Investigation On Microstructure Manipulation And Macroscopic Performance Based On Shape Memory Effect Of PLLA

Microstructure is an important parameter that determines the properties of materials.It is necessary to control the microstructure actively and precisely.There have been some reports about this.Most of them,however,focused on parameter optimization during membrane preparation.A few works tried to adjust the microstructure after preparation based on stimuli-responsive material.These methods face kinds of challenge,e.g.,poor accuracy,complex trigger mechanism and high cost.Therefore,it is urgent to develop new strategy to manipulate microstructures continuously and precisely.In this work,a novel “post-treatment” strategy has been proposed to tailor the microstructure by uniaxial/biaxial tension in polylactic acid(PLLA),a typical shape memory polymer.Composite surface/vertically-penetrative membrane/nano-patterned surface with a regular morphology have been fabricated based on the polylactic acid(PLLA)with the help of swelling or Si pillar templates.(1)PVDF@PLLA Composite Surface: Fabrication,Microstructure Manipulation and Programmable Transition between Adhesive/Anti-adhesive Performances: By loading porous poly(vinylidene fluoride)(PVDF)spheres on a shape memory polylactic acid(PLLA)film,a quasi-superhydrophobic surface of composite film(PVDF@PLLA)with the ability to tailor its surface structures/composition and related adhesive behaviors was fabricated.The as-prepared surface is covered by porous PVDF spheres.The combination of hydrophobicity of PVDF and hierarchical roughness resulted from porous spheres contributing to the high contact angle and low sliding angle,corresponding to Cassie state and lotus leaves effect.Upon uniaxial or biaxial tension,the distance among hydrophobic spheres is so high that more and more hydrophilic defects(PLLA film)have been exposed to water droplets,accounting for the quasi-superhydrophobic surface with a higher sliding angle.This is the reason for the Wenzel state and rose petals effect.After heating,PLLA film recovers to its original state.The porous PVDF spheres cover the whole film again,leading to the enhanced mobility of water droplets on the surface.The transition between the rose petals effect and the lotus leaves effect is programmable and reversible.Our result provides a novel strategy to tailor adhesive behaviors by combining(quasi-)superhydrophobic surface with shape memory effect.(2)PLLA Vertically-penetrative Membrane: Fabrication,Microstructure Manipulation and the Analysis of Mass Transfer: Based on shape memory effect,PLLA vertically-penetrative membranes were prepared by reverse replication of silicon templates.The pore diameter of vertically-penetrative membrane was tailored via biaxial tension.The mass transfer was divided into three stages,i.e.,entering,transfer and detaching.Our results indicate that in the hydrophilic membranes,it is not contact angle between solid and liquid but the interfacial tension between them that determines the mass transfer through membrane.Inspired by the key role of interfacial tension on mass transfer,a Janus membrane with asymmetrical pore diameter was designed to improve the permeability.(3)PLLA Nano-patterned Surface: Fabrication,Microstructure Manipulation and Programmable Transition between Structural Colors: Surface nano-stripes and reflective grating have been fabricated on shape memory polymers(SMPs)with the help of Si pillar template.The structural color resulted from the interference of reflected lights exhibits high-saturation and can be regulated continuously based on shape memory effect.Uniaxial tension along stripes at high temperature produces remarkable blueshift of the resultant color(from red to green and blue)which can switch back to red after shape recovery upon heating.The evolution of structural color can be attributed to the lower and higher magnitudes of nano-structure periods in temporary(deformed)and permanent(recovery)states respectively.Based on the combination of angle and deformation dependences of structural color,a“colorful” product code has been fabricated.It exhibits enhanced ability to hide and display information which plays an important role in anti-counterfeiting.