DMA−FTIR Creep−Recovery Study of a Poly(ester urethane) Elastomer with Molecular-Level Viscoelastic Modeling

The combination of a research-grade DMA (dynamic mechanical analyzer) and an FTIR (Fourier transform infrared) spectrometer is demonstrated in simultaneous mechanical analysis and dynamic IR spectral measurement. In such a study, molecular-level responses to the deformation can be observed in the dynamic IR spectra and used to better understand the macroscopic viscoelastic behavior of the polymer sample characterized by monitoring the stress and strain. The measurements are performed on polymer films, with the DMA in the tensile geometry and IR measurement in the transmission mode. The DMA−FTIR technique is demonstrated in the creep−recovery study of an industrially important elastomer, Estane 5703. Differential orientation of various segments of the macromolecule during the creep and recovery process is observed. In a novel molecular-level application, Burger's model, a viscoelastic model that is often used to explain the bulk properties of materials, is applied to the analysis of the orientation of individual infrared dipoles. By replacing strain with orientation functions, contributions from separate molecular moieties to the macroscopic elasticity and viscosity are differentiated. Permanent damage observed after a large displacement is attributed to the irreversible alteration of the microscopic network structure of the elastomer and is discussed in light of IR spectral changes during the creep−recovery process.