Effect of tacticity on the structure and glass transition temperature of polystyrene adsorbed onto solid surfaces

We have carried out atomistic (all-atom) molecular dynamics simulations to investigate the effect of tacticity on the structure and glass transition temperature (T_g) of polystyrene (PS) thin films adsorbed on two distinct types of solid substrates. The systems consist of thin films made of atactic, isotactic, and syndiotactic PS chains supported by graphite or hydroxylated α-quartz substrates, which are known to be atomically flat but chemically and structurally different. We have observed a marked dependence of the film structure on substrate type as well as on tacticity. For instance, rings' orientation near substrate surfaces depends on substrate type for atactic PS and isotactic PS films, while no such dependence is observed for syndiotactic PS films whose interfacial structure seems to result from their propensity to adopt the trans conformation rather than their specific interaction with the substrates. Moreover, our results indicate that glass transition temperatures of substrate supported polystyrene films are higher compared to those of the corresponding free-standing films. More specifically, PS films on graphite exhibit larger T_g values than those on α-quartz, and we have noticed that syndiotactic PS has the largest T_g irrespective of the substrate type. Furthermore, the local T_g in the region of the film in contact with the substrates shows a strong tacticity and substrate dependence, whereas no dependencies were found for the local T_g in the middle of the film. Substrate-film interaction energy and chains' dynamics near substrate-film interfaces were subsequently investigated in order to substantiate the obtained T_gs, and it was found that films with higher T_gs are strongly adsorbed on the substrates and/or exhibit smaller interfacial chains' dynamics essentially due to steric hindrance.