Isothermal crystallization and melting of isotactic polypropylene analyzedby time- and temperature-dependent small-angle X-ray scattering experiments

Structure evolution during isothermal crystallization of isotactic polyprop ylene and the changes during a subsequent heating to the melt were analyzed by time- and temperature-dependent small-angle X-ray scattering experiment s. Results demonstrate that isotactic polypropylene, when forming the a-pol ymorph, is governed by the same general laws for crystallization and meltin g as syndiotactic polypropylene. Crystal thicknesses are inversely proporti onal to the supercooling under a characteristic temperature T-c(infinity) w hich is located above the equilibrium melting point of a sample. The crysta llization line describing this dependence can be understood as representing the stability limit of a well-defined initial state with higher free energ y. Its transformation into the final lamellar two-phase structure provides the stabilization expressed in the difference between the crystallization a nd melting temperature. Observations and data from other sources can be see n as indicating that (i) crystal thicknesses are independent of the isotact icity and that (ii) the initial state is composed of crystal blocks in plan ar assemblies, in agreement with corresponding observations on syndiotactic polypropylene. The small-angle X-ray scattering experiments provide a deta iled insight into the structure changes during the continuous melting. For high crystallization temperatures no recrystallization occurs. Crystals wit h different stabilities, all having the same thickness, melt consecutively. Structures produced at low crystallization temperatures are less stable. H ere heating leads to repeated melting-recrystallization processes associate d with jumplike changes in the length scale of the structure.