Biomedical Engineering Reference
In-Depth Information
15.3
Schematic representation of a polymer chain segment and MgO
particle interaction.
phenomenon is reflected by an increase in conductivity during isothermal
stabilization at low temperatures.
The degree of polymer chain and ceramic particle interaction can be
reduced by a heavier, fewer, and lower weight or volume percent of MgO.
For example, if the weight percent of MgO is maintained constant and the
particle size is reduced from 5
m to 20 nm, over 15 million additional
polymer-ceramic interaction sites are created and conductivity enhance-
ments occur. Table 15.1 shows that reducing particle size from 5
μ
mto
20 nm reduces the mass of a particle by seven orders of magnitude. These
lighter MgO particles become far more receptive to the segmental chain
motion of the polymer, leading to improved polymer chain and MgO
particle interaction. Such an interaction is reflected by an enhanced and
time-dependent conductivity.
It is imperative (from equation 15.5) that conductivity is directly related
to the mobility and concentration of ions. Because the concentration is
maintained constant in a given specimen, the mobility must increase to
account for the conductivity enhancement. It has been suggested that the
mobility of charge carriers in nanocomposites is increased by the annealing-
induced structural changes that occur because of an interaction between
polymer chains and ceramic particles (Kumar et al., 2001).
The fact that motions of polymer chains contribute to the transport of
ions in polymer electrolytes also has deleterious effects on the transport
number. The chain motion also facilitates transport of larger anionic species
and thus measured conductivity includes a contribution from both cationic
and anionic species. Lithium in PEO-based electrolytes is coordinated with
five oxygen atoms, and the probability of finding a similar site for lithium
transport is low. The polymer electrolytes have a lithium ion transport
number that may be as small as 0.3. As a result, when such materials are
used in a lithium cell, extensive concentration gradients are set up during use
and these affect their performance.
μ
