场流分离理论的研究现状及发展趋势

场流分离是一种方便快捷的分析分离技术,它具有设备简单,应用广泛,效率高等优点。该文介绍了场流分离原理及理论,描述了场流分离设备的主要结构,着重讲述了电场流分离、热场流分离、沉降场分离、流场流分离的方法及应用。比较了不同场流分离技术的差异,展望了场流分离发展的方向。引用文献44篇。     

Polystyrene-equivalent molecular weight versus true molecular weight in size-exclusion chromatography

The evaluation of the size-exclusion chromatography (SEC) concentration elution curves by means of a calibration dependence obtained in a given SEC set for a polymer different from the polymer to be analyzed results in an error in the determination of both molecular weight and molecular-weight distribution (MWD). The problem is analyzed assuming the validity of the universal-calibration concept. The differences between the true and apparent values of molecular weight, MWD and M_w/M_n depend on and are expressed in terms of the parameters of the Mark-Houwink-Kuhn-Sakurada equation, describing the molecular-weight dependence of intrinsic viscosity, for the polymer to be analyzed and the polymer used for calibration. The differences in molecular weight and the M_w/M_n ratio are typically tens of percent and, in extreme cases, can amount up to a factor of three for molecular weight and a factor of two for the M_w/M_n ratio.     

Accurate molecular weight distribution of polymers using thermal field-flow fractionation with deconvolution to remove system dispersion

Thermal field-flow fractionation (ThFFF) is an elution process that separates polymeric materials by molecular weight. Elution profiles thus provide approximations to the molecular weight distributions of polymers. The accuracy of such approximate distributions is expected to be improved by accounting for the effect on the elution profile of band-broadening processes in the FFF system. Fortunately this intrinsic band broadening, referred to as system dispersion, is theoretically well-defined in ThFFF. In this article we present an algorithm that corrects ThFFF elution profiles by removing system dispersion. The program is applied to ThFFF fractograms of standard polymers having both narrow and broad molecular weight distributions. The increased accuracy obtained by accounting for system dispersion is demonstrated. For the narrow standard, deconvolution shows that the polydispersity (weight/number-average mol. wt.) is only 1.004. For the broad standard, NBS 706, the molecular weight distribution and parameters obtained agree well with previously published results. Application to a simulated fractogram resulting from mixing five narrow standards helps define the conditions under which accurate molecular weight information can be recovered.     

Experimental investigation of flow induced molecular weight fractionation during extrusion of HDPE polymer melts

In this work, two different HDPEs with virtually identical number, M_n, and weight, M_w, average molecular weights were investigated from rheological as well as die drool phenomenon point of view. It has been revealed that long-chain branching, low polymer melt elasticity and shear viscosity significantly reduce die drool phenomenon at the die exit region. It has been concluded that die drool phenomenon of HDPE polymer melts can be explained by the flow induced molecular weight fractionation.     

聚丙烯熔体流动指数与分子量及其分布的关系

研究了聚丙烯(PP)的熔体流动指数(MI)与聚合物不同分子量之间的关联性,对于分子量分布较窄的PP,数均分子量(Mn)、重均分子量(Mw)和粘均分子量(Mv)均能较好的关联;反之,MI与Mn关联性下降,而MI与Mn和Mv的关联性仍很好,尤其是MI与Mv的关联性受分子量分布的影响很小;MI与Z均分子量的关联性很差。同时．确定了MI与各种分子量之间的关联式,该式用于本体PP工艺反应器内氢气浓度的计算和MI的预测,与实验测量结果吻合良好。     

共溶剂对聚乳酸相对分子质量及热性能的影响

以超临界二氧化碳(SC-CO2)为载体,分别以丙酮、无水乙醚、六氟异丙醇为共溶剂,合成聚乳酸(PLLA),并采用FT-IR、GPC和DSC等对所获得的聚合物进行了相对分子质量测定和结构性能表征,探讨了3种共溶剂对用SC-CO2为溶剂合成的PLLA相对分子质量及热性能的影响。结果表明,共溶剂对PLLA的相对分子质量、相对分子质量分布指数以及热性能影响较大;以丙酮为共溶剂时,可以获得最高相对分子质量为47 690且热性能较好的PLLA。     

Characterization of fine coal particles by steric field-flow fractionation

Steric field-flow fractionation (steric FFF) is described as a method for the separation and characterization of particulate materials of particle diameter 1 μm. The theoretical basis of steric FFF is described and the concept of apparent particle radius is introduced. Experiments are described using coal ground to a few μm mean diameter. Fractionation is demonstrated by collecting fractions and reinjecting them into the device, after which they re-emerge in a position close to that of the original fraction. A size-distribution curve is obtained and compared to those generated by other means. Possible sources of discrepancies are described. The advantages and potential applications of steric FFF to process control and coal research are discussed.     

Effects of molecular weight and molecular weight distribution on creep properties of polypropylene homopolymer

The molecular weights of the industrial-grade isotactic polypropylene (i-PP) homopolymers samples were determined by the melt-state rheological method and effects of molecular weight and molecular weight distribution on solid and melt state creep properties were investigated in detail. The melt-state creep test results showed that the creep resistance of the samples increased by M_w due to the increased chain entanglements, while variations in the polydispersity index (PDI) values did not cause a considerable change in the creep strain values. Moreover, the solid-state creep test results showed that creep strain values increased by M_w and PDI due to the decreasing amount of crystalline structure in the polymer. The results also showed that the amount of crystalline segment was more effective than chain entanglements that were caused by long polymer chains on the creep resistance of the polymers. Modeling the solid-state viscoelastic structure of the samples by the Burger model revealed that the weight of the viscous strain in the total creep strain increased with M_w and PDI, which meant that the differences in the creep strain values of the samples would be more pronounced at extended periods of time.     

Evolution of molecular weight and molecular weight distribution in ab initio styrene emulsion polymerizations using series of rigid rodlike cationic amphiphiles as surfactants

The emulsion polymerization of styrene is carried out using a series of unconventional rigid rodlike cationic surfactants (1‐[ω‐(4′‐methoxy‐4‐biphenylyloxy)alkyl]pyridinium bromides, PCX) of different lengths. The evolution of the molecular weight (M) and molecular weight distribution of the polymers is analyzed to obtain information about the chain stopping mechanism. Our results indicate that the M is strongly dependent on the initial surfactant concentration and is not dependent on the alkyl chain length. The Clay and Gilbert model [ln P(M) versus M plots] yields a concave‐up region at low molecular weights and a linear region that extends to high values. The slope of the linear region, which is related to the rate coefficient of the chain transfer to the monomer versus the propagation rate coefficient ratio, decreases as the PCX concentration increases. This behavior indicates that as the PCX concentration increases the chain transfer to monomer becomes the dominant chain stopping mechanism. On the other hand, the ln P(M) versus M plots of polymer samples taken at low and high conversions show differences in slope, particularly at low PCX concentration. It is likely that at low conversion the chain transfer to monomer competes with other chain stopping mechanisms that could be associated with a coagulative nucleation process. The formation of a high molecular weight fraction at low conversion supports this explanation. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1513–1523, 2002; DOI 10.1002/app.10489     

超高分子量聚丙烯的制备

超高分子量聚丙烯（UHMWPP）是一种黏均分子量百万以上,具有超高的强度、超高的耐磨性、较强的抗氧化能力的热塑性工程塑料,可用于制备高强度、高模量、耐腐蚀、抗冲击、耐应力开裂的聚丙烯产品。本工作的目的在于制备出分子量超过200万的聚丙烯,将其用作3D打印材料来解决由于分子链较长引起高熔体黏度和低流动性而导致加工难成型问题。本工作基于传统的Ziegler-Natta催化剂,对主催化剂进行金属离子和有机物的负载,通过控制丙烯的链转移来控制聚丙烯的分子量,并且在聚合反应过程中不加入氢气（带有活性氢的物质）,以防止其成为聚合反应的终止剂。研究了聚合反应温度、聚合反应时间、助催化剂和外给电子体对聚丙烯分子量的影响。采用黏度法、升温淋洗分级法等表征了制备的聚丙烯分子量。通过聚合工艺优化,在聚合反应温度70℃、聚合反应时间60min、助催化剂三异丁基铝、外给电子体P Donor下,最终制备出了黏均分子量超过204万的超高分子量聚丙烯。     

Determining molecular weight scale and molecular weight distribution of ethylene-tetrafluoroethylene alternating copolymer via a rheological technique

A self-scaling rheology-based technique was developed to determine the molecular weight (MW) and molecular weight distribution (MWD) of ethylene-tetrafluoroethylene alternating copolymer (ETFE). The self-scaling technique makes determining MW and MWD with isolated completely rheological method possible. Moreover, the two key parameters (the plateau modulus and zero-shear viscosity) were obtained by more robust numerical technique, which let determining MW and MWD via rheological method initiated by Tuminello [Macromolecules 1993, 26, 499] being building on more robust and rigid basis. Our case overcomes the shortage of Tuminello's method and gives more practical and simply mean to analyze the MW scale and MWD in the production and application of ETFE. It is found that the peak MW of a ETFE (commercial grade: EP541) is 1.73 × 10⁵ g/mol, the MWD curve is a pattern with a slightly raised “shoulder” at high-molecular mass end, and a high peak on the median and the polydispersity is broad (the polydispersity index is near to 10.3). The wide polydispersity indicates the commercial ETFE combining good processability of relative lower MW molecules with physical properties of high MW ones. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

木质素分级对其应用性能的影响

木质素是一种结构复杂的天然芳香类聚合物,功能基团丰富,具有良好的开发应用前景。工业碱木质素是当前木质素转化利用的主体,主要源于制浆黑液。由于原料来源、制浆工艺的影响,木质素多分散性高、结构与性能不一,致使相应的木质素基产品的均一性与稳定性较差。通过木质素分级可以获得具有不同分子量或特定化学结构特征的木质素级分,促使各级分的多分散性降低、均一性提高,进而针对特定性能的级分进行改性或者直接利用可进一步提高产品的稳定性。对现行木质素的主要分级方式进行了总结,重点探讨了工业碱木质素的分级方式对其后续利用的影响及存在问题。最后,针对木质素不同级分的特点对木质素未来分级的方向及应用前景提出了展望。     

Z-N催化剂生产宽/双峰MWD聚乙烯的研究进展

介绍了Ziegler-Natta(Z-N)催化剂在单反应器中生产宽,双峰相对分子质量分布(MWD)聚乙烯(PE)的最新研究进展,概括了Z-N催化剂制备宽／双峰MWDPE的主要方法及特点,即传统的Ti系催化体系生产的双峰PE的MWD较窄,改进的Ti-V双金属催化体系活性高,可以调节MWD,但反应操作难度大。通过加入不同的配合物,调节并改进PE的MWD,取得了较好的实验结果。宽/双峰MWDPE的生产和开发是PE工业发展的一个重要方向。     

Effects of frequency,molecular weight and thermal oxidation on the dynamic mechanical response of poly (ethylene oxide)

The technique of dynamic mechanical thermal analysis (DMTA), operated in the dual cantilever mode, was used to characterize the effects of frequency, crystallinity, molecular weight (MW) and the extent of thermal oxidation on the dynamic mechanical response of poly(ethylene oxide) (PEO). The glass transition temperature (T_g) of PEO (MW = 9 × 10⁵ Dalton) was found to be ?44°C. For PEO (MW = 1 × 10⁵ Dalton) the T_g is ?39°C and this value increases by 2–9°C for every decade increase in the measuring frequency. Two minor, second-order transitions of PEO are also discernible at ?33 and 32°C. An inverse dependence of T_g on molecular weight was found in the molecular weight range studied and this is contrary to the Fox-Flory theory. It was also found that a partially crystalline sample is obtained despite very rapid quenching of PEO from the melt into liquid nitrogen. Thermal oxidation of PEO before processing leads to an increase in the amplitude of the loss tangent peak. This reflects the effect of oxidation products in restricting polymer crystallization and the subsequent increase in the amorphous fraction of the polymer. The position of the T_g peak in PEO remains reasonably fixed with progressive ageing and this was attributed to crosslinking having occurred in addition to chain scission during thermal oxidation.     

Studies of thermal oxidative degradation of polypropylene impact copolymer using the temperature rising elution fractionation method

Thermal oxidative degradation of polypropylene impact copolymer has been studied with its fractions obtained using the temperature rising elution fractionation method. The fractions were analyzed using ¹³C NMR, Fourier transform infrared and differential scanning calorimetry measurements, and the chemical structure, isotacticity, conformation and melting point were investigated. It is found that these fractions are composed of a homopolymer or a polymer blend of polypropylene, polyethylene and ethylene–propylene copolymer. The thermal oxidative degradation of each fraction was carried out at 130 °C, and the degradation progress was estimated by the change of molecular weight distribution (from gel permeation chromatography curves). The rate of degradation is found to be dominated by the content of tertiary C? H bonds (propylene unit) and the existence of 3₁ helix conformation corresponding to a crystalline polypropylene part in each fraction. The latter is more evident leading to the degradation reaction path with a lower activation energy. Copyright © 2007 Society of Chemical Industry     

Synthesis and thermal cure of high molecular weight polybenzoxazine precursors and the properties of the thermosets

High molecular weight polybenzoxazine precursors have been synthesized from aromatic or aliphatic diamine and bisphenol-A with paraformaldehyde. The precursors were obtained as soluble white powder. Molecular weight was estimated from the size exclusion chromatography to be several thousands. The structure of the precursors was confirmed by IR, ¹H NMR and elemental analysis, indicating the presence of cyclic benzoxazine structure. The ratio of the ring-closed benzoxazine structure and the ring-opened structure in the high molecular weight precursor was estimated from ¹H NMR spectrum and also from the exotherm of DSC, showing that the ratio of the ring-closed benzoxazine structure was 77–98%. The precursor solution was cast on glass plate, giving transparent and self-standing precursor films, which was thermally cured up to 240 °C to give brown transparent polybenzoxazine films. The toughness of the crosslinked polybenzoxazine films from the high molecular weight precursors was greatly enhanced compared with the cured film from the typical low molecular weight monomer. Tensile measurement of the polybenzoxazine films revealed that polybenzoxazine from aromatic diamine exhibited the highest strength and modulus. While, polybenzoxazine from longer aliphatic diamine had higher elongation at break. The viscoelastic analyses showed that the glass transition temperature of the polybenzoxazines derived from the high molecular weight precursors were as high as 238–260 °C. Additionally, these novel polybenzoxazine thermosets showed excellent thermal stability.     

宽分子量分布聚合物的制备

分别对采用物理共混法、自由基聚合、配位聚合和离子聚合法制备宽分子量分布聚合物进行了综述。指出目前应用较为广泛的是物理共混法，配位聚合容易实现分子量和分子量分布宽度可控，近几年已成为研究热点。     

Simulation of nanotube separation in field-flow fractionation (FFF)

A Brownian dynamics simulation based on a prolate spheroid particle model has been developed to model the separation of nanotubes in cross flow driven, field-flow fractionation (FFF). The particle motions are governed by stochastic forms of a linear momentum balance with orientation dependent drag and diffusion coefficients, and the Jeffrey equation with rotational diffusion. The simulation shows that nanotube scale particles would be expected to elute by a normal mode mechanism up to aspect ratios of about 1000, based on a particle diameter of 1 nm. Separation of nanotubes of different length is governed by the value of the retention variable for each component in agreement with theory. Elution profiles and average velocity through the device as a function of particle size, and the flow rates in the throughput and cross-flow directions are examined. The simulation shows that clean separations between components of different size is achieved when the ratio of the retention values is greater than 2.     

A critical evaluation of thermal fractionation of butter oil

Butter was fractionated on the basis of temperature (17–29°C) without agitation using slow cooling of melted anhydrous fat in conjunction with gentle vaccum filtration to produce four solid and four liquid fractions. Each of the fractions was analyzed for fatty acid composition, triglyceride profile, and characterized by gel permeation high performance liquid chromatography and differential scanning calorimetry thermograms. Fatty acid analysis indicated that the solid fractions had slightly higher amounts of palmitic and stearic acid and lower levels of oleic acid, while the remaining analyses did not indicate any substantial compositional differences between the fractions. Although the 29°C solid fraction (∼10%) could be said to be somewhat unique, the natural variation in the normal seasonal composition of butterfat was almost equal to that obtained by fractionation. The experimental physicochemical data obtained for the fractions in this study extend and verify previous work on butteroil fractionation, and indicate that thermal fractionation has marginal merit. On the other hand, literature describing more positive thermal butteroil fractionation results obtained by the properietary Tirtiaux process (Fleurus, Belgium), indictes that it may be a more expedient avenue to explore and let market forces determine whether fractionation has a future in Canada and North America.