temperature highly stereoregular samples prepared with metallocene catalysts.
Finally, a form IV was obtained by exposing fiber specimens in the zigzag transplanar form to various organic solvents (e.g., benzene, toluene) at room temperature.
This form is characterized by helices with (T 6 G 2 T 2 G 2 ) n conformation. A monoclinic
unit cell has been proposed for this modification with a ¼ 5.72 A ˚ , b ¼ 7.64 A ˚ , and
c ¼ 11.6 A ˚ (chain axis), α ¼ 73.1
, β ¼ 88.8
, and ϒ ¼ 112.0
. The hypothetical
X-ray diffraction powder spectrum of this form would present main peaks at
d ¼ 6.84, 5.26, and 4.46 A ˚ (2θ ¼ 29
, 16.8
, and 19.9
, Cu Kα).
The formation of the four crystalline modifications depends, as mentioned
above, on the conditions of crystallization and the stereoregularity of the polymer
sample. Form I, the thermodynamically stable form of s-PP is obtained under the
most common conditions of crystallization, from the melt and/or precipitation from
solution, in powder samples and single-crystals of s-PP. Form II is a metastable
polymorph of s-PP and has been obtained only in oriented fibers of s-PP and,
recently, by melt-crystallization at elevated pressure [203–212]. It can be also
obtained by room temperature stretching of compression-molded specimens of
low stereoregular s-PP samples prepared with vanadium-based Ziegler–Natta
catalysts [10–17] or upon releasing the tension in stretched fibers of highly stereoregular s-PP samples initially in form III.
In fact, when highly stereoregular samples, prepared with homogeneous
metallocene catalysts, are stretched at room temperature, the trans-planar form III
is obtained, which transforms into the isochiral helical form II by releasing the
tension. In powder of unoriented s-PP samples, only disordered modifications of
form II, characterized by conformationally disordered chains containing transplanar sequences (kink band defects) [213–215], can be obtained at atmospheric
pressure, for instance, by precipitation from solutions of low stereoregular samples
and in copolymers of s-PP with ethylene as the co-monomeric units. The metastable
polymorphic forms II, III, and IV generally transform into the stable form I by
annealing.
The complex polymorphic behavior of s-PP influences its macroscopic
properties. For instance, oriented fibers of s-PP show an elastic behavior that is
related to the structural organization. Unoriented compression-molded samples of
s-PP behave like a typical highly crystalline material showing a plastic deformation
upon stretching at room temperature. The crystalline domains, with chains in
helical conformation, tend to assume a preferred orientation along the stretching
direction to originate a plastic, irreversible, deformation. High orientation of the
crystalline phase is generally achieved. Along with this plastic deformation, a phase
transition from the most stable helical form into form III, with chains in the transplanar conformation, gradually occurs. The phase transition is reversible: after
release of the tension, the crystalline domains remain nearly oriented with the
c-axis parallel to the preferred (stretching) direction, and the trans-planar form III
transforms again into the more stable helical form. As a result, for previously
unoriented material, a partial recovery of the macroscopic dimensions of the sample
is attained. Therefore, unoriented samples show only fair or poor elastic properties.
Stress-relaxed fibers show instead very good elastic behavior upon successive
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
101
Finally, a form IV was obtained by exposing fiber specimens in the zigzag transplanar form to various organic solvents (e.g., benzene, toluene) at room temperature.
This form is characterized by helices with (T 6 G 2 T 2 G 2 ) n conformation. A monoclinic
unit cell has been proposed for this modification with a ¼ 5.72 A ˚ , b ¼ 7.64 A ˚ , and
c ¼ 11.6 A ˚ (chain axis), α ¼ 73.1
, β ¼ 88.8
, and ϒ ¼ 112.0
. The hypothetical
X-ray diffraction powder spectrum of this form would present main peaks at
d ¼ 6.84, 5.26, and 4.46 A ˚ (2θ ¼ 29
, 16.8
, and 19.9
, Cu Kα).
The formation of the four crystalline modifications depends, as mentioned
above, on the conditions of crystallization and the stereoregularity of the polymer
sample. Form I, the thermodynamically stable form of s-PP is obtained under the
most common conditions of crystallization, from the melt and/or precipitation from
solution, in powder samples and single-crystals of s-PP. Form II is a metastable
polymorph of s-PP and has been obtained only in oriented fibers of s-PP and,
recently, by melt-crystallization at elevated pressure [203–212]. It can be also
obtained by room temperature stretching of compression-molded specimens of
low stereoregular s-PP samples prepared with vanadium-based Ziegler–Natta
catalysts [10–17] or upon releasing the tension in stretched fibers of highly stereoregular s-PP samples initially in form III.
In fact, when highly stereoregular samples, prepared with homogeneous
metallocene catalysts, are stretched at room temperature, the trans-planar form III
is obtained, which transforms into the isochiral helical form II by releasing the
tension. In powder of unoriented s-PP samples, only disordered modifications of
form II, characterized by conformationally disordered chains containing transplanar sequences (kink band defects) [213–215], can be obtained at atmospheric
pressure, for instance, by precipitation from solutions of low stereoregular samples
and in copolymers of s-PP with ethylene as the co-monomeric units. The metastable
polymorphic forms II, III, and IV generally transform into the stable form I by
annealing.
The complex polymorphic behavior of s-PP influences its macroscopic
properties. For instance, oriented fibers of s-PP show an elastic behavior that is
related to the structural organization. Unoriented compression-molded samples of
s-PP behave like a typical highly crystalline material showing a plastic deformation
upon stretching at room temperature. The crystalline domains, with chains in
helical conformation, tend to assume a preferred orientation along the stretching
direction to originate a plastic, irreversible, deformation. High orientation of the
crystalline phase is generally achieved. Along with this plastic deformation, a phase
transition from the most stable helical form into form III, with chains in the transplanar conformation, gradually occurs. The phase transition is reversible: after
release of the tension, the crystalline domains remain nearly oriented with the
c-axis parallel to the preferred (stretching) direction, and the trans-planar form III
transforms again into the more stable helical form. As a result, for previously
unoriented material, a partial recovery of the macroscopic dimensions of the sample
is attained. Therefore, unoriented samples show only fair or poor elastic properties.
Stress-relaxed fibers show instead very good elastic behavior upon successive
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
101