Chemistry Reference
In-Depth Information
Figure 5.
MTF fractogram of 3-arm star sample as detected by 90
8
light scattering. The assignments of the four modes
observed in the fractogram are discussed in the text. The modes are numbered 1 through 4 with 1 being the
earliest eluter and 4 being the latest eluter.
its presence is now clearly revealed in the
MTF experiment. Formation of the 4-arm
star is discussed below.
Considering the SEC data, it would
appear that a small amount of uniarm
material and a relatively large amount of
two-arm material was present in the sample.
Both uniarm and two-armmaterial are linear
species, and as such, their MTF elution
behavior should match that of linear PS
standards. A comparison of the relative
peak elution volumes (or times) of linear PS
standards and the peaks of the 4 modes in
the 3-arm star sample is shown in Figure 6.
The molecular weights of the first two
modes were taken from the SEC data and
assigned based on the known elution order
of MTF. As can be observed, the elution
volumes of the first two modes are
consistent with those of linear PS standards,
and thus, these two modes appear to
represent the uniarm and 2-arm species
present in the sample. It is clear that the
latter two modes were retained far longer
than the corresponding linear chains and,
the LALLS/UV ratios suggest that the
molecular weights of these modes are 3 and
4 times the molecular weight of mode 1,
respectively.
The light scattering detector used in
these studies employed detection angles of
7 degrees and 90 degrees. Although it is
possible to estimate the radius of gyration
(R
G
) from two angle light scattering data,
the expected R
G
values for a multi-arm
species having arm molecular weights of
1,250 kg/mol exceed the values that can be
determined reliably from two angle data.
[6]
Calculations of the 7 degree particle
scattering form factor, P(7), for PS stars
[7]
having arm molecular weights of 1,250 kg/
mol suggest that M
W
values obtained from
7 degree light scattering data should
provide reasonably accurate molecular
weights of the potential species present in
the fractogram.
To improve the signal to noise ratio for
both the concentration detector response
and the light scattering response, the injec-
ted mass of the 3-arm star sample was
increased by a factor of five over the
standard conditions. The improved signal
to noise enabled a reasonable estimate of
the absolute M
W
values across the MTF
elution profile of modes 3 and 4. This
analysis revealed that the third mode
included molecular weights ranging from
3,100 kg/mol to 3,900 kg/mol. A 3-arm
