Lattice imaging in melt crystallized polypropylene thin films

Melt crystallized isotactic polypropylene thin films of thickness between 30 and 100 nm have been investigated by high-resolution transmission electron microscopy at room temperature. The c-axis projection of the 2*31 helices and their packing in the lattice were clearly visible in flat-on lamellae of the α-phase following reconstruction from the components of the image Fourier transform corresponding to the (110) and (040) lattice planes, and the image power spectra also indicated contributions from (130) and (060) relfections, corresponding to a line resolution of about 0.35 nm. These results are discussed in terms of Bloch wave calculations based on the generally accepted structure for the α-phase. Attempts to obtain lattice images of the β-phase in isotactic polypropylene and melt crystallized syndiotactic polypropylene under similar operating conditions are also briefly discussed, although these provided relatively little structural informatio

Physical, chemical and mechanical aspects of the durability of (semicrystalline) thermoplastics

The term durability designates the capacity of a material to withstand for extended periods of time (aggressive) environmental action. Depending on the intended use of a material, durability can be expressed in terms of time to failure, creep deformation or residual strength or discoloration. In all cases it is highly desirable to know the mechanisms involved in ageing and their kinetics. In this presentation we discuss the mechanical behaviour of stabilised and aged polypropylene. Using different methods of characterisation we try to identify the physical mechanisms occurring during ageing (chain scission, recristallisation)

Time Lapse in Vivo Visualization of Developmental Stabilization of Synaptic Receptors at Neuromuscular Junctions

The lifetime of nicotinic acetylcholine receptors (AChRs) in neuromuscular junctions (NMJs) is increased from <1 day to >1 week during early postnatal development. However, the exact timing of AChR stabilization is not known, and its correlation to the concurrent embryonic to adult AChR channel conversion, NMJ remodeling, and neuromuscular diseases is unclear. Using a novel time lapse in vivo imaging technology we show that replacement of the entire receptor population of an individual NMJ occurs end plate-specifically within hours. This makes it possible to follow directly in live animals changing stabilities of end plate receptors. In three different, genetically modified mouse models we demonstrate that the metabolic half-life values of synaptic AChRs increase from a few hours to several days after postnatal day 6. Developmental stabilization is independent of receptor subtype and apparently regulated by an intrinsic muscle-specific maturation program. Myosin Va, an F-actin-dependent motor protein, is also accumulated synaptically during postnatal development and thus could mediate the stabilization of end plate AChR