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Shape memory polymers possess some characteristics that make them attractive to use in textiles,
such as light weight, corrosion resistant, formability and low cost. SMPs are available for example in
form of pellets. They can easily be compounded and formed by extrusion or injection moulding.
Recently developed memory polymers belong to the group of polyurethanes. They are thermoplastic
polymers having their glass transition temperature at a range of – 30°C till 100°C. Mitsubishi Heavy
Industries commercialises these polymers in the form of hydrophilic vapour-transmitting membranes.
There are several advantages of SMPs compared to SMAs, such as easy processability,
comparatively low production costs, biocompatibility, substantially easier shaping procedure and high
shape stability. A disadvantage of SMP in comparison to SMA is the much lower recovery force.
Compared with other types of shape memory materials, such as alloys, literature for understanding the
characteristics and the detailed mechanisms responsible for the properties of shape memory polymers
are still very fragmented and restricted. The Hong Kong Polytechnic University is doing investigations
into their detailed characteristics.
A. Block copolymers
Shape memory polymers are block copolymers consisting of two segments: a hard and a soft
segment. The soft segments should be large enough to allow a considerable free rotation when the
temperature changes and to result in the recovery of the deformations previously exerted on the
structure.
The hard segments include the urethane bond and chain lengtheners 7 . They are present in a sufficient
quantity to be able to form a continuous crystalline phase 8 , the fixed phase. The soft segments,
consisting of polyether or polyester dioles, form the glass phase 9 or the reversible phase. During the
first formation of an object from a shape memory polymer via a thermoplastic process, both phases
are in the melted position. The extrusion temperature of the films lies between 170 and 200 °C. When
the object is cooled down to a temperature where both phases are in the solid state, the definite form
of the object is obtained. Next, the object is re-heated to a temperature at which only the glass phase
turns soft. Now, it is possible to transform the object under pressure. The object is cooled down again
in this temporary form. A second heating under negligible pressure results into a recovery of the
permanent shape.
7
chain lengtheners : chemicals which can react with the end groups of two or more polymer chains resulting into a longer
polymer.
8
continuous crystalline phase: in certain parts of a polymer, the atoms are arranged in such a way that it is possible to
assume a disposition in a crystal lattice;
9
glass phase : parts of the polymer where no crystallisation can occur, can have two positions. A hard, brittle position below
a certain temperature and a viscous, rubbery position above this temperature. The temperature at which this transition
occurs, is called the glass transition temperature, the rubbery state is the glass phase.
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