92168.pdf

252 Rheology

viscosity and relative molecular mass can be expressed by the general

equation proposed by Mark and Houwink:

to] = KM? (9,6)

K and a are constants characteristic of the polymer-solvent system (a
depends on the configuration of the polymer chains) and approxim-
ately independent of relative molecular mass.

Table 9.1 Values of K and a for some polymer-solvent systems

System K/m\g~'

Cellulose acetate in acetone (25°C)

Polystyrene in toluene (25°C)

Poly(methyl methacrylate) in benzene (25°C

Poly(vinyl chloride) in cyclohexanone (25°C)

1.49 x 1(T^5

3.70 x 1(T^5

0.94 x 10~^5

0.11 X HT^5

0.82

0.62

0.76

1.0

In view of experimental simplicity and accuracy, viscosity measure-

ments are extremely useful for routine relative molecular mass

determinations on a particular polymer-solvent system. K and a for

the system are determined by measuring the intrinsic viscosities of

polymer fractions for which the relative molecular masses have been

determined independently - e.g. by osmotic pressure, sedimentation

or light scattering.

For polydispersed systems an average relative molecular mass

intermediate between number-average (a = 0) and mass-average (a

= 1) usually results.

Non-Newtonian flow

Steady-state phenomena

Shear-thinning

Shear-thinning, as the term suggests, is characterised by a gradual
(time-independent) decrease in apparent viscosity with increasing
rate of shear, and can arise from a number of causes.