The surface segregation of binary athermal polymer blends confined in a nanoscale thin film was investigated by dissipative particle dynamics. The polymer blend included linear/linear, star/linear, bottlebrush/linear, and rod-like/linear polymer systems. The segregation was driven by purely entropic effects and two different mechanisms were found. For the linear/linear and star/linear polymer blends, the smaller sized polymers were preferentially segregated to the boundary because their excluded volumes were smaller than those of the matrix polymers. For the bottlebrush/linear and rod-like/linear polymer blends, the polymers with a larger persistent length were preferentially segregated to the boundary because they favored staying in the depletion zone by alignment with the wall. Our simulation outcome was consistent with experimental results and also agreed with theoretical predictions - that is, a surface excess dictated by the chain ends for the branch/linear system. These consequences are of great importance in controlling the homogeneity and surface properties of polymer blend thin films.