Origin of the crossover in dynamics of the sub-Rouse modes at the same temperature as the structural α-relaxation in polymers

Abstract

The nature of the liquid-glass transition remains an unsolved fundamental problem. One aspect is the striking change in dynamics of the structural α-relaxation generally observed at a temperature T(B) above Tg in all glass-formers. More intriguing in the case of polymers is that the change of dynamics occurs not only in the structural α-relaxation but also in the sub-Rouse modes, i.e. chain modes in between the α-relaxation and the Rouse modes. However, the nature of the dynamic crossover of the sub-Rouse modes remains unclear. In this paper, the dynamics of a series of poly(n-alkyl methacrylates) with different molecular weights and microstructures studied by mechanical spectroscopy are reported. We demonstrate that the sub-Rouse modes exhibit a similar crossover of dynamics at the same T(B) as the α-relaxation. This property shared by the two viscoelastic mechanisms is remarkable. By invoking the results from the studies using positron annihilation spectroscopy and adiabatic calorimetry, we show that both viscoelastic mechanisms are coupled to density, correlated with the change of the configuration entropy, and are intermolecularly cooperative. The time scale of sub-Rouse modes at T(B) of the polymers studied is approximately independent of molecular weight and micro-structure of the polymers studied. The findings enhance the understanding of the sub-Rouse modes and their manifestation in the viscoelasticity of polymers in the glass-rubber transition region.

A perspective on experimental findings and theoretical explanations of novel dynamics at free surface and in freestanding thin films of polystyrene

“…Observed in the 1984 paper is that the mean characteristic times of the two processes approach each other at low temperatures and high pressures, which is the behavior exhibited by the sub-Rouse modes and the segmental relaxation as we now know in several polymers. The explanation of this thermorheological complex behavior was given two years later in 1986 [38], and the explanation is based on n sR being smaller than n  , consistent with papers published in the the next decade [6,22,28,31,33,36]. The creep compliance data [17] of PMPS with MW=5000 g/mol enable the determination of the range of compliance contributed by the segmental -relaxation, which is given by = 5.39 × 10 −10 Pa −1 ≤̂( ) ≤ ≈ 2.82 × 10 −9 Pa −1 .…”

“…Experiments in bulk polymers have shown that the sub-Rouse modes are also cooperative but to a lower degree than the segmental -relaxation [17,22,[28][29][30][31][32][33], and have smaller bulk coupling parameter, n sR , than n  of the segmental -relaxation. Like the segmental -relaxation, the sub-Rouse modes are also intermolecularly cooperative, albeit to the lesser degree and with a smaller coupling parameter , than , .…”

Ordering kinetics in two-dimensional hexagonal pattern of cylinder-forming PS- b -PMMA block copolymer thin films: Dependence on the segregation strength

“…[15] In this context, a value above ≈ 1.2•TG sets the threshold where the inverse relationship between diffusivity and viscosity holds. [56][57][58][59] In conclusion, reported data demonstrate connections between the BCP equilibrium state and the ordering kinetic of the system. In this framework, a model system (PS-b-PMMA) along with a suitable process (SARTA) emerge as a platform for fundamental studies where a collection of various universal behavior can be simultaneously scrutinized.…”

Opposite Effects of SiO2 Nanoparticles on the Local α and Larger-Scale α’ Segmental Relaxation Dynamics of PMMA Nanocomposites

“…Generally, it could be well fitted by the Vogel–Fulcher–Tamman (VFT) equation [49,50,51]: τ=τ0exp[1αf(T−T0)] where τ 0 is the pre-exponential factor, α f is the thermal expansion coefficient of the fractional free volume, and T 0 is the critical temperature (or “Vogel temperature”) at which τ diverges. To reduce the uncertainty of VFT equation fitting in the limited frequency range, according to previous works, the values of ln τ 0 of the α process, sub-Rouse modes, and Rouse modes are assumed about to be −14, −10, and −6.9 s, respectively [52,53]. As shown in Figure 10, the temperature dependences of relaxation time τ for different relaxation modes are well-described by a single VFT equation.…”