ABSTRACT
Whether a material behaves as viscous or elastic depends on the properties of its intermolecular bonds, notably their strengths and lifetimes relative to the strength and duration of the applied force. It is not surprising that many materials behave partly viscous and partly elastic. Such materials are referred to as viscoelastic. Interactions between structural elements in soft condensed matter such as concentrated dispersions, coagulated and flocculated colloidal particles, and particle and polymer gels often give rise to viscoelastic
behavior. By way of example, the chains of the polymer molecules in semidilute and concentrated solutions (see Sections 12.4 and 12.7) are able to slide along each other but the chains may become entangled to form temporary crosslinks. A certain amount of time is needed for the disentanglement allowing the material to flow. Therefore, the rheological behavior of the system depends on the timescale of the measurement, that is, on the duration of the applied force.
