SynopsisA description is given of an analytical temperature-rising elution fractionation (TREF) system for the purpose of determining short-chain branching (SCB) or copolymer distributions in polyethylenes and ethylene copolymers. The system achieves fractionation on the basis of crystallhbility and is shown to be very little influenced by molecular weight in the normal high polymer range. Sample preparation by slow cooling from relatively dilute solution followed by continuous elution with a simultaneous and fairly rapid rate of temperature rise proves to be an efficient fractionation process. An on-line detector and data system allows application of a calibration curve to give realistic SCB distribution data in a convenient manner. The potential value of the TREF technique for providing structural information is illustrated by examples which include low-density high-pressure resins made by both tubular and autoclave reactors, high-and low-density resins made by lowpressure processes, and copolymers of ethylene with vinyl acetate and ethyl acrylate.

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Melt How data has been determined for a series of fractionated and whole low density polyethylenes which has been characterized in terms of their molecular weights and degree of long‐chain branching, (LCB). The resulting data indicate that low LCB influences melt flow both through a reduction in molecular size and an increased level of intermolecular interaction. Die swell measurements on whole polymers indicate an increase in melt elasticity with increase in degree of LCB for samples of similar melt flow (MI). Comparison of GPC data with observed die swell characteristics indicates that die swell is a molecular size dependent property and independent of intermolecular entanglement effects, suggesting that the measurement of elastic properties of LDPE melts will provide a means of determining relative degrees of LCB for commercial resins.

A method is described and a computer program outlined whereby gel‐permeation chromatographic analysis of fractions from gradient‐elution fractionation of branched polyethylenes provides a complete molecular weight evaluation of each fraction and the parent resin. The procedure involves the use of the universal calibration concept of Benoit et al. in a way that eliminates the ambiguities present in attempts to apply it directly to whole polymers. The resultant molecular weight data for fractions, when related to their solution viscosity and low‐shear melt viscosity and to their infrared analysis, provides a total structural evaluation of a branched polyethylene resin, including molecular weight molecular weight distribution and the distribution of both long‐and short‐chain branching. The potential of this method for providing a comprehensive structural evaluation of branched polyethylene is illustrated by examples of its application in the analysis of some commercial resins.

The development of a fractional crystallization technique for characterization of polypropylenes with respect to stereoregularity is described. It is a simple technique which is attractive for routine analysis and under suitable conditions yields quantitative data with good reproducibility. Separation by fractional crystallization from hot xylene solution is shown to take place according to polymer crystallizability and is relatively independent of molecular weight. It thus represents an alternative and in some ways superior approach to the more commonly used fractional extraction method. Preliminary work indicates that the fractional crystallization method may prove of value in establishing correlations between the stereoregular nature of polypropylenes and their physical properties.

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.

SynopsisThe effect of long-chain branching on the size of lowdensity polyethylene molecules in solution is demonstrated through solution viscosity and molecular weight measurements on fractionated samples. These well-characterized fractions are analyzed by gel permeation chromatography (GPC), and it is shown that the separation of the polymer molecules by this technique is sensitive to the presence of long-chain branching, By using fractions of branched polyethylene possessing differing degrees of branching, one observes that a single curve is adequate in relating elution volume to molecular weight.This calibration curve is applied in the GPC analysis of a variety of commercial lowdensity polyethylene resins and it is shown, by comparison with independent osmometric and gradient elution chromatographic data, that realistic values for molecular weight and molecular weight distribution are obtained. The replacement of molecular weight M by the parameter [VIM as a function of elution volume, leads to a single relationship for both linear and branched polyethylenes. This indicates that GPC separation takes place according to the hydrodynamic volumes of the polymer molecules. The comparison of data for polyethylene and polystyrene fractions suggests that this volume dependence of the separation will be observed for other polymer-solvent systems.

SynopsisA comparison is made of two methods by which one may derive molecular weight distribution and degree of long-chain branching using only the measured solution viscosity of a branched polyethylene whole polymer and its GPC trace. These are (a) Drott and Mendelson method and (b) Ram and Miltz procedure. In each case, the purpose of the method is to devise a means by which one may establish a relationship between solution viscosity and molecular weight for use in conjunction with the GPC universal calibration relationship of Benoit et al. The effectiveness of these theoretical approaches is evaluated by comparison with the true D and degree of long-chain branching data obtained using our complete iterative analysis method. Such a detailed comparison using low, moderate, and highly branched resins leads to a conclusion that both the techniques provide very good MWD and branching data and, further, that they may be considered interchangeable for most resins. For highly branched resins, the Ram and Miltz method, which is slightly more sensitive to the presence of a high degree of long-chain branching, is preferred. In practice, the Drott and Mendelson method has the advetage of using less computer time and providing a direct measure of degree of long-chain branching, and thus is likely to be used most frequently.