In polymer chemistry, the gel point is an abrupt change in the viscosity of a solution containing polymerizable components. At the gel point, a solution undergoes gelation, as reflected in a loss in fluidity. Gelation is characteristic of polymerizations that include crosslinkers that can form 2- or 3-dimensional networks. For example, the condensation of a dicarboxylic acid and a triol will give rise to a gel whereas the same dicarboxylic acid and a diol will not. The gel is often a small percentage of the mixture, even though it greatly influences the properties of the bulk.^[1]

Mathematical definition

An infinite polymer network appears at the gel point. Assuming that it is possible to measure the extent of reaction, ${\displaystyle p}$, defined as the fraction of monomers that appear in cross-links, the gel point can be determined.^[2] The critical extent of reaction ${\displaystyle p_{c}}$ for the gel point to be formed is given by:

${\displaystyle p_{c}={\frac {1}{N-1}}\approx {\frac {1}{N}}}$

For example, a polymer with N≈200 is able to reach the gel point with only 0.5% of monomers reacting. This shows the ease at which polymers are able to form infinite networks. The critical extent of reaction for gelation can be determined as a function of the properties of the monomer mixture, ${\displaystyle r}$, ${\displaystyle p}$, and ${\displaystyle f}$:^[3]

${\displaystyle p_{c}={\frac {1}{\sqrt {r+rp(f-2)}}}}$

See also

• Pour point
• Cold filter plugging point
• Petroleum

References

Further reading

• Rudin, Alfred; Choi, Phillip (2012). The Elements of Polymer Science and Engineering, 3rd Edition. Elsevier Science. p. 410. ISBN 978-0-12-382178-2.