Glass Transition Temperature of Nanometer Ultra-thin Polymer Films

United-atom molecular-dynamic simulations were performed to study the nature of the glass transition(T_g) in free-standing polymeric films. Experimentally it has been found that the glass transition dependence on film thickness varies considerably with the polymeric chemistry. We are investigating linear and cyclic polystyrene(PS) as well as poly(alpha-methylstyrene). These are polymers with very similar monomers, yet they behave differently when cooled towards T_g. Both static and dynamic data will be presented. We find that for linear PS a larger number of end groups inhabit the interfacial layer rather than the middle of the film. This increases mobility and yet it does not explain the T_g dependence on film thickness observed for cyclic polymers. Hence, we hypothesized* that the observed deficit of phenyl groups in the interfacial film layers, which weakens the phenyl-phenyl aromatic(π-π) interactions, contributes to the lower T_g for thinner films of PS. In addition, local orientation mobility of the phenyl bonds is studied with the help of Legendre polynomials of the second order.
*Alexey V. Lyulin, Nikolav K. Balabaev, Arlette R. C. Baljon, Gerardo Mendoza, Curtis W. Frank, and Do Y. Yoon, , J. Chem. Phys. 146, 203314 (2017)