Molecular Glass Research Articles

Design and preparation of glassy molecular precursors by adjusting molecular structures for facile processing of high-performance polymers

By taking the advantages of molecular glass and BCB crosslinking, a new design of amorphous micromolecular precursors was prepared for balance of processability and performance of the corresponding high-performance polymers.

Light‐Responsive Bilayer Cell Culture Platform for Reversible Cell Guidance

In vivo, cells are surrounded by a constantly changing microenvironment, which regulates many cell functions such as differentiation, migration, and cell death. Stimuli‐responsive biomaterials aim to mimic this interaction between cells and extracellular matrix in vitro. However, reproducing dynamic signaling noninvasively without affecting the cell viability remains a challenge. Herein, a dynamic cell culturing platform consisting of a light‐responsive azobenzene molecular glass film and a protective polydimethylsiloxane (PDMS) coating is developed. By tuning the PDMS layer thickness, surface relief gratings (SRGs) can be efficiently photoinscribed on the platform surface. The SRGs can also be erased with light in the presence of PDMS, i.e., the topography can be reversibly photomodulated. The inscribed SRGs can guide epithelial cell orientation along the topography. The erasure parameters are targeted toward cell culturing environment, enabling experiments with live cells. Finally, the photoresponsive platform is patterned with proteins by microcontact printing, allowing its biofunctionalization and the combination of microtopography and protein patterns. This study paves the way for using reconfigurable cell culture platforms for the dynamic control of cell–material interactions. The PDMS coating has potential to protect underneath material, broadening the spectrum of possible materials for dynamic cell culture platforms.

Surfactant-mediated synthesis of monodisperse Poly(benzyl methacrylate)-based copolymer microspheres

Polymeric microspheres (MSs) have broad applications. Poly(benzyl methacrylate) (PBzMA) have various important applications, such as optical materials, electrochromatography, and photoresists. However, previous studies on PBzMA were on fabricating bulk polymers or nano-sized (<500 nm) spheres. In this study, by introducing a cationic surfactant, cetyltrimethylammonium bromide (CTAB), into the processing, we were the first to successfully synthesize PBzMA-based (pristine and co-polymer) highly monodisperse microspheres. In the synthesis, poly(vinyl pyrrolidone) (PVP) is used as the stabilizer to prevent particle aggregation, and cetyltrimethylammonium bromide (CTAB) is used to assist dispersion polymerization. The sizes of PBzMA-based MSs can be tuned between 1.0 and 10.0 µm with a high monodispersity by controlling the CTAB and monomer concentrations. The morphology, molecular structure, molecular weight distribution, thermal stability, and glass transition behavior of the MS samples are determined. Particularly, the glass transition temperature can be tuned continuously and smoothly between 59 °C–120 °C by adjusting the monomer ratio in the synthesis of the copolymer MSs. This study provides insights into a controlled synthesis of size- and property-tunable polymeric and copolymer MSs for broad applications.

Derivatives of Bis(trifluoromethyl)biphenyl and Various Donor Noieties Exhibiting Dual State Emission

A series of bis(trifluoromethyl)biphenyl derivatives bearing carbazole, dimethylacridan, and phenothiazine donor moieties were designed and synthesized via Ullman coupling reaction. The compounds were found to be capable of forming molecular glasses with glass transition temperatures ranging from 152 °C to 244 °C. Significant dual-state emission was revealed for the synthesized compounds. Radiative decay of singlet and triplet states emerged from restricted molecular motion in solid state. The phenothiazinyl-substituted bis(trifluoromethyl)biphenyl derivative induces particular interest, as it exhibits room-temperature phosphorescence. This makes it promising for oxygen sensing applications. The theoretical work revealed that carbazolyl disubstituted bis(trifluoromethyl)biphenyl has the largest deviation of the planes of donor and acceptor fragments from 90°, which leads to larger overlap of electron density in the ground and excited states and larger oscillator strength. The calculated values of dipole moment and charge transfer during excitation were found to be the lowest for phenothiazinyl disubstituted bis(trifluoromethyl)biphenyl. This result explains the experimental fact that the solvent polarity has the least effect on the fluorescent spectrum of this compound.

Synthesis and Thermal, Photophysical, Electrochemical Properties of 3,3-di[3-Arylcarbazol-9-ylmethyl]oxetane Derivatives.

Novel oxetane-functionalized derivatives were synthesized to find the impact of carbazole substituents, such as 1-naphtyl, 9-ethylcarbazole and 4-(diphenylamino)phenyl, on their thermal, photophysical and electrochemical properties. Additionally, to obtain the optimized ground-state geometry and distribution of the frontier molecular orbital energy levels, density functional theory (DFT) calculations were used. Thermal investigations showed that the obtained compounds are highly thermally stable up to 360 °C, as molecular glasses with glass transition temperatures in the range of 142–165 °C. UV–Vis and photoluminescence studies were performed in solvents of differing in polarity, in the solid state as a thin film on glass substrate, and in powders, and were supported by DFT calculations. They emitted radiation both in solution and in film with photoluminescence quantum yield from 4% to 87%. Cyclic voltammetry measurements revealed that the materials undergo an oxidation process. Next, the synthesized molecules were tested as hole transporting materials (HTM) in perovskite solar cells with the structure FTO/b-TiO2/m-TiO2/perovskite/HTM/Au, and photovoltaic parameters were compared with the reference device without the oxetane derivatives.

4-Arylethynyl-5-fluorobenzoyl-1-methylimidazole Exhibiting Self-Recovering Mechanofluorochromism and Forming Fluorescence Molecular Glass

Abstract 4-Arylethynyl-5-fluorobenzoyl-1-methyl-1H-imidazoles exhibited self-recovering mechanofluorochromism and formed a glass state. Although the solid sample of the anisyl-substituted 1a and dimethoxyphenyl-substituted 1b exhibited no and blue fluorescence, respectively, they exhibited bluish-green fluorescence after grinding. These compounds recovered the color of their fluorescence after approximately 20 s and after 20 h, respectively. Both compounds exhibited non-crystalline samples after melting and rapid cooling to room temperature. Based on thermal analysis, it was found 1a and 1b were supercooling liquid and glass, respectively, at room temperature. Both non-crystalline samples exhibited fluorescence at a wavelength longer than that of the solid state. DFT calculations indicated that the bluish-green fluorescence of the ground and non-crystalline samples was derived from the twisted intramolecular charge transfer excitation state.

Azo Molecular Glass Patterning from Chiral Submicron Pillar Array to Self‐Organized Topographic Transition via Irradiation with Circularly Polarized Light

AbstractThis work reports chiral structure formation and self‐organized surface patterning upon submicron pillar arrays of an azo molecular glass, induced by irradiation with a circularly polarized laser beam at 488 nm. After the irradiation for 2–3 min, the chiral spiral structures form on the wall of every pillar in the hexagonal array. The spiral direction induced by the right‐handed circularly polarized light is mirror‐symmetrical to that caused by the left‐handed circularly polarized light. Upon further irradiation and gradual erasure of the original pillars, which is caused by mass transfer along the electric vibration direction of the circularly polarized light, a new set of pillars emerges through splitting every original pillar into four equivalent pillars with smaller size. A series of the well‐organized surface patterns appear in the intermediate stage of the topographical transition process. In the final stage, the highly regular pillar array with quadruple area density is formed and stabilized. The structure formation and self‐organized topographic transition are thoroughly investigated by experiments and optical simulation. These intriguing observations are found to be correlated with several unique light–matter interaction effects. The discoveries can provide a new approach to create hierarchical surface patterns and detect the handedness of light.

Dynamic resonant waveguide crossed gratings for wavelength-selective polarization conversion and optical modulation

Two-dimensional Resonant Waveguide Crossed Gratings (RWCG) were fabricated on azobenzene molecular glass thin films and their resonance behavior was studied once placed in between orthogonally aligned polarizers. Normally-incident polychromatic light was transmitted and/or reflected from these RWCGs only in narrow positive peaks. In addition, the central wavelength and transmitted intensity of these positive peaks were actively modulated by an external light source. Furthermore, a dynamic volume birefringence behavior related to the photomechanical effect of the azobenzene chromophores was observed. A mechanism to explain the polarization conversion of the resonant light using RWCGs at normal incidence was also proposed.

Mechanical performance analysis of polyurethane‐modified asphalt using molecular dynamics method

AbstractIn this study, the mechanical performance of polyurethane (PU)‐modified asphalt was investigated. To this end, the molecular dynamics method was adopted to study the influence of the PU modifier on the compatibility, mechanical properties, and structure of asphalt. Using the Materials Studio software, a four‐component molecular model of asphalt, a PU molecular model, and an asphalt–PU mixed system model were constructed, and the molecular density and glass transition temperature () were used to verify the performance of the aforementioned models. Molecular dynamics simulations of the three molecular models were conducted under different temperatures, and the solubility parameter (), interaction energy, mechanical properties, and radial distribution functions (RDFs) were obtained. The results showed that at 413.15 K (140°C), the difference between the solubility parameters of the asphalt and PU molecules () is the smallest, and their interaction reaches the most optimal phase state. The effects of the PU modifier on the Young's modulus, bulk modulus, and shear modulus of asphalt were studied. Finally, the RDF analysis showed that the PU modifier has a direct effect on the degree of swelling and dispersion between the aromatic–aromatic and saturate–saturate components in the asphalt, reducing the aggregation of both molecules. This verifies the PU modification effect.

Prediction of the Synergistic Glass Transition Temperature of Coamorphous Molecular Glasses Using Activity Coefficient Models.

The glass transition temperature (Tg) of a binary miscible mixture of molecular glasses, termed a coamorphous glass, is often synergistically increased over that expected for an athermal mixture due to the strong interactions between the two components. This synergistic interaction is particularly important for the formulation of coamorphous pharmaceuticals since the molecular interactions and resulting Tg strongly impact stability against crystallization, dissolution kinetics, and bioavailability. Current models that describe the composition dependence of Tg for binary systems, including the Gordon-Taylor, Fox, Kwei, and Braun-Kovacs equations, fail to describe the behavior of coamorphous pharmaceuticals using parameters consistent with experimental ΔCP and Δα. Here, we develop a robust thermodynamic approach extending the Couchman and Karasz method through the use of activity coefficient models, including the two-parameter Margules, non-random-two-liquid (NRTL), and three-suffix Redlich-Kister models. We find that the models, using experimental values of ΔCP and fitting parameters related to the binary interactions, successfully describe observed synergistic elevations and inflections in the Tg versus composition response of coamorphous pharmaceuticals. Moreover, the predictions from the NRTL model are improved when the association-NRTL version of that model is used. Results are reported and discussed for four different coamorphous systems: indomethacin-glibenclamide, indomethacin-arginine, acetaminophen-indomethacin, and fenretinide-cholic acid.

Internal molecular conformation of organic glasses: A NEXAFS study.

The origin of the exceptional stability of molecular glasses grown by physical vapor deposition (PVD) is not well understood. Differences in glass density have been correlated with thermodynamic stability for thin films of N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) grown by PVD at specific substrate temperatures below the glass transition temperature. However, the relationship between the internal conformation of glass molecules and the thermodynamic properties of molecular glasses is not well studied. We use carbon 1s near edge x-ray absorption fine structure (NEXAFS) spectroscopy to examine different TPD sample preparations in which differences in the thermodynamic stability of the glass are known. Density functional theory simulations of the NEXAFS spectra of TPD allow us to attribute spectroscopic differences to changes in the internal conformation of the TPD molecule and relate this conformation to the stability of the TPD glass. This provides a direct experimental measurement of the internal conformation of molecules forming an organic glass.

Metal Organic Frameworks Derived Sustainable Polyvinyl Alcohol/Starch Nanocomposite Films as Robust Materials for Packaging Applications.

Bio-nanocomposites-based packaging materials have gained significance due to their prospective application in rising areas of packaged food. This research aims to fabricate biodegradable packaging films based upon polyvinyl alcohol (PVA) and starch integrated with metal-organic frameworks (MOFs) or organic additives. MOFs offer unique features in terms of surface area, mechanical strength, and chemical stability, which make them favourable for supporting materials used in fabricating polymer-based packaging materials. zeolitic imidazolate frameworks (ZIFs) are one of the potential candidates for this application due to their highly conductive network with a large surface area and high porosity. Present research illustrates a model system based on ZIF-67 (C8H10N4Co) bearing 2–10 wt.% loading in a matrix of PVA/starch blend with or without pyrolysis to probe the function of intermolecular interaction in molecular packing, tensile properties, and glass transition process. ZIF-67 nanoparticles were doped in a PVA/starch mixture, and films were fabricated using the solution casting method. It was discovered through scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR) that addition of ZIF-67 and pyrolyzed ZIF-67 changed and enhanced the thermal stability of the membrane. Moreover, 2–10 wt.% loading of ZIF-67 effected the thermal stability, owing to an interlayer aggregation of ZIF-67. The membranes containing pyrolyzed ZIF-67 showed mechanical strength in the order of 25 MPa in a moderate loading of pyrolyzed ZIF-67 (i.e., at 4 wt.%). The crystallinity enhanced by an increment in ZIF-67 loading. On the other hand, pyrolyzed ZIF-67 carbon became amorphous because of the inert environment and elevated temperature. The surface area also increased after the pyrolysis, which helped to increase the strength of the composite films.

Crystal Growth Kinetics in GeS2 Glass and Viscosity of Supercooled Liquid.

The crystal growth kinetics and morphology in germanium disulfide bulk glass and glass surface is described. The structural relaxation taking place below the glass transition is slow and the corresponding volumetric change is negligible. Therefore, it does not affect substantially the crystal growth process. The crystal growth rate of low temperature β-GeS2 and high temperature α-GeS2 polymorphs in the bulk glass is comparable, being slightly decoupled from the shear viscosity below the glass transition. The crystal growth rate of β-GeS2 in an amorphous thin film of the same composition is several orders of magnitude faster than that at the surface of bulk glass. This fast surface crystal growth is strongly decoupled from viscosity. Such behavior resembles the glass-to-crystal fast growth mode observed by several authors in some organic molecular glasses. Taking into account previously reported viscosity and heat capacity data, the crystal growth kinetics of both polymorphs can be quantitatively described by the 2D surface growth model for low and high supercooling. The nonisothermal differential scanning calorimetry experiments are analyzed, providing evidence of a complex nature of the overall crystallization process with apparent activation energy comparable to that obtained from isothermal microscopy measurement of crystal growth in the same temperature range.

Relationship between crystal growth and surface/volume mobilities in Se95Te5 bulk glasses and thin films

The mobility of structural units in amorphous materials has a significant influence on crystal growth. In this study, we show that the motion of structural units from the amorphous matrix to the growing crystals in the volume of the Se95Te5 bulk samples can be adequately described by viscosity. Nevertheless, surface crystal growth results in a much faster growth in bulk and thin films samples, which can be attributed to a high surface mobility, similar to the behavior of organic molecular glasses. Based on current theories, the crystal growth in Se95Te5 bulk and thin film was described by standard crystal growth models, with the kinetic mobility represented in terms of viscosity and surface diffusion. Furthermore, the surface crystal growth in Se95Te5 was monitored at temperatures down to the glass transition temperature and below, and exhibited a similar behavior to that observed in molecular glasses. In addition, the near-surface viscosities estimated from the smoothing process of a periodic sinusoidal surface grating embossed onto the surface of thin film and bulk samples exhibited similar values to the bulk viscosities.

Directional Growth of Human Neuronal Axons in a Microfluidic Device with Nanotopography on Azobenzene‐Based Material

AbstractAxonal dysfunction and degeneration are important pathological features of central nervous system (CNS) diseases and traumas, such as Alzheimer's disease, traumatic brain injury, ischemic stroke and spinal cord injury. Engineered microfluidic chips combined with human pluripotent stem cell (hPSC)‐derived neurons provide valuable tools for targeted in vitro research on axons to improve understanding of disease mechanisms and enhance drug development. Here, a polydimethylsiloxane (PDMS) microfluidic chip integrated with a light patterned substrate is utilized to achieve both isolated and unidirectional axonal growth of hPSC‐derived neurons. The isolation of axons from somas and dendrites and robust axonal outgrowth to adjacent, axonal compartment, is achieved by optimized cross‐sectional area and length of PDMS microtunnels in the microfluidic device. In the axonal compartment, the photoinscribed nanotopography on a thin film of azobenzene‐containing molecular glass efficiently guides the growth of axons. Integration of nanotopographic patterns with a compartmentalized microfluidic chip creates a human neuron‐based model that supports superior axonal alignment in an isolated microenvironment. The practical utility of the chip by studying oxygen‐glucose deprivation‐induced damage for the isolated and aligned axons is demonstrated here. The created chip model represents a sophisticated platform and a novel tool for enhanced and long‐term axon‐targeted in vitro studies.

Preparation and Structure of the Ion-Conducting Mixed Molecular Glass Ga2I3.17

Modern functional glasses have been prepared from a wide range of precursors, combining the benefits of their isotropic disordered structures with the innate functional behavior of their atomic or molecular building blocks. The enhanced ionic conductivity of glasses compared to their crystalline counterparts has attracted considerable interest for their use in solid-state batteries. In this study, we have prepared the mixed molecular glass Ga2I3.17 and investigated the correlations between the local structure, thermal properties, and ionic conductivity. The novel glass displays a glass transition at 60 °C, and its molecular make-up consists of GaI4– tetrahedra, Ga2I62– heteroethane ions, and Ga+ cations. Neutron diffraction was employed to characterize the local structure and coordination geometries within the glass. Raman spectroscopy revealed a strongly localized nonmolecular mode in glassy Ga2I3.17, coinciding with the observation of two relaxation mechanisms below Tg in the AC admittance spectra.

Identifying the structural relaxation dynamics in a strongly asymmetric binary glass former.

Here, we provide calorimetric and dielectric studies in asymmetric binary mixtures constituted by 2-picoline and triphenylethylene. Extreme broadening of the calorimetric glass transition is observed in the mixtures, which is accompanied by a large mismatch of the glass transition temperatures defined by the two techniques. As large broadening in the relaxation dispersion is identified in the mixtures of intermediate concentrations, strong temperature dependence of the relaxation dispersion is detected. The relation between the stretching exponent and non-linear factor derived from the Tool-Narayanaswamy-Moynihan-Hodge model shows a remarkable shift from the one established by pure molecular glass formers and symmetric mixtures. The unusual behaviors suggest an extreme dynamical decoupling mode imposed by the occurrence of strong concentration fluctuation.

Inline holographic inscription of diffractive lenses in azobenzene molecular glass thin films.

A simple inline holographic setup is used to fabricate holographic diffractive lenses using off-the-shelf components. The resulting surface relief gratings are inscribed directly in azobenzene-functionalized thin films with pitches that agree well with a theoretical Fresnel zone plate. The annular gratings have an outer radius of approximately 9 mm and an inner radius of less than 4 mm. Interfering laser beams, circularly polarized in the same direction, generally produce poor-quality gratings in azo-films, but the addition of a reference beam lens greatly improved their consistency and produced quality gratings with depths up to 400 nm. Multiple exposures produce multifocal diffractive lenses, while angling the sample resulted in focal lines, instead of focal points.

Topographical transition of submicron pillar array of azo molecular glass induced by circularly polarized light

The well-aligned submicron patterns on surfaces have attracted wide attention from scientific curiosity to practical applications. Understanding their formation and transition is highly desirable for efficient manufacture of the patterns for many usages. Here, we report a unique observation on self-organized topographical transition of submicron pillar array of an azo molecular glass, induced by irradiation with circularly polarized light. During gradual erasure of the patterns upon exposure to the light, which is a property of this material, a new set of pillars unexpectedly emerge with new one in middle of each triangle cell of the original array. The highly regular pillar array with triple area density is formed and finally stabilized in the process, as revealed by thorough investigation reported here. This unusual observation and its rationalization will be of benefit for deep understanding of the light–matter interaction and can be expected to be applied in different areas.

Correlation between kinetic fragility and Poisson's ratio from analysis of data for soft colloids

We consider the link between fragility and elasticity that follows from the analysis of the data for a set of soft colloid materials consisting of deformable spheres reported by [Mattsson et al., Nature, 2009, 462, 83]. The present work makes a quantitative analysis through an explicit definition for fragility index m in terms of density dependence, extending the corresponding formula of m for molecular systems in terms of temperature dependence. In addition, we fit the data for the high-frequency shear modulus for the respective soft-colloid to a corresponding theoretical expression for the same modulus. This expression for the elastic constant is in terms of the corresponding pair correlation function for the liquid treated as of uniform density. The pair correlation function is adjusted through a proper choice of the parameters for the two body interaction potential for the respective soft-colloid material. The nature of correlation between the fragility and Poisson ratio observed for the soft colloids is qualitatively different, as compared to the same for molecular glasses. The observed link between fragility of a metastable liquid and its elastic coefficients is a manifestation of the effects of structure of the fluid on its dynamics. The present work thus analyses the data on soft colloids and by introducing definitions from statistical mechanics obtains a correlation between kinetic fragility and Poissons's ratio for the soft material.