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Thermo-adaptive functionality of graphene/polydimethylsiloxane nanocomposites

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2012 Smart Mater. Struct. 21 105032

(http://iopscience.iop.org/0964-1726/21/10/105032)

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IOP PUBLISHING SMART MATERIALS AND STRUCTURES

Smart Mater. Struct. 21 (2012) 105032 (6pp) doi:10.1088/0964-1726/21/10/105032

Thermo-adaptive functionality ofgraphene/polydimethylsiloxanenanocomposites

Liqiang Ren1, Jingjing Qiu2 and Shiren Wang1

1 Department of Industrial Engineering, Edward E Whitacre Jr College of Engineering, Texas TechUniversity, Lubbock, TX 79409, USA2 Department of Mechanical Engineering, Edward E Whitacre Jr College of Engineering, Texas TechUniversity, Lubbock, TX 79409, USA

E-mail: Shiren.Wang@ttu.edu

Received 21 May 2012, in final form 1 August 2012Published 30 August 2012Online at stacks.iop.org/SMS/21/105032

AbstractThermo-adaptive materials exhibit transformative structures/properties in response to anexternal thermal stimulus and thus are very promising for many industrial applications. In thispaper, we present a new strategy to develop a thermo-adaptive material using polymer-coatedgraphene sheets. Nanocomposite platelets were fabricated through multilayer deposition, laserpatterning, and lift-off. The resultant micro-platelets rolled up to a microtube structure afterthey were released from the substrate. They can evolve from a tube to a flat plain sheetstructure under heating, and can also roll up to a tube again under cooling. Both experimentaland modeling results indicated that the geometrical evolution under a thermal stimulus wassignificantly dependent on the structure and morphology of the graphene/polymernanocomposites. The reversible geometric evolution demonstrates a great potential for drugdelivery, sensors, and other functional devices.

S Online supplementary data available from stacks.iop.org/SMS/21/105032/mmedia

(Some figures may appear in colour only in the online journal)

Smart materials can transit between different structuresand properties in response to single or multiple externalstimuli, including pH, temperature, light, electromagneticfield, mechanical force and ultrasonic waves, etc [1,2]. Smart materials and systems have been investigatedfor numerous applications, such as sensors [3], artificialmuscles, actuators, [4] microgrippers [5], drug delivery [6],microfluidic valve control [7], and smart coating [8]. Dueto the ease of applying stimuli, thermally adaptive materialshave been intensively investigated. Thermally adaptivematerials can alter their structure/properties significantlyin response to a change in the temperature of theenvironment induced by direct heating, high-intensity focusedultrasound, electrical current, or light illumination [9–11].Currently, the most popular thermally adaptive materialsare acrylamide-based hydrogels, especially poly[N-isopropylacrylamide] (PNIPAAm) and polypeptides (ELPs) [9, 10].Pedron [1] designed and fabricated a rectangular smart

bilayer-hydrogel film that can roll and unroll at the lowercritical solution temperature (LCST) of 37 ◦C. The filmconsists of a thermosensitive layer, PNIPAAm, at thebottom and a non-thermosensitive layer of poly(methylmethacrylate) PMMA composite at the top. The bilayerfilm was initially sandwiched between two pre-treated glassslides. Subsequently, 0.5 mm× 1 mm patterns were producedby a photolithographic process. Due to the thermosensitiveproperties of the bottom layer, the thin microscale bilayercan roll or unroll. In addition, biological tissues can beadhered and grown on the smart thin film by virtue ofmodifying the non-thermoresponsive material surface. Thisresults in efficient tissue transplantation for tissue engineeringand therapy. Simpson et al developed a smart bilayerparticle consisting of polydimethylsiloxane (PDMS) and goldfilms [12]. Due to the development and release of themismatched strain at the interface of the bilayer, the particlecan be tubular at room temperature and flat at the elevated

10964-1726/12/105032+06$33.00 c© 2012 IOP Publishing Ltd Printed in the UK & the USA

Smart Mater. Struct. 21 (2012) 105032 L Ren et al

Figure 1. (a) Schematics of a cross section of a smart platelet on a PAA/glass substrate and a smart platelet. (c) Schematic of manufacturingof the PDMS/GO bilayer. An optical image of the multilayer assembly and laser cutting results are also illustrated in (d) and (e) respectively.

temperature [13]. The shape and size of the smart tubeswere dependent on the material elastic modulus, thicknessratio and lateral dimension. Brain et al [12] also showedthat these particles can capture/release fluorescently labeledpoly(ethylene glycol). These efforts indicated that the smartparticles have a great potential for drug delivery in thebio-medical field. However, the biocompatibility of the Aufilm in the PDMS/Au smart particle may hinder potentialbio-medical applications.

Some researchers have also tried to employ carbonnanomaterials in thermo-adaptive systems. Carbon nanomate-rials, such as carbon nanotubes, graphene and nanodiamonds,are non-toxic for loading and transporting molecules, cellsand tissue [14]. Their excellent thermal properties, likehigh thermal conductivity and low coefficient of thermalexpansion, make them a good candidate for thermoresponsivesmart materials. Wang and Chen [15] fabricated carbon nan-otubes/PNIPAAm and carbon nanotubes/poly-L-lysine (PLL)colloids, and found the complex colloids are responsive toboth temperature and pH stimuli. Carbon nanotubes/polymercomplex solutions can be used as sensors. Liu et al [16]utilized polyethylene glycol (PEG) functionalized grapheneoxide (GO) sheets (<50 nm) to attach water-insoluble drugs,such as aromatic molecule SN38, through a non-covalentvan der Waals interaction. The non-covalent binding forcebetween SN38 and GO makes SN38 soluble in the biologicalsolution. Therefore, the PEG-GO hybrid may be used as anefficient drug delivery vehicle. Recently, Zhang et al [17]employed a PNIPAM and single-walled carbon nanotubescomposite as a thermally and optically responsive actuator.The PNIPAM showed a dramatic volume change abovethe LCST as a result of the hydrogel’s transition from ahydrophilic to a hydrophobic state. Millimeter-scale smartstructures with cubic and ‘flowers’ geometries, in whichthe hinges were made with PNIPAM/SWNT actuators, weredemonstrated.

An innovative graphene-based smart material is presentedin this work. Three thin organic films, including a scarifyinglayer, a substrate layer and a top thin layer, were deposited

on a cleaned glass slide sequentially. After thermal curing,laser cutting was employed as a high throughput and lowcost patterning method to generate micro-platelets. Due to themismatched strain developed at the interface of the bilayer, thesmart platelet can alter its geometry between the tube and theplanar platelet at a critical temperature of 60 ◦C, which wasreal-time monitored via an optical microscope. The responsivebehavior was also modeled and verified by experiments.The graphene is capable of adhering various chemicalsand cells [14], making our PDMS/GO bilayer capable ofdelivering molecules, cells and drugs. The smart materialdeveloped in this work is promising for many applications,including drug delivery, sensors, smart solutions, smart tissuesand smart solar tracking systems where temperature can beused as a stimulus.

1. Results and discussion

1.1. Fabrication of smart platelets

The bilayer smart platelets were prepared by depositingpolydimethylsiloxane (PDMS) on a poly(acrylic acid) (PAA)coated glass substrate. The GO film was prepared by aflow-directed assembly method and subsequently depositedonto the PDMS surface by the contact printing method,as shown in figures 1(a) and (d) [19]. The multilayerassembly was patterned by laser assisted precision machining(figure 1(e)) [20–22]. It is worth noting that the othermicrofabrication tools, which are able to create patterns onmultiple polymer layers, can be employed in the fabricationprocess as substitutes for the laser cutting machine.

Finally the individual smart platelet was released bydissolving the PAA film in water. A cross section of thePDMS/GO is illustrated in figure 1(b). The step-by-stepfabrication process is illustrated in figure 1(c). A detaileddescription of the manufacturing process and a discussion ofthe laser cutting can be found in the methods section and inthe supporting information (available at stacks.iop.org/SMS/21/105032/mmedia), respectively.

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Figure 2. Optical images of a thermo-adaptive square platelet in glycerol (a) heating process; (b) cooling process; insets in (a) show thediameter and edge length of the smart platelet, respectively.

1.2. Thermo-adaptive behavior

The mismatched strain, generated due to the mismatch of thelattice constant or coefficient of thermal expansion (CTE),can bend the thin film when the substrate is released [23].When the strain is large enough, the bilayer film may rollup to a scroll or a tube. Figure 2 demonstrates the geometrychange of a ∼800 µm × 800 µm square smart platelet.It consists of a 4 µm thick PDMS film covered with a50 nm-thick GO film. After laser cutting and dissolvingthe PAA film, the platelet was suspended in water, andsubsequently transferred to glycerol solution for opticalmicroscope characterization. Glycerol was used rather thanwater for in situ characterization because it helps to get aclearer observation. At room temperature, the smart plateletrolled up to a tube after the glass substrate was removed and itturned into a flat platelet at about 100 ◦C. After cooling downto room temperature, it returned to the tubular structure again.It rolled up in the diagonal direction due to the uneven strainat the interface.

In order to evaluate the rolling/unrolling process, wedefine the swelling ratio (SR) as the ratio of tube diameter(D) to edge length (L) of the platelet, as shown in the insetsof figure 2(a). The SR of the samples was plotted againsttemperature for a heating and cooling process in figure 3. Acritical temperature for SR change was found at 60 ◦C. Thus,we set the phase change LCST of the smart platelet as 60 ◦C.Although the LCST should vary with film thickness andplanar size, we did not notice any significant LCST variationfor other samples. When the solution temperature was withinthe range of 60–80 ◦C, the platelet transformed from a tube toa flat structure within about 20 s. When cooled down, it rolledup to a tube again. It was also found that the unrolling (rolling)rate of the tube is positively proportional to the heating(cooling) rate, allowing us to precisely control the unrolling(rolling) process through tailoring the temperature and heating(cooling) rate. In addition, according to the thermal adaptivemechanism of the PDMS/graphene oxide (GO) bilayer, theGO film is necessary for developing the interface strainmismatch. Without the GO film, there is no strain to drive theshape change of the thin film, as shown in figure S3 (available

Figure 3. Swelling ratio (tube diameter (D)/planar edge length (L))versus solution temperature of the sample shown in figure 4, heating(solid line) and cooling (dot line) process.

at stacks.iop.org/SMS/21/105032/mmedia). As a result, thepure PDMS film did not show any geometry change uponheating.

In addition, the SR in the heating process did notmatch the one in the cooling process, indicating that thethermal history affects the thermal responsive functionality.In particular, the SR at room temperature seems larger afterthe heating–cooling cycle, indicating a smaller curvature ofthe composite platelets. This phenomenon is attributed to thechange of the residual strain generated at the interface. Afterthe heating–cooling cycle, the residual strain is mainly fromthe disparity between the thermal expansion of the PDMSand the GO film. The mismatched strain generated beforethe thermal cycles was developed from various stages in thewhole manufacturing process.

At the z direction defined in figure 1(b),

εm = εf,internal − εs,thermalcuring + εf,lasercutting − εs,lasercutting

= εf,internal − αs1Tcuring + (αf − αs)1Tcutting (1)

where εm is the residual strain generated in the samples.εf,internal is the residual strain after drying the GO film onPDMS after the contact printing process. εs,thermalcuring isthe strain developed from thermal curing of the PDMS film,

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Smart Mater. Struct. 21 (2012) 105032 L Ren et al

originating from the cross linking of PDMS. εf,lasercutting

and εs,lasercutting are from the laser cutting process, which isintrinsically a thermal heating process. αf and αs are the CTEof the GO film and the PDMS film, respectively.

In equation (1), the αs1Tcuring is quite large due to thePDMS-cross-linkage induced shrinkage. Obviously, the freshplatelet has a lower SR compared with the platelet afterthe heating–cooling process. In addition, the strain directiondetermines the rolling direction of the composite layer afterit was released from the substrate. If εm is negative, thecomposite layer rolls toward the GO film so that the GO filmserves as an inside wall. Otherwise, the composite layer rollstoward the PDMS film so that the PDMS film serves as aninside wall [13]. Our results demonstrate that the tubes showan outside wall of PDMS film due to negative residual strain.The energy-dispersive x-ray spectroscopy (EDXS) results ofthe unreleased bilayer in figure S4 (available at stacks.iop.org/SMS/21/105032/mmedia) show that the pure GO area hasa carbon to oxygen atomic ratio of about 2.3, indicating anoxidized graphene sheet [35]. A very small silicon signal wasdetected in the GO film. However, the intensity of the siliconin the selected PDMS film is much stronger than that of theother elements (figure S4(b) available at stacks.iop.org/SMS/21/105032/mmedia). Meanwhile, the outside of a wrappedtube shows a strong silicon intensity and a similar spectrumto a pure PDMS film (figure S5 available at stacks.iop.org/SMS/21/105032/mmedia), suggesting that the outside wall ofthe tube is the PDMS film.

1.3. Mechanism of the thermal responsive functionality

In order to further understand the thermal responsivefunctionality, we employed a physical model to predict theplatelet geometry below its LCST. When a thin film isdeposited on a substrate, the system will be curved dueto the mismatched strain at the interface developed in themanufacturing process. Stoney [24] developed a mathematicalmodel to understand the relationship between the curvatureand residual strain. Stoney’s formula has been widely usedto calculate thin film’s mechanical properties with a givencurvature [25]. Generally, the bilayer curvature is determinedby the following equation, which is derived from thestationary potential energy [26].

κ =1ρ=

6εm

hsmn

[1+ m

1+ mn(4+ 6m+ 4m2)+ m4n2

]m =

hf

hsn =

Mf

Ms=

Ef1−νf

Es1−νs

(2)

where κ is the curvature; ρ is the curvature radius; εm standsfor the mismatch strain at the interface; hs, hf and m stand forthe substrate thickness, film thickness and hf/hs, respectively;M, E and ν represent the biaxial elastic modulus, elasticmodulus and Poisson ratio, respectively; The subscripts f ands stand for film and substrate, respectively.

Figure 4. Smart tube/platelet curvature radius versus (a) mismatchstrain, (b) platelet radius, (c) PDMS thickness and (d) GO filmthickness calculated from equation (3), the remaining parameters foreach figure are shown in its inset.

When the deformation is large, and hf � hs, equation (2)can be changed to the following formulation: [26, 27]

3εmR2hfMf

2h3s Ms

= K[1+ (1− νs)K2]

K = R2κ/4hs

(3)

where R is the radius of the circular sample.Equation (3) illustrates the bilayer’s large curvature as a

function of its geometry parameters, such as film thicknessand lateral dimension. The curvature was predicted at varioussituations and the results are shown in figure 4. For theGO/PDMS bilayer, Ef and Es were assumed to be 32×109 Paand 1.5×106 Pa [28, 29], respectively. υf and υs were assumedto be 0.165 and 0.5 [30, 31], respectively.

The geometry of the free-suspended platelet is dependenton the diameter (D) of the circular sample and edge length(L) of the rectangular sample. If D ≥ 2πρ, the bilayer willform a tube structure, where 2πρ is the circumference of thecross-section cycle of the single layer tube exactly made fromthe platelet. If D < 2πρ, arc structures will be formed [13].

The relationship between the curvature radius and themismatch strain is shown in figure 4(a), indicating that alarger strain results in a smaller curvature radius of the tubestructure. To estimate the mismatch strain in the GO/PDMSbilayer, we simplified the εm calculation equation (1) by onlycounting the mismatch strain developed from the thermalcuring process in laser cutting (αs − αf)1T . Since thePDMS was melted in the cutting process, 1T was assumedas 200 ◦C, which is close to the PDMS film’s meltingtemperature. αf and αs were assumed to be 7.1 × 10−6 ◦C−1

and 3.14 × 10−4 ◦C−1 [32, 33], respectively. As a result,εm was around 0.06, indicating a formation of ∼300 µmdiameter arcs or tubes according to figure 4(a). As an example,for the 800 µm diameter disk sample shown in figures3(a) and (b), the platelet would become an approximatelytubular structure since its diameter (D) is slightly less thanthe tube circumference 2πρ(942 µm). Compared with theexperimental result (∼170 µm), the error of the modelprediction (∼150 µm) is around 13%.

Figure 4(b) shows that at a specific thin film’s thicknessand mismatch strain, a smaller size of the bilayer results

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Figure 5. Optical microscope images of the PDMS/GOtube/platelet at room temperature with different PDMS and GOthicknesses.

in a smaller curvature radius. This is probably because thesmaller sample has less resistance to the bending driven bythe mismatch strain. Furthermore, when the circular radiusof the sample (R) increased from 50 to 500 µm, the SR(curvature radius/platelet radius) only increased graduallyfrom 0.24 to 0.51. In addition, for the 50 µm sample, D =100 > 2πρ = 75.3, whereas, D = 1000 < 2πρ = 1614 forthe 500 µm sample. Therefore, the small sample can a formmore obvious tubular structure while the large sample willform scrolls rather than a closed tube. As an example, the SRfor the 400 µm diameter sample is 0.48, which is close to ourobservation as shown in figure 3. However, we cannot get anobvious clear scroll or tubular structure for the sample with a300 µm edge length (diameter). This is probably due to thepoor interface quality and defects in the GO film, losing theconstraining of the strain for bending the film.

Although a thicker PDMS film may lead to a largercurvature, the PDMS film’s thickness does not show asignificant effect on the curvature radius from the modelsimulation (figure 4(c)). Experimental results also confirmedthat the platelet with the 2 µm thick PDMS film showeda smaller curvature radius than that of the 10 µm sample(figure 5). The thicker PDMS film is more rigid because ofits higher stiffness. Since the mismatch strain is only relatedto the material’s intrinsically physical properties such as CTE,the ‘hard’ thick PDMS film will have a small curvature.

The GO film’s thickness can alter the curvature radiusdramatically, according to the simulation result as shownin figure 4(d). For a thin GO film, the bilayer has a largecurvature radius due to the loose constraining of the interfacestrain. With a GO thickness from 10 to 200 nm, the curvatureradius approaches 100 µm. The GO film thickness effect onthe curvature radius was also investigated experimentally, asshown in figure 5. Bonded with the same thickness of PDMSfilm, a thin GO film platelet presents a small curvature radius.The laser cutting can exfoliate the GO film at the peripheralPDMS/GO interface, resulting in loose adhesion between theGO and PDMS film. Consequently, a thick GO sample mayhave a complicated peripheral interface and loose constrainingof the mismatch strain at the peripheral PDMS/GO interface,resulting in more resistance and a small stress for the bilayerrolling up.

2. Conclusions

PDMS was coated on a graphene film surface and thethermoresponsive functionality was investigated. The criticaltemperature for a substantial geometric transformationoccurred at 60 ◦C within less than 20 s. The thermallystimulated rolling and unrolling behaviors were boththeoretically and experimentally investigated, and were foundto be significantly dependent on the platelet size, filmthickness and heating/cooling rate. The platelets produced inthis work have potential applications in drug delivery, smartsolution fillers, actuators in smart solar tracking systems andother functional devices. Future work will be carried out toengineer smart platelets with various material combinationsand geometries, as well as to downsize the platelets to thenanometer scale.

3. Experimental details

3.1. Sample preparation

The glass substrates (VWR Labshop) were successivelycleaned in detergent, distilled water, acetone and isopropylalcohol (10 min per step). Then they were dried on a hotplate at 130 ◦C for 30 min in air. A 4 µm thick water solublepoly(acrylic acid) (PAA, Sigma-Aldrich) film was spin coatedon a 1′′×1′′ glass substrate at 1000 rpm for 1 min from 20 wt%aqueous solution. After the PAA film was dried at 80 ◦C for30 min on a hot plate in air, polydimethylsiloxane (PDMS,Dow Corning Sylgard Elastomer 184) films with thicknessesof 2 µm, 4 µm and 10 µm were deposited by spin coating the10 wt%, 20 wt% and 50 wt% solution, respectively. PDMSwas prepared by mixing prepolymer (base) and a curing agentat a weight ratio of 10:1 in chloroform solution. Subsequently,the PDMS film was cured at 120 ◦C for 15 min.

Graphene oxide (GO) was synthesized through ultrason-ically exfoliating graphite oxide which was prepared via amodified Brodie’s method [34]. GO film was prepared bya flow-directed assembly method [18]. Generally, GO wasdispersed in water with the concentration of 360 mg/L viahorn sonication (Misonix 3000) ∼2 h. 0.2 and 0.4 ml GOsolution were mixed with 200 ml water first and then thebulk solution was filtered through a filtration membrane (poresize: 220 nm, Millipore Corporation), resulting in a 50 nm and100 nm GO film, respectively.

Due to the high surface energy of PDMS, the GOfilm was easily attached onto the PDMS by the contactprinting method [19]. GO/filter assembly was applied on thePDMS/PAA/glass substrate. Mild stress was then applied bya rubber roller ∼5 min continuously. Finally, the filter waspeeled off from the GO film which was adhered on the PDMSsurface. The resultant glass substrate/PAA/PDMS/GO waspatterned by laser cutting machining (ILS9.150D, UniversalLasers System Inc.) with a custom pre-designed feature shapeand size in an AutoCAD 2010. The laser power, cuttingspeed and points per inch (PPI) were controlled for anefficient pattern. After laser cutting, the multilayer materialwas immersed in water for 2 h to dissolve the PAA layer andthen to release the micro-platelet.

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3.2. Sample characterization

The micro PDMS/GO platelet suspended in aqueous solutionwas transferred to glycerol (Sigma-Aldrich) by a woodentoothpick and in situ characterized under an opticalmicroscope (Eleitz, wetzlar, Nr. 518910). Optical images andvideo were real-time collected by a digital camera (AmScopeMD900). The temperature at the sample solution wasmonitored and calibrated by a handheld laser thermometer.The bilayer graphene/PDMS material was also characterizedby a scanning electron microscope (SEM, Hitachi S-4300) andenergy-dispersive x-ray spectroscopy (EDXS).

Acknowledgments

This work was partially supported by funds from AFOSR andan NSF CAREER grant. We thank Mr Mike West from theCollege of Architecture Texas Tech University for helpingus with laser cutting. We also thank Dr Brandon Weeks andMr Sanjoy Bhattacharia from the Department of ChemicalEngineering Texas Tech University for helping us with themicroscope imaging.

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