The objective of this work has been to investigate how molecules are organized in nematic polymers within the temperature range between melting of the solid phase and complete isotropization. The main experimental technique in this study has been broad-line proton NMR and the polymers used were a sequentially aperiodic aromatic-aliphatic copolymer and its isomeric periodic compound. The study included also calculations of conformational energy and molecular shape in seven-mer chemical sequences present in the polymers investigated. NMR measurements as a function of temperature on the aperiodic copolymer provided evidence for the segregation of chains by chemical composition in the nematic and isotropic regions of biphasic patterns observed at high temperatures. The more flexible chains of the system congregate in isotropic regions whereas nematic regions retain the more rigid ones. Experiments at lower temperatures where biphasic patterns are no longer observed by optical microscopy suggest that nematic polymers form orientationally ordered phases that tolerate isotropic motion in their molecular segments. We propose that these motions can occur through 'nematic-isotropic fluctuations' with spatial and temporal periodicities which depend on temperature and molecular structure. The physical origin of these fluctuations is not known but molecular weight distribution and the aperiodicity of chemical sequences could be important factors.
|Number of pages||9|
|Publication status||Published - Jan 1 1991|
ASJC Scopus subject areas
- Materials Chemistry