Polypyrrole ( PPy ) : a carry oning polymer. Research work around the Earth on the synthesis, electronic and chemical construction, belongingss and applications of PPy has been exhaustively reviewed several times in the literature [ 1-3 ] . In most instances, PPy has been considered as a unidimensional polymer holding a poly-conjugated anchor. Different spectroscopic techniques such as UV-photoelectron spectrometry ( UPS ) , electron energy loss spectrometry ( EELS ) , electron paramagnetic resonance spectrometry ( EPR ) , and near edge X-ray soaking up all right construction spectrometry ( NEXASS ) have been used to look into the chemical and electronic construction of PPy [ 4-7 ] . Although there is still some uncertainness about the proposed theory of conductivity mechanism in electronic set construction of PPy, it is widely accepted that the conduction is achieved by bring forthing intermediate provinces besides known as nonlinear excitements, or polarons and bipolarons [ 8-11 ] .
First of wholly, the electronic and chemical constructions of PPy are briefly described in the undermentioned subdivision of this chapter. X-ray photoelectron spectrometry ( XPS ) , infrared ( IR ) , and atomic magnetic resonance ( NMR ) spectroscopic surveies in the literature indicated that the chemical construction of PPy is non additive but instead distorted and branched with several types of defects ( discussed in the undermentioned subdivision ) [ 12 ] .
Furthermore, assorted man-made paths of PPy including most normally used methods like electrochemical polymerisation and chemical polymerisation, and less studied methods like photopolymerization, enzyme catalyzed polymerisation and plasma polymerisation have been discussed in footings of their process, advantages and disadvantages. In add-on, the polymerisation of pyrrole, which is assumed to follow a mechanism proposed by Hsing et al. , has besides been described in this chapter. Since the present research work in this thesis involves the readying of complexs or coatings of PPy with nonconductive polymers, a brief literature reappraisal on up-to-date research on carry oning PPy complexs with nonconductive polymers is given following by the literature reappraisal on the usage of UV polymerisation for the readying of nonconductive polymer coatings ( Section 2.2 ) .
Electronic and chemical construction of PPy
The conductivity mechanism in carry oning polymers can be elucidated by understanding the electronic energy set construction. Using several different spectroscopic techniques, the set constructions of assorted carry oning polymers have been demonstrated in the literature. For illustration, the set construction of PPy is shown in Figure 2.1 [ 2, 13 ] .
Figure 2.. Electronic energy set constructions of impersonal PPy, PPy with one polaron, PPy with one bipolaron and to the full doped PPy ( with many bipolarons and polarons ) [ 2 ]
In impersonal PPy with the benzenoid construction, there is a set spread of 3.2 electron volt between the valency set ( VB ) and conductivity set ( CB ) . Due to high energy set spread, it is about impossible for negatrons to reassign from the VB to CB at room temperature. Therefore, impersonal PPy behaves like an insulating polymer with about negligible electrical conduction. However, upon an negatron extraction from polymer concatenation by oxidative procedure, a partial positive charge is created, which causes a local distortion in the concatenation taking to the quinoid construction ( Figure 2.2 ) . The partial charge formation is called doping and usually occurs during polymerisation. In order to derive chemical neutrality and thereby stableness of PPy concatenation, counteranions in the reaction medium are incorporated into the polymer concatenation by adhering with the positive charges [ 2, 13 ] .
Figure 2.. Electronic constructions of impersonal PPy, polaron and bipolaron in doped PPy [ 2 ]
A polaron is a province associated with the intermediate energy degree formed in the set spread of PPy due to the formation of a positive charge, an odd spin, and a counteranion. As shown in Figure 2.1, as a consequence of the formation of a polaron, two new intermediate provinces that are adhering and antibonding are formed within the set spread. Further oxidization removes another negatron from PPy concatenation, ensuing 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 favorite than the formation of two polarons holding two positive charges and two odd spins. As the oxidization continues, more bipolarons and polarons are generated, ensuing in the imbrication of energy provinces with the formation of many new intermediate set degrees. Therefore, extremely doped province contains closely jammed set construction that allows facile negatron transportation from VB to CB [ 13 ] .
Figure 2.. Possible chemical construction of PPy with conformational and chemical defects [ 12 ]
Typically, the doping degree in PPy can be achieved up to 40 mol % at which bipolarons serve as prevailing charge bearers. It is theoretically proved that the electrical conduction in a stuff is straight relative to the merchandise of the concentration of charge bearers and their mobility [ 14 ] . Although the concentration of charge bearers in PPy at this degree is four to five orders of magnitudes higher than that of inorganic semiconducting materials, the electrical conduction of PPy is found to be in the scope of semiconducting materials merely. The low conduction occurs as a consequence of decreased mobility of charge bearers due to low grade of crystallinity and other defects. Therefore, the mobility of charge bearers must be enhanced in order to accomplish higher electrical conduction.
The entire conduction of a conducting PPy depends on the combination of intra-chain ( charge transportation along the polymer concatenation ) and inter-chain ( charge transportation from one concatenation to neighbouring concatenation ) mobility of charge bearers. However, the mobility can sometimes be restricted by the conformational agreements and chemical defects, for case ?-? ( 2-2I„ ) , ?-? ( 2-3I„ ) , or ?-? ( 2-4I„ ) matching with non-linear rotary motion, and formation of carbonyl and hydroxyl groups within the polymer ironss due to over-oxidation ( Figure 2.3 ) [ 12 ] .
Synthesis of PPy
Although assorted methods have been proposed so far for the readying of PPy, two methods, viz. chemical and electrochemical oxidative polymerisations, have been widely used and investigated. It is believed and widely accepted that the polymerisation procedure in both methods follows the same mechanism in which the yoke occurs between extremist cations that are generated from the oxidization of monomers [ 3, 15-16 ] .
As shown in Figure 2.4, the oxidization of a pyrrole monomer produces a extremist cation. Matching between two of these extremist cations generates bipyrrole by let go ofing two protons. Since this bipyrrole is more easy oxidizable than monomer, it is easy oxidized to give rise to extremist cations, and thereby couple with another extremist cation of monomer or bipyrrole to bring forth an oligomer. These stairss continue to happen until the formation of PPy. The driving force for the polymerisation procedure is the lower oxidization potencies of polymeric and oligomeric species than that of the monomer. The polymerisation procedure ceases to happen when the concatenation length of polymer exceeds the solubility bound of the dissolver ensuing in the precipitation of PPy. Various possible givens about the expiration measure such as the nucleophilic onslaught on the active polymer concatenation and extremist cation onslaught on the monomer have been proposed but non to the full confirmed [ 16 ] .
Figure 2.. Mechanism of pyrrole polymerisation via yoke of cationic groups [ 16 ]
In electrochemical polymerisation, PPy movies are by and large prepared on the anodal working electrode from a solution incorporating pyrrole monomer and an electrolyte salt. The electrochemical polymerisation of PPy was foremost performed by Dall’Ollio in aqueous sulfuric acid on a Pt electrode [ 17 ] . In a subsequent work, Diaz et Al. prepared electrochemically free-standing PPy movies on Pt electrode with first-class electrical and mechanical belongingss [ 18-19 ] . Generally, PPy obtained from the electrochemical method shows better carry oning belongingss. Therefore far the highest conduction reported for PPy movie doped with hexafluorophosphate prepared via electrochemical method at room temperature was 2 Ten 103 S/cm [ 20 ] .
Although PPy movies can be easy synthesized electrochemically on inert electrodes such as Pt, gold, glassy C and chromium steel steel in aqueous or organic dissolvers, electrochemical polymerisation of pyrrole on the oxidizable metals was one of the disputing jobs during recent decennaries [ 2 ] . Since the oxidization potencies of these metals are more negative than that of pyrrole, the disintegration of metal occurs before the polymerisation of pyrrole takes topographic point. Significant research has been contributed to decide this job by obtaining passivation of the metal, which slows down disintegration of the metal without impeding the polymerisation of pyrrole [ 21 ] . To day of the month, several probes on the consequence of a broad assortment of polymerisation conditions by changing different parametric quantities such as pH, dissolvers, temperature, current denseness and applied potency on the formation and belongingss of end point PPy movies have been reported [ 22-24 ] .
PPy was foremost synthesized by Angeli et Al. in 1916 via chemical oxidization of pyrrole with H2O2 to bring forth a black formless pulverization [ 25 ] . However, it was found to be indissoluble in any common organic dissolvers. Subsequently, many research workers have used assorted oxidising agents such as FeCl3, Fe ( NO3 ) 3, Fe ( ClO4 ) 3, HNO3, PbO2, CuCl2, CuBr2, ( NH4 ) 2S2O8 and many others to fix PPy and reported the conduction of PPy and the reaction rates. Among all the oxidising agents, ferrous salts produced high conductive PPys. On the other manus, cuprous salts produced PPy with considerable conduction next to ferrous salts. The chief advantage of chemical oxidative polymerisation over the electrochemical method is that the PPy can be easy produced in big measures with low cost. By and large, the electrical conductions of PPys obtained from chemical polymerisation are lower than those of PPys prepared from electrochemical polymerisation. Several research workers have reported the effects of assorted reaction parametric quantities such as the type of oxidizer, the concentration of oxidant, reaction clip, temperature, stoichiometry and dissolver on the concluding belongingss such as conduction, stableness, and morphology of chemically synthesized PPy [ 1, 26 ] .
Chemical polymerisation in the presence of wetting agents
The physical belongingss of the conducting polymer are extremely influenced by the method of readying, the features of other additives in the reaction mixture, and the reaction conditions. For case, the consequence of wetting agent on the morphology, conduction and thermic stableness of the chemically synthesized PPy has been reported by Omastova et Al. [ 27 ] . In their probe, assorted wetting agents of anionic, cationic and non-ionic types were studied with consequences reported for the output, conduction and thermic stableness of the PPy when doped with dodecylbenzenesulfonic acid ( DBSA ) and sodium dodecylsulfate ( SDS ) . While anionic wetting agents interact with positively charged PPy through strong ionic bonding, the interaction of cationic and non-ionic wetting agents is much weaker. It was proposed that the betterment in the conduction in the instance of DBSA and SDS could be attributed to the presence of the manner of in-plane distortion quiver of N+H2 on protonated N found in FTIR particularly for the incorporation of bulky anionic wetting agents.
Kudoh et Al. reported that the usage of wetting agent increased the rate of polymerisation. Further, it was found that the presence of phenolic derived functions incorporating electron-withdrawing groups such as 3-nitrophenol along with the anionic wetting agent showed enhanced conduction, thermic and air stableness likely due to the interactive interaction of phenol derivative with pyrrole and oligomers during the polymerisation [ 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 consequence 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 thermic stableness [ 29 ] . In another survey by Lee et Al. for the readying of soluble PPy, the addition in the concentration of wetting agents such as DBSA, which contains bulky alkyl groups, was determined to take to an addition in the doping degree. This was found to do doped PPy to be soluble in m-cresol, NMP and chloroform [ 30 ] . Soluble PPy was besides prepared by functionalizing PPy with functional substituents via the interpolation of chlorosulfonyl and sulfonic acid groups [ 31 ] .
Colloidal atoms of PPy were prepared via micro-emulsion-polymerization utilizing several emulsifiers by Moon et Al. in order to clarify the relationship between the morphology and photoluminescence ( PL ) [ 32 ] . It was observed that a clear tendency in the morphology of PPy atoms with the addition in the concentration of cationic wetting agents occurred. It was besides mentioned that the morphology was greatly influenced by the oxidant chosen. Finally, it was concluded that the highest PL strength could be obtained when the atoms were little and uniformly dispersed in the medium. Using a fresh 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 acerb semen wetting agent, and SDS, which is an anionic wetting agent. The consequences 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 severally [ 33 ] .
Even though photopolymerization of functional monomers and oligomers such as propenoates and thiol-ene systems has been in pattern for several decennaries, this technique is considered as one of the less evolved for the polymerisation of carry oning polymers [ 26 ] . Despite several advantages of this technique including faster rates of polymerisation, low energy ingestion and decreased VOC, merely a really few studies on photopolymerization of carry oning polymers have been found upon literature study. One of the major advantages of photopolymerization is that it can used to fix movies and coatings on both conducting and nonconductive substrates where electrochemical and chemical procedures have restrictions and drawbacks.
Recent surveies on photopolymerization of pyrrole in the presence of Cu, gold and Ag salts as negatron acceptors under the UV visible radiation have shown interesting consequences for the incorporation of metal nanoparticles into PPy [ 34-38 ] . Omowunmi Sadik and his colleagues investigated the destiny and function of metal nanoparticles incorporated into photopolymerized PPy [ 35 ] . It was shown that the rate of movie formation of PPy depends upon the metal salt used. Aqueous pyrrole solutions incorporating CuSO4 and AgNO3, which generated movies with the exclusion of UV visible radiation after 48h, could really bring forth movies in the presence of UV visible radiation within 2 hours. On the other manus, the solutions incorporating AuCl3 showed immediate blackening in the presence of UV visible radiation and yielded the black precipitate within few proceedingss upon the add-on of monomer to the solutions. The conductions of movies prepared from these metal salts on fibreglass were in scope from 1 ten 10-2 to 1.1 ten 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 movies observed from scanning negatron micrograph contained metal atoms, the mechanism of photopolymerization appeared to affect decrease of the metal salts. Equations 1 and 2 shown in Figure 2.5 in the instance of AuCl3 indicated the loss of two negatrons from each mole of pyrrole ( Py ) , ensuing in the decrease of 2 moles of gold cations to gold metal. Consequently, the yoke of Py cation groups, farther oxidization of polymer by anions in the electrolyte, and association of the anions with the polymer concatenation would take topographic point [ 35 ] .
Photopolymerized PPy movies incorporating Ag nanoparticles were derived from preparations dwelling of pyrrole as monomer, silver salt as negatron acceptor/dopant, and a extremist or cationic photoinitiator [ 39 ] . In this work, it was found that Ag nitrate ( AgNO3 ) is far superior negatron acceptor for photopolymerization of pyrrole to other Ag salts such as Ag perchlorate ( AgClO4 ) , silver nitrite ( AgNO2 ) , and silver tosylate ( AgTs ) in footings of the rate of reaction, output, and conduction. In the instance of photoinitiators, both extremist ( Irgacure 784 ) and cationic ( Iracure 261, Cyracure 6974, and Cyracure 6990 ) types were used to look into the consequence of photoinitiator on the rate of photopolymerization procedure. Besides it is well-known from the literature that the polymerisation procedure of pyrrole returns via the formation of cation groups, dimmers, and finally polymer ironss. From the experimental consequences, it was found that cationic photoinitiators, particularly Irgacure 261, demonstrated faster bring arounding rates than extremist photoinitiators as indicated by the faster rate of formation of dark and solid movie. Although increasing the sum of photoinitiator in the preparations increased the hardening rate, it caused a additive lessening in conduction bespeaking the possible loss of junction in PPy ironss.
Despite the higher standard decrease potency of Ag+ ( 0.8V vs. SHE ) than Fe3+ ( 0.771V vs. SHE ) and Cu2+ ( 0.153V vs. SHE ) , the oxidization of pyrrole by Ag+ cations in the absence of UV visible radiation, in which instance it is considered to be merely chemical oxidization, is really slow as compared to the oxidization rates of pyrrole by Fe3+ and Cu2+ [ 40 ] . In fact, the rates of chemical oxidization of pyrrole lessening in the order: Fe3+ & A ; gt ; Cu2+ & A ; gt ; Ag+ . Although the existent ground for this phenomenon is non yet wholly understood, it has been suggested that these disagreements in the reaction rates may originate due to the differences in the hydration energy of different metal cations ( Fe3+ & A ; lt ; Ag+ & A ; lt ; Cu2+ ) .
Previously, there were studies on multiphoton-sensitized polymerisation and self-sensitized polymerisation of pyrrole [ 41-42 ] . From these surveies, it has been proposed that pyrrole monomers absorb visible radiation in the close UV part to organize photoexcited pyrrole molecules. The aroused pyrrole molecules are so instantly quenched by unexcited pyrrole molecules, giving rise to both cationic and anionic pyrrole groups. The pyrrole cationic and anionic groups react with pyrrole molecules to organize dimers, oligomers, finally polymer. However, it was shown the attendant brown movie exhibits really low values of conduction due to either little figure of negative dopant molecules or lower intermolecular interactions between polymer ironss.
A few studies have discussed the likely mechanisms of photopolymerization of pyrrole in the presence of an negatron 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 instance of other metal cations such as Fe3+ and Cu2+ every bit good [ 45 ] . However, some of these studies mentioned that upon UV light the photoinitiation measure may affect the publicity of Ag+ cation into a photoexcited province, which becomes more reactive, therefore taking to faster oxidization rate of pyrrole ; whereas other studies claimed that the photoexcitation may happen in monomer foremost, and so the Ag+ accepts the negatron signifier monomer by oxidising the aroused monomer.
Figure 2.. Conventional representation of photopolymerization procedures [ 46 ]
However, harmonizing to Kobayashi et al. , photopolymerization taking to carry oning polymer synthesis can by and large be divided into two classs: ( 1 ) photopolymerization with photocatalytic system dwelling of sensitiser and negatron acceptor ; ( 2 ) photopolymerization via photo-excitation of monomer itself and its oxidization in the presence of negatron acceptor ( Figure 2.6 ) [ 46 ] . Therefore, as discussed before, the photopolymerization procedure affecting AgNO3 can be considered as either of those two classs where AgNO3 can function as either both sensitiser and acceptor or merely acceptor.
Photopolymerization of pyrrole has besides been performed by utilizing Ru, Co and Cu composites. Here the mechanism is assumed to follow the first class in which these composites may move as photosensitizers which upon UV light accepts negatrons 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 seeable 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 oxidizer [ 47 ] . However, this pulverization merchandise showed lower conduction ( 3 X 10-4 S/cm ) than the 1s prepared from traditional chemical and electrochemical oxidization methods.
As shown in Figure 2.7, the mechanism of this photochemically initiated polymerisation was thought to continue via photo-excitation of [ Ru ( bipy ) 3 ] 2+ to ( [ Ru ( bipy ) 3 ] 2+ ) * , oxidization extinction of ( [ Ru ( bipy ) 3 ] 2+ ) * to [ Ru ( bipy ) 3 ] 3+ by Co ( III ) composite, and followed by the oxidization of pyrrole by [ Ru ( bipy ) 3 ] 3+ . Other research workers used Cu2+ composites such as [ Cu ( dpp ) 2 ] 2+ ( dpp = 2,9-diphenyl-1,10-phenanthroline ) as the photosensitizer and p-nitrobenzyl bromide as the sacrificial oxidizer to lodge carry oning PPy forms on assorted substrates such as paper and glassy C [ 48 ] .
Figure 2.. Photochemically initiated polymerisation of pyrrole [ 47 ]
Photoelectrochemical deposition of PPy on assorted semiconducting materials such as n-type GaAs and n-type Si wafers, and every bit good as TiO2 particles has been attempted to forestall photodegradation of the semiconducting material surfaces and better photo-efficiency in their applications for solar cells [ 49-51 ] . In these efforts, an external prejudice was employed to the semiconducting material to originate the polymerisation. In add-on to the applied electric electromotive force, the application of UV visible radiation generates holes in the semiconducting material by forcing the negatron into conductivity set ( LU ) ( Figure 6 ) , and the photogenerated hole in semiconducting material can oxidise pyrrole to originate the polymerisation. Then, Ag+ ion that is present in the solution can function as negatron acceptor to accept the negatron from the conductivity set of semiconducting material.
Figure 2.. ( a ) Conventional representation of active image formation of carry oning polymer induced by photoillumination ( B ) micrograph of photopolymerized polyaniline [ 46 ]
The incorporation of TiO2 atoms into PPy was performed by utilizing photoelectrochemical procedure in which the combination of both electrolysis of pyrrole and photo-oxidation of pyrrole by UV illuminated TiO2 scatterings was utilised [ 52 ] . In this method, the valency set spread of TiO2 was sensitized utilizing UV irradiation in the presence of O as sacrificial negatron acceptor and fluoroborate as a counteranion. This was presumed to do the photochemical transition of pyrrole to PPy. Further, the as-synthesized PPy coated TiO2 nanoparticles were found to demo improved long-run seeable light photoactivity for the coevals of H. 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 atoms to successfully integrate into positively charged PPy.
Recently, photopolymerization has been utilized to manufacture functional carry oning polymeric stuffs with optical and electrical belongingss for image formation [ 46 ] . As can be seen in Figure 2.8, the image formation has been obtained via the polymerisation and subsequent electrochromism of carry oning polymer, both induced by photoillumination. Electrochromism is a phenomenon in which the spectral alterations of a stuff can be processed by the application of external stimulations. Photopolymerization of pyrrole by AgNO3 has besides been employed in bring forthing humidness detectors by surfacing polyester-based substrate with PPy/TiO2 complexs to analyze the consequence of photopolymerized complex on the electrical and humidness detection belongingss [ 53-54 ] .
Similar to chemical and electrochemical polymerisations, the photopolymerization procedure can besides let integrating assorted foreign molecular species such as flexibilizers and stabilizers into photopolymerization preparations to better the processibility and mechanical belongingss of PPy. Generally, the incorporation of big amphiphilic ( surfactant ) organic anions such as Na dodecyl sulphate ( DDS ) and sodium dodecylbenzne sulfonate ( DDBS ) as dopants into polypyrrole matrices is found to better the mechanical belongingss and solubility of chemically and electrochemically prepared PPy. The movies incorporating these additives showed greater flexibleness and every bit good as first-class attachment to Mylar and Teflon substrates [ 55 ] .
The usage of enzymes such as horseradish peroxidase ( HRP ) as accelerators with oxidizers such as peroxides ( H2O2 ) has late shown considerable involvement for the synthesis of polyaniline and PPy [ 56-58 ] . Polymers produced from this method are by and large of low molecular weights and extended concatenation ramification. However, it has besides been proved in a recent survey that the job of concatenation ramification can be resolved through a sophisticated attack utilizing polyelectrolytes such as polystyrenesulfonate ( PSS ) as templets [ 59 ] . In this method, PSS can non merely assist monomer molecules to aline prior to polymerisation but besides provide the counterions for doping the synthesized polymer, and enable polymer to be soluble in H2O.
One of the major advantages of the enzyme-catalyzed polymerisation is the well higher pH reaction solutions as compared to the chemical and electrochemical polymerisations. There are few cases found in the literature where biological polyelectrolytes such as Deoxyribonucleic acid have besides been used as templets [ 60-61 ] . In this instance, the alliance required for aminobenzine monomers occurs through the electrostatic interactions between the DNA phosphate groups and protonated aminobenzine monomers.
Plasma polymerisation has been considered as an interesting procedure to obtain thin movies of assorted polymers including carry oning polymers [ 62-63 ] . This polymerisation can be done in gas stage without any chemical oxidizers. Since the mechanism involves fragment formation and at bay groups, the plasma-polymerized PPy is believed to hold a different chemical construction from that of chemically, electrochemically and photochemically polymerized PPy. Figure 2.9 shows the proposed chemical construction of plasma-polymerized PPy [ 64 ] .
Figure 2.. Proposed chemical construction of plasma-polymerized PPy [ 64 ]
The synthesis of nano and meso spherical I doped atoms of plasma-polymerized PPy utilizing glow discharges of pyrrole has been reported [ 65 ] . The conduction in these PPy/I atoms has been found to increase from 10?9 to 10?6 S/m with the addition in humidness from 80 % to 90 % RH. Cruz et Al. has deposited the PPy/I movies that have shown addition in conduction from 10-7 to 10-3 S/m at comparative humidness of & A ; gt ; 90 % . The addition in conduction can be attributed to the improved mobility of PPy ironss and the interactions of dissolved iodine ions with H2O molecules [ 66 ] .
Conducting PPy complexs and coatings with nonconductive polymers
Complexs of per se carry oning polymers are by and large stuffs that are prepared by incorporating the carry oning polymer such as PPy with at least one secondary constituent that can be organic, inorganic or biologically active stuffs. The major map of the secondary constituent is to better any of chemical, physical, optical, electrical and mechanical belongingss of carry oning polymer, depending on the demands for end-use application of the composite. In general, nonconductive polymers such as poly ( vinyl intoxicant ) ( PVA ) , polystyrene ( PS ) , poly ( methyl methacrylate ) ( PMMA ) , and poly ( vinyl ethanoate ) ( PVAc ) are combined with the carry oning polymers in order to get the better of some of the physical and chemical restrictions of carry oning polymers, including the processibility and mechanical and thermic stableness [ 67 ] .
The cardinal factors in optimising the belongingss of attendant complexs are the ratio of PPy versus the insulating polymer, the processibility of two constituents in solution and the grade of scattering of PPy inside the insulating polymer [ 68 ] . Nevertheless, the grade of scattering depends on the solubility of PPy and the insulating polymer matrix in the dissolver and the ability of dissolver in swelling the matrix to organize homogenous spheres for the incorporation of PPy.
Since it may be hard to obtain unvarying scattering of PPy atoms in most of the common dissolvers, it was shown that the polymerisation of pyrrole in the presence of dissolved polymer matrix can ease the procedure of facile incorporation and proper scattering of PPy in the polymer matrix [ 26 ] . For illustration, the complex of PPy/polycarbonate has been synthesized through the polymerisation of pyrrole with FeCl3 as oxidizer in a solution incorporating dissolved polycarbonate in CHCl3. The attendant complex was observed to incorporate uniformly dispersed PPy throughout the polycarbonate matrix [ 69 ] .
Similarly the complex of PPy/poly ( ethylene-co-vinyl ethanoate ) [ PEVA ] has been produced by fade outing host polymer matrix and pyrrole in methylbenzene and so oxidising the pyrrole with FeCl3 [ 70 ] . The conduction of this complex was reported to be about 5 S/cm. The processing of these complexs into movies and other molded objects was performed by hot-pressing at about 100-150 & A ; deg ; C and 15-20MPa force per unit area for 1 hr. In another study by the same group, the complex of PPy/poly ( alkyl methacrylate ) has been prepared by scattering poly ( alkyl methacrylate ) and pyrrole in an aqueous surfactant solution and further adding the oxidizer for pyrrole polymerisation. The same process of hot-pressing was followed to obtain movies from these complexs. These movies exhibited conductions up to 2 S/cm. A similar process was used to bring forth the complexs of PPy/poly ( cinnamene ) and PPy/chlorinated co-polymers from latexes of matching insulating polymers. Although the conductions of these movies were well reduced from that of pure PPy, the thermic stableness and mechanical belongingss were enhanced [ 71 ] .
The readying of complexs of PPy with nonconductive polymers utilizing electrochemical oxidization method has besides been reported [ 67 ] . PPy/polyurethane complexs have been prepared first by projecting polyurethane on indium-tin oxide electrodes ; where upon electrochemical polymerisation of pyrrole has been carried out. PPy/poly ( vinyl-methylketone ) ( PVMK ) complexs have besides been prepared electrochemically by first dip-coating the insulating polymer and using subsequent electrochemical polymerisation of pyrrole.
Among nonconductive polymers, assorted hydrophilic and water-soluble polymers such as poly ( vinyl intoxicant ) ( PVA ) , poly ( vinyl ethanoate ) ( PVAc ) , poly ( methyl methacrylate ) ( PMMA ) , poly ( N-vinyl pyrrolidin-2-one ) ( PVP ) have besides been used to fix PPy complexs in aqueous solutions. Interestingly, complexs of PPy/poly ( vinyl ethanoate ) with 27 % of PPy exhibited electrical conduction about comparable to that of pure PPy and mechanical belongingss similar to those of poly ( vinyl ethanoate ) . Furthermore, the environmental stableness of this complex was besides significantly improved as compared to that of pure PPy movies [ 71 ] .
The usage of UV radiation in polymerisation or hardening of nonconductive polymers
Radiation hardening is a procedure in which a liquid becomes solid by the irradiation of UV visible radiation or negatron beam. It causes important alterations to physical belongingss of stuff such as viscousness, solubility, adhesion, colour, and electrical conduction [ 72 ] . This subdivision briefly nowadayss different facets and recent progresss in the UV-curing engineering.
UV radiation hardening has grown into an indispensable and permeant engineering which is the footing of legion applications because of its alone advantages such as rapid procedure, low energy demands, low temperature runing conditions, environmental friendly characteristic, and possible application onto assorted substrates [ 73-74 ] .
UV radiation is well-known for its hurtful effects on organic affair which decomposes upon drawn-out sunshine exposure. However, the good consequence of UV radiation is its ability to originate chemical reactions such as polymerisation by exciting the molecular bonds. A cardinal demand for successful UV radiation polymerisation or hardening is the lucifer between the emanation from the beginning and the soaking up spectra of the stuff under the photochemical procedure. In order to accomplish maximal efficiency with low cost, it is required that the UV beginning should bring forth high strength UV radiation without coevals of inordinate infrared radiation. Normally used UV radiation beginnings for commercial application are average force per unit area quicksilver vapour lamps [ 75-76 ] .
Typically, a UV curable preparation consists of photoinitiator, functionalized oligomer, and monomer moving as a reactive dilutant. UV curable systems can be classified into two classs, depending on whether the chemical reaction follows via either the extremist type or cationic type mechanism. In the extremist type mechanism, by exposing the system to UV radiation, a big sum of free groups will be generated to originate the polymerisation of oligomers and monomers through measure growing add-on mechanism. In the cationic type mechanism, a proton acid is generated by the exposure of UV radiation via photolysis of photoinitiator to originate the polymerisation of oligomers and monomers [ 73, 76 ] . The undermentioned subdivisions briefly depict the free group and cationic UV radiation bring arounding systems.
Free extremist UV hardening system
Unsaturated rosins incorporating high reactive groups such as vinyl dual bonds are most frequently used in this type of system. The reactive vinyl bonds in these rosins react with free groups and lead to polymerisation or hardening. The polymerisation procedure consists of three different stairss: ( 1 ) induction, ( 2 ) extension, and ( 3 ) expiration. In the induction measure, a photoinitiator efficaciously absorbs the incident UV visible radiation and produces extremely reactive free groups by the cleavage of electronically aroused bonds. However, in order to obtain maximal efficiency of photoinitiator in bring forthing free groups, photoinitiator should normally hold a high molar soaking up coefficient in the scope of UV radiation beginning spectrum and besides have high quantum output for extremist coevals [ 77 ] .
In the extension measure, the reaction of the originating species with the monomer and oligomer functional groups leads to the formation of polymer ironss. Polymer ironss incorporating active species at the terminals become inactive by stoping polymerisation reaction via either recombination or decomposition mechanisms in the expiration measure. Therefore this consequences in the formation of three-dimensionally cross-linked, indissoluble and stiff macromolecular web. In general, the complete procedure of this web formation happens within seconds or less than 2nd, depending on the responsiveness of functional groups in oligomers and monomers. Figure 2.10 shows the mechanism of free extremist UV polymerisation [ 77 ] .
Figure 2.. Mechanism of free extremist UV polymerisation [ 77 ]
Presently, propenoates in the extremist type system occupy the highest market portion. They are the most widely used rosins due to high responsiveness and big sum of available propenoate functionalized oligomers and monomers. In add-on, propenoates are preferred instead than methacrylates particularly for publishing inks due to rapid remedy at room temperature and besides less oxygen suppression consequence. While propenoates with aliphatic construction produce low-modulus elastomers, acrylates incorporating aromatic construction bring forth difficult and glassy stuffs. The basic physical belongingss of coatings are obtained by taking proper oligomers. Most popular oligomers include acrylated epoxy, acrylated urethane, acrylated polyethers, and acrylated polyesters. The monomers, besides known as reactive dilutants, affect the viscousness, bring arounding velocity, concluding movie belongingss, cost, shrinking, and shelf life of the concluding coating preparation [ 76 ] .
Based on the type of induction 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 ) severally. As shown in Figure 2.11, the cleavage takes topographic point at C-C bonds in the three province of a typical cleavage type photoinitiator. Some of the commonly used unimolecular Type I or cleavage type photoinitiators are benzoin quintessences, hydroxyl alkyl phenyl ketones, dialkoxy acetophenones, benzoyl cyclohexanol, benzyl dimethyl ketals, and trimethyl benzoyl phosphine oxides [ 77-78 ] .
Figure 2.. Cleavage type mechanism of benjamin acetal photoinitiator [ 78 ]
Bimolecular type photoinitiators produce groups in the presence of a H giver such as aminoalkanes, intoxicants, and thiols. As shown in Figure 2.12, in a typical abstraction type or bimolecular type photoinitiator, the abstraction of H from the H giver by the photoinitiator generates the free groups, which will originate the polymerisation procedure. Some of known abstraction type photoinitiators are benzophenones, camphorquinones, ketocoumarins, thioxanthones, and benzyls [ 77-78 ] .
Figure 2.. Abstraction type mechanism of benzophenone extremist photoinitiator
Oxygen suppression is one of the major jobs in free extremist UV hardening procedure. Since coatings by and large have high surface country to volume, it causes high O exposure taking to greater possibility of O suppression. Oxygen is a diradical and can respond with the terminal free group of a propagating concatenation or photoinitiator originating species to organize a peroxy free group, which does non readily take part in the reaction ( Figure 2.13 ) . Therefore, it leads to the expiration of the growing of propagating ironss. Several attacks such as utilizing an inert ambiance, paraffin wax, O scavengers, high strength UV beginnings, and high concentration of photoinitiator have been used to cut down this job [ 73-74, 79-80 ] .
Figure 2.. Conventional representation of the reactions possible for O suppression [ 80 ]
Cationic UV bring arounding system
Cationic UV hardening has late become well-received engineering due to its typical features such as the absence of O suppression, low shrinking, low monomer toxicity, and good adhesion [ 81 ] . In add-on to surfacing and publishing ink applications, cationic UV hardening is besides being employed for assorted other applications such as stereo- and photolithography [ 82 ] . Some of the typically used photoinitiators for cationic UV hardening system are triarylsulfonium, diaryliodonium, and aryldiazonium salts. The photolysis of these salts under UV radiation generates the protons of ace acids, which act as initiating species for the polymerisation. The general conventional representation of mechanism is shown in Figure 2.14.
Figure 2.. Conventional representation of mechanism of cationic UV polymerisation utilizing diaryliodonium salt as photoinitiator [ 74, 83-84 ]
It involves the photoexcitation of the photoinitiator, for illustration diaryliodonium salt, followed by its decay from vest province with both homolytic and heterolytic cleavage at the carbon-iodine bond. Subsequently, it releases cations, free groups, and cationic-radicals. Since these aryl and aryliodine cations are extremely reactive species, they can respond with dissolvers, monomers, or drosss to bring forth protonic acids, HMtXn. These protonic acids are the chief instigators for the cationic polymerisation. As seen in Figure 2.14, free groups that are generated along with the formation of the cationic initiating species can besides originate free extremist polymerisation ; therefore it causes the addition in the rate of polymerisation procedure taking to the formation of intercrossed systems [ 83-84 ] .
Most normally used monomers for cationic UV polymerisation are vinyl quintessences, oxiranes, cyclic aliphatic epoxides and oxetanes. The responsiveness of these monomers is strongly influenced by the steric and electronic factors along with the consequence of functional groups present in the molecule [ 85 ] . Figure 2.15 shows some illustrations of these monomers. It is observed that cyclic aliphatic epoxides are far more reactive than the acyclic epoxides due to the strain in the rings in the cyclic aliphatic epoxides. Silicon-containing cycloaliphatic epoxides are by and large used to optimise the mechanical belongingss of the cationic UV hardening system.
Figure 2.. Chemical constructions of some of normally used cationic UV utilizing monomers: ( I ) and ( II ) – vinyl quintessences ; ( III ) – cycloaliphatic epoxide ; ( IV ) – Si incorporating epoxide ; ( V ) – oxetane