TIT# Nanostructure Evolution in Polypropylene During Mechanical
     Testing
AUT# Stribeck, Norbert; Nöchel, Ulrich; Funari, Sérgio S.; Schubert, Tom;
     Timmann, Andreas;
SOU# Macromol. Chem. Phys. (2008), ~, submitted
LOC# xv107
CLA#
COM#
APP#
MAT#
ABS# We monitor slow mechanical tests of highly oriented, hard-elastic
     polypropylene by in situ X-ray scattering. For the analysis
     two-dimensional small-angle X-ray scattering (SAXS) and
     ultra-small-angle X-ray scattering (USAXS) patterns are combined.
     Direction of strain is the normal to the crystalline lamellae of
     the polymer. The scattering patterns are transformed into a
     representation of nanostructure in real space. This is the
     multidimensional cord distribution function (CDF). From the CDF
     topological parameters of the semicrystalline nanostructure are
     extracted and discussed in conjunction to the mechanical data.
     In a continuous-strain experiment we observe fracture of an
     ensemble of weak lamellae into blocks already at an elongation of
     2%. These blocks are completely dissolved during further
     stretching up to 10% (yield point). At higher elongations we
     observe continuous transformation from crystalline lamellae into
     an ensemble of rigid needles with 8 nm thickness. The material
     breaks, as all the lamellae are consumed.
     In a fatigue test a load-reversal experiment is carried out
     between elongations of 10% and 35%. At the beginning of every
     straining branch strain-induced crystallization generates
     extended lamellae until a stress of 20 MPa is reached. Thereafter
     breakup of the lamellae sets in. In the further course of the
     straining branch an unclear superposition of various mechanisms
     is observed (e.g. the melting of weak blocks as well as the
     strain crystallization of new and strong blocks). In the
     relaxation branch of each load cycle we first observe fast
     relaxation-induced melting, which is continuously slowing down.