Dispersion characteristics and rheology of organoclay nanocomposites based on a segmented main-chain liquid-crystalline polymer having pendent pyridyl group

The dispersion characteristics and rheology of organoclay nanocomposites based on a thermotropic liquid-crystalline polymer (TLCP) having pendent functional group were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), and oscillatory shear rheometry. For the study, a segmented main-chain TLCP having pendent pyridyl group (PyHQ12) was synthesized and, for comparison, another segmented main-chain TLCP having pendent phenylsulfonyl group (PSHQ12) was synthesized, both TLCPs having the identical chemical structure for the main-chain backbone. The differences in phase transition temperature and rheological behavior observed between PyHQ12 and PSHQ12 are explained in terms of the self-associating characteristics of the pendent pyridyl group in PyHQ12. PyHQ12 and PSHQ12 were used to prepare nanocomposites with two commercial organoclays: one (Cloisite 30B) was natural clay (montmorillonite) treated with a surfactant (MT2EtOH) having hydroxyl groups, and the other (Cloisite 20A) was montmorillonite treated with a surfactant (2M2HT) having no polar group. The following observations have been made. (i) PyHQ12/Cloisite 30B nanocomposite has featureless XRD patterns and a very high degree of dispersion of Cloisite 30B aggregates as determined from TEM. (ii) PyHQ12/Cloisite 20A nanocomposite has a conspicuous XRD reflection peak giving rise to an increase of gallery distance by 0.3 nm from the gallery distance of 3.1 nm for Cloisite 20A and intercalation of Cloisite 20A aggregates as determined from TEM. (iii) PSHQ12/Cloisite30B nanocomposite also has a conspicuous XRD reflection peak, giving rise to an increase of gallery distance by 0.2 nm from the gallery distance of 1.9 nm for Cloisite 30B and intercalation of Cloisite 30B aggregates as determined from TEM. The observed highly dispersed Cloisite 30B aggregates in PyHQ12/Cloisite 30B nanocomposite are attributed to the formation of hydrogen bonds, as determined by Fourier transform infrared spectroscopy, between the pendent pyridyl group in PyHQ12 and the hydroxyl groups in the surfactant MT2EtOH residing at the surface of Cloisite 30B. Oscillatory shear measurements of the three nanocomposites support the observations made from XRD and TEM.