Crystallization of low molecular weight atactic polystyrene

We observe and characterize the crystallization of atactic polystyrenes (PS) of nearly oligomeric M_w using atomic force microscopy. We find that the low M_w polystyrene exhibits observable crystals on the surface. The crystals appear to be a few nm thick and nm to microns wide. These crystals grow at all temperatures less than ∼290 K. Melting of crystals was probed over an extended temperature range, and some fraction of the crystals start to melt at 302 K, but some fraction persist to higher temperatures and do not exhibit complete melting until 343 K. The tacticity of the molecules is tested with NMR spectroscopy and found to be atactic. We suggest that the crystals form due simply to the distribution of isomerism along the molecule which necessarily leaves some fraction of the molecules with uniform stereoregularity. This natural crystallinity may be related to previously observed and not definitively explained gel formation in atactic PS (a-PS), as well as cluster formation. The measurements are compared with the theory by Semenov (Macromolecules, 2009, 42, 6761) and together suggest that such crystallinity is possible over a wide range of polymerization index (N), and is limited only by the vanishingly small volume fractions and sluggish growth.