Abstract (Buckley Crist Seminar--Nov. 20, 1998)
It is well known that short chain branching decreases polymer crystallinity, crystal thickness and melting temperature, and has derivative effects on properties such as elastic modulus, etc. The recent development of single site catalysts has renewed interest in ethylene-alpha olefin copolymers. Hydrogenated polybutadienes, which emulate random ethylene-butene copolymers, are used to compare experiment to the predictions of Flory's equilibrium theory for copolymer crystallization. While equilibrium theory presupposes the crystallization of long ethylene segments, this does not occur for understandable kinetic reasons. Experimental melt crystallization does occur at a temperatures low enough to stabilize crystals corresponding to populous segment lengths, and the observed melting range is substantially below that from theory. While the final melting temperature can be used to estimate the thickest of the crystals present in observable quantity, the shape of the melting curve provides no direct information on the distribution of crystal thickness in copolymers. Isothermal crystallization results in a bimodal distribution of crystal thicknesses. A heuristic model underscores important similarities and differences in crystallization of homopolymers and copolymers.