Usage: Polyethylene (PE) is one of the most widely used thermoplastic polymers primarily used in packaging (bags, containers, bottles) but also has uses in pipe lining, wire coating and insulation applications. PE’s are classified into different categories based on their density and long hydrocarbon chain branching, which strongly influence physical properties such as chemical resistance and mechanical strength. The properties of the final polyethylene material are highly dependent on the molecular weight; hence, analysis of this polymer by gel permeation chromatography (GPC) is needed for research and quality control of the product to be used for specific applications. The challenge associated with analysis of PE is that it is insoluble at room temperature. This is overcome by using a High Temperature GPC system well suited to operation at temperatures up to 160oC.
Manufacture: Ethane is separated from natural gas using distillation columns and then converted to ethylene followed by polyethylene.
Chemical Formula: (C2H4)n
Figure 1: The repeating unit of Polyethylene (PE)
The polyethylene sample was dissolved in TCB (1,2,4-Trichlorobenzene) stabilized with 500 ppm Butylated hydroxytoluene (BHT), and heated for 4 hours at 160oC. Analysis was completed on a Viscotek High Temperature Triple Detection GPC (HT-GPC 350) equipped with a Refractive Index (RI) detector, Viscometer, Right Angle (90o) Light Scattering (RALS) and Low Angle (7o) Light Scattering (LALS). Manual filtration is not necessary since this system is equipped with an in-line, self-cleaning filter system which filters the small amount of sample that is transferred to the columns. Separation was performed using four PolyAnalytik HiTempTM columns (PAUHT-M and 1 x PAHT-G) connected in series. Samples were injected at a volume of 200 µL and eluted through the system at flow rate of 1 mL/min in stabilized TCB. A temperature of 160oC was maintained during separation and detection.
An overlay of the chromatogram from the different detectors for a sample of polyethylene can be seen in Figure 2. The signal to noise ratio is good and the molecular weight parameters as well as physical parameters of the reference and samples can be seen in Table 1. The values reported are an average of three injections with percent relative standard deviation (%RSD).
Figure 2. A typical multi detector overlay of chromatograms for a sample of polyethylene
Table 1. Average molecular weight and physical parameters of the sample of polyethylene.
Branching analysis of the polyethylene backbone is possible using the viscometer. Mark-Houwink plots of log intrinsic viscosity as a function of log molecular weight was calculated for polyethylene sample and for a linear reference material that is known to contain no branching, as shown in Figure 3. While it is apparent that the sample is branched, the curve on the Mark-Houwink-Sakurada plot is below that of the linear reference.
Figure 3. Mark Houwink plot of linear reference and one polyethylene sample.
High Temperature Triple Detection GPC can be used as a quality control tool in the production of polyethylene materials with well-defined and characterized molecular weight and branching distributions for specific applications. This is of significant interest as different molecular weights dictate different properties appropriate to particular applications, such as the rigidity and stability of the polymer.