Flow Injection Analysis-Static Light Scattering: Two Minute Determinations of Weight
Average Molecular Weight
L. Mark DeLong
Infrastructure Engineering, Operations and Central Technology Union Carbide Corporation
South Charleston, WV
Molecular weight is perhaps the most important physical property to polymer end-use application. Two of the most common means of determining molecular weights are the indirect methods of gel permeation chromatography (GPC) and capillary viscometry (CV). While these techniques are valuable tools to the polymer analyst, GPC and CV are both techniques that perturb the polymer under study. Static light scattering (SLS), on the other hand, is a non-perturbing technique.
The biggest drawback to SLS is that it has tended to be more time consuming than either CV or GPC. Flow injection analysis (FIA) can be combined with SLS detection to eliminate the multiple concentration, multiple angle approach to light scattering when only weight average molecular weights are required.
The basic concepts of FIA-SLS along with the advantages and limitations are discussed. In brief, FIA-SLS system is little more than a flowing polymer stream passing through a light scattering detector. Since the concentration of the solution is known apriori and its scattered intensity is measured, the polymer weight average molecular weight can be determined. Feasibility of this approach has been tested on a polymer with a range of known weight average molecular weight. This range, covering 12,000 to 220,000 daltons, can be measured accurately (to within approximately 5%) and with less than 1.5% relative standard deviation. Futhermore, these molecular weight determinations can be made in a theta solvent. This has the added benefit of eliminating concentration dependence of the light scattering signal; thus a single defined polymer concentration range for a given polymer/solvent system will produce accurate results over an extremely broad molecular weight range. Additionally, the use of the FIA-SLS system coupled to a concentration detector will be demonstrated on a water soluble polymer.