SYNTHESIS OF HBPS-PEO MULTI-ARM STAR POLYMER ELECTROLYTES AND THEIR IONIC CONDUCTIVITY

In order to obtain polymer electrolytes with high ionic conductivity at room temperature, multi-arm star polymers (HBPS-PEO) containing polyoxyethylene arms of different molecular weights (PEO750, PEO1000 and PEO1900) were successfully synthesized by click chemistry of hyperbranched polystyrene with azide groups (HBPS-N-3) and polyoxyethylene with an alkynyl group (ay-PEO). The structures of the obtained star polymers were characterized by ATR-FTIR,H-1-NMR, GPC, respectively. Ionic conductivities of polymer electrolytes composed of HBPS-PEO and lithium trifluoromethane-sulfonimide (LiTFSI) were investigated via electrochemical impedance spectroscopy. The influences of the arm length of the prepared star polymer, the content of lithium salt in polymer electrolyte and the structure of the polymer matrix on ionic conductivity were also studied. The results showed that star polymer electrolyte with arms of PEO1000 exhibited the highest ionic conductivity,while star polymer electrolyte with arms of PEO1900 had the lowest ionic conductivity, indicating the star polymer with shorter PEO arms possessing lower crystallinity was benefit for the transport of ions. The content of lithium salt also affected the ionic conductivity. The result suggested that ionic conductivity first increased and then decreased gradually with the change of the content of lithium salt. The maximum was achieved when n(EO)/n(Li) = 40. Besides, DSC results showed that the star-shaped structure of the polymer could inhibit the formation of crystals. Lower degree of crystallinity of star polymer and star polymer electrolyte than corresponding linear PEO was observed, and higher ionic conductivity at low temperatures was found for electrolyte based on star polymer than that of corresponding linear PEO electrolyte. The conductivity of the star polymer electrolyte was 6.7 x 10(-5) Scm(-1) at 30 degrees C, and could reach 1.2 x 10(-4) SCM-1 at 40 degrees C. TGA results showed that all of the prepared star polymers exhibited good thermal stability with T-onset above 360 degrees C. It is concluded that star-branched structure can inhibit the formation of crystals,which facilitates the improvement of ionic conductivity at low temperatures, and proper length of PEO chains is necessary to achieve high ionic conductivity.