Study On Properties Of Biodegradable Poly(Lactic Acid) And Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) Carbon-based Nanocomposites

Poly(Lactic acid)(PLA) and Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)], as the most promising biodegradable materials, have been successfully applied into various fields ranging from domestic and medical necessities to the aerospace industry because of their excellent biocompatibility, complete biodegradability and easy processability. However, the disadvantages of PLA and P(3HB-co-4HB), such as slow crystallization rate, bad mechanical properties and slow biodegradation rate, etc. have restricted their practical applications. In the present dissertation, based on the defects of PLA and P(3HB-co-4HB), we selected the carbon nanofiller as additives and prepared PLA and P(3HB-co-4HB) nanocomposites to study their crystalliization properties, mechanical properties and enzymatic degradation. Main contents and conclusions are as follows:(1) Two sets of Poly(L-lactide)(PLLA)/mutiwalled carbon nanotubes(MWCNTs) nanocomposites and PLLA/graphene nanosheets(GNSs) nanocomposites with various MWCNTs and GNSs loadings prepared by a melt blending approach were investigated in terms of both nonisothermal and isothermal crystallization behaviors. The results indicated that MWCNTs and GNSs not only acted as heterogeneous nucleating agents for PLLA crystallization but also restricted the mobility and diffusion of PLLA chains. At low MWCNTs and GNSs concentrations, the nucleation effect of MWCNTs and GNSs was dominant to achieve accelerated overall crystallization kinetics. As the MWCNTs and GNSs concentration rose up to 2.0 wt% and 2.5 wt%, respectively, the MWCNTs and GNSs network structures were formed in the PLLA matrix, which were proved by solid-like rheological behavior at low frequencies in rheological measurement. With further increasing concentration of MWCNTs above the critical concentration, an enhanced nucleation density but an almost unchanged overall crystallization rate for the PLLA/MWCNTs nanocomposites indicated that the expected increase of the crystallization promoting effect from more MWCNTs was offset by some confining effect. However, for GNSs, the formed network structure provided a more severely confined space for PLLA crystal nucleation and growth in contrast to MWCNTs, resulting in the decreased nucleation density and retarded crystallization rate.(2) Herein, through a solution and coagulation method, biodegradable poly(3-hydroxybutyrate-co-4-hydroxybutyrate)[P(3HB-co-4HB)]/graphene nanocomposites were prepared. The nonisothermal melt crystallization peak temperature values and overall isothermal melt crystallization rates are greater in the nanocomposites than in neat P(3HB-co-4HB), indicative of a nucleating agent effect of graphene; however, the crystallization mechanism of P(3HB-co-4HB) remains unchanged in the nanocomposites, regardless of graphene loading and crystallization temperature. The nonisothermal and melt crystallization processes of the nanocomposites are found to vary with the graphene loading, exhibiting a maximum at 1 wt% graphene loading. The incorporation of graphene obviously affects the spherulites nucleation density values of the nanocomposites. Graphene does not modify the crystal structure of P(3HB-co-4HB) in the nanocomposites. The storage modulus has been apparently improved in the nanocomposites with respect to neat P(3HB-co-4HB). Moreover, the enzymatic hydrolytic degradation rates have been enhanced obviously in the P(3HB-co-4HB)/graphene nanocomposites than in neat P(3HB-co-4HB).