Determination of MWD and chemical composition of polymers by chromatographic techniques

Complex polymers are distributed in more than one direction of molecular heterogeneity. In addition to the molar mass distribution, they are frequently distributed with respect to chemical composition, functionality, and molecular architecture. For the characterization of the different types of molecular heterogeneity it is necessary to use a wide range of analytical techniques. Preferably, these techniques should be selective towards a specific type of heterogeneity. The combination of two selective analytical techniques is assumed to yield two-dimensional information on the molecular heterogeneity.

Hyphenated Techniques in Liquid Chromatography of Polymers

Polymerization control and fast characterization of the stereo-defect distribution of heterogeneous Ziegler–Natta isotactic polypropylene

This review covers the fundamental developments in the field of column liquid chromatography (LC) during the period of 1984-1985. In an attempt to conserve space, patents and theses have been excluded. Only articles published in English, French, German, and Russian with some exceptions have been included. As compared to the previous review (Anal. Chem. 1984, 56, 300R-340R), all of the tables dealing with selected applications have been deleted and sections on automation and data handling, ion chromatography, trace analysis and sample preparation, and novel applications have been included. New sections that have been added are hydrophobic interaction chromatography, micellar mobile phases, fast LC, and indirect photometric direction. This review is not a comprehensive coverage of all LC literature. The attempt has been made to critically select only those references that reflect fundamental developments in LC theory, methodology, and instrumentation. The main data base for this review was CA Selects (HPLC and GPC) from October 31, 1983, to December 2, 1985. This represented over 8000 abstracts of which 1200 were used. In addition, each author used other search routines to augment coverage from CA Selects. 195 references.

Column liquid chromatography

In situ compatibilization of polypropylene–polyethylene blends: a thermomechanical and spectroscopic study

This paper is concerned with the theoretical aspects of the gel permeation chromatography of copolymers and physical blends, such as polypropylene homopolymer and copolymers. Such considerations have become important with the advent of modern UV and IR chromatographic detectors. The behavior of the compositional variation observed as a function of elution volume is investigated by a simulation technique. As an example, ethylene–propylene copolymers having a bivariate normal distribution for molecular weight and composition are taken for calculations. We deal with the effects of the molecular weight distribution, the compositional distribution, and the correlation coefficient between molecular weight and composition on the detected compositional variation. It is concluded that these three factors are equally important in evaluating the curves fo the variation. Therefore, the difference between compositional distributions of copolymers is detectable only under limited conditions where the other factors are kept constant.

Compositional variation as a function of elution volume in gel permeation chromatography of copolymers and physical blends

A simple method to calculate average molecular weights and D (≡Mw/Mn) value from the GPC chromatogram of copolymerization products, especially for ethylene–propylene copolymerization products, was investigated by simulation technique. The method is based on the use of the calibration curve determined by the average ethylene content of the products. In addition to this method, the calibration curve prepared for polypropylene was also applied to determined the D value. Average molecular weights and D values were determined, with small errors, for narrow distribution samples with respect to molecular weight and chemical composition.

Gel permeation chromatography of ethylene–propylene copolymerization products

An acrylonitrile-styrene copolymer having 51.4 wt % acrylonitrile content was fractionated using dimethylformamide and a toluene–n-propanol mixture as solvent and nonsolvent, respectively. From characterization of the fractions, it is shown that the copolymer was fractionated principally in accordance with composition. The weight distribution curve of the copolymer was expressed as a function of both molecular weight and chemical composition on a two-dimensional contour map. The same plotting technique was applied to a copolymer having nearly the azeotropic composition.

Column fractionation of acrylonitrile-styrene copolymers

Ethylene–propylene (EP) copolymers were fractionated according to chemical composition, molecular weight, or monomer sequence length, which are fundamental distribution factors. Cloud points of polyethylene (PE), polypropylene (PP), and EP copolymer were determined prior to the fractionation. From the results, it was estimated that xylene–butyl cellosolve and tetralin–ethyl carbitol systems were suitable for the fractionations according to chemical composition and molecular weight, respectively. EP random-type copolymers were fractionated using a xylene–butyl cellosolve system. Separations according to chemical composition were obtained as expected. Then, the above polymer fractions were further fractionated in a tetralin–ethyl carbitol system. and the dependence on molecular weight was observed fairly well. Furthermore, fractionation according to monomer sequence length was satisfactorily achieved by solvent extraction using ethyl ether, n-hexane, cyclohexane, and n-heptane. Therefore, it is concluded that the more detailed characterizations of EP copolymers are made possible by a combination of these techniques.

Fractionation of ethylene‐propylene copolymers

Investigations on the molecular weight distribution of cyrstalline polypropylene were carried out by column fractionation and gel-permeation chromatography for the purpose of comparing the determination methods. Average molecular weight, standard deviation, skewness, and kurtosis were calculated as statistical parameters from the distribution curve of each, and the number-average and weight-average molecular weights were determined by osmometry and light scattering. The molecular weight distribution curves obtained from column fractionation were found to be narrower than those from gel-permeation chromatography, and it was confirmed that the molecular weight distribution curve obtained from GPC was more accurate and reliable than that from column fractionation by the fact that the average molecular weight from GPC agreed fairly well with that from the absolute methods. On the other hand, no clear difference between these determination methods was observed with respect to skewness and kurtosis of the data, while the standard deviation from GPC seemed to be greater than that from column fractionation.

Toshio Ogawa

Papers

Compositional variation as a function of elution volume in gel permeation chromatography of copolymers and physical blends

Gel permeation chromatography of ethylene–propylene copolymerization products

Column fractionation of acrylonitrile-styrene copolymers

Fractionation of ethylene‐propylene copolymers

Determination of molecular weight distribution for polypropylene by column fractionation and gel-permeation chromatography