Chemistry Reference
In-Depth Information
Requirements for rubber toughening of glassy polymers include (1) good
adhesion between the elastomer and matrix, (2) cross-linking of the elastomer,
and (3) proper size of the rubber inclusions. These topics are reviewed briefly in
the order listed:
1.
Rubber-matrix adhesion. If adhesion between the glassy polymer and
elastomer is not good, voids can form at their interfaces and can grow into a
crack. The required adhesion is provided by grafting. Affinity between the
matrix and rubber is not needed in such cases. Thus, polybutadiene, which has
less affinity for polystyrene than styrene
butadiene copolymer, is a better
rubbery additive for polystyrene. The butadiene homopolymer has a lower
glass transition temperature and remains rubbery at faster crack propagation
speeds than the styrene
butadiene copolymer. The inherently poorer adhesion
of the polybutadiene and the matrix is masked by the effectiveness of
polystyrene
polybutadiene grafts.
2.
Cross-linking of rubber. A moderate degree of cross-linking in the rubbery
phase of the graft copolymer is required to optimize the contribution of the
rubbery phase in blends with glassy polymers. Inadequate cross-linking can
result in smearing out of the rubbery inclusions during mechanical working of
the blend, while excessive cross-linking increases the modulus of the
inclusions and reduces their ability to initiate and terminate the growth of
crazes.
3.
Particle size. In general a critical particle size exists for toughening different
plastics. The impact strength of the blend decreases markedly if the average
particle size is reduced below this critical size. The decrease in impact
strength is not as drastic when the particle size increases beyond the optimum
value, but larger particles produce poor surfaces on molded and extruded
articles and are of no practical use.
Block Copolymers. Block and graft copolymers have generally similar effects
of collecting at interfaces and stabilizing dispersions of one homopolymer in
another. Most graft copolymers are made at present by free radical methods
whereas most commercial block copolymers are synthesized by ionic or step
growth processes. As a result, the detailed architecture of block copolymers is
more accurately known and controlled.
Many block copolymers segregate into two phases in the solid state if the
sequence lengths of the blocks are long enough. Segregation is also influenced by
the chemical dissimilarity of the components and the crystallizability of either or
both components. This two-phase morphology is generally on a microscale with
domain diameters of the order of 10
2
6
10
2
5
cm.
The critical block sizes needed for domain formation are greater than those
needed for phase separation in physical mixtures of the corresponding homopoly-
mers. This is because the conformational entropy of parts of molecules in the block
domains is not as high as in mixtures, since placement of segments is restricted by
the unlike components to which they are linked. Thus, the minimum molecular
