Diffusivity of alkanes in polystyrene

Gravimetry is used to study the diffusivity of a homologous series of linear alkanes (Cn, with n=8, 10, 12,14 and 16) in amorphous polystyrene at temperatures ranging from 45 °C to 145 °C, i.e. both below and above the polymer glass transition temperature (100 °C). All the mass uptake results obtained are well described by a simple Fickian model (for t<t1/2) and are used to calculate the corresponding diffusion coefficients (D) using the thin-film approximation of the Fickian equation. For all the alkanes considered, the temperature dependency of the diffusion coefficients is non-Arrhenius in character, over the broad temperature intervals considered. For any particular temperature log(D) varies linearly (R2>0.993, for all temperatures) with respect to the number of carbon atoms (n) in the alkyl chain, log(D) decreasing when n increases. For each liquid penetrant, over the temperature intervals considered, its log (D) also increases linearly (R2>0.996 for all the systems) with the decrease in the penetrant’s liquid viscosity.Gabriel Bernardo acknowledges financial support from the IPC's (Institute for Polymers and Composites) strategic project: "PEst-C/CTM/LA0025/2011" (Projecto Estrategico -LA 25 - 2011-2012 - Strategic Project - LA 25 - 2011-2012)