Toluene, ethanol, from Labsynth, Sulfuric acid (H₂SO₄) from Merck, and styrene from Huntsman Ltda were used as received. Fluoropolymers with thickness 125 µm (FEP and ETFE) were supplied by Goodfellow. UK.

Fluoropolymers were cut into 5 × 2.5 cm pieces, washed in isopropanol and then dried at 60˚C under vacuum for 8 h. Cleaned polymers were weighed and irradiated by Co⁶⁰ gamma radiation source using pre-irradiation oxidative method at room temperature (dose rate 10 kGy/h). Low pH was studied by addition of acid at different molar acid concentrations (from 0.05 to 1 M H₂SO₄) and acid percentage (from 1% to 10%), in solutions of monomer/solvent. Pre-irradiated fluoropolymers was put in glass tube (30 mL) with monomer-acid-solvent solutions and sealed under a nitrogen atmosphere. Sealed glass tubes were placed in a thermostatic bath at different temperatures and reaction times. After completed the reaction time, the residual unreacted monomer and homopolymer were removed from by Soxhlet system in toluene for 12 h. Then glass tubes with grafting copolymers and toluene were put in sonication bath for 6 h, and finally dried at 60˚C under vacuum. Grafting degree (GD) was calculated using the following equation:

G D = 100 × ( ( G w g − G w 0 ) / G w 0 ) (1)

where G_wg andG_w0 are grafted and initial weights respectively.

Copolymers were characterized by FTIR-ATR spectra using previously dried films (Nexus 6700 FTIR Thermo Scientific, Nicolet 6700 FTIR-MID) and differential scanning calorimetry (DSC) (822 Mettler-Toledo under nitrogen atmosphere at a heating rate of 10˚C/min, in the temperature range of 24˚C to 350˚C). Swelling degree was used to check modification of grafting copolymers after sulfonation process (grafted films were previously sulfonated with 2 vol.% chlorosulfonic acid in dichloromethane at room temperature for 8 h, washed with distillated water for 24 h and dried at 60˚C). Modified samples were immersed in distillated water for 24 h, removed from the solvent at different time intervals, their surface blotted with a piece of paper, weighed and placed again in water at room temperature. Swelling degree was calculated gravimetrically as follows:

S w d = ( W s f − W 0 ) / W s f × 100 (2)

where W₀ and W_Sf are dried and swollen weights of grafted copolymers, respectively.

Styrene grafting onto FEP and ETFE films was done using oxidative pre-irradiation method. Absorbed dose, temperature, monomer concentration, acid-enhancing and reaction time were studied by tuning grafting degree. Grafting degree was calculated with the Equation (1). Monomer concentration was adjusted to graft styrene onto ETFE (ETFE-g-St) at 20%, 40% 60% and 80% (V/V styrene/ethanol) without acid. pH of these solution was between 5 and 6. Figure 1 shows an increase in grafting degree with increase of styrene concentration as expected. Grafting degree does not show significant changes at higher concentrations because of homopolymer formation predominates over grafted into fluoropolymer, and there is not apparently dependence of dose in the grafting degree.

Figure 2 shows grafting degree as a function of molar acid concentration H₂SO₄. Grafting of styrene onto ETFE (ETFE-g-St) and FEP (FEP-g-St) was calculated with the Equation (1). 10% of different molar acid concentrations were added at solution styrene/ethanol (40/50 V/V). Figure 2 shows an increase at 0.05 M in grafting degree, after 0.05 M grafting degree decreased and remained without appreciable changes until 0.8 M. Increase-decrease behavior could depend of diffusion of styrene into fluoropolymer backbone promoted by dissolution of H₂SO₄ in the bulk grafting solution and influence of pH due to at low pH radical yields are higher to react with monomer and fluoropolymer. Grafting degree after 1 M decreased anew because of less monomer available close to active sites where grafting carried out and homopolymer inhibition. Buzanowski and coworkers probed that incorporation of strong acids retarded or reduced homopolymerization rates, which could permit diffusion of styrene trough matrix before homopolymerization [48] . Due to monomer diffusion by interaction acid-monomer-fluoropolymer at 0.05 M, grafting degree was more favorable than without acid or superior molar concentrations of H₂SO₄. Most active radical species might be formed at low pH. Active radical species rapidly decompose and interact with monomer-solvent that promotes a lot of free radicals to enhance grafting degree.

Figure 3 shows grafting degree as a function of different percentage of added acid. Grafting process took place at 2 kGy using H₂SO₄ 0.05 M, 70˚C reaction

temperature for 24 h in thermostatic bath. For both grafting systems with ETFE and FEP, were presented maximum grafting degree at 2% of added acid (pH ~3.3). Higher acid percentage (1 < pH < 4) could reduce homopolymerization and pH > 4 could increase competition with generated stable radicals and contribute to decrease of grafting degree. In the case of ETFE, is possible to generate more active sites available to grafting due to chemical structure and interaction of C-C, C-H and C-F bonds during gamma irradiation. In the case of FEP, C-C and C-F are more difficult to produce radicals available for grafting.

Figure 4 shows grafting degree as a function of pH. Grafting reaction was carried out at constant reaction time (24 h), temperature 70˚C, dose (2 kGy) and concentration (40/60 styrene/ethanol). pH was changed from 1 to 5 with incorporation of sulfuric acid 0.05 M. Grafting degree showed an increase with

increases pH of solutions until pH > 3, where grafting degree decreases again. Both fluoropolymers had similar behavior in pH range studied. Not all pH have an enhanced effect in the grafting degree due to at low pH < 1 homopolymerization could be retarded and pH > 4 could occur first homopolymerization over grafting.

Figure 5 shows a grafting degree as a function of temperature. Grafting degree increased with increase of temperature due to activation of radicals. During grafting process, hydroperoxides are more active than peroxides with increasing temperature, which causes its decomposition and reaction with available monomer [49] . An increase in temperature promoted monomer diffusion into fluoropolymer increasing grafting degree. Differences in chemical structure between ETFE and FEP promoted higher radical formation in ETFE due to bond C-H is easier to break than C-C and C-F during irradiation (Scheme 1). The radical formation in FEP is due to bond break C-C or C-F, but is more probably that bond break C-C occurs because of bond in C-F is stronger than C-C [50] .

Figure 6 showed grafting degree as a function of reaction time. Grafting degree increase with reaction time as expected. Both grafting systems (FEP-g-St and ETFE-g-St) have not significant changes after 15 h.

Figure 7 shows FTIR spectra of fluoropolymers unmodified and modified with styrene. For ETFE, symmetric and asymmetric vibrations CH₂ are at 2850 - 2920 cm⁻¹ respectively. At 1460 cm⁻¹ and 1490 cm⁻¹ represent C-H and C-F-C deformation respectively. 3100 - 3000 cm⁻¹ can be assigned to stretching vibrations of the C-H of aromatic ring. 2850 cm⁻¹ is asymmetric stretching of CH₂ of grafting styrene. At 1625 cm⁻¹ corresponds to aromatic C=C bonds stretching vibrations. The bands at 1670-1950 cm⁻¹ are attributed to harmonic bands C-H of the aromatic ring out of plane deformation. For FEP spectrum the bands are observed

Scheme 1. Radiation effects in ETFE and FEP.

the same than ETFE spectrum, but the Figure 6 shows the difference between unmodified FEP and modified with styrene.

Figure 8 shows DSC thermograms of initial fluoropolymer (FEP and ETFE) and grafting with styrene. DSC analysis was useful to find thermal transition temperatures and checked final grafting onto fluoropolymers. Membranes modified with styrene exhibited an apparently glass transition temperature (T_g) at 87˚C and 100˚C for ETFE and FEP system respectively. T_g and melting point temperature (T_m) of polystyrene has been reported at ~100˚C and 240˚C respectively [51] [52] . T_m of grafting systems with styrene did not show an evident variation respect to initial fluoropolymers. This temperature variation was due to grafting process (radiation, reaction temperature, styrene diffusion). Thermogram could be an evidence of the grafting copolymer and confirm infrared analysis.

After sulfonation of grafting fluoropolymers with styrene, swelling degree was studied by immersion of copolymers in distilled water as a function of time and grafting degree (Figure 9(a) and Figure 9(b)). Figure 9(a) shows that swelling degree has not significant variations up to 12 h. Figure 9(b) shows that swelling

degree increases with increase of grafting degree as expected after sulfonation process. Swelling degree remains constant after several cycles (drying at 60˚C for 48 h and anew immersion in distilled water several times).