Lec-16
Crystallization, Morphological Structure and Melting
Behavior of Immiscible Polymer Blends
Introduction
From a commercial point of view, semicrystalline polymers are of
prime importance. Among the four mostly used commodity plastics (PE,
PS, PVC and PP), only PS is completely amorphous.
The three semicrystalline polymers account for the largest volume of
the commercial polymer blends. A majority of the polymer blends
contains at least one crystalline component. Most polymer blends are
immiscible.
The immiscible semicrystalline polymer blends may be classified in
terms of:
1-Crystalline/crystalline systems in which both components are
crystallizable, and
2-Crystalline/amorphous systems in which only one component can
crystallize, being either the matrix or the dispersed phase.
The properties of the finished articles made from immiscible blends are
governed by the morphology created as a result of the interplay of
processing conditions and inherent polymer characteristics, including
crystallizability.
Therefore, a scientific understanding of the crystallization behavior in
immiscible polymer blends is necessary for the effective manipulation
and control of properties by compounding and processing of these blends .
In table below, an overview is given of a number of important
immiscible crystallizable blend systems.
Factors Affecting the Crystallization Behavior of Immiscible
Polymer Blends:
The discussion on the crystallization behavior of neat polymers would
be expected to be applicable to immiscible polymer blends, where the
crystallization takes place within domains of nearly neat component,
largely unaffected by the presence of other polymers.
However, although both phases are physically separated, they can exert
a profound influence on each other. The presence of the second
component can disturb the normal crystallization process, thus
influencing crystallization kinetics, spherulite growth rate, semicrystalline
morphology, etc.
►Important factors are:
1- Molecular structure and molecular mass of the components
2- Blend composition
3- Type and degree of dispersion of the phases in T the melt state
4- Phase interactions (nature of the interface, migration of nuclei, etc.)
5- Melt history (Tmelt , tmelt , etc.),
6- Crystallization conditions (for example Tc, cooling rate, etc.),
7- Physical crystallization conditions (surrounded by melt or solidified
material).
These factors influence the crystalline morphology development,
resulting in changes of crystallization parameters such as:
1- Nucleation density (N)
2- Spherulite growth rate (G)
3- Overall crystallization rate (K)
4- Total degree of crystallinity (Xc)
5- Semicrystalline morphology, i.e., shape, size and texture of the
spherulites, interspherulitic boundaries, etc.
To discuss these topics in a systematic way, a distinction will be made
between three main blend categories, namely:
1. Blends with a crystallizable matrix and an amorphous dispersed phase,
2. Blends with an amorphous matrix and a crystallizable dispersed phase,
3. Blends containing two crystallizable components
1-Blends with a Crystallizable Matrix & an Amorphous
Dispersed Phase:
In immiscible blends, the phases are separated in the molten state,
before crystallization of the matrix starts. The dispersed amorphous phase
is assumed to be homogeneously distributed in the melt in droplet-like
domains.
1.1 Nucleation Behavior of the Crystallizable Matrix
When a crystallizable component forms the matrix phase in a polymer
blend, nucleation can occur via heterogeneous nucleation by
heterogeneities in a similar way as in the pure component.
The heterogeneities, available in the melt, can be residual catalysts,
fillers, impurities, crystalline residues (due to incomplete melting), etc.
Since in immiscible blends the phases are physically separated, the same
heterogeneities that nucleate the homopolymer at Tc,pure may nucleate the
crystallizable matrix. As a result, the crystallization temperature, Tc, of
the blend during cooling from the melt will in general not differ that
much from the Tc of the pure component.
Some general principles governing the crystallization behavior of
homopolymers also remain valid for immiscible polymer blends in which
the crystallizable component forms the continuous phase.
♦ The premelting temperature, Tmelt, may have a profound influence on
the crystallization temperature of the matrix, Tc , during cooling from the
melt .
The higher the temperature at which the blend is kept in the melt prior to
crystallization, the less residual crystalline parts (otherwise leading to
self-seeded nucleation) remain in the melt. As a result, fewer nuclei are
available to nucleate the melt phase, thus leading to the formation of
fewer, but larger spherulites.
♦ Another, less important factor, is the isothermal crystallization
temperature, Tc,iso, when the crystallization is carried out at a constant
temperature . When a crystallization experiment is performed at lower
temperatures, the activation energy for nucleation of several types of
heterogeneities can be overcome. At that Tc,iso, more nuclei become
active, leading to the formation of a larger number of smaller spherulites.
NOTE:
Nucleation rate together with growth determine the size and number of
spherulites: slow nucleation and fast growth result in a small number of
large spherulites, whereas fast nucleation and slow growth result in a
large number of small spherulites
♦♦ Although most principles for the crystallization of homopolymers
remain valid for immiscible blends with a crystallizable matrix, the
crystallization behavior can be altered by two phenomena, inherently
correlated with immiscible two-phase systems, namely:
1- Migration of impurities during melt-mixing:
During the melt-mixing process, heterogeneous impurities can migrate
across the interface between both blend phases.
Several factors determine the ‘possibility’ for the impurities to migrate
from one phase to the other phase during the melt-mixing process.
The longer the mixing or the more intense the mixing, the higher the
probability that nuclei find themselves somewhere at an interface, where
they can easily migrate.
2-Nucleating activity of the interface between two phases:
In immiscible polymer blends with a high degree of immiscibility such
as PP/PS, it has been shown that nucleation at the interface affects the
crystallization behavior.
However, not all interfaces can produce additional nucleating centers.
For immiscible and highly incompatible polymer blends, since their
interfacial tension is higher, the interface is very sharp. Such interfaces
can rarely induce new nuclei.
Only an interface which wets well with the crystallizable matrix, so that
a crystalline chain can deposit on it, can cause heterogeneous nucleation .
The wetting ability between two melt phases can be calculated from the
spreading coefficient F12.
Blends with a Crystallizable Dispersed Phase in an Amorphous
Matrix
In immiscible polymer blends, the minor component often forms the
dispersed phase, whose shape and size are complex functions of:
1- The blend composition
2- The melt-viscosity of the dispersed phase and the matrix
3- The viscosity ratio
4- The interfacial tension
5- The processing conditions
It has been reported by several authors that crystallizable dispersed
droplets can exhibit the phenomenon of “fractionated crystallization”
originating from the primary nucleation of isolated melt particles by
species with different nucleating activity (heterogeneities, local chain
ordering)
Several factors can influence the fractionated crystallization behavior:
1- Thermal history of the sample
2- The degree of dispersion of the minor phase plays a crucial role.
Important factors here are the blend composition, the interfacial tension
between both components, the melt-viscosity of both components, the
processing device and mixing conditions, the blend preparation method,
etc.
In this context, it is interesting to evaluate also the influence of
compatibilization on the crystallization behavior of the dispersed
phase.
Since compatibilization reduces the droplet size of the minor phase
even more drastically, it can be expected that this can lead to a serious
shift of the crystallization temperature towards lower temperatures,
resulting in more pronounced fractionated crystallization, or even in a
homogeneous crystallization.
However, this issue is more complex due to numerous other factors
involved in the nucleation process. Some examples from the literature are
listed in Table below. They illustrate how differently the
compatibilization can influence the crystallization behavior of the
dispersed phase.
Conclusions:
It can be stated that the crystallization behavior of a semicrystalline
polymer phase, dispersed into an amorphous matrix, is characterized by:
1- Fractionated crystallization or homogeneous nucleation if the minor
phase is finely dispersed.
2- A decrease in the overall degree of crystallinity, Xc, after cooling from
the melt, most pronounced in finely dispersed blend morphologies.
3- A slight decrease of the melting temperature due to the formation of
less perfect crystalline lamellae at higher undercoolings.
Binary Polymer Blends Containing Two Crystallizable Phases
A large number of polymer blends consists of two crystallizable phases ,
hence more studies have been carried out on the thermal behavior of
crystalline/crystalline polymer blends.
The morphology of a polyblend consisting of two crystallizable
polymers can vary depending on :
1-The processing conditions
2- The relative rates
3- Temperature of crystallization of the constituent polymers.
These can either crystallize at the same time (coincident crystallization)
or separately in a sequential manner, leading to different morphologies
and hence different properties.
As such, in blends of two semicrystalline polymers, the physical
properties may be altered not only by the blend composition and the
phase morphology, but also by changing their relative crystallization
behavior. Therefore, it is important to study the effect of blending on the
crystallization behavior of each component in the blend, to understand the
structure development as influenced by melt-processing.
Nucleation rate together with growth determine the size
and number of spherulites: slow nucleation and fast growth result in a
small number of large spherulites, whereas fast nucleation and slow
growth result in a large number of small spherulites