Advances in Science and Technology Vol. 60 (2008) pp 1-10 Online available since 2008/Sep/02 at www.scientific.net © (2008) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AST.60.1
The Investigation about the Shape Memory Behavior of Wool Jinlian Hu 1, a, Zheng-E Dong2, b, Yan Liu1 and Yijun Liu1 1
Institute of Textile and Clothing, The Hong Kong Polytechnic University, Hong Kong 2
Donghua University, Library, Shanghai, 201620 Peoples Republic of China a
[email protected]
b
[email protected]
Keywords Shape memory polymers; Stimuli-responsive materials; Wool; Thermal and hygrothermal effects; Shape memory behavior
Abstract Shape memory polymers are a promising class of stimuli-responsive materials that have dual-shape capability. This kind of materials can recover their shape in a predefined way from temporary shape to desired permanent shape when exposed to an appropriate stimulus. In the development and extensive application of synthetic shape memory polymers on textile industrials, the thermal and hygrothermal effects of wool materials have attracted considerable attention. In this article the fundamental concept of the shape memory polymers and the fundamental aspects of the shape-memory effect were reviewed. The thermal and hygrothermal effects of wool materials were also summarized to discuss the shape memory behavior of wool materials. Besides the effects of synthetic shape memory polymers on the thermal and hygrothermal of the woven wool fabrics were introduced to show the shape memory behavior of treated wool further. 1. Introduction Shape Memory Polymers (SMPs) are a promising type of stimuli-responsive materials and have the characteristics such as large recoverability, lighter weight, superior molding property and lower cost [1]. Owing to their valuable advantages, many synthetic shape memory polymers have attracted considerable attention [2] and found application in the different fields such as smart fabrics [3-4], heat-shrinkable tubes or films [5] and intelligent medical devices etc. [6]. But are there natural shape memory materials? In textile industry the wool fiber as a class of natural proteinaceous polymer has many interesting and surprising thermal and hygrothermal effects, which provide manufacture processing and care methods of these products for a number of applications. For example, pine cone clothes as a new type of ‘smart’ clothing consist of a top layer of tiny spikes of water-absorbent material, wool, and wool clothing can be refreshed by hanging in a steamy bathroom [7]. Most of these applications are often adapted to the existing properties induced by thermal and water or moisture. In the application of synthetic shape memory polymers on the wool fabrics and garments for smart textiles [8], the shape of wool fabrics such as wrinkling and crease retention is highly susceptible to external environmental conditions, especially changes in humidity. Numerous investigations have dealt with attempts to analysis the thermallyinduced structural changes in wool [9] and demonstrated the effects of humidity with regard to changes in physical properties such as swelling, glass transition temperature, Tg [11], melting point temperature, Tm [12], initial modulus and elongation at break and so on. It is well known that the wrinkle recovery of wool deteriorates under very high humidity although the ability of wool to recover from
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unwanted wrinkles is superior to that of most other textile fibers [13]. It is also observed that Wrinkle behavior is different at different temperature and humidity conditions including preconditions, pleat insertion, recovery conditions [14]. This article reviews the general concept of shape-memory polymers, the fundamental aspects of the shape-memory effect. The investigation progress of thermal and hygrothermal effects on wool also was presented. And the shape memory behavior of wool material was discussed. 2. General concept of SMPs SMPs are dual-shape materials belonging to the group of ‘actively moving’ polymers [6]. They have the capacity to actively recover from a temporary shape to another permanent shape. The former shape is obtained by mechanical deformation and subsequent fixation of that deformation, while the latter one is the desired shape that must first be formed by processing such as molding and curing [18]. It is worth noting that recovery was required to take place in a much shorter time interval and polymers allow a much higher deformation rate between permanent shape and temporary shape. 2.1 Molecular mechanism of the shape memory effect of polymers It will behave the shape memory functionality if an elastomer can be stabilized in the deformed state in a temperature range. According to literatures [6], the shape memory polymers were regarded as elastic polymer networks that equipped with suitable stimuli sensitive switches. The polymer network is made up of netpoints and molecular switches on the molecular level (shown in Fig. 1). Permanent shape
Temporary
Heating
Extension & cooling
T > Ttrans Netpoint
Recovered shape
T < TTrans
T > Ttrans
Switching segment
Fig. 1 Molecular mechanism of the thermally induced shape memory effect The former determined the permanent shape of the SMPs, was named as hard segment and come from physical crosslinks (Entanglements of certain polymer chains) or chemical crosslinks (Intermolecular interaction in the form of covalent bonds). The later was relevant for the stabilization in the deformed stage in a temperature range and was named as soft segment. In chemically cross-links the permanent shape of SMPs is stabilized by the covalent netpoints. One possibility for a switch function is a thermal transition ( Ttrans ) of the network chains in the temperature range. At temperature above Ttrans the chain segments are flexible, whereas the flexibility of the chains below Ttrans is limited. Thus the flexibility of the entire segment is limited
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when a transition from the rubber-elastic or viscous stage to the glassy state occurs. The crystallites formed prevent the segments from immediately reforming the coillike structure and from spontaneously recoverying the permanent shape that is defined by the netpoints. There are two kinds of SMPs according to the Ttrans of the particular switching segment [19]. (1) Ttrans is a melting temperature( Tm ). In this case there is a relatively sharp transition. (2) Ttrans is a glass transition temperature ( Tg ). In this case glass transitions always extend over a broad temperature interval. 2.2 Different actuations During the recovery from any temporary shapes to its initial permanent shape an external stimulus is needed. It means that SMPs can response to changes in the external stimulus or conditions such as temperature, ionic strength, pH, electromagnetism, solvent composition, etc [20]. According to the difference of the stimuli-sensitive implant polymers there are several kinds of shape memory polymers as follows [19]. 1. Temperature-induced shape memory polymers (Thermo-responsive shape memory polymers) The shape change initiated by the temperature change is defined thermally induced shape Memory effect [19]. For these kinds of thermally induced SMPs, the polymer switches from its actual, temporary shape to its desired, permanent shape when it exceeds the switching transition temperature ( Ttrans ). The Ttrans can be either a glass transition temperature ( Tg ) or a melting temperature ( Tm ) of polymer [20]. 2. Indirect actuation of the thermally induced shape memory effect According to the literature [6], there are two different methods to obtain the indirect actuation of the shape memory effect. One method includes indirect heating, e.g. by irradiation. The other one is to lower Ttrans by diffusion of low molecular weight molecules into the polymer, which works as a plasticizer. This allows the triggering of the shape memory effect while the sample temperature remains constant. Many methods can be used to heat the thermally induced shape memory polymers without increasing the environment temperature. They are as follows: (1) Infrared light was illuminated to enhance heat transfer by some approach like incorporation of conductive fillers (2) An electrical current was applied to trigger the shape memory effect by increasing the sample temperature (3) Magnetic fields were used to enable remote actuation of the thermally induced shape memory effect by incorporation of magnetic nano-particles into shape memory thermoplastics. In this case inductive heating of the nano-particles in an alternating magnetic field was used to increase the sample temperature. (4) The moisture uptake was used to lower Ttrans (e.g. Tg ). When immersed in water, moisture diffuses into the polymer sample and acts as a plasticizer, resulting in shape recovery. It is noticed that this shape memory polymer still has to be understood as a
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polymer and not as a hydrogel even if the maximum moisture uptake has been realized. 3. Light-induced shape memory effect (Photo-responsive shape memory polymers) Light-induced stimulation of shape memory polymers can be achieved by the incorporation of reversible photo-reactive molecular switches [6]. This stimulation is irrespective of any temperature effects and must be different from the indirect actuation of the thermally induced shape memory effect. To introduce light sensitivity into shape memory polymers, light-triggered switches such as cinnamic acid (CA) or cinnamylidene acetic acid (CAA) moieties can be incorporated into the polymer architecture by a graft polymer and an interpenetrating polymer. Irradiation with light of suitable wavelength will results in cleavage of the newly formed bonds. 4. Water-actuated SMP [2] A kind of water-actuated shape memory polymers was achieved in polyetherurethane polysilesquisiloxane block copolymers [6]. In this system low molecular weight poly (ethylene glycol) or PEG, has been used as the polyether segment. Upon immersion in water, the PEG segment dissolves, resulting in the disappearance of Tm and recovery of the permanent shape. 2.3 The fundamental aspects of the shape memory effect (SME) The shape memory effect is not an intrinsic property. That is to say polymers do not display this effect by themselves. It is considered that the shape memory is the results of a combination of polymer morphology and specific processing, and can be regarded as a polymer functionalization [6]. Fig. 2 demonstrates the schematic representation of four steps of the shape memory effect in SMPs. Softening and free deformation
Permanent shape 错误！
Heating
Desired shape
Shape fixity
Cooling
Above Tg with applied force
Shape recovery
Heating
Below Tg
Above Tg
Cooling
Fig. 2. Schematic representation of the shape memory effect with four steps [1,18] Taken the shape memory polymers of thermo-elastic phase transformation for example, their shape memory effect reveals at the temperatures above and below Tg . [1] The details of the four steps are as follows: (1) To form the permanent shape. The SMPs were heated at fluxing temperature over Tg and formed into a desired shape by specified processing.
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(2) To deform into an arbitrary shape above the Tg under an applied force. It is considered that there exist two phases, stationary phase and reversible phase, which correspond to the crystal portion with bridging construction and the amorphous portion, respectively. The amorphous portion shows the rubber elasticity by heating above Tg and is easy to deform. (3) Shape fixity by cooling below Tg . In this situation, the deformed shape was fixed and cooled below Tg , and then constrain was removed from SMP. (4) Shape recovery by heating over Tg under free load condition. In this step, the SMPs were heated above Tg to recover original shape. 3. Thermal and hygrothermal effects of wool materials Wool is a semi-crystalline, natural polymer being made up of crystalline micro-fibrils embedded in an amorphous matrix, which is cross-linked by disulphide bonds. 3.1 Tg of wool and its effects on shape memory behaviour The glass transition temperature ( Tg ) in amorphous polymers marks the boundaries in time or temperature where the behavior of a material changes from glass-like to rubber-like. The wool can be characterized as glassy/elastic below Tg and as plastic above this point [17]. Tg of the wool is an important parameter that is related to the viscoelastic properties of wool fibers [11]. The visco-elastic behaviour of wool is profoundly affected by the regain of the wool. However, the actual Tg depends on the water content of the wool shown in Fig. 3 [11]. Fig.3 makes it clear why wool garments are de-aged repeatedly during use. It reveals that dry wool has a Tg of about 170℃, while when wet, the Tg is about -10℃. Many other textiles are plasticized by water shown in Table. 1. From Table.1, it is possible to speculate that in the extreme case of moist garments bent, then dried while on the wearer, nylon 6.6, nylon6, orlon, acetate and silk would also wrinkle severely. In the wet state only polyester and Qiana nylon have Tg which are well above body temperatures (about 35℃). As the Tg can be greatly reduced by moisture, a novel feature, namely, the water actutable recovery of wool is proposed. The effects of moisture on Tg of wool fabrics reveal its two new features. One is the functionally gradient in Tg , and the other is the actuation of the material triggered by water. The investigation shows that the properties of wool are highly susceptible to changes in humidity. For example, on going from completely dry to wet, wool takes up 34% to 37% by mass of water and shows a decrease of 180 ℃ in the Tg , a decrease of 70℃ in the Tm , a decrease of 60% in the initial modulus, and an increase of 80% in elongation at break. At 65%RH, with a water content of wool of about 15%, the glass transition temperature is around 50-60℃, so that under the conditions of normal climate wool is a semi-crystaline polymer with a glassy matrix .
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180 160 140 120
o
Tg ( C)
100 80 60 40 20 0 0
5
10
15
20
25
30
Water content (%)
Fig. 3 Effect of water content on Tg of wool [11] Table. 1. The Tg of different textiles at different conditions
Tg , ℃
∆ Tg , ℃
Dry
Conditioned
Wet
(Conditioned Wet)
Contrast with body temperature， ≈ 35℃
Polyester (PET)
73
71
57
14
>
Nylon 6.6
59
40
29
11
