Chemistry Reference
In-Depth Information
have different viscosities, the more fluid component will take up most or all of
the imparted strain, particularly if it is the major ingredient of the mixture. Thus
it is easy to melt blend a minor fluid component with a major, more viscous
ingredient, but a minor viscous component may swim in a more fluid sea of the
major component without being dispersed. Similar considerations apply if there
are serious mismatches in the melt elasticities of the components of a mixture.
It is also well known in compounding technology that the quality of a disper-
sion may be sensitive to the condition of the blend that is fed to the mixing
machine and in some cases also to the order in which the ingredients are added to
the mixer.
Some mixers provide dispersive as well as laminar mixing. In dispersive mix-
ing, the volume elements of the compound are separated and shuffled. Dispersive
mixing processes can be added to laminar mixing operations by introducing mix-
ing sections into extruder screws or installing stationary mixers in the extruder
discharge sections.
The correlation between quality of a laminar mixture and the total shear strain
that the material has undergone applies particularly to blends of polymers. When
hard or agglomerated components are being mixed, however, it is necessary to
subject such materials to a high shear stress gradient, and special equipment and
processes have been developed for such purposes. The rubber and coatings indus-
tries in particular abound with examples of such techniques.
Special note should be taken of the difficulty of forming intimate mixtures of
some semicrystalline polymers, and particularly of polyethylenes. Experimental
and theoretical studies have shown that local structure persists in such polymers
even at temperatures above the T
m
measured by differential scanning calorimetry
(DSC). These structures consist of folded chain domains in polyethylenes and of
helical entities in polypropylene. That is to say, in these polymers, at least, the
lowest energy states of the uncrystallized material are characterized by minima in
free energy, rather maxima in entropy. Molecular dynamics simulations of mix-
tures of linear polyethylene and isotactic polypropylene indicate that the two spe-
cies will segregate into distinct domains in the melt even when the initial state
was highly interpenetrating
[24]
. Such domains also form in the mixtures of poly-
ethylenes with different branching characteristics
[25,26]
. The formation of
locally ordered regions is expected to be more significant for longer polymer
chains. From a theoretical point of view, such observations imply that the mixing
of polymers cannot be described adequately by a purely statistical model as in the
original Flory
Huggins formulation. This theory has been generalized by some
researchers to decompose the interaction parameter,
, into enthalpic and entropic
terms, where the latter may be construed as reflecting “local structures”
[27]
.
Practically, the foregoing phenomena indicate the difficulty, or perhaps the
impossibility, of forming molecular level mixtures of polyethylenes with other
polymers, or with other polyethylenes, by conventional techniques which operate
on polymers in which some local order has already been established during poly-
merization. In a sense, then, “polyethylene is not compatible with polyethylene,”
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