Study On Continuous Preparation And Defects Of PVA Nanocoatings

Polymer nanocoatings have attracted much attention in many fields,such as electronic devices and biomedicine due to their simple and versatile preparation methods and multi-functional properties.The common preparation methods of polymer nanocoatings include spin-coating,dipcoating and blade-coating methods,etc.However,only nanocoatings with small area could be prepared by these techniques,which limits the further development of large-size thin film devices.Therefore,how to obtain high-quality nanocoatings continuously and in large area has attracted the attention of many scholars.In addition,the performance of the final device is also determined by the appearance of surface defects of the nanocoatings obtained from the deposition of dilute solution due to the temperature difference and pressure changes caused by the gas flow in the process of evaporation,as well as impurities in the air.Therefore,in this paper,we independently designed and built a device based on the combination of roll-to-roll technology and dip-coating method to prepare polyvinyl alcohol(PVA)nanocoatings,systematically studied the effects of different process parameters on the morphology and structure of nanocoatings,and revealed the source of surface morphology defects of nanocoatings by observing the formation process of nanocoatings online.With the defects caused by impurities,we further use numerical simulation to study the Pinning phenomenon during the preparation of polymer nanocoatings.The research is mainly divided into three parts,with specific research contents and results including:(1)The influence of the withdraw speed(4-48 mm/min)and concentration of solution(10-20mg/m L)on the thickness and morphology of the nanocoating was studied on the self-designed equipment,and the morphology and structure of the nanocoating under different drying temperatures were studied by heating the solidifying area.The results show that the continuous preparation(50 cm)of the polymer nanocoating can be achieved successfully through the continuous equipment designed and built by ourselves.It is found that the relationship between the thickness of the nanocoating and the withdraw speed is consistent with the Landau-Levich theory at high speed,but the scale coefficient is different,and the scale coefficient of our device is 1.9.At low speed,the thickness of the nanocoating is independent of the withdraw speed.The nanocoatings with the required thickness can be prepared by controlling the withdraw speed and solution concentration.When the drying temperature is lower than 125 ℃,the morphology and structure of the nanocoating are constant,and the preparation efficiency can be improved by increasing the temperature within this range.When the drying temperature is higher than 125 ℃,crystallization occurred and the degree of crystallinity increases with the increase of temperature.When the drying temperature reaches 200 ℃,the degree of crystallinity could up to 27%.Our research on the preparation of nanocoatings on continuous equipment is helpful to provide reference for further expanding the production of nanocoatings.(2)The film forming process of polymer nanocoating was observed online and the influence of different processing conditions on the morphology of PVA nanocoating was studied.The results show that strip defects appeared in the late stage of continuous preparation of nanocoatings,and the formation of defects can be eliminated by stirring solution before dipping.This is due to that the polymer in the surface layer of solution is pulled away,and the polymer in the bulk solution has no time to migrate to the surface layer,resulting in the surface tension difference in the liquid film,which drives the Marangoni flow and leads to the formation of strip defects.The thickness is uneven,and stirring solution can make the solution reach thermodynamic equilibrium.This is proved by the fact that the strip defects of the nanocoating appear earlier and the area is larger after further compression of the surface area of the solution.The edge uniformity of the nanocoating can be improved by introducing inorganic salt additives,which is caused by the affinity between the inorganic salt ions and the surface of silicon substrate;the homogeneity of the nanocoating can be improved by introducing surfactants,which reduce the surface tension of the solution.Our research on the formation mechanism of strip defects on the surface of polymer nanocoatings is helpful to guide the preparation of homogeneous nanocoatings with no obvious defects.(3)With the method of numerical simulation,we attempt to conduct a simulation research about Pinning on the liquid film at the bulge defect,aiming at exploring the dynamic behavior of liquid film at a single bulge defect.By constructing different types of substrate defects,we studied the state of liquid film under different retraction rates,different defect sizes and heterogeneous defects.The results show that,after meeting defects,liquid film will stay at the defect,presenting the Pinning phenomenon.The larger the defect size,the longer the liquid film Pinning time.After encountering defects,the liquid film will present three different situations,which are(Ⅰ)direct crossing,(Ⅱ)residual solution,and(Ⅲ)complete fixation.The different states of the liquid film when encountering defects are related to the capillary number Ca,and the larger the Ca,the easier it will pass through the defect.When the substrate and the defect are different substances,the smaller the defect contact Angle is,the longer the nailing time will be.This is because the contact Angle between the substrate and the defect is different,and the liquid film will tend to spread on the surface with small contact Angle.Based on Comsol,we simulated the dynamic behavior of the bulge defects in the process of liquid film retraction,which is helpful to understand the source of defects after nanocoating processing,and can be used to control the liquid film retraction by bulge,so as to achieve accurate control of the coating morphology.