Fabrication And Structure Control In Porous PVDF By Crystallization Templating

Porous materials have attracted tremendous attention due to their wide applications such as gas storage,separation and controlled release of drugs.The fabrication and pore-size control remain as great challenges in the past decades.In this paper,therefore,the poly(vinylidene fluoride)/poly(1,4-butylene succinate)(i.e.PVDF/PBSU)blend with miscible windows in phase diagram has been employed as a model to achieve the precise localization of“inter-spherulites/inner-spherulites","inter-lamellar/inter-fibrillar" and a inter-lamellar inclusion/inter-lamellar exclusion" structures.Subsequently,the PBSU was selectively removed by extraction,resulting in the formation of porous PVDF membranes.Furthermore,the pore morphology as well as pore-size and the mechanical performance of the resultant porous PVDF membranes were controlled by means of stretching,composition of the blend and the thermal history.(1)Precise inter-lamellar/inter-fibrillar localization and consequent fabrication of porous membranes with crystallization-modulated pore-size:In PVDF/PBSU blend,the precise inter-lamellar or inter-fibrillar localization of PVDF crystal lamellae has been achieved.In the specimen with PVDF 70wt%,comparable PVDF and PBSU volume fractions result in the inter-lamellar structures.When PVDF acts as the minority phase(PVDF 30 wt%),its crystal lamellae locate between PBSU crystal stacks,accounting for the formation of inter-fibrillar structures.By extracting the continuous PBSU,nano-porous PVDF membranes exhibiting excellent mechanical performance were fabricated.The extractions in the membranes with inter-lamellar and inter-fibrillar structures produce smaller and bigger pores,respectively.Our results provide a facile and effective strategy to manipulate pore-size by crystallization template.(2)Inter-lamellar inclusion and inter-lamellar exclusion structures in PVDF/PBSU blend and consequent fabrication of oriented PVDF porous membranes:The flux and separation efficiency are important parameters for the separation performance of porous membranes.In order to improve the flux with the constant separation efficiency,the oriented PVDF porous membranes were fabricated with the combination of isothermal crystallization and stretching.Our results indicate that the PVDF and PBSU lamellae are incorporated into each other,producing the inter-lamellar inclusion structures.Upon stretching,the PBSU lamellae were expelled out from the PVDF lamellae,leading to the formation of its crystal stacks.This is the reason for the occurrence of inter-lamellar exclusion structures.After the selective removal of PBSU by chloroform,the oriented PVDF porous membranes with various sizes have been fabricated.(3)Thermal history dependence of crystallization of PVDF/PBSU and resultant PVDF porous membrane:The programmable structure control is significant for the developments of fabricating strategy and applications of porous materials.In this part,therefore,the thermal annealing temperature and the subsequent isothermal crystallization temperature were employed to manipulate the pore structure and the mechanical performance of the porous PVDF membranes.Our results indicate that the phase-separated mixture resulted from the two-phase region in phase diagram produce the localization of PBSU crystals in "inter-spherulites" regime,corresponding to the bigger pores and the lower elongation at break in stress-strain curve of the resultant membranes.On the contrary,the miscible blends(one-phase region in phase diagram)account for the“inner-spherulites" structures,the smaller pores and better mechanical performance.The crystallization temperature generates remarkable influence on the crystal structures too.