Chemistry Reference
In-Depth Information
solvent exchange that causes a change in solvent polarity
10
,pH
11
and ionic
strength
12
. An example of a physical trigger is an electrical field, which is
one of the most attractive triggers for use. Polyelectrolyte gels contain ionic
species. These species can move inside the gel by diffusion (due to a con-
centration gradient) and/or by migration (due to the presence of an elec-
trical field). A reorganisation of positively and negatively charged ions
occurs to resist against the presence of the electrical field. A drawback of
ionic species is that they are not only sensitive to an electrical field but are
also electrochemically active if they are in contact with the electrodes used
for the application of the electrical field. Such electrochemical reactions are
often undesirable if an actuating property is wanted.
The main electroactive polymers, known for their actuating properties,
are listed below:
•Polyacrylic acid gel was the first polyelectrolyte investigated as an elec-
troactive polymer gel. Polyacrylic acid gel can be deformed by a DC
electrical field application
13
.A polyacrylic acid gel rod was immersed in
a saline aqueous solution while a DC field was applied at both sides of
the gel, using platinum electrodes in contact with the gel. A slow bending
motion was observed in the gel with the magnitude being dependent on
the type and concentration of the salt. An asymmetric deformation of
the gel occurs when the electrical field is applied at both cross sections
of the gel rod. This is because the gel shrinks at one end and swells at
the other. The negatively charged ions move to one end and positively
charged ions to the other. The positively charged ions take a consider-
able amount of solvent with them because of their solvated form. This
explains why the mass is moved more with the positive ions than with
the negative ions, resulting in the asymmetrical deformation mentioned
above.
•Poly(2-actylamido-2-methylpropanesulphonic acid) (PAMPS) gel was
found to undergo worm-like mobility
14
.The principle of deformation is
based on an electrokinetic molecular assembly reaction of surfactant
molecules in the hydrogel, caused by both electrostatic and hydropho-
bic interactions and resulting in anisotropic contraction to give bending
towards the anode. In order to have a complex formation ratio at less
than 1 ¥ 10
-3
, surfactants like
N
-dodecylpyridinium chloride (Cl
2
PyCl)
were used. This low ratio value explains that mainly surface complex
formation occurs to bend the gel in an electrical field
15
.
•The hydrogel of perfluorosulphonate ionomer is also an effective elec-
troactive material. This material already shows actuating properties as
a film of 0.2 mm in a DC field, with application of only 3 V
16
.The prin-
ciple of the deforming mechanism is similar to that for other polyelec-
trolyte gels. The response time and durability are much better than for
