Chemistry Reference
In-Depth Information
is prereacted with an excess of phenol in the presence of the nylon, but without any catalyst, at
temperatures high enough to cause condensation. This is followed by addition of toluenesulfonic acid
at lower temperatures. At that point, when free formaldehyde is no longer present in the reaction
mixture to cause gelation, the novolac molecules attach themselves to the nylon backbones. The
excess phenol is washed away, leaving pure graft copolymers [
347
].
Yagci and coworkers reported a special preparation of perfectly alternating poly(
-phenylene)
amphilitic graft copolymers by combination of controlled free-radical polymerization and Suzuki
coupling process [
345
].
p
9.7 Block Copolymers
These polymers consist of two or more strands of different polymers attached to each other. There
does not appear to be any general stipulation to the minimum size of each block. There does appear to
be, however, a general agreement that each sequence should be larger than just a few units. In
describing a block copolymer, it is helpful if the following structural parameters are available to
characterize the material:
1. Copolymer sequence distribution as well as the length and the number of blocks.
2. The chemical nature of the blocks.
3. The average molecular weight and the molecular weight distribution of the blocks and of the
copolymer.
Block copolymers, particularly of the A—B—A type, can exhibit properties that are quite different
from those of random copolymers and even from mixtures of homopolymers. The physical behavior
of block copolymers is related to their solid state morphology. Phase separation occurs often in such
copolymers. This can result in dispersed phases consisting of one block dispersed in a continuous
matrix from a second block. Such dispersed phases can be hard domains, either crystalline or glassy,
while the matrices are soft and rubber-like.
An interesting example of block copolymers is work by de Ruijter et al. [
348
], who prepared a
series of block copolymers that contain rigid liquid crystal forming blocks of poly(
-phenylene
terephthalamide) and flexible blocks of hexamethylene adipamide. The polymers have been prepared
in a one-pot procedure by a low-temperature polycondensation reaction in
p
N
-methyl-2-pyrrolidone.
9.7.1 Block Copolyesters
Two polyester homopolymers can react and form block copolymers in a molten state at temperatures
high enough for ester interchange [
414
]. As the reaction mixtures are stirred and heated, the
interchanges initially involve large segments. With time, however, smaller and smaller segments
form as the transesterifications continue. To prevent eventual formation of random copolymers, the
reactions should be limited in time.
Ester interchange can be retarded, particularly when esterification catalysts like zinc or calcium
acetate are present by addition of phosphorous acid or triphenyl phosphite [
415
]. This improves the
chances of forming block copolymers. The procedure can be applied to preparation of block
copolymers of poly(ethylene terephthalate) with poly(ethylene maleate), poly(ethylene
citraconate), and poly(ethylene itaconate) [
416
]. With ester interchange catalysts, like titanium
alkoxides or their complexes, melt randomization may be inhibited by adding arsenic pentoxide
that deactivates them [
417
].
