Chemistry Reference
In-Depth Information
another low-molecular-weight polymer. For example, rigid poly(vinyl chloride) is
a hard solid material used to make credit cards, pipe, house siding, and other arti-
cles. Mixing with about 50
100 parts by weight of phthalate ester plasticizers
converts the polymer into leathery products useful for the manufacture of uphol-
stery, electrical insulation, and other items. Plasticizers in surface coatings
enhance the flow and leveling properties of the material during application and
reduce the brittleness of the dried film.
Some degree of solvency of the plasticizer for the host polymer is essential
for plasticization. Not surprisingly, a match of solubility parameters of the plasti-
cizer and polymer is often a necessary but not a sufficient condition for compati-
bility. In the case of PVC, the dielectric constants of the plasticizer should also be
near that of the polymer.
It is often useful to employ so-called “secondary plasticizers,” which have
limited compatibility with the host polymer. Thus, aliphatic diesters are poorly
compatible with PVC, but they can be combined with the highly compatible
phthalate ester plasticizers to improve low-temperature properties of the blend.
Continued addition of a plasticizer to a polymer results in a progressive reduc-
tion in the glass transition temperature of the mixture. This suggests that the plas-
ticizer acts to facilitate relative movement of macromolecules. This can happen if
the plasticizer molecules are inserted between polymer segments to space these
segments farther apart and thus reduce the intensity of polymer
polymer interac-
tions. Such a mode of action is probably characteristic of low-temperature plasti-
cizers for PVC, like dioctyl adipate. Plasticizers with more specific interactions
with the polymer will reduce the effective number of polymer
polymer contacts
by selectively solvating the polymer at these contact points. PVC plasticizers like
diisoctyl phthalate seem to act in the latter fashion.
Rubbers are plasticized with petroleum oils, before vulcanization, to improve
processability and adhesion of rubber layers to each other and to reduce the cost
and increase the softness of the final product. Large quantities of these “oil-
extended” rubbers are used in tire compounds and related products. The oil con-
tent is frequently about 50 wt% of the styrene
butadiene rubber. The chemical
composition of the extender oil is important. Saturated hydrocarbons have limited
compatibility with most rubbers and may “sweat-out.” Aromatic oils are more
compatible and unsaturated straight chain and cyclic compounds are intermediate
in solvent power.
5.4
Fractionation
The properties of a polymer sample of given composition, structure, and average
molecular weight are not uniquely determined unless the distribution of molecular
weight about the mean is also known. Methods to determine this distribution
include gel permeation (size exclusion) chromatography and various fractionation
techniques. Fractionation is a process for
the separation of a chemically
