25
The MW values in the last column of
Table A.2-1
are calculated by a different approach
that incorporates the universal calibration method and only the SEC viscometer data, but not the
RALLS data. The close agreement of the MW values of the last two columns, which are
determined by two totally different methods, gives the strong evidence for the accuracy of the
triple detector approach in determining polymer MW and size. The most amazing feature of the
triple detector technique is its capability of measuring polymer
R
g
values down to a value as low
as one nanometer, which is an equivalent of 10 angstroms, a value approaching the
measurement of chemical bond dimensions! (See the third column of
Table A.2-1
.)
In certain situations, however, the dissymmetry correction becomes important. Rod-like
materials of high molecular weight can exhibit a large angular dissymmetry that can alter the
correction algorithm used by the RALLS detector. The low angle (LALLS) detector can be used
for these materials, or can be used on any other sample with molecular shapes having
undefined size where very accurate values of molecular weight are desired.
In the case of a light scattering detector that can measure responses below 10 degrees, the
particle scattering function P(
θ
) approaches a value of unity. Therefore, no assumptions are
needed, and the calculated molecular weight is highly accurate. The Viscotek LALLS detector
measures signal response at an angle of 7 degrees having virtually no background noise and a
high signal-to-noise ratio compared to earlier low angle instruments.
The remarkable ability of triple detector SEC to detect polymer branching has been
illustrated for an experimental bimodal polymer. The RALLS detected a very high MW species
in the prepeak while the viscometer showed a weak signal for a relatively small molecular size.
This is indicative of a highly branched structure. The presence of this branched structure would
not have been detected by the conventional SEC using a single RI or UV detector.
The quantitative determination of polymer branching and chain conformation relies on
the measurement of the Mark-Houwink exponent
a
in the viscosity-to-MW relationship shown
earlier in Equation [A.2-15]. Because [
η
] and MW values in SEC-Viscometry-RALLS/LALLS are
measured directly from detector signals, the precision of determining this Mark Houwink
a
value
by triple detector SEC is significantly better than any other technique that exists today. As
shown in
Table A.2-2
below, with triple detector SEC, the precision of the
a
value determination
is now quite good, reliable to two significant digits or better. This degree of precision is
necessary for the meaningful use of this
a
value for the investigation of polymer branching and
conformational differences. The standard deviation of the
a
value determined by the triple
detector is almost two orders of magnitude better than that by the multi-angle laser light
scattering approach.
a
values, where
[ ]
η
=
KM
a
