Studies On Preparation And Properties Of Poly(Lactic Acid)/Lignin Composites

Poly(Lactic Acid)(PLA)is a widely used polymeric material in all walks of life due to its extraordinary biodegradability,renewability and eco-friendly nature to environment.However,the brittleness of PLA prohibits it from being utilized extensively,as well as the low resistance to elevated temperature.Many efforts have been given to improve its thermal/mechanical properties and bestow it with a higher value.In this research,Lignin,a naturally born polymer with a complex three-dimensional structure,is chosen to blend with PLA to fabricate a renewable biocomposite.Effects of the content of components,the interaction between composition phases and the particular composition structure on the tensile properties,thermal properties,crystallization and anti-UV light properties of the as-prepared PLA/Lignin composites are discussed thoroughly.At first a renewable PLA/ESO/Lignin composite is prepared through blending the PLA,maleic anhydride modified Lignin(Lignin-Ma,LM)and epoxidized soybean oil(ESO)in which the ESO and LM works as the toughing component and enhancing component,respectively.The reactive blending between ESO and LM in the composites is assumed to improve the miscibility of these two materials and PLA matrix.It is evidenced from the results of the differential scanning calorimetry(DSC)characterizations that the ternary composite shows a fast crystallization than that of pure PLA as the additive agents can serve as the heterogeneous nucleating agent of PLA phase accelerating its crystallization process by cutting down the energy barrier it needs.The thermogravimetric analysis(TGA)experiments demonstrate that the decomposition temperature of PLA/ESO/Lignin composite has increased by 35 ?C or so compared to the pure PLA.The optimal composition for the ternary composition is achieved at a blending ratio of 0.5 wt % LM and 20 wt % ESO(PLA/20ESO/0.5LM),which features tensile strength of 55 MPa,elongation at break of 198 %.The soaring elongation at break of PLA composite is about 20 times of that of pure PLA.In order to achieve a better miscibility between Lignin and PLA along with adding more value to PLA,two different molecular structures of Lignin-g-polyester including Lignin-gPDVL-ran-PLLA and Lignin-g-PDVL-b-PLLA are designed and synthesized through ringopening polymerization of δ-valerolactone(DVL)and L-lactone(L-LA)by the hydroxyl groups of Lignin structure.Effects of the various structures and content of Lignin-g-polyester in PLA/Lignin composites on thermal properties,crystallization and anti-UV light properties of composites are detailed studied.The DSC results show that the added Lignin-g-polyester could boost the crystallization behavior and improve the degree of crystallization of PLA.Compared with the Lignin-g-PDVL-b-PLLA,the Lignin-g-PDVL-ran-PLLA structure is apt to facilitate the crystallization of PLA.The results of field emission scanning electron microscope(FESEM)characterizations indicate that the Lignin-g-polyester additives can be dispersed evenly in PLA matrix showing an enhanced miscibility of Lignin and PLA.Furthermore,mechanical tests reveal that the elongation at break for PLA/Lignin-g-polyester composite is up to 249%.That results from the multiple effect of the formation of holes and wrinkles between Lignin-g-polyester additives and PLA matrix during the stretch.In addition,corresponding UVshield experiments evidence that Lignin-g-polyester can endow PLA/Lignin-g-polyester composites with an excellent UV-shield property with an ultraviolet protection factor(UPF)value of 52.Last the particle size of Lignin is constrained to 210 nm via hydrothermal method.Two bio-based monomers,1,10-sebacic acid and 1,4-butanediol,are utilized to synthesis a novel toughing unit poly(butylene sebacate)(PBS).The hexamethylene diisocyanate(HDI)is applied to bond the Lignin and PBS unit chemically and a semi-interpenetrate network is aimed to construct in the PLA polymer chain and the crosslinked network of Lignin and PBS.The IPN structure is expected to modify the miscibility of Lignin and PLA.Effects of the IPN structure on the comprehensive properties of the PLA composites are covered completely.According to the Fourier transform infrared spectroscopy(FTIR)and Nuclear magnetic resonance(NMR)characterizations,PBS is proved to be synthesized successfully with a molecular weight about 10 KDa referring to the gel permeation chromatograph(GPC)result.DSC results illustrate that the PLA composites have a lower cold crystallization temperature(Tcc)and a boosted crystallization ability compared to the pure PLA.TGA experiments find the PLA composite has a start decomposition temperature of 330 ?C,better than pure PLA.In addition,the tensile strength of the PLA composite is up to 58 MPa,elongation at break of the composite increased to 41%.In conclusion,series of PLA/Lignin composites are been fabricated in this thesis.By altering the size and structure of Lignin,variety of components,interaction between composition phases,the brittleness of PLA has been solved partly with a moderate tensile strength and increasing elongation at break.The thesis offers a new sight in the design and fabrication of PLA composites with an outstanding and comprehensive performance and has a little contribution to the research and preparation of biobased degradable PLA composites theoretically and practically.