Usage: Polyurethane formulations are primarily found as flexible foams in bedding, packaging and furniture cushions. Other polyurethane applications include high performance adhesives, surface coatings on naval vessels or automobiles and rigid foam insulation panels. While chemical properties are often controlled by monomer selection, physical properties of polyurethanes are dictated by molecular weight. Characterization of molecular weight is therefore essential for quality control of polyurethane-based commercial products.
Manufacturing: Polyurethane is composed of several organic monomers joined by urethane or carbamate groups. Often formed by the reaction of isocyanate and polyol monomers, polyurethane shown in Figure 1 contains on average two (or more) repeating alkyl groups which were originally present in the monomers.
Chemical Formula: C25H42N2O6
Figure 1. Generic structure of polyurethane
Three polyurethane samples were dissolved in tetrahydrofuran (THF) at a concentration of 2 mg/mL. Analysis was completed on a Viscotek Triple Detector Array (TDA) 302 equipped with a Refractive Index (RI) detector, Viscometer, Right Angle (90o) Light Scattering (RALS) and Low Angle (7o) Light Scattering (LALS). Separation was performed using two PolyAnalytik SupeResTM Columns, PAS 105L and PAS 104L, connected in series. Samples were injected at a volume of 100 µL and eluted through the system at flow rate of 1 mL/min in THF. A temperature of 30oC was maintained during separation and detection.
An overlay of the RI signal for the three samples can be seen in Figure 2. A good signal to noise ratio was obtained for each injection as well as a symmetric mono-modal distribution. Using the information from all three detectors, as shown in Figure 3 for sample C, molecular weight parameters and size data was derived using triple detection. The dn/dc values were calculated based on 100% mass recovery of each injection. The data in Table 1 is an average value obtained from 3 injections.
Figure 2. An overlay plot of the RI chromatograms of three polyurethane samples
Figure 3. An overlay of the RI, RALS, LALS and Viscometer response signals of sample C
Table 1. Molecular weight parameters and size data derived from triple detection using Omni-Sec software.
Triple detection allows for reproducible analysis of the polyurethane samples as a standard deviation of less than 5 percent was obtained for the average molecular weight of all samples. Furthermore, a Mark-Houwink plot was constructed as shown in Figure 4, using the intrinsic viscosity determined by the viscometer and the molecular weight determined by the RI and light scattering detectors. Branching information can be obtained from the Mark-Houwink plot; a decrease in the slope on the plot indicates branching is present. The samples are different with respect to branching because they do not completely overlap. Sample A has the largest change in slope suggesting a larger branching ratio. A known linear sample would be needed in order to generate the branching ratio for each sample.
Figure 4. An overlay of the Mark-Houwink plots of three polyurethane samples
Polyurethane has a number of commercial uses including foams, clothing, sports equipment and protective coatings. Advanced GPC multi-detection method provides a simple procedure to determine molecular weight parameters and degree of branching which can assist in predicting properties such as flexibility, melting point or stability.