Anomalous periodic structure of polypropylene chains observed with scanning tunneling microscopy

Using low-current scanning tunneling microscopy (STM), isolated strands of the insulating polymer isotactic polypropylene (iPP) supported, on highly o riented pyrolytic graphite (HOPG) have been studied. STM imaging can clearl y identify a 2.0 nm periodicity along the strand, which indicates an aggreg ation of polymer chains, known to exist in the alpha and beta crystalline f orms for iPP, which both exhibit, a lattice constant in one dimension near 2.0 nm. However, a structural conformation with repeat units of 4.0 nm is m ore commonly found. Tunneling spectroscopy on this conformation finds varia tions in the electronic structure across the repeat units which can be asso ciated with specific structural features an the chain. We suggest that this conformation derives from substrate-polymer interactions and is a modifica tion of the 3-1 helix.