Molecular weight distribution broadening

Figure 22 presents the GPC curves of polystyrenes obtained in concentrated emulsions at various temperatures. The molecular weight distribution broadens because of a greater amount of low molecular weight polymers generated in the bulk as the polymerization temperature increases. The greater the temperature, the greater is the coalescence and hence the amount of bulk phase formed. 

If initiation is slow in an otherwise ideal system, the molecular weight distribution broadens up to a limiting value of DP ,/DP = 1.3 [272], More dramatic broadening is caused by transfer and termination. Polydispersity depends on whether transfer and/or termination involves monomer, counterion, polymer, or a transfer agent, and is affected by other variables such as the extent of consumption of the transfer/terminating agent this is reviewed extensively in Ref. [273]. 

High temperature GPC studies who that the 20 C PE sample is monomodal and has a narrow molecular weight distribution. The polydispersity Mw/M of 2.4 indicates a single site insertion mechanism. With increasing polymerization temperature the molecular weight distribution broadens. 

A particularly interesting result in Table V is seen for the solutions containing about 66% methylene chloride. At this point a sharp break occurred in several properties (1) the syndiotactic content decreased and the isotactic content increased, (2) the yield dropped drastically, (3) the molecular weight decreased, and (4) the molecular weight distribution broadened. These changes all point to a change in the reaction mechanism, probably from one of solvated ion pairs to one of predominantly contact ion pairs at that solvent concentration. Nevertheless, the tacticities of all the polymers formed in this series were found to fit on a Bovey plot, which indicates that they were formed by a Bernoullian process and no penultimate effect was present. 

Impossible and the molecular weight distribution broadens. Our best results were obtained at -40°C with Initial monomer concentrations between 1 and 1.5 mol. 1 (20). 

The linear a,co-hexamethyl(dimethyl)diethylsiloxanes obtained included from 0 to 27 mol% dimethylsiloxane units. An increase in dimethylsiloxane unit content results in a drop in the solidification temperature from -66 to -118 °C, a refractive index drop, a density increase, a molecular weight distribution broadening, and a decrease in the viscosity dependence on temperature. 

It was subsequently shown by various researchers [42 to 45] that the j/Zj/q -rjoy (or rjlrjg - o i2) relationship depended upon the breadth of the molecular weight distribution with the shear viscosity falling oif more rapidly as the molecular weight distribution broadens. Table 1.6, taken fromYamane and White [45], relates the values of t]lr]Q vs. 7]g y for different polymer melts. At high shear rates, the shear viscosity of polymer melts can be represented by a power law... 

The dependence of on <7i2 is very molecular weight distribution dependent. As the molecular weight distribution broadens, Nj increases at fixed 0 2 [44,49,50]. 

Property As average molecular weight increases As molecular weight distribution broadens As degree of long chain branching increases... 

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