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Semi-Crystalline Polypyrrole:Polystyrene Sulfonate Synthesized through the Pores of Filter Paper.

INTRODUCTION

Polypyrrole (PPy) has been attracted considerable interest because of its good electrical conductivity, environmental stability, and easy synthesis process [1-5]. On the other hand, pristine PPy is insoluble, infusible, brittle, and amorphous due to the rigidity of its molecular chains from the quinoidal and aromatic states. Poor conductivity and solubility not only retarded further analysis of its structure but also provided only limited commercial applications in solid devices and electronics [6, 7], Several studies have been performed to overcome the limitations by adding the appropriate side groups [8-11] and altering the ion types of the dopants used [12, 13]. Determining the relationship between charge transport and polymeric morphology is also a key factor to improve the electronic conductivity of [pi]-conjugated polymers. The electrical conductivity of a conducting polymer must involve not only charge transport along the polymer backbone, but also charge hopping between the neighboring polymer chains [14]. Intermolecular hopping may be related to the overlap of polymer chains, which could be strongly dependent on how the neighboring chains are packing with each other. The packing of polymer chains is much more important to transport charges because there is insufficient charge migration along the [pi]-conjugated chain before hopping to the neighboring chains. The ordered chain packing in the crystalline regions produces a high degree of local chain orientation, which is likely to result in higher conductivity [15, 16]. A small change in the amount of crystallinity has a large effect on the conductivity due to the reduction of energy required for interchain charge transport [17], On the other hand, there are no reports of the relationship between the crystallinity of the [pi]-conjugated polymer and its electric conductivity. Therefore, dependence of the electrical conductivity on the degree of crystallinity has become an important issue that needs to be addressed.

In this article, semi-crystalline PPy:PSS composites with different crystallinity were synthesized through the micro-pores of filter paper used as a channel to control the diffusion speed of the initiator and to select the molecular weight of the PSS template. The crystalline region of the resulting PPy:PSS composites was increased gradually by increasing the pore size of the filter paper. The synergetic effect between the degree of crystallinity and the electric conductivity was observed as a function of the pore size used. Several trials have been examined in basic research on the relationship between the crystallinity of [pi]-conjugated polymer and its electric conductivity, but these have not yet led to any successful result.

EXPERIMENTAL

Materials

Pyrrole was purchased from Aldrich and distilled prior to use. Fe[Cl.sub.3] from Aldrich was used without further purification as an oxidant. The sodium salt of polystyrene sulfonate (PSS, [M.sub.w]=70,000) from Aldrich was used as a template. De-ionized water (Human power. Water Purification System) was used as the solvent for chemical oxidative polymerization.

Synthesis of Semi-Crystalline PPy :PSS Composites

As shown in Scheme 1, microporous filter paper was used as a dividing wall in a two-compartment cell to control the diffusion speed of the Fe[Cl.sub.3] initiator and to separate selectively the molecular weight of the PSS template. In the left compartment, 10 mM pyrrole was stirred in 100 mL of de-ionized water at pH 1.5. The oxidant of 10 mM Fe[Cl.sub.3 and the 10 mM PSS template were allowed to diffuse slowly through the filter paper from the right compartment. Fe[Cl.sub.3] and PSS were diffused toward the pyrrole in the left side and was reacted with pyrrole to yield a semi-crystalline PPy:PSS. A series of experiments adopted filter papers with various pore sizes of 0.2,0.45, 0.8, and 1.0 [micro]m for a 24 h reaction time. The filter paper was removed from the cell and rinsed with de-ionized water to remove the residual initiators and unreacted monomers. The resulting PPy: PS S composites were reacted selectively at only the left compartment cell. Chemical oxidative polymerization was performed at 0[degrees]C of an ice bath at pH 1.5.

Characterization of the Synthesized PPy.PSS

Fourier transform infrared (FT-IR, CARY-640, Agilent) spectroscopy of the obtained PPy:PSS composites was carried out to confirm the chemical structures. The optical properties of PPy:PSS synthesized at various pore sizes were investigated using a Jasco V-630 UV-Vis spectrophotometer over the wavelength, 200-1,100 nm. The electrical conductivity of the semicrystalline PPy:PSS composites were evaluated using a standard four-point probe technique, called A.I.T. CMT-SR 1000N. The crystallite size of the individual PPy:PSS composites prepared using different filter papers was evaluated in the angular range, 10[degrees] [less than or equal to] 2[theta] [less than or equal to] 70[degrees], by X-ray diffraction (XRD, Rigaku Miniflex) with continuous intensity sampling every 0.05[degrees] at a detector speed l[degrees]/min and a detector slit width of 0.5 mm. The weight change of PPy:PSS composites with different crystallinity was determined by thermogravimetric analysis (TGA, TA instruments Q50) at a heating rate of 10[degrees]C/min.

RESULTS AND DISSCUSSION

Figure 1 shows the FT-IR absorption spectrum of the PPy:PSS composites synthesized in the presence of filter paper. The molecule showed located at five major absorption bands: 1,528, 1,467, 1,285, 1,142, and 1,047 [cm.sup.-1]. Four characteristic peaks could be assigned to C=C stretching vibration of the pyrrole ring, C-N stretching vibration in the ring, and C-H in-plane vibrations, respectively. The characteristic peak at 1,047 [cm.sup.-1] could be associated with the -S[O.sup.-.sub.3] group, which indicated that PPy had been doped with PSS.

Figure 2 presents the optical behaviors of the PPy:PSS composites synthesized using various pore sizes of 0.2, 0.45, 0.8, and 1.0 [micro]m. In all reactions, the bipolaron absorption peaks of PPy:PSS were observable at approximately 456 nm. As pore size is increased, the optical behaviors of all reaction mixtures were shifted slightly to a high wavelength. The results suggest that the characteristic peaks assignable to the bipolaron absorption of the sulfonate-doped PPy chains were observed in all final products, regardless of the pore size. In addition, the individual optical density of all PPy:PSS composites was stabilized after 24 h and increased with increasing pore size.

XRD was carried out to determine the quantitative crystallinity of the individual PPy:PSS composites synthesized through different pore sizes. Figure 3 presents XRD patterns of the PPy:PSS composites as a function of the pore size. The PPy:PSS composites obtained with the 1.0 [micro]m filter paper had much higher crystallinity than those of the PPy:PSS composites produced from the filter papers with the 0.2-0.8 [micro]m pore size. The broad peak at 2[theta] = 25[degrees] in the overall samples could be attributed to the amorphous nature due to scattering from the PPy chains at the interplanar spacing [17, 18]. On the other hand, the prominent peaks with sharpness in the XRD pattern could reveal the crystalline nature of the PPy:PSS composites synthesized through micro-pores. The sharp patterns at 2[theta] = 17.7[degrees], 20.2[degrees], and 22.8[degrees] may be assigned to the spacing between the ring planes of the benzene ring in the adjacent PPy chains. Similarly, the sharp patterns observable in region between 20 = 32.4[degrees] and 58[degrees] may be due to the closed packing of benzene rings in the crystalline domains of the PPy:PSS composites. Table 1 lists the percentage crystallinities calculated from the XRD curves in all samples. The crystalline region of PPy:PSS composites increased with increasing pore size of the filter paper. The PPy:PSS composites prepared through 1.0 [micro]m pores formed a more crystalline region by 16.76%. The increase in crystallinity may be indicative of the accommodation of PPy chains with a similar length to the separated PSS templates, apparently strengthening the order of the crystalline regions.

Figure 4 shows the TGA curves to estimate the weight change of the individual PPy:PSS composites synthesized using different micro-pores. The thermal stability of the PPy:PSS composites in the range, 30-700[degrees]C, increased gradually with increasing pore size. The reduction of weight loss could be indicative of the increase in crystallinity during synthetic processing through micro-pores. The distinct enhancement of thermal stability at the 1.0 [micro]m micro-pores could also be indicative of the dramatic increase in the orderliness of the PPy chains formed compared to the different micro-pores in the range from 0.2 to 0.8 [micro]m.

Figure 5 shows the dependence of electrical conductivity and crystallinity of the PPy:PSS composites on the size of the micro-pores in the filter paper used for synthesis and Table 2 shows the relationship between conductivity and pore size. The results suggest that the enhancement of conductivity could be due to the initial linear increase in crystallinity between 0.2 and 0.8 [micro]m. At a pore size of 1.0 [micro]m, the conductivity increased dramatically with increasing crystallinity. This suggests that the crystalline region may provide sufficient paths to the charge transport process of the [pi]-conjugated polymer. In addition, the regular space of the separated PSS template could lead to more ready crystallization of the PPy chains.

CONCLUSIONS

Semi-crystalline PPy:PSS composites were synthesized successfully by chemical oxidative template polymerization through the micro-pores of filter paper. The crystalline influence on the conductivity was investigated by XRD and conductivity measurements. The enhanced crystallinity of the PPy chains provides the strengthened thermal stability of the PPy:PSS composites. The electrical conductivity of the PPy:PSS composites was varied by a factor of 1 within the range of crystallinities studied. The conductivity dependence of the crystalline portion could lead a dramatic role in the charge transport process.

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Kyung Seok Kang, (1) Chanhyuk Jee, (1) Ji-Hong Bae, (1) Eunyoung Kim, (1) Hyo Jin Jung, (1) Jung Yup Yang [C],2 PilHo Huh [iD] (1)

(1) Department of Polymer Science and Engineering, Pusan National University, Busan, South Korea

(2) Department of Physics, Kunsan National University, Gunsan, South Korea

Correspondence to: P. Huh; e-mail: pilho.huh@pusan.ac.kr or J. Yup Yang; e-mail: jungyup.yang@kunsan.ac.kr

Contract grant sponsor: The Ministry of Trade, Industry and Energy; contract grant sponsor: Korea Institute for Advancement of Technology (KIAT) through the Research and Development for Regional Industry.

DOI 10.1002/pen.24659

Caption: FIG. 1. FT-IR spectrum for polymerization of pyrrole with PSS in the presence of filter paper.

Caption: FIG. 2. UV-vis-NIR spectra of PPy:PSS composites prepared at various pore sizes of 0.2, 0.45, 0.8, and 1.0 [micro]m.

Caption: FIG. 3. XRD curves of PPy:PSS composites obtained through various pore sizes of 0.2, 0.45, 0.8, and 1.0 [micro]m.

Caption: FIG. 4. TGA curves of PPy:PSS composites synthesized through different micro-pore size.

Caption: FIG. 5. The crystallinity dependence of electrical conductivity of semicrystalline PPy:PSS composites.

Caption: SCHEME: Schematic representation of the two-compartment cell used to perform the chemical oxidative polymerization of PPy: PSS.
TABLE 1. Percentage crystallinity of different PPy:PSS
composites calculated from XRD curves.

PPy:PSS at pore
size ([micro]m)    Crystallinity (%)

0.2                      1.16
0.45                     2.63
0.8                      3.01
1.0                      16.76

TABLE 2. Electrical conductivities of semi-crystalline
PPy:PSS composites as a function of pore size.

Sample ([micro]m)    Conductivity (S/cm)

0.2                         0.007
0.45                        0.011
0.8                         0.015
1.0                         0.062
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Author:Kang, Kyung Seok; Jee, Chanhyuk; Bae, Ji-Hong; Kim, Eunyoung; Jung, Hyo Jin; Yang, Jung Yup; Huh, Pi
Publication:Polymer Engineering and Science
Article Type:Report
Date:Jul 1, 2018
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