| The thesis includes there parts:I. Denpending on ab initio calculations, the conformational properties of poly (vinylidene chloride) (PVDC) have been investigated. The following is the main results:The geometries and conformational energies of model segments for PVDC are determined at MP2 level using a 6-311++g** basis set. Vibriational frequences are calculated in order to verify the stationary points at the same level. All quantum chemistry calculations are performed with the quantum chemistry package Gaussian 03. By fully optimizing the original molecule geometries, we obtained 4 stationary conformers for 2, 2, 4, 4-tetrachloropentane (TCP) and 13 stationary conformers for 2, 2, 4, 4, 6, 6-hexachloroheptane (HCH). Other conformers having higher energies are not stable. All the conformational energies are given with respect to the lowest energy conformation of each model molecule. For PVDC, the energies of trans (t) conformation are higher than gauche (g) conformation due to the electronstatic repulsion basing on quantum chemisty calculations. Additionally, the strong electronic effects of -CH2- group influenced by Cl atom makes the conformer unstable. The center dihedral angles deviate from the standard values in order to alleviate the steric and electrostatic repulsion, which make the model segement become a helix.II. According to the results of quantum chemistry calculations, the conformational populations and characteristic ratio ( C∞) for poly (vinylidene chloride) have been studies based on the rotational isomer state theory.Recently, with the development of electronic structure theory, the studies on the rotational isomerc state (RIS) model attract the extensive attention again. In past, because of the restriction of the computer and electronic structure theory, it is popular to study conformational energies and geometry properties of model molecules by molecular mechanics (MM) calculations. But the MM calculations do not explicitly treat the electrons in a molecular system. Therefore, the MM method can not accurately describe the molecule systems where the electronic effects are predominant. In order to better quantify the conformational energies and geometry properties, many researchers try to employ ab initio electronic structure (quantum chemistry) calculations on the model segment of polymer where electronic effects are important. In this part, the geometry properties and conformational energies of model molecules TCP and HCH for PVDC are used to construct a rotational isomeric state model. According to RIS theory, The conformational population and C∞can be determined using the rotational isomeric state model. Furthermore, we analyze the influences of the statistical weight parameters on C∞. The calculated characteristic ratio is in agreement with available experimental values. The following is the main results:Based on the quantum chemistry calculations, we have obtained the average of C-C skeleton bond length and C-C-C skeleton valence angle of model molecules for PVDC, which will be used in RIS model. Because of the C-C-C-C skeleton torsion angles deviate generally from the standard values, a six-order statistical weight matrix is required for PVDC according to RIS theory.By means of the geometry properties and conformational energies of stationary conformers of TCP and HCH, we can obtain the interaction energies and then get the statistical weight parameters. Fist, we represent the RIS energies of the stationary conformers using interaction energies. Through the comparison between the RIS representation energies and the quantum calculation energies of stationary conformers, the interaction energies can be determined. The interaction energies obtained basing on quantum calculations show that steric and electrostatic repulsion are both important in PVDC. Generally, MM method can not treat the electrostatic interacitons.According to the interaction energies, we can obtain statistical weight parameters and then construct the six-order statistical weight matrices for PVDC. Using the statistical weight matrices, we have calculated C∞and the conformational populations for PVDC with–CH2- center or -CCl2- centers, respectively. The values of C∞are in reasonable agreement with theoretical values.Additionally, it is worth to calculate the values of ?C∞?lnw, where w denotes the parameter that is being varied, and all the other parameters keep constant values. According to the values of ?C∞?lnw, it can be seen that Cl-CH2 pentane interaction and Cl-Cl long range interaction are important in second-order interactions. However, the influences of first-second order interactions are all small.III. The phase behaviors of bimodal molecular weight high density polyethylene (BHDPE) have been studies by molecular dynamics simulations. Polyethylene has been widely used and studies. In this part, BHDPE has been chosen as molecular models. The blends composed with a series of the branch polyethylene with different branch content and linear polyethylene have been studies by molecular dynamics and mesodyn simulations. The main results are shown as following.Hildebrand solubility parameters (δ) are calculated for the models of higher molecular weight branch polyethylene (HBPE) with different branch content and lower molecular weight linear polyethylene (LLPE), respectively, by using molecular dynamics simulations. Theseδvalues are then used to calculate the corresponding Flory-Huggins interaction parameter (χ) between HBPE and LLPE models. The influences of branch content on the HBPE and LLPE blends are studied according toχobtained above.In order to better understand the influences of compositions on the compatibility between LLPE and HBPE, Mesodyn simulations are used to give the density profiles of the blends at different compositions. The results indicate that the phase behavior of BHDPE is influenced by both the global branch content of the system and the local branch content, i.e., the branch content of HBPE. |
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