Polysulfone with lower levels of cyclic dimer: use of MALDITOF in the study of cyclic oligomers

Solutions of polysulfone in certain solvents can become cloudy due to a slight and partial precipitation of a cyclic oligomer, which has been characterized to be crystalline cyclic dimer of polysulfone containing two repeat units of polysulfone. This precipitation has been a source of manufacturing problems in a number of solution processing industries such as membranes, films and coatings. MALDI-TOF mass spectroscopy was successfully employed to establish the presence of not only of cyclic dimer but also of other cyclic oligomers such as trimer, tetramer, pentamer, hexamer and so on in commercially available polyarylsulfones. Solvay Advanced Polymers has developed a proprietary process for making polysulfone with lower levels of cyclic dimer and other cyclic oligomers. This commercially available low-cyclic dimer polysulfone resin offers a number of advantages to the solution processing industries such as long self-life, and less insoluble material that needs to be filtered. Reduction in cyclic oligomeric content also fortuitously reduced the polydispersity of the resin. For a given weight average molecular weight, increased number average molecular weights have been obtained for the low-cyclic dimer resin which is expected to increase the mechanical properties of the membranes.

We speculate that many condensation polymers that are assumed to contain linear polymer molecules, in general, may contain oligomeric cyclic polymer molecules and perhaps even macrocyclic molecules. Reduction of cyclic dimer in UDEL® polysulfone results in improve-ment in number of average molecular weight. Commercially available LCD grades from Solvay are better suited for solution processing industries.