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How does the cooling rate affect the glass transition temperature of a material?

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The glass transition temperature ( $T_{g}$ ) is not a fixed point but rather shifts in response to the rate at which a material is cooled. Specifically, $T_{g}$ shifts downward slightly when the cooling rate is reduced.1

This phenomenon occurs because of the steep temperature dependence of the molecular response time. As a material cools, its molecules rearrange to maintain equilibrium. When the cooling rate is high, the material reaches a state where the molecular response time becomes significantly longer than the experimental timeframe, causing the atomic configuration to "freeze" into a solid state at a higher temperature.1

Conversely, when the cooling rate is slower, the material remains in equilibrium for a longer period as it cools, allowing the structure to continue adjusting until it reaches a lower temperature before the molecular mobility effectively disappears. Consequently, slower cooling results in a lower $T_{g}$ and a denser glass, as the material has more time to contract before the structure becomes rigid.12