Fabrication And Manipulation Of Porous Membranes Based On Exclusion Behaviors In PLLA/PVAc Blend System

Porous materials exhibit advantages of low density,large specific surface area,adsorption capacity,high specific strength,adjustable pore structure and so on.Therefore,they play important roles in the fields of catalytic loading,drug release,separation,gas adsorption and storage,and electrochemical devices.The fabrication strategy of them have attracted much attention.In our previous work,a novel strategy has been developed to prepare porous materials based on crystallization templates.However,there are still some open problems in this method.On the one hand,due to the high crystallization rate(e.g.POM,PVDF adopted in our previous work),it is impossible to track the crystallization on-line.The exclusion behavior of amorphous component remains obscure;on the other hand,the universality of this strategy needs to be validated,especially in various blend system and crystallization conditions.For instance,how about the selective enrichment of amorphous component during cold crystallization which is benefit to the preparation of polymer porous material relative to melting crystallization.In this work,therefore,the miscible poly(L-lactic acid)(PLLA)/poly(vinyl acetate)(PVAc)system is employed to investigate the melting and cold crystallization behaviors of them with the combination of SAXS with DSC.Based on the selective distribution of them,PLLA porous material is constructed according to the crystallization template.Moreover,the pore size of the PLLA porous film is manipulated by adjusting the composition of the blend and the crystallization temperature.The main research contents are as follows:(1)Study on crystallization process of miscible PLLA/PVAc blend system:A new parameter?_s'is introduced.The(melt/cold)crystallization process of the PLLA/PVAc blend system has been clarified with the help of?_s'and?_s.?_s and?_s'have been employed to describe the exclusion/inclusion behaviors of PVAc in different scales.The former can act as a good parameter to show the selective distribution of PVAc as a whole,while the latter only focuses on the crystal lamellar regions.During the melting and cold crystallization,the increase in?_s indicates that more and more PVAc has been included in the inter-lamellar regions due to the growth of PLLA crystal lamellae.However,the decrease in?_s'makes it clear that only part of PVAc(not all PVAc)locate in the regions discussed above.The distribution of PVAc can be clarified well with the combination of?_s and?_s'.(2)Preparation and pore size manipulation of PLLA porous material based on melt crystallization template:Based on the selective distribution of PVAc during the crystallization of PLLA,bi-continuous structures of them has been obtained.The three-dimensional PLLA porous material has been fabricated successfully by etching with selective solvent of PVAc.Furthermore,the precise manipulate of distribution of PVAc can be achieved by adjusting the composition of PLLA/PVAc blend and melt crystallization temperature.Our results suggest that higher weight fraction of PVAc located in the inter-fibrillar region correspond to the higher magnitude of pore size.(3)Preparation and pore size manipulation of PLLA porous material based on cold crystallization template:According to the exclusion behaviors of PVAc during the cold crystallization of PLLA,bi-continuous structures of them have been obtained.It is facile to fabricate three-dimensional PLLA porous materials by etching PVAc with selective solvent methanol.The porous structures are also under the control of weight ratio of PLLA/PVAc ratio as well as the cold crystallization temperature.The increase in weight fraction of PVAc located in the inter-fibrillar region is benefit to the formation of porous structures with higher pore size.