Structural phase transformation in tetrakis(4,4′-(2,2-diphenyl-vinyl)-1,1′-biphenyl)methane

An irreversible structural phase transformation is observed in tetrakis(4,4′-(2,2-diphenyl-vinyl)-1,1′-biphenyl)methane, C(DPVBi)4, upon increasing the temperature. The PI phase of the as-synthesized material is crystalline with tiny (typically 10–20 nm) crystallites. The PI phase has hexagonal symmetry (space group P6̄) with the following lattice parameters a=2.068 nm and c=2.194 nm. When PI is annealed above its melting point (Tm=260 °C), the structure transforms irreversibly into a different hexagonal phase, PII, (space group P6/m) with lattice parameters a=2.102 nm, c=3.370 nm. The PII phase melts at around Tm=278 °C. When heated above 278 °C, C(DPVBi)4 becomes amorphous with random molecular packing. The amorphous phase has a glass transition temperature, Tg, around 138 °C. The PI phase transforms directly into the amorphous phase if the sample is continuously heated above the melting point (Tm=278 °C) of PII.