Porous Shape Memory Polymers:fabrication By Complicated Phase Behavior In Polymer Blend Containing Crystallizable Component And Structure/Performance Manipulation

Porous shape memory polymers(PSMPs)play an important role in many applications.So far,there are two kinds of PSMPs including foams and scaffolds.The closed pores in the former does limit its applications as functional materials while the mechanical properties declines remarkably in the latter due to the high porosity.The fabrication of PSMPs with both interconnected pores and good mechanical performance is greatly desired but still challenging.In this work,therefore,we developed a new strategy for this purpose based on the combination of crystallization and phase separation.On one hand,tiny crystals(or chemical cross-linked net-points)and amorphous parts act as shape fixed phase and reversible phase respectively,contributing to shape memory performance;On the other hand,the bicontinuous structures resulted from the crystallization or phase separation template produce interconnected porous structures by means of selective etching.In the obtained PSMPs,the programmable and continuous geometry manipulation of micro-pores as well as macro-scale performance has been achieved by means of uniaxial or biaxial tensions.(1)Fabrication of PSMPs with both interconnected pores and high strength based on the combination of crystallization and phase separation: In PLLA/PVAc/PEO blend,phase separation between PLLA and PVAc/PEO takes place and results in bicontinuous structures in microscale including a PLLA-rich phase and a mixed PVAc/PEO phase.On one hand,the tiny crystals and amorphous parts of PLLA contribute to shape memory performance,in which serve as shape fixed phase and reversible phase,respectively.One the other hand,the crystallization of PEO in the miscible PVAc/PEO blend produced submicron bicontinuous structures.The interconnected nanopores have been obtained by selective etching of the PEO by water.Our strategy opens a new avenue for fabricating PSMPs with both interpenetrated channels and high strength.(2)Preparation of PSMPs with both interconnected pores and good mechanical properties and micropore geometry manipulation by macroscopic deformation: Phase separation takes place in PVDF/PBSu/BMG(with crosslinker)produced bicontinuous structures including a mixed PVDF/PBSu phase and a BMG-rich phase.In the former,the crystallization of PVDF expels the amorphous PBSu chains to its interfibrillar regions,resulting in the formation of interconnected pores after selectively etching.In the latter,chemical cross-linked net points and amorphous parts act as shape fixed phase and reversible phase,respectively.The obtained PSMPs exhibit excellent shape fixity and good mechanical properties owing to PVDF crystals and cross-linked BMG.The formation mechanism of PSMPs has been investigated in detail.Based on the shape memory effect,the geometry manipulation of micro-pores by macroscopic deformation has been achieved.(3)Preparation of PSMPs with interconnected isotropous pores and the precise micropore geometry manipulation by macroscopic deformation: In PLLA/PEO membrane fabricated by blade-coating,phase separation takes place and the phase-separated PEO-rich phase connected with each other because of the confinement and shear field.The PLLA membrane with isotropous pores has been prepared by water etching,in which the tiny crystals and amorphous parts of PLLA serve as shape fixed phase and reversible phase,respectively.The proportional deformations on two scales have been achieved by uniaxial or biaxial tension,providing a facile way to manipulate continuously the size and the orientation degree of pores on microscale.The enhanced separation performance has been validated by taking polystyrene colloids with varying diameters as an example.