Triple detector SEC is ideal for polymer structure analysis as it directly detects changes in polymer structure as well as molecular weight, whereas conventional single concentration detector SEC detects only differences in hydrodynamic size
In this analysis, a bimodal polymer was submitted in four phases of backbone modification. The backbone of lower molecular weight species was modified without affecting that of the higher molecular weight species.
Triple detection gel permeation chromatography (TD-GPC) was used to study the molecular weight and structure of the polymer. The polymer was subjected to a backbone modification in 3 steps. The original polymer and the polymer after the first and second modifications were measured. The final product was also measured. All the polymers were dissolved in toluene at a concentration of approximately 0.5% w/v. Injections were 100 µL and the flow rate was 1 mL/min. Separation was performed on 2 mixed bed columns in series. The refractometer (RI) was used to obtain the concentration while the light scattering was used to measure the molecular weight of the sample. Finally the differential viscometer was used to measure the intrinsic viscosity.
Figure 1: Triple detection of the bimodal polymer. Red trace is the RI, green is the Light Scattering while the blue curve is from the viscometer.
Figure 1 shows the triple detector chromatogram of the sample. It is clear that this sample is bimodal as there are two peaks. The leftmost peak is from the high molecular weight polymer while the peak eluting at a later retention volume is from the lower molecular weight species. The starting polymer and the final polymer after complete backbone modification are compared in figure 2. The Mark-Houwink is composed by plotting the intrinsic viscosity versus the molecular weight in a log-log graph.
Structural information can be determined from overlaying Mark-Houwink plots. The two different structures corresponding to the two peaks of the starting material are clearly seen by the change in intercept of the Mark-Houwink plot below. After some modification, the final product is shown to have a more consistent structure (same intercept) throughout the molecular weight range, indicating that the lower molecular weight species had modification.
Samples that possess higher values of intrinsic viscosity at the same molecular weight are less dense. The intercept value tells us about the density of the backbone structure per repeat unit. The Mark-Houwink plot in figure 2 at lower molecular weight shows that the starting material has a lower intrinsic viscosity at the same molecular weight. Intrinsic viscosity is inversely proportional to molecular density, therefore we can assume the final product is a less dense species.
Figure 2: Mark-Houwink plot overlay of the starting material (red) and the final product (blue).
The four phases of modification are shown in the plot overlay of the lower molecular weight region in figure 3. With each modification, the intrinsic viscosity is increased, showing a decrease in density. The lower intercept confirms the presence of backbone modification.
Figure 3: Mark-Houwink plot overlay of the four stages of polymer modification. Starting material in red, first modification in green, second modification in purple and the final product in blue.
The inverse relationship between intrinsic viscosity and molecular density is well demonstrated using the triple detector chromatography system. TD-GPC is ideal for polymer structure analysis because it directly detects changes in polymer structure as well as molecular weight, whereas conventional methods detect only differences in hydrodynamic size.