| Abstract: | Calculations of dipole moments of poly(vinylidene fluoride)(PVDF) and its model compounds were performed by the MNDO method. 2,2-difluoropropane as a model compound was found to have a dipole moment of 8.97 × 10?30 C m (2.69 D). It was in satisfactory agreement with a previously obtained experimental value, 8.01 × 10?30 C m. Dipole moments of two other model compounds, tetrafluoropentane and octafluorononane, were calculated to be 1.70 and 3.24 × 10?29 C m, respectively. Ratios of repeat unit moments of the second and third compounds to the first compound moment were equal to 0.95 and 0.90, respectively. These were nearly identical with a theoretical ratio, 0.96, derived from the free rotation model of a polymer chain. The calculated dipole moments were considered to reflect the molecular structures in which free rotations of nearly tetrahedral bond angles might be allowed around C—C links. Dipole moments of each monomer unit for three polymorphs of PVDF, Form I, II, and III were calculated to be 7.64, 5.40, and 5.07 × 10?30 C m, respectively. Ratios of the three moments to the first model compound moment were found to be 0.85, 0.60, and 0.57. The decreasing order of the three factors suggests that orientations of monomer unit dipoles are more and more interdependent, and free internal rotations around skeletal bonds are more and more hindered, when the conformation varies from Form I to III. Also it was confirmed that the atomic charge distributions of the three polymorphs were very similar, and that the difference in dipole moments were primarily caused by conformation changes of the polymer chain. |
