7.5.1 The use of the functional properties
Moisture permeability, volume expansion and refractive index vary significantly above and below Tg/Tms according to the difference of the kinetic properties of molecular chains [34] between temperatures above and below Tg/ Tms. Temperature dependency of water vapor permeability is an important factor to be considered for the effective utilization of this type of smart materials.
One of the applications of moisture permeability of a thin film is in sports wear. Moisture permeability is high above Tg/Tms and low below Tg/Tms (Fig. 7.6). For sports wear, the heat retaining property at low temperature and gas permeability at high temperature are excellent, yielding high quality for the sportswear. Imagine a coat that detected rain. It could change shape to keep the moisture out, but always be comfortable to wear. When the weather becomes cooler, it could change again to keep the wearer warm.
The shape-deformable matrix material contains an elastomeric, segmented block copolymer, such as a polyurethane elastomer or a polyether amide elastomer. The elastomeric polymer provides the force for dimensional change when the moisture-absorbing polymer softens and relaxes as a result of moisture absorption. The inclusion of a non-elastomeric polymer provides a degree of recovery when exposed to humidity. The activation process for the shape-deformable materials does not require substantially increasing the
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Stress (MPa)
2
1
0
(a)
N = 1
N = 2 N = 3 N = 4
ep en em
0 20 40 60 80 100 120
Strain (%)
Stress (MPa)
2
1
0
(b)
N = 1 N = 2
N = 3 N = 4
0 20 40 60 80 100 120
Strain (%)
Stress (MPa)
2
1
0
(c)
N = 1 N = 2 N = 3
N = 4
0 20 40 60 80 100 120
Strain (%)
7.5 Cyclic tensile behavior of (a) PMCP, (b) PMCP-PE1, and (c) PMCP- PE2 (elongated at 45 ∞C and recovered at 85 ∞C), N represents number of cycles, ethylene segment in PE1 and PE2 are 8.1 and 17.0% (wt%) respectively. (Adapted with permission from Jeong et al., Polymer International, 51, 275, 2002, (c) 2002 SCI, John Wiley and Sons Ltd. [33]).
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temperature of the materials, which helps in preventing leakage from the absorbent product. Also, by adjusting the chemical structure of the shape- deformable polymer, a specific polymer can be tailored to interact at a selected level of humidity. These kinds of shape memory materials would be applicable in disposable hygiene products, such as diapers, training pants, and incontinence products [35].
7.5.2 The use of changes of physical properties related to the phase transformation
Physical properties of materials are related to the crystal structure and microstructure of the involved phases. In spite of the diffusionless character of the phase transformation, the lattice parameter, crystal structure and microstructure of the parent phase and the reverse phase, differ significantly.
As a consequence, during transformation, values of E-modulus, internal friction, electrical resistance, and hardness might change [36]. A typical example of changing phase change from temporary shape to permanent shape is suture.
The technique of keyhole surgery minimizes scarring, speeds healing and reduces the risk of infection. However, it is extremely difficult to carry out delicate surgical procedures accurately in a confined space, such as implanting a bulky device or knotting a suture with the right amount of tension. In the latter case, if a knot is pulled too tight, necrosis of the surrounding tissue can occur, but if it is too loose, the incision will not heal properly and scar tissue develops [37].
7.6 Temperature dependency water vapor permeability of shape memory polymers.
WVP
Temperature
Low Transition region High permeability permeability (Tms/Tg)
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Technology has developed a shape memory polymer that has been designed by MnemoScience to act as a smart suture that ties itself into a perfect knot.
The suture’s self-knotting action occurs when it is heated a few degrees above normal body temperature. The suture can therefore be used to seal difficult wounds where access is limited [11]. The material is a multi-block copolymer, in which block-building segments are linked together in linear chains. Specifically, the polymer they created contains a hard segment and a
‘switching’ segment, both with different thermal properties. One segment melts, or makes another kind of transition, at a higher temperature than the other. By manipulating the temperature and stress applied to the overall material, Langer and Leindlin end up with a material that forms a temporary shape at one temperature, and a permanent shape at a higher temperature.
After increasing the temperature, the suture material shrunk, creating a knot with just the right amount of tension on the surrounding tissue. It is difficult to create such a knot in the confined spaces associated with endoscopic surgery. They demonstrated this by creating the first ‘smart’ degradable suture.
When SMPs change from glassy state to rubbery state, Young’s modulus, tensile and elastic properties vary greatly, which would be useful for garments.
For example, when the SMPs-based garments were washed at higher temperature or worn at body temperature, they could recover to the original state (wrinkle free). Shape memory fabrics regain the flatness of their appearance and retention of their creases after the fabrics have been immersed in hot water [6, 38].