Calibration of thermal field-flow fractionation using broad molecular weight standards

Thermal field-flow fractionation (ThFFF) is a new elution-based separation method for determining molecular weight distributions of polymers. Calibration can be achieved using monodisperse standards of the specific polymer of interest. In order to expand the range of polymer types for which absolute molecular weight data can be obtained using ThFFF a calibration procedure has been developed and tested which uses only broad molecular weight polymer samples. The method requires two polydisperse molecular weight standards of the required polymer whose average molecular weight (normally M?_w) is measured by an independent method (e.g. light scattering). From the average molecular weight data and the ThFFF elution pattern (fractogram) the required calibration constants can be calculated. The method has been tested using the polystyrene-tetrahydrofuran system and gave satisfactory results when checked against a series of monodisperse polystyrene standards. This calibration approach should expand the applicability of ThFFF to include a wide range of polymer types.     

Calibration of size-exclusion chromatography columns with polydisperse polymer standards. II. Applications

New methods for calibrating SEC columns by means of polydisperse polymer samples with known M_n and M_w have been tested with computer-generated chromatograms and with experimental data of high-performance SEC. Calculations with the artificial chromatograms show that accurate calibration dependences can be recovered even when polymers with broad and/or bimodal molecular weight distributions are used as standards. Polystyrene calibration calculated by the proposed method from chromatograms of five polydisperse polystyrenes follows closely the curve obtained in a conventional manner from nine narrow polystyrene standards. The dependence log M vs. ν for PMMA determined from chromatograms of six PMMA samples with moderately broad molecular weight distributions agrees well with the curve obtained by shifting the dependence for polystyrene using the universal calibration concept. The new method is particularly useful when SEC columns are to be calibrated for dextrans in water, where only a few standards having a rather broad molecular weight distribution are available, and can considerably improve the accuracy of molecular weight determination by SEC.     

High performance gel permeation chromatography of polystyrene

Microparticulate crosslinked polystyrene packings in short columns have been investigated with high performance liquid chromatography instrumentation. Reliable molecular weight data for six polystyrene standards having narrow molecular weight distributions and for a polystyrene having a broad distribution have been obtained by optimizing the injection procedure, using a constant flow pump, and incorporating an internal standard into each injected solution. Experimental determinations of the dependence of the polydispersity for polystyrene standards on eluent flow rate and polymer diffusion coefficient were in agreement with a relation predicted from theoretical considerations of chromotogram broadening. Because of the dependence of chromatogram broadening on polystyrene molecular weight, high efficiency separations for high polymers were only obtained at low eluent flow rates. For low polymers, high efficiency separations may be performed at fast eluent flow rates. It was concluded that accurate molecular weight distributions can only be determined from chromatograms obtained at low eluent flow rates, which was supported by experimental measurements of polydispersity on polystyrene sample prepared by a radical polymerization at low monomer conversion. A differential weight distribution calculated from an experimental chromatogram for the polydisperse polystyrene determined at the lowest eluent flow rate (0.1 cm³min^?1) was compared with distributions predicted theoretically for polystyrenes prepared by radical polymerization. It was concluded that the experimental distribution contained a small contribution from chromatogram broadening and that most of the radicals in the polymerization of styrene terminated by combination.     

Molecular weight distribution and environmental stress cracking of linear polyethylene

Molecular weight distributions of both Phillips and Zieglertype high density polyethylenes were determined following fractionation of the polymer samples using a column elution technique. The Wesslau log normal distribution function was used to describe the distributions of the resins investigated. Resistance to environmental stress rupture of speciments cut from compression molded plaques of these samples was measured by the constant tensile loading procedure. Data are presented showing annealed resins with “broad” molecular weight distributions, characterized particularly by a quantity of low molecular weigt material and a high molecular weight “tail,” to have poorer stress crack resistance than samples having a “narrow” molecular weight distribution. Stress crack resistance of specimens quenched from the melt, however, tends to improve for “broad” distribution resins, while decrasing for those polyethylenes having a “narrow” molecualar weigt distribution. Differences in crystal structure are used to explain the physical bassis for these effects.     

Molecular weight effects on phase transitions of side-chain liquid crystalline poly(vinyl ether)s with terminal alkoxybiphenyl groups

Two vinyl ether monomers with alkoxybiphenyl groups, 2-(4′-ethoxy-4-biphenyloxy)ethyl vinyl ether (EtOVE) and 2-(4′-hexyloxy-4-biphenyloxy)ethyl vinyl ether (HexOVE), were polymerized by living cationic polymerization reactions using either the hydrogen iodide/iodine (HI/I₂) or the hydrogen iodide/zinc iodide (HI/ZnI₂) initiator systems, or both. These initiators yield polymers with narrow molecular weight distributions (MWDs) and for comparison broad MWD polymers were also prepared by using the BF₃OEt₂ initiator. The thermal properties and phase transitions of these polymers were determined by differential scanning calorimetry, by visual observations of samples on a hot stage on a polarizing microscope, by polarized light transmission intensity measurements and by wide angle X-ray diffraction analysis. The polymer from EtOVE, P(EtOVE), with a weight average molecular weight, , of less than about 8000 formed both smectic and nematic liquid crystalline, LC, phases after one heating cycle. In contrast, the polymer from this monomer which had an of more than about 8000 exhibited only a nematic LC phase. For the polymer from HexOVE, P(HexOVE), both the narrow and broad MWD samples showed only a nematic LC phase over the range from 2600 to 7300. The phase transitions of both types of polymers are discussed in relation to the molecular weight and MWD of the samples. The effect of the terminal group attached to the biphenyl group in the polymer is considered in relation to its possible steric effects.     

Ultrasonic degradation of poly(methyl methacrylate

Ultrasonic degradation studies on a variety of molecular weights and molecular weight distributions of poly(methyl methacrylate) are reported. The extent of degradation was measured using gel permeation chromatography. Polydispersity decreased as a function of irradiation time for polymers with initial broad distributions. In contrast, polymers with an initial narrow distribution increased in polydispersity, passed through a maximum and then gradually decreased in polydispersity. Results appear to show no limiting degree of polymerization for poly(methyl methacrylate).     

Number‐average molecular weight of branched polymers from SEC with viscosity detection and universal calibration

BACKGROUND: Number‐average molecular weight, M?_n, is an important characteristic of synthetic polymers. One of the very few promising methods for its determination is size‐exclusion chromatography (SEC) using on‐line viscometric detection and assuming the validity of the universal calibration concept. RESULTS: We have examined the applicability of this approach to the characterization of statistically branched polymers using 22 copolymers of styrene and divinylbenzene as well as 3 homopolymers of divinylbenzene with various degrees of branching. SEC with three on‐line detectors, i.e. concentration, light scattering and viscosity, enables us to evaluate experimental data by various computational procedures yielding M?_n and weight‐average molecular weight, M?_w. Analysis of the results has shown that the universal calibration theorem has limited validity, apparently due to the dependence of the Flory viscosity function on the molecular shape, the molecular weight distribution and the expansion of molecules. CONCLUSION: For complex polymers, the universal calibration, i.e. the dependence of the product of intrinsic viscosity and molecular weight, [η]M, on elution volume, can differ in values of [η]M from those obtained for narrow molecular weight standards by 10–15%. The method studied is helpful for the determination of M?_n of polymers, in particular of those with very broad molecular weight distribution, such as statistically highly branched polymers. Copyright © 2008 Society of Chemical Industry     

Gel permeation chromatography of polyethylene. I. Calibration

An accurate GPC calibration is essential if computer techniques are to be utilized in obtaining the molecular weight distribution and degree of long-chain branching from an intrinsic viscosity and GPC trace of a polymer. The use of the National Bureau of Standards Linear Polyethylene Standard Reference Material, SRM 1475, to calibrate GPC is described. Employing this calibration, the Mark–Houwink relationship for linear polyethylene in 1,2,4-trichlorobenzene was established utilizing narrow molecular weight fractions derived through fractionation of SRM 1475 and other polymers. This Mark–Houwink equation was subsequently employed for the evaluation of high molecular weight fractions which were then used to extend the GPC calibration to the high molecular weight region not covered by SRM 1475. An iterative technique was used to obtain coincidence of the measured intrinsic viscosity and the viscosity calculated from the GPC data. The accuracy of the GPC calibration was demonstrated by obtaining coincidence of the measured and calculated viscosity of high and low molecular weight polymers of both narrow and broad polydispersity.     

Structural Effects on the Adhesive Properties of Butadiene/Styrene Radial Teleblock Copolymers

The effects of butadiene/styrene ratio, monomer distribution, and molecular weight distribution and branching on the pressure sensitive adhesive properties of butadiene/ styrene radial teleblock copolymers are reported. Styrene content of polymers with varying structures shows a close relation with tack response, and styrene content and structure affect solution viscosity and shear adhesion. When part of the styrene is incorporated into the polybutadiene segment to yield a block progressively enriched in styrene (tapered block), solution viscosity and shear adhesion are reduced. When the butadiene segment is replaced by a block of randomly copolymerized butadiene and styrene, the polymers provide lower solution viscosities and shear adhesion but unchanged tack.

The molecular weight distribution of the radial teleblock polymers can vary from broad, highly branched compositions to narrow molecular weight distributions of almost Iinear polymers. The latter have relatively high solution viscosity and low shear adhesion, whereas the former polymers produce moderate solution viscosity but high shear adhesion. Tack is generally unaffected.     

Structural Effects on the Adhesive Properties of Butadiene/Styrene Radial Teleblock Copolymers

The effects of butadiene/styrene ratio, monomer distribution, and molecular weight distribution and branching on the pressure sensitive adhesive properties of butadiene/ styrene radial teleblock copolymers are reported. Styrene content of polymers with varying structures shows a close relation with tack response, and styrene content and structure affect solution viscosity and shear adhesion. When part of the styrene is incorporated into the polybutadiene segment to yield a block progressively enriched in styrene (tapered block), solution viscosity and shear adhesion are reduced. When the butadiene segment is replaced by a block of randomly copolymerized butadiene and styrene, the polymers provide lower solution viscosities and shear adhesion but unchanged tack.

The molecular weight distribution of the radial teleblock polymers can vary from broad, highly branched compositions to narrow molecular weight distributions of almost Iinear polymers. The latter have relatively high solution viscosity and low shear adhesion, whereas the former polymers produce moderate solution viscosity but high shear adhesion. Tack is generally unaffected.     

Calibration procedures in gel permeation chromatography

Procedures for the determination of the log molecular weight vs. elution volume calibration relation are reviewed for linear homopolymers. The calibration curve is readily established with narrow molecular weight distribution fractions. For broad molecular weight distribution fractions the peak molecular weight at the peak elution volume of a fraction's gel permeation chromatogram has to be calculated from average molecular weights. When well-characterised fractions of the polymer requiring analysis are unavailable, a calibration curve can be established by procedures which assume that the hydrodynamic volume of a polymer molecule in solution controls fractionation in gel permeation chromatography. These universal calibration procedures require information on Mark-Houwink viscosity constants or polymer unperturbed dimensions. The validity of hydrodynamic volume as the universal calibration parameter is discussed with special reference to the goodness of the solvent for a polymer and polymer polarity. Examples are given of the various calibration procedures which are employed in the determination of molecular weight distribution and average molecular weights for poly(methylmethacrylate), polyethylene and polyisoprene.     

Calibration of gel permeation chromatography columns using polydisperse polymer standards

Two methods of calibrating gel permeation chromatography columns are given. The first method uses polymer standards that may have broad molecular weight distributions. Either the weight-, number-, or viscosity-average molecular weight of each standard must be known. This method neglects column peak spreading. The second method requires polymer standards of moderately narrow molecular weight distributions for which both the weight- and number-average molecular weights are known. However, the second method determines both the column peak spreading and calibration of the column. The second method is applied to calibration of a column using polystyrene standards. The column peak spreading is found to be small and independent of molecular weight for this column.     

The Instrument Spreading Correction in GPC. II. The General Shape Function Using the Fourier Transform Method with a Nonlinear Calibration Curve

Abstract

The instrument spreading function suggested in Part I of this series is investigated for use with the Fourier transform method for generating corrected elution volume chromatograms. The instrument spreading parameters are obtained using linear theory on narrow molecular weight distribution standards, as indicated in Part I. The corrected chromatogram is then combined with a nonlinear molecular weight calibration curve which was fit with a function suggested by Yau and Malone to generate true values of the number- and weight-average molecular weights.

The instrument spreading function is shown to qualitatively and quantitatively describe the dispersion, skewing, and flattening effects ordinarily found in GPC chromatograms due to imperfect resolution by the GPC columns. The Yau-Malone function is shown to be a very useful function for fitting nonlinear molecular weight vs elution volume calibration data. Although the Fourier transform method is shown to work well with analytically generated data, it is shown that a number of numerical problems must be overcome before it can quantitatively produce corrected elution volume chromatograms. Some of these numerical problems are discussed.     

Mechanical properties of ethylene/1-hexene copolymers with tailored short chain branching distributions

Recently, we have investigated a metallocene catalyst system that can produce polyethylene and ethylene/α-olefin copolymers with tailored molecular weight and short chain branching distributions (SCBD). Ethylene/α-olefin copolymers produced with this system have narrow molecular weight distributions as expected from metallocene catalysts. However, these copolymers are quite unique in that their SCBDs are broad and sometimes bimodal, similar to Ziegler-Natta LLDPE.To examine the effect of these broad SCBDs on the polymer properties, a series of poly(ethylene-co-1-hexene) resins with very distinct, and in some cases bimodal crystalline distributions, were synthesized. The attractive feature of the resins in this study is that their molecular weight distributions are similar but each possesses a different SCBD, thus effectively minimizing the effect of molecular weight on the properties investigated.It was found that the tensile properties of a copolymer could be controlled by the ratio of the crystalline species present in the sample. In this study, a balance of stiffness and toughness was exhibited by a copolymer containing a large proportion of crystalline material and a small fraction of material of lower crystallinity.     

Temperature drop turbidimetry as a means for the rapid determination of the polydispersity of polymers ii polyethylene oxide

The molecular weight averages of narrow and broad distribution commercial polyethylene oxide have been estimated by controlled precipitation from iso-octane/benzene solutions by cooling from 333 to 273 K. Estimates of the distributions, which compared favourably with those determined by gel permeation chromatography (gpc), were obtained from a calibration procedure between the temperature of initial onset of precipitation and the viscosity average molecular weight. The chief advantages of this procedure were that it enabled molecular weight characteristics to be determined rapidly and with small amounts of materials when g.p.c. was not readily available. The method is used to assess various fractionation procedures.     

Polymerization of vinyl ethers using titanium catalysts containing tridentate triamine ligand of the type N[CH2CH(Ph)(Ts)N]2

Titanium complexes having tridentate triamine of the type N[CH₂CH(Ph)(Ts)N]₂²⁻ in combination with methylaluminoxane (MAO) was able to polymerize ethyl vinyl ether in good yields. The polymers obtained in general were having molecular weight in the order of 10⁵ with narrow molecular weight distributions. Polymerization conditions had an impact on the molecular weight and the polydispersity index (PDI). Using chlorobenzene as the solvent the polymer had an M_n of 350?000 and PDI of 1.21, where as under neat conditions the M_n was 255?000 with PDI of 1.21. The type of solvent and the temperature dictated the polymerization rate and the polymer stereo regularity. The molecular weight of the polymer is distinctly governed by the polymerization temperature. Temperature ranging between −50 and ambient (30 °C) resulted in high molecular weight polymers and vice versa at a temperature of 60-70 °C resulted in low molecular weight polymers in moderate yields. The polymers obtained below 30 °C are highly stereo-regular compared to that of the ones produced at and above ambient temperature. The polymerization of iso-butyl vinyl ether (IBVE) was faster than that of linearly substituted n-butyl vinyl ether (BVE) and less bulky ethyl vinyl ether (EVE). The order of isotacticities of the polymers obtained are polyIBVE > polyBVE > polyEVE. The use of borate cocatalyst for activation generated narrow molecular weight polymers with a linear increase in the yield and molecular weight over time suggesting the living nature of the catalyst system.     

Shear properties of polystyrenes in the transition region

Shear properties of commercially available polystyrenes with narrow molecular weight distribution have been measured in the transition region from 100°C to 150°C and over a frequency range from 50 Hz to 1000 Hz. The effect of molecular weight or shear properties is established with four polymers ranging in molecular weight from 20,000 to 860,000. A broad, bimodal distribution is also studied. The properties of these polymers, with two different diluents added, illustrate the rather marked qualitative difference in effects caused by diluents.     

Correction method for a concentration effect in the calculation of molecular weight averages from GPC chromatograms

In the calculation of molecular weight averages by GPC, the traditional method uses the calibration curve obtained at the same concentration as the samples, which results in a large degree of disagreement between molecular weight averages at several concentrations. Because of the concentration dependence of peak elution position in gel permeation chromatography of polymers, correct molecular weight averages cannot be obtained if calibration concentrations are the same as sample concentrations. A computation approach which uses calibration curves at finite and zero concentrations and can correct concentration effects is shown. The concentration used varied from 0.1% up to 0.4%. The elution chromatogram was divided into several parts, and concentration of species at each elution point was obtained from a concentration–peak height calibration curve. Molecular weight at the point was obtained from a molecular weight–elution volume calibration curve corresponding to a concentration of species at the point, and molecular weight averages were calculated by using the usual method. Nearly identical values for molecular weight averages could be obtained at different concentrations, and additional support for this approach is that these values for molecular weight averages were in fair agreement with NBS data.     

阻聚剂对自由基聚合的活性化影响

研究了阻聚剂对自由基聚合活性化的影响,用1H NM R表征了聚合物的结构。发现在甲基丙烯酸甲酯(MM A)自由基聚合体系中加入少量的加成型阻聚剂2,4,6-三硝基苯酚(TNP)或1,4-苯醌(BQ)时,体系转化率降低,引发效率降低,呈现出一定的缓聚性质;同时得到的聚合物分子量变小,分子量分布变窄,具有一定的“活性”/可控的特征。链转移型阻聚剂如对羟基苯甲醚(4-M P)和对苯二酚(HQ)则不能起到类似的效果。     

Living cationic polymerization of vinyl monomers by organoaluminum halides

Summary Well-defined living polymers of isobutyl vinyl ether were obtained in the polymerization initiated with ethylaluminum dichloride (EtAlCl₂) in conjunction with a stoichiometric excess of dioxane (5–10 vol%) in n-hexane at 0°C. Under these conditions, the number-average molecular weight of the polymers increased in direct proportion to monomer conversion, while the molecular weight distribution stayed narrow (M_w/M_n = 1.1–1.25). In sharp contrast, the EtAlCl₂-initiated polymerization in the absence of dioxane led to non-living polymers with a broad molecular weight distribution. It was concluded that the propagating carbocation is stabilized not by the counteranion but by an externally added basic compound (dioxane) that strongly interacts with the active end.