Neutron scattering investigations on methyl group dynamics in polymers

Abstract

Among the different dynamical processes that take place in polymers, methyl group rotation is perhaps the simplest one, since all the relevant interactions on the methyl group can be condensed in an effective mean-field one-dimensional potential. Recent experimental neutron scattering results have stimulated a new revival of the interest on methyl group dynamics in glasses and polymer systems. The existence of quantum rotational tunnelling of methyl groups in polymers was expected for a long time but only very recently (1998), these processes have been directly observed by high-resolution neutron scattering techniques. This paper revises and summarizes the work done on this topic over last ten years by means of neutron scattering. It is shown that the results obtained in many chemically and structurally different polymers can be consistently described in the whole temperature range -from the quantum tunnelling limit to the classical hopping regime -as well as in the librational spectrum, in terms of the Rotation Rate Distribution Model (RRDM), which was first proposed in 1994. This model introduces a distribution of potential barriers for methyl group rotation, which is associated to the disorder present in any structural glass. The molecular and structural origin of the barrier distribution in polymers is discussed on the basis of a huge collection of investigations reported in the literature, including recent fully atomistic molecular dynamics simulations.

Recent progress on polymer dynamics by neutron scattering: From simple polymers to complex materials

“…This implies that the enhancement of conductivity is not due to an increased polymer mobility. The QENS spectra revealed a rotation in both filled and unfilled samples that would be consistent with rotation of (PEO) 6 :LiClO 4 , a channel-like structure that is more conductive than the amorphous equivalent. The authors suggested that the NP surface would stabilize the conductive (PEO) 6 :LiClO 4 structure, allowing them to persist long enough for conduction to occur.…”

“…The three main goals were (i) to study the local dynamics associated to small molecular groups (e.g., methyl groups, phenylene rings, etc; see, e.g., 22,[24][25][26][27] ), (ii) to characterize the dynamics at the interchain level, related to the structural relaxation in connection to the glass-transition phenomenon (see, e.g. ref.…”

“…These processes are usually localized and activated, and, due to the structural disorder in the system, broad barrier distributions are found. To this kind of dynamics belong, among others, vibrations including the Boson peak, 5 dynamics associated to side-groups like, for example, methyl group rotations or tunneling processes 6 and secondary relaxations like the Johari-Goldstein or -relaxation. 7 A "relaxation map" showing the temperature dependence of the characteristic timescales of the different processes in the case of 1,4-polyisoprene (PI) is included in Figure 1.…”

Introduction to Quasielastic Neutron Scattering

“…This glass transition is also observed in some liquid alloys and one of the questions to be answered is the role of the alloy's composition in the glass transition. In many binary or ternary melts the cooling rate needed to vitrify the systems is in the order of 10 6 Ks -1 , while in alloys like Pd 43 Ni 10 Cu 27 P 20 cooling rates less than 0.1 Ks -1 are sufficient to bring the alloy into a glassy state [28]. For this special alloy it was found, that the self-diffusivity close to the liquidus temperature as measured by QENS is about one order of magnitude less than for simple metallic liquids at their melting temperature, while on the other hand the diffusivity is larger as compared to good metallic glass formers.…”

“…Methyl group dynamics plays an important role in many polymer systems. In [43] this dynamical process is explained in great detail and provides a good starting point for the phenomenon of the so-called tunneling. In addition, the number of reports on experiments on micellar systems is increasing continuously.…”