Note: This is the first article of a three-part series covering smart polymer (1) trends, (2)material/process advances and (3) applications.
In the past century, polymers served elementary functions, such as being structural, insulative or optical. In the new millennium, polymers are increasingly intelligent and capable of adapting to their environment.
Smart polymers ensure optimization of their characteristics or that of the system of which they are a part. Two early examples of smart polymers and their applications are:
- Photochromic polycarbonate (PC) in eyeglass lenses where the photochromic polycarbonate lens material reversibly darkens in response to sunlight.
- Thermal shrinkable polyolefin film in packaging applications where heat is applied to the polymer film, and it irreversibly shrinks tightly over whatever it is covering. This material is commonly used as an overwrap on many types of packaging.
An ever-increasing spectrum of smart polymers based on various stimulus/adaptive property combinations is being developed to satisfy a growing range of applications in automotive, aerospace, medical, consumer, industrial and other market sectors.
There are multiple routes to smart polymer development, namely:
- Additives, such as magnetic oxide powder in thermochromic, photochromic or electrochromic dyes/pigments
- Block copolymers and evolutionary morphologies, where shape memory polymers as an actuator are used to trigger shape change via crystallization and the fusion of segments, for example, with caprolactone dimethacrylate copolymerized with segments of butylacrylate
- Stabilization of networks crosslinked in a stretched state as is the case, for example, in the development of heat-shrinkable plastics
- Polymer alloys
Reversibility or irreversibility is of primary consideration. Reversible materials can operate multiple cycles of response/adaptation to a stimulus, whereas irreversible materials must be reset to repeat their reaction to a new cycle of stimulation.
Irreversibility allows traceability of a history. An irreversible thermochromic polymer, for example, makes known that a temperature threshold has been exceeded during transportation or storage.
Reversibility makes it possible to use the same device multiple times, for example, a medical thermometric band for external control of human temperature. Reversibility may not be useful, and is therefore of no interest in certain applications, for example, in heat-shrinkable film for packaging.
Smart polymers find applications spanning the gamut of the high-tech, the sophisticated, the novel and the mundane. Smart polymers have properties that can be altered by temperature, moisture, electric or magnetic fields, pH and stress. They can change shape and color, become stronger or produce voltage as a result of external stimuli.
By using smart polymer materials, instead of adding components and mass, engineers can endow products/structures with built-in responses in myriad contingencies. In their various forms, these materials can perform as actuators that can adapt to their environments by changing characteristics such as shape and stiffness, or as sensors that provide actuators with information about environmental or structural changes.
Key to the development of smart polymers is our growing understanding of the world at the molecular level and our ability to manipulate it at that level, too. Smart polymers engineered for special qualities and capable of interacting with the larger environment are expected to proliferate in the coming decade, as scientists learn about the chemistry and triggers that induce conformational changes in polymer structures and devise ways to take advantage of and control them.
Properties inherent in shape-memory polymers and other smart polymers have the potential to be game-changers in the automotive advanced materials field, eventually leading to vehicle subsystems that can self-heal in the event of damage, or that can be designed to change color or appearance.
In the medical field, smart polymers are being chemically formulated to sense specific environmental changes in biological systems, and adjust in a predictable manner making them useful tools for drug delivery or other metabolic control mechanisms.
Smart fabrics and intelligent textiles that make use of smart polymers and clever electronics currently in embryonic stages are poised for tremendous growth. The global market for smart fabrics and interactive textiles is projected to reach $1.5 billion by 2015.
Smart polymers offer considerable promise in clean-energy generation. The global market for electroactive polymer (EAP) actuators and sensors is projected to grow to $275 million by 2015.
Let's take a closer look at a shape memory polymer (SMP) systems to give us a starting point of understanding in this emerging plastics technology field. Veriflex, produced by CRG Inc., is a fully cross-linked, two-part styrene-based thermoset SMP resin system.
The fully-formable thermoset SMP, said to process like wet-layup epoxy, is also available in a thermochromic version that changes color at its transition temperature to signal readiness to be molded, shaped and formed. Both standard and thermochromic Veriflex resins have a glass transition temperature (Tg) of 62 degrees C.
In its elastic state, Veriflex can be reshaped and elongated up to 200 percent. If cooled while constrained in the new shape, the polymer hardens and can maintain its deformed configuration indefinitely.
The material is characterized by triggering segments. At a temperature above Tg (Tform), the material can be easily deformed. The deformed shape will be maintained when the material is cooled below the Tg (Tcool). The material will "remember" or return to its original shape when it is heated to a temperature above the Tg again. The material should be manipulated while in its fully elastic state, which is above the temperature transition range illustrated below.
Plastics Institute of America
Shape memory polymer (SMP) Tg glass transition temperature curve (left), and a self-healing SMP (right).
Customizable Veriflex with tailored thermal transition temperatures from minus-30 C to 260 C is available with extremely high temperature and cryogenic range also capabilities. CRG is also investigating possible future systems that may respond to stimuli such as light, electric field or magnetic field.
Veriflex E (Epoxy) has the SMP properties of standard Veriflex with the toughness/strength of epoxy. Veriflex E comes as either a one-part or two-part, fully-formable thermoset SMP resin system. One-part Veriflex E has an activation temperature of approximately 90 degrees C and two-part Veriflex E at 104 C.
Both are open-mold curable. Over 95 percent (one-part resin) and 100 percent (two-part resin) elongation is possible in their elastic state.
Veriflex EH (Epoxy, Healing) is a self-healing version of epoxy-based SMP. Veriflex EH can be processed using vacuum-assisted resin transfer molding (VARTM), resin transfer molding (RTM), or wet lay-up techniques. CRG has also developed a prepreg formulation.
Two healing mechanisms, shape recovery and polymer diffusion are used to restore up to 85 percent of the original mechanical performance. Shape recovery brings the cracked or damaged area back into intimate physical contact.
The second healing mechanism, polymer diffusion, allows long polymer chains to diffuse across the failure line and restore mechanical integrity. Both healing mechanisms are thermally activated and are achieved in three to seven minutes.
