Diffusion coefficients of variable size amphiphilic additives in a glass forming polyethylene matrix

CAMMARATA, MARÍA DEL MAR; CONTIN, MARIO DANIEL; NEGRI, RICARDO MARTÍN; FACTOROVICH, MATIAS

JOURNAL OF PHYSICAL CHEMISTRY B

American Chemical Society

Año: 2024 vol. 128 p. 312 - 328

1089-5647

Diffusion of additives in polymers is an important issue in the plastics industry since migratory type molecules are widely used to tune the properties of polymeric composites. Predicting the diffusional behavior of new additives can minimize the need for repetitive experiments. This work presents molecular dynamics simulations at the microsecond time scale and using the MARTINI force-field to estimate diffusion coefficients, D, of six mono-unsaturated amides and their analogues carboxylic acids in polyethylene matrices (PE). The diffusion results are deeply influenced by the glass forming properties of the polymer matrix, for which we identified three characteristic temperatures upon cooling. The beginning of the metastability region at T below 325K, the glass transition temperature (Tg = 256-260K) and the end of the glass transition at approximately 200 K. Diffusion mechanisms are inferred from the results of the dependence of D with the molecular mass of the additive at different fixed temperatures, observing a change from Rouse-like behaviour at high temperatures to a "tube-like" mechanism for T inside the metastability region of the matrix. Interestingly, our results show that for additives of equal size, the values of D are non-sensitive to the nature of the considered polar head. Regarding the diffusion dependence on temperature at fixed additive size the systems display a linear-Arrhenius behaviour at high temperatures, and super-Arrhenius trend at lower temperatures, this last region, is well represented with a power law equation as predicted by the Mode Coupling Theory (MCT). We find that the mobility of the matrix strongly influences the response of the Diffusion of the solutes on the temperature in both regimes, getting equal activation enthalpies for the Arrhenius region, or the same power law parameters for the super-Arrhenius regime. Finally, we conceptually explain the super-Arrhenius behavior in terms of Truhlar & Kohen interpretation of available initial at the transition state energies.