Semidilute

Semidilute refers to a concentration regime in polymer solutions where polymer chains overlap and interpenetrate, but the solution dynamics are not yet dominated by entanglements. It lies above the overlap concentration c*, signaling the onset of inter-chain contact, and below concentrations where entanglements control viscoelastic response (the entangled semidilute regime or higher).

In semidilute solutions, a characteristic length scale called the correlation length or mesh size, ξ, emerges. ξ signifies the size of a region over which monomer concentration fluctuations are correlated and decreases as concentration increases. A useful conceptual picture is the blob model: each chain can be viewed as a sequence of blobs of size ξ, within which statistics resemble those of a dilute solution, while at larger scales the chain samples many blobs connected in a network.

Physical properties change markedly with concentration in the semidilute regime. Viscosity increases with concentration, often strongly, and the relaxation times of macromolecules lengthen. The diffusion of polymer chains slows, and the linear viscoelastic response shows characteristic dependence on frequency that reflects the blob-like structure of the solution. The exact behavior depends on solvent quality (good solvent versus theta conditions), temperature, and whether the system is in the unentangled or entangled sub-regime.

Applications and relevance include polymer rheology, coatings, and processing, where semidilute solutions are common. The concept is central to scaling theories of polymer solutions and provides a framework for understanding how concentration governs structure and dynamics in crowded macromolecular environments.