Incorporation of Shape Memory Polymers in Interactive Design

Jessica Berry, Jinsil Hwaryoung SeoTexas A&M University

College Station, TX USAjlb70@tamu.edu, hwaryoung@viz.tamu.edu

Abstract

This paper seeks to explore the question of how to incorpo-rate smart materials into a design to aesthetically demonstratescience and engineering concepts in an interactive way. Thiswork introduces the development of interactive artworks usinga shape memory polymer (SMP) material that changes shapebased on an external thermal stimulus. In this paper we explorehow interdisciplinary work between engineering and scienceis needed to create an artwork that mimics natural phenom-ena around us. This paper discusses fabrication and electronicimplementation challenges associated with utilizing the shapememory properties of the material. Specifically, the paper ex-plores some ways to obtain different geometric shapes of theSMP and to utilize different sources of thermal stimulus to cre-ate a shape memory effect (SME) in artworks.

Keywordssmart materials, shape memory polymer, interactive artwork,interdisciplinary collaboration

IntroductionCreating artistic models that allow children to experiencescience, technology, engineering, and mathematics (STEM)fields, may increase their propensity towards choosing thesefields to career paths. Smart materials provide a uniquemedium to present scientific information, because they arematerials that exhibit a change based on an applied stimu-lus. Some examples of smart materials are shape memoryalloys (SMAs), shape memory polymers, thermo-chromaticpowders and photo-chromatic powders. Solar Color Dust isa brand of thermo-chromatic and photo-chromatic powdersthat change from a given color to a white color with heat,body temperature, or light stimulus [1].

SMAs and SMPs exhibit what is known as the SME, theability to hold a temporary structure and geometric configu-ration until a stimulus reverses it structure back to its originalconfiguration. For SMAs, this effect is due to a phase changein the metal. This phase change is a change in the crystal lat-tice structure from a low temperature phase called martensiteto a high temperature phase called austenite. By holding a de-sired shape at a high temperature, called the Austenite finishtemperature, Af , and then quenching it to a low temperaturecalled the Martensite finish temperature, Mf , a shape may betrained into the material to create the SME [2]. Once an SMA

is trained, SME occurs by applying force to the metal it in thelower temperature martensitic phase causing a shifting in thecubic lattice structure that results in macroscopic deformationof the object. Afterwards, the force is removed and the ma-terial is heated to above Af . At this temperature the metalreturns to the austenitic cubic structure which causes it to re-turn to its trained shape [3]. Once, the SMA is cooled again,it can be deformed again to another shape.

SMPs that have a shape change from a thermal stimulusare of interest to this paper. In general polymers have dif-ferent properties above and below a temperature known asthe glass transition temperature or Tg . Below this tempera-ture the material is described as stiff or rubbery, and aboveit is is described as glassy. Polymers are made of networksof chains of molecules, that are connected at points. Typi-cally the chains within an SMP are described as either frozensegments or switching segments and the connection betweenthese two types of segments is called a ”netpoint” [4] [5].When heated above the Tg , the switching segments are ableto move to create a new shape, while the frozen segmentsremain rigid. If force is applied at T > Tg then the switch-ing segments move to accommodate the force, creating a newshape. If the temperature is lowered to T < Tg , the switchingsegments are frozen in their new configuration and the newdeformed shape is held. If the temperature is again raised toT > Tg the switching segments move back to the originalshape. This can be seen in Figure 1.

Figure 1: Shape Memory Cycle in an SMP: The temporaryshape is formed by applying heat plus mechanical deforma-tion and rapidly cooling it. The original shape is recoveredby applying heat again. [6]

Prior ArtworksArtists have explored the area of using smart materials as amedium. Elaine Ng Yan Ling is a textile artist creating de-signs she describes a ”naturology” the combination of na-ture and technology. Her work incorporates shape memorypolymer and alloys and wood [7]. Among her pieces she hascreated a Living Furnishing Fabric with uses shape-memorypolymer to allow the textile to respond to humidity. Also, ashape memory polymer and wood veneer piece for seasonaldepression disorder. This work is designed to allow light dur-ing the winter months a projects interact shadows indoors inresponse to heat [8]. Other artists include Jie Qi whose mainfocus is on creating activation and movement of origami us-ing SMAs. Among some of her projects she has tested howmaking origami cranes with incorporated SMA can be usedas an educational tool with children ages 9-15 [9]. Charo-lette Lelieveld investigate the use of smart materials for anadaptive architecture concept. She investigated SMA embed-ded into SMP composite system on a small scale to prototypefor larger shape morphing building applications. With herprototype she created a structure which exhibited a wrinkledappearance when activated by heat. These wrinkles occurredbecause when the SMA was heated it changed shape, and theSMP became rubbery to accommodate the force of the SMA.Her work noted some possible drawbacks of work with SMAsand SMPs including fatigue of the SMA and issues related toenvironmental heating of the system [10]. Yvonne Y.F. ChanVili has done work in incorporating SMAs and SMPs intosmart textile applications to enhance their appearance. Shecreated SMA and SMP yarns for woven applications. Chancreated interesting aesthetics by weaving the shape memoryyarns into patterns to create a 3D surface effect. Her workprovides a proof of concept for extending the shape memoryyarns to create window treatments, partitions, or wall hang-ings to chance the essence and function of a space [11]. TheTexas Institute for Intelligent Materials and Structures (Ti-iMS) at Texas A&M University have installed a ’Pop-Op’which is a shape memory alloy wall installation composedof resin and C-glass fiber. The installation contains severalcomponents like flowers and flaps that are actuated by SMAand controlled by an Arduino [12].

Fabrication of Shape Memory PolymerThe polymer used in this study is based on the work ofXie and Rosseau [13]. This polymer consists of three com-ponents, diglycidyl ether bisphenol A epoxy (Epon 826),poly(propylene glycol) bis(2-aminopropyl) ether (JeffamineD-230), and neopentyl glycol diglycol ether (NGDE). TheSME is created due to the chemical structural differencesand reactivity of NGDE and Epon 826. Jeffamine D-230 isthe cross-linking agent in this structure, meaning its aminegroups, −NH2, react with the epoxide groups of Epon 826and NGDE to create the structure. This reaction creates thecross-links in the polymer, which are the connection pointsbetween the chains called ”netpoints.” Attached to these ”net-points” are either the NGDE chains or the Epon 826 chains.The NGDE chains are considered the switching segments be-cause it is a flexible aliphatic diepoxide, meaning it contains

carbon atoms forming open chains as its backbone. The Epon826 are the frozen segments because it is a rigid aromaticepoxide, meaning it contains carbon rings on its backbone.Therefore, the shape memory in this polymer occurs becausewhen the polymer is heated, above the Tg , the NGDE seg-ments become flexible and bendable [13]. These segmentsare able to be rotated around their cross-links to form a newshape. Once, the shape deformed and cooled to below the Tg ,the new shape is set. If the material is heated again, the shapewill return to its pre-deformed shape.

The polymers used in this study were fabricated accordingto the Xie and Rosseau formulations given in Table 1. To mixthe samples initially, some of the Epon 826 was placed in aglass vile and heated on a hot plate until the viscosity was re-duced and there was no appearance of bubbles in the mixture.The Solar Color Dust was weighed based on the appropriateamount for the recommended mix ratio of 10 g powder to 1pint of solution [1]. This dust was placed in a glass jar. Theamount of Jeffamine D-230 for the desired Tg was measuredwith a graduated cylinder and placed into the same glass jar.This mixture was then sonicated. The NGDE was then mea-sured and added to the Jeffamine and the mixture was againsonicated. After the Epon 826 was sufficiently heated, it wasmeasured and then added to the mixture, stirred, and soni-cated. The mixture was vacuumed with a for approximately20 minutes and then placed in a silicone rubber mold. Thenthe mixture was cured for 1.5 hours at 100◦C and post-curedfor 1 hour at 130◦C [13].

Table 1: Shape Memory Polymer Volumetric Mix Ratios [13]

Samples Epon826 (ml)

JeffamineD-230(ml)

NGDE(ml)

Tg (◦C)

1 4.7 2.43 1 60-802 3.14 2.43 2.08 40-603 1.57 2.43 3.12 20-404 0 2.43 4.15 0-10

Working with Shape Memory PolymerCreating interactive artworks using shape memory polymersprovides some challenging components. However, it pushesthe boundary of material aesthetics and helps us to focus oninteractions with the material. Since, additional thermal en-ergy is needed to cure the polymer, the temperature of theoven make it difficult to incorporate electronic components orsensors into the polymer itself. Also, the shape memory poly-mer must be heated and deformed at a high temperature andthen cooled quickly. Therefore, the environment for an instal-lation should be considered when determining which Tg is ap-propriate. Molding of a SMP can also require time to obtainthe correct mold for the artistic structure desired. To make themold, first a plastic female mold is made with a 3D printer,then a silicone rubber male mold is cast from the 3D printedmold. This silicone mold is used to cast the SMP. Therefore,it can be time consuming to create new shapes with the SMP.Also, incorporation internal heating means, such as wires,can create a stress concentration, which causes the material

to break at the interface when deformed. As mentioned pre-viously, the artworks focus on scientific concepts in our dailylife.

AuxesisThis work was made to help describe the mechanism in whichchameleons change color. Chameleons change color to sendvisual signals to other animals regarding their mood, aggres-sion, territorial instincts, or for mating purposes. The changeof color in a chameleon occurs at the microscopic level basedon cells called chromatophores. These chromatophores be-have as flexible bags of color that are either stretched out tocover a large flat area or retracted back to a small, retractedpoint [14]. Each chromatophore is attached to radial musclefibers at various points along its edge controlled by a nervefiber. When a nerve impulse is sent, it causes these mus-cles to contract and expand the chromatophore. When themuscles relax, the chromatophore returns to a small, compactshape, thus reducing its area and making the pigmented areashrink [14].

Figure 2: Contraction of Chiral Structure: Expanded (left)andCompressed States (right)

This art piece explored the use of SMP materials to mimicthe cells of the chameleon. In order to accomplish this task,the SMP was molded into a rectangular mold and then lasercutter was used to explore different shapes. Through trial anderror a chiral shape with is a type of auxetic shape, mean-ing the shape has a negative Poisson’s ratio (the lateral widthincreases when stretched from either end), was chosen. Theidea of SMP based that utilizes the geometry of the auxeticshape was based on the work of Rossiter et. al. [15]. A pic-ture of the chiral shape in the curled and uncurled states aregiven in Figure 2.

In laser-cutting the chiral shape, several iterations are donein order to maintain the correct thickness for the struts of thechiral to avoid breaking the chiral. Also, the heat from theCO2 laser resulted in a gummy polymer, if the Tg of thepolymer was lower. The polymer performed better duringlaser cutting if it was taped and place in a freezer prior tolaser-cutting.

In order to mimic the color expansion and retraction ofchromatophores, Solar Color Dust and Castin’ Craft dye wasincorporated into the SMP matrix and fabric was added to theinside of the SMP chirals. Photos of these color and fabricchirals are given in Figure 3.

These shapes were mounted on a canvas and attached toservo motors. A Lilypad thermal sensor was placed in closeproximity to the shape. Once the thermal sensor detected a

Figure 3: Left: Chiral changes from purple to blue based onheat stimulus, Right: Colored Shape Memory Chirals withFabric Incorporated

sufficient temperature change, above the Tg , due to the hairdryer stimulus a command was sent from the Arduino to ro-tate the servo motor. The motor curled in the SMP chiralsuntil the thermal sensor detected that the temperature was re-duced to below Tg . Then the motors were told to release theirhold on the SMP chirals. The chirals remained in the de-formed until the temperature raised to return them back totheir original shape. The artwork is named ”Auxesis” whichis from the Greek meaning growth, which describes the mech-anism of the chromatophore and the structure that the chro-matophore uses. A depiction of the artwork is given in Fig-ure 4.

Figure 4: Front of ”Auxesis” contains the color changingcomponents

The Secret GardenThis artwork seeks create a piece where the observer can re-veal the beauty in nature. The artwork contains three com-ponents that can be revealed based on thermal stimulus thecomponents utilize SMP and Solar Color Dust. The first ofwhich is the blossoming of a flower. The hidden flower con-cept is inspired by the Chinese flowering tea, which is createdby needle tea leaves being wrapped and compressed arounda flower into a small ball. When the tea is heat the ball isopened to reveal a beautiful flower. To mimic the revealingof the flower, a shape memory polymer was laser-cut froma 2D sheet into the shape of a flower. The behavior of theflowering shape in hot water is given in Figure 5. In an in-stallation setting, the observer will place the flowers in clearbowl of water on a hot plate to open the flower.

The other two aspects of the artwork are the revealing ofhidden properties, beneath the surface. Specifically, moss iscast with clear resin and Solar Color Dust. When the materialis cool only some parts of visible to the observer, however asit is heated the viewer can see the intricate structure of themoss. From experimentation with different thicknesses of the

Figure 5: Left: SMP Flower Before Exposed to Hot Water,Right: SMP Flower After Exposed to Hot Water

resin with the moss, it was found that a thin layer of resinprovided the most ideal aesthetic, allowing the user to viewmore of the intricate details of the structure and requiring lessthermal energy to heat to reveal the moss. The third aspectof the artwork focuses on the changing of the leaves with theseasons. This aspect utilizes Solar Color Dust, clear resin,and heat pad. The leaves are coated with a black thermo-chromatic powder in a medium are cast into a clear epoxyresin. When the heat source is applied, the beautiful colors offall appear. Initially, conductive thread was used as the heatsource, however the thread only heats a small area around thethread, so for more through heating, a larger source is needed.

Conclusion and Future WorkSMP materials provide a unique challenge for interactive art-work due to the nature how the SME must be activated. Someof the issues in working with this specific SMP include, dif-ficulty in testing new shapes. Even though, the laser-cutterprovides a much faster method to test out new geometries, theheat from the CO2 laser does induce some thermal residualstress by breaking cross-links in the polymer and weakeningthe material. In the laser-cut chiral structure, often the strutsof the structure broke down due to thermal stress concentra-tions. Future work will explore the direct casting of the chiralshape. Also, some current work has been done on machiningthe SMP using a drill press. When measuring the hole createdby drilling the size of the hole and the size of the tool are ap-proximately the same size. This result, could mean that thereis some mild thermal residual stress due to covalent cross-links in the SMP breaking by the drilling action and causingopen chains of NGDE to reconfigure at the high temperature.From the qualitative preliminary testing, it appears that ma-chining provides a lower stress on the SMP than laser-cutting,resulting in a better end-product. Exploring the pros and consof machining, casting, and laser-cutting will provide valuableinformation on effective ways to create new shapes. Also, forthe ”Auxesis” work, a restructuring of the visual aesthetics togreater visualize the action of the SMP would create a workthat is a better representation of the chromatophores move-ment.

For ”The Secret Garden” piece, experimentation with amore complex layered 3D flowering shape is required. Also,the method of casting the hidden moss into thin sheets needsto be modified to create a consistent thin size of the cast. Fur-ther testing with a heating pad is needed on the changing leaveaspect of this work.

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