Tricritical points in bimodal polymer solutions

The tricritical point of polymer solutions composed by two polymer homologs of different molecular weight in a solvent is studied using the single-chain mean-field theory. The tricritical point is found for a ratio of molecular weights of the two polymers r=N₁/N₂, which decreases as a function of the short chain length N₂ from 51 to approximately 40 for N₂ increasing from 1 to 8. This last value is still much larger than the experimentally measured value for mixtures of polystyrene in cyclohexane, where it was found that r≃25. Although a further reduction of r as a function of N₂ seems likely, the chain lengths of the corresponding long chains are beyond the reliability limit of the SAW's simulation involved. The addition of a small effective repulsion between the polymer homologs, is found to bring down the predicted value to 25.