Preparation And Structure Study Of PVDF Ultrathin Films By Solution Deposition

With the rapid development of nanotechnology in microelectronics,new energy and other fields,polymer nano film materials with broad application prospects have attracted more and more attention.Polyvinylidene fluoride(PVDF)ultra thin films have been widely used in sensing,medicine,environmental protection and other fields because of their good piezoelectric effect and excellent physical and chemical properties.At present,the research mainly focuses on the influence of solvent,mechanical stretching,temperature and other preparation conditions on the crystalline structure of PVDF films with micron thickness,but the influence of film forming conditions on the surface structure and crystalline structure of PVDF nano ultra thin films is not sufficient.In this paper,the formation process,surface morphology and crystal form of PVDF ultra thin films were systematically studied by controlling different preparation conditions such as humidity,temperature,immersion time and fluorescence assisted In-situ film-forming observation.The main results are as follows:(1)The cloud point curves of PVDF-DMF-H₂O and PVDF-Acetone-H₂O systems at different temperatures were determined by cloud point titration.The liquid-liquid coexistence line in the full concentration range and the ternary phase diagram of polymer,good solvent and bad solvent were obtained by LCP extrapolation.Based on the phase diagram,the phase separation process and phase separation structure in the process of PVDF film formation can be qualitatively analyzed.(2)By systematically changing the environmental humidity conditions during the preparation of PVDF ultra thin films and the immersion time before film formation,we found that with the increase of film formation humidity and immersion time,the visible light transmittance of the prepared PVDF ultra thin films gradually decreased,and the surface roughness,porosity,pore diameter and?The crystal content increases gradually.This is because the moisture in the environment can diffuse into the PVDF liquid membrane under high humidity environment or long immersion time,which changes the solidification process from the volatilization and solidification of good solvent to the mode of phase separation and solidification,resulting in the non-uniformity of polymers and the formation of holes in the liquid membrane.At the same time,water molecules with strong polarity and a large number of phase interfaces formed in the process of phase separation promote more?Phase crystal formation.(3)The effect of film forming temperature on the structure of PVDF ultra thin films was systematically studied.We found that with the increase of the film forming temperature,the visible light transmittance of the prepared PVDF ultrathin films gradually increased,the surface roughness decreased,and the grain size?The crystal content also decreased gradually.This is because at high temperature,the curing path of PVDF is far away from the phase separation region,and the surface of PVDF is smoother.However,the rapid volatilization of solvent caused by high temperature also causes dissipation structure in the film,which limits the size of spherulites.At the same time,higher film forming temperature and more uniform solution system contribute to the formation of thermodynamically more stable?Crystal,in film?The relative content of crystal decreased.(4)The process of PVDF ultra thin film formation was studied by fluorescence assisted on-line.We found that the relationship between the real-time thickness and time of liquid film evaporation and curing can be divided into two different stages:the first stage is the stable solvent evaporation stage,and the film thickness decreases linearly with time.In the second stage,the film thickness decreases exponentially with time,because the curing rate changes from the control of solvent volatilization at low concentration to the control of solvent diffusion in the gel like liquid film at high concentration when the liquid film increases a certain concentration.When the mixed solvent of DMSO and DMF is selected,a large number of bulges appear on the surface of PVDF ultra thin films,which is mainly due to the difference of volatilization rate and surface tension of different solvents driving the Marangoni flow on the surface of the films to form undulating structure,which is finally fixed by surface gel.