| Poly lactic acid(PLA)is a kind of thermoplastic polymer with great mechanical properties,high transparency,processing simplicity,and biocompatibility.It has great potential in the widely application of agricultural production,biomedicine,and packaging.In addition,PLAis biodegradable,and its degradation products are CO2 and H2O,without any pollution.Therefore,PLA is considered to be a type of bio-based plastic with the greatest potential to replace traditional petroleum-based plastics(PP,PS,etc.).However,PLA has a high brittleness,which greatly limits the application in structural parts,such as agricultural films,medical stents.In addition,the mechanical properties of PLA will be significantly reduced when the temperature beyond glass transition temperature.Therefore,it has always been a hot field in academia and industry that to produce PLA with high strength,high toughness,and high heat resistance without damaging their unique properties such as high transparency,biodegradability,and biocompatibility.Therefore,this article focuses on the research of PLA composites and microcellular foams with high strength,high toughness,and high heat resistance.First,this dissertation proposes a green method for strengthening and toughening PLA by CO2 treatment at room temperature.The crystalline behaviors,the thermodynamic properties,and the mechanical properties after CO2 treatment were studied.The study found that all mechanical properties of PLA have been greatly improved,and its heat resistance has also been greatly improved.Under the most identical CO2 pressure of 3.5 MPa,the tensile strength,modulus and toughness were increased by 25%,70%and 1214%,respectively,and impact strength was increased by 134%.In addition,the Vicat softening temperature increased sharply from 55.8? to 155.3?.The strengthening and toughening mechanism is related to CO2-induced crystallization.Under low-pressure CO2(1.7 MPa)treatment,a mesophase composed of micro-ordered structures was formed in the PLA,and the crystallinity increased only slightly.The mechanical properties of the treated PLA did not show any significant improvement.However,under high-pressure CO2(3.5 MPa)treatment,many fine crystals like ?" crystals or PLA-CO2 crystals were produced in the PLA,and the crystallinity was increased to about 24%.Since these small crystals can promote shear slide and plastic deformation,which can help prevent cracks from developing and expanding during deformation.Secondly,PLA/PMMA composites were prepared by melt blending,and their two-phase morphology was characterized by SEM.It was found that PMMA could be uniformly dispersed in the PLA matrix at a very small scale,but the two phases were not completely miscible.The addition of PMMA did not increase or decrease the tensile strength and Young's modulus of PLA,but the elongation at break increased significantly with the increase of PMMA content,which increased by 290%at 20%PMMA content.After annealing at 90? and 120?,the tensile strength of the samples did not change much,but the modulus was positively related to the increase in crystallinity,which increased by 150%and 200%respectively.The elongation at break and impact strength were decreased however,which is related to the huge crystals induced by the heat treatment.The tensile strength,Young's modulus,elongation at break,and impact strength of all the samples treated with CO2 were all improved,and the elongation at break increased by up to 500%.This is due to the enhanced toughening effect of a large number of nanoscale crystals treated with CO2.The addition of PMMA has a significant effect on the crystallization ability of PLA.It was found that the PMMA has heterogeneous nucleation effects,but inhibit the crystal growth.In the case of CO2-induced crystallization,it was found that there was no significant difference in the solubility of CO2 in PMMA/PLA,but the crystallinity of PLA decreased.Finally,PLA/PMMA foams were produced by two-step foaming at 2 MPa and 3.5 MPa.First of all,at 2 MPa,PLA/PMMA did not have obvious foaming phenomenon when PMMA content was low,and when PMMA content was high,many tiny cells were observed,but the expansion ratio was still low.At 3.5 MPa,the cell size increased significantly,and the expansion ratio increased significantly.At the same time,as the PMMA content increased,the expansion ratio and cell nucleation density of PLA/PMMA also increased,and the cell structure was more uniform Orderly.It is found that the increase of PMMA content can not only inhibit the crystallization,but also its immobilization can improve the damping of PLA/PMMA system.Under the action of high temperature and stretching in the foaming process,the crystallinity of the foam is significantly improved than before the foaming. |
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