Glass Transition Range Research Articles

Effect of adhesion promoters on rheokinetics and roll peel strength of electrical steel stacks

Epoxy varnishes for stacked electrical steel are of high relevance for renewable energy and electric mobility technologies. Water-borne epoxy varnish systems are still under development, especially with regards to the improvement of the adherence to the steel substrate. The main objective of this study was to assess the potential of low molar mass, silane-based adhesion promoters (AP) with amine or glycidyl functional groups as to their effect on the crosslinking kinetics of coatings and the mechanical performance of electrical steel laminates.Model formulations with different types and contents of silane AP or in combination with a pre-crosslinker were based on a Bisphenol A diglycidyl ether, a dicyandiamide (DICY) crosslinking agent and an emulsifier. The amount of water was fixed to 45 m%. Model varnishes were prepared by shear emulsification at elevated temperatures. Varnishes were applied onto electrical steel sheets and partly crosslinked to the B-stage. Finally, coated steel sheets were stacked and laminated in a heated press.To examine the crosslinking kinetics under non-isothermal conditions, rheokinetic measurements were performed. While the curing onset temperature of formulations with amine-based AP dropped by ∼10 °C, the effect of the epoxysilane AP was less pronounced (drop by ∼5 °C). Pre-conditioning of the varnishes in A-stage revealed a further reduction of the curing onset temperature associated with significantly higher initial viscosity. This was a clear indication for pronounced pre-crosslinking in A-stage and hence, a limited shelf life of silane-modified varnishes. The glass transition range in the fully cured state was solely affected by the diamine-based AP. Interestingly, the investigated low molar mass silane APs had no significant positive effect on the roll peel strength of the laminates.

Correlations between shadow glass transition, enthalpy recovery and medium range order in a Pd40Ni40P20 bulk metallic glass

Two distinct enthalpic states were induced in a Pd40Ni40P20 bulk metallic glass through cyclic annealing at 473 K (0.81Tg) and 558 K (0.96Tg), targeting individual β or α relaxation modes, here referred to as the so-called shadow glass transition (SGT) and enthalpy recovery, respectively. Fluctuation electron microscopy (FEM) was employed to systematically investigate the medium-range order (MRO) within the amorphous structure in the post-annealed state and subsequent quenching after the SGT. Comparing the results with literature data from the same glass quenched from SCL samples, a significant increase in MRO was observed after annealing at 0.81Tg. Conversely, samples quenched directly after the SGT exhibit a reduced MRO, similar to those quenched from the supercooled liquid (SCL). These findings suggest a correlation between the SGT and a decrease in MRO within the glass, a phenomenon discussed in the context of underlying structural rearrangements associated with β relaxation, encompassing the SGT. Additionally, prolonged annealing at 0.96Tg led to glass separation in the material, with remixing observed as an endothermic transformation in the SCL. The origin of this transformation is discussed as either a potential liquid–liquid or glass–liquid transition and their connection to modifications in medium-range order.

Synthesis and properties of modified thermoplastic polyimide films with good dielectric properties at high frequency and enhanced thermal stability via incorporation of rigid ester and biphenyl structural units

AbstractA series of ester‐ and fluoro‐containing thermoplastic polyimide (PI) films, PEsI‐1 ~ PEsI‐4 have been prepared by the two‐step thermal imidization procedure via the soluble poly(amic acid) (PEsAA) precursors from the copolymerization of four monomers, including the dianhydrides of bisphenol A dibenzoate‐3,3′,4,4′‐tetracarboxylic acid dianhydride (TMBPA) and 3,3′, 4,4′‐biphenyl tetracarboxylic acid dianhydride (BPDA) and the diamine monomers of 4‐aminophenyl‐4′‐ aminobenzoate (APAB) and 2,2′‐bis(trifluoromethyl)‐4,4′‐diamino biphenyl (TFMB) in the polar aprotic solvent of N‐methyl‐2‐pyrrolidinone (NMP). The PI films obtained from the high‐temperature dehydration of the PEsAA solutions exhibited the good thermal stabilities with the glass transition temperatures (Tg) over 269°C, the 5% weight loss temperatures (T5%) higher than 480°C, and the linear coefficients of thermal expansion (CTE) values in the range of 17.7 × 10−6/K–33.6 × 10−6/K in the temperature range of 50–250°C. Meanwhile, the PI films showed good thermoplasticity in the dynamic mechanical analysis (DMA) tests, in which the storage modulus of the PI films dropped sharply in the temperature range of glass transition of the polymers. At last, the PI films exhibited the good dielectric properties with the breakdown voltages (Vb) over 5200 V, the dielectric strength (Ds) higher than 210 V/μm, the dielectric constants (Dk) in the range of 3.26 ~ 3.32 at the frequency of 10 GHz, and the dielectric dissipation factors (Df) of 0.0038 ~ 0.0047. Compared with the commonly used PEsI‐ref film for high‐frequency flexible copper clad laminates (FCCL), the currently developed PI films exhibited the obviously increased Tg values, decreased CTE values, and comparable dielectric properties.

Water release from dacitic melts near the glass transition and implications for volcanic eruptions

To get insights in the mechanisms of magma degassing and fragmentation, we performed dehydration experiments at ambient pressure with dacitic glasses containing between 1.42 and 5.28 wt% H2O. The focus was on the degassing behavior in the range of glass transition, i.e., from temperatures 50 K below to 50 K above the glass transition temperature Tg. At each T the duration of the experiments was varied between 5 and 125 h. Water contents and initial water speciation was determined by FTIR spectroscopy. Diffusion depth profiles were measured by Raman spectroscopy. We developed an equation to calculate the total water content along the depth profiles from the ratio of the fundamental OH stretching vibration band at 3550 cm−1 and the T-O-T bending band near 500 cm−1. Glasses with high water content (4.66–5.28 wt%) showed huge alterations, i.e., crack formation below Tg and crystallization at and above Tg, but glasses with water contents up to 2.5 wt% were visually unchanged and suitable for water diffusion analysis.The measured profiles of total water content vs. distance can be fitted well by an error function, assuming that the water content in the glass does not decrease to zero at the surface. However, the apparent water concentration at the surface and the total water diffusivity derived from the error function fitting decrease with increasing duration of the experiment. This clearly shows that degassing of dacitic melts is not a simple out-diffusion mechanism. The model of Coumans et al. (2020) that explicitly considers the kinetics of the interconversion of water species can reproduce the temporal evolution of the total water concentration profiles very well. Our results show the importance of considering explicitly the interconversion kinetics of water species when modelling water diffusion in melts at high viscosities. Due to the slow interconversion of OH groups to much more mobile water molecules, dehydration is much slower in the range of glass transition than predicted by diffusion models based on experiments performed on stable melts at magmatic temperatures.

Analysis method of binary concentration-inhomogeneous systems

A method for the analysis of binary concentration-inhomogeneous polymer-monomer systems, in which the concentration of components varies continuously from point to point, is proposed. As a characteristic of inhomogeneity serves the distribution function of the polymerization mass over concentrations. The method is based on measuring the dependence of the heat capacity of the polymerization mass on temperature in the temperature range, which includes the glass transition range, and comparing it with the dependence of the heat capacity of homogeneous systems on temperature and concentration, followed by the solution of the Fredholm integral equation of the first kind.

Hygromechanical Behavior of Polyamide 6.6: Experiments and Modeling.

This paper investigates water absorption in polyamide 6.6 and the resulting hygroscopic swelling and changes in mechanical properties. First, sorption and swelling experiments on specimens from injection molded plates are presented. The observed swelling behavior is dependent on the melt flow direction of the injection molding process. Additionally, thermal analysis and mechanical tensile tests were performed for different conditioning states. The water sorption is accompanied by a decrease in the glass transition temperature and a significant reduction in stiffness and strength. Next, a sequentially coupled modeling approach is presented. A nonlinear diffusion model is followed by mechanical simulations accounting for swelling and concentration-dependent properties. For the mechanical properties, the notion of a "gap" temperature caused by the shift of the glass transition range due to water-induced plasticization is employed. This model enables the computation of local moisture concentration fields and the resultant swelling and changes in stress-strain behavior.

Ergodicity breaking of an inorganic glass aging near Tg probed by elasticity relaxation

We performed a series of aging experiments with an inorganic glass (${\mathrm{As}}_{2}{\mathrm{Se}}_{3}$) at a temperature ${T}_{2}$ near the glass transition point ${T}_{g}$ by first relaxing it at ${T}_{1}$. The relaxations of Young's modulus were monitored, which were (almost if not ideally) exponential with ${T}_{1}$-dependent relaxation time \ensuremath{\tau}, corroborating the Kovacs' paradox in an inorganic glass. Associated with the divergence of \ensuremath{\tau}, the quasiequilibrated Young's modulus ${E}_{\ensuremath{\infty}}$ does not converge either. An elastic model of relaxation time and a Mori-Tanaka analysis of ${E}_{\ensuremath{\infty}}$ lead to a similar estimate of the persistent memory of the history, illuminating ergodicity breaking within the accessible experimental time, as described in the Gardner transition theory. Experiments with different ${T}_{2}$ exhibit a critical temperature ${T}_{p}\ensuremath{\sim}{T}_{g}$, i.e., when ${T}_{2}>{T}_{p}$, both \ensuremath{\tau} and ${E}_{\ensuremath{\infty}}$ converge. The results unveil a long-expected phenomenon that structural glass transition could be a zero-to-nonzero transition, manifested by a nonvanishing structural memory in aging when the temperature is below ${T}_{p}$ in the glass transition range. This demonstrates the existence of the ergodicity breaking deep in the glass state and ${T}_{p}$ could be the Gardner transition point of the structural glass.

Review: Elaboration, structure, and mechanical properties of oxynitride glasses

AbstractOxynitride glasses are glasses where threefold coordinated nitrogen atoms substitute for twofold oxygen ones, hence resulting in a larger interatomic cross‐linking degree. Such glasses were first observed at the grain boundary in silicon nitride ceramics, where they govern the high‐temperature behavior. Later, they were prepared as bulk materials and motivated numerous researches, thanks to their large viscosity, glass transition range, elastic moduli, hardness, and fracture toughness among inorganic and non‐metallic glasses. In different chemical systems that were investigated, the synthesis routes and the sources for these exceptional mechanical properties are reviewed. Oxynitride glasses are not easy to process and suffer from the loss of transparency as nitrogen is incorporated over some critical content. Nevertheless, they are attractive “specialty” glasses in various niche areas, thanks to their large refractive index and dielectric constant, improved chemical durability, high softening point, etc., and majorly to their exceptional mechanical properties.

Long-Term Stability of Emitter Orientation in Organic Light-Emitting Diodes at Temperatures in the Range of the Active Layer Glass Transition

Long-Term Stability of Emitter Orientation in Organic Light-Emitting Diodes at Temperatures in the Range of the Active Layer Glass Transition

Alternating Methyl Methacrylate/n-Butyl Acrylate Copolymer Prepared by Atom Transfer Radical Polymerization.

Poly(methyl methacrylate/n-butyl acrylate) [P(MMA/BA)] copolymer with an alternating structure was synthesized via an activator regenerated by electron transfer (ARGET) atom transfer radical (co)polymerization (ATRP) of 2-ethylfenchyl methacrylate (EFMA) and n-butyl acrylate (BA) with subsequent postpolymerization modifications (PPM). Due to the steric hindrance of the bulky pendant group of EFMA, as well as the low reactivity ratio of BA in copolymerization with methacrylates, copolymerization of EFMA and BA generated a copolymer with a high content of alternating dyads. A subsequent PPM procedure of the alternating EFMA/BA copolymer was comprised of the hydrolysis of a tertiary ester by trifluoroacetic acid and methylation by (trimethylsilyl)diazomethane. After the modifications, the architecture of the obtained alternating MMA/BA copolymers was compared with gradient and statistical copolymers with overall similar compositions, molecular weights, and dispersities. 13C NMR indicated the absence of either MMA/MMA/MMA or BA/BA/BA sequences, in contrast to an abundance of homotriads in either the statistical or especially in the gradient copolymer. All three copolymers had similar glass transition temperatures, as measured by differential scanning calorimetry (DSC), but the alternating copolymer had the narrowest range of glass transition.

Influence of Fe2+/Fetot on the viscosity of melts from the Colli Albani Volcanic District (Italy) – foidite to phonolite

Viscosity of a melt strongly depends on melt composition and the ratio of network modifiers to network formers. In silicate melts, Fe exists as Fe2+ and Fe3+, both affecting the viscosity in different manners: Fe2+ acts as a network modifier and lowers the viscosity whereas Fe3+ behaves as a network former and increases the viscosity. The Fe speciation strongly depends on the oxygen fugacity fO2.In this study, the viscosities of Fe-rich synthetic tephri-phonolitic, foiditic and tephritic melts from the Colli Albani Volcanic District (Italy) are investigated as a function of Fe2+/Fetot in the glass transition range. Base glasses made in air at 1600 °C at log10fO2 = −0.68 are re-equilibrated at log10fO2 = −4 and − 7 in a gas mixing furnace at 1250 °C. Melts made in air and at log10fO2 = −4 have nearly identical Fe2+/Fetot due to the different experimental temperatures. At 700 °C, the viscosity of these melts drops by ~2, ~1.5 and ~ 0.4 log10 Pa s with increasing Fe2+/Fetot from ~0.45, ~0.39 and ~ 0.44 to ~0.76 for the tephri-phonolite, the foidite and the tephrite, respectively. These melts have H2O concentrations up to 0.03 wt%. It is shown that the addition of ~0.23 wt% H2O reduces the viscosity by the same amount as the increase in Fe2+/Fetot.In addition, the viscosities of a phonolite from Mt. Vesuvius are studied over a range of Fe2+/Fetot values (~0.41 to 0.87). In contrast to the other melts of this study, viscosity appears to increase with increasing Fe2+/Fetot. However, this change in viscosity results from a depletion of ~14 mol% in network modifying alkali and alkaline earths elements and a loss of approx. 50 atom% Fe both affecting the viscosity of this melt more effectively than the increase in Fe2+/Fetot.

Relaxation behavior of an Al-Y-Ni-Co metallic glass in as-prepared and cold-rolled state

Dynamical mechanical behavior of the Al85Y8Ni5Co2 metallic glassy ribbon samples in the as-prepared state and subjected to cold rolling with 35 % size reduction is studied in the present work and compared with the results of thermal mechanical analysis performed at a constant load. Deformation of the samples was also modeled by different values (such as reaction force, viscosity, speed of sound and so on) in frames of the same physic-mechanical model. The values of the above mentioned parameters, predicted by calculations, are found to be in good agreement with the experiment. The influence of different external conditions on the properties of the alloy was analyzed upon dynamic mechanical analysis at the different temperatures (up to the glass transition range). The structural mechanisms leading to the observed effects are described in frames of "shear bands" and "free volume" models.

Comment on Comment on “Anomalous structural recovery in the near glass transition range in a polymer glass: Data revisited in light of temperature variability in vacuum oven‐based experiments”*

AbstractCangialosi, Alegría, and Colmenero have made a comment on a paper of ours [Polym. Eng. Sci. 2022:1–13], in which we discussed the concern that the enthalpy recovery data reported by Cangialosi and co‐workers [Phys. Rev. Lett. 2013;111(9):095701] for polystyrene aged up to 15 K below glass transition temperature was anomalous and contradicted existing experimental results from the literature over a similar range of aging conditions. Their response shifts the focus away from the raised questions about their experimental results and attempts to invalidate the data that we cited in support of our argument. Here we respond to the comment and add additional analysis that suggests the structural recovery response of glassy materials exhibits smooth behavior over the full range of measurements, up to 7 or 8 logarithmic decades. We do this by referring to the work on the intrinsic isotherm down‐jump and memory responses of poly(vinyl acetate) between 40°C and 15°C over six logarithmic decades by Kovacs [Fortsch. Hochpolym. Fo. 1963;3(1/2):394–508], the small‐strain tensile creep of poly(vinyl chloride) quenched from 90°C to 40°C (approximately 40°C below Tg) over seven logarithmic decades from Struik [Polym Eng. Sci., 1977;17:165–173], and the volume recovery behavior for aging times up to 3 months in a temperature range between 95°C and −50°C by Greiner and Schwarzl [Rheol. Acta. 1984;23(4):378–395]. We also add discussion that an isothermal aging procedure using a vacuum oven is highly vulnerable to temperature errors due to the problem of good temperature control when the heat transfer mechanism is primarily radiative.

Comment on “Anomalous structural recovery in the near glass transition range in a polymer glass: Data revisited in light of temperature variability in vacuum oven‐based experiments”

AbstractRecent efforts, fostered by a pioneering work by us, have shown the of multiple steps in the recovery of equilibrium of glasses. Jin and McKenna raise concerns regarding the validity of such scenario alleging that the multiple recovery steps would be an artifact arising from poor temperature control in the oven used for isothermal glass equilibration. We critically discuss Jin and McKenna arguments from both the viewpoints of scrutinizing previous literature data and that of the temperature control in the oven. In doing so, we provide compelling arguments that Jin and McKenna conjectures are unjustified and point out the need for efforts to describe glass dynamics significantly below the glass transition temperature, Tg, by accounting for the presence of different relaxation mechanisms active in glass equilibration.

DSC investigation of chain cooperativity rearrangements and critical length scale of polybutadiene networks in glass transition range

DSC investigation of chain cooperativity rearrangements and critical length scale of polybutadiene networks in glass transition range

On-Demand Tailoring between Brittle and Ductile of Poly(methyl methacrylate) (PMMA) via High Temperature Stretching.

Dog-bone shaped poly(methyl methacrylate) (PMMA) samples were pre-stretched at different temperatures (within the glass transition range and slightly above) to different strains. Subsequently, these pre-stretched samples were aged at 40 °C for up to three months, and finally, all samples were uniaxially stretched to fracture. The Young’s modulus, ultimate stress and toughness of the samples were obtained and plotted as a function of the temperature, and strain in pre-stretching in the contour format. The influence of aging was revealed when the contours of different aging times were compared. One of the most interesting findings was that the toughness of this PMMA can be tailored via controlling the temperature and strain in pre-stretching. The toughness of the pre-stretched samples ranged from 1.317 MJ/m3 to 23.281 MJ/m3 (without aging) and from 1.476 MJ/m3 to 27.532 MJ/m3 (after three months of aging). Based on the results of a series of additional experiments, a mechanism was proposed to reveal the fundaments behind the influence of the temperature and strain in pre-stretching and aging.

Anomalous structural recovery in the near glass transition range in a polymer glass: Data revisited in light of temperature variability in vacuum oven‐based experiments*

AbstractThere is considerable interest in data reported by Cangialosi et al. [Cangialosi, D., Boucher, V. M., Alegría, A., & Colmenero, J. (2013). Physical Review Letters, 111(9), 095701] that purportedly showed unusual irregular responses in the isothermal structural recovery behavior of glasses because the observation challenges the general view of the dynamics being smooth functions of time as observed in extensive dilatometric experiments by Kovacs [Kovacs, A. J. (1964). In Fortschritte der Hochpolymeren‐Forschung, 3, 394–507. Springer, Berlin, Heidelberg.] in the 1960s. The reported data show an apparent two mechanism structural (enthalpy) recovery at aging temperatures ranging from 6 to 17 K below which is also controversial as it could not be reproduced in work from Koh and Simon [Koh, Y. P., & Simon, S. L. (2013). Macromolecules, 46(14), 5815–5821.] where a second plateau observed in the enthalpy loss curve was not reproduced in nearly 1‐year‐long experiments at 15 K below . Therefore, it is important to determine what might be possible reasons for the apparent non‐smooth relaxations. Here, we examine the possibility that the poor temperature control of a typical vacuum oven could explain the anomalous results. We used the Tool–Narayanaswamy–Moynihan (TNM) model of structural recovery to calculate the effect of typical vacuum oven temperature variation on the structural recovery of polystyrene and find that we can reproduce the reported experimental results. The issue of temperature control using a vacuum oven is discussed, and data from thermocouple measurements of temperature at various locations inside a vacuum oven are shown.

Modeling the impact of glass transition on the frequency-dependent complex conductivity of CNT-polymer nanocomposites

While it is well known that glass-transition process tends to lead to significant reduction of Young's modulus in polymers, it is not commonly known how it affects the electrical conductivity and dielectric permittivity of carbon nanotube (CNT)-polymer nanocomposites, especially under AC loading. In this paper, a novel two-scale effective-medium approximation with complex conductivity is presented to answer these questions. The theory is developed by considering the irreversible thermodynamics of phase transition from the glassy to rubbery state and the thermal degradation of the interphase layer. In addition, the temperature-affected interfacial connection, electronic tunneling, Maxwell-Wagner-Sillars polarization, and the frequency-dependent electron hopping and dielectric relaxation, are also considered. It is demonstrated that the predicted results are in close agreement with the experimental data of MWCNT/polyester nanocomposites across the glass-transition range from 240 K to 380 K and the frequency spectrum from 102 Hz to 106 Hz. It is also found that both conductivity and permittivity decrease and then increase nonlinearly with temperature, with the minimum detected around the glass-transition point. Several other novel features of the theory are also presented.

Crystal Nucleation and Growth in Cross-Linked Poly(ε-caprolactone) (PCL).

The crystal nucleation and overall crystallization kinetics of cross-linked poly(ε-caprolactone) was studied experimentally by fast scanning calorimetry in a wide temperature range. With an increasing degree of cross-linking, both the nucleation and crystallization half-times increase. Concurrently, the glass transition range shifts to higher temperatures. In contrast, the temperatures of the maximum nucleation and the overall crystallization rates remain the same, independent of the degree of cross-linking. The cold crystallization peak temperature generally increases as a function of heating rate, reaching an asymptotic value near the temperature of the maximum growth rate. A theoretical interpretation of these results is given in terms of classical nucleation theory. In addition, it is shown that the average distance between the nearest cross-links is smaller than the estimated lamellae thickness, which indicates the inclusion of cross-links in the crystalline phase of the polymer.

Influence of CO2 on the rheology of melts from the Colli Albani Volcanic District (Italy): foidite to phonolite

In this study, the influence of CO2 on the rheology of silica poor and K-rich melts from highly explosive eruptions from the Colli Albani Volcanic District (Italy) (CAVD) is measured for the first time. The investigated melts range from foidite to tephri-phonolite to tephrite from the CAVD to a phonolite from the Vesuvius (Italy) with CO2 concentrations up to 0.50 wt%. Viscosity and calorimetric measurements are performed in the glass transition range Tg between 600 and 780 °C. Although nominally anhydrous, the investigated melts contain H2O concentrations up to 0.23 wt%. The data exhibit a decrease in viscosity of approx. 100.40 Pa s for the phonolitic composition with ~ 0.07 wt% CO2 and a Tg reduced by approx. 14 °C. For the tephritic composition, Tg is approx. 5 °C lower and has a viscosity reduced by 100.25 Pa s for the sample containing ~ 0.5 wt% CO2. Calorimetric measurements of the tephri-phonolite show lowered onset of Tg by approx. 6 °C for the melt with ~ 0.11 wt% CO2 and Tg of the foidite appears not to be influenced by a CO2 concentration of ~ 0.37 wt% CO2. However, these tephri-phonolitic and foiditic melts foamed during calorimetric measurements preventing a reliable measurement. It would appear that most of this overall drop in viscosity is caused by the small amounts of H2O in the melts with CO2 slightly reducing the viscosity or having no effect on viscosity. Additionally, it is shown that the reduction in viscosity decreases with an increasing degree of the depolymerisation for the investigated melts. Consequently, the explosive style of the CAVD eruptions is mainly caused by crystals and bubbles which form and rise during magma storage and ascent which increases the magma viscosity whereas the CO2 in the melt slightly reduces the viscosity.