Dynamics of the phase transition of the fully polymerized and deuterated diacetylene 2,4-hexadiynylene bis(p-toluenesulfonate)

Coherent neutron scattering and Raman scattering with IR sources were used to make a dynamical study of the structural transition of the disubstituted and fully deuterated diacetylene 2,4-hexadiynylene bis(p-toluenesulfonate) (pTS-D) in the monocrystalline polymer state. This compound may undergo a progressive polymerization in the crystalline state, which gives a quasiunique opportunity to obtain large size monocrystals of conjugated polymer. A second-order-like transition takes place at {~}Tc=182 K; it is associated to motions of the polar side groups. We achieved a dynamical study of a soft mode which is clearly observed in the high-temperature phase (neutron) as in the low-temperature phase (Raman) and is characterized by overdamping on approaching Tc. In addition the growth of a central peak is observed above Tc. We show that this structural instability presents an intermediate character between order disorder and displacive, with a more pronounced displacive character than in the monomer crystal.