Swelling And Dewetting Behavior Of Langmuir-Blodgett Films Of PS-b-PMMA Induced By Selective Solvent Vapor

It is well known that block copolymers can self-assemble into nanometer-sized domains,which can be used to produce well-ordered Langmuir-Blodgett?LB?films.Solvent vapor annealing?SVA?can be used to further modify the LB films,therefore,it is meaningful to study the swelling and dewetting behavior of LB films induced by solvent vapor.In this paper,atomic force microscopy?AFM?was used to study the morphology evolution of LB films of two kinds of diblock copolymers polystyrene-block-poly?methyl methacrylate??PS-b-PMMA,coded as SMMA34K and SMMA51K?induced by selective solvent,which included acetone,tetrahydrofuran?THF?and carbon disulfide?CS₂?.Effects of surface pressure,layers of LB films and volume of solvent on the dewetting behavior of LB films were studied.In addition,X-ray photoelectron spectroscopy?XPS?and droplet shape analyzer were used to characterize the surface properties of thin films.SMMA34K and SMMA51K show the mixed structure of predominant circular micelles and few rod-like aggregates and the isolated circular micelles at higher surface pressure,respectively.This phenomenon is due to the length of PMMA block in the later is longer than that in the former,which restricts the contact and coalescence between PS blocks.The morphologies in the multilayer LB films of SMMA34K are similar to those in the monolayer films,and defects increased obviously with the increase of the number of layers of LB films.Similar morphology was also obtained at lower surface pressure,which indicates the effect of surface pressure on the surface morphology is small.With the annealing of acetone vapor,the monolayer LB films of SMMA34K and SMMA51K undergo a complicated process including swelling and coalescing,bicontinuous spinodal-like pattern,rupturing into droplets,and periodic increase and decrease of the droplet diameters.The emergence of bicontinuous spinodal-like pattern indicates that the dewetting occurs via the spinodal dewetting mechanism.The last step is a novel phenomenon observed for the first time during the solvent-induced dewetting processes of thin polymer films,which is different from the single increase and decrease of the droplet diameters of spin-coated films.The variations of droplet diameters are mainly attributed to the PS component with the lower surface energy and the preferential affinity of acetone vapor for PMMA blocks.The droplet diameters of SMMA34K are larger than those of SMMA51K at the same annealing time.It demonstrates that the swelling degrees of the LB films of SMMA51K are smaller than those of SMMA34K due to the stronger interaction between the longer PMMA blocks and the substrate in the former.The more solvent is used,the faster dewetting behavior occurs and the larger droplet diameters become.Furthermore,the results of XPS and contact angle indicate that the content of PMMA blocks in the surface of droplets increase with the increase of annealing time.In addition,the morphology evolution of the multilayer LB films of SMMA34K is similar to the monolayer ones,and droplet diameters increase with the increase of the number of layers of LB films.The dewetting behavior is observed in the monolayer LB films of SMMA34K induced by THF vapor,in which the morphology evolution is similar to that in acetone vapor,however,the dewetting mechanisms in the two kinds of solvent are different.The flat structure with hole is obtained in the monolayer LB films of SMMA34K induced by CS₂ vapor,which indicates that the LB films cannot occur dewetting behavior annealed in CS₂ vapor.