The role of co-units in polymer crystallization and melting: New insights from studies on syndiotactic poly(propene-co-octene)

The effect of noncrystallizable units on the crystallization and melting was studied for two octene copolymers of syndiotactic polypropylene, in an extension of previous investigations on a homopolymer sample with 3% meso dads. Using time and temperature dependent SAXS experiments and DSC, we determined the dependencies of the crystal thickness, the rate of crystallization, and the melting point on the chosen crystallization temperature. With an increase in the content of noncrystallizable units (octene units or meso dads; both show equal effects), we observed, as expected, a shift of the melting points to lower temperatures and similar shifts of the growth rate versus temperature curves, but surprisingly, no effect at all on the crystal thickness. The thicknesses of all three samples show a common temperature dependence, being inversely proportional to the supercooling below the equilibrium melting point of perfect syndiotactic polypropylene. The latter is located at 196 degrees C, as determined by an extrapolation based on measured melting points. Data demonstrate that the popular Hoffman-Weeks plot when applied to random copolymers does not yield the respective equilibrium melting points; it can only be used for perfect homopolymers. Crystal thicknesses and growth rates are, according to the observations, independent properties. The thicknesses are those of a specific native crystal form with high surface free energy. DSC experiments indicate that all crystals first form this native state and then become stabilized by relaxation processes that decrease the surface energy. These stabilization processes, which produce the difference between the temperatures of crystallization and melting, leave the crystallite thickness unchanged.