Parameter-free predictions of the viscoelastic response of glassy polymers from non-affine lattice dynamics

Abstract

We study the viscoelastic response of amorphous polymers using theory and simulations. By accounting for internal stresses and considering instantaneous normal modes (INMs) within athermal non-affine theory, we make parameter-free predictions of the dynamic viscoelastic moduli obtained in coarse-grained simulations of polymer glasses at non-zero temperatures. The theoretical results show very good correspondence with rheology data collected from molecular dynamics simulations over five orders of magnitude in frequency, with some instabilities that accumulate in the low-frequency part on approach to the glass transition. These results provide evidence that the mechanical glass transition itself is continuous and thus represents a crossover rather than a true phase transition. The relatively sharp drop of the low-frequency storage modulus across the glass transition temperature can be explained mechanistically within the proposed theory: the proliferation of low-eigenfrequency vibrational excitations (boson peak and nearly-zero energy excitations) is directly responsible for the rapid growth of a negative non-affine contribution to the storage modulus.

In silicobroadband mechanical spectroscopy of amorphous tantala

“…More explicitly, we performed nonequilibrium molecular-dynamics (NEMD) simulations of glassy tantala created from the melt with different cooling rates. Following the protocol of mechanical spectroscopy adopted in experiments [60][61][62] and simulations [46,[63][64][65][66], the glass is subjected to periodic tensile elongation along one single direction perpendicular to a face with frequency f , and the storage and the dissipative parts of the dynamic elastic modulus, E ( f ) and E ( f ), respectively, are evaluated in the linearresponse regime (500 MHz f 1 THz), see Fig. 1.…”

Novel approach to promote the hydrophobic association: Introduction of short alkyl chains into hydrophobically associating polyelectrolytes

“…The dependences of the storage ( G ′) and loss ( G ″) moduli on the angular frequency are used to characterize the viscoelastic behavior. G ′ and G ″ reflect the elastic and viscous behaviors of a polymer solution, respectively . Viscoelastic experiments were carried out to investigate the relationship of G ′ and G ″ with the angular frequency in 2 g/dL polymer solutions.…”

Nonaffine lattice dynamics with the Ewald method reveals strongly nonaffine elasticity of α-quartz

“…The resulting framework is known as the nonaffine response theory or nonaffine lattice dynamics and has recently been applied to various systems and materials, from packings (where it recovers the ∼ (z − 2d) jamming scaling, with z the coordination number [3]) to polymers [4], and to analyze dissipation in high-frequency oscillatory rheology [5]. The framework also provides quantitative predictions of dynamic viscoelastic moduli of coarse-grained (Kremer-Grest) glassy polymers [6].…”