gelpunkt

Gelpunkt, or gel point, is the stage in a polymerizing or crosslinking system at which an infinite, macroscopic network first forms. At this point the material transitions from a viscous liquid or sol into a gel, a solid-like phase that does not flow freely on long timescales. The gel point marks a fundamental change in the connectivity of the system and is a key reference in understanding the mechanical and transport properties of the resulting polymer network.

Mechanism and theory: Gelation results from the formation of a continuous network through reactions of multifunctional monomers. As the reaction proceeds, clusters grow and eventually percolate across the sample, creating a macroscopic network. In Flory–Stockmayer theory, the gel point depends on the average functionality of the reacting species and is influenced by cyclization and concentration. In ideal systems, a critical extent of reaction or conversion p_c exists at which an infinite network appears; deviations from ideality can raise or lower p_c.

Detection and criteria: The gel point can be identified rheologically or chemically. A common rheological criterion is the crossing of the storage modulus G′ and the loss modulus G″, or a sustained tan δ around unity, indicating balanced elastic and viscous responses. At the gel point, both moduli often follow a power-law frequency dependence, reflecting critical dynamics. Alternative methods include swelling measurements and network fraction analysis.

Practical context: The gel point is important in polymer design and manufacturing, affecting processing, curing, and final properties of coatings, adhesives, elastomers, and hydrogels. Controlling the gel point allows tuning of workability, solvent content, and mechanical performance.

See also: gelation, crosslinking, percolation theory, Flory–Stockmayer theory, rheology.