Controlled Structure And Performance Of The Poly (Butylene Adipate-Ran-Terephthalate) (PBAT)/Poly (Butylene Succinate) (PBS) Biodegradable Cocontinuous Blends Via External Field During Processing

Most of the polymer blends are thermodynamic incompatible systems,which are difficult to reach the compatibility at molecular level,so the polymer blends are often heterogeneous systems.Generally,the structure obtained after mixing of immiscible polymer pairs can be categorized into matrix-dispersed particle structure and cocontinuous structure.Cocontinuous structure of polymeric system can offer an improved combination of two components properties,with advantageous properties such as enhanced processability and mechanical properties.Compared with the discrete structure,the cocontinuous structure is beneficial to stress transfer and matrix deformation,which gives rise to a remarkable improvement in the toughness of two-phase system.Despite its mechanical properties,the cocontinuous structure imparts functions to materials such as hierarchically porous materials and electrically conductive composites.In addition,the reasonable design of the co-continuous structure can enhance the mechanical properties.Hence,understanding and tuning the cocontinuous structure have a great potential to significantly enhance mechanical properties and broaden the application range of polymer materials.Notably,the unique structure of naturally occurring materials provides a novel and powerful strategy to achieve extraordinarily performance such as stiffness-toughness balance,structural colors and solar-to-vapor transition.The prominent characteristics of natural materials include gradient and orientation.For instance,bones and bamboos exhibit an oriented outer layer and an isotropous inner layer.In nature,gradients play an essential role in guiding the function of a wide range of tissues,including tendon,cartilage,and the central nervous system.Moreover,the appropriate orientation of phase field can enhance the fracture toughness without major compromise to stiffness and.Pioneering works indicate that the cocontinuous structure is sensitive to the content of composition,temperature,compatibilization component and shear stress,and this provides a good basis for the manipulation of the cocontinuous structure.The cocontinuous structure can be formed within a range of content with a high level continuity,and the phase size of cocontinuous structure varies at different contents.Furthermore,the cocontinuous structure can be effectively controlled using thermal treatment.The phase size can increase from several micrometers to hundreds of micrometers by using thermal treatment above the melting point（semi-crystalline polymer）or the glass transition temperature（amorphous polymer）.The compatibilization component can improve the interfacial action and reduce the phase size.Moreover,the shear stress has a profound effect on the orientation of cocontinuous phase.At intense shear stress,the isotropic cocontinuous structure can be elongated to oriented cocontinuous structure such as continuous fibrils or cocontinuous microlayers.Meanwhile,shear stress can also induce the orientation ranging from sub-micro scale to nanometer scale such as oriented crystallites and molecular chains,leading to the hierarchical orientation.In this study,the poly（butylene adipate-ran-terephthalate）（PBAT）/poly（butylene succinate）（PBS）blends with full biodegradability are chosen as a typical system to obtain the gradient and oriented cocontinuous structure.When the content of PBAT reaches 50 wt%,the cocontinuous structure is observed and owns excellent performance.The precision control of the continuous structure during the processing field was studied from the following aspects:（1）the gradient temperature and shear stress imposed on the PBAT/PBS blends with co-continuous structure during injection molding,obtainning gradient and oriented cocontinuous structure with excellent properties;（2）the co-continuous PBAT/PBS casting films were imposed controllable stretched stress by"casting-thermal stretching"method,which has excellent strength,tear resistance and prickle resistance;（3）the chain extender ADR containing epoxy functional groups can react with terminal carboxy groups and hydroxyl groups of PBAT and PBS,forming graft polymers,which can effectively improve the interaction between components;（4）quantitatively analyse the evolution of hierarchical structure of cocontinuous structure in shear-temperature field;（5）quantitatively analyse the evolution of hierarchical structure in cocontinuous structure under pressure-shear-temperature field.These elaborately designed roadmaps allow us to make some important achievements,as listed following:1.Imparting gradient and oriented characters to cocontinuous structure for improving integrated performance.Inspired by the oriented,gradient and heterogeneous characters of natural materials,an oriented and gradient cocontinuous structure is designed to obtain excellent mechanical performance via a gradient shear stress and temperature during melt processing.The investigation of hierarchical structure containing phase morphology,crystalline morphology and lamellae in the cocontinuous structure can clarify the role of gradient shear stress and temperature in promoting gradient and oriented features.Given such oriented and gradient features in the cocontinuous structure,a notable enhancement in the tensile strength,tensile modulus,elongation at break and impact strength is achieved in the model system of poly（butylene adipate-ran-terephthalate）（PBAT）/poly（butylene succinate）（PBS）biodegradable cocontinuous blends.Compared with the isotropic cocontinuous structure,homogenous structure and discrete phase structure,oriented and gradient cocontinuous structure provides a promising balance of strength,hardness and toughness.Local reinforcement is achieved by highly oriented cocontinuous phase,crystal and lamellae in skin layer.Meanwhile,the toughness is achieved by weakly oriented cocontinuous phase and improved interfacial interactions between the phases owing to epitaxial growth of PBS in the vicinity of PBAT fibrils.The tuning of cocontinuous structure with gradient and oriented features can be regarded as a novel strategy for the development of“materials systems”with excellent mechanical performance.2.Formation of in-situ interconnected multi-microlayers induced by extensional stress:achieving remarkable properties.Different from conventional multilayer structure,a cocontinuous multilayer film is fabricated by uniaxial stretching of poly（butylene adipate-ran-terephthalate）（PBAT）/poly（butylene succinate）（PBS）biodegradable cocontinuous blends via“casting-thermal stretching”.The investigation of phase morphology,crystalline morphology and lamellae in the cocontinuous multilayer films can clarify the role of extensional stress in designing hierarchical structure.The oriented PBAT layers can be beneficial to low interfacial tension and availably regulate PBS crystals to grow along the PBAT layers,improving interfacial adhesion.Furthermore,the intensely extensional stress can also promote the formation of crystallites and thicker lamellae.Given such cocontinuous features in the multilayer films,a notable enhancement in the tear strength,tensile strength and puncture resistance is achieved.In this work,the fabricate method of cocontinuous multilayer structure is preferable and scalable,and it can be regarded as a novel strategy for the development of“structural systems”with excellent mechanical performance.3.Preparation and properties of compatibilized in-situ interconnected multi-microlayers induced by extensional stress by extensional stress.In the previous work,the interconnected multi-microlayers of PBAT/PBS were prepared by extensional stress.The tensile strength and tear strength along the stretched direction were greatly improved,and the elongation at break was maintained at a high level.On the basis of the preparation of interconnected multi-microlayers,we further introduced the chain extender ADR containing multi-epoxy functional groups to react with the carboxy group（-COOH）and hydroxyl group（-OH）at the ends of PBAT and PBS to form block polymers,thus improving the compatibility of PBAT and PBS,enhancing the interfacial adhesion,and forming an isotropic cocontinuous structure with low phase region size and high continuity.It is found that the introduction of ADR has little contribution to the properties along the stretched direction.The PBAT-50-0 and PBAT-50-5 at draw ratio of 5.1 with excellent properties as examples,the tensile strength,elongation at break and tear strength are107.7 MPa,96.1%,171.1 k N.m^-1 and 117.4 MPa,100.1%,166.5 k N.m^-1,respectively.However,the introduction of 5%ADR can simultaneously increase the tensile strength,elongation at break and tear strength perpendicular to shear direction of PBAT-50-0 from 25.7 MPa,706.9%,115.9 k N.m to 33.1MPa,919.3%and 131.3k N.m^-1,which provides a new strategy for the preparation of bidirectional high performance films.4.Formation and evolution of cocontinuous structure in shear-temperature field.Usually,in the processing process,polymer melt will suffer temperature field and shear field.Its morphology and hierarchical structure will change accordingly.For example,when the film is prepared by casting,the shear flow is forming in the interface between slit die and polymer melt,inducing the orientation of phase structure.In addition,there are complex shear field and temperature field in injection molding,which have great influence on phase structure and crystal structure.When designing the cocontinuous structure,the shear field and temperature field will also have a great effect on the co-continuity index.Hence,it is of great significance to study the evolution of the hierarchical structure in the shear-temperature field for the design and regulation of the cocontinuous structure.In this work,we impose accurate shear rate to the cocontinuous PBAT/PBS blends by high pressure capillary rheometer at different temperatures.The hierarchical orientation ranging from phase structure at microscale and lamellae at nanoscale to molecular scale is also characterized by FE-SEM and X-ray scattering（diffraction）.It has important guiding significance for the regulation and control of cocontinuous structure in the actual processing process.5.Formation and evolution of cocontinuous multilayered structure in pressure-shear-temperature field.In the process of polymer processing,the polymer melt often faces complex pressure-shear-temperature coupling field.Therefore,it is very important to study the pressure-shear-temperature coupling field for the design of the hierarchical structure.Usually,the existence of pressure can reduce the free volume of polymer melt,increasing the viscosity of the system and reducing the movement ability of the chain segments.In addition,the equilibrium melting point will increase under pressure,which will also have a great impact on the crystallization behavior.In this section,we analyze the evolution of the cocontinuous structure in the pressure-shear-temperature field.In the pressure-shear-temperature field,the cocontinuous structure can evolve into a multilayer structure at a low shear rate,and the thickness of the lamellar layer can be as thin as 150 nm.In addition,when the temperature is lower,the cocontinuous structure can be induced to form oriented lamellar structure at lower shear rate.However,enough shear stress is needed to obtain the oriented lamellar structure at high temperature.In addition to the effect on the phase structure,the pressure-shear-temperature field has a great influence on the lamellar structure and the orientation of the chain segments.In this section,the evolution of hierarchical structure of cocontinuous structure under pressure-shear-temperature field is discussed,which provides important guidance for the design of hierarchical oriented cocontinuous structure in the processing process.