Tat Hung Tong

Light-activated shape memory polymers and associated applications

Continuous product development and technology integration efforts using shape memory polymers (SMPs) have uncovered a need for faster response times. As with most smart materials, SMP responds to a specific stimulus. Traditionally SMP is triggered by thermal stimulus; increasing the temperature of the SMP above a Tg will transition the polymer from a glassy state to a rubbery state. The transition is reversible upon cooling below the Tg. It has been determined that many SMP applications can be significantly enhanced with non-thermal triggering. Non-thermal triggering eliminates the need for heating mechanisms and reduces cycle time. Furthermore, it has been found that with a faster response time many new applications become viable. Previous successful attempts have been made to improve response time of SMP by increasing its thermal conductivity with various thermally conductive additives1. However, thermal heating and cooling of polymers and composites of substantial thickness, thermally conductive or not, takes time. In an effort to facilitate system integration and increase the response time of SMP, researchers at Cornerstone Research Group, Inc. (CRG) have sought to eliminate the thermal dependency of SMP by developing light-activated shape memory polymer (LASMP). In this work, monomers which contain photo-crosslinkable groups in addition to the primary polymerizable groups were developed. These monomers were formulated and cured with other monomers to form LASMP. The mechanical properties of these materials, the kinetics, and the reversibility of the light-activated shape memory effect were studied. The near-, mid-, and far-term potential of this new material technology for system level applications is discussed.

Towards Novel Light-Activated Shape Memory Polymer: Thermomechanical Properties of Photo-responsive Polymers

The basic principle for the operation of a thermally stimulated shape memory polymer (SMP) is a drastic change in elastic modulus above the glass transition temperature ( T g ). This change from glassy modulus to rubbery modulus allows the material to be deformed above the T g and retain the deformed shape when cooled below the T g . The material will recover its original shape when heated above the T g again. However, thermal activation is not the only possibility for a polymer to exhibit this shape memory effect or change of modulus. This paper discusses results of an alternative approach to SMP activation. It is well known that the T g of a thermosetting polymer is proportional to its crosslinking density. It is possible for the crosslinking density of a room temperature elastomer to be modified through photo-crosslinking special photo-reactive monomer groups incorporated into the material system in order to increase its T g . Correspondingly, the modulus will be increased from the rubbery state to the glassy state. As a result, the material is transformed from an elastomer to a rigid glassy photoset, depending on the crosslinking density achieved during exposure to the proper wavelength of light. This crosslinking process is reversible by irradiation with a different wavelength, thus making it possible to produce light-activated SMP materials that could be deformed at room temperature, held in deformed shape by photo-irradiation using one wavelength, and recovered to the original shape by irradiation with a different wavelength. In this work, monomers which contain photo-crosslinkable groups in addition to the primary polymerizable groups were synthesized. These monomers were formulated and cured with other monomers to form photo-responsive polymers. The mechanical properties of these materials, the kinetics, and the reversibility of the photo-activated shape memory effect were studied to demonstrate the effectiveness of using photo-irradiation to effect change in modulus (and thus shape memory effect).

Applications of liquid crystal network polymers (Invited Paper)

Cornerstone Research Group Inc. (CRG) will present a brief overview on our recent efforts in developing liquid crystal (LC) network polymers for various applications. These efforts cover different aspects of the LC network polymer material technology in optical, structural and other novel applications. Liquid crystal network polymers have demonstrated great potential in producing high-performance optical components and smart materials, such as actuators. We will discuss the potential applications of these materials as optical filters, reflectors for lightweight space-based mirrors, and structural resins to improve toughness. The potential to capitalize on the templating capability of these materials to produce novel all-polymer conducting composites will also be discussed. Various possibilities and directions for future research and applications of liquid crystal network polymers will also be presented.