When a DAWN or miniDAWN and a concentration detector are used to make molar mass and radius measurements, ASTRA can assess the shape of the molecule based on the measured molar mass and rms radius. A plot of the log of rms radius as a function of the log of the molar mass, called a Conformation Plot, shows the relationship between the two quantities. The slope of this graph gives a good estimate of the shape:
- Sphere: slope is approximately 0.33
- Random Coil: slope is near 0.5 - 0.6
- Rod: slope is approximately 1.0
When molecule sizes are above 10 nm we can measure both molar mass and rms radius simultaneously using only a light scattering instrument and a concentration source.
This conformation plot compares two aqueous polymer samples (hyaluronic acid). HA2 has a slope of 0.49, which indicates that it is a random coil. HA4 has a slope of 0.18, which indicates a much more compact structure. ASTRA's branching analysis could be used with these polymers to assess HA4's highly branched structure more closely.
Polyethylene in TCB
This conformation plot compares two polyethylene samples in TCB at 135°C. PE_linear has a slope of 0.62, which indicates that it is a linear random coil. PE_branched has a slope of 0.47. The difference in slope indicate that the PE_branched sample is more compact, and likely has a more branched structure.
Light Scattering with Viscometry
If we wish to determine the conformation of molecules with rms radius below 10 nm, a ViscoStar can be added to the analysis to determine conformation. A plot of the log of intrinsic viscosity as a function of molar mass, called a Mark-Houwink-Sakurada (MHS) Plot, shows the relationship between the two quantities. As with the Conformation Plot, the slope of this graph (the MHS a term) gives a good estimate of the molecular shape:
- Solid Sphere: a is approximately 0.0
- Linear Random Coil in Theta Conditions: a is appropximately 0.5
- Linear Polymers in Good Solvent: a is approximately 0.5 to 0.8
- Rigid Rod: a is approximately 1.8 to 2.0
This Mark-Houwink-Sakurada plot shows Alginate, a natural polymer occurring in algae. Its slope is approximately 0.99, which indicates that it is a linear polymer.
This Mark-Houwink-Sakurada Plot shows two polystyrene samples in THF. The slope of PS_linear is 0.68, while PS_branched is 0.51. While both are random coils, the branched sample is clearly more densely packed.