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The Lauritzen and Hoffman secondary nucleation theory was applied to linear growth rate data of syndiotactic polypropylene taken from the literature. Observation of the distinctive upward change of slope in plots of log G+U~* /2.3...
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The Lauritzen and Hoffman secondary nucleation theory was applied to linear growth rate data of syndiotactic polypropylene taken from the literature. Observation of the distinctive upward change of slope in plots of log G+U~* /2.303R(T_c-T_infinity) versus 1/2.303T_c(triangle openT)f suggested the regime II->III transition at the crystallization temperature of 110deg C. Based on the input parameters judged to the the most accurate, the ratios of K_(g.III)/K_(g.II) were found to range from 1.7 to 2.2. Regardless of the crystal structure, if the growth is assumed to occur on the bc plane, the lateral surface free energy, #sigma#=11.3 erg cm~(-2) and the fold surface free energy #sigma#_e=63.7+-7.1 erg cm~(-2) were found. The latter leads to the average work of chain folding of anti q=9.6+-1.1 kcal mol~(-1) is found. These values are applicable to both regimes II and III, A detailed evaluation of the effects of changes in input parameters was also carried out.
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The application of a nephelometric probe for investigation of nucleation kinetics is reported. First a check of direct proportionality of the signal with respect to the surface of the solid suspension was performed, followed by an...
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The application of a nephelometric probe for investigation of nucleation kinetics is reported. First a check of direct proportionality of the signal with respect to the surface of the solid suspension was performed, followed by an experimental analysis of the nucleation of potassium sulfate crystals from aqueous solutions by applying both polythermal and isothermal methods. The adopted instrument allowed to accurately detect the metastable zone width and the induction time and to monitor the first period of time after nucleation. These latter measurements give an indication of the overall crystal mass evolution at the starting of the crystallization operation.
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The kinetics of nucleation and crystal growth in drowning-out crystallization using a T-mixer were estimated using crystal size distribution, taking into account a size-independent growth. At the conditions of the feed composition...
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The kinetics of nucleation and crystal growth in drowning-out crystallization using a T-mixer were estimated using crystal size distribution, taking into account a size-independent growth. At the conditions of the feed compositions investigated, the product weight mean size changed from 7-29 mu m when the residence time varied between 0.32 and 0.61 s. Nucleation and growth rates were expressed simply as a function of the residence time. The T-mixer can be used to generate high levels of supersaturation due to inducing micro-mixing effects. The particle size correlated well with the ratio of growth rate to nucleation rate. Finally, the particle size obtained in drowning-out crystallization using a T-mixer was found to be proportional to the 1.69th power function of the residence time.
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Non-isothermal crystallization kinetics has been traditionally predicted using the Nakamura crystal growth model, which is an integral form of the Avrami equation under the assumption that the non-isothermal kinetics can be explai...
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Non-isothermal crystallization kinetics has been traditionally predicted using the Nakamura crystal growth model, which is an integral form of the Avrami equation under the assumption that the non-isothermal kinetics can be explained using a series of infinitesimal isothermal steps. However, the Nakamura model often overpredicts crystal fraction near the end of crystallization because of ignoring slower secondary crystallization kinetics. In this study, the dual Nakamura model is proposed to capture both primary and secondary crystallization kinetics of poly(ether ether ketone) (PEEK). First, non-isothermal crystallization kinetics of PEEK is monitored at five constant cooling rates of 10, 30, 50, 70, and 90 K/min via differential scanning calorimetry. The dual Nakamura model, which contains a smaller secondary Avrami exponent (ns) to account for the secondary crystallization, is fitted to the exothermal heat flow curves. The dual Nakamura model successfully describes the final stage of crystallization at all cooling rates by reducing the crystal growth dimension from np = 3 for primary crystallization to ns approximately equal to 1.
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Nylon 6 was prepared by adiabatic anionic polymerization of e-caprolactam using hexamethylene epsilon-dicarbamoyl dicaprolactam (HDC), cyclohexyl carbamoyl caprolactam (CCC), or phenyl carbamoyl caprolactam (PCC) as activators and...
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Nylon 6 was prepared by adiabatic anionic polymerization of e-caprolactam using hexamethylene epsilon-dicarbamoyl dicaprolactam (HDC), cyclohexyl carbamoyl caprolactam (CCC), or phenyl carbamoyl caprolactam (PCC) as activators and sodium caprolactamate (NaCL) as a catalyst at various initial reaction temperatures ranging from 130 to 160 degrees C. Adiabatic temperature rise was recorded as a function of polymerization time to investigate polymerization ion kinetics. Kinetic parameters for polymerization, which are more accurate than data reported to date, could be obtained by fitting the temperature rise data with a new polymerization kinetic equation involving crystallization exotherm and thermal conduction. The polymerization rate highly depended on the chemical structure of the activator used, which indicates that the initiating step where the activator is attacked nucleophilically by NaCL is a very important reaction step, affecting the overall polymerization rate. CCC showed the fastest polymerization rate, whereas HDC and PCC showed the medium and the slowest rate, respectively. The contributions of crystallization exotherm and thermal conduction to the resultant temperature rise during polymerization were significant, when the initial reaction temperature was lower than 140 degrees C. In all cases, the molecular weight obtained from intrinsic viscosity measurement was greater than the expected molecular weight. This may be attributed to the branching and/or crosslinking reaction through Claisen-type condensation reactions. (C) 1995 John Wiley & Sons, Inc. [References: 20]
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The para-ferroelectric tricritical phase transition of a single crystal of triglycine selenate (TGSe) has been studied in the neighbourhood of the transition temperature, under weak electric fields, E, using a highly sensitive cal...
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The para-ferroelectric tricritical phase transition of a single crystal of triglycine selenate (TGSe) has been studied in the neighbourhood of the transition temperature, under weak electric fields, E, using a highly sensitive calorimetric technique. The specific heat, c(E), under fields in the range between 5 and 175 V cm(-1) and close to transition temperature (0.2 K), shows different behaviour on cooling and on heating at a temperature variation rate of +/- 0.03 K h(-1); for T < T-c -0.5 K all sets of data match each other. The experimental data have been fitted to the 2-6 Landau potential obtained for E = 0; a term -xi Q, taking into account the coupling between E (xi being dependent on E) and the order parameter Q, is included. This potential fits well with the experimental data in the ferroelectric phase. The different relation between xi and E obtained on heating and on cooling runs is discussed and it is concluded that data on heating correspond to the thermal equilibrium.
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In the present work, a large number of growth rate data for alpha-phase isotactic polypropylene, taken from literature, have been described by the Lauritzen and Hoffmann equation. A general procedure to predict nuclei density by s...
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In the present work, a large number of growth rate data for alpha-phase isotactic polypropylene, taken from literature, have been described by the Lauritzen and Hoffmann equation. A general procedure to predict nuclei density by simple DSC measurements is described and applied successfully to a commercial iPP.
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Oxides-based glass-ceramics have been intensively studied and while they exhibit exceptional thermo-mechanical properties, their transparency in the infrared is limited to the 3 l-art region. In this paper we describe a new type o...
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Oxides-based glass-ceramics have been intensively studied and while they exhibit exceptional thermo-mechanical properties, their transparency in the infrared is limited to the 3 l-art region. In this paper we describe a new type of glass-ceramics which are transparent up to 11 mum and based on the controlled nucleation and crystallization of cesium chloride sub-micron particles inside a Ge-Sb-S glass matrix. The evolution of the optical transmission versus annealing time and temperature has been investigated. Observations under scanning electronic microscopy as well as X-ray diffraction indicate that the crystalline phase is a primitive cubic cell with a parameter slightly inferior to that of pure CsCl and that the grain sizes are about 100 nm. A preliminary test on fracture propagation shows a much better resistance of glass-ceramics to cracks than the corresponding pure glass matrix. (C) 2004 Elsevier B.V. All rights reserved.
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Poly(vinyl alcohol) (PVA) was found to form complexes with PbCl2, Pb(NO3)(2), and Pb(CH3COO)(2). Different complex compositions were prepared by casting technique. Microdomains were observed in the case of the PbCl2-PVA system. Co...
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Poly(vinyl alcohol) (PVA) was found to form complexes with PbCl2, Pb(NO3)(2), and Pb(CH3COO)(2). Different complex compositions were prepared by casting technique. Microdomains were observed in the case of the PbCl2-PVA system. Complexes of PbCl2-PVA system are semicrystalline over the entire composition range. PbCl2-PVA complexes are stable to temperatures greater than 350 degrees C in comparison to 225 degrees C of pure PVA. The general features of the microdomains depends upon the temperature and the salt concentration. The hardness properties of the binary mixture were found to change in a nonlinear mode exhibiting a maximum value with the increase of the PbCl2 content and in a linear mode for the other salts. (C) 1995 John Wiley and Sons, Inc. [References: 21]
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