Rheological studies on polymer networks with static and dynamic crosslinks

The three-dimensional structures of polymer networks are formed through two types of crosslinks: static and dynamic. Although the characteristics of crosslinks strongly influence the rheological properties of polymer networks, the understanding of the molecular origin of these properties remains incomplete owing to structural complexity. The relationship between the structure and dynamics of polymer networks and their rheological behavior can be elucidated based on multiple experimental methods. From this viewpoint, this review introduces rheological studies on static and dynamic polymer networks. Investigations of the nonlinear elastic behaviors of model static networks (Tetra gels) through general biaxial stretching revealed that a cross-effect of strains in different directions is present, even in networks without defects, which is not considered in conventional molecular models. Linear viscoelastic and spectroscopic studies on dynamic networks (host–guest gels and hydrophobically modified ethoxylated urethane) have shown that conventional molecular models are not suitable for describing rheological properties. Dynamic heterogeneities, such as the different mobilities of rotaxane-like crosslinks and micelles, strongly influence the rheological properties. These results demonstrate the remaining issues to be investigated to obtain a molecular understanding of the rheological properties of polymer networks.