Influence of Tacticity on the Glass-Transition Dynamics of Poly(methyl methacrylate) (PMMA) under Elevated Pressure and Geometrical Nanoconfinement

The physicochemical properties of polymers can be modified by spatial configuration. Here, dielectric spectroscopy (DS) was employed to investigate the effect of the tacticity on the glass-transition dynamics of confined polymer films. We have also carried out dielectric relaxation studies at ambient and elevated pressure conditions. High-pressure measurements provide information on the sensitivity of the glass-transition dynamics to density changes, which can be helpful to better understand the confinement effect. For our study, we have chosen a glass-forming polymer, poly(methyl methacrylate) (PMMA), with two different tacticities (isotactic and syndiotactic) and an approximately equal molecular weight. We have shown that the segmental relaxation for the bulk syndiotactic PMMA is observed at a higher temperature range compared to the bulk isotactic PMMA. The experimental data analysis has also revealed that the molecular order influences the behavior of the polymer under increased pressure. The glass- transition dynamics of isotactic PMMA is more sensitive to the pressure/density changes than syndiotactic PMMA. Finally, we have also demonstrated that tacticity has a significant impact on the segmental dynamics of the confined polymer films. Syndiotactic PMMA has a stronger interaction with the substrate than isotactic PMMA, which is indicated by the higher amount of irreversibly adsorbed polymer chains. For this reason, the segmental dynamics of s-PMMA thin films follows the bulk behavior. In contrast, for iPMMA, the confinement effects are pronounced, and the glass-transition dynamics accelerate with the reduction of the thickness of thin films.