Graftıng Of Polystyrene To Poly(Ethylenealt- Tetrafluoroethylene) (Etfe) Fılms By Controlled Raft Polymerızatıon In The Presence Of A Crosslınkıng Agent

Abstract

Controlling the molecular weights and distributions of polymers is not possible in syntheses carried out by conventional Free Radical Polymerization (FRP). This hinders the widespread use of FRP techniques in the synthesis of polymers with well-designed molecular structures. On the other hand, Controlled Free Radical Polymerization (CRP) methods have emerged as promising methods in which the polymer weight and distribution can be controlled. Among the CRP methods, Reversible Addition-Fragmentation Chain Transfer (RAFT), which is the only method that can be carried out under radiation, seems quite advantageous since it is suitable for the polymerization of almost all vinyl monomers and can be carried out using a wide range of solvent and temperature alternatives [1]. As the polymers synthesized by Controlled Free Radical Polymerization methods have well-designed molecular structures, they can be used effectively in many different fields such as preparation of sensor materials, polymer-protein conjugates, development of polymeric materials with cylindrical, spherical, hyper-branched architectures, and pH or temperature responding smart polymers, etc. [2].

Polymer Electrolyte Membrane Fuel Cells (PEMFC, also knowns as Proton-Exchange Membrane Fuel Cells) that directly convert the energy of the fuel into electrical energy through a series of electrochemical reactions have an important place among the various fuel cells as they can be used in transportation and small-sized power generation systems due to benefits such as increased power density, immediate response to changes in power demand, and low operating temperatures. Since the first electrically powered automobile was developed using the PEMFC system, many different types and models of vehicles have used these systems, which use hydrogen as the fuel and convert it into electricity. One of the most important factors determining the cost is the membranes that are needed during passage of substances in the electrolyte system. The most common material used as Polymer Electrolyte Membrane PEM is Nafion produced by DuPont. However, due to the high cost of this membrane, it is challenging for electric vehicles to replace existing ones that use fossil fuels. For this reason, many studies aiming to synthesize new membranes to increase the performance of existing systems and reduce costs have gained great momentum, especially over the last few decades.

One of the most widely applied methods to develop low-cost and high-performance alternative PEMs is to provide the functionality required for proton conductivity to fluorinated or partially fluorinated polymer films with the desired properties by conventional radiation-induced grafting (RIG) method. Conventional Free Radical Polymerization technique has been used for the grafting process in the studies carried out so far. However, it is not possible to achieve the desired structural control with this method. The advantages of the CRP methods for the preparation of PEMFC have been demonstrated in some studies. However, these studies are insufficient and the application of controlled polymerization methods in the presence of a crosslinking agent and using the Radiation Induced Grafting Technique for the preparation of PEM is not available in the literature.

In this thesis, by applying radiation-induced and RAFT-mediated graft copolymerization of polystyrene from the cost-efficient ETFE films using a crosslinker (divinyl benzene, DVB) for the first time, a well-defined PEM will be obtained. Compared to PEMs synthesized by conventional methods, it is aimed to synthesize unique PEMs with superior properties, especially in terms of proton conductivity, thanks to the structural control and homogeneity to be achieved by the RAFT mechanism.

Membranes with different degrees of grafting prepared within the scope of the thesis were characterized by ATR-FTIR, SEM-EDX, AFM, TGA, XPS, and DMA techniques. These extensive characterizations were used to confirm the presence of grafted polystyrene (PS) chains in copolymer compositions and the success of sulfonation. In summary, it was seen from the ATR-FTIR results that the syntheses were performed successfully, and when the AFM images were examined, it was observed that the surface roughness increased because of grafting. The results from DMA and TGA provided significant and promising details regarding the mechanical and thermal performance of the membranes. It was determined that the chemical resistance of the membranes synthesized in the presence of DVB increased approximately 4 times compared to those synthesized without DVB. Although there was a decrease in proton conductivity due to the use of DVB, a significant increase in chemical stability emerged as a result of cross-linking reactions. Membranes with 45% and 67% degrees of grafting exhibited higher proton conductivity than many alternatives in the literature, especially commercial Nafion samples.