UV and Raman study of thermochromic phase transition in poly(di-n-hexylgermane)

The thermochromic phase transition of poly(di-n-hexylgermane) (1) was reinvestgated using low-molecular-weight (lmw) and high-molecular-weight (hmw) polymer samples. The equilibrium phase transition temperature Tc of 1, determined by variable temperature Raman and UV spectroscopy as well as by DSC, was found to lie not in the interval from +15 to −10°C, as was reported previously, but in the region 45–55 °C, that is, very close to Tc of the Si and Sn analogues. Thus, the phase behaviour of the polymetallanes [nHex2M]n, M=Si, Ge, Sn is governed not by the nature of the main chain but by intermolecular interactions of the substituent groups. The nature of the order–disorder phase transition of 1 depends on the polymer molecular weight and the sample thermal history. Above Tc, hmw polymer 1 exists as hexagonal columnar mesophase while lmw1 is in amorphous state, both with disordered backbone. On cooling to room temperature, hmw polymer transforms to a crystalline modification with an all-anti conformation of the backbone and ordered hexyl groups, whereas the process of ordering of the main chain for lmw sample is more complicated.