Corrected response for
‘smart’ material
0 100
Input parameter
T3
T2
T1
Perceived output
100
0
7.2 The response of a ‘smart’ material. The uncorrected response is a function of the parameter T, but after correction in the material, an unambiguous output is obtained, independent of T. (Adapted with permission from Smart Structures and Materials, B. Culshaw, © 1996, Artech House Publisher [7]).
Temperature sensitive shape memory polymers 107 with the behavior of a shape memory polymer (SMP), with the potential for application as a smart material and also as a new functional material. For example, the well-known use as a control for knotting by a smart suture made with shape memory polymer which ties itself into a perfect knot where access is limited [11]. Its self-knotting action occurs when it is heated a few degrees above normal body temperature. At the start, a key characteristic of polymeric materials, which are good candidates for development into smart intelligent materials, is the ability to change properties, such as structure or composition, and function in a controlled response to a change in environment or operating conditions. The next level is the promise for development of some level of built-in intelligence, such as graded reaction to stimuli and ability to recognize or discriminate shapes or forms [12].
Stimuli sensitive polymers (SSPs) or shape memory polymers (SMPs) yield intelligent textiles that exhibit unique environmental responses. The molecular design of SSPs [13] facilitates phase change behavior in response to environmental stimuli and allows SSP textiles to change structures and properties. SSPs yield fabrics with such properties as air permeability, hydrophilicity, heat transfer, shape, and light reflectance that are responsive to such environmental stimuli as temperature, pH, moisture, light, and electricity. Shape memory polymers (SMPs) offer greater deformation capacities, easier shaping, and greater shape stability, and small changes to the chemical structure and composition of SMPs result in a wide variety of transition temperatures and mechanical properties.
7.3.1 Principle of temperature sensitive shape memory polymer
Shape-memory materials are stimuli-responsive materials. They have the capability of changing their shape upon application of an external stimulus.
Shape memory may be triggered by heat, light, electric field, magnetic field, chemical, moisture, pH and other external stimuli [5–6]. Change in shape caused by a change in temperature is called a thermally induced shape- memory effect. These are materials which are stable at two or more temperature states. While in these different temperature states, they have the potential to be different shapes once their ‘transformation temperatures’ (Tx) have been reached. Shape memory alloys (SMAs) and Shape Memory Polymers (SMPs) are materials with very different shape changing characteristics. While exposed to their Tx, devices made from SMAs have the potential to provide force such as in the case of actuators. Devices made from SMPs in contrast, while exposed to their Tx, provide mechanical property loss as in the case with releasable fasteners. The shape memory polymers described in this chapter are all thermosensitive shape memory polymers.
Temperature sensitive shape memory polymers are a special class of adaptive
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materials which can convert thermal energy directly into mechanical work.
This phenomenon, known as the shape-memory effect (SME) occurs when one of these special polymers is mechanically stretched at low temperatures, then heated above a critical transition temperature, which results in the restoration of the original shorter ‘memory’ shape of the specimen [14]. The proposed theory expresses well the thermomechanical properties of thermoplastic polymer, such as shape fixity, shape recovery, and recovery stress [1]. The mechanism of shape memory behavior with temperature stimuli can be shown as outlined in Fig. 7.3. These materials have two phase structures, namely, the fixing phase which remembers the initial shape and the reversible phase which shows a reversible soft and rigid transition with temperature.
At temperatures above the glass transition temperature (Tg), the polymer achieves a rubbery elastic state (Fig. 7.4) where it can be easily deformed without stress relaxation by applying external forces over a time frame t < t, where t is a characteristic relaxation time. When the material is cooled below its Tg, the deformation is fixed and the deformed shape remains stable.
The pre-deformation shape can be easily recovered by reheating the material
Deformation
T > Tg/Tms T > Tg/Tms T > Tg/Tms Permanent
shape
Temporary shape Permanent
shape
Fixation Recovery
7.3 Typical temperature stimulating shape memory behaviors.
Modulus
Glassy state Rubbery state Flow
Tg Temperature
7.4 Temperature dependency elasticity of thermoplastic polymer.
Temperature sensitive shape memory polymers 109 to a temperature higher than the Tg [15]. Therefore, admirable shape memory behavior requires a sharp transition from glassy state to rubbery state, a long relaxation time, and a high ratio of glassy modulus to rubbery modulus. The micromorphology of SMPs strongly affects its mechanical properties. There are many factors that can influence these SMPs: chemical structure, composition, and sequence-length distribution of the hard and soft segment in segmented polymer, overall molecular weight and its distribution. An elastomer will exhibit a shape-memory functionality if the material can be stabilized in the deformed state in a temperature range that is relevant for the particular application. The shape is deformed under stress at a temperature near or above Tg or the segment crystal melting temperature, Tms. The deformed shape is fixed by cooling below Tg or Tms. The deformed form reverts to the original shape by heating the sample above Tg or Tms.
7.3.2 Architecture of the shape memory polymer-network
Shape memory effect is not related to a specific material property of single polymers; it rather results from a combination of the polymer structure and the polymer morphology. Shape memory behavior can be observed for several polymers that may differ significantly in their chemical composition and structure. The shape memory polymers that show a thermally induced shape- memory effect are network structure. The netpoints, which can have either a chemical or physical nature, determine the permanent shape. The net chain shows a thermal transition in the temperature range which the shape-memory effect is supposed to trigger. Segmented polymer with shape-memory effect properties requires a minimum weight fraction of hard-segment-determining blocks to ensure that the respective domains act as physical netpoints [1].
Formation of a network structure with rigid point/segments (fixed phase) and amorphous/flexible segment/regions (reversible phase) are the two necessary conditions for their good shape memory effect [16]. In general, interpolymer chain interactions are so weak that one-dimensional polymer chains cannot keep a certain shape above Tg.
To maintain a stable shape, polymer chains should have a three-dimensional network structure. Interpolymer chain interactions useful for constructing the polymer network are crystal aggregate or glassy state formation, chemical cross-linking, and chain entanglement [5]. The latter two interactions are permanent and used for constructing the original shape. The other interactions are thermally reversible and used for maintaining the transient shapes. This can be reached by using the network chains as a kind of molecular switch.
For this purpose the flexibility of the segments should be a function of the temperature. One possibility for a switch function is a thermal transition (Ttrans) of the network chains in the temperature range of interest for the
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particular applications. The crystallites formed prevent the segments from spontaneously recovering the permanent shape that is defined by the netpoints.
The permanent shape of the shape memory network structure is stabilized by covalent netpoints, whereas the permanent shape of the shape memory thermoplastics is fixed by the phase with the highest thermal transition at Tperm [1].