Solvent vapor annealing(SVA)is known to be a simple,low-cost and highly efficient way to annihilate defects in diblock copolymer(BCP)thin films and it facilitates the formation of highly ordered microdomains.SVA has been widely used in defects removal,however its underlying mechanism is still not well understood.In this work,a three-fluid model is presented to study the dynamic coupling between BCP segments and solvent during SVA.As an example of its applications,we consider very thin BCP films with fast relaxation through the film thickness such that the variation of solvent fraction and order parameter can be both neglected throughout the film thickness.In this case,the three-fluid model reduces to be a 2D Cahn-Hilliard type model in the BCP plane.We carry out numerical simulations and examine systematically the effects of several key parameters and focus on swelling ratio and the evaporation rate of solvent,on the defect removal in perpendicular lamellar BCP films by SVA.It is pointed out that the solvent not only dilutes the segmental interactions but also varies the periodic spacing of BCP microdomains.It is found that the solvent-induced variation of lamellar spacing facilitates more efficient removal of defects in BCP films.Moreover,the final morphology of the dried BCP film is shown to depend not only on the morphology in the swollen state but also on the evaporation rate of solvent.We also give some suggestions to improve the efficiency of defect removal by SVA in thin BCP films.