7.4 Syndiotactic Polypropylene: Processing and Rheology
Syndiotactic polypropylene has proven to process very differently from conventional i-PP, both in melt-phase processes and in solid-phase processes. At high
shear rates, s-PP is more viscous and has higher elastic modulus than conventional
i-PP, regardless of the degree of the tacticity. s-PP, has also a much smaller melt
flow than that of the corresponding i-PP with the same molecular weight. s-PP has a
lower crystallization rate than i-PP, and minor modifications are necessary when
existing processing equipment designed for conventional i-PP is used for s-PP,
e.g., s-PP needs a longer cycle time.
With a melting point of approximately 130
C (versus 160
C for i-PP)
and crystallinity of approximately 30% (versus 55% for i-PP) for commercially
produced s-PP, the discrepancies and the differences in solid-phase properties and
processing seem obvious. As for the melt phase, the narrow molecular weight
distribution of 2 for metallocene catalyst-based s-PP provides only part of the
explanation for the observed differences in melt-phase properties. A cursory
comparison with controlled-rheology i-PP (an ex-situ, extruder tailormade i-PP of
very narrow MWD) makes one realize that must still be other reason(s), for the
observed melt behavior. Polymer microtacticity could be one parameter to
considered. When considering the molecular networks in high molecular weight
polymer melts, the molecular weight and MWD are usually the only parameters
compared at length for such linear polymers (long-chain branching does not usually
exist in ZN polypropylenes). The effects of polymer chain tacticity are widely
ignored in these rheological treatments. One reason is probably that it does not seem
obvious why there should be a difference between amorphous polymer melts of
i-PP and s-PP, each containing freely rotating molecules of similar chemistry
(equal monomer units) in motion. However, polymer microtacticity appears to be
the only other parameter that could provide the necessary explanations for the
observed melt property differences for s-PP and i-PP.
Rheological studies have confirmed [219–221] that s-PP forms significantly
more entanglements in the melt than its isotactic counterpart. Melt property effects
are thus justified with this inherent difference in molecular chain flexibility and
entanglement in mind. It is generally accepted that above a minimum (critical)
molecular weight for each polymer type, there exists an equilibrium level
(or distribution) of molecular entanglements in the molten phase. These chain
entanglements contribute in expected ways to the observed rheological behaviors.
In particular, they are imagined to act as minor “crosslinks,” providing resistance to
flow beyond the normal friction of molecule against molecule. In addition, each
polymer backbone defines a characteristic flexibility (or rigidity) for interaction
with its neighboring molecules. This also contributes to a polymer molecule’s
ability to move and fold around and amongst other like molecules in the melt.
These two properties are obviously related.
The calculations performed by Wheat [220] for i-PP and s-PP also give
surprising differences in these quantities. Essentially, based on this study, s-PP is
104
A. Razavi
Syndiotactic polypropylene has proven to process very differently from conventional i-PP, both in melt-phase processes and in solid-phase processes. At high
shear rates, s-PP is more viscous and has higher elastic modulus than conventional
i-PP, regardless of the degree of the tacticity. s-PP, has also a much smaller melt
flow than that of the corresponding i-PP with the same molecular weight. s-PP has a
lower crystallization rate than i-PP, and minor modifications are necessary when
existing processing equipment designed for conventional i-PP is used for s-PP,
e.g., s-PP needs a longer cycle time.
With a melting point of approximately 130
C (versus 160
C for i-PP)
and crystallinity of approximately 30% (versus 55% for i-PP) for commercially
produced s-PP, the discrepancies and the differences in solid-phase properties and
processing seem obvious. As for the melt phase, the narrow molecular weight
distribution of 2 for metallocene catalyst-based s-PP provides only part of the
explanation for the observed differences in melt-phase properties. A cursory
comparison with controlled-rheology i-PP (an ex-situ, extruder tailormade i-PP of
very narrow MWD) makes one realize that must still be other reason(s), for the
observed melt behavior. Polymer microtacticity could be one parameter to
considered. When considering the molecular networks in high molecular weight
polymer melts, the molecular weight and MWD are usually the only parameters
compared at length for such linear polymers (long-chain branching does not usually
exist in ZN polypropylenes). The effects of polymer chain tacticity are widely
ignored in these rheological treatments. One reason is probably that it does not seem
obvious why there should be a difference between amorphous polymer melts of
i-PP and s-PP, each containing freely rotating molecules of similar chemistry
(equal monomer units) in motion. However, polymer microtacticity appears to be
the only other parameter that could provide the necessary explanations for the
observed melt property differences for s-PP and i-PP.
Rheological studies have confirmed [219–221] that s-PP forms significantly
more entanglements in the melt than its isotactic counterpart. Melt property effects
are thus justified with this inherent difference in molecular chain flexibility and
entanglement in mind. It is generally accepted that above a minimum (critical)
molecular weight for each polymer type, there exists an equilibrium level
(or distribution) of molecular entanglements in the molten phase. These chain
entanglements contribute in expected ways to the observed rheological behaviors.
In particular, they are imagined to act as minor “crosslinks,” providing resistance to
flow beyond the normal friction of molecule against molecule. In addition, each
polymer backbone defines a characteristic flexibility (or rigidity) for interaction
with its neighboring molecules. This also contributes to a polymer molecule’s
ability to move and fold around and amongst other like molecules in the melt.
These two properties are obviously related.
The calculations performed by Wheat [220] for i-PP and s-PP also give
surprising differences in these quantities. Essentially, based on this study, s-PP is
104
A. Razavi