The Microscopic Basis of the Glass Transition in Polymers from Neutron Scattering Studies

Recent neutron scattering experiments on the microscopic dynamics of polymers below and above the glass transition temperature T(g) are reviewed. The results presented cover different dynamic processes appearing in glasses: local motions, vibrations, and different relaxation processes such as alpha- and beta-relaxation. For the alpha-relaxation, which occurs above T(g), it is possible to extend the time-temperature superposition principle, which is valid for polymers on a macroscopic scale, to the microscopic time scale. However, this principle is not applicable for temperatures approaching T(g). Below T(g), an inelastic excitation at a frequency of some hundred gigahertz (on the order of several wave numbers), the "boson peak," survives from a quasi-elastic overdamped scattering law at high temperatures. The connection between this boson peak and the fast dynamic process appearing near T(g) is discussed.

[Hemobilia as a rare cause of gastrointestinal bleeding]

In a 67-year-old man with upper abdominal and gastrointestinal bleeding gastroscopy revealed a duodenal ulcer which was initially taken to be the source of the bleeding. But subsequent retrograde cholangiography demonstrated nearly complete occlusion of the right hepatic duct and a suspicious tumour-like structure. At laparotomy a tumour was excluded and the supplying artery to the ulcer was ligated to arrest the bleeding. Gastrointestinal bleeding recurred postoperatively so that selective angiography of the hepatic artery had to be performed. This demonstrated an aneurysm of a branch of the hepatic artery near the hilus. No further bleeding occurred after its embolization. Haemobilia is a rare cause of upper abdominal bleeding and may be difficult to diagnose, except by selective imaging techniques.