共查询到20条相似文献,搜索用时 3 毫秒
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Howard W. Starkweather Mark C. Han 《Journal of polymer science. Part A, Polymer chemistry》1992,30(13):2709-2713
Emulsion polymerizations of ethylene were run using potassium persulfate as the initiator and either an anionic or a cationic surfactant, usually at five times the critical micellar concentration. The molecular weight distributions had at least two major peaks. The broadly distributed lower molecular weight component is formed in the early stages of polymerization. The higher molecular weight component has a narrow distribution characteristic of termination by the combination of chain radicals. The weight-average molecular weight of this component was about 400,000 when the surfactant was sodium laurate and 33,600,000 when the surfactant was dodecyl amine hydrochloride. © 1992 John Wiley & Sons, Inc. 相似文献
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This article presents a method that provides the molecular weight distribution (MWD) of polymeric material from rheological data. The technique has been developed to deal with linear polymers with a log‐normal molecular weight distribution. The rheological data must include the shear storage modulus, G′(ω), and the shear loss modulus, G″ (ω), ranging from the terminal zone to the rubberlike zone. It was not necessary to achieve the relaxation spectrums via the extremely unstable problem of inverting integral equations. The method has been tested with different polymers (polydimethylsiloxane, polyisoprene, random copolymer of ethylene and propylene, and polystyrene) and the calculated MWDs were in good agreement with experimental data. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1539–1546, 2000 相似文献
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R.A. Hutchinson P.J. McLellan J.O. Ramsay H. Sulieman D.W. Bacon 《Macromolecular Symposia》2004,206(1):495-508
Molecular weight distributions (MWDs) are inherently functional observations in which differential weight fraction is expressed as a function of chain length. Conventional approaches for analyzing and predicting MWDs include discretization and treatment as multi-response estimation problems, characterization using moments, and detailed mechanistic modeling to predict fractions for each chain length. However, these approaches can be sensitive to loss of information, complexity and problem conditioning. An alternative is to treat the MWDs as functional observations, and to use techniques from Functional Data Analysis (FDA), notably functional regression. The objective of this paper is to develop and apply empirical modeling techniques based on functional regression for investigating the impact of operating parameters on MWDs. 相似文献
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Luigi D'Agnillo Joo B. P. Soares Alexander Penlidis 《Journal of polymer science. Part A, Polymer chemistry》1998,36(5):831-840
A critical look at the possibility of controlling the molecular weight distribution (MWD) of polyolefins by combining metallocene/methylalumoxane (MAO) catalysts is offered. Catalysts investigated were bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2), its titanium and hafnium analogues (Cp2TiCl2 and Cp2HfCl2), as well as rac-ethylenebis(indenyl)zirconium dichloride (Et(Ind)2ZrCl2). As observed by other researchers, the MWD of polyethylene can be manipulated by combining soluble catalysts, which on their own produce polymer with narrow MWD but with different average molecular weights. Combined in slurry polymerization reactors, the catalysts in consideration produce ethylene homopolymer just as they would independently. Unimodal or bimodal MWDs can be obtained. This effect can be mimicked by blending polymers produced by the individual catalysts. We demonstrate how a variability in catalyst activity translates into a variability in MWD when mixing soluble catalysts in polymerization. Such a variability in MWD must be considered when setting goals for MWD control. We introduce a more quantitative approach to controlling the MWD using this method. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 831–840, 1998 相似文献
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Joo B. P. Soares J. D. Kim 《Journal of polymer science. Part A, Polymer chemistry》2000,38(9):1408-1416
Metallocene and other single‐site catalysts can be combined to produce polyolefins with broadened distributions of molecular weight, chemical composition, and long‐chain branching. These resins are finding increasing applications because of their enhanced properties compared to ones made with conventional Ziegler–Natta catalysts. Resins with bimodal molecular weight distributions (MWDs) have especially attractive mechanical and rheological properties. Although the use of these resins is expected to increase, there are very few studies available to quantify MWD bimodality or to decide a priori which combinations of metallocene catalysts will lead to the formation of polyolefins with bimodal MWDs. In this article, a necessary condition for the production of polymer with bimodal MWD using two single‐site‐type catalysts is derived. Additionally, a bimodality index is defined to quantify MWD bimodality. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1408–1416, 2000 相似文献
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Jessica E. Coughlin Andreas Reisch Marie Z. Markarian Joseph B. Schlenoff 《Journal of polymer science. Part A, Polymer chemistry》2013,51(11):2416-2424
Sulfonation of narrow polydispersity polystyrene, PS, standards remains the method of choice for generating polystyrene sulfonate, PSS, samples with defined composition. Although a variety of sulfonation techniques have been described, relatively little is reported on the material obtained, which is used for so many studies on the fundamental behavior of polyelectrolytes. Here, we show that powdered polystyrene treated with concentrated sulfuric acid (96%) at 90 °C without catalyst yields fully sulfonated PSS. Extensive characterization with 1H and 13C NMR as well as size exclusion chromatography coupled with static and dynamic light scattering shows no evidence of sulfone crosslinking or chain degradation under the conditions used. Though mono‐sulfonated as soon as it dissolves in the acid, the PSS contains about 6% meta substitution. Sulfonation kinetics for this heterogeneous reaction depend strongly on particle size, sulfuric acid content and temperature. For preparing perdeuterated PSS from the corresponding PS it is essential to employ D2SO4, as about half of the aromatic units undergo H/D exchange during sulfonation. The remaining ortho H/D may be exchanged with extended exposure to the concentrated sulfuric acid, but the meta site is deactivated. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2416–2624 相似文献
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Timothy R. Dargaville Felix N. Guerzoni Mark G. Looney Devon A. Shipp David H. Solomon Xiaoqing Zhang 《Journal of polymer science. Part A, Polymer chemistry》1997,35(8):1399-1407
The number- and weight-average molecular weights of several statistical and high ortho novolac resins were determined using gel permeation chromatography (GPC). The standards used were pure compounds having between 2 and 12 phenol units bridged via methylene linkages. Three series of compounds were studied: (i) those with methylene linkages substituted in only the ortho positions relative to the phenolic hydroxyl group; (ii) those in which all para positions, together with sufficient ortho positions, were used to synthesize the compounds; and (iii) those in which the methylene linkages were substituted at a mixture of ortho and para positions. Such compounds, having known molecular architecture and units similar to the segments of industrial novolac resins, provide for a more exact measurement of the molecular weight than do the commonly used poly(styrene) standards. Using these new standards the number average molecular weights of the resins determined by GPC were in good agreement with the average molecular weight obtained by 1H-NMR spectra of the resins, particularly for low molecular weight resins. GPC analysis of higher molecular weight resins tends to underestimate the molecular weights because of complications introduced by hydrogen bonding. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1399–1407, 1997 相似文献
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Christopher G. Robertson Csar A. García‐Franco Srivatsan Srinivas 《Journal of Polymer Science.Polymer Physics》2004,42(9):1671-1684
We present new results and examine literature data concerning the linear viscoelastic behavior of polyethylene with sparse to intermediate levels of long‐chain branching (LCB). These branched polymers displayed a common rheological signature, namely, a region of frequency‐independent loss tangent along with the corequisite scaling of the storage and loss moduli to the same frequency exponent. This apparent power‐law response occurred within a finite frequency window and bore resemblance to the behavior of physical gels. The appearance of this region, however, was the consequence of the presence of two distinct, yet partially overlapping, terminal relaxation processes. After considering the analogous relaxation behavior of wholly linear polymers with bimodal molecular weight distributions, we considered the polymers with LCB as blends of linear and branched species to develop a simple method of quantifying the extent of LCB. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1671–1684, 2004 相似文献
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采用新型茂钛化合物Cp^*Ti(O-Ph-OCH3)3(A)/CpTi(O-Ph-OCH3)3(B)和Cp^*Ti(O-Ph-OCH3)3(A)/Cp^*Ti(O-Ph-F),(C)作为复合催化剂,在甲基铝氧烷(MAO)、三异丁基铝(TIBA)激活下进行苯乙烯间规聚合。采用A/B复合催化剂可通过调节两种茂钛催化剂的摩尔配比,得到宽分子量分布的间规聚苯乙烯(s-PS);而采用A/C复合催化剂得到的产物分子量分布基本不变。对不同聚合条件下得到的s-PS产物进行了GPC分析。 相似文献
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Yonggyu Han Min Hyung Lee Junseong Lee Youngkyu Do 《Journal of organometallic chemistry》2009,694(26):4216-4222
Reactions of the dilithium salt of aminosilylene-bridged ligands with (Me2N)2ZrCl2(THF)2 followed by the treatment of Me3SiCl are found to be an efficient synthetic route to aminosilylene-bridged ansa-zirconocenes, R2N(Me)Si(η5-C5H4)2ZrCl2 (R = Me (1), Et (2)) and Me2N(Me)Si(η5-C5H4)(η5-C5Me4)ZrCl2 (3). Crystal structure of 3 determined by X-ray diffraction study reveals the presence of π-bonding interaction between N and Si atoms, which is further supported by DFT calculation results. These complexes are very active (>1 × 103 Kg/(mol Cat.·atm·h)) for homopolymerization of ethylene in the presence of methylalumoxane (MAO) cocatalyst, generating polyethylenes that contain branches as well as bimodal molecular weight distribution (MWD). Methyl, ethyl, butyl, and other longer branches (n ≥ 6) are observed in the resulting polyethylenes. The polyethylenes from 1, 2 and 3/MAO show a broad MWD range (6.3-42.2, 3.5-4.0 and 2.6-3.4, respectively). 相似文献
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Douglas Poland 《Journal of Polymer Science.Polymer Physics》2001,39(13):1513-1518
Using polypropylene as an example, we applied a method we have recently developed to calculate the probability distribution of enthalpy from the temperature dependence of the heat capacity. The method involves the use of local temperature expansions of the heat capacity to calculate a set of moments of the enthalpy distribution. Using the maximum‐entropy method, one can then construct the enthalpy distribution for the system. The method is completely model free. The enthalpy distribution so obtained is the analogue of the Maxwell–Boltzmann distribution of kinetic energies for a gas, and like that function, tells one the distribution of enthalpies that an average unit in the polymer chain can have, a quantity that is crucial to understanding the chemical and physical properties of a polymer. Given the enthalpy distribution, one can then calculate the Gibbs free energy and the density of states that correspond to a particular value of enthalpy, thus giving one an expanded thermodynamics of the system in addition to the usual average quantities. We illustrate the fact that the Gibbs free‐energy distribution for this system scales as a simple function of temperature and that the density‐of ‐states function yields a simple empirical partition function for the system giving both the average thermodynamics and the distribution functions. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1513–1518, 2001 相似文献
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The molecular weight distribution (MWD) of high-molecular-weight isotactic polypropylene is measured by a new method developed for polyethylene (PE) using the turbidity at the lower critical solution temperature (LCST). The attenuation of transmitted light hi is measured at Ti as each fraction i is separated from the solution during a temperature increase ΔTi. From the thermogram, the set of hi (Ti), the MWD can be calculated through a relation Mi = f(Ti). The solvent chosen, a mixture of butane and 2-methylbutane, optimizes resolution and relatively rapid dissolution. Homogenization and absence of degradation are verified by different methods including calorimetry in solution. © 1992 John Wiley & Sons, Inc. 相似文献
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《大分子反应工程》2018,12(1)
Polyolefins are commercially produced in continuous reactors that have a broad residence time distribution (RTD). Most of these polymers are made with heterogeneous catalysts that also have a particle size distribution (PSD). These are totally segregated systems, in which the catalyst/polymer particle can be seen as a microreactor operated in semibatch mode, where the reagents (olefins, hydrogen, etc.) are fed continuously to the catalyst/polymer particle, but no polymer particle can leave. The reactor RTD has a large influence on the PSD of the polymer particles leaving the reactor, as well as in polymer microstructure and properties, polymerization yield, and composition of reactor blends. This article proposes a Monte Carlo model that can describe how particle RTD in a single or a series of reactors can affect the PSD of polymer particles made under a variety of operation conditions. It is believed that this is the most flexible model ever proposed to model this phenomenon, and can be easily modified to track all properties of interest during polyolefin production in continuous reactors with heterogeneous catalysts. 相似文献