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Polycaprolactone-Polyaniline Blend: Effects of the Addition of Cysteine on the Structural and Molecular Properties Sergio Kogikoski Jr, Michelle da Silva Liberato, Irina Marinho Factori, Emerson Rodrigo da Silva, Cristiano Luis Pinto de Oliveira, Romulo A. Ando, and Wendel Andrade Alves J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10011 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 15, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Polycaprolactone-Polyaniline Blend: Effects of the Addition of Cysteine on the Structural and Molecular Properties Sergio Kogikoski Jr,a Michelle S. Liberato,a Irina M. Factori,a Emerson R. da Silva,b Cristiano L. P. Oliveira,c Rômulo A. Ando,d Wendel A. Alvesa*

a

Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Santo André

09210-580, Brazil. b

Departamento de Biofísica, Universidade Federal de São Paulo, São Paulo 04023-

062, Brazil. c

Instituto de Física, Universidade de São Paulo, São Paulo 05314-970, Brazil.

d

Instituto de Química, Universidade de São Paulo, C.P. 26077, 05513-970 São Paulo,

SP, Brazil. * [email protected]

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ABSTRACT: Organic molecule conjugates usually arrange themselves into complex multiphase systems that are sensitive to processing and local chemical environment. Moreover, these conjugates are able to self-assemble at different scales, varying from nano to macroscale. In addition, intermolecular interactions introduce structural variations in the molecular packing and large-scale ordering, which directly affects different physicochemical properties of the materials. Herein, we study the synthesis of polymer blends based on poly-(ε)-caprolactone (PCL) with polyaniline (PANI) doped with the amino acid N-Acetyl-L-Cysteine (NAC).Samples were prepared either through solution casting or via electrospinning methods. The materials were characterized regarding their morphological, structural and molecular properties at different length scales. From the results obtained, the relationship between changes in blend properties and different NAC concentrations was determined. Deep structural details have been unveiled by using different characterization techniques, including X-rays micro computed tomography (Micro-CT), small- and wide-angle scattering (SAXS and WAXS) and differential scanning calorimetry (DSC). Our findings indicated that NAC enhances

organization and

crystallinity index of the blends and that a close

relationship appears between the synthesis method and the internal micro-nanostructure. It is also shown that NAC modifies the molecular properties of PANI. Through spectroscopic techniques (UV-Vis and Resonant Raman), it was shown that NAC favors the formation of different forms of PANI. In addition, thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) showed the formation of a supramolecular structure maintained by sulfur-π (SH-π) intermolecular interactions between PANI and NAC.


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INTRODUCTION Phenomena related to the supramolecular organization of colloidal systems have attracted much attention from the materials chemistry community in recent decades.1 The fine control of intermolecular interactions (hydrogen bonding, van der Waals, πinteractions, etc.) and their repercussions on the spatial organization of these systems are central issues because they are crucial for the proper design of shapes, orientation, surface and electronic properties, among other characteristics, that ultimately drive the macroscale behavior of materials. The addition of guest compounds to colloidal matrixes also opens a broad horizon for developing new materials because the interplay between these molecules and the host material is capable of generating unique characteristics that are not observed in the individual counterparts.2 The development of new scaffolds for biomedical applications is envisaged based on the inclusion of biomolecular groups into soft matter platforms.3, 4 Recently, our group investigated the effects of incorporating peptide-based nanostructures into poly-(e)-caprolactone matrices that were intended for drug delivery and demonstrated a strong brittle-to-ductile transition, which improves both the release capabilities and biodegradability properties of the composites.5 Additionally, we have observed the interplay between poly(allylamine hydrochloride) and L,L-diphenylalanine nanostructures that leads to the decrease of the band gap, which results in a smaller charge transfer resistance of peptide assemblies;6 we have also observed the interaction between L,L-diphenylalanine and polyfluorenes, which enables the production of biodegradable organic light-emitting diodes.7, 8 Although much effort has been made in these studies, there is still a lack of detailed information on the 3D structure of scaffolds designed from mixtures of polymers matrixes. Specifically, these polymer mixtures 3 ACS Paragon Plus Environment


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enable matrixes with different spatial organizations to be obtained depending on a variety of parameters ranging from the composition to preparation procedure.9 In this work, we contribute to this research by investigating both the structural behavior and electronic properties of polymer blends made with poly-(e)-caprolactone (PCL) and polyaniline (PANI) doped with N-Acetyl-L-Cysteine (NAC). PANI is one of the most studied conductive polymers. Due to the ease of synthesis, it has been widely used in a range of applications, from electronics10 to tissue engineering.11,

12

PANI can be prepared with different morphologies and nanoscopic

shapes, including nanotubes,13 nanofibers,14 nanoparticles,15 etc., by using different synthesis approaches. One of the interesting properties of PANI is that its conductivity is dependent on the acid moiety that protonates the polymeric chain, with HCl and H2SO4 being the most commonly used acids.16 In addition, many other dopants have been combined with PANI to determine the conducting properties, such as sulphonic acids,17 porphyrins,18 and amino acids.13 However, many of these dopants are not suitable for biomedical applications due to their strong acidity, which can leak out of the polymeric matrix and acidify the local environment, thereby causing some toxicity.11 Herein, we describe the doping process of PANI with N-Acetyl-L-Cysteine (NAC), which is a modified amino acid that can act as a proton donor with lower toxicity compared to strong acids. The spatial organization of different structures formed between PANI and other non-conductive polymers is still not completely understood. On the other hand, PCL is a well-known polymer commonly used in many applications, especially for producing biocompatible materials. It is suitable for such applications due to its good mechanical properties, biodegradability and strong affinity with biological tissues.19 The spatial organization of pure PCL has been studied for controlled nanocylinders that are 4 ACS Paragon Plus Environment

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prepared by the casting crystallization method

20

and nanofibers that are produced via

electrospinning processes.21 A hybrid material formed between PANI doped with camphorsulphonic acid and PCL using electrospinning has previously been reported.22, 23

The authors studied the effect of the PANI concentration on the conductivity,

diameter of the fibers, and alignment of the fibers in the cellular growth of fibroblasts, but the molecular characteristics of the formed material and the nanoscopic structures of the matrixes were not well established. In this study, we report the characterization of the supramolecular assembly formed between PCL and PANI doped with NAC, which is prepared by solution casting and electrospinning methods. In order to obtain information about the mesoscopic structure, degree of crystallization and internal organization of the polymer mixture at different scales, the materials obtained were extensively characterized using SEM, SAXS, WAXS, micro computed X-ray tomography (micro-CT), and DSC techniques. In addition, a wide range of spectroscopic techniques, including UV-Vis, Raman, XPS and TGA, was used to study the interaction between PANI and NAC. Our results indicate the formation of sulfur-π (S-π) interactions, providing new insight into the interplay between PANI and NAC and illuminating the interactions involved in the stabilization of other supramolecular structures, such as proteins and enzymes. EXPERIMENTAL METHODS Materials Poly-(ε)-caprolactone with an average MW between 70000 and 90000, a polyaniline emeraldine base with an average MW of ~65000, and N-Acetyl-L-Cysteine were purchased from Sigma-Aldrich (US). Chloroform and methanol were purchased



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from Synth (Brazil). All chemicals were of the highest purity and used without further purification. All the obtained data were analyzed using the OriginPro® 2016. Preparation of PCL/PANI Blends The mixtures were prepared in solution. First, 100 mg of PANI was added into 15 mL of chloroform and the mixture was sonicated using a probe sonicator for 10 minutes. After that, 1.6 g of PCL was added into the solution and the mixture was magnetically stirred for 24 hours to complete mixing of the two polymers. During that time, NAC was solubilized in 5 mL of methanol. In this study, we used six different molar ratios of NAC, which depended of the amount of polyaniline used. We considered that each monomer contributes one nitrogen atom that could be doped (the molecular mass of the monomer used was 93.15 g mol-1); therefore, 100 mg of PANI had ~1x10-3 mol of nitrogen, and when used in a molar ratio of 100%, it was added to 1x10-3 mol of NAC in solution. After the mixing of the PCL with PANI was completed, the NAC in methanol solution was added, and the color of the solution instantly changed from dark blue to dark green. The solution was stirred for 24 h prior to its usage. Preparation of the casting and electrospun matrixes From the 20 mL polymer blend prepared, 15 mL was used for electrospinning, and the remaining 5 mL was placed inside a 5 mL petri dish and covered to allow the slow evaporation of the solvent for 24 hours. We used a setup that was previously described; 5 the solution was transferred to 15 mL syringes with 15G needles. Syringes were placed in a vertical position and dripping flow was obtained due to gravity. Square


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15 x 15 cm2 plates, covered with aluminum foils, were positioned 15 cm from the needle tip, and a voltage of 22 kV was applied. The collection time was approximately 2 hours. SEM, Micro-CT and XPS measurements Scanning electron microscopy images of the hybrid materials were obtained on a high-resolution JSM 6330F instrument (SEM-FEG) at the Center for Research in Energy and Materials (CNPEM, Campinas, Brazil). Secondary back-scattered electrons were collected from Au-coated samples impinged by electron beams with an energy of 5kV. X-ray micro-CT measurements were performed using a Bruker SkyScan 1272 instrument (placed in CNPEM, Campinas, Brazil). The X-ray source was operated at a voltage of 20 kV and a current of 175 µA. Data were recorded using a 2D X-ray detector with 16 megapixels, with a pixel size of 3.37 µm. The dimensions of each sample were approximately 8.5 x 0.2 x 0.15 mm3, and the samples were positioned along the vertical axis of a goniometer and rotated at angular steps of 0.4°. In this configuration, a z-resolution of approximately 400 nm was achieved and 450 images were acquired to cover an angular range of 180°. Image treatment and 3D reconstructions were performed using the manufacturer’s DataViewer and CTVox software. The porosity and density histograms were obtained using the CTAn software, with a selected voxel coefficient. X-ray photoelectron spectroscopy was conducted using the obtained polymeric matrixes. The superficial analysis was performed using a K-Alpha XPS, Thermo Fisher Scientific (placed in CNPEM, Campinas, Brazil) with an Al Kα emission, the applied vacuum was 0.8 nm-1, the behavior is also characterized by a linear descent, which carries information about the general shape of nanometer-sized scattering centers. The length scale probed at this region is below ~8 nm, and it is related to local structure of the blends, in contrast to data from the low q-range.

Figure 5. SAXS profiles from samples containing different NAC ratios prepared by the casting or electrospinning technique. Solid red lines (color online) are least-square fits using the function shown in Equation 1. Data were shifted for better visualization.

The scattering profiles shown in Figure 5 exhibit noticeable changes upon the addition of NAC, and they indicate that the preparation method strongly affects the structural organization of the polymeric matrix. Unfortunately, the multilevel organization found across the data, with different arrangements and the appearance of a strong correlation peak in the intermediate q-range, makes it too complex to devise a detailed model to provide full-range fitting of the scattering curves obtained in this work. Therefore, we have adopted a shape-independent approach by using an empirical 21 ACS Paragon Plus Environment


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model, which enables the extraction of quantitative information associated with the general features of the curves. Therefore, we have fitted the curves using the following functional formula to perform quantitative analyses of SAXS data:

"

$ ' + + + , + '-. 1 % " 1 + |" − " |* "

Equation 1 is a summation of empirical models that are widely applied for describing small-angle scattering from soft matter systems.29,

32-34

A, B and C are

multiplying factors used to adjust the contribution of each term to the final intensity profile. A constant background, Bkg, is added to the model to account for incoherent scattering. The first and third terms are scaling laws used to fit the initial and final parts of the q-range, and valuable information that arises from them is embodied in the Porod exponents, α and β, from which the dimensionality of scattering centers can be ascertained.33,

34

The second term accounts for the broad peak observed at the

intermediate q-range. The m exponent is left free during the fitting process, when it is equal to 2, the peak assumes the usual form of Lorentzian functions. The q0 parameter is the peak position and ξ is a correlation length associated with the local structure. In Table 2, we show the best fitting parameters obtained from least-square fits conducted with Equation 1 and implemented in the SASFit program.35 Changes noticed in the outline of the curves upon the addition of NAC or the use of different preparation techniques are now quantitatively described. At the low-q region, one observes that the Porod exponents α tends to be smaller in samples prepared by casting deposition than in membranes obtained by electrospinning. In particular, except for the formulation containing 25% NAC, the samples prepared by casting show low-q slopes that are lower than those observed in electrospun membranes. The numerical values of these


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exponents indicate that structures at these length scales exhibit fractal-like features, which is consistent with clustering at dimensions of tens of nanometers.33 Specifically, α values less than 3 are an indication of mass fractal scatterers in the medium, a behavior that is usually attributed to the formation of entangled networks.36 On the other hand, α values closer to 4 indicate the presence of surface fractals,29, 34 which points to the formation of more compact structures with smooth fractal surfaces. This latter behavior is clearly observed in membranes prepared by electrospinning, and it could indicate that applying electric fields would promote the stretching of polymer chains and result in a better accommodation of the blends at the nanoscopic scale. It should be noted that compactness at the nanoscale is related to the inner part of clusters and fibers, whereas the porosity revealed by micro-CT is associated with the macroscale arrangement of clusters and fibers. Therefore, the internal structure of fibers can be more compact than clusters without resulting in a macroscopic arrangement of fibers that are more compact than an arrangement of clusters.

Table 2. Best fitting parameters obtained with Equation 1. The direct-space distance associate with the correlation peak q0 that is calculated using the Bragg law, d = 2π/q0. NAC % 0 10 25 50 75 100 0 10 25 50 75 100

q0 (nm-1) d (nm) ξ (nm) β CASTING 2.8 0.395 15.9 10.3 0.16 3.3 0.389 16.2 10.3 0.17 3.75 0.397 15.8 10.5 0.08 2.75 0.433 14.5 10.2 0.07 2.7 0.453 13.9 10.3 0.003 2.63 0.472 13.3 11.1 0.017 ELECTROSPINNING 3.84 0.394 15.9 9.6 0.19 3.87 0.391 16.1 10.0 0.24 3.71 0.433 14.5 12.0 0.43 3.8 0.457 13.7 11.1 0.21 3.9 0.486 12.9 11.9 0.16 3.83 0.499 12.6 11.2 0.097 α



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At the intermediate range, the peak position exhibits an appreciable shift upon the addition of NAC to the polymer blend. The peaks move from q0 ~ 0.39 nm-1 in formulations without NAC to q0 ~ 0.5 nm-1 in the sample with 100% NAC. These shifts reveal that the mean distance associated with the correlation peaks decrease from d ~16 nm to d ~ 13 nm, which could indicate that NAC promotes condensation of the polymer matrix at the nanoscopic level. The same behavior is found to be independent of the preparation method for the samples, which indicates that phenomena involved in condensation have a chemical origin. The same behavior was observed earlier in the micro-CT, WAXS and DSC data. It was observed that the organization or the crystallinity index of the blends was enhanced due to the presence of NAC. Therefore, we believe that NAC is present in the crystalline part of the matrix since its addition caused an increase in the crystalline index and a decrease in the distance related to the non-crystalline part of PCL. The correlation lengths associated with the peaks are around ξ~ 10.5 nm in the casting membranes and ξ~ 11 nm in the electrospun matrices, and apparently, neither the NAC content nor the preparation strategy affects the behavior of the correlation lengths. In addition, the low values of x (even smaller than the average separation) show that scattering domains in the inner part of the blends are not highly correlated, which is consistent with fractal and clustering features. At high-q values, structural information is provided by the β parameter in the empirical formula shown in Equation 1. Similar to data observed in the low-q region, the value of the Porod exponent at this range provides information on the general shapes of scattering structures. A Porod exponent β= 0 is associated with spherical shapes,


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whereas β= 1 is related to the presence of cylinder structures.37 Intermediate values indicate the presence of elongated shapes with an ellipsoidal geometry; therefore, inspecting Table 2, one determines that the local structure of our polymer matrices is characterized by elongated scattering centers and, as a general trend, those prepared by the electrospinning technique exhibit higher anisotropy compared to those obtained by casting. An additional feature that appears in SAXS data from samples prepared by casting deposition, Figure 5A, is a diffuse oscillation around q ~0.1 nm-1, which is ascribed to lower polydispersity of sizes in the scattering structures.38 This higher homogeneity observed in casted membranes may be related to a slow solvent drying process, which likely leads to more regular nano-sized clusters. On the other hand, fast evaporation during electrospinning could induce higher size dispersity at the nanoscale, whereas the stretching induced by an oriented electric field promotes elongation of the inner structure of the fibers. Molecular and Electronic Aspects UV-Vis electronic spectroscopy is a versatile technique used to study PANI doping and to observe the contribution of different species in the molecular structure. The structure of PANI undergoes oxidation and reduction processes that change the electronic absorption of the materials. The majority of the reaction occurs by proton and electron transfer between the nitrogen atoms of the structure and external agents, which are usually acids that donate protons to the PANI chains. The absorption occurs in two components of the molecule: UV absorption by the aromatic rings and Vis-NIR absorption by the conjugated double bonds of the structure. The characteristic absorption bands for each PANI species are well characterized in the literature.16



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Initially, we studied the reaction between PANI and NAC in the casting materials, as shown in Figure 6A. Prior to the reaction with NAC, the spectrum shows only the contribution of the non-conductive form of PANI emeraldine base (PANI-EB), with the bands centered at 310 and 600 nm. Upon adding 10% of NAC, the band at 349 shifts towards 350 nm, and the formation of polarons is observed by the continuous band starting at 500 nm and with a maximum at 860 nm, indicating the protonation of the PANI chain that forms emeraldine salt (PANI-ES). However, the presence of a shoulder at 620 nm indicates that PANI-EB still exists. Upon the addition of 25% of NAC, the bands assigned to the aromatic rings shift to 360 nm, the polaronic absorption remains at the same position, but the shoulder assigned to the PANI-EB disappears. By increasing the concentration of NAC to 50, 75 and 100%, the band at 360 nm shifts to 380 nm, and the polaron absorption red-shifts from 800 to 1000 nm. An interesting thing occurs with the formation of a band at approximately 490 nm in the 50% NAC spectrum; this can be attributed to the bipolaronic structure of pernigraniline base (PANI-PB), which forms a structure called a pernigraniline salt (PANI-PS).39, 40 When the NAC concentration increases, this band shifts towards 550 nm, which is characteristic of PANI-PB. For the electrospun materials, the results are less expressive, as observed in Figure 6B. Without NAC, the two bands, characteristic of PANI-EB, appear at 350 and 650 nm. However, upon the addition of NAC, the spectrum shifts and remains unchanged until excess NAC is added, at which point the three characteristic bands of PANI-ES appear at 360, 420 and 820 nm. These results show that the force applied during the electrospinning process changes the PANI structure. A direct molecular correlation between the casting and electrospinning process was expected, however, this is not the observed result. The application of potential during the electrospinning results in the 26 ACS Paragon Plus Environment

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same final structure for PANI with the doping agent. The only distinct behavior is observed in the spectrum of the matrix containing 50% of PANI, which exhibits a high intensity 820 nm band (however, this band is narrower band), indicating that the polymer networks are highly localized within the polymeric structure.



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Figure 6. UV-Vis spectra for the A) casting solutions, and for the B) electrospun matrixes. C) Proposed doping mechanism of PANI by increasing the concentration of the added NAC.

From the UV-Vis results of the casting materials, we can propose a mechanism of the reaction, as shown in Figure 6C. Starting with the PANI-EB, upon the addition of NAC (10, 25 and 50%), two protons are donated, and after the internal redox reaction, the PANI-ES form is obtained. After adding NAC in excess (50 and 75%), NAC is probably oxidized by capturing two electrons, and it subsequently forms a mixture of PANI-ES and PANI-PS. After 100% of NAC is added, two more protons are captured by NAC, forming a mixture of PANI-PB and PANI-ES. To determine if the process observed by UV-Vis is plausible, we performed resonance Raman experiments to observe the different structures of PANI. Resonance Raman spectroscopy is one of the most important techniques for characterizing PANI structures; since changing the incident radiation the bands assigned to vibrations of different forms of PANI are selectively enhanced, and therefore probe the presence of different structures within the polymer chain can be probed. Herein, we used three different wavelengths: 532, 633 and 785 nm. From the UV-Vis results, the presence of shoulders in the spectra with increased NAC concentrations was observed, and the use of resonance Raman can provide information about the molecules that absorb within that specific wavelength. The first wavelength, 532 nm, should give primary information about the pernigraniline base and salt that was observed, as shown in Figure 7A. The 633 nm wavelength is used to obtain information about the emeraldine forms, where the bands of characteristics vibrational modes of the aromatic rings are enhanced, as observed in Figure 7B. The 785 nm wavelength was used to


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obtain information about the charge carriers of the structures, polarons and bipolarons, as shown in Figure 7C.

Figure 7. Resonance Raman spectra of the casting matrixes for different excitation wavelengths A) 532 nm, B) 632.8 nm and C) 785 nm. The spectra names are given as a function of the NAC concentration (a) 0%, (b) 10%, (c) 25%, (d) 50%, (e) 75% and (f) 100%. Intensity relationship of bands obtained from the Raman data in each wavelength D) 532 nm, E) 632.8 nm and F) 785 nm.



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The presented results show that the spectra vary considerably with the addition of NAC. Initially, in all three experiments without the addition of NAC, the characteristics bands of PANI-EB are observed and the bands are positioned at 1160 (β C-H of benzenoid rings), 1220 (υ C-N of benzenoid rings), ~1480 (υ C=N) and ~1590 (υ C-C of quinoid rings) cm-1.41-44 This is a good proof that the experiments started with the PANI-EB form, and it confirms the UV-Vis results. At 532 nm, by increasing the concentration of NAC, an increase of the band at 1604 cm-1 is first observed, which is characteristic of υ C-C of the semi-quinoid rings present in PANI-ES. This band is broadened for 50 and 75% (spectra d and e) of NAC, which is probably due to the formation of charged pernigraniline. The presence of the band at 1320 cm-1 is attributed to the formation of charged species in the polymer chain, with its highest intensity for 25, 50 and 75% of NAC, which indicates the formation of the PANI-PS species and corroborates the UV-Vis spectrum again. The presence and relative population of the protonated species in the material can be observed by the relationship between the intensities of two different pairs of bands: the doublet at 1170 (non-protonated ring β CH) and 1190 (polaron, β C-H) cm-1, and the doublet at 1490 (non-protonated υ C=N) and 1510 (β N-H of polarons) cm-1.44, 45 The intensity ratios of the 1170/1190 cm-1 and 1490/1510 cm-1 bands are shown in Figure 7D. It is observed that initially (10 and 25%), the area is dominated by the 1170 and 1490 cm-1 bands, which indicates the formation of non-protonated pernigraniline species, however, increasing the concentration of NAC causes an increase in the intensity of the bands at 1190 and 1510 cm-1, which is due to the formation of the protonated pernigraniline species, PANI-PS. We observe that the maximum conversion to PANI-PS occurs with 50% of NAC, and it decreases with an additional increase in the concentration of NAC, which is attributed to


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the formation of PANI-PB. These results corroborates with the proposed mechanism of Figure 6C. Using the 633 nm wavelength, the bands assigned to modes of the bipolaronic species in the emeraldine salt are enhanced. We can observe the presence of the 1604 cm-1 band that is characteristic of the protonated form of PANI in all the cases studied. For 10% of NAC, there is a convolution of many bands that show the presence of a mixture of both PANI-EB and PANI-ES but is still dominated by the PANI-EB form. After adding NAC, the doublet at 1170 and 1190 cm-1 appears and the 1490 and 1510 cm-1 doublet peak is resolved, which indicates the formation of polarons in the structure. From a 10% concentration of NAC, the 1335 cm-1 band emerges, which is also formed due to the presence of polarons. The intensity ratios of the 1490/1550 cm-1 and 1490/1190 cm-1 bands are presented in Figure 7E. Initially, the ratio of the 1490/1510 cm-1 bands is high, ~3.5, and adding NAC into the system decreases its value below 1, which indicates that the polarons are present in higher amount than the bipolarons and non-protonated species; however, at an NAC concentration of 100%, this ratio starts to increase again likely due to the formation of PANI-PB. The ratio of the 1490/1190 cm-1 bands shows other forms of the relationship between polarons and bipolarons. At a 50% concentration of NAC, the maximum amount of polarons formed, which indicates the formation of other forms of PANI, probably PANI-PS, as indicated by the previous result. The 785 nm wavelength is in resonance with the polaronic absorption, so the bands assigned to polaron vibrations are all enhanced. Such enhancement is observed in the doublet band at 1325 and 1370 cm-1, which is characteristic of the polaronic segments. However, for two concentrations, 10 and 50%, the band at 1370 cm-1 shifts towards a higher wavenumber of 1385 cm-1, and the shift can indicate the formation of 31 ACS Paragon Plus Environment


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bipolaronic segments. One can observe that the intensity of the doublet at 1150 and 1170 cm-1 is close to 1 in those cases, which indicates the formation of the two charged species. The same thing is observed in the intensity ratios presented in Figure 7F. A similar shift was observed for the 25% sample using 633 nm, where there is the appearance of the band at 1390 cm-1. These results show that a mixture of polarons and bipolarons is formed for small concentrations of NAC, a mixture of different PANI species is formed with a higher concentration of NAC, which is similar to the proposed mechanism of Figure 6C. However, some peculiar trends were observed from the UV-Vis and Raman results. First in the UV-Vis spectra, the bands at 420 nm, characteristic of the excitation of polarons to the aromatic ring orbitals of PANI, have a low intensity that is usually higher than the intensity of the band at 380 nm. In addition, the 1604 cm-1 band in the Raman experiments are shifted towards smaller wavenumber as it usually appears at approximately 1625 cm-1. We believe that this shift is due to another type of interaction between PANI and NAC. This interaction was studied using TGA, where the degradation rate of cysteine changed due to the interaction with the matrix, as presented in Figure 8A, and XPS using the data collected in the sulfur 2p region, as shown in Figure 8D. The thermal degradation of the matrix shows one degradation process for the matrix without NAC and two processes when NAC is present, with one related to the PCL. The PCL degradation occurs at ~400 °C, and it is indicated by a continuous weight loss and by the first derivative of the weight loss curve (Figure 8B). The second signal is attributed to the degradation of NAC; however, the expected temperature for the maximum weight loss of NAC is ~170 °C but the observed degradation of NAC in the experiments occurs at a higher temperature of approximately 215 °C. In addition, 32 ACS Paragon Plus Environment

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the first derivative shows that this process involves two species, one that is first degraded at 205 °C and the second one that is degraded at 225 °C (Figure 8C). Therefore, it is possible that a certain amount of NAC interacts strongly with PANI and increases the degradation temperature to higher temperatures, while another amount of NAC does not interact with the same strength. Those two species were observed again using XPS.

Figure 8. A) TGA weight loss due to thermal degradation of the casting matrixes. B) DTG plot of the TGA data presented in A. C) Zoom-in of the degradation region of NAC. D) XPS spectra of the casting matrixes containing different amounts of NAC, two different signals are observed, they increase due to increase in the concentration of NAC.



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XPS spectroscopy can be used to study intermolecular interactions or processes that involves the retro-donation of electronic density. The binding energy is strongly related to the interaction of the atoms with exterior electrons; therefore, the addition of electron density to external orbitals of the element will decrease the binding energy of the electron, while the donation of electrons to external molecules will increase the binding energy.46 The sulfur spectrum occurs at approximately 165 eV, and it is characterized by an S 2p3/2 doublet signal. From the spectra given in Figure 8D, there is the appearance of two doublet signals, one centered at 164 eV and a second one at approximately 168 eV. The first signal is characteristic of non-bound SH groups in cysteines, and it appears throughout the range of concentrations, which indicates that the cysteine is present in the matrix and is non-bound.47,48 However, the second doublet is not typical for thiolate compounds but is typical for sulfonated groups; however, we do not believe that this conversion is energetically favorable in the studied case.49 We believe that this second doublet is related to a possible electronic density donation of PANI to NAC by an intermolecular interaction of the SH-π type. In the literature, computational studies have shown that the charges in PANI are distributed through the polymer chain.50 The aromatic rings of PANI exhibit a maximum partial positive charge every four rings,51 in such a way that it can accommodate 2.5 molecules of NAC that interact directly with the charged aromatic ring for every decamer of PANI. We show that the maximum intensity of the doublet at ~168 eV is obtained for 25% of NAC with respect to PANI and that it decreases in intensity for 50% of NAC and disappears for 100% of NAC, which is likely due to the favorable interaction between NAC molecules, instead of the PANI-NAC interaction. We believe that using NAC to dope PANI promotes the formation of the SH-π


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interaction, as shown in Scheme 1, and because of this, many of the PANI experimental results were anomalous compared to results in the literature. From the obtained results, it is now possible to develop a model for the interaction between PANI and NAC. The model development starts with the absence of the absorption band at 420 nm in the UV-Vis spectra (Figure 6A): this band appears due to the presence of delocalized positive charges on the PANI chain.50, 51 This distribution of charges induces an electric dipole in the polymer chain. Therefore, we expect the formation of a partial positive charge that is localized on the aromatic rings of PANI and has a maximum intensity every four rings. On the other hand, the nitrogen is slightly negatively charged, and all nitrogen atoms in the PANI structure have a similar energy.

Scheme 1. Proposed model for the interaction between NAC and PANI. Two types of interactions are observed in the model, the proton donation of the nitrogen in the PANI chain to form the charged polaronic species and the delocalization of the positive charges to the aromatic ring with the coupling of the thiol group of cysteine, which forms the SH-π interaction.

The presence of the positive charge localized in the aromatic ring makes the system appropriate for interaction with the thiol group of cysteine. In biological systems, amino acids containing aromatic groups (principally phenylalanine and 35 ACS Paragon Plus Environment


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tryptophan) interact with a variety of different chemical structures, such as anions, cations, carboxylic groups, and thiols, and in all these cases, the interaction occurs due to the presence of opposite residual charges.52 In the case of the SH-π interactions, the energy is usually approximately 4 kcal mol-1, and the distance between the ring and the thiol is approximately 3.5 Å.53 The studied system is favorable to that type of interaction, and we believe that this type of interaction is feasible within the studied system. This interaction is therefore proposed as the model presented in Scheme 1. The interaction presented in Scheme 1 occurs many times compared to the dipoleinduced interactions; in other words, the thiol group is negatively charged, and it induces the formation of a positive charge density on the aromatic ring of PANI. Because of the protonic doping of PANI, delocalized positive charges already exist on the polymer chain and they can easily be trapped by the negative dipole of the thiol group. The proposed model corroborates the experimental results, and moreover, it provides new information about the properties of SH-π interactions, which are very difficult to recognize and usually require very sophisticated and relatively inaccessible techniques.54-56 CONCLUSION Based on the results, we can conclude that a PCL/PANI-NAC blend was obtained. SEM showed that the diameter of the electrospun fibers depends on the amount of NAC in the solution and that the diameter of the fiber decreases. Unique micro-CT results were obtained in this work. The technique was used for more than visualizing the samples in three-dimension; instead, it provided information about the porosity and variation in the ordering within the matrix because of better molecular packing of the polymers chains. This corroborates the results obtained by WAXS, DSC and SAXS,


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which showed that increasing the concentration of NAC enhanced the crystallinity index of the material. The results also showed the ordering of the system due to the larger enthalpy associated with the melting and crystallization processes, the higher intensity of the Bragg’s peaks, and the scattering in small angles of the matrixes. The spectroscopic studies provided information about the structure in the molecular and electronic scale of the obtained casting matrixes, which enabled the generation of an experimental model to describe the interaction between PANI and NAC. The UV-Vis and Resonant Raman spectroscopy results showed that the doping process of PANI by NAC involves the presence of many polyaniline forms, among which are the conductive emeraldine salt and the oxidized pernigraniline salt and base. The results showed that the matrix contributes to the stabilization of different chemical structures, in particular the pernigraniline salt form of PANI, which, as far as we know, was only stabilized in the hydrophobic ionic liquids and during electrochemical control;40 in addition, in this work, it appears naturally in the presence of 50% of NAC in the matrix. However, some anomalous phenomena, such as the absence of the electronic transition of PANI at 420 nm, were not observed, which led us to use the TGA and XPS techniques to further analyse the polymeric matrix. Both techniques showed the presence of two NAC species: one that had a free side chain of NAC and another that was interacting with other species. The interacting species degrades at higher temperatures and has a higher binding energy than the free NAC molecules. Therefore, we propose an interaction between PANI and NAC that follows the concept of SH-π intermolecular interactions, which corroborates the observed experimental results. We showed that the presence of NAC in the polymeric matrix can lead to the formation of different structures depending on the concentration of NAC, which enables the control of porosity, crystallinity, and the polyaniline state. Obtaining a material with 37 ACS Paragon Plus Environment


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such controlled functions is particularly attractive for the development of biosensors, bioelectronic interfaces, vehicle for nitric-oxide release and biomaterials with controlled interface properties. ACKNOWLEDGEMENTS This work was supported by FAPESP (grant no. 2013/12997-0, 2015/24018-1) and CNPq (grant no. 302923/2015-2, 400239/2014-0). INCT in Bioanalytics (FAPESP grant no. 08/57805-2 and CNPq grant no. 573672/2008-3) is kindly acknowledged for the grants. S.K.Jr., M.S.L. and E.R.d.S. acknowledge FAPESP for fellowships (Procs. no.: 2012/01933-8, 2012/15481-1, and 2013/12674-6). The Staff at LNNano and LNLS are kindly recognized for invaluable help and access to SEM, XPS, XRPD and X-ray micro-CT facilities (proposals SEM - 18306, XPS – 18563, XRD1 - 14395, and Micro CT – 18458). SUPPLEMENTARY MATERIAL Micro-CT images and 3D reconstruction REFERENCES

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