Preparation And Properties Of Functional Poly(lactic Acid) Nanocomposites

Poly?L-lactic acid??PLA?is a kind of environment-friendly polymer material,which has good bicompatibility,biodegradation,excellent mechanical strength,rigidity and processability.But the slow crystallization rate,poor heat resistance,low toughness and lack of function of PLA badly limit its wide application.Therefore,stereocomplex structures,multi-walled carbon nanotubes?MWCNTs?,nano zinc oxide?ZnO?,nano titanium dioxide?TiO₂?were used to improve the crystallization ability,in the meanwhile,given conductivity,antibacterial activity and ultraviolet resistance to PLA,with the aim of expanding the application scope of PLA.MWCNTs-g-PLLA nanohybrids were prepared successfully through ring-opening polymerization of L-lactide initiated by the hydroxyl groups of MWCNTs,and the grafting weight of PLLA was 17%.The PDLA/PHA/MWCNTs and PDLA/PHA/MWCNTs-g-PLLA nanocomposites were prepared via solution casting.And the crystallization behavior of PDLA/PHA/MWCNTs and PDLA/PHA/MWCNTs-g-PLLA nanocomposites were studied by DSC and XRD,with the increase of MWCNTs content up to 1.2 wt%,the crystallization rates of both PHA and PDLA in the PDLA/PHA/MWCNTs nanocomposites increased.Compared to MWCNTs,the MWCNTs-g-PLLA showed stronger effect toward acceleration of the crystallization of PDLA;however,crystallization behavior of the PHA component in the PDLA/PHA-based nanocomposites was not much affected.According to the statistical percolation theory,the values of?_c were 0.56 and 0.32 wt%for the PDLA/PHA/MWCNTs and PDLA/PHA/MWCNTs-g-PLLA nanocomposites,respectively.The MWCNTs were located in both the PDLA matrix and PHA domains in the PDLA/PHA/MWCNTs nanocomposites.However,the MWCNTs-g-PLLA nanohybrids were selectively distributed in the PDLA matrix in the PDLA/PHA/MWCNTs-g-PLLA nanocomposites due to the formation of stereocomplex structures between the grafted PLLA chains and the PDLA matrix.When the content of MWCNTs reaches 1.2 wt%,the tensile strength of PDLA/PHA/MWCNTs and PDLA/PHA/MWCNTs-g-PLLA nanocomposites increases from 32.1 to 45.1 and 50.2 MPa,respectively.Secondly,Zn O-g-PLLA was synthesized by silane coupling agent and in situ aminolysis reaction at the surface of Zn O.The formation of ZnO-g-PLLA nanohybrids was confirmed by FTIR,XRD,TEM,AFM and TGA,and the grafting weight of PLLA was 19.2%.The PDLA/Zn O and PDLA/Zn O-g-PLLA nanocomposites were prepared by solution casting route.ZnO inhibits the crystallization of PDLA,while the crystallization of PDLA is improved by adding Zn O-g-PLLA nanohybrids studied by DSC and XRD.The formation of stereocomplex between the PDLA matrix and the Zn O-g-PLLA induces the rapid crystallization of PDLA.The antimicrobial activity and ZnO leaching behavior of PDLA/Zn O and PDLA/Zn O-g-PLLA nanocomposites were studied by the plate count method and disc diffusion test.The incorporation of 3 wt%of ZnO and ZnO-g-PLLA reduced the bacteria?E.coli?count by 99.8%and 99.9%,respectively.Three percent of the ZnO and the ZnO-g-PLLA led to complete elimination of S.aureus.The PDLA/ZnO?3 wt%?nanocomposite show antibacterial activity with notable inhibitory against E.coli and S.aureus,respectively.Clearly,the ZnO nanoparticles leached out from the PDLA matrix.In sharp contrast,the PDLA/ZnO-g-PLLA?0.5,3 wt%?nanocomposites did not show any inhibitory zones.This was due to the grafted PLLA chains on the surface of Zn O and PDLA matrix formed stereocomplex structures at the interface.At the same filler content,PDLA/Zn O-g-PLLA nanocomposites can shield ultraviolet radiation better.Finally,The silicon dioxide?SiO₂?coated titanium dioxide?TiO₂?were prepared by ethylsilicate?TEOS?through its hydrolysis process.The hydroxyl groups of TiO₂@SiO₂ were used to initiate ring-opening polymerization of D-lactide to prepare TiO₂@SiO₂-g-PLLA nanohybrids.The formation of TiO₂@SiO₂ and TiO₂@SiO₂-g-PLLA nanohybrids were confirmed by FTIR,XRD,TEM,AFM and TGA,and the grafting weight of PDLA was20.5%.PLLA/TiO₂ and PLLA/TiO₂@SiO₂-g-PLLA nanocomposites were prepared via solution casting.TiO₂ inhibits the crystallization of PLLA,while the cold crystallization temperature of PLLA decreased from 125.2?to 118.2?by adding 0.5 wt%TiO₂@SiO₂-g-PLLA nanohybrids.The data of DSC and XRD confirmed the stereocomplex formation between the PLLA matrix and the TiO₂@SiO₂-g-PLLA nanohybrids.After UV irradiation?such as 72 h?,the surface of PLLA/TiO₂ nanocomposites cracks and the mechanical properties significant decreased due to the pyrolysis reaction between PLLA chains and the free radicals on the surface of the TiO₂.For PLLA/TiO₂@SiO₂-g-PLLA nanocomposites,the produced free radicals on the surface of TiO₂ can not contact with PLLA and attack PLLA chains,therefore,the UV resistance of PLLA/TiO₂@SiO₂-g-PLLA nanocomposites are good.