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Copolymerization 245

Miscible blend Immiscible blend

Star polymer
IPN

FIGURE 7.2 Polymer mixtures and special shapes.

such materials are generally not viewed as copolymers, though they are covered in this chapter
along with blends and dendrites. Many alloys contain a matrix that is a mixture of polymer types
with many of them containing ABS. ABS thermoplastics have a two-phase morphology consisting
of elastic particles dispersed in a styrene-acrylonitrile (SAN) copolymer matrix.
Interpenetrating polymer networks (IPNs) are described by Sperling to be an intimate combination
of two polymers, both in network form, where at least one of the two polymers is synthesized and/or
cross-linked in the immediate presence of the other. This is similar to taking a sponge cake; soaking in
it warm ice cream, and refreezing the ice cream, resulting in a dessert (or a mess) that has both spongy
and stiff portions. Such IPNs, grafts, blocks, and blends can offer synergistic properties that are being
widely exploited. Some of these special polymer mixtures and special shapes are given in Figure 7.2.

7.15 DENDRITES
Along with the varying structures given in the previous sections, there exist other structurally com-
plex molecules called dendrites developed by a number of scientist, including Tomali and Frechet.
These molecules can act as “spacers,” “ball bearings,” and as building blocks for other structures.
Usually, they are either wholly organic or they may contain metal atoms. They may or may not be
copolymers depending on the particular synthetic rout employed in their synthesis.
While some make a distinction between dendrimers and hyperbranched polymers, we will not
do so here. In essence, hyperbranched polymers are formed under conditions that give a variety of
related but different structures while dendrimers are formed one step at a time giving a fairly homo-
geneous product.
Dendrites are highly branched, usually curved, structures. The name comes from the Greek
name for tree “dendron.” Another term often associated with these structures is “dendrimers”
describing the oligomeric nature of many dendrites. Because of the structure, dendrites can contain

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