Spectral Density Function Analysis Reveals Coupled Relaxation and Resonance Modes in Fluorinated Elastomers: Comparison With Semicrystalline Poly(tetrafluoroethylene)

We reveal contrasting behaviors in molecular motion between the two materials, including the identification of resonance-enhanced dynamic features in elastomers. We present a depth-resolved analysis of molecular dynamics in semicrystalline polytetrafluoroethylene (PTFE) and fully amorphous fluorinated elastomer (SIFEL) films using static-gradient solid-state ¹⁹F NMR imaging. By measuring spin-lattice relaxation rates ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>R</mml:mi> <mml:mn>1</mml:mn></mml:msub> </mml:math> ) at multiple frequencies and evaluating the corresponding spectral density functions, we reveal distinct dynamic behaviors between the two materials. PTFE exhibits pronounced depth dependence in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>R</mml:mi> <mml:mn>1</mml:mn></mml:msub> </mml:math> , indicating enhanced molecular motion near the surface due to a structurally disordered amorphous layer. In contrast, the fluorinated elastomer shows spatially uniform <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>R</mml:mi> <mml:mn>1</mml:mn></mml:msub> </mml:math> values, reflecting its homogeneous molecular mobility. Notably, the elastomer's spectral density function contains resonance-like peaks at finite frequencies, suggesting the presence of intrinsic vibrational modes superimposed on stochastic motion. This hybrid dynamic signature, captured through nuclear magnetic resonance (NMR) relaxation, offers a unique fingerprint of the elastomer's viscoelastic behavior. Our results demonstrate that static-gradient NMR imaging can probe subtle spatial variations in polymer dynamics noninvasively and with high sensitivity, enabling direct comparison between crystalline and amorphous systems. The findings provide new insights into nanoscale surface dynamics and contribute to the development of advanced materials with tailored thermomechanical properties.