An Approach of Domain Polymorph Component Design

International audienceHeterogeneous modelling and design tools allow the design of software systems using several computation models. The designed system is built by assembling components that obey a computation model. The internal behavior of a component is specified either in some programming language or by assembling sub-components that obey a possibly different computation model. When the same behavior is used in several computation models, it must be implemented in as many components as there are models, or, if the design platform supports it, it may be implemented as a generic component. Model-specific components require the recoding of the same core behavior several times, and generic components may not take model- specific features into account. In this paper, we introduce the notion of domain-polymorph component. Such a component is able to adapt a core behavior to the semantics of several computation models. The core behavior is implemented only once and is automatically adapted to the semantics of different computation models. Domain-polymorph components can be chosen by a system designer and integrated in a computation model: they will benefit from an appropriate execution environment and their semantics will be adapted to the host model. The designer will have the choice for several parameters of the adaptation. Contrary to generic components, such components adapt their behavior to the host model instead of letting the host model interpret their generic behavior. We also present an implementation of the concept of domain-polymorph component in the Ptolemy~II framework

Heterogeneous Nucleation in Semicrystalline Polymers

Nucleation of polymer crystals is a key issue in polymer science and technology. Indeed, it is of outmost importance for industrial application of semicrystalline polymers, since the nucleation rate often dictates the processing time of the products, and strongly affects the resulting mechanical or optical properties of the material. Despite this a comprehensive understanding of the phenomenon is still lacking, as testified for instance by the fact that the scouting of new heterogeneous nucleating agents for polymers is still mostly driven by empiricism. With this thesis, we aim to provide novel quantitative approaches to quantify the heterogeneous nucleation efficiency of different surfaces in various, industrially relevant, systems, such as polymer/fiber composites or nucleated polymers. The presented results are a contribution towards the clarification of the mechanism of heterogeneous nucleation in semicrystalline polymers.The nucleation process of biodegradable poly(lactide) on a series of fibers, including commercially available fibers, natural fibers and a custom-spun fiber of stereocomplex enantiomeric PLA blend, was studied by polarized optical microscope during crystallization. The nucleation ability of different fiber substrates was derived and compared in the light of classical heterogeneous nucleation theory, by considering the interfacial free energy difference parameter, \u394\u3c3, directly related to the nucleation barrier. The role of fiber surface topography and chemical interactions between the fiber substrate and the crystallizing polymer in promoting the nucleation was investigated and discussed in detail. While a general effect of surface roughness on lowering the heterogeneous nucleation energy barrier can be deduced, the role of chemical interactions between the fiber substrate and the crystallizing polymer cannot be neglected.Furthermore, a novel approach was proposed to quantitatively evaluate the nucleation efficiency of several additives for isotactic polypropylene (i-PP), using droplets containing nucleating agents (i.e., sodium benzoate, NA11, quinacridone quinone) dispersed in an immiscible polystyrene matrix. The crystallization was investigated by isothermal step crystallization and melting with Differential Scanning Calorimetry (DSC). Moreover, self-nucleation of neat i-PP droplets is also studied in detail, enabling to derive an \u201cintrinsic\u201d nucleation efficiency scale based on the ratio of the free energy barrier, \u394G*, of heterogeneous nucleation on different substrates with respect to that of self-nucleation, which is found equivalent to the secondary nucleation barrier for crystal growth. Having established the interfacial free energy difference parameter, governing the heterogeneous nucleation kinetics of i-PP onto different substrates, a simple correlation curve useful to describe non-isothermal fractionated crystallization of i-PP/PS blends with droplet morphology was constructed.Finally, we propose a Differential Scanning Calorimetry (DSC) approach for the quantitative investigation of isotactic poly(1-butene) Form II-on-Form I cross-nucleation. Seeds of trigonal Form I crystal were produced in PB1 samples, and their amount and characteristic size were varied by using different crystallization conditions. DSC isothermal and non-isothermal crystallization measurements of Form I seeded samples were performed, highlighting a clear nucleation effect of the stable polymorph on Form II. Moreover, the nucleating efficiency is highly dependent on the content of Form I seeds, more specifically, on the area of Form I spherulites per unit of sample volume. Depending on the seeding and crystallization conditions, Form II crystallization is controlled either by nucleation on foreign heterogeneous surfaces or on Form I crystals

New in situ solid-state NMR strategies for exploring materials formation and adsorption processes: prospects in heterogenous catalysis

Solid-state NMR spectroscopy is a powerful technique for studying structural and dynamic properties of solids and has considerable potential to be exploited for in situ studies of chemical processes. However, adapting solid-state NMR techniques and instrumentation for in situ applications are often associated with technical challenges, and for this reason, the opportunities remain underexploited. This paper highlights two experimental strategies that we have developed in recent years for in situ solid-state NMR investigations of solid-state processes. One technique is focused on probing details of the time evolution of materials formation processes, and the other technique is focused on understanding the time evolution of adsorption processes in microporous and mesoporous solid host materials. Each of these in situ solid-state NMR techniques has significant prospects for applications in areas relating to heterogeneous catalysis

Template induced polymorphic selectivity in pharmaceutical crystallisation

Polymorphism in pharmaceutical drug crystals causes differences in their bioavailability, stability and processability. Hence, identifying different crystal polymorphs of an active ingredient during the early stages of drug development and controlling crystal polymorphism during the manufacturing process are important aspects of pharmaceutical crystallisation. Nucleation and growth of different polymorphs in a crystallising solution are regulated by a delicate balance between thermodynamic and kinetic factors. Crystal nucleation predominantly occurs via heterogeneous nucleation pathway as it is energetically favourable than homogeneous nucleation. Template-induced nucleation approach aims to utilise the advantage of heterogeneous nucleation to induce nucleation of specific crystal polymorphs through interfacial interactions between a preformed solid surface and solute molecules at the nucleation stage. In template-induced crystallisation, templates with specific surface properties that can act as heterogeneous nucleation sites are introduced in contact with the crystallising solution. Specific interactions between the template surface and solute molecules are known to influence nucleation and growth of crystal polymorphs. However, the effects of template surface chemistry and other operating conditions such as temperature and supersaturation on template-induced crystallisation is not clearly understood. Hence, the aim of this study is to probe the combined effects of surface chemistry, crystallisation temperature, supersaturation, and solvent on template-induced crystallisation experimentally and consequent development of a molecular modelling approach to study template-induced nucleation. This could help in establishing template-induced nucleation as a method to achieve preferential nucleation of crystal polymorphs and to support template chemistry as a novel parameter for polymorph screenings. Carbamazepine (CBZ) was selected as the model drug compound and silanised glass vials were chosen as the template surfaces. CBZ crystallisation from ethanol solutions on templates with cyano functional surface groups led to selective nucleation of metastable form II crystals while the control surfaces resulted in concomitant nucleation of both form II and stable form III crystals. On mercapto and fluoro templates, CBZ crystallised preferentially as form III polymorph. These variations in the polymorphic outcome with template chemistry, temperature and supersaturation were mapped on to template-induced polymorphic domain (TiPoD) plots. The analysis of TiPoD plots showed that the template-induced nucleation mechanism was prominent within a narrow range of supersaturation across the temperature range studied. The influence of solvents on template-induced nucleation of CBZ polymorphs was also investigated by constructing TiPoD plots in five different solvents. These studies revealed that the templates were less effective in altering polymorphic outcome in highly polar solvent in comparison with the less polar solvents. Interfacial interactions between the template surface and CBZ crystal polymorphs were calculated through molecular modelling. The simulation results suggest that those templates exhibiting favourable interaction energies with the dominant crystal facets of a specific polymorph preferentially induce nucleation of that crystal form.Open Acces

Template and Temperature Controlled Polymorph Formation in Squaraine Thin Films

Controlling the polymorph formation in organic semiconductor thin films by the choice of substrate and deposition temperature is a key factor for targeted device performance. Small molecular semiconductors such as the quadrupolar donor-acceptor-donor (D-A-D) type squaraine compounds allow both solution and vapor phase deposition methods. A prototypical anilino squaraine with branched butyl chains as terminal functionalization (SQIB) has been considered for photovoltaic applications due to its broad absorption within the visible to deep-red spectral range. Its opto-electronic properties depend on the formation of the two known polymorphs adopting a monoclinic and orthorhombic crystal phase. Both phases emerge with a strongly preferred out-of-plane and rather random in-plane orientation in spincasted thin films depending on subsequent thermal annealing. Upon vapor deposition on dielectric and conductive substrates, such as silicon dioxide, potassium chloride, graphene and gold, the polymorph expression depends on the choice of growth substrate. In all cases the same pronounced out-of-plane orientation is adopted, but with a surface templated in-plane alignment in case of crystalline substrates. Combining X-ray diffraction, atomic force microscopy, ellipsometry and polarized spectro-microscopy we identify the processing dependent evolution of the crystal phases, correlating morphology and molecular orientations within the textured SQIB films.Comment: 10 pages, 7 figure

Merging Data-Driven And Computational Methods to Understand Ice Nucleation

Heterogeneous ice nucleation (IN) is one of the most ubiquitous phase transitions on earth and impacts a plethora of fields in industry (e.g. air transport, food freezing and harsh-weather operations) and science (e.g. freeze avoidance of animals, cryobiology, cloud research). Still to date, we are lacking reliable answers to the question: What is it at the molecular scale that causes an impurity to facilitate the freezing process of supercooled liquid water? In this thesis we make headway towards identifying such microscopic principles by performing computational studies combined with data-driven approaches. In chapter 3 we screen a range of model substrates to disentangle the contributions of lattice match and hydrophobicity and find that there is a complex interplay and an enormous sensitivity to the atomistic details of the interface. In chapter 4 we show that the heterogeneous setting can alter the polymorph of ice that forms and introduce the concept of pre-critical fluctuations, yielding new ideas to design polymorph-targeting substrates. Chapter 5 deals with the liquid dynamics before and during the nucleation event, an aspect of nucleation that mostly goes unrecognized. We show that the homogeneous nucleation event happens in relatively immobile regions of the supercooled liquid, a finding that opens new avenues to understand and influence heterogeneous nucleation by targeting dynamics rather than structure. Finally, Chapter 6 builds on the large amount of data created during this project in that we combine all previously simulated systems and devise a machine-learning approach to find the most important descriptors for their ice nucleation activity (INA). With this we identify new microscopic guidelines and demonstrate that the quantitative prediction of heterogeneous INA is in reach. The unveiling of a computational artifact that potentially affects many computational interface studies is also part of this thesis

Towards More Efficient Enhanced Sampling Methods To Study Phase Transitions

The most familiar phase transitions observed in nature are associated with a change in the state of matter (solid, liquid, and gas). In some rare cases this may involve the plasma phase. Such transitions are often referred to as first order phase transitions and often occur commonly such as during the melting of snow or freezing of lakes and rivers during winter. This project focuses on the most ubiquitous phase changes such as, liquid-solid and vapor-liquid as well as the less prevalent vapor-solid transitions. These types of phase transitions are also known as classical phase transitions. They usually involve symmetry breaking and can be identified by a singularity in the free energy or one of its derivatives. More modern classification of phase transitions relies on the order parameters as exemplified by the Landau\u27s theory. An order parameter is a quantity that takes a value of zero in the disordered phase and assumes finite values in the ordered phase. In the case of liquid-vapor transition, the order parameter is the density. The study of phase transitions is often complicated by the amount of time required by these phase changes and the presence of a high free energy barrier. Consequently, changes occurring close to coexistence are hard or even impossible to follow via conventional experimental techniques. Molecular simulation is therefore the method of choice to study these processes. Molecular simulations are numerical experiments carried out on model systems and have a number of advantages over traditional experiments. Simulations do not have any limitation as to the type of molecules or conditions under which they can be applied. Current simulation methods used to accomplish this task, such as the grand canonical and Gibbs ensemble Monte Carlo methods, employ the concept of particles insertion and deletion moves or requires the knowledge of at least one point at coexistence. These types of moves are extremely inefficient when dense fluids are involved and limit the accuracy of these methods. To circumvent these difficulties, non-Boltzmann sampling methods such as the umbrella sampling and Wang-Landau sampling techniques, have been employed to study these phase transitions. Vapor-solid and liquid-solid phase transitions were studied using a combination of hybrid Monte Carlo (HMC) and the umbrella sampling on a system of C60 molecules. The crystallization process occurs in two steps, nucleation and growth. The nucleation step is an activated process that involves a high free energy barrier. The free energy barrier is overcome through a series of HMC steps. The growth step on the other hand is studied by means of unconstrained molecular dynamics (MD). This study illustrates that the body centered cubic structure plays no role in the crystallization of C60. This is because only the face centered cubic and the hexagonal closed parked crystal structures were observed in both the nucleation and growth steps. In addition, the growth process is observed to follow a complex mechanism known as cross nucleation. The process of cross nucleation has also been observed in model fluids such as Lennard-Jones fluid and in the experimental study of D-mannitol. Hybrid Monte Carlo and configurational bias Monte Carlo (CBMC) were combined with the Wang-Landau (WL) sample method to study the vapor-liquid equilibria of Polycyclic aromatic hydrocarbons (PAHs) with four fused benzene rings and &alpha-olefins (C2 - C6 respectively. These studies are conducted in the isothermal-isobaric (NPT) ensemble to avoid the particle insertion and deletion moves that resulted in low acceptance rates in previous simulations. These studies led to the prediction of the critical temperatures, pressures and densities of both systems

Nature of catalytically active sites in the supported WO3/ZrO2 solid acid system: a current perspective

Tungstated zirconia (WO3/ZrO2) is one of the most well-studied solid acid catalyst systems and continues to attract the attention of both academia and industry. Understanding and controlling the properties of WO3/ZrO2 catalysts has been a topic of considerable interest over almost the past three decades, with a particular focus on discovering the relationship between catalytic activity and the molecular structure of the surface acid site. Amorphous tungsten oxide (WOx) species on ZrO2 surfaces were previously proposed to be very active for different acidic reactions such as alcohol dehydration and alkane isomerization. Recent developments in electron optical characterization and in situ spectroscopy techniques have allowed researchers to isolate the size, structure, and composition of the most active catalytic species, which are shown to be three-dimensional distorted Zr-WOx clusters (0.8–1.0 nm). Complementary theoretical calculations of the Brønsted acidity of these Zr-WOx clusters have confirmed that they possess the lowest deprotonation energy values. This new insight provides a foundation for the future characterization and theory of acidic supported metal oxide catalytic materials that will, hopefully, lead to the design of more active and selective catalysts. This perspective presents an up-to-date, comprehensive summary of the leading models of WO3/ZrO2 solid acid catalysts