Glass Transition Peak Research Articles

Synthesis, characterization and devitrification behaviour of an yttrium containing boroaluminate glass

The glass forming region in the B2O3-Al2O3-Y2O3 composition diagram has been determined by a melting and quenching procedure at temperatures up to 1800°C. Different physical characteristics (density, coefficient of thermal expansion, glass transition and crystallization peak temperatures) have been determined for a 35B2O3-40Al2O3-25Y2O3 glass composition (in mol.%). By using a predictive model and some NMR structural data, different elastic moduli (Young's modulus, bulk modulus, shear modulus and Poisson's ratio) have been calculated. The devitrification behaviour has also been studied. Internal crystallization is the dominant mechanism and a new (Y, Al)BO3 ternary phase has been characterized by X-ray powder diffraction. The temperature and time nucleation dependence have been determined from DTA experiments as well as the crystallization kinetics (i.e. the Avrami exponent and the activation energy for crystal growth).

Morphology and mechanical and dynamic mechanical properties of linear and star poly(styrene-b-isobutylene-b-styrene) block copolymers

The morphologies of solution cast linear and star poly(styrene-b-isobutylene-b-styrene) (SIBS) block copolymers, of ∼0.3–0.33 polystyrene (PS) volume fraction, were investigated using transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) methods. The linear (L) and 4-arm star materials have hexagonal packed cylinder morphologies. TEM indicates more continuous domains for the 3-arm SIBS and SAXS indicates lamellar morphology. The diameters of the PS cylinders are 25–30nm in all cases, but inter-domain distances increase in the order: L→3-arm star →4-arm star SIBS. Dynamic mechanical analysis shows a decrease of plateau modulus in the order L→3-arm →4-arm star. Tg for the PIB phase is independent of star-branching. Tg increases for the PS block phase from L- to 3-SIBS although Tg(PS) for 4-SIBS is not significantly greater than that of L-SIBS. A high temperature shoulder on the PIB block glass transition peak is associated with sub-Rouse motions. Tensile modulus is essentially the same for L- and 3-SIBS but considerably greater for 4-SIBS. Stress levels and energy-to rupture increase in the order: L-SIBS <3-SIBS <4-SIBS and this behavior is rationalized in terms of the covalent junction points, entanglements, and hard block domain reinforcement. The results of cyclic deformation studies of hysteresis vs. maximum percent strain per cycle are interpreted in terms of irreversible morphological rearrangements, chain slippage through entanglements, and the influence of degree of star branching on motional constraints. There is a permanent set that increases while the stress required to extend the sample to a given strain decreases with each cycle. For cycles of lower maximum strain, and before PS block domains are disrupted, microscopic deformation is viewed as somewhat reversible, accompanied by a measure of energy dissipated as a consequence of slow relaxation kinetics of chain rearrangement vs. rate of bulk deformation. At high strain, a major fraction of the energy loss is attributed to irreversible, longer ranged disruption of the PS domains leading to permanent morphological change and permanent set. Percent hysteresis for all three cases decreases—then increases—with increasing maximum percent strain per cycle. The initial decrease is thought of in terms of a progressive decrease in the entropy of stretched chains with consecutive extensions where the chains are increasingly less able to contribute to hysteresis until the PS domains become disrupted on a large scale, after which the curves begin to rise.

Ageing behaviour of Pd40Cu30Ni10P20 bulk metallic glass during long-time isothermal annealing

In this paper, positron annihilation spectroscopy and differential scanning calorimetry were applied to study the structural relaxation of Pd40Cu30Ni10P20 bulk metallic glass during long-time annealing at 523 K. It was found that the relaxation of the alloy follows a two-stage process. In the initial stage, with increasing annealing time, the height of the glass transition peak (namely Δcp) increased, while the average positron lifetime decreased. The isothermal relaxation kinetics obeys a Kohlrausch law with exponent of β = 0.5± 0.03. However, when the annealing time is beyond 32 h, the height of the glass transition peak (Δcp) decreased, and the average positron lifetime increased. This is possibly due to the enhancement of the topological short-range ordering, which results in the generation of new defects on the interfaces between the solid-like regions and the liquid-like regions in amorphous phase. The experimental results were tentatively discussed in terms of free-volume theory and percolation theory.

Physical aging effects on high temperature relaxations of poly(DTH carbonate) and poly(DTO carbonate) monitored by TSPC and TSDC

Thermal stimulated polarization and depolarization experiments with blocking electrodes are performed on two tyrosine-derived polycarbonates: poly(DTH carbonate) and poly(DTO carbonate). Aging effects are studied in a high temperature range that involved the contribution of the glass transition relaxation peak and the charge redistribution peak originated by the conduction through the sample and the presence of blocking electrodes. A reliable new models are used to described these high temperature relaxations and the results are presented as a function of aging times, polarization conditions and changing structure. For each sample the glass transition temperature, the parameter related with the shape of the glass transition peak and the contribution between the two modeled peaks, are found to be constant and independent of aging time. The variations in shape and temperature position of the maximum of the charge redistribution peak show logarithmic time dependence with aging time. These results are compared to the ones previously reported on a particular tyrosine-derived polyarylate, and the analysis suggests that structural differences in mobility and flexibility of the polymeric chains originate the observed variations. The examination of the experimental data indicates that charge redistribution and glass transition peaks share the same origin, and that the conduction aging effects are triggered by the onset of the glass transition.

Investigation of phase behavior during melt processing of novel inorganic‐organic polymer hybrid material

AbstractThe phase behavior of novel, binary organic‐inorganic hybrids consisting of an ultra‐low Tg tin‐based phosphate glass (Pglass) and polystyrene (PS) was investigated. Dynamic mechanical analysis (DMA) revealed that the glass transition peaks of the PS changed slightly with Pglass volume fraction, leading to a broad peak at the phase inversion point. The phase inversion and degree of phase continuity of the hybrid were studied through solvent extraction, optical/scanning electron microscopy, and dynamic rheology. The Jordhamo and Utracki viscosity ratio models provided reliable estimates of the inversion point. Torque rheometry revealed a trend toward linear additivity within the temperature range 200°C–230°C. Small‐angle neutron scattering experiments gave further evidence of the hybrid phase incompatibility. The results of this study point to a promising new class of blend materials with the potential to present a unique combination of properties impossible to achieve with classical polymer blends. Polym. Eng. Sci. 44:1692–1701, 2004. © 2004 Society of Plastics Engineers.

Glass-transition temperature of water: a simulation study.

We report a computer simulation study of the glass transition for water using the extended simple point charge potential. To mimic the difference between standard and hyperquenched glass, we generate glassy configurations with different cooling rates, and we calculate the temperature dependence of the specific heat on heating. The absence of crystallization phenomena allows us, for properly annealed samples, to detect in the specific heat the simultaneous presence of a weak prepeak ("shadow transition") and an intense glass transition peak at higher temperature. Our results support the view-point that the glass transition temperature is higher than the conventionally accepted value 136 K. We also compare our simulation results with the Tool-Narayanaswamy-Moynihan phenomenological model.

Characterization of free volume changes associated with shear band formation in Zr- and Cu-based bulk metallic glasses

The free volume model for flow in metallic glasses predicts a significant increase in free volume at the onset of plastic deformation. The details of these structural changes are unclear, however, particularly during strain localization in shear bands. In this study, the free volume changes associated with inhomogeneous plastic deformation of a Cu-based bulk metallic glass were examined using positron annihilation spectroscopy (PAS). PAS results indicated that there was a distribution of free volume site sizes in both the as-quenched and rolled glasses, and that the concentration of larger sites increased with deformation. Differential scanning calorimetry was also used to observe the glass transition behaviors of Cu- and Zr-based glasses after rolling and annealing. Annealing resulted in an increase in the height of the endothermic glass transition peak, consistent with structural relaxation relative to the as-quenched material. Deformation resulted in both a lower endothermic peak height and an earlier and deeper exothermic peak associated with structural relaxation, indicating a more disordered structure with more free volume.

Applying Kissinger analysis to the glass transition peak in amorphous metals

Some amorphous metals show a glass transition peak in a linear heating differential scanning calorimetry (DSC) experiment before crystallisation occurs. The temperature at which this peak is observed is a function of the heating rate used in the DSC experiment. From this dependence, an effective activation energy can be obtained using Kissinger analysis. The literature suggests that the glass transition is a kinetic phenomenon, caused by the production and annihilation of structural defects. Starting from the differential equation that describes the rate of change of these defects, an analytic expression is derived for this effective activation energy. The energy is found to have two contributions: one corresponds to the migration of structural defects and the other is related to their equilibrium concentration. The analysis shows the relevance of experimentally determining the slope of a Kissinger plot of the glass transition peak in amorphous metals.

Effect of Co60 gamma ray irradiation for carbon fibre on interfacial properties in epoxy resin composites

In order to improve the interfacial adhesion between carbon fibre and resin matrix in composite materials, it is necessary to treat the surface of the carbon fibre. In this paper, γ-ray irradiation technique was used to modify polyacrylonitrile based carbon fibre. Laser Raman spectrum and X-ray photoelectron spectroscopy were used to investigate and analyse the structure and chemical composition near the surface of the carbon fibre. The influence of irradiation parameters on the interlaminar shear strength (ILSS) of carbon fibre reinforced epoxy composite materials and the bundle tension strength of carbon fibre was studied. The interfacial adhesion behaviour of composites was characterised using torsional braid analysis. The results show that after irradiation the ILSS of the composite was increased by 20%, while the glass transition peak of the specimen, determined from torsional braid analysis, shifts towards a higher temperature compared with an unirradiated specimen. The value of the glass transition temperature Tg is increased from 416.8 to 424.3 K. After irradiation there was no apparent change in the bundle tensile strength of carbon fibre. Investigations indicate that after irradiation the decrease of microcrystal size, the increase of surface free energy of carbon fibre surface and the active chemical function group formed from unsaturated carbon atoms improve the interface adhesion between the carbon fibre and the matrix in the composites.

Molecular design of damping rubber based on polyacrylate‐containing silicone

AbstractA series of damping rubbers based on poly(meth)acrylate and poly(meth)acrylate‐containing silicone rubbers has been prepared. The dynamical mechanical properties were evaluated by using a dynamic mechanical viscoelastometer (DMA). A detailed investigation is reported on the relationship of the damping capability of the poly(meth)acrylate rubbers with their composition and macromolecular architecture. Also discussed is the effect of two kinds of silicone elastomers on the damping performance of the polyacrylate‐containing silicone rubber. The results indicate that, in vulcanized rubber systems, both statistical copolymerization for multiple monomers and blending between immiscible polymers with close Tgs facilitate broadening the glass transition peaks. Furthermore, the molecular design is quite an effective approach in which the multiple monomers whose polymers have a ladder‐changing Tg are used to synthesize the damping rubber with the broad effective functional area. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 746–751, 2002

Free Volume Evolution in Bulk Metallic Glass during High Temperature Creep

ABSTRACTMetallic glasses lack long-range translational symmetry and have excess volume trapped within their amorphous structure, which has a direct bearing on their physical properties including deformation characteristics. Moreover, the trapped excess free volume is directly correlated to the defect concentration facilitating the possibility to model the temperature and time dependence of the free volume changes during creep as a trade off between defect generation and annihilation. Using differential scanning calorimetry (DSC) analysis the residual free volume of a metallic glass can be characterised based on the glass transition peak height (Δcp). In the present work constant strain rate tests were carried out at the ‘onset’ (Tgon = 685 K) and ‘point of inflection’ (Tgp = 705 K) of the calorimetric glass transition to study the time dependent flow behaviour in Zr55Cu30Al10Ni5 bulk metallic glass. Modelling based on DSC analysis and positron lifetime spectroscopy on samples creep deformed to different plastic strain values corroborate the stress decrease after the peak stress (‘stress overshoot’) occurring in bulk metallic glasses with increasing plastic strain to be associated with a small increase in free volume.

Dynamic mechanical signatures of a polyester‐urethane and plastic‐bonded explosives based on this polymer

AbstractThe complex shear moduli of the segmented polyurethane Estane 5703p, Livermore explosive (LX)‐14, and plastic bonded explosive (PBX)‐9501, which use this polymer as a binder, have been investigated. Segmented polyurethanes, such as Estane 5703, contain microphase‐separated hard segments in a rubbery matrix of soft segments. LX‐14 is composed of 95.5% 1,3,5,7‐tetranitroazacyclooctane (HMX) explosive with 4.5% Estane 5703 binder. PBX‐9501 is composed of 94.9% HMX, 2.5% Estane 5703p binder, 2.5% nitroplasticizer (NP), and about 0.1% antioxidant Irganox 1010. In the temperature range from −150 to 120°C, two relaxations were observed as peaks in the loss modulus and tangent delta in Estane 5703p and LX‐14. A third relaxation was found in PBX‐9501. The low temperature relaxation associated with vitrification of the poly(ester urethane) soft segment occurred in the shear loss modulus (G″) at −29 and −26°C in Estane and LX‐14, respectively, at 1 Hz. In PBX‐9501 the Estane soft segment glass transition peak, Tg(SS), in the loss modulus occurred at −40 ± 3°C at 1 Hz. The reduction in soft segment glass transition in PBX‐9501 is clear evidence of plasticization of the soft segment by NP. The apparent activation energy of the maximum in the loss modulus for LX‐14 and PBX‐9501 over the frequency range from 0.1 to 10 Hz was 230 kJ/mole (55 kcal/mole). The hard segment glass transition, Tg(HS), was observed as a peak in the loss modulus at about 70°C. In LX‐14 the transition was observed at lower temperatures (56–58°C at 1 Hz) depending on thermal history. There was a low temperature shoulder on the Tg(HS) of Estane 5703 associated with soft segment crystallinity. Modulated differential scanning calorimetry (MDSC) was used to verify the Tg(HS) in Estane and 50/50 mixtures of Estane with NP. In PBX‐9501 the hard segment glass transition occurred between 65 and 72°C. The presence of NP in PBX‐9501 gave rise to a new transition, Teu(NP), between 8 and 15°C. This peak is believed to be associated with the eutectic melting of the plasticizer. Returns of fielded PBX‐9501 that were 6 and 11 years old were also measured. Small variations in Tg(SS) and the rubber plateau modulus were observed in these aged samples, consistent with migration of plasticizer and/or very low levels of chain scission. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1009–1024, 2002

Glass Transition Temperature and the Nature of the Amorphous Phase in Semicrystalline Polymers: Effects of Drawing, Annealing and Hydration in Polyamide 6

The influence of the organization of the amorphous chains segments on the glass transition temperature (Tg) in semicrystalline polymers is analysed by studying the effects of drawing, annealing and hydration in polyamide 6 fibers. We consider the role of three of the features of the amorphous phase: orientation (configurational entropy), density (free volume) and confinement (segmental mobility). Three classes of amorphous phases are identified; two of these are constrained in the intercrystaline regions, at the fold surfaces (between the lamellae within the lamellar stack) and at the stem surface (growth surface of the lamellae or between the fibrils). The third species is the bulk amorphous phase outside the lamellar stacks, and constitutes a large fraction of the amorphous phase especially at low crystallinities. Because the small fraction of the amorphous chain segments in the intercrystalline regions, and because they are in confined spaces, we suggest that these interlamellar and the interfibrillar components do not contribute significantly to the observed major glass transition peak. Rather, it is the amorphous region outside the lamellar stack that determines the Tg. Tg increases upon drawing and decreases upon annealing (heat setting). Our data suggest that orientation has a direct influence on Tg and can easily be measured whereas the influence of crystallinity is more complex. The influence of orientation of Tg can be understood in terms of a two Tg model in which the oriented amorphous component has a higher Tg than the unoriented component.

Effect of fast neutrons on dielectric properties of pure and gelatin doped-poly (vinyl alcohol) films

Cast thin films of pure and gelatin doped poly(vinyl alcohol) (PVA) with concentrations of 2, 5, 7, 10, and 15 wt% were prepared and subjected to fast neutron fluences in the range of 105–108 n/cm2. The dielectric constant (e′), dielectric loss tangent (tanδ) and a.c. electrical conductivity (σa,c) were investigated as a function of field frequency and temperature for all samples before and after irradiation. A clear glass transition peak was observed in e′(T) and tanδ(T) curves. The irregular shift in the position of the glass transition temperature (Tg) in tanδ spectra with increasing neutron fluence can be attributed to either degradation or the predominance of the crosslinking process. The electric dipole moment and activation energy were calculated for pure and gelatin doped PVA samples before and after neutron irradiation.

Structural relaxation and plastic flow in amorphous La 50Al 25Ni 25

The tensile deformation of amorphous La 50Al 25Ni 25 is studied in an extensive set of tensile tests at temperatures just below the glass transition temperature. Depending on the pre-annealing treatment, in many cases strain softening occurs during the tensile tests. Only in the case of a very short pre-annealing treatment does apparent strain hardening occur. After deformation, up to ε=0.3, all samples are tested by differential scanning calorimetry, which reveals that the glass transition peak depends on the temperature and strain rate during the tensile test, but not on the pre-annealing treatment. The entire set of experimental evidence can adequately be explained by the free volume concept. The conclusion is that during plastic deformation additional free volume is created as compared with thermal equilibrium, and the structure reaches a temperature and strain rate dependent steady state of disorder, i.e. amount of free volume. A quantitative description of this process, which should be envisaged as the production of a small amount of free volume during each flow event, is presented.

Stiffness Improvements and Molecular Mobility in Epoxy-Clay Nanocomposites

ABSTRACTConventional composites filled with clay as well as intercalated nanocomposites, and exfoliated nanocomposites based on a glassy epoxy matrix have been synthesised. Flexural moduli of these materials were measured in three-point bending at various clay contents. For a given clay content, stiffness improvements depended not only on the dispersion of the clay on the microscale, but also on the exfoliation of the clay layers at the nanolevel. Dynamic mechanical measurements indicated a decrease of intensity in the glass transition peak with the extent of exfoliation of the clay and the clay content, suggesting a restriction of the molecular mobility of the polymer in the vicinity of the clay layers. A shift in Tg of 20°C towards lower temperature for the epoxy resin cured at 160°C was possibly caused by thermal degradation of compatibilizing agents at high temperature.

Conductivity and high-temperature relaxation of tyrosine-derived polyarylates measured with thermal stimulated currents

Thermal-stimulated polarization and depolarization experiments without blocking electrodes are performed on tyrosine-derived polyarylates with different backbone lengths. The experiments on the different samples are carried out using the same thermal history throughout the entire characterization process. The high-temperature current rise caused by the conductivity of the samples is studied with a simple model that utilizes an approximation of the Williams–Landel–Ferry (WLF) relaxation time. The conductivity data is well reproduced except for temperatures well below the glass-transition temperature and for small currents. The glass-transition peak is modeled with a phenomenological expression valid near Tg, which is able to describe the glass relaxation with a minimum number of parameters. The conduction and the glass-transition relaxation are studied versus the structural changes for the different samples. It is found that the conductivity and the glass-transition temperature shift to lower temperatures as the methylene groups in the backbone increase. Furthermore, if the experimental data is presented as a function of the reduced temperature, the shape of the glass-transition relaxation for the different samples is independent of the polymer backbone length. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3504–3511, 1999

SYNTHESIS OF WATER-SOLUBLE URETHANE ACRYLATE CATIONOMERS AND THEIR ULTRAVIOLET COATING PROPERTIES

Water-soluble urethane acrylate cationomers were synthesized by incorporating N-methyldiethanolamine (MDEA) into the molecular backbone for an application of ultraviolet (UV) curing. It was found that they were easily dissolved in, because of the sufficient hydration ability of the incorporated cationic groups. Differing from conventional urethane acrylate derivatives, when UV-cured, the films of the urethane acrylate cationomers showed significantly improved coating properties. This was because of the formation of the ionic hard domains in the urethane acrylate network. In dynamic mechanical analysis (DMA), the formation of the ionic hard domain could be confirmed by detecting the glass transition peak Tgh, around 23.4°C. The dyeing of the urethane acrylate cationomer films was considerably favorable because of their positively-charged network structure.

Veneering composites - a thermoanalytical examination.

Differential thermal analysis and thermal gravimetric analysis were used to characterize veneering composites. Samples weighing 4-20 mg, made from the composite materials Visio-Gem (Espe, Germany), Sinfony (Espe, Germany), Artglass (Kulzer, Germany), Dentacolor (Kulzer, Germany) and Targis (Ivoclar, Liechtenstein), were examined. The samples were subjected to various thermal curing times of between 4 s and 25 min, using the relevant devices of the manufacturers. As a control group, samples of all materials were examined unreacted. In order to avoid post-curing during storage, all samples were subjected, immediately after manufacture, to the appropriate dynamic temperature programme of the thermoanalytical unit at a heating rate of 10 degrees C min(-1). The materials showed specific material characteristics which can, for instance, be used to analyse the curing behaviour of the materials. The position of the glass transition, polymerization and post-polymerization peaks at temperatures between 30 and 100 degrees C, as well as 150 and 300 degrees C, and the filler and matrix content, allow the classification of the veneering composites.

Pressure-Induced Structural Relaxation in AmorphousPd40Ni40P20: The Formation Volume for Diffusion Defects

Structural relaxation induced by hydrostatic pressure is experimentally observed for the metallic glass ${\mathrm{Pd}}_{40}{\mathrm{Ni}}_{40}{\mathrm{P}}_{20}$. The pressure dependence of the structural state is displayed in the shape of the glass-transition peak that is measured by means of differential scanning calorimetry after annealing treatments at 563 K and pressures up to 0.75 GPa. Quantification of the effect using the free-volume theory yields a formation volume of $5.9\ifmmode\pm\else\textpm\fi{}0.5{\AA{}}^{3}$ for defects in this glass. In combination with the previously determined pressure dependence for the Au diffusivity, a value of $5\ifmmode\pm\else\textpm\fi{}4{\AA{}}^{3}$ is derived for the migration volume for Au diffusion.