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.