Calibration transforming molecular weight

The calibration technique used in conventional SEC does not always give the correct MWD, however. The molecular size of a dissolved polymer depends on its molecular weight, chemical composition, molecular structure, and experimental parameters such as solvent, temperature, and pressure ( ). If the polymer sample and calibration standards differ in chemical composition, the two materials probably will feature unequal molecular size/weight relationships. Such differences also will persist between branched and linear polymers of identical chemical composition. Consequently, assumption of the same molecular weight/V relation for dissimilar calibrant and sample leads to transformation of the sample chromatogram to an apparent MWD. 

Simple homopolymers, where monodisperse standards and suitable solvents are available, are easily characterized by SEC. Homopolymers for which no monodisperse standards are available additionally require some more elaborate detection system for transformation of the retention time into molecular weight. This can be done by, e.g., universal calibration. Alternatively, an absolute molar mass detector, like an on-line light scattering detector or mass spectrometer, can be used. 

Size exclusion chromatography (SEC) polymer elution profiles yield information regarding the molecular size distributions of polydisperse macromolecules. Polymer molecular weight distribution (MWD) represents an intrinsic property which provides direct correlation with many end-use physical properties and a universal criterion for polymer characterization (1). In order to convert elution profiles or chromatograms into MWD information proper calibration methods are required. SEC molecular weight calibration techniques represent experimental approaches for transformation of polymer elution profiles into MWD information and are dependent upon instrumentation, columns, and the polymer/solvent system under study. 

The Q-factor approach is based upon the weight-to-size ratios (Q-factors) of the calibration standard and the polymer to be analyzed. The Q-factors are employed to transform the calibration curve for the chemical type of the standards (e.g. polystyrene) into a calibration curve for the chemical type of polymer under study. The inherent assumption In such a calibration approach is that the weight-to-size ratio is not a function of molecular weight but a constant. The assumption is valid for some polymer types (e.g. polyvinylchloride) but not for many polymer types. Hence the Q-factor method is generally referred to as an approximation technique. 

The differential refractive index detector response on the ordinate of the SEC chromatogram in Fig. 3-8 can be transformed into a weight fraction of total polymer while suitable calibration permits the translation of the elution volume axis into a logarithmic molecular weight scale. 

Two methods that determine the correct Mp values of the standards for molecular weight-sensitive detectors are discussed (3). These methods are based on transforming the MWD of the calibration standards into the responses of the molecular weight-sensitive detectors. The first... 

A Nicolet Magna 550 Fourier Transform Infrared Spectrometer (FTIR) and a Bruker MW 250 MHz proton NMR were used to verify the chemical structure of all monomers and polymers. Optical activity of the compounds was measured at 25 on a Perkin-Elmer Polarimeter in chloroform. A Waters Gel Permeation Chromatograph with 440 UV absorption detector and R401 differential refructometer was used to determine the molecular weights of the polymers tetrahydrofuran was used as the mobile phase at 1.0 mL/min, and the Waters polystyrene gel columns were calibrated with monodisperse polystyrene standards. Polarizing optical microscopy was used to identify liquid crystalline phases using a Leitz optical microscope with a CCD camera attachment... 

If a different polymer needs to be analyzed, a different calibration curve needs to be established. However, if the relationship between the molecular weight and the size, or more precisely, the hydrodynamic volume, of a molecule is known for both the stanchud polymer and the unknown polymer, one can simply transform the molecular-wei t axis. The procedure for this is called the universal calibration, and its principles are discussed below. 

Polymerization Polystyrene-block-polyethyleneoxide samples were prepared by sequential anionic polymerization in THF with cumyl potassium as the initiator. The PEO block lengths were varied from 1 to 25 mole%. Samples were characterized by SEC and H-NMR. Results are summarized in Table I. The monomer to initiator ratio was chosen so that molecular weights of about 50.000 g/mole could be expected for all polymers. The actual molecular weights which were found by SEC were higher. This can be explained by partial precipitation of the initiator in the stock solution. It must be noted also, that SEC elution volumes had been transformed to molecular weights by means of a polystyrene calibration curve. For this reason, some deviation of the actual molecular weight of the styrene/ethyleneoxide block copolymers is expected. 

Today, data acquisition and processing are usually computer controlled. There are four transformations required of the raw chromatographic data to provide results as usually reported see Figure 3.20 (79) (a) conversion of elution time to elution volume, (b) conversion of elution volume to molecular weight, (c) conversion of detector response to polymer concentration, and (d) conversion of polymer concentration to weight fraction. Quantification of plate count and resolution, as well as calibration are discussed further in ASTM D5296-97 (89). 

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