A Literature Review Of Synthesis And Chemical Structure Biology Essay

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Polypyrrole (PPy): a conducting polymer. Research work around the globe on the synthesis, electronic and chemical structure, properties and applications of PPy has been thoroughly reviewed several times in the literature [1-3]. In most cases, PPy has been considered as a one-dimensional polymer having a poly-conjugated backbone. Different spectroscopic techniques such as UV-photoelectron spectroscopy (UPS), electron energy loss spectroscopy (EELS), electron paramagnetic resonance spectroscopy (EPR), and near edge X-ray absorption fine structure spectroscopy (NEXASS) have been used to investigate the chemical and electronic structure of PPy [4-7]. Although there is still some uncertainty about the proposed theory of conduction mechanism in electronic band structure of PPy, it is widely accepted that the conductivity is achieved by generating intermediate states also known as nonlinear excitations, or polarons and bipolarons [8-11].

First of all, the electronic and chemical structures of PPy are briefly described in the following section of this chapter. X-ray photoelectron spectroscopy (XPS), infrared (IR), and nuclear magnetic resonance (NMR) spectroscopic studies in the literature indicated that the chemical structure of PPy is not linear but rather distorted and branched with several types of defects (discussed in the following section) [12].

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Furthermore, various synthetic routes of PPy including most commonly used methods like electrochemical polymerization and chemical polymerization, and less studied methods like photopolymerization, enzyme catalyzed polymerization and plasma polymerization have been discussed in terms of their procedure, advantages and disadvantages. In addition, the polymerization of pyrrole, which is assumed to follow a mechanism proposed by Hsing et al., has also been described in this chapter. Since the present research work in this thesis involves the preparation of composites or coatings of PPy with non-conducting polymers, a brief literature review on up-to-date research on conducting PPy composites with non-conducting polymers is given following by the literature review on the use of UV polymerization for the preparation of non-conducting polymer coatings (Section 2.2).

Electronic and chemical structure of PPy

The conduction mechanism in conducting polymers can be elucidated by understanding the electronic energy band structure. Using several different spectroscopic techniques, the band structures of various conducting polymers have been demonstrated in the literature. For example, the band structure of PPy is shown in Figure 2.1 [2, 13].

Figure 2.. Electronic energy band structures of neutral PPy, PPy with one polaron, PPy with one bipolaron and fully doped PPy (with many bipolarons and polarons)[2]

In neutral PPy with the benzenoid structure, there is a band gap of 3.2 eV between the valence band (VB) and conduction band (CB). Due to high energy band gap, it is almost impossible for electrons to transfer from the VB to CB at room temperature. Therefore, neutral PPy behaves like an insulating polymer with nearly negligible electrical conductivity. However, upon an electron extraction from polymer chain by oxidative process, a partial positive charge is created, which causes a local deformation in the chain leading to the quinoid structure (Figure 2.2). The partial charge formation is called doping and normally occurs during polymerization. In order to gain chemical neutrality and thereby stability of PPy chain, counteranions in the reaction medium are incorporated into the polymer chain by binding with the positive charges [2, 13].

Figure 2.. Electronic structures of neutral PPy, polaron and bipolaron in doped PPy [2]

A polaron is a state associated with the intermediate energy level formed in the band gap of PPy due to the formation of a positive charge, an unpaired spin, and a counteranion. As shown in Figure 2.1, as a result of the formation of a polaron, two new intermediate states that are bonding and antibonding are formed within the band gap. Further oxidation removes another electron from PPy chain, resulting in the formation of a bipolaron, which is the combination of two positive charges over about four pyrrole rings (Figure 2.2). The formation of bipolaron is energetically more favored than the formation of two polarons having two positive charges and two unpaired spins. As the oxidation continues, more bipolarons and polarons are generated, resulting in the overlapping of energy states with the formation of many new intermediate band levels. Thus, highly doped state contains closely packed band structure that allows facile electron transfer from VB to CB [13].

Figure 2.. Possible chemical structure of PPy with conformational and chemical defects [12]

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Typically, the doping level in PPy can be achieved up to 40 mol% at which bipolarons serve as predominant charge carriers. It is theoretically proved that the electrical conductivity in a material is directly proportional to the product of the concentration of charge carriers and their mobility [14]. Although the concentration of charge carriers in PPy at this level is four to five orders of magnitudes higher than that of inorganic semiconductors, the electrical conductivity of PPy is found to be in the range of semiconductors only. The low conductivity occurs as a result of reduced mobility of charge carriers due to low degree of crystallinity and other defects. Therefore, the mobility of charge carriers must be enhanced in order to achieve higher electrical conductivity.

The total conductivity of a conducting PPy depends on the combination of intra-chain (charge transfer along the polymer chain) and inter-chain (charge transfer from one chain to neighboring chain) mobility of charge carriers. However, the mobility can sometimes be restricted by the conformational arrangements and chemical defects, for instance α-α (2-2΄), α-β (2-3΄), or α-γ (2-4΄) coupling with non-linear rotation, and formation of carbonyl and hydroxyl groups within the polymer chains due to over-oxidation (Figure 2.3) [12].

Synthesis of PPy

Although various methods have been proposed so far for the preparation of PPy, two methods, namely chemical and electrochemical oxidative polymerizations, have been widely used and investigated. It is believed and widely accepted that the polymerization process in both methods follows the same mechanism in which the coupling occurs between radical cations that are generated from the oxidation of monomers [3, 15-16].

As shown in Figure 2.4, the oxidation of a pyrrole monomer produces a radical cation. Coupling between two of these radical cations generates bipyrrole by releasing two protons. Since this bipyrrole is more easily oxidizable than monomer, it is easily oxidized to give rise to radical cations, and thereby couple with another radical cation of monomer or bipyrrole to produce an oligomer. These steps continue to occur until the formation of PPy. The driving force for the polymerization process is the lower oxidation potentials of polymeric and oligomeric species than that of the monomer. The polymerization process ceases to occur when the chain length of polymer exceeds the solubility limit of the solvent resulting in the precipitation of PPy. Various possible presumptions about the termination step such as the nucleophilic attack on the active polymer chain and radical cation attack on the monomer have been proposed but not fully confirmed [16].

Figure 2.. Mechanism of pyrrole polymerization via coupling of cationic radicals [16]

Electrochemical polymerization

In electrochemical polymerization, PPy films are generally prepared on the anodic working electrode from a solution containing pyrrole monomer and an electrolyte salt. The electrochemical polymerization of PPy was first performed by Dall'Ollio in aqueous sulphuric acid on a Pt electrode [17]. In a subsequent work, Diaz et al. prepared electrochemically free-standing PPy films on Pt electrode with excellent electrical and mechanical properties [18-19]. Generally, PPy obtained from the electrochemical method displays better conducting properties. Thus far the highest conductivity reported for PPy film doped with hexafluorophosphate prepared via electrochemical method at room temperature was 2 X 103 S/cm [20].

Although PPy films can be easily synthesized electrochemically on inert electrodes such as platinum, gold, glassy carbon and stainless steel in aqueous or organic solvents, electrochemical polymerization of pyrrole on the oxidizable metals was one of the challenging problems during recent decades [2]. Since the oxidation potentials of these metals are more negative than that of pyrrole, the dissolution of metal occurs before the polymerization of pyrrole takes place. Significant research has been contributed to resolve this problem by obtaining passivation of the metal, which slows down dissolution of the metal without hindering the polymerization of pyrrole [21]. To date, several investigations on the effect of a wide variety of polymerization conditions by varying different parameters such as pH, solvents, temperature, current density and applied potential on the formation and properties of resultant PPy films have been reported [22-24].

Chemical polymerization

PPy was first synthesized by Angeli et al. in 1916 via chemical oxidation of pyrrole with H2O2 to produce a black amorphous powder [25]. However, it was found to be insoluble in any common organic solvents. Subsequently, many researchers have used various oxidizing agents such as FeCl3, Fe(NO3)3, Fe(ClO4)3, HNO3, PbO2, CuCl2, CuBr2, (NH4)2S2O8 and many others to prepare PPy and reported the conductivity of PPy and the reaction rates. Among all the oxidizing agents, ferric salts produced high conductive PPys. On the other hand, cupric salts produced PPy with considerable conductivity next to ferric salts. The main advantage of chemical oxidative polymerization over the electrochemical method is that the PPy can be easily produced in large quantities with low cost. Generally, the electrical conductivities of PPys obtained from chemical polymerization are lower than those of PPys prepared from electrochemical polymerization. Several researchers have reported the effects of various reaction parameters such as the type of oxidant, the concentration of oxidant, reaction time, temperature, stoichiometry and solvent on the final properties such as conductivity, stability, and morphology of chemically synthesized PPy [1, 26].

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Chemical polymerization in the presence of surfactants

The physical properties of the conducting polymer are highly influenced by the method of preparation, the characteristics of other additives in the reaction mixture, and the reaction conditions. For instance, the effect of surfactant on the morphology, conductivity and thermal stability of the chemically synthesized PPy has been reported by Omastova et al. [27] . In their investigation, various surfactants of anionic, cationic and non-ionic types were studied with results reported for the yield, conductivity and thermal stability of the PPy when doped with dodecylbenzenesulfonic acid (DBSA) and sodium dodecylsulfate (SDS). While anionic surfactants interact with positively charged PPy through strong ionic bonding, the interaction of cationic and non-ionic surfactants is much weaker. It was proposed that the improvement in the conductivity in the case of DBSA and SDS could be attributed to the presence of the mode of in-plane deformation vibration of N+H2 on protonated nitrogen found in FTIR especially for the incorporation of bulky anionic surfactants.

Kudoh et al. reported that the use of surfactant increased the rate of polymerization. Further, it was found that the presence of phenolic derivatives containing electron-withdrawing groups such as 3-nitrophenol along with the anionic surfactant showed enhanced conductivity, thermal and air stability probably due to the synergistic interaction of phenol derivative with pyrrole and oligomers during the polymerization [28].

Kim et al. synthesized rod-type PPy doped with p-Toluenesulfonic acid (pTSA), via micelle formation. Doped PPy samples were prepared with different concentrations of pTSA. It was found that the best result could be obtained when the ratio of pTSA to pyrrole monomer was 2.0. It was concluded that at this ratio, PPy would exhibit high crystallinity, dispersity and thermal stability [29]. In another study by Lee et al. for the preparation of soluble PPy, the increase in the concentration of surfactants such as DBSA, which contains bulky alkyl groups, was determined to lead to an increase in the doping level. This was found to cause doped PPy to be soluble in m-cresol, NMP and chloroform [30]. Soluble PPy was also prepared by functionalizing PPy with functional substituents via the insertion of chlorosulfonyl and sulfonic acid groups [31].

Colloidal particles of PPy were prepared via micro-emulsion-polymerization using several emulsifiers by Moon et al. in order to elucidate the relationship between the morphology and photoluminescence (PL) [32]. It was observed that a clear trend in the morphology of PPy particles with the increase in the concentration of cationic surfactants occurred. It was also mentioned that the morphology was greatly influenced by the oxidant chosen. Finally, it was concluded that the highest PL intensity could be obtained when the particles were small and uniformly dispersed in the medium. Using a novel oxidizing agent, Benzoyl Peroxide (BPO) and through inverted-emulsion-polymerization, PPy was synthesized by Palaniappan et al. under the presence of both pTSA, which is an acid cum surfactant, and SDS, which is an anionic surfactant. The results showed that the high conductivite PPy was obtained when the ratios of concentrations of BPO, SDS and pTSA with pyrrole are 1.2:1, 1:3 and 2:1 respectively [33].

Photopolymerization

Even though photopolymerization of functional monomers and oligomers such as acrylates and thiol-ene systems has been in practice for several decades, this technique is considered as one of the less evolved for the polymerization of conducting polymers [26]. Despite several advantages of this technique including faster rates of polymerization, low energy consumption and reduced VOC, only a very few reports on photopolymerization of conducting polymers have been found upon literature survey. One of the major advantages of photopolymerization is that it can used to prepare films and coatings on both conducting and non-conducting substrates where electrochemical and chemical processes have limitations and drawbacks.

Recent studies on photopolymerization of pyrrole in the presence of copper, gold and silver salts as electron acceptors under the UV light have shown interesting results for the incorporation of metal nanoparticles into PPy [34-38]. Omowunmi Sadik and his co-workers investigated the fate and role of metal nanoparticles incorporated into photopolymerized PPy [35]. It was shown that the rate of film formation of PPy depends upon the metal salt used. Aqueous pyrrole solutions containing CuSO4 and AgNO3, which generated films with the exclusion of UV light after 48h, could actually generate films in the presence of UV light within 2 hours. On the other hand, the solutions containing AuCl3 showed immediate darkening in the presence of UV light and yielded the black precipitate within few minutes upon the addition of monomer to the solutions. The conductivities of films prepared from these metal salts on fiberglass were in range from 1 x 10-2 to 1.1 x 10-6 S/cm, which are much lower than those of prepared from chemically and electrochemically.

Figure 2.. Mechanism of pyrrole photopolymerization in the presence of AuCl3 [35]

Since the films observed from scanning electron micrograph contained metal particles, the mechanism of photopolymerization appeared to involve reduction of the metal salts. Equations 1 and 2 shown in Figure 2.5 in the case of AuCl3 indicated the loss of two electrons from each mole of pyrrole (Py), resulting in the reduction of 2 moles of gold cations to gold metal. Consequently, the coupling of Py cation radicals, further oxidation of polymer by anions in the electrolyte, and association of the anions with the polymer chain would take place [35].

Photopolymerized PPy films containing silver nanoparticles were derived from formulations consisting of pyrrole as monomer, silver salt as electron acceptor/dopant, and a radical or cationic photoinitiator [39]. In this work, it was found that silver nitrate (AgNO3) is far superior electron acceptor for photopolymerization of pyrrole to other silver salts such as silver perchlorate (AgClO4), silver nitrite (AgNO2), and silver tosylate (AgTs) in terms of the rate of reaction, yield, and conductivity. In the case of photoinitiators, both radical (Irgacure 784) and cationic (Iracure 261, Cyracure 6974, and Cyracure 6990) types were used to investigate the effect of photoinitiator on the rate of photopolymerization process. Also it is well-known from the literature that the polymerization process of pyrrole proceeds via the formation of cation radicals, dimmers, and eventually polymer chains. From the experimental results, it was found that cationic photoinitiators, especially Irgacure 261, demonstrated faster curing rates than radical photoinitiators as indicated by the faster rate of formation of dark and solid film. Although increasing the amount of photoinitiator in the formulations increased the curing rate, it caused a linear decrease in conductivity indicating the possible loss of conjugation in PPy chains.

Despite the higher standard reduction potential of Ag+ (0.8V vs. SHE) than Fe3+ (0.771V vs. SHE) and Cu2+ (0.153V vs. SHE), the oxidation of pyrrole by Ag+ cations in the absence of UV light, in which case it is considered to be only chemical oxidation, is very slow as compared to the oxidation rates of pyrrole by Fe3+ and Cu2+ [40]. In fact, the rates of chemical oxidation of pyrrole decrease in the order: Fe3+ > Cu2+ > Ag+. Although the actual reason for this phenomenon is not yet completely understood, it has been suggested that these discrepancies in the reaction rates may arise due to the differences in the hydration energy of different metal cations (Fe3+ < Ag+ < Cu2+).

Previously, there were reports on multiphoton-sensitized polymerization and self-sensitized polymerization of pyrrole [41-42]. From these studies, it has been proposed that pyrrole monomers absorb light in the near UV region to form photoexcited pyrrole molecules. The excited pyrrole molecules are then immediately quenched by unexcited pyrrole molecules, giving rise to both cationic and anionic pyrrole radicals. The pyrrole cationic and anionic radicals react with pyrrole molecules to form dimers, oligomers, ultimately polymer. However, it was shown the resultant brown film exhibits very low values of conductivity due to either small number of negative dopant molecules or lower intermolecular interactions between polymer chains.

A few reports have discussed the probable mechanisms of photopolymerization of pyrrole in the presence of an electron acceptor such as AgNO3 [34, 39, 43-44]. They all have shown that the mechanism involves the formation of a complex between Ag+ cation and two pyrrole monomers, as observed in the case of other metal cations such as Fe3+ and Cu2+ as well [45]. However, some of these reports mentioned that upon UV illumination the photoinitiation step may involve the promotion of Ag+ cation into a photoexcited state, which becomes more reactive, thus leading to faster oxidation rate of pyrrole; whereas other reports claimed that the photoexcitation may occur in monomer first, and then the Ag+ accepts the electron form monomer by oxidizing the excited monomer.

Figure 2.. Schematic representation of photopolymerization processes [46]

However, according to Kobayashi et al., photopolymerization leading to conducting polymer synthesis can generally be divided into two categories: (1) photopolymerization with photocatalytic system consisting of sensitizer and electron acceptor; (2) photopolymerization via photo-excitation of monomer itself and its oxidation in the presence of electron acceptor (Figure 2.6) [46]. Therefore, as discussed before, the photopolymerization process involving AgNO3 can be considered as either of those two categories where AgNO3 can serve as either both sensitizer and acceptor or only acceptor.

Photopolymerization of pyrrole has also been performed by using ruthenium, cobalt and copper complexes. Here the mechanism is assumed to follow the first category in which these complexes may act as photosensitizers which upon UV illumination accepts electrons from monomers. Shimidzu and his co-researchers reported that the deposition of PPy/Cl occurred on Nafion membranes from an aqueous pyrrole solution under the visible light irradiation in the presence of [Ru(bipy)3]2+ (bipy = 2,2´- bipyridine) as the photosensitizer and [CoCl(NH3)5]3+ as a sacrificial oxidant [47]. However, this powder product showed lower conductivity (3 X 10-4 S/cm) than the ones prepared from traditional chemical and electrochemical oxidation methods.

As shown in Figure 2.7, the mechanism of this photochemically initiated polymerization was thought to proceed via photo-excitation of [Ru(bipy)3]2+ to ([Ru(bipy)3]2+)*, oxidation quenching of ([Ru(bipy)3]2+)* to [Ru(bipy)3]3+ by Co(III) complex, and followed by the oxidation of pyrrole by [Ru(bipy)3]3+. Other researchers used Cu2+ complexes such as [Cu(dpp)2]2+ (dpp = 2,9-diphenyl-1,10-phenanthroline) as the photosensitizer and p-nitrobenzyl bromide as the sacrificial oxidant to deposit conducting PPy patterns on various substrates such as paper and glassy carbon [48].

Figure 2.. Photochemically initiated polymerization of pyrrole [47]

Photoelectrochemical deposition of PPy on various semiconductors such as n-type GaAs and n-type Si wafers, and as well as TiO2 particles has been attempted to prevent photodegradation of the semiconductor surfaces and improve photo-efficiency in their applications for solar cells [49-51]. In these attempts, an external bias was employed to the semiconductor to initiate the polymerization. In addition to the applied electric voltage, the application of UV light generates holes in the semiconductor by pushing the electron into conduction band (LU) (Figure 6), and the photogenerated hole in semiconductor can oxidize pyrrole to initiate the polymerization. Then, Ag+ ion that is present in the solution can serve as electron acceptor to accept the electron from the conduction band of semiconductor.

Figure 2.. (a) Schematic representation of active image formation of conducting polymer induced by photoillumination (b) micrograph of photopolymerized polyaniline [46]

The incorporation of TiO2 particles into PPy was performed by using photoelectrochemical process in which the combination of both electrolysis of pyrrole and photo-oxidation of pyrrole by UV illuminated TiO2 dispersions was utilized [52]. In this method, the valence band gap of TiO2 was sensitized using UV irradiation in the presence of oxygen as sacrificial electron acceptor and fluoroborate as a counteranion. This was presumed to cause the photochemical conversion of pyrrole to PPy. Further, the as-synthesized PPy coated TiO2 nanoparticles were found to show improved long-term visible light photoactivity for the generation of hydrogen. In fact, TiO2 particles in suspensions became negatively charged upon the application of UV radiation in the presence of hole scavengers such as fluoroborate. The formation of negative charge helped the TiO2 particles to successfully incorporate into positively charged PPy.

Recently, photopolymerization has been utilized to fabricate functional conducting polymeric materials with optical and electrical properties for image formation [46]. As can be seen in Figure 2.8, the image formation has been obtained via the polymerization and subsequent electrochromism of conducting polymer, both induced by photoillumination. Electrochromism is a phenomenon in which the spectral changes of a material can be processed by the application of external stimuli. Photopolymerization of pyrrole by AgNO3 has also been employed in producing humidity sensors by coating polyester-based substrate with PPy/TiO2 composites to study the effect of photopolymerized composite on the electrical and humidity sensing properties [53-54].

Similar to chemical and electrochemical polymerizations, the photopolymerization process can also allow incorporating various foreign molecular species such as flexibilizers and stabilizers into photopolymerization formulations to improve the processibility and mechanical properties of PPy. Generally, the incorporation of large amphiphilic (surfactant) organic anions such as sodium dodecyl sulfate (DDS) and sodium dodecylbenzne sulfonate (DDBS) as dopants into polypyrrole matrices is found to improve the mechanical properties and solubility of chemically and electrochemically prepared PPy. The films containing these additives showed greater flexibility and as well as excellent adherence to Mylar and Teflon substrates [55].

Enzyme-catalyzed polymerization

The use of enzymes such as horseradish peroxidase (HRP) as catalysts with oxidants such as peroxides (H2O2) has recently shown considerable interest for the synthesis of polyaniline and PPy [56-58]. Polymers produced from this method are generally of low molecular weights and extensive chain branching. However, it has also been proved in a recent study that the problem of chain branching can be resolved through a sophisticated approach using polyelectrolytes such as polystyrenesulfonate (PSS) as templates [59]. In this method, PSS can not only help monomer molecules to align prior to polymerization but also provide the counterions for doping the synthesized polymer, and enable polymer to be soluble in water.

One of the major advantages of the enzyme-catalyzed polymerization is the considerably higher pH reaction solutions as compared to the chemical and electrochemical polymerizations. There are few instances found in the literature where biological polyelectrolytes such as DNA have also been used as templates [60-61]. In this case, the alignment required for aniline monomers occurs through the electrostatic interactions between the DNA phosphate groups and protonated aniline monomers.

Plasma polymerization

Plasma polymerization has been considered as an interesting process to obtain thin films of various polymers including conducting polymers [62-63]. This polymerization can be done in gas phase without any chemical oxidants. Since the mechanism involves fragment formation and trapped radicals, the plasma-polymerized PPy is believed to have a different chemical structure from that of chemically, electrochemically and photochemically polymerized PPy. Figure 2.9 shows the proposed chemical structure of plasma-polymerized PPy [64].

Figure 2.. Proposed chemical structure of plasma-polymerized PPy [64]

The synthesis of nano and meso spherical iodine doped particles of plasma-polymerized PPy using glow discharges of pyrrole has been reported [65]. The conductivity in these PPy/I particles has been found to increase from 10−9 to 10−6 S/m with the increase in humidity from 80% to 90% RH. Cruz et al. has deposited the PPy/I films that have shown increase in conductivity from 10-7 to 10-3 S/m at relative humidity of >90%. The increase in conductivity can be attributed to the improved mobility of PPy chains and the interactions of dissolved iodine ions with water molecules [66].

Conducting PPy composites and coatings with non-conducting polymers

Composites of intrinsically conducting polymers are generally materials that are prepared by integrating the conducting polymer such as PPy with at least one secondary component that can be organic, inorganic or biologically active materials. The major function of the secondary component is to improve any of chemical, physical, optical, electrical and mechanical properties of conducting polymer, depending on the requirements for end-use application of the composite. In general, non-conducting polymers such as poly (vinyl alcohol) (PVA), polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly (vinyl acetate) (PVAc) are combined with the conducting polymers in order to overcome some of the physical and chemical limitations of conducting polymers, including the processibility and mechanical and thermal stability [67].

The key factors in optimizing the properties of resultant composite are the ratio of PPy versus the insulating polymer, the processibility of two components in solution and the degree of dispersion of PPy inside the insulating polymer [68]. Nevertheless, the degree of dispersion depends on the solubility of PPy and the insulating polymer matrix in the solvent and the ability of solvent in swelling the matrix to form homogeneous domains for the incorporation of PPy.

Since it may be difficult to obtain uniform dispersion of PPy particles in most of the common solvents, it was shown that the polymerization of pyrrole in the presence of dissolved polymer matrix can facilitate the process of facile incorporation and proper dispersion of PPy in the polymer matrix [26]. For example, the composite of PPy/polycarbonate has been synthesized through the polymerization of pyrrole with FeCl3 as oxidant in a solution containing dissolved polycarbonate in CHCl3. The resultant composite was observed to contain uniformly dispersed PPy throughout the polycarbonate matrix [69].

Similarly the composite of PPy/poly(ethylene-co-vinyl acetate) [PEVA] has been produced by dissolving host polymer matrix and pyrrole in toluene and then oxidizing the pyrrole with FeCl3 [70]. The conductivity of this composite was reported to be approximately 5 S/cm. The processing of these composites into films and other shaped objects was performed by hot-pressing at approximately 100-150°C and 15-20MPa pressure for 1 hour. In another report by the same group, the composite of PPy/poly(alkyl methacrylate) has been prepared by dispersing poly(alkyl methacrylate) and pyrrole in an aqueous surfactant solution and further adding the oxidant for pyrrole polymerization. The same procedure of hot-pressing was followed to obtain films from these composites. These films exhibited conductivities up to 2 S/cm. A similar procedure was used to produce the composites of PPy/poly(styrene) and PPy/chlorinated co-polymers from latexes of corresponding insulating polymers. Although the conductivities of these films were considerably reduced from that of pure PPy, the thermal stability and mechanical properties were enhanced [71].

The preparation of composites of PPy with non-conducting polymers using electrochemical oxidation method has also been reported [67]. PPy/polyurethane composites have been prepared first by casting polyurethane on indium-tin oxide electrodes; where upon electrochemical polymerization of pyrrole has been carried out. PPy/poly(vinyl-methylketone) (PVMK) composites have also been prepared electrochemically by first dip-coating the insulating polymer and employing subsequent electrochemical polymerization of pyrrole.

Among non-conducting polymers, various hydrophilic and water-soluble polymers such as poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc), poly(methyl methacrylate) (PMMA), poly(N-vinyl pyrrolidin-2-one) (PVP) have also been used to prepare PPy composites in aqueous solutions. Interestingly, composites of PPy/poly(vinyl acetate) with 27% of PPy exhibited electrical conductivity almost comparable to that of pure PPy and mechanical properties similar to those of poly(vinyl acetate). Moreover, the environmental stability of this composite was also significantly improved as compared to that of pure PPy films [71].

The use of UV radiation in polymerization or curing of non-conducting polymers

Radiation curing is a process in which a liquid becomes solid by the irradiation of UV light or electron beam. It causes significant changes to physical properties of material such as viscosity, solubility, adhesion, color, and electrical conductivity [72]. This section briefly presents different aspects and recent advances in the UV-curing technology.

UV radiation curing has grown into an essential and pervasive technology which is the basis of numerous applications because of its unique advantages such as rapid process, low energy requirements, low temperature operating conditions, environmental friendly feature, and possible application onto various substrates [73-74].

UV radiation is well-known for its deleterious effects on organic matter which decomposes upon prolonged sunlight exposure. However, the beneficial effect of UV radiation is its ability to initiate chemical reactions such as polymerization by exciting the molecular bonds. A key requirement for successful UV radiation polymerization or curing is the match between the emission from the source and the absorption spectra of the material under the photochemical process. In order to achieve maximum efficiency with low cost, it is required that the UV source should produce high intensity UV radiation without generation of excessive infrared radiation. Commonly used UV radiation sources for commercial application are medium pressure mercury vapor lamps [75-76].

Typically, a UV curable formulation consists of photoinitiator, functionalized oligomer, and monomer acting as a reactive diluent. UV curable systems can be classified into two categories, depending on whether the chemical reaction follows via either the radical type or cationic type mechanism. In the radical type mechanism, by exposing the system to UV radiation, a large amount of free radicals will be generated to initiate the polymerization of oligomers and monomers through step growth addition mechanism. In the cationic type mechanism, a proton acid is generated by the exposure of UV radiation via photolysis of photoinitiator to initiate the polymerization of oligomers and monomers [73, 76]. The following sections briefly describe the free radical and cationic UV radiation curing systems.

Free radical UV curing system

Unsaturated resins containing high reactive groups such as vinyl double bonds are most often used in this type of system. The reactive vinyl bonds in these resins react with free radicals and lead to polymerization or curing. The polymerization process consists of three different steps: (1) initiation, (2) propagation, and (3) termination. In the initiation step, a photoinitiator effectively absorbs the incident UV light and produces highly reactive free radicals by the cleavage of electronically excited bonds. However, in order to obtain maximum efficiency of photoinitiator in generating free radicals, photoinitiator should usually have a high molar absorption coefficient in the range of UV radiation source spectrum and also have high quantum yield for radical generation [77].

In the propagation step, the reaction of the initiating species with the monomer and oligomer functional groups leads to the formation of polymer chains. Polymer chains containing active species at the ends become inactive by ending polymerization reaction via either recombination or decomposition mechanisms in the termination step. Thus this results in the formation of three-dimensionally cross-linked, insoluble and rigid macromolecular network. In general, the complete process of this network formation happens within seconds or less than second, depending on the reactivity of functional groups in oligomers and monomers. Figure 2.10 shows the mechanism of free radical UV polymerization [77].

Figure 2.. Mechanism of free radical UV polymerization [77]

Currently, acrylates in the radical type system occupy the highest market share. They are the most widely used resins due to high reactivity and large amount of available acrylate functionalized oligomers and monomers. In addition, acrylates are preferred rather than methacrylates especially for printing inks due to rapid cure at room temperature and also less oxygen inhibition effect. While acrylates with aliphatic structure produce low-modulus elastomers, acrylates containing aromatic structure produce hard and glassy materials. The basic physical properties of coatings are obtained by choosing proper oligomers. Most popular oligomers include acrylated epoxy, acrylated urethane, acrylated polyethers, and acrylated polyesters. The monomers, also known as reactive diluents, affect the viscosity, curing speed, final film properties, cost, shrinkage, and shelf life of the final coating formulation [76].

Based on the type of initiation reaction mechanism, the photoinitiator can be classified into two types: (1) unimolecular or cleavage type (Type I), and (2) bimolecular or abstraction type (Type II) respectively. As shown in Figure 2.11, the cleavage takes place at C-C bonds in the triplet state of a typical cleavage type photoinitiator. Some of the commonly used unimolecular Type I or cleavage type photoinitiators are benzoin ethers, hydroxyl alkyl phenyl ketones, dialkoxy acetophenones, benzoyl cyclohexanol, benzyl dimethyl ketals, and trimethyl benzoyl phosphine oxides [77-78].

Figure 2.. Cleavage type mechanism of benzoin acetal photoinitiator [78]

Bimolecular type photoinitiators produce radicals in the presence of a hydrogen donor such as amines, alcohols, and thiols. As shown in Figure 2.12, in a typical abstraction type or bimolecular type photoinitiator, the abstraction of hydrogen from the hydrogen donor by the photoinitiator generates the free radicals, which will initiate the polymerization process. Some of known abstraction type photoinitiators are benzophenones, camphorquinones, ketocoumarins, thioxanthones, and benzyls [77-78].

Figure 2.. Abstraction type mechanism of benzophenone radical photoinitiator

Oxygen inhibition is one of the major problems in free radical UV curing process. Since coatings generally have high surface area to volume, it causes high oxygen exposure leading to greater possibility of oxygen inhibition. Oxygen is a diradical and can react with the terminal free radical of a propagating chain or photoinitiator initiating species to form a peroxy free radical, which does not readily participate in the reaction (Figure 2.13). Thus, it leads to the termination of the growth of propagating chains. Several approaches such as using an inert atmosphere, paraffin wax, oxygen scavengers, high intensity UV sources, and high concentration of photoinitiator have been used to reduce this problem [73-74, 79-80].

Figure 2.. Schematic representation of the reactions possible for oxygen inhibition [80]

Cationic UV curing system

Cationic UV curing has recently become well-received technology due to its distinctive characteristics such as the absence of oxygen inhibition, low shrinkage, low monomer toxicity, and good adhesion [81]. In addition to coating and printing ink applications, cationic UV curing is also being employed for various other applications such as stereo- and photolithography [82]. Some of the typically used photoinitiators for cationic UV curing system are triarylsulfonium, diaryliodonium, and aryldiazonium salts. The photolysis of these salts under UV radiation generates the protons of super acids, which act as initiating species for the polymerization. The general schematic representation of mechanism is shown in Figure 2.14.

Figure 2.. Schematic representation of mechanism of cationic UV polymerization using diaryliodonium salt as photoinitiator [74, 83-84]

It involves the photoexcitation of the photoinitiator, for example diaryliodonium salt, followed by its decay from singlet state with both homolytic and heterolytic cleavage at the carbon-iodine bond. Subsequently, it releases cations, free radicals, and cationic-radicals. Since these aryl and aryliodine cations are highly reactive species, they can react with solvents, monomers, or impurities to produce protonic acids, HMtXn. These protonic acids are the principal initiators for the cationic polymerization. As seen in Figure 2.14, free radicals that are generated along with the formation of the cationic initiating species can also initiate free radical polymerization; thus it causes the increase in the rate of polymerization process leading to the formation of hybrid systems [83-84].

Most commonly used monomers for cationic UV polymerization are vinyl ethers, oxiranes, cyclic aliphatic epoxides and oxetanes. The reactivity of these monomers is strongly influenced by the steric and electronic factors along with the effect of functional groups present in the molecule [85]. Figure 2.15 shows some examples of these monomers. It is observed that cyclic aliphatic epoxides are far more reactive than the open-chain epoxides due to the strain in the rings in the cyclic aliphatic epoxides. Silicon-containing cycloaliphatic epoxides are generally used to optimize the mechanical properties of the cationic UV curing system.

Figure 2.. Chemical structures of some of commonly used cationic UV using monomers: (I) and (II) - vinyl ethers; (III) - cycloaliphatic epoxide; (IV) - silicon containing epoxide; (V) - oxetane