Network-forming Materials Research Articles

Ring-originated anisotropy of local structural ordering in amorphous and crystalline silicon dioxide

Rings comprising chemically bonded atoms are essential topological motifs for the structural ordering of network-forming materials. Quantification of such larger motifs beyond short-range pair correlation is essential for understanding the linkages between the orderings and macroscopic behaviors. Here, we propose two quantitative analysis methods based on rings. The first method quantifies rings by two geometric indicators: roundness and roughness. These indicators reveal the linkages between highly symmetric rings and crystal symmetry in silica and that the structure of amorphous silica mainly consists of distorted rings. The second method quantifies a spatial correlation function that describes three-dimensional atomic densities around rings. A comparative analysis among the functions for different degrees of ring symmetries reveals that symmetric rings contribute to the local structural order in amorphous silica. Another analysis of amorphous models with different orderings reveals anisotropy of the local structural ordering around rings; this contributes to building the intermediate-range ordering.

Molecular Rotations, Multiscale Order, Hyperuniformity, and Signatures of Metastability during the Compression/Decompression Cycles of Amorphous Ices.

We model, via large-scale molecular dynamics simulations, the isothermal compression of low-density amorphous ice (LDA) to generate high-density amorphous ice (HDA) and the corresponding decompression extending to negative pressures to recover the low-density amorphous phase (LDAHDA). Both LDA and HDA are nearly hyperuniform and are characterized by a dynamical HBN, showing that amorphous ices are nonstatic materials and implying that nearly hyperuniformity can be accommodated in dynamical networks. In correspondence with both the LDA-to-HDA and the HDA-to-LDAHDA phase transitions, the (partial) activation of rotational degrees of freedom activates a cascade effect that induces a drastic change in the connectivity and a pervasive reorganization of the HBN topology which, ultimately, break the samples' hyperuniform character. Key to this effect is the rapid rate at which changes occur, and not their magnitude. The inspection of structural properties from the short- to the long-range shows that signatures of metastability are present at all length-scales, hence providing further solid evidence in support of the liquid-liquid critical point scenario. LDA and LDAHDA differ in terms of HBN and structural properties, implying that they are distinct low-density glasses. Our work unveils the role of molecular rotations in the phase transitions between amorphous ices and shows how the unfreezing of rotational degrees of freedom generates a cascade effect that propagates over multiple length-scales. Our findings greatly improve our basic understanding of water and amorphous ices and can potentially impact the field of molecular network-forming materials at large.

Anomalous density, sound velocity, and structure of pressure-induced amorphous quartz

The study of quartz and other silica systems under pressure is one of the most prolific domains of research over the past 50 years because of their applications in material science and fundamental relevance to planetary interiors. The characterization of the amorphous state is essential for the comprehension of pressure-induced amorphization of minerals, the metamorphism observed in shocked materials, and the study of melt structures under pressure. Here, we measured in situ, under static compression the density, sound velocities, and electronic structure of quartz as it passes through its pressure-induced amorphization transition. The transition pressure could be derived from the abrupt increase in density and sound velocity at 24 GPa, and from strong changes in the silicon ${L}_{2,3}$ edge and oxygen $K$ edge between 22 and 27 GPa observed in x-ray Raman scattering data, confirming previous results from x-ray diffraction. Above this pressure, our data show an anomalous behavior in density, sound velocity, and electronic fine structure compared to the cold compressed glass and other silica polymorphs. The pressure-induced amorphous quartz has a lower density relative to that of the compressed glass, consistent with the lower average coordination inferred from a different signature in the Si ${L}_{2,3}$ and O $K$ electronic absorption edges measured by x-ray Raman scattering spectroscopy. This behavior sheds light on the pressure limit of tetrahedral units in ${\mathrm{SiO}}_{2}$ components and the existence of polyamorphism in network-forming materials, and highlights the possibility to discriminate between different amorphous states with x-ray Raman scattering spectroscopy.

Location of the gel-like boundary in patchy colloidal dispersions: Rigidity percolation, structure, and particle dynamics

During the past decade, there has been a hot debate about the physical mechanisms that determine when a colloidal dispersion approaches the gel transition. However, there is still no consensus on a possible unique route that leads to the conditions for the formation of a gel-like state. Based on gel states identified in experiments, Valadez-Pérez etal. [Phys. Rev. E 88, 060302(R) (2013)PLEEE81539-375510.1103/PhysRevE.88.060302] proposed rigidity percolation as the precursor of colloidal gelation in adhesive hard-sphere dispersions with coordination number 〈n_{b}〉 equal to 2.4. Although this criterion was originally established to describe mechanical transitions in network-forming molecular materials with highly directional interactions, it worked well to explain gel formation in colloidal suspensions with isotropic short-range attractive forces. Recently, this idea has also been used to account for the dynamical arrest experimentally observed in attractive spherocylinders. Then, by assuming that rigidity percolation also drives gelation in spherical colloids interacting with short-ranged and highly directional potentials, we locate the thermodynamic states where gelation seems to occur in dispersions made up of patchy colloids. To check whether the criterion 〈n_{b}〉=2.4 also holds in patchy colloidal systems, we apply the so-called bond-bending analysis to determine the fraction of floppy modes at some percolating clusters. This analysis confirms that the condition 〈n_{b}〉=2.4 is a good approximation to determine those percolating clusters that are either mechanically stable or rigid. Furthermore, our results point out that not all combinations of patches and coverages lead to a gel-like state. Additionally, we systematically study the structure and the cluster size distribution along those thermodynamic states identified as gels. We show that for high coverage values, the structure is very similar for systems that have the same coverage regardless the number or the position of the patches on the particle surface. Finally, by using dynamic Monte Carlo computer simulations, we calculate both the mean-square displacement and the intermediate scattering function at and in the neighborhood of the gel-like states.

Probing the network topology in network-forming materials: The case of water

Rings statistic has been widely used to investigate the network topology in numerically simulated network-forming materials in order to rationalize their physical and mechanical properties. However, different topologies arise depending on how rings are counted, leading to incomplete or even contrasting physical interpretations. Solving this critical ambiguity is of primary importance for the correct assessment of material properties. Here, we show how such differences emerge in water, a complex network-forming material endowed with polyamorphism and a directional network of hydrogen bonds whose topology is correlated with the anomalous behavior of water. We probe the network in the liquid state at several thermodynamic points under equilibrium conditions, as well as during the out-of-equilibrium first-order-like low density to high density amorphous transformation. We study three schemes for counting rings and show that each of them provides complementary insightful information about the network, suggesting that a single counting scheme may not be sufficient to properly describe network topologies and to assess material properties. Our results provide a molecular description of the rings in supercooled water and of the amorphous-to-amorphous transformation kinetics, hence shedding light on the complex nature of water. Nonetheless, our results expose how delicate the proper choice of method for counting rings is, an issue with important consequences for rationalizing the properties of network-forming materials at large.

Molecular Simulations of Thermoset Polymers Implementing Theoretical Kinetics with Top-Down Coarse-Grained Models

Thermoset polymers are examples of chemically cured, network-forming materials whose bulk properties depend sensitively on formulation chemistry and reaction conditions. In this work, we employ mol...

Self-Assembly of Metal-Organic Frameworks into Monolithic Materials with Highly Controlled Trimodal Pore Structures.

We present a two-step template-free approach toward monolithic materials with controlled trimodal porous structures with macro-, meso-, and micropores. Our method relies on two ordering processes in discrete length scales: 1) Spontaneous formation of macroporous structures in monolithic materials by the sol-gel process through the short-range ordered self-assembly of metal-organic frameworks (MOFs), and 2) reorganization of the framework structures in a mediator solution. The Zr-terephthalate-based MOF (UiO-66-NH2 ) was adopted as a proof of concept. The self-assembly-induced phase separation process offered interconnected macropores with diameters ranging from 0.9 to 1.8 μm. The subsequent reorganization process converted the microporous structure from low crystalline framework to crystalline UiO-66. The resultant mesopore size within the skeletons was controlled in the range from 9 to 21 nm. This approach provides a novel way of designing spaces from nano- to micrometer scale in network-forming materials.

Influence of End-Block Dynamics on Deformation Behavior of Thermoresponsive Elastin-like Polypeptide Hydrogels

End-block association dynamics are known to influence deformation behavior in block copolymer systems. The ability to obtain precisely sequence controlled systems can help inform this deformation behavior, which is often influenced by dispersity in sequence and molar mass. Elastin-like polypeptides (ELPs) are a class of protein that consists of a sequence of five amino acids (XPGVG) that thermoresponsively aggregate in solution. These ELPs can be used as end-blocks in triblock fusion proteins with coiled-coil associating midblock domains to result in dual-associating, network-forming materials. By modifying the standard glycine-containing ELP sequence (XPGVG) to instead contain alanine in the third position of the repeat sequence (XPAVG), it is possible to improve the properties of the material in both shear and extension. In extension at 50 °C, the alanine-containing triblock (A10P4A10) and the glycine-containing triblock (G10P4G10) have similar Young’s moduli. However, while G10P4G10 yields and breaks within 5 strain units, A10P4A10 plastically deforms to an ultimate strain at break of over 15 strain units and a tensile stress of almost 90 kPa. In shear, G10P4G10 exhibits a clear stress overshoot at less than one strain unit before plateauing at a steady-state shear stress of 0.43 kPa, while the shear stress of A10P4A10 monotonically increases to a shear stress of 48.4 kPa after 2.4 strain units. In shear, these differences in behavior correlate to rearrangement of G10P4G10’s nanostructure during viscous-dominated flow, while the nanostructure of A10P4A10 initially elastically deforms. These differences are a function of the relative network relaxation time scales, where the presence of the faster glycine-containing ELP end-block sequence decreases the network relaxation time such that it occurs on the same order of magnitude as the imposed flow.

Impact of dispersion forces on the atomic structure of a prototypical network-forming disordered system: The case of liquid GeSe2.

A set of structural properties of liquid GeSe2 are calculated by using first-principles molecular dynamics and including, for the first time, van der Waals dispersion forces. None of the numerous atomic-scale simulations performed in the past on this prototypical disordered network-forming material had ever accounted for dispersion forces in the expression of the total energy. For this purpose, we employed either the Grimme-D2 or the maximally localized Wannier function scheme. We assessed the impact of dispersion forces on properties such as partial structure factors, pair correlation functions, bond angle distribution, and number of corner vs edge sharing connections. The maximally localized Wannier function scheme is more reliable than the Grimme-D2 scheme in reproducing existing first-principles results. In particular, the Grimme-D2 scheme worsens the agreement with experiments in the case of the Ge-Ge pair correlation function. Our study shows that the impact of dispersion forces on disordered chalcogenides has to be considered with great care since it cannot be necessarily the same when adopting different recipes.

Structure, dynamics and multiple length-scales in network-forming materials

Relationships between the structural and dynamical properties of network-forming materials are investigated. A generic model is utilised for systems of stoichiometry MX2 which are linked in the sense that they can all be usefully considered as constructed from linked MX4 tetrahedra. A single model parameter (the anion polarizability) is varied systematically to control the mean MXM bond angles (and hence the network topologies). The networks evolve from those dominated by corner-sharing units to those dominated by edge-sharing structural motifs. These changes are accompanied by changes in the characteristic length-scales, with the emergence of ordering on intermediate length-scales. Key dynamical properties (the liquid relaxation just above the melting point and the liquid fragility) are studied and their relationship to the underlying static structure analysed.

Pressure-driven transformation of the ordering in amorphous network-forming materials

The pressure-induced changes to the structure of disordered oxide and chalcogenide network-forming materials are investigated on the length scales associated with the first three peaks in measured diffraction patterns. The density dependence of a given peak position does not yield the network dimensionality, in contrast to metallic glasses where the results indicate a fractal geometry with a local dimensionality of $\ensuremath{\simeq}5/2$. For oxides, a common relation is found between the intermediate-range ordering, as described by the position of the first sharp diffraction peak, and the oxygen-packing fraction, a parameter that plays a key role in driving changes to the coordination number of local motifs. The first sharp diffraction peak can therefore be used to gauge when topological changes are likely to occur, events that transform network structures and their related physical properties.

Dynamical arrest in adhesive hard-sphere dispersions driven by rigidity percolation

One major goal in condensed matter is identifying the physical mechanisms that lead to arrested states of matter, especially gels and glasses. The complex nature and microscopic details of each particular system are relevant. However, from both scientific and technological viewpoints, a general, consistent and unified definition is of paramount importance. Through Monte Carlo computer simulations of states identified in experiments, we demonstrate that dynamical arrest in adhesive hard-sphere dispersions is the result of rigidity percolation with coordination number 〈n(b)〉 equal to 2.4. This corresponds to an established mechanism leading to mechanical transitions in network-forming materials [Phys. Rev. Lett. 54, 2107 (1985)]. Our findings connect the concept of critical gel formation in colloidal suspensions with short-range attractive interactions to the universal concept of rigidity percolation. Furthermore, the bond, angular, and local distributions along the gelation line are explicitly studied in order to determine the topology of the structure at the critical gel state.

Design of polymeric microparticles for pH-responsive and time-sustained drug release

We described the design of uniform microencapsulates with almost 100% encapsulation efficiency, synthesized without organic solvents, via microfluidic spray drying of water-based dispersions of pH-responsive methacrylic acid polymers (Eudragit® L 30D-55). The effects of incorporating water-based network-forming materials in the formulations on pH-responsiveness and controlled release patterns of enteric microparticles were observed. Acid hydrolysed tetraethoxysilane (TEOS) was used to form an interpenetrating, rigid framework of silica, whereas Eudragit® NE (a copolymer based on ethyl acrylate and methyl methacrylate) was added to produce a more flexible polymeric network. The spray-dried microparticles generally displayed crumbled or buckled morphologies dependent on drying temperatures, due to large hydrodynamic sizes of solutes in feed dispersions. The drug release kinetics of microparticles were sensitive to the type and the added amount of network-forming materials, due to different colloidal interactions between Eudragit® L and either silica or the copolymer. This study demonstrated a strategy to design enteric microparticles with different microstructural properties and drug release behaviours through understanding of colloidal interactions between constituents of matrix materials.

The effects of topology on the structural, dynamic and mechanical properties of network-forming materials

The effects of network topology on the static structural, mechanical and dynamic properties of MX2 network-forming liquids (with tetrahedral short-range order) are discussed. The network topology is controlled via a single model parameter (the anion polarizability) which effectively constrains the inter-tetrahedral linkages in a physically transparent manner. Critically, it is found to control the balance between the stability of corner- and edge-sharing tetrahedra. A potential rigidity transformation is investigated. The vibrational density of states is investigated, using an instantaneous normal model analysis, as a function of both anion polarizability and temperature. A low frequency peak is seen to appear and is shown to be correlated with the fraction of cations which are linked through solely edge-sharing structural motifs. A modified effective mean atom coordination number is proposed which allows the appearance of the low frequency feature to be understood in terms of a mean field rigidity percolation threshold.

Model investigations of network-forming materials

Recent advances in the study of network-forming materials are described for systems dominated both by ionic and covalent interatomic interactions. Modelling strategies are described which focus both on describing specific systems of interest and on modelling the systematic evolution of network topology. The effect of network topology on the presence of ordering both on intermediate- and extended-length-scales is discussed. The effect of the topology on the mechanical rigidity is also described and analysed in terms of a mean coordination model. In addition, the isomorphology between amorphous silicon and the silicon sub-lattice in SiO(2) is described. Polyamorphism in Si and ZnCl(2) is analysed and discussed. Finally, the study of reduced (two) dimensional systems is discussed for carbon, silicon and germanium.

UV–vis spectroscopic study of room temperature ionic liquid–water mixtures: N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate

N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate, [DEME][BF4], and H2O mixtures were investigated in optical properties by UV–visible spectroscopy. The concentration region of H2O in the mixtures was 0–100mol%. At room temperature, the mixtures of H2O exhibit absorption peaks at around 280nm and 750nm, which depend on H2O concentrations. At 90–95mol% H2O, the optical transmittance decreased over the whole region of UV–visible light. In X-ray diffraction, correlation over the medium-range such as network-forming materials develops at 90–95mol%. Also, in quenched glass, low glass transition temperature appeared at these H2O concentrations. The specific glass is induced by different molecular aggregation in liquid. Therefore, an optical anomaly is connected with the specific water network over the medium-range order.

Slow dynamics and internal stress relaxation in bundled cytoskeletal networks

Crosslinked and bundled actin filaments form networks that are essential for the mechanical properties of living cells. Reconstituted actin networks have been extensively studied not only as a model system for the cytoskeleton, but also to understand the interplay between microscopic structure and macroscopic viscoelastic properties of network-forming soft materials. These constitute a broad class of materials with countless applications in science and industry. So far, it has been widely assumed that reconstituted actin networks represent equilibrium structures. Here, we show that fully polymerized actin/fascin bundle networks exhibit surprising age-dependent changes in their viscoelastic properties and spontaneous dynamics, a feature strongly reminiscent of out-of-equilibrium, or glassy, soft materials. Using a combination of rheology, confocal microscopy and space-resolved dynamic light scattering, we demonstrate that actin networks build up stress during their formation and then slowly relax towards equilibrium owing to the unbinding dynamics of the crosslinking molecules.

Anomalous Mixing State in Room-Temperature Ionic Liquid-Water Mixtures: N, N-diethyl-N-methyl-N-(2-methoxyethyl) Ammonium Tetrafluoroborate

The mixing states of room-temperature ionic liquid (RTIL) H2O mixtures (x = 0.0 mol pct to 99.5 mol pct H2O) were investigated using wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and optical absorption in an ultraviolet and visible (UV-vis) region. The RTIL is N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate, [DEME][BF4]. In a “prepeak” region of the WAXS, the scattered intensities increased at 85 mol pct to 95 mol pct. A medium-range order (MRO) in the liquid structure as observed in network-forming materials developed markedly. In the SAXS experiments, we can detect nanoscale fluctuations relating to polar and nonpolar regions. At 65 mol pct to 85 mol pct, the SAXS intensity increased unexpectedly. Furthermore, entirely different optical absorption spectra in the UV-vis region were observed as a macroscopic property from 90 mol pct to 95 mol pct. We suppose that these anomalies relate to the MRO of the liquid structure. All anomalies probably are induced by an intrinsic property in [DEME][BF4]-H2O mixtures.

Defective to fully coordinated crossover in complex directionally bonded nanoclusters

Defect-free fully coordinated (FC) structures are well known to be highly stable for a number of materials exhibiting three-coordinated bonding at the nanoscale (e.g., ${\text{C}}_{60}$). For topologically more complex nanosystems with higher bonding connectivities the structures and stabilities of the lowest energy FC structures with respect to low-energy defective isomers are unknown. Herein, we describe a general method to thoroughly search through the low-energy geometries of only those nanoclusters that possess FC atomic connectivities. As a pertinent example of our approach we investigate four-connected ${\text{SiO}}_{2}$, a fundamentally important network-forming material used in many applications at the nanoscale. Using our method we predict that a structurally complex stability crossover from defective to FC nanoclusters occurs in ${\text{SiO}}_{2}$ at a size of $\ensuremath{\sim}100$ atoms. At variance with previous works, based on constructing FC ${\text{SiO}}_{2}$ cagelike nanoclusters by hand, we also show that cagelike clusters are only favored for smaller cluster sizes with dense FC topologies becoming energetically favored with increasing system size.

Dynamics and energy landscape in a tetrahedral network glass-former: direct comparisonwith models of fragile liquids

We report molecular dynamics simulations for a new model of tetrahedral networkglass-former, based on short-range spherical potentials. Despite the simplicity of theforcefield employed, our model reproduces some essential physical properties of silica, anarchetypal network-forming material. Structural and dynamical properties, includingdynamic heterogeneities and the nature of local rearrangements, are investigated in detailand a direct comparison with models of close-packed, fragile glass-formers is performed.The outcome of this comparison is rationalized in terms of the properties of the potentialenergy surface, focusing on the unstable modes of the stationary points. Our resultsindicate that the weak degree of dynamic heterogeneity observed in network glass-formersmay be attributed to an excess of localized unstable modes, associated with elementarydynamical events such as bond breaking and reformation. In contrast, the morefragile Lennard-Jones mixtures are characterized by a larger fraction of extendedunstable modes, which lead to a more cooperative and heterogeneous dynamics.