SYNTHESIS AND THEIR THERMOTROPIC LIQUID CRYSTALLINE BEHAVIOR OF CYCLOTRIPHOSPHAZENE BASED A3B3 MIKTOARM AZOBENZENE-CONTAINING STAR-SHAPED COPOLYMERS

A new type of A3B3 six-arm azobenzene-containing star-shaped copolymers based on cyclotriphosphazene was prepared by stepwise nucleophilic substitution of hexachlorocyclotriphosphazene, using the sodium salt of methoxy poly(ethylene glycol) (MPEG-Na) and the sodium salt of 4-methoxy-4'-(6-hydroxyhexyloxy)azobenzene (AZO-Na) in sequence. Structures of the resultant copolymers [NP(MPEG550)(AZO)](3) and [NP(MPEG1100)-(AZO)](3) were characterized by Fourier transform infrared spectroscopy (FT-IR) proton nuclear magnetic resonance spectroscopy (H-1-NMR) and gel permeation chromatography (GPC) Thermal properties of both six-arm azobenzene-containing copolymers were investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). DSC result showed that [NP(MPEG550) (AZO)](3) showed reversible thermotropic liquid crystalline (LC) behavior (TS-N = 60.5 degrees C; TN-I = 104.7 degrees C by the second heating DSC curve) and was an enantiotropic LC polymer. Nematic droplets texture was observed by POM, but smectic-to-nematic translation was not clear. This might be related to the complex LC phase transition process with the unique molecular structure of [NP ( MPEG550) (AZO)](3). It was not observed the thermotropic LC transition of [NP(MPEG1100) (AZO)](3) by DSC and POM. This might be the result of the difficulty of crystallization of azobenzene segment linked to the long and flexible PEG chains with big volume fraction in the [NP(MPEG1100) (AZO)](3). Thermotropic liquid crystalline behavior of such A3B3 polymers was mainly relevant to the chemical property and volume fraction of different arms.