Methyl Group Dynamics in Glassy Polyisoprene: A Neutron Backscattering Investigation

Abstract

Energy-resolved, elastic neutron backscattering was employed to investigate the methyl group dynamics in polyisoprene between T = 2 K and room temperature. The use of partially deuterated samples (PI-d6, PI-d3, and PI-ds) and of a fully protonated sample (Pl-hg) allowed the separation of the dynamics arising from the methyl group and from the backbone. A two-step relaxation is observed and attributed to the methyl group rotation at low temperatures and to the main-chain relaxation close to the glass transition.An Arrhenius-like increase of the methyl group rotational correlation time r = ro exp(EtcJkT), with E^Jk = 1550 K ~12 kJ/mol and 0 ~1/ 0 = 23.5 meV (r0 ~1.76 X 10-13 s) describes well the midposition of the first elastic intensity decrease but not its breadth. A 3-fold jump model with a broad Gaussian distribution of activation energies (dE/E ~25%) around 1500 K can account for the observed temperature decrease. Inconstancies in the Q-dependence might be due to disorder effects. The torsional mode of the methyl group rotation is directly observed at = 23.5 meV by time-of-flight. Near the glass transition temperature a further decrease of the elastic scattering is observed due to the onset of a fast dynamics of the backbone in the picosecond range.

Anharmonic transitions in nearly dry l-cysteine I

“…Figure 1 shows the X-ray powder diffraction pattern of the studied sample. The Rietveld refinement indicated that the predominant phase is orthorhombic (P 2 1 2 1 2 1 space group) with unit cell parameters a = 8.09758 (8) findings are consistent with others previously reported in the literature [13,14]. Figure 2 shows the Raman spectra of L-cysteine I at 30 and 300 K. A clear energy threshold at 500 cm −1 (60 meV) could be observed.…”

“…The microscopic nature of this transition is still an object of debate. The discussed possibilities include quantum effects near a zero point vibration [7], and methyl group rotation associated with anharmonicity [2,8]. Vibrational modes play a very important role in the conformational arrangement of different functional states and also in the energetic balance of the chemical bonds [9].…”

Neutron scattering investigations on methyl group dynamics in polymers

“…In the seminal works of Refs. [25] and [26], QENS data for methyl group hopping in polymers were successfully interpreted by introducing a broad distribution of rotational barriers, resulting from the highly disordered nature of the glassy state. In last years, a noticeable amount of experiments in different systems have supported this picture (see Sections V and VI).…”

Application de la diffusion inélastique aux systèmes désordonnés

“…Dans le même temps, on fait varier un paramètre physique extérieur, en général la température, parfois la pression. L'apparition d'un mouvement dont la dynamique se situe dans la fenêtre expérimentale se manifeste par un élargissement quasiélastique et conduit à une baisse de l'intensité élastique détectée [30,31]. Cette technique permet une inspection rapide de la dynamique de l'échantillon dans un vaste domaine de température.…”

“…On observe également une différence notable entre les deux pentes, ce qui s'explique par l'évolution exacte de la composante quasiélastique avec la température et par la nature plus complexe du terme <u 2 (T) > à haute température. Cette analyse est décite en détails dans les références [30,31] 0. z 'o 820 9'…”