Interfacial engineering of polypropylene/graphene nanocomposites: improvement of graphene dispersion by using tryptophan as a stabilizer

Abstract

In this work, graphene nanosheets functionalized with polypropylene (PP) chains through non-covalent π–π interactions, PP-f-graphene, were prepared and their reinforcing effect on PP was investigated. With the aid of tryptophan, graphene can stably disperse in water due to the non-covalent π–π interactions between graphene and tryptophan. By mixing the aqueous dispersion of tryptophan-functionalized graphene and the xylene solution of polypropylene-graft-maleic anhydride (MAPP), a binary phase system was obtained. The graphene nanosheets continuously transferred from water phase into xylene phase as the reaction between MAPP and tryptophan at the water/oil interface proceeded. The structure of the resultant PP-f-graphene nanosheets was carefully characterized by spectroscopy and microscopy methods. The results confirmed that MAPP chains have been successfully bonded onto graphene surfaces. PP/PP-f-graphene nanocomposites were fabricated by melt blending. A well-dispersed PP-f-graphene in PP matrix was realized due to the strong adsorption between graphene and functionalized PP. The addition of only 0.6 wt% of PP-f-graphene increased the tensile strength and flexural modulus of PP by 20.8% and 44.6%, respectively. The crystallization temperature and rate of PP were also elevated by the incorporation of PP-f-graphene. Making use of the interaction between graphene and functionalized PP is a facile method to improve the dispersion of graphene in the PP matrix.

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1. Introduction

Polymer nanocomposites have drawn much attention because they often exhibit remarkable enhancement in material properties with a very low level of nano-filler loading when compared to pristine polymers. Various nanoparticles have been investigated as nano-fillers to improve the mechanical, electrical or thermal properties of polymers, for instance, calcium carbonate nanoparticles are widely used to reinforce polyolefins,¹ carbon nanotubes are often applied to improve the electrical conductivity or thermal conductivity of polymers.² Graphene, a typical two-dimensional carbon nano-material, has been regarded as a promising star material due to its prominent performance.^3–5 Its unique structure, high electrical mobility (250 000 cm² V⁻¹ s⁻¹), and high Young's modulus (∼1100 GPa)⁶ make it very attractive for fabricating high performance polymer nanocomposites.⁷ Indeed, graphene-filled polymer nanocomposites have attracted significant interest from both industry and academia during past few years.⁸

It is well-known that the dispersion state of nano-fillers in polymeric matrix plays a crucial role in the final performance of nanocomposites. For graphene-filled nanocomposites, large efforts have been devoted to achieve the homogeneous dispersion of graphene nanosheets in different polymer matrix.^9–11 However, it is still a big challenge to realize the uniform dispersion of graphene in polyolefins till now due to the hydrophobic nature and low polarity of polyolefin chains.¹² Polyethylene (PE) and polypropylene (PP) are the two most important and common polyolefins which have been widely used in many fields. When graphene nanosheets is incorporated to the polyolefin matrix, the mechanical properties of their nanocomposites are usually rather poor owing to the aggregation of graphene and the weak interfacial interaction between graphene and polymer chains. Generally, the interfacial interaction between graphene and polyolefins can be enhanced through surface functionalization of graphene nanosheets or chemical modification of polyolefin chains. For instance, Cao et al. have grafted graphene nanosheets with long alkyl chains and achieved remarkably improved dispersion of the alkyl-functionalized graphene in PP matrix;¹³ Song et al. has also fabricated PP/graphene nanocomposites with good dispersion of graphene nanosheets via first coating graphene using PP latex and then melt-blending the coated graphene with PP pellets, an obvious reinforcing effect was achieved by adding only 0.42 vol% of graphene.¹⁴ Kim et al. have modified PE chains to improve their miscibility with graphene by using the ring-opening metathesis polymerization technique, and more homogeneous dispersion of graphene nanoplatelets in modified PE was observed.¹⁵ Furthermore, covalently bonding polyolefins onto graphene is another effective way to enhance the interfacial adhesion between them.¹⁶ Due to the promising applications of polyolefin/graphene nanocomposites, more effective approaches to improve the interfacial interactions between graphene and polyolefins are strongly needed.

With the aid of the non-covalent π–π interactions between graphene and aromatic rings of some organic molecules, the uniform dispersion of graphene and improved performance of graphene-filled polymer nanocomposites can be achieved more simply.¹⁷ Tryptophan, an important amino acid to human beings, has been used by Rajesh et al. to functionalize the surface of graphene nanosheets through non-covalent π–π interaction.¹⁸ In their study, they demonstrated that the π–π interaction between the aromatic ring of tryptophan and graphene is the highest among different amino acids. The tryptophan modified-graphene can be well dispersed in poly(vinyl alcohol) (PVA), the thermal stability and mechanical property of resultant PVA/graphene nanocomposites can be obviously enhanced by adding only 0.2 wt% of such graphene nanosheets.¹⁹ On the other hand, polyolefin-based nanocomposites filled with using such amino acid functionalized graphene have not been reported yet.

In this paper, a type of tryptophan-functionalized PP(tryptophan-functionalized PP-graft-maleic anhydride, MAPP–tryptophan) was prepared and used as a compatibilizer to improve the interfacial interaction between graphene nanosheets and PP matrix. On one hand, the MAPP–tryptophan can be adsorbed onto graphene surfaces through the π–π interactions between tryptophan and graphene. On the other hand, the MAPP–tryptophan tends to disperse in PP matrix due to the similar nature of MAPP and PP. Thus this type of “amphiphilic” macromolecules can improve the interfacial interactions between graphene nanosheets and PP and offer PP/graphene nanocomposites with better performance. The fabrication procedures, mechanical properties and crystallization behavior of PP/graphene nanocomposites compatibilized by MAPP–tryptophan are discussed in detail.

2. Experimental

2.1. Materials

Graphite oxide was synthesized from natural graphite (Sinopharm Chemical Reagent Co., Ltd) with the particle size of 50–74 μm according to a method recently reported by Marcano et al.²⁰ MAPP (1.5 wt% grafting ratio of maleic anhydride) with an average M_n of 20 000 was prepared in our laboratory. Polypropylene (K1008, MFI = 10 g per 10 min) was obtained from Yanshan Petro-Chemical limited Company (China). All other reagents and solvents were obtained as analytical grade products and used without further purification.

2.2. Synthesis of tryptophan functionalized graphene (f-graphene)

The f-graphene was synthesized by reducing graphene oxide (GO) in water in the presence of tryptophan. Briefly, 0.60 g of tryptophan was dissolved in 300 mL of deionized (DI) water, then 0.30 g of graphite oxide powder was added, followed by sonication for 2 h, allowing the exfoliation of graphite oxide into GO nanosheets. Thereafter, 3 mL of hydrazine hydrate was slowly added into the suspension and the mixture was heated at 95 °C under vigorous mechanical stirring for 8 h. The obtained f-graphene suspension was stored in the flask for further use.

2.3. Preparation of MAPP–tryptophan functionalized graphene (PP-f-graphene)

PP-f-graphene was fabricated through the strong absorption between graphene and functionalized PP (MAPP–tryptophan). Firstly, MAPP solution was prepared by dissolving 0.4 g of MAPP pellet in 160 mL of xylene solvent at 125 °C. Then the MAPP solution was cooled to 95 °C and poured into 300 mL of freshly prepared f-graphene suspension. A binary phase of water–xylene system was observed and the two-phase system was heated at 95 °C for 8 h under mechanical stirring. The f-graphene nanosheets gradually transferred from water phase into xylene phase as the reaction between the amino groups of tryptophan and the maleic anhydride groups of MAPP chains proceeded. Finally, the two-phase system was cooled to room temperature, filtered, and washed with excess DI water to remove the free tryptophan. The obtained black powder was dried at 80 °C under vacuum for 48 h to offer a MAPP/PP-f-graphene master batch. The weight ratio of PP-f-graphene (w_{PP-f-graphene}) in the resultant master batch can be calculated by the following equation.where, m_GO and m_MAPP are the mass of GO and MAPP, respectively. k₁ and k₂ are the weight ratio of the absorbed tryptophan and the absorbed MAPP–tryptophan on graphene surfaces to graphene, which can be obtained from thermal gravimetric analysis results. w_{mass loss} is the mass loss of GO after hydrazine reduction, which is determined to be 0.4 by comparing the weight of GO before and after reduction.

2.4. Fabrication of PP/PP-f-graphene nanocomposites

The PP/PP-f-graphene nanocomposites were fabricated by melt blending PP with certain amount of MAPP/PP-f-graphene master batch using a microcompounder (Haake miniLab, Thermo Fisher Scientific) at 180 °C with a speed of 60 rpm for 8 minutes. Two nanocomposites with different graphene contents (0.2 wt% and 0.6 wt%) were prepared, which were denoted as PP/PP-f-G (0.2) and PP/PP-f-G (0.6), respectively. For comparison, a nanocomposite sample containing 0.6 wt% of unfunctionalized reduced graphene oxide (PP/G (0.6)) was also prepared through similar procedures, identical content of MAPP was also incorporated to improve the dispersion of graphene in PP matrix.

2.5. Characterization and measurement

Atomic force microscopy (AFM) images of f-graphene were collected on a Dimension™ 3100 system (Bruker Corporation, Germany) in tapping mode. Ultraviolet-visible (UV-vis) absorption spectra were recorded on a Lambda 35 (PerkinElmer Corporation, USA) spectrometer. Infrared spectra were measured by using a Nicolet 6700 spectrometer (Thermo Fisher Scientific, USA). Thermal gravimetric analysis (TGA) was performed on a HCT-3 system (Beijing Henven Scientific, China) with a heating rate of 10 °C min⁻¹ from 25 °C to 600 °C under nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) of each sample was recorded on a Kratos AXIS ULTRADLD system (Japan), using monochromatic Al Kα radiation as exciting source. X-ray diffraction (XRD) patterns of samples were collected by using a D/MAX 2500VB2+/PC system (Rigaku, Japan) with a Cu K_α radiation source (40 kV, 150 mA). The f-graphene samples for UV-vis, IR, TGA, XPS and XRD measurements were prepared by filtering the f-graphene suspension over a nylon membrane with the pore size of 1.20 μm and then repeatedly rinsing the filtrate with DI water to remove free tryptophan. The PP-f-graphene samples for the above characterization were isolated from free MAPP by exhaustive extraction. Typically, certain amount of PP-f-graphene master batch was put in a cellulose thimble and placed into the Soxhlet extractor, refluxed with xylene at 130 °C for 4 days.

The fracture surfaces of neat PP and PP/PP-f-G(0.6) nanocomposites were sputter-coated with gold and observed by using a S-4700 field emission scanning electron microscopy (FESEM, HITACHI Corporation, Japan) with an accelerating voltage of 30 kV. The crystallization behavior of PP and PP/PP-f-graphene nanocomposites was investigated by using a differential scanning calorimetry (DSC-60, SHIMADZU, Japan) under nitrogen atmosphere. Samples were first heated from room temperature to 200 °C at a rate of 10 °C min⁻¹ and held at this temperature for 3 min to erase the thermal history, then cooled to room temperature at the same rate. Crystalline morphology of PP and the nanocomposites was observed by using a polarizing optical microscopy (POM, Leitz Corporation, Germany). The isothermal crystallization procedure for POM observation is described as follows: samples were heated to 200 °C at a rate of 20 °C min⁻¹ and held at this temperature for 5 min, then cooled to 130 °C at the same rate and maintained at this temperature for observation. Photos of the crystalline morphologies of the samples at 130 °C were recorded by a digital camera with a time interval of 1 min. The mechanical properties of specimens were tested on an AG-IC Electronic Testing machine (SHIMADZU, Japan) at room temperature. The standard mechanical testing samples of PP and its nanocomposites were injection molded at 195 °C in a Haake Mini Jet (Thermo Fisher Scientific, USA). The dumb-bell dimensions of the tensile specimens were 70 × 4 × 2 mm³ and the crosshead speed was 20 mm min⁻¹. Flexural specimens with dimensions of 80 × 10 × 4 mm³ were measured under 3-point bending mode at a flexural rate of 2 mm min⁻¹. The span length was 60 mm. All mechanical tests were conducted with six specimens for each sample.

3. Results and discussion

3.1. Chemical structure of tryptophan functionalized graphene (f-graphene)

Graphene nanosheets tend to restack to a graphite structure due to the strong intermolecular interactions among them. Many methods have been developed to suppress the aggregation of graphene nanosheets during the reduction process from GO.^11,21 Tryptophan is essential amino acid for human and the amino or carboxyl groups on it can react with many chemical compounds. More importantly, The indole structure of tryptophan allows it to improve the dispersion of graphene through non-covalent π–π interactions.¹⁸ In this work, we also choose tryptophan to modify the surface properties of graphene nanosheets. The characterization results of obtained f-graphene nanosheets are summarized in Fig. 1. After functionalized with tryptophan, the f-graphene nanosheets still showed a two-dimensional profile with the thickness of ca. 1.6 nm which is much higher than that of graphene (ca. 1.0 nm by AFM measurement), indicating the successful absorption of tryptophan molecules (Fig. 1a). In addition, the dispersion stability of graphene in water was remarkably enhanced. The f-graphene nanosheets could be stably dispersed in water in a high concentration (0.6 mg mL⁻¹) for at least 15 days (Fig. 1a inset). Therefore, tryptophan can be regarded as a good dispersing agent for graphene.

Fig. 1 Structure characterizations of tryptophan (Try), reduced graphene oxide (graphene), and reduced graphene oxide functionalized with tryptophan (f-graphene): (a) typical tapping-mode AFM image of f-graphene and the height profile of the cross-section analysis. Inset: the f-graphene dispersion in water (0.6 mg mL⁻¹) after storage for 15 days; (b) IR spectra; and (c) UV-vis absorption spectra.

The functionalization should be attributed to the non-covalent π–π interactions between the basal planes of graphene nanosheets and tryptophan molecules. The absorption effect was confirmed by IR spectra (Fig. 1b). In the spectrum of f-graphene, a sharp absorption peak at 3430 cm⁻¹ can clearly be observed, which should be ascribed to the stretching vibration of amino groups of tryptophan and indicates the existence of tryptophan molecules on graphene surfaces.

The absorption effect between tryptophan and graphene can be further verified by comparing their UV-vis spectra (Fig. 1c). The spectrum of graphene exhibited a characteristic peak at 268 nm which is assigned to the absorption of aromatic bonds.²² Tryptophan shows three absorption peaks at 271 nm, 279 nm and 288 nm. In the absorption spectrum of f-graphene, two peaks (279 nm and 288 nm) corresponding to the characteristic absorptions of tryptophan can be observed (Fig. 1c inset), implying some tryptophan molecules were bonded onto graphene nanosheets through non-covalent π–π interactions.

3.2. Chemical structure of MAPP–tryptophan functionalized graphene (PP-f-graphene)

MAPP is often used to improve the dispersion of inorganic fillers in PP matrix due to strong interactions can be achieved between strong polar maleic anhydride groups and the surface of inorganic fillers. In this study, the chemical reaction between MAPP and tryptophan was employed to assist the dispersion of f-graphene nanosheets in PP matrix. As shown in Fig. 2, a binary-phase system appeared by mixing the dispersion of f-graphene in water and the solution of MAPP in xylene: the upper is the xylene phase and the lower is the water phase. An interesting phenomenon was observed by heating the binary-phase system at 95 °C: f-graphene nanosheets migrated from water phase into xylene phase. Generally, graphene nanosheets show rather poor dispersion in xylene.²³ Therefore, the homogenous dispersion of PP-f-graphene in xylene (Fig. 2b) in our work is unique and amazing. A reasonable explanation is that a high level functionalization of MAPP chains on graphene surfaces can be realized with the aid of tryptophan. The amidation between the amino groups of tryptophan and the maleic anhydrides of MAPP can easily occur at the water/oil interface and induce the migration of tryptophan from water to xylene phase. Meanwhile, the high coverage density of tryptophan on graphene surface and the strong π–π interactions between them provide a sufficient driving force for the migration of graphene nanosheets from water into xylene phase. Subsequently, the dispersion of graphene in xylene was greatly improved. The mechanism was described in Fig. 2c. Parallel experiments of the dispersion of graphene or f-graphene in the binary-phase system were also performed and the results are shown as Fig. S1 (in ESI†). It can be observed that, for graphene–water and MAPP–xylene binary-phase system, some graphene nanosheets immigrated into xylene phase because of the interactions between graphene and MAPP chains. While no graphene was found in xylene phase for the f-graphene–water and PP–xylene system.

Fig. 2 The binary phase mixture of MAPP in xylene (upper) and f-graphene in water (lower) before (a) and after (b) heating at 95 °C for 8 h; (c) schematic description of the migration of graphene during the process.

Fig. 3 shows the IR spectra of MAPP and PP-f-graphene. Compared to MAPP, a new absorption peak at 3410 cm⁻¹ appeared in the spectrum of PP-f-graphene after the functionalization. This peak should be ascribed to the stretching vibration of N–H bonds, which implies the reactions between tryptophan-functionalized graphene and MAPP.

Fig. 3 IR spectra of MAPP and PP-f-graphene.

The XRD patterns of tryptophan, graphene, and PP-f-graphene are given in Fig. 4. It can be observed that the pristine graphene platelets show a broad peak at 2θ = 24.7° (corresponding to a d-spacing of 0.361 nm) which can be attributed to the restacking of reduced graphene oxide nanosheets.²⁴ In XRD pattern of tryptophan, multiple peaks at 2θ = 4.8°, 9.8°, 14.7°, 19.8°, 23.0°, 24.7° and 35.0° can be observed. For PP-f-graphene sample, both the characteristic peaks of MAPP (2θ = 13.8°, 16.6°, 18.3°, 21.0°, 21.6°, 25.2° and 28.7°) and the peaks corresponding to tryptophan appeared in its XRD pattern, revealing that MAPP–tryptophan molecules has been successfully non-covalently bonded onto graphene surfaces. This observation is consistent with the above-mentioned IR results. The chemical structure of PP-f-graphene was further confirmed by XPS. As shown in Fig. S2,† C 1s peak at 284.8 eV and O 1s peak at 532.8 eV can always be seen in graphene, f-graphene, and PP-f-graphene curves. A new peak corresponding to the nitrogen sp³ of C–N appeared at 400.0 eV for f-graphene and PP-f-graphene,²⁵ which can be attributed to the –CO–NH group of the absorbed tryptophan or MAPP–tryptophan molecules on graphene surfaces.

Fig. 4 X-ray diffraction patterns of tryptophan (Try), graphene, and PP-f-graphene.

The TGA curves in Fig. 5 were performed to obtain the weight absorption ratio of tryptophan (k₁) and MAPP–tryptophan (k₂) to graphene. Most of tryptophan has been decomposed at 500 °C, therefore, k₁ can be calculated by comparing the mass loss of graphene, tryptophan and f-graphene at this temperature. The mass loss of graphene, tryptophan and f-graphene at 500 °C are 11.6%, 73.2% and 23.4%, respectively. Thus k₁ is calculated to be 0.23. k₂ can be obtained through similar calculation method and is calculated to be 0.47. Consequently, the weight fraction of PP-f-graphene in the MAPP/PP-f-graphene master batch (w_{PP-f-graphene}) is 0.43 which is determined through eqn (1).

Fig. 5 TGA curves of graphene, tryptophan (Try), f-graphene and PP-f-graphene.

3.3. Mechanical properties and crystalline behavior of PP/PP-f-graphene nanocomposites

In this section, the effect of PP-f-graphene as a filler on the mechanical properties and crystallization behaviors of PP matrix is discussed in detail. It is widely accepted that a good dispersion of fillers in polymer matrix is crucial to prepare polymer nanocomposites with high performance. For non-polar PP matrix, how to realize the homogeneous dispersion of graphene nanosheets throughout the space is a severe challenge. Here we employed tryptophan as an intermedium to anchor some MAPP chains on graphene surfaces. The resultant hybrid PP-f-graphene nanosheets with “PP hair” would facilitate their dispersion in PP matrix. Typical optical photos of PP/graphene and PP/PP-f-graphene nanocomposite films are shown in Fig. 6. It can clearly be seen that some black agglomerations appeared in the PP/graphene composite film, indicating the poor dispersion graphene in PP matrix (Fig. 6a). In comparison, there is nearly no agglomerations of graphene in the PP/PP-f-graphene film, implying improved dispersion behavior of PP-f-graphene in PP matrix.

Fig. 6 Optical photos of (a) PP/graphene and (b) PP/PP-f-graphene nanocomposite films, the content of graphene in both films was controlled to be 0.6 wt%.

The effect of unfunctionalized graphene and PP-f-graphene on the mechanical properties of PP was compared in Fig. 7 and Fig. 8. For PP/graphene nanocomposites with 0.6 wt% of graphene, the tensile strength, flexural strength and flexural modulus are all higher than those of the neat PP, indicating graphene has a reinforcing effect on PP. Compared to PP/graphene nanocomposites, the tensile strength of PP/PP-f-graphene nanocomposites with 0.6 wt% of PP-f-graphene further increased to 43.0 MPa, which is about 20.8% higher than that of neat PP. The flexural strength and flexural modulus of PP can also be remarkably improved by the introduction of PP-f-graphene. Compared to neat PP, the increment in flexural strength and flexural modulus for PP/PP-f-graphene nanocomposites with 0.6 wt% of graphene are 21.1% and 44.6%, respectively. Considering the weight fraction of PP-f-graphene is only 0.6 wt%, this reinforcing effect is rather obvious. Furthermore, the enhancement in tensile strength, flexural strength and flexural modulus of PP/PP-f-graphene nanocomposites is clearly higher than that of PP/graphene. On the other hand, the elongation at break of both PP/graphene and PP/PP-f-graphene nanocomposites decreased after filled with graphene nanosheets. As expected, PP/PP-f-graphene nanocomposites show smaller loss in failure strain than PP/graphene nanocomposites. More details of reinforcing effect of PP-f-graphene and pristine graphene on the mechanical properties of PP matrix are listed in Table 1. The increment in the tensile strength and Young's modulus of PP induced by the filling of PP-f-graphene is at a high level compared to other literature.^16,26,27 Furthermore, the rate of the increase of tensile strength and Young's modulus with the volume fraction of PP-f-graphene (dσ_T/dV_f and dY/dV_f) is much higher than those of nanocomposites filled with graphene through simple melt blending. Coleman et al.²⁸ have used dY/dV_f as a yardstick to compare the reinforcement effect of carbon nanotubes (CNTs) on polymer. The mean and median dY/dV_f values are 23 and 11 GPa for polymer/CNTs composites fabricated by melt blending, while the two values are higher at 128 and 38 GPa for polymer/CNTs composites fabricated by solution blending. In this paper, the dY/dV_f values of PP/PP-f-graphene nanocomposites with two different contents are all higher than the mean and median dY/dV_f values for polymer/CNTs fabricated by solution blending, indicating that PP-f-graphene exhibits a better reinforcing effect than CNTs on PP. The better reinforcing effect can be ascribed to the excellent dispersion of PP-f-graphene in PP matrix and effective stress transferring between PP-f-graphene and PP matrix. This can be confirmed by SEM observations. Fig. 9 shows the typical SEM graph of fractured surface of PP/graphene and PP/PP-f-graphene nanocomposites. The fracture surfaces of PP/graphene is smooth, indicating the poor interfacial adhesion between graphene and PP, which will dissipate little energy when fractured. However, large graphene nanosheets with a wrinkled morphology can be clearly seen in PP/PP-f-graphene nanocomposites, revealing the functionalized graphene has good interfacial interaction with PP. Therefore, the fabrication technique by using the non-covalent interaction between graphene and functionalized PP (MAPP–tryptophan) is a facile method to reinforce PP.

Fig. 7 Tensile strength and elongation at break of neat PP, PP/graphene and PP/PP-f-graphene nanocomposites.

Fig. 8 Flexural strength and flexural modulus of neat PP, PP/graphene and PP/PP-f-graphene nanocomposites.

Table 1 Mechanical properties of PP and PP-based nanocomposites with different filler contents

Content			Tensile strength, σT (MPa)	%Increase of σT		Elongation at break, %	Flexural strength, σF (MPa)	%Increase of σF		Flexural modulus, Y (MPa)	%Increase of Y
Filler	Wf, %	Vf^a, %
a The volume content of fillers in matrix (Vf) was converted from the weight content (Wf). The density of PP-f-graphene is calculated to be 1.83 g cm^−3 and the density of graphene is 2.25 g cm^−3.
PP-f-graphene	0	0	35.6	—	—	816.1	37.0	—	—	1049.2	—	—
	0.2	0.11	39.0	9.6	3145.0	679.2	42.6	15.1	5180.0	1381.6	31.7	307.5
	0.6	0.32	43.0	20.8	2281.6	589.6	44.8	21.1	2437.5	1517.3	44.6	144.3
Graphene	0.6	0.27	39.4	10.7	1424.9	413.5	41.9	13.2	1837.6	1280.1	22.0	86.6

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Fig. 9 SEM graph of fractured surface of PP/graphene with 0.6 wt% of graphene (a) and PP/PP-f-graphene nanocomposite with 0.6 wt% of PP-f-graphene (b).

It has been reported that graphene or graphene oxide can affect the crystallization behaviors of polymer.²⁹ They can be served as heterogeneous nucleating centers to facilitate the crystallization of PP. The crystallization curves of PP with different PP-f-graphene loadings are displayed in Fig. 10. The crystallization temperature (T_c) of neat PP was 117.3 °C. The T_c increased to 118.9 °C by filled with 0.2 wt% of PP-f-graphene and further increased to 119.3 °C by filled with 0.6 wt% of PP-f-graphene. Namely, the crystallizability of PP can be improved by graphene. Fig. 11 shows the isothermal crystallization of PP and PP/PP-f-graphene at 130 °C. Commonly, the crystallization of polymer can be divided into two steps, the first step is the generation of crystal nucleus and the second step is the growth of crystal grain. In Fig. 11, very few small crystal grains can be seen for neat PP and PP/PP-f-graphene nanocomposites with 0.2 wt% of PP-f-graphene at t = 0 min, while the quantity of crystal grains increased when the PP-f-graphene loading reached 0.6 wt%. After 1 min, more and larger crystal grains appeared for neat PP and PP/PP-f-G (0.2) nanocomposites, while some obvious spherulites were present for PP/PP-f-G (0.6) nanocomposites. As time proceeds the number of spherulites further increased and the size of spherulites kept growing for all three samples. At t = 6 min, large number of big spherulites formed for all three samples. Therefore, the incorporation of PP-f-graphene can accelerate the crystallization of PP due to the heterogeneous nucleating effect of graphene.

Fig. 10 DSC curves of PP/graphene and PP/PP-f-graphene nanocomposites with different graphene contents during cooling process.

Fig. 11 Polarized optical micrographs of PP and PP/PP-f-graphene nanocomposites during isothermally crystallizing at 130 °C: (a) neat PP, (b) PP/PP-f-G (0.2) nanocomposites and (c) PP/PP-f-G (0.6) nanocomposites.

It should be mentioned that the increase of crystallizability (X_c) of PP may also play an important role in enhancing the mechanical properties of its nanocomposites. Therefore, the X_c of PP/graphene and PP/PP-f-graphene nanocomposites with various graphene loadings was compared. The X_c was calculated based on the crystallization enthalpy in Fig. 10. The X_c of neat PP is 31.5%. This value increased to 34.5% for PP/graphene nanocomposite with 0.6 wt% of graphene and 36.6% for PP/PP-f-graphene nanocomposites with 0.6 wt% of PP-f-graphene. The difference of the X_c between PP/graphene and PP/PP-f-graphene is small. In addition, though PP-f-graphene can accelerate the crystallization of PP, the spherulites size PP/PP-f-graphene nanocomposites did not change greatly with PP-f-graphene loadings (Fig. 11). Therefore, the increase of X_c resulted from graphene is not the main reason for the obvious reinforcing effect on PP. By considering the large differences in the mechanical properties of PP/graphene and PP/PP-f-graphene nanocomposites, the improved interfacial interaction between PP-f-graphene and PP matrix should be the most possible reason for the remarkable reinforcement on PP.

4. Conclusions

Graphene nanosheets functionalized with PP chains, PP-f-graphene, were successfully prepared by mixing the xylene solution of MAPP and the aqueous dispersion of reduced graphene oxide with tryptophan adsorbed on surfaces. The reaction between MAPP and tryptophan at oil/water interface offered a driving force to transfer graphene from water phase to xylene phase. The resultant PP-f-graphene nanosheets can be stably dispersed in xylene due to the surface functionalization with MAPP–tryptophan through π–π interaction. Such PP-f-graphene nanosheets can be well dispersed in PP matrix to improve its mechanical properties. By adding only 0.6 wt% of PP-f-graphene, a 20.8% improvement in tensile strength and a 44.6% increase in flexural modulus of PP were realized. In addition, DSC and POM results revealed that PP-f-graphene can enhance the crystallization temperature and crystallization rate of PP. The functionalization technique reported in this work, adsorbing MAPP–tryptophan chains onto graphene surfaces through non-covalent π–π interactions, is an efficient approach to improve the dispersion of graphene nanosheets in PP matrix and their reinforcement effect on PP.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47112h

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