Study of the correctness of the solubility parameters obtained from indirect methods by molecular dynamics simulation

Conformational energies of models of polyethylene (PE), isotactic-polypropylene (i-PP), atactic-polypropylene (a-PP) and polyisobutylene (PIB) with different chain lengths in their unperturbed liquid state (E-l), unperturbed isolated state (E-iu) and perturbed isolated state (E-ip) at 190 degreesC were computed using molecular dynamics simulation. The computed E-l, E-iu and E-ip were then used to calculate the solubility parameters of the polymers (delta). Our results indicate that there exists a cutoff chain length (similar to 20 backbone carbons) above which the differences between the solubility parameters (Deltadelta) computed based on (E-iu - E-l) and (E-ip - E-l), respectively, become significant. In addition, it was found that the higher the degree of shrinkage of the polymer in vacuum is, the larger the Deltadelta is. Since PE and PIB exhibited considerable shrinkage in vacuum, their Deltadelta are much higher than those of the PPs. It seems that, at 190 degreesC, vacuum acts more or less like a Theta solvent for both types of polypropylenes but a bad solvent for PE and PIB. Our results also suggest that the characteristic ratio of the polymer has little effect on Deltadelta and that it was the long range attractive interactions between the monomers distant along the backbone contour of the same molecule that led to the formation of fairly compact globule in the cases of PE and PIB. Since delta of the high molecular weight models computed from (E-iu - E-l) agree with experiment better than those from (E-ip - E-l), it implies that delta obtained from indirect measurements correspond to a hypothetical vaporization process where the conformations of polymer coils do not undergo significant changes. In other words, delta of a polymer that is determined by the currently available indirect methods may deviates significantly from their 'true' value if the polymer exhibits a high degree of swelling or shrinkage in vacuum at the temperature of the experiment. (C) 2004 Elsevier Ltd. All rights reserved.