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polymer blends prepared via aqueous dispersions
Synthetic Metals 110 Ž2000. 189–193 www.elsevier.comrlocatersynmet
Polyaniline–DBSArpolymer blends prepared via aqueous dispersions Y. Haba a , E. Segal a , M. Narkis a , G.I. Titelman a b
b,)
, A. Siegmann
b
Department of Chemical Engineering, Technion — IIT, Haifa 32000, Israel Department of Materials Engineering, Technion — IIT, Haifa 32000, Israel Received 16 June 1999; accepted 13 October 1999
Abstract Stable polyaniline–dodecyl benzene sulfonic acid ŽPANI–DBSA. aqueous dispersions were obtained by a unique method of aniline polymerization in the presence of DBSA, through an anilinium–DBSA complex appearing as solid needle-like particles, in an aqueous medium. The average size of the PANI primary particles, determined by small angle X-ray scattering ŽSAXS., is 18.7 nm. These primary particles form aggregates, which further cluster into ; 50 mm agglomerates. PANI–DBSArpolymer blends were obtained by mixing an aqueous PANI–DBSA dispersion with an aqueous emulsion of the matrix polymer, followed by water evaporation. These blends exhibit electrical conductivity already at a very low PANI–DBSA content Ž0.5 wt.%.. The conductivity level of the various blends depends on the PANI content, on the surfactant present in the polymer matrix emulsion, and it is practically independent of the polymer matrix nature. Thus, a similar structuring mechanism prevails in these blends, irrespective of the polymer matrix Žcontrary to solution and melt blends.. The PANI–DBSA particles strongly segregate within the polymer matrix, already in the combined aqueous dispersion, and upon drying, a very fine conductive network is formed. This strong segregation tendency leads to a conductive network formation already at low PANI–DBSA contents, thus generating the conductive blends. q 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Polyaniline; Polymerization; DBSA; Aqueous dispersions; Blends
1. Introduction Intrinsically conductive polymers, including polyaniline ŽPANI., have the utilization potential in a large number of applications: rechargeable batteries, conducting paints, conducting glues, electromagnetic shielding, antistatic formulations, sensors, electronic devices, light-emitting diodes, coatings etc. However, the major problem in applying these polymers is their poor processability. Several methods were used to overcome this problem including N-substitutions w1–3x or ring substitutions w4,5x of aniline by aliphatic large radicals, block copolymers w6x and doping PANI base with a functionalized protonic acid w7,8x. Colloidal particles have the potential to be finely dispersed in a polymer medium due to their small size. Therefore, when colloidal particles of an intrinsically conducting polymer are used to form, for example, PANIrpolymer blends, fine conductive paths may generate and high conductivity levels may be realized already at low PANI contents. Thus, the synthesis of colloidal PANI ) Corresponding author. IMI ŽTAMI., P.O.B. 10140, Haifa 26111, Israel
particles w9–14x and their application in PANIrpolymer blends w6,13,15–20x have been of great interest. Beadle et al. w15x obtained electrically conductive polyanilinercopolymer blends by polymerizing aniline in the presence of a chlorinated copolymer latex. Xie et al. w20x used one-step in situ emulsion polymerization to obtain polyanilinerSBS blends by polymerizing xylene solution of aniline and polyaniline–dodecyl benzene sulfonic acid ŽDBSA. in the presence of xylene solution of SBS. The use of polymeric steric stabilizer to stabilize PANI colloidal particles and form conductive blends has been reported w6,16–19x. Gospodinova et al. w13x, used colloidal PANI–DBSA dispersions for the preparation of blends with both, water soluble ŽPVAL. and water insoluble ŽEVA. host polymers. The PANI–DBSA dispersion was mixed with the matrix polymer solution and then cast. In this article a unique method of aniline polymerization w21x in the presence of DBSA, in an aqueous medium, is presented. In this polymerization method DBSA, a functionalized protonic acid, acts both, as a surfactant and a dopant. First aniline and DBSA at a stoichiometric ratio react to form a fine dispersion of solid needle-like anilinium–DBSA complex which upon the addition of an oxi-
0379-6779r00r$ - see front matter q 2000 Published by Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 2 8 0 - 5
Y. Haba et al.r Synthetic Metals 110 (2000) 189–193
190
Table 1 Compression molding conditions of blends containing PANI Matrix polymer
Sintering temperature Ž8C.
Final temperature Ž8C.
PS PMMA Commercial polyacrylate
110 120 80
180 160 120
dizer polymerize forming a PANI–DBSA stable dispersion in the aqueous medium. The stable aqueous dispersions obtained can be used to form various low PANI content conductive blends, by blending with other polymer latices.
2. Experimental
PMMA: MMA Ž135 g., potassium persulphate Ž0.25 g. and water Ž1200 g. were mixed in a reactor at 708C under nitrogen atmosphere for 6 h. 2.3. Blend preparation PANIrpolymer blends were obtained by simple mixing of the aqueous PANI–DBSA dispersion with an aqueous emulsion of the matrix polymer, followed by either water evaporation or film casting. Surface morphology of the PANI blends Žpowders or films. and the neat matrix polymers Žpowders or films. was studied using a LEO-982 high-resolution scanning electron microscope ŽHRSEM., at an accelerating voltage of 1 kV, using the SE IN-LENS technique. The samples were placed on a carbon tape for observation. 2.4. Compression molding of PANI–DBSA r polymer blends
2.1. Polymerization of aniline Aniline Ž6 g. was mixed with DBSA Ž21.6 g. in water Ž1200 g. for 3 h to form a homogeneous white dispersion of anilinium–DBSA complex. The dispersion was cooled Ž; 08C., and a solution of ammonium peroxydisulfate ŽAPS. Ž15 g dissolved in 50 g water. was added dropwise. Polymerization was carried out at ; 08C for 5 h. The dispersion’s color changed from white through blue to dark green. At the end of polymerization a green and stable PANI–DBSA dispersion was obtained, containing ; 0.8 wt.% doped PANI Ždetermined by complete precipitation with methanol.. The PANI–DBSA particle’s size was measured using a Coulter LS230 size analyzer. Small angle X-ray scattering ŽSAXS. of the PANI–DBSA dispersion was used to characterize the size of the primary PANI particles and the distance between PANI–DBSA layers. A Philips X-ray generator equipped with a Kratky compact camera collimator ŽA. Paar., and a position-sensitive linear detector ŽRaytech. were used for the SAXS analysis.
PANIrpolymer blends Žpowders or films. were sintered under pressure followed by heat treatment of the free samples and fast cooling. Table 1 details the sintering and final treatment temperatures for each of the PANIrpolymer blends. The conductivity of the compression-molded strips and films was measured using the four-probe technique ŽASTM D-991-89..
3. Results and discussion 3.1. PANI–DBSA dispersion A stable PANI–DBSA aqueous dispersion was obtained at the end of the polymerization process. Since the molar ratio of aniline:DBSA fed into the reactor was 1:1, while the maximum molar ratio in doped PANI:DBSA is 1:0.5, at the end of the polymerization process excess DBSA is present in the reactor w21x. This excess DBSA forms
2.2. Matrix polymer aqueous emulsions A commercial aqueous acrylic latex Ž48% solid content, ; room temperature Tg . was supplied by B.G. Polymer ŽKibbutz Beit-Govrin, Israel.. Polystyrene ŽPS. and polyŽmethyl methacrylate. ŽPMMA. were synthesized according to the following procedures. PS: Styrene Ž525 g., sodium lauryl sulphate Ž0.3 g., Triton X-100 Ž6 g., Na 4 P2 O 7 P H 2 O Ž0.4 g. and water Ž500 g. were mixed in a 2-l reactor at 708C under nitrogen atmosphere. A mixture of sodium lauryl sulphate Ž1.3 g., Triton X-100 Ž22.6 g., potassium persulphate Ž1 g. and water Ž450 g. was added dropwise. The polymerization process proceeds 6 h.
Fig. 1. A schematic model of aqueous dispersions of polyaniline with DBSA.
Y. Haba et al.r Synthetic Metals 110 (2000) 189–193
191
Fig. 2. Conductivity vs. PANI–DBSA content of the various blends.
micellar structures and also acts as a surfactant, which stabilizes the dispersion of the PANI–DBSA particles. The hydrophobic tails of free and bonded DBSA molecules are arranged in a way that they all turn to each other, while the hydrophilic groups of the free DBSA turn to the aqueous phase ŽFig. 1.. The molecular conformation of PANI– DBSA chains is affected both by the molecule–molecule Žintramolecule. interactions and molecule–aqueous medium interactions as suggested by Levon et al. w22x. Using SAXS, it was calculated that the average size of the doped primary PANI particles is 18.7 nm, in accordance with Wessling’s results w23x. These primary particles generate aggregates, which further cluster into agglomerates. These agglomerates, about 50 mm in size, are located within gel-like units, which are structured due to formation of hydrogen bonds between free DBSA molecules. These hydrogen bonds form an infinite network which stabilizes the PANI–DBSA dispersion. 3.2. Blends containing PANI An extremely important result of the present study is that upon mixing a PANI–DBSA dispersion with a matrix
Fig. 4. A high-resolution SEM micrograph of PANI–DBSArpolystyrene powder Ž4.5% PANI..
polymer emulsion, the 50 mm PANI–DBSA agglomerates disintegrate and a visually homogeneous mixture is obtained. The conductivity of the various PANI– DBSArpolymer blends vs. PANI–DBSA content is depicted in Fig. 2. The conductivity of these blends gradually increases with PANI–DBSA content, without exhibiting a sharp percolative insulator–conductor transition. At 0.5 wt.% PANI–DBSA, the conductivity is already 10y6 Srcm and it tends to level off above 2 wt.% PANI–DBSA. The absence of a sharp percolative insulator–conductor transition Ža gradual conductivity rise is observed. in the studied PANI blends is ascribed herein to a very significant and fast segregation process taking place already in the combined PANI–DBSArpolymer aqueous dispersions. This strong segregation stems from the different surface characteristics of the PANI–DBSA particles and the matrix polymer particles Žrefer to the different surfactants used to stabilize the particles, as given in the experimental part.. Recall also that segregation in the present systems takes place in a very low viscosity aqueous medium, thus very likely a fast process, contrary to a segregation phenomenon in solution cast films, or within a polymer melt. High-resolution SEM depicts 100–200 nm spherical emulsion PS particles ŽFig. 3., whereas the PANI–DBSA particles in the PANI–DBSArPS blend form very thin layers, Table 2 Conductivity of films and compression-molded films of PANIrpolyacrylate based blends at various PANI–DBSA contents
Fig. 3. A high-resolution SEM micrograph of the polystyrene particles obtained by emulsion polymerization.
PANI–DBSA content Ž%.
Film conductivity ŽSrcm.
Compressionmolded film conductivity ŽSrcm.
0.5 1.0 1.5 4.5
1.4ey4 1.2ey3 7.9ey3 3.3ey2
1.5ey6 5.6ey4 3.8ey3 2.3ey2
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Y. Haba et al.r Synthetic Metals 110 (2000) 189–193
Fig. 5. Conductivity of compression-molded strips of PANIrPS blends made of unwashed and washed powders.
much thinner than the PS particle’s size, covering the PS particles and occasionally forming some elongated structures ŽFig. 4.. The strong segregation and formation of the thin PANI–DBSA layers yield conductive blends already at very low PANI–DBSA contents. The similarity of the conductivity curves for the various polymer matrices is a further proof that a key factor playing role in the segregation process is the particle surface characteristics Žeach polymer particle is coated with its surfactant., rather than the character of the polymer particle itself. This segregation mechanism again is extremely different from the one in solution and melt blending where the PANI–DBSA particles ‘‘feel’’ the matrix polymer and segregate accordingly. Using a specially synthesized PS emulsion, stabilized by DBSA in a blend with the PANI–DBSA aqueous dispersion has resulted in lower conductivity levels due to a remarkably reduced level of segregation, Židentical surface characteristics of the PS and PANI particles.. The conductivity values of cast and compression-molded PANI–polyacrylate blends at various PANI–DBSA contents are compared in Table 2. The compression molding
Fig. 6. A high-resolution SEM micrograph of the neat acrylic polymer film.
Fig. 7. A high-resolution SEM micrograph of PANIracrylic polymer film Ž4.5% PANI..
process reduces the conductivity level of the blends, especially at the lower PANI–DBSA contents. Water rinsing of
Fig. 8. High-resolution SEM micrographs of PANIracrylic polymer film Ž4.5% PANI. after 7 months at Ža. high, and Žb. low magnification.
Y. Haba et al.r Synthetic Metals 110 (2000) 189–193
the blends before compression molding increases the blends’ conductivity ŽFig. 5., especially for the lower PANI content blends. Water rinsing may partially remove some excess free DBSA, thus improving contact among the conducting particles, which is more significant for the lower PANI–DBSA contents. In the latter, the quality of the conductive networks is still low and becomes even lower upon application of pressure and heat. Blends of a PANI–DBSA dispersion with the polyacrylate aqueous emulsion are film forming at room temperature due to the low Tg of the acrylic polymer Žcontrary to the PS-based blends which are non-film forming at room temperature.. Upon water evaporation, the submicron polymer particles are getting closer, till maximum contact is achieved, forming hexagonal structures which subsequently coalescence into an homogeneous film w24,25x. In the last stage of this film-forming mechanism, an immiscible emulsion blend will not mix perfectly, thus leading to significantly poor properties of the resulting blend w26x. The surface morphology of a cast neat acrylic film is rather smooth ŽFig. 6., while the surface morphology of a cast PANI–DBSAracrylic film ŽFig. 7. includes some hexagonal-like structures. Interestingly, it was found that the conductivity of this film slowly decreases with time; a decrease of two orders of magnitude in 7 months. In parallel, the film surface morphology has also changed ŽFig. 8.. The small hexagonal-like structures have coalesced, forming much larger hexagonal units Žan average hexagonal size has changed from 2 to 40 mm.. It seems that during this coalescence process of the acrylic particles, PANI particles have been ‘‘squeezed’’ out and formed isolated spherical particles Žblack particles in Fig. 8b.. These structural changes taking place at the ambient temperature are due to progression of the coalescence process. The observed conductivity decrease is related to gradual destruction of the PANI–DBSA network resulting from the structural changes in the acrylic matrix. 4. Conclusions Polymerization of aniline in the presence of DBSA in an aqueous medium, through an anilinium–DBSA complex, forms a stable PANI–DBSA dispersion. The average size of a doped primary PANI particle is ; 18.7 nm. These primary particles generate aggregates, which further cluster into agglomerates ; 50 mm in size. The stability of the aqueous PANI–DBSA dispersion is caused by the presence of excess DBSA molecules which form strong hydrogen bonding with the PANI–DBSA particles, creating agglomerates. Upon mixing the PANI– DBSA aqueous dispersion with an aqueous emulsion of another polymer Žstabilized with a different surfactant., the PANI agglomerates disintegrate to form a visually homogeneous dispersion. Compression-molded strips and cast films of PANI– DBSArpolymer blends become conducting at a very low
193
PANI–DBSA content Ž0.5 wt.%. and attain conductivity levels of 10y2 to 10y3 Srcm at 1—2% PANI–DBSA content for the different polymers studied. These blends exhibit similar conductivity levels at a given PANI content, indicating a common mechanism of the conductive structure formation, irrespective of the polymer matrix. The PANI–DBSA particles segregate within the polymer matrix, already in the combined aqueous dispersions and upon drying form a very fine conductive network. A low-Tg matrix with PANI–DBSA shows conductivity reduction with time due to structural changes caused by coalescence progression with time at room temperature. Acknowledgements The authors are grateful to the Israel Ministry of Science for partially supporting this research. References ¨ w1x W.Y. Zheng, K. Levon, J. Laakso, J.E. Osterholm, Macromolecules 27 Ž1994. 7754. w2x J.-W. Chevalier, J.-Y. Bergeron, L.-H. Dao, Macromolecules 25 Ž13. Ž1992. 3325. w3x S.E. Chapman, N.C. Billingham, S.P. Armes, Synth. Met. 55–57 Ž1993. 995. w4x Y.H. Liao, M. Angelopoulos, K. Levon, J. Polym. Sci., Part A: Gen. Pap. 33 Ž1995. 2725. w5x Y. Wei, R. Hariharan, S.A. Patel, Macromolecules 23 Ž1990. 758. w6x C. Dearmitt, S.P. Armes, J. Colloid Interface Sci. 150 Ž1. Ž1992. 134. w7x Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 48 Ž1992. 91. w8x A.J. Heeger, Synth. Met. 55–57 Ž1993. 3471. w9x N. Kuramoto, E.M. Genies, Synth. Met. 68 Ž1995. 191. w10x J. Stejskal, P. Kratochvil, N. Gospodinova, L. Terlemezyan, P. Mokreva, Polymer 33 Ž1992. 4857. w11x B. Vincent, J. Waterson, J. Chem. Soc. Chem. Commun. Ž1990. 683. w12x N. Gospodinova, P. Mokreva, L. Terlemezyan, J. Chem. Soc. Chem. Commun. Ž1992. 923. w13x N. Gospodinova, P. Mokreva, T. Tsanov, L. Terlemezyan, Polymer 38 Ž3. Ž1997. 743. ¨ w14x J.-E. Osterholm, Y. Cao, F. Klavetter, P. Smith, Synth. Met. 55–57 Ž1993. 1034. w15x P. Beadle, S.P. Armes, S. Gottesfeld, C. Mombourquette, R. Houlton, W.D. Andrews, S.F. Agnew, Macromolecules 25 Ž1992. 2526. w16x J. Stejskal, P. Kratochvil, N. Radhakrishnan, Synth. Met. 61 Ž1993. 225. w17x B. Vincent, Polym. Adv. Technol. 6 Ž1994. 356. w18x P. Banerjee, M.L. Digar, S.N. Bhattacharyya, B.M. Mandal, Eur. Polym. J. 30 Ž4. Ž1994. 499. w19x S.P. Armes, M. Aldissi, J. Chem. Soc. Chem. Commun. Ž1989. 88. w20x H.-Q. Xie, Y.-M. Ma, J.-S. Guo, Polymer 40 Ž1998. 261. w21x Y. Haba, E. Segal, M. Narkis, G.I. Titelman, A. Siegmann, Synth. Met., in press. w22x K. Levon, K.-H. Ho, W.-Y. Zheng, J. Laakso, T. Karna, ¨ ¨ T. Taka, ¨ J.-E. Osterholm, Polymer 36 Ž1995. 2733. w23x B. Wessling, J. Phys. II 6 Ž1996. 395. w24x D.M.C. Heymans, M.F. Daniel, Polym. Adv. Technol. 6 Ž1995. 291. w25x E.M. Boczar, B.C. Dionne, Z. Fu, A.B. Kirk, P.M. Lesko, A.D. Koller, Macromolecules 26 Ž1993. 5772. w26x L.M. Robeson, M.S. Vratsanos, ACS Div. Polym. Mater.: Sci. Eng., Proc. 80 Ž1999. 565.
Polyaniline–DBSArpolymer blends prepared via aqueous dispersions Y. Haba a , E. Segal a , M. Narkis a , G.I. Titelman a b
b,)
, A. Siegmann
b
Department of Chemical Engineering, Technion — IIT, Haifa 32000, Israel Department of Materials Engineering, Technion — IIT, Haifa 32000, Israel Received 16 June 1999; accepted 13 October 1999
Abstract Stable polyaniline–dodecyl benzene sulfonic acid ŽPANI–DBSA. aqueous dispersions were obtained by a unique method of aniline polymerization in the presence of DBSA, through an anilinium–DBSA complex appearing as solid needle-like particles, in an aqueous medium. The average size of the PANI primary particles, determined by small angle X-ray scattering ŽSAXS., is 18.7 nm. These primary particles form aggregates, which further cluster into ; 50 mm agglomerates. PANI–DBSArpolymer blends were obtained by mixing an aqueous PANI–DBSA dispersion with an aqueous emulsion of the matrix polymer, followed by water evaporation. These blends exhibit electrical conductivity already at a very low PANI–DBSA content Ž0.5 wt.%.. The conductivity level of the various blends depends on the PANI content, on the surfactant present in the polymer matrix emulsion, and it is practically independent of the polymer matrix nature. Thus, a similar structuring mechanism prevails in these blends, irrespective of the polymer matrix Žcontrary to solution and melt blends.. The PANI–DBSA particles strongly segregate within the polymer matrix, already in the combined aqueous dispersion, and upon drying, a very fine conductive network is formed. This strong segregation tendency leads to a conductive network formation already at low PANI–DBSA contents, thus generating the conductive blends. q 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Polyaniline; Polymerization; DBSA; Aqueous dispersions; Blends
1. Introduction Intrinsically conductive polymers, including polyaniline ŽPANI., have the utilization potential in a large number of applications: rechargeable batteries, conducting paints, conducting glues, electromagnetic shielding, antistatic formulations, sensors, electronic devices, light-emitting diodes, coatings etc. However, the major problem in applying these polymers is their poor processability. Several methods were used to overcome this problem including N-substitutions w1–3x or ring substitutions w4,5x of aniline by aliphatic large radicals, block copolymers w6x and doping PANI base with a functionalized protonic acid w7,8x. Colloidal particles have the potential to be finely dispersed in a polymer medium due to their small size. Therefore, when colloidal particles of an intrinsically conducting polymer are used to form, for example, PANIrpolymer blends, fine conductive paths may generate and high conductivity levels may be realized already at low PANI contents. Thus, the synthesis of colloidal PANI ) Corresponding author. IMI ŽTAMI., P.O.B. 10140, Haifa 26111, Israel
particles w9–14x and their application in PANIrpolymer blends w6,13,15–20x have been of great interest. Beadle et al. w15x obtained electrically conductive polyanilinercopolymer blends by polymerizing aniline in the presence of a chlorinated copolymer latex. Xie et al. w20x used one-step in situ emulsion polymerization to obtain polyanilinerSBS blends by polymerizing xylene solution of aniline and polyaniline–dodecyl benzene sulfonic acid ŽDBSA. in the presence of xylene solution of SBS. The use of polymeric steric stabilizer to stabilize PANI colloidal particles and form conductive blends has been reported w6,16–19x. Gospodinova et al. w13x, used colloidal PANI–DBSA dispersions for the preparation of blends with both, water soluble ŽPVAL. and water insoluble ŽEVA. host polymers. The PANI–DBSA dispersion was mixed with the matrix polymer solution and then cast. In this article a unique method of aniline polymerization w21x in the presence of DBSA, in an aqueous medium, is presented. In this polymerization method DBSA, a functionalized protonic acid, acts both, as a surfactant and a dopant. First aniline and DBSA at a stoichiometric ratio react to form a fine dispersion of solid needle-like anilinium–DBSA complex which upon the addition of an oxi-
0379-6779r00r$ - see front matter q 2000 Published by Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 2 8 0 - 5
Y. Haba et al.r Synthetic Metals 110 (2000) 189–193
190
Table 1 Compression molding conditions of blends containing PANI Matrix polymer
Sintering temperature Ž8C.
Final temperature Ž8C.
PS PMMA Commercial polyacrylate
110 120 80
180 160 120
dizer polymerize forming a PANI–DBSA stable dispersion in the aqueous medium. The stable aqueous dispersions obtained can be used to form various low PANI content conductive blends, by blending with other polymer latices.
2. Experimental
PMMA: MMA Ž135 g., potassium persulphate Ž0.25 g. and water Ž1200 g. were mixed in a reactor at 708C under nitrogen atmosphere for 6 h. 2.3. Blend preparation PANIrpolymer blends were obtained by simple mixing of the aqueous PANI–DBSA dispersion with an aqueous emulsion of the matrix polymer, followed by either water evaporation or film casting. Surface morphology of the PANI blends Žpowders or films. and the neat matrix polymers Žpowders or films. was studied using a LEO-982 high-resolution scanning electron microscope ŽHRSEM., at an accelerating voltage of 1 kV, using the SE IN-LENS technique. The samples were placed on a carbon tape for observation. 2.4. Compression molding of PANI–DBSA r polymer blends
2.1. Polymerization of aniline Aniline Ž6 g. was mixed with DBSA Ž21.6 g. in water Ž1200 g. for 3 h to form a homogeneous white dispersion of anilinium–DBSA complex. The dispersion was cooled Ž; 08C., and a solution of ammonium peroxydisulfate ŽAPS. Ž15 g dissolved in 50 g water. was added dropwise. Polymerization was carried out at ; 08C for 5 h. The dispersion’s color changed from white through blue to dark green. At the end of polymerization a green and stable PANI–DBSA dispersion was obtained, containing ; 0.8 wt.% doped PANI Ždetermined by complete precipitation with methanol.. The PANI–DBSA particle’s size was measured using a Coulter LS230 size analyzer. Small angle X-ray scattering ŽSAXS. of the PANI–DBSA dispersion was used to characterize the size of the primary PANI particles and the distance between PANI–DBSA layers. A Philips X-ray generator equipped with a Kratky compact camera collimator ŽA. Paar., and a position-sensitive linear detector ŽRaytech. were used for the SAXS analysis.
PANIrpolymer blends Žpowders or films. were sintered under pressure followed by heat treatment of the free samples and fast cooling. Table 1 details the sintering and final treatment temperatures for each of the PANIrpolymer blends. The conductivity of the compression-molded strips and films was measured using the four-probe technique ŽASTM D-991-89..
3. Results and discussion 3.1. PANI–DBSA dispersion A stable PANI–DBSA aqueous dispersion was obtained at the end of the polymerization process. Since the molar ratio of aniline:DBSA fed into the reactor was 1:1, while the maximum molar ratio in doped PANI:DBSA is 1:0.5, at the end of the polymerization process excess DBSA is present in the reactor w21x. This excess DBSA forms
2.2. Matrix polymer aqueous emulsions A commercial aqueous acrylic latex Ž48% solid content, ; room temperature Tg . was supplied by B.G. Polymer ŽKibbutz Beit-Govrin, Israel.. Polystyrene ŽPS. and polyŽmethyl methacrylate. ŽPMMA. were synthesized according to the following procedures. PS: Styrene Ž525 g., sodium lauryl sulphate Ž0.3 g., Triton X-100 Ž6 g., Na 4 P2 O 7 P H 2 O Ž0.4 g. and water Ž500 g. were mixed in a 2-l reactor at 708C under nitrogen atmosphere. A mixture of sodium lauryl sulphate Ž1.3 g., Triton X-100 Ž22.6 g., potassium persulphate Ž1 g. and water Ž450 g. was added dropwise. The polymerization process proceeds 6 h.
Fig. 1. A schematic model of aqueous dispersions of polyaniline with DBSA.
Y. Haba et al.r Synthetic Metals 110 (2000) 189–193
191
Fig. 2. Conductivity vs. PANI–DBSA content of the various blends.
micellar structures and also acts as a surfactant, which stabilizes the dispersion of the PANI–DBSA particles. The hydrophobic tails of free and bonded DBSA molecules are arranged in a way that they all turn to each other, while the hydrophilic groups of the free DBSA turn to the aqueous phase ŽFig. 1.. The molecular conformation of PANI– DBSA chains is affected both by the molecule–molecule Žintramolecule. interactions and molecule–aqueous medium interactions as suggested by Levon et al. w22x. Using SAXS, it was calculated that the average size of the doped primary PANI particles is 18.7 nm, in accordance with Wessling’s results w23x. These primary particles generate aggregates, which further cluster into agglomerates. These agglomerates, about 50 mm in size, are located within gel-like units, which are structured due to formation of hydrogen bonds between free DBSA molecules. These hydrogen bonds form an infinite network which stabilizes the PANI–DBSA dispersion. 3.2. Blends containing PANI An extremely important result of the present study is that upon mixing a PANI–DBSA dispersion with a matrix
Fig. 4. A high-resolution SEM micrograph of PANI–DBSArpolystyrene powder Ž4.5% PANI..
polymer emulsion, the 50 mm PANI–DBSA agglomerates disintegrate and a visually homogeneous mixture is obtained. The conductivity of the various PANI– DBSArpolymer blends vs. PANI–DBSA content is depicted in Fig. 2. The conductivity of these blends gradually increases with PANI–DBSA content, without exhibiting a sharp percolative insulator–conductor transition. At 0.5 wt.% PANI–DBSA, the conductivity is already 10y6 Srcm and it tends to level off above 2 wt.% PANI–DBSA. The absence of a sharp percolative insulator–conductor transition Ža gradual conductivity rise is observed. in the studied PANI blends is ascribed herein to a very significant and fast segregation process taking place already in the combined PANI–DBSArpolymer aqueous dispersions. This strong segregation stems from the different surface characteristics of the PANI–DBSA particles and the matrix polymer particles Žrefer to the different surfactants used to stabilize the particles, as given in the experimental part.. Recall also that segregation in the present systems takes place in a very low viscosity aqueous medium, thus very likely a fast process, contrary to a segregation phenomenon in solution cast films, or within a polymer melt. High-resolution SEM depicts 100–200 nm spherical emulsion PS particles ŽFig. 3., whereas the PANI–DBSA particles in the PANI–DBSArPS blend form very thin layers, Table 2 Conductivity of films and compression-molded films of PANIrpolyacrylate based blends at various PANI–DBSA contents
Fig. 3. A high-resolution SEM micrograph of the polystyrene particles obtained by emulsion polymerization.
PANI–DBSA content Ž%.
Film conductivity ŽSrcm.
Compressionmolded film conductivity ŽSrcm.
0.5 1.0 1.5 4.5
1.4ey4 1.2ey3 7.9ey3 3.3ey2
1.5ey6 5.6ey4 3.8ey3 2.3ey2
192
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Fig. 5. Conductivity of compression-molded strips of PANIrPS blends made of unwashed and washed powders.
much thinner than the PS particle’s size, covering the PS particles and occasionally forming some elongated structures ŽFig. 4.. The strong segregation and formation of the thin PANI–DBSA layers yield conductive blends already at very low PANI–DBSA contents. The similarity of the conductivity curves for the various polymer matrices is a further proof that a key factor playing role in the segregation process is the particle surface characteristics Žeach polymer particle is coated with its surfactant., rather than the character of the polymer particle itself. This segregation mechanism again is extremely different from the one in solution and melt blending where the PANI–DBSA particles ‘‘feel’’ the matrix polymer and segregate accordingly. Using a specially synthesized PS emulsion, stabilized by DBSA in a blend with the PANI–DBSA aqueous dispersion has resulted in lower conductivity levels due to a remarkably reduced level of segregation, Židentical surface characteristics of the PS and PANI particles.. The conductivity values of cast and compression-molded PANI–polyacrylate blends at various PANI–DBSA contents are compared in Table 2. The compression molding
Fig. 6. A high-resolution SEM micrograph of the neat acrylic polymer film.
Fig. 7. A high-resolution SEM micrograph of PANIracrylic polymer film Ž4.5% PANI..
process reduces the conductivity level of the blends, especially at the lower PANI–DBSA contents. Water rinsing of
Fig. 8. High-resolution SEM micrographs of PANIracrylic polymer film Ž4.5% PANI. after 7 months at Ža. high, and Žb. low magnification.
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the blends before compression molding increases the blends’ conductivity ŽFig. 5., especially for the lower PANI content blends. Water rinsing may partially remove some excess free DBSA, thus improving contact among the conducting particles, which is more significant for the lower PANI–DBSA contents. In the latter, the quality of the conductive networks is still low and becomes even lower upon application of pressure and heat. Blends of a PANI–DBSA dispersion with the polyacrylate aqueous emulsion are film forming at room temperature due to the low Tg of the acrylic polymer Žcontrary to the PS-based blends which are non-film forming at room temperature.. Upon water evaporation, the submicron polymer particles are getting closer, till maximum contact is achieved, forming hexagonal structures which subsequently coalescence into an homogeneous film w24,25x. In the last stage of this film-forming mechanism, an immiscible emulsion blend will not mix perfectly, thus leading to significantly poor properties of the resulting blend w26x. The surface morphology of a cast neat acrylic film is rather smooth ŽFig. 6., while the surface morphology of a cast PANI–DBSAracrylic film ŽFig. 7. includes some hexagonal-like structures. Interestingly, it was found that the conductivity of this film slowly decreases with time; a decrease of two orders of magnitude in 7 months. In parallel, the film surface morphology has also changed ŽFig. 8.. The small hexagonal-like structures have coalesced, forming much larger hexagonal units Žan average hexagonal size has changed from 2 to 40 mm.. It seems that during this coalescence process of the acrylic particles, PANI particles have been ‘‘squeezed’’ out and formed isolated spherical particles Žblack particles in Fig. 8b.. These structural changes taking place at the ambient temperature are due to progression of the coalescence process. The observed conductivity decrease is related to gradual destruction of the PANI–DBSA network resulting from the structural changes in the acrylic matrix. 4. Conclusions Polymerization of aniline in the presence of DBSA in an aqueous medium, through an anilinium–DBSA complex, forms a stable PANI–DBSA dispersion. The average size of a doped primary PANI particle is ; 18.7 nm. These primary particles generate aggregates, which further cluster into agglomerates ; 50 mm in size. The stability of the aqueous PANI–DBSA dispersion is caused by the presence of excess DBSA molecules which form strong hydrogen bonding with the PANI–DBSA particles, creating agglomerates. Upon mixing the PANI– DBSA aqueous dispersion with an aqueous emulsion of another polymer Žstabilized with a different surfactant., the PANI agglomerates disintegrate to form a visually homogeneous dispersion. Compression-molded strips and cast films of PANI– DBSArpolymer blends become conducting at a very low
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PANI–DBSA content Ž0.5 wt.%. and attain conductivity levels of 10y2 to 10y3 Srcm at 1—2% PANI–DBSA content for the different polymers studied. These blends exhibit similar conductivity levels at a given PANI content, indicating a common mechanism of the conductive structure formation, irrespective of the polymer matrix. The PANI–DBSA particles segregate within the polymer matrix, already in the combined aqueous dispersions and upon drying form a very fine conductive network. A low-Tg matrix with PANI–DBSA shows conductivity reduction with time due to structural changes caused by coalescence progression with time at room temperature. Acknowledgements The authors are grateful to the Israel Ministry of Science for partially supporting this research. References ¨ w1x W.Y. Zheng, K. Levon, J. Laakso, J.E. Osterholm, Macromolecules 27 Ž1994. 7754. w2x J.-W. Chevalier, J.-Y. Bergeron, L.-H. Dao, Macromolecules 25 Ž13. Ž1992. 3325. w3x S.E. Chapman, N.C. Billingham, S.P. Armes, Synth. Met. 55–57 Ž1993. 995. w4x Y.H. Liao, M. Angelopoulos, K. Levon, J. Polym. Sci., Part A: Gen. Pap. 33 Ž1995. 2725. w5x Y. Wei, R. Hariharan, S.A. Patel, Macromolecules 23 Ž1990. 758. w6x C. Dearmitt, S.P. Armes, J. Colloid Interface Sci. 150 Ž1. Ž1992. 134. w7x Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 48 Ž1992. 91. w8x A.J. Heeger, Synth. Met. 55–57 Ž1993. 3471. w9x N. Kuramoto, E.M. Genies, Synth. Met. 68 Ž1995. 191. w10x J. Stejskal, P. Kratochvil, N. Gospodinova, L. Terlemezyan, P. Mokreva, Polymer 33 Ž1992. 4857. w11x B. Vincent, J. Waterson, J. Chem. Soc. Chem. Commun. Ž1990. 683. w12x N. Gospodinova, P. Mokreva, L. Terlemezyan, J. Chem. Soc. Chem. Commun. Ž1992. 923. w13x N. Gospodinova, P. Mokreva, T. Tsanov, L. Terlemezyan, Polymer 38 Ž3. Ž1997. 743. ¨ w14x J.-E. Osterholm, Y. Cao, F. Klavetter, P. Smith, Synth. Met. 55–57 Ž1993. 1034. w15x P. Beadle, S.P. Armes, S. Gottesfeld, C. Mombourquette, R. Houlton, W.D. Andrews, S.F. Agnew, Macromolecules 25 Ž1992. 2526. w16x J. Stejskal, P. Kratochvil, N. Radhakrishnan, Synth. Met. 61 Ž1993. 225. w17x B. Vincent, Polym. Adv. Technol. 6 Ž1994. 356. w18x P. Banerjee, M.L. Digar, S.N. Bhattacharyya, B.M. Mandal, Eur. Polym. J. 30 Ž4. Ž1994. 499. w19x S.P. Armes, M. Aldissi, J. Chem. Soc. Chem. Commun. Ž1989. 88. w20x H.-Q. Xie, Y.-M. Ma, J.-S. Guo, Polymer 40 Ž1998. 261. w21x Y. Haba, E. Segal, M. Narkis, G.I. Titelman, A. Siegmann, Synth. Met., in press. w22x K. Levon, K.-H. Ho, W.-Y. Zheng, J. Laakso, T. Karna, ¨ ¨ T. Taka, ¨ J.-E. Osterholm, Polymer 36 Ž1995. 2733. w23x B. Wessling, J. Phys. II 6 Ž1996. 395. w24x D.M.C. Heymans, M.F. Daniel, Polym. Adv. Technol. 6 Ž1995. 291. w25x E.M. Boczar, B.C. Dionne, Z. Fu, A.B. Kirk, P.M. Lesko, A.D. Koller, Macromolecules 26 Ž1993. 5772. w26x L.M. Robeson, M.S. Vratsanos, ACS Div. Polym. Mater.: Sci. Eng., Proc. 80 Ž1999. 565.