Calculation of the lifetime of positronium in polymers via molecular dynamics simulations

Multicomponent Quantum Mechanics/Molecular Mechanics Study of Hydrated Positronium

We propose a model for solvated positronium (Ps) atoms in water, based on the sequential quantum mechanics/molecular mechanics (s-QM/MM) protocol. We developed a Lennard-Jones force field to account for Ps-water interactions in the MM step. The repulsive term was obtained from a previously reported model for the solvated electron, while the dispersion constant was derived from the Slater-Kirkwood formula. The force field was employed in classical Monte Carlo (MC) simulations to generate Ps-solvent configurations in the NpT ensemble, while the quantum properties were computed with the any-particle molecular orbital method in the subsequent QM step. Our approach is general, as it can be applied to other liquids and materials. One basically needs to describe the solvated electron in the environment of interest to obtain the Ps solvation model. The thermodynamical properties computed from the MC simulations point out similarities between the solvation of Ps and noble gas atoms, hydrophobic solutes that form clathrate structures. We performed convergence tests for the QM step, with particular attention to the choice of basis set and expansion centers for the positronic and electronic subsystems. Our largest model was composed of the Ps atom and 22 water molecules in the QM region, corresponding to the first solvation shell, surrounded by 128 molecules described as point charges. The mean electronic and positronic vertical detachment energies were (4.73 ± 0.04) eV and (5.33 ± 0.04) eV, respectively. The latter estimates were computed with Koopmans' theorem corrected by second-order self-energies, for a set of statistically uncorrelated MC configurations. While the Hartree-Fock wave functions do not properly account for the annihilation rates, they were useful for numerical tests, pointing out that annihilation is more sensitive to the choice of basis sets and expansion centers than the detachment energies. We further explored a model with reduced solute cavity size by changing the Ps-solvent force field. Although the pick-off annihilation lifetimes were affected by the cavity size, essentially the same conclusions were drawn from both models.

Free Volume Element Sizes and Dynamics in Polystyrene and Poly(methyl methacrylate) Measured with Ultrafast Infrared Spectroscopy

The size, size distribution, dynamics, and electrostatic properties of free volume elements (FVEs) in polystyrene (PS) and poly(methyl methacrylate) (PMMA) were investigated using the Restricted Orientation Anisotropy Method (ROAM), an ultrafast infrared spectroscopic technique. The restricted orientational dynamics of a vibrational probe embedded in the polymer matrix provides detailed information on FVE sizes and their probability distribution. The probe's orientational dynamics vary as a function of its frequency within the inhomogeneously broadened vibrational absorption spectrum. By characterizing the degree of orientational restriction at different probe frequencies, FVE radii and their probability distribution were determined. PS has larger FVEs and a broader FVE size distribution than PMMA. The average FVE radii in PS and PMMA are 3.4 and 3.0 Å, respectively. The FVE radius probability distribution shows that the PS distribution is non-Gaussian, with a tail to larger radii, whereas in PMMA, the distribution is closer to Gaussian. FVE structural dynamics, previously unavailable through other techniques, occur on a ∼150 ps time scale in both polymers. The dynamics involve FVE shape fluctuations which, on average, conserve the FVE size. FVE radii were associated with corresponding electric field strengths through the first-order vibrational Stark effect of the CN stretch of the vibrational probe, phenyl selenocyanate (PhSeCN). PMMA displayed unique measured FVE radii for each electric field strength. By contrast, PS showed that, while larger radii correspond to unique and relatively weak electric fields, the smallest measured radii map onto a broad distribution of strong electric fields.

From Mesoscale Back to Atomistic Models: A Fast Reverse-Mapping Procedure for Vinyl Polymer Chains

This paper introduces a systematic procedure to obtain well-relaxed atomistic melt structures from mesocale models of vinyl polymers based on sequences of diads. Following the methodology introduced by Milano and Müller-Plathe [J. Phys. Chem. B. 2005, 109, 18609], coarse-grain models consisting of sequences of superatoms of two different types meso and racemo have been used to relax mesocale melts of atactic and syndiotactic polystyrene. The proposed method, based on a fully geometrical approach, does not involve expensive potential energy and force evaluations and allows a very fast and efficient reconstruction of the atomistic detail. The method, successfully tested against experimental data, allows us to obtain all atom models of both stereoregular and stereoirregular polymers and opens the possibility of relaxing large molecular weight melts of vinyl chains.

Mapping Atomistic Simulations to Mesoscopic Models: A Systematic Coarse-Graining Procedure for Vinyl Polymer Chains

The paper introduces a systematic procedure to coarse-grain atomistic models of the largest family of synthetic polymers into a mesoscopic model that is able to keep detailed information about chain stereosequences. The mesoscopic model consists of sequences of superatoms centered on methylene carbons of two different types according to the kind of diad (m or r) they belong to. The corresponding force-field contains three different bonds, six angle and three nonbonded terms. Recently developed analytical potentials, based on sums of Gaussians for bond and angle terms of the mesoscale force field have been used. For the nonbonded part, numerical potentials optimized by pressure-corrected iterative Boltzmann inversion have been used. As test case we coarse-grained an atomistic all-atom model of atactic polystyrene. The proposed mesoscale model has been successfully tested against structural and dynamical properties for different chain lengths and opens the possibility of relaxing melts of high molecular weight vinyl polymers.

Positronium lifetime in polymers

A model describing the relationship between the orthopositronium lifetime and the volume of a void, located in a synthetic zeolite, is analyzed. Our idea, which allows us to take into account the effects of temperature, comprises the introduction of a non-Hermitian term in the Hamiltonian, which accounts for the annihilation of the orthopositronium. The predictions of the present model are also confronted against an already known experimental result.

The distribution of the unoccupied volume in glassy polymers

The aim of this paper is to present an improved method to describe the unoccupied volume in glassy polymers. The method is able to treat atoms with non-spherical symmetry. The fineness of the raster points scanning the unit cell can be as small as 0.01 nm. This method was used to determine the unoccupied volume of molecular dynamic simulations of poly(amide imide) unit cells by probing it with a tracer atom. Since the reachable unoccupied volume strongly depends on the tracer radius, we used tracer radii between 0.03 and 0.17 nm. The poly(amide imide)s were used in the present work because they are well characterized, especially there already exist free volume data determined by positron lifetime experiments.