Handbook of Solvents - George Wypych - ChemTech - Ventech!

692 George Wypych
Specific viscosity
1000
100
10
1
0.1
1 10
Concentration, g l -1
Figure 12.1.19. Specific viscosity of PVC in
bromobenzene solution vs. concentration. [Adapted, by
permission, from Hong Po-Da, Chen Jean-Hong Chen,
Polymer, 40, 4077-4085, (1999).]
I 560 /I 524
1
0.1
0.01
0.01 0.1 1 10 100
Concentration, g dl -1
Figure 12.1.21. Fluorescence intensity ratio vs. concentration
of polyimide in chloroform. [Data from H Luo,
L Dong, H Tang, F Teng, Z Feng, Macromol. Chem.
Phys., 200, No.3, 629-34 (1999).]
Association number, p
1
0.001 0.01 0.1 1
strands between knots occurs much later. 2
A similar course of events occurs with the
molecular aggregation of PVC solutions. 5
Figure 12.1.18 gives more details on
the mechanism of aggregate formation. 5
PVC dissolved in bromobenzene, BrBz,
undergoes a coil to globule transition
which does not occur in dioxane, DOA, solution.
Bromobenzene has a larger molar
volume than dioxane and the polymer
chain must readjust to the solvent molar
volume in order to interact. Figure 12.1.19
shows that two regimes of aggregation are
involved which are divided by a certain
critical value. Below the critical value
chains are far apart and do not interact due
to contact self-avoidance. At the critical
point, knots, similar to shown in Figure
12.1.17, begin to form and the resultant aggregation
increases viscosity. As the polymer
concentration increases, the
association number, p, also increases, but then levels off (Figure 12.1.20).
Fluorescence studies (Figure 12.1.21) show that there are two characteristic points relative
to concentration. Below the first point, at 0.13 g/dl, in very dilute solution, molecules
are highly expanded and distant from one another and fluorescence does not change. Between
the two points, individual chain coils begin to sense each other and become affected
100
10
Concentration, mg cm -3
Figure 12.1.20. Diblock copolymer association number
vs. its concentration. [Adapted, by permission, from
D Lairez, M Adam, J-P Carton, E Raspaud,
Macromolecules, 30, No.22, 6798-809 (1997).]