Effect of Nanopore Geometry in the Conformation and Vibrational Dynamics of a Highly Confined Molecular Glass
Haonan Wang, Kenneth L. Kearns, Aixi Zhang, Ahmad Arabi Shamsabadi, Yi Jin, Aaron Bond, Steven M. Hurney, Carlos Morillo, and Zahra Fakhraai*
Nano Lett. 2021, 21, 4, 1778–1784
The effect of nanoporous confinement on the glass transition temperature (Tg) strongly depends on the type of porous media. Here, we study the molecular origins of this effect in a molecular glass, N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD), highly confined in concave and convex geometries. When confined in controlled pore glass (CPG) with convex pores, TPD’s vibrational spectra remained unchanged and two Tg’s were observed, consistent with previous studies. In contrast, when confined in silica nanoparticle packings with concave pores, the vibrational peaks were shifted due to more planar conformations and Tg increased, as the pore size was decreased. The strong Tg increases in concave pores indicate significantly slower relaxation dynamics compared to CPG. Given TPD’s weak interaction with silica, these effects are entropic in nature and are due to conformational changes at molecular level. The results highlight the role of intramolecular degrees of freedom in the glass transition, which have not been extensively explored.
nanoconfinement, pore geometry, molecular conformation, glass transition, Raman spectroscopy, organic glass