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Effect of liquid additives on graft copolymerization of styrene onto perirradiated poly(ethylene-co-tetrafluoroethylene) films.

Introduction

Radiation induced graft copolymerization is an interesting method for preparation of ion exchange membranes for various fields including chemical, biochemical and biomedical technologies. The attractiveness of this method lies in its versatility to combine virtually unlimited number of base polymers and monomers under controlled reaction conditions to achieve membrane compositions for specific requirements [1].

The role of additives such as solvent and mineral acids is important in radiation grafting reactions since appropriate composition of grafting solution can enhance the grafting yields, and as a result, the monomer consumption and the radiation dose required to achieve a particular grafting level can be lowered in the presence of suitable additives [2,3]. This concept is of particular interest when grafting is carried out using highly expensive monomers (e.g. trifluorostyrene) and also in case of grafting onto polymer substrates of high radiation sensitivity such as poly(tetrafluoroethylene) (PTFE). Lower radiation doses may also decrease the likelihood of deterioration of mechanical properties of the graft copolymers, which may be undesirable in membranes applications [4,5].

Grafting reactions are often carried out in pure monomers or monomer solutions. The grafting yield is well known to be dependent on the concentration of monomer [6] and the type of diluting solvent when the grafting reaction is carried out in bulk solution [7]. This is because solvents presumably bring about swelling of the bulk or surface layers of the base polymer causing an enhancement of monomer accessibility to active sites and eventually determining the penetration depth of the grafts into the substrate [8]. Grafting onto polyolefines in poor-swelling solvents often lead to surface grafting while good-swelling solvents boost chances for obtaining bulk grafting with a homogenous distribution, which is very essential when ion exchange membranes/resins are sought [9]. On the other hand, bulk grafting onto fluoropolymers, which scarcely swell in solvents is achieved through front mechanism where grafting front starts at layers close to the surface, then progress inward towards the core of the film by a consecutive diffusion of the monomer solution through the swollen grafted layers [9].

The effect of addition of good solvents on the radiation graft copolymerization of vinyl monomers onto polyolifines and fluoropolymers has been studied in various occasions and recently reviewed in literature [1,8,10]. Unlikely, investigations of the effect of non-solvents on grafting of vinyl monomers onto fluoropolymers can be barely found in literature [4,11].

The addition of mineral acids has been also found to enhance grafting reactions onto polyolifines as concluded from studies on grafting of vinyl monomers onto polyethylene (PE), polypropylene (PP) and cellulose, which were reviewed in Refs [8,12]. Unlikely, investigation of the effects of addition of acids on grafting onto fluoropolymers has received relatively little attention in the literature. Dworjanyn et al reported a slight but definite increase in the degree of grafting of a mixture of styrene and 2-hydroxyethyl methacrylate in methanol onto PTFE using the simultaneous method on addition of an unspecified acid [13]. In contrast, Nasef [9] found almost no acid effects on the degree of grafting upon addition of specific amount of diluted mineral and organic acids during grafting of styrene diluted in methanol, benzene or dichloromethane onto PTFE, FEP and poly(tetrafluoroethylene-co-perfluorovinyl ether) (PFA) using the simultaneous method.

Recently, grafting of styrene onto ETFE films has been carried out and the obtained graft copolymers were used as precursors for preparation of proton exchange membranes for fuel cell after being sulfonated [14,16]. The properties of the obtained membranes [17,18] and their performance in fuel cells were evaluated and proved to be promising cheap electrolyte materials [19-23]. However, a study on the role of solvents and the mineral acids addition on the grafting reaction for styrene/ETFE system have not been reported.

The objective of the present study is to investigate the effect additives namely diluents (solvents and non-solvents) and mineral acids on the degree of grafting of styrene onto electron irradiated ETFE films.

Experimental

Materials

ETFE films expressed by the molecular formula [(-[CF.sub.2][CF.sub.2]- C[H.sub.2]C[H.sub.2]-).sub.n] with thickness of 125 [micro]m and density of 1.7 [gcm.sup.-3] were obtained from Goodfellow (Cambridge, England) and used as polymer substrates in all experiments. Styrene of purity more than 99% (Fluka) was used without further purification. Other solvents such as acetone, methanol, toluene, isopropanol and dimethylformamide (DMF) were research grade and used as received.

Electron Beam Irradiation of ETFE Films

Clean and vacuum dried ETFE samples were placed in a tray on a conveyer and irradiated on a stepwise basis using an electron beam (EB) accelerator (Curetron, EBC-200-AA2, Japan) under N2 atmosphere. The doses were varied from 20 to 100 kGy at 10 kGy per pass and verified using cellulose triacetate dosimeter (ASTM E1650-97).

Graft Polymerization of Styrene onto ETFE Films

The irradiated ETFE film was placed in the glass ampoule equipped with threaded joint and vacuum stopcock, which was tightly sealed then evacuated to remove air using a vacuum pump (10 mbar). Simultaneously, a cold trap-type ampoule containing the monomer solution was bubbled with purified [N.sub.2] gas for 10 minutes to remove air. The air free grafting solution was then transferred to the evacuated glass ampoule containing ETFE film through a tri-way vacuum stopcock and the glass ampoule was carefully sealed under [N.sub.2] atmosphere. The ampoule was then placed in a thermostatic oil bath at specified temperature to allow the graft copolymerization reaction to proceed.

After completion of the reaction, the grafted films were removed, washed with toluene and rinsed therein while ultrasonically cleaned for one day to remove the excess monomer and the homopolymer occluded in their surfaces. The grafted films were dried under vacuum (10 mbar) at 70[degrees]C until a constant weight was obtained. The grafted films were then weighed and the degree of grafting (G%) was calculated by considering the percent of weight increase in the grafted film according to equation 1.

G% = [W.sub.g] - [W.sub.o]/[W.sub.o] x100 (1)

where, [W.sub.g] and [W.sub.o] are the weights of original and grafted ETFE films, respectively.

Swelling in Solvents and Monomer/Solvent Mixtures

The swelling measurement of original and grafted ETFE films was conducted by soaking dry samples in desired solvents or monomer/solvent mixtures at room temperature for 24 h. The samples were then removed and blotted quickly with absorbent paper and weighed. The percent of swelling was calculated using the following equation:

Swelling% = [W.sub.w] - [W.sub.d]/[W.sub.d] x100 (2)

Where, [W.sub.w] is the weight of the wet samples and [W.sub.d] is the weight of dry samples.

Verification of Grafting

FTIR measurements were carried out using a Nicolet (Magna-IR 560) spectrometer equipped with attenuated total reflection, ATR, (Thunder dome-HATR) having Ge spherical crystal. The spectra were measured in a transmittance mode in a wave number range of 4000-700 [cm.sup.-1].

Results

Swelling Behavior of Original ETFE Films

Figure 1 shows the swelling-time curves for sorption of various solvents in pristine ETFE films at room temperature. It can be seen that the amount of sorbed solvents expressed as degree of swelling increases rapidly at the early hours for all solvents and tend to reach plateaus after achieving swelling equilibriums at 24 h. The highest degree of swelling for ETFE films was recorded with toluene followed by DMF. Conversely, alcohols caused low degrees of swelling in ETFE films according to the sequence: isopropanol>ethanol>methanol i.e. methanol is the least sorbed solvent in ETFE films among alcohols.

[FIGURE 1 OMITTED]

films at room temperature.

Table 1: Sorption data of ETFE films in various monomer/solvent mixtures (50:50 v/v) and the corresponding data in pure solvents. Swelling time is 24 h at room temperature.

Table 1 presents swelling data of ETFE films in various monomer/solvent mixtures (50:50 v/v) in comparison with the corresponding swelling data in pure solvents. The degree of swelling of ETFE film in styrene/toluene is found to be higher than that of styrene/DMF mixtures and in close range with ETFE swelling data in pure toluene and DMF. Unlikely, the degree of swelling in mixture of styrene with methanol, ethanol or isopropanol was found to be higher than their swelling in the corresponding pure solvents.

Swelling Behaviour of Grafted ETFE Films

Table 2 shows the swelling data of polystyrene grafted ETFE films with different degrees of grafting in various solvents. All solvents were found to swell the grafted films to various extents depending on the degree of grafting and the type of the solvent. For instance, toluene and DMF showed greatly higher degrees of swelling compared to the three alcoholic solvents at all grafting levels under the same swelling conditions. The alcoholic solvents were found to have close swelling values at all degrees of grafting. Among alcohols, methanol swells grafted ETFE films to slightly higher values at all degrees of grafting.

Solubility Parameters of Components of Grafting Mixtures

Table 3 presents the solubility parameters of pure and mixed components involved in the grafting mixtures. The solubility parameters of pure components such as styrene, solvents and ETFE film were obtained from literature [24,25]. The solubility parameters of the mixtures of styrene with various solvents are calculated according to the following equation (26):

[[delta].sub.Mix] = [([[phi].sub.1][[delta].sub.M.sup.2] + [[phi].sub.2][[delta].sub.S.sup.2]).sup.1/2] (3)

Where, [[delta].sub.Mix], [[delta].sub.M] and [[delta].sub.S] are the solubility parameters of the grafting mixture, the monomer and the solvent, respectively. [[phi].sub.1] and [[phi].sub.2] are the volume fractions of the monomer and the solvent, respectively. The same equation was used to calculate the solubility parameters of the polystyrene grafted ETFE films (presented in Table 3) taking the solubility values of pure ETFE and polystyrene and their volume fractions in the grafted films into account.

The difference in solubility parameters between ETFE and solvents is much smaller with toluene than that with methanol. The solubility parameter difference is in the sequence of toluene<DMF<isopropanol<ethanol<methanol. Also, the difference in the solubility parameters between ETFE and grafting mixtures is found to follow the same sequence i.e. the highest difference between ETFE and grafting mixtures is with styrene/methanol while the lowest one is with styrene/toluene mixture. In addition, the solubility parameter difference of the grafted ETFE films (G% = 15.5-93.0%) was found to be smaller compared to those of pure solvents and the monomer/solvents mixtures with the increase in the degree of grafting.

Effect of Solvents on The Degree of Grafting

Figure 2 shows variation of the degree of grafting of styrene from various solvents onto ETFE films with the composition of grafting solution. The degree of grafting increases with the increase in styrene concentration in all styrene/solvent mixtures until it reaches a critical concentration at which grafting maxima are achieved. Further increase in monomer concentration causes the degree of grafting to decline remarkably (except with toluene). The maximum degree of grafting and the critical concentration of styrene in the grafting mixtures were found to vary depending on the type of solvent. For instance, methanol was found to produce the highest degree of grafting with the lowest critical concentration (40:60 v/v) while toluene produced its maximum, although inferior, degree of grafting at a styrene concentration of 80:20 v/v. The variation in the maxima of degree of grafting and the corresponding critical concentrations of styrene in the grafting mixtures is found to be in the sequence of styrene/methanol (40:60 v/v)>styrene/ethanol (60:40 v/v)>styrene/isopropanol (80:20 v/v)>styrene/DMF (80:20 v/v)>styrene/toluene (80:20 v/v). These results shows that dilution of styrene with alcoholic solvents i.e. methanol, ethanol and isopropanol permit higher degrees of grafting compared to non-polar toluene at the same grafting parameters with highest G% obtained with alcohols in the sequence methanol>ethanol>isoproanol. Therefore, methanol and ethanol are selected as solvents to dilute styrene during grafting onto ETFE films. The criterion for the selection of both methanol and ethanol is based on achieving desired degrees of grafting together with the high quality of the obtained grafted films in addition to the cost of solvent.

[FIGURE 2 OMITTED]

Grafting Kinetics

The grafting kinetics were determined for two grafting systems having styrene diluted with methanol and ethanol to various concentrations and grafted onto ETFE film irradiated at 100 kGy and the results are plotted in Figure 3 (a and b). It can be obviously seen that the degree of grafting in both grafting systems initially increases rapidly with time at all monomer concentrations over a period of 4 h beyond which, it tends to level off gently with the increase in the reaction time giving limiting value called final degree of grafting at 48 h. The final degree of grafting was found to increase for mixtures of styrene/methanol in the sequence of 40/60>60/40>80/20>20/80>100/0 (v/v) with the increase in the monomer concentration. Correspondingly, the increase in the final degree of grafting in the mixture of styrene/ethanol was found to be in the sequence of 60/40>40/60>80/20>100/0>20/80 (v/v). The initial rates of grafting for styrene diluted with methanol and ethanol obtained graphically from Figure 3 a and b, are presented in Table 4. The initial rate of grafting in both mixtures increases until critical styrene concentration is reached beyond which, it tends to decrease. The initial rate of grafting of styrene/methanol mixtures achieves its highest value at 40:60 v/v whereas that of styrene/ethanol was reached at 60:40 v/v.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Figure 4 shows degree of grafting-time curves at various irradiation doses for grafting of styrene diluted with methanol and ethanol onto ETFE films. In styrene/methanol system, the degree of grafting initially increased drastically with time over a period of 4 h then slowed down as the reaction time prolonged to 8 h followed by a second increase at 8-24 h beyond, which it levelled off. Correspondingly, the degree of grafting in styrene/ethanol systems increased rapidly at the first 4 hours beyond which, it tends to level off with the time increase. The final degree of grafting in both grafting system was found to increase with the increase in irradiation dose. The initial rate of grafting obtained at a dose range of 20-100 kGy and presented in Table 5 was also found to increase with the increase in the irradiation dose. The initial rates of grafting from styrene diluted with methanol seem to be lower than that of styrene diluted with ethanol.

Effect of Acid Addition

Figure 5 shows degree of grafting-time curves for grafting of styrene/methanol and styrene/ethanol mixtures containing acid additives onto ETFE films. Mineral acids such as sulfuric and nitric acids were used at a volume of 10 vol% of solutions having concentration of 0.2 M. As can be seen, the degree of grafting in styrene/methanol mixture increased rapidly with time over a period of about 4 h beyond which it started to level off gently. Similar behaviour was observed for styrene/ethanol mixture with the degree of grafting levelling off after 8 hours. The addition of both sulfuric and nitric acids was found to increase the degree of grafting in both grafting systems compared to no acid addition. However, the addition of these two acids seems to be more pronounced in styrene/ethanol system. Moreover, the magnitude of acid enhancement to the degree of grafting seems to be higher with nitric acid in both grafting systems.

[FIGURE 5 OMITTED]

Discussion

The results of this work shows that solvents play a significant role in affecting grafting of styrene onto electron beam irradiated ETFE films. To understand this role, the swelling behaviour of the ETFE films was systematically investigated with time and the obtained data were presented in Figure 1. The degree of swelling that was found to increase rapidly at the early hours for all solvents until achieving different values of swelling equilibrium after 24 h suggests that swelling in ETFE films depends on the solvent type. For instance, non-polar toluene recorded the highest degree of swelling followed by the aprotic DMF. Unlikely, alcohols, which are polar solvents, produced lower degrees of swelling according to the following sequence: isopropanol>ethanol>methanol. Despite the appreciable value of toluene sorption compared to other solvents, the swelling of ETFE films in the present solvents remain very scarce and this is in accord with other fluoropolymers behaviour towards most of solvents.

To further illustrate the swelling behaviour of the polymer matrix in solvents when used to dilute the monomer during grafting, mixtures of 50:50 v/v styrene/solvents were used to swell ETFE films. The overall degree of swelling in 50:50 v/v of styrene with alcohols produced data close to those of swelling in pure toluene and DMF as shown in Table 1. This behaviour is possibly due to the reduction in the solubility parameter of pure alcoholic solvents by mixing with styrene. Based on these results one would expect high degrees of grafting to be achieved when using solvents such as toluene and DMF are used to dilute styrene during its grafting onto ETFE films. To further illustrate this trend, the effect of the type of solvent on the grafting of styrene onto ETFE film was investigated at various styrene/solvent mixtures as depicted in Figure 2. The degree of grafting was found to depend on the type of solvent, which dictates the critical concentration where maximum degree of grafting is obtained. Alcohols i.e. methanol, ethanol and isopropanol, which are non-solvents of styrene were found to produce grafting maxima (extremely high degrees of grafting) compared to toluene and DMF, which are good solvents for styrene. Among alcohols, methanol produced the highest degree of grafting with the lowest critical concentration of styrene (40:60 v/v) followed by ethanol at 60:40 v/v and isopropanol at 80:20 v/v. This observation is not in accord with the solubility parameter data of the three alcohol solvents and their mixtures with styrene (shown in Table 3), which suggests an opposite sequence of isopropanol>ethanol>methanol. Also, toluene and its mixtures with styrene, which have the closest solubility parameters to ETFE films, did not bring about any grafting facilitation. Hence it can be concluded that the effect of solubility parameters does not seem to be essential to the grafting reaction in the present system.

The behaviour of alcohols in boosting the degree of grafting can be reasonably attributed to the presence of Trommsdorff-type effect, which depends on the insolubility of the formed grafts in the diluting solvent. Since, methanol, ethanol and isopropanol are non-solvents for styrene and polystyrene, the polystyrene graft growing chains become immobilized in away that hinder termination by recombination without disturbing the styrene diffusion. This allows the formation of longer polystyrene grafts followed by the increase in the degree of grafting. On contrary, using toluene, which is a good solvent for both of styrene and polystyrene enhance termination of the graft growing chains by recombination leading to low degrees of grafting. In addition, the presence of resonance stabilization effect of benzene rings in toluene, which acts as an energy transfer agent is likely to reduce the its radicals reactivity and therefore, reduce their contribution to copolymerization reaction [9]. The difference in the degree of grafting of styrene in various alcohols onto ETFE film can be also ascribed to the variation in the degree of swelling of the polystyrene grafted films despite its small values as shown in Table 2.

Elmidaoui et al [14] observed grafting maxima upon grafting styrene (30% concentration) in methanol into ETFE films using pre-irradiation method in air and in presence of an inhibitor. These authors suggested that the interactions between the monomer diffusivity and the viscosity of the monomer solution dictate the grafting yield. The increase in the monomer concentration from one hand increases the diffusivity of the monomer through the grafted layers, and from the other hand hinders the diffusivity by increasing the viscosity. Accordingly, the maximum degree of grafting was attributed to an increase in the viscosity in the grafted layers at high monomer concentration leading to lower diffusion of monomer to the internal layers of the films, and hence causing a sharp drop in the grafting yield. Grafting maxima were also observed when grafting styrene diluted with dichloromethane into PTFE [27], FEP [28], and PFA [29] using simultaneous grafting. Since dichloromethane is a good solvent for polystyrene, the observed maxima were attributed to the increase in the styrene diffusion and hence the concentration in the grafting region, which reaches maxima at approximately 60% styrene concentration in three grafting systems. Above this concentration there was significant homopolymer formation, resulting in a hindrance in the monomer diffusion to the grafting sites that lowers the grafting yield [9]. Dargaville et al [30] have also reported maxima for the grafting of styrene onto PFA in dichloromethane, toluene and methanol. They concluded that the grafting yields did not appear to be dependent on the solvent power for polystyrene alone and suggested that the solvent and polymer chain transfer constants, as well as the polymer matrix viscosity, are also important factors.

Grafting in the present system is well known to proceed by front mechanism where it starts at layer close to the surface and proceeds progressively towards the middle of the film by continuous diffusion in the swollen grafted layers. Therefore, to explain the synergetic effect of alcohols in the present grafting system the results of swelling of polystyrene grafted ETFE films in diluting solvents was presented in Table 2. The extreme swelling of the grafted films in toluene, followed by DMF, compared to isopropanol, ethanol and methanol at all grafting levels suggest that alcohols, which are non-solvents for styrene or polystyrene, tend to enhance their concentration at the surfaces of ETFE film than in its bulk leaving styrene to diffuse independently inside the swollen polystyrene grafted layers formed at the surface and as a result faster rate of grafting and higher degrees of grafting were obtained. Conversely, diluting of styrene with a good styrene and polystyrene solvent such as toluene or DMF causes a competitive diffusion through the grafted layers of the ETFE film and as a result the availability of styrene in the grafting layers is reduced leading to lower degrees of grafting. These results confirm that alcohols such as methanol and ethanol are potential solvent candidates for diluting styrene during its radiation induced grafting onto ETFE films to achieve high degrees of grafting.

Similar behaviour towards polar and non-polar solvents was observed for grafting of styrene in isopropanol or toluene onto PVDF film by Walsby et al [11]. These authors found that toluene recorded lower degrees of grafting compared isopropanol and this was attributed to the lower viscosity of the grafted zone in the styrene/toluene system, which favours termination. It was suggested that the use of a solvent that contributes to the swelling of the grafted moiety is far from being essential to the reaction. Similar finding was earlier reported for grafting of styrene into FEP [31]. Rager [4] also reported that grafting of styrene onto FEP films was enhanced to three folds by the dilution with isoporanol compared to toluene. He attributed such behaviour to Trommsdorff-type effect without ignoring the role of the tiny variation in the degree of swelling. This author suggested that other parameters such as type of solvent, the amount of sorption and the individual solvent exchange rate have an impact on the local viscosity, radical mobility, swelling equilibrium and reaction rate and eventually the degree of grafting.

Since methanol and ethanol have been found to produce wide range of degrees of grafting, which have desired values for making membranes for fuel cell application, both solvents were considered for diluting styrene during grafting onto ETFE film and their kinetics were investigated with respect to variation of monomer concentration as presented in Figure 3 (a and b). Trommsdorff-type effect was found to be the main cause for maximizing the degree of grafting, which was reached in styrene/methanol mixture at 40:60 (v/v) and in styrene/ethanol at 60:40 (v/v). This observation is in a complete agreement with the data of the initial rates of grafting presented in Table 4, which prevails that lower monomer concentration can be used with methanol to obtain higher degrees of grafting than that with ethanol. The higher initial rate of grafting with methanol dilution can be attributed to the slightly higher swelling of original and grafted ETFE films in methanol as seen in Tables 1 and 2.

The increase in the initial rate of grafting and the final degree of grafting with the irradiation dose for styrene/methanol and styrene/ethanol grafting systems presented in Figure 4 (a and b) and Table 5 can be generally ascribed to the increase in the amount radicals formed in the irradiated polymer film, which enhances the grafting reaction in both systems when these radicals are thermally activated in presence of monomer molecules.

The second increase seen in Fig. 4 (a) for styrene/methanol system after the first 4 h may be attributed the role of crystallinity in the irradiated films as ETFE crystallinity is reported to be a dose dependent [32]. Irradiation of semicrystalline polymer such as ETFE is well known to form radicals in both amorphous and crystalline regions with a polymer of high crystallinity is most likely to contain higher amount of trapped radicals than a polymer of low crystallinity. Assuming adequate supply of monomer, it is expected that grafting is initiated by radicals in the amorphous region followed by propagation of growing chains, which continue dominating until consuming most of the radicals in the amorphous region causing a grafting slow down after 4 h. As the time prolonged with the monomer unable to diffuse to the crystallites, the trapped radicals in crystalline region would migrate to the lamella surface and participate in initiation and chain transfer causing a second increase in G% until termination reaction become more frequent above 24 h.

The lower initial rates of grafting of styrene observed in methanol compared to ethanol with the variation of the irradiation dose can not be due to transfer to solvent as the transfer constant of ethanol is higher than that of methanol. Nevertheless, the high values of the initial grafting rates observed upon dilution of styrene with both methanol and ethanol could make it possible to reduce the doses necessary to achieve desired degrees of grafting for applications such as fuel cell.

The increase in the degree of grafting caused by the addition of sulfuric and nitric acids to grafting mixtures of styrene/methanol and styrene/ethanol (Figure 5 a and b) can be attributed to the partitioning effect of such acids, which increases not only the rate of diffusion of the monomer but also the equilibrium concentration of the monomer within the grafted layers of the polymer substrate and eventually results in enhanced grafting rates. The enhancement seems to be working faster at the early hours with methanol than ethanol due to its lower chain transfer constant. These results are in a complete agreement with the work by Garnett et al [33] and suggest that acids can be used to reduce the monomer consumption during grafting of styrene onto ETFE films without scarifying the desired degrees of grating. This would help to improve the economy of grafting by reducing the monomer consumption.

The features shown in FTIR spectra of ETFE-g-PS films having various degrees of grafting and presented in Fig. 6 clearly indicate the formation of polystyrene grafts in ETFE films. These features are in form of additional peaks representing benzene rings such as the stretching vibration of =C-H at 3010-3100 [cm.sup.-1], the skeletal C=C in-plane stretching vibrations in the range of 1500 to 1600 [cm.sup.-1] and out-of-plane C-H deformation band at 700[cm.sup.-1] which is assigned for mono-substitution benzene rings.

[FIGURE 6 OMITTED]

Conclusions

Graft copolymerization of styrene onto electron beam irradiated ETFE films was studied in various solvents and with acid addition. The dilution of the styrene with non-solvents (alcohols) has been found to synergize its grafting onto ETFE films compared to that with good solvents (toluene and DMF). The role of the close proximity of the solubility parameters of the individual components in the grafting mixture and the corresponding solubility parameters of their mixtures in the diluting solvents has been marginalized during grafting reaction in the present system. Methanol followed by ethanol has been found to be most preferable diluents for grafting of styrene onto ETFE because they allow wide range of degrees of grafting. The addition of sulfuric and nitric acid has been also found to further enhance the degree of grafting at low monomer concentrations. Finally, it can be suggested that the synergetic effects caused by dilution of styrene with methanol or ethanol and the addition of acids can be used to reduce monomer consumption as well as processing doses in a way that would improve the economy of the grafting process and the mechanical properties of the graft copolymer.

Acknowledgement

The authors wish to acknowledge the financial support for this work by Malaysian Ministry of Science, Technology and Innovation (MOSTI) through IRPA mechanism.

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[22] Gubler, L., Prost, N., Gursel, S.A. and Scherer, G.G., 2005, Proton Exchange Membranes Prepared by Radiation Grafting of Styrene/divinylbenzene onto Poly(ethylene-alt-tetrafluoroethylene) for Low Temperature Fuel Cells, Solid State Ionics 176, pp. 2849-2860.

[23] Chen, J., Asano, M., Yamaki T., and Yoshida, M., 2006, "Preparation and Characterization of Chemically Stable Polymer Electrolyte Membranes by Radiation-induced Graft Copolymerization of Four Monomers into ETFE films," J. Membr. Sci., 269, pp. 194-204.

[24] Ueda A., Nagai, S., 1999, In: Polymer Handbook, Bandrup, J. Immergut, E.H., Grulke EA, Eds.: Wiley Interscience: New York, 1999 Vol. II.

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[26] Omichi, H. and Okamoto, J., 1982, "Solvent Effects on the Synthesis of Ion- exchange Membranes by Radiation-induced Graft Polymerization of Methyl [alpha],[beta],[beta]-trifluoroacrylate," J. Appl. Polym. Sci., 20, pp. 1559- 1568.

[27] Nasef, M.M., Saidi, H., Dessouki, A.M. and El-Nesr, E.M., 2000, "Radiation- induced Grafting of Styrene onto Poly(tetrafluoroethylene) (PTFE) Films. I. Effect of Grafting Cconditions and Properties of the Grafted Films," Polym. Int., 49, pp. 399-406.

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[30] Dargaville, T.R., Hill, D.J.T., Perera, S., 2002, "Grafted Fluoropolymers as Supports for Solid-phase Organic Chemistry: Preparation and Characterization. Aust. J. Chem., 55, pp. 439-441.

[31] Gupta, B., Buchi, F., Scherer, G., 1994, "Cation Exchange Membranes by Pre- irradiation Grafting of Styrene into FEP Films. I. Influence of Synthesis Conditions. J. Polym. Sci.: Part A: Polym. Chem., 32, pp. 1931-1938.

[32] Nasef, M. M., Saidi, H. and Dahlan, K., 2003, "Electron Beam Irradiation Effects on Ethylene- tetrafluoroethylene Copolymer Films," Radiat. Phys. Chem., 68, pp. 875-883.

[33] Garnett, J.L, Jankiewicz, S.V. and Sangster, D.F., 1990, "Mechanistic Aspects of the Acid and Salt Effect in Radiation Grafting," Radiat. Phys. Chem., 36, pp. 571-579.

Mohamed Mahmoud Nasef (a) *, Rosiah Rohani (a), Hamdani Saidi (a) and Khairul Zaman Mohd Dahlan (b)

(a) Chemical Engineering Department, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia

(b) Radiation Processing Technology Division, Malaysian Nuclear Agency, Bangi, 43000 Kajang, Selangor, Malaysia

Corresponding Author E-mail: mahmoudeithar@fkkksa.utm.mymahmoudeithar@fkkksa.utm.my
Table 1: Sorption data of ETFE films in various monomer/solvent
mixtures (50:50 v/v) and the corresponding data in pure solvents.
Swelling time is 24 h at room temperature.

Solvents              Degree of swelling (%)

              Pure solvent   Solvent/monomer (50:50 v/v)

Toluene       1.91           2.10
DMF           1.59           1.90
Isopropanol   0.24           1.60
Ethanol       0.25           1.55
Methanol      0.26           1.65

Table 2: Sorption data of polystyrene grafted ETFE films having
different degrees of grafting in various solvents. Swelling time is 24
h at room temperature.

Degree of                        Swelling (%)
grafting (%)   Toluene   DMF     Isopropanol    Ethanol   Methanol

0.0            2.60      1.59    0.24           0.25      0.26
15.5           6.50      7.30    1.00           1.05      1.10
43.0           16.00     11.20   1.22           1.30      1.46
64.0           22.50     16.60   1.82           2.20      2.94
93.0           37.50     26.60   3.20           3.60      4.10

Table 3: Solubility parameters of various components involved in the
grafting mixtures in pure and mixed forms.

Components of grafting mixtures   Solubility, [delta] ([Mpa.sup.1/2])

Toluene                           18.3 (a)
DMF                               24.7 (a)
Isopropanol                       23.5 (a)
Ethanol                           26.2 (a)
Methanol                          29.7 (a)
Styrene                           19.0 (b)
Polystyrene                       17.5 (b)
ETFE                              13.7 (b)
Toluene/styrene                   18.6 (c)
DMF/styrene                       22.0 (c)
Isopropanol/styrene               21.4 (c)
Ethanol/styrene                   22.9 (c)
Methanol/stryrene                 24.9 (c)
ETFE/polystyrene (15.5%)          14.4 (c)
ETFE/polystyrene (43.0%)          15.2 (c)
ETFE/polystyrene (64.0%)          16.2 (c)
ETFE/polystyrene (93.0%)          16.9 (c)

(a) Obtained from Handbook of solubility parameters, CRC Press, 1991.

(b) Obtained from Polymer handbook

(c) Calculated as reported in Refs [9,25] .

Table 4: Initial rate of grafting for grafting of styrene diluted with
methanol and ethanol onto ETFE films. The  irradiation dose is 100 kGy
and the rest of grafting conditions as in Figure 3 (a and b).

Monomer/solvent   Initial rate of grafting (G%/h)
(v/v)             Methanol   Ethanol

20/80             6.91       3.96
40/60             28.83      24.80
60/40             21.13      31.68
80/20             19.22      16.35
100/0             5.49       5.49

Table 5: Initial rate of grafting at various irradiation doses for
grafting of styrene diluted with methanol and ethanol onto ETFE films.
The mononor/solvent concentration is 40:60 (v/v) and the rest of
grafting conditions as in Figure 4 (a  and b).

Irradiation dose   Initial rate of grafting (G%/h)
(kGy)              Methanol    Ethanol

20                 5.44        14.80
40                 15.65       18.75
60                 23.33       22.25
80                 27.78       34.00
100                30.41       41.66
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Author:Nasef, Mohamed Mahmoud; Rohani, Rosiah; Saidi, Hamdani; Dahlan, Khairul Zaman Mohd
Publication:International Journal of Applied Chemistry
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Date:Sep 1, 2008
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