共查询到20条相似文献,搜索用时 15 毫秒
1.
Francisco Lpez María P. Calcagno Jesús M. Contreras Zulay Torrellas Katania Felisola Franco M. Rabagliati Issa A. Katime 《Polymer International》1991,24(2):105-112
The diphenylzinc-butanone system was used as polymerization catalyst for some oxiranes in benzene solution at 60°C. This system is greatly influenced by the molar ratio of butanone to diphenylzinc, and the maximum catalytic activity for propylene oxide and ethylene oxide was found for a ratio of unity. GPC results strongly suggest the presence of more than one active species for the system. 13C NMR analysis indicates that the resulting poly(propylene oxide) has a head-to-tail arrangement. For the polymerization of propylene oxide with butanone/diphenylzinc = 1, after an initial induction period, the reaction was first-order with respect to monomer with k = 2·51 × 10?5 s?1. Ethylene oxide polymerizations using butanone/diphenylzinc = 1 and 5 were also first-order with respect to monomer after an initial induction period with k = 7·80 × 10?6 s?1 and k = 5·71 × 10?6 s?1, respectively. The diphenylzinc-butanone system was not an effective catalyst for styrene oxide polymerization. 相似文献
2.
The termination mechanism in gamma-radiation-initiated polymerization of methyl methacrylate was investigated. The termination mechanism and the associated rate constants were determined by comparing the theoretical molecular weight distribution (MWD) and the experimental MWD, which was determined by gel permeation chromatography. Disproportionation occurs 1.2 times as frequently as combination at 30°C and 0.5 times as frequently at 0°C. The activation energy for disproportionation is 5.5 kcal/mole greater than that for combination. The rates of initiation were determined by a new method and were found to compare well with those reported in the literature. It is also suggested that it is necessary for the theoretical MWD to have the same shape as the experimental MWD before a mechanism can be authenticated. 相似文献
3.
K. K. Chee 《应用聚合物科学杂志》1995,58(11):2057-2062
The swelling behavior of the networks of natural rubber, an epoxidized natural rubber, low-density polyethylene, polystyrene, and poly(methyl methacrylate) are studied in six classes of solvents at 60°C. The data of the swelling coefficient are successfully treated by a modified version of the empirical equation proposed by Gee, as to determine the solubility parameter of a polymer, δ2. It is found that increasing the temperature would decrease the δ2's of these polymers with a common rate of 0.02 (J/mL)1/2/K. A semiempirical model is proposed to rationalize the present finding satisfactorily. © 1995 John Wiley & Sons, Inc. 相似文献
4.
Jorge C. Ramirez Jorge Herrera-Ordonez Hortensia Maldonado-Textle 《Polymer Bulletin》2005,53(5-6):333-337
Summary The effect of stirring speed (SS) on the kinetics of styrene emulsion polymerization at 70 °C, using SDS as surfactant and KPS as initiator, was studied. The conversion (x) and weight average molecular weight (Mw) results are higher in the optimum SS range, which seems to be strongly dependent on the hydrodynamic characteristics of the reactor and the polymerization conditions. Above the optimum SS range, the number of particles and Mw are lower, therefore limited coagulation lead to a decrease in Mw caused by the increment of the free-radical entry frequency. These radicals not only come from the aqueous phase (oligomeric radicals), but also come from the radicals contained in the particles subject of coagulation (coagulative entry). 相似文献
5.
Rate and molecular weight distribution modeling of syndiospecific styrene polymerization over silica-supported metallocene catalyst 总被引:2,自引:0,他引:2
The kinetics of syndiospecific polymerization of styrene over silica-supported Cp∗Ti(OCH3)3/MAO catalyst has been investigated through experimentation and theoretical modeling. At low monomer concentrations, the polymerization rate increases almost linearly with monomer conversion, but the reaction rate becomes independent of monomer concentration at high bulk phase monomer concentrations. A kinetic model that incorporates the monomer partition effect between the solid and the liquid phases has been proposed. The model simulations show that the observed non-linear kinetics can be adequately modeled by the monomer partition model. The polymer molecular weight has also been found to increase with the monomer concentration and the polymer molecular weight distribution (MWD) is quite broad, suggesting that the catalytic behavior deviates from the single site catalytic polymerization model. The MWD broadening is modeled by a two-site kinetic model and a good agreement between the model and the experimental data has been obtained. 相似文献
6.
A. Glover Bailie Karel Bouzek Petr Lukek Ivo Rouar Anthony A. Wragg 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1996,66(1):35-40
The solubility of potassium ferrate (K2FeO4) was measured in aqueous solutions of NaOH and KOH of total concentration 12 M containing various molar ratios of KOH:NaOH in the range 12:0 to 3:9. Several analytical methods were tested for the determination of ferrate concentration. The final method chosen consisted of potentiometric titration of the ferrate sample with an alkaline solution of As2O3. The assumption was made that ferrate dissociates in concentrated KOH solutions predominantly to KFeO4−. The solubility constant, S, defined as the product of the molar concentration of the potassium ion, K+, and the ferrate anion, KFeO4−, was found to be 0·044 ± 0·006 mol2 dm−6 for 20°C, 0·093 ± 0·004 mol2 dm−6 for 40°C and 0·15 ± 0·09 mol2 dm−6 for 60°C. From these results the heat of dissolution of K2FeO4 was calculated as −14·3 kJ mol−1. At 60°C the enhanced decomposition of the ferrate at the higher temperature led to a greater deviation in solubility values compared with data for either 20°C or 40°C. 相似文献
7.
Beads of SAN copolymer produced by suspension polymerization have sizes distributed over the range of 0.2–3.0 mm; 70%–80% of these are in the range of 0.4–0.8 mm diameter. Studies on molecular weight and molecular weight distribution showed that the average molecular weight of the polymer was higher with increasing sizes of beads. Fractionation of small-, medium-, and large-sized beads as well as determination of molecular weight by GPC provided clear evidence that there is also an increase of polydispersity and higher molecular weight ends with increasing diameter of beads. This increase in molecular weight and polydispersity in larger beads is probably due to slow thermal polymerization in near absence of initiator toward the end of polymerization. 相似文献
8.
Ehsan Vafa Mohammad Shahrokhi Hossein Abedini 《Polymer Engineering and Science》2010,50(12):2306-2320
In this work, simultaneous inferential control of particle size distribution (PSD) and molecular weight distribution (MWD) in a semi‐batch emulsion polymerization reactor of styrene has been addressed. Using a comprehensive dynamic model for PSD and MWD predictions and performing a sensitivity analysis, it has been revealed that free surfactant and chain transfer agent (CTA) concentrations in the reactor are the most suitable candidates for inferential control of PSD and MWD, respectively. To control concentrations of these species in the reactor, their inlet feed flow rates are used as manipulated variables. It is assumed that the concentration of CTA is measured infrequently and therefore an open‐looped observer, based on the reaction calorimetry, has been designed to estimate the CTA concentration. The infrequent measurements of CTA concentration are used to correct its estimation. As the online measuring of the ionic free surfactant concentration is also difficult, solution conductivity which is a good indication of free surfactant concentration is used for control purposes. Simulation results show that the performance of the proposed control scheme is satisfactory even in the presence of model mismatch. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers 相似文献
9.
A new method is presented for modeling and controlling polymer molecular weight distribution (MWD) and tensile strength in a batch suspension polymerization of styrene. The molecular weight distribution is modeled by computing the weight fraction of the polymer in different chain length intervals. Tensile strength is then related to the modeled molecular weight distribution using a correlation available in the literature and based on the concept of a threshold molecular weight. This method enables the design of operating conditions for a batch suspension polymerization reactor, which will theoretically yield amorphous polystyrene with a desired tensile strength. Two numerical examples are presented to illustrate the feasibility of the proposed method. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1017–1026, 1998 相似文献
10.
A computational scheme is presented for the calculation of molecular weight distribution in styrene polymerization initiated by a binary initiator system. In this paper, the method of finite molecular weight moment is presented by calculation of the polymer chain length distribution. This method is compared with the method of integrating an infinite number of polymer population balance equations. The results of the two methods show a reasonably good agreement. It is possible to produce polymer having the same molecular weight distribution. 相似文献
11.
This paper describes the nonoxidative thermal degradation of an epoxide resin based on the diglycidyl ether of bisphenol A crosslinked with p,p′-diaminodiphenyl methane. Temperatures of degradation lay between 200 and 310°C. and the process was followed concurrently by three means: changes in dielectric properties, changes in infrared spectra, and weight loss. Dielectric properties support the contention that there is a dehydration step during degradation. It is proposed that vacuum curing at high temperature can produce optimum crosslinking. Evidence of phenol and N-methyl aniline as degradation products is advanced, and possible degradation mechanisms are discussed. 相似文献
12.
Yang Zhang Guotian Ye Wenjing Gu Dafei Ding Liugang Chen Lingling Zhu 《Journal of the American Ceramic Society》2018,101(7):2712-2717
There have been different hypotheses about the transformation mechanisms of calcium aluminate cement hydrates and this work aims to clarify the long‐running debate about the conversion approaches. In this work, CAH10 and C2AH8 were produced from the pastes of calcium aluminate cement (CAC) cured for 24 hours at 10 and 20°C separately. And the cured pastes were continually cured at 60°C for 3 days with water and without water, respectively. The hydration of the pastes was halted by freeze‐drying, and the phases and microstructure of hydrates were investigated by XRD and SEM, respectively. The results indicate that CAH10 and C2AH8 converted into C3AH6 and AH3 in water presence at 60°C, but did not transform into C3AH6 and AH3 without water. It is confirmed that the conversion of CAH10 and C2AH8 to C3AH6 and AH3 happens through preceding solution of CAH10 and C2AH8 and subsequent precipitation of C3AH6 and AH3. 相似文献
13.
Commercial grade alumina along with 5, 10, 15 and 20 wt.% zirconia-dispersed aluminas were tested for their wear resistance at ambient temperature, 140 °C and 250 °C using a pin on disk tribotester fitted with a hot stage. The sample suite was investigated for physical characteristics including hardness, fracture toughness, bulk density, alumina grain size and zirconia grain size. The wear track and wear debris were investigated using profilometry, SEM as well as TEM.
The 5 wt.% zirconia-dispersed aluminas had the lowest wear volume loss over the temperature range. The alumina sample exhibited a wear dependence with relative humidity which is attributed to the formation of a tribochemical layer. Investigation of the tribochemical layer using SEM/EDS and TEM electron diffraction showed the tribochemical layer to be aluminium hydroxide. The major wear mechanism for all samples was brittle fracture. 相似文献
14.
Jonathan W. Martin Brian Dickens David Waksman Dale P. Bentz W. Eric Byrd Edward Embree Willard E. Roberts 《应用聚合物科学杂志》1987,34(1):377-393
Small but significant numbers of chain scissions occur in a commercial poly(methyl methacrylate) sheet exposed to temperatures between 50 and 125°C. The scission rate is initially high and then levels off to a constant rate. The short-time rate of chain scissions is temperature dependent, while the long-time rate of chain scissions appears to be temperature independent. Four possible sources of random chain scission initiation were considered: (1) the presence of unreacted initiators of polymerization, (2) free radicals generated from additives in the commercial film, (3) weak links in the polymer chain, and (4) free radicals generated from the thermal decomposition of an oxidation product of methyl methacrylate (MMA) monomer. The source most consistent with our results is the one involving free radicals generated from the oxidation product of MMA monomer. 相似文献
15.
Available solubility constants indicate that ettringite should be the stable form of calcium aluminate sulfate hydrate with respect to monosulfate in cement porewater. However, monosulfate is generally present in mature cement pastes, usually in the absence of ettringite. The objectives of this study were to determine the solubility product of ettringite under equilibrium conditions and to examine the solubility data used in predictive thermodynamic models. Solubility products were calculated for ettringite prepared from both supersaturated and undersaturated solutions with a pH range between 10.4 and 13.7. The mean solubility product for ettringite dissolution: Ca6Al2O6(SO4)3 · 32H2O → 6Ca2+ + 2Al(OH)4− + 3SO42− + 4OH− + 26H2O was 10−44.91, i.e. Log Ksp = -44.91 ± 1.06 (2 S.D.). Activity coefficients were calculated using the specific ion interaction approach. The mean solubility product was close to other values calculated from concentrations reported elsewhere for the solubility of ettringite. As is the case for all solubility products, this value cannot be inserted directly into the databases of other thermodynamic models because of differences in the methods used to calculate activity coefficients and the manner in which ion-pairing is handled by different models. However, raw solubility data are provided for recalculation of the solubility product for use in other models. 相似文献
16.
The emulsion polymerization of MMA was explored for the BDE/CuCl coordinated catalyst. The Mn of PMMA linearly increased both with increasing the monomer conversion and the proceeding polymerization time, which means that the MMA polymerized in “living”/controlled characters with zero order kinetics under BDE/CuCl‐catalyzed emulsion conditions. The apparent polymerization rate constants of MMA were k = 0.765 mol/min, k = 0.760 mol/min at 80°C, while k = 0.228 mol/min at 50°C, respectively. Slight differences of polymerization results were obtained when emulsifier lauryl phosphate (ADP) and Polyoxyethylene nonyl phenyl ether (OP‐10) were adapted in the polymerization. Based on the “coordinated radical cage” mechanism proposed particularly to the BDE/CuCl catalyzed polymerization, reversible equilibrium between common free radical and the coordinated “living” species should exist in this system. Increasing the amount of catalyst must affect the fast equilibrium between those two species, thus, also affecting the relative content in the emulsion circumstance. Therefore, PMMA, with bimodal molecular weight distribution, was achieved through this approach. The formation of PMMA with bimodal distribution was affected by concentration of catalyst and polymerization temperature. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 3076–3081, 2002; DOI 10.1002/app.2336 相似文献
17.
Kinetic constants for decomposition of nitrocellulose in the 50 °C to 500°C range are analyzed. At T < 100°C, three processes (depolymerization, peroxide formation, and hydrolysis) are consistent with the reported kinetics. For T = 100°C–200°C, 28 of 30 previously reported kinetic measurements can be organized clearly into two categories by the use of the kinetic compensation effect. These two groups fit the first-order and autocatalytic processes. Conflicting interpretations are reconciled by this approach. At T > 200°C, the kinetics are consistent with the existence of the first-order step and desorption of the products as two parallel processes which, together, control the rate. Time-to-exotherm and mass burning rate kinetics are compared as temperature-dependent reaction-desorption events. 相似文献
18.
A method is described which allows the precise estimation of molecular weight distribution parameters p and A which pertain to free-radical polymerizations. Thus, expressions are developed which allow the estimation of (1 ? p) by means of a programmable calculator using molecular weight distribution data derived from GPC. Values of A may subsequently be calculated by means of a plot of one of the expressions given. Values of (1 ? p) and A obtained in this paper were checked using theoretical values as well as by a comparison between calculated and observed values. The agreement between calculated and theoretical or experimental values indicates that the method presented for the estimation of (1 ? p) and A is both reliable and relatively rapid. Previously reported methods for the estimation of (1 ? p) and A have involved curve-fitting trial calculations as well as more precise interpolation procedures. However, the latter are based on single-peak maximum values. 相似文献
19.
Polystrene (PS) particles, generated from soap-free emulsion polymerization of styrene monomer, water, and potassium persulfate, were investigated by photon correlation spectroscopy (PCS) and TEM. The particle size distribution (PSD) was quite uniform. From the data of PCS, it could be said that lots of particles flocculated in the final stage of reaction. It was also deduced from the molecular weight distribution (MWD), measured by GPC, that, during the early stage of reaction, molecules with low molecular weight (<4000) might exist and the particles were perhaps formed through micellar-type nucleation mechanism. When initiator concentration increased, reaction rate increased but weight average molecular weight, tensile modulus, and elongation decreased. The number density of particles was found to be proportional to 0.49 power of initiator concentration. However, monomer concentration did not seem to have any great effect on all of them above. 相似文献
20.
A mathematical model of the molecular weight distribution (MWD) based on a multilayer model and an improved intrinsic kinetics model was proposed to simulate the MWD of the gas‐phase polymerization of butadiene with a heterogeneous catalyst. Intrinsic kinetics and heat and mass‐transfer resistances based on the multilayer model of a polymeric particle were considered in the modeling of the MWD. The effects of the reaction conditions, catalyst particle size, mass‐transfer resistance, deactivation of active sites, and transfer of the polymer chain on the molecular weight and MWD were simulated. The results show that the effects of the deactivation of active sites and transfer of the polymer chain on the average molecular weight are significant and that the effect of the catalyst particle size on the MWD is not significant. The simulation results of the molecular weight and MWD are compared with the experimental results. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 88–103, 2003 相似文献