Bulletin Summer 1975

Set-upfor thermal conductivity experiments on polymers The polymer research work undertaken by a group of members of the Physics Department is concerned with a wide spectrum of the physical characteristics of polymers, so as to give a deeper understanding of their basic structures. For instance, polymer is widely used as insulating materials in electrical cables and appliances, so its dielectric response under the influence of a strong varying external electric field is of great industrial importance. Such behaviour is experimentally studied at one of the physics laboratories, at temperatures between - 1 95 and 200°C and covering a wide frequency range. One of the aims is to determine the dipole moments in various structural regions of the polymers. The thermal characteristics of a polymer is closely connected with its utility as fabric and construction materials. Its thermal properties, i.e. heat capacity and conductivity, are therefore also of much practical interest. They are a measure of the energy absorption and flow arising from the wiggling of the spaghetti-like giant molecules under thermal agitation, and hence provide a key to understanding the structures and interactions of such molecules. Considerable work has been done in the Department on the measurement of heat capacities down to liquid nitrogen temperature (-196°C), and this measurement is being extended down to liquid helium temperature (-269°C). Work on both the dynamic and static measurements of thermal conductivity of polymers over a wide temperature range has also been initiated. Apart from electrical and thermal characteristics, the mechanical properties of polymers are also of much practical interest, not only because of one's natural concern with its strength and shock absorption capability as a commonly used material but also due to the recently revealed possibility that it could attain extremely high tensile strength (with an elastic modulus comparable to steel) by a drawing process. There are at present two separate experiments on the mechanical properties. One measures the strength and energy absorption of polymers under slowly varying tension, whereas the other studies the mechanical properties from the velocity and absorption of ultra-sound (that is, mechanical vibrations at frequencies of about 10 million cycles per second) transmitted through polymer samples. Work in the mechanical area will shortly be expanded by the addition of a programmable Instron tensile machine, which would make possible the measurement of the stress-strain relation at different rates of stretching. Recent Advances in Holographic Fourier-Transform Spectroscopy Since the first successful operation of lasers some fifteen years ago, physicists all over the world are exploring the many exciting experiments with lasers, and one example is holography. Within a few years the scope of holography grew larger and larger as many applications were found in various fields such as optical data processing, photography, vibration and stress analysis, and microscopy, etc. In 1965, G. Stroke pioneered a new branch: holographic fourier-transform spectroscopy (abbreviated HFTS). HF TS has a short history and its performance is still below that of grating spectroscopy. Stroke's result on Hg spectrum gave only the strongest lines, and among these the yellow doublet 5770, 5790A was unresolved. Later experiments by Japanese groups did not resolve this doublet either. Mr. L . K. Su started repeating Stroke's experiment in 1974, and discovered that he could obtain a resolved yellow doublet in the Hg spectrum, while the theory of Stroke predicted that this doublet would not be resolved under the experimental conditions of Su. Dr. S.Y. Feng later joined Mr. Su in the effort to find a satisfactory explanation. They discovered that Stroke's theory contained an error and they constructed a simple theoretical model which could explain the experimental result. However, their experimental result was a very marginal one ； for example, a light touch on the bench, or a very slight misalignment of the spectrometer, would readily change the resolved doublet into an unresolved single line. At this juncture, the progress and difficulty of HF TS drew the attention of Prof. S.T. Hsue of the University of Northern Iowa, who was then a visitor 12