Bulk Graft Modification Of γ-Ray Pre-irradiated Polymer In Supercritical Carbon Dioxide

Functional graft-modification of general-purpose polymer materials such as polypropylene or polyethylene is one of important routes for providing the desired properties in connection with higher added value applications. Usually, grafting reaction is carried out in organic solvent or melts extrusion. The limitation in these methods is that the need for organic solvent or/and the requirement for high temperature lead to the formation of ungrafted homopolymer, which along with organic solvent and unreacted monomer trapped in polymer substrate is very difficult to remove.In this thesis, a green and versatile route for uniform bulk graft-modification of vinyl monomers to polymer materials at relatively low temperature was presented by combining gamma (γ)-ray preirradiation-induced graft copolymerization and supercritical CO₂-swelling polymerization techniques. The polymer substrates were first irradiated withγ-rays originated from cobalt-60 resource under nitrogen atmosphere at ambient temperature, and thereby leading to uniform formation of trapped radicals on polymer backbone. Then, the produced polymer trapped-radicals were utilized to initiate graft-polymerization of vinyl monomers dissolved in supercritical CO₂ within polymer substrates. Finally, the un-reacted monomers trapped in polymer matrix were further removed via supercritical CO₂ extraction. Altering irradiation dose, reaction time and temperature can control the grafting process. The present method was succeeded in applying to bulk chemical graft-modification of various vinyl monomers such as styrene, N-vinylpyrrolidone (N-VP), methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) to polypropylene, low-density polyethylene and silicon rubber membranes. The grafting process did not require any organic solvent, and the resulting modified polymer materials did not remain unreacted monomer as well as ungrafted homopolymer. The bulk graft-modification of polymer materials were confirmed by fourier transform infrared spectroscopy (FTIR), element analysis, dynamic contact angles determination, scanning electron microscope (SEM) and transmission electron microscopy (TEM) analyses. SEM and TEM micrographs indicated that the side graft-chains in nano size were uniformly dispersed in the polymer matrix throughout the thickness of the modified sample. Differential scanning calorimetry (DSC) measurements showed that the graft predominately occured in amorphous part of the polymer substrate and the resulting graft-chains were also amorphous.In consideration of their low stability of polymer trapped-radicals, a novel method for bulk graft-modification of polymer materials was presented by combining gamma (γ)-ray preirradiation-induced polymer-diperoxide graft-copolymerization and supercritical CO₂-swelling polymerization techniques. With styrene as model vinyl monomer, the influences of reaction time, temperature, pressure and monomer concentration on grafting yield were investigated in the graft-modification of styrene to polypropylene and low-density polyethylene membranes initiated by polymer-diperoxide with supercritical CO₂ both as solvent and swelling agent. As comparing with the former route, this method has some advantages such as easily controlling the reaction process and obtaining relative high grafting yield. Although ungrafted homopolymer was found in the modified polymer materials, its content was usually less than 5% with regard to that of the side graft-chains. FTIR analyses for comparing characteristic peak intensity of side graft chain of the modified polymer membranes at outer and inner layer, as well as SEM and TEM observations of a cross-section all demonstrated the uniformly bulk grafting property.On the basis of the work mentioned above, bulk living graft-copolymerization modification of styrene or MMA to polyolefin membranes was initially performed by combining the advantages of nitroxide-mediated living free radical polymerization.