Novel liquid crystalline hyperbranched polyesters with interior 4,7-diphenyl-2,1,3-benzothiadiazole units

Abstract

Novel hyperbranched (HB) polyesters with interior 4,7-diphenyl-2,1,3-benzothiadiazole (DBT) units were prepared by melt polycondensation of a dimethyl ester derivative of DBT (BMBT) (A₂ monomer) with 2-hydroxymethyl-2-methyl-1,3-propanediol (HMP) (B₃ monomer) at various mole ratios (A₂/B₃) via an A₂ + B₃ approach. The liquid crystalline (LC) and optical properties were investigated. FT-IR and ¹H NMR spectroscopies revealed that the HB polymers have good solubility in chloroform, and can be synthesized without gelation during the polymerization with a degree of branching (DB) of 36–94%. The branched structures of HB polymers are dependent on the feed mole ratio. The HB polymer prepared from a mole ratio of A₂/B₃ = 1/3 had the highest number average molecular weight (M_n) and a more linear structure than the other HB polymers (A₂/B₃ = 3/1, 2/1, 3/1 and 1/2), which had the lowest DB values. DSC measurements, polarizing microscopy observations and X-ray analyses suggested that the HB polymer (A₂/B₃ = 1/3) forms the smectic A phase, whereas the others formed the smectic C phase. UV-vis and photoluminescent (PL) spectra in the solutions and in films indicated that the HB polymers show maximum absorbances and yellow-light emission attributable to the DBT unit. The PL spectra of HB polymers in film showed another broad peak, which was likely to be due to interchain interactions or aggregations, at longer wavelengths. Relative quantum yields (Φ) of the HB polymers in the chloroform solutions were lower than those of HB polymers comprising the 2-phenylbenzothiazole unit. Polarized absorption and fluorescent spectra revealed that the HB polymers have poor orientation properties and the luminescent properties are independent of the orientation structures of HB polymers.