Facile Preparation Of Graphene/Polylactide Composites With Tunable Structure And Properties

Polylactic acid（PLA）is a bio-based,completele degradable polymer with excellent gas permeability and biocompatibility.Up to now,PLA has been widely used as medical suture,coating,textile,and packaging materials.However,some disadvantages,i.e.,low toughness,inferior heat resistance,and slow crystallization rate,severely limit its potential applications in the emerging fields,such as flexible electronics and intelligent wear.Graphene（Gr）,a two-dimensional lamellar material,has excellent electrical,optical,and mechanical properties.Now,Gr has been extensively used to modify polymers.However,whether Gr can disperse uniformly and stablely in matrix polymers determines its modification effectiveness.The water-soluble polyethylene glycol（PEG）is a kind of non-toxic and non-irritating plasticizers,which was used to improve the dispersion uniformity and stability of nano-components in various polymers.In this case,it is important to develop new PLA composites modified with Gr and Gr/PEG in order to realize the simultaneous toughening and strengthening of PLA,and further to reveal the synergistic mechanism of inorganic nanomaterials and organic polymers on PLA.In this thesis,binary Gr/PLA blends and ternary Gr/PEG/PLA composites with 0.1,0.2,0.5,1.0 and 2.0 phr of Gr were prepared via a facile solution casting method.Then,the effects of Gr contents on the microstructure,crystalline properties,UV-Vis shielding property,thermal characteristics,mechanical behavior,and non-isothermal crystallization kinetics of PLA composites were investigated by x-ray diffraction analysis（XRD）,fourier transform infrared spectroscopy（FTIR）,differential scanning calorimeter analysis（DSC）,polarized light microscopy（POM）,thermogravimetric analysis（TGA）and mechanical properties tests.Meanwhile,ternary Gr/PEG/PLA composites were annealed to investigate the effect of annealing time on their crystalline and mechanical properties.The main findings are as follows:（1）XRD and DSC tests show that,PLA used in this work has bothαandβcrystalline forms.High-quality PLA composite films can be easily prepared under mild conditions via the solution casting method developed in this research.With the increasing Gr content,the number of PLA spherulite increases,while the spherulite size decreases under POM.Within the wavelength range of 200-800 nm,the transmittance of the Gr/PLA films increases with wavelength,it indicates that the binary Gr/PLA blends has better shielding effect on UV light.At wavelength of 800 nm,its transmittance gradually decreases with the increasing Gr content.With 0.5 phr of Gr,the binary Gr/PLA blends show multiple percolation transitions:lowest cold crystallization temperature,highest heat resistance,highest tensile strength（33.65 MPa,31.9%higher than that of pure PLA）,and highest elongation at break reached 4.91%（8.6%higher than that of pure PLA）.It means that PLA was simultaneously toughened and strengthened.When the Gr content is higher than 0.5 phr,partial Gr aggregates within the PLA matrix,which leads to lower mechanical properties,slower crystallization rate,and worse heat resistance of the rersultant Gr/PLA blends.Non-isothermal crystallization results show that the crystallization rate constant（k_a）of the binary Gr/PLA blends increases with the increasing heating rate（）and the half-crystallization time(t_1/2)decreases accordingly.For the same heating rate,the k_a of the binary Gr/PLA blends increases and then decreases,while the t_1/2decreases and then increases with the increasing Gr content.（2）The sizes of PLA spherulites increase from 10-40μm to 80μm,and glass transition temperature（T_g）reduce from 62.8°C to 54.2°C for the PEG/PLA blends.The tensile stress of the PEG/PLA blends reduce to 20.2 MPa（20.7%lower than that of neat PLA）.Excitingly,its elongation at break reaches up to 5.75%（27.2%higher than that of neat PLA）.For the Gr/PEG/PLA composites,the superimposition between adjacent spherulites becomes more obvious and grain refinement effect appears with the increasing Gr content.Also,their cold crystallization peak temperature reduction and the crystallization rates are accelerated.After PEG addition,on the one hand,it enables the uniform and stable dispersion of Gr in the PLA matrix;on the other hand,it significantly increases the migration rate of PLA macromolecular chains.Thus,the motilities of PLA chains are enhanced,which means PEG can enable synergistic effect with Gr.With 1.0 phr of Gr,the ternary Gr/PEG/PLA composite achieves PLA’s concurrent toughening and strengthening with tensile stress of 27.2 MPa and elongation at break of 7.1%.With the increasing heating rate,both the cold crystallization peak temperature and crystallization enthalpy increase.For PLA composites with the same Gr content,ternary Gr/PEG/PLA composites crystallize faster than their binary counterparts（i.e.,the Gr/PLA composites）.Under the same heating rates,the ternary Gr/PEG/PLA composites with 1.0 phr of Gr show fastest crystallization rate and more perfect spherulite morphology.（3）The locations of XRD diffraction peaks are basically unchanged for the ternary Gr/PEG/PLA ternary composites.However,the intensity of its diffraction peak increases significantly,when the annealing time increased from 30 min to 60 min.It shows that annealing treatment enhances the interactions between the lamellar Gr and the PLA matrix,and makes PLA spherulites more perfect.DSC testing results show that the crystallinity of the Gr/PEG/PLA composites with 1.0 phr of Gr reaches up to 39.7%,when it was subjected to annealing treatment at 100℃for 60 min.In this case,PLA still shows spherulites.Under POM,the number of PLA spherulites increase significantly and their diameters decrease after Gr and PEG were added into the PLA matrix.The tensile stress of the annealed Gr/PEG/PLA composites with 1.0 phr of Gr reaches to 32.5 MPa（19.3%higher than their unannealed counterparts）after annealing treatment at 100°C for 60 min,while their elongations at break decreases to 3.8%（46.2%lower than their unannealed counterparts）.Their tensile stress further reaches up to 33.7 MPa（23.8%higher than their unannealed counterparts）by increasing the annealing time to 90 min,while their elongations at break reduce to 3.4%（52.1%lower than their unannealed counterparts）.