Ag/AgCl in KCl (3 M) electrode. The elecropolymerization was performed by applying a constant potential of 1 V versus Ag/AgCl. In

Experiments have been performed using a standard AFM apparatus (Solver P47H, NT-MDT, Russia) in air and at room temperature. In order to verify the reliability of the results, two sets of measurements have been carried out by using two standard Si cantilevers (NSG10, NT-MDT, Russia), each with nominal length l = 100 ␮m, width w = 35 ␮m, and thickness t = 2 ␮m (namely cantilever 1 and 2, respectively). The spring constants of the cantilevers have been determined as high as kc1 = 18 ± 2 N/m and kc2 = 15.0 ± 1.5 N/m, respectively, by analyzing their first resonance in air by the method of Sader et al. [30]. To convert the cantilever deflection electric signal (expressed in nanoampere) into the effective cantilever deflection (expressed in nanometer), calibration had to be performed by collecting d(z) curves on infinitely stiff materials, on which indentation can be considered negligible: kA1 = 51 ± 3 nm/nA and kA2 = 79 ± 1 nm/nA have been obtained for cantilever 1 and 2, respectively, the former evaluated on an ITO coated glass substrate while the latter on a polycrystalline Al2 O3 sample. For each force curve collected on hard reference sample, the slope of the loading d(z) portion is slightly steeper compared to the unloading one of about 1% due to hysteresis effect of the piezoelectric actuator [31,32]. Such an effect can be therefore neglected in the scan rate ranges involved in our measurements. 4. Results and discussion In each measurement session, a number of d(z) curves have been collected at different points of the surface of both the PANI samples and the reference materials without imaging their surfaces, in order to avoid any accidental modification of the AFM tip and thus allowing us to reasonably suppose the same geometry of the apex during the whole experiment. On each sample, measurements have been performed by imposing different values of dlim , in order to experience different values of penetration depth. Due to the not ideal geometry of the contact, parasitic torsion of the cantilever can be experienced in the loading phase, resulting in the apparent reduction of the deflection [32–34]. To quantify such an effect, the lateral deflection signal (LS) has been monitored as a function of the vertical deflection signal (VS) during indentation on the polymeric and stiff reference samples [33]. No variation of LS is observed on LDPE,

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D. Passeri et al. / Synthetic Metals 161 (2011) 7–12

Table 1 Summary of the sample characteristics and of the experimental details of the performed measurements. For each PANI sample (A, B, and C) the applied deposition time  d and the measured mean thickness tf are reported. For each group of measurements, the used cantilever, the range of applied maximum load values Fmax and the corresponding obtained range of maximum penetration depth values hmax are indicated. Sample

 d (s)

tf (nm)

Cantilever

Fmax (␮N)

hmax (nm)

Fmax (nN)

hmax (nm)

A B C

300 600 900

150 240 280

1 1 1 2

1.5–2.4 1.3–2.7 1.7–2.8

25–60 35–90 100–230

450–900 650–750 800–900 300–600

15–55 30–50 30–150 30–60

while it is lower than 10% of VS on PC. Finally, LS is lower than 19% of VS on the stiff reference samples, leading to a systematic error in the evaluation of kA . Although such errors are relatively high on each measurement, they less severely affect the estimated value of the indentation modulus of the investigated sample due to the performed calibration using the polymeric reference samples. From each d(z) curve, dD , zD , SdZ , d˙ h , d˙ u have been obtained and hmax , Fmax , and Se have been evaluated. The low values of hmax experienced suggest that the very end of the tip is involved in the measurements on both the PANI films and the reference samples. Such a consideration, together with both the analysis of the unloading F(h) curves and the reconstruction of the AFM apex geometry by imaging a inverted tip array sample, indicated the parabolical model as the most suitable for describing the tip shape. According to such a model, for each measurement hc , f, and g have been evaluated using Eqs. (9), (10), and (14), respectively. We observed a slight inverse dependence of both f and g versus hc , thus suggesting that the real shape of the tip is intermediate between the parabolical and the flat one. For each reference material, the values of f collected at the different points of the sample surface and for the different values of dlim have been averaged. For each experimental session, measurements on the reference samples have been used to obtain both the f(M) and the g(H) calibration curves and to evaluate the calibration coefficients cfM and cgH , respectively. As an example, Fig. 2 reports the calibration curves obtained using cantilever 2. Fig. 2(a) shows the mean value of f and its standard deviation for each polymer as a function of the corresponding value of M measured by standard DSI. The good linear dependence between f and M allow us to confirm the reliability of the technique and to calculate the calibration coefficient cfM = 2.1 ± 0.2 nm0.5 . Analogously, Fig. 2(b) reports the mean value of g and its standard deviation for each polymer as a function of the correspondent value of H retrieved by DSI. Also in this case, a good linear dependence between g and H is observed and the calibration coefficient cgH = 101 ± 7 nm is calculated. After the linearity of both the f(M) and the g(H) calibration curve is verified, the values of cfM and cgH have been used for evaluating M and H of the PANI samples. To this aim, particular care has to be used to take into account the effect of the finite thickness of the films, which implies that the values of M and H measured on the PANI samples are affected by the mechanical properties of the ITO substrates. To investigate such an effect, the dependence of both the values of M and H retrieved from each d(z) curve should be analyzed on the correspondent value of hmax /tf (or hc /tf ). The significative deviations in the local value of film thickness from its average value reported in Table 1, that may be produced by the variations in the surface height of the PANI samples, and the not ideal shape of the tip, that produce the slight dependence of f and g (and thus of the calculated M and H) on hc reduce the effective use of such an approach. Consequently, for investigating the effect of the substrate and extracting the values of ‘film-only’ indentation modulus and hardness (namely, Mf and Hf ), two sets of measurements have been performed on each PANI sample by using cantilever 1 for applying loads higher than 1.3 ␮N or lower than 900 nN (which are referred to in the following as high or low loads, respectively). In Table 1, the ranges of high and low loads value are reported for

Fig. 2. (a) Average values of the parameter f (open squares), measured for the LDPE, TPX, and PC reference samples using cantilever 2, as a function of the corresponding indentation modulus M, obtained by standard DSI tests, with superimposed the linear fit (solid line) form which the calibration coefficient cfM is deduced. (b) Average values of the parameter g (open squares), measured for the LDPE, TPX, and PC reference samples using cantilever 2, as a function of the corresponding hardness H, obtained by standard DSI tests, with superimposed the linear fit (solid line) form which the calibration coefficient cgH is deduced.

each sample, together with the obtained ranges of hmax . Moreover, a further set of measurements has been performed on sample C by using cantilever 2 for applying loads lower than 600 nN, which is intended to supply an independent evaluation of M and H with lower penetration depths on the thickest film, and thus less affected by the mechanical properties of the substrate, in order to verify the reliability of the results. As for the reference materials, each d(z) curve has been analyzed in order to obtain the corresponding values of f and g from those measured of dD , zD , Sdz , d˙ h , and d˙ u . As far as the value of the latter is concerned, it is worth noting that the d˙ u = 1 nm/nm in d(z) curves retrieved on an infinitely stiff material, while d˙ u < 1 nm/nm on compliant samples. Nevertheless, the uncertainty in the determination of the current-deflection conversion factor (±6% and ±1% for kA1 and kA2 , respectively) produce an relative uncertainty in the d˙ u value for a infinitely stiff sample of the same amount, i.e., 1.00 ± 0.06 and 1.00 ± 0.01 for cantilever 1 and 2, respectively. From all the d(z) curves obtained on each reference (thick) sample, values of d˙ u lower than 0.94 and 0.99 (for

D. Passeri et al. / Synthetic Metals 161 (2011) 7–12

where



L=

h

1+A

max

11

C −1 ,

tf

(17)

being A and C adjustable parameters. Analogously, Hmeas can be expressed as Hmeas = Hs

 H N f

Hs

where



N=

Fig. 3. As-measured indentation modulus Mmeas (a) and hardness Hmeas (b) of the PANI samples, obtained by using either cantilever 1 (open squares) or 2 (open circles), as a function of the ratio hmax /tf between the average maximum penetration depth hmax and the thickness of the film tf with superimposed the corresponding best fitting theoretical curves (solid lines).

cantilever 1 and 2, respectively) thus indicating that samples surfaces were always indented by the AFM tip. Conversely, on the PANI film samples, values of d˙ u higher that 0.94 and 0.99 (for cantilever 1 and 2, respectively) have been occasionally observed, especially for the sample A, i.e., the thinnest films: in such cases, the indentation of the sample surface cannot be definitively asserted, thus suggesting the local thickness of the film was so low that the tip penetrated the entire polymeric layer and reached the stiff substrate. In such cases, the measurements were not further used in the analysis, in order to avoid erroneous interpretation of the experimental data. After such a preliminary selection, for each measurement the values of M and H have been evaluated from those of f and g, respectively, through the calibration coefficients cfM and cgH , respectively. Finally, for each group of measurements summarized in Table 1, the average indentation modulus and hardness values (Mmeas and Hmeas , respectively) and their standard deviations have been calculated. Fig. 3(a) and (b) showssuch values of Mmeas and Hmeas , respectively, as a function of the ratio between the average value of hmax (reported in Table 1) and tf . Both Mmeas and Hmeas are observed to increase with the increasing of hmax /tf . Such a result has to be attributed to the effect of the mechanical properties of the stiff ITO substrate (whose indentation modulus and hardness are Ms = 107 GPa and Hs = 6.5 GPa, respectively [35]), which produces an higher increase of Mmeas and Hmeas with respect to the ‘film-only’ ones Mf and Hf the higher is the value of hmax /tf . In order to extract the values of Mf and Hf , the approach proposed by Jung et al. [36]. According to such an approach, the value of Mmeas can be expressed as Mmeas = Ms

 M L f

Ms

,

(16)

1+B

h

max

tf

,

(18)

D −1 ,

(19)

being B and D adjustable parameters. By fitting Mmeas data using Eqs. (16) and (17) by imposing Ms = 107 GPa for the ITO substrate [35], the corresponding curve being reported in Fig. 3(a) as a continuous line, the value Mf = 4.6 ± 0.9 GPa is determined. Analogously, by fitting Hmeas data using Eqs. (18) and (19) by imposing Hs = 6.5 GPa for the ITO substrate [35], the corresponding curve being reported in Fig. 3(b) as a continuous line, the value Hf = 163 ± 114 MPa is obtained. The estimated value of Mf is coherent with data reported in literature, within the experimental uncertainty. Indeed, PANI Young’s modulus is reported to vary in the range 2–4 GPa [14,37,38] (which roughly corresponds to the the indentation modulus values range 2.4–4.8 GPa, if  = 0.4 is assumed), although lower values have been occasionally found [10,19]. PANI hardness values reported in literature vary in a quite broad range (from tens of megapascals to 1 GPa), while surface microhardness of thermoplastics generally do range between 0.1 and 0.6 GPa [2,19–21]. Therefore, we consider the Hf values obtained in this study a reasonable estimation of PANI film hardness, within the experimental uncertainty. Although a relatively high scattering of indentation modulus measurements by AFM is quite common [39], the uncertainty in the determination of Mf (±20%) is significatively higher than that of cfM (±10%). Moreover, the uncertainty of Hf (±70%) is dramatically higher than that of cgH (±7%). These results suggest that the technique is reliable on relatively thick films, while it supplies only a semi-quantitative estimation of mechanical parameters when characterizing ultra thin films. This has to be mainly ascribed to the relatively high roughness of the films, which does not allow a reliable evaluation of the local film thickness. To overcome such limitation, when characterizing ultra-thin films with high roughness, ultra low loads should be recommended in order to obtain penetration depth values lower than 15% of the film thickness, thus reducing the effect of variation of the local film thickness. 5. Conclusions We reported the characterization of indentation modulus and hardness of PANI films deposited by electrochemical polymerization on ITO coated glass substrates. We used a recently proposed AFM based nanoindentation technique which takes into account the viscoelastic behavior of polymers. Cantilever deflection versus sample vertical displacement curves have been acquired for both the investigated samples and three polymeric reference materials, the latter used for calibrating the technique. After calibration, the apparent indentation modulus and hardness have been measured for different penetration depth values. By applying to AFM measurements an approach proposed for standard DSI, the dependence between the measured indentation modulus (and hardness) and the penetration depth versus film thickness ratio have been analyzed, taking into account the effect of stiff ITO substrates, thus obtaining the indentation modulus (and hardness) of the only PANI.

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