Dynamics of propylene glycol and its 7-mer by neutron scattering

Abstract

The dynamics of propylene glycol (PG) and its 7-mer have been investigated by quasielastic neutron scattering (QENS). In the case of PG we used two techniques; ordinary QENS and neutron spin-echo (NSE). The QENS and NSE experiments were carried out at the temperature ranges 300–420 K and 260–340 K, respectively. The QENS results on both the monomer and the 7-mer showed three clear dynamical processes; a weakly temperature dependent fast local motion of hydrogens in the polymeric backbone, an almost temperature independent rotational motion of the methyl groups, and a strongly temperature dependent translational process. This latter motion exhibits a clear quasi-elastic broadening proportional to Q2 at high temperatures (T⩾380 K). At lower temperatures the diffusion becomes more difficult to characterize due to the contribution from the methyl group rotation. However, despite this difficulty it is no doubt about that the diffusion is faster for the 7-mers than for the monomers at low temperatures (T⩽340 K). The reason for this anomalous behavior may be that the OH end groups of the monomers are linked together to a network at lower temperatures, which would slow down the translational diffusion. Another possible explanation can be that the dynamics of the 7-mers is dominated by segmental motions, which might be faster than the translational diffusion of single monomers at low temperatures. The NSE data on deuterated PG were described by the Kohlrausch–Williams–Watts (KWW) stretched exponential function, with a temperature independent stetching parameter βKWW. The reason for the stretched behavior of the coherent intermediate scattering function I(Q,t) is most likely that several dynamical processes, such as the primary α-relaxation and secondary β-relaxations, are observed in the experimental time window (3–1500 ps), in accordance with the QENS results given above. An interesting observation is that the dynamics is slowest and the stetching parameter βKWW is largest for Q=1.4 Å−1, corresponding to the position of the first diffraction peak in the static structure factor S(Q), a phenomenon known as de Gennes narrowing.