Chapter 9
Morphology of Polymer Chains by Transmission Electron Microscopy 1
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Yvon P. Carignan , Masanori Hara , and Frederic Cosandey l
U.S. Army Armament Research, Development and Engineering Center, Picatinny Arsenal, NJ 07806-5000 Department of Mechanics and Materials Science, Rutgers, The State University of New Jersey, Piscataway, NJ 08855-0909
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Thin films of tagged (Iodinated) polystyrene dispersed in a polystyrene matrix have been studied by Transmission Electron Microscopy. Photomicrographs reveal the presence of random dark spots on an otherwise structureless background. These spots are interpreted as being the 2-D projection of the random coil chains of tagged polystyrene. Their count per unit area of the photomicrograph i s directly proportional to the ratio of tagged polystyrene to polystyrene in the films. Varying the time of exposure under the electron beam (0 to 120 minutes) indicates that the polymer chains are not affected by the electron beam. Information is presented on the dimensions of the polymer coil and the occurrence of aggregation of the polymer chains at high mixing ratios (1:1) of the tagged polystyrene to polystyrene. Great advances have been made in the study of polymers during the last twenty years principally because of the introduction and development of three techniques based on interference phenomena, namely, smallangle neutron (SANS), x-ray (SAXS) and light (SALS) scattering. The fraction of the radiation scattered is in SANS related to the nature of the scattering nucleus, in SAXS to the electron densities and in SALS to polarizabilities or refractive indices. These techniques have proven v i t a l , from both experimental and theoretical points of view in addressing polymers either as homophase amorphous systems, heterophase amorphous or crystalline systems. For instance one of the most significant accomplishment i n modern polymer science has been the confirmation of Flory's fundamental hypothesis through the application of SANS, namely, polymer chains exist as random coils in amorphous bulk polymers (1).Flory's hypothesis has been used extensively to explain various properties of amorphous polymers. One example of its successful application can be found in Kramer's work on the deformation behavior of glassy polymers, where he uses the end-to-end distance of the random coil as the basic parameter (2). 0097-6156/90AM4(M)136$06.00A) © 1990 American Chemical Society Sacher et al.; Metallization of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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CARIGNANETAL.
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Transmission Electron Microscopy
In spite of a l l the recent success which can be traced to the three techniques, SANS, SAXS, SALS, there are serious l i m i t a t i o n s i n t h e i r scope (3). For instance only average values, such as root-meansquare end-to-end distance and radius of gyration of a polymer chain can be extracted from these techniques. In other words, these techniques cannot provide information on the actual conformation or dimension of the i n d i v i d u a l polymer chain. Also since a l l three techniques are i n d i r e c t , the analysis of the raw data i s not straightforward and requires various manipulations and f a i t h i n e x i s t i n g theories to draw conclusions. F i n a l l y with regard to SANS, i t must be noted that only a l i m i t e d number of f a c i l i t i e s i n the U.S.have t h i s c a p a b i l i t y , thus r e s t r i c t i n g i t s use. In the present work we have taken the challenge of developing a d i r e c t technique which could provide the ultimate information on the actual morphology and dimensions of the i n d i v i d u a l polymer chains. We were convinced that i f successful, an experimental technique which would give such detailed information on polymer chains would not only be received by researchers and technologists as a major breakthrough towards the s i m p l i f i c a t i o n of their e f f o r t s i n understanding and improving the performance of current and future polymeric materials but would also sort out the best, most r e a l i s t i c theory and model among the many being offered today to explain the properties of polymers. Towards t h i s objective, we have turned to the concept of mass thickness contrast microscopy, which could be exploited by tagging the polymer under study with a heavy atom, and a f t e r forming t h i n films of the tagged polymer dispersed i n a matrix of untagged polymer, perform examination of the films by Transmission Electron Microscopy (TEM). Polystyrene (PS) was chosen as the polymer mainly because of i t s a v a i l a b i l i t y over a wide range of molecular weights (M - 1x10 to 2xl0 ) and very low p o l y d i s p e r s i t y (M /M
