Stearic acid infused polyurethane shape memory foams

Stearic acid infused polyurethane shape memory foams

Accepted Manuscript Stearic acid infused polyurethane shape memory foams Marcos Pantoja, Tommy Alvarado, Mukerrem Cakmak, Kevin A. Cavicchi PII: S003...

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Accepted Manuscript Stearic acid infused polyurethane shape memory foams Marcos Pantoja, Tommy Alvarado, Mukerrem Cakmak, Kevin A. Cavicchi PII:

S0032-3861(18)30703-1

DOI:

10.1016/j.polymer.2018.08.002

Reference:

JPOL 20809

To appear in:

Polymer

Received Date: 22 May 2018 Revised Date:

16 July 2018

Accepted Date: 4 August 2018

Please cite this article as: Pantoja M, Alvarado T, Cakmak M, Cavicchi KA, Stearic acid infused polyurethane shape memory foams, Polymer (2018), doi: 10.1016/j.polymer.2018.08.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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GRAPHICAL ABSTRACT Marcos Pantoja, Tommy Alvarado, Mukerrem Cakmak, Kevin A. Cavicchi

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Stearic Acid Infused Polyurethane Shape Memory Foams

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Stearic Acid Infused Polyurethane Shape Memory Foams Marcos Pantoja1, Tommy Alvarado2, Mukerrem Cakmak1,3, Kevin A. Cavicchi1* 1

Department of Polymer Engineering, University of Akron, Akron, OH 44325-0301

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NIHF STEM High School, Akron, OH 44308 Departments of Materials and Mechanical Engineering, Purdue University, West Lafayette, IN 47907

*Corresponding Author. (E-mail: [email protected]) Keywords: Shape memory polymer, foam, polymer blend

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Abstract

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Shape memory polymer (SMP) foams were fabricated by immersing a polyurethane foam inside stearic acid-isopropyl alcohol solutions of varying concentration. Samples were programmed using a compression press or a dynamic mechanical analyzer (DMA). It was determined that fixity increases with increasing wt% stearic acid, reaching 95% fixity with 29 wt% (1 vol%) stearic acid. Pre- and postsubmersion volume measurements reveal that the foam volume increases after the solution treatment. Optical microscopy images illustrate that the stearic acid coats the foam struts, eventually filling the pores at higher stearic acid loading forming a percolating network whose strength increases with increasing stearic acid concentration. The ease of fabrication and the large volume compression that is fixed in the SMP foams is useful for space-filling applications stimulated by heating. Introduction

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Shape memory polymers (SMPs) are a class of responsive materials that can fix a deformed shape and recover to their original shape once exposed to an external stimulus [1-4]. The activating stimulus can include temperature [1-4], electricity [5,6], magnetism [7], humidity [8], light [9], or pH change [10,11], with temperature being the most common. The morphology of SMPs should contain at minimum two components, a permanent network and a reversible network, where the composition and distribution of these two networks directly influences the ability of the polymer to fix and recover from a deformation [12]. The permanent network is a chemically or physically crosslinked polymer enabling elastic recovery to the original shape. Intermediate fixed shapes result from the presence of the reversible network, which may be a separate network or superimposed on-top of the original network (e.g. the crystallization or vitrification of an elastomer network). This superimposed reversible network is sometime described in terms of reversible netpoints, but the structure-spanning nature of the reversible network defined by these netpoints is crucial for shape memory. For example, if these glassy or crystalline domains are discontinuous and do not percolate, then elastic retraction will occur between these domains lowering the ability to fix the deformation of the programmed shape memory polymer [13]. SMPs possess many attractive attributes including ease of fabrication, processability, and the ability to recover from relatively large strains [2], making them highly attractive in a wide range of industries including aerospace [14], health care [15-18], and textiles [19]. SMPs are also a promising insulation or space-filling material if processed into foams. First patented in 1991 by Hayashi et al. [20], SMP foams, or porous SMPs, are a relatively novel SMP variation which has recently received much

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attention. Unlike conventional solid SMP materials which are deformed primarily by extension and bending with conserved volume, SMP foams possess porous structures allowing for large compressive deformations and volume changes. A number of techniques have been used to fabricate SMP foams. Some common techniques involve gas foaming [20-24], syntactic foaming [25-27], particulate leaching [28-30], and electrospinning [31-34]. Most SMP foams are derived from three central polymers: polyurethane [20-23,29], polystyrene [25-27,35,36], and epoxy resin [37,38]. Typical commercially available SMP foams include the CHEM and MF polyurethane foam series from Mitsubishi Heavy Industries and Jet Propulsion Laboratories, the TEMBO® epoxy foam series from Composite Technology Development, and the Veriflex® polystyrene syntactic foam series from CRG Industries. On the other hand, non-commercially available SMP porous materials have been synthesized from various polyurethanes [29,39-41] and poly(ε-caprolactone) (PCL) [24,28,42-45] containing structures. These SMP foams have a glass transition temperature, Tg, above room temperature, except for the PCL SMPs whose reversible networks result from the melting/crystallization of the semi-crystalline PCL. These studies have been summarized in SMP foam reviews provided by Hasan et al. [46], Hearon et al. [47], and Santo et al. [48]. Finally, recent studies have prepared shape memory aerogels from a variety of materials via supercritical or freeze drying [49-54]. A limitation of all of these systems is that they are inherently constrained in their application by the transition temperature determined by the Tg or Tm of the reversible network within the parent foam. Studies have modified the transition temperature of shape memory polyurethanes by rearranging the block copolymer morphology or by varying the soft block transition temperature [55-57]. However, each transition temperature variation requires synthesizing a new formulation, targeting one transition temperature per synthesis.

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An alternative approach to fabricate shape memory polymers is to mix an elastomer with a crystalline small molecule capable of forming a structure-spanning solid network [58-68]. This strategy decouples the synthesis of the permanent and reversible networks and allows variation of the SMP transition temperature through the choice of the small molecule melting temperature. This study applies this blending approach to generate a SMP foam by immersing a commercial polyurethane foam in a stearic acid solution. This fabrication process was used to vary the stearic acid content and evaluate how the foam’s 1) shape memory behavior and its 2) physical properties (volume and extent of swelling) vary as a function of stearic acid content. Overall, this method results in shape memory foams achieving fixity and recovery values greater than 95% with only 29 wt% (1 vol%) stearic acid.

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Experimental Methods Materials:

Polyurethane foam cubed sheets (McMaster-Carr®) with ca. 25.4 mm (1 inch) cube dimensions and a density of 0.025 g/cm3, stearic acid (TCI), hexane (VWR Analytical BDH®) , soybean oil (Giant Eagle), toluene (VWR Analytical BDH®), dioctyl phthalate (Sigma-Aldrich®), benzene (Sigma-Aldrich®), tetrahydrofuran (THF) (Sigma-Aldrich®), acetone (VWR Analytical BDH®), diethyl phthalate (Alfa Aesar®), dimethyl phthalate (Alfa Aesar®), isopropyl alcohol (IPA) (MacronTM), and methanol (VWR Analytical BDH®) were all used as received. Deionized water was prepared using a reverse osmosis desalination system. A polyurethane foam cylinder (McMaster-Carr®) with a diameter of 50 mm and a density of 0.016 g/cm3 was cut along its cross section into pieces 110 mm in length.

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Shape Memory Foam Fabrication:

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Stearic acid solutions were prepared in a 100 mL beaker (ca. 70 mm height and 50 mm diameter) by adding 0.25, 0.5, 1, 2, 3, 4, 5, 10, or 15 g of stearic acid to 37 g of IPA (47 mL). Solutions were stirred at 50 °C for 10 minutes using a magnetic stir bar to reach thermal equilibrium. The solution had to be heated since stearic acid is largely insoluble in IPA at room temperature. The beaker was covered with aluminum foil to prevent the solvent from evaporating. However, the aluminum foil was punctured to insert the hotplate’s thermocouple into the solution. A foam cube was fully submerged inside the 50 °C solution and was left to soak for 1 minute. The soaked cube was patted dry with paper towels without squeezing the cube. The cube was placed on a new paper towel every 30 seconds, resting the cube on the same face after each transfer. After six transfers, the cube was rested on the face opposite the drying face and was left to dry inside a fume hood overnight. Three foam cubes were individually soaked in separate solutions for each solution concentration to obtain average measurements. Foam cylinders were soaked following the same procedure. Stearic acid solutions for foam cylinder soaking were prepared inside a 3,500 mL beaker (ca. 25 cm height and 15.5 cm diameter) by adding 42 g of stearic acid to 785 g (1,000 mL) of IPA. Finally, a single cube was immersed for one minute inside a jar (ca. 60 mm height and 50 mm diameter) containing 100 mL of pure molten stearic acid. The temperature of the stearic acid was kept constant at 80 °C using an oil bath and a thermostated hotplate.





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The weight and volume of the foams were measured before and after solution treatment using a digital balance and a digital caliper, respectively. Volume was calculated from the length of the cubes’ three faces, where the length of the individual faces was measured by placing the end of the calipers on the center of the faces. Three separate foam cubes were measured for each solution concentration treatment to obtain average weight and volume values. Weight percent acid (wt%) was calculated as,



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where Wi and Wt are the foam’s pre- and post-treatment weight, respectively. Similarly, volume percent acid (vol%) was given as:





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Here, ρSA and Vt are the density of stearic acid (0.94 g/cm3) and the bulk volume of the treated foam, respectively. Finally, percent volume increase was calculated as,

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where Vi is the initial foam bulk volume. The weight and volume measurements were also used to calculate density. Compression Press Shape Memory Cycling:

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Stearic acid treated foam cubes were compressed for 5 minutes between two metal plates. A Carver® hydraulic compression press was used to set the compression temperature and pressure to 80 °C and 3,000 psi, respectively. A compression temperature of 80 °C was chosen since that is above the melting temperature of stearic acid (70 °C), which serves as the transition temperature of the shape memory foam. The applied strain was controlled by placing the samples between two rectangular 1 or 4 mm thick metal spacers set 35 mm apart. While under load, the samples were cooled to room temperature using the press’ water cooling system. Once cooled, the samples were left compressed for 30 minutes to ensure complete stearic acid crystallization. The samples were then removed from the compression press and were left under ambient conditions for 15 hours. Finally, the samples were submerged for 2 minutes inside a water bath set to 100 °C. Foam cylinders were fixed and recovered using the same procedure but the metal spacers were set 60 mm apart. Prior to recovery, the fixed cylinder was inserted inside a polypropylene cylinder 50 mm long and 25 mm in diameter. The sample dimensions during shape memory cycling were measured using a digital caliper by placing the end of the calipers on the center of the cube face. These measurements included the initial sample height (li), the height after equilibrating for 15 hours under ambient conditions (lf), and the height after being removed from the hot water bath (lr). The height during compression (lc) was kept constant at 1 or 4 mm. Compressed samples were marked on either of the compressed faces prior to being placed in the hot water bath to ensure that lr was measured using the same cube face used for the li, lf, and lc measurements. Using these measurements, fixity and recovery were calculated as follows [1]:

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Here, initial strain (εi), compressed strain (εc), fixed strain (εf), and residual strain (εr) are given by,

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where lx is li, lc, lf, or lr, with the same subscript as εx. Dynamic Mechanical Testing: Shape memory analysis was also conducted via dynamic mechanical measurements performed using a dynamic mechanical analyzer (DMA, TA Instruments Q800) with a compression clamp fixture (diameter = 40 mm) operating in Controlled Force mode. The sample shape and preload force were

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specified to “square disk” and “0.005 N”, respectively. The samples were first heated to 80 °C at a rate of 10 °C/min. After stabilizing at the set temperature for 10 minutes, the force was ramped to 7 N at a rate of 7 N/min. Upon reaching the target applied force, the samples were then cooled to 25 °C at a rate of 10 °C/min and were held under load at that temperature for 10 minutes. The force was then lowered to 0.005 N at a rate of 7 N/min and the samples were held in that state for 10 minutes. Finally, the samples were reheated to 80 °C at a rate of 10 °C/min and were held isothermally for 10 minutes to allow shape recovery. This procedure was repeated two more times for a total of three shape memory cycles. Since the maximum length between the compression clamp disks is 11 mm, the foams were cut parallel to one of the faces, resulting in samples with base areas of 25.4 x 25.4 mm2 and heights of ca. 10 mm.

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Compression testing was also performed using DMA equipped with the compression clamp fixture operating in Controlled Force mode. Samples were equilibrated at 25 °C for 5 minutes before being compressed by ramping the compression force to 18 N at a rate of 7 N/min.

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Solvent Swelling:

Optical Microscopy:

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Foams were swelled for 53 hours at room temperature inside jars (ca. 60 mm height and 50 mm diameter) filled with 100 mL of hexane, soybean oil, toluene, dioctyl phthalate, benzene, THF, acetone, diethyl phthalate, dimethyl phthalate, IPA, methanol, or water, or at 80 °C inside a jar filled with 100 mL of molten stearic acid. The polyurethane foam readily sank to the bottom of the jar when using hexane, toluene, benzene, THF, acetone, IPA, or methanol. However, when using soybean oil, stearic acid, dioctyl phthalate, diethyl phthalate, dimethyl phthalate, or water, the foam had to be squeezed inside the solvent to expel any air from inside the foam. Once completely filled with solvent, the solvent absorbed by the foam provided enough weight for the foam to remain within the solvent. Foams were individually swelled inside each jar and were completely submerged inside the solvent. A single foam cube was used to calculate the volume of solvent swollen samples using the volume measuring method described above.

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Optical microscopy images of neat foams and foams containing stearic acid were obtained at 5X and 10X magnification using an Olympus BX51 Optical Microscope. Pictures of foams swelled in solvent were only taken at 5X magnification. The foams were cut into sheets ca. 4 mm thick in order to facilitate the focusing of the microscope and further increase the resolution of the images. Microscopy images of the low volatility diethyl phthalate and water swollen foams were taken as part of the solvent swelling analysis. The microscope’s QCapture Pro® v5.1 image processing software was used to stamp the 5X and 10X magnified images with a 1,000 and 500 μm scale bar, respectively. ImageJ v1.8.0.77, a public domain image processing and analysis software in Java (National Institute of Health), was used to calculate the width of the foam struts by using the pictures’ stamped scale bar as a calibration to convert pixels to length (0.735 pixels/1,000 µm stamped scale bar length). Twenty strut width measurements were taken per image to obtain average strut width values. Differential Scanning Calorimetry (DSC):

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A DSC (8500 PerkinElmer) set to a standby temperature of 25 °C was used to investigate the thermal transitions of the neat polyurethane foam via the following temperature profile: heating ramp at 10 °C/min to 100 °C, isothermal for two minutes, cooling ramp at 10 °C/min to 25 °C, isothermal for two minutes, heating ramp at 10 °C/min to 100 °C, isothermal for two minutes, and cooling ramp at 10 °C/min to 25 °C.

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Results and Discussion 1. Shape Memory Behavior:

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Figure 1 shows the wt% stearic acid uptake of foams soaked in stearic acid-IPA solutions calculated using Equation 1. The amount of stearic acid contained within the foam increases linearly with solution concentration and begins to plateau at higher concentrations. Optical microscopy images of foams varying in stearic acid concentration are seen in Figure 2. Comparing the neat foam in Figure 2a and the stearic acid-containing foams in Figure 2b-d, the stearic acid appears to first coat the foam struts (Figure 2b) and forms a percolating structure (Figure 2c-d) once sufficient stearic acid is present. Figure S1 in the Supporting Information contains 10X magnified images which show individual stearic acid crystals. Compression tests of the foams are shown in Figure 3. The neat foam shows stress-strain curves typical of an elastic cellular solid with three regions. First is an initial rise due to elastic bending of the foam walls, followed by a plateau corresponding to wall buckling, and finally a sharp rise in the stress due to densification once the cell walls meet [69]. The formation of stearic acid crystals on the walls and in the pores of the foam are shown to generally increase the initial modulus and plateau stress and decrease the strain at which densification occurs with increasing stearic acid content. Stearic acid forms platelet crystals, which will also form cellular solid networks by coating the foam walls and/or interlocking ‘house of cards’ networks across the pores [70-75]. Therefore additional force will be required to bend the foam walls and stearic acid crystals, raising the initial modulus, while wall buckling will require yielding and or fracture of the wax crystals, raising the plateau stress. The positive scaling of the modulus and plateau stress with stearic acid content is consistent with the mechanical behavior of cellular solid networks as a function of increasing foam density. At the same time the strain where densification occurs should decrease with increasing stearic acid content due to the filling of the pore volume with stearic acid reducing the distance required for solid-solid contact. Beyond these general trends the complete stress-strain behavior appears complex with a number of additional features including the similarity of the curves in the initial elastic region in the neat and 18% stearic acid samples; the overlap of the curves in the densification region of the 18 and 43 wt% samples; and the double plateaus in the 62 and 85 wt% samples. Further study of the reproducibility and variance of these features would be needed to completely understand the mechanical behavior of these materials.

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Figure 1: Weight percent stearic acid in polyurethane foam as a function of solution concentration.

Figure 2: Optical microscopy images of a) a neat foam and of foams with b) 18, c) 43, and d) 97 wt% stearic acid taken at 5X magnification. Scale bar represents 1,000 μm.

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The fixity and recovery results obtained through programming on a compression press and DMA and analyzed via Equations 4 and 5 are summarized in Figure 4a-b. Figure 5a-c contains the DMA shape memory curves of foams with 0, 18, and 43 wt% stearic acid. Similar to shape memory polymers analyzed using DMA under extension, Figure 5a-c shows some loss in recovery after the first cycle which has been attributed to residual forces from processing history and creep of the sample during the shape memory cycle [64]. As a result, the reported fixity and recovery values obtained using DMA are based on the second cycle. Some residual stearic acid was observed on the plates during compression molding and DMA testing. Additionally the expulsion of stearic acid was observed during the recovery in water. The amount of stearic acid lost under heating appears to be small as the change in DMA fixity from the second to third cycle was 0.9% and 0.2% for the 18 wt% and 43 wt% samples, respectively. Therefore, while there will be a decay in the fixity as stearic acid is expelled for these samples, it should not affect their properties in one-time use applications, such as space-filling of gaps or holes. Furthermore, these samples could be regenerated if a large loss of stearic acid is observed over multiple cycles.

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Figure 5: Compression DMA shape memory cycle of polyurethane foams with a) 0, b) 18, and c) 43 wt% stearic acid.

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The control experiment at zero weight percent acid (i.e. neat polyurethane foam) shows fixity and recovery values of 45 and 100% when using the compression press with a 1 mm spacer and 47% fixity and 97% recovery when using DMA. As seen in Figure 5a, the neat foam shows slow recovery at room temperature after releasing the applied force. This was further explored by monitoring the fixity of the neat foam obtained using compression molding past the intial 15-hour wait time interval described in the Experimental Procedures section. As shown in Figure S2 in the Supporting Information, the foams gradually recover under ambient conditions. The apparent fixity of the neat foam is not due to a Tginduced temporary network as DSC shows no transition temperatures below 80 °C (Figure S3 in the Supporting Information), but rather the slow viscoelastic recovery of the compression set obtained during the heating and cooling treatment during shape programming. Performing the compression molding shape memory cycle at room temperature resulted in no compression set (i.e. ~0% fixity). Once treated with stearic acid-IPA solutions, the fixity obtained using compression molding with a 1 mm spacer reaches 80% with only 4.5 wt% stearic acid and continues increasing until it plateaus at 98% with 43 wt% stearic acid, indicating a wt% saturation threshold past which additional stearic acid does not greatly improve the fixity. Investigating the 7 – 35 wt% range with a 4 mm spacer resulted in lower fixities compared to the 1 mm spacer except for the 35 wt% sample which has a fixity ≥ 95%, closely matching the fixities of the samples programmed using a 1 mm spacer and the DMA. The fixities of the samples programmed using a 4 mm spacer and of the 18 wt% sample programmed using DMA were lower due to their smaller compressive strain (εc) compared to the εc obtained using the 1 mm spacer. As seen in Figure 5b, the compression length (lc) obtained using DMA is ca. 2 mm. Having a

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smaller lc reduces the foam’s bulk volume and facilitates the ability of the stearic acid network to percolate therefore allowing for a stronger network, which better fixes the compressed foam. While the stored force in the network would also increase with compression, the strength of the network must increase at a faster rate over this measured range of strain. This effect is not seen at ≥ 35 wt% since the strength of the stearic acid network at that concentration is already high enough to fix any deformation. Meanwhile, recovery remains greater than 95% at all stearic acid concentrations regardless of shape programming method. The overlap of the fixity from the 4 mm spacer data and the DMA data is attributed to the different cooling rates of the experiment, which likely also affect the detailed structure of the stearic acid solid network. Figure 6 shows a picture of treated and fixed foams whereas a video demonstrating shape recovery can be found in the Supporting Information. The video shows the shape recovery at elevated temperature of a flattened cylindrical foam placed inside a hollow cylinder, showcasing the potential application of shape memory foams as space-filling materials.

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Figure 6: Stearic acid treated (left) and shape memory fixed (right) polyurethane foams containing 18 wt% stearic acid.

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Previous studies involving SMPs from elastomer/molecular crystal blends have attributed the shape memory mechanism to the arrangement of the additive’s crystalline platelets into threedimensional load-bearing “house of cards” network structures [70-75]. As shown from the optical microscopy images in Figure 2 at higher wt% stearic acid a fraction of the pores are filled with stearic acid. When heating the treated foam above the melting temperature of stearic acid and applying a compression force, the molten stearic acid will flow and become distributed throughout the foam. Under compression, the relative volume occupied by the stearic acid in the foam is now significantly larger than in the underformed sample. Therefore, percolating stearic acid crystal networks can form through the open-cell structure at low wt% stearic acid, resulting in fixed shapes. Increasing the stearic acid concentration forms stronger load-bearing networks more capable of restraining the foam’s expansion as evidenced by the higher plateau stress and modulus with increasing weight percent acid in Figure 3. An important feature of these foams is that good shape memory properties are achieved at low overall loading of stearic acid, when a significant fraction of air filled pores remain. This can be observed by considering the content of acid in the foam in terms of its volume percentage (vol%) given by Equation 2, which takes into account the occupying air in the foam. Figure 7 shows the foam’s vol% acid as a function of its wt% acid. At 85 wt% stearic acid the foam only contains 11 vol% stearic acid. Directly immersing the foam in molten stearic acid to reach 97 wt% stearic acid only results in 71 vol% stearic

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acid. Based on the average weight (0.34 g) and volume (16 cm3) of ten foam cubes and assuming a bulk density of 1 g/cm3 for the polyurethane, the vol% of air in the initial foam cube was calculated to be 98%. These results indicate that the foams exhibit porosity even at 97 wt% stearic acid consistent with Figure 2d. As the foam expands with the uptake of stearic acid the vol% acid in a fully filled foam would be even larger than 98 vol% as the polyurethane is incompressible. Figure S4 in the Supporting Information contains plots of foam density vs weight percent acid which further emphasizes the low amount of stearic acid required for these systems to achieve good shape memory properties while maintaining significant porosity.

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Figure 7: Volume percent acid of polyurethane foams as a function of wt% acid. The data point at 97 wt% acid corresponds to the foam immersed in molten stearic acid for 1 minute.

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Another question is, are the shape memory properties entirely due to the stearic acid crystallizing in the pores or does the stearic acid also swell the polyurethane? As seen in Figure 8, treating foams in stearic acid-IPA solutions also increased the volume of the foams, where the percent volume increase given by Equation 3 was larger in foams with more stearic acid (i.e. foams treated in more concentrated solutions). The respective pre- and post-treatment volumes for each solution concentration are shown in Figure S5 in the Supporting Information.

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Figure 8: Percent volume increase of polyurethane foams as a function of weight percent acid. The data point at 97 wt% acid corresponds to the foam immersed in molten stearic acid for 1 minute.

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Examining the optical microscopy images in Figure 2 of foams varying in stearic acid concentration show that the width of the foam’s struts decrease once treated in the stearic acid-IPA solutions. This result is opposite to what one would expect if the polymer struts were swollen with stearic acid. For example, in non-porous elastomer samples, the samples swell due to the uptake of stearic acid in the polymer leading to an increased volume [63,64]. Therefore, while the foam expands it appears that this is only due to the stearic acid filling the pores and not swelling the polyurethane. These results are illustrated in Figure 9. It was hypothesized that the neat foam is slightly contracted due to Laplace pressure in the pores (P ~ γ/R), which would be counterbalanced by the elastic deformation of the foam network [76,77]. This contraction increases the strut width due to the incompressibility of the foam. The uptake of stearic acid could drive a reduction in the surface tension of the foam reducing the Laplace pressure and therefore drive foam expansion and a reduction in the strut width without swelling the polymer network.

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Figure 9: Average strut width of neat, stearic acid, water, and diethyl phthalate swollen polyurethane foams as a function of swollen foam volume obtained from Figures 2 and 11.

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To examine this idea further, the swelling extent of the polyurethane foam in various solvents was measured and is plotted vs. solubility parameter in Figure 10, while the average strut width vs. bulk foam volume are plotted for water and diethyl phthalate and are shown in Figure 9. Of the solvents tested, THF yields the maximum swollen foam volume, with an overarching peak from solvents with similar solubility parameters, and downturns at the polarity extremes of the solubility parameters. The respective pre- and post-swelling volumes are shown in Figure S6 in the Supporting Information. Optical microscopy images of foams swollen in diethyl phthalate and water are shown in Figure 11. As seen in Figure 9, the average strut width of the water swollen sample is smaller than the average strut width of the neat foam whereas the average strut width of the diethyl phthalate swollen sample is much larger than the average strut width of the neat foam. Diethyl phthalate appears to be a good solvent for the polymer, which expands the pores and swells the polyurethane while water, being a poor solvent, only expands the foam pores. The concave-like swelling behavior in Figure 9 summarizes how these different solvents affect strut width, where the negative slope region corresponds to expansion of the foam due to the reduction in Laplace pressure, while in good solvent both the reduction of the Laplace pressure and the swelling of the polymer increase the foam volume.

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Hexane Soybean Oil Stearic Acid Toluene Dioctyl Phthalate Benzene THF Acetone Diethyl Phthalate Dimethyl Phthalate IPA Methanol Water

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Figure 10: Percent volume increase of solvent swollen-foams as a function of solvent solubility parameter [78-80]. The stearic acid data point corresponds to the foam immersed in molten stearic acid for 53 hours.

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Figure 11: Optical microscopy images of a) diethyl phthalate and b) water-swollen foams taken at 5X magnification. Scale bar represents 1,000 μm. Conclusion

Shape memory polymer (SMP) foams were fabricated by immersing polyurethane foam inside stearic acid-isopropyl alcohol solutions varying in concentration. The shape memory properties were analyzed using a compression press and DMA. Both methods resulted in fixities ≥ 95% with 29 wt% (1 vol%) stearic acid compared to an instantaneous fixity of ca. 46% for the neat foam. The fixity at the lower wt% region was found to depend on the amount of strain applied to the foam, where more compressed samples had higher fixities due to the ability of the stearic acid to better fill the volume and form a stronger network. It was determined that the foam’s volume increases once treated with the stearic acid solution. Furthermore, optical microscopy images reveal that the stearic acid coats the

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foam’s struts and eventually forms a percolating network at higher concentrations. These images also reveal that the strut width decreases upon solution treatment and that the foams remain highly porous. Examination of the foam morphology indicate that direct swelling of the polymer network is not required for shape fixing in these open-celled foams. Therefore, a wide range of molecular crystals, with different melting temperatures could be used to generate reversible solid networks for shape memory. The simple processing technique converts a readily available elastic foam into a shape responsive material useful for space-filling applications. Acknowledgement

Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund, the American Chemical Society, and the Project SEED endowment for support of this research.

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Appendix A. Supplementary Data

Supplementary data related to this article can be found at [URL pending].

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Shape memory polymer foams are fabricated by immersing foam in molten stearic acid Excellent fixation and recovery of shapes at low acid concentration Effective, versatile method to convert elastic foam into shape memory material

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