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Composite truss structures are being developed that can be compacted for stowage and later deploy themselves to full size and shape. In the target applications, these smart structures will precisely self-deploy and support a large, lightweight space-based antenna. Self-deploying trusses offer a simple, light, and affordable alternative to articulated mechanisms or inflatable structures. The trusses may also be useful in such terrestrial applications as variable-geometry aircraft components or shelters that can be compacted, transported, and deployed quickly in hostile environments. The truss technology uses high-performance shape-memory-polymer (SMP) thermoset resin reinforced with fibers to form a helical composite structure. At normal operating temperatures, the truss material has the structural properties of a conventional composite. This enables truss designs with required torsion, bending, and compression stiffness. However, when heated to its designed glass transition temperature (Tg), the SMP matrix acquires the flexibility of an elastomer. In this state, the truss can be compressed telescopically to a configuration encompassing a fraction of its original volume. When cooled below Tg, the SMP reverts to a rigid state and holds the truss in the stowed configuration without external constraint. Heating the materials above Tg activates truss deployment as the composite material releases strain energy, driving the truss to its original memorized configuration without the need for further actuation. Laboratory prototype trusses have demonstrated repeatable self-deployment cycles following linear compaction exceeding an 11:1 ratio (see figure).

Composite truss structures are being developed that can be compacted for stowage and later deploy themselves to full size and shape. In the target applications, these smart structures will precisely self-deploy and support a large, lightweight space-based antenna. Self-deploying trusses offer a simple, light, and affordable alternative to articulated mechanisms or inflatable structures. The trusses may also be useful in such terrestrial applications as variable-geometry aircraft components or shelters that can be compacted, transported, and deployed quickly in hostile environments. The truss technology uses high-performance shape-memory-polymer (SMP) thermoset resin reinforced with fibers to form a helical composite structure. At normal operating temperatures, the truss material has the structural properties of a conventional composite. This enables truss designs with required torsion, bending, and compression stiffness. However, when heated to its designed glass transition temperature (Tg), the SMP matrix acquires the flexibility of an elastomer. In this state, the truss can be compressed telescopically to a configuration encompassing a fraction of its original volume. When cooled below Tg, the SMP reverts to a rigid state and holds the truss in the stowed configuration without external constraint. Heating the materials above Tg activates truss deployment as the composite material releases strain energy, driving the truss to its original memorized configuration without the need for further actuation. Laboratory prototype trusses have demonstrated repeatable self-deployment cycles following linear compaction exceeding an 11:1 ratio (see figure).

Shape Memory Polymers (SMPs) are a class of polymers, which can undergo deformation in a flexible state at elevated temperatures, and when cooled below the glass transition temperature, while retaining their deformed shape, will enter and remain in a rigid state. Upon heating above the glass transition temperature, the shape memory polymer will return to its original, unaltered shape. SMPs have been reported to recover strains of over 400%. It is important to understand the stress and strain recovery behavior of SMPs to better develop constitutive models which predict material behavior. Initial modeling efforts did not account for large deformations beyond 25% strain. However, a model under current development is capable of describing large deformations of the material. This model considers the coexisting active (rubber) and frozen (glass) phases of the polymer, as well as the transitions between the material phases. The constitutive equations at the continuum level are established with internal state variables to describe the microstructural changes associated with the phase transitions. For small deformations, the model reduces to a linear model that agrees with those reported in the literature. Thermomechanical characterization is necessary for the development, calibration, and validation of a constitutive model. The experimental data reported in this paper will assist in model development by providing a better understanding of the stress and strain recovery behavior of the material. This paper presents the testing techniques used to characterize the thermomechanical material properties of a shape memory polymer (SMP) and also presents the resulting data. An innovative visual-photographic apparatus, known as a Vision Image Correlation (VIC) system was used to measure the strain. The details of this technique will also be presented in this paper. A series of tensile tests were performed on specimens such that strain levels of 10, 25, 50, and 100% were applied to the material while it was above its glass transition temperature. After deforming the material to a specified applied strain, the material was then cooled to below the glass transition temperature (Tg) while retaining the deformed shape. Finally, the specimen was heated again to above the transition temperature, and the resulting shape recovery profile was measured. Results show that strain recovery occurs at a nonlinear rate with respect to time. Results also indicate that the ratio of recoverable strain/applied strain increases as the applied strain increases.

This paper investigates the thermomechanical behavior of a thermosetting shape memory polymer (SMP) by using a high temperature nanoindentation technique. The nanoindenter is equipped with a microheater and a sophisticated temperature control and monitoring system. This allows the SMP to be activated at elevated temperatures enabling proper implementation of the thermomechanical cycle typically used to quantify the shape memory behavior. The load-depth curves of the SMP were obtained at various temperatures, from which the instantaneous moduli were calculated with a revised indentersample contact depth formula. The moduli from nanoindentation are consistent with those obtained from dynamic mechanical analysis on bulk samples. When activated at elevated temperatures, the SMP exhibits surface profiles different from those obtained when activated at room temperature. A large amount of sink-in is observed at the SMP surface when activated at temperatures above its glass transition temperature (Tg). It is seen that the large-strain elastic deformation is almost fully recoverable when recovery takes place at a recovery temperature, Tr is greater than Tg.

Reconfigurable multifunctional structures, which allow combined changes of shape, functionality and mechanical properties on demand, require new adaptive materials and novel chemistry that permit reversible modulation of mechanical properties in effective manner. They also demand the development of robust modeling and design tools based on a fundamental understanding of the complex and time-variant properties of the material and mechanization structure in diverse environments. In this research, we: 1) developed two new classes of two-way shape memory polymers (SMPs); 2) fostered these two SMPs to free-standing SMP composites with enhanced reversible modulation through novel composite design; 3) pursued a fundamental understanding of underlying physics of the proposed two-way SMPs and composites; 4) established modeling and simulation-design tools for applications of these novel materials for reconfigurable aerospace structures; and 5) explored design, fabrication and testing of novel SMP devices enabling for Air Force applications.

A research grade Dynamic Mechanical Analyzer (DMA) was acquired in 2007 through a successful Department of Defense grant under the DURIP program. This equipment has played a key role in the evaluation of candidate polymeric materials for developing reconfigurable, or morphing), aerospace structures. In particular, shape memory polymers (SMP) in filled and unfilled form have been investigated with particular emphasis on the recovery time and force as the materials undergo transformation. Response time and recovery force are performance characteristics essential to the design of SMP based actuators and reconfigurable structures, and new testing protocols have been developed to quantity these merits of performance. While favorable response times have been observed, the low recovery force measured in experiments, and the bulky size of the triggering mechanism have presented themselves as challenges in the development of efficient and lightweight structures. However, the provision of the equipment is enabling more research into the modification of the polymer's properties to overcome these shortcomings. Ongoing approaches include alternation of the SMP's chemistry and the use of different fillers such as carbon black, aniline, and micron sized iron and copper particles.

physical conformation changes when exposed to an external stimulus, such as a change in temperature. Such materials have a permanent shape, but can be reshaped above a critical temperature and fixed into a temporary shape when cooled under stress to below the critical temperature. When reheated above the critical temperature (Tc, also sometimes called the triggering or switching temperature), the materials revert to the permanent shape. The current innovation involves a chemically treated (sulfonated, carboxylated, phosphonated, or other polar function group), high-temperature, semicrystalline thermoplastic poly(ether ether ketone) (Tg .140 C, Tm = 340 C) mix containing organometallic complexes (Zn++, Li+, or other metal, ammonium, or phosphonium salts), or high-temperature ionic liquids (e.g. hexafluorosilicate salt with 1-propyl-3- methyl imidazolium, Tm = 210 C) to form a network where dipolar or ionic interactions between the polymer and the low-molecular-weight or inorganic compound forms a complex that provides a physical crosslink. Hereafter, these compounds will be referred to as "additives". The polymer is semicrystalline, and the high-melt-point crystals provide a temporary crosslink that acts as a permanent crosslink just so long as the melting temperature is not exceeded. In this example case, the melting point is .340 C, and the shape memory critical temperature is between 150 and 250 C. PEEK is an engineering thermoplastic with a high Young fs modulus, nominally 3.6 GPa. An important aspect of the invention is the control of the PEEK functionalization (in this example, the sulfonation degree), and the thermal properties (i.e. melting point) of the additive, which determines the switching temperature. Because the compound is thermoplastic, it can be formed into the "permanent" shape by conventional plastics processing operations. In addition, the compound may be covalently cross - linked after forming the permanent shape by S-PEEK by applying ionizing radiation ( radiation, neutrons), or by chemical crosslinking to form a covalent permanent network. With respect to other shape memory polymers, this invention is novel in that it describes the use of a thermoplastic composition that can be thermally molded or solution-cast into complex "permanent" shapes, and then reheated or redissolved and recast from solution to prepare another shape. It is also unique in that the shape memory behavior is provided by a non-polymer additive.

Shape memory polymers (SMPs) are materials that have properties, such as Young's modulus, that change in response to an external stimulus. As such they are one of a number of materials, including shape memory alloys (SMAs) and ceramics, that can be used as adaptive materials in intelligent systems. In this memorandum, the literature pertaining to shape memory polymers is reviewed. Topics covered include the history of the development and commercialization of SMPs, the basis of the shape memory effect in polymers, the advantages and disadvantages of SMPs, applications of SMPs, the description of linear and nonlinear constitutive models proposed for SMPs, and the potential to develop poly(urethane) based SMPs with tailored properties.