Effect of the linking structure of nonlinear optical side groups on the phase behavior of an aromatic polyester backbone

The phase behavior of poly(p-phenylene terephthalate)s (PPT) with pendant side groups, N-(4-nitrophenyl)-ethylaminoethanol (NPE) and N-(4-nitrophenyl)-L- prolinol (NPP) has been studied by using differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and second harmonic generation (SHG). PPT-NPE showed a layered liquid crystalline morphology while PPT-NPP showed a completely amorphous structure. Comprehensive or shear stress applied on the polymer melt surface at 210 °C induced a more prominent layered structure of PPT-NPE whereas the amorphous structure of PPT-NPP remained unchanged under the stress. In order to understand this phase difference in terms of the repeat structure, we attempted theoretical ab initio Hartree-Fock and DFT calculations for the monomers and molecular dynamics for the bulk state. The result indicated that molecular configurations are a good way of microspically understanding the phases of rigid backbone polymers with functional side groups: The NPT (constant particle number, pressure, and temperature) simulation data at 210 °C agree qualitatively with the experimental data and the difference PPT-NPE and PPT-NPP could be understood using rotational energy barrier, steric hindrance and inter-chain interaction. X-ray diffractometer (XRD) simulation patterns for the oligomers are also in qualitative agreement with the experimental WAXS data and the structural parameters of stacks of PPT-NPE chains are estimated to be layer distance (4.6 Å), backbone distance (21.5 Å), and side distance (12 Å).