| Polylactic acid(PLA)porous materials have the advantages of low relative density,high specific surface area,good heat insulation,light weight,excellent adsorption performance,good permeability,biodegradability,and biocompatibility,among others.They are used in oil-water separation,adsorption,heat insulation,electromagnetic shielding,biological scaffolding,phase change energy storage,and many other fields with broad application prospects.However,PLA porous materials' preparation process is complicated,the surface micro-nano structure is difficult to effectively construct and other shortcomings severely limit porous PLA in superhydrophobic,oil-water separation,and other fields of application.The experiment in this thesis is based on non-solvent-induced phase separation(NIPS).By controlling the volatilization rate of the solvent,a method of slow evaporation-induced precipitation has been developed,and a series of hydrophobic PLA materials have been prepared,which is different from general non-solvent-induced phase separation methods(such as immersion precipitation).Compared with the solvent and non-solvent direct mixing method),this method is designed to slowly cover the surface of the polymer solution with a low-density non-solvent,use the volatilization ability of the solvent to realize the replacement between the solvent and the non-solvent,and promote the polymer solution.Different phase separation processes occur on the surface and inside,and parameters such as polymer solution concentration and molding temperature that affect the phase separation kinetics are discussed.Various micro-nano structures were constructed,which enables hydrophobic modified.Expand the contact area between the solvent and non-solvent to control the phase separation rate of the solution.Both the molding time of the material and the growth time of the crystal were reduced,which obtain the low size crystal particle structure on the surface,and further improve the hydrophobicity.With the introduction of crystallization gel strategy,the rapid crystallization of surface molecular chains was induced by heterogeneous nucleation or steric recombination to promote the rapid solidification of the surface double continuous structure,so that the surface can achieve superhydrophobic effect.The Breath-figure method was introduced to change the solubility parameters of the non-solvent,and a mild method was used to make the water molecules in the air solidify on the surface of the solution and combine with the non-solvent to improve the solubility parameters of the non-solvent,so as to realize the controllable construction of the surface microstructure and achieve the superhydrophobic effect on the surface without any additives.The main work of this paper is as follows:First,in a one-step method,slow evaporation-induced precipitation is used to prepare a hydrophobic PLA foam material with a porous structure on the surface containing a micro-nano structure.As the polymer solution concentration increases,the surface morphology changes from rough spheres to petal-like,porous pits.The morphology of the plane,which is rich in micro-nano structures,is transformed.The cross-sectional morphology realizes the transformation from the fiber network structure to the micro-spherical morphology.Affected by the entanglement of molecular chains,the increase in solution concentration limits the movement of the molecular chains and causes the crystallinity to gradually decreased from 82.9% to58.7%.Restricting the growth of polymer lean phase volume promotes the decrease of foam porosity and reduces the oil absorption of the foam.The increase of the surface microstructure size combined with the reduction of porosity encourages the surface hydrophobicity to gradually decreased from 135° to 77°.Indeed,the removal of foam thermal stability indirectly proves the decrease of foam hydrophobicity.Enlarges the contact area of solvent and non-solvent,and increasing the forming rate of the material.It(What can prepare this material)can prepare a hydrophobic PLA foam material with a micro-nano structure on the surface and a sponge pore structure inside.The crystallinity of the film,contact angle,size of the internal pores,oil absorption,and other properties gradually decrease with increasing concentration.However,compared with foam materials,the size of the surface microstructure and the membrane's porosity significantly increase,which allows the membrane to have higher hydrophobicity.The maximum contact Angle is 138°,the maximum porosity and the maximum oil absorption are 90.8% and 24.1g/g,respectively.The molding temperature has a significant effect on the phase separation rate in the phase separation process.A lower molding temperature inhibits the solvent's volatilization,and the molecular chains have enough time to arrange regularly,which helps to build the surface micro-nano structure.Therefore,the molding temperature can be adjusted to control surface microscopic properties and internal porous structure.As the molding temperature increases,the surface morphology changes from agglomerates rich in lamellar burrs to a rough,flat morphology.When the molding temperature is higher than the boiling point of the solvent,the surface forms a number of round,convex ellipsoid structures on the surface(cluster aggregates).Moreover,the section changes from a sponge-like,porous structure to a loose-layered structure.A higher molding temperature promotes the acceleration of the phase separation rate and reduces the growth time of the polymer-poor phase,the foam porosity,and the oil absorption of the foam,The surface water contact Angle decreased from 112° to 71°with the increase of surface microstructure size and the decrease of porosity,and the thermal stability of the foam also decreased.Enlarges the contact area of solvent and non-solvent,and increasing the forming rate of the material.It(What can prepare this material)can prepare a hydrophobic PLA foam material with a micro-nano structure on the surface and a sponge pore structure inside.The crystallinity of the film,contact angle,size of the internal pores,oil absorption,and other properties gradually decrease with increasing temperature.However,compared with foam materials,the size of the surface microstructure and the membrane's porosity significantly increase,which allows the membrane to have higher hydrophobicity.The maximum contact Angle is 142.3°,the maximum porosity and the maximum oil absorption are 78.5%and 16.89g/g,respectively.In addition to temperature,the non-solvent content can be adjusted to further explore the relationship between phase separation and membrane surface wettability.As the non-solvent content increases,the surface microstructure gradually forms,and properties such as crystallinity,porosity,and oil absorption are improved and eventually stabilized,The porosity is stable in the range of 80.1% to82.7%,and the oil absorption is in the range of 12.41g/g-13.43g/g..The rise in number combined with the increase in porosity promotes the hydrophobicity of the membrane surface,the maximum contact Angle is 139.6°.As a semi-crystalline polymer,PLA has difficulty to forming crystal nuclei during the crystallization process.The molecular chain can easily grow into large-sized crystals on fewer crystal nuclei.Increasing the number of crystal nuclei can effectively reduce the crystal size and affect the phase separation.Therefore,based on the slow evaporation-induced precipitation method,the strategy of introducing heterogeneous nucleating agents and stereocomplex crystals is designed to induce crystal gelation on the surface and build a fiber network structure.First,hydrophobically modified ZnO nanoparticles are introduced as nucleation points to promote preferential crystallization.The fiber network structure is obtained by controlling the additional amount of ZnO nanoparticles.The connection of the fibers forms a granular assembly.With the increase of nanoparticles' content,the network shape changes: the pore wall-size gradually becomes smaller,and the particle size gradually decreases.However,properties such as crystallinity,porosity,and oil absorption increase progressively with the escalation of the content of nanofillers,the crystallinity was between 36.9% and 41.1%,the highest porosity and oil absorption were 94.1% and 11.4g/g,respectively.The surface network structure,combined with high porosity,promotes the improvement of the hydrophobic properties of the membrane surface,and the maximum contact Angle is 146°.On the other hand,the molecular chains of left-handed polylactic acid(PLLA)and right-handed polylactic acid(PDLA)are combined by hydrogen bonding to form a special stereocomplex crystal,promoting the formation of a fiber network structure on the surface of the phase-separated PLA membrane.With the increase of content,the body is transformed from a stacked,spherical particle structure into a network structure composed of spherical particles and fibers.The content,porosity,and oil absorption of stereo composite crystals gradually increase with additional PDLA content,and the surface particle structure is combined,the maximum porosity and oil absorption were85.5% and 13.28g/g,respectively.The high porosity promotes the hydrophobicity of the membrane surface and achieves the superhydrophobic effect,and the maximum contact Angle is 155°.Based on the slow evaporation-induced precipitation method,the Breath-Figure method is introduced to control the air humidity on the surface of the solution.In addition,changing the solubility parameter of the non-solvent and building a multi-level micro-nano structure on the surface and explains the formation mechanism of the micro-topography.At 60% humidity,it can promote rapid phase separation and build a fiber network structure.With the increase of PLLA solution concentration,the fiber network structure is gradually improved.The maximum contact Angle reaches147.1°,the phase separation interacts with the crystallization process,the crystallinity ranges from 54.6% to 63.3%,the maximum porosity and the maximum oil absorption reach 80.5% and 15.99g/g,respectively Under 99% humidity,the surface forms a flower-like combination of micron-level flowers and nano-level lamellas.Morphology,crystallinity,porosity,and oil absorption decrease progressively with the increase of PLLA solution concentration.The practical construction of the surface microstructure combined with the expansion of porosity improves the hydrophobicity of the membrane surface and achieves a superhydrophobic effect.The maximum contact Angle reaches 152.1°,the phase separation interacts with the crystallization process,the crystallinity ranges from 43.7% to 53.1%,the maximum porosity and the maximum oil absorption reach 87.5% and 17.88g/g,respectively.Finally,ZnO nanoparticles are used as nucleation points to explore the relationship between the formation of microstructure and crystallization.Under 60% humidity,a uniform granular assembly structure is formed on the surface,the maximum contact Angle reaches 156.7°,the phase separation interacts with the crystallization process,the crystallinity ranges from 51.9% to 61.2%,the maximum porosity and the maximum oil absorption reach 91.4% and 9.78g/g,respectively.A crystal-shaped cluster structure is formed on the surface under 99% humidity.The surface micro-nano structure gradually decreases with the increase of nanoparticle content,the crystallinity nanoparticle content increases slowly,and the porosity and oil absorption properties gradually increase with the growth of nanoparticle content.The surface micro-nano structure combined with high porosity leads to good superhydrophobic properties in the membrane surface.The maximum contact Angle reaches 153.7°,the phase separation interacts with the crystallization process,the crystallinity ranges from 47.3% to 60.5%,the maximum porosity and the maximum oil absorption reach86.2% and 11.71g/g,respectively. |
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