In this Fickian diffusion regime, the diffusion coefficient is simply determined as the slope of the linear fit (black dashed lines) of the ε(t)/εmax curves as a function of t 1/2 /h0 using equation ( ). In this way, the diffusion coefficient of TMA in PMMA at 70°C is determined to be 1.8  0.1 x 10 -14 cm 2 /s, in perfect agreement with previous results in the literature. In the case of PLA the fitting of the ε(t)/εmax curves with equation ( ) is somehow questionable because the polymer films exhibit a complex swelling evolution. Nevertheless, the fitting of the initial stages of the swelling returns a rough indication of the effective diffusivity of TMA in PLA that is found to be  4.9  0.3 x 10 -13 cm 2 /s.