| Chemical modification of pre-existing polymer materials by functional monomers has always been an object of great interest as it offers an effective route to address some low cost commodity shaped-materials to higher added value applications. In traditional solution method, organic solvent is used to swell or dissolve the polymer matrix, and then the dissolved initiators initiate polymerization of monomers in polymer matrix. The high viscosity leads to the solvent, un-reacted monomers and initiators not be completely removed after the reaction. An alternative method is modification of polymers in the melt, but strong shear fields may be needed to conquer high viscosity polymers in extruders. In this research work, a novel process for graft copolymerization is studied by combining gamma (y)-rays pre-irradiation-induced graft copolymerization with supercritical carbon dioxide swelling polymerization techniques. Firstly, the trapped radicals on the polyolefin backbones would be uniformly distributed by y-rays radiation under nitrogen atmosphere. Subsequently, the produced polymer trapped-radicals were utilized to initiate graft-polymerization of vinyl monomers dissolved in scCO2 within polymer substrates. Finally, the un-reacted monomers trapped in polymer matrix were further removed via scCO2 extraction. The process offers many unique advantages over conventional methods for grafting reactions. Also, supercritical carbon dioxide serves dual purpose, being a solvent for monomers as well as the swelling agent for the polymer.The bulk grafting modification of styrene onto polypropylene is the focus of the first project. Supercritical carbon dioxide is utilized as both solvent and swelling agent to promote this heterogeneous reaction and lead to successful grafting. The grafting composition is controlled by controlling reaction parameters. After polymerization reaction, the un-reacted monomers are further extracted via supercritical CO2 without any residues. Of importance, it is beneficial to diffuse the vinyl monomers to the internal layers of pre-existing polymer materials with the aid of supercritical CO2, which does not interfere with the chain-growth process during polymerization. Thereby, grafting component can be easily accomplished throughout the polymer matrix without homopolymer. Characterization of the composites shows polystyrene is covalently bonded to polymeric backbone in nanometer scale and the graft chains are uniformly dispersed throughout the thickness of polymer membranes. The crystalline domains of the polypropylene are unaffected. Styrene infuses into polypropylene and polymerizes within only the amorphous domains of polypropylene.In the final project, our strategy takes advantage of the fact that well-defined alternating poly(styrene-alt-maleic anhydride) can be synthesized in supercritical CO2 and y-ray has high penetration depth to various polymers so that the pure graft copolymers with alternating trend can be obtained. The results indicate that styrene/maleic anhydride binary monomers synergistically promote the bulk grafting process. The maximum of grafting yield can be always attained at 1:1 molar feed ratio of maleic anhydride to styrene, indicating that the grafting process is attributed to the formation of charge transfer complex (CTC) between the binary monomers. DSC and element analysis support the conclusion that an alternative graft copolymer may be achieved. |
