Porous Polymer Membranes: Manipulation Of Porous Structures In Micro-scale Based On Deformation In Macro-scale And Performance

Pore size and pore geometry play important roles in porous materials.There have been some reports concerning the precise manipulation of them.Most of them,however,focused on the optimization during membrane preparation.The lack of efficient strategy for "continuous and quantitative manipulation of porous structure" has been a major barrier to further improvement of the performance of porous materials.In this work,it has been proposed to manipulate the pore geometry and pore size in micro-scale by means of uniaxial/biaxial tension in macro-scale in shape memory membranes.Surface/vertically-oriented pores(VOPs)membranes with a uniform morphology have been constructed based on the shape memory effect of poly(lactic acid)(PLLA)with the help of Breath Figure or Si pillar templates.In membranes with surface pores,uniaxial tension has been adopted to tailor the pore geometry and the resultant water wettability,which is an efficient method to achieve directional transport of droplets.In membranes with VOPs,our investigations focus on the novel solution of Trade-off effect based on narrow pores,the high-accuracy separation according to size-selective porous membranes with narrow pore size distribution,and the anti-fouling performance of separation membranes with the help of recovery behaviors of shape memory materials.(1)Switchable Isotropic/Anisotropic Wettability and Programmable Droplet Transportation on a Shape-Memory Honeycomb: Breath figure has been employed to fabricate shape-memory PLLA honeycombs.Upon uniaxial tension,circle pores from the breath figure are deformed to elliptical pores,producing contact angle differences and anisotropic wetting behaviors in two directions.Both pore geometry and anisotropic wettability can be tailored via the draw ratio.On the PLLA honeycomb surface with a lower draw ratio,the contact angle difference is too small to induce droplet transportation along the desired direction.In the case of a higher draw ratio,however,the movement of water droplets has been controlled absolutely along the tension direction.The transition between them can be achieved reversibly during uniaxial tension and recovery processes based on the shape-memory effect.The enhanced flow control,which can be attributed to the synergism between optimal hydrophobicity and enlarged anisotropic wetting behaviors,endows water droplets with the ability to turn a corner spontaneously on a V-shaped surface including two regions exhibiting different oriented directions.(2)Tunable Pore Geometry in VOPs Membranes based on Shape Memory Effect: Maximizing Selectivity and Permeability in Separation: Trade-off effect between permeability and selectivity in separation remains as a great challenge in membrane filtration.The membranes with uniform VOPs have been prepared in shape memory PLLA.The pore geometry can be tailored from square pores to anisotropic pores by uniaxial tension.As a result,membrane deformation and formation of narrow pores contribute to a simultaneous increase in permeability and selectivity relative to the reference.On one hand,the membranes exhibit the higher flux,which can be attributed to the higher magnitude of pore length;on the other hand,the rejection ratio has been enhanced remarkably(almost 22%),resulting from the smaller pore width.Therefore,anisotropic porous structures provide an efficient strategy for maximizing selectivity and permeability in separation.(3)Tunable Pore Size in VOPs Membranes based on Shape Memory Effect:High-Accuracy Separation and Anti-fouling Performance: Pore size has been adjusted according to uniaxial/biaxial tension.The resultant membranes can be used in the separation of polystyrene(PS)microspheres with various diameters at lower concentration.The separation accuracy reaches 2 microns.In the case of high concentration,the separation process suffers from the effects of membrane pore blockage and the formation of filter cake layers,leading to the obvious decrease of separation flux.The solid pollutions can be removed in membranes with VOPs according to the program of “increase of pore size at high temperature-shape fixing by cooling down-rinsing-recovery at high temperature” with the help of shape memory effect,which can act as novel solutions for high-accuracy separation and anti-fouling of membranes.