Branching Viscosity curve, effect

The value of the MW exponent for the low shear-rate melt viscosity, usually accepted as 3.4—3.5 for linear polymers, has been called in question for PE. Schreiber and Bagley (164) reported a value of 4.22 for linear PE Porter and Johnson (165) had earlier reported values up to 8 for LDPE. For branched polymers an effect of this kind could be due to the effect mentioned in Section 5, in which a cross-over point is found on a log-log plot of viscosity vs. MW when linear and slightly branched polymers are compared, since the curve for the branched polymers must be steeper than that for the linear ones near the cross-over point. 

The family of curves found shows a minimum value of intrinsic viscosity around the value of 10 at. The descending branch of curves p = 5-10 at) evidences a decrease of [rj] with pressure, the effect being induced, obviously, by the intensification of the shearing stress at the capillary wall. 

Ether sulphates show a strong salt effect - that is an increase in viscosity on addition of salt (or other electrolyte). The response to electrolyte (the salt curve ) can be very different between ether sulphates, even from different suppliers of the same product. Generally, the more soluble the surfactant, the lower the salt response but higher degrees of ethoxylation reduce salt response, as does branching in the alcohol as shown in Figure 4.20. 

The application of refractive index and differential viscometer detection in SEC has been discussed by a number of authors [66-68]. Lew et al. presented the quantitative analysis of polyolefins by high-temperature SEC and dual refractive index-viscosity detection [69]. They applied a systematic approach for multidetector operation, assessed the effect of branching on the SEC calibration curve, and used a signal averaging procedure to better define intrinsic viscosity as a function of retention volume. The combination of SEC with refractive index, UV, and viscosity detectors was used to determine molar mass and functionality of polytetrahydrofuran simultaneously [70]. Long chain branching in EPDM copolymers by SEC-viscometry was analyzed by Chiantore et al. [71]. 

Branching reduces the hydrodynamic volume relative to the mass of the coil. The intrinsic viscosity of branched macromolecules is thus lower than that of their unbranched (linear) counterparts. The effect is particularly marked in the case of long-chain branching. If the number of branch points in a polymer homologous series increases with the molar mass, then the [rj] values also decrease relative to those of linear molecules. Thus, the slope of the log f (log 2) curve continuously decreases with increasing molar mass,... 

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