Investigation Of The Relationship Between Structures And Properties In Poly(lactic Acid)Film

With the acceleration of modernization,social life is developing towards the direction of convenience and hygiene.Disposable film packaging materials have emerged,which bring great convenience to life,but also led to a large number of"white pollution" with serious harm to China's environment and economy.Therefore,it is necessary to develop biodegradable film materials.Polylactic acid(PLA)coming from biomass is a thermoplastic aliphatic polyester.It has good biodegradability,high physical properties,mechanical properties and good permeability of oxygen and carbon dioxide,but also has the characteristics of odor isolation and antibacterial.And so,PLA is an excellent environment-friendly packing material.However,the severe brittleness of PLA material greatly limits its application in the field of thin film packaging.In this dissertation,the effect of polymer structure at different levels(including PLA chain structure,the condensed state structure of PLA and phase structure of PLA based blend)on the mechanical properties of the its film products has been systematically explored,combineing the characteristics of multistage morphological structure in polymer material and applying the relationship between the structure and properties.Besides,a new type of PLA film extrusion blow molding technology has been put forward in order to improve the toughness of PLA thin film.The main contents and results obtained in this dissertation are as follows:(1)Adjusted take-up ratio and blow-up ratio so as to change the molecular chain orientation of PLA in the film;used acetyl tributyl citrate(ATBC)to improve the mobility of the molecular chains of PLA,applied epoxy type chain extender(ADR)increasing the molecular mass of PLA,used dicumyl peroxide(DCP)change the morphology of PLA molecular chain,and then explored the effect of PLA chain structure on its mechanical properties in film products.The results showed that simply changing the orientation of the molecular chain of the PLA thin film by adjusting the take-up ratio and blow-up ratio could not improve the tensile toughness of the film.ATBC could destroy the physical entanglement(cohesive entanglements and topological entanglements)between PLA molecules and improve the movement of chain segments,which reduced the yield stress of PLA and helped to improve the tensile toughness of the thin film products.However,the plasticizing effect could be significantly observed only when the mass fraction of ATBC reached to a certain value(greater than or close to 15.00 wt%).At the same time,the addition of ATBC will reduced the tensile strength and tear strength of the film.ADR could increase the molecular mass of PLA and increased the density of physical entanglement between molecular chains.DCP changed the molecular chain structure of PLA from linear structure to network structure.The fracture stress of PLA could be increased by introducing an appropriate number of physical/chemical entanglements.DCP also led to degradation of PLA,and low-molecular-weight PLA could act as a plasticizer to reduce yield stress.When the fracture stress was greater than the yield stress,the brittle-ductile transition was observed in PLA thin film.Meanwhile,the tensile strength and tear strength of the film also increased finitely.(2)An improved PLA film blowing molding process was presented.Pure PLA films with high toughness and tensile strength was prepared by adjusting the condensed structure of PLA films.The technological process was as follows:first isothermal crystallization at a specific temperature(90,110 or 120?),and then blow molding at 155?(near the melting point of PLA crystals).It was found that the amorphous PLA material changed to semi-crystalline state after isothermal crystallization.When the blowing molding temperature was lower than the full melting temperature of the PLA crystals,the crystals were not completely destroyed and still existed in the PLA melt in the form of residual microcrystals.The cohesive entanglements increased the yield stress of PLA and so becomed a main reson of brittle fracture.These residual microcrystals could be used as regulators to adjust the condensed matter structure of PLA thin film in the bubble forming stage and induce the formation of the mesophase.The generation of mesophase inhibited the formation of cohesive entanglements and reduced the yield stress of PLA.Thus high-toughness pure PLA thin films(the elongation at break in the transverse direction and machine direction reached up to 113.87%and 126.76%,respectively)were prepared.At the same time,the appropriate amount of microcrystals contributed to the stress transfer and increased the stress hardening ability of PLA thin films.As a result,the tensile strength of the film was also enhanced(the maximum tensile strength in the transverse direction and machine direction was 45.42 MPa and 78.01 MPa).The tear resistance of PLA films was also improved because the microcrystals impeded crack development.The lower the isothermal crystallization temperature was,the smaller the crystals size and more the number of PLA crystals were.So the lower isothermal crystallization temperature easily induced mesophase,which greater inhibited cohesive entanglements.Due to that,the lower isothermal crystallization temperature was,the higher the tensile toughness of the film was.(3)The ethylene-acrylate-glycidyl methacrylate ternary copolymer(EAGM)was used to improve the brittleness of PLA,and the relationship between the phase structure and properties of PLA/EAGM blends and blends thin films also was investigated.It was found that when the mass fraction of EAGM was 15.00 wt%,the dispersed phase presented a uniform sea-island structure in the blend with a size of about 1.13um,and the tensile toughness of PLA/EAGM blends reached the maximum(elongation at break up to 134.00%).The toughening mechanism was cavitation and shear yielding theory.After blow molding,a lamellar structure was formed in the PLA based film(the lamellar structure was dominated by the EAGM phase).The tensile toughness of film in the transverse direction and machine direction was difference,and the maximum elongation at break up to 19.80%and 242.20%,respectively.Although the EAGM phase increased the toughness of the PLA films,the tensile strength of the blend film products decreased due to the lower strength of the EAGM.Both the tensile toughness and tensile strength of the PLA films determined the tearing strength of the material.The tearing strength of PLA films in the transverse direction and machine direction was 197.80 KN/m and 137.70 KN/m,respectively.