Chemistry Reference
In-Depth Information
can decrease the number of parts required and extend the life expectancy
due to the favourable fatigue properties of the alloy
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.
8.2.3
Chromic materials
The term chromic materials is a general one relating to materials that
radiate, lose or change colour because of an induction caused by the exter-
nal stimuli. Chromic is a suffix that means colour. Depending on the stimuli
affecting the chromic materials, they can be classified as:
•
photo-chromic:
external stimuli energy is light;
•
thermo-chromic:
external stimuli energy is heat;
•
electro-chromic:
external stimuli energy is electricity;
•
piezo-chromic:
external stimuli energy is pressure;
•
carsol-chromic:
external stimuli energy is an electron beam.
Organic materials are specifically used in products having chromic proper-
ties because of the wide variety of colours due to the presence of small or
large chromoforic groups (conjugated electron systems). Thermo-chromic
materials are the most common. Two types of thermo-chromic system that
have been used successfully in textiles are liquid-crystal and molecular
rearrangement
5
.Thermo-chromic textiles are also called chameleon fibres
because of their colour change as a function of ambient temperature.
8.2.4
Electroactive polymers
Materials that have actuating properties are based mainly on polymers.
Since these materials respond to a stimulus signal, they can be defined as
intelligent textiles. Historically, actuator materials have been investigated
with mainly inorganic compounds
8
. However, with the development of
polymers, this field of R&D has grown intensively, despite the initial limi-
tation of polymer materials as actuators, because they generate much lower
strain than inorganic materials. Despite this, however, polymer materials,
such as polymer gels, generate a large strain by variation of triggers, such
as solvent exchange, pH and temperature, though their response and dura-
bility is rather poor, and they have not really been used in actuators.
A polymer gel is an electroactive material which differs in various ways
from solid polymer materials
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.The polymer chains in the gel are usually
considered to be chemically or physically cross-linked and form a three-
dimensional network. The gels also have various actuating modes, such as
change of volume due to swelling and de-swelling, symmetric deformation
and asymmetric deformation. Various triggers for actuating polymer gels
have been described. Triggers are classified into two categories: chemical
and physical triggers. A well-known example of a chemical trigger is a
