functionalized graphene nanocomposites with improved mechanical and thermal properties

Composites Science and Technology 72 (2012) 702–707

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High performance polyurethane/functionalized graphene nanocomposites with improved mechanical and thermal properties Dongyu Cai, Jie Jin, Kamal Yusoh, Rehman Rafiq, Mo Song ⇑ Department of Materials, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK

a r t i c l e

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Article history: Received 11 July 2011 Received in revised form 22 January 2012 Accepted 24 January 2012 Available online 8 February 2012 Keywords: A. Polymer–matrix composites (PMCs) A. Nano composites B. Mechanical properties B. Thermal properties

a b s t r a c t This communication reported the substantial improvement in the mechanical and thermal properties of a polyurethane (PU) resulting from the incorporation of well-dispersed graphene oxide (GO). The stress transfer benefited from the covalent interface formed between the PU and GO. The Young’s modulus of the PU was improved by 7 times with the incorporation of 4 wt% GO, and the improvement of 50% in toughness was achieved at 1 wt% loading of GO without losing elasticity. Significant improvements were also demonstrated in the hardness and scratch resistance measured by nano-indentation. Thermogravimetric analysis revealed that the decomposition temperature was increased by 50 °C with the addition of 4 wt% GO. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Polyurethanes (PUs) are one of most versatile synthetic soft materials that have been popular commercial polymeric products with diverse industrial application for a long period. The synthesis of PUs basically takes the advantage of the reaction between polyols and polyisocyanates [1]. Various selections of raw materials and synthetic routes provide very flexible solutions to tailor the microstructure of PUs and achieve desirable properties. Nanotechnology has been considered as a new approach to develop next generation PU materials for broader industrial applications, in which nanomaterials such as nanoclays and carbon nanotubes have shown strong ability to comprehensively upgrade the physical properties of PUs [2–7]. Very recently, the benefits of graphene in polymer nanocomposites have been realized in fast track since the discovery of this magic two-dimensional material in 2006 [8,9]. Currently, the graphene used in composite materials consists of pristine graphene, functionalized graphene (FG) and chemically reduced graphene (CRG). Pristine graphene can be made by mechanical exfoliation of graphite in organic solvents [10]. Khan et al. reported a highly stiff PU yielded by adding 55 wt% graphene [11]. But the elasticity of the PU was seriously deteriorated. Graphene oxide (GO) is the most popular FG with oxygen groups on the surface of carbon sheets. The GO can be simply fabricated

⇑ Corresponding author. Tel.: +44 (0)1509223160; fax: +44 (0)1509223949. E-mail addresses: [email protected] (D. Cai), [email protected] (M. Song). 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2012.01.020

via exfoliating graphite oxide in organic solvents by sonication [12]. It paves the way to achieve good dispersion of the graphene sheets in polymeric matrix via a solution method. Our previous work reported that the stiffness and scratch resistance of a PU could be significantly enhanced by the addition of GO [13]. In order to repair the honeycomb structure of carbon sheets damaged by oxidization, chemical approaches are used to reduce the amount of oxygen groups to form semi-conductive CRG [14]. The CRG is capable of improving both of mechanical and electrical properties of PUs. It was believed that the ideal interface for stress transfer was supposed to be the covalent bonds formed between graphene and PU matrices [15,16]. A study by Nguyen et al. disclosed that non-covalent interface led to poor reinforcing effect of the GO [17]. Overall, it can be seen that the graphene can enhance the stiffness of PUs. But it is still not clear how to switch on the toughening effect of the graphene in PU matrices. In this work, a linear PU was designed to hold isocyanate (ANCO) groups in the ends, which could react with the oxygen groups on the GO to form covalent bonds. Compared with our previous work [13], it was found that toughening effect compromised with the intrinsic network of PU matrices. In a crosslinked system, the toughness and strength of the PU decreased with the incorporation of GO although huge improvement was achieved in the stiffness [13]. Here, this communication reported that, in a linear PU, the addition of GO substantially enhanced the stiffness and toughness without deteriorating elasticity. In addition, it was found that the GO could improve the thermal stability of the PU. All these findings pointed out that the GO is promising nanofiller to yield high performance PU nanocomposites.

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2. Experimental

3. Results and discussion

2.1. Materials

3.1. Morphology and mechanical properties of the GPUNs

Expandable graphite (EG) was purchased from Chinese Qingdao Graphite Company. Difunctional polycaprolactone (PCL) with a molecular weight of 2000 was a Solvay product (CAPA2205). 4,4-methylenebis (phenyl isocyanate) (MDI) was provided by Hyperlast Ltd (UK). 1,4-Butanediol (BDO), dibutyltin dilaurate (DBTL) and N,N-dimethylformamide (DMF) were purchased from Sigma–Aldrich Ltd (UK). All materials used for the oxidation of EG, such as H2SO4, HNO3, HCl, KMnO4, 30% H2O2 solution and BaCl2, were purchased from Fisher Ltd (UK).

Fig. 1a shows typical stress–stain curves of the GPUNs, which apparently demonstrates the improvements in the mechanical performance of the GPUNs. The area under stress–strain curves is usually used to evaluate the toughness of polymers. Fig. 1b presents that the Young’s modulus of the PU increases by 2 and 7 with the incorporation of 1 t% and 4 wt% GO, respectively, indicating the significant enhancement in the stiffness of PUs. Meanwhile, it can be seen that the toughness of the PU is increased by nearly 50% and 30% with the incorporation of 1 wt% and 4 wt% GO, respectively. Investigating the dispersion level of the GO in the PU matrix and the interfacial interaction between the GO and PU matrix is a key to understand the improvement in the stiffness of GPUNs. Both of the dispersion and interfacial interaction are decisive factors affecting the efficiency of stress transfer. The SEM images shown in Fig. 2 reveals that the average thickness of the GO strips is less than 50 nm in the GPUN with 4 wt% GO, indicating nano-scale dispersion of the GO in the PU matrix. Our previous work reported that the GO could be exfoliated to the mixture of mono- and multi-layers in DMF. It was believed that, to some extent, the operations of ‘‘mixing’’ and ‘‘drying’’ applied to make the nanocomposites could result in re-stacking of exfoliated graphene sheets obtained in DMF into thicker nano-flakes (650 nm) in the PU matrix. In terms of the interface, it was assumed that urethane

2.2. Preparation of the GO/PU nanocomposites (GPUNs) Graphite oxide was prepared by oxidizing EG following the procedure as reported in reference [12]. 0.1 g graphite oxide was exfoliated in 10 g DMF by using ultrasonication with a power of 300 W for 30 min at room temperature. 10 g PCL, 0.45 g BDO, 3.25 g MDI and 0.07 g DBTL (catalyst) were stirred in 20.6 g DMF at 60 °C for 24 h in a four-necked flask protected by N2 gas, by which a PU/ DMF solution was obtained with the solid content of 40 wt%. In this formulation, the PU was designed to have linear chains with the hard segment of 27 wt%, and NCO/OH molar ratio was fixed as 1.3 to have excess NCO group terminating linear PU. Afterward, calculated amount of the GO/DMF dispersion was mixed with the PU/DMF solution at 80 °C for another 1 h. Finally, the solution was dried at 50 °C for 2 weeks to have solid GPUNs after the evaporation of DMF.

(a)

15 4Wt% GO

2.3. Characterization

Stress (MPa)

PU

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The scanning electron microscopy (SEM) images of fracture surface were taken on field emission gun scanning electron microscopy (FEGSEM) (LEO 1530VP instrument). The PU nanocomposites were fractured in liquid nitrogen. The samples with fracture surfaces on the top were placed on specimen holder using doublesided carbon conductive tape. Modulated differential scanning calorimetry (MDSC) analysis was conducted on a TA Instrument 2920 MDSC calorimeter with a heating rate of 4 °C/min. The amplitude and period of oscillation was 1 °C and 60 s, respectively. Thermogravimetric analysis (TGA) was performed on a DSC-TGA 2950 instrument. The samples were heated from room temperature to 700 °C at a heating rate of 10 °C/min. The rate of gas (air) was 50 ml/min. Infrared spectra were recorded using a Mattson 3000 Fourier Transform Infrared Spectroscopy (FTIR) for 128 scans at a resolution of 4 cm1. The reflection and transmission mode was used for the films of PU nancomposites and pure GO powders, respectively. Swelling tests were carried out by immersing each sample with same size into same amount of DMF. Tensile test was carried out using a Hounsfield test machine at a crosshead rate of 500 mm/min. Five specimens were tested for each sample. Nanoindentation was performed on a small piece of film (2 cm  2 cm  1 mm) using a Nano TestTM (Micro Materials, UK) equipped with a Berkovich (three sided pyramidal) diamond indenter tip at room temperature. The maximum load and initial load placed on the indenter tip was 2.5 mN and 0.15 mN, respectively. The loading and unloading rate of indenting was 0.05 mN/s. The holding time at maximum load was 180 s. Nano-scratch test was also conducted on the film samples using a Nano TestTM (MicroMaterial, UK) equipped with Rockwell indenter tip at room temperature. The scratch load and length was 0.05 mN/s and 100 lm, respectively. Two scratch rates used were 3 lm/s and 5 lm/s.

1Wt% GO

12

30

0

2

4

Wt%GO Fig. 1. (a) Typical stress–strain curves of the GO/PU nanocomposites, and (b) the Young’s modulus and toughness of the GO/PU nanocomposites as a function of GO loading.

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Fig. 2. SEM images of the GO/PU nanocomposites. The dark area represents the PU matrix. The bright strips inside are the graphene nano-flakes with a thickness of 30–50 nm.

bonds (ANHACOA) could be formed to serve as covalent interface, which resulted from the reaction between the hydroxyl groups (AOH) on surface of the GO and ANCO groups on the ends of PU chains. FTIR spectrum (1) in Fig. 3 shows the typical bands belonging to the PU. The bands of 1017 cm1 and 1077 cm1 represent the in-plane vibration of AOCOANHA group. The band of 1731 cm1 belongs to stretching vibration of free AC@O group. The band of 3319 cm1 and 1700 cm1 is attributed to stretching vibration of hydrogen bonded ANHA and C@O group, respectively. Fig. 3 also informs the negligible effect of the GO on the spectra of the PU, indicating that it is difficult to directly identify the formation of urethane bonds between the PU and GO because PU backbones have the same chemical bonds. However, in comparison with the spectrum of GO, it can be clearly seen that the band of 3455 cm1 for hydroxyl groups in the GO disappears in the spectra of GPUNs. It is the evidence to convince the formation of urethane bonds between the PU and GO. The solubility of the GPUNs in DMF was further tested in order to confirm this conclusion. As shown in Fig. 4, the PU is completely dissolved in DMF in one day, while the GPUNs exhibit less solubility in DMF. Apparently, the addition of 4 wt% GO make the PU swallowed with DMF heavily but not dissolved in DMF. It is another evidence to support the role of the GO as crosslinkers for the linear PU. In terms of the toughness, it should be noted that it was a failure of our previous attempt to use 4 wt% GO to toughen a crosslinked PU, in which the route for filling the PU matrix with the GO was exactly same as revealed in this study [13]. It led us to believe that the toughening performance of the GO was closely relevant with the crosslinking degree of PU network.

1: PU 2: PU/1wt%GO 3: PU/4wt%GO 4: GO

Intensity

3 2 1 4 -1

3455cm

4000

3500

3319cm

3000

-1

a 2500

2000

1500

1000

500

Wavelength (cm-1) Fig. 3. FTIR spectra of the GO/PU nanocomposites (refection mode) and pure GO powders (transmission mode).

PU is a segmented polymer consisting of soft and hard segments. The crosslinking as mentioned above resulted in the covalent link between the GO and the hard segment. Thermal analysis performed on a MDSC was used to study the effect of the GO on the glass transition temperature (Tg) of the soft segment. Fig. 5 shows the dCp/dT signals against temperature as the function of GO concentration. The peaking points reflect the Tg of 45 °C for the soft segment, and the addition of the GO imposed little influence on the Tg and Cp of the soft segment. This implied that the interaction between the GO and the soft segment was weak, and no confinement took place in the soft segment caused by layered structured GO. It has been found that, generally, the Tg of the soft segment was affected when functionalized nanofillers joined in the polymerization [15]. In this case, the PU matrix with NCO groups was synthesized without the involvement of the GO. Thus, the covalent interaction was mainly formed between the GO and the hard segment, and the soft segment was hardly affected. This was consistent with our previous findings [13].

3.2. Subsurface mechanical properties of the GPUNs Fig. 6 shows the typical indentation loading–unloading curves for the GPUNs. The maximum indentation depth of the PU is about 7500 nm under the maximum load of 2.5 mN. The maximum depth slightly decreases to around 6500 nm when the PU is incorporated with 1 wt% GO. At 4 wt% GO, the maximum depth is reduced by 50%, to be 4000 nm, which directly indicates that the resistance of the nanocomposites to the penetration by the indenter is stronger than the pure PU. Fig. 7a and b presents the hardness and modulus of the GPUNs as a function of GO concentration, respectively, which are obtained from nano-indentation test. The hardness and modulus of the PU is improved by 100% and 200%, respectively, with the incorporation of 4 wt% GO. It was clear that the GO made the PU much stiffer, which coincided with the results from tensile tests. Furthermore, scratch tests were performed via moving the intender on the surface of the GPUNs within a specified scratch length. The scratch depth of the indenter was recorded against the scratch length, by which wear/scratch resistance could be assessed. Fig. 8a and b shows the scratch depth profiles at the scratch rate of 3 lm/s and 5 lm/s, respectively. At a scratch rate of 3 lm/s, it can be seen that the scratch depth is 3000 nm for the PU at the scratch length of 20 lm. For the GPUNs, the scratch depth undergoes a significant reduction to 2000 nm. At a higher scratch rate of 5 lm/s, the scratch depth of the PU observes a value of 2000 nm within the same scratch length because there is less time for the indenter to penetrate into deeper position in the material. The scratch depth of the GPUNs is also observed to be nearly half of

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Fig. 4. The picture of the GO/PU nanocomposites after being immersed in DMF for one day. The scheme in the left illustrates the route for the formation of the covalent bonds between the GO and PU matrix.

0.03

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Fig. 5. MDSC dCp/dT signals of the GO/PU nanocomposites against temperature.

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3.0 PU01 PU02 GO 1wt% 01 GO 1wt% 02 GO 4wt% 01 GO 4wt% 02

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Displacement (nm) Fig. 6. Typical nanocomposites.

indentation

loading–unloading

curves

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GO/PU

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Fig. 7. Hardness (a) and modulus (b) of the GO/PU nanocomposites as a function of GO loading.

Q ¼k that of the PU. At the scratch length of 60 lm, the reduction in the scratch depth reaches 65% for the GPUN with 4 wt% GO. These results could be understood from two aspects. A model has been established to describe the link between the wear resistance and the hardness of material surface as follows [18],

0.9

Wt% GO

8000

P H

ð1Þ

in which Q, k, P and H is wear rate, wear coefficient, applied load and the hardness of materials, respectively. It can be seen that the wear resistance is linearly inversely proportional to the hardness, which means higher hardness results in lower wear resistance.

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Scratch length (µm) Fig. 8. Scratching profiles of the GO/PU nanocomposites at the scratching rate of 3 lm/s (a) and 5 lm /s (b), respectively.

From another aspect, highly improved scratch resistance might result from geometrical consequence. The nanofillers played as hard particles to resist the scratch of the indenters on the surface of the soft polymeric matrix. Once the interspaces among nanofillers become smaller, the indenter has less chance to scratch the polymeric matrix. As a result, the scratching depth should be reduced. It might be imagined that the indenter could hardly create the scratching damages to the surface of the nanocomposites once the interspaces among the nanofillers are smaller than the tip size of the indenter. The SEM images in Fig. 2 shows that the size of interspaces are nearly same for the GPUNs with 1 wt% and 4 wt% GO, which are less than the tip size of the indenter. The increasing loading of the GO resulted in the formation of thicker nanoflakes instead of reducing the size of interspaces. That was the reason why the scratching depths were independent of the GO concentration at two different scratching rates although the hardness presented to be much improved with increasing addition of the GO. 3.3. Thermal stability of the GPUNs Thermal stability of the GPUNs was assessed by a TGA technique. According to the degrading mechanism of PUs [19], the depolycondensation also called chain scission occurs around 170–200 °C due to the breakdown of urethane bonds. After that, the polyols and polyisocyanates start to decompose respectively, which divides the non-isothermal degradation of PUs into two stages. Generally, the polyisocyanates decompose at higher temperature than the polyols since the polyisocyanates tend to form

1: PU 2: 1Wt% GO 3: 4Wt% GO

1 2

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Deriv.weight (%/oC)

Scratch depth (nm)

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D. Cai et al. / Composites Science and Technology 72 (2012) 702–707

3

1.0 0.8 0.6 0.4 0.2 0.0 -0.2 200

300

400

500

600

700

o Temperature ( C)

Fig. 9. (a) Thermogravimetric analysis (TGA) and (b) differential thermal analysis (DTA) thermographs of the GO/PU nanocomposites.

carbodiimide and urea. The TGA thermograms shown in Fig. 9a disclose that, the dominant decomposition temperature of the PU is increased by 50 °C, and the rate of decomposition slightly decreases as 4 wt% GO is incorporated. As stated above, this dominant weight loss should be mainly attributed to the decomposition of the PCL polyol. The onset decomposition temperature of the GPUNs was observed to be lower than that of the PU. It could be attributed to the elimination of the free oxygen groups on the surface of the GO [20]. Fig. 9b shows differential thermal analysis (DTA) thermograms associated with TGA thermograms, in which the peak point gives the temperature that causes main weight losses during decomposing process. In the second stage, the incorporation of 4 wt% GO leads to the shift of the onset decomposition temperature of the polyisocyanate from 430 °C to 500 °C, and the addition of 1 wt% GO hardly affects the decomposition of the polyisocyanate. These results continued to support the conclusion that the GO can act as a barrier to reduce the diffusion of the heat in polymeric matrices [21,22].

4. Conclusions In this study, the incorporation of the GO led to substantial enhancement in both stiffness and toughness of a PCL-based linear PU matrix with elasticity interestingly maintained. Nano-indentation tests showed that these nanocomposites had excellent protective

D. Cai et al. / Composites Science and Technology 72 (2012) 702–707

ability with improved hardness and scratch resistance. The mechanism of reinforcement was investigated and discussed. It was found that the chemical bonds were formed between the graphene sheets and PU matrix, which made a big contribution to stress transfer. The crosslinking degree of PU network should be controlled to minimize the negative effect of the GO on the elasticity. TGA analysis revealed that the GO was capable of acting as barrier to reduce thermal diffusion across the PU matrix, and thus enhanced the thermal stability of the PU. It could be concluded that the GO exhibited great potential to yield high performance PU nanocomposites with greatly enhanced physical properties.

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