graphene nanocomposites

Reactive & Functional Polymers 88 (2015) 1–7

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Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react

Electroactive shape memory performance of polyurethane/graphene nanocomposites J.T. Kim a, H.J. Jeong a, H.C. Park b, H.M. Jeong c, S.Y. Bae a, B.K. Kim a,⇑ a

Department of Polymer Science & Engineering, Pusan National University, Busan 609-735, Republic of Korea School of Materials Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea c Department of Chemistry, University of Ulsan, Ulsan 680-749, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 5 March 2014 Received in revised form 18 November 2014 Accepted 23 January 2015 Available online 2 February 2015 Keywords: Polyurethane nanocomposites Thermally reduced graphene (TRG) Electroactive Shape memory behavior

a b s t r a c t A series of electroactive shape memory polyurethane (SMPU) nanocomposites were synthesized from poly(tetramethylene ether) glycol (PTMG), 4,4-methylenebis(phenyl isocyanate) (MDI) and 1,3-butandiol (1,3-BD) with the addition of various amounts of thermally reduced graphenes (TRG) which were chemically modified with allyl isocyanate (iTRG). The effects of iTRG on electroactive shape recovery behaviors as well as the conventional direct heat actuated SMPU material have been studied in terms of morphological, thermal, mechanical, electrical properties and thermomechanical cyclic behavior. It was found that significant increases in electrical conductivity and temperature were obtained high iTRG contents (>2%) to electrically actuate the nanocomposite, along with large increases in glass transition temperature (Tg) and initial modulus with a dramatic drop in elongation at break. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Polyurethanes (PUs) are most versatile engineering materials which are synthesized by a simple polyaddition reaction of polyol, isocyanate and chain extender. They find a variety of industrial applications including coatings, adhesives, sealants, elastomers, primer, sports goods, medical devices, textile finish aside from the various foam products [1–3]. Shape memory polymers (SMPs) are smart materials that respond to external stimuli, typically heat, and their applications have expanded steadily [4–8]. Aside from the current applications in textile, films, smart toys, etc., vast applications have opened for biomedical devices using biocompatible polymers such as PU and space materials with enhanced mechanical performance [9–11]. Among the many SMPs, shape memory polyurethanes (SMPUs) have found the broadest applications because of their ample degree of freedom in property design. Design variables include various types and molecular weights of soft segment (polyol), type of hard segment, and soft segment/hard segment composition. Depending on the molecular design, SMPU could be crystalline or amorphous showing the actuation temperature in a broad range of 20 to 150 °C. ⇑ Corresponding author. Tel.: +82 51 510 2406; fax: +82 51 514 1726. E-mail address: [email protected] (B.K. Kim). http://dx.doi.org/10.1016/j.reactfunctpolym.2015.01.004 1381-5148/Ó 2015 Elsevier B.V. All rights reserved.

Mechanical reinforcement and functionalization are largely achieved by the hybridization of SMPs with fillers in the form of particles, fibers, platelets or tubes [12–15]. Generally, chemical hybridization on the nanoscale is superior to physical blending on the macro-scale due to the fine dispersion and improved interfaces between the polymer and filler. Chemically-incorporated fillers provide multifunctional crosslinks that augment the rubber elasticity and strain recovery [16–18] as well as the conventional reinforcement. Graphene is a fascinating two-dimensional nanomaterial with superior thermal and electrical conductivity and modulus. Graphene sheets can be produced in bulk by the thermal expansion of sufficiently oxidized graphite oxide (GO). Deformation of the planar structure by oxidation and pyrolysis prevents restacking of layers [19]. Because of the enormous surface area (700–1500 m2/g), surface polarity, and high electrical conductivity, the thermally reduced graphene oxide (TRG) sheets can be used to produce polymer composites with excellent mechanical, electrical, and thermal properties [20]. TRG carries some oxygen containing groups, which can be used for chemical modification. Chemically-modified TRG can be incorporated into the polymer by covalent bonding [18], which generally makes fine dispersion of TRG in polymer matrix and augments the electrical conductive of the composite [21]. On the other hand, little has been evolved from the literature survey for

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the use of graphene as electroactive shape memory materials [22,23]. Presently, TRG was prepared and modified chemically with allyl isocyanate (iTRG). The iTRG was reacted with the hydroxyethyl acrylate (HEA) termini of PU at various compositions by UV curing to synthesis PU/iTRG nanocomposites. The effects of graphene and chemical hybridization were analyzed in terms of the particle dispersion, mechanical, thermal, electrical and shape memory properties of the nanocomposites.

2. Experimental 2.1. Raw materials Polytetramethylene ether glycols (PTMG, Mn = 300 g/mol) and 1,3-butandiol (1,3-BD, Aldrich) were dried and degassed at 80 °C under vacuum for 3 h before use. 4,40 -Methylene diphenyl diisocyanate (MDI, BASF), 2-hydroxyethyl acrylate (HEA, Aldrich), allyl isocyanate (Aldrich), phenylglyoxylic acid methyl ester as the photoinitiator (Darocur MBF, BASF) and dibutyltin dilaurate (DBTDL, Aldrich) were used as received. Expandable graphite (ES350 F5, average particle size of 280 lm) purchased from Qingdao Kropfmuehl Graphite (China) was used for the preparation of graphene.

Scheme 1. Modification of TRG by allyl isocyanate.

Table 2 Formulations to solid = 30 g) (g). Series

2.2. Preparation of TRG GO was prepared using the Brodie method [24]. A reaction flask with 200 mL of fuming nitric acid was cooled in an ice bath to 0 °C, and 10 g of graphite powder was added with stirring. Over a 1 h period, 85 g of potassium chlorate was added slowly with stirring at 25 °C. After 24 h, the mixture was poured into 3 L of distilled water. The GO was filtered and washed with distilled water until the pH of the filtrate was neutral. The resulting GO filtrate was then dried in a vacuum oven at 100 °C. Elemental analysis showed a GO composition of C10O3.45H1.58. To obtain graphene, dried GO was placed in a quartz tube and flushed with nitrogen for 5 min. The quartz tube was then quickly inserted into a furnace at 1100 °C for 1 min to split the GO into individual sheets by evolving CO2. Elemental analysis (Table 1) showed that some oxygen-containing functional groups, such as epoxide or hydroxyl groups, remained even after thermal reduction.

2.3. Preparation of iTRG GO (1.5 g) was loaded into a 500-mL round bottom flask equipped with a stirrer under nitrogen atmosphere. Anhydrous DMF (400 mL) was added to create an inhomogeneous suspension. Allyl isocyanate (3.45 g) was then added and stirred for 5 days (Scheme 1) at 30 °C. Then, the reaction mixture was poured into methylene chloride (1.5 L) to coagulate the product. The product was filtered, washed with additional methylene chloride (1.5 L) and dried under vacuum [18].

Table 1 Relative atomic concentrations of TRG and iTRG from XPS.

C1s N1s O1s

TRG

iTRG

91.15 – 8.85

63.83 0.72 9.43

EP00 EP10 EP15 EP20 EP25

prepare

PU/iTRG

nanocomposites.

(Mn = 6000 g/mole,

Polyurethane

total

iTRG (Phr)

PTMG300

MDI

1,3-BD

HEA

15.80

14.64

0.79

1.16

– 1.0 1.5 2.0 2.5

2.4. Synthesis of PU/iTRG nanocomposites Table 2 and Scheme 2 respectively show the formulation to prepare the PU/iTRG nanocomposites and overall reaction scheme to prepare the PU/iTRG nanocomposites, respectively. PTMG300 and 1,3-BD was placed in a 500-mL round-bottom flask with a mechanical stirrer, thermometer and condenser with a drying tube. The reactions were carried out in a constant temperature oil bath. Molar excess of MDI was reacted in DMF at 70 °C until the theoretical isocyanate values were obtained. The isocyanate terminated PU was then end capped with HEA at 50 °C to obtain PU with a molecular weight of approximately 6000 g/mol. Then, the iTRG particles dispersed in DMF (concentration = 3.0 mg/mL) were fed into the PU solutions at various compositions, as listed in Table 2. To improve the dispersion of iTRG particles in DMF, the particles were added to DMF in a vial and ultrasonicated for 2 h at 40 °C before adding the mixture to the polymer solution. After mixing for 30 min, additional ultrasonication was done for sufficient dispersion of iTRG. The mixture was cast on a polyethylene film and partially dried before being cured under a UV lamp (365 nm, 8 W, Crosslink) for 2 h in air. Finally UV cured film was fully dried at 70 °C.

2.5. Characterizations The progress of the HEA capping reaction was monitored by Fourier transform infrared (FT-IR, Mattson Satellite) spectroscopy using the characteristic isocyanate peak. Standard X-ray photoelectron spectroscopy (XPS) measurements were done using a VG-Scientific ESCALAB 250 spectrometer with an AL Ka X-ray source. Thermal properties of the cast films were determined using

J.T. Kim et al. / Reactive & Functional Polymers 88 (2015) 1–7

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Scheme 2. Overall reaction scheme to prepare the PU/iTRG nanocomposites.

a differential scanning calorimetry (DSC, Q100). The samples were first heated to 100 °C to erase their thermal history and then cooled to below 50 °C at 20 °C/min under nitrogen. Glass transition temperature (Tg) was measured during the second heating cycle at 10 °C/min. Morphology of the film was examined using a scanning electron microscopy (Zeiss FE-SEM SUPRA25). Sample was cryogenically fractured before viewing. The tensile properties of the cast film at room temperature were measured with a universal testing machine (UTM, Lloyd LRX) at a crosshead speed of 200 mm/min using specimens prepared according to ASTM D1822. The direct current conductivity of graphene papers was measured using a four-point probe system (CMT-SR 1000N, AIT Co. Ltd, Korea). The shape memory properties were characterized using the UTM attached with a heating chamber. The sample was first heated to the glass transition temperature (Tg) + 20 °C and stretched uniaxially to a maximum strain (em) of 100%, followed by cooling and unloading at Tg  20 °C. Upon unloading, a small part of the strain (em  eu) was recovered instantaneously, leaving an unload strain (eu). The sample was reheated to Tg + 20 °C to recover the strain, leaving a substantial amount of permanent strain (ep). These three steps completed one thermomechanical cycle. The shape fixity (Rf) and shape recovery (Rr) ratios for the cycle are defined as follows:

Shape fixity ratio; Rf ð%Þ ¼

eu  100 em

Shape recovery ratio; Rf ð%Þ ¼

er  100 em

3. Results and discussion 3.1. Characterization of TRG, iTRG and HEA capping of PU The formation of iTRG was confirmed by XPS. Table 1 lists the atomic compositions of TRG and iTRG. The nitrogen content increased from 0% (TRG) to 0.72% (iTRG). This confirms the formation of a urethane linkage between the allyl isocyanate and hydroxyl group of the TRG. Fig. 1 shows the typical FT-IR spectra obtained from the isocyanate-terminated PU and HEA capped PU. The absorption peak at approximately 2270 cm1, corresponding to the stretching vibration of the isocyanate group completely disappeared upon capping with HEA.

3.2. Morphologies and properties of PU/iTRG nanocomposites 3.2.1. SEM morphology Fig. 2 shows the SEM morphology obtained from the cryogenically fractured surface of the PU/iTRG nanocomposites. It is seen that primary iTRG particles are well dispersed in polymer matrix

ð1Þ

ð2Þ

where er = emep is the recovered strain. For electroactive shape memory performance, the sample was heated to the Tg + 20 °C and bended to 90°, followed by cooling to Tg  20 °C to freeze the temporally shape. The electroactive shape memory effect was observed by applying constant voltage (50 V) current to the films while monitoring the temperature rise by an infrared thermometer and shape change with a semicircular protractor. The shape recovery (Rro) ratio was calculated as follow:

Shape recovery ratio; Rf ð%Þ ¼

Ar   100 90

where Ar is the recovered angle.

ð3Þ Fig. 1. FTIR spectra of the isocyanate-terminated PU and HEA capped PU.

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Fig. 2. SEM images of PU/iTRG nanocomposites; EP10 (a), EP15 (b), EP20 (c), EP25 (d).

at low contents (EP00-EP20). However, particles are agglomerated at high content of iTRG (EP25). 3.2.2. Thermal and mechanical properties Fig. 3 and Table 3 show the stress–strain behavior of the nanocomposites at 26 °C. The Young’s modulus of the nanocomposites significantly increased with the addition of iTRG showing a maximum at 2.0 phr iTRG (EP20) beyond which (EP25) it is decreased. The effect of iTRG incorporation was most pronounced with EP20. On the other hand, the elongation at break monotonically decreased with the addition and increasing amount of iTRG suggesting that the nanoparticles disturb the polymer chain orientations at high elongations [24]. At high iTRG contents, the particles are vulnerable to aggregation, as noted from the morphology measurements, and auto-inhibition of the allyl groups [25] which reduces crosslinking sites and filler effect. Fig. 4 shows the DSC thermograms of the nanocomposites and the detailed data are provided in Table 3. The glass transition temperature (Tg) increased with the addition of iTRG with a maximum at 2.0 phr loading. The Tg of PU is increased due to the restricted

Table 3 Glass transition nanocomposites.

EP00 EP10 EP15 EP20 EP25

temperature

(Tg)

and

tensile

properties

of

PU/iTRG

Tg (°C)

Young’s modulus E (MPa)

Break strength rb (MPa)

Elongation at break eb (%)

29.03 33.74 35.57 36.37 35.87

180.46 466.47 512.13 600.23 565.65

46.43 35.65 27.80 48.10 45.49

461.86 140.09 25.06 9.02 8.64

Fig. 4. DSC curves of PU/iTRG nanocomposite films.

motion of polymer chains in the presence of iTRG particles. In addition, the increase in Tg is related to the crosslinking effect of iTRG as described by the Dibenedetto’s equation [26]:

T g ¼ K  T g;0 Fig. 3. Stress–strain behavior of the PU/iTRG nanocomposites at 26 °C.

Xc þ T g;0 1  Xc

ð4Þ

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Fig. 5. Thermomechanical cyclic behavior of EP00 (a), EP10 (b), EP15 (c), EP20 (d), EP25 (e).

Table 4 Shape fixity and shape recovery of PU/iTRG nanocomposites. (N = number of thermomechanical cycle). Shape fixity (%)

N=1 N=2 N=3 N=4

Shape recovery (%)

EP00

EP10

EP15

EP20

EP25

EP00

EP10

EP15

EP20

EP25

97 97 98 97

97 97 96 96

97 97 98 98

97 98 98 98

96 96 97 96

96 96 95 95

95 92 89 89

92 89 85 84

92 90 89 89

89 86 84 84

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where Tg,0 is the Tg of the uncrosslinked polymer, Xc is the mole fraction of crosslinker, and K is a constant. The iTRG provides the materials with multifunctional crosslinks and the effect was most pronounced with EP20. At high iTRG content (EP25), the combined effects of particle aggregation and auto-inhibition of the allyl groups seem to make the effect less pronounced.

Fig. 6. Electrical conductivity of PU/iTRG nanocomposite films.

3.2.3. Shape memory behavior Fig. 5 shows the cyclic loading and unloading behavior of the PU and nano-composites for the first four cycles, and the detailed data are provided in Table 4. The virgin PU shows shape fixity ratios over 97% and shape recovery ratios over 95%. Both ratios are already high due to the crosslinkined structure of PU, where the crosslinkings were introduced by the trifunctional HEA. With the addition of iTRG, the shape fixity ratios are essentially the same or a bit improved with EP20 while the shape recovery ratios are slightly decreased showing a minimum decrease with EP20. However, the elastic strain energy (area under the stress–strain curve) is increased over up to 400% with the addition of iTRG at high content. Since the strain energy corresponds to the shape recovery force, the retraction force is expected to increase with the chemical incorporation of iTRG. For EP25, elongation at break was less than 100%, and SM measurements were measured at 50% strain. 3.2.4. Electrical conductivity and electroactive shape memory behavior Fig. 6 shows electrical conductivities of the nanocomposites which increase from 6.7  1014 S/cm to 2.5  102 S/cm with the addition and increasing amount of iTRG. The threshold content seems around 1% iTRG. Fig. 7 shows temperature rise of the films during electric current application. Up to 1.5% iTRG, there was no temperature change for the film. With only 2% and 2.5 % iTRG, film temperature increases asymptotically to 46 °C for EP20 and 64 °C for EP25. High conductivity composite provided high electric current to produce significant Joule heat (H) according to

Fig. 7. Temperature rise vs time for PU/iTRG nanocomposite films at 50 V.

H ¼ RI2 t;

ð5Þ

Fig. 8. Electroactive shape memory behavior of EP00 (a), EP10 (b), EP15 (c), EP20 (d), EP25 (e). The as cast straight line is deformed (left) and recovered (right) partially with EP20 and almost completely with EP25. EP00 and EP10 do not respond to the electrical current.

J.T. Kim et al. / Reactive & Functional Polymers 88 (2015) 1–7 Table 5 Electroactive shape memory performance of PU/iTRG nanocomposites.

EP00 EP10 EP15 EP20 EP25

Voltage (V)

Time (min)

Temperature (°C)

Shape recovery (%)

50

2

17 17 18 46 64

0 0 0 44.4 96.7

R is electrical resistance, I is current and t is time. The shape recovery behavior is visualized in Fig. 8 which shows the photographs for the electroactive shape memory performance. At low iTRG contents (EP00 - EP15), electric current does not induce any shape change due to the insufficient electrical dissipation. However, at high iTRG contents (EP20, EP25), film temperature increased over the Tg within 2 min of current application, and induced shape recovery over 44% (EP20) and 97% (EP25) of the original shape (Table 5). 4. Conclusions PU/iTRG nanocomposites were synthesized to reinforce and electrically actuate the shape memory behavior. The nanocomposites showed fine and uniform particle dispersion up to 2.0% (EP20) beyond which aggregation of particles occurred. Consequently, the glass transition temperature and Young’s modulus remarkably increased with the addition of iTRG and the effects were most pronounced with 2.0%. Consequently, the elastic strain energy increased over 400% at this content of iTRG. On the other hand, the break strain monotonically decreased with increasing iTRG content implying that iTRG particles disturb the polymer chain orientation. With the addition of iTRG, the shape fixity ratio was essentially not changed due to the already high value of virgin PU, while the shape recovery ratio slightly decreased. Among all the samples, the highest shape fixity and smallest shape recovery decrease was obtained with EP20 with remarkably increased recovery force in terms of elastic strain energy. Electrical conductivity increased sigmoidally with the addition and increasing amount of iTRG, which raised the film temperature up to 46 (EP20) and 64 °C

7

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