Surface forces play a paramount role in most aspects of Nanoimprint Lithography In particular, subjecting nanoimprinted patterns to moderate heating allows surface tension to smooth out undesirable roughness and defects in the patterns, but this "thermal reflow" treatment can induce structural decay or even collapse of the patterns by capillary instability if this process is not carefully controlled. Adhesion between the mold and polymer film can also cause the imprinted structure to tear or fracture. Fluorinated surfactants (FS) are attractive for reducing mold adhesion, yet the effects of these additives on nanostructure stability during thermal reflow are not well understood. Here we present thermal stability studies of linespace grating patterns created by Thermal Embossing Nanoimprint, Lithography (TENIL) on model polystyrene (PS) films with FS additives. As expected by energy considerations, FS segregates to the air interface, where it seems to facilitate mold release. This also reduces the surface energy and thus reduces the driving force for pattern "slumping" (height decay) However, the beneficial effects of the surfactant are counterbalanced by the fact that the FS decreases the effective film viscosity, which accelerates nanopattern leveling. The net effect is that the pattern height decay is strongly a function of FS concentration. This enhanced film fluidity in the presence of FS also makes the pattern more susceptible to an undulatory capillary instability under thermal reflow conditions. Surface phase segregation of FS and PS is also observed in conjunction with both slumping and lateral capillary instabilities, which may be useful for producing chemically patterned surfaces. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B, Polym Phys 47 2591-2600, 2009