Preparation And Crystallization Behavior Of Poly(L-Lactic Acid)/graphene Nanosheets Nanocomposites

As a novel one atom thick two dimensional material, graphene hasreceived more and more attention because of its remarkable thermal,electronic and mechanical properties; moreover, graphene can significantlyimprove the physical properties of host polymers at low loading. In thisdissertation, biodegradable poly(L lactide) based nanocomposites have beenprepared successfully using graphene oxide (GO) and thermally reducedgraphene (TRG). The main results are as follows.For the PLLA/GO nanocomposite, transmission electron microscopyand wide angle X ray diffraction results indicate that a relatively finedispersion of GO is achieved in the PLLA matrix. Nonisothermal meltcrystallization peak temperatures are slightly higher in the nanocompositesthan in neat PLLA. The overall isothermal melt crystallization rates arereduced for both neat PLLA and the PLLA/GO nanocomposites withincreasing crystallization temperature; moreover, the overall isothermal meltcrystallization rates are significantly greater in the nanocomposites than inneat PLLA although the crystallization mechanism remains unchanged. The crystal structure of PLLA is not modified in the PLLA/GO nanocomposites.Both the nonisothermal and isothermal melt crystallization of PLLA in thenanocomposites are influenced apparently by the GO loading. In addition,crystallization behaviors of PLLA/GO nanocomposites at different GOloadings from the amorphous state were also investigated in detail. During thenonisothermal cold crystallization, the crystallization peak temperature ofPLLA shifts to low temperature range in the nanocomposites with increasingthe GO loading relative to neat PLLA despite heating rate; moreover, theoverall nonisothermal cold crystallization has been accelerated with increasingheating rate for both neat PLLA and its nanocomposites. Relative to neatPLLA, the overall isothermal cold crystallization rate of PLLA is increasedwith increasing the GO loading in the nanocomposites despite crystallizationtemperature. The crystallization rate becomes faster with increasingcrystallization temperature, while the crystallization mechanism and crystalstructure remain unchanged for both neat PLLA and its nanocomposites.For the PLLA/TRG nanocomposite, the overall isothermal meltcrystallization rates are significantly greater in the nanocomposites than inneat PLLA although the crystallization mechanism remains unchanged. Thecrystal structure of PLLA is not modified in the PLLA/TRG nanocomposites.During the nonisothermal cold crystallization, the crystallization peaktemperature of PLLA shifts to low temperature range in the nanocompositesrelative to neat PLLA despite heating rate; moreover, the overall nonisothermal cold crystallization has been accelerated with increasingheating rate for both neat PLLA and its nanocomposites. Dynamic mechanicalproperties of PLLA/TRG nanocomposites have also been improved apparentlybecause of the presence of TRG.