Amber J. W. McClung

Photomechanical bending mechanics of polydomain azobenzene liquid crystal polymer network films

Glassy, polydomain azobenzene liquid crystal polymer networks (azo-LCNs) have been synthesized, characterized, and modeled to understand composition dependence on large amplitude, bidirectional bending, and twisting deformation upon irradiation with linearly polarized blue-green (440–514 nm) light. These materials exhibit interesting properties for adaptive structure applications in which the shape of the photoresponsive material can be rapidly reconfigured with light. The basis for the photomechanical output observed in these materials is absorption of actinic light by azobenzene, which upon photoisomerization dictates an internal stress within the local polymer network. The photoinduced evolution of the underlying liquid crystal microstructure is manifested as macroscopic deformation of the glassy polymer film. Accordingly, this work examines the polarization-controlled bidirectional bending of highly concentrated azo-LCN materials and correlates the macroscopic output (observed as bending) to measured ...

Fatigue Cycling of Shape Memory Polymer Resin

Shape memory polymers have attracted great interest in recent years for application in reconfigurable structures (for instance morphing aircraft, micro air vehicles, and deployable space structures). However, before such applications can be attempted, the mechanical behavior of the shape memory polymers must be thoroughly understood. The present study represents an assessment of viscoelastic and viscoplastic effects during multiple shape memory cycles of Veriflex-E, an epoxy-based, thermally-triggered shape memory polymer resin. The experimental program is designed to explore the influence of multiple thermomechanical cycles on the shape memory performance of Veriflex-E. The effects of the deformation rate and hold time at elevated temperature on the shape memory behavior are also investigated.

Novel Bismaleimide-Based Shape Memory Polymers: Comparison to Commercial Shape Memory Polymers

A series of novel shape memory polymers, synthesized from 4-4-bismaleimidodiphenyl-methane, an extended chain aliphatic diamine, and a bis-isocyanate, have been created and characterized with the aim of providing a family of robust high temperature shape memory polymers with tailorable transition temperatures for use in reconfigurable aerospace structures. In the present study, three of the polymers are chosen for more detailed study of their thermomechanical properties. These materials are compared to commercial resins Veriflex® and Veriflex-E® which are styrene- and epoxy-based proprietary formulations, respectively. The thermal and mechanical properties are determined utilizing thermogravimetric analysis and dynamic mechanical analysis. The temperatures at which 2% weight loss is observed in dry air ranges from 272 to 305 °C for the synthesized polymers, and occurs at 242 and 317 °C for the commercial Veriflex® and Veriflex-E® respectively. The glass transition temperatures, as measured by the peak in the tan(δ) curve, for the synthesized polymers range from 110 to 144 °C which is a higher than the Veriflex® and Veriflex-E® achieve at 84.3 and 100 °C respectively. With operation temperatures of subsonic structural aircraft components often reaching 121 °C (250 °F), the transition temperatures of the bismaleimide-based shape memory polymers are clearly desirable to ensure that shape memory polymers used in aircraft structures will not be prematurely triggered by the existing heat loads. In addition, the shape memory performance of the bismaleimide-based shape memory polymers compares well with the Veriflex® and Veriflex-E® resins.Copyright

The Strain Rate- and Temperature-Dependent Mechanical Behavior of Veriflex-E in Tension

In this study, the inelastic deformation behavior of Veriflex-E, a thermally-triggered shape memory polymer resin, was investigated. The experimental program was designed to explore the influence of strain rate on monotonic loading at various temperatures. In addition, the creep behavior of specimens at various temperatures was evaluated. The time-dependent mechanical behavior of the Veriflex-E resin is strongly influenced by the temperature as well as the deformation rate. Thermally-actuated shape memory polymers can be thought of as having two phases separated by the glass transition temperature (Tg ). At temperatures well below the Tg (room temperature), the Veriflex-E exhibits a high elastic modulus and positive, nonlinear strain rate sensitivity in monotonic loading. Likewise, the room temperature creep response is significantly influenced by the prior strain rate. The Poisson’s ratio at room temperature is independent of the strain rate, but dependent upon the strain magnitude. As the temperature is increased, the strain rate sensitivity in monotonic loading decreases. Well above the Tg , the elastic modulus drops by several orders of magnitude, and strong strain rate sensitivity is no longer observed in the path of the stress-strain curve. In this high temperature region, the material achieves strain levels well above 100% and the Poisson’s ratio is constant at 0.5 regardless of strain rate or strain magnitude. The creep strain, on the other hand, is significantly influenced by the prior strain rate at the elevated temperature. A slight hysteresis is observed during unloading, while recovery following unloading shows a permanent strain.

The relationship between constituent property and bending actuation of shape memory composites

Shape memory composites (SMCs) made of shape memory alloy (SMA) wires embedded in a shape memory polymer (SMP) matrix show great promise in adaptive structures due to their ability to combine strong actuation force of SMAs with the large deformation of SMPs. However, in order to advance these SMCs past the proof-of-concept stage, methods for optimizing the material selection and volume fraction balance between the alloy and the polymer must be developed. Before such multi-objective optimization can take place, sensitivity studies must be conducted to determine which parameters are most impactful in determining the ultimate behavior of the SMC. The present study focuses on thermal and mechanical characterization of a group of SMCs composed of Nickel-Titanium SMA and a styrene based SMP in order to elucidate the critical material properties for a functioning SMC. Here, the SMA wires have been trained with a one-way shape memory effect (SME) and integrated on the surface of the SMP. A morphing system has been obtained by introducing SMA wires on the SMP system in the volume fraction range of 0.5% to 1.2%. The results show that the proper modulus balance between the SMP and SMA as well as the balance of activation forces are important features to consider when developing a SMC. Also important is a proper pairing of glass transition temperature for the SMP with the Austenite and Martensite transition temperatures for the SMA. In addition, the current study has shown that during the heating process of the SMC, the thermal expansion of the SMP appears to overcome the actuation forces of the SMA wires, showing that the thermal expansion is also a critical variable for the composite performance. Based on these results, both the SMA and the SMP constituent parameters of temperatures, elastic modulus, actuation force, and thermal expansion are considered with regards to future tailoring the SMC performance.

Optimizing the photomechanical performance of glassy azobenzene liquid crystal polymer networks

Glassy, polydomain azobenzene liquid crystal polymer networks (azo-LCN) have been synthesized, characterized, and modeled to understand composition dependence on large amplitude, bidirectional bending and twisting deformation upon irradiation with linearly polarized blue-green (440-514 nm) light. These materials exhibit interesting properties for adaptive structure applications in which the shape of the photoresponsive solid state structure can be rapidly reconfigured with light. The basis for the photomechanical output observed in these materials is absorption of actinic light by azobenzene, which upon photoisomerization dictates an internal stress within the local polymer network. The photoinduced disruption of the order/orientation of the local polymer network accompanying photoisomerization is manifested in a macroscopic deformation. Accordingly, this work examines the polarization-controlled bidirectional bending of highly concentrated azo-LCN materials and compares the macroscopic bending to a nonlinear photoshell model. The resulting photomechanical output is highly dependent on the concentration of crosslinked azobenzene mesogens employed in the formulation. The model comparisons illustrate differences in internal photostrain and deformation rates as a function of composition.

Photoresponsive Azobenzene Liquid Crystal Polymer Networks: In Situ Photogenerated Stress Measurement

Azobenzene liquid crystal polymers and polymer networks are adaptive materials capable of converting light into mechanical work. Often, the photomechanical output of the azobenzene liquid crystal network (azo-LCN) is observed as a bending cantilever. The response of these materials can be either static (e.g. a simple bending cantilever) or dynamic (e.g. oscillating cantilever of 20–270 Hz). The resulting photomechanical output is dependent upon the domain orientation of the polymer network and the wavelength and polarization of the actinic light. Polydomain azobenzene liquid crystal polymer networks, which have the capability of bending both backwards and forwards with the change of polarization angle, are of particular interest. In the current study, three azo-LCNs are compared — two of them are equivalent in all respects except for one contains pendant azobenzene mesogens (1azo, azo-monoacrylate) and the other contains crosslinked azobenzene mesogens (2azo, azo-diacrylate). The third specimen has a combination of both mesogens. The mechanical behavior at different temperatures and examination of structure-property relationships in the polymerization process, including curing temperatures and liquid crystal cell alignment rubbing methods, were explored. Using dynamic mechanical analysis (DMA) the mechanical properties and the photogenerated stress and strain in the polymer are examined. It is found the differences in chemistry do correlate to small variation in the speed of photodirected bending, elastic modulus, and glass transition temperature. Despite these differences, all three azo-LCNs display nearly equivalent photogenerated stresses.Copyright

Non-Contact Technique for Characterizing Full-Field Surface Deformation of Shape Memory Polymers

Thermally-actuated shape memory polymers (SMPs) typically display two phases separated by the glass transition temperature (Tg ). At temperatures well below the Tg , the polymer exhibits a relatively high elastic modulus. Well above the Tg the elastic modulus drops by several orders of magnitude. In this high temperature region, SMP materials can achieve strain levels well above 100 %. The complex behavior of SMPs (stiffnesses dropping to the order of 1 GPa and extremely high strain levels) precludes the use of traditional strain gages and low-contact force extensometers. The present study presents a detailed expansion of state-of-the-art thermomechanical testing techniques used to characterize the material behavior of SMPs. An MTS environmental chamber with an observation window allows for non-contact optical measurements during testing. A laser extensometer is used for measurement and active control of axial strain. The upper limit on the strain rate capability of the laser extensometer is established. In addition, the photographic strain measurement method known as digital image correlation (DIC) is incorporated, allowing for full field measurement of axial and transverse strains of SMPs over a range of temperatures and strain rates. The strain measurements of the DIC and laser extensometer are compared to each other as well as to clip-on extensometers and strain gages. The comparisons provide insight into the limitations of the traditional strain measurement systems. A series of tensile tests are performed on a commercial SMP from 25 °C up to temperatures of 130 °C and strain levels above 100 %. The laser extensometer provides a robust method for controlling the strain in the gage section of the samples. In addition, results show that the full field measurements of both the axial and the transverse strain are essential for characterizing the constitutive response of SMPs at room and elevated temperatures.© 2010 ASME