Studies of Polymers with Radiation
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oduction Research into polymers began slowly. While polymers have long been considered to be very useful materials, today they are receiving greater recognition as subjects for conducting good science. Polymers are interesting materials to study by physical means. We can study their size, miscibility, deformation, aggregation and crystallization, and the nature of their surfaces. All of these properties are important in their application. Polymers, like many other materials, exhibit several types of transitions. They can phase-separate in the amorphous stage, and they can crystallize. In this article, I will primarily talk about the crystallization of polymers. As an undergraduate student in polymer science, I wrote my first paper,1 which is now 55 years old. It was fun to learn about radiation at that stage, and I profited very much by doing that research. I was inspired by a visit to Brooklyn Polytechnic University by Nobel Laureate Peter Debye, for one of the exciting Saturday-morning lectures arranged by Herman Mark, the pioneer in polymer education in the United States. Debye described the use of lightscattering from polymer solutions for the determination of molecular weights and dimensions.2 Such studies arose from the early scattering observations by Tyndall, the theory and experiments by Lord Rayleigh, and their extension to fluctuating liquids and solids by Einstein. Much of the early work was carried out by the Indian school
MRS BULLETIN/OCTOBER 2000
of Raman and Bhagavantam. Debye was involved in the American “crash program” during World War II to develop synthetic rubber. He realized that this technique could provide a much-needed means for characterizing the new rubbers that were being synthesized. My thesis advisor, Paul Doty (whose 80th birthday I recently helped celebrate), immediately changed the topic of my thesis, and we went on to carry out the first measurement of the size of a polymer molecule by this method, which is based on the fact that the phase of the scattered wave is dependent on the distance it travels. If we look at the variation of scattering with angle, the phase difference increases with increasing angle at a rate dependent on the size of the particle (Figure 1). To measure particle size by this method, we must use radiation having a wavelength commensurate with the size of what one is trying to measure. Just as we use radar waves to look at airplanes, we can use x-rays to examine small molecules and crystals, and de Broglie waves for nuclear and atomic phenomena.
Light scattering is a very good technique for studying big polymer molecules, since their dimensions are comparable with the wavelength of visible light. Generally, a polymer molecule does not have definite dimensions; it is statistical—a wormlike entity that is moving around—so one must describe its dimensions statistically, in terms of an average size such as its radius of gyration. One can get a measure of this by studying the variation in the intensity of the scattering with scattering angle. The bigger the molecul
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