Universal calibration procedure

As pointed out by Hamielec and Ouano (8), the separation of a branched polymer by size in the SEC process results in molecular species of different molecular weights eluting at the same volume. Thus, the molecular weight of these species is not monodispersed and MCV) must be replaced by the instantaneous number average molecular weight when using the universal calibration procedure for branched polymers. 

Mark-Houwink relationships are also important for the application of the universal calibration procedure in GPC-SEC of polymer molecules, where the... 

The chromatograph is usually calibrated with polystyrene standards, although a limited range of other polymer standards is available (see Appendix 2). If standards are not available for the polymer of interest, and if the Mark-Houwink constants K and a are known, then the universal calibration procedure can be employed, making use of the following equation ... 

This universal calibration procedure is widely used for many different polymer types [9,10] the procedure can be used to allow for differences in morphology as well as differences of chemical type. However, it is important to note that the procedure cannot be applied to all polymer types exceptions are often found when the polymer molecule has some structure when in solution, or when there are unwanted interactions between the solute and the column packing. 

This universal calibration procedure, using hydrodynamic volume to relate the elution behaviour of a variety of polymers, has been shown to be applicable to a very wide range of polymers. A list of frequently encountered Mark-Houwink parameters is included in Table 3.2. Also, a considerable number of proven applications are mentioned in Chapters 4 and 5, and a more comprehensive list of parameters is included as Appendix 4. 

To apply the universal calibration procedure, it is necessary to make some important assumptions or to carry out extensive preliminary testing with respect to the general applicability of the procedure and on the suitability of... 

The measurement of polymer molecular masses by dilute solution viscometry is not an absolute technique. However, the parameters used to relate solution viscosity to molecular mass are the same as those used in the universal calibration procedure (section 3.2.3). Earlier it was pointed out that one of the weaknesses associated with the universal calibration procedure was the use of literature values for these parameters. Alternatively, if some absolute molecular mass data is available, dilute solution viscometry can be used to determine these Mark-Houwink parameters. The choice is between extensive preliminary practical work and in making wide-ranging assumptions, often with quite old data on Mark-Houwink parameters. The use of a viscosity detector, whilst not removing the need for conventional calibration, does potentially widen the range of the applicability of the calibration, without the use of any assumptions. 

In addition to replacing the universal calibration procedure when used with conventional calibration methods, viscosity detectors can also be used to enhance the calibration with broad-MMD calibrants. In section 3.3.2, the use of a broad-MMD calibrant with known weight and number averages was mentioned. If a viscosity detector is used, there is no prior requirement for... 

The universal calibration procedure outlined by Benoit [18] offers an alternative method of establishing a molecular mass calibration for HDPE chromatograms. The procedure assumes that, in the SEC analysis, the chain molecules separate and elute according to their hydrodynamic volumes, and this in turn is directly proportional to the product of [ /] M. (see Chapter 3, section 3.2.3). 

Coll and Gilding [39] reviewed the theoretical justification for comparing M[rj] values for different polymers at the same retention volume, i.e. the universal calibration procedure, in analysing the SEC of i-PP samples in TCB at 135°C. The universal calibration line for polypropylene was derived from polystyrene and linear polyethylene standard materials, and molecular mass averages determined from viscosity measurements in decalin solutions and the... 

Rudin and Hoegy [43] have considered the assumption, inherent in the universal calibration procedure, that the hydrodynamic volume of a polymer at the concentration range adopted in SEC analysis is that which pertains at infinite dilution, and discuss whether this can account for apparent failures in some instances. A model is presented to estimate hydrodynamic volumes of polymers at finite concentrations and provide a universal calibration. Polypropylene is one of the examples used to illustrate the size of the effect. 

This list of Mark-Houwink parameters is not exhaustive and is intended only as a guide. The values in this appendix are believed to be reasonable to use in application of the universal calibration procedure however, the limitations on this procedure, as described in Chapter 3, should be considered. The values for Mark-Houwink parameters referred to in the text of this book are reproduced in this table. 

Mark-Houwink relationships are also important for the application of the universal calibration procedure in GPC/ SEC of polymer molecules, where the calibration curve is expressed in terms of the size of fractionated molecules against retention volumes. The intrinsic viscosity of a polymer (expressed in cm /g) is, in practice, a measure of the... 

In the case of propylene homopolymer, the universal calibration procedure established by Grubisic et al. [2] can be applied using anion-ically polymerized polystyrene molar mass standards with narrow MMD for TCB at 135°C, the following values of the parameters K and a in the equation ... 

A new universal calibration procedure, dubbed the Southern Method by its... 

This indicates that the product [77]Mis proportional to the hydrodynamic volume for a given polymer and solvent. This relationship is the basis of the universal calibration procedure used to determine molecular weight distribution from a GPC elution curve (see Section 2.6.3). 

In some cases, monodisperse samples of the polymer of interest are not available. In such a case, use can be made of the universal calibration procedure. This involves the measurement of intrinsic viscosity. As is shown by Eq. 2.87, the product of intrinsic viscosity and molecular weight is proportional to molecular size, i.e., hydrodynamic volume, which is the actual basis of the separation in a chromatography column. 

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