An Off-Lattice Flory-Huggins Approach of the Partitioning of Bulky Solutes between Polymers and Interacting Liquids (original) (raw)

2000, International Journal of Chemical Reactor Engineering

The desorption of additives and polymer residues from materials (P) is of significant concern for a wide range applications, including polymer ageing, offodors, safety of materials in contact with food or water. . . With diffusion coefficients, partition coefficients between P and hydrogen-bonding liquids (L), denoted Ki,L/P for a solute i, are fundamental quantities to assess the loss of plastics constituents. For a polymer with a crystallinity c, they are defined as ln(Ki,L/P) -ln(1-c) =(uexi,P-uexi,L)/(kBT), where {uexi,k}k=L,P are the excess chemical potentials and where kB is the Boltzmann constant. Our ambition was to relate calculations of {uexi,k}k=L,P at atomistic scale to measurable partition coefficients obtained for bulky solutes with different stiffness and shape, such as hindered phenolic antioxidants, n-alkanes and n-alcohols. For large solutes in dense and cohesive phases (P or L), promising computation techniques involve free energy perturbation and non-equilibrium methods, thermodynamic integration, extended ensembles (Gibbs or osmotic ensembles). Because the free energy landscape of the coupled i+k system contains several minima and highly heterogeneous barriers, a low convergence is generally achieved. To overcome such complications, we calculated {uexi,k}k=L,P at atomistic scale in the framework of the Flory-Huggins approximation. The main advantage is that the mixing energy for a wide range of conformers was derived from pairwise interactions and a continuous approximation of the packing of molecules without representing explicitly large molecular systems. The translational entropy in L was inferred from a mesoscopic representation of the liquid medium matching the radial distributions calculated by isothermal and isobaric molecular dynamics simulations. The corresponding predictions were satisfactory compared with 38 experimental Ki,L/P values between polyethylenes, polystyrene and different alcohols (isopropanol, ethanol).