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Relaxation and chain dynamics of polymer nanostructures after release of spatial confinement were studied using line gratings as small as 33 nm on polystyrene surface fabricated by nanoimprint lithography. The temperature for “slumping”rapid line height relaxationdecreased as the line width diminished for all molecular masses (MWs), but a simple explanation based on enhanced surface mobility fails to explain the results. When MW was low and the structure was large, the line height monitored with an AFM reduced as surface tension overcame the polymer viscosity. Interesting and complex behaviors were observed when the radius of gyration (Rg) of the polymer molecules was not small compared with the dimension of the nanostructure. Careful examination of the surface viscosity shows that confinement of polymer chains into space comparable to or even smaller than its Rgappears to enhance relaxation, which is the major factor for the surprisingly low temperature at which nanostructures of high MW slumps. [ABSTRACT FROM AUTHOR]