Polymer solutions average molecular weight

The viscosity of dilute polymer solutions is considerably higher than that of the pure solvent. The viscosity increase depends on the temperature, the nature of the solvent and polymer, the polymer concentration, and the sizes of the polymer molecules. This last dependence permits estimation of an average molecular weight from solution viscosity. The average molecular weight which is measured is the viscosity average A/v, which differs from those described so far in this text. Before viscosity increase data are used to calculate Afv of the solute it is necessary, however, to eliminate the effects of solvent viscosity and polymer concentration. The methods whereby this is achieved are described in this section. 

The predicted intrinsic width of the order-disorder transition of a mono-disperse, flnite-molecular-weight polymer solution was also tested. The average molecular weights of dialkyl-substituted polysilylenes are in the order of 6 X 10, which implies that N is 3000-5000 silicon atoms. With equation 9, the theory predicts that ATq/Tc is 0.004-0,006, which for Tc = -30 corresponds to an intrinsic width of roughly 1 or 2 C. This result is in good agreement with the experimental observations summarized in Table II. 

A 200,000-amu average molecular weight polymer is contaminated with 0.5% of a 100-amu impurity, presumably the monomer. Determine the error in the molecular weight determination if a 1.000 x lO molal aqueous solution is used. Assume a temperature of 25.0°C. 

M. P. Vega, E. L. Lima, J. C. Pinto, In-line monitoring of weight average molecular weight in solution polymerization using intrinsic viscosity measurements , Polymer, 2001, 42, 3909. 

Polymer solutions are often characterized by their high viscosities compared to solutions of nonpolymeric solutes at similar mass concentrations. This is due to the mechanical entanglements formed between polymer chains. In fact, where entanglements dominate flow, the (zero-shear) viscosity of polymer melts and solutions varies with the 3.4 power of weight-average molecular weight. 

Among the techniques employed to estimate the average molecular weight distribution of polymers are end-group analysis, dilute solution viscosity, reduction in vapor pressure, ebuUiometry, cryoscopy, vapor pressure osmometry, fractionation, hplc, phase distribution chromatography, field flow fractionation, and gel-permeation chromatography (gpc). For routine analysis of SBR polymers, gpc is widely accepted. Table 1 lists a number of physical properties of SBR (random) compared to natural mbber, solution polybutadiene, and SB block copolymer. 

These values are given for polymers of narrow molecular-weight distribution, with number-average molecular weights (M ) of about 20,000 prior to chlorination. Chlorination reactions are carried out under homogeneous conditions in CCl solutions at temperatures between 90 and 110°C with viscosities at about 5 Pa (50 P). 

Attempts were made to determine number average molecular weights (Mn) by osmometry (Mechrolab Model 502, high speed membrane osmometer, 1 to 10 g/1 toluene solution at 37 °C), however, in many instances irreproducible data were obtained, probably due to the diffusion of low molecular weight polymer through the membrane. This technique was abandoned in favor of gel permeation chromatography (GPC). 

The diffusion coefficients of entangled polymers in solution will most certainly depend on the viscosity of the medium and vice versa. It is reasonable therefore to expect that the diffusion coefficient would correlate well with the weight average molecular weight of the polymer. M is therefore used with equation (lO) giving... 

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