Preparation,Structure And Electrochemical Properties Of Hierarchical Porous Carbons Based On Vinylidene Chloride Block Copolymers

Porous carbons exhibit a great number of pores and great specific surface area, and have wide applications in many fields. Hierarchical porous carbons (HPCs) possess a multimodal pore size distribution of micro-, meso-and/or macro-pores, and combine the feature of high surface area of the micro-porous carbons and the large pore diameter of meso-/macro-porous carbon. Thus, HPCs have excellent application properties as they used in the electrodes for electrochemical double layer capacitors, catalyst supports, and macromolecules adsorption and separation. Up to now, HPCs have been generally prepared by the methods of multi-templating carbonization, catalytic activation, carbonization of polymer blend and organic gel. However, needing of complicate process and low controllability on pore structure have limited their applications.This dissertation presented a facile, novel self-template approach to fabricate HPCs with high surface area and large pore volume by direct carbonization of micro-phase separated block copolymers composed of poly(vinylidene chloride)(PVDC) and pyrolyzable polymer blocks. Micro-porous carbon skeleton would formed by the carbonization of PVDC block, and the properly selected pyrolyzable blocks would transfer to the mesopores or meso-/macro-pores after their pyrolysis. Thus, porous carbons exhibited a multimodal pore size distribution of micro-, meso-and/or macro-pores would be obtained.A series of block copolymers comprising the PVDC block and the pyrolyzable block including polyethylene glycol (PEG), polybutyl acrylate (PBA), polyacrylic acid (PAA) and polystyrene (PS) were synthesized via reversible addition fragmentation-chain transfer (RAFT) polymerization. RAFT copolymerization of VDC was successfully carried out using2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (TTCA) as a chain transfer agent (CTA). The kinetics studies showed the obvious living/controlling radical polymerization behavior. The significant transfer reaction of macro-radicals to VDC monomer was considered to address the relatively broad polydispersity index (PDF) and low molecular weights. PVDC-b-PBA, PVDC-b-PAA and PVDC-b-PS copolymers were synthesized via reinitiation reactions of corresponding monomers.PVDC-b-PEG-b-PVDC copolymers were synthesized via RAFT polymerization of VDC using PEG-TTCA macro-CTA prepared by the esterification of PEG and TTCA. PS-b-PVDC-b-PS was synthesized by using S,S’-bis(α,α’-dimethyl-α"-acetic acid)-trithiocarbonate (BDMAT) as CTA. The variations of the molecular weight of block copolymers with conversion indicated that the chain growth depended on the molecular weight of macro-CTAs and the phase separation during the solution RAFT copolymerization of VDC, and the molecular weight of block copolymers were relatively low in the solution copolymerization.Considering disadvantages of the solvent polymerization, an amphiphilic PAA-b-PS-TTCA copolymers containg trithiocarbonate reactive groups were used in the ab initio RAFT emulsion copolymerization of VDC. Effects of macro-RAFT agent structure and concentration, neutralization policy of PAA-b-PS-TTCA macro-RAFT agent, initiator type and polymerization temperature on the kinetics and controllability of polymerization, the stability and particle size distribution of latexes were investigated. It was found that the RAFT emulsion copolymerization of VDC showed greater polymerization rates than the solution polymerization, and PVDC with high molar masses (25kg-mol-1) and low PDI could be obtained. The determined molecular weights of PVDC were increased continuously and were in good agreement with the corresponding theoretical values, indicating well controllability of polymerization. The as-prepared PVDC latexes were further used as seeds in the emulsion polymerization of styrene, enabling the preparation of novel PVDC-b-PS copolymers with a high molar mass and a relatively low PDI.The micro-phase separation of block copolymers were investigated by using atomic force microscopy (AFM), transmission electron microscope (TEM) and small-angle x-ray scattering (SAXS) methods. Due to the thermodynamic incompatibility between pyrolyzable polymer block and PVDC block, all block copolymers showed micro-phase separation structures. The dimensions of the micro-dispersed phase in PVDC-b-PEG-b-PVDC, PVDC-b-PAA and PVDC-b-PS copolymers were10～20nm,30-70nm and20～100nm, respectively. The differential scanning calorimetry (DSC) results indicated the block copolymers exhibited two glass transition temperatures corresponding to the glass transition temperatures of PVDC and pyrolyzable polymer block. The thermogravimetric analysis (TGA) indicated that PVDC block and PBA (or PS) block were decomposed independently, while the degradation temperatures of PEG and PAA blocks were closed to that of PVDC block. Furthermore, PAA block showed an incomplete pyrolysis at even higher temperature, which might jam the pores of the corresponing porous carbons.The micrographs and microstructure parameters of the carbons prepared from the block copolymers were characterized by field emission scanning electron microscopy and N2absorption/desorption analysis. The results indicated that hierarchical porous structures could be accomplished via well-designed self-templates, Multimodal pore size dimensions could also be obtained via adjusting ratio of two blocks structures. The carbons prepared from PVDC-b-PEG-b-PVDC copolymers exhibited a maximum specific surface area (SBET) of1242 m2/g, a maximum total pore volume (Vtotai) of0.49cm3/g and a low mesoporosity of14.5%. The carbons prepared from PVDC-b-PBA copolymers exhibited a maximum SBET of957m2/g, a maximum Vtptal of0.52cm3/g and a mesoporosity of44.2%. PVDC-b-PAA based carbon exhibited a maximum SBET of1093m2/g, a maximum Votal of0.51cm3/g and a mesoporosity of22.6%. Due to complete pyrolysis of PS block, the carbons with higher SBET and mesoporosity could be obtained from PVDC-b-PS copolymers. PVDC-b-PS copolymers based carbon exhibited a maximum SBET of1220m2/g, a maximum Vtotai of0.92cm3/g and a mesoporosity of57.5%. PS-b-PVDC-b-PS copolymers based carbon exhibited a maximum SBET of839m2/g, a maximum Vtotal of0.42cm3/g, and a mesoporosity of54%. The carbon prepared from the seeded emulsion polymerized PVDC-b-PS copolymer exhibited a maximum SBET of1226m2/g, a maximum Vtotai of1.86cm3/g, and a mesoporosity of77.9%.The electrochemical performances of as-prepared HPCs used as supercapacitor electrodes were studied via galvanostatic cycling and cyclic voltammetry. The results showed that the hierarchical porous structures played a key role in electrochemical performances. The specific capacitances of the electrodes were increased with the specific surface areas of HPCs. The presence of larger pore size in HPCs could improve the ion transportation, keep the stability of specific capacitance under high current density and enhance electrodes performance. The highest specific capacitance values of252F/g,233F/g,216F/g,218F/g and216F/g were obtained at0.5A/g for HPCs prepared from PVDC-b-PEG-b-PVDC. PVDC-b-PAA, PVDC-b-PS, PS-b-PVDC-b-PS and seed emulsion polymerized PVDC-b-PS copolynmers, respectively. With increase of current density and scan rate, larger mesoporosity led to less decrease of specific capacitance, which indicated meso-pore structure could obviously enhance performance under high current.