Chemistry Reference
In-Depth Information
In the coextrusion and lamination of polymers the individual layers are some-
times inherently nonadhering. An expedient to improve the strength of such multi-
layer structures involves the use of intermediate “glue” layers between surfaces that
do not adhere well. Copolymers are often useful in such glue layers, particularly
when the copolymer contains a comonomer that adheres well to one of the surfaces
and a comonomer that interacts or is miscible with the other polymer to be bonded.
Acid-containing copolymers are often prepared for this purpose. Ionomers consist-
ing of partially neutralized ethylene/methyl methacrylate copolymers have been
employed to bond polyethylene with nylons and poly(butylene terephthalate). In
this case the acid component of the copolymer is capable of hydrogen-bonding
interactions with the nylon or polyester. There is also the potential for some inter-
change between functional groups in the two polymers during melt processing.
Graft Copolymers. Graft copolymers themselves may exhibit a two-phase mor-
phology and this influences their behavior in blends. The morphological structure
that is observed depends on the relative volume fractions of the backbone and
graft polymers and their mutual affinity. If separation occurs it will be on a
microphase scale because of the chemical linkages between the two polymer
types. Amorphous graft copolymers often have good transparency (if there is no
crystalline component) because of the small scale of segregation.
The structure of graft copolymers is generally more complex than that of
block polymers in that the trunk polymer may be joined to more than one grafted
branch and the nature of the production of such copolymers is such that cross-
linking also may occur. For this reason the microphase separation that is observed
in graft copolymers alone is less distinct and regular than that seen with block
copolymers of the same species.
The component of the graft copolymer that is present in the larger concentra-
tion will normally form the continuous phase and exert a strong influence on the
physical properties of the unblended material. If both phases are present in nearly
equal volume fractions, fabrication conditions will determine which component
forms the continuous phase.
Graft copolymers decrease the particle size of the dispersed phase in a binary
homopolymer mixture and improve the adhesion of the dispersed and continuous
phases. The copolymers accumulate at interfaces because parts of the graft are
repelled by the unlike component of the blend. They do not necessarily form
optically homogeneous mixtures with homopolymers for this reason.
The major application of graft copolymers is in high-impact polystyrene
(HIPS), ABS, and other rubber-toughened glassy polymers. The morphology of
such blends depends on their synthesis conditions. They are normally made by
polymerizing monomers
in which the elastomer
is dispersed. The elasto-
mer
monomer mixture will tend to form the continuous phase initially but stir-
ring in the early stages of the polymerization of the glassy polymer produces a
phase inversion with a resulting dispersion of monomer-swollen rubber in a poly-
mer/monomer continuous phase. When polymerization is completed, the result is
a dispersion of rubbery particles in the rigid matrix.
