Note: This is the second article of a three-part series covering smart polymer (1) trends, (2) material/process advances and (3) applications.
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 serve purposes from helping/protecting us to conserving energy.
These new polymers 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.
Let's start with quantum tunneling composites (QTCs), a new class of electrically conductive smart polymer composites that advances pressure switching and sensing material technology. QTCs operate using quantum tunneling: Without pressure, the conductive elements are too far apart to conduct electricity. When pressure is applied, they move closer and electrons can tunnel through the insulator.
The patented technology was developed in the U.K. by Peratech Ltd. The material is a composite of metal filler particles combined with an elastomeric rubber (typically silicone). QTC has the unique ability to smoothly change from an electrical insulator to a metal-like conductor when placed under pressure.
While in an unstressed state, the QTC material is a near-perfect insulator. Yet with any form of deformation, the material starts to conduct and, with sufficient pressure, metallic conductivity levels can be achieved.
Peratech
Composite structures: conventional (left), QTC (center) and scanning electron microscope of QTC (right).
In conventional electrically conductive thermoplastic compounds, conductive particles are always in contact with one another, creating a constant conduction path. In these compounds made of resins modified with various conductive additives, increasing the conductive additive concentration reduces the interparticle separation distance.
Then, at a critical concentration unique to each conductive additive (the percolation point), a constant conduction path is formed causing resistance to decrease dramatically allowing electrons to move rapidly. These traditional filled conductive polymers always show some conduction.
QTC makes use of metal particles that are given an irregular structure with a wet, spiked metal surface and electrically insulated by the silicone rubber. The wetting allows the metal particles to get close but not touch, even when the QTC material is squeezed or densely loaded.
Spikes on the metal filler particle surface allow a higher concentration of electron charge to build with localized high electric fields at their tips. The increased charge on the spikes effectively decreases the distance and energy required for the electron charge to tunnel through and allow conduction to occur. This is known as field-assisted tunneling.
As the QTC is compressed, the metal filler particles are brought closer together allowing electrical conduction. Under QTC compression, barrier widths are further reduced, leading to an exponential increase in the probability of tunneling and an exponential decrease in electrical resistance. The ability to vary the width of the barrier through compression, tension or torsion gives QTC its uniquely controllable electrical properties.
QTC can be tailored to suit different force, pressure or touch-sensing applications from sensing feather-light or finger operation to heavy-pressure applications. With its unique properties, QTC can be made into pressure-sensitive switches of any shape or size. The QTC switches and switch matrices can be screen-printed allowing for development and integration of switches that are as thin as 15 microns.
QTC is also low power, and interfaces can be designed with no start resistance. Without pressure, the switch draws no power and passes no current. When pressure is applied, the resistance drops in proportion to the amount of pressure. This enables sophisticated human machine interface designs to be created that react to variations in pressure. Potential applications include smart clothing, robotics and RFID/card security.
Next, let's take a look at shape memory polymer (SMP) rapid composite manufacturing process advances. A patented process from the Cornerstone Research Group (CRG) replaces expensive metal molds with its Veriflex SMP. CRG's SMP system is capable of being thermally formed into a precise negative image of a master part, cooled, and made to retain the new shape.
Molds of SMP offer multiple advantages over metal:
- Relatively inexpensive
- Capability of fabricating many molds
- Easier prototype production
- Gentle demolding (Veriflex can withstand the elevated temperatures needed to cure composite parts without deformation.)
After the composite part has cured, the mold is raised above the glass transition temperature (Tg), which allows it to retract to its memorized shape. This tooling system also possesses versatility in size variations, including being capable of micro (nanometers) to macro (meters) replication.
SMP mandrels are also of interest from a smart polymer processing standpoint. An object with a diameter larger in the center than at either end is a difficult shape for current fabrication techniques, because rigid mandrels cannot be extracted after the composite is cured.
Conventional approaches to this problem include multipiece or water-soluble mandrels. Rigid multipiece mandrels are complex and labor-intensive, and water-soluble mandrels require the disposal of waste materials and are costly and time-consuming.
Veriflex SMP can be placed in a clamshell mold, shaped into a complex curved mandrel and cooled. The SMP smart mandrels can be filament-wound and cured, and then the mandrels heated and removed.
Advantages to an internal thermoset mandrel system include:
- Speeds up fabrication process
- Reduces cost of developing and removing mandrels
- Offers a simple extraction process
- Allows for reusable mandrels with shape versatility
Finally, let's review an energy-absorbing foam composite smart polymer material. British firm D3O has a patented energy-absorbing gel that is a shear-thickening material based on a complex elastomeric compound.
The gel works at a molecular level. When moved slowly, the "intelligent" molecules will slip freely past each other, allowing the material to be soft and flexible. When forced to move quickly — as in a high-energy impact — the material stiffens as the molecules snag and lock together in a netlike structure, and in doing so both absorb and distribute the force of impact.
The material is incorporated it into a soft foam matrix. Once synthesized, the mixture of viscous liquid and polymer is poured into a shaped mold to form a self-supporting energy-absorbing material that is resistant to compressive set. The composite comprises:
- a solid foamed synthetic polymer, suitably an elastic, preferably an elastomeric matrix;
- a polymer-based dilatant, different from the preceding solid foamed synthetic polymer, distributed through the matrix and incorporated therein during manufacture of the preceding solid foamed synthetic polymer; and
- a fluid distributed through the matrix, the combination of matrix, dilatant and fluid being such that the composite is resiliently compressible, and preferably also flexible.
Resilient compressibility of the composite is provided by the fluid dispersed throughout the matrix, with recovery occurring in 2-5 seconds or less.
One class of particularly preferred dilatants comprises the borated siloxane-based material, marketed by Dow Corning as polyborondimethylsiloxane (PBDMS), that constitutes the base polymer. Potential applications include protective pads for elbows, knees, hips and shins; or clothing, including gloves, headwear and sports gear; and energy-absorbing zones in vehicles.
D3O
Impact absorption D3O vs. polyurethane-PU (left), D3O's net-like absorption mechanism (center), D3O energy-absorbing protective fabric padding (right).
