Rheological and mechanical properties of poly(methyl methacrylate) doped with lithium salts

In this study, we compared the effects of adding two salts with different carbon chain lengths––LiCF3SO3 (lithium trifluoromethanesulfonate; LFMS) and LiC4F9SO3 (lithium nonafluorobutanesulfonate; LFBS)––on the uniaxial tensile properties of poly(methyl methacrylate) (PMMA). The samples were broken in a brittle fashion according to the salt concentration. The decrease in the entanglement density estimated from the viscoelastic properties and the longer average relaxation times in the flow region in the PMMA-salt systems indicate that the salts have a “pinning” effect on PMMA chains, causing them to be brittle, and the PMMA chains tend to be disentangled. This pinning effect was more pronounced for the LFMS sample with a shorter carbon chain length. The tensile toughness of the LFMS sample (with a shorter carbon chain length) was slightly reduced. Doping with LFMS enhanced ductility, indicating that the stronger pinning effect between the PMMA chains effectively modified brittle fracture. We found that the rheological behavior at the compression-molding temperature affects the brittle fracture behavior in tension.