Functionalised Graphene

Properties of Thermoplastic Polyurethane/Functionalised Graphene Sheet Nanocomposites Prepared by the in Situ Polymerisation Method

Properties of Thermoplastic Polyurethane/Functionalised Graphene Sheet Nanocomposites Prepared by the in Situ Polymerisation Method Duc Anh Nguyen, Anjanapura V. Raghu, Jin Taek Choi and Han Mo Jeong* Department of Chemistry, University of Ulsan, Ulsan 680-749, Korea Received: 7 May 2009, Accepted: 16 April 2010

SUMMARY Thermoplastic polyurethane (TPU)/functionalised graphene sheet (FGS) nanocomposites were prepared by an in situ polymerisation method. The synthesised TPU/FGS nanocomposites were fully characterised by FTIR, TEM, TGA, DSC, DMA, and by measurements of electrical conductivity and mechanical properties. The results indicated that the chemical and/or physical interactions between FGS and TPU were enhanced in the nanocomposites prepared by the in situ polymerisation method, and were compared to those of an earlier study which were prepared by physical mixing. Thermogravimetry showed that the degree of TPU adherence onto the FGS increased when the nanocomposite was prepared by the in situ polymerisation method. There were pronounced modulus enhancements and decreases in tensile strength and elongation at break as a result of incorporating FGS. The FGS, well dispersed in the TPU matrix, effectively improved the electrical conductivity; the nanocomposite containing 2 parts FGS per 100 parts TPU had an electrical conductivity of 2.07×10-3 S/cm, about 108 times higher than that of the pristine TPU.

1. INTRODUCTION The carbon-based additives like graphite or carbon black have led to various types of electrical and thermal applications, and can also improve the physical as well as mechanical properties of original polymers. These improvements can be substantially enhanced in the nanocomposites where high aspect ratio nano-size fillers are used, because the interfacial contact between the filler and matrix polymer is maximised and the filler can make interconnected channel in the polymer matrix at an extremely low content. Nanocomposites containing carbon nanotube, fibrous nano-size filler with a diameter less than 100 nm and an aspect ratio greater than 100 have been extensively investigated in the past few decades1-7. Graphene, a one atom thick sheet of hexagonal carbon rings, can be viewed

as an unrolled sheet of single-wall carbon nanotube (SWCNT) which was scissored along the longitudinal plane. So graphene has excellent electrical and thermal conductivity, and a high modulus, comparable to those of SWCNT8. Graphite has a layered crystal structure composed of graphene sheets. It shows a sharp X-ray diffraction peak at 2θ = 26.5° due to the typical 3.35 Å interlayer spacing between the graphene layers. Graphite oxide (GO), prepared from the oxidation of graphite, also has a layered structure. GO has a broad X-ray diffraction peak at smaller angles than graphite, normally at 2θ ~ 10-15°, because polar groups such as hydroxyl, epoxide, ether, and carboxylate are present on the surface of its graphene sheets as a result of oxidation and expanded interlayer spacing9-11.

*Correspondence author: Prof. Han Mo Jeong, Department of Chemistry, University of Ulsan, Ulsan 680-749, Korea. Tel: +82-(0)52-259-2343, Fax: +82-(0)52-259-2348, E-mail: [email protected]

Smithers Rapra Technology, 2010

©

Polymers & Polymer Composites, Vol. 18, No. 7, 2010

Recently, it has been reported that the exfoliation of graphite into a single graphene sheet can be achieved from sufficiently oxidised GO, when the inter-graphene spacing associated with native graphite is completely eliminated in the oxidation stage, and that the heating is sufficiently rapid to assure adequate pressure build up at the gallery between the GO sheets12,13. The pressure results from CO2 evolved during the thermal decomposition of the functional groups. This exfoliated graphite has an affinity for polar solvents and polymers, as well as good conductivity, because this exfoliated material is functionalised graphene sheets (FGSs) having remnant polar functional groups that remain still after thermal treatment14. Some research groups have recently begun to report experimental research on FGS/ polymer nanocomposites15-18. Thermoplastic polyurethane (TPU) can be designed with a wide range of physical and chemical properties; various kinds of monomeric materials can be combined to meet the highly

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Duc Anh Nguyen, Anjanapura V. Raghu, Jin Taek Choi and Han Mo Jeong

diversified demands of modern technologies 19,20 . Because TPU composites containing nano-size conductive filler can be utilised effectively in electronic devices for antistatic or electromagnetic shielding, many studies have been devoted to TPU/CNT nanocomposites 21-23. However we are not aware of any paper about FGS nanocomposites of TPU except that of our group24. In our previous study, we prepared TPU/ FGS nanocomposites from a mixture of TPU solution and FGS suspended in methyl ethyl ketone and examined their morphology and physical properties. The FGS was dispersed on a nano-scale throughout the TPU matrix without any further surface treatment, and effectively enhanced the conductivity. The nanocomposite containing 2 parts FGS per 100 parts TPU had an electrical conductivity of 5.41×10-4 S/cm, a 107 times increase over that of pristine TPU24. The physical and/or chemical interactions between FGS and TPU can be increased, if FGS is added together with monomeric materials in the polymerisation reactor of TPU to prepare the nanocomposites by an in situ polymerisation method, because the FGS has reactive oxygencontaining polar groups such as epoxy or hydroxyl groups on the surface14. In this study, we prepared the TPU/ FGS nanocomposites using the in situ polymerisation method to examine the effect of the preparative method on the properties of the TPU/FGS nanocomposites. The mechanical and physical properties, including conductivity, are discussed herein.

2. EXPERIMENTAL Materials Natural graphite (HC-908) with an average particle size of 8 μm was purchased from Hyundai Coma Co. Ltd., Korea. Polycaprolactone diol (PCL; 2000g/mol; Solvay S. A.,

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Brussels, Belgium) was dried and degassed at 80 °C under vacuum for 3 h. 4,4’-methylenebis(cyclohexyl isocyanate) (H 12 MDI; BASF), 1,4-butanediol (BD; BASF), dioctyltin dilaurate (DOT; CNA Co., Ltd., Chungnam, Korea), methylethylketone (MEK; Aldrich), concentrated H2SO4 (96%, Matsunoen Chemicals Ltd., Osaka, Japan), fuming HNO3 (Matsunoen Chemicals, Ltd.), N,Ndimethylformamide (DMF; Aldrich), and KClO3 (Samchun Pure Chemical Co., Ltd., Seoul, Korea) were used as received.

2.1 Preparation of the FGS The GO was prepared by using a Staudenmaier method25,26. A 500 mL round bottom flask containing 10 g of graphite powder and 270 mL of a concentrated H2SO4/fuming HNO3 mixture (2/1 v/v) was cooled to 0 °C and agitated. Then, 110 g of KClO3 was slowly added to the reaction mixture, while maintaining the temperature of the mixture below 20 °C. The reaction mixture was then allowed to reach room temperature and stirred for 120 h before being poured into 10 L of deionised water. The GO was filtered and washed with double distilled water to pH 6.5. The resulting GO was dried at 80 °C, and then pulverised and screened with a 100 mesh sieve to obtain fine particles. Elemental analysis showed that the composition of the GO was C10O3.68H2.48. In order to prepare the FGS, the dried GO particles were placed in a quartz tube, and the tube was flushed with argon for 10 min. The quartz tube was then quickly inserted into a furnace preheated to 1100 °C. The tube was

left in the furnace for 5 min while the evolution of CO2 split the GO into individual graphene sheets12-14,25. The apparent specific volume of the FGS was 410 cm3/g, and elemental analysis showed that the composition of the FGS was C10O0.50H0.51.

2.2 Preparation of the TPU/ FGS Nanocomposite A 500 mL round-bottomed, fournecked separation flask was equipped with a mechanical stirrer, a nitrogen inlet, a thermometer, and a condenser with a drying tube. To prepare the TPU/FGS nanocomposite by the in situ polymerisation method, the FGS was immersed in MEK and than sonicated for 1 hour. This sonicated FGS mixture, about 1% solid content by weight, was mixed with PCL diol (0.01277 mol) in a round-bottom reactor and agitated at 80 °C for 1 h. The excess MEK solvent was removed by evaporation during this agitation. This mixture was reacted with H12MDI (0.02554 mol) in the presence of DOT (0.03 phr based on total solids) for 2 h at 80 °C under dry N2 atmosphere. BD (0.01277 mol) was then added and the reaction continued for another 3 h at 80 °C. As the reaction progressed, the increasing viscosity of the mixture was controlled by the addition of MEK solvent. Upon completion of the reaction, the solid content of the polymer solution was about 45%. The recipe calculations placed the soft PCL segment content of the TPU at 70% by weight. The synthesised TPU/FGS nanocomposite films were cast onto polypropylene plates at 25 °C for 24 h, and then at 60 °C for 24 h. The sample designation code in Table 1 provides

Table 1. Characteristics of TPU/FGS nanocomposites. Sample

Molecular weight of TPU

TPUNC-0

17,101

Mw

38,826

Tm (°C)

ΔHm (J/g)

Conductivity (S/cm)

37.1

2.85×10-11

TPUNC-1

18,186

42,978

43.7

36.0

6.81×10-10

TPUNC-2

17,059

41,404

45.0

39.5

2.07×10-3

TPUNC-3

22,393

54,145

45.3

38.3

2.77×10-3

Mn

Thermal properties 44.9



information regarding the amount of FGS included in the TPU samples. For example, TPUNC-3 contains 3 parts FGS per 100 parts polymer.

2.3 Measurements The molecular weights of the synthesised TPU samples were evaluated at 43 °C by gel permeation chromatography (GPC, Waters M510) using tetrahydrofuran (THF) as an eluent. The nanocomposite was dissolved in THF and the solution was filtered with a 0.45 μm membrane filter before measurement. The Fourier-transform infrared (FTIR) spectrum was recorded with an FTS 2000 FT-IR (Varian) using a KBr tablet that was made by the compression moulding of KBr powder mixed with a small amount of TPU/ FGS sample. Thermogravimetric analysis (TGA) was performed (Mettler Toledo, SDTA 851e) to measure the residual weight after thermal degradation by heating up to 700 °C at a heating rate of 10 °C /min under a N2 atmosphere with 22 mg of TPU/FGS sample in a platinum crucible. The morphology of the TPU/FGS nanocomposites was examined with a transmission electron microscope (TEM, Hitachi H-8100). In order to obtain samples for TEM observation, the cast films of the TPU/FGS nanocomposites were cryogenically pulverised. The TPU/ FGS nanocomposite powder was then mixed with epoxy resin and cured in a vacuum at 70 °C for 24 h. The cured material was microtomed into slices. The differential scanning calorimetry (DSC) was carried out (DSC 823e Mettler Toledo) at a 10 °C /min temperature change rate, with 7 mg of TPU/FGS sample. The TPU/FGS was loaded at 30 °C and cooled to -60 °C; the thermal properties were measured in a subsequent heating scan.

The dynamic mechanical properties of 0.5 mm thick cast films were analyzed using a dynamic mechanical analyzer (DMA, TA Instrument, DMA-Q800). The testing was carried out in bending mode at 1 Hz with a heating rate of 5 °C /min. The mechanical properties were measured by using a tensile tester (OTU-2, Oriental TM Co., Korea). The prepared TPU/FGS cast films were cut into micro-tensile specimens 25 mm in length, 5 mm in width, and 0.5 mm in thickness. The specimens were elongated at a rate of 200 mm/ min at 25 °C. The direct current conductivity at room temperature across a 0.5 mm thick cast film was measured with a picoammeter (Keithley 237). Circular silver electrodes measuring 0.28 cm2 were attached to both surfaces of the film. Silver paste was used to ensure good contact between the film surface and the electrode.

3. RESULTS AND DISCUSSION 3.1 Characterisation The TPU/FGS nanocomposites were prepared by using 0 to 3 parts of FGS per 100 parts TPU; tenacious flexible cast films were unachievable when the content of FGS was more than 3 parts per 100. The number-average molecular weight (Mn) and weightaverage molecular weight (Mw) of TPU are shown in Table 1, illustrating that the molecular weights of matrix TPU do not vary significantly with the presence of FGS. The TPU/FGS (100:3 by weight) nanocomposites were dissolved with a 100-fold amount of DMF; the dispersed FGS was then filtered through 1 μm filter paper. The filtered FGS was dispersed in a second 100fold amount of DMF, agitated at room temperature for 24 h to wash out any TPU molecules adhered on the FGS,


and then filtered again. This washing operation was repeated up to nine times. Table 2 shows the TGA results of the FGS washes. Because the weight of pristine FGS does not reduce below 700 °C and the residual weight of TPU is only 1.0% by weight, the residual weight shown in Table 2 may be that of the FGS and the weight reduction is likely to have been caused by the loss of TPU. The date presented in Table 2 show that the amount of adhered TPU in washed FGS is more than 50% by weight, even after washing, although it decreases with repeated washes. It was similarly reported that some polymer molecules strongly adhere onto carbon nanotubes (CNT) and that these strong interactions allow the molecules to surround CNT, even in the solution state27-29. Our results in Table 2 show that the physical interactions between FGS and TPU molecules are strong enough that they cannot be separated easily by a solvent. The amount of adsorbed TPU increased when the nanocomposites were prepared by the in situ polymerisation method compared to those prepared by physical mixing, as shown in Table 2. These results indicate that the interactions between FGS and TPU molecules were enhanced by the intimate mixing or grafting of the in situ polymerisation method. Table 2. Residual weights of washed FGS after thermal degradation Number Residue (wt.%) of Physical In situ washing mixing polymerisation method method 1

32.7

26.5

2

38.7

33.9

3

43.4

37.3

6

46.5

39.3

9

50.3

41.1

As shown in Figure 1a, FGS has broad IR absorption bands around 1540 cm-1 and 1233 cm-1 which are assignable to a C=C bond and a C-O bond, respectively30-33. However, the

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Figure 1. FT-IR spectra of (a) FGS, (b) FGS of the physical mixing method and washed once, (c) FGS of the physical mixing method and washed nine times, (d) FGS of the in situ polymerisation method and washed once, (e) FGS of the in situ polymerisation method and washed nine times, and (f) TPU

Figure 2. TEM micrograph of TPUNC-3

FT-IR spectrum of the FGS obtained from the nanocomposite prepared by the physical mixing method with only a single wash (Figure 1b) shows additional absorption bands at 1739 cm-1 and 1167 cm-1. These absorption bands are due to the ester C=O stretching vibration and the ester C-O-C stretching vibration of TPU (Figure 1f)30-33, respectively, representing the TPU that is adhered onto the FGS. The heights of these TPU IR peaks relative to those of FGS are reduced in Figure 1c, illustrating that the adhered TPU was reduced with repeated rinses. The absorption bands assigned to TPU are stronger in Figure 1d than in Figure 1b, and in Figure 1e compared to Figure 1c. These results indicate that higher amounts of TPU are adhered on FGS obtained from the nanocomposite prepared by the in situ polymerisation method compared to that produced using the physical mixing method, in agreement with the TGA results. However, Figures 1d and 1e do not have any evident peaks that are correspondingly absent in Figures 1b and 1c. This suggests that no new kind of chemical bond between FGS and TPU (that would present a new FT-IR absorption band which is absent in the nanocomposites prepared by the physical mixing method), is created by the in situ polymerisation method. The results of TGA and FT-IR show that the interactions between the FGS and TPU molecules were enhanced when the nanocomposites were prepared by the in situ polymerisation method. However, it was unclear whether or not any grafting reaction was induced by the in situ polymerisation method.

3.2 Morphology and Physical Properties Figure 2 shows the TEM image of TPUNC-3, illustrating that the wrinkled thin FGS sheets are dispersed in the TPU matrix without any agglomeration. This morphology demonstrates that the compatibility of

354



FGS with TPU is sufficient to get fine dispersion of the FGS sheets.

Figure 3. Dynamic mechanical properties of TPU/FGS nanocomposites

The DSC thermogram showed a sharp endothermic melting peak of the PCL segment; however, no melting peak for the hard segment was observed. The melting temperature (Tm) and the heat of fusion (ΔHm) are shown in Table 1, where it is shown that these thermal properties do not vary significantly with FGS content. The dynamic mechanical properties were measured by using a DMA and the data are shown in the Figure 3. When heated from -130 °C, TPUNC-0 slowly decreases in storage tensile modulus (E’), due to thermal expansion. This E’ reduction becomes evident at Tg, and another sudden drop of E’ occurs at Tm. Figure 3 also shows that, throughout the temperature range, E’ generally increases as the content of FGS in the nanocomposite is increased. This demonstrates that FGS effectively reinforces the TPU. Also evident is the decrease in height of the tan δ peak at Tg as the content of FGS is increased. Because tan δ is correlated with the energy dissipated during cyclic deformation, this result indicates that, due to molecular interactions, the viscous flow of the molecules is reduced in the presence of FGS34,35. The tensile properties of the TPU/ FGS nanocomposite made by the in situ polymerisation method are summarised in Table 3. The tensile modulus is effectively improved by the reinforcing effect of FGS, as in the results of DMA. However, tensile strength and elongation at break, which were measured at large deformation, decrease as the content of FGS is increased. When TPUs are highly elongated, hard segments break apart when phase mixed with soft segments; the hard and soft segments orient in the direction of elongation, resulting in maximum intermolecular interaction36,37. The evident lowering of tensile properties measured at large deformation suggests that these

Table 3. Tensile properties of TPU/FGS nanocomposites Sample

Modulus (MPa)

Tensile strength (MPa)

Elongation at break (%)

TPUNC-0

59±19

7.4±0.5

1368±55

TPUNC-1

138±5

4.3±0.4

898±7

TPUNC-2

150±28

4.8±0.4

653±73

TPUNC-3

171±9

3.7±0.4

428±15

molecular rearrangements were interrupted in the presence of FGS. In our previous report, when TPU was physically mixed with 3 phr of FGS, the increase of modulus and the decrease of tensile strength and elongation


at break were 43%, 23%, and 15%, respectively, of that of pristine TPU24. In contrast, Table 3 shows that these respective changes are 190%, 50%, and 69% in the nanocomposites made by the in situ polymerisation

355


method. These results suggest that the interactions between FGS and TPU are increased when prepared by the in situ polymerisation method, and that the increased interactions reduce the chain mobility for realignment. The electrical conductivities of TPU/ FGS nanocomposites made using the in situ polymerisation method are

displayed in Table 1 and Figure 4. Figure 4 also illustrates our previous data on the conductivities of TPU/ FGS nanocomposites made using the physical mixing method, for comparison24, it is shown that the conductivities of nanocomposites at high loadings of FGS made using the in situ polymerisation method are slightly higher than those of nanocomposites

Figure 4. Conductivities of TPU/FGS nanocomposites made by the in situ polymerisation method (o) and by the physical mixing method (D)

Figure 5. TGA thermograms of TPU/FGS nanocomposites

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made by physical mixing. This suggests that the dispersion of FGS in a TPU matrix is improved by the in situ polymerisation method. According to percolation theory, the electrical conductivity of a filled material follows a power law relationship according to the following equation38-41: σ ∝ σo ( P - Pc )t

(1)

where σ is the electrical conductivity of the nanocomposite, σo is a scaling factor related to the intrinsic electrical conductivity of FGS itself, P is the volume fraction of the filler, Pc is the volume fraction of the percolation threshold, and t is the conductivity exponent controlled by the dimensions and orientation of the conductive fillers38. In this study, because the density of the FGS can only be estimated, we used the weight fraction from Figure 3. The change in log σ reaches a maximum when P approaches Pc 39. The Figure 3 shows that Pc is about 1.5% by weight. The thermal degradation behaviours of TPU/FGS nanocomposites are shown in Figure 5, where it can be noted that the degradation temperature increases at low FGS content and decreases at higher FGS content. These results are similar to those of our previous study of TPU/ FGS nanocomposites made by a physical mixing method 24 . In polymer/clay nanocomposites, the thermal degradation is retarded by the presence of clay because the thin silicate lamellae with a high aspect ratio provide a barrier that hinders the diffusion of the volatile decomposition products and enhances the char formation42,43. The improved resistance to thermal degradation at low levels of FGS shown in Figure 5 can be similarly explained by the barrier effect of the thin FGS. However, the high thermal conductivity of FGS can promote thermal degradation39,44, which is a possible cause of the disadvantageous effect at high FGS content.



4. CONCLUSIONS In this article, the experimental results indicated that FGS could play a role as a conductive reinforcing nanofiller more effectively when TPU/ FGS nanocomposites were prepared by an in situ polymerisation method, compared to those prepared using the physical mixing method. About a 2.5-fold enhancement of the modulus was achieved when 100 parts of TPU was reinforced with 2 parts of FGS, whereas it was only 1.4-fold in the physical mixing method. The FGS finely dispersed in the TPU effectively improved the conductivity; the nanocomposite containing 2 parts FGS per 100 parts TPU had a conductivity of 2.07×10-3 S/cm, which is about 108 times higher than pristine TPU. The gravimetric analysis also indicated that the chemical and/or physical interactions between FGS with TPU were increased by the intimate mixing or grafting of the in situ polymerisation method.

ACKNOWLEDGEMENTS This research was financially supported by the Ministry of Education, Science Technology (MEST) and Korea Institute forAdvancement of Technology (KIAT) through the Human Resource Training Project for Regional Innovation. And this work was supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0093818).

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