Investigation of Microstructure and Mechanical Properties of Poly (Lactic Acid) and Its Multicomponent System via Pre-stretching
Author:Chen Yun Jing
Supervisor:dong li song han li jing
Poly(lactic acid)(PLA)is a thermoplastic aliphatic polyester derived from 100%renewable resources,such as corns and sugar beets.PLA has been widely used in biomedical materials and packaging applications due to its excellent biocompatibility and biodegradability,coupled with high strength and modulus,and good processability.The development and application of PLA can not only alleviate the shortage of oil resources,but also reduce the white pollution caused by traditional plastics.However,the inherent brittleness of PLA greatly blocks its wide application of engineering materials.In this context,uniaxial pre-stretching process was applied to enhance the toughness of PLA.That is,the PLA was pre-stretched to different pre-stretching ratio(PSR)around the glass transition temperature of itself,and then cooled down to room temperature for tensile testing.Three systems,namely pure PLA system,PLA toughening system and PLA strengthening system,were selected to study the changes of microstructure and mechanical properties of the samples before and after pre-stretching,in order to explore the origin of the brittleness of PLA and the toughening mechanism of pre-stretching.The innovative results and conclusions of the dissertation are listed as follows:(1)Pre-stretched PLA(ps-PLA)with different PSR were obtained by uniaxial pre-stretching at 60℃ in order to study the effect of pre-stretching on the microstructure and mechanical properties of PLA.It was found that ps-PLA transformed from a brittle fracture to a ductile fracture at PSR=0.2.Then the toughness of ps-PLA was improved and reached its maximum at PSR=0.4.With further increment of PSR,the mechanical strength of ps-PLA was enhanced while toughness was weakened.We believe that a network structure consisting of cohesional entanglements was formed in PLA during the usual compression molding process,leading to the brittleness of PLA.The network structure consisting of cohesional entanglements could be destroyed by pre-stretching,leading to the toughness of PLA.Moreover,the oriented molecular chains could inhibit the formation of crazing in the tensile-testing,leading to further improvement of toughness.With increasing PSR,the degree of orientation increased along with the occurrence of mesophase,which reduced the activity of molecular chains,therefore,the modulus and strength increased,while the elongation at break decreased.However,the elongation at break was still larger than that of undrawn PLA.(2)PL As with 2%,4%,and 12%D-isomer content were selected to investigate the effect of stereoregularity on the microstructure and mechanical properties of uniaxially pre-stretched PLAs.Results show that PLAs fractured in a brittle way,and the more D-isomer the smaller mechanical strength.After pre-atretching,ps-PLAs transformed from a brittle fracture to a ductile fracture,and the more D-isomer the better toughness of ps-PLAs.With the increment of PSR,the mechanical strength was improved while the toughness was reduced,and the more D-isomer the smaller increase of mechanical strength as well as smaller decrease of toughness.These findings indicated that the more D-isomer in PLAs,the better toughening effect of pre-stretching.Reasons for these phenomena were that the introduction of D-isomer brought in stereoregularity defects,which disturbed the ordered arrangement of molecular chains,and then the activity of molecular chains was improved.As a result,the network structures consisting of cohesional entanglements was more likely to be destroyed in the prestretching process,moreover,the mesophase was less likely to form.Therefore,the toughness of ps-PLAs with more D-isomer was more excellent.(3)PLA/20 wt%poly[(ethylene oxide)-block-(amide-12)](PEBA)blends with high strength and excellent toughness were prepared through melt compounding and later uniaxial pre-stretching.The microstructure,thermal and mechanical properties of PLA/PEBA blend before and after pre-stretching process at 60℃ and 25℃ were investigated.Research findings showed that the toughness of PLA/PEBA blend increased while the mechanical strength decreased dramatically,which might because the addition of PEBA greatly reduced the content of cohesional entanglements in PLA and then destroyed the network structures composed of cohesional entanglements.After pre-stretching at 60℃,the mechanical strength of ps-PLA/PEBA blend was notably improved,and the toughness remained at a high level.This might due to the formation of orientation,mesophase,and even defective a crystal developed during pre-stretching.However,at the same PSR,the mechanical strength of ps-PLA/PEBA blend pre-stretched at 25℃ increased less,meanwhile,the toughness decreased a lot.The possible reason for this result was that except for orientation,mesophase and a’ crystal,a larger number of crack and pore defects were also generated during pre-stretching process at 25℃.These results indicate that pre-stretching temperature was the key factor for improving the mechanical properties of PLA/PEBA blend.(4)PLA/SiO2 nanocomposites with high strength and excellent toughness were prepared through melt compounding and later uniaxial pre-stretching.The microstructure,thermal properties,mechanical properties and thermal insulation properties of PLA/SiO2 nanocomposites before and after pre-stretching process were studied.It was found that ps-PLA/SiO2 nanocomposites transformed from a brittle fracture to a ductile fracture after pre-stretching,since the network structures composed of cohesional entanglements might be destroyed by pre-stretching,and then the mobility of molecular chains could be improved.With increment of PSR,the modulus and tensile strength of ps-PLA/SiO2 nanocomposites were improved a lot,but elongation at break was decreased gradually,resulting from the formation of mesophase.Moreover,the SiO2 nanoparticles exhibited reinforcement,hence,the modulus and tensile strength of ps-PLA/SiO2 nanocomposites were larger than those of ps-PLA under the same pre-stretching condition.When the PSR was larger than 3.5,a great number of micropores were generated in psPLA/SiO2 nanocomposites,resulting in decrease of modulus and tensile strength.In addition,micropores could notably improve the thermal insulation properties of PLA/SiO2 nanocomposites.