Conformational analysis and molecular modeling of cholesteric liquid crystal polyesters based on XRD, Raman, and transition thermal analysis

Molecular modeling of the cholesteric liquid crystal polyester poly[oxy(1,2 -dodecane)oxycarbonyl-1,4-phenyleneoxycarbonyl-1,4-phenylene-carbonyloxy-1, 4-phenylenecarbonyl] (PTOBDME), [C34H36O8](n), synthesized in our laborator y and thermally characterized by differential scanning calorimetry (DSC), w as performed to explain both its cholesteric mesophase and 3D crystalline s tructure. Conformational analysis (CA) was run for the monomer both by syst ematic search and with molecular dynamics (MD) simulations. Minima energy c onformers were "polymerized" with Cerius(2) and helical, cholesteric molecu les were obtained in all cases. Our models agree with the chiral behavior o bserved by X-ray diffraction (XRD), thermooptical analysis (TOA) and circul ar dichroism (CD) experiments. Crystal packing of the polymer molecules wer e simulated in cells with parameters a and b obtained from experimental pow der X-ray diffraction patterns and c calculated from the translational repe titive unit during the theoretical polymerization. Recalculated X-ray powde r diffraction patterns of our models matched the observed ones. Morphology simulation from those crystal models is in good agreement with the crystals observed by optical microscopy. We have also modeled the self-associating nature of those polyesters when dispersed in aqueous media. Simulation of o ur models surrounded by different solvents, such as water and chloroform, w ere performed by calculating their interaction energies, coordination numbe rs, and mixing energies, applying Monte Carlo simulation techniques based o n the Flory-Huggins theory. These results were compared with their experime ntal vibrational Fourier transform (FT)-Raman spectra in the regions in whi ch structural marker bands of the polymer appear.