Preparation And Characterazation Of Plla-Co-Bis A/MVMTS Nanocomposites

In this thesis, in situ melt polyeondensation of L-LA in the presenee of epoxy-modified vermiculites (MVMTs) has been proposed as a novel approach to prepare Poly(L-lactic acid)-co-bisphenol-A epoxy resin/modified vermiculite (PLLA-co-bis A/MVMTs) nanocomposites for the first time.Vermiculates containing epoxy groups were successfully prepared by reaching KH-560 with vermiculites which had previouly been exfoliated by cation exchange and mechanical force. Then carboxyl-terminated PLLA/MVMTs prepolymer and PLLA-co-bis A/MVMTs chain extended products were synthesized by in situ polycondensation. The effect of MVMTs on the molecular weight, nanostructure, crystallization behaviors, thermal stability and mechanical properties of the PLLA-co-bis A/MVMTs nanocomposite was investigated in detail.Laser particle size analyzer, X-ray diffraction (XRD) and transmission electron microscopy (TEM) were introduced to characterize vermiculite exfoliation. The results showed that a particle size of 100 nm, thickness of 20 nm around exfoliated vermiculite was successfully prepared. We attached epoxy groups onto vermiculite surface via siloxane linkage by the use of KH-560. It could be confirmed by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA) showed about 4.01% coupling agents were grafted to MVMTs surface.The introduction of modified vermiculite had no effect on the molecular weight of PLLA/MVMTs prepolymer, but the molecular weight of PLLA-co-bis A/MVMTs matrix decreased. As the percentage of modified vermiculite content increased, the molecular weight continued to declined. The molecular weight decreased by 14%,30%,43% when the modified vermiculite content of 0.5%,1%,1.5%, respectively. Vermiculite grafted with polymer (g-VMT) in PLLA-co-bis A/MVMTs nanocomposites was isolated from free polymer via repeated dispersion/centrifugation processes and characterized. Formation of chemical bonding between PLLA-co-bis A chains and surface epoxy groups from MVMTs was detected by FT-IR. Calculations based on TGA revealed that more than 53.3% polymers were successfully grafted into MVMTs. SEM showed that the g-VMT had special'rod' and 'pie' forms. Morphological images showed a novel assembly of MVMT nanocrystals in the PLLA-co-bis A matrix and demonstrated a strong interfacial interaction between them. This special interface structure will affect crystallization behavior, thermal stability and mechanical properties of PLLA-co-bis A/MVMTs nanocomposites.Differential scanning calorimetry calorimetry (DSC) and Polarizing optical microscopy (POM) were introduced to characterize the crystallization behavior of PLLA-co-bis A/MVMTs nanocomposites. The results showed that overall crystallization rate of nanocomposite elevated and spherulite size decresed in the presence of vermiculite particles. PLLA-co-bis A/MVMTs nanocomposites exhibited increased crystallinity and glass transition temperature (Tg), compared with pure PLLA-co-bis A.TGA tests showed that PLLA-co-bis A/MVMTs nanocomposites had better thermal stability, compared to pure PLLA-co-bis A copolymer. When MVMTs content of 1.5%, the nanocomposite Tonset, Tend,Tmax increased by 13.8℃,9.3℃,2.1℃. Modified vermiculiate did not play a negative role in improving the breaking strength and elongation at break of the PLLA-co-bis A copolymers.