Molecular modeling of the glass transition of stereoregular PMMAs

Abstract

Molecular dynamics simulations were performed on PMMA chains of different tacticity to investigate the glass transition phenomenon. Since the difference in the simulated T _gs between the two stereomers is accurately expressed, microscopic investigations accountable for such a variation can be performed. In this paper, energetic and local dynamics analyses are carried out. From energetic studies, it is shown that the principal energy difference between the two stereomers stays on intermolecular interactions and on the bending energy term associated with the intra-diad angle. A more important opening of this angle was observed for isotactic chains. Such a result has actually to be correlated with the local dynamics behavior of both stereomer chains. For that purpose, relaxation studies were carried out. The isotactic side-chains are shown to rotate with a higher frequency than the syndiotactic ones. Actually, from experimental NMR data, it was shown that the side-chain rotation greatly influences the backbone mobility. Consequently, according to the free volume theory, simulation results perfectly agree with the difference in T _gs between the two configurations.

Keywords:

• Molecular dynamics
• Tg
• Relaxation time
• Local dynamics
• VFT

Acknowledgments