Modeling elastic properties of polystyrene through coarse-grained molecular dynamics simulations

This paper presents an extended coarse-grained investigation of the elastic properties of polystyrene. In particular, we employ the well-known MARTINI force field and its modifications to perform extended molecular dynamics simulations at the $\mu$s timescale, which take slow relaxation processes of polystyrene into account, such that the simulations permit analyzing the bulk modulus, the shear modulus, and the Poisson ratio. We show that through the iterative modification of MARTINI force field parameters it turns out to be possible to affect the shear modulus and the bulk modulus of the system, making them closer to those values reported in the experiment.Comment: 29 pages, 8 figure

Anisotropic relaxation in NADH excited states studied by polarization-modulation pump-probe transient spectroscopy

We present the results of experimental and theoretical studies of fast anisotropic relaxation and rotational diffusion in the first electron excited state of biological coenzyme NADH in water-ethanol solutions. The experiments have been carried out by means of a novel polarization-modulation transient method and fluorescence polarization spectroscopy. For interpretation of the experimental results a model of the anisotropic relaxation in terms of scalar and vector properties of transition dipole moments and based on the Born-Oppenheimer approximation has been developed. The results obtained suggest that the dynamics of anisotropic rovibronic relaxation in NADH under excitation with 100~fs pump laser pulses can be characterised by a single vibration relaxation time $\tau_v$ laying in the range 2--15~ps and a single rotation diffusion time $\tau_r$ laying in the range 100--450~ps a subject of ethanol concentration. The dependence of the times $\tau_v$ and $\tau_r$ on the solution polarity (static permittivity) and viscosity has been determined and analyzed. Limiting values of an important parameter $\langle P_2(\cos\theta(t))\rangle$ describing the rotation of the transition dipole moment in the course of vibrational relaxation has been determined from experiment as function of the ethanol concentration and analyzed.Comment: 14 pages, 13 figure

Probe Beam Dichroism and Birefringence in Stumulated Raman Scattering of Biologically Relevant Polyatomic Molecules

The dichroism and birefringence effects in stumulated Raman scattering (SRS) in polyatomic molecules were studied theoretically. General expressions have been derived describing the change of the intensity and polarization of the probe pulse after transmission through a solution of arbitrary polyatomic molecules for any initial polarization of each of the laser pulses. The expressions were written in terms of spherical tensor operators that allowed for separation of the both beams polarization matrices and the material part containing three scalar values of nonlinear optical susceptibility where that rank K is limited to the values K=0,1,2. The expressions are valid for arbitrary directions of propagation of both pump and Stokes light beams and arbitrary polarizations of both beams. The expressions contain contributions from linear dichroism and birefringence in the molecular excited states. It was shown that, under certain conditions, both effects can be observed simultaneously. The geometry of almost collinear propagation of the pump and Stokes pulse beams through the molecular sample was considered, and it was shown that the contributions from linear dichroism and birefringence to the signal can be completely separated in the experiment by means of an appropriate choice of a probe beam polarization analyzer placed in front of the photodetector. The expressions obtained were used to describe the signals obtained using the polarization-modulation technique developed recently by the authors (Gorbunova et al, Phys. Chem. Chem. Phys. 2020, Vol. 22, 18155-18168). It was shown that the modulated dichroism and birefringence signals could be observed in quadrature to the second harmonic of the modulated reference signal.Comment: 11 pages, 2 figure

Exaltation et anticipation du crossover de Ioffe-Regel dans des copostes amorphe/nanocristallins

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Linear and Nonlinear Elastic Properties of Polystyrene-Based Nanocomposites with Allotropic Carbon Fillers and Binary Mixtures

We report measurements of linear and nonlinear elastic properties of polystyrene-based nanocomposites with six types of nanofillers, including single and binary mixtures of allotropic carbon nanoparticles. Composite samples were fabricated by the same technology and contained the same filler concentration (5% wt.), which allowed for a direct comparison of their properties. It was shown that the most significant variations of linear and nonlinear elastic properties occur in different nanocomposites. In particular, the most pronounced enhancements of linear elastic moduli (in about 50%) obtained in tensile and flexural tests and in dynamic mechanical analysis were recorded in the sample filled with spherical fullerene nanoparticles. While the most profound rise of absolute values of nonlinear elastic moduli (tens of times) was obtained in the sample filled with the mixture of carbon nanotubes and graphene. The observed tendencies demonstrated the synergistic effect of fillers of different dimensionality on the elastic properties of nanocomposites