The particle size and shape of polyaniline in aqueous solutions of poly-(n-vinylpyrrolidone)

Plasticheskie Massy, No. 1, 2013, pp. 10 14 The particle size and shape of polyaniline in aqueous solutions of poly-(n-vinylpyrrolidone) S.V. Osadchenko, Ya.O. Mezhuev, Yu.V. Korshak, and M.I. Shtil man D.I. Mendeleev Russian Chemico-Technological University, Moscow Selected from International Polymer Science and Technology, 4o, No. 8, 2013, reference PM 13/01/10; transl. serial no. 17107 Translated by P. Curtis Summary Certain rheological properties of aqueous dispersions of polyaniline in the presence of poly-(n-vinylpyrrolidone) as a stabiliser were investigated. The influence of the molecular weight of poly-(n-vinylpyrrolidone) and also the concentration of aniline and ammonium peroxydisulphate on the size and shape of the polyaniline particles was shown. INTRODUCTION It is known that polyaniline (PANI) is soluble only in basic form in certain bipolar aprotic solvents such as dimethyl sulphoxide and N-methylpyrrolidone, which limits the possibilities of its use and processing [1]. The possibility of producing stable dispersions of polyaniline in aqueous solutions of a number of polymers such as poly-(n-vinylpyrrolidone) (PVP), polyvinyl alcohol, and polyethylene glycol has been reported [2]. At the same time, the synthesis and physicochemical properties of such systems have not been studied adequately. The present work is devoted to determining the size and shape of polyaniline particles in aqueous solutions of PVP, and also certain rheological characteristics of this system. EXPERIMENTAL Rheological investigations were conducted in the Griffin beaker of a rotational viscometer in the process of the forming of a polyaniline dispersion during its oxidative polymerisation in aqueous solutions of PVP of different molecular weights. The beaker was filled with 200 ml of a 16% solution of PVP in water, and to this solution was added 1.03 g of aniline chloride. In 50 ml of a 16% aqueous solution of PVP was dissolved 2.28 g of ammonium peroxydisulphate, and the prepared solution was mixed with the monomer solution in the Griffin beaker. The change in viscosity in time was recorded at a temperature of 298 K using the rotational viscometer at 100 rpm. Data on the dependence of the viscosity of the system in time during oxidative polymerisation of aniline were obtained for PVP of four molecular weights: 12 500, 24 000, 40 000, and 160 000. The particle shape of dispersed-phase PANI was determined by the capillary viscometry method on the basis of the dependence of the viscosity of the PANI-PVP system on the dilution for PVP of four different molecular weights: 8000, 12 500, 24 000, and 40 000. For preparation of polyaniline-pvp systems, in 50 ml of twicedistilled water was dissolved 5.55 g of PVP, and then 0.643 g of aniline chloride. In 50 ml of twice-distilled water were successively dissolved 5.55 g of PVP and 2.28 g of ammonium peroxysulphate. The prepared solutions were mixed and heated for 24 h. Polyaniline- PVP systems were obtained by the procedure described above for PVP of four different molecular weights: 8000, 12 500, 24 000, and 40 000. Solutions of 5.55 g of PVP of corresponding molecular weights in 100 ml of twice-distilled water were also prepared, and these were used to dissolve the obtained polyaniline-pvp dispersions. The viscosities were determined for all the investigated molecular weights of PVP and dilutions with the use of a capillary viscometer. 2014 Smithers Information Ltd. T/41

The influence of the molecular weight of PVP on the size of the formed polyaniline particles in aqueous solutions of PVP of different molecular weights was investigated according to the following procedure: four specimens of PVP with molecular weights of 8000, 12 500, 24 000, and 40 000, taken in a quantity of 1.11 g each, were dissolved in four reactors each containing 50 ml of twice-distilled water. In each of the prepared solutions was dissolved 0.129 g of aniline chloride. Four solutions of 0.456 g of ammonium peroxydisulphate in 50 ml of twice-distilled water were prepared separately. All solutions were heated for 30 min at 298 K and then mixed and held for 24 h at 298 K in an oven. In order to establish the influence of the oxidant to monomer ratio on the size of the polyaniline particles formed, the oxidative polymerisation of aniline was carried out in aqueous solutions with a different concentration of ammonium peroxydisulphate according to the following procedure: 1.11 g of PVP 40 000 is dissolved in five reactors in 50 ml of twice-distilled water. In each prepared PVP solution was dissolved 0.129 g of aniline chloride. Five solutions containing 0.285, 0.342, 0.399, 0.456, and 0.912 g of ammonium peroxydisulphate in 50 ml of twice-distilled water were prepared separately. All solutions were heated for 30 min at 298 K and then mixed and held for 24 h at 298 K in an oven. The influence of the concentration of formed polyaniline on the size of its particles was investigated according to the following procedure: 1.11 g of PVP 40 000 was dissolved in four reactors in 50 ml of twice-distilled water. In each prepared solution of PVP was dissolved 0.129, 0.194, 0.258, and 0.516 g of aniline chloride. Four solutions containing 0.456, 0.684, 0.912, and 1.824 g of ammonium peroxydisulphate in 50 ml of twice-distilled water were prepared separately. All solutions were heated for 30 min at 298 K and then mixed and held for 24 h at 298 K in an oven. For preparation of a specimen of the polyaniline-pvp 40 000 system, in order to prevent the precipitation of polyaniline, oxidative polymerisation of aniline was conducted in a tenfold molar excess of PVP in terms of constituent recurrent units in relation to aniline chloride. Chemically pure aniline chloride weighing 0.645 g (0.005 mol) was dissolved in 100 ml of aqueous solution containing 5.55 g of PVP with a molecular weight of 40 000. Analytical-grade ammonium peroxydisulphate weighing 1.425 g (0.00625 mol) was dissolved in four measuring beakers in 100 ml of distilled water. The prepared aqueous solutions of aniline chloride and ammonium peroxydisulphate were heated at 25 C for 30 min, and they were then mixed and held for 24 h. A quantity of 10 ml of obtained solution was subjected to dialysis for 3 days and then lyophilic drying (Christ Alpha 1-4 LD freeze dryer). The obtained solid polyaniline- PVP specimens were additionally dried in an oven at 500 C for 7 h. IR spectra of polyaniline, PVP 40 000, and also polyaniline-pvp specimens were taken from pellets obtained by pressing 1 mg of each specimen with 100 mg of potassium bromide (Nicolet-380 IR spectrometer). The particle size of the polyaniline formed in the aqueous solution of PVP was determined by dynamic laser light scattering (HORIBA LB-550 nanoparticle size analyser). RESULTS AND DISCUSSION The experimentally found changes in the viscosity of the reaction system in time during oxidative polymerisation of aniline with ammonium peroxydisulphate in an aqueous solution of PVP are given in Figure 1. In all cases, a sharp increase was observed in the viscosity of the reaction medium in time, which is due to the autocatalytic nature of the oxidative polymerisation of aniline, which has been noted by a number of authors [3-5]. In addition, the time of the start of increase in viscosity decreases slightly with increase in the molecular weight of PVP in aqueous solution. Given that, with increase in the molecular weight of PVP in solution, the local density of segments approaches the average density, which is equivalent to increase in the intensity of overlapping of the macromolecular coils [6], it follows that the formation of lower oligomers of aniline is sufficient for structure formation of the system in solutions of PVP of high molecular weight. With increase in the molecular weight of PVP in aqueous solution there is a linear increase in the contribution of polyaniline to the viscosity of the system (Dη) with a constant concentration of polyaniline and PVP (Figure 2). The contribution of polyaniline to the viscosity of the polyaniline-pvp system (Dη) was assessed as the difference in the viscosities of the system at the end of polymerisation of aniline and before the start of polymerisation: Δη = η f η i (1) where h f is the final viscosity of the reaction system (cp) and h i is the initial viscosity of the reaction system (cp). The dependences of the initial and final viscosity of the reaction system on the molecular weight of PVP are given in Figure 3. A characteristic feature of the presented dependences is their symbatic relationship, and also their linearity at molecular weights of PVP above a certain value. Such a nature of the obtained dependences indicates the identical mechanism of structure formation of the system for PVP of all the investigated molecular weights. T/42 International Polymer Science and Technology, Vol. 41, No. 4, 2014

Deviations of the experimental dependences from the linear in the region of low values of the molecular weight of PVP are connected, it seems, with pronounced nonuniformity of the distribution of PVP segments in solution. This is consistent with the existence of isolated coils of PVP of relatively low molecular weight, separated by the solvent, and overlapping coils of PVP of high molecular weight. The results of IR spectral study indicate the formation of hydrogen bonds between secondary amino groups of polyaniline and the amide carbonyl of PVP (Figure 4), which in our opinion is the reason for stabilisation of polyaniline in an aqueous solution of PVP. It is well known that the reduction in the energy of the system by the formation of a C=N---H-N hydrogen bond with the participation of iminium nitrogen of quinone diimine fragments is lower than with the emergence of a C=O---H-N hydrogen bond with the participation of the carbonyl group of PVP. When polyaniline and PVP interact, bound amino groups and water are transferred from imine groups of quinone diimine fragments of polyaniline to the amide carbonyl of PVP, which is accompanied with increase in the wave number corresponding to stretching vibrations of the C=N bonds of polyaniline from 1591 cm 1 for Figure 2. The dependence of the viscosity difference of the reaction system at the end of the oxidative polymerisation of aniline and before its start on the molecular weight of PVP in aqueous solution Figure 1. The time dependence of the viscosity of the reaction system during the oxidative polymerisation of aniline in an aqueous solution of PVP. Molecular weights of PVP: (a) 12 400; (b) 24 000; (c) 40 000; (d) 360 000 Figure 3. The dependences of (1) the initial viscosity of the reaction system and (2) the final viscosity of the reaction system on the molecular weight of PVP 2014 Smithers Information Ltd. T/43

polyaniline to 1623 cm 1 for the polyaniline-pvp system. A second signal of amide carbonyl taking part in the formation of the hydrogen bond also appears at 1639 cm 1, but the absorption band of the amide carbonyl of PVP (1658 cm 1 ) unbound by hydrogen bonds remains, as the complex was obtained in a tenfold excess of PVP. Thus, in the process of formation of the polyaniline-pvp system, breakdown of the C=N---H-N hydrogen bonds of quinone diimine and aminobenzoic fragments of the initial polyaniline and the formation of C=O---H-N bonds occur (Figure 5). It is the reduction in energy of the polyaniline-pvp system by the formation of stronger C=O---H-N hydrogen bonds that governs its stabilisation. The shape of polyaniline particles stabilised in an aqueous solution of PVP was determined by capillary viscometry. The dependences of the viscosity of the polyaniline-pvp system on its dilution by the aqueous solution of PVP were obtained for PVP of four different molecular weights: 8000, 12 400, 24 000, and 40 000. In accordance with Einstein s law, the viscosity of the diluted suspensions increases linearly with the volume fraction of dispersed phase in accordance with the equation: η/η 0 = 1+ kϕ where h 0 is the viscosity of the dispersion medium (cp), η is the viscosity of the dispersed system (cp), k is the shape factor, and ϕ is the volume fraction of the dispersed phase. Experimental data on the dependence of the viscosity of the polyaniline-pvp system on the dilution in linear coordinates η/h 0 - ϕ are given in Figure 6. From the slope tangent of the obtained linear dependences, the shape factors of polyaniline particles in aqueous solutions of PVP of different molecular weights were determined. According to Kuhn and Simha, the (2) Figure 4. IR spectra of (1) polyaniline, (2) PVP, and (3) the polyaniline-pvp system Figure 6. The η/h 0 -ϕ dependences for polyaniline-pvp systems containing PVP of different molecular weights: 1-8000; 2-12 400; 3-24 000; 4-40 000 Figure 5. The redistribution of hydrogen bonds when polyaniline reacts with PVP T/44 International Polymer Science and Technology, Vol. 41, No. 4, 2014

geometric shape of the particles of dispersed phase can be described by an ellipsoid of rotation with a ratio of lengths of the major and minor half-axes p [7]. Parameter p is clearly connected with the shape factor by the equation: k = 14/15 + (p 2 /5) 1 3(ln2p λ) +1 (ln2p λ +1) { } where p is the ratio of the lengths of the major and minor half-axes, and λ is a parameter depending on the proposed shape of particles, equal to 1.5 for an ellipsoid of rotation and 1.8 for a cylindrical rod. The shape factors and also the ratios of the lengths of the major and minor half-axes of the ellipse (the height and diameter of the base of the cylindrical rod) calculated by equations (2) and (3) are given in Table 1. With all investigated molecular weights of PVP in aqueous solution, the polyaniline particles have a rod shape, and here modelling of the particles both by ellipsoids of rotation and by a cylindrical rod lead to similar results. At the same time, increase in the molecular weight of PVP leads to increase in the ratio of the length of the cylinder to the diameter of the base. With account taken of the high thermodynamic rigidity of the polyaniline chain, the length of the polyaniline aggregates probably cannot change considerably, and increase in p is due to decrease in the aggregate diameter. This is equivalent to decrease in the size of the aggregates with increase in the molecular weight of the PVP. The latter effect is due to increase in surface activity of PVP with increase in its molecular weight, which leads to partial deaggregation of the polyaniline chains. Thus, polyaniline in aqueous solutions of PVP exists in the form of rod-shaped aggregates stabilised by PVP by the formation of hydrogen bonds. If these suggestions are valid, then, with increase in the molecular weight of PVP in aqueous solution, the particle size should also decrease. The latter assumption is entirely consistent with the dynamic light scattering data in Table 2. Likewise, the particle size of polyaniline in aqueous solutions of PVP evidently should depend on the volume (3) fraction and the level of oxidation of the polyaniline chain, which is determined by the proportion of quinone diimine fragments. With increase in the volume fraction of polyaniline in the system, the size of its particles regularly increases, and especially rapidly in the region of high concentrations of dispersed phase, as shown in Table 3. Such a sharp increase in the size of the polyaniline aggregates with increase in its volume fraction in the system indicates the presence of pronounced forces of attraction between individual polyaniline chains and their aggregates. Such pronounced affinity is probably due to the formation of hydrogen bonds between aminobenzoic and quinone diimine fragments of the polyaniline chains that is noted in Figure 5. This assumption is confirmed by an increase in the size of the polyaniline aggregates with increase in the molar excess of oxidant in relation to the monomer up to a certain limit. With increase in the molar excess of oxidant, the proportion of quinone diimine fragments in the polyaniline chain increases, which leads to an increase in the energy of attraction between the chains on account of increase in the number of hydrogen bonds between the aminobenzoic and quinone diimine fragments. At the same time, high excesses of oxidant promote the occurrence of oxidative degradation of polyaniline, which, conversely, promotes a reduction in the polyaniline particle size. Data on the dependence of the particle size of polyaniline on the molar excess of oxidant in the system are entirely consistent with the assumptions made and are given in Table 4. Table 2. The dependence of the effective diameter of the polyaniline particles (d) on the molecular weight of PVP in aqueous solution M (PVP) 8000 12 400 24 000 40 000 d (µm) 3.39 2.08 0.475 0.106 Table 3. The dependence of the particle size of polyaniline on its volume fraction in aqueous PVP 40 000 at a temperature of 298 K ϕ x 10 3 0.86 1.293 1.72 3.44 (polyaniline) d (µm) 0.106 0.162 0.48 2.48 Table 1. The shape factors of polyaniline particles and the ratios of the lengths of the major and minor half-axes of the ellipse (height and diameter of the base for the cylindrical rod model) in aqueous solutions of PVP of different molecular weights M (PVP) k p (model of ellipsoid of rotation; λ = 1.5) p (model of cylindrical rod; λ = 1.8) 8000 50.5 23.5 22.3 12 400 53.7 24.5 23.3 24 000 75.2 30 28.3 40 000 85.5 32.7 30.7 2014 Smithers Information Ltd. T/45

Table 4. The dependence of the effective diameter of the polyaniline aggregates in the aqueous solution of PVP 40 000 on the molar ratio of oxidant to monomer in the initial reaction mixture (ϕ = 0.86 x 10 3 ; T = 298 K) [Ox]/[Anil] 1.25 1.5 1.75 2.0 4.0 d (µm) 0.101 0.115 0.14 0.106 0.096 account of increase in the proportion of chain quinone diimine fragments capable of forming hydrogen bonds with aminobenzoic fragments. Further increase in the ratio of ammonium peroxydisulphate to aniline leads to a reduction in the polyaniline particle size on account of the development of the oxidative degradation of polyaniline. CONCLUSIONS 1. Increase in the viscosity of aqueous solutions of poly- (N-vinylpyrrolidone) under conditions of oxidative polymerisation of aniline, governed by structure formation of the system formed by polaniline, has been shown. Here, the increase in viscosity of the solution is proportional to the molecular weight of poly-(n-vinylpyrrolidone). 2. It was established that, with increase in the molecular weight of poly-(n-vinylpyrrolidone) in aqueous solution, the size of the polyaniline particles decreases considerably, while the shape of the polyaniline particles is rod-like. 3. It was shown that increase in the concentration of polyaniline particles in solution leads to an increase in the particle size of polyaniline. 4. It was established that increase in the molar ratio of ammonium peroxydisulphate to aniline leads initially to increase in the particle size of polyaniline on REFERENCES 1. Wessling B., Adv. Mater., 6(3):226-228 (1994). 2. Armes S.P. and Aldissi M., Mater. Res. Soc. Symp. Proc., 173:311 (1990). 3. Wei Y. et al., J. Polym. Sci., 27:2385-2396 (1989). 4. Mezhuev Ya.O. et al., Plast. Massy, (3):26-31 (2011). 5. Mav I. and Zigon M., J. Polym. Sci., 39(2471-2481) (2001). 6. Budtov V.P., Methods for Investigating Polymers. Khimiya, St Petersburg, 383 pp. (1992). 7. Allen P.V., Methods for Investigating Polymers, ed. by Pravednikov A.N. Izdatel stvo Inostrannoi Literatury, Moscow, 335 pp. (1961). T/46 International Polymer Science and Technology, Vol. 41, No. 4, 2014