AN IMPROVED METHOD FOR RETAINING POLYANILINE ON GLASS SURFACE

Abstrct: An improved method for obtaining polyaniline retained glass plates having considerably higher electrical conductivity is described. In this method, precleaned glass plates were reflwed wit11 conc. HC1 and then iinsed wit11 distilled water. The plates we1.e heated at 40O0C, and the surface l~ydroxyl groups were replaced wit11 NHC$15 groups. The deposition of polyadine on such modified surfaces results in a polyaniline layer chemical bondcd to glass plates. The maximum resistance of 1 cm distance on the conducting surface was 9 kn that is an order of magnitude greater than that prcpared without the refluxing step. Only the use of oxidants, S2OO2-(aq) and Feh(aq) gave polyaniline layers in their conducting state. Tlle molar ratio of the monomer to oxidant that yields the highest conductivity is 1:2. The oxidants such as Cr20,"(aq) and Mn0,-(aq) gave polyaniline layers in their pel-nigraniline foim at the same molar ratios. Loweling the pH of the polymeiisation medium pinduced increased conductivity in the polyaniline coated glass plates. The polyaniline layer hydrolyses when treated with concentrated alkali.


INTRODUCTION
Polyaniline can exist in six different structural forms depending upon the extent of oxidation and protonation of the polymer backbone. The fully reduced form of the polymer, which can exist only in a neutral or basic medium, is leucoemeraldine(a).
This contains only phenyl groups and -NHlinkages. In a sufficiently acidic medium, some of the amine units of leuco-emeraldine can be protonated resulting in a quaternary ammonium salt type of structure. The latter is known as leuco-emeraldine salt(b). These two forms of the polymer are transparent yellow in colour. The partial oxidation of leuco-emeraldine salt results in a green polymer known as emeraldine salt(c). In this form, one out of four phenyl rings have converted to quinoid form with = Nattached to it on both sides. Due to partial oxidation, the polymer contains some cation radicals known as polarons. Neutralisation of the emeraldine salt gives emeraldine base(d), which is blue in colour. Futher oxidation'of the emeraldine salt gives a violet polymer called pernigraniline salt(e). Neutralisation of pernigraniline salt gives pernigraniline base(fll. While the first four forms of the polymer are remarkably stable in air and in solution, the fully oxidised form is prone to various ' Coiresponding author chemical reactions and thereby degradationY. Only the emeraldine salt form of the polymer is an electrical conductor with maximum recorded conductivity of 100 S -1 cm while all other forms are electrical insulators. Forms (a) and (c) can be interconverted by chemical redox agents or electrochemically by switching applied potential between -0.2 V to + 0.5 V with respect to the saturated calomel electrode (wrt SCE). This process is known as redox switching. The redox switching can be achieved in a millisecond time scale with concomitant colour change (from yellow to green or vice versa). Hence the application of the material in electrochromic devices has already been suggested by many authors. In fact, a recent report shows that the 3 device has become a reality .
The forms (c) and (d) or (a) and (b) may be interconverted by the acidbase treatment. The concomitant colour change has been utilised in devising sensors for non-redox basic gases such as NH and for non-redox acidic gases such as HC1 in the form of fumes4. Further the 2onductivity and colour change associated with redox switching of polyaniline have been utilised in fabricating sensors for both gaseous and solution phase redox agents4. The counter ion or proton influx or egress associated with redox switching has been utilised in devising sensors for anions and for pH",". Polyaniline was found to be a good electrocatalyst for L-ascorbic acid oxidation7. Precious metal doped polyaniline has been used in electrocatalytic oxidation of methanol, ethylene glycol, glucose etc." Polyaniline based various electronic and electrical devices including rechargeable batteries have already been fabricated!',"'.
Most of the above technological applications of the polymer undoubtedly denland polyaniline coated glass plates. The usual procedure for this is solution casting. One of the problems with polyaniline and such other conducting polymers is their extreme insolubility in common solvents. However, recent advances in conducting polymer research enabled the development of various processing techniques for them. For instance, when polyaniline is doped with campour sulpl~onic acid or such other surfactants, the resulting materials are soluble in N-methyl pyrrolidinonel'. By modifying the polymer to possess pendant or interacting polar groups, water soluble-polyanilines were also developed12. The polymer films obtained on glass surfaces by solvent casting are, however, not firmly attached to glass surfaces. These films peel off on abrasion.
Recently, we reported a method to retain polyaniline onto ordinary glass surfaces functionalised with aniline1" In this procedure, pre-cleaned ordinary glass pieces were pre-heated at 400 T and the surface -OH groups were replaced by -C1 groups by SOC1, treatment at 400 "C. The Si-CI groups were the11 converted to Si-NHC,<H, groups. This has resulted in aniline functionalised glass. The polymerisation of aniline on such surfaces by S,0,2-in an acidic aqueous solution generated polyaniline grafted onto glass surfaces. The lowest resistance of 1 cm 1.ength of the polymer coat was 306 kL2. The material was found to be a good template for aligning liquid crystalline molecules in liquid crystal display devices14. The material has also shown to be attractive in fabricating gas sensors. However, the conductivity of the material js insufficient for use as an electrochromic device with a fast switching time. The application of the material as electrodes in rechargeable batteries is not possible due to its low conductivity. These obstacles led us to devise an improved protocol to graft polyaniline onto glass plates to give a higher electrical conductivity.

METHODS AND MATERIALS
All chemicals used were ofAnalaR grade and were purchased from Aldrich Chemical Company, UK. The aniline used was distilled under nitrogen and other chemicals were used without further purification. Glass plates used were high-quality pre-cleaned plates.
In the present work, glass plates ( 1 cm x 1 cm) were refluxed in concentrated HCI for varying lengths of times (2, 4, 6, 7, 8, 10, 24 h). The treated glass plates were rinsed well with distilled water, and allowed to dry. They were then heated on a pre-heated hot plate at ca. 400 "C for 30 min in a fume cupboard. SOCl, was then sprayed onto their exposed surfaces under the above conditions, for 2 mjn. This spray was then followed by a spray of dry, distilled aniline for further two min. In certain experiments, the spraying of SOC1, and aniline was repeated twice. The treated glass plates were then rinsed with distilled water several times and placed in a solution containing aniline, S,OS2-(aq) and an acid (HC1, H,SO,, H.,PO,, HNO, or HC10,) whose concentrations were selected as shown in Table 2. In order to investigate the best oxidant that gives poly'miline coated glass plates with highest conductivity, oxidants sucli as acidic aqueous solutions of Fe", Mn0,-, Cr,O," or H,O, were used in place of S,O," ( Table 3). The effect of pH of the polymerisation solution on the conductivity of the polymer coat obtained was also studied by varying the HCI concentration but keeping all other variables constant ( Table 4). The polymer bo~uid glass plates were air-dried and stored in a dry desiccater. The resistance of a 1 cin length of the polymer coat on the glass plate was measured using a high impedance multirneter at different locations of the polymer coat and found to be almost constant. The polynier coats were uniform when observed through an optical microscope.
In order to investigate the side of attachment of aniline (i.e., via Si-N or via Si-C linkages) on the glass surface, the ordinary pre-cleaned glass pieces, aniline attached glass pieces <and polymer attached glass pieces were placed in a diazotisation mixture at 0-5 "C for several hours. The glass plates were then removed from the mixture, rinsed well with doubly distilled water and placed in a solution containing P-naphthol and NaOH. The colour changes and other effects of the above chemical treatments were investigated. Table 1 shows the dependence of the surface conductivity of the polyaniline bound glass surfaces on the length of refluxing the glasses with conc. HCI prior to polyaniline attachment. In all these experiments, the composition of the polymerisation solution was fned at 1 mol dm-WC1, 0.1 mol dm-"aniline and 0.2 mol dm-3 S2OS2-. As can be seen from this Table, the conductivity of the polymer coat increases as the refluxing time is increased from 2 h to 6 h and then it remains almost unchanged.   Table 2. Results show that the nature of the acid used has an effect on the conductivity of the polymer coat. Among the acids chosen use of HNO, or HC1 gives the polymer coats with lowest resistance per 1 cm length at 1:2 monomer: S20,2concentration ratio. Also the use of oxidants such as MnO,, Cr20," or H202 always resulted in non-conducting polymer coats while Fe" and S2OS2-gave conducting polymer coats ( Table 3). Fe" seems to be a better oxidant than S2OX2in this respect. Thus when Fe3+ was used as the oxidant at the optimum monomer to oxidant ratio of 1:2, the decrease in pH of the polymerisation solution has resulted in increased conductivity for the polymer coat obtained ( Table 4). The monomer bound and polymer bound glass plates were exposed to various acids, bases and redox agents and their effects are depicted in Table 5.

RESULTS
In the P-naphthol coupling experiment to diasotized amine groups, the characteristic pink colouration was immediately observed with polymer modified glass plates. The polymer layer that had turned pink was peeled off from the glass surface upon ageing. There was no immediate colour change in the aniline modified glass plates. However, pink colouration was observed in this case also with ageing and the coating on the glass then peeled off. There was no pink colouratjon when ordinary pre-cleaned glass plates were subjected to the above chemical treatments.    pH of the pol.ymerisation solution Resistance of 1 cm length of the polymer coatAcC2 The film did not peel off but colour MnOetc.) changed from green to violet as

Conc. NaOH
Colour changed from green to blue but the film did not peel off.
Colour changed from green to blue and eventually the polyaniline layer peeled off

DISCUSSION
We have recently shown that anili.ne-functionalised glass surface retains polyaniline14. The solubility tests and FT-IR studies of such materials have shown that the polyaniline molecules are most lilcely to be covalently attached to the glass sudace. The attachment should have taken place at positions of the glass p1.ate where surface hydroxyl. groups were present. Hence if one could enhance the n~xmber of surface hydroxyl groups of glass, one could then attach more polyaniline molecules onto its surface. In a recent study of grafting of silica membrane on glass surfaces, Chentoufi et a1 have refluxed glass pieces with conc. HCl for several hours1? In addition to hydroxyl groups, the glass surface contains an appreciable aniount of Si-0-Si groups. The refluxing then hydrolyses these groups to resu1.t in two Si-OH groups per each Si-0-Si group. The chemical processes accompanying the refluxing glass with conc. HCl are shown in Figure 1. The replacement of Si-OH by Si-C1 using SOC1, at an elevated temperature would make the glass surface more reactive than the ordinary one. Aniline then reacts with it to form Si-NHC,,H, with the elimination of HCI ( Figure 2). As Table 1 depicts, the refluxing helps to bind more polyaniline molecules onto the glass surface resulting in increased conductivity of the final material. We see again that refluxing for approximately G h is required to hydrolyse a maximum possible number of Si-0-Si groups and beyond that there is not much of a change of number of surface OH groups. The maximum conductivity of the glass functionalised with polyaniline prepared according to this improved procedure is an order of magnitude greater than that without pre-refluxing.
conc. HCI number of surface -OH According to the results shown in Table 2, it is clear that whatever the acid used, among the set selected, the surface conductivity of the final product is enhanced when [anilinel:[S,O,"l reaches 1:2. This is in accordance with our previous results14. The counter ion introduced by the acid al.so has a weak effect on the conductivity of the final product. When HCl was used as the acid, the conductivity of the plates were a little higher than those prepared using other acids. It may be explained by considering the necessity of counter ion influx for the emeraldine salt form of the polymer, in order to counterbalance the positive charge of its backbone. The electrochemical quartz crystal microbalance studies have revealed that counter ion influx is occurring during the redox switching of leuco-emeraldine to emeraldine salt form. During this process, unlike other anions, which are trapped in their dehydrated forms, the C1-ion carries its hydration sphere to some extent1? Hence, the polymer always contains some water molecules in it when C1-ion is used as the counter ion and as a result one could expect higher conductance.

conc. HC1 for at least 6 hours.
Glass with polyaniline retained on its surface. The surface conductivity of this glass is an order of magnitude greater than that prepared without refluxing step.  As can be seen from Table 3, only S,0,2-and Fe3+ ions give conducting glass plates. Although polymer coats were obtained with other oxidants they were in their non-conducting states. With Cr20,2-and MnO; the polymer coats obtained were violet in colour. They were in their pernigraniline state despite the lower concentrations of oxidants used by considering the possibility for 6e and 5e oxidation by these ions, respectively. Despite the highest redox potential of S20,", it gives the polymer in its conducting emeraldine salt form. The conductivities of polyaniline coated glass plates, obtained using S20," as the oxidant, were comparable to those obtained with Fe' as the oxidant. Finally, the pH of the polymerisation solution has a decisive effect on the polymer coat obtained. The lower the pH of the polymerisation solution the higher the conductivity of the final product ( Table 4).

Surface
The stability of Si-N groups will undoubtedly determine the shelf-life of the devlces fabricated using these materials. The results in Table 5 clearly show that the polymer coated glass plates can stand for robust conditions and are stable towards strong acids, reducing and oxidismg agents. Although the polymer does not peel off when treated by exposing to a weak base the results show that the polymer peels off when treated with a concentrated strong base for a sufficiently long period of time. Th~s can be explained by considering the cllemistry of Sj-N linkages. The Si-N bond nlay undergo base hydrolysis under drastic conditions and hence the covalent, attachment site of the polymer to the gIass surface breaks resuIting in the removal of the polymer from the glass surface. This experiment gives an additional support to the retention of polymer on the glass by covalent attachment. The P-naphthol experiment reveals that there are no free amine groups in aniline attached glass surfaces. I-Ience, aniline molecules must have attached to the glass surface through Si-N linkages with hanging phenyl groups. With time however, Si-N bond undergoes base hydrolysis releasing free aniline molecules on the glass sudace and this explains the pink colouration and peeling off the coat upon ageing. When the polymer is formed on such surfaces, one cannot expect all the polymer molecules to join in the linear chain starting from the monomer unit attached to the glass surface. Although there are more tendencies for the formation of such attached polymer molecules, the presence of polymer chains deposited in random manner is inevitable. This would leave some end NH, groups in the polymer coated glass plate to give the characteristic pink colouration with P-naphthol. With ageing, the Si-N bonds undergo base hydrolysis and hence the pink layer peels off from the glass surface. The ordinary pre-cleaned glass surface does not contain any amine group and hence no response to this chemical treatment.