Chemistry Reference
In-Depth Information
The value of
M
n
cannot be calculated in many cases of practical interest
because
r
in
Eq. (3-36)
is not known. This is particularly true when the branching
character or composition of the polymer is uncertain. End group analysis is very
useful, up to a point, in calculating how to react the polymer further. Thus, it is
common practice to use parameters like the saponification number (number of
milligrams of KOH that react with the free acid groups and the ester groups in
1 g of polymer), acid number (number of milligrams of KOH required to
neutralize 1 g of polymer), acetyl number (number of grams of KOH that react
with the acetic anhydride required to acetylate the hydroxyl groups in 1 g of poly-
mer), and so on.
3.2
Light Scattering
Data obtained from light scattering
m
easurements can give information about the
weight average molecular weight
M
w
, about the size and shape of macromole-
cules in solution, and about parameters that characterize the interaction between
the solvent and polymer molecules.
Light may be regarded as a periodically fluctuating electric field associated
with a periodic magnetic field. The electric and magnetic field vectors are in
phase with each other and are perpendicular to each other and to the direction
of propagation of the light. If the light wave travels in the
x
direction, then the
electric and magnetic vectors would vibrate in the
z
and
y
directions, respectively
(see
Fig. 3.3a
, for example).
y
Locus of
identical I
′
θ
Instantaneous
electric field
vector
I
′
θ
Incident
light
θ
x
x
Magnetic
field vector
z
Scattering
dipole
(a)
(b)
FIGURE 3.3
(a) Light depicted as a transverse wave. (b) Scattering envelope for point scatterer with
unpolarized incident light.
