Supaphol et al. [216–218] reviewed a number of hypotheses that have been
proposed to explain the occurrence of multiple melting endotherm phenomena.
In the studies on isothermal crystallization under quiescent conditions (i.e., crystallization is only a function of temperature), the multiple melting behavior of these
semicrystalline polymers have been attributed to the following four phenomena:
(1) the presence of two (or more) crystal modifications in the sample; (2) the
presence of two (or more) crystalline morphologies; (3) the presence of two
populations of crystal lamellae of different thicknesses; and (4) the simultaneous
melting, recrystallization, and re-melting of the lamellae initially formed at the
crystallization conditions.
7.3 Syndiotactic Polypropylene: Physical and Mechanical
Properties
Compared to conventional i-PP produced with ZN catalysts, the s-PP produced
commercially with supported metallocene catalyst has, due to its relatively low
crystallinity, on the one hand, lower density, hardness, tensile strength, and flexural
modulus and, on the other hand, higher clarity or transparency, impact strength,
toughness (or elasticity), and stiffness. The melting point, density, hardness,
crystallization temperature, heat of fusion, tensile strength at break, flexural
modulus, and percent haze of s-PP all increase with increasing syndiotacticity of
the polymer. The crystallization rate (defined customarily as the inverse of the time
needed to attain one half of the final crystallinity multiplied by 100), crystallization
temperature, and melting point are also all affected by the degree of stereoregularity
of the polymer chains. The low temperature impact strength of s-PP decreases with
decreasing temperature, but at 0
C it has still about two to three times higher impact
strength than that of i-PP. Syndiotactic polypropylene also has different thermal
properties to conventional i-PP and its glass transition temperature and heat of fusion
are lower than those of i-PP. Unique properties of s-PP include good γ-ray internal
stability and very low melting point for heat sealing. s-PP has also different thermal
properties than the conventional i-PP. Its heat of fusion is lower than that of i-PP.
Syndiotactic polypropylene also has different rheological behavior to i-PP; for
example, regardless of the degree of its tacticity, s-PP has a much smaller melt
flow than i-PP with comparable molecular weight. The rheological behavior of s-PP
seems to indicate that, owing to its more flexible chain backbone structure, it
contains a significantly higher number of molecular entanglements per unit volume
than i-PP (see following section).
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
103
proposed to explain the occurrence of multiple melting endotherm phenomena.
In the studies on isothermal crystallization under quiescent conditions (i.e., crystallization is only a function of temperature), the multiple melting behavior of these
semicrystalline polymers have been attributed to the following four phenomena:
(1) the presence of two (or more) crystal modifications in the sample; (2) the
presence of two (or more) crystalline morphologies; (3) the presence of two
populations of crystal lamellae of different thicknesses; and (4) the simultaneous
melting, recrystallization, and re-melting of the lamellae initially formed at the
crystallization conditions.
7.3 Syndiotactic Polypropylene: Physical and Mechanical
Properties
Compared to conventional i-PP produced with ZN catalysts, the s-PP produced
commercially with supported metallocene catalyst has, due to its relatively low
crystallinity, on the one hand, lower density, hardness, tensile strength, and flexural
modulus and, on the other hand, higher clarity or transparency, impact strength,
toughness (or elasticity), and stiffness. The melting point, density, hardness,
crystallization temperature, heat of fusion, tensile strength at break, flexural
modulus, and percent haze of s-PP all increase with increasing syndiotacticity of
the polymer. The crystallization rate (defined customarily as the inverse of the time
needed to attain one half of the final crystallinity multiplied by 100), crystallization
temperature, and melting point are also all affected by the degree of stereoregularity
of the polymer chains. The low temperature impact strength of s-PP decreases with
decreasing temperature, but at 0
C it has still about two to three times higher impact
strength than that of i-PP. Syndiotactic polypropylene also has different thermal
properties to conventional i-PP and its glass transition temperature and heat of fusion
are lower than those of i-PP. Unique properties of s-PP include good γ-ray internal
stability and very low melting point for heat sealing. s-PP has also different thermal
properties than the conventional i-PP. Its heat of fusion is lower than that of i-PP.
Syndiotactic polypropylene also has different rheological behavior to i-PP; for
example, regardless of the degree of its tacticity, s-PP has a much smaller melt
flow than i-PP with comparable molecular weight. The rheological behavior of s-PP
seems to indicate that, owing to its more flexible chain backbone structure, it
contains a significantly higher number of molecular entanglements per unit volume
than i-PP (see following section).
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
103