Usage: Polyvinyl chloride (PVC) is one of the most widely produced plastics because of its relatively low cost compared to rubber and durability. PVC is commonly used for sewerage pipes instead of metal pipes because it is less vulnerable to corrosion. As well, PVC is used in the insulation of electric wire, waterproof clothing and in housing as vinyl siding.
Manufacturing: Produced from polymerization of Vinyl chloride (VCM) in an aqueous medium under controlled conditions is unmodified PVC that requires the incorporation of other additives before it can be made into finished products. The properties of the final PVC products is strongly affected by the additive’s molecular weight distribution and content which are important parameters controlled by the performance requirements of the end use product. Different molecular weights would be appropriate to particular applications and triple detection analysis by GPC is the ideal analytical tool for characterization.
Chemical Formula: (C2H3Cl)n
Figure 1. Conversion of PVC to CPVC by Chlorination
Two PVC samples were dissolved in tetrahydrofuran (THF) at a concentration of 5 mg/mL. The samples were shaken at 200 rpm for 48 hours. 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 PolyAnalytik SupeResTM Columns (3 x PAS 104L). The sample was injected at a volume of 200 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.
While CPVC is formed from PVC, two chains of the same length will have the same hydrodynamic radius. Using triple detection, and more importantly light scattering, the two samples can be differentiated from each other. Furthermore small differences in molecular weight can be determined between samples. An RI overlay of two PVC samples can be seen in Figure 2. All samples have a good signal to noise ratio and a mono-modal distribution.
Figure 2. Overlay of RI of different PVC sample injections.
Figure 3. An overlay of the RI, RALS, LALS and Viscometer response signals of sample A.
Using data from the light scattering detector and viscometer, the molecular weight and physical parameters can be seen in Table 1.
Table 1. Molecular weight and size derived from triple detection method.
Using the viscometer, along with the other detectors, information on the polymer backbone can be determined. A plot of Log Intrinsic Viscosity and Log Molecular Weight of the two samples as seen in Figure 4 do not overlay indicating that there are differences in the backbone structure. Sample A and B overlap at lower molecular weight but at higher molecular weight B is lower on the plot indicating a higher degree of branching as compared to A. The degree of branching in the polymer backbone can be measured using a known linear reference sample.
Figure 4. Mark Houwink plot of three PVC samples.
Polyvinyl chloride has a number of uses in liquid handling, insulation of electric wires, and clothing among numerous industrial applications. The triple detection method described here allows for molecular weight determination not dependent on conventional techniques but rather absolute molecular weight as well as determination of branching of the polymer backbone. The use of this method allows for a more accurate prediction of polymer characteristics such as thermal stability, electrical resistance or flexibility of PVC products.