2,583 research outputs found
Modifying Fragility and Collective Motion in Polymer Melts with Nanoparticles
We investigate the impact of nanoparticles (NP) on the fragility and
cooperative string-like motion in a model glass-forming polymer melt by
molecular dynamics simulation. The NP cause significant changes to both the
fragility and the average length of string-like motion, where the effect
depends on the NP-polymer interaction and the NP concentration. We interpret
these changes via the Adam-Gibbs (AG) theory, assuming the strings can be
identified with the "cooperatively rearranging regions" of AG. Our findings
indicate fragility is primarily a measure of the temperature dependence of the
cooperativity of molecular motion.Comment: To appear in Physical Review Letter
A quantitative theoretical model of the boson peak based on stringlet excitations
The boson peak (BP), a low-energy excess in the vibrational density of states
over the phonon Debye contribution, is usually identified as one of the
distinguishing features between ordered crystals and amorphous solid materials.
Despite decades of efforts, its microscopic origin still remains a mystery and
a consensus on its theoretical derivation has not yet been achieved. Recently,
it has been proposed, and corroborated with simulations, that the BP might stem
from intrinsic localized modes which involve string-like excitations
("stringlets") having a one-dimensional (1D) nature. In this work, we build on
a theoretical framework originally proposed by Lund that describes the
localized modes as 1D vibrating strings, but we specify the stringlet size
distribution to be exponential, as observed in independent simulation studies.
We show that a generalization of this framework provides an analytically
prediction for the BP frequency in the temperature regime well
below the glass transition temperature in both 2D and 3D amorphous systems. The
final result involves no free parameters and is in quantitative agreement with
prior simulation observations. Additionally, this stringlet theory of the BP
naturally reproduces the softening of the BP frequency upon heating and offers
an analytical explanation for the experimentally observed scaling with the
shear modulus in the glass state and changes in this scaling in cooled liquids.
Finally, the theoretical analysis highlights the existence of a strong damping
for the stringlet modes at finite temperature which leads to a large
low-frequency contribution to the 3D vibrational density of states, as observed
in both experiments and simulations
Parallel Emergence of Rigidity and Collective Motion in a Family of Simulated Glass-Forming Polymer Fluids
The emergence of the solid state in glass-forming materials upon cooling is
accompanied by changes in both thermodynamic and viscoelastic properties and by
a precipitous drop in fluidity. Here, we investigate changes in basic elastic
properties upon cooling in a family of simulated polymer fluids, as
characterized by a number of stiffness measures. We show that
can be expressed quantitatively both in terms of measures of the material
``stiffness'', and , and the extent of
cooperative particle exchange motion in the form of strings, establishing a
direct relation between the growth of emergent elasticity and collective
motion. Moreover, the macroscopic stiffness parameters, , , and , can all be expressed quantitatively in terms of the molecular scale
stiffness parameter, with
being Boltzmann's constant, and we discuss the thermodynamic
scaling of these properties. We also find that is related to the cohesive
energy density , pointing to the critical importance of
attractive interactions in the elasticity and dynamics of glass-forming
liquids. Finally, we discuss fluctuations in the local stiffness parameter as a
quantitative measure of elastic heterogeneity and their significance for
understanding both the linear and nonlinear elastic properties of glassy
materials.Comment: 69 pages, 18 figure
- β¦