Limitation observed in the application of the three dimensional solubility parameters to the coating formulation of poly (3-hydroxybutyrate-hydroxyvalerate) systems

Purpose: Poly (3-hydroxybutyrate-hydroxyvalerate) displayed high dipole-dipole interaction, a high hydrogen bonding but low polar interaction, and was therefore expected to be miscible with solvents/plasticizers that exhibit similar pattern of cohesive interaction. To determine the applicability, or otherwise of the theory of the three dimensional solubility parameters to the formulation of poly (3-hydroxybutyrate-hydroxyvalerate) polymeric coating system, and hence identify any limitation in the application of the theory. This aspect was investigated in the study. Method: The structural group contribution method was employed to compute the partial and total solubility parameters of the compounds – the biopol polymer, a series of organic solvents and plasticizers. The computed partial solubility parameters included: dipole-dipole (δd), polar (δp) and hydrogen bonding (δh). Following a standard procedure in the literatures, the δd and δp values were combined to form a composite solubility parameter, δv: where δv =sqrt{δd^2+δp^2}. A plot of δh versus δv gave the energy maps, which depicted the energy levels of the various compounds and from which the miscibility of the compounds were predicted. The closer the position of the solvent or plasticizer to the polymer in the map, the greater, the probability of mixing. Cast films of the various polymeric formulations were made and examined for homogeneity by scanning electron microscopy. Results: It was possible to select suitable plasticizers that were miscible with the polymer by applying theory of solubility parameters. The prediction for the solvents was, however, erroneous and this may be attributable to the inability of the δv parameter to clearly reflect the differences between the δd and δp interactions of the polymer on the one hand and those of the various solvents in all situations on the other hand. This means that in certain instances, the δv values of the polymer and the solvents were similar even though their δd and δp interactions were dissimilar. Conclusion: The analysis of the data showed that the composite solubility parameter δv of compounds could be similar even though the actual energies of δd and δp interactions are different. This is a limitation in the application of the theory of the three dimensional solubility parameters.