Bulletin Number Four 1985

Department in 1967 by Professor Baysung Hsu and Dr. S.H. Kwan (now at the Hong Kong Polytechnic). The work was concentrated on polyethylene terephthalate (popularly known as Dacron or Mylar), an important polymer commonly used as textile fibres and insulating films in electronic components such as capacitors. The mechanical strength o f this material is often enhanced by drawing along one direction (in the case o f fibres) or in two perpen dicular directions (for films). The drawing process induces preferential alignment o f molecular chains, thereby leading to significant changes in the mechan ical and dielectric properties. These properties were studied as a function o f the extent o f stretching and were correlated w ith the degree o f chain orientation determined from X-ray and birefringence measure ments. Since 1969 several staff members, including Drs. C.L. Choy, F.C. Chen, K. Young and W.P. Leung have also undertaken research in polymer physics, extending the work in various directions. Brief summaries o f present projects are given below: (1) Strong Materials w ith High Thermal Conduc tiv ity Polymers have numerous advantages, including light weight, fle xib ility , chemical inertness and ease o f processing but they suffer from a major drawback —they aremuch weaker than metals. Consequently, an important goal o f many research institutes is to improve the stiffness and strength o f polymers through appropriate processing techniques. A commonly used method for strengthening polymers along one direction is drawing, by means o f which we have produced polyethylene fibres stronger than aluminium. This ultradrawn material has other interesting properties. For example, its ability to conduct heat is as good as that o f metallic alloys, an extremely rare attribute for electrical insulators. Therefore it would be a useful material in electronic systems requiring both electrical insulation and large dissipation o f heat. Structural measurements were made to pinpoint the origin o f the unusual properties and it was found that they arose from the presence o f perfectly aligned needle-like crystals produced during ultradrawing. (2) Control o f Thermal Expansion by Chain Orientation Many objects are made by joining poly mers to other types o f materials w ith lower expansion coefficient. As a result o f differential thermal contraction the jo in t may break when the temperature o f the environment changes. We have found that the expansion coefficient is sensitively dependent on the degree o f orien­ tation o f polymer chains and so are able to control the amount o f expansion through prior drawing. (3) Theoretical Prediction o f Ultimate Properties We have seen that the remarkable behav­ iour o f ultradrawn polymers merely reflects the properties o f needle-like crystals in these materials. To find out how far we can improve the thermal properties it is imperative to know the ultimate lim its. Our theoretical calculation shows that the thermal conductivity o f poly­ ethylene crystals along the chain axis is as high as that o f copper, one o f the best thermal conductors known. Surprisingly, the expansion coefficient along the chain direction has a negative value, meaning that the crystal con­ tracts in this direction when it is heated. (4) Composite Materials It was mentioned that very strong fibres can be prepared by drawing. However, in many applications, it is necessary to use objects o f various shapes and sizes. These can be made by embedding fibres in another polymer medium, forming what are known ascomposite materials. In collaboration w ith the Institute o f Chemistry o f Academia Sinica we have studied the physical properties o f composites containing glass or carbon fibres. In the future, we hope to replace these fibres by the strong organic fibres pro duced in our laboratory. (5) Non-destructive Testing by Ultrasonic Tech nique Ultrasonics is a well-established technique for the non-destructive testing o f a large variety o f materials but its application to oriented polymers has been hampered by the small sample size and strong attenuation o f ultrasonic waves. We have bu ilt an equipment which allows complete characterization o f small-strain mech anical properties over a wide temperature range from -60° to 150°C. Through the use o f this unique facility we have collaborated w ith several institutions, including the Univer sity o f Leeds, University o f Massachusetts and Japan Telegraph and Telephone Corpor ation. It is worth mentioning that poly- oxymethylene tubes o f small diameter are now used by Japan Telegraph and Telephone Corporation to strengthen optical fibre cables and we have been able to determine all the independent mechanical moduli o f these tubes. (6) Gas and Vapour Diffusion through Polymer Films The passage o f gas or vapour through 26 RECENT DEVELOPMENTS