Meaning
Chemical reaction rates governed by the physical mobility of molecules rather than the chemical activation energy describe a specific regime in polymer science. Diffusion controlled kinetics take over when the viscosity of the reacting medium increases to the point that reactive groups cannot find each other easily. This state occurs most frequently in the late stages of thermoset curing as the network density increases.
Mobility Constraint
Molecular transport becomes the primary bottleneck for conversion progress. As the network cross-links, diffusion controlled kinetics reduce the effective reaction rate below the levels predicted by standard Arrhenius equations. The transition from a chemically controlled to a diffusion controlled regime marks a shift in the governing physics of the process.
Glassy State
Vitrification induces a sudden drop in the rate of molecular motion within a thermosetting resin. When the glass transition temperature of the resin exceeds the current isothermal curing temperature, diffusion controlled kinetics become dominant. The reaction does not stop but proceeds at a pace several orders of magnitude slower than in the liquid phase.
This slow progression continues until the system reaches a frozen equilibrium.
Rate Deviation
Kinetic modeling requires a correction factor to account for the slowing of the reaction as the free volume decreases. Most models for diffusion controlled kinetics apply a Rabinowitch adjustment to the chemical rate constant. Failing to include this adjustment results in a notable overestimation of the final degree of cure.
Accurate predictions depend on knowing when the molecular motion begins to lag behind the chemical potential.
