Molecules Weight Dependence of Melt Viscosity

The flow behavior of low molecular weight polymers differs significantly from that of high molecular weight polymers. As one might expect, melts and solutions of (very) low molecular weight behave like Newtonian liquids, that is, the viscosity is shear rate independent. This is not necessarily the case for polymers of high molecular weight where entanglement of polymer molecules takes place, that is, entangled polymers show a much stronger resistance to flow, and since polymers tend to disentangle when subjected to shear above a critical value ý_c, the viscosity depends on the shear rate ý whereas below ý_c the viscosity is more or less constant. The shear rate dependence (shear thinning) is usually expressed in a power law:

η = const.    for  ý ≤ ý_c

η = Φ ý^n-1   for  ý ≥ ý_c

where  ý_c is the critical shear rate for the transition from Newtonian to shear thinning behavior, Φ is an interaction constant which depends on the polymer structure, and n is the power-law coefficient describing the shear thinning behavior.⁶

To be able to compare the rheology of polymers with different MW, the dependence on shear rate has to be eliminated. This can be achieved by using the zero shear-rate viscosity, η₀, which is the viscosity in the limit of infinite small shear rate. As the name suggest, this viscosity can not be measured but has to be extrapolated. At this shear rate, Newtonian or quasi Newtonian behavior is observed, and when the molecular weight drops below a certain threshold where entanglements between chains become unlikely or even impossible, the viscosity is directly proportional to the molecular weight M. The critical molecular weight for entanglement coupling is about two times the entanglement weight, M_c = 2M_e. Above this value, the zero shear rate viscosity can be described by a simple power law:^1,2,5

η₀  = k_l · M,  M < M_c

η₀  = k_m · M^α,  M > M_c

The power-law coefficient α of polymer melts has a value of about 3.4 ± 0.2.

As has been shown by Nichetti eta al. (1998)³, the critical shear rate ý_c is a function of the molecular weight.  At ý = ý_c the zero shear rate viscosity η₀ has to be equal to the power law viscosity:

η₀ = k_m M^α = Φ ý_c^n-1 

or

ý_c = (Φ / k_m M^α)^1/(n-1) 

The parameter α is the inverse of n: n = α^-1. This is not surprising since n describes the disentanglement (shear-thinning) an α the entanglement state at zero shear rate.

The critical shear rate ý_c can also be derived from the characteristic disentanglement or relaxation time τ_R which is inverse proportional to the critical shear rate:^3,4

ý_c = τ_R^-1

The relaxation time depends on the polymer concentration and type of solvent (if present). For example, in a melt τ_R is proportional to M^3.4 whereas for polymers in a dilute theta-solvent τ_R ∼ M^3/2 and in a bad solvent τ_R ∼ M.

References and Notes

1. T.G. Fox, P.J. Flory, J. Am. Chem. Soc., 70, 2384-2395 (1948)
2. T.G. Fox, P.J. Flory, J. Polym. Sci., 14, 315-319 (1954)
3. D. Nichetti, I. Manas-Zloczowera, J. Rheol., 42 (4), 951-969 (1998)
4. M. Bouldin, W.-M. Kulicke, H. Kehler, Coll. and Poly. Sci., Vol. 266, 9, 793–805 (1988)
5. Lewis J. Fetters, David J. Lohse, and Scott T. Milner, Macromolecules, 32, 6847-6851 (1999)

6. For n = 1, the viscosity is shear rate independent. Newtonian behavior is observed at both very high and very low shear rates. The first Newtonian limit is often denoted as η₀ and the second as η_∞.