Polyacrylamide's hydrophilic and swelling properties make it a popular as a functional hydrogel component. Viscotek's advanced triple detector can be used to correlate molecular weight and size properties to critical performance parameters
Polyacrylamide (PAAm) is difficult to characterize because it possesses a non-polar, hydrocarbon backbone with a highly polar amide side chain capable of both ionic character and hydrogen bonding capabilities. This unique set of properties has made this material very useful as hydrogel in applications such as waste water treatment, oil well fracturing, gel electrophoresis, soil conditioners, as well as a thickening agent and filler.
Table 1: Molecular weight and size data derived from the triple detection chromatograms using OmniSEC software.
The PAAm sample was acquired pre-diluted from a gel electrophoresis manufacturing company. Viscotek's model TDA305 complete SEC/GPC system was used to analyze the samples. The unit is equipped with a differential refractometer, four-capillary viscometer, right angle light scattering (RALS), and low angle light scattering detectors (LALS). Each sample was injected twice at a volume of 100 uL and eluted in a mixed aqueous/organic mobile phase at 1.0 mL/min. Separation was performed on two ViscoGEL C-series columns while maintaining a constant temperature of 30 °C during chromatography and detection.
To achieve a size only separation of PAAm, both the column technology and mobile phase must be optimized. Due to the partial cationic nature of amide functionalities, we have employed a polycationic porous packing material for reverse-phase ionic repulsion column separation. To ensure that the system is fully ionized as well as capable of maintaining dissolution of the amphiphilic PAAm, a mixed mobile phase was used. Figure 1 shows the triple detection chromatograms for an approximately 1 million Da sample of PAAm resin. Good signal-to-noise is achieved on all four detectors along with good molecular weight distribution resolution. The data derived from the detector signals is listed in Table 1. The repeatability of the molecular weight moments between injections shows the excellent precision of this technique.
Figure 1: Triple detector chromatogram of PAAm obtained on Viscotek's Triple Detection SEC/GPC model TDA305.
Figure 2: The Mark-Houwink overlay plot of the two injections of PAAm. The y-axis is LogIV obtained from the viscometer and the x-axis is LogMW obtained from the light scattering.
Using advanced detection capabilities, structural information is obtained from the intrinsic viscosity and hydrodynamic radius measurements. Figure 2 shows the Mark-Houwink overlay plot of the two injections. This plot shows the change in intrinsic viscosity across the molecular weight distribution. The parameters derived from this plot provide insight into the structure, shape, and density of the polymer. Good repeatability is observed. Using this plot, advanced branching analysis can be completed using OmniSEC software (Figure 3). The unknown sample is compared to a linear PAAm standard with the Mark-Houwink parameters provided. The initial overlap of the two plots indicates a lack of branching at low molecular weights. At higher molecular weights, the unknown curve begins to flatten, indicating a more dense structure at these molecular weights. This is suggests that a small quantity of branching is present when the polymerization advances to the high molecular weight chains.
It can also be indicative of cross-linking, co-monomer feed ratio, or a change in the polymerization mechanism, all of which can be monitored and controlled by the synthesis procedure.
Figure 3: Branching analysis of the PAAm sample. The black line is a theoretical Mark-Houwink plot using parameters obtained from a linear PAAm sample under the similar conditions. The term "Branches" refers to the average number of branches per polymer chain.
Polyacrylamide is a widely used commodity polymer in various industrial and pharmaceutical applications. Its hydrophilic and swelling properties make it a popular choice as a functional hydrogel component. Analyzing the PAAm resins using advanced SEC/GPC detection has traditionally been difficult due to the amphiphilic and ionic nature of the polymer causing non-ideal chromatographic separation. Viscotek's triple detection SEC/GPC can provide good size exclusion chromatography as well as advanced detection capabilities to correlate molecular weight and size properties to critical performance parameters.