Optimization of solution fractionation of poly(vinyl chloride)

The optimization of solution fractionation of poly(vinyl chloride) (PVC) is described with regard to the variation of several parameters. The best separation efficiency is obtained by a single‐step discontinuous solution fractionation at the following conditions: solvent: cyclohexanone (CHN); nonsolvent: ethylene glycol (EG); temperature: 130°C, isothermal; contact‐time: 2 h per fraction; amount of polymer: 2 mg/g support (glass beads). An essential improvement with respect to the separation of high molecular weight species can be achieved, applying a re‐deposition method of the polymer remaining in the column after half of the fractionation steps. This diminishes the unfavorable formation of a molecular weight‐ and concentration gradient along the column, occurring during fractionation. The optimally controlled single‐stage discontinuous solution fractionation of PVC turns out to be superior to multistage processes. This is attributed to the very critical problem of formation of insoluble microcrystalline aggregates with increasing duration of stay of the polymer in the column. The efficiency of the fractionation technique can be established by the characterization of fractions (center fractions: M̄w/M̄n < 1.1) and by confirmation of small differences in molecular weight distributions, shown by other methods.