stretching and relaxation; the helical form transforms by stretching into the transplanar form, which transforms again into the helical form by releasing the tension,
and a nearly total recovery of the initial dimensions of the fiber samples is observed.
When the crystalline domains are already oriented along the stretching direction,
i.e., when the sample has already undergone the plastic deformation, the fibers show
good elastic properties. Of course, during the mechanical cycles the chains in the
amorphous regions are also subjected to a reversible conformational transition from
the “random coils” into extended conformations, and vice versa.
This reversibility of the transition is possibly assisted and is, somehow, favored
by the polymorphic transition occurring in the crystalline regions. It has been
suggested that although the driving force that induces the recovery of the initial
dimensions in common elastomers upon release of the stress is mainly entropic, in
the case of s-PP it is basically linked to the enthalpy gain achieved when the sample
is relaxed due to the metastability of the trans-planar form III. On the other hand, it
has been also hypothesized that the elastic behavior is mainly due to the conformational transition occurring in the amorphous phase for chains connecting the
crystalline domains.
7.2 Thermal Properties and the Origin of Multiple Melting
Behavior of s-PP
Differential scanning calorimetry (DSC) studies that were performed on the
isothermal bulk crystallization and subsequent melting behavior of various s-PP
resins from laboratory and commercially produced samples, indicate that most DSC
endothermic traces exhibited two (or three) distinct melting endotherms, depending
on the temperature at which the samples were crystallized. The DSC scans
performed at different heating rates on as-polymerized or melt-crystallized forms
of such samples generally present endotherms with a double peak shape. The
increase in the enthalpy of the high-melting peak with decreasing heating rate is
indicative of the occurrence of a recrystallization phenomenon during the heating
process, whereby the less-ordered form II is partially transformed to the more
ordered form I. This observation was made mainly for s-PP samples with stereoregularity lower than 90% (rrrr < 90%); disordered modifications of form I
are always obtained by melt crystallization. For highly stereoregular samples
(rrrr > 93%), only a single endothermic peak is observed, independent of the
crystallization temperature of the isothermally melt-crystallized samples and the
heating rate. The equilibrium melting temperature, T m of the fully s-PP estimated to
be 182
C, which is close to the value of 186
C found for i-PP [128]. Multiple
melting behavior is not an exclusive phenomenon for s-PP. In fact, similar
observations have been made on a number of other semicrystalline polymers,
including some flexible polymers and some semi-stiff polymers.
102
A. Razavi
and a nearly total recovery of the initial dimensions of the fiber samples is observed.
When the crystalline domains are already oriented along the stretching direction,
i.e., when the sample has already undergone the plastic deformation, the fibers show
good elastic properties. Of course, during the mechanical cycles the chains in the
amorphous regions are also subjected to a reversible conformational transition from
the “random coils” into extended conformations, and vice versa.
This reversibility of the transition is possibly assisted and is, somehow, favored
by the polymorphic transition occurring in the crystalline regions. It has been
suggested that although the driving force that induces the recovery of the initial
dimensions in common elastomers upon release of the stress is mainly entropic, in
the case of s-PP it is basically linked to the enthalpy gain achieved when the sample
is relaxed due to the metastability of the trans-planar form III. On the other hand, it
has been also hypothesized that the elastic behavior is mainly due to the conformational transition occurring in the amorphous phase for chains connecting the
crystalline domains.
7.2 Thermal Properties and the Origin of Multiple Melting
Behavior of s-PP
Differential scanning calorimetry (DSC) studies that were performed on the
isothermal bulk crystallization and subsequent melting behavior of various s-PP
resins from laboratory and commercially produced samples, indicate that most DSC
endothermic traces exhibited two (or three) distinct melting endotherms, depending
on the temperature at which the samples were crystallized. The DSC scans
performed at different heating rates on as-polymerized or melt-crystallized forms
of such samples generally present endotherms with a double peak shape. The
increase in the enthalpy of the high-melting peak with decreasing heating rate is
indicative of the occurrence of a recrystallization phenomenon during the heating
process, whereby the less-ordered form II is partially transformed to the more
ordered form I. This observation was made mainly for s-PP samples with stereoregularity lower than 90% (rrrr < 90%); disordered modifications of form I
are always obtained by melt crystallization. For highly stereoregular samples
(rrrr > 93%), only a single endothermic peak is observed, independent of the
crystallization temperature of the isothermally melt-crystallized samples and the
heating rate. The equilibrium melting temperature, T m of the fully s-PP estimated to
be 182
C, which is close to the value of 186
C found for i-PP [128]. Multiple
melting behavior is not an exclusive phenomenon for s-PP. In fact, similar
observations have been made on a number of other semicrystalline polymers,
including some flexible polymers and some semi-stiff polymers.
102
A. Razavi