1,721,014 research outputs found
Self-Nucleation in Stereodefective Isotactic Polypropylene: The Impact of Stereodefects on the Melt Memory
No full textThe 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
ncrease 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
argely 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
Crystal structures and polymorphism of polymers: Influence of defects and disorder
Full text linkThe crystal structures and the polymorphism of polymers are described on the basis of the main principles that define the conformation of polymer chains in the crystalline state and the modes of packing of macromolecules. We show that the presence of defects and disorder in the crystals influences the polymorphic behavior. The cases of polymorphism of isotactic poly(butene) (iPB) and syndiotactic poly(styrene) (sPS) are ilustrated as examples. In the case of iPB, the effect of the presence of defects of stereoregularity and of comonomeric units on the crystallization of form I and form II is described as an example of alteration of the crystallization behavior because of the modification of both thermodynamic stability and crystallization kinetics from the melt of the polymorphic forms. The case of sPS is taken as an example of a very complex polymorphic behavior arising from the presence and development of structural disorder
Structure and mechanical properties of copolymers of syndiotactic polypropylene with branched monomers.
No full textCrystallization and elastic properties of syndiotactic propene-branched 1-olefins copolymers.
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Crystallization and elastic properties of syndiotactic propene-branched 1-olefins copolymers.
No full textAssessment of the Crystallographic Model for Initiation of Plastic Flow in Random Copolymers of Isotactic Polypropylene.
No full textMelting and crystallization behavior of binary blends of syndiotactic polypropylenes of different stereoregularity
No full textBinary blends of samples of syndiotactic polypropylene of different stereoregularity
have been prepared by mixing a more stereoregular and crystalline sample sPP1 having
[rrrr] = 78%, with low stereoregular and poorly crystalline samples samPP1 and samPP2
having [rrrr] = 54.6 and 45.8%, respectively, and a nearly atactic fully amorphous sample
amPP1 with [rrrr] = 26.5%. The blends show separate melting of the two components with
the more stereoregular component sPP1 melting at high temperature and the poorly
syndiotactic component melting at lower temperature. Crystals of sPP1 are formed by
cooling from the melt whereas more defective crystals of samPP1 and samPP2 are formed
by cold crystallization during aging at room temperature. The presence of the
strereoirregular component disturbs the melt crystallization of the stereoregular
component sPP1 and depression of the crystallization and melting temperatures of
sPP1 in the blends is observed. On the other hand, the cold crystallization at room
temperature of the irregular component is accelerated and favored by the presence of
the more stereoregular component sPP1, whose crystals act as crystallization nuclei. All
data indicate interactions between the components in the melt and amorphous state,
compatible with a good degree of mixing
Elastomeric properties and stress-induced phase transformations of syndiotactic poly(propylene-co-eicosene)
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