| The development of biodegradable polymer materials is one of the effective strategies to solve plastic pollution.Polyglycolic acid（PGA）,the simplest structured biodegradable polyester with excellent mechanical strength,barrier properties,compostability and seawater degradability,is highly promising candidate for various applications such as biomedicine,eco-packaging,and agricultural films.In recent years,production capacity and demand for PGA have been constantly increasing with the development of coal-based synthetic gas polymerization routes and the establishment of industrial plants.However,current performance deficiencies such as inherent brittleness（elongation at break<10%）and inferior hydrolysis resistance seriously limits its application.The improvement of PGA performance properties by traditional modification methods such as copolymerization,blending and composite modification usually accompanied by the sacrifice of other properties.Hence,achieving excellent comprehensive performance is still a key challenge for expanding PGA applications.Drawing field is one of the most frequently encountered external fields during polymer processing,especially in film processing.Under drawing field,the polymer condensed structure can evolve at multi-scale structural evolution such as crystallization,orientation,crystal slip,and crystalline transition,which significantly impact on the macroscopic properties of the materials.Nevertheless,a comprehensive understanding of the structural evolution of PGA under drawing has not been established yet.In this thesis,the PGA-based films with improved mechanical strength,ductility,hydrolysis resistance and barrier properties were fabricated by using solid-state drawing,combined with crystalline modification and chain mobility regulation,blending and reactive compatibilization.The structural evolution of orientation,crystalline,polymorphic structure,entanglement network,phase morphology and cavitation of PGA films have been systematically investigated.The mechanism of microstructure-driven property regulation has been elucidated,leading to a preliminary understanding of the relationship between“process-structure-property”in drawn PGA films.The main research of this thesis are as follows:（1）The high temperature melting and quenching process was employed to obtain low crystallinity PGA,which was subjected to solid-state drawing and annealing to prepare uniaxially drawn PGA films.A new crystalline modification of PGA,named asβform was firstly observed in uniaxially drawn PGA films.The requirements for the formation and transformation ofβform by the synergistic effect of multiple fields were determined.The refined unit cell parameter ofβform is a=8.72（?）,b=5.02（?）,c=20.5（?）,α=γ=90o,β=88o,and the chains in the cell are arranged in a 7/2 helix,which fit for the P-1 space group.The reliability factor of the crystal model is 0.18.Theβform can be selectively induced at high drawing ratios and low annealing temperatures.Theβform is stable at room temperature but will be transformed intoαform when heated.To obtain more stable structures and properties,drawn PGA films withαform were prepared.The oriented crystals,rigid amorphous chains（RAF）and robust chain entanglement network enhanced the tensile strength（335 MPa）,heat resistance and ductility.（2）To further improve the ductility,poly（butylene adipate-co-terephthalate）（PBAT）,was used as toughening component to fabricate uniaxially drawn PGA/PBAT films.The finding indicate that the crystallization and orientation induced by drawing only leads to a little improvement on the film properties.To achieve excellent modulus and tensile strength of PGA/PBAT films,a great number of oriented crystals constructed by further annealing is required.Stress-induced strengthened entanglement network contributes to stabilization of craze growth and slipping,thus leading to the ductile behavior.The elongation at break and tensile toughness of the films were increased from 73%and 25 MJ/m3 to 131%and 93 MJ/m3,respectively.The abundant oriented crystallites and RAF structure result in close chain packing that restricts chain motility and hinder the diffusion of water molecules,thus leading to improved heat and hydrolysis resistance of the films.（3）On the basis of uniaxially drawn PGA/PBAT films,the temperature dependent of the structure and properties evolution were investigated.The drawing temperature was divided into three regions according to the characteristics of chain motility and properties.At low temperatures（35-40°C）,the poor chain mobility leads to a predominance of stress-induced amorphous chain orientation,and the drawn films exhibit high tensile strength.However,the poor chain mobility also induces the nanoscale cavities,which result in a decreased ductility.Then,the chain relaxation becomes pronounced due to the increased chain mobility at a temperature range of 45-50°C,resulting in a low orientation,low crystallinity and consequently high ductility.At high temperature region（55-60°C）,further enhanced chain mobility facilitates the formation of oriented PGA crystallites and robust entanglement network,which provide more strengthened elements and restrict chain relaxation.As a result,PGA-based films with excellent strength,stiffness and ductility（e.g.,103 MPa,2800 MPa and 220%,respectively）are achieved.（4）To address the performance degradation perpendicular to the drawing direction and potential cavitation of uniaxially drawn films,isotropic high-performance PGA/PBAT films were prepared by combining reactive compatibilization and biaxial drawing processes.The abundant oriented crystallites induced by biaxial drawing increased the modulus and strength of the film,while also acting as a gas barrier unit to impede the diffusion and dissolution of gas molecules.In addition,by regulating the interfacial adhesion and chain motility through reactive compatibilization,the cavitation was suppressed and the robust entanglement network was maintained,which further improved the mechanical and barrier properties of films.Consequently,the tensile strength and ductility of biaxially drawn PGA/PBAT films were increased by 224%and 78%,respectively,while the oxygen permeability(0.54×10-15cm3·cm·cm-2·s-1·Pa-1)was reduced by 88%. |
