Conformational and dynamical properties of high performance polymers designed for novel applications

Official URL: http://risc01.sabanciuniv.edu/record=b1063531 (Table of Contents)

The conformational and dynamical properties of newly designed high performance polymers that may be utilized in a variety of industrial applications, are studied. In the first part, molecular dynamics simulations under different conditions and Rotational Isomeric States calculations are performed to understand the local and global conformational properties of the bacterial polyester, poly(3-hydroxyundecanoate). The rotational isomeric state model is incorporated with Monte Carlo simulations to calculate the dimensions and the characteristic ratio of the infinitely long chain. These calculations, performed in vacuum, predict a Gaussian distribution for the end-to-end vector with a non-zero mean and a value of 5.5 for the characteristic ratio. Molecular dynamics simulations predict the characteristic ratio of the single chain in good solvent, and of chains in the bulk state as 23 and 18, respectively. The role of temperature on the overall dimensions and on the distribution of dihedral angles is discussed. Radii of gyration and helix formation propensities at different temperatures and in different media are compared. The chain in solution is found to have the largest persistence length with the highest helical persistence. Results are compared with experimental and theoretical studies conducted on poly(3-hydroxybutyrate), which has the same backbone structure, but a different type of side-chain. In the second part, novel block co-oligomers are designed as candidate surfactants in near supercritical CO2 environment, with the CO2 . phobic block consisting of ethyl propionate and 10 different types of ethylene monomers, flanked on either side by eight repeat unit fluorinated CO2 . philic blocks. Single chain molecular dynamics simulations are performed to understand their conformational and dynamic properties. Depending on the side chain type, the CO2 . phobic blocks are prone to shrinkage in the CO2 environment, while the CO2 . philic blocks preserve their vacuum dimensions. The overall chains form U-shaped planar structures with flapping motion of the fluorinated arms; thus, we expect bilayer micelle formation under these conditions. The origin of the CO2 . oligomer interactions are investigated and van der Waals interactions are found to dominate over electrostatic interactions in the CO2 environment. Calculations of the radial distribution function for the solvent molecules around the oligomer backbone show a solvation shell around 5 - 6 Å, irrespective of the oligomer type; density of the solvent around the oligomer, on the other hand, varies with type of side chain due to the interactions between the CO2 molecules and the oligomer, and the available volume around the side chain. The local chain dynamics are investigated by orientational autocorrelation functions, and the characteristic time of the relaxation of selected C-H and C-F bonds is found to depend on the local friction experienced by the fluctuating atoms and the energy barrier that needs to be surmounted during the relaxation process. The simple exponential decay of the correlation functions for the C-H bond is common for all oligomer types, whereas the stretched exponents take on smaller values depending on the side chain for the C-F bond vector, implicating that the fluorinated blocks are exposed to more complicated dynamical processes.