Graft Copolymerization Of High-molecular-weight Atactic Polypropylene In Supercritical CO₂

High-molecular-weight atactic Polypropylene(aPP) is one of polymers which is usedmore and more widely. However, its non-polar chemical structure brings on limitation inproperty such as low surface energy, poor hydrophilicity, chromaticity and adhesiveproperty. Thus, it is difficult to blend with other polar polymers or fillings in order to obtainbetter performances by using alloyed or composite technique. Graft copolymerization byintroducting a polar group into aPP offers an effective approach to modify itsperformances. Grafting different monomers onto aPP have been successfully achievedby using conventional methods such as solution grafting, melt grafting, solid phasegrafting, but they all have some drawbacks to some extent. Recently, supercriticalCO₂(SC-CO₂) technigue has become a new method in polymer modifications. SC-CO₂fluid provides an environmentally benign, inexpensive, and nonflammable alternative tothe aqueous and organic solvents employed traditionally.The free-radical double grafting of maleic anhydride (MAH) and styrene (St) onto agranular atactic Polypropylene (aPP) using supercritical CO₂ as a solvent and swellingagent has been studied to prepare aPP-g-MAH and aPP-g-(MAH-St). The effects ofMAH content, initiator content, molar ratio of monomer, swelling temperature and time,reaction time and pressure on the graft degree are discussed. The chemical structures ofgraft copolymers are characterized by FTIR, the phase transition temperatures aredetected by using DSC. The crystalline are detected by X-ray diffraction. The molecularweights of aPP and the grafted aPP are measured.It is found that St as a co-monomer in the grafting reaction system could significantlyenhance the graft degree of aPP for MAH and St. The maximum of the grafting degreefor aPP-g-(MAH-St) in supercritical CO₂ fluid under the optimal reaction condition was10.8% which is one times larger than that of aPP-g-MAH (5.10%). The structure ofgrafting copolymers characterized by FTIR indicates that MAH and St have been graftedonto aPP. The thermal properties characterized by DSC show that the introduction of Stunit into the branch chains of aPP has little influence on the crystallization behavior ofaPP-g-(MAH-St). Furthermore, the processes of the operation and separation are simpleto show that supercritical CO₂ technique is better than other grafting ways. APP-g-MAHmembranes which contain cobaltous ions have good intensity. And the membranes havehigh permeability coefficients and separation factors.