Influence of the spacer and molecular weight on the phase behavior of side-chain liquid crystalline polymers containing triphenylene discotic mesogen units as side groups

Abstract

Two series of side chain liquid crystalline polymers (SCLCPs), poly{[3,6,7,10,11-pentakis(hexyloxy)-2-oxytriphenylene]methacrylate} (PMTS with no flexible spacer) and poly{6-[3,6,7,10,11-pentakis(hexyloxy)-2-oxytriphenylene]hexyl methacrylate} (PMT6S with 6 methylene units as a flexible spacer), with different molecular weights (M_n) and low molecular weight distributions have been successfully synthesized via atom transfer radical polymerization (ATRP). The phase behavior of the polymers has been investigated by a combination of techniques, including differential scanning calorimetry (DSC), polarized optical microscopy (POM), and 1D and 2D wide-angle X-ray diffraction (1D and 2D WXRD). The experimental results revealed that the LC phase structures of PMTS were found to be strongly M_n dependent. The PMTS polymers were amorphous when the M_n was lower than a critical M_n of approximately 2.03 × 10⁴ g mol⁻¹. When the M_n was higher than this critical value, the PMTS polymers displayed a rectangular (Φ_R) columnar phase. On the contrary, the PMT6S polymers displayed a higher symmetry hexagonal (Φ_H) columnar phase, which was independent of M_n. A comparison between PMTS and PMT6S suggested that the spacer plays an important role in the construction of liquid crystalline (LC) ordered structures. For the former, PMTS polymers without the spacer formed rectangular Φ_R phases caused by the Tp moieties and the main chain as a whole, due to the strong coupling effect between the Tp moieties and the main chain. For the latter, PMT6S polymers with 6 methylene units as a flexible spacer gave a higher symmetry Φ_H phase owing to the decoupling and self-organization of the Tp moieties.