Relaxation of polymers in 2 nm slit-pores: confinement induced segmental dynamics and suppression of the glass transition

Molecular Dynamics (MD) simulations are used to explore the structure and d ynamics of polystyrene confined in 2 nm slit pores, between parallel, cryst alline, mica-type surfaces. The systems simulated resemble experimentally s tudied intercalated nanocomposites, where polystyrene is inserted between l ayered-silicate layers. The molecular modeling perspective complements the experimental findings and provides insight into the nature of polymers in n anoscopic confinements, especially into the molecular origins of their macr oscopic behavior. Namely, a comparison between simulation and NMR studies s hows a coexistence of extremely faster and much slower segmental motions th an the ones found in the corresponding bulk polymer at the same temperature . The origins of these dynamical inhomogeneities are traced to the confinem ent induced density modulations inside the 2 nm slits. Fast relaxing phenyl and backbone moieties are found in low density regions across the film, an d preferentially in the center, whereas slow relaxing moieties are concentr ated in denser regions in the immediate vicinity of the confining surfaces. At the same time, the temperature dependence of the segmental relaxations suggests that the glass transition is suppressed inside the confined films, an observation confirmed by scanning calorimetry. (C) 2001 Elsevier Scienc e B.V. All rights reserved.