Basic HTML Version
145
The argument goes like this:
Under
θ
-conditions, the excluded volume of monomers is exactly balanced by
weak attraction forces. The total effect is therefore as if the monomers did not
have a physical volume at all. Hence, the expansion factor
α
becomes equal
to 1. The same applies to entire polymer chains. Weak attraction forces
‘cancel’ the excluded volume effects, and the chains behave as if they have
no physical volume. Hence, they behave as ideal solutions, and A
2
= 0.
(Compare Table 10.1 in textbook)
3.2.9.
Finding θ-‐conditions by experiment
We take advantage of the fact that A
2
can be determined by light scattering or
by osmometry:
a)
Determine A
2
for a series of solvents (or T, I)
b)
Find (by interpolation) conditions where A
2
= 0, which correspond to
θ
-
conditions:
0
Solvent parameter
θ
-Solvent
A
2
2