Poly(ethylene-alt-tetra-fluoroethylene (ETFE) and poly(tetrafluoroethylene-cohexafluoropropylene) (FEP) were pre-irradiated under air using a Co 60 gamma source to graft styrene at low pH. Grafting copolymers were tuned by study of different parameters (monomer, reaction time, temperature, and pH with addition of sulfuric acid (H 2SO 4)). The maximum degree of grafting was 80% and 40% for ETFE and FEP respectively at dose 2 kGy. Influence of low pH in grafting degree by adding sulfuric acid was studied. Grafting degree was examined by infrared (FTIR-ATR), differential scanning calorimetry (DSC) and swelling behavior analysis after sulfonation process.
As is known, fluoropolymers are used in a great variety of applications due to excellent physicochemical properties: chemical and thermal stability, hydrophobic, low surface energy, and environment resistance. Fluoropolymers have been modified by etching method or by ionizing radiation technology. Radiation technology is used as a grafting method to modify surfaces of materials in several areas (biomaterials, protective materials, wastewater treatment, fuel cells, alkaline cells, etc.) [
Acid enhanced could be due to irradiation of the solvent increases hydrogen atom yield (G(H)), low pH of grafting solutions and partitioning effect (partitioning effect consists in increase of monomer diffusion through substrate improving homogenous grafting). The hydrated electrons are scavenged by hydrogen ion (H+) and transformed to hydrogen radical at low pH (H+ + e→H●). In solutions at low pH, radical yields are rather higher than in neutral or basic solutions. Formed hydrogen radicals are able to abstract hydrogen atoms from polymeric matrix and create additional sites for grafting initiation. Increasing of additional grafting sites and diffusion of more monomer onto substrate promote increase in the grafting degree at low dose.
To study the behavior of low pH in the grafting degree sulfuric acid (H2SO4) as an additive was added. In comparison with other authors, we pretend to obtain grafting degree at low dose (0 to 5 kGy). In this work, influence of low pH in the creation of additional grafting sites or optimization of monomer diffusion through fluoropolymer (ETFE and FEP), was studied. Styrene grafting degree into FEP and ETFE by gamma radiation was examined by infrared (FTIR-ATR), differential scanning calorimetric analysis (DSC) and swelling analysis.
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 Gwg andGw0 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 WSf 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.
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.
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.
Scheme 1. Radiation effects in ETFE and FEP.
the same than ETFE spectrum, but the
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 (
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).
Grafting of styrene onto FEP and ETFE by pre-irradiation method in air was done and increased by tuning of pH with addition of H2SO4. The grafting of styrene onto FEP and ETFE was increased by optimization of monomer concentration, reaction time, reaction temperature and dose at 2 kGy using gamma radiation. The best grafting degree of styrene onto FEP and ETFE was found ~40% and 80% respectively. Optimal conditions were styrene/ethanol concentration 60/40 (V/V), reaction temperature of 60˚C and reaction time of 15 h. Grafting degree was optimized at pH 3.5, where the most active radical species might be formed to graft enhancing into fluoropolymers, increasing reliability and application of these grafting films. FTIR-ATR confirmed grafting process onto fluoropolymers due to new band associated to grafting styrene and not only a presence of styrene. DSC technique confirmed the modification with styrene due to variation of thermal behavior of unmodified and modified copolymers with styrene. Different swelling degree of modified fluoropolymers reported in this work, confirmed the presence of grafting styrene. Grafting films with styrene were produced several times to confirm its reproducibility. Different grafting degrees for both systems could be employed in several applications as hydrogen membrane cells, encapsulation in solar cells devices or alkaline cells; in the case of alkaline developments, our group will use our modified fluoropolymers to research it in future.
The authors wish to express their thanks to Paulo de Souza Santos from CTR-IPEN-USP, for technical assistance in the irradiation, and FAPESP-2013/13144-0 for economic support.
González-Pérez, G. and Lugao, A.B. (2017) Influence of pH to Increase Grafting Degree into Fluoropolymers. Open Journal of Polymer Chemistry, 7, 43-56. https://doi.org/10.4236/ojpchem.2017.73004