Chemistry Reference
In-Depth Information
these materials reveal phase separation but the phases vary in size, shape, and
sharpness of boundaries depending on the basic miscibility of the component
polymers, the cross-link density in the two polymers, and the polymerization
method. Some affinity of the components is needed for ordinary interpenetrating
networks because they must form solutions or swollen networks during synthesis.
This may not be required for IPNs based on latex polymers, where the second
stage monomer is often soluble in the first, cross-linked latex polymer.
Blends of elastomers are routinely used to improve processability of unvulca-
nized rubbers and mechanical properties of vulcanizates like automobile tires.
Thus, cis-1,4-polybutadiene improves the wear resistance of natural rubber or SBR
tire treads. Such blends consist of micron-sized domains. Blending is facilitated if
the elastomers have similar solubility parameters and viscosities. If the vulcanizing
formulation cures all components at about the same rate the cross-linked networks
will be interpenetrated. Many phenolic-based adhesives are blends with other poly-
mers. The phenolic resins grow in molecular weight and cross-link, and may react
with the other polymers if these have the appropriate functionalities. As a result,
the cured adhesive is likely to contain interpenetrating networks.
Mechanical Interlocking of Components. In some instances the polymers in a
blend may be prevented from demixing because of numerous mutual entangle-
ments produced by mechanical processing or the polymerization history of the
blend.
If the melt viscosities of polypropylene and poly(ethylene terephthalate) poly-
mers are reasonably matched under extrusion conditions, a finely dispersed blend
may be produced in fiber form. Orientation of such fibers yields strong filaments
in which microfibrils of the two partially crystallized polymers are intertwined
and unable to separate. Similar fibers with a sheath of one polymer surrounding a
core of the other have no mechanical integrity
[30]
.
Enhanced hydrophilicity or dyeability can be conferred on some acrylonitrile-
based polymers by polymerizing them in aqueous media containing polyacryl-
amide. In this case, also, two separate phases exist but the zones of each compo-
nent are too highly interpenetrating to permit macro separation and loss of
mechanical strength.
Thermoplastic polyolefins (TPOs) are based on blends of polypropylene with
ethylene-propylene rubbers. Many perform well as hose, exterior automotive trim,
and bumpers without chemical linking of the main polymeric components.
5.5.3.4
Use of “Compatibilizing Agents”
[20]
Mixtures of immiscible polymers can be made more stable by the addition of
another material that adheres strongly to the original components of the blend.
Plasticizers perform this function if a single plasticizer solvates the dissimilar
major components of a blend. Phthalate esters help to stabilize mixtures of poly
(vinyl chloride) and poly(methyl methacrylate), for example. These materials are
also plasticizers for polystyrene, and stable blends of this polymer with poly(vinyl
