2016 Volume 65 Issue 8 Pages 598-605
The present study attempted to model dynamic viscoelastic properties of polyurethane block copolymers to understand the effect of molecular properties on shape change functions of the materials. For this purpose, we proposed dynamic viscoelastic simulations to obtain stress response in frequency domain against oscillating strain history, using molecular mechanics models consisting of hard and soft segments. The simulations employed coarse-grained bead-spring model based on the force fields given by bond-stretch potential for covalent bonding and Lennard-Jones potential for intermolecular interaction. The rigidity and aggregability of hard segments (HS) were represented by varying the molecular mechanics parameters for these force fields. The proposed simulation could reproduce a clear glass transition from glassy state to rubbery state in frequency domain. The rigidity of HS increased the stiffness at glassy state and tanδ as an indicator of energy dissipation as well, which led to the sharpening of glass transition range. The aggregability of HS increased the stiffness at rubbery state and it also shifted glass transition point to a higher frequency and broadened glass transition range by changing the mobility of soft segments (SS). Finally, a spherically aggregated HS domain was found to degrade shape change functions from the viewpoint of storage modulus. We concluded that the shape change functions of the materials were closely related to energy storage ability of HS and mobility of SS, and that the aggregability of HS yielded by hydrogen bonding in polyurethanes plays an important role in shape change functions.
