Investigation On The Preparation And Characterization Of Poly(Lactic Acid)-based Nanocomposites

Poly （lactic acid）（PLA）, as an ideal green and excellent biodegradable polymermaterial, has excellent biocompatibility and biological absorbability. It is thereforewidely used not only for a variety of applications in the pharmaceutical andbiomedical field but for packaging materials, which has different requirements forthermal stability or antibacterial property. This thesis includes two parts of work: thefirst work is to synthesize PLA/graphene oxide （GO） nanocomposites by in-situ meltpolycondensation for improving the thermal stability, the second is that to parparePLA/nanosilver-loaded silica （Ag-SiO₂） nanocomposites by melt compounding forimproving antibacterial property. Molecular weight, structure, morphology,crystallization properties, thermal properties, mechanical properties, crystallizationproperties and rheological behavior of the PLA and its composites were characterizedin terms of,,, scanning electron microscopy （SEM）, differential scanning calorimeter（DSC）, thermogravimetric analysis（TGA）, dynamical mechanical analysis （DMA）and rotary rheometer, respectively. The main research work and results were listed asfollowing:Part â… : Investigation on the in-situ synthesis and properties of PLA/GOnanocomposites（1） Graphene oxide is prepared using the modified Hummers method, andPLA/GO nanocomposites are in-situ synthesized using GO aqueous solution andlactic acid. The molecular weight, chemical structure and morphology of the purePLA and its nanocomposites are characterized in terms of gel permeationchromatography, Fourier-transform Infrared （FTIR）,1H nuclear magnetic resonance（¹H NMR） and electron microscopy, respectively. The FTIR and1H NMR spectrashow that the incorporation of GO has no obvious influence on the chemical structurebut reduces the molecular weight of PLA, especially in the case of0.5%GOloading level. SEM and TEM micrographs indicate that GO nanosheets dispersehomogeneously in the PLA matrix and a fraction of the GO nanosheets is reduced tographene. （2） Thermal stability of the nanocomposites is characterized by differentialscanning calorimetry （DSC） and thermogravimetric analysis （TGA）. It is found thatthe cold crystallization temperatures of the nanocomposites shift toward to the lowertemperatures as compared to the pure PLA, and the crystallinity of thenanocomposites is increased with the increasing GO nanosheets amounts. The TGAdata indicate that initial decomposition temperature of PLA-0.10and PLA-0.25wasincreasd by12oC and10oC with comparison with pure PLA, respectively, and themaximum decomposition temperature of the nanocomposites is higher than that of thepure PLA, especially it is increased by10oC in the case of0.25%GO amounts.Therefore, the incorporation of GO nanocomposites can improve significantly thethermal stability of the PLA.（3） The rheological behaviors of the PLA and its nanocomposites are studied byrotational rheometer. The experimental results show that PLA and its nanocompositesare pseudoplastic fluid and exhibit shear thinning behaviors. The curve of lgG’-lgωrevealed the sensitivity of the viscoelastic-function to the structural change ofcomposites at low frequency, and the curves of lgG’-lgG" indicate that the phaseseparation occurs at160℃.（4） The isothermal crystallization behaviour and melting behavior of the purePLA and its nanocomposite are investigated by DSC. The isothermal crystallizationkinetics are analyzed using the Avrami equation, and the Avrami index are calculatedin the range from2.17to3.74, indicating that t indicating that the model of nucleationand growth of PLA is a mixture of two-dimensional discotic growth andthree-dimensional spherulite growth. Moreover, the equilibrium melting temperaturesare determined according to the Hoffman-Weeks theory. The results illustrated that thethe melting temperature of the nanocomposites is lower than that of the pure PLA butis increased with the increasing GO amounts.Part â…¡: Investigation on the preparation and properties of PLA/Ag-SiO₂nanocomposites（1） The PLA/Ag-SiO₂nanocomposites are parpared by melt compounding on aninteral mixer, and their morphology and antibacterial property are investigated. TheTEM images show that the Ag-SiO₂nanoparticals disperse homogeneously in thePLA matrix and do not aggregate. It was proved that Ag-SiO₂nanoparticalseffectively improve the antimicrobial property of PLA, as indicating by that Ag-SiO₂ nanoparticals display sensitively antibiotic property to S. aureus and completely killthe S. aureus. It is enough amount to kill the S. aureus and colon bacterium at10000ppm Ag-SiO₂loading level.（2） Dynamic mechanical analysis shows that the storage modulus decreasestremendously near the glass transition while increases slightly after the coldcrystallization. The experimental results of rheological behavior show that PLA andits nanocomposites are pseudoplastic fluid and exhibit shear thinning behaviors. Thecurves of lgG’-lgG" and Cole-Cole indicate that the nonuniform dispersion occurs tofor Ag-SiO₂nanoparticals.