PU) blends

Synthetic Metals 156 (2006) 1187–1193

Effect of temperature and moisture on electrical conductivity in polyaniline/polyurethane (PANI/PU) blends Hitoshi Yoshikawa a,b , Tetsuo Hino a , Noriyuki Kuramoto a,∗ a

Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa City, Yamagata Prefecture 992-8510, Japan b Material Technology Research and Development Laboratories, Tokai Rubber Industries, Ltd., 1 Higashi 3-chome, Komaki City, Aichi Prefecture 485-8550, Japan Received 5 May 2006; received in revised form 2 August 2006; accepted 19 August 2006 Available online 4 October 2006

Abstract Miscible blend of conductive polyaniline/polyurethane (PANI/PU) showed preferable electrical property at low percolation threshold compared to immiscible blend of PANI/polystyrene-isoprene-copolymer (PANI/SIS) and carbon black/PU composite (CB/PU). The time dependence of the electrical conductivity was investigated with these samples aged under different humidity and temperatures. The electrical conductivity of PANI/PU (11.5/88.5, v/v) decreased with aging time and the morphology changed with time in the coexistence of high moisture and high temperature. After the aging treatment, the film of the miscible blend was re-dissolved and re-cast. The morphology and electrical conductivity were found to recover to the same state as the original film. In addition, the recovery mechanism of the morphology and the conductivity was also proposed here. © 2006 Elsevier B.V. All rights reserved. Keywords: Polyaniline; Dopant; Blend; Moisture; Aging

1. Introduction Conducting polymers such as doped polyacetylene, polyaniline (PANI), and polythiophene have generated considerable interest in applied and fundamental researches since the 1970’s. These materials offer a unique combination of properties (e.g. processability and electrical conductivity) that make them very attractive in the electronic area [1–3]. Among all these conducting polymers, PANI seems to be the most promising candidate due to its interesting electrical property as well as its simplicity of synthesis. Since 1992 [4], it has been possible to process doped PANI into thin films using spin casting. In addition, the resulting PANI film exhibited a high electrical conductivity (σ > 300 S/cm) and a good environmental stability. Furthermore, in order to improve mechanical properties of these films, PANI can be blended with classical polymers (polystyrene, cellulose acetate, and so on) [5–7] to form films with high electrical conductivity and good mechanical proper-

∗

Corresponding author. Tel.: +81 238 26 3051; fax: +81 238 26 3051. E-mail address: [email protected] (N. Kuramoto).

0379-6779/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2006.08.007

ties. The blend films with conductivity varying between 10−11 and 300 S/cm can be easily fabricated and be used for antistatic applications (ESD) and electromagnetic shielding (EMI) applications [8,9]. Electrical conductivity between 10−7 and 10−5 S/cm is enough for ESD, while conductivity between 10−3 and 10−1 S/cm is necessary for EMI shielding. The conductivity of these blends drastically depends on the PANI content in the blend and a percolative component can be observed with the existence of a very low percolation threshold [5–7]. Recently, Ho et al. have reported that the miscibility between PANI and polyurethane (PU) could influence the mechanical properties remarkably [10]. Barra and coworkers have presented the improvement of the miscibility between PANI and nonpolar copolymer; polystyrene-block-poly(ethylene-ran-butylene)block-polystyrene copolymer (SEBS) [11]. On the other hand, the stability factor (i.e., time dependence of electrical properties) is a crucial parameter in case of industrial applications. Many conducting polymers are air unstable, polyacetylene being the more striking example [12]. Some works have shown that conducting polymers with aromatic rings (polypyrrole, PANI, polythiophene, etc.) seemed to be more stable in air [13]. Some studies on the electrical aging have been undertaken for polypyrrole samples [14–18], for doped

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Scheme 1. Structure of polyurethane (PU).

PANI samples [19–21], and for conducting polymer blends [22]. However, the stability of the blends which were aged under high humidity and high temperature has not been reported yet, as far as we are concerned. Such investigations might play an important role in the development of practical conducting products. The aim of this work is to investigate the electrical variation of the miscible PANI/polyurethane blend (PANI/PU) which is aged under the above-mentioned condition compared with the immiscible PANI/polystyrene-block-polyisoprene-blockpolystyrene copolymer blend (PANI/SIS) and carbon black/PU composite (CB/PU). The recovery mechanism of morphology and conductivity of the miscible blend (PANI/PU) is also discussed. 2. Experimental 2.1. Materials Aniline (Kanto Chemical Co.) was distilled under vacuum and stored at −10 ◦ C before used. Pentadecylbenzenesulfonic acid (PDBSA) was commercially supplied from Tayca Co., Osaka-fu, Japan and was dried under vacuum at 40 ◦ C for 2 h prior to use. Ammonium peroxodisulfate (APS, Kanto Chemical Co.) was used as purchased. Poly[4-4 -methylenebis(phenyl isocyanate)-alt-1,4butanediol/di(propylene glycol)/polycaprolactone] (PU, glass transition temperature = −40 ◦ C) shown in Scheme 1 was purchased from Aldrich Co. Polystyrene-block-polyisopreneblock-polystyrene copolymer was also purchased from Aldrich Co. Carbon black was the semi-reinforcing furnace (SRF) grade named Seast S commercially supplied by Tokai Carbon Co. Unless otherwise stated, other reagents and solvents were of analytical grade from Kanto Chemical Co. and were used without further purification. 2.2. Preparation of PANI The emulsion polymerization of aniline was performed by adapting the procedure described in the literature [23,24]. In a typical polymerization, 0.1 mol of PDBSA and 0.1 mol of aniline were dissolved in 200 mL of deionized water under stirring. The polymerization was initiated by drop-wise addition of 0.1 mol of APS under constant stirring at 0–5 ◦ C. After complete addition of the oxidizing agent, the reaction mixture was further stirred for 24 h. The resulting precipitate was filtered and washed with deionized water until the filtrate was colorless, and then the obtained PANI was dried under reduced pressure at room temperature for 12 h. Finally, the PANI was dissolved in tetrahydrofuran (THF) under ultrasonic irradiation.

2.3. Preparation of polymer blend and composite PANI/PU and PANI/SIS blends were prepared by using co-dissolution method in tetrahydrofuran, respectively. CB/PU composite as the reference was prepared in THF by mixing in a mortar. The blends and the composite were cast on the glass substrates and stainless steel, and the solvent was evaporated slowly at 60 ◦ C under ambient atmosphere for 1 h. Free standing films with thickness around 20 ␮m were obtained. 2.4. Measurement techniques The aging experiments were performed on samples in a temperature and humidity chamber (Tabai Espec Corp. model PL2FP) in the temperature range from 25 to 80 ◦ C and the humidity range from 50 to 95%. The electrical conductivity and its time dependency were measured at 25 ◦ C and ambient atmosphere using the two-point method (Keithley 237 High-Voltage Source Measurement Unit) as in the previous work [25]. Prior to such measurements, circular silver electrodes were deposited on the surface of the sample. The conductivity as a function of mechanical elongation was estimated by using the previous method [26]. Samples of 20 ␮m thickness were cut into a rectangular shape (100 mm × 10 mm), the ends of the samples were coated uniformly with silver paste as electrodes, and connected with two copper wires using an adhesive. Polytetrafluoroethylene film was sandwiched between the sample and clamps. The samples were elongated using the tensile tester with an elongation rate of 50 mm/min. The electric potential was kept at high voltage (100 V) to permit the measurement of low conductivity. The UV/vis/near-IR spectra of the blend films were recorded by using a JASCO V-570 spectrophotometer. Morphological studies were performed by optical microscope and transmission electron microscope (TEM). Optical microscopy observation was carried out using a Keyence VH8000 at 3000× magnification. The sample sheet for the TEM observation was cut by a microtome at −140 ◦ C to get an ultra-thin section (100 nm thick) and stained with osmium tetraoxide and ruthenium oxide. TEM observation was carried out at 100 keV by JEOL JEM-1010 at 50,000× magnification. 3. Results and discussion Fig. 1 shows the conductivity as a function of the volume fraction of the conductive component (PANI or CB) in (a) PANI/PU, (b) CB/PU, and (c) PANI/SIS. The abrupt increase of conductivity at a certain content of a conductive component is defined as the percolation threshold, which implies that certain conductive routes are formed in the insulating matrix. The percolation threshold value can be obtained by intersecting two tangent lines on the curve, one for the quick-change

H. Yoshikawa et al. / Synthetic Metals 156 (2006) 1187–1193

Fig. 1. Conductivity as a function of the volume fraction of the conductive component in the blends (a) PANI/PU, (b) CB/PU, and (c) PANI/SIS.

section and the other for the slow-change section. The blends and composite showed different conductivity as a function of volume fraction of conducting component (PANI or CB). At the same fraction, the conductivity increased in the order of (c) PANI/SIS, (b) CB/PU, and (a) PANI/PU up to 17.5% by volume fraction, and in case of more than 17.5% it increased in the order of (c) PANI/SIS, (a) PANI/PU, and (b) CB/PU. The values of percolation threshold of PANI/PU, CB/PU, and PANI/SIS were 7, 12.5, and 18%, respectively. PANI/PU should be most favorable to the formation of a conductive route by the self-assembly of the conductive component, in the case of lower conductivity region [<10−6 S/cm]. Fig. 2 shows the optical microscopy images of films prepared by casting; (a) PANI/PU (20/80, v/v), (b) CB/PU (16/84, v/v),

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and (c) PANI/SIS (20/80, v/v). PANI/PU revealed a homogeneous structure and the good miscibility of the blend, whereas PANI/SIS revealed typical phase separation morphology. In CB/PU the aggregations of CB were seen by the presence of isolated black “islands”. Thus, these results strongly suggested that the interactions between carboxyl groups in PU and imine groups in PANI could induce the miscibility of PANI/PU blend, in addition to the surfactant effect of long alkyl chain in PDBSA. The nanostructures of PANI/PU and CB/PU are observed more clearly in TEM images as shown in Fig. 3(a and b), respectively. The dark region corresponds to the conducting PANI phase in PANI/PU and CB particles in CB/PU. These TEM images suggested that the continuity of conductive component (PANI or CB) imbedded in insulating PU affected the electric conductivity of the blend. In PANI/PU as shown in Fig. 3(a), darker regions about 20 nm widths seemed to be connected like a net. This indicated that the polymer chains of PANI were in the expanded coil-like conformation and became self-assembled into continuous conductive pathways for the conduction of charge carriers. However, in CB/PU as shown in Fig. 3(b) the CB particles flocked and formed larger aggregates with size of >500 nm. The CB regions of the conductive component made much larger aggregates than that in PANI/PU and have the clear borders between dark and bright regions. This morphological feature of CB/PU was similar to the PANI/SIS case in Fig. 2(c). Fig. 4 shows the conductivity as a function of elongation of (a) PANI/PU, (b) CB/PU, and (c) PANI/SIS. For the samples (b) CB/PU and (c) PANI/SIS, the conductivity decreased more than two orders of magnitude after 100% elongation. This was because the distance between each conductive component

Fig. 2. Optical microscopy images of films (a) PANI/PU (20/80, v/v), (b) CB/PU (16/84, v/v), and (c) PANI/SIS (25/75).

Fig. 3. TEM images of (a) PANI/PU (20/80, v/v) and (b) CB/PU (16/84, v/v).

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Fig. 4. Conductivity as a function of elongation: (a) PANI/PU (11.5/88.5, v/v), (b) CB/PU (16.2/83.8, v/v), and (c) PANI/SIS (25/75, v/v).

increased with the increase of the elongation of the film. On the other hand, interestingly, for the miscible blend (a) PANI/PU, the conductivity did not change with the increase of the elongation. This should imply that the homogeneous structure kept the conductive pathways connected during the elongation. In addition, this phenomenon should be of importance from a viewpoint of development of soft conducting materials. Fig. 5 shows the electrical conductivity variation as a function of aging time at 80 ◦ C and humidity of 95% for conductive components. The decrease of electrical conductivity of PANI was slightly more than that of CB. This was assumed to have little influence on that of PANI/PU blend, because it was negligibly small. Fig. 6 shows the electrical conductivity variation versus aging time at 80 ◦ C and humidity of 95% for samples of (a) PANI/PU, (b) CB/PU, and (c) PANI/SIS. The CB/PU and PANI/SIS were found to possess excellent electrical stability for duration up to 120 days. By contrast, the PANI/PU blend showed substantial decrease only for 4 days duration. Fig. 7 shows the electrical conductivity variation for PANI/PU versus aging time under different conditions, (a) 80 ◦ C and humidity of 95%, (b) 80 ◦ C and ambient atmosphere, (c) 50 ◦ C and humidity of 95%, and (d) 25 ◦ C and ambient atmosphere. PANI/PU was found to have excellent stability under the condition (b and d) for duration up to 120 days, whereas it

Fig. 5. Time dependency curves of the electrical conductivity of the conductive components (PANI and CB) at 80 ◦ C and humidity of 95%.

Fig. 6. Time dependency curves of the electrical conductivity at 80 ◦ C and humidity of 95% for samples: (a) PANI/PU, (b) CB/PU, and (c) PANI/SIS.

showed poor stability under the conditions with high humidity, (a and c). The present results indicated that the high humidity coupled with high thermal annealing temperature significantly affected the decrease of electrical conductivity of PANI/PU. On the basis of the above results, the following processes might be considered to have taken place in PANI/PU during aging under high humidity and high temperature: loss and/or migration segregation of dopant, oxidation of the polymer chains, hydrolysis, or crosslinking [27]. The changes of the morphologies of the samples were investigated in order to elucidate the aging process in the miscible PANI/PU films. Fig. 8(a-1) indicates the TEM image of PANI/PU after the aging, and was compared to the blend before aging as shown in Fig. 3(a). The dark regions correspond to the PANI phase. Before the aging process, PANI/PU presented a very homogeneous morphology composed of continuous conducting pathways. This type of morphology explained the relatively moderate conductivity (10−5 to 10−9 S/cm). After aging the microstructure of the miscible blend of PANI/PU revealed typical phase separating morphology. The conducting pathways were evidently disconnected, which explained the decrease of electrical conductivity to 2 × 10−12 S/cm corresponding to that of insulating PU. In case of immiscible samples such as CB/PU and PANI/SIS, it is assumed that the conductive pathways formed by the large aggre-

Fig. 7. Time dependency curves of the electrical conductivity of PANI/PU (11.5/88.5, v/v) under different aging conditions: (a) 80 ◦ C and humidity of 95%, (b) 80 ◦ C and ambient atmosphere, (c) 50 ◦ C and humidity of 95%, and (d) 25 ◦ C and ambient atmosphere.

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Fig. 8. TEM image of PANI/PU (20/80, v/v): (a-1) after aging and (a-2) re-dissolved and re-cast the aged sample.

Scheme 2. The schematic representation of phase separation phenomenon of PANI/PU during aging under high humidity and high temperature.

gates of conductive component could be maintained throughout the whole aging process. The schematic representation of phase separation phenomenon is shown in Scheme 2. At the initial state, free PDBSA (not doped to PANI) acts as surfactant for PANI and induces the formation of expanded coil-like conformation of the PANI in the PANI/PU blend. The free PDBSA absorbs moisture and moves to the hydrophilic region under high humidity and high temperature, and the loss of PDBSA causes aggregation of PANI in PU matrix because PANI has intrinsic self-assembly nature. The increase of aggregation of PANI eventually leads to phase separation of the sample. On the other hand, CB/PU and PANI/SIS showed excellent stability as compared to PANI/PU. For CB/PU, the excellent stability should be due to the absence of PDBSA that was hygroscopic in nature. Even though PDBSA existed in PANI/SIS, it had little effect of dispersing the PANI in SIS matrix homogeneously, as shown by the presence of large aggregates in Fig. 2(c). Fig. 8(a-2) shows the morphology of the PANI/PU film that was re-dissolved and re-cast after aging at 80 ◦ C and humidity of 95% for 7 days, and the time dependency curves of the electrical conductivity are presented in Fig. 9. The morphology was interestingly found to be similar to that of before aging, and the electrical conductivity was almost same as the blend before aging (10−7 S/cm). In addition, the conductivity curve as a function of elongation of the re-dissolved sample was almost same as that of the original sample shown in Fig. 4(a). It is evident that the re-dissolving process made the aged blend recover to the original state with continuous conducting pathways.

Fig. 10 shows UV–visible spectra of PANI/PU (11.5/88.5, v/v) and PU polymer before and after aging. Those of PANI/PU exhibited the absorption band near 800 nm corresponding to the presence of localized polarons, responsible for the electrical conduction. On the other hand, those of PU did not contribute any optical bands along the UV spectra from 400 to 1100 nm. The absorption band near 800 nm was maintained throughout the aging process. The bands around 600–700 nm, due to the de-doped state of PANI, was undetected in the samples before and after aging. In addition, the peak of the sample appeared at 825 nm after aging, which was slightly red-shifted from 801 nm (before aging). This peak was remarkably different from the

Fig. 9. Time dependency curves of the electrical conductivity of PANI/PU under 80 ◦ C and humidity of 95% for 7 days and then re-dissolved and re-cast.

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Scheme 3. The schematic representation of recovering phenomenon by dissolving the blend after aging.

decrease of conductivity caused by the discontinuity of the conducting pathways. By re-dissolving and re-casting, the morphology and the conductivity of the blend after aging were recovered to almost similar to the original state before aging. As predicted, the free PDBSA should act as a surfactant for PANI and also play a significant role on the recovery of the morphology during the phase transition. Further investigations are now in progress to study the stability of PANI/PU blends under high humidity and high temperature, and the results will be reported elsewhere.

Fig. 10. UV–visible spectra of PANI/PU (11.5/88.5, v/v) and PU polymer before and after aging.

peak of de-doped state at 590 nm. The localized polarons in the PANI/PU blend, therefore, are presumed to be unaffected by aging at 80 ◦ C and humidity of 95%. These phenomena support our speculation that the decrease of electrical conductivity was caused by the phase separation of PANI in the blend. Based on the above results, the schematic presentation of the recovering phenomenon by dissolving the blend after aging is as illustrated in Scheme 3. At the first stage of the re-dissolving process, the aged blend exists in the state of separated phases composed of the aggregated PANI and the free PDBSA. However, in re-dissolving in THF, they are mixed together, and the free PDBSA acts as the surfactant for PANI again. Thus, the aggregated PANI in the blend is homogeneously re-dispersed again into the PU matrix. After casting and drying, the morphological state of the blend should recover to the initial state with the homogeneous conductive PANI pathways. 4. Conclusion In this article, electrical conductivity study of miscible PANI/PU blend aging under high humidity and temperature was presented compared with immiscible PANI/SIS blend and CB/PU composite. The aging study has been undertaken to clarify the mechanism of the electrical aging for the blends. Under the aging conditions, only the miscible blend with homogenous morphology showed phase separation, consistent with the

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