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Introduction
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Conductive polymers have been the subjects of study for many decades as possible synthetic metals. Many of these polymers, especially those with a conjugated p -bond system, often yield higher conductivity once having undergone the doping process. However, practical uses of conducting polymers are not very likely because of their poor mechanical properties (such as strength and processibility) that rarely meet industrial requirements. Thus, the unique combination of electronic and mechanical properties of blends of conductive polymers with conventional polymers seems to have great promise for many applications (Laska et al, 1997). Since the conducting polyblends are stable and retain the mechanical properties of the host polymer, films, fibers, and coatings can be fabricated by solvent evaporation or by melt-processing for use in anti-static applications, for electromagnetic shielding and/or absorption, for transparent conducting films, etc (Heeger, 1995).A blend is a mixture of two polymers. Sometimes, a bifunctional linker is used, as in the case of this project, to increase the force of attraction between the two polymeric components. In much of the research done in this field, it has been found that the intrinsic properties of the components of the blends were changed. Sometimes, the properties, as shown by Kim and Levon, were enhanced (Kim and Levon, 1996), but for the most part, they were diminished. For example, in an A/B blend system, the conductivity and the mechanical properties of the blend is usually poorer than those of the respective polymers. Thus, the objective of this research is to study the effects of para-hydroxybenzenesulfonic acid (PHBSA), which serves as a bifunctional linker (or the dopant) between polyaniline (PANi) and polycaprolactone (PCL), on the doped PANi/PCl blends (Figure 1). |
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Figure 1: Schematic of PANi/PCL interpolymeric complex, with PHBSA as the bifunctional linker The purpose of a bifunctional linker is to increase the system's conductivity and to promote stronger interaction between PANi and PCL. Blends of various PANi and PCL ratios were prepared to study the influences a bifunctional linker would have on the final properties of a blend system. Polyaniline is one of the oldest conductive polymers known. It was first prepared by Letheby in 1862 by anodic oxidation of aniline in sulphuric acid (Kumar and Sharma, 1998). It has been known as an electrically conductive polymer (ECP) for the past thirty years. ECPs are able to conduct electricity because of their conjugated p -bond system, which is formed by the overlapping of carbon p orbitals and alternating carbon-carbon bond lengths. In some systems, such as polyaniline, nitrogen pz orbitals and CPANi was chosen for this experiment because it offers the possibility of providing properties to end products that are very difficult to obtain by existing commercial methods (Virtanen et al, 1997). In addition, it has a conjugated double bond structure, the benzenoid ring, between the quinoid imine and the benzenoid amine structures, which renders the polymer a candidate as an ECP (Figure 2). |
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Figure 2: Structural formula of undoped PANi (EB) PANi exists in three forms of oxidation states: leucoemeraldine (fully reduced or only benzenoid amine structures), emeraldine (neutral or partially reduced and partially oxidized), and pernigraniline(fully oxidized or only quinoid imine structures). The emeraldine-based (EB) form of PANi was used for this research because only doped EB PANi is conductive among the three oxidation states. The emeraldine-based form of PANi is also the most stable of the three states because leucoemeraldine is easily oxidized when exposed to air and pernigraniline is easily degraded. PANi was doped with PHBSA in a 1:2 ratio or for every repeat unit of PANi, two molecules of PHBSA was used (Figure 3). According to Kumar and Sharma, conductivity of ECPs can be increased several-fold by doping them with oxidative/reductive substituents or by donor/acceptor radicals (Kumar and Sharma, 1998). Doping is the process by which polymers that are insulators or semi-conductors as synthesized are exposed to charge transfer agents (dopant) in the gas or solution phase or through appropriate electrochemical oxidation or reduction. This process will increase the polymer's ability to conduct electricity because of the increased concentration of charge carriers (McAndrew, 1997). |
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Figure 3: Structural formula of doped PANi and PCL blend system |
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Pure PANi, in the undoped state, is a poor semiconductor with conductivity of about 10-8 S/cm. However, once it is doped, its conductivity could increase by a factor of 10 |
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The new benzenoid ring, although more stable than it was previously, still has high energy because of the repulsion force from the adjacent positive charges. In order to stabilize this structure, the positive charge of one of the hydrogen ions will attract electrons from the neighboring benzene ring, neutralizing the charge. This will create a new positively charged nitrogen group with a neutral nitrogen atom in between the two positive ones. The increased distance between the two positive charges results in the polaron structure, which has a lower energy level than the bipolaron structure (Figure 5). Since initially, the ionic interactions between PANi and PHBSA anions are weak, the PHBSA anions are able to migrate to the newly formed positively charged nitrogen atom as the doped PANi complex stabilizes itself by transforming into the polaron structure. The migration of PHBSA ion tends to be easier when in solution form. |
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PHBSA anion has one hydroxy functional group. This functional group (-OH) can, theoretically, be used to establish weak hydrogen bonds between doped PANi and the carbonyl functional groups of PCL molecules; thus, one of the reasons PCL was chosen for this project (Figure 3). In addition to having a carbonyl functional group, PCL also possesses relatively better mechanical properties than PANi EB. These mechanical properties include tensile, compressive, and flexural strength, impact resistance, hardness, abrasion resistance, and tear resistance (Stevens, 1990). It is hoped that hydrogen bonding would exist between PANi and PCL in the presence of a bifunctional linker and that the blends would possess both the conductive property of PANi and the mechanical properties of PCL. The existence of such an interaction would greatly improve the possibility of applying PANi/PCL blends to their various applications. In this paper, the various properties of the blends will be analyzed using a variety of equipment and techniques. Optical microscopy and the hot stage have been applied for studying the true homogeneity and morphology of the blend systems. The conductivity of PANi/PCL solvent cast films has been measured by the two-probe method; the effects PANi has on the d-spacings of PCL have been analyzed using X-ray diffraction. The doping level of PANi was analyzed using ultraviolet-visible (UV-vis) spectroscopy. In addition, Fourier Transform Infrared Spectroscopy (FTIR) has been used to find evidence of interpolymer interactions. The glass transition points and the melting points of PANi, PCL, and their blends have been measured using differential scanning calorimetry (DSC) and the decomposition temperature of PANi has been measured using thermogravimetric analysis (TGA). |
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Figure 4: Bipolaron structure of PANi EB
Figure 5: Polaron structure of PANi EB