Shape Memory Polymers (SMP)

What is Shape Memory Polymer?

First introduced in Japan and then the United States 1984, shape memory polymers are polymers whose qualities have been altered to give them dynamic shape "memory" properties. Using thermal stimuli, shape memory polymers can exhibit a radical change from a rigid polymer to a very elastic state, then back to a rigid state again. In its elastic state, it will recover its "memory" shape if left unrestrained. However, while pliable it can be stretched, folded, or otherwise conformed to other shapes, tolerating up to 200% elongation.

Unlike shape memory alloys, SMP exhibits a radical change from a normal rigid polymer to a very stretchy elastic and back on command, a change which can be repeated without degradation of the material. The "memory," or recovery, quality comes from the stored mechanical energy attained during the reconfiguration and cooling of the material.

Above its transition temperature, SMP goes from a rigid, plastic state to a flexible, elastic state. When cooled, it becomes rigid again and can be constrained in its new shape configuration. Shape memory characteristics can be engineered into most polymers. Some of CRG's current SMP formulations are based on styrene acrylate, cyanate ester, and epoxy polymer systems.

There are many activation methods for thermally responsive SMP:

  • Resistive heating
  • Embedded heaters (for example, stretchy heaters, nichrome wires)
  • Contact heating (MRE heaters)
  • Induction heating
  • Dielectic heating
  • Microwave heating
  • Infrared radiant heating

Some of these methods may be enabled by fillers such as conductive fillers, CNT, CNF, iron and ferrite.

Transition Temperature

A polymer engineered with shape memory characteristics provides a unique set of material qualities and capabilities that enhance the traits inherent in the polymer system itself. SMP changes between rigid and elastic states by way of thermal stimuli. The change takes place at what is referred to as the glass transition temperature (Tg). SMP can be formulated with a Tg that matches an application need. Current SMP systems have been demonstrated with Tgs from –30°C to 260°C (–22°F to 500°F).

Above its transition temperature, which can be custom-engineered, SMP goes from a rigid, plastic state to a flexible, elastic state. When cooled below that temperature, it becomes rigid again, with high specific strength. The SMP can be manipulated and cooled into a variety of new shapes; when heated above its transition temperature, it will return to its "memorized" shape.

Shape memory polymers are characterized by triggering segments that have a specific Tg. At a temperature above Tg, the material can be easily deformed. The deformed shape will be maintained when the material is cooled below the Tg. 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 in its fully elastic state and not in the temperature transition range.

 

While in its elastic state, SMP will recover its cast, or cured, shape if left unrestrained. Otherwise, it may be reconfigured or manipulated into other shapes, tolerating up to 200% elongation. SMP heated above its transition temperature can be stretched, folded, rolled, twisted, or bent. At any point, the SMP can be cooled to maintain its altered shape indefinitely, until once again it is brought above its transition temperature. Above its transition temperature, SMP recovers its "memorized" shape very quickly.

SMP is not simply an elastomer, nor simply a plastic. It exhibits characteristics of both materials, depending on its temperature. While rigid, it demonstrates the strength-to-weight ratio of a rigid polymer. While pliable, it has the flexibility of a high-quality, dynamic elastomer. While elastic, SMP can be manipulated in many ways. It can be reshaped many times without losing material integrity.

SMP can be cast and cured into any "memorized" shape, from a thick sheet to a concave dish, to a complicated open honeycomb network. SMP can also be used as a coating on another material. This versatility makes SMP ideal for applications such as dynamic configurable parts, deployable components, and inexpensive, reusable custom molds.

SMP Composites

Shape memory polymer (SMP) resin is an integral component of a shape memory composite system. A shape memory composite acquires some SMP characteristics, making it a unique material for use in dynamic structures and other applications requiring both load strength and "shape-shifting" flexibility.

Under thermal controls, shape memory composites can be temporarily softened, reshaped, and rapidly hardened to function as structures in a variety of configurations. An example of a use for this versatility is the capability for space-efficient stowing and then later deployment to the operational shape. The composites can be fabricated with nearly any fiber type, and creative reinforcements permit dramatic shape changes in functional structures. Shape memory composites are also machinable.

Applications include lightweight, rigid deployable structures (an alternative or enhancement to current inflatable structures), rapid manufacturing, and dynamic reinforcement.

What is Veriflex?

This coupon of Veriflex begins with a rectangular memory shape. When heated above its transition temperature, it becomes elastic and can be manipulated into a different shape and then cooled to maintain the new shape in a rigid state. When reheated above its transition temperature, it will return to its memory shape if left unrestrained."

The registered trademark for CRG's family of shape memory polymer resin systems, Veriflex currently functions on thermal activation customizable from –30°C to 260°C (–20°F to 500°F). Extremely high temperatures and cryogenic ranges may be possible. CRG is also investigating possible future systems that may respond to stimuli such as light, electric field, or magnetic field. Some examples of applications include custom reusable mandrels, reusable molds, replica optics, and deployment mechanisms for outer space.

CRG has been awarded several patents based on this technology: Shape Memory Styrene Copolymers, US Pat. No. 6,759,481; Structural and Optical Application for SMP, US Pat. No. 6,986,855; Maleimide-Based High-Temp SMP, US Pat. No. 7,276,195; and Shape Memory Mandrel, US Pat. No. 7,422,714.

There are many activation methods for thermally responsive SMP:

  • Resistive heating
  • Embedded heaters (for example, stretchy heaters, nichrome wires)
  • Contact heating (MRE heaters)
  • Induction heating
  • Dielectic heating
  • Microwave heating
  • Infrared radiant heating

Some of these methods may be enabled by fillers such as conductive fillers, CNT, CNF, iron and ferrite.

Veriflex® E

Cured Veriflex E combines shape memory properties with the high toughness and strength of epoxy. When heated above its activation temperature, Veriflex E changes from a rigid plastic to an elastic state. While elastic, it can be twisted, pulled, bent, and stretched. When cooled, the polymer hardens and can maintain its deformed configuration indefinitely. When heated again, the polymer returns to the shape in which it was cured. This process can be repeated for many cycles without loss of the memory shape or degradation of the material.

Veriflex E comes as either a one-part or two-part, fully formable, thermoset shape memory polymer (SMP) resin system. Engineered with an activation temperature of approximately 90°C (194°F) for one-part Veriflex E, and 104°C (219°F) for two-part Veriflex E, the resin can be cured in an open mold.

Epoxy is a well-understood and commonly used resin system. It provides convenient processing, usage, and property modification. Veriflex E is a high-performance material formulated from commercially available resins and curing agents. It can be cast and cured into a variety of “memorized” shapes, depending on the application requirements. Epoxy SMP has a very sharp modulus transition curve, yielding a quick recovery to the “memory” shape upon heating above the transition temperature.

There are many activation methods for thermally responsive Veriflex E:

  • Resistive heating
  • Embedded heaters (for example, stretchy heaters, nichrome wires)
  • Contact heating (MRE heaters)
  • Induction heating
  • Dielectic heating
  • Microwave heating
  • Infrared radiant heating

Some of these methods may be enabled by fillers such as conductive fillers, CNT, CNF, iron and ferrite.

Benefits

  • Toughness
  • Unique shape memory properties
  • Recovery to memorized shape after repeated deformation
  • Ability to change from a rigid polymer to rubbery elastomer
  • Over 95% (one-part resin) and 100% (two-part resin) elongation possible in elastic state
  • Low viscosity for easy processing (RTM or VARTM) (two-part resin)
  • Open-mold curable
  • Aesthetic clarity
  • Machinability once cured

Applications

  • Customized, reusable molds
  • Deployable mechanisms and structures
  • Adjustable furniture
  • Reformable toys
  • Customized containers, adjustable shipping and packaging
  • Actuators
  • Sensors
  • Space-qualifiable applications
  • Removable mandrels
  • Automotive components

Veritex™ Composites

CRG's Shape Memory Composite Systems

Veritex is the trademark name for CRG's family of dynamic shape memory composites. Dynamic polymer composites are like other high-performance composites, except CRG's shape memory polymer, Veriflex®, is used as the matrix. Fabrication with Veriflex resin allows easy manipulation of the composite above its activation temperature and high strength and stiffness at lower temperatures.

Veritex composites capitalize on the ability of the shape memory resin to quickly soften and harden repeatedly. Because of this property, the composites can be temporarily softened, reshaped, and rapidly hardened to function as structures in a variety of configurations. They can be fabricated with nearly any fiber type, and creative reinforcements allow dramatic shape changes in functional structures. Veritex is also machinable. Some possible applications include rapid manufacturing, dynamic structures, composite patching, and adaptable reinforcement.

The photos below show composite Veritex material. These composite structures can be temporarily manipulated for storage and/or transportation, or their configurations changed in real-time to fulfill varying applications.

Veritex is the base material found in Rec'Repair and Rubbn'Repair patch material.