Modification of various thermoplastic polymers through graft copolymerization using supercritical carbon dioxide as a solvent: Synthesis and characterization

The graft copolymerization process is commonly used for chemical modification of polymers to impart superior properties to the main polymer backbone. They are also used as compatibilizers in polymer blending. Since most of the polymers used in blending are incompatible with each other in the absence of a compatibilizer, these graft copolymers serve as important intermediate ingredient.; In this research work, a novel process for carrying out graft copolymerization reactions is studied. The reaction is carried in supercritical {dollar}rm COsb2{dollar} solvent medium. The process offers many unique advantages over conventional processes used for carrying out such grafting reactions. Also, supercritical {dollar}rm COsb2{dollar} serves dual purpose, being a solvent for the monomer as well as the swelling agent for the polymer being modified. Various thermoplastic polymers such as polypropylene (PP), poly(vinyl chloride) (PVC), polycarbonate (PC) and poly (acrylic acid) (PAAc) have been modified by grafting monomers having different properties. Monomers grafted onto polymers included styrene, acrylic acid and butyl acrylate. The reactions were carried out in a high pressure stainless steel Autoclave reactor. Effects of different process variables such as time, temperature, pressure, initiator as well as monomer concentration were studied. The resulting grafted polymers were characterized using different analytical techniques such as FTIR, DSC, TGA and NMR. A quantitative method was developed based on absorption law to find out the weight percent grafting in the resulting copolymers.; The results obtained on various graft copolymers showed that weight percent grafting increased with time, temperature and monomer concentration but found to be decreasing with pressure well above the critical point of the solvent-monomer mixture. The successful grafting was also confirmed by thermal analysis using DSC and TGA. DSC results concluded that there was a marked decrease in crystallinity of the grafted copolymers (PP-g-PS and PP-g-AAc) compared with homopolymer isotactic PP. The glass transition temperature {dollar}rm (Tsb{lcub}g{rcub}){dollar} was found to increase in comparison with starting polymers PP and PVC. TGA analyses showed that PVC-g-PS and PP-g-AAc were more stable than their respective homopolymer but PP-g-PS showed a decreased thermal stability compared to PP due to PP being inherently more stable than PS. SEM pictures of the film cross-sections showed a decrease in phase separation of the blend when some quantity of the grafted product was added onto it. {dollar}rmsp{lcub}13{rcub}C{dollar} NMR and {dollar}rmsp1H{dollar} NMR analyses of the graft copolymer PP-g-PS also confirmed the successful addition of PS to PP. Absolute confirmation of side chain grafts onto main chain polymer still needs to be done by doing some 2D- and 3D-NMR analyses of the samples.