On the scaling law of the evolution of lap-shear strength with healing temperature at amorphous polymer−polymer interfaces and surface glass transition

The evolution of lap-shear strength (σ) with healing temperature T _h at symmetric and asymmetric amorphous polymer−polymer interfaces formed of the samples with vitrified bulk has been investigated. It has been found that the square root of the lap-shear strength behaves with respect to healing temperature as σ ^1/2 ~ T _h both at symmetric and asymmetric interfaces. Basing on this scaling law between σ and T _h, the values of the surface glass transition temperature ( T_g^surface ) \left( {T_{\rm{g}}^{\rm{surface}}} \right) have been estimated for a number of amorphous polymers by the extrapolation of the experimental curves σ ^1/2 ~ T _h for symmetric polymer−polymer interfaces and, in some cases, for asymmetric, both compatible and incompatible, polymer−polymer interfaces, to zero strength. A significant reduction in surface glass transition temperature T_g^surface T_{\rm{g}}^{\rm{surface}} with respect to the glass transition temperature of the polymer bulk ( T_g^bulk ) \left( {T_{\rm{g}}^{\rm{bulk}}} \right) , reported earlier, has been confirmed by the use of the new proposed approach. The quasi-equilibrium surface glass transition temperature T_g^surface T_{\rm{g}}^{\rm{surface}} of amorphous polystyrene (PS) has been predicted in the framework of an Arrhenius approach using the plot “logarithm of healing time − reciprocal surface glass transition temperature T_g^surface￠￠ T_{\rm{g}}^{\rm{surface}}\prime \prime and the activation energy of the surface alpha-relaxation of PS has been calculated.     

Resolving the controversy on the glass transition temperature of water?

We consider experimental data on the dynamics of water (1) in glass-forming aqueous mixtures with glass transition temperature T(g) approaching the putative T(g) = 136 K of water from above and below, (2) in confined spaces of nanometer in size, and (3) in the bulk at temperatures above the homogeneous nucleation temperature. Altogether, the considered relaxation times from the data range nearly over 15 decades from 10(-12) to 10(3) s. Assisted by the various features in the isothermal spectra and theoretical interpretation, these considerations enable us to conclude that relaxation of un-crystallized water is highly non-cooperative. The exponent β(K) of its Kohlrausch stretched exponential correlation function is not far from having the value of one, and hence the deviation from exponential time decay is slight. Albeit the temperature dependence of its α-relaxation time being non-Arrhenius, the corresponding T(g)-scaled temperature dependence has small steepness index m, likely less than 44 at T(g), and hence water is not "'fragile" as a glassformer. The separation in time scale of the α- and the β-relaxations is small at T(g), becomes smaller at higher temperatures, and they merge together shortly above T(g). From all these properties and by inference, water is highly non-cooperative as a glass-former, it has short cooperative length-scale, and possibly smaller configurational entropy and change of heat capacity at T(g) compared with other organic glass-formers. This conclusion is perhaps unsurprising because water is the smallest molecule. Our deductions from the data rule out that the T(g) of water is higher than 160 K, and suggest that it is close to the traditional value of 136 K.     

Is adhesion between amorphous polymers sensitive to the bulk glass transition?

Two bulk samples of one and the same or of different amorphous polymers were brought into contact and held for a chosen period of time at a constant healing temperature (T) over the interval of T from below the bulk glass transition temperature (T _g ^bulk) by ~50 °C to above T _g ^bulk by ~10 °C. As formed adhesive joints were shear-fractured in tension at room temperature, and lap-shear strength (σ) was measured as a function of T. It has been found that σ develops with T as logσ?~?1/T both at symmetric and asymmetric interfaces of polystyrene, poly (methyl methacrylate) and poly (2,6-dimethyl-1,4-phenylene oxide). This behaviour implies that there is no discontinuity in the evolution of σ when going through T _g ^bulk, and that this process is controlled by one and the same diffusion mechanism both below and above T _g ^bulk. The results obtained indicate that the contact layer of the polymers investigated is in the viscoelastic state at T well below T _g ^bulk and support the concept of a decrease in the T _g of a near-surface layer with respect to T _g ^bulk.     

Influence of macroinitiator’s glass transition temperature on the response times of polymer dispersed liquid crystals

Glass transition temperature (T_g), an important parameter of polymer, was reported to have great influence on the electro-optical properties of polymer dispersed liquid crystals (PDLCs). In this study, macroinitiators with different T_g were synthesised by reversible addition fragmentation chain transfer polymerisation, and used to prepare PDLCs with different T_g block chains. The effect of different T_g of the block chains on response times was investigated. It was found that rise time decreased and decay time increased with the decrease of the block chain’s T_g. We proposed a possible mechanism by which T_g of the block chains influence response times.     

Studies of temperature influence on adsorption behaviour of nonionic polymers at the zirconia–solution interface

The temperature influence (15–35 °C) on the adsorption mechanism and conformation of nonionic polymers (polyethylene glycol (PEG), polyethylene oxide (PEO) and polyvinyl alcohol (PVA)) on the zirconium dioxide surface was examined. The applied techniques (spectrophotometry, viscosimetry, potentiometric titration and microelectrophoresis) allowed characterization of the changes in structure and thickness of polymer adsorption layers with the increasing temperature. The rise of temperature favours more stretched conformation of polymer chains on the ZrO₂ surface, which results in higher adsorption and thicker adsorption layer. Moreover, these conformational changes of adsorbed macromolecules affect the electric (solid surface charge density) and electrokinetic (zeta potential) properties of the zirconia–polymer interface. The obtained data indicate that the polyvinyl alcohol adsorption has a greater influence on zirconia properties in comparison to that of PEG and PEO. It is due to the presence of acetate groups in the PVA macromolecules (degree of hydrolysis 97.5%), which undergo dissociation.     

Twinkling fractal theory of the glass transition: Rate dependence and time–temperature superposition

The twinkling fractal theory (TFT) of the glass transition temperature T_g provides a new method of analyzing rate effects and time–temperature superposition in amorphous materials. The rate dependence of T_g was examined in the light of new experimental and theoretical evidence for the nature of the dynamic heterogeneity near T_g. As T_g is approached from above, dynamic solid fractal clusters begin to form and eventually percolate rigidity at T_g. The percolation cluster is a solid fractal and to the observer, appears to “twinkle” as solid and liquid clusters interchange in dynamic equilibrium with a vibrational density of states g(ω) ∼ ω. The solid-to-liquid twinkling frequencies ω_TF are controlled by the Boltzmann population of intermolecular oscillators in excited energy levels of their anharmonic potential energy functions U(x) such that ω_TF = ω exp −B(T*² − T²)/kT in which T* ≈ 1.2T_g. An oscillator changes from a solid to a liquid when a thermal fluctuation causes it to expand beyond its inflection point in the anharmonic potential. This leads to a continuous solid fraction P_s near T_g given by P_S ≈ 1−[(1 − p_c) T/T_g] where p_c ≈ 1/2 is the rigidity percolation threshold. Since g(ω) is continuous from very low to very high frequencies, the complex twinkling dynamics existing near T_g produces a continuous relaxation spectrum with many different length scales and times associated with the fractal clusters. The twinkling frequencies control the kinetics of T_g such that for a given observation time t when the rate γ > 1/t, only those parts of the twinkling spectrum with ω > γ can contribute to relaxation or percolation upto time t. The most important results in this article are as follows: The TFT describes the rate dependence of T_g, both for DSC thermal heating/cooling rates and DMA frequencies as the classic T_g − lnγ law as T_g(γ) = T_go + (k/2B) ln γ/γ_o in which the constant B = 0.3 cal/mol K². The constant B appears quite universal for the 17 thermoset polymers investigated in this study and 18 linear polymers investigated by others. Many other amorphous metal and ceramic glass materials exhibited the same rate law but required a new B value approximately half that for polymers. The same B = 0.3 value was also used to successfully describe the TTS shift factors using the twinkling fractal frequencies ω_TF = ωexp −B(T*² − T²)/kT, as ln a_T(TFT) = exp B(T_R² − T²)/kT, which gave comparable results with the classical WLF equation, log a_T = [−C₁(T − T_R)]/[C₂ + (T − T_R)]. The advantage of the TFT over the WLF is that C₁ and C₂ are not universal constants and must be determined for every material, whereas the TFT uses one known constant B which appears to be the same for all polymers. The TFT has also been found to describe the strong and fragile nature of the viscosity behavior of liquids and the rate and temperature dependence of the yield stress in polymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2578–2590, 2009     

Effect of copper addition on glass transition of silicate–phosphate glasses

Glass transformation effect of mixed SiO₂?CP₂O₅?CK₂O?CMgO?CCaO?CCuO glasses was studied by DSC, XRD, SEM, and Raman spectroscopy methods. The relationship between the parameters characterizing glass transformation effect and an amount of phosphorous and copper forming the glassy structure was discussed. It was shown that an increasing content of phosphorous increased solubility of copper in the structure of the studied glasses which was the result of P?CO?CCu bonds formation. Degree of changes of T _g, ?c _p, and time of relaxation values were higher in glasses with higher content of P₂O₅ and CuO. The observed relations were explained on the basis of the local atomic interactions in the structure of glass.     

Effect of hydrofluoric acid treatment on the crystallisation behaviour of Zr–Ti–Cu–Al–Ni metallic glass

The surface of the metallic glass Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ has been modified by hydrofluoric acid (HF) etching treatment. The devitrification and crystallisation process has been mainly studied by isothermal differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The nucleation-growth process is the mechanism of crystallisation; nevertheless the JMA-model is not applicable in every situation. Alternative methods are used to interpret the data. Results show the devitrification process is strongly affected by surface nucleation, which depends on the surface topology. Zr₃Al₂ is the first phase formed on the concave areas whereas the quasicrystalline and Zr₂Ni-based phases appears in the flatter ones. Nevertheless, the presence of an oxide surface layer acts upon the surface nucleation and dwarfs the consequences of such topological differences. Moreover, the quasicrystalline formation appears also to be in competition with the parallel formation of ZrO₂ due to thermal re-oxidation during the DSC experiments.     

On the microelectrode behaviour of graphite–epoxy composite electrodes

Electrodes fashioned from conducting particles embedded in insulating binder show some properties commonly associated with arrays of microelectrodes, viz. independence of current from convective flow and sigmoidal current–voltage curves. We have systematically investigated the electrochemical behaviour of a range of composite formulations. The fraction of the surface area that is conducting and the size of the conducting features have been quantified using the novel technique of conducting atomic force microscopy (C-AFM). We have shown that flow sensitivity and voltammetric behaviour are correlated with results from the C-AFM. The more dilute formulations behave like arrays of microelectrodes, though exhibiting large time constants. The origin of this behaviour is discussed.     

Study of phase behaviour for the aqueous two-phase polymer–polymer systems using the modified UNIQUAC-NRF model

A modified form of the UNIQUAC-NRF activity coefficient model was used to study the phase behaviour of the aqueous two-phase systems (ATPSs) of polyethylene glycol (PEG) and dextran (DEX) with different molecular weights at various temperatures. In the proposed model, a ternary interaction parameter was added to the expression for the excess Gibbs free energy and, in turn, to the corresponding activity coefficient rendered by the UNIQUAC-NRF model. The combinatorial part of the new model takes the same form as that of the original UNIQUAC model and the residual part considers the nonrandomness and also the binary and the ternary interactions among the molecules in mixtures of PEG, DEX, and water. The results show that the new model can more accurately correlate the experimental data for the systems studied in this work than those obtained from the original UNIQUAC and the UNIQUAC-NRF activity coefficient models. In order to favourably compare the results the same minimisation procedure and the same experimental data were used for the models studied in this work. The results for the Root Mean Square Deviations (RMSD) produced by the three UNIQUAC-based models are also reported.     

Thermal analysis of cadmium addition on the glass transition and crystallization kinetics of Se–Te–Sn glassy network

Journal of Thermal Analysis and Calorimetry - Calorimetric measurements have been performed in quaternary glassy system Se78−xTe20Sn2Cdx (x = 0, 2, 4, and 6) to study the...     

Measuring the optical chirality of molecular aggregates at liquid–liquid interfaces

Some new experimental methods for measuring the optical chirality of molecular aggregates formed at liquid–liquid interfaces have been reviewed. Chirality measurements of interfacial aggregates are highly important not only in analytical spectroscopy but also in biochemistry and surface nanochemistry. Among these methods, a centrifugal liquid membrane method was shown to be a highly versatile method for measuring the optical chirality of the liquid–liquid interface when used in combination with a commercially available circular dichroism (CD) spectropolarimeter, provided that the interfacial aggregate exhibited a large molar absorptivity. Therefore, porphyrin and phthalocyanine were used as chromophoric probes of the chirality of itself or guest molecules at the interface. A microscopic CD method was also demonstrated for the measurement of a small region of a film or a sheet sample. In addition, second-harmonic generation and Raman scattering methods were reviewed as promising methods for detecting interfacial optical molecules and measuring bond distortions of chiral molecules, respectively.     

The gel-like structure of polymer in thin films: an explanation of the thickness dependent glass transition?

The understanding of the origin of the thickness (h) dependent glass transition temperature, T_g(h), reported over the last decade for supported and freely standing thin polymer films, is still unclear. Indeed, the spin-coating process, the interfacial adsorption as well as the freezing-in of non-equilibrated chain conformations and orientations caused by fast solvent evaporation could result in partially disentangled chains which can be depicted as a gel-like structure. The effect of PMMA stereoregularity on the chain conformation and orientation and its persistence length in thin films is discussed. Moreover, striking evolutions of T_g(h) by changing the nature of the solvent support the assumption of a specific organisation of the chains in thin films which can hold over thickness far above Rg.     

Isochronal temperature–pressure superpositioning of the α-relaxation in type-A glass formers

Dielectric spectra were obtained at ambient and elevated pressures for three ‘type-A’ glass formers, which exhibit excess intensity on the high frequency side of the structural relaxation peak. The response to pressure of the peak maximum and the excess wing suggests categorization of such glass formers into two groups: associated liquids, in which the α-relaxation and the excess wing have a different pressure dependence, and van der Waals liquids, which at fixed value of the α-relaxation time, conform to temperature–pressure superpositioning. This distinction is believed to arise from the change in the number of intermolecular bonds (non-dispersive interactions) with volume.     

Radiationless transitions in the excited aromatics—oxygen systems adsorbed on porous glass at low temperature

The efficiency of fluorescence quenching by oxygen has been extensively studied for a number of aromatic hydrocarbons adsorbed on porous Vycor glass plates immersed in liquid oxygen at 77 K. Quite unexpected is the fact that the fluorescences of most aromatic hydrocarbons were detectably strong even in the presence of liquid oxygen. It has been found that the intrinsic rate constants for the oxygen-enhanced radiationless transitions in the excited aromatics—oxygen pairs are reasonably explained by taking account of the Franck—Condon factors, the location of the second triplet states, and the symmetries of the related electronic states. It has been concluded that in some aromatics, the radiationless transition from the excited singlet state to the second triplet state (S₁ → T₂) is possible, leading to high intrinsic quenching rate constants by large Franck—Condon factors. In other aromatics, the major process is S₁ → T₁ and the rate constants in these cases depend on the symmetry of the S₁ and T₁ electronic wave functions. A relatively strong phosphorescence of benzophenone has also been observed in the presence of liquid oxygen. The results of the phosphorescence quenching efficiencies for benzophenone and naphthalene by oxygen are also discussed.     

On the analysis of ionic surface conduction to unravel charging processes at macroscopic soft and hard solid–liquid interfaces

The electrohydrodynamics of soft interfaces and the processes underlying interfacial charge formation by, for example, unsymmetrical ion adsorption are important aspects of current research on the electrosurface phenomena. In particular, the recent progress in both fields greatly benefits from the now-possible accurate evaluation and quantitative interpretation of (ionic) excess conductivities at solid surfaces and in 3D polyelectrolytic architectures. Achievements in the proper formulation of the conceptual and theoretical framework and in the improvement of measurement capabilities have been tightly connected to the work of Johannes (Hans) Lyklema (1930–2017). Considering his valuable contributions, we herein summarize the theoretical basis of surface conductivity analyses, review the experimental options for the quantification of the surface conductivity at macroscopic planar solid–liquid interfaces, and discuss exemplary surface conductivity case studies for soft and hard interfaces permeable or not to ions and fluid flow.Dedication: Johannes Lyklema (November 23, 1930–October 31, 2017) was a key scientist in colloid and interface science. He completed his doctorate at the University of Utrecht in 1957 under the supervision of Professor J. Th. G. Overbeek with a thesis entitled ‘Adsorption of counterions.’ In 1963, he was appointed a Professor of Physical and Colloid Chemistry at Wageningen Agricultural College (later named Wageningen University), a position he kept until his retirement in 1995. Under his directorate, the Laboratory of Physical and Colloid Chemistry at the Wageningen University became a world-leading research center known for its key research in the fields of interfaces, macromolecules, and electrochemistry. Johannes Lyklema published almost 400 articles and wrote the five-volume text book ‘Fundamentals of Interface and Colloid Science’; he gave lectures in five languages and in five continents, received honorary doctorates in Belgium, Sweden, and Spain, and has been awarded with the Order of the Dutch Lion, to quote only a few of the distinctions and awards he received during his career. In his research, Johannes Lyklema paid particular attention to the analysis and interpretation of the electrosurface phenomena. He was the founder and chairman of the International Advisory Board of the conference series Electrokinetic Phenomena (ELKIN) and section editor on electrokinetics in this journal (Current Opinion in Colloid and Interface Science). We will remember Johannes Lyklema as an eminent scientist as well as a warm-hearted and outstanding person.