Molecular weight direct measurements

The number average molecular weight is required. This is obtained directly from measurements of a colligative property, such as the osmotic pressure, of dilute polymer solutions (see Chap. VII). It is often more convenient to establish an empirical correlation between the osmotic molecular weight and the dilute solution viscosity, i.e., the so-called intrinsic viscosity, and then to estimate molecular weights from measurements of the latter quantity on the products of polymerization. 

Viscosity measurements alone cannot be directly used in the Mark-Houwink-Sakurada equation to relate absolute viscosity and polymer molecular weight, since additional unknowns, K and a must be determined. Therefore, viscometry does not yield absolute molecular weight values it rather gives only a relative measure of polymer s molecular weight. Viscosity measurements based on the principle of mechanical shearing are also employed, most commonly with concentrated polymer solutions or undiluted polymer these methods, however, are more applicable to flow properties of polymers, not molecular weight determinations. 

A LALLS detector from which the response Als can be used to measure molecular weight directly according to... 

A plot of (rjsp/c) 1 vs c1/2 is shown in Figure 3, from which extrapolation to c = 0 seems justifiable. If so, the constant A represents the intrinsic visocity nsp/c when c -> 0. From measurements on other polyelectrolytes (see, e.g., Reference 18) this constant increases with the second power of the molecular weight as measured by a direct method. 

Montaudo, G., Scamporrino, E., Vitalini, D., and Mineo, P., Novel Procedure for Molecular Weight Averages Measurements of Polydisperse Polymers Directly from Matrix-assisted Laser Desorption/Ionization Time-of-flight Mass Spectra, Rapid Comm. Mass Spectrom., 10,1551,1996. 

Montaudo, G. Scamponino, E. Vitalini, D. Mineo, P. Novel procedure for molecular weight averages measurement of polydisperse polymers directly from matrix-assisted laser desorption/ionization time-of-flight mass spectra. Rapid Commun. Mass Spectrom. 1996,10,1551. 

In prachce, molecular weight is measured by solu-hon viscomefry. Dissolving a precise amount of resin in a solvent and measuring the change in viscosity of the solution versus pure solvent is correlated with resin molecular weight. While soluhon viscomefry does not give a direct measure of molecular weight distribution, the MJM of PVC is very nearly two in all cases as a result of the polymerizahon and chain transfer mechanism. 

When direct measurement is not available, the molecular weight can be estimated by two different means ... 

Next let us consider the light scattered by liquids of low molecular weight compounds. We are actually not directly interested in this quantity per se, but in scattering by solutions-polymer solutions eventually, but for now solutions of small solute molecules. The solvent in such a solution does scatter, but, in practice, the intensity of light scattered by pure solvent is measured and subtracted as a blank correction from the scattering by the solution. 

In its simplest form, a mass spectrometer is an instmment that measures the mass-to-charge ratios ml of ions formed when a sample is ionized by one of a number of different ionization methods (1). If some of the sample molecules are singly ionized and reach the ion detector without fragmenting, then the ml ratio of these ions gives a direct measurement of the molecular weight. The first instmment for positive ray analysis was built by Thompson (2) in 1913 to show the existence of isotopic forms of the stable elements. Later, mass spectrometers were used for precision measurements of ionic mass and abundances (3,4). 

Controlled stress viscometers are useful for determining the presence and the value of a yield stress. The stmcture can be estabUshed from creep measurements, and the elasticity from the amount of recovery after creep. The viscosity can be determined at very low shear rates, often ia a Newtonian region. This 2ero-shear viscosity, T q, is related directly to the molecular weight of polymer melts and concentrated polymer solutions. 

Molecular weights of PVDC can be determined directly by dilute solution measurements in good solvents (62). Viscosity studies indicate that polymers having degrees of polymerization from 100 to more than 10,000 are easily obtained. Dimers and polymers having DP < 100 can be prepared by special procedures (40). Copolymers can be more easily studied because of thek solubiUty in common solvents. Gel-permeation chromatography studies indicate that molecular weight distributions are typical of vinyl copolymers. 

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