Possible causes of the change of dynamics in glass-forming materials subjected to reduced dimension

There is currently many ongoing investigations of the change in the glass transition temperature when a material is reduced in dimension from the normal bulk state. Tile reduction in dimension can be accomplished by casting the material as thin films with or without a substrate or putting it in nanometer size pores. In this work, we explore possible causes of the change in dynamics of the bulk material when the glass-former is subjected to such modifications. The existence of a growing cooperative length scale L(T) with decreasing temperature in bulk fragile glass-forming liquids reaching the size of approximately 1.5-2.0 nm at the glass transition temperature is the basis of our consideration. When the reduced dimension is comparable to L(T-g), cooperative dynamics within a lengthscale equal to L(T-g) can no longer be maintained in all three dimensions throughout the sample. The imposed reduction of the cooperative length scale speeds up the dynamics and causes a reduction of the glass transition temperature. For polymeric glass-formers particularly at higher molecular weights, reduction of one dimension in thin films engenders orientation of the polymer chains when their radius of gyration becomes comparable to the film thickness. The latter is known to cause also a reduction of the glass transition temeperature.