고분자산이 도핑된 폴리아닐린의 물성에 미치는

고분자산 구조의 영향

문 혁 수, 박 정 기

한국과학기술원 화학공학과

Structural Effect of Polymeric Acid on the Properties of

Polymeric Acid Doped Polyaniline

Hyuk-Soo Moon, Jung-Ki Park

Department of Chemical Engineering,

Korea Advanced Institute of Science & Technology

INTRODUCTION

Polyaniline (PANi) is one of the most important conducting polymers mainly due to its good environmental stability, and thus has attracted considerable interest in recent years.  However, up to now, commercial applications of PANi have been limited.  One of the obstacles to practical use of PANi is its poor processability when it was doped with conventional small molecule organic or inorganic acids.  With a rigid □-conjugated backbone molecular structure, PANi decomposes without melting when heated and few proper solvents have been found for PANi.  The conducting form of PANi powder synthesized in aqueous HCl reffered to as PANi hydrochloride is insoluble in common organic solvents and even in 1-methyl-2-pyrrolidone (NMP), which is the only organic solvent having been found so far that can dissolve high molecular weight PANi.  Moreover, these small molecular acid dopants for PANi may evaporate at room or higher temperature, causing a depression on the conductivity of the acid-doped PANi.  These kind of obstacles can be overcome by an appropriate way, that is, by using polymeric acid dopants.  In addition, polymeric acid dopants can be expected to enhance the flexibility of the PANi composite since the polymeric acid usually has lower glass transition temperature (Tg) than that of PANi.

However it has been seldom known about the micro-structural feature of the polymeric acid doped PANi including how much amount of the acid groups are capable of doping PANi and how is the dopability affected or changed by the molecular structure of polymeric acid such as copolymer composition and/or chemical structure of component monomers.  In addition, neither how the dopant interacts with PANi nor how the doped PANi subchains align have been investigated yet.  It is expected that not all of the acid groups in polymeric acid dope PANi and that nonuniform distribution of the acid groups would occur due to the confinement of protonic acid groups on polymer chains and due to the conformational hinderance created by the flexible chain.

This study has been devoted to investigate systematically the effect of the composition and comonomer structure of three kind of polymeric acid, i.e., poly (methyl methacrylate-co-p-styrenesulfonic acid) (PMMA-co-SSA), poly(styrene-co-p- styrenesulfonic acid) (PS-co-SSA), and poly(methyl methacrylate-co-2-acrylamido 2-methyl 1-propane sulfonic acid) (PMMA-co-AMPS), on the conductivity of PANi and on their dopability and subsequently on the doping level of PANi.

EXPERIMENTALS

Preparation of Materials.	The PANi hydrochloride powder was synthesized by the oxidative polymerization of aniline in 1M aqueous HCl with (NH4)2S2O8 as oxidant at 5℃, which is similar to the method used by MacDiarmid et al.  The powder was then converted to PANi base by treatment with 0.5M aqueous NH4OH followed by washing with water/methanol several times and dynamically evacuating at 40℃ for 2 days.  The PANi base obtained as above described has a ratio of IR absorption intensity at 1589 cm-1 (due to the quinoid ring) to that at 1496 cm-1 (due to the benzenoid ring) nearly the same as that of the emeraldine base with a 60 mol% oxidation level.  And thus the synthesized PANi base is an emeraldine base and becomes emeraldine salt after acid doping.

Two kind of styrenesulfonic acid based polymeric dopants, PMMA-co-SSA and PS-co-SSA, were respectively obtained by solution polymerization of poly(methyl methacrylate-co- styrene) (PMMA-co-St) and polystyrene using 2,2'- azobisisobutyro-nitrile (AIBN) as an initiator in tetrahydrofuran (THF) at 60℃, and followed by sulfonation.  The sulfonation was conducted at 50℃ in ethylene dichloride using acetyl sulfate as a sulfonation agent, which had been prepared by the reaction of acetic anhydride and concentrated sulfuric acid.  The acid compositions in PMMA-co-SSA were controlled by varying the feeding mole ratio of styrene over methyl methacrylate during the polymerization of PMMA-co-St, that is all the styrene in PMMA-co-St was converted to styrenesulfonic acid, and those in PS-co-SSA were determined by the amount of acetyl sulfate added during the sulfonation reaction.  The other type of sulfonic acid in this study, AMPS based polymeric dopant, PMMA-co-AMPS was also synthesized using solution method, where AIBN and water were used as an initiator and reaction medium respectively.

Doping of Polyaniline.  The polymeric acid-doped PANi was prepared by mixing appropriate volumes of the 0.22M PANi (based on the approximate repeat unit -C6H4NH-) in NMP solution and 0.22M polymeric acid (based on its acid content) solution.  The used solvent for PMMA-co-SSA and PS-co-SSA was NMP, whereas dimethyl formamide (DMF) was selected for PMMA-co-AMPS because NMP was a poor solvent for PMMA-co-AMPS, especially for high-AMPS content one.  The mixed solutions of polymeric acid-doped PANi were then agitated vigorously and casted on a evaporating dish under dynamic vacuum at 50-60℃ for more than 48h.

Properties Measurements.	Electrical dc conductivities measurements of the compressed pellets of three kind of polymeric acid-doped PANi were made using conventional four-point probe technique at room temperature.

RESULTS AND DISCUSSION

The conductivities of pellets of PSSA and PAMPS doped PANi with the compositions 1/4, 1/2, 1/1, and 2/1 are shown in Figure 1, where the compositions  are expressed in mole ratios of the sulfonic acid units of polymeric acid to those of PANi.  It is noticeable that the conductivities of PSSA doped PANi are higher than those of PAMPS doped PANi for all ranges of compositions examined in this study execpt for the composition of 1/4 which seems to contain insufficient amount of polymeric acid dopants to elucidate the structural effect of polymeric acid dopants on the conductivities of the polymeric acid doped PANi.  Anyway, it is clear from Figure 1 that the room temperature dc conductivitiy of PSSA doped PANi is higher than that of PAMPS doped PANi of the same composition up to about 2 orders of magnitude.  This indicate that the conductivity of polymeric acid doped PANi are strongly dependent on the structural feature of the polymeric acid dopant, and thus the large difference of the conductivities of PSSA and PAMPS doped PANi are able to be explained as follows under the consideration of chemical structure of polymeric acid dopant.  The PSSA doped PANi may align more perfectly than the PAMPS doped PANi because of the styrenic phenyl group of PSSA capable of aligning with aromatic backbone of PANi which PAMPS does not have.  It is also proposable that the conductivity difference between PSSA and PAMPS doped PANi is attributable to the dopability differnce of PSSA and PAMPS which is again a function of compatibility with PANi, structure of dopant such as steric hinderance upon the moving of pendant acid group, and desociation energy of acid proton in the acid groups.

Figure 1 shows another important result, that is, there exists an optimum composition of polymeric acid to PANi showing maximum conductivity, and the optimum composition is the same for PSSA doped PANi and PAMPS doped one, 1/1.  As the polymeric acid content increases, the doping level of PANi increases, however volume fraction of PANi in the polymeric acid/PANi composite decreases. Although the conductivity may be enhanced with the doping level, there is a limit in doping level and the polymeric dopant extraneously added may lower the volume

fraction of PANi in the composite and substantially reduce the conductivity.  Thus there exists a optimum composition, 1/1, which means that this composition is sufficient to dope PANi completely, at least apparently.

Figure 2. shows the composition effect of PMMA-co-SSA, PS-co-SSA, and PMMA-co-AMPS on the conductivities of PMMA-co-SSA, PS-co-SSA, and PMMA-co-AMPS doped PANi, respectively.  For all the polymeric acid, the conductivity of polymeric acid doped PANi decreases as the acid content in the polymeric dopant decreases.

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