CONNECTION BETWEEN POLYMER MOLECULAR-WEIGHT, DENSITY, CHAIN DIMENSIONS, AND MELT VISCOELASTIC PROPERTIES

One of the main goals of polymer science has been to relate the struct ure of macromolecular chains to their macroscopic properties. In parti cular, it has been hoped that one could relate the sizes of polymer co ils to the degree to which they entangle with one another and thus to their viscoelasticity in the melt. In recent years, the availability o f model polymers with nearly monodisperse molecular weight distributio ns and precisely controlled chemical structures has allowed for improv ed data both on rheology and on the dimensions of the chains. This has now allowed us to determine the correlations between such properties as chain dimensions, density, and plateau modulus and to show that som e quite simple relations exist between them. The main body of these da ta is on polymers that can be considered to be models for polyolefins. These have been made by the hydrogeneration of polydienes synthesized by anionic polymerization techniques. In this way the molecular weigh t distribution can be made to be nearly monodisperse (M(w)/M(n) < 1.1) and the chemical structure is well controlled. For example, models of a wide range of ethylene-butene copolymers have been made by the satu ration of polybutadienes with a range of vinyl content. Such polymers can be made at many molecular weights as well. The viscoelastic proper ties of these polymers have been measured very precisely, and their ch ain dimensions have been determined by small-angle neutron scattering. To a high degree of correlation, we find that the mean-square unpertu rbed end-to-end distance, <R2>0, the density, rho, and molecular weigh t, M, are related to the plateau modulus, G(N)0, G(N)0 is-proportional -to {<R2>0rho/M}3 a finding in accord with that of Ronca. This simple relationship gives us a deep understanding of what controls the rheolo gy of these polyolefins and of how we might be able to predict the pro perties of as yet unsynthesized polymers.