Morphology, Crystallization And Properties Of Biodegradable Polymers/multi-walled Carbon Nanotubes Nanocomposites

In this work, preparation, morphology, crystallization, and mechanicalproperties of three kinds of biodegradable polymers/carboxyl-functionalizedmulti-walled carbon nanotubes (f-MWCNTs) nanocomposites were studiedwith various techniques, which should be of great interest and importance tomodify the physical properties and to extend the practical application forbiodegradable polymers from both academic and industrial viewpoints.1. In this work, crystallization behavior of biodegradablepoly(L-lactide)(PLLA) and its nanocomposites at different f-MWCNTscontents from the amorphous state was studied in detail. For the isothermalcold crystallization, the presence of f-MWCNTs enhances the isothermal coldcrystallization rate of PLLA in the nanocomposites compared with that of neatPLLA at the same crystallization temperature; moreover, the overall coldcrystallization rate of PLLA increases with increasing the f-MWCNTs contentwhile the crystallization mechanism does not change. For the nonisothermalcrystallization, the f-MWCNTs also accelerate the crystallization process of PLLA. The Ozawa method failed to describe the nonisothermal coldcrystallization process of neat PLLA and its nanocomposites. In addition, theactivation energies of nonisothermal cold crystallization process werecalculated using both the Kissinger and Friedman methods. The coldcrystallization activation energies of PLLA are higher in the nanocompositesthan in neat PLLA, indicating that the addition of f-MWCNTs into the PLLAmatrix acts as a physical hindrance to retard crystallization.2. Biodegradable poly(butylene succinate-co-butylene carbonate)(PEC)/f-MWCNTs nanocomposites were prepared through solution castingmethod at low f-MWCNTs loadings. Scanning electron microscopyobservations reveal a fine dispersion of f-MWCNTs in the PEC matrix.Differential scanning calorimetry and optical microscopy were used to studythe nonisothermal melt crystallization behavior, overall isothermal meltcrystallization kinetics, and spherulitic morphology of neat PEC and itsnanocomposites. The presence of f-MWCNTs has a significant heterogeneousnucleation effect on the crystallization and morphology of PEC in thenanocomposites. Moreover, the overall isothermal melt crystallization rate ofPEC is enhanced significantly, while the crystallization mechanism remainsunchanged in the nanocomposites. The dynamical mechanical properties havealso been improved in the nanocomposites. Upon incorporation of only1wt%f-MWCNTs, the storage modulus of PEC is improved by about30%from2281to2958MPa at–60oC. In addition, the glass transition temperature of PEC is increased slightly in the nanocomposites relative to neat PEC.3. The effect of low f-MWCNTs loading on the crystallization behaviorof poly(butylene adipate)(PBA) was studied with various techniques in thiswork. PBA may crystallize in the α-or β-form under different conditions.Relative to the β-form, the α-form crystals are thermodynamically more stableand show a faster biodegradation rate; however, they are formed during thenonisothermal melt crystallization at only relatively low cooling rate.F-MWCNTs were incorporated into the PBA matrix at low f-MWCNTsloading to regulate the polymorphic behavior of PBA crystals. It is found thatthe formation of polymorphic crystals of PBA is affected not only by coolingrate during the crystallization process but also by the f-MWCNTs loading. Theformation of α-form crystals can be induced by f-MWCNTs at high coolingrate, providing an effective way of controlling the polymorphic crystalsformation and biodegradation rate of PBA for its wider practical application.In addition, for the nonisothermal melt crystallization, f-MWCNTs acceleratethe crystallization process of PBA apparently due to the heterogeneousnucleation effect. The Ozawa method fails to describe the nonisothermalcrystallization process of neat PBA and its nanocomposite. Isothermal meltcrystallization kinetics of neat PBA and its nanocomposite was analyzed bythe Avrami equation. The overall isothermal crystallization rate of neat PBAand its nanocomposite increases with increasing crystallization temperature.The addition of f-MWCNTs accelerates the isothermal crystallization of PBA as compared with that of neat PBA at a given crystallization temperature,indicative of the nucleating agent effect of f-MWCNTs; however, thecrystallization mechanism does not change. The crystal structure of PBAremains unchanged in the PBA/f-MWCNTs nanocomposite despite thepresence of f-MWCNTs.