Crystal Nucleation in Soda–Lime–Silica Glass at Temperatures below the Glass Transition Temperature

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.     

Time-resolved SAXS studies on ordering kinetics of block copolymers: Further investigation of two-step ordering from disorder-sphere to hex-cylinder

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, T_OOT, 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.     

DSC and DMTA study of annealed cold-drawn PET: a three phase model interpretation

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.     

Heat Capacity and Structural Relaxation of Enthalpy in As2Se3 Glass

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.     

Kinetics of nucleation and crystallization in poly(?-caprolactone) (PCL)

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.     

Thermal-induced recovery of small deformations and degradation defects on epoxy coating surface

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 (T _g). 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.     

High-Temperature Dynamic Mechanical Thermal Analysis of a Lithium Zinc Silicate Glass-Ceramic

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.     

一种简便的评价TiO2晶核剂效果的方法

使用差热分析方法测定玻璃的转变温度Ｔｇ和液化温度Ｔｍ。假定与形核方程和生长方程相联系的几个参数是固定不变的,作样品的形核和生长速率曲线,发现样品中ＴｉＯ２的含量越高,在形核和生长速率曲线间的距离越大,同时相应的最大形核的生长速率的相对温度越低。     

Rigid amorphous phase and low temperature melting endotherm of poly(ethylene terephthalate) studied by modulated differential scanning calorimetry

The rigid amorphous phase, the low temperature melting endotherm, and their development with thermal treatment in poly(ethylene terephthalate) (PET) were investigated by means of modulated differential scanning calorimetry. The differential of the reversing heat capacity and nonreversing heat flow signals were used to analyze the behavior of the glass transition and the low temperature melting endotherm. With increasing annealing time, the increment of the heat capacity at the glass‐transition temperature decreased and the increment of heat capacity at the annealing temperature increased. It was suggested that the origin of the low temperature melting endotherm mainly resulted from the transition of the rigid amorphous fraction for the PET used. The glasslike transition of the rigid amorphous fraction occurred between the glass transition and melting. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2779–2785, 2001     

Li取代对LAS系微晶玻璃核化行为的影响

以Li2O-Al2O3-SiO2三元系(LAS)微晶玻璃为研究对象,采用DTA、XRD等测试技术研究了不同Li2O含量LAS玻璃的核化行为,分析了Li取代对LAS玻璃核化行为的影响。研究结果表明,随着Li含量的降低,LAS系玻璃的相转变温度、析晶峰温度逐步增大,最佳核化温度逐渐升高,最佳核化时间没有明显的变化。     

Energy Release in Isothermally Stretched Silicate Glass fibers

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.     

Slow crystallization process from amorphous PbTiO3

The effect of isothermal annealing of amorphous PbTiO₃ at a temperature below the crystallization temperature, T_crys, 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, T_g, and the steadiness of dielectric constant at room temperature have been established.     

Interdiffusion in microlayered polymer composites of polycarbonate and a copolyester

The interdiffusion of two miscible polymers, polycarbonate (PC) and a copolyester (KODAR), was studied at temperatures from 200 to 230°C. The two polymers were coextruded as microlayer composites with up to 3713 alternating layers. The microlayer structure provided a large area of intimate contact between the two polymers with minimal mixing. Initially, two glass transition temperatures were observed by DSC that were intermediate between the glass transition temperatures of the pure components. Upon annealing, the glass transition temperatures shifted closer together, reflecting the extent to which inter-diffusion had occurred. After no more than 2 h of annealing, a single glass transition temperature was observed. A model was formulated based on Fick's law of diffusion that related the mutual diffusion coefficient, D, to the change in the glass transition temperatures. The model also incorporated an “equivalent” residence time to account for diffusion that occurred during the coextrusion process. It was not necessary to consider the concentration dependence of D to satisfactorily describe the data with this model. For the temperature range from 200 to 230°C, the value of D varied from 4.0 × 10^?16 to 1.6 × 10^?15 m²/s. The activation energy of interdiffusion was determined to be 95 kJ/mol. © 1994 John Wiley & Sons, Inc.     

药品冷冻干燥过程的退火机理分析

运用热力学和动力学理论分析了药品冷冻干燥过程中的退火操作改变冻结药品结构的机理。通过对化学势随晶粒曲率半径变化、重结晶过程中冰晶半径随时间变化、非晶态基质的粘度随温度变化几个关系式的分析,得出退火温度必须要高于冻结药品的玻璃化转变温度;最佳退火持续时间与重结晶的速率紧密相关,且是退火温度的函数;退火后再冷却过程的冷却速率不影响升华干燥速率等结论。     

Influence of crosslinking on crystallization,rheological, and mechanical behaviors of high density polyethylene/ethylene‐vinyl acetate copolymer blends

High density polyethylene (HDPE)/ethylene‐vinyl acetate copolymer (EVA) blends with selective crosslinking the EVA phase were prepared and the crystallization, rheological, and mechanical behaviors were studied. Selective crosslinking of EVA component could greatly improve both tensile and impact strengths of the HDPE‐rich blends and influence melting enthalpy at different annealing temperature in successive self‐nucleation and annealing procedure. Dynamic mechanical analysis reveals that glass transition temperatures of both the HDPE and EVA components are lowered upon blending and are raised upon crosslinking. The uncrosslinked HDPE/EVA blends are unstable in the melt and show increment in storage modulus (G′) and decay in loss tangent (tanδ) with annealing time associated with phase coarsening. However, morphology of selectively crosslinked blends in the melt state is highly unstable, characterized by a fast migration of uncrosslinked HDPE component out of the crosslinked EVA phase to the surface resulting in a rapid decay in G′ and an increment in tanδ at the early stage of annealing. POLYM. ENG. SCI., 54:2848–2858, 2014. © 2014 Society of Plastics Engineers     

The relationship between microstructure and toughness of biaxially oriented semicrystalline polyester films

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.     

Morphology and properties control on rubber-epoxy alloy systems

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.     

Glass Formation and Polyamorphism in Rare-Earth Oxide–Aluminum Oxide Compositions

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.     

Nucleation and Self‐Nucleation of Bio‐Based Poly(ethylene 2,5‐furandicarboxylate) Probed by Fast Scanning Calorimetry

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 (T_g) results in nuclei formation whose rate decreases when the nucleation temperature approaches PEF T_g. 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.