Empirical potential structure refinement of semi-crystalline polymer systems: polytetrafluoroethylene and polychlorotrifluoroethylene

Modeling the Structure of Ketoprofen-Poly(vinylpyrrolidone) Amorphous Solid Dispersions with Empirical Potential Structure Refinements of X-ray Scattering Data

The metastability of amorphous formulations poses barriers to their safe and widespread commercialization. The propensity of amorphous solid dispersions (ASDs) to crystallize is directly linked to their molecular structure. Amorphous structures are inherently complex and thus difficult to fully characterize by experiments, which makes structural simulations an attractive route for investigating which structural characteristics correlate with ASD stability. In this study, we use empirical potential structure refinement (EPSR) to create molecular models of ketoprofen-poly(vinylpyrrolidone) (KTP/PVP) ASDs with 0-75 wt % drug loading. The EPSR technique uses X-ray total scattering measurements as constraints, yielding models that are consistent with the X-ray data. We perform several simulations to assess the sensitivity of the EPSR approach to input parameters such as intramolecular bond rotations, PVP molecule length, and PVP tacticity. Even at low drug loading (25 wt %), ∼40% of KTP molecules participate in KTP-KTP hydrogen bonding. The extent of KTP-PVP hydrogen bonding does not decrease significantly at higher drug loadings. However, the models' relative uncertainties are too large to conclude whether ASDs' lower stabilities at high drug loadings are due to changes in drug-excipient hydrogen bonding or a decrease in steric hindrance of KTP molecules. This study illustrates how EPSR, combined with total scattering measurements, can be a powerful tool for investigating structural characteristics in amorphous formulations and developing ASDs with improved stability.

Structural Analysis of Molecular Materials Using the Pair Distribution Function

This is a review of atomic pair distribution function (PDF) analysis as applied to the study of molecular materials. The PDF method is a powerful approach to study short- and intermediate-range order in materials on the nanoscale. It may be obtained from total scattering measurements using X-rays, neutrons, or electrons, and it provides structural details when defects, disorder, or structural ambiguities obscure their elucidation directly in reciprocal space. While its uses in the study of inorganic crystals, glasses, and nanomaterials have been recently highlighted, significant progress has also been made in its application to molecular materials such as carbons, pharmaceuticals, polymers, liquids, coordination compounds, composites, and more. Here, an overview of applications toward a wide variety of molecular compounds (organic and inorganic) and systems with molecular components is presented. We then present pedagogical descriptions and tips for further implementation. Successful utilization of the method requires an interdisciplinary consolidation of material preparation, high quality scattering experimentation, data processing, model formulation, and attentive scrutiny of the results. It is hoped that this article will provide a useful reference to practitioners for PDF applications in a wide realm of molecular sciences, and help new practitioners to get started with this technique.

On the Solubility and Stability of Polyvinylidene Fluoride

This literature review covers the solubility and processability of fluoropolymer polyvinylidine fluoride (PVDF). Fluoropolymers consist of a carbon backbone chain with multiple connected C-F bonds; they are typically nonreactive and nontoxic and have good thermal stability. Their processing, recycling and reuse are rapidly becoming more important to the circular economy as fluoropolymers find widespread application in diverse sectors including construction, automotive engineering and electronics. The partially fluorinated polymer PVDF is in strong demand in all of these areas; in addition to its desirable inertness, which is typical of most fluoropolymers, it also has a high dielectric constant and can be ferroelectric in some of its crystal phases. However, processing and reusing PVDF is a challenging task, and this is partly due to its limited solubility. This review begins with a discussion on the useful properties and applications of PVDF, followed by a discussion on the known solvents and diluents of PVDF and how it can be formed into membranes. Finally, we explore the limitations of PVDF's chemical and thermal stability, with a discussion on conditions under which it can degrade. Our aim is to provide a condensed overview that will be of use to both chemists and engineers who need to work with PVDF.

The Use of Scattering Data in the Study of the Molecular Organisation of Polymers in the Non-Crystalline State

Scattering data for polymers in the non-crystalline state, i.e., the glassy state or the molten state, may appear to contain little information. In this work, we review recent developments in the use of scattering data to evaluate in a quantitative manner the molecular organization of such polymer systems. The focus is on the local structure of chain segments, on the details of the chain conformation and on the imprint the inherent chemical connectivity has on this structure. We show the value of tightly coupling the scattering data to atomistic-level computer models. We show how quantitative information about the details of the chain conformation can be obtained directly using a model built from definitions of relatively few parameters. We show how scattering data may be supplemented with data from specific deuteration sites and used to obtain information hidden in the data. Finally, we show how we can exploit the reverse Monte Carlo approach to use the data to drive the convergence of the scattering calculated from a 3d atomistic-level model with the experimental data. We highlight the importance of the quality of the scattering data and the value in using broad Q scattering data obtained using neutrons. We illustrate these various methods with results drawn from a diverse range of polymers.

Using In-Situ Laboratory and Synchrotron-Based X-ray Diffraction for Lithium-Ion Batteries Characterization: A Review on Recent Developments

Renewable technologies, and in particular the electric vehicle revolution, have generated tremendous pressure for the improvement of lithium ion battery performance. To meet the increasingly high market demand, challenges include improving the energy density, extending cycle life and enhancing safety. In order to address these issues, a deep understanding of both the physical and chemical changes of battery materials under working conditions is crucial for linking degradation processes to their origins in material properties and their electrochemical signatures. In situ and operando synchrotron-based X-ray techniques provide powerful tools for battery materials research, allowing a deep understanding of structural evolution, redox processes and transport properties during cycling. In this review, in situ synchrotron-based X-ray diffraction methods are discussed in detail with an emphasis on recent advancements in improving the spatial and temporal resolution. The experimental approaches reviewed here include cell designs and materials, as well as beamline experimental setup details. Finally, future challenges and opportunities for battery technologies are discussed.

Coarse-grained empirical potential structure refinement: Application to a reverse aqueous micelle

Conventional atomistic computer simulations, involving perhaps up to 10⁶atoms, can achieve length-scales on the order of a few 10s of nm. Yet many heterogeneous systems, such as colloids, nano-structured materials, or biological systems, can involve correlations over distances up 100s of nm, perhaps even 1μm in some instances. For such systems it is necessary to invoke coarse-graining, where single atoms are replaced by agglomerations of atoms, usually represented as spheres, in order for the simulation to be performed within a practical computer memory and time scale. Small angle scattering and reflectivity measurements, both X-ray and neutron, are routinely used to investigate structure in these systems, and traditionally the data have been interpreted in terms of discrete objects, such as spheres, sheets, and cylinders, and combinations thereof. Here we combine the coarse-grained computer simulation approach with neutron small angle scattering to refine the structure of a heterogeneous system, in the present case a reverse aqueous micelle of sodium-dioctyl sulfosuccinate (AOT) and iso-octane. The method closely follows empirical potential structure refinement and involves deriving an empirical interaction potential from the scattering data. As in traditional coarse-grained methods, individual atoms are replaced by spherical density profiles, which, unlike real atoms, can inter-penetrate to a significant extent. The method works over an arbitrary range of length-scales, but is limited to around 2 orders of magnitude in distance above a specified dimension. The smallest value for this dimension is of order 1nm, but the largest dimension is arbitrary. This article is part of a Special Issue entitled "Recent Advances in Bionanomaterials" Guest Editor: Dr. Marie-Louise Saboungi and Dr. Samuel D. Bader.

Radical re-appraisal of water structure in hydrophilic confinement

The structure of water confined in MCM41 silica cylindrical pores is studied to determine whether confined water is simply a version of the bulk liquid which can be substantially supercooled without crystallisation. A combination of total neutron scattering from the porous silica, both wet and dry, and computer simulation using a realistic model of the scattering substrate is used. The water in the pore is divided into three regions: core, interfacial and overlap. The average local densities of water in these simulations are found to be about 20% lower than bulk water density, while the density in the core region is below, but closer to, the bulk density. There is a decrease in both local and core densities when the temperature is lowered from 298 K to 210 K. The radical proposal is made here that water in hydrophilic confinement is under significant tension, around -100 MPa, inside the pore.