Self-Nucleation in Stereodefective Isotactic Polypropylene: The Impact of Stereodefects on the Melt Memory

Abstract

The memory of crystals in the melt of stereodefective samples of isotactic polypropylene (iPP), characterized by different concentrations of rr stereodefects from 0.49 to 10.5 mol %, was analyzed. Experiments of self-nucleation and annealing have demonstrated that high contents of rr stereodefects, largely incorporated in the crystals of iPP, produce a significant memory of crystals in the melt that persists up to high temperatures well above the melting temperature. For low stereodefect concentrations (lower than 2−3 mol %), the memory of the crystals is erased at temperatures (T s,DI-DII ) only few degrees above the end of the melting endotherm (T m,end ), whereas for contents of rr defects higher than 3−4 mol %, the memory of crystals persists even upon heating at temperatures much above the end of the endothermic signal. The width of the heterogeneous melt Domain II, in terms of range of temperatures in the melt in which the memory exists and self-nucleation takes place, and the difference between the temperature at which the isotropic melt begins T s,DI-DII and the end of the melting endotherm T m,end increase with the increase of defects concentration. The higher the amount of stereodefects and the lower the melting temperature of iPP, the higher the temperature at which the self-nuclei must be heated to cancel the memory of crystals. These results indicate that a significant memory of iPP crystals exists in the melt not only in copolymers of iPP with noncrystallizable comonomeric units but also for iPPs containing small defects largely incorporated in the crystals. During crystallization of these stereodefective iPPs, the selection of the crystallizable segments of suitable length, which has been considered responsible for the formation of the heterogeneous melt and self-nuclei, should be less demanding thanks to the incorporation of stereodefects in the crystallizable sequences. However, upon successive heating to melt at low temperatures these highly irregular produced crystals, the diffusion and homogenization of all long and short sequences is in any case not easy, also considering the low temperature, and portions of partitioned sequences are left in the melt acting as efficient self-nuclei upon cooling and crystallization from the melt. The melt-memory attributed to these self-nuclei and the process of self-nucleation induce crystallization of the γ form, while crystallization from the isotropic melt induces crystallization of the α form, also in the case of samples with high concentrations of stereodefects that should crystallize in the γ form.

Melt memory in random ethylene-1-alkene copolymers

“…In addition, De Rosa et al reported a remarkable melt memory in propene–pentene isotactic copolymers 21 and stereodefective iPP. 35 Their results indicated that the melt memory effect of iPP, which persists in the melt up to high temperatures, is evident not only in copolymers with non-crystallizable comonomers, but also in those with small defects largely included in the crystals. Recently, the melt memory effect above T 0 m in rigid poly(arylene ether sulfone ketone) homopolymers has been reported.…”

Unusual Memory Effect of Crystallization Raised by 4-Phenyl-1-butene and 4-Anthryl-1-butene Comonomers in 1-Butene-Based Copolymers

“…Consequently, the thermal stability of the condensed structures in polymers extends beyond the crystal melting point and resists the formation of other possible structures associated with original crystallization. , The crystallization of polymers depends on the thermal properties. The residual structures retained in the molten state owing to the memory effect influence subsequent recrystallization, − accelerating the kinetics of crystallization. ,− Thus, the underlying mechanism governing the memory effect of the condensed structures of polymers should be studied for controlling the morphology and properties of the final crystallites.…”