Chemistry Reference
In-Depth Information
chromatograms with indication of equally spaced retention volumes. These can be every 2.5 or
5.0 mL of volumes. The resultant artificial fractions are characterized by their heights
h
i
, their solute
concentrations
C
i
, and by the area they occupy within the curve
A
i
. The cumulative polymer weight
values is calculated according to:
.
A
T
X
IðVÞ¼
1
A
i
After conversion of the retention volumes
V
i
into molecular weights (using the calibration curve),
M
w
,
M
n
, and
M
z
can be calculated:
the molecular weights,
.
.
.
X
X
X
h
i
M
I
X
X
2
X
M
n
¼
h
I
h
I
=M
i
;
M
w
¼
h
i
;
M
z
¼
h
i
M
i
h
i
M
I
If the chromatogram is not equipped with a microcomputer for data treatment, one can easily
determine results on any available PC. Programs for data treatment have been written in various
computer languages. They are available from many sources.
Recently, there were several reports in the literature on combining size exclusion with high
pressure liquid chromatography for more comprehensive characterization of polymers. Thus, Gray
et al. reported that a combination of high pressure liquid chromatography with size exclusion
chromatography allows comprehensive structural characterization of macromolecules [
71
].
On the other hand, Chang et al. reported on using a modified form of high pressure liquid
chromatography analysis, referred to as
interaction chromatography
for polymer characterization.
The process utilizes enthalpic interaction of polymeric solutes with the stationary phase. Such
interaction depends on both, the chemical composition and on molecular weight. It is claimed to be
less sensitive to chain architecture and to offer superior resolution to SEC. The typical HPLC
instrument is modified to precisely control the temperature of the column. The temperature of the
column and the mobile phase is controlled by circulating a fluid through the column jacket from a
programmable bath/circulator. The mobile and stationary phases require careful choices to adjust the
interaction strength of the polymer solutes with the stationary phase so that the polymer solutes elute
out in a reasonable elution time. The process depends upon variations of the column temperature for
precise control of the solute retention in the isocratic elution mode. Mixed solvent system of a polar
and a less polar solvent are often employed to adjust the interaction strength [
72
].
2.8 Optical Activity in Polymers
Optical activity in biopolymers has been known and studied well before this phenomenon was observed
in synthetic polymers. Homopolymerization of vinyl monomers does not result in structures with
asymmetric centers (The role of the end groups is generally negligible). Polymers can be formed and
will exhibit optical activity, however, that will contain centers of asymmetry in the backbones [
73
].
This can be a result of optical activity in the monomers. This activity becomes incorporated into the
polymer backbone in the process of chain growth. It can also be a result of polymerization that
involves asymmetric induction [
74
,
75
]. These processes in polymer formation are explained in
subsequent chapters. An example of inclusion of an optically active monomer into the polymer chain
is the polymerization of optically active propylene oxide. (See Chap.
5
for additional discussion). The
process of chain growth is such that the monomer addition is sterically controlled by the asymmetric
portion of the monomer. Several factors appear important in order to produce measurable optical
activity in copolymers [
76
]. These are: (1) Selection of comonomer must be such that the induced
asymmetric center in the polymer backbone remains a center of asymmetry. (2) The four substituents
on the originally inducing center on the center portion must differ considerably in size. (3) The location
