Entropic Surface Segregation from Athermal Polymer Blends due to Conformational Asymmetry

Surface segregation of polymers based on conformational asymmetry is a subtle effect, and the literature is divided regarding questions as basic as whether the stiffer or more flexible polymer segregates to the surface. This work uses Monte Carlo simulations to investigate the surfaces of athermal blends of stiff and flexible polymers as the bending modulus of the stiff polymers, κ, is increased from zero to the point where the bulk undergoes an isotropic−nematic transition. For hard walls characteristic of polymer/solid surfaces, the flexible polymers segregate to the surface and follow the universal concentration profile predicted by mean-field theory [Wu et al., J. Chem. Phys. 1996, 104, 6387]. This prediction breaks down for higher κ, where packing becomes important and a thin nematic layer rich in stiff polymers forms next to the wall. This appears to be followed by a second surface transition where the degree of order abruptly increases, which, in turn, causes a rapid thickening of the nematic layer that nucleates the bulk nematic phase.