Molar mass distribution of polymers

Huber, A. (1992). In Analysis of Polymers/Molar-Mass and Molar-Mass Distribution of Polymers, Polyelectrolytes and Latices (W.-M. Kulicke, ed.), Hiithig Wepf Verlag, 61, 248. 

Medium Effect on Molar-Mass Distribution of Polymers Produced by Initiation and Propagation at 250 Using t-Butylmagnesium... 

Gel permeation chromatography (GPC) is a widely used technique for determining molar mass and molar mass distribution of polymers. In its usual form it is not an absolute method, though by making the appropriate measurements it may be made to be so. 

Gel permeation chromatography (GPC) is the established method for the determination of molar mass averages and the molar mass distributions of polymers. GPC retention is based on the separation of macromolecules in solution by molecular sizes and, therefore, requires a molar mass calibration to transform elution time or elution volume into molar mass information. This kind of calibration is typically performed with narrow molecular mass distribution polymer standards, universal, or broad calibration methods or molar-mass-sensitive detectors like light-scattering or viscosity detectors. 

Size-exclusion chromatography (SEC) has become the technique of choice in measuring the molar-mass distributions of polymers that are soluble in easily handled solvents (Dawkins, 1989). The technique as widely practised is not an absolute method and a typical SEC system must be calibrated using chemically identical polymers of known molar mass with a narrow distribution unless a combined detector system (viscosity, light scattering and refractive index) is employed. 

One should keep in mind that mole fraction-based activity coefficients /a become very small values for common polymer solutions and reach a value of zero forMs ->oo, which means a limited applicability at least to oligomer solutions. Therefore, the common literature provides only mass fraction-based activity coefficients for (high-molecular) polymer -i- (low-molecular) solvent pairs. The molar mass Mb of the polymeric liqitid is an average value (M ) according to the usual molar-mass distribution of polymers. 

S.T., and Owens, K.G. (2005) Molar mass distributions of polymers from size exclusion chromatography and matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry methods for comparison. J. Appl. Polym. Set, 97, 627-539. 

Draft International Standard ISO/DIS 16014 Plastics— Determination of average molar masses and molar mass distribution of polymers using size exclusion chromatography. Part 1 General Principles. Part 2 Measurement at lower temperatures. Part 3 Measurement at higher temperatures—draft under consideration, 1999... 

Gel permeation chromatography (GPC) or size exclusion chromatography (SEC) has become one of the most important techniques for characterizing the molar mass, and molar mass distribution of polymer samples because of the relative ease and rapidity of measurement. There are, however, problems to consider, such as accurate calibration, correction for peak broadening effects, concentration dependence of elution volumes, non-exclusion effects, and inadequate peak separation, which continue to occupy attention. 

Size exclusion chromatography (SEC) is used to determine the molar mass distribution of polymers. SEC is not an absolute method. It requires calibration. Narrow molar mass fractions of atactic polystyrene... 

The molar mass distribution of hyperbranched polymers is, therefore, always larger than diat of titeir linear homologues and tends toward infinity when conversion becomes close to 1. The use of a B3, comonomer, acting as a chain limiter and core molecule, helps in reducing polydispersity and controlling the molar mass of the final polymer.197... 

Polymers in solution or as melts exhibit a shear rate dependent viscosity above a critical shear rate, ycrit. The region in which the viscosity is a decreasing function of shear rate is called the non-Newtonian or power-law region. As the concentration increases, for constant molar mass, the value of ycrit is shifted to lower shear rates. Below ycrit the solution viscosity is independent of shear rate and is called the zero-shear viscosity, q0. Flow curves (plots of log q vs. log y) for a very high molar mass polystyrene in toluene at various concentrations are presented in Fig. 9. The transition from the shear-rate independent to the shear-rate dependent viscosity occurs over a relatively small region due to the narrow molar mass distribution of the PS sample. 

For a free-radical polymerization and a condensation polymerization process, explain why the molar mass distribution of the polymer product will be different depending on whether a mixed-flow or a plug-flow reactor is used. What will be the difference in the distribution of molar mass ... 

Reaction mechanisms and molar mass distributions The molar mass distribution of a synthetic polymer strongly depends on the polymerization mechanism, and sole knowledge of some average molar mass may be of little help if the distribution function, or at least its second moment, is not known. To illustrate this, we will discuss two prominent distribution functions, as examples the Poisson distribution and the Schulz-Flory distribution, and refer the reader to the literature [7] for a more detailed discussion. 

Copper bromide and pentakis-A-(heptadecafluoroundecyl)-l,4,7-triazeheptane (1 in Figure 10.9), along with an initiator, ethyl-2-bromoisobutyrate (2 in Figure 10.9), in a perfluoromethylcyclohexane-toluene biphase efficiently catalyse the polymerization of methyl methacrylate, with a conversion of 76 % in 5 h at 90 °C. The resultant polymer has a Mn = 11100 and a molar mass distribution of 1.30. After polymerization, the reaction was cooled to ambient temperature, the organic layer removed and found to contain a copper level of 0.088 % (as opposed to 1.5% if all the catalyst were to have remained in the polymer). A further toluene solution of monomer and 2 could be added,... 

Note 2 The LCST depends upon pressure and the molar-mass distributions of the constituent polymer(s). 

Recently, a miniaturized thermal apparatus, [t-ThFFF, was developed and applied to characterize the molar mass distribution of synthetic polymers in organic solvent as well to determine the particle size distribution of nanoparticles (PSs latex) in aqueous carrier. This 4-ThFFF proved to performed well in both macromolecule and particle analysis [48]. 

Other important pitfalls lie again in the low selectivity of SEC, which does not allow identifying small amounts of the macromolecular admixtures that is the minor components of polymer blends. The bell-shaped chromatograms with a broad base and a slim upper part are often erroneously proclaimed to signalize the narrow molar mass distribution of sample. On the other hand, the accumulation peaks due to presence of macromolecules excluded from the packing pores (Section 16.8.1) are interpreted as the sign of sample bimodality. The absolute detectors may also contribute to erroneous conclusions concerning sample polydispersity (Section 16.8.1). 

The excessive peak broadening. Under critical conditions, the effect of molar mass distribution of sample disappears. Therefore, the peak width reflects only the chromatographic processes and these seem to be rather slow in the case of LC CC. The peak broadening in LC CC raises with the increasing polymer molar mass and with the decreasing packing pore size [180]. 

Since the molar mass distribution of polydisperse polymers easily spans 3 to 4 decades, the low molar masses are masked by the strong scattering of the high-... 

Fig. 1. Weight fraction Wn of backbone polymer with n branches for most probable molar mass distribution of the mother polymer parameter is the average number of branches in one mother molecule, Nt. The points in the curves denote Wn at n = 0,1,... 8...

The determination of compositional changes across the molar mass distribution of a polymer or the detection of a specific component in a complex polymer mixture is of considerable interest. This information allows the prediction of physical properties and ultimately the performance of the polymer. Several analytical techniques are of use in determining these properties. Mass spectrometry, NMR, and infrared spectroscopy can be used to provide data about the compositional details of the sample. 

Size exclusion chromatography (SEC) is normally used for determining molar mass distribution in polymer and oligomer homologeous mixtures. Values for average molecular masses can be obtained. But it is difficult to get these results of specimen with different chemical heterogeneity SEC cannot provide... 

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