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 dis ubstituted and fully deuterated diacetylene 2,4-hexadiynylene bis(p-to luenesulfonate) (pTS-D) in the monocrystalline polymer state. This com pound may undergo a progressive polymerization in the crystalline stat e, which gives a quasiunique opportunity to obtain large size monocrys tals of conjugated polymer. A second-order-like transition takes place at approximately T(c) = 182 K; it is associated to motions of the pol ar 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 appr oaching T(c). In addition the growth of a central peak is observed abo ve T(c). We show that this structural instability presents an intermed iate character between order disorder and displacive, with a more pron ounced displacive character than in the monomer crystal.