On the relaxation theory of glass transition in liquids

A method for calculating the constant in the main equation of glass transition (which relates the relxation time and the cooling rate near the glass transition temperature) with consideration given to the temperature dependence of the activation energy in this region is proposed. A modification of the main glass transition equation is considered. Application of this equation to the relaxation spectrometry of amorphous polymers and inorganic glasses is discussed.     

Thermodynamics of viscous flow and elasticity of glass forming liquids in the glass transition range

The elastic moduli of glasses from different chemical systems, including oxide, chalcogenide, oxynitride, and metallic, were investigated through the glass transition (T(g)), typically from 0.4 to 1.3 T(g). These data were used to interpret the temperature sensitivity of the shear viscosity coefficient obtained on the same materials. The relevant Gibbs free activation energy was estimated from the apparent heat of flow by means of the temperature dependence of the shear elastic modulus. The activation entropy associated with the viscous flow was also derived and was found to correlate with the fragile versus strong character of the glass forming liquids. Finally, the physicochemistry of the flow process was described on the basis of the glass network de-structuration which shows up through the temperature dependence of Poisson's ratio, and an expression for the shear viscosity coefficient is proposed which is chiefly based on the high temperature elastic behavior.     

Confinement effects in the hydrogen adsorption on paddle wheel containing metal-organic frameworks

The confinement effects upon hydrogen adsorption in Cu(II)-paddle wheel containing metal-organic frameworks (MOFs) were evaluated and rationalized in terms of the structural properties (cavity types and pore diameters) of PCN-12, HKUST-1, MOF-505, NOTT-103 and NOTT-112. First-principles calculations were employed to identify the strongest adsorption positions at the paddle wheel inorganic building unit (IBU). The adsorption centres due to confinement were located through analysis of 3D occupancy maps obtained from the hydrogen trajectories computed via molecular dynamics simulations. It was found that the confinement enhances the adsorption on the weakest adsorption centres around the IBU in regions close to the narrowest windows and promotes the formation of new adsorption regions into the small cavities. Our results indicate that at low pressure, the high H(2) uptake in these materials is partly due to the presence of small cavities (5.3-8.5 ?) or narrow windows where the long-range contribution to the adsorption becomes important. Conversely, confinement effects in cavities with diameters >12 ? were not observed.     

Polymer–nanoparticle interfacial interactions in polymer nanocomposites: Confinement effects on glass transition temperature and suppression of physical aging

The effects of confinement on glass transition temperature (T_g) and physical aging are measured in polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-vinyl pyridine) (P2VP) nanocomposites containing 10- to 15-nm-diameter silica nanospheres or 47-nm-diameter alumina nanospheres. Nanocomposites are made by spin coating films from sonicated solutions of polymer, nanofiller, and dye. The T_gs and physical aging rates are measured by fluorescence of trace levels of dye in the films. At 0.1–10 vol % nanofiller, T_g values can be enhanced or depressed relative to neat, bulk T_g (T_g,bulk) or invariant with nanofiller content. For alumina nanocomposites, T_g increases relative to T_g,bulk by as much as 16 K in P2VP, decreases by as much as 5 K in PMMA, and is invariant in PS. By analogy with thin polymer films, these results are explained by wetted P2VP–nanofiller interfaces with attractive interactions, nonwetted PMMA–nanofiller interfaces (free space at the interface), and wetted PS–nanofiller interfaces lacking attractive interactions, respectively. The presence of wetted or nonwetted interfaces is controlled by choice of solvent. For example, 0.1–0.6 vol % silica/PMMA nanocomposites exhibit T_g enhancements as large as 5 K or T_g reductions as large as 17 K relative to T_g,bulk when films are made from methyl ethyl ketone or acetic acid solutions, respectively. A factor of 17 reduction of physical aging rate relative to that of neat, bulk P2VP is demonstrated in a 4 vol % alumina/P2VP nanocomposite. This suggests that a strategy for achieving nonequilibrium, glassy polymeric systems that are stable or nearly stable to physical aging is to incorporate well-dispersed nanoparticles possessing attractive interfacial interactions with the polymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2935–2943, 2006     

On the role of frustration on the glass transition and polyamorphism of mesoscopically heterophase liquids

The model of heterophase fluctuations is developed accounting frustration of the mesoscopic solidlike fluctuons. Within the framework of this model, the glass transition and polyamorphous transformations are considered. It is shown that the frustration increases the temperature range in which the heterophase liquid state exists. the upper and lower boundaries of this temperature range are determined. These boundaries separate different phase states-amorphous solid, heterophase liquid, and fluid phases. Polyamorphous liquid-liquid transitions in the liquid are investigated. Frustration can call forth continuous fluid-solid phase transformation avoiding the first- or second-order phase transition. Conditions under which the first-order phase transition fraction takes place are formulated. Two scenarios of the first-order liquid-liquid polyamorphous transformation are described. As an example the glacial phase formation and the first-order liquid-liquid phase transition in triphenyl phosphate are considered and discussed. Impact of frustration on the liquid crystallization and crystallinity of the glassy state is studied.     

Small molecule‐mediated glass transition of acrylic copolymers: Effect of hydrogen bonding strength on glass transition temperature

Poly(styrene‐co‐ethyl acrylate) [P(St‐co‐EA)] with different ratios of St/EA was mixed with the small molecule 4,4′‐thio‐bis(6‐tert‐butyl‐m‐methyl phenol) (AO300) to investigate the influence of hydrogen bonding strength on the glass transition behavior. The glass transition temperature (T_g) linearly increased after adding AO300, and the slope value decreased with increased St/EA ratio. All lines could be extended to 62 °C, demonstrating that T_g of the small molecule in situ detected by the polymer chain was much higher than that by small molecule itself (29 °C). Fourier transform infrared spectroscopy analysis showed that the small molecules began to be self‐associated at a concentration where the hydrogen bonded carbonyl ratio of the bulk polymer was approximately 0.5 and irrespective of the St/EA ratio. Above the critical loading, the mixture's T_g negatively deviated from the linearly extended lines because of self‐association of the small molecules. The apparent T_g of AO300 was found to strongly depend on intermolecular hydrogen bonding number and strength. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 400–408     

Solvent effects on the thermodynamics of the formation of hydrogen bonded complexes ofm-cresol III

Thermodynamic parameters for the hydrogen bonded complexes of m-cresol with various bases in the solvent benzene have been determined from calorimetric and spectroscopic data. These data were analyzed by linear solvation energy relationships. When combined with data previously determined for the same complexes in CCl₄ and cyclohexane solvents, it is shown that solvent effects on the thermodynamics of hydrogen bond formation are due to solvation of the free m-cresol and base through dipolar and perhaps donor-acceptor interactions.     

Kinetic interpretation of the glass transition: Glass temperatures of n-alkane liquids and polyethylene

On the basis of an isoviscosity criterion for the glass transition (η_g ? 10¹³ poise) in liquids of low molecular weight, theoretical T_g values were calculated for the n-alkane series by the equation log η = log A + B/(T ? T₀), with the use of values reported by Lewis for the parameters. The T_g/T₀ ratio reaches a limiting value of 1.25 and ?_g = (T_g ? T₀)/2.3B = 0.027, a constant. Extrapolation to (CH₂)_∞ gives T_g = 200°K., T₀ = 160°K., and B = 640°K. This T_g is consistent with other estimates for poly-ethylene, and T₀ coincides with the temperature at which the “excess” liquid entropy for (CH₂)_∞ becomes zero from thermodynamic data. For polymer liquids it is proposed that E₀ = 2.3RB is determined by the internal barriers to rotation for the “isolated” polymer chains. Thus, E₀ = 2.9 kcal./mole for polyethylene, 3.0 kcal./mole for polystyrene, 5.7 kcal./mole for polyisobutylene, and 1.9 kcal./mole for polydimethylsiloxane.     

Nuclear quantum effects and hydrogen bonding in liquids

We have employed ab initio path integral molecular dynamics simulations to investigate the role of nuclear quantum effects on the strength of hydrogen bonds in liquid hydrogen fluoride. Nuclear quantum effects are shown to be responsible for a stronger hydrogen bond and an enhanced dipole-dipole interaction, which lead, in turn, to a shortening of the H...F intrachain distance. The simulation results are analyzed in terms of the electronic density shifts with respect to a purely classical treatment of the nuclei. The observed enhanced hydrogen-bond interaction, which arises from a coupling of intra- and intermolecular effects, should be a general phenomenon occurring in all hydrogen-bonded systems.     

A simple calorimeter for the heats of mixing study of associated liquids: Enthalpy of hydrogen bonded ethanol-butylamine complex

A simple dewar type calorimeter has been constructed to determine the enthalpy of mixing in dilute concentration range and its performance checked by determining the heats of mixing of cyclohexane (l)-n-hexane (2) and ethanol (1)n-hexane (2) systems. The heats of mixing ofn-butylamine withn-hexane and ethanol have been determined at 30° C. The enthalpy of ethanol-butylamine complex calculated by a thermochemical cycle was found to be-40.3 kJ/mol. NCL Communication No. 2492     

Confinement of hydrogen atom with Dirac equation

The problem of a particle confined in a spherical cavity is studied with the Dirac equation. A hard confinement is obtained by forcing the large component to vanish at the cavity radius. It is shown that the small component cannot vanish simultaneously at this radius. In the case of a confined hydrogen atom, the energies are given by an implicit equation. For some values of the radius, explicit analytical expressions of the energy exist like in the nonrelativistic case. Very accurate energies and wave functions are obtained with the Lagrange-mesh method with few mesh points. To this end, two differently regularized Lagrange-Jacobi bases associated with the same mesh are used for the large and small components. The importance of relativistic effects is discussed for hydrogen-like ions. The validity of this definition of hard confinement is discussed with a soft-confinement model studied with the R-matrix method.     

Quantum design of ionic liquids for extreme chemical inertness and a new theory of the glass transition

In many modern technologies (such as batteries and supercapacitors), there is a strong need for redox-stable ionic liquids. Experimentally, the stability of ionic liquids can be quantified by the voltage range over which electron tunneling does not occur, but so far, quantum theory has not been applied systematically to this problem. Here, we report the electrochemical reduction of a series of quaternary ammonium cations in the presence of bis(trifluoromethylsulfonyl)imide (TFSI) anions and use nonadiabatic electron transfer theory to explicate the results. We find that increasing the chain length of the alkyl groups confers improved chemical inertness at all accessible temperatures. Simultaneously, decreasing the symmetry of the quaternary ammonium cations lowers the melting points of the corresponding ionic liquids, in two cases yielding highly inert solvents at room temperature. These are called hexyltriethylammonium TFSI (HTE-TFSI) and butyltrimethylammonium TFSI (BTM-TFSI). Indeed, the latter are two of the most redox-stable solvents in the history of electrochemistry. To gain insight into their properties, very high precision electrical conductivity measurements have been carried out in the range +20 °C to +190 °C. In both cases, the data conform to the Vogel-Tammann-Fulcher (VTF) equation with “six nines” precision (R ²?>?0.999999). The critical temperature for the onset of conductivity coincides with the glass transition temperature T _g. This is compelling evidence that ionic liquids are, in fact, softened glasses. Finally, by focusing on the previously unsuspected connection between the molecular degrees of freedom of ionic liquids and their bulk conductivities, we are able to propose a new theory of the glass transition. This should have utility far beyond ionic liquids, in areas as diverse as glassy metals and polymer science.     

Moisture effects on the glass transition and the low temperature relaxations in semiaromatic polyamides

The influence of moisture absorption on the primary (glass) transition (T_a or T_g) and the low temperature relaxations of semiaromatic amorphous polyamides synthesized by isomeric aliphatic diamine and metha or para oriented phthalicdiacids has been investigated by means of differential scanning calorimeter (DSC) and dynamic mechanical thermal analyser (DMTA). The glass transition of semiaromatic polyamides was lowered due to the water absorption, and the β and the γ relaxations were as well. From the observed T_g and the difference in the heat capacity, the calculated T_g depression per 1 wt % water content was 12.3 K and the result was in good agreement with the experimental data. The depression of the glass transition may be expressed by the same manner as the plasticization of nylon 6 by water. The depressed β relaxation observed in the specimen containing a few percent of moisture was splitted into two transitions due to the reduction of water content, of which one was the elevation of the T_β and another was the simultaneous appearance of the T_γ, and then the single T_γ solely was observed for the completely dried specimen. The T_γ seemed to be merged into or not to be observed by the large and broad T_β transition when the sample was governed by a few percent of water, then it was emerged from the T_β due to water desorption. Thus, the T_β is believed to arise from the intermolecular hydrogen bonding between water molecules or between water and amide groups in wet polyamides. In addition, the γ relaxation originated from the peptide groups is attributable to the inter- and intramolecular hydrogen bonding between amide groups. © 1997 John Wiley & Sons, Inc. J Polyn Sci B: Polym Phys 35: 807–815, 1997     

The glass transition and the distribution of voids in room-temperature ionic liquids: a molecular dynamics study

The glass transition in prototypical room temperature ionic liquids has been investigated by molecular dynamics simulations based on an Amber-like empirical force field. Samples of [C(4)mim][PF(6)], [C(4)mim][Tf(2)N], and [C(3)mim][Tf(2)N] have been quenched from the liquid phase at T = 500 to a glassy state at T ～ 0 K in discontinuous steps of 20 K every 1.2 ns. The glass temperature estimated by simulation (T(g) = 209 K for [C(4)mim][PF(6)], T(g) = 204 K for [C(4)mim][Tf(2)N], and T(g) = 196 K for [C(3)mim][Tf(2)N]) agrees semi-quantitatively with the experimental values (T(g) = 193÷196 K for [C(4)mim][PF(6)], T(g) = 186÷189 K for [C(4)mim][Tf(2)N], and T(g) = 183 K for [C(3)mim][Tf(2)N]). A model electron density is introduced to identify voids in the system. The temperature dependence of the size distribution of voids provided by simulation reproduce well the experimental results of positron annihilation lifetime spectroscopy reported in G. Dlubek, Y. Yu, R. Krause-Rehberg, W. Beichel, S. Bulut, N. Pogodina, I. Krossing, and Ch. Friedrich, J. Chem. Phys. 133, 124502 (2010), with only one free parameter needed to fit the experimental data.     

Thermodynamics of the glass transition

A thermodynamic analysis of the glass transition in polymers is presented, which predicts, with a minimum of assumptions, three of the four inequalities which have consistently been observed experimentally: i.e., that specific heat and isothermal compressibility of the glass are lower than those of the rubber, and that the Prigogine-Defay ratio is larger than unity. The analysis cannot predict the fact that the thermal expansion coefficient of the glass is also lower than that of the rubber, and indeed the converse situation is not a thermodynamic impossibility.     

Size effects on miscibility and glass transition temperature of binary polymer blend films

After determining the size dependent miscibility of binary polymer blend films using molecular dynamics simulation and thermodynamics, the size dependent glass transition temperatures Tg(w,D) of several polymer blend films in miscible ranges are determined by computer simulation and the Fox equation where w is the weight fraction of the second component and D denotes thickness of films. The Tg(w,D) function of a thin film can decrease or increase as D decreases depending on their surface or interface states. The computer simulation results are consistent with available experimental results and theoretical results for polymer blend films of PPO/PS [poly(2,6-dimethyl-1,4-phenylene oxide)/polystyrene] and stereoregular PMMA/PEO [poly(methyl methacrylate)/poly(ethylene oxide)]. The physical background of the above results is related to the root of mean square displacement of thin films in their different regions.     

Environmental effects on hydrogen bonded systems: A quantum chemical study of proton relay model systems

In this paper an application of a reaction field theory of solvent effects has been made to study proton transfer mechanisms in hydrogen bonded systems coupled to an environment. The latter is simulated with reaction fields having variable strength and direction (defined with respect to the supermolecule's total dipole moment direction), together with superposed uniform external electric fields. Changes in proton potential curves and some other properties of a model water dimer and a water trimer are reported. The results are discussed in relation to relevant phenomena in biology and biochemistry, namely proton relay systems in enzymatic catalysis.     

Theoretical study on stabilities of multiple hydrogen bonded dimers

A series of self-constituted multiple hydrogen bonded (MHB) complexes has been investigated systematically by density functional theory (PBE1PBE /6-31G**), the Morokuma energy decomposition method (HF/6-31G**) and MP2 (6-31G** and 6-311++G**) calculation. We have discovered that (i) for doubly hydrogen bonded (DHB) complexes, both the interaction energy and stability increase with the charge transfer energy; (ii) for quadruple hydrogen bonded (QHB) complexes, cooperativity is the most important factor determining stability of the complex: stronger cooperative energy correlates well with larger interaction energy and thus more stable complex and vice versa; (iii) correlation energy plays an important role in intermolecular interactions. The correlation energy, mainly consisting of dispersive energy, also exhibits cooperativity in MHB dimers: positive for M-aadd and generally negative for other complexes.     

Determination of the glass transition temperature

The definition of the glass transition temperature, T _g, is recalled and its experimental determination by various techniques is reviewed. The diversity of values of T _g obtained by the different methods is discussed, with particular attention being paid to Differential Scanning Calorimetry (DSC) and to dynamic techniques such as Dynamic Mechanical Thermal Analysis (DMTA) and Temperature Modulated DSC (TMDSC). This last technique, TMDSC, in particular, is considered in respect of ways in which the heterogeneity of the glass transformation process can be quantified.