Polymer Mixtures - Polymer Science and Engineering - page 256

2. Many synthetic polymers are essentially nonpolar and do not participate in
specific interactions like acid base reactions, hydrogen-bonding, or

dipole dipole interactions. In that case, intermolecular interactions are of the

van der Waals type and Δ H m in Eq. (5-1) is positive. The only contribution to

a negative S m term.

If specific interactions do occur between the components of a polymer
blend, the mixing process will be exothermic (

Δ G m then comes from the small Δ

H m negative) and
miscibility can be realized. Water-soluble polymers are often miscible with
each other, for example, because they participate in hydrogen bonding.
3. The most widely used method for predicting mixture stability relies on the
selection of ingredients with matching solubility parameters and hydrogen-
bonding tendencies, as outlined earlier. A small or negative Flory

Δ

Huggins
interaction parameter value is also characteristic of a stable mixture.
Predictions of blend stability can be made quickly from tabulations of
solubility and Flory

Huggins parameters. Although such calculations are very
useful, they cannot be expected to be universally accurate because the
solubility parameter model does not take account of polymer molecular
weight and the Flory

Huggins parameter may be concentration dependent.

5.5.3.2 Reliance on Slow Diffusion Rates
High-Molecular-Weight Polymers. A given blend of two or more polymers can be
made more stable by decreasing the molecular weights of the components to the
level of oligomers, as mentioned above in connection with polymer miscibility.
When a particular blend is not sufficiently stable, it can also paradoxically be
improved in this regard by increasing the molecular weights of the ingredients.
Since demixing is a diffusional process, it can be reduced to an acceptable level
by using higher molecular weight, more viscous polymers. The difficulty of dis-
persing such materials to a fine level is correspondingly increased, of course, but if
this can be achieved the rate of segregation will also be retarded.
Cocrystallization. An additional factor that is operable in some cases involves
the ability of the ingredients of a mixture to cocrystallize. These components can-
not then demix since portions of each are anchored in the ordered regions in
which they both participate. This may be particularly useful for hydrocarbon poly-
mers where favorable enthalpies of mixing do not exist.
Copolymers of ethylene, propylene, and unconjugated diene (EPDM) poly-
mers vary in their usefulness as blending agents for polyethylene. It has been
shown that EPDMs with relatively high levels of ethylene can cocrystallize
with branched polyethylene or high-ethylene-content copolymers of ethylene
with vinyl acetate or methyl methacrylate [28] . Such blends are stable and
may have particularly good mechanical properties. Ethylene/propylene copoly-
mers can serve as compatibilizing agents for blends of polypropylene and low
density polyethylene. Those copolymers which have residual crystallinity
because of

longer ethylene sequences are preferable to purely amorphous