26
SEC-MALLS
SEC-Visc-RALLS
0.47
0.729
0.41
0.726
0.23
0.731
0.59
0.727
1.19
0.723
0.38
0.739
1.52
0.735
0.44
0.724
Average
0.62
0.729
Std. Dev.
0.044
0.005
Table A.2-2 Unprecedented Precision of SEC-Viscometry-RALLS in Determining Polymer
Conformation, Repeated Runs of Polystyrene Standard (NBS 706) in Toluene
The
Mark-Houwink
a
value is a polymer conformation parameter. The
a
value
decreases for molecules of more compact structure. This
a
parameter can have a near zero
value for globular proteins. For random coil molecules, it can have a value between 0.5 for poor
solvent conditions and 0.8 for good solvent conditions. An
a
value of 1.0 or greater is possible
for extended rod-like molecules. For polymers containing long chain branching, the
a
value can
fall significantly below 0.5, depending on the degree of branching. The
g’
branching index is
calculated as the ratio of sample [
η
]
B
value against its linear reference [
η
]
L
value of same
molecular weight:
[A.2-21]
[ ]
[ ]
g
B
L
'
=
η
η
In conclusion, SEC-Viscometry-RALLS/LALLS is capable of determining accurate
molecular weight and molecular distribution of polymer samples over the widest range of
molecular weights. The technique provides extra sensitivity at low molecular weights. The triple
detector combination also eliminates the need of SEC column calibration and instrumental band
broadening correction.
14
In addition, the low angle LALLS detector can confirm the molecular
weight calculations of odd shaped species where the angular dissymmetry correction factor is
altered. Therefore, the technique can provide the most accurate determinations today for the
polymer MW, intrinsic viscosity [
η
], Mark Houwink exponent, and branching factors. The triple
detector SEC system represents the most powerful tool existing today to meet the challenges of
the polymer and biopolymer characterization needs. Viscotek’s SEC-Viscometry-RALLS/LALLS
instrument is supported with a smart A/D Data Manager and the fully integrated PC-based
Windows TriSEC software.
A.2.b
Components
The Light Scattering detector is composed of a cell block and Laser diode mount.
Figure 7
is an exploded view of the cell block. It is contained inside the heated oven
compartment. It contains both the 90
°
and the 7
°
detectors housed in separate optical blocks.
14
W. W. Yau, J. J. Kirkland, and D. D. Bly, “Modern Size Exclusion Liquid Chromatography”, Wiley, New
York, 1979.