Predict the glass transition temperature of glycerol-water binary cryoprotectant by molecular dynamic simulation

Vitrification is proposed to be the best way for the cryopreservation of organs. The glass transition temperature (T_g) of vitrification solutions is a critical parameter of fundamental importance for cryopreservation by vitrification. The instruments that can detect the thermodynamic, mechanical and dielectric changes of a substance may be used to determine the glass transition temperature. T_g is usually measured by using differential scanning calorimetry (DSC). In this study, the T_g of the glycerol-aqueous solution (60%, wt/%) was determined by isothermal-isobaric molecular dynamic simulation (NPT-MD). The software package Discover in Material Studio with the Polymer Consortium Force Field (PCFF) was used for the simulation. The state parameters of heat capacity at constant pressure (C_p), density (ρ), amorphous cell volume (V_cell) and specific volume (V_specific) and radial distribution function (rdf) were obtained by NPT-MD in the temperature range of 90–270 K. These parameters showed a discontinuity at a specific temperature in the plot of state parameter versus temperature. The temperature at the discontinuity is taken as the simulated T_g value for glycerol–water binary solution. The T_g values determined by simulation method were compared with the values in the literatures. The simulation values of T_g (160.06–167.51 K) agree well with the DSC results (163.60–167.10 K) and the DMA results (159.00 K). We drew the conclusion that molecular dynamic simulation (MDS) is a potential method for investigating the glass transition temperature (T_g) of glycerol–water binary cryoprotectants and may be used for other vitrification solutions.     

Water Content�CWater Activity�CGlass Transition Temperature Relationships of Spray-Dried Boroj�� as Related to Changes in Color and Mechanical Properties

The water content–water activity–glass transition temperature relationships of commercial spray-dried borojó powder, with and without maltodextrin, have been studied as related to changes in color and mechanical properties. The GAB and Gordon and Taylor models were well fitted to the sorption and glass transition data, respectively. The Boltzman equation adequately described the evolution of the mechanical parameter characterized in the samples with the difference between the experimental temperature and the glass transition temperature (T _g) of the sample. The color of the samples showed a sigmoid change with water activity. The changes in the mechanical properties of borojó powder related to collapse development started when the sample moved to the rubbery state and began to be significant at about 10 °C above T _g. The increase in the molecular mobility from this point on also favors browning reactions. Maltodextrin presence slows the caking kinetics but induces color changes to spray-dried borojó powder.     

Evaluation of thermo-mechanical properties of graphene/carbon-nanotubes modified asphalt with molecular simulation

A comparative study was conducted to determine the effects of graphene and carbon nanotubes on the thermo-mechanical properties of asphalt binder using molecular simulations and experiments. Micro-morphology of graphene and carbon nanotubes was measured by scanning electron microscopy. Thermal stability and glass transition temperature were investigated by differential scanning calorimeter. Simulation results indicated that the T_g had slightly changed for graphene-modified asphalt (GMA) and carbon nanotubes-modified asphalt (CNsMA) and that the thermal expansion coefficients and thermal conductivity increased along with the adding amount of graphene or carbon nanotubes. The T_g calculated by density–temperature method was closer than the experimental T_g and the T_g decreased in the order of CNsMA, GMA and asphalt. Young’s modulus of asphalt, GMA and CNsMA were 9.2658, 25.7563 and 17.8249 GPa at 298 K, respectively, which indicated that thermo-mechanical properties of asphalt showed considerable improvements after the addition of graphene or carbon nanotubes, and carbon nanotubes-modified asphalt and GMA were promising candidates for the future modified asphalt.     

Characterization of the hidden glass transition of amorphous cyclomaltoheptaose

An amorphous solid of cyclomaltoheptaose (β-cyclodextrin, β-CD) was formed by milling its crystalline form using a high-energy planetary mill at room temperature. The glass transition of this amorphous solid was found to occur above the thermal degradation point of the material preventing its direct observation and thus its full characterization. The corresponding glass transition temperature (T_g) and the ΔC_p at T_g have, however, been estimated by extrapolation of T_g and ΔC_p of closely related amorphous compounds. These compounds include methylated β-CD with different degrees of substitution and molecular alloys obtained by co-milling β-CD and methylated β-CD (DS 1.8) at different ratios. The physical characterization of the amorphous states have been performed by powder X-ray diffraction and differential scanning calorimetry, while the chemical integrity of β-CD upon milling was checked by NMR spectroscopy and mass spectrometry.     

Glass transition investigations on highly crosslinked epoxy resins by molecular dynamics simulations

Molecular dynamics simulations at the atomistic level were performed to investigate the glass transition of a highly crosslinked thermoset epoxy resin system composed of diglycidyl ether bisphenol A and isophorone diamine. The crosslinked model was first constructed using a cyclic dynamic method, and extended by investigating the effect of conversion degree on the static properties of local structure, internal energy and volume shrinkage. Based on this model, a systematic investigation on volume, energy and dynamic properties against temperature was made, which determined the glass transition temperature (T_g). The T_gs obtained from various volumetric and energy properties agree well with the differential scanning calorimetry experimental data available, yet a dynamic T_g obtained from the diffusion coefficient is relatively higher. Moreover, the investigation on epoxy segmental dynamics confirmed that the glass transition of the highly crosslinked epoxy resin has a strong dependence on the backbone bond torsional kinetics.     

A combined experimental and computational study on the material properties of shape memory polyurethane

A type of shape memory polyurethane with 60 wt% hard segments (SMPU60) was prepared. Its material properties were tested by dynamic mechanical analysis (DMA) and Instron, and simulated using fully atomistic molecular dynamics (MD). The glass transition temperature (T _g) of SMPU60 determined by DMA is 316 K, which is slightly lower than that estimated through MD simulations (T _g = 328 K) , showing the calculated T _g is in good agreement with experimental data. A complex hydrogen bonding network was revealed with the calculation of radial distribution functions (RDFs). The C═O⋯H bond is the predominant hydrogen-bonding interaction. With increasing temperature, both the hydrogen bonding and the moduli decreased, and the dissociation of intermolecular hydrogen bonding induced the decrease of the moduli.     

Glass transition behavior of the vitrification solutions containing propanediol, dimethyl sulfoxide and polyvinyl alcohol

Knowledge of the glass transition behavior of vitrification solutions is important for research and planning of the cryopreservation of biological materials by vitrification. This brief communication shows the analysis for the glass transition and glass stability of the multi-component vitrification solutions containing propanediol (PE), dimethyl sulfoxide (Me₂SO) and polyvinyl alcohol (PVA) by using differential scanning calorimetry (DSC) during the cooling and subsequent warming between 25 and −150 °C. The glass formation of the solutions was enhanced by introduction of PVA. Partial glass formed during cooling and the fractions of free water in the partial glass matrix increased with the increasing of PVA concentration, which caused slight decline of glass transition temperature, T_g. Exothermic peaks of devitrification were delayed and broadened, which may result from the inhibition of ice nucleation or recrystallization of PVA.     

The effect of alumina nanoparticles on the thermal properties of PMMA: a molecular dynamics simulation

Metal oxides, as one of the most promising flame retardant additives, improve the fire retardant and the thermal stability properties of polymers. In the present study, molecular dynamics (MD) simulations based on the united atom model were applied to study the effect of alumina nanoparticles on the density, thermal conductivity, heat capacity, and thermal diffusivity of isotactic poly(methyl methacrylate) (is-PMMA). Thermal diffusivity of PMMA and PMMA/alumina nanocomposite were investigated through calculating thermal conductivity, density and heat capacity in the range of 300–700?K. Heat capacity can be calculated using fluctuations properties of energy. Thermal conductivity was calculated through the nonequilibrium molecular dynamics (NEMD) simulation by Fourier’s law approach. Our results show that the addition of alumina nanoparticles decreases the heat capacity and increases the glass transition temperature (T_g), thermal conductivity and thermal diffusivity of the PMMA. Therefore, the addition of alumina nanoparticles to PMMA improves the fire retardancy of the polymer. In addition, we illustrate the links between the intermolecular and bulk properties of PMMA in the presence of the alumina nanoparticles.     

The Roles of Vitrification of Stabilizers/Matrix Formers for the Redispersibility of Drug Nanocrystals After Solidification: a Case Study

To elucidate the roles of vitrification of stabilizers/matrix formers for the redispersibility of drug nanocrystal powder after solidification at storage stress, the influence of different drying methods and storage stresses on stability of drug nanocrystals was systemically investigated. A poorly soluble drug, baicalin, used as model drug was converted into baicalin nanocrystals (BCN-NC). The residual moisture contents of BCN-NC were applied at two different stress conditions defined as “conservative” (<1%) and “aggressive” (>1%), respectively. The influence of different stabilizers, matrix formers, and storage stresses on the redispersibility of BCN-NC powder was systemically investigated, respectively. The results showed that storage stresses had significantly influence the redispersibility of BCN-NC. Aggressive storage temperature and residual moisture could be unfavorable factors for stability of drug nanocrystals, due to the exacerbation of aggregation of BCN-NC induced by vitrification. It was demonstrated that vitrification of spray-dried BCN-NC was dependent on temperature and time. The polymeric stabilizers hydroxypropylmethylcellulose (HPMC) and sodium carboxymethyl starch (CMS-Na) with high glass transition temperature (T _g) played more important role in protecting the BCN-NC from breakage during storage, compared to the surfactants Tween 80, d-α-tocopherol acid polyethylene glycol 1000 succinate (TPGS), or RH 40. Besides, the polyvinylpyrrolidone K30 (PVP K30) and lactose with high T _g were effective matrix formers for preserving the redispersibility of BCN-NC. It was concluded that the vitrification transition of stabilizers/matrix formers could be responsible for aggregation of drug nanocrystals during storage, which was a time-dependent process. The suitable residual moisture contents (RMC) and T _g were very important for preserving the stability of drug nanocrystals during storage.     

Practical Considerations for Determination of Glass Transition Temperature of a Maximally Freeze Concentrated Solution

Glass transition temperature is a unique thermal characteristic of amorphous systems and is associated with changes in physical properties such as heat capacity, viscosity, electrical resistance, and molecular mobility. Glass transition temperature for amorphous solids is referred as (T _g), whereas for maximally freeze concentrated solution, the notation is (T _g′). This article is focused on the factors affecting determination of T _g′ for application to lyophilization process design and frozen storage stability. Also, this review provides a perspective on use of various types of solutes in protein formulation and their effect on T _g′. Although various analytical techniques are used for determination of T _g′ based on the changes in physical properties associated with glass transition, the differential scanning calorimetry (DSC) is the most commonly used technique. In this article, an overview of DSC technique is provided along with brief discussion on the alternate analytical techniques for T _g′ determination. Additionally, challenges associated with T _g′ determination, using DSC for protein formulations, are discussed. The purpose of this review is to provide a practical industry perspective on determination of T _g′ for protein formulations as it relates to design and development of lyophilization process and/or for frozen storage; however, a comprehensive review of glass transition temperature (T _g, T _g′), in general, is outside the scope of this work.     

Molecular dynamics simulations of a cyclic-DP-240 amylose fragment in a periodic cell: Glass transition temperature and water diffusion

Molecular dynamics simulations using AMB06C, an in-house carbohydrate force field, (NPT ensembles, 1 atm) were carried out on a periodic cell that contained a cyclic 240 glucose residue amylose fragment (c-DP-240) and TIP3P water molecules. Molecular conformation and movement of the amylose fragment and water molecules at different temperatures were examined. The periodic cell volume, density, and potential energy were determined at temperatures above and below the glass transition temperature (T_g) in 25 K increments_. The amorphous cell is constructed through successive dynamic equilibration steps at temperatures above the assumed T_g value and the temperature successively lowered until several temperature points were obtained below T_g. Molecular dynamics simulations were continued for at least 500 ps or until the volume drift stopped and remained constant for several hundred picoseconds. The T_g values were found by noting the discontinuity in slope of the volume (V), potential energy (PE), or density (ρ) versus 1/T. The changes in flexibility and motion of the amylose chain as well as differences in self diffusion coefficients of water molecules are described. The final average T_g value found (316 K) is in agreement with experimental values, i.e. 320 K.     

Effect of Water Content on the Glass Transition Temperature of Calcium Maltobionate and its Application to the Characterization of Non-Arrhenius Viscosity Behavior

To understand the fundamental physical properties of calcium maltobionate (MBCa), its water sorption isotherm, glass transition temperature (T _g), and viscosity (η) were investigated and compared with those of maltobionic acid (MBH) and maltose. Although amorphous maltose crystalized at water activity (a _w) higher than 0.43, MBCa and MBH maintained an amorphous state over the whole a _w range. In addition, MBCa had a higher T _g and greater resistance to water plasticizing than MBH and maltose. These properties of MBCa likely originate from the strong interaction between MBCa and water induced by electrostatic interactions. Moreover, the effects of temperature and water content on η of an aqueous MBCa solution were evaluated, and its behavior was described using a semi-empirical approach based on a combination of T _g extrapolated by the Gordon-Taylor equation and a non-Arrhenius formula known as the Vogel–Fulcher–Tammann equation. This result will be useful for understating the effect of MBCa addition on the solution’s properties.     

Morphological properties and thermoanalysis of micronized cassava starch

The granule morphology, microstructure, and thermal properties of micronized cassava starch prepared by a vacuum ball-grinding machine were investigated. Scanning electron microscopy (SEM) analysis indicated that the morphology of starch granule changes during the ball-grinding treatment. Differential scanning calorimetry (DSC) analysis indicated that the maximum peak temperature (T_p) of the gelatinization process, the glass transition (T_g), and peak height index (PHI) for the starch granules decreased when the size of micronized starch granules was reduced. When the size of starch granules was reduced beyond 9.11 μm, they have a tendency to agglomerate and their ΔH were increased. The granule size has a significant effect on the gelatinization properties of cassava starch. This study will provide useful information of the micronized starch for its potential industrial application.     

Isolation and Identification of γ-l-Glutamyl-l-glutamic Acid from Tobacco Cells in Suspension Culture

The Maillard Reaction (MR) rate below the glass transition temperature (T_g) for various model glassy food systems was studied at temperatures between 40 °C and 70 °C. As a sample, freeze-dried glucose and lysine systems embedded in various glassy matrices (e.g., polyvinylpyrrolodone and trehalose) were used, and the MR rate below the T_g was compared among the various glassy matrices. The extent of MR was estimated spectrophotometrically from the optical density at 280 nm (OD₂₈₀), and the MR rate (k₂₈₀) was determined as a pseudo zero order reaction rate from the time course of OD₂₈₀. Although k₂₈₀ was described by the Arrhenius plot, the temperature dependence of k₂₈₀ was almost the same and the intercept was different among the matrices. From the comparison of k₂₈₀, it was suggested that the MR rate in glassy matrix was affected not only by the T_g, but also by the hydrogen bonding between MR reactants and glassy matrix.     

Computation of densities,bulk moduli and glass transition temperatures of vinylic polymers from atomistic simulation

The aim of molecular modeling is to mimic reality by considering approximations appropriate to the scale at which the simulation is carried out. At the atomic level, forcefields that represent average atomic interactions are used. However, the phase space has to be adequately explored in order to compare successfully computed and experimental properties. The procedure exposed in this article considers an initial selection of relevant configurations on which a simulated annealing process is applied using the first generation forcefield OPLS, followed by a uniform hydrostatic compression using the second generation forcefield COMPASS©. The resulting data are fitted by an equation of state, from which density and bulk modulus are determined. The glass transition is then simulated and T _gs are computed. Our approach is tested using a series of vinylic polymers, which differ from each other by small variations in atomic interaction combinations. The excellent agreement with experimental data shows the validity of the procedure exposed. Moreover, a clear linear relationship between simulated and experimental T _gs is revealed.     

Glass transition behavior of octyl β-D-glucoside and octyl β-D-thioglucoside/water binary mixtures

The lyotropic behavior and glass-forming properties of octyl β-d-glucoside (C8Glu) and octyl β-d-thioglucoside (C8SGlu)/water binary mixtures were evaluated using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The results clearly indicate that the mixture forms a glass in the supercooling state of liquid crystalline phases such as cubic, lamellar, and smectic. The glass transition temperature (T_g) of the mixture was strongly dependent on solute concentration, with a higher concentration correlating with a higher T_g. The experimental T_g was consistent with the predicted value calculated using the Couchman-Karasz equation in both the C8Glu and C8SGlu/water mixtures. The change of heat capacity at T_g showed the two bending points under variation of concentrations. And the highest temperature of phase transition from lamellar to isotropic solution was observed at around 50% molar concentration. It was expected that non-percolated state of water existed in extremely higher concentration ranges.     

Refinement of a coarse-grained model of poly(2,6-dimethyl-1,4-phenylene ether) and its application to blends of PPE and PS

A coarse-grained (CG) molecular simulation model has been refined for poly(2,6-dimethyl-1,4-phenylene ether) (PPE). This was successfully validated against atomistic simulation and experimental data. Particularly, the glass transition temperature (T_g) of PPE was studied using both atomistic and CG models and compared favourably to experimental data. In addition, we used the CG model together with an existing Martini CG model of polystyrene (PS) to study the blending behaviour of these two polymers. We solved the problem to mix the different potentials and molecular dynamics of high-molecular-weight blends of PPE/PS was performed in detail.     

Application of the Kwei equation to model the Tg behavior of binary blends of sugars and salts

Vitrification of sugar-based solutions plays an important role in cryopreservation, lyophilization, and the emerging field of anhydrous preservation. An understanding of the glass transition characteristics of such formulations is essential for determining an appropriate storage temperature to ensure an extended shelf life of vitrified products. To better understand the effect of salts on the glass transition temperature (T_g) of glass-forming sugars, we investigated several data-fitting models (Fox, Gordon–Taylor and Kwei) for sugar–salt formulations using data from the literature, as well as new data generated on blends of trehalose and choline dihydrogen phosphate (CDHP). CDHP has recently been shown to have promise as a stabilizing agent for proteins and DNA. The Kwei equation, which has a specific parameter characterizing intermolecular interactions, provides good fits to the T_g data for sugar–salt blends, and complements other commonly used models that are frequently used to model T_g data.     

Structure and properties of carboxymethyl cellulose/soy protein isolate blend edible films crosslinked by Maillard reactions

Edible films based on carboxymethyl cellulose (CMC) and soy protein isolate (SPI), compatibilized by glycerol, were prepared by solution casting. The effects of CMC content on blend structure, thermal stability, water solubility and water sorption, and mechanical properties were systematically investigated. Fourier transform infrared (FTIR) spectra showed that Maillard reactions occurred between CMC and SPI, and X-ray diffraction (XRD) scans indicated that the Maillard reactions greatly reduced the crystallinity of SPI. According to differential scanning calorimetry (DSC) analysis, CMC/SPI blends had a single glass transition temperature (T_g) between 75 and 100 °C, indicating that CMC and SPI form one phase blends. Increasing the CMC content improved the mechanical properties and reduced the water sensitivity of blend films. The results indicate that the structure and properties of SPI edible films were modified and improved by blending with CMC.     

Melting, glass transition, and apparent heat capacity of α-d-glucose by thermal analysis

The thermal behaviors of α-d-glucose in the melting and glass transition regions were examined utilizing the calorimetric methods of standard differential scanning calorimetry (DSC), standard temperature-modulated differential scanning calorimetry (TMDSC), quasi-isothermal temperature-modulated differential scanning calorimetry (quasi-TMDSC), and thermogravimetric analysis (TGA). The quantitative thermal analyses of experimental data of crystalline and amorphous α-d-glucose were performed based on heat capacities. The total, apparent and reversing heat capacities, and phase transitions were evaluated on heating and cooling. The melting temperature (T_m) of a crystalline carbohydrate such as α-d-glucose, shows a heating rate dependence, with the melting peak shifted to lower temperature for a lower heating rate, and with superheating of around 25 K. The superheating of crystalline α-d-glucose is observed as shifting the melting peak for higher heating rates, above the equilibrium melting temperature due to of the slow melting process. The equilibrium melting temperature and heat of fusion of crystalline α-d-glucose were estimated. Changes of reversing heat capacity evaluated by TMDSC at glass transition (T_g) of amorphous and melting process at T_m of fully crystalline α-d-glucose are similar. In both, the amorphous and crystalline phases, the same origin of heat capacity changes, in the T_g and T_m area, are attributable to molecular rotational motion. Degradation occurs simultaneously with the melting process of the crystalline phase. The stability of crystalline α-d-glucose was examined by TGA and TMDSC in the melting region, with the degradation shown to be resulting from changes of mass with temperature and time. The experimental heat capacities of fully crystalline and amorphous α-d-glucose were analyzed in reference to the solid, vibrational, and liquid heat capacities, which were approximated based on the ATHAS scheme and Data Bank.