Study On Mechanical Activation Enhancing Starch Graft Modification

Starch is a sort of natural polymer materials with wide source and low priceand can be completely degraded. Due to its poor mechanical performance, poordispersion property at low temperature and weak seepage force, native starchcan only find limited applications. After being initiated by physical or chemicalmethods, starch could be grafted with monomers such as acrylonitrile,acrylamide and acrylic acid and form graft copolymers with different properties,which may have potential applications in polymeric flocculants, super absorbentmaterials, paper manufacturing, petroleum chemical materials, degradable films,plastics and etc. However, starch granule usually has a semicrystalline structureand the graft reaction can only occur on the granule surface due to the hindranceof crystalline region, which makes it very difficult to prepare copolymer withhigh grafting ratio (G) and high grafting efficiency (GE). Therefore, how todecrease the ratio of crystalline region in starch granule and improve thereaction activity of starch, has become an important and hot research topic. In this thesis, maize and cassava starch were mechanically activated by astirring-type ball mill. Then the activated starches were used to fabricate graftcopolymer in the presence of monomer of acrylamide and combined initiators ofammonium persulfate and sodium bisulfate. Using G and GE as evaluatingparameters, the influences of activation time, reaction time, starch concentration,initiator concentration, monomer concentration and reaction temperature on thegraft reaction were investigated. Using grafted mass (G_m) as an evaluatingparameter, the kinetics of graft copolymerization was also studied. The graftcopolymers were characterized by Fourier transform infrared (FTIR)spectroscopy, differential scanning calorimeter (DSC), X-ray diffraction (XRD)and scanning electron microscopy (SEM). It was found that:(1) For the maize starch, the G values of the non-activated (aftergelatinization) starch, activated starch with activation time of 0.5h and 1.0h are93.68%, 108.60% and 143.71%, respectively; while the GE values are 58.93%,62.67% and 86.27%, respectively. For the cassava starch, the non-activated(after gelatinization) starch, activated starch with activation time of 0.5h and1.0h have G values of 84.90%, 134.01%, 111.94% and GE values of 48.36%,82.61%, 71.36%, respectively. It can be seen that the mechanical activationpretreatment could obviously enhance the starch graft reactivity. However, whenthe cassava starch is over-activated, the G and GE decrease slightly due to itsexcessive degradation.(2) For the graft copolymerization of maize starch, the native starch and activated starch with activation time of 0.5h and 1.0h have apparent activityenergies (E_g) of 34.92, 31.26, 17.38 kJ.mol^-1, reaction orders of 0.49, 0.44, 0.40,monomer reaction orders of 1.05, 0.99, 0.87 and initiator reaction orders of 0.50,0.46, 0.39, respectively. For the graft copolymerization of cassava starch, thenative starch and activated starch with activation time of 0.5h and 1.0h have E_gof 36.33, 22.86, 24.61 kJ.mol^-1, reaction orders of 0.50, 0.44, 0.46, monomerreaction orders of 1.00, 0.94, 0.96 and initiator reaction orders of 0.50, 0.45,0.47, respectively. These results clearly indicate that the mechanical activationprocessing could obviously decrease the reaction orders and activity energy, andtherefore enhance the starch graft copolymerization. The mechanical activationprocessing also weakens the dependence of the graft copolymerization on thereaction temperature, initiator concentration and monomer concentration.(3)The acrylamide is successfully involved in the starch graftcopolymerization, as is verified by the FTIR analysis. DSC results demonstratethat the thermal stability of the graft copolymers improves with the increase ofG and the crystallinity of the product decreases. XRD and SEM analysis revealsthat the graft reaction occurs not only in the amorphous region, but also in thecrystalline region of the starch. And the graft reaction has notable influence onthe crystal structure of the starch granule.