Surface and interface are the important windows for us to learn the unique physical chemistry of polymer materials. Recently

Professor Biao Zuo and his collaborators used atomic force microscope to observe the creep relaxation of surface wrinkle at the edge of ionic liquid droplet on the smooth surface of polystyrene. They found that in the low temperature region nearby body glass transition temperature of polystyrene

the creep relaxation could not reproduce the time-temperature superposition law as observed in the high temperature region. Since surface polymers hold relatively high mobility

there exists a mobility gradient from the surface to the body of polystyrene. They performed molecular dynamics simulations to demonstrate that the body glassy state brings constraint to the surface polymers

and it expands the modulus plateau of the rubbery states of surface polymers in the low temperature region. Leaving away from the surface

the glassy part of surface polymers plays the role of physical crosslinking

restricts the mobility of surface polymers

and thus enhances their rubbery behaviors in the low temperature region. This work challenges the current hypothesis of entanglement network

causing the rubbery state of bulk amorphous polymers. Once linear polymers make the partial frozen of the glassy states in their bulk amorphous phase for physical cross-linking

they could attain the unique rubbery states

distinguished from small molecules.