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Molecular dynamics study of the linear viscoelastic shear and bulk relaxation moduli of poly(tetramethylene oxide) (PTMO)

Here we report the linear viscoelastic properties of amorphous poly(tetramethylene oxide) (PTMO), which is one of the key components in synthesizing segmented polyurethane (PU) elastomers. The temperature and molecular weight dependent viscoelastic behavior is investigated in detail by computing the shear relaxation modulus G(t) and the bulk relaxation modulus K(t), using the Green-Kubo relationship with correlation function. Our results provide new data for PTMO melt from the united atom model and also extend the existing knowledge of viscoelastic properties of polymers in general. The predicted viscoelastic behavior range is shifted on a master curve using the time-temperature superposition principle (TTSP) with horizontal and vertical shift factors. The emerging shift factors agree with the Williams-Landel-Ferry (WLF) equation. For the validation of the united-atom model of PTMO using the TraPPE-UA force field we explored the transport properties and observed a position-dependent diffusion dynamics throughout the polymer chain, which subsequently influences the scaling laws for chain dynamics. These findings are discussed in terms of emerging experimental evidence on position dependent displacement for different chain portions along the chain length.