Interpreting the dynamics of nano-confined glass-formers and thin polymer films: Importance of starting from a viable theory for the bulk

The changes of the dynamic properties of the nanoconfined materials vary greatly depending on the nature of the interfaces, the chemical structure of the nanoconfined glass-former, the experimental methods used, and, in the case of polymers, the length-scale of the dynamics probed. Just for the glass transition temperature (T-g) alone, it can decrease, increase, or remain the same depending upon the experimental or simulation conditions. The conventional theories of T-g are unable to explain the range of behaviors seen at the nanometer size scale, and some of the theories give even conflicting predictions on the effect of small size or nanoconfinement on T-g. These problems of conventional theories orginate from the neglect or inadaquate treatment of the many-molecule relaxation, showing up already when applied to the bulk for not being able to explain some general properties of glass transition. Thus, it is not surprising to find the conventional theories fail to explain the range of behaviors of the more complicated case of materials in nanoconfinement. On the other hand, based on concepts and parameters that capture the essentials of many-molecule relaxation, the Coupling Model is not only consistent with the general properties of bulk glass-formers but can also explain the range of behaviors found in materials subjected to nanoconfinement. (c) 2006 Wiley Periodicals, Inc.