Deep Impact of the Template on Molecular Weight, Structure, and

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The Deep Impact of the Template on Molecular Weight, Structure and Oxidation State of the Formed Polyaniline Nikolay A. Ogurtsov, Yurij V. Noskov, Kateryna Yu. Fatyeyeva, Vladimir G. Ilyin, Galina V. Dudarenko, and Alexander A. Pud J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp311898v • Publication Date (Web): 27 Mar 2013 Downloaded from http://pubs.acs.org on April 2, 2013

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The Journal of Physical Chemistry

The Deep Impact of the Template on Molecular Weight, Structure and Oxidation State of the Formed Polyaniline

Nikolay A. Ogurtsov,*,† Yuriy V. Noskov,† Kateryna Yu. Fatyeyeva,†,‡ Vladimir G. Ilyin,§ Galina V. Dudarenko,¶ Alexander A. Pud*,† †

Institute of Bioorganic Chemistry and Petrochemistry, NAS of Ukraine, 50 Kharkivske shose, Kyiv, 02160, Ukraine, ‡

Laboratoire Polymères, Biopolymères et Surfaces, UMR 6270 & FR 3038 CNRS, Université de Rouen, Bd. Maurice de Broglie, 76821 Mont Saint Aignan Cedex, France, §

¶

Institute of Physical Chemistry, NAS of Ukraine, 31 pr. Nauki, Kyiv, 03028, Ukraine,

Institute of Macromolecular Chemistry, NAS of Ukraine, 48 Kharkivske shose, Kyiv, 02160, Ukraine * Corresponding authors. E-mail: [email protected]; [email protected]

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Abstract In this work we find that polyaniline (PANI), synthesized by aniline chemical polymerization at a surface of template polycarbonate (PC) particles, is significantly different in molecular weight, structural order, oxidation state and conductivity from a neat PANI. Molecular weight of the PANI phase in the composite (Mw = 158000) is 1.6 times higher than that of the neat PANI synthesized in the absence of the template particles. Moreover, XRD analysis shows that crystallinity of the PANI phase in the composite is three times higher than that of the neat PANI. Raman spectroscopy indicates that the oxidation level of PANI in the PC/PANI composite is lower than that of the neat PANI. These noticeable changes of the PANI phase properties suggest specific interactions of reagents in the polymerization medium and formed PANI with the template phase as well as an orientation effect of the latter surface. FTIR spectroscopy reveals that hydrogen bonding in the neat doped PANI is weaker than one between –NH– of PANI and C=O of PC at their interface. The discovered differences are supported with the fact that conductivity of the PANI phase in the composite is more than three times higher than that of the neat PANI. Keywords: Polyaniline; Polycarbonate; Hydrogen bond; Polarons; Crystallinity; Percolation threshold


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1. Introduction Intrinsically conducting polymers (ICPs) are among the most successful and important discoveries in modern polymer chemistry and physics. These polymers have been successfully used in new advanced materials for photovoltaics, optoelectronics, sensorics, antistatic protection, catalysis, corrosion protection, etc.1-4 However, a development and enhancement of ICP properties through creation of their composites and blends with common polymers are needed for some of these important applications.5,6 One of effective methods to form these materials is a synthesis of ICP in a presence of other components of the composite. This approach facilitates an interaction of all components of the system.7 However, though knowledge and understanding of what happens in the system with the forming ICP are the keys to control properties of the final composite material, it is little known how a template phase affects properties of this ICP phase in the composite material. To fill this gap in our knowledge, additional studies of this aspect on an example of the composites with specific interactions are still needed to create the ICP based materials with properties suitable for the innovative technologies. Among ICPs, polyaniline (PANI) has attracted a particular interest due to a set of unique properties which open wide possibilities of its applications.8-10 Its composites with conventional polymers not only simply combine properties of the components but demonstrate synergetic properties due to specific interactions and new morphology of the composite material as a whole and components themselves.11 A suitable component for such a composite of PANI is bisphenol A polycarbonate (PC), which has high mechanical strength, heat resistance, and transparency favorable for well-known applications of different scale (electronic, automotive, aircraft components, construction materials, data storage, medical applications, etc.). However, high melting temperature of PC hinders favorable for industry melt mixing of the both polymers to produce the conducting PC/PANI composite. Therefore, there have been developed a number of other methods namely the emulsion and inverted emulsion


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polymerization methods,12-14 the solution blending method,15 the template synthesis at a surface of a porous polycarbonate film,16 electrochemical and chemical polymerization of aniline in/on a PC films.17-19 In the case of other polymer matrixes like polystyrene, poly(methyl methacrylate), polyvinylidene fluoride, etc., a simple one-pot method has been frequently used to synthesize their composites with PANI through the aniline polymerization in aqueous dispersions of particles of these polymers.11,20-22 This method allows preparation of high quality PANI containing composite materials due to probably the fact that during the PANI synthesis an intimate contact appears between all participants of the system.7 This approach often leads to formation of hybrid particles with PANI shell and a core of other material due to precipitation of a thin PANI layer at any surface being in contact with the reaction medium.23-27 Importantly that kinetics of the aniline polymerization, yield and intrinsic structure of PANI strongly depend on a presence and nature of a dispersion phase, temperature, pH, type of acid, loading and nature of reagents.7,28-30 Naturally, this dependence is important for homogeneity, thickness and morphology of the PANI shell7,23,31 and, thus, can affect the shell transformations into PANI clusters of different size, shape and quality when the core-shell composite is processed by melting techniques.7 In turn, different condition of these PANI clusters indirectly influences properties of the PANI percolation network formed inside the ultimate composite material. Therefore, understanding different aspects of the PANI overlayer (shell) formation is a necessary step in developing the methods to control the composite properties. On our way to this understanding we choose as the core material PC which contains carbonyl groups forming hydrogen bonds >С=О···Н−N< at the interface between the PANI and PC. The possibility of such interactions has been previously confirmed earlier for PC/PANI composites prepared by other methods.12,14,16 Obviously, these interactions can be one of the factors controlling quality of the formed PANI overlayer. The question arises as to what extent the interface interactions and their nature affect the ordering of the macromolecular organization and other properties of PANI? Perhaps the answer to this question will help us to understand why blending of PANI with other polymers sometimes leads to ACS Paragon Plus Environment

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unexpected increase of conductivity32 and why PANI tubes synthesized inside pores of membrane show enhanced conductivity compared to PANI bulk materials.33 That is why we consider herein properties of such the composite materials on the example of conducting PC/PANI-p-toluenesulfonic acid (PC/PANI) composites obtained in this one-pot polymerization method. The special interest is focused on the comparison of the properties of the neat PANI and PANI phase in the composite synthesized under the same conditions. The effects of the PC-PANI interaction on PANI properties (such as oxidation state, electronic structure, crystalline structure, and molecular weight), conductivity of the compression molded PC/PANI composite films and parameters of the equation of percolation theory are discovered. Features of aniline polymerization in the PC dispersion in dependence on the nature of acid dopant and oxidant have been presented elsewhere.34

2. Experimental 2.1. Materials Aniline (Merck) was vacuum-distilled prior to use. Other chemicals were of analytical grade and used as received. PC powder was prepared by the precipitation method. In short, PC granules (Lexan) were dissolved in chloroform. This solution (20 wt% of PC, 50 ml) was added dropwise under stirring to acetone (500 ml). After 3 h the precipitate was filtered, washed 3 times with acetone and dried under vacuum at 50-60 °C for 24 h. Powdered PC was sieved with a 0.1 mm sieve allowing a mixture of irregular shape PC particles with sizes less than 100 µm. 2.2. Preparation of PC/PANI composites In a typical experiment, 17 ml of distilled water, 0.1572 g (0.83 mmol) of p-toluenesulfonic acid (TSA) (monohydrate, Aldrich), 0.0513 g (0.55 mmol) of aniline monomer were mixed in a 50-ml round-bottom glass flask. 2 g of the PC powder was added to this flask after dissolution of aniline. The mixture was cooled down to 7 °C and maintained by a thermostat (CC1-K6, Huber) under stirring for 30 min. Ammonium persulfate (0.1571 g, 0.69 mmol) was dissolved in 3 ml of distilled water. This


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oxidant solution was cooled down to 7 °C and then was dropwise added to the reaction mixture followed by stirring for 24 h at this temperature. The final product PC/PANI was filtered out, thoroughly washed with distilled water and dried under vacuum at 50-60 °C for 24 h. The neat PANI was synthesized under the same conditions but without the PC powder. 2.3. Preparation of the composite films The free-standing flexible PC/PANI films (∼100 µm) were prepared by hot pressing of the dry composite powders under 5 MPa (SPECAC press) at 240 °C for 1 min. 2.4. Measurements The real content of PANI in the PC/PANI composites was found from the analysis of their solutions in N-methyl-2-pyrrolidone (NMP) by the UV-Vis spectroscopy method35 using spectrophotometer Cary 50 (Varian). In short, the synthesized composite was dedoped by the typical dedoping procedure in surplus of 0.3 wt% ammonia aqueous solution for 24h,36 then dried. The fixed portion of the dedoped powder composite was dissolved in NMP and mixed with ascorbic acid solution in NMP to reduce the dissolved PANI in emeraldine base state to leucoemeraldine base. The concentration of the latter was determined by absorbance of its solution in 1 mm quartz cuvette at 343 nm compared with the calibration curve previously built using UV-Vis spectra of different concentrations of leucoemeraldine base solution in NMP. This concentration was then used to calculate the dedoped PANI content in the analyzed composite.35 Whereas we studied mainly the doped form of the PC/PANI composite (except the molecular weight characterization), the dedoped PANI percentage was then recalculated for the doped PANI one in the form of the salt PANI(TSA)0.5, i.e. for the stoichiometric ratio of TSA and the imine nitrogens of PANI. The detailed description of the method of determining the PANI content in the PC/PANI composites is given in the Supporting Information. Fourier transform infrared (FTIR) spectra of the samples in pellets with KBr were recorded with a resolution of 1 cm–1 with the help of Bruker Vertex 70 spectrometer.


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The Raman spectra of the samples were measured with a multi-channel Jobin-Yvon T64000 spectrometer connected to liquid nitrogen cooled CCD detector and equipped with a confocal microscope. The measurements were performed with λ = 514.5 nm line (Ar-Kr laser). In order to avoid any local degradation and perturbation of the composite samples, the laser beam power was limited to 0.05 mW on a sample surface and the spectra were collected with an integration time of 450 s. The XRD spectra were recorded in step-scan mode with the 2θ step of 0.05° using diffractometer DRON-3M with the CuKα line irradiation (λ = 1.541 Å). The molecular weight (MW) of PANI was estimated by size-exclusion chromatography (SEC) using the Du Pont LC System 8800 with an ultraviolet detector and Azorbax bimodal exclusion columns. The temperature of the column was held at 50 °C. NMP containing 0.02 wt% LiCl was used as the eluent. Polystyrene standards with narrowly distributed Mw values were used to calibrate the columns. All MW measurements were made for both neat PANI and PC/PANI samples previously dedoped, dried, then dissolved in NMP and reduced directly in these solutions by ascorbic acid from emeraldine base to leucoemeraldine base. The reduction procedure was run in accord with the known procedure.37 The prepared solutions contained additionally LiCl (0.02 wt%) and used for the further MW measurements. In order to discriminate leucoemeraldine macromolecules from PC macromolecules in the reduced PC/PANI solution, the ultraviolet detector was tuned to the spectral wavelength 343 nm. This wavelength corresponds to the leucoemeraldine base absorption band while PC does not absorb under these conditions (see the Supporting Information, Fig. S1). The dc conductivity of the PC/PANI-TSA films was measured at room temperature by standard two-probe (for σ10-7 S/cm) techniques. Microphotographs of the composite powder and film were made using a Nikon Eclipse E200-F optical microscope equipped with a digital camera.


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3. Results and discussion It is well-known that PANI properties strongly depend on its molecular weight, oxidation degree, doping level and structural order, which in their turn can be controlled by conditions/methods of preparation and subsequent treatment. Therefore, to estimate changes in this polymer when transition from its neat state to the composite one we used a number of characterization methods that allowed a general understanding of what is happening to the system under study. 3.1. The molecular weight of the PANIs One of the key characteristics of the polymer is its molecular weight, which in the case of PC/PANI can be affected by aniline polymerization conditions (see Introduction). Therefore, we compared this property for the net PANI and PANI from the composite synthesized under the same conditions. As it is known, the mass-average molecular weight (Mw) of polyaniline typically synthesized in HCl solution is about 53 000 (polydispersity Mw/Mn=2.08).38 It can be increased to about 90 000 and 156 000 by carrying out the aniline polymerization with pernigraniline or in the presence of water-soluble polymeric acid instead of HCl, respectively; or to about 385 000 (Mw/Mn=3.56) by decreasing the polymerization temperature to about −40 °C.38 It is interesting to compare these data with the characteristics of the PANI synthesized in this study. Specifically, the molecular weight of the neat PANI showed unimodal molecular weight distribution (Mw = 98 000, Mw/Mn=2.06). The polymerization of aniline in the presence of PC powder also gave PANI with unimodal molecular weight distribution and a close value of polydispersity (Mw/Mn=2.36), but the molecular weight of the resulting PANI is 1.6 times higher than that of neat PANI (Mw = 158 000). (SEC chromatograms for the net PANI and PANI in the composite are given in the Supporting Information, Fig. S3). This significant increase in molecular weight of the PANI can be probably caused by a self-organization of the monomer and products of its polymerization as well as by their ordering interactions in the adsorption layer at the surface of the PC particles.


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3.2. The hydrogen bonding between PANI and PC To confirm the specific interactions between the PANI and PС components the FTIR spectrum of the PC/PANI composite (10.5 wt% PANI) is compared with that of the neat PANI and PC (Fig.1). The main features observed in the PANI spectrum agree well with published data.28,39-41 As one can see from Fig.1A, bands of the net doped PANI at about 1574 and 1300 cm-1 assigned to stretching vibrations of quinoid rings and C–N in the aromatic amine units respectively, can be observed also in the composite spectrum. At the same time, stretching vibrations of benzene rings and the Q=NH+–B structure at about 1488 and 1145 cm-1 are masked by the strong bands of PC.

Fig. 1. FTIR spectra (A) of PANI, PC, PC/PANI composite and enlarged parts of these spectra (B) related to stretching vibrations of Н−N groups (left) and С=О (right).


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FTIR spectrum of PС is similar to that of obtained by other authors.15-18 It contains a strong band at 1768 cm-1, which is assigned to stretching vibrations of С=O groups.42,43 This band shifts to 1760 cm-1 and becomes broader in the PC/PANI composite spectrum (Fig.1B, right). Such a shift of the carbonyl stretching mode has been reported previously for the PC/PANI doped with hydrochloric and dodecylbenzene sulfonic acids.12,14,16 In the net PANI spectrum (Fig. 1B, left), we observe two peaks at 3440 and 3227 cm-1, which can be assigned to free N-H and hydrogen-bonded N-H stretching vibrations of secondary amines, respectively.42,43 The latter appears due to hydrogen bonding between amine group of PANI (hydrogen-bonding donor/acceptor) and oxygen atom belonging to sulfonate group of TSA dopant (hydrogen-bonding acceptor, O···H−N

Deep Impact of the Template on Molecular Weight, Structure, and

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