posted on 2024-02-11, 17:04authored byAlessandra Cicolella, Fabio De Stefano, Miriam Scoti, Claudio De Rosa
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 (Ts,DI‑DII) only few degrees above the
end of the melting endotherm (Tm,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 Ts,DI‑DII and the end of the melting endotherm Tm,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.