The effect of crystal nuclei formed during the production of glass on the kinetics of nucleation of crystals in it at temperatures below the glass transition temperature is studied. A glass composition of 22.4Na2O ⋅ 28.0CaO ⋅ 49.6SiO2 (mol %) is studied (glass transition temperature 540°C). Using the development method, the dependence of the number of nucleated crystals on the heat treatment time is determined at a temperature of 500°C. The nuclei of crystals formed during the production of glass lead to an unusual, in the form of a step, initial section of the curve N(t). The rate of stationary nucleation of crystals with a long time of nucleation heat treatment exceeding the induction period of crystal nucleation is determined. The reasons for the unusual initial nature of the dependence N(t) are discussed. The data obtained make it possible to expand the temperature dependences of the stationary rate and the induction period of crystal nucleation for the given glass toward low temperatures. 相似文献
We have conducted the systematical investigations for the ordering process and kinetics of both hexagonally packed cylindrical microdomains (hex-cylinder) and spheres in a body-centered cubic lattice (bcc-sphere) from spherical microdomains with a short-range liquid-like order (disorder-sphere) over a wide range of quench depth for a polystyrene-block-polyisoprene-block-polystyrene triblock copolymer, having order-order transition (OOT) temperature, TOOT, at 190 °C between hex-cylinder and bcc-sphere, by using time-resolved small-angle X-ray scattering. As a result, we clearly assessed a range of temperature over which the two-step ordering from disorder-sphere to hex-cylinder occurs. A possible interpretation for the two-step ordering process was proposed in the text. The following three kinds of ordering processes are observed. (i) The single-step ordering of hex-cylinder: when the system is quenched into the temperature below 183.2 °C, hex-cylinder is directly transformed from disorder-sphere via the nucleation and growth process. The incubation time for the nucleation and the transition rate largely depend on the quench depth. (ii) The ordering of bcc-sphere: when the system is quenched into the temperature above 190 °C in the bcc-sphere phase, bcc-sphere is formed from disorder-sphere via the nucleation and growth process. The quench-depth dependence of the incubation time is very small, compared with that for hex-cylinder, and the transition rate is independent of the quench depth. (iii) The two-step ordering of hex-cylinder: when the system is quenched into the temperature range from 183.2 to 190 °C, the volume-filling metastable bcc-sphere is first developed by nucleation and growth, followed by the OOT from metastable bcc-sphere to hex-cylinder via the nucleation and growth process. The incubation time for the formation of metastable bcc-sphere is very close to that for the formation of equilibrium bcc-sphere, and the transition rate for the formation of metastable bcc-sphere is independent of the quench depth. Thus, the ordering pathways for the volume-filling of metastable bcc-sphere are almost identical to that for the volume-filling equilibrium bcc-sphere. The incubation time for the OOT from metastable bcc-sphere to hex-cylinder is longer than that for the formation of metastable bcc-sphere, and the transition rate for the former is also slower than that for the latter. Moreover, the incubation time for the OOT from metastable bcc-sphere to hex-cylinder is longer than that for that from equilibrium bcc-sphere to hex-cylinder, and the transition rate for the former is also slower than that for the latter, because metastable bcc-sphere is considered to be more distorted than the equilibrium bcc-sphere, which suppress the epitaxial growth of hex-cylinder along a [111] direction of bcc-sphere. 相似文献
Cold-drawn poly(ethylene terephthalate) (PET) samples annealed at different undercoolings are studied by means of differential scanning calorimetry and dynamic mechanical thermal analysis. When heating from room temperature, the onset of the glass transition region in cold-drawn, un-annealed samples is found to be significantly lower than in the case of un-oriented PET. On the contrary, the presence of crystalline lamellae in oriented PET cause a shift (and spread out) of the glass transition region towards higher temperatures. The crystal thickening process caused by heating above the annealing temperature, is suggested to take place after a rigid amorphous phase linked to the basal surface of the lamellae has softened. It is found that the low-temperature (between 100 and 140 °C) annealed samples have a glass dispersion region ranging significantly above the annealing temperature itself. This circumstance leads to envisage vitrification as a possible mechanism able to limit lamellar thickening during the annealing process at these low temperatures. 相似文献
The evolution of enthalpy of As2Se3 glass during structural relaxation in the glass transition region was measured via differential scanning calorimetry for two types of time-temperature programs: rate-heating at 10 K/min following a cool at a constant rate (-20 to -0.31 K/min) and isothermal annealing following a temperature step from an equilibrium state. The rate-heating data yield kinetic parameters for the structural relaxation which predict accurately the evolution of the enthalpy during isothermal annealing. The glass heat capacities were independent of cooling rate within experimental precision (≤0.2%). In this respect, As2Se3 is unlike previously studied glasses whose heat capacities are more dependent on thermal history. 相似文献
The recently developed differential fast scanning calorimetry (DFSC) is used for a new look at the crystal growth of poly(?-caprolactone) (PCL) from 185 K, below the glass transition temperature, to 330 K, close to the equilibrium melting temperature. The DFSC allows temperature control of the sample and determination of its heat capacity using heating rates from 50 to 50,000 K/s. The crystal nucleation and crystallization halftimes were determined simultaneously. The obtained halftimes cover a range from 3 × 10−2 s (nucleation at 215 K) to 3 × 109 s (crystallization at 185 K). After attempting to analyze the experiments with the classical nucleation and growth model, developed for systems consisting of small molecules, a new methodology is described which addresses the specific problems of crystallization of flexible linear macromolecules. The key problems which are attempted to be resolved concern the differences between the structures of the various entities identified and their specific role in the mechanism of growth. The structures range from configurations having practically unmeasurable latent heats of ordering (nuclei) to being clearly-recognizable, ordered species with rather sharp disordering endotherms in the temperature range from the glass transition to equilibrium melting for increasingly perfect and larger crystals. The mechanisms and kinetics of growth involve also a detailed understanding of the interaction with the surrounding rigid-amorphous fraction (RAF) in dependence of crystal size and perfection. 相似文献
Surface defects, which are dependent on the surface properties, determine the appearance, toughness, and other properties
of coatings; thus, changes to the morphology of a damaged epoxy coating surface were investigated on annealing. Changes in
mechanically produced indentations and scratches with annealing indicate a surface transition temperature about 10 degree
lower than the measured bulk glass transition temperature (Tg). Polymer molecules at a free surface, even in a crosslinked coating, may have higher mobility, and thus allow different
relaxation activity from the bulk material. The eventual extent of deformation is a function of the annealing temperature
and time. Results showed that the deformation profile diminished, driven by viscoelastic deformation and surface energy; the
effect of structural relaxation will be further studied. When the surface features were generated by photodegradation, the
roughness increased initially when temperature was increased, possibly due to phase coarsening, and then the roughness diminished
during extended annealing. 相似文献
High-temperature dynamic mechanical thermal analysis (DMTA) has been used to study a lithium zinc silicate glassceramic. The DMTA technique was capable of measuring the glass transition temperature of the original starting glass composition, as well as being able to follow crystallization and structural changes within the glass. The greater sensitivity of DMTA compared to differential thermal analysis, or differential scanning calorimetry, enabled the detection and measurement of the glass transition temperature of the residual glass phase in a largely crystalline glassceramic. An activation energy of 542 kJ·mol-1 for this glass transition was obtained from a multifrequency experiment. A second transition was found in the crystallized glassceramics, and this is thought to be due to a crystal phase transition. Some evidence was also found supporting the view that in these glasses nucleation occurs by an amorphous phase separation mechanism. 相似文献
Three types of silicate glass fibers are annealed, simultaneously stretched in the glass transition region for certain time lengths, then slowly cooled to room temperature under load, and subsequently scanned by differential scanning calorimetry (DSC). During the DSC scanning, a broad exothermic peak (representing energy release) occurs in the stretched fibers well below the glass transition temperature, while it does not occur in the non-stretched fibers. The peak indicates that mechanical stretching can result in an energy enhancement in the fibers. It also confirms that the energy released during reheating of the fibers formed using an industrial continuous fiber drawing process originates not only from thermal quenching but also from mechanical stretching. However, the mechanical stretching-induced energy is much lower than the thermal hyperquenching-induced energy in glass fibers. The effect of annealing temperature and time on the energy release behavior is discussed in terms of viscoelasticity. 相似文献
The effect of isothermal annealing of amorphous PbTiO3 at a temperature below the crystallization temperature, Tcrys, has been investigated. Time dependent dielectric constant in the annealing process has been observed. From the experimental results, both the existence of the glass transition temperature, Tg, and the steadiness of dielectric constant at room temperature have been established. 相似文献
The relationship between microstructure and toughness of biaxially stretched semicrystalline polyester films was investigated. Optically transparent films were prepared by simultaneous biaxial stretching of melt-cast sheets near the glass transition temperature. Copolyesters of polyethylene terephthalate (PET) with different compositions of two diols: ethylene glycol (EG) and cyclohexane dimethanol (CHDM), and stoichiometrically matched terephthalic acid were used to produce films with different degrees of crystallinity. In addition, the PET films with different crystalline morphologies were produced by constrained high temperature annealing of biaxially oriented films. The toughness, degree of crystallinity and crystalline morphology/molecular ordering were studied using mechanical testing, synchrotron small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD) techniques, and differential scanning calorimetry (DSC). The results indicate that the toughness of a semicrystalline polymeric film is determined by the interconnectivity of the crystalline phase within the amorphous phase and is greatly influenced by the degree of crystallinity and the underlying crystalline morphology. 相似文献
Morphology and properties of polymer alloys can be controlled by thermody-namlcally reversible (structure freeze-in) or irreversible (structure lock-in) processes by simultaneously manipulating miscibility, mechanisms of phase separation, glass transition temperature (structural relaxation), and cure kinetics of polymer systems. Using phase diagrams consisting of binodal and spinodal curves, the morphology of epoxy/CTBN (carboxyl-terminated butadiene acryloni-trile copolymer) systems can be controlled by the mechanism of nucleation and growth or by spinodal decomposition via simultaneously manipulating the kinetic processes of phase separation and curing reactions. We have found that the particle size of the rubber reinforcement in epoxies is affected by the mechanisms of phase separation. Phase separation by nucleation and growth gives larger rubber particles than the corresponding phase separation by spinodal decomposition. This contrast in the morphology development is the consequence of controlling phase separation through chemorheological behavior. Medication of the phase separation kinetics in epoxy/CTBN systems was extremely effective at altering both morphology and properties of these alloys. This technique offers a means to shift the glass transition temperature of the rubber-rich phase drastically without reducing the glass transition temperature of the epoxy-rich phase significantly. Such control over morphology is the key to ultimately controlling material properties. This morphology manipulation allows us to tailor the mechanical properties of alloy systems. 相似文献
We report formation of single- and two-phase glasses from rare-earth oxide–alumina materials. Liquids with the Y3Al5O12 and Er3Al5O12 compositions underwent a liquid–liquid phase transition which resulted in glasses with a cloudy appearance due to spheroids of one glass in a matrix of a second glass. The two glasses were isocompositional within the limits of experimental error. Clear, brilliant, single-phase glasses were obtained from La3Al5O12, ErLaYAl5O12, and compositions containing ≥5 mol% La2O3 substituted for the other rare-earth oxides. Formation of two glasses is attributed to nucleation and growth of the second liquid at a temperature below the equilibrium liquid–liquid transition temperature. Addition of lanthanum depresses the phase transition temperature below the glass transition temperature and the liquid–liquid phase transition is not observed. The results are discussed in the context of first-order liquid–liquid phase transitions (polyamorphism) and formation of single-phase glass from liquids that contain a high proportion of 4-coordinate aluminum ions. 相似文献
The present work focuses on the influence of nucleation processes on the crystallization of bio‐based poly(ethylene 2,5‐furandicarboxylate) (PEF). Nuclei formation has been studied by means of fast scanning calorimetry (FSC) both when cooling from the melt (nonisothermal conditions) and when annealing at either low‐ or high‐temperatures (isothermal conditions). FSC results show that nucleation on cooling can be prevented by using fast rates allowing to keep the polymer in its amorphous state; whereas cooling at moderate rates results in sample nucleation with a subsequent increase of the crystallization rate. Isothermal pretreatment just above the PEF glass transition temperature (Tg) results in nuclei formation whose rate decreases when the nucleation temperature approaches PEF Tg. On the other hand, annealing below the PEF melting point allows determination of the sample self‐nucleation behavior which occurs in a very narrow temperature range, i.e., between 195 and 198 °C.