Plastics with a Memory

Thermoplastics can be bent, pulled, or squeezed into various useful shapes. But eventually, especially if heat is added, they return to their original form. This behavior, known as plastic memory, can be annoying. If properly applied, however, plastic memory offers interesting design possibilities for all types of fabricated parts (Chapter 6, PLASTICS WITH A MEMORY). 

A plastic s molecular orientation can be developed during processing. Initially the melted molecules are relaxed. During processing the molecules tend to be more 

Molecular orientation results in increased stiffness, strength, and toughness (Table 8-12) as well as resistance to liquid and gas permeation, crazing, microcracks, and others in the direction or plane of the orientation. The orientation of fibers in reinforced plastics causes similar positive influences. Orientation in effect provides a means of tailoring and improving the properties of plastics. 

Considering a fiber or thread of nylon-66, which is an unoriented glassy polymer, its modulus of elasticity is about 2,000 MPa (300,000 psi). Above the Tg its elastic modulus drops even lower, because small stresses will readily straighten the kinked molecular chains. However, once it is extended and has its molecules oriented in the direction of the stress, larger stresses are required to produce added strain. The elastic modulus increases. 

High-Energy Fatigue Impact (55 lb. weight 50-in. height) (24.9 kg weight at 127-cm height)  

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As has been mentioned, the patient is often an unreliable witness concerning previous experience in industry. However, a solution to this problem lies in new developments allied to the availability of computers in the doctor s surgery. In France and in the United Kingdom experiments have taken place with smart cards , small plastic cards with a memory chip embedded in them. A patient s history can be recorded on the card and, when he goes to the surgery, he will present his card to the doctor, who will read it from his compute, update it as necessary during the visit, and return it to the patient If such a card carried an internationally coded job history as well as the medical facts, this would be a major contribution to chemical safety. 

Apparently, the formation of the microporous structure within the PVdF—HFP copolymer was of critical importance to the success of Bellcore technology, and the ion conductivity was proportional to the uptake of the liquid electrolyte. To achieve the desired porosity of PVdF film, Bellcore researchers prepared the initial polymer blend of PVdF with a plasticizer dibutylphthalate (DBP), which was then extracted by low boiling solvents after film formation. Thus, a pore-memory would be left by the voids that were previously occupied by DBP. However, due to the incomplete dissolution of such high-melting DBP during the extraction process, the pore-memory could never be restored at 100% efficiency. Beside the total volume of pores thus created by the plasticizer. 

PKC signaling is also involved in the plasticity underlying fear learning and memory. Inhibition of PKC in rats impairs acquisition of fear conditioning (Goosens et al. 2000 Li et al. 2002) and mice with a knockout of the isoform of protein kinase C have deficient cued and contextual fear conditioning (Weeber et al. 2000). Inhibitors of PKC also block LTP (MaUnow et al. 1989 Reyman et al. 1988). 

A good elastomer should not undergo plastic flow in either the stretched or relaxed state, and when stretched should have a memory of its relaxed state. These conditions are best achieved with natural rubber (ds-poIy-2-methyl-1,3-butadiene, ds-polyisoprene Section 13-4) by curing (vulcanizing) with sulfur. Natural rubber is tacky and undergoes plastic flow rather readily, but when it is heated with 1-8% by weight of elemental sulfur in the presence of an accelerator, sulfur cross-links are introduced between the chains. These cross-links reduce plastic flow and provide a reference framework for the stretched polymer to return to when it is allowed to relax. Too much sulfur completely destroys the elastic properties and produces hard rubber of the kind used in cases for storage batteries. 

Most plastic products can be produced with a built-in memory. That is, the tendency to move into a new shape is included as an integral part of... 

As one would expect, there are a number of applications that currently use shape-memory alloy materials many more are projected for the future. The earliest application was for greenhouse window openers, with the metal serving as an actuator to provide temperature-sensitive ventilation. Some commercial faucets/showerheads are already equipped with this material that shuts off the water if a certain temperature is reached, which effectively prevents scalding. An intriguing future application will be for automobile frames as we will see later, some plastics may also be designed with shape memory. Someday soon, your car may reshape itself in front of your eyes within minutes after an accident ... 

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