Coupling of Caged Molecule Dynamics to JG β-Relaxation II: Polymers

• DOI: 10.1021/acs.jpcb.5b07293
• Publication Date: 2015-08-28T13:10:39Z
• Authors: K. L. Ngai; S. Capaccioli; D. Prevosto; Li-Min Wang
• Keywords: Quasielastic neutron scattering; Quasielastic scattering; Atmospheric temperature range

At temperatures below the nominal glass transition temperature Tgα, structural α-relaxation and Johari-Goldstein (JG) β-relaxation are too slow to contribute susceptibility measured at frequencies higher than 1 GHz. This is particularly clear in neighborhood of secondary Tgβ, which can be obtained directly by positronium annihilation lifetime spectroscopy (PALS) adiabatic calorimetry, or deduced from JG time τβ reaches 1000 s. The fast process such high comes vibrations caged molecules dynamics manifested as nearly constant loss (NCL) measurements, elastic scattering intensity, I(Q, T), mean-square-displacement, ⟨u(2)(T)⟩, quasielastic neutron experiment. Remarkably, we find for many different glass-formers that NCL, I, ⟨u(2)⟩ glassy state changes its dependence THF near Tgβ. In paper I (Capaccioli, S.; et al. J. Phys. Chem. B 2015, 119 (28), 8800-8808) have made known this property case polyalcohols a pharmaceutical former, flufenamic acid studied THz dielectric spectroscopy, explained it coupling NCL β-relaxation, density these processes. II, extend consideration frequency response broader range 100 MHz polymers observed light scattering, Brillouin GHz-THz relaxation. all cases, T-dependence some temperature, THF, approximately same latter independently determined PALS, low mechanical spectroscopy. property, ≈ had not been pointed out before others any studies various amorphous van der Waals small molecular over past three decades. generality fundamental importance novel revitalize data previous publications, making necessary reckoned with attempt solve problem. our rationalization, arises first fact both depends on entropy. Second, terminator dynamics, hence two processes inseparable effectively coupled. Consequently, occurrence Tgβ necessarily accompanied corresponding change