Thermal reactions of solid-state amorphized borates and borate glasses

The properties and course of phase transition of amorphous borates obtained by solid-state thermal and mechanical treatment of hydrated borates and melt quenched glasses with the compositions of these borates were studied. Processes of structural relaxation and crystallization were considered.It was found that different methods of preparation of amorphous borates and the differences in their structural ordering resulting from these methods affect the rate and the mechanism of transformations occurring during their heating.Support for this work was provided by the Polish Committee of Scientific Research (KBN) — grant No. 3 P407 034 06.     

Structural Characterization of Silver Doped Silica Prepared by Two Different Wet Chemical Methods

Two sets of silver doped silica samples were prepared, one using the traditional sol-gel method and the other from colloidal silica. In samples prepared by the first method, the addition of Ag promotes crystallization of the SiO₂ matrix when annealed at temperatures below those marked by the phase diagram. Before crystallization of the silica glass into the -cristobalite phase occurs, the silver diffuses throughout the amorphous network to form silver colloidal particles at annealing temperatures that depend on the silver concentration. In samples obtained from the colloidal silica, larger Ag particles are formed at lower annealing temperatures. Further annealing at higher temperatures crystallizes the glass into the already mentioned phase. The reason of having larger Ag particles at lower temperatures in the latter set of samples is probably because they have a more open structure, produced by a wider distribution in the Si—O—Si bridging angle.     

The isothermal crystallization of engineering polymers — POM and PEEK

The isothermal crystallization of two engineering polymers — POM and PEEK — was studied, both theoretically and experimentally. The experiments were performed by means of differential scanning calorimetry (DSC) and polarized light optical microscopy (OM).Building on previously developed theoretical formalisms (Avrami/Evans, Hillier and Tobin), a new procedure is presented, based on Tobin's model coupled with a modification of Hillier's calculation technique, to accurately describe the kinetics and mechanism of the crystallization of polymers from quiescent melts. First, it is shown that Tobin's model alone, without modification, is more accurate than Avrami/Evans model to describe single-mechanism processes, for a wide range of materials and for longer crystallization times, despite having exactly the same nature and number of parameters (the kinetic, nucleation and growth rate-related, parameterK and the dimensionalityn). Then, Hillier's formalism is modified and combined with Tobin's model, to accurately predict the kinetics of dual mechanism crystallization processes; a clear contrast is drawn with Hillier's Avrami-based, original procedure which uses the same number and nature of parameters, but cannot adequately predict the experimental behavior.The parameter values predicted by the model(s) and procedure presented in this work are all given, are then physically interpreted and, in the case of POM, related to independent morphological observations by polarized light optical microscopy. They are also consistent with electron microscopy observations made by other authors on the detailed morphology of the spherulitic crystallization of polymers.Presented at the EPS-Meeting Solidification Processes in Polymers in Stockholm May 1991     

Studies of the enthalpy relaxation and the “multiple melting” behavior of semicrystalline poly(arylene ether ether ketone) (PEEk)

We report the results of an investigation by differential scanning calorimetry (DSC) of two mobility controlled processes in the amorphous phas e of semicrystalline PEEK — enthalpy relaxation below the glass transition (T _g) and secondary crystallization aboveT _g. Both result in the observation of an endothermic peak just above the annealing temperature in the DSC scan of the polymer — the enthalpy recovery peak and the low temperature melting peak, respectively. There is a striking similarity in the time and temperature dependence of the endothermic peak for these two processes. These results are reminiscent of those obtained from small strain creep studies of physical aging of semicrystalline PEEK below and aboveT _g.We gratefully acknowledge support of this work by the National Science Foundation, Science and Technology Center for High Performance Polymeric Adhesives and Composites under DMR grant 91-2004 and by an NSF Young Investigator Award (DMR 93-57512).     

Effect of amorphous seeds on the crystallization kinetics of Na-A zeolite

The formation of crystal nuclei occurs when alkaline aluminosilicate gels are treated at temperatures below the crystallization temperature or during crystallization of gels rather than during precipitation of the gels. The linear growth rate of the crystals of Na-A zeolites yielded by alkaline aluminosilicate gels in the presence of amorphous seeds is a function of the product of the concentrations of the silicate and aluminate ions in the intermicellar liquid.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 799–804, May, 1994.     

Crystallization in PHB/DGEBA blends

Real‐time experiments using small‐angle X‐ray scattering and differential scanning calorimetry on blends of the semicrystalline polyester poly(3‐hydroxybutyrate) (PHB) and amorphous monomer epoxy DGEBA (diglycidyl ether of bisphenol A) were performed. Differences in the processes of melting and re‐crystallization were observed in blends relative to pure PHB. The results obtained in this study indicated that re‐crystallization is more important in blends with 50% DGEBA than in pure PHB. Moreover, segregation toward the interfibrillar region would facilitate re‐crystallization. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 882–886     

Effect of molecular weight and temperature on the isothermal crystallization of poly(oxetane)

Poly(oxetane) fractions ranging in number-average molecular weights from 7800 to 157000 have been isothermally crystallized in the temperature range from –50 to 19 C, using dilatometric and calorimetric techniques. In both cases, reproducible isotherms were obtained with an Avrami exponent equal to three. The crystallization rate against crystallization temperature presents a maximum at –30 C. The level of crystallinity changes with molecular weight and the influence of this parameter on the rate of crystallization is pronounced. The crystallization temperature coefficient was studied using nucleation theory and it was found an slight increase in the basal interfacial free energy for the lowest molecular weight fraction. For the analysis of the temperature coefficient at the higher undercoolings, different approximations for the free energy of fusion and the transport term have been considered. The conclusion of this analysis is that, independently of these approximations, the obtained temperature coefficients are the same.     

Anhydrous Amorphous Calcium Oxalate Nanoparticles from Ionic Liquids: Stable Crystallization Intermediates in the Formation of Whewellite

The mechanisms by which amorphous intermediates transform into crystalline materials are not well understood. To test the viability and the limits of the classical crystallization, new model systems for crystallization are needed. With a view to elucidating the formation of an amorphous precursor and its subsequent crystallization, the crystallization of calcium oxalate, a biomineral widely occurring in plants, is investigated. Amorphous calcium oxalate (ACO) precipitated from an aqueous solution is described as a hydrated metastable phase, as often observed during low‐temperature inorganic synthesis and biomineralization. In the presence of water, ACO rapidly transforms into hydrated whewellite (monohydrate, CaC₂O₄ ? H₂O, COM). The problem of fast crystallization kinetics is circumvented by synthesizing anhydrous ACO from a pure ionic liquid (IL‐ACO) for the first time. IL‐ACO is stable in the absence of water at ambient temperature. It is obtained as well‐defined, non‐agglomerated particles with diameters of 15–20 nm. When exposed to water, it crystallizes to give (hydrated) COM through a dissolution/recrystallization mechanism.     

Morphology and crystallization behavior of poly(l‐lactide)/poly(butylene oxalate) blends

The isothermal crystallization of poly(l ‐lactide) (PLLA) in blends with poly(butylene oxalate) (PBOX) is investigated by time‐resolved small‐angle X‐ray scattering, differential scanning calorimetry, and optical microscopy. We focus on the temperatures at which only PLLA crystallizes while PBOX is amorphous. It is obtained that the addition of PBOX causes a reduction of the melting temperature of PLLA. The lamellar thickness of PLLA crystals decreases whereas the amorphous layer thickness increases with blend composition, suggesting the occurrence of the interlamellar incorporation upon the addition of PBOX. The crystal growth rate and morphology of PLLA/PBOX blends are analyzed by polarized optical microscopy. The spherulite growth rate of PLLA is found to increase with the addition of PBOX. Analysis of the isothermal crystallization in terms of the Lauritzen and Hoffman equation give the reduction of the fold surface free energy upon the addition of PBOX in PLLA, indicating that the mobility of the PLLA chains is significantly improved due to the presence of PBOX. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 192–202     

Effect of the Heating Rate on Crystallization Kinetics of Mg84Ni12.5Y3.5 Amorphous Ribbons

Crystallization kinetics of Mg₈₄Ni_12.5Y_3.5 amorphous ribbons produced by the melt-spinning method, was studied by DSC analysis and X-ray diffraction. The effect of heating rate (from 2 to 240 K min^–1) was investigated in the temperature range from 298 to 673 K. The results showed that the crystallization process took place in two stages: a) crystallization of part of the amorphous matrix to an intermediate phase and hcp-Mg, and b) transformation of the intermediate phase and the remaining amorphous material to Mg₂Ni+Mg (solid solution of Y in Mg). Increasing the heating rate from 2 to 240 K min^–1 results in increases of the temperature difference between the two-step crystallization of the first stage transformation processes from 33 to 56 K and in increases of the temperature difference between the two-stage transformation from 62 to 97 K.This revised version was published online in November 2005 with corrections to the Cover Date.     

Crystallization kinetics of virgin and processed poly(lactic acid)

Poly(lactic acid) (PLA) is an emerging material mainly because it can be synthesized from renewable resources and is thus environmentally and ecologically safe. The mechanical properties, above all the thermal resistance of PLA are determined by the crystalline content: the heat deflection temperature of crystalline PLA can reach 100 °C, whereas amorphous PLA loses mechanical properties at temperatures slightly higher than 60 °C. However, PLA has a low crystallization rate, so that after processing it remains mostly amorphous. This characteristic heavily limits the use of PLA for commercial applications. Many studies have been recently published on the crystallization kinetics of PLA. The effect of processing on this feature is however often neglected. In this work, the significance of processing on the crystallization kinetics of a commercial PLA was investigated. Two processing methods were explored: extrusion and injection moulding. The obtained materials, and the starting pellets of virgin polymer, were analyzed by calorimetry in order to obtain the crystallization kinetics. Two protocols were adopted to determine the crystallization rates during cooling from the melt or heating from the solid. The parameters of a kinetic equation were determined for all the materials and protocols adopted and it was thus possible to describe the evolution of crystallinity during heating and during cooling.     

Determination of Crystallization and Melting Behaviour of Poly-lactic Acid and Polypropyleneblends as a Food Packaging Materials by Differential Scanning Calorimeter

The food packaging materials commonly used is made from polymers synthetic base on petroleum derivatives. However, the use of synthetic polymers has negative impacts on the environment, because it is difficult to degrade naturally either by biotic or abiotic process. This is a problem for the environment and therefore it needs to do the assessment of the technology to reduce the degree of difficulty on its degradation or to find a new material that can be degradable naturally. One of the most important properties of food packaging materials is the polymer crystallinity. This refers to the overall level of crystalline component in relationship to its amorphous component. It is directly related to many of key properties exhibited by a semi-crystalline polymer including brittleness, toughness, stiffness or modulus, optical clarity, creep or cold flow, barrier resistance and long-term stability. Thus, in this study, PP blends with the PLA and meltsat 250^oCfor 4 hours, and investigates their crystallization and melting behavior using DSC at cooling rate of 10 and 40^oC/min. The results show that base on their thermograms, with increasing the cooling rate will decreasing the crystallinity or increasing amorphous area underthe peaks.     

Melt-crystallization, glass transition and morphology of a (R)-3-hydroxybutyrate pentamer

The melt-crystallization of an oligo[(R)-3-hydroxybutyrate] with five repeating units has been analyzed using standard and temperature-modulated calorimetry, optical microscopy, and atomic force microscopy. Specimens of different crystallinity and supermolecular structure were generated by variation of the rate of cooling of a quiescent melt, or by variation of the temperature of isothermal crystallization. Completely amorphous samples can be obtained by cooling of the melt at a rate of 40 K min⁻¹, or faster, to a temperature lower than the glass transition. The crystallinity depends on the crystallization temperature. The maximum enthalpy-based crystallinity of about 40-45% is obtained by crystallization at temperatures lower than the temperature of the maximum crystallization rate, which is between 310 and 320 K. Analysis of the apparent heat capacity in metastable structural equilibrium reveals reversible melting at temperatures between 320 and 370 K by observation of an excess heat capacity above the level of the vibrational heat capacity, i.e., in the temperature range of irreversible reorganization and melting. The reversible melting is discussed in the context of coupling of the crystalline and amorphous phases, and compared to earlier studies on oligoethylene and oligo(oxyethylene). The presence of crystals causes formation of a rigid amorphous fraction of about 30% at a crystallinity of 40%. Optical and atomic force microscopy reveal spherulitic crystallization. At relatively high crystallization temperature, and in the early stage of the crystallization process, dendrites are observed which finally yield spherulites of decreased perfection. Larger spherulites of higher perfection grow at relatively low crystallization temperature, as deduced from the appearance of the Maltese cross, and the regularity of banding. The band spacing is less than 5 μm, as is accurately determined by atomic force microscopy. The temperature dependence of the spherulitic growth rate is in accord with the calorimetric analysis of the crystallization rate.     

Small-angle x-ray scattering of isotactic polystyrene

Isothermal crystallization process of isotactic polystyrene at 167°C has been studied by smallangle x-ray scattering. The observed SAXS intensities consist of the twophase lamellar structure component, the density fluctuation, and the foreign particle components. The profile of lamellar structure component remains unchanged during crystallization while its intensity increases with crystallization. The lamellar structure of isotactic polystyrene is investigated on the basis of the interface distribution function. An interface distribution function is obtained from the lamellar structure component after correcting the effect of the finite thickness of boundary regions between crystalline and amorphous phases. In order to obtain the structure parameters, the Gaussian correlation model is used, in which the correlation between the distributions of neighboring crystal and amorphous thicknesses is taken into account. Agreement is satisfactory between the experimental results and the calculations. The structure parameters of isotactic polystyrene are determined for isothermal crystallization at 167°C as follows: the average and the standard deviation of crystal thickness are 40 A and 10 A, respectively, those of amorphous thickness are 70 A and 23 A, and the standard deviation of long period is 31 A.     

Melt-crystallization behavior and isothermal crystallization kinetics of crystalline/crystalline blends of poly(ethylene terephthalate)/poly(trimethylene terephthalate)

The melt-crystallization and isothermal melt-crystallization kinetics of poly(ethylene terephthalate)/poly(trimethylene terephthalate) blends (PET/PTT) were investigated by differential scanning calorimetry (DSC) and polarized optical microscopy. Although PET and PTT in the binary blends are miscible at amorphous state, they will crystallize individually when cooled from the melt. In the DSC measurements, PET component with higher supercooling degree will crystallize first, and then the crystallite of PET will be the nucleating agent for PTT, which induce the crystallization of PTT at higher temperature. On the other hand, in both blends of PET80/PTT20 and PET60/PTT40, the PET component will crystallize at higher temperature with faster crystallization rate due to the dilute effect of PTT. So the commingled minor addition of one component to another helps to improve the crystallization of the blends. For blends of PET20/PTT80 and PET40/PTT60, isothermal crystallization kinetics evaluated in terms of the Avrami equation suggest different crystallization mechanisms occurred. The more PET content in blends, the fast crystallization rate is. The Avrami exponent, n = 3, suggests a three-dimensional growth of the crystals in both blends, which is further demonstrated by the spherulites formed in all blends. The crystalline blends show multiple-melting peaks during heating process.     

Elaboration de poudres de yig par coprecipitation

This paper deals with an original method for the preparation of ferrites with the garnet structure by coprecipitation.The different transformations of the coprecipitated hydroxide during heat treatment are studied and reported.YIG crystallization from amorphous oxide is significantly dependent of some parameters.However, pure homogeneous materials can be obtained at low temperature (800–850C) for Al substituted YIG Y₃Al_xFe_5–xO₁₂ with 0.5x1.

     

Phase Behavior of Polymer Blends

The present report deals with some results on phase behavior, miscibility and phase separation for several polymer blends casting from solutions. These blends are grouped as the amorphous polymer blends, blends containing a crystalline polymer or two crystalline polymers. The blends of PMMA/PVAc were miscible and underwent phase separation at elevated temperature, exhibited LCST behavior. The benzoylated PPO has both UCST and LCST nature. For the systems composed of crystalline polymer poly(ethylene oxide) and amorphous polyurethane, of two crystalline polymers poly(-caprolactone) and poly[3,3,-bis-(chloromethyl) oxetane], appear a single T_g, indicating these blends are miscible. The interaction parameter B's were determined to be –14 J cm^–3, –15 J cm^–3 respectively. Phase separation of phenolphthalein poly(ether ether sulfone)/PEO blends were discussed in terms of thermal properties, such as their melting and crystallization behavior.This revised version was published online in November 2005 with corrections to the Cover Date.     

Kinetic study of zirconia crystallization from amorphous ZrO2-SiO2 composite precursors processed by sol-gel chemistry

Powder precursor gels with composition xZrO₂·(100–x)SiO₂, with selected values of x=8, 24, 43 and 75 mol%, were processed by sol-gel chemistry. Differential thermal analysis (DTA) was used to study crystallization in (cubic/tetragonal)-ZrO₂ during the heating of the reactive amorphous precursors. Kinetic parameters such as activation energy, Avrami's exponent and frequency factor have been simultaneously calculated from the computed DTA data using a previously reported kinetic model. The crystallization temperature decreases relative to the increase in the amount of ZrO₂, the value of the kinetic parameter of the crystallization being related to the value of x.     

聚醚醚酮及其碳纤维复合材料——恒温结晶动力学的研究

本文研究了聚醚醚酮(PEEK)和以PEEK树脂为基体的碳纤维复合材料(APC-2)在恒温条件下的结晶行为。采用差示扫描量热仪(DSC)测定从熔体和橡胶体结晶过程中热焓的变化。利用Avrami方程分析了PEEK和APC-2试样的恒温结晶动力学。PEEK从熔体和橡胶体结晶的活化能分别为532.1和531.7KJ/mol,Avrami指数的平均值分别为5.0和3.9;而APC-2则分别为444.2和432.5KJ/mol,3.5和2.3。这些实验结果表明,APC-2试样中碳纤维表面对PEEK树脂基体具有显著的成核作用,能降低其结晶活化能,因而导致PEEK树脂基体结晶速率加快和促进其结晶更加完整。     

Complexation and eutectic crystallization in poly(2-vinyl pyridine)-block-poly(ε-caprolactone) and pentadecylphenol mixtures

Crystallization behavior via hydrogen bonding interaction in amphiphilic block copolymer/surfactant mixtures consisting of poly(2-vinyl pyridine)-block-poly(ε-caprolactone) (P2VP-PCL) and 3-pentadecylphenol (PDP) were investigated by differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. The P2VP-PCL/PDP mixtures exhibit eutectic crystallization behavior; the eutectic composition is approximately at 70 wt.% PDP. Scanning probe microscopy (SPM) observation revealed the microphase structure in the P2VP-PCL/PDP mixtures and the unique eutectic morphology at the eutectic composition, which was further confirmed by small angle X-ray scattering (SAXS) results. To our knowledge, this is the first example of eutectic crystallization observed in amphiphilic block copolymer/surfactant systems. The FTIR study proved that there are competitive hydrogen bonding interactions between P2VP block/PDP and PCL block/PDP pairs in the P2VP-PCL/PDP mixtures. On the basis of the SPM results and FTIR study, a model describing the microstructure of the P2VP-PCL/PDP eutectic mixtures is proposed. The amorphous P2VP blocks are expelled from the ordered eutectic lamellae formed by the crystalline PCL blocks and PDP, which deviates remarkably from the existing structural model proposed by other authors for poly(vinyl pyridine)/PDP and poly(styrene-block-4-vinyl pyridine)/PDP mixtures.