Recasting the wobbling-in-a-cone model for the rotational anisotropy of phenylselenocyanate in poly(methyl methacrylate): Effect of internal bond rotation and polymer segmental motion.

This article focuses on refining the wobbling-in-a-cone (WIAC) model to better describe the rotational anisotropy of phenylselenocyanate (PSC) probes embedded in poly(methyl methacrylate) (PMMA) polymers. Using classical molecular dynamics simulations on PSC and its variants with different internal rotational barriers, as well as on p-chlorobenzonitrile (ClBZN) which lacks internal rotation, the study finds that the faster (~10 ps) anisotropy decay component arises from combined internal bond rotation of the –SeCN group and WIAC motion, while the slower (~200 ps) component is minimally influenced by polymer segmental motion and reflects wobbling within static free volume elements (FVEs) of the polymer matrix. The authors propose a modified WIAC model that separates contributions from inertial dynamics, internal bond rotation, and wobbling motion, enabling more accurate quantification of FVEs associated with different rotational timescales. The study highlights the importance of using vibrational probes without internal rotational freedom to isolate the true polymer free volume characteristics probed by long-time WIAC motion.