Ionic Polymer–Metal Nanocomposites(IPMNCs): Fundamentals - Artificial Muscles - page 40

2.5

Sample dimension: L eff = 20 mm and W = 5 mm
Cation: Li +
1 V step @1/2 Hz

Improved IPMC/Nafion-117
(treated by additives)
Conventional IPMC/Nafion-117

2.0

1.5

1.0

0.5

0.0

0 1 2 3 4 5

T (sec)

FIGURE 2.20(a) Force response characteristics of the improved IPMNC versus the conven-
tional IPMNC. Note that the improved IPMNC is one treated by an effective dispersing agent.

Figure 2.21(a) depicts an SEM, an IPMNC film treated with a dispersing agent
(top), and its x-ray line-scan (bottom), while figure 2.21(b) depicts the profiles of
platinum concentration versus penetration for different permeability coefficients.
Note that figure 2.20(b) depicts TEM micrographs of two samples of IPMNC
with and without PVP treatment, showing how the addition of PVP causes the
nanoparticles of platinum to not coalesce and create a uniform and fairly homoge-
neous distribution of particles. This is believed to create more uniform internal
electric fields and cause the increased force capability of IPMNCs and IPCNCs.
As can be seen, the Pt penetration is increasingly homogeneous and consistent
using a dispersant during the reduction process. Note from figures 2.21(a) and 2.21(b)
that a good platinum penetration is achieved, meaning that an effective additive
enhances platinum dispersion and leads to better penetration in the polymer. A
convenient way to handle this situation (free diffusion into finite porous slab or
membrane) is to use an effective diffusivity, D eff , and then to consider it as a one-
dimensional problem. Assuming fast kinetics for the metal precipitation reaction of
(Pt(NH3)4) 2+ + 2e - ≥ Pt 0 + 4NH3, the precipitated platinum concentration, N x , can

be expressed as

CL
C

() δ δ

Pt t t

N = = 1 − erf ( ) (2.7)

x

4 Dt

Pt i ,

eff

δ t are the platinum concentration, the platinum
concentration at the interface, and the particle penetration depth, respectively.
For a typical reduction time of t = 15 min (in fig. 2.21(a) and 2.21(b)), equation
(2.7) is plotted for values of D eff = 1

where notations C Pt ( δ t ), C Pt,i , and

10 -8 cm 2 s -1 , respectively.
The effective diffusivity, D eff , could be estimated to be of the order of 1

× 10 -10 , 1 × 10 -9 , and 1 ×

× 10 -8 cm 2 s -1