Chemistry Reference
In-Depth Information
CHAPTER
5
Polymer Mixtures
Nature is a rag merchant who works up every shred and ort and end into
new creations; like a good chemist whom I found the other day, in his
laboratory, converting his old shirts into pure white sugar.
—Ralph Waldo Emerson, Conduct of Life: Considerations by the Way, Chapter 5
5.1
Compatibility
The term compatibility is often assumed to mean the miscibility of polymers with
other polymers, plasticizers, or diluents. Decisions as to whether a mixture is
compatible are not always clear-cut, however, and may depend in part on the par-
ticular method of examination and the intended use of the mixture.
A common criterion for compatibility requires the formation of transparent
films even when the refractive indices of the components differ. This means that
the polymer molecules must be dispersed so well that the dimensions of any seg-
regated regions are smaller than the wavelength of light. Such a fine scale of seg-
regation can be achieved most readily if the components are miscible. It is
possible, however, that mixtures that are otherwise compatible may appear not to
be, by this standard, if it is difficult to produce an intimate mixture. This may
happen, for example, when two high-molecular-weight polymers are blended or
when a small quantity of a very viscous liquid is being dispersed in a more fluid
medium.
Another criterion is based on the observation that miscible polymer mixtures
exhibit a single glass transition temperature. When a polymer is mixed with com-
patible diluents the glass
rubber transition range is broader and the glass transi-
tion temperature is shifted to lower temperatures. A homogeneous blend exhibits
one T
g
intermediate between those of the components. Measurements of this prop-
erty sometimes also show some dependence on mixing history or on solvent
choice when test films are formed by casting from solution.
Heterogeneous blends with very fine scales of segregation may have very
broad glass transition regions and good optical clarity. It is a moot point, then,
whether such mixtures are compatible. If the components are not truly miscible,
