1,721,081 research outputs found
A microscopic insight into the deformation behavior of semicrystalline polymers: the role of phase transitions
No full textThe deformation behavior of semicrystalline polymers associated with polymorphic transformations
under tensile deformation is discussed in the case of syndiotactic polypropylene. We report a phase
diagram of this polymer where the regions of stability of the different polymorphic forms are defined as a
function of the degree of stereoregularity and deformation. The values of critical strain corresponding to
the structural transformations depend on the stereoregularity that affects the relative stability of the
involved polymorphic forms and the state of the entangled amorphous phase
The role of crystals in the elasticity of semicrystalline thermoplastic elastomers
No full textThe role of crystals in the elasticity of semicrystalline polymers is discussed in the case of syndiotactic
polypropylene, which provides an example of a thermoplastic elastomer with a degree of crystallinity
that can be tailored by changing and controlling the stereoregularity. This can be achieved using metallocene
catalysts with different structures and stereoselectivity. The comparison of crystallization and physical
properties of samples of syndiotactic polypropylene of different stereoregularity, with rrrr pentad
concentrations being variable in the wide range 26-96%, prepared with different catalysts, has shown
that syndiotactic polypropylenes present different types of elastic behavior, depending on the degree of
crystallinity. For the most-stereoregular and crystalline samples with high melting temperatures, crystals
actively participate to the elastic response of the material and elasticity has a mainly enthalpic character
attributable to the metastability of the trans-planar form III that transforms into the more-stable helical
form II during elastic recovery. For less-crystalline samples, with low melting temperatures, elasticity
has instead a pure entropic origin as in conventional thermoplastic elastomers, and crystals act only as
knots of the physical elastomeric network
Miscibility study in fluorinated tetrafluoroethylene copolymer-copolymer blends
No full textThe miscibility of random fluorinated tetrafluoroethylene copolymers is investigated by
differential scanning calorimetry (DSC) and X-ray diffraction (XRD). In particular, mixtures composed
of poly(tetrafluoroethylene)-co-(hexafluoropropylene) (FEP) containing 1 mol % of comonomer and poly-
(tetrafluoroethylene)-co-(perfluoromethylvinyl ether) (PFMVE) containing from 2 to 10 mol % of comonomer
have been examined. The miscibility has been found to depend more upon the difference in the defect
(counit) concentration between the two components than upon the blend composition and the crystallization
conditions
Miscibility study in fluorinated tetrafluoroethylene copolymer-copolymer blends
No full textThe miscibility of random fluorinated tetrafluoroethylene copolymers is investigated by
differential scanning calorimetry (DSC) and X-ray diffraction (XRD). In particular, mixtures composed
of poly(tetrafluoroethylene)-co-(hexafluoropropylene) (FEP) containing 1 mol % of comonomer and poly-
(tetrafluoroethylene)-co-(perfluoromethylvinyl ether) (PFMVE) containing from 2 to 10 mol % of comonomer
have been examined. The miscibility has been found to depend more upon the difference in the defect
(counit) concentration between the two components than upon the blend composition and the crystallization
conditions
Conformational-analysis of Poly(methylene-1,3-cyclopentane) and Chain Conformation In the Crystalline Phase
No full textA conformational analysis for poly(methylene-1,3-cyclopentane) (PMCP) for different microstructures is presented. Independently of the microstructure of PMCP, extended conformations suitable for the disordered crystalline phase are geometrically and energetically feasible. However, strong restrictions to the conformational freedom, both of the rings and of the backbone, are required. The similarity between the experimental diffraction profiles of a PMCP sample and the Fourier transform calculations for isolated PMCP extended chains confirms that the crystalline phase of PMCP consists of parallel extended chains (configurationally and conformationally disordered) with a pseudohexagonal arrangement of the chain axes but with a nearly complete intermolecular rotational and translational disorder along the chain axes. The large increases of the entropy of melting with all the examined kinds of configurational order (cis or trans or isotactic) would be due to the related higher conformational order in the disordered crystalline phase
Phase diagram of syndiotactic polypropylene: influence of stereoregularity and temperature on the polymorphic behavior
No full textA method for building the phase diagram of syndiotactic polypropylene in stretched fibers as a
function of stereoregularity, degree of deformation, and temperature is reported. The method is based on the
analysis of the X-ray fiber diffraction patterns of samples of different stereoregularity, prepared with different
metallocene catalysts and stretched at different degrees of deformation and stretching temperatures. For all the
analyzed samples the boundary lines between the different regions of stability of the different polymorphic forms
have been determined by the emergence or disappearance of X-ray reflections typical of the different forms.
Although the phase diagram is given from a dynamic rather than a thermodynamic perspective, the polymorphic
forms observed by X-ray diffraction correspond to the equilibrium modifications, thermodynamically stable at
those values of stereoregularity and deformation
Structure and polymorphic behavior of high molecular weight poorly syndiotactic polypropylene
No full textAn analysis of the structure and polymorphic behavior of high molecular weight, low
stereoregular, poorly crystalline syndiotactic polypropylene (sam-PP), prepared with heterocycle-fused
indenylsilylamidodimethyltitanium complexes is presented. The samples do not crystallize by cooling
the melt to room temperature but slowly crystallize if they are kept at room temperature for several
days. Disordered modifications of the helical form I of syndiotactic polypropylene (s-PP) are obtained,
and small degrees of crystallinity (16-20%) are achieved. The stretching of compression-molded films of
sam-PP samples produces oriented crystalline fibers in the trans-planar mesomorphic form of s-PP. The
low stereoregularity ([rrrr] ) 40-55%) prevents the formation of the ordered trans-planar form III of
s-PP, which instead is obtained in stretched fibers of the highly stereoregular and crystalline s-PP. The
trans-planar mesomorphic form, in turn, transforms into the helical form I upon release of the tension in
stretched fibers of sam-PP samples. The isochiral helical form II of s-PP, which is generally obtained in
s-PP fibers initially in the trans-planar form III by removing the tension, has never been observed in the
sam-PP fiber samples. These data confirm that the isochiral form II of s-PP can be obtained only starting
from the trans-planar form III through a spontaneous cooperative crystal-crystal transformation when
the tension in stretched fibers is removed
Influence of regio- and stereo-regularity of propene insertion on crystallization behavior and elasticity of ethene-propene copolymers
No full textCocrystallization in blends of random tetrafluoroethylene fluorinated copolymers: the effects of the chain structure and crystallization conditions
No full textThe possibility of the cocrystallization of random fluorinated tetrafluoroethylene
copolymers was investigated with differential scanning calorimetry and wideangle
X-ray scattering. In particular, mixtures composed of poly(tetrafluoroethylene)-
co-(hexafluoropropylene) containing 8 or 1 mol % comonomer or poly(tetrafluoroethylene)-
co-perfluoromethylvinylether (2–10 mol%comonomer) were examined. The extent
of cocrystallization was determined by the difference in the comonomer content, being
higher when the difference was lower, and it was favored when quenching fromthe melt
state was adopted. Nevertheless, a key to determining the extent of cocrystallization
was the behavior of counits with respect to inclusion or exclusion from the crystal
lattice: when the components were different with respect to this behavior, they were not
likely to be miscible in the crystal state even if the difference in the comonomer content
was low. Moreover, the similarity in the crystallization rates between the components
played an important role: the cocrystallization decreased as the difference in the
crystallization rate increased until, when the difference became high enough, the blend
became immiscible
Molecular organization in the pseudo-hexagonal crystalline phase of ethylene-propylene copolymers
No full textA detailed X-ray diffraction pattern of an oriented sample of the ethylene-propylene copolymer (75 mol % of ethylene) in the pseudo-hexagonal form is presented. The comparison between the experimental diffraction intensities with the calculated intensities for ordered and disordered pseudohexagonal chain aggregates has allowed us to clarify some features of the structure of the pseudo-hexagonal form of the ethylene-propylene copolymers: (i) the methyl groups of the propylene monomeric units are included in the crystalline phase; (ii) the pseudo-hexagonal packing of the nearly trans-planar copolymer chains corresponds locally to relative shifts of neighboring chains nearer to those observed in the monoclinic and in the more common orthorhombic form of polyethylene; (iii) the ratio between the integrated intensities of the main peaks on the first layer line and on the equator can be accounted for by conformational disorder, implying a waviness of the nearly trans-planar chains and departures from strictly 180 degrees of internal rotation angles in the backbone chains; (iv) some further partial disorder (intermolecular translational along c and intermolecular rotational around c) should be introduced to account for the broadness of the nonequatorial peaks. A comparison between the presently proposed structure for the pseudo-hexagonal form of the ethylene-propylene copolymer with other disordered structures of homopolymers and copolymers with a hexagonal arrangement of the chain axes is also presented
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