Revisiting the Ultrafast IR Spectroscopic Study of Free Volume Elements in Polymers: The Role of Probe Molecule's Internal Rotational Fluctuation in Anisotropy Decays

Free volume in amorphous polymer materials is a crucial factor that dictates their macroscopic properties. By probing the orientational relaxation dynamics of an infrared probe embedded in the polymer matrix, the free volume element size and its probability distribution can be derived. Two distinct timescales of anisotropy decay of the nitrile probe in phenyl selenocyanate at ~10 and ~200 ps were assigned to two different wobbling-in-a-cone (WIAC) motions of the probe. In this study, using ultrafast infrared pump-probe (IR-PP) spectroscopy and molecular dynamics (MD) simulation, we investigate the rotational dynamics of phenyl selenocyanate (PSC), 2-methylphenyl selenocyanate (MPSC), and 2,2-dimethylphenyl selenocyanate (DMPSC) embedded in poly (methyl methacrylate) (PMMA), poly (2-phenoxyethyl acrylate) (PPEA) and poly (2-methoxyethyl acrylate) (PMEA) polymers to unravel the role of the internal rotational fluctuation of the nitrile group around the C(Ph)-Se(CN) bond vector on the rotational anisotropy as well as their biocompatible properties. We show that with increasing the steric effect by one or two methyl groups substituted at the ortho positions of the phenyl ring, the short-time rotational anisotropy dynamics of the nitrile group becomes 5-6 times slower in MPSC (28.7 ± 1.4) and DMPSC (31.9 ± 1.8) compared to that in PSC (4.8 ± 0.5 ps), and the corresponding amplitude decreases. These experimental observations indicate that the faster (~10 ps) rotational anisotropy component of phenyl selenocyanate in polymers originates from the internal rotational fluctuation of a nitrile group instead of a WIAC motion. The simulated dihedral fluctuation time-correlation functions of PMMA show that its segmental motions are substantially slower than the long-time component of rotational anisotropy by a factor of ~30-40, suggesting that the slow anisotropy decay component (~200 ps) reflects the static free volume element sizes in these polymers. This approach further reveals a larger free volume element size of 4.90 ± 0.04 Å for PPEA and 5.07 ± 0.04 Å in PMEA, having a longer sidechain length than that of PMMA (3.3 ± 0.03 Å), which could potentially impact the water permeability and hence biocompatibility of the respective polymers. Overall, the combined approach of IR pump-probe experiments, the discrete wavelet transform analysis method, and MD simulations employed in this study hold significant promise for investigating FVEs in various polymeric materials for potential biomedical applications.