Chemistry Reference
In-Depth Information
CHAPTER
6
Diffusion in Polymers
Acquire new knowledge whilst thinking over the old and you may become a
teacher of others.
—Confucius, about 551 B.C.
6.1
Introduction
Diffusion of low-molecular-weight compounds in polymers is an old topic of sig-
nificance to a wide variety of industrial processes. Such processes encompass sep-
aration of gas mixtures using polymeric membranes
[1]
, drying of polymeric
coatings
[2]
, removal of undesired volatiles and non-reacted monomers from
freshly made polymers
[3]
, use of packaging films with certain barrier properties
[4]
, and more recently, delivery of hydrophobic drugs in a controlled manner
using block copolymers
[5]
, to name a few. The topic entitled Diffusion in
Polymers by J. Crank and G. S. Park, published in 1968, is a classical reference
for the workers in the field
[6]
. However, as researchers have accumulated more
fundamental understanding of such diffusion processes at the molecular level, one
is able to design polymers for the intended applications in a much more efficient
manner. And diffusivity of small molecules in polymers, which is at the heart of
many such applications, is no longer a phenomenological constant that can only
be obtained experimentally but also can be predicted using various theoretical
models.
This chapter will focus on the study of the diffusion of small molecules in
polymers above their glass transition temperatures (T
g
). However, some of the
theories introduced here are not limited to T
T
g
(e.g., the Darken equation for
mutual diffusion and the free volume theory for self-diffusion). Diffusion of unen-
tangled polymer chains in solutions and melts will also be discussed. However,
diffusion of entangled chains will not be discussed as the subject requires the
understanding of the reptation theory, which is more complex and beyond the
scope of this volume. A detailed description of the reptation theory is given in
reference
[7]
. Only diffusion involving binary mixtures is of interest here, as mul-
ticomponent systems increase the mathematical complexity while not providing
much further insight into the fundamental concepts involved.
.
