The influence of heat treatment on the semi-crystalline structure of polyaniline Emeraldine-salt form

Accepted Manuscript The influence of heat treatment on the semi-crystalline structure of Polyaniline Emeraldine-salt form Lilian R. de Oliveira, Lizandro Manzato, Yvonne P. Mascarenhas, Edgar A. Sanches PII:

S0022-2860(16)30971-1

DOI:

10.1016/j.molstruc.2016.09.044

Reference:

MOLSTR 22956

To appear in:

Journal of Molecular Structure

Received Date: 12 May 2016 Revised Date:

15 September 2016

Accepted Date: 16 September 2016

Please cite this article as: L.R. de Oliveira, L. Manzato, Y.P. Mascarenhas, E.A. Sanches, The influence of heat treatment on the semi-crystalline structure of Polyaniline Emeraldine-salt form, Journal of Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.09.044. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT 1 The Influence of Heat Treatment on the Semi-crystalline Structure of Polyaniline Emeraldine-salt Form

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Lilian R. de Oliveira1, Lizandro Manzato2, Yvonne P. Mascarenhas3, Edgar. A. Sanches1

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Laboratório de Polímeros Nanoestruturados (NANOPOL), Programa de Pós-graduação em Física (PPGFIS), Universidade Federal do Amazonas (UFAM), Manaus/AM, Brasil. 2 Instituto Federal do Amazonas (IFAM), Campus Manaus - Distrito Industrial, Manaus/AM, Brasil. 3 Universidade de São Paulo (USP), Instituto de Física de São Carlos (IFSC), São Carlos/SP, Brasil.

ABSTRACT

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Emeraldine-salt polyaniline form (ES-PANI) was chemically synthesized using hydrochloric acid and subjected to heat treatment for 1 h at 50, 100, 200 and 300 °C. X-ray Diffraction (XRD), Le Bail method, Infrared-transform Fourier Spectroscopy (FTIR), Small-angle X-ray Scattering (SAXS), Scanning Electron Microscopy (SEM) and Electrical Conductivity measurements were used to evaluate the influence of heat treatment on the semi-crystalline structure of PANI. The heat treatment has resulted in a progressive decrease of crystallinity from 50 to 22%. A crosslinking process during heat treatment was observed by FTIR at 200 °C, revealing some chemical changes in molecular structure of PANI such as elimination of HCl on the imino groups and the simultaneous chlorination of the aromatic rings. Le Bail method showed that unheated ES-PANI is strongly dependent on the molecular size of the counter ion, so the unit cell volume needed to be increased for their accommodation in polymer structure. The refined parameters suggested decomposition from tetrameric to dimeric-folded chains, accompanied by a decrease in the crystallite anisotropy and average size and shape, which reduced from 36 Å to 16 Å and acquired oblate shape. The pair-distance distribution function (p(r)) curves suggested particles tending from oblate to prolate form over heat treatment. Well-defined nanofibers were observed in unheated ES-PANI, which decreased and lost progressively their initial morphology over heat treatment. Electrical conductivity showed a decreasing of about 90% due to the loss of emeraldine sequences and to the removal of chloride ions.

Keywords: Heat treatment, Polyaniline; XRD; SAXS; Le Bail Method

ACCEPTED MANUSCRIPT 2 1. Introduction Intrinsically Conductive Polymers (ICPs) have attracted attention due to its potential in technological applications. Their properties are dependent on the manufacturing process and

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crystallinity [1,2,3]. For this reason some physical-chemical properties of solids are directly related to their microstructure. Therefore, crystallite size, morphology, and imperfections in the crystal structure can have important effects in the material performances [4].

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Polyaniline (PANI) has been used in many technological applications [5,6,7,8]. This ICP is known as a para-linked phenylene amineimine which possesses well defined oxidation states, as shown

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in Fig.1 [9]. Polyaniline Emeraldine-base form (EB-PANI, blue, undoped) can be doped by protonic acids, leading to Emeraldine-salt form (ES-PANI, green, doped). This doping effect promotes an increase in its electrical conductivity and crystallinity [10,11].

Electric conductivity and crystallinity can be considerably reduced by heat treatment. Thermal

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stability of PANI [12,13,14] is an relevant parameter for commercial applications, since it is important to understand the effect temperature on the oxidation state and structure of PANI powders. During heat treatment, chemically synthesized PANI undergoes several stages of mass loss due to the release of

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water and dopant as well as changes in chain packing and stacking [15]. Furthermore, a crosslinking reaction arising at about 250 °C and an irreversible thermal transition lead to very low solubility and

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conductivity [16].

Structural and morphological aspects in semi-crystalline materials continue to be an interesting researched topic [17,18,19,20,21,22]. Understanding of the regular arrangement of polymer chains is essential for the prediction of processing methods and new properties. This work presents a structural and morphological study of unheated and heated PANI in order to provide a systematic interpretation of the crystalline phase through line broadening analysis of diffraction patterns and a real description of the microstructural features of heated and unheated PANI. X-ray diffraction technique (XRD) was used

ACCEPTED MANUSCRIPT 3 as input data for Le Bail method performance and for estimative of crystallinity. Le Bail method was performed using the Fullprof program [23] to refine cell parameters and to obtain crystallite size. Peak broadening with linear combinations of spherical harmonics has been applied to evaluate the

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anisotropic crystallite shape; SAXS was used to estimate size and shape of the scattering particles which are composed of several crystallites. SEM was applied for the determination of the powder

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polymer morphology. Then, these results were correlated with electrical properties.

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2. Experimental

2.1. Polyaniline Emeraldine –salt form (ES-PANI) synthesis

Aniline (Aldrich) was used after distillation. HCl (aq), acetone and (NH4)2S2O8 (Aldrich) were used without further purification. ES-PANI synthesis was based on the method described by Bhadra et

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al. [24] and Sanches et al. [25]. Two different solutions were prepared. Solution I was prepared by dissolving 0.2 M of distilled aniline monomer in 500 mL of 1.0 M hydrochloric acid (HCl) at room temperature. Solution II, on the other hand, was obtained by adding a stoichiometric calculated amount

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of ammonium persulfate to 200 mL of 1.0 M HCl. Solution II was added drop by drop to Solution I. The system remained under constant stirring for 3 h. Then, the green dispersion was vacuum filtered

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and washed with acetone.

2.2. Thermal treatment

Equal mass of ES-PANI were subjected to the thermal treatment process. Each portion remained for 1 hour in a tube oven at 50 °C (TT50), 100°C (TT100), 200 °C (TT200) and 300 °C (TT300). A programming temperature was made using the Flycon software.

ACCEPTED MANUSCRIPT 4 2.3. XRD analysis and crystallinity percentage XRD data were obtained at the Laboratory of X-ray Crystallography of IFSC/USP using a Rigaku Rotaflex diffractometer equipped with graphite monochromator and rotating anode tube,

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operating with CuKα, 50 kV and 100 mA. Powder diffraction patterns were obtained in the range 2θ = 5 - 60°, step of 0.02° and 5 seconds/step. Peak Fitting Module program [26,27] was used for the peak decomposition of the semi-crystalline pattern. Crystallinity percentage was estimated using the

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deconvolution method. The ratio between the sums of the peak areas to the area of non-crystalline

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broad hallo due to the non-crystalline contribution was applied to assess the crystalline phase.

2.4. Fourier-Transformed Infrared Spectroscopy (FTIR).

FTIR spectra were carried out in Nanomed Inovação em Nanotecnologia (São Carlos/SP, Brazil) in a spectrophotometer Bomem-MB Series Hartmann and Braun in the range of 400–2000

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cm−1. Samples were prepared in KBr pellet with mass ratio of 1:100.

2.5. Le Bail whole powder pattern decomposition method

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Le Bail method was applied in order to extract cell parameters and crystallite size and shape from XRD patterns. The software package Fullprof [23] was used to perform the method. All

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parameters were refined by the least-squares method [28]. The pseudo-Voigt function modified by Thompson-Cox-Hastings was used as peak profile function [29]. Instrumental resolution function parameters were obtained from a lanthanum hexaborate standard, LaB6. The end-capped aniline tetramer single crystal parameters obtained by Evain et al. [30] were used as initial parameters (triclinic _

– P 1; a = 5.7328 Å; b = 8.8866 Å; c = 22.6889 Å; α = 82.7481°; β = 84.5281 ° and γ = 88.4739 °). Linear combination of Spherical harmonics (SHP) [31] were used to evaluate the crystallite anisotropy.

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2.6. SAXS Measurements SAXS experiments were performed at the National Synchrotron Light Laboratory (LNLS), Campinas, Brazil, using a monochromatic X-ray beam (λ = 1.488 Å), which focuses the beam

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horizontally and a bidimensional position-sensitive X-ray detector. Powder samples were placed in a parallel window cell and the scattering curves were normalized with respect to the decreasing intensity of the incoming synchrotron beam and to the sample absorption [32]. The distance between sample and

(4π/λ)sin θ, where 2θ is the scattering angle [33,34,35].

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detector was 1010.6 mm. The scattering intensity was measured over the scattering vectors, q =

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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) [36].

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I ( q ) = Ie( q ) Np ( ∆ρυ ) 2 exp(

− Rg 2 2 q ) 3

Eq.1

In a Guinier plot, ln I (q) vs q2(I (q)→I (0)), the slope of the linear region allows to obtain the Rg

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[37]. It was considered the monodisperse system, to obtain de average Rg and particles radii. In the absence of interference effects, a Fourier transform connects the normalized particle form factor (and

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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 [38]: p (r ) = (

1 2π

∞

) I (q ) qr sin( qr ) dq 2 ∫

Eq. 2

0

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. Moreover, the particle radius of gyration, Rg, value is given by [37]:

ACCEPTED MANUSCRIPT 6 D max

∫ p(r )r dr 2

Rg2 =

Eq. 3

0 D max

2

∫ p(r )dr

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0

In this work, we make use of the GNOM program [39] to calculate p(r) and estimate the radii of

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gyration (RgGNOM) for unheated and heated polyaniline.

2.7. SEM Analysis

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SEM experiments were performed using a Supra 35, Carl Zeiss, 1.0 kV. Powder samples were deposited on a carbon tape and the surface morphology was obtained at room temperature.

2.8. DC Electrical conductivity measurements

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DC electrical conductivity measurements were carried out at room temperature. The resistivities of unheated and heated PANI were measured at the Laboratório de Polímeros Nanoestruturados (NANOPOL/UFAM), Brazil, using a Keithley Model 2612 A from 0.5 to 2 V. Powder samples were

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processed into pellets coated with silver ink on both sides. Electrical contacts were made in pellets

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using metal wires and silver paste.

3. Results and discussion

3.1. XRD Analysis and crystallinity percentage X-ray powder diffraction is a powerful technique for the microstructural analysis of solids even if deviations from the ideal structure are slightly observed in diffraction pattern. These deviations involve a large number of microstructural parameters that do not have a one to one correlation with the

ACCEPTED MANUSCRIPT 7 different peak profile features. For this reason, the full characterization of semi-crystalline materials is always a complex task. ES-PANI form is significantly less stable than the Emeraldine base form (EB-PANI) and the

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instability of the former is related to the protonic acid [40]. According to previous reports [41,42,43] the degradation temperature of EB-PANI powder is higher than ES form and exhibited lower weight loss. The violent degradation observed for ES-PANI should be attributed to the oxidation by HCl, the

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only source of oxidizing agent at elevated temperature in the nitrogen atmosphere.

Fig.2 shows the XRD patterns of unheated and heated PANI powder. These patterns showed the

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evolution of the ES-PANI degradation over heat treatment. For unheated ES-PANI form, the XRD pattern showed peaks at 2θ = 6.3°, 9.2°, 14.7°, 20.7°, 25.2 and 26.9°. A broad peak is located between 2θ = 28.9° and 32.2°, and another between 2θ = 32.2° and 39.5°.

The XRD pattern of TT50 is very similar to TT100, with broader peaks observed in the last,

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which are located at 2θ = 6.3°, 9.8°, 15.2°, 20.2° e 24°. For TT200 and TT300, XRD patterns showed even more loss of crystallinity. For TT200 a broad peak was observed at 2θ =11.5°, and another centered at approximately 2θ = 19.9°. Similar pattern was observed for TT300 but with loss of intensity

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related to the peak located at 2θ = 11.5º.

The heat treatment promoted loss of crystallinity in ES-PANI. The patterns obtained after heat

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treatment have less number of peaks, which are broader when compared to the as-synthesized ESPANI. This fact is related to the increasing of the non-crystalline contribution and decreasing of crystallite size.

To estimate the crystallinity percentage of semi-crystalline polymers, it is assumed that the polymer is constituted of well-defined mixture of non-crystalline and crystalline regions. Many methods have been applied for the estimation of crystallinity percentage in semi-crystalline materials.

ACCEPTED MANUSCRIPT 8 These methods do not always give the same result. However, all current methods assume implicitly a two phase model of crystallites embedded in a non-crystalline matrix [44,45]. The most used method to access the crystalline phase of a given material using XRD pattern is

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the deconvolution method [46]. Some assumptions such as the shape and number of peaks and an appropriate function have to be made. An important hypothesis for this analysis is that increased noncrystalline contribution is the main contributor to peak broadening. However, in addition to non-

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crystalline content, there are other intrinsic factors that influence peak broadening, such as crystallite size and non-uniform crystal strain. The peak deconvolution of XRD patterns for unheated and heated

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PANI is showed in Fig.3 (a-e).

Assuming the non-crystalline contribution, the Chebyshev polynomial was subtracted from the total pattern allowing the crystalline peaks assessment. Comparing the experimental area with those of the polynomial curve, the estimative of crystallinity percentage is showed in Fig.4. Peaks were best

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fitted using Gaussian curves, which is typical of isotropic structures. A decreasing in crystallinity percentage from 50 to 22% was observed after heat treatment.

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3.2. FTIR Analysis

Chemical changes associated with the observed reduced crystallinity were investigated by

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FTIR. Some investigations concerning structural changes in ES-PANI after heat treatment above 200 °C have already been reported in literature, which suggest the formation of crosslinked chains [47]. Fig.5 shows the FTIR spectra for ES-PANI, TT50, TT100, TT200 and TT300. Unheated ES-

PANI showed vibrational bands related to the normal modes of polyaniline: absorption bands at 1561 cm-1 and 1470 cm-1 are assignable to the ring stretching vibration of quinoid (Q) and benzenoid (B), respectively [43,48,49]. The band located at 1298 cm-1 corresponds to π-electron delocalization induced in the polymer by protonation [50].The band characteristic of the conducting protonated form

ACCEPTED MANUSCRIPT 9 is observed at 1238 cm-1 and assigned as a C–N–C stretching vibration in the polaron structure [51]. The band at 1130 cm-1 is related to the vibration mode –NH+= which is formed during protonation [30]. Chemical changes observed at elevated temperature include the elimination of HCl on the imino

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groups and the simultaneous chlorination of the aromatic rings. A blue shift in the band at 1561 cm-1 to 1590 cm-1 was observed. This fact is even more clear for TT200 and TT300 samples, which the same peak is located at 1610 cm-1. Trchová et al. (2004) [52] have reported this shift, attributing to the

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removal of dopant counter ion. Bhadra and Khastgir (2008) [53] have reported that there is loss of HCl after heat treatment above 200 °C due to the partial removal or modification of the HCl concentration.

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In the band at 1470 cm-1 is also observed a similar change. The modification of the band at 1130 cm-1 to 1106 cm-1 is observed with the increase of temperature due to the decrease in the emeraldine sequence.

The crosslinking process of PANI-EB occurs during heat treatment. Longer heat treatment time

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leads to more pronounced crosslinking which, in turn, lead to reduced electrical conductivity of posttreatment protonated polymer [Error! Bookmark not defined.]. It is important to stress that there are changes in intensity of the peaks (Q) to (B) from PANI-ES to TT50, TT100, TT200 and TT300, which

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resulted from the changes in concentration of (Q) and (B). When PANI-ES is treated at 200 °C, the ratio of (Q) to (B) rings is increased compared to the unheated PANI-ES. This indicates that there has

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been a conversion of (B) to (Q) rings, which is characteristic of PANI-ES oxidation. For TT300 the ratio of (Q) to (B) rings decreases, i.e., (Q) rings are converted into (B) rings, which suggested that the characteristics of quinone rings had been considerably lost and the residual dopant had been further removed. This result is due to the crosslinking reaction in the polymer chains. Other researchers also reported this phenomenon and set up similar models to elucidate the crosslinking reaction. Fig.6 is adapted from Luo et al. [42] to present the structure of crosslinked polyaniline chains. When PANI-ES was heated between 200 and 300 °C, the absorption bands located

ACCEPTED MANUSCRIPT 10 at 1299 cm-1 and 1239 cm-1 disappeared from the spectrum. This indicates that there is a large decrease in the electron π delocalization due to the decreased of the protonation level and emeraldine sequence, resulting in the destruction of the polaronic structure. On the other hand, the bands associated with

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multi-substitution at 1051 and 935 cm-1 on the benzene rings were increased, and the characteristic band for the 1,2-disubstituted benzene rings at 700 cm-1 were also well developed, which meant the

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crosslinking among the PANI molecules was formed after heating.

3.3. Le Bail Method

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Le Bail method [54] has recently been performed to obtain structural information of semicrystalline materials [55,56,57,58,59]. This process uses iteratively the Rietveld decomposition formula for whole powder pattern decomposition (WPPD) in Fullprof program package [23]. A fair approximation to the observed integrated intensity can be made by separating the peaks according to the calculated values of the integrated intensities:

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{

I K ( obs ) = ∑ w j .k ⋅ S K2 ( calc) ⋅ y j ( obs ) / yi ( calc) j

}

Eq. 4

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where wj.k is a measure of the contribution of the Bragg peak at position 2θk to the diffraction profile yj at position 2θj. The sum is the overall yj(obs), which can theoretically contribute to the integrated

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intensity Ik(obs). Le Bail method allows to refine the unit cell parameters, and what would be the effectively measured Sk(calc) intensities are only the intensities suggested by the profile fitting based on a distribution of intensities using a profile function for each overlapped reflection constituting the very broad peaks. These are due to the low crystallinity and small crystallite size. Thus, this method allows obtaining very reasonable unit cell parameters and an estimative of crystallite size and shape. The most widely used method to determine the size of the coherent diffraction domains is based on the integral breadth of line profiles, from which the apparent size can be obtained [4]. Anisotropic

ACCEPTED MANUSCRIPT 11 size broadening can be written as a linear combination of spherical harmonics (SHP) and it is supposed that anisotropic size contributes only to the Lorentzian component of the total Voigt function. The explicit formula of the intrinsic integral breadth using the SPH treatment of size broadening is given

βh =

λ Dh cos θ

=

λ cos θ

∑a lmp

lmp

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by:

ylmp (Θ h Φ h )

Eq. 5

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where β h is the size contribution to the integral breadth of reflection (hkl), y lmp (Θ h Φ h ) are real

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spherical harmonics (arguments Θ h and Φ h are the polar angles of the vector h with respect to a Cartesian crystallographic frame), [4] and a lmp are refinable coefficients, depending on the Laue class. After refinement of the coefficients a lmp , the program calculates the apparent size (Dh, in Å) along each reciprocal lattice vector if the parameters (U,V,W)instr. are fetched to the program from an external

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Instrumental Resolution Function file. It was possible to visualize the crystallites in the directions [100], [010] and [001] using the GFourier Program [60]. Structural refinement of unheated and heated PANI started using as initial parameters those

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regarding the end-capped aniline tetramer reported by Evain et al. [30]. Some studies have reported the peak located at about 2θ = 6° [20,25,61] in the XRD pattern of unheated ES-PANI, as showed in Fig.2.

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This peak is strongly dependent on the molecular size of the respective counter ion used to induce crystallinity and electrical conductivity in polyaniline. For this reason, the unit cell volume need to be increased for accommodation of the counter ion in the polymer structure. A good fitting was obtained through Le Bail method by introducing a new "b" unit cell parameter value around 17.83 Å. For heated samples, the refined parameters suggested two sets of values: there are structural similarities among the samples TT50 and TT100, and between the samples TT200 and TT300. Table 1 shows the refined parameters for unheated and heated PANI. The observed (Iobs) and calculated (Icalc)

ACCEPTED MANUSCRIPT 12 diffractograms as well as the residual lines (Iobs – Icalc) are shown in Fig.7(a-e). For TT50 and TT100 refinement, one of the main variations was the value of the "c" parameter, which has become smaller in about 4 Å. This value is similar to the length of an aniline monomer. Supposing the chains lying along

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this direction in unit cell, we suggest that the heat treatment allowed the chains decomposition from tetrameric to trimeric-folded chains. Based on the phenyl-end-capped tetramer of aniline [30], Fig.8 shows the speculative model of the unit cell for (a) unheated ES-PANI, (b) TT50 and TT100, and (c)

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TT200 and TT300.

As observed in the XRD patterns, the increase of the non-crystalline regions due to heat

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treatment influenced the crystallites average size. A decrease of approximately 40%, from 36 (unheated form) to 22 Å (TT100) was observed. A decrease in the crystallite size anisotropy also was verified. The crystallite shape obtained by GFourier [60] program proposes even more oblate shapes for TT50 and TT100 when compared to the unheated PANI.

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The second set is constituted for TT200 and TT300. The elimination of the counter ion was observed by the disappearance of the peak located in the 2θ = 6.3° and thus the value of "b" parameter was decreased to its original value. A decrease in "c" parameter around 4.5 Å was also observed,

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suggesting the progressive decomposition of chains, but now from a trimeric to dimeric-folded chain. Average crystallite size is almost equivalent.

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Thus, the complete heat treatment, from 50 to 300 °C, has caused a decreasing of approximately 60% in the average crystallite size. The refined average crystallite size projections for unheated and heated PANI are showed in Fig.9. Unheated PANI showed average crystallite size of 36 Å with anisotropy of 10 Å. The crystallite

shape can be described as a prolate ellipsoidal shape with its longer axis roughly parallel to [110] [25]. There is a smaller apparent size of 25 Å in the [001] direction and an equivalent value of 43 Å along [010] and [100]. After complete the heat treatment, TT300 presented average crystallite size of 16 Å

ACCEPTED MANUSCRIPT 13 with anisotropy of 3 Å. The crystallite shape can be described as an oblate shape. There is a smaller apparent size of 18 Å in the [001] direction and almost equivalent along [010] and [100], respectively of 12 and 18 Å. Thus, Le Bail method allowed an assessment of the PANI molecular structure, and the

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evaluation of crystallite average size and shape.

3.4. SAXS analysis

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The calculation of the pair-distance distribution function p(r) is a key mechanism for the observation of any change in particle size and shape during heat treatment, as well as to obtain the radii

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of gyration (Rg). Fig.10 shows the p(r) curves obtained from the scattering curves using the GNOM program for unheated and heated samples. This analysis was made based on the p(r) shapes and in their Dmax/2 position [62].

The p(r) curve for unheated ES-PANI suggests prolate shape particles. However, the Dmax/2

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positions has moved to increasingly smaller values of r. For this reason, these curves suggest particles tending to oblate form over the heat treatment. Table 2 shows the Dmax values for unheated and heated samples. There is a more significant change in the Dmax value for ES-PANI and TT50, in the first stage

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of heating, representing an average reduction of 23% in diameter particle. For TT100 and TT200 a decrease of 10% was observed. This decrease in Dmax values is directly related to the particle size.

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Thus, particles are decreasing after heat treatment, which is in agreement with the XRD and Le Bail analysis. The Rg values obtained from p(r) for heated and unheated samples are also shown in Table 2. These values are in agreement with those obtained for Dmax values. These results suggest a decreasing in average particle diameter over heat treatment. Once the particle size and shape are predicted by observing the shape of the p(r) curves and Dmax values, a three-dimensional display was performed using a suitable computer program. To propose

these models, a fitting of the scattering curves using the DAMMIN program was performed to obtain a

ACCEPTED MANUSCRIPT 14 model that minimizes the discrepancy between the theoretical and experimental curves [63]. These proposed models were visualized in the MASSHA program [64,65]. Fig.11 shows the template for the particle shape of unheated and heated samples. Through visualization of particle shapes, it is possible

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to note that they are consistent with those results provided by the p(r) curves [62,66]. Thus, particles tend to the prolate form over heat treatment, losing the original oblate particle shape of ES-PANI.

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3.5. DC Electrical Conductivity Measurements

The electrical conductivity of protonated PANI decreases during thermal treatment at elevated

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temperatures. The conductivity stability can be related to intrinsic factors, such as the macromolecular structure, the morphology, and the type of acid used for protonation, and to extrinsic factors, such as the surrounding atmosphere and temperature. The changes at the molecular level are the main process for controlling electrical conductivity [52].

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One of the difficulties associated with the processing of conjugated polymers is the poor solubility in common, volatile organic solvents. EB-PANI is commonly processed by dissolving the polymer in N-methylpyrrolidone (NMP) or m-cresol, both high boiling point solvents, resulting in cast

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films containing a non-negligible amount of residual solvent, often associated with residual water from the polymerization reaction. Thus, these results usually contain a considerable amount of NMP [67].

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The effect of residual solvent on the thermal properties of PANI has been reported by several authors [68,69]. Residual solvent also affects electrical conductivity [70] and degree of crystallization and crystalline structure [71] apart from mechanical and thermal properties. Several studies have been devoted to study the electrical conductivity of PANI in pellets both in protonated and non-protonated forms [72,73,74,75]. These studies are devoted to obtain the resistivity values and then reaching the electrical conductivity values. All studies have indicated that protonated PANI has its electrical conductivity slowly decreased when treated at temperatures below 200°C. At

ACCEPTED MANUSCRIPT 15 above temperatures the electrical conductivity decreases very rapidly and the material eventually becomes an insulator [76,77]. These studies indicate that electrical conductivity decay is associated with different chemical degradation mechanisms [14]. During this chemical process the conducting

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protonated PANI is converted into a non-conducting PANI base by the loss of molecules of acid. Complete deprotonation corresponds to a reduction in the conductivity, chain degradation and aromatic-ring substitution.

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Our results showed a decreasing in electrical conductivity verified after heat treatment as observed by the changes in the molecular structure of PANI (as described by FTIR spectra and Le Bail

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method). During the heat treatment, the crystalline regions in the unheated PANI-ES decreased gradually due to the loss of sequences of emeraldine and removal of chloride ions. The more the polymer chains are ordered, the easier the counter ions have to move along the chains in the presence of an external electric field [78].

The electric conductivity of PANI-ES and TT50, TT100 and TT200 are shown in Table 3. The

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electrical conductivity values decreased with increasing temperature in about 90%, from 0.3 x 10-4 to 0.4 x 10-5. The electrical conductivity value for the TT300 was not obtained due to the difficult to pellet

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the sample. Sanches et al. [43] showed the TG curve of ES-PANI and reported the removal of all internal water of hydration below 300 °C, so this sample does not have the minimum of needed

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moisture to pellet. Sambhu et al. [79] have reported this decrease in electrical conductivity of PANI, obtaining values with magnitude order of 10-3. Sanches et al. [25] have reported values with magnitude order of 10-4. Electrical conductivity values of the same order of magnitude were found in scientific literature [80,81,82,83].

ACCEPTED MANUSCRIPT 16 3.6. SEM analysis

SEM technique was used to observe the morphologies of unheated and heated PANI, as showed in Fig.12(a-e). Well-defined nanofibers are observed in unheated ES-PANI (Fig.12a). Nanofibers seem

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to be formed by interconnected nanospheres, which size cannot be measured precisely. It can be seen that the nanofiber sizes decreased progressively over heat treatment. Thus, the TT300 sample no longer presents well-defined nanofibers. Through evaluation of such morphologies, it is possible to suggest

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that the increase of temperature have caused the partial degradation of chains and consequently the nanofibers degradation, leading to decrease of crystallinity and electrical conductivity. Electrically

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conductive PANI-ES has been depicted as a heterogeneous structure in which highly conducting crystalline "islands" are separated by less conducting, non-crystalline parts [84,85]. The degradation of conductivity has been explained in terms of the size of these conducting "islands" [86], and directly

Conclusions

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related to the decrease in chain sizes an in conductivity.

ES-PANI was synthesized by oxidative polymerization in the presence of hydrochloric acid

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(HCl) and successfully subjected to heat treatment. Information obtained by XRD showed that the crystallinity decreased over heat treatment of ES-PANI. Le Bail method allowed the obtaining of the

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cell parameters and crystallite size and shape. ES-PANI unit cell volume was increased for accommodation of the counter ions derived from the protonic acid chloride to the polymer backbone. For TT200 and TT300, the cell volume was decreased due to the chain decomposition. FTIR spectra were important for structural characterization and also showed the crosslinking reaction at 200 °C, however, without increasing in crystallinity. SAXS technique indicated that the heated samples have their particle shapes modified by temperature, which lost their globular shape. SEM images showed that the morphology of PANI-ES presented nanofiber composed of nanospheres interconnected and after heat treatment, samples showed no more nanofiber morphology, a result that may be associated

ACCEPTED MANUSCRIPT 17 with the loss of crystallinity and electrical conductivity. Electrical conductivity measurements showed that the heat treatment caused a decrease about 90% in their values due to the proposed chemical degradation mechanisms. Thus, this work is expected to provide a systematic structural and

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morphological data of heated and unheated PANI and may contribute to the scientific community in the area of semi-crystalline materials characterization.

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Acknowledgments

Authors are grateful to the National Synchrotron Light Laboratory (LNLS) - Campinas/Brazil for the

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SAXS measurements and to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (Processo 308169/2014-0) for the financial support.

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FIGURE CAPTIONS

Fig.1. PANI structure and schematic representation of protonic doping process. Fig.2. XRD patterns of unheated and heated forms of PANI powder.

Fig.4. Crystallinity evolution for unheated and heated PANI. Fig.5. FTIR spectra of unheated and heated PANI.

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Fig.3. Peak deconvolution of XRD patterns.

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Fig.6. Crosslinking reaction of PANI adapted from Luo et al.[42] Fig.7. Le Bail method applied to unheated and heated PANI.

Fig.8. Speculative model of the unit cell for (a) unheated ES-PANI, (b) TT50 and TT100, and (c)

TT200 and TT300 based on the phenyl-end-capped tetramer of aniline.

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Fig.9. Crystallite size projections for unheated and heated PANI.

Fig.10. Pair-distance distribution function p(r) of unheated and heated PANI. Fig.11. Particle shape of unheated and heated PANI.

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Fig.12. SEM images of unheated and heated PANI.

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TABLE CAPTIONS

Table 1: Le Bail Method for unannealed and annealed PANI using Fullprof program: cell parameters,

cell volume, average size and anisotropy, crystallite apparent size and agreement factors. Table 2: Dmax and Rg for unheated and heated samples

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Table 3: Electrical conductivity for unheated and heated samples

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Table 1: Le Bail Method for unheated and heated PANI using Fullprof program: cell parameters, cell volume, average size and anisotropy,

a (Å)

5.7328

5.7180

b (Å)

8.8866

17.8344

c (Å)

22.6889

23.0154

α (º)

82.7481

83.3509

β (º)

84.5281

γ (º)

88.4739

V (Å3)

1141.3

Global Average Size (anisotr.) (Å)

--------

Crystallite Apparent size 100 (Å)

--------

Crystallite Apparent size 010 (Å) Crystallite Apparent size 001 (Å) Rwp (%) Rp (%)

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Unheated ES-PANI

PANI heated at 100 °C

PANI heated at 200 °C

PANI heated at 300 °C

5.72402

5.80363

5.79667

5.59972

18.47208

18.87048

8.83773

8.95020

19.04559

19.09448

14.38766

14.57734

79.52648

79.58224

82.45613

83.98209

84.8781

83.53752

83.84959

85.45853

85.13715

88.2333

88.18578

87.60662

87.55589

89.04167

2321.3

1967.513

2044.331

728.010

723.932

36 (10)

22 (5)

22 (4)

19 (4)

16 (3)

43

24

25

19

18

--------

43

21

19

12

12

--------

25

20

20

20

18

11.3

1.69

2.23

2.42

8.18

2.20

4.9

1.34

1.52

1.67

5.55

1.51

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PANI heated at 50 °C

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Initial Parameters

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Refined Parameters

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crystallite apparent size and agreement factors

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Dmax (Å) 650 ± 12 500 ± 12 500 ± 12 450 ± 12 450 ± 12

Rg (Å) 217 ± 9 146 ± 9 146 ± 9 139 ± 9 139 ± 9

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Sample ES-PANI TT50 TT100 TT200 TT300

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Sample ES-PANI TT(50) TT(100) TT(200)

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Heat treatment has resulted in a progressive decrease of crystallinity, from 50 to

•

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22%. Le Bail method suggested decomposition from tetrameric to dimeric-folded chains, accompanied by a decrease in the crystallite anisotropy and average size. The pair-distance distribution function (p(r)) curves suggested particles tending

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Electrical conductivity showed a decreasing of about 90% due to the loss of

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emeraldine sequences and to the removal of chloride ions.