Viscosity Molecular weight distribution, effect

Several prelreatmcnt techniques have been practiced in conjunction with the use of organic polymeric membranes. The choice of a particular technique to apply depends to a great extent on the chemical nature of the feed, membrane resistance toward the feed stream and the product requirements. The characteristics of the feed such as the panicle size or molecular weight distribution, effective fluid viscosity and pH (and therefore the... 

In SEC, universal calibration is often utilized to characterize a molecular weight distribution. For a universal calibration curve, one must determine the product of log(intrinsic viscosity molecular weight), or log([7j] M). The universal calibration method originally described by Benoit et al. (9) employs the hydro-dynamic radius or volume, the product of [tj] M as the separation parameter. The calibration curves for a variety of polymers will converge toward a single curve when plotted as log([7j] M) versus elution volume (VJ, rather than plotted the conventional way as log(M) versus V, (5). Universal calibration behavior is highly dependent on the absence of any secondary separation effects. Most failures of universal calibration are normally due to the absence of a pure size exclusion mechanism. 

It is well known that LCB has a pronounced effect on the flow behavior of polymers under shear and extensional flow. Increasing LCB will increase elasticity and the shear rate sensitivity of the melt viscosity ( ). Environmental stress cracking and low-temperature brittleness can be strongly influenced by the LCB. Thus, the ability to measure long chain branching and its molecular weight distribution is critical in order to tailor product performance. 

Han, C. D. and Lamonte, R. R. Studies on melt spinning I. Effect of molecular structure and molecular weight distribution on elongational viscosity, Trans. Soc. Rheol., 16, 447 (1972). 

Knowledge of the molecular weight and of the molecular-weight distribution of a polymeric material is indispensable for scientific studies as well as for many technical applications of polymers. They effect the solution and melt viscosity, the processability, and the resulting mechanical properties tremendously. Therefore, we will give a short introduction into methods that allow us to determine the required information. Roughly, the methods developed for the determination of molecular weights are subdivided into absolute and relative methods ... 

The classical solvent precipitation fractionation technique provides reproducible fractionations for determining molecular weight distributions of CTPB and almost 100% recovery of the sample from the column. A solvent-nonsolvent combination which has been used effectively is the toluene—acetone-methanol system, where acetone and methanol are used as the nonsolvents. The precipitating fractions are required to stand approximately 24 hours to ensure complete separation. Each fraction is vacuum stripped of solvent at approximately 30 °C., and the molecular weight of each fraction is then determined by either VPO or intrinsic viscosity. 

Theories based on the uniformly effective medium have the practical advantage that they can be extended quite easily to polydisperse systems (227). Viscosity master curves can be predicted from the molecular weight distribution, for example. The only new assumption is that the entanglement time at equilibrium for a chain of molecular weight M in a polydisperse system has the form suggested by the Rouse theory (15) ... 

Graessley,W.W., Segal,L. Effect of molecular weight distribution on the shear rate dependence of viscosity in polymer systems. A-I.Ch.E. J. 16,261-267 (1970). 

Middlemans. Effect of molecular weight distribution on viscosity of polymeric fluids. J. Appl. Polymer. Sci. 11,417-424 (1967). 

Rheological measurements are of central importance in the processing of siloxane polymers. Typical studies would include determination of the dependence of the bulk viscosity of the material on the average molecular weight, molecular weight distribution, and rate of shear. Characterization of the effects of any branched chains or reinforcing fillers present is also of great importance.16... 

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