Kinetics and time-temperature equivalence of polymer degradation.

We studied the hydrolysis kinetics of amorphous polylactide. It was found the hydrolysis rate had a slow-to-fast transition at a certain molecular weight (Mn). This transition was not correlated with the mass loss and water uptake of samples, nor the pH values of testing media. We speculated that this transition was due to the slow diffusion of polymer chain ends. The chain ends did not significantly promote the hydrolysis of samples until their concentrations (∼1/Mn) reached a critical value. The degradation tests were also conducted over a temperature range from 37 to 90 °C. A time−temperature equivalent relationship of degradation processes was established and a master curve spanning a time range equivalent to 3−5 years at 37 °C was constructed. This master curve can be used to predict polymer degradation processes based on accelerated tests. The functional time and disappearance time of degradable polymers were also discussed.