Study On The Properties Of Poly(Lactic Acid)Modified By Graphene Nanoplatelets With Polyethylene Oxide

Poly(lactic acid)(PLA)is widely used in the fields of food packaging and biomedical applications as a bio-based biodegradable aliphatic polyester.However,PLA has disadvantages like intrinsic brittleness,slow crystallization rate,and low thermal stability,which hinder its wider applications.Graphene nanoplatelets(GNPs)have been used to modify PLA due to its outstanding mechanical properties,thermal conductivity,electrical conductivity and electromagnetic interference(EMI)shielding performance.Nevertheless,dispersion of GNPs in polymer matrix is still a key factor in determining the properties of the nanocomposites.Polyethylene oxide(PEO)is a water-soluble polyester,which has good miscibility with PLA.Therefore,PEO has been used usually as plasticizer or compatiblizer of PLA nanocomposites.In this thesis,firstly,the effect of molecular weights of PEO on the properties of PLA modified by GNPs with PEO was investiagted.Then,the effect of GNPs content on the properties,especially EMI performance,of PLA modified by GNPs with PEO was studied.The main results are as follows:(1)The effect of molecular weights of PEO on the properties of PLA modified by GNPs with PEO was investiagted.GNPs were dispersed into PEO aqueous solution and freeze-dried to form PEO/GNPs masterbatch,followed by melt-mixed with PLA to prepare the nanocomposites.Three grades of PEO with different molecular weights were evaluated.It is found that adding a very low loading of PEO(1 wt%)enhances the dispersion of GNPs in PLA matrix,resulting in remarkable improvement in elongation at break of PLA/GNPs without obviously sacrificing its modulus and tensile strength.Meanwhile,PEO reveals a synergistic effect with GNPs in promoting PLA crystallization.Moreover,GNPs enhances significantly the thermal stability of PLA,while further addition of PEO does not apparent compromise this effect.In addition,PEO with low molecular weight presents the best effect in improving the properties of PLA/GNPs nanocomposites.(2)The effect of GNPs content on the properties,especially EMI performance,of PLA modified by GNPs with PEO was studied.PEO/GNPs masterbatch was prepared by solution method,followed by melt-mixing with PLA to prepare PLA/PEO/GNPs nanocomposites.As reference,PLA/GNPs nanocomposites and PLA/PEO/GNPs/CNTs nanocomposites with 1 wt%CNTs were prepared.It is found that GNPs compounded with PEO can improve the dispersion of GNPs to a certain extent,increase the electrical conductivity,and then improve EMI shielding effectiveness(SE_T).Liquid-like rheological behavior changes into a more solid-like behavior and SE_T of the nanocomposites increases with the increase of GNPs loading,which can be attributed to the improvement of the GNPs conductive 3D network as confirmed by fracture surface morphology.SE_T can reach 13 d B for the nanocomposites with GNPs loading of up to 15 wt%,and reflection is the dominant shielding mechanism.The increase in GNPs loading leads to decreased crystallinity of PLA in nanocomposites,but significantly improves the thermal stability,tensile strength and modulus of the materials.For PLA/PEO/GNPs nanocomposites containing 6 wt%GNPs,the further addition of 1 wt%CNTs completes the conductive path,and the SE values can reach nearly 10 d B,which is higher than that of nanocomposites containing 9 wt%GNPs.Similarly,the thermal stability,tensile strength and modulus of the nanocompsoites also increase in the same way.This indicates that the addition of nanofillers with two different structures into polymer matrix is a favorable way to improve the properties of polymer nanocomposites.