The Molecular Structure Of Polyacrylonitrile

As basic knowledge, it is very important to reveal the conformational nature of PAN for predicting the properties of PAN either in its solid state or in its solution state. But until now the conformational structure of PAN is still uncertain. Therefore our job is to study the structure of PAN and make it clear.According to the Flory's theory of molecular conformation, we, using molecular mechanical method, compute the conformational energies of pentane, dimethylpentane, dicyanopentane in the DREIDING FORCE FIELD. The model compound, dicyanopentane, includes meso- and racemic-dicyanopentane, which are isotactic and syndiotactic dimmer model compounds for PAN. It is the same to dimethylpentane. And we draw their energy contours and meshs by MATLAB software, finding that the conformational changes of dimethylpentane and dicyanopentane are very similar. The meso-2,4-dicyanopentane has the same minimum energy conformation tg- or g+t as the meso-2,4-dimethylpentane.But racemic-2,4-dicyanopentane has only one minimum energy conformation tt whose energy is just a little higher than the conformation g+g+, while racemic-2,4-dimethylpentane has two minimum energy conformations tt and g+g-. And the minimum energy of racemic-2,4-dicyanopentane and meso-2,4-dicyanopentane are lower than the energy of racemic-2,4-dimethylpentane and meso-2,4-dimethylpentane in the same conformation. So we conclude that there are electrostatistic interaction between the groups of -CN which decrease the conformational energy to make tt, tg- and g+t conformation more stable. But when the distance between the -CN groups is a little far, the electrostatistic will become small and the conformational energy of dicyanopentane will higher than that of dimethylpentane which has the same conformation.And we also compute the minimum energy conformations of tricyanoheptane and trimethylheptane. The minimum energy conformation of mm-2,4,6- tricyanoheptane is tg-tg- which is the same to mm-2,4,6-trimethylheptane. And the minimum energy conformation of rr-2,4,6-tricyanoheptane is tttt which is the same tomm-2,4,6-trimethylheptane. But the conformation g g g'g" is not the minimum energy conformation of mm-2,4,6-trimethylheptane which is the different from the result of dyads. And the energy of conformation g+g+g-g- of rr-2,4,6-tricyanoheptane is still a little higher than that of conformation tttt. Therefore we draw a conclusion that dyads could represent polymer well, respectively. But they could not represent the polymer thoroughly.Finally, the NMR spectra of 2,4-DCP(2,4-dicyanopentane) are measured. The results are in accord with the results we compute.Therefore we propose that pure isotactic PAN, the chain conformation would adopt the nearly, but not exactly, 3i helical structure; While assuming pure syndiotactic PAN, the chain would adopt roughly zigzag conformation.