Determination of refractive index increment of synthetic polybutadienes and microstructural control of grafting density and liquid crystalline properties†
Polybutadienes (PBs) with microstructural control, i.e., 1,2-olefins ranging from 8% to 94% (mole fractions), were readily synthesized using living anionic polymerization (LAP) by varying the polar additives and their mole ratios over BuLi. Herein, the determination of refractive index increment (dn/dc) and weight-average molecular weight (Mw) of PBs using gel permeation chromatography equipped with differential refractive index (RI) and low angle laser light scattering (LALLS) detectors (GPC-RI/LALLS) is shown, and as expected, the values of dn/dc have a significant dependence on 1,2-olefins of microstructures which is relevant to the glass transition temperature (Tg) of PBs. These PB backbones were subsequently used as precursors for the synthesis of PB-based liquid crystal polymers (LCPs) (PB-M1 and PB-M2) with a microstructural control of grafting densities of 15%, 30% and 50% (moles) after the longitudinal attachment of azobenzene moieties (Azo-M1 and Azo-M2) along PBs. More importantly, the unchanged grafting density (almost 50%) with increasing 1,2-olefin fractions was observed for both PB-M1 and PB-M2 (≥50% 1,2-olefins), which explained the 1,2-olefin microstructural control in terms of phase transitions (mesogenic formation) regardless of whether Azo-M1 or Azo-M2 was used as a side attachment, while DSC curves using different processing temperatures and X-ray analysis with a domain spacing shift over temperature demonstrate a much more variable mesogenic phase and wider ΔT values of PB-M2 compared to PB-M1.