Thermoresponsive Shape-Memory Aerogels from Thiol–Ene Networks

Article pubs.acs.org/cm

Thermoresponsive Shape-Memory Aerogels from Thiol−Ene Networks Brian T. Michal,† William A. Brenn,† Baochau N. Nguyen,§ Linda S. McCorkle,§ Mary Ann B. Meador,‡ and Stuart J. Rowan*,† †

Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland, Ohio 44106-7202, United States ‡ NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135, United States § Ohio Aerospace Institute, 22800 Cedar Point Road, Cleveland, Ohio 44142, United States S Supporting Information *

ABSTRACT: Thermoresponsive shape-memory polymer aerogels have been produced from thiol−ene networks of 1,6-hexanedithiol, pentaerythritol tetrakis(3-mercaptopropionate), and triallyl-1,3,5-triazine-2,4,6-trione. The thiol−ene networks form organogels with either acetonitrile or acetone as the solvent, which can be subsequently removed using supercritical CO2 extraction. The resulting aerogels have nearly quantitative shape fixing and shape recovery with a glass transition temperature ranging from 42 to 64 °C, which serves as the thermal transition trigger for the shape-memory effect. The aerogels have a porosity of 72% to 81% but surface areas of only 5−10 m2/g.

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INTRODUCTION Aerogels are an intriguing class of materials which are formed by the extraction of the solvent from a gel while preserving the porous structure. This gives them a range of attractive properties such as low density, high surface area, low thermal conductivity, and low dielectric constant.1−3 Some of the first aerogels were produced from silica and as a result were very brittle, requiring, for example, the addition of polymers to provide elasticity and mechanical toughness.4−6 More recently, aerogels have been prepared entirely from organic materials such as polyimides,7−12 polyamides,13 polyurethanes,14,15 and polystyrene.16,17 As the skeletal materials for aerogels have moved from inorganic to polymeric they have been shown to have increased toughness and deformability as well as an increased ability to recover from deformation. Such properties suggest the possibility of accessing shape-memory aerogels where the aerogel can be held in a compressed state and recover to its open porous structure upon demand. There are a number of examples in the literature of porous materials that have been designed to feature a compressive shape-memory response. 1 8 These include foams, 1 9 − 2 3 electrospun meshes,24−26 and scaffolds produced from salt leaching.27−29 These materials have largely been focused on biomedical30 and aerospace31 applications. Recently, aerogels based on cellulose nanofibrils32,33 and nanocrystals34 have been reported to show shape-memory properties where the shape recovery is triggered by a solvent. However, to date there have been no reports of a thermally responsive shape-memory aerogel. © XXXX American Chemical Society

With this in mind, we sought to create an aerogel from a thermal shape-memory polymer (SMP) that can hold a temporary shape and, upon application of heat, recover to its original shape. One class of thermal SMPs are cross-linked networks that use a physical transition such as a melting transition (Tm) or glass transition (Tg) to inhibit elastic recovery after deformation and removal of heat. Simply heating above the thermal transition results in a drop in the modulus, and the material is able to recover to its original shape.35−39 We report herein our studies on using an SMP as the basis for producing a polymer aerogel, with the goal of accessing a material that could hold a compressed shape and then, upon application of heat, return to its original shape and density (Figure 1a).

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EXPERIMENTAL SECTION

Materials. 1,6-Hexanedithiol (HDT), pentaerythritol tetrakis(3mercaptopropionate) (PETMP), 1,3,5-triallyl-1,3,5-triazine-2,4,6-trione (TTT), benzoyl peroxide (BPO), and N,N,4-trimethylaniline (TMA) were purchased from Sigma-Aldrich. All other reagents were purchased from Fisher Scientific. All chemicals were used without further purification. General. Dynamic mechanical analysis (DMA) was carried out on a TA Instruments Q800 in tension mode using a heating rate of 3 °C/ min and a frequency of 1 Hz. For DMA testing, organogels were Received: February 1, 2016 Revised: March 20, 2016

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observed in approximately 15 s. The cylindrical gel was then ejected from the syringe into a jar of acetone or acetonitrile. The gel was allowed to soak overnight after which the solvent was replaced with fresh solvent. This soaking procedure was repeated three times. Supercritical CO2 Extraction. Thiol−ene organogels were immersed in either acetone or acetonitrile in a 1 L sealed chamber. The chamber was pressurized to 75 bar at a temperature of 25 °C, and the gel was continuously washed with liquid CO2 at a flow rate of 9 g/ min for 90 min. The gel was then allowed to soak for 30 min. This wash/soak procedure was repeated three more times. The pressure and temperature of the vessel were then raised to 90 bar and 35 °C, respectively, and the gel was continuously washed with supercritical CO2 at a flow rate of 9 g/min for 90 min followed by 30 min of soaking. This wash/soak was repeated one time, and then the pressure in the vessel was reduced to atmospheric pressure by venting CO2 at a rate of 10 g/min. The resulting aerogel was then dried overnight in vacuo to remove any remaining solvent. Shape Memory Testing. Shape memory testing was carried out on a TA Instruments Q800 DMA equipped with a compression fixture. A cylindrical sample with approximate length and diameter of 3 and 10 mm, respectively, is loaded into the fixture. The temperature is then raised to 100 °C, and the sample is compressed with a force of 1.0 N. The deformation is then held while the sample is cooled to room temperature. Once cooled, the compressive strain is measured (εm, maximum strain). The force on the sample is then released, and the compressive strain (εf, fixed strain) is measured. The sample is then heated again to 100 °C and allowed to recover, with a compressive strain measurement again taken (εr, recovered strain).

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Figure 1. (a) Heating a thermally responsive SMP aerogel followed by compression and subsequent cooling results in the material “fixing” a compressed, denser shape. Upon reheating, the material returns to its original shape and density. (b) SMP aerogels are produced via a thiol− ene polymerization of PETMP, HDT, and TTT in either acetone or acetonitrile to afford a thiol−ene organogel. The gel is subsequently dried via supercritical CO2 extraction to produce an aerogel.

RESULTS AND DISCUSSION To access shape-memory aerogels, highly cross-linked amorphous thiol−ene networks with a Tg above room temperature were targeted. These were based on triallyl-1,3,5triazine-2,4,6-trione (TTT), a triene monomer, and two thiolcontaining monomers: hexanedithiol (HDT), a dithiol, and pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), a tetrathiol (Figure 1b). A similar material based on TTT and PETMP had been reported by Bowman and co-workers as a potential dental restorative material and had a reported Tg of 64 °C.41 By introducing HDT as a difunctional linear component it was expected that it would be possible to tailor the Tg of the materials by simply altering the HDT:PETMP ratio. The Tg is important for these SMPs because this is the temperature at which the shape-memory response will occur, therefore being able to control this parameter is advantageous in order to produce SMPs for specific applications. Thiol−ene organogels were synthesized from PETMP, HDT, and TTT in either acetonitrile or acetone (Figure 2a). These solvents were chosen for their compatibility with the supercritical CO2 extraction process. [PETMP]:[HDT] in the samples was either 1:0, 2:1, 1:1, or 1:2 with the [TTT] set such that overall [thiol]:[ene] was 1:1. To understand the mechanical properties of the resulting polymers, nonaerogel

produced in acetone in a film mold according to the below procedure and then dried over 48 h with the first 24 h under ambient conditions and the second 24 h in vacuo. Film specimens for DMA testing were cut to approximate dimensions of 0.3 mm × 6 mm × 15 mm. Skeletal densities (ρs) were determined by helium pycnometry using a Micromeritics Accupyc 1340 gas pycnometer. Bulk density (ρb) was determined by measuring the length and diameter of cylindrical samples to geometrically calculate the volume and then dividing the mass by this result. Porosity (p) was then calculated by p = 100% × (1 − (ρb/ρs)). Surface area measurements were conducted using ASAP 2000 surface area/pore distribution analyzer (Micromeritics Corp.). All samples were degassed for 8 h at 80 °C under vacuum prior to surface area measurements. Scanning electron microscope (SEM) imaging was performed on a Hitachi S-4700 Field Emission Microscope. All samples were sputter coated with platinum prior to imaging. Synthesis of Thiol−Ene Organogel. Thiol−ene organogels were prepared by a redox-initiated thiol−ene polymerization of PETMP, HDT, and TTT following a procedure from Bowman and coworkers.40 A redox-initiated system was chosen to avoid the issues of light penetration (for a photoinitiated system) and difficulty maintaining even heating in the syringe molds (for a thermo-initiated system). [PETMP]:[HDT] was set at 1:0, 2:1, 1:1, or 1:2, and an appropriate amount of TTT was used such that [thiol]:[ene] was 1:1. As an example, for [PETMP]:[HDT] = 1:1 with acetone as the solvent, the procedure is as follows: PETMP (258 mg, 0.527 mmol), HDT (0.081 mL, 0.527 mmol), TTT (0.227 mL, 1.05 mmol), and BPO (15 mg, 0.063 mmol) were dissolved in 3 mL of acetone. For samples in acetonitrile, the mass of PETMP, HDT, and TTT was adjusted such that the total mass of polymer in the gel was 900 mg. TMA (0.032 mL, 0.253 mmol) was then added to the reaction mixture, and the mixture was then immediately transferred to a 5 mL plastic syringe which had the tip removed. Gelation of the mixture was

Figure 2. (a) Thiol−ene acetone organogel with 1:1 [PETMP]: [HDT] soaking in acetone and (b) the thiol−ene aerogel after supercritical CO2 extraction. B

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Figure 3. DMA temperature sweeps of nonaerogel films made by the thiol−ene reaction of TTT with different ratios of [PETMP]:[HDT]. Figure 4. Porosity and surface area measurements for all aerogel samples. Aerogels produced from acetone organogels had a porosity of ca. 81% and those produced from acetonitrile organogels had a porosity of ca. 72%. All aerogels had a surface area in the range of 5− 10 m2/g.

films were prepared by drying the acetone gels under ambient conditions. Figure 3 shows DMA temperature sweeps of the four films and confirms that the Tg of these materials can be systematically controlled by the PETMP:HDT ratio, with a ratio of 1:0, 2:1, 1:1, and 1:2 yielding Tg values of ca. 66, 59, 50, and 44 °C, respectively. Thus, as expected, increasing the amount of PETMP in the gel increases the degree of crosslinking resulting in a higher Tg material. Likewise, increasing the amount of PETMP in the material also increases the rubbery plateau modulus. To access the aerogels, the thiol−ene organogels (either acetonitrile or acetone) were dried using supercritical CO2 extraction. The resulting aerogels were white, lightweight solids (Figure 2b). ATR-FTIR was used to compare aerogels that had been dried ambiently against those dried using supercritical CO2 extraction (Figure S1). No differences were observed in the two spectra confirming that chemical structure of the material was unaffected by the supercritical CO2 extraction process. DSC thermograms of the aerogels did not reveal any crystallinity in these materials (Figure S2), and TGA revealed an onset of thermal decomposition at ca. 300 °C (Figure S3). The bulk density (ρb) of the materials was determined by measuring the length and diameter of cylindrical samples, calculating the volume and then dividing the mass by this result. The skeletal density (ρs) of the materials was determined by gas pycnometry. The porosity (p) was then calculated by p = 100% × (1 − (ρb/ρs)). As can be seen from Figure 4, the porosity of the aerogels that were produced did not depend on the PETMP:HDT ratio. All the aerogels produced from the acetonitrile organogels have a porosity of ca. 72%, whereas the porosity of all the aerogels produced from acetone organogels was ca. 81%. The aerogels produced from acetone organogels had a higher porosity presumably due to the fact that the original gel had a lower polymer concentration (200 g/ L). For organogels produced in acetonitrile a higher polymer concentration (300 g/L) was required in order to obtain gelation. Conversely, acetone aerogels could not be produced at this higher concentration on account of the greater reaction exotherm from having more monomer present in solution, which along with the lower boiling point of acetone relative to acetonitrile resulted in bubbling of the solvent. The surface area for all aerogels tested generally range from 5 to 10 m2/g (Figure 4), and no trend was observed. This result is low for aerogels, which typically have surface areas >100 m2/ g and may be related to the limited solvent−polymer compatibility resulting in larger pore sizes in the gel. The fact that the organogels are opaque is consistent with both the

structures and pores being large. Although synthesizing the aerogels using a better solvent for the polymer (e.g., methylene chloride or tetrahydrofuran) may have resulted in lower pore sizes, the effect of these solvents could not be studied due to their incompatibility with the supercritical CO2 extraction process. To better confirm the nature of the aerogel morphology, SEM analysis of these materials was carried out. Figure 5a,b show SEM images of the aerogel produced from acetonitrile

Figure 5. (a) and (b) SEM images of neat aerogels produced from acetonitrile (a) and acetone (b) organogels with [PETMP]:[HDT] = 1:1 showing a macroporous structure. (c) and (d) SEM images of aerogels after heating, compressing to ca. 20% strain and then cooling to room temperature, showing the compressed microstructure in the fixed shape. The aerogels were produced from acetonitrile (a) and acetone (b) organogels, respectively. (e) and (f) SEM images of aerogels after five shape-memory cycles produced from acetonitrile (e) and acetone (f) organogels with [PETMP]:[HDT] = 1:1 showing that the microstructure is preserved after shape-memory cycling. C

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Chemistry of Materials and acetone organogels, respectively, with a [PETMP]:[HDT] = 1:1. The images clearly show that both aerogels have macroporous structures with pore sizes of >1 μm consistent with the fact that both acetone and acetonitrile are relatively poor solvents for the polymer. The macroporous structure of these materials (along with the relatively low surface area) is similar to aerogels prepared via freeze-drying42,43 or pyrolysis methods.44 The average polymer structure size does appear to be slightly smaller for the aerogels prepared from acetone relative to those prepared from the acetonitrile gels. Specifically, the average width of the polymer skeleton decreases from ca. 0.7 to 0.5 μm going from the acetonitrile to acetone base aerogels, which is consistent with the higher porosity aerogels prepared from the lower concentration acetone gels. The DMA temperature sweeps shown in Figure 3 suggest that at 100 °C all the materials are clearly within the rubbery plateau region, and as such, this would be a sufficient stimulus temperature for the shape-memory experiments. Cylindrical samples were therefore heated to 100 °C in the DMA and then subsequently compressed with a force of 1 N followed by cooling to room temperature. The samples were compressed to approximately 15−22% compressive strain (εm, maximum strain). This resulted in porosities of 65−67% for acetonitrilebased aerogels and 75−77% for acetone-based aerogels. The compressive force was released, and the compressive strain under zero load was measured (εf, fixed strain). SEM images of compressed samples are given in Figures 5c,d. The sample was then reheated under no load and allowed to recover, and the strain after recovery was measured (εr, recovered strain). This procedure was carried out a total of five times. Figure 6 shows the results of these compressive shape-memory experiments for the aerogel produced from an acetone organogel with [PETMP]:[HDT] = 1:1. The material showed excellent shape-memory properties with a fixing ratio (defined as (εfεi)/(εm-εi), where εi is the initial strain at the start of the shapememory cycle) of 99.4% ± 0.1% and a recovery ratio (defined as (εr-εm)/(εi-εm)) of 98.0% ± 2.2%. In general, it appears that most of the permanent compression set on the material occurs after the first shape-memory cycle, with nearly quantitative shape recovery observed thereafter. All other samples showed similarly high fixing and recovery ratios (see Supporting Information for shape-memory graphs of all other samples). To ensure that no shape recovery was occurring at room temperature, a sample was subjected to the same heat/ compress/cool cycle and allowed to sit at room temperature for 24 h under zero load while the strain was recorded. After 24 h, 99% for each cycle, the recovery ratio consistently decreased as the compressive load was increased, suggesting that there is greater compression set at higher loads. Figure 7b shows the recovery ratio and recoverable strain for the sample at each compressive load. As can be seen from the graph, there is an inherent trade-off between recoverable strain and recovery. By going to higher loads, higher compressions can be realized but at the cost of decreased recovery. This means that the density of D

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Table 1. Summary of Thermal, Physical, and Shape Memory Properties (for a 1 N Compressive Force) of the Investigated Aerogels [PETMP]:[HDT]

solvent

porosity (%)

surface area (m2/g)

Tg (°C)

1:0 1:0 2:1 2:1 1:1 1:1 1:2 1:2

acetone acetonitrile acetone acetonitrile acetone acetonitrile acetone acetonitrile

81.2 72.1 81.2 71.6 81.3 71.2 81.2 71.8

5.6 8.3 7.5 10.2 10.9 5.7 4.7 5.1

66.2 58.7 49.8 43.6

fixing ratio (%) 99.3 99.5 99.4 99.8 99.4 99.2 99.8 99.7

± ± ± ± ± ± ± ±

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

recovery ratio (%) 98.3 99.1 98.6 98.9 98.0 98.0 99.4 98.7

± ± ± ± ± ± ± ±

1.9 1.0 1.3 0.5 2.2 1.9 1.4 0.5

recoverable strain (%) 16.1 14.2 17.8 14.6 18.6 15.3 21.1 16.3

± ± ± ± ± ± ± ±

0.6 0.2 0.4 0.4 0.5 0.4 0.3 0.4

Figure 8. Acetone-based aerogel with 1:1 [PETMP]:[HDT] is compressed with a load of 2.0 N at 80 °C and then cooled to r.t. in the compressed state. The aerogel is then reheated to 80 °C and recovers its original shape. The same aerogel is then heated to 80 °C followed by compression with a load of 9.8 N and subsequent cooling to r.t. in the compressed state. The aerogel is again heated to 80 °C and recovers its original shape.

Figure 8 shows images of an acetone-derived aerogel with 1:1 [PETMP]:[HDT] undergoing two shape-memory cycles. In the first cycle, the aerogel was heated to 80 °C and a load of 2.0 N was manually applied followed by cooling to r.t. under compression. The gel is then heated to 80 °C (by placing in 80 °C water) under no load to demonstrate the shape recovery (see SI video). While the video (see Supporting Information) shows the shape recovery is observed in less than 1 min, it is important to note that thermal conductivity will limit the rate of the shape recovery. It is therefore expected that larger samples would require larger recovery times. The aerogel is then heated to 80 °C again, and a load of 9.8 N is applied, resulting in much greater compression. The aerogel is cooled to r.t. under load followed by heating under no load to demonstrate shape recovery. In the second test, the diameter of the aerogel did increase, showing that under sufficiently large compression, the aerogel cannot continue to densify and will increase in diameter in response to further compression.

Figure 7. (a) Thermal compressive “step” shape-memory graph for the aerogel material produced from an acetone organogel with [PETMP]: [HDT] = 1:1. The compressive load is increased by 0.5 N with each shape-memory cycle. (b) The corresponding recovery ratios and recoverable strain for each compressive load. Increasing the compressive load results in larger compressions but decreased shape recovery.

“remembered” shape will be higher for higher compressive loads. For certain applications, however, this could be acceptable if maximum space-filling is desired and a return to the original density is not required. SEM imaging of the aerogels after shape-memory testing show that the pore structure is preserved after the shapememory tests. Figure 5e,f show SEM images aerogel materials after five shape-memory cycles (at a compressive force of 1N) produced from acetonitrile and acetone organogels respectively with [PETMP]:[HDT] = 1:1. The images show that the microstructure remains microporous and essentially the same as the as-processed aerogel.

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CONCLUSIONS Thermoresponsive shape-memory polymer aerogels have been produced from the thiol−ene polymerization of 1,6-hexanedithiol, pentaerythritol tetrakis(3-mercaptopropionate), and triallyl-1,3,5-triazine-2,4,6-trione. The resulting aerogels have porosities between ca. 72% and 81% and surface areas between 5 and 10 m2/g. The materials display thermoresponsive shape memory with nearly quantitative shape fixing and recovery for E

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(2) Pierre, A. C.; Pajonk, G. M. Chemistry of aerogels and their applications. Chem. Rev. 2002, 102, 4243−4266. (3) Du, A.; Zhou, B.; Zhang, Z.; Shen, J. A Special Material or a New State of Matter: A Review and Reconsideration of the Aerogel. Materials 2013, 6, 941−968. (4) Maleki, H.; Duraes, L.; Portugal, A. An overview on silica aerogels synthesis and different mechanical reinforcing strategies. J. Non-Cryst. Solids 2014, 385, 55−74. (5) Meador, M. A. B. Improving elastic properties of polymerreinforced aerogels. In Aerogel Handbook; Aegerter, M. A., Leventis, N., Koebel, M. M., Eds.; Springer: New York, 2011; pp 315−334. (6) Randall, J. P.; Meador, M. A. B.; Jana, S. C. Tailoring Mechanical Properties of Aerogels for Aerospace Applications. ACS Appl. Mater. Interfaces 2011, 3, 613−626. (7) Meador, M. A. B.; Malow, E. J.; Silva, R.; Wright, S.; Quade, D.; Vivod, S. L.; Guo, H.; Guo, J.; Cakmak, M. Mechanically Strong, Flexible Polyimide Aerogels Cross-Linked with Aromatic Triamine. ACS Appl. Mater. Interfaces 2012, 4, 536−544. (8) Guo, H.; Meador, M. A. B.; McCorkle, L.; Quade, D. J.; Guo, J.; Hamilton, B.; Cakmak, M. Tailoring Properties of Cross-Linked Polyimide Aerogels for Better Moisture Resistance, Flexibility, and Strength. ACS Appl. Mater. Interfaces 2012, 4, 5422−5429. (9) Meador, M. A. B.; Wright, S.; Sandberg, A.; Nguyen, B. N.; Van Keuls, F. W.; Mueller, C. H.; Rodriguez-Solis, R.; Miranda, F. A. Low Dielectric Polyimide Aerogels As Substrates for Lightweight Patch Antennas. ACS Appl. Mater. Interfaces 2012, 4, 6346−6353. (10) Leventis, N.; Sotiriou-Leventis, C.; Mohite, D. P.; Larimore, Z. J.; Mang, J. T.; Churu, G.; Lu, H. Polyimide Aerogels by Ring-Opening Metathesis Polymerization (ROMP). Chem. Mater. 2011, 23, 2250− 2261. (11) Meador, M. A. B.; Aleman, C. R.; Hanson, K.; Ramirez, N.; Vivod, S. L.; Wilmoth, N.; McCorkle, L. Polyimide aerogels with amide cross-links: a low cost alternative for mechanically strong polymer aerogels. ACS Appl. Mater. Interfaces 2015, 7, 1240−1249. (12) Nguyen, B. N.; Cudjoe, E.; Douglas, A.; Scheiman, D.; McCorkle, L.; Meador, M. A.; Rowan, S. J. Polyimide Cellulose Nanocrystal Composite Aerogels. Macromolecules 2016, 49, 1692− 1703. (13) Williams, J. C.; Meador, M. A. B.; McCorkle, L.; Mueller, C.; Wilmoth, N. Synthesis and Properties of Step-Growth Polyamide Aerogels Cross-linked with Triacid Chlorides. Chem. Mater. 2014, 26, 4163−4171. (14) Rigacci, A.; Marechal, J. C.; Repoux, M.; Moreno, M.; Achard, P. Preparation of polyurethane-based aerogels and xerogels for thermal superinsulation. J. Non-Cryst. Solids 2004, 350, 372−378. (15) Bang, A.; Buback, C.; Sotiriou-Leventis, C.; Leventis, N. Flexible Aerogels from Hyperbranched Polyurethanes: Probing the Role of Molecular Rigidity with Poly(Urethane Acrylates) Versus Poly(Urethane Norbornenes). Chem. Mater. 2014, 26, 6979−6993. (16) Venditto, V.; Pellegrino, M.; Califano, R.; Guerra, G.; Daniel, C.; Ambrosio, L.; Borriello, A. Monolithic Polymeric Aerogels with VOCs Sorbent Nanoporous Crystalline and Water Sorbent Amorphous Phases. ACS Appl. Mater. Interfaces 2015, 7, 1318−1326. (17) D’Aniello, C.; Daniel, C.; Guerra, G. ε Form Gels and Aerogels of Syndiotactic Polystyrene. Macromolecules 2015, 48, 1187−1193. (18) Hearon, K.; Singhal, P.; Horn, J.; Small, W.; Olsovsky, C.; Maitland, K. C.; Wilson, T. S.; Maitland, D. J. Porous Shape Memory Polymers. Polym. Rev. 2013, 53, 41−75. (19) Chevalier, E.; Chulia, D.; Pouget, C.; Viana, M. Fabrication of porous substrates: A review of processes using pore forming agents in the biomaterial field. J. Pharm. Sci. 2008, 97, 1135−1154. (20) Kang, S. M.; Lee, S. J.; Kim, B. K. Shape memory polyurethane foams. eXPRESS Polym. Lett. 2012, 6, 63−69. (21) Lee, S. H.; Jang, M. K.; Hee Kim, S.; Kim, B. K. Shape memory effects of molded flexible polyurethane foam. Smart Mater. Struct. 2007, 16, 2486−2491. (22) Xu, T.; Li, G. A shape memory polymer based syntactic foam with negative Poisson’s ratio. Mater. Sci. Eng., A 2011, 528, 6804− 6811.

compressive loads of 1 N, and the porous structure was shown to be preserved after five cycles of shape-memory testing. Going to higher compressive loads resulted in higher compressions of the material but did result in a slight decreased in shape recovery. Nonetheless even at a compression force of 3N > 85% recovery was observed. In addition, the associated Tg that triggers the shape-memory response can be tailored by adjusting the [PETMP]:[HDT] ratio and could be varied from ca. 42 to 64 °C. The relatively low surface areas of these porous materials are attributed to poor solvent−polymer interactions leading to relatively large pore sizes in the organogel and consequently relatively large pore sizes and low surface areas in the resulting aerogels. Future work in this area will include the investigation of polymer−solvent systems with better compatibility that are still amenable to the super critical CO2 extraction process. Thermally responsive shape-memory materials with higher surface areas would be very attractive materials to use as expandable insulation materials in, for example, aerospace applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b00474. ATR-FTIR of the acetone organogel ([PETMP]:[HDT] = 1:1) after ambient drying and after supercritical CO2 extraction. DSC and TGA of aerogels produced from the acetone organogels ([PETMP]:[HDT] = 1:0, 2:1, 1:1 and 1:2). Five cycle shape-memory graphs for aerogels made from acetone organogels ([PETMP]:[HDT] = 1:0, 2:1 and 1:2) and aerogels made from acetonitrile organogels ([PETMP]:[HDT] = 1:0, 2:1, 1:1 and 1:2). Room temperature creep test for compressed aerogel made from acetone organogel ([PETMP]:[HDT] = 1:1) (PDF) Video showing the shape recovery of the 1:1 [PETMP]: [HDT] aerogel (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

National Aeronautics and Space Administration (Grant No. NNX11AN50H) and the Kent H. Smith Charitable Trust. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Aeronautics and Space Administration (Grant No. NNX11AN50H to BTM) and the Kent H. Smith Charitable Trust.

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REFERENCES

(1) Gesser, H. D.; Goswami, P. C. Aerogels and related porous materials. Chem. Rev. 1989, 89, 765−788. F

DOI: 10.1021/acs.chemmater.6b00474 Chem. Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.chemmater.6b00474 Chem. Mater. XXXX, XXX, XXX−XXX