Effect of Alkyl Chain Length Grafted to Graphene Nanoplatelets on the Characteristics of Polypropylene Nanocomposites.
Polymer nanocomposites, which have been attracting great attention from polymer engineers for last two decades, are generally composed of a polymer matrix and a type of nanofiller that has a size of at least one dimension <100 nm [1-5]. When compared with traditional polymer composites filled with typical micron-sized fillers like silica, talc, titanium dioxide, and so on, polymer nanocomposites can accomplish much greater improvements in various physical properties despite containing much less amount of nanometer-sized fillers like graphene [6-8], graphene nanoplatelets (GNPs) [9,10], carbon nanotubes [2,11-13], carbon nanofibers [2,14], nanoclay [15-17], metal oxide nanoparticles , and cellulose nanocrystals . The addition of nanofillers into polymer matrices results in an improvement in the properties of polymers including enhanced mechanical, thermal, barrier and physicochemical properties [20-22].
Polypropylene (PP) is a representative commodity plastic that has been widely used to make bottles, packaging films, fiber products like carpets, car parts, toys, housewares, rechargeable battery separators, and so on. Though PP has a glass transition temperature that is observed in amorphous region considerably lower than room temperature (-10 to -18[degrees]C) it is quite stiff at room temperature because it has crystalline region that has a quite high melting transition temperature (176[degrees]C) . The heat distortion temperatures of PP products range from 40 to 152CC depending on the formulations of the products . When PP is used as a polymer matrix in designing a high-performance polymer nanocomposite the incompatibility problem between the nonpolar polymer and nanofillers should be solved first. There have been many studies on improving interfacial interactions between PP and nanofillers [24-27].
Graphene is a 2D, one-atom-thick carbon monolayer sheet with a planar honeycomb lattice. Graphene has received a great deal of attention as a type of nanofiller because it has a facile functionalization capability with excellent physical properties [9,28,29]. Graphene has been known to be more effective than ID carbon nanotubes or nanofibers in improving various physical properties of polymers because 2D graphene has much larger specific surface area than the 1D nanocarbons [9,30]. Oxidized graphenes with oxygen-based functional groups like carboxyl and epoxy groups on their surfaces could be easily modified via simple chemical reactions [31-33]. The polymer nanocomposites comprising the modified graphenes resulted in superior physical properties [9,34].
Graphene nanoplatelets (GNPs) are generally composed of several graphene monolayer sheets stacked together and they can be produced much more easily than graphene. It would be much more practical and economical if incorporating GNPs instead of graphene is enough to accomplish required properties improvements of a polymer. Therefore, GNPs with oxygen-based functional groups on their surfaces were functionalized first then used to reinforce a PP matrix in this study. The functionalization of the GNPs was performed by grafting alkyl (alkyl-GNPs) chains to the GNPs' surfaces via chemical reactions between the amine groups of alkyl amines and the carboxyl and epoxy groups of the GNPs. Hexylamine, dodecylamine and octadecylamine were used respectively to investigate the effect of the grafted alkyl chain length on the physical properties of the PP/alkyl-GNP nanocomposites.
A granule-shaped PP (Y-120) was supplied by Lotte Chem., Korea. The PP has the mol. wt. of 425,800 g/mol, the melt flow index of 1 g/10 min and the density of 0.90 g/[cm.sup.3]. A type of GNPs (xGnP-M-5) was supplied by XG Science, USA. The technical data sheet supplied by the company reports that the GNPs have the average diameter of 5 [mu] with 6- to 8-nm thick, the specific surface area of 120-150 [m.sup.2]/g, the residual acid content of <0.5% and the oxygen content of <1%. Hexylamine (mol wt = 101.19 g/mol, melting point [mp] = -19[degrees]C and boiling point [bp] = 130[degrees]C) was purchased from Alfa Aesar, Korea. Dodecylamine (mol wt = 185.35 g/mol, mp = 29[degrees]C and bp = 248[degrees]C) was purchased from Junsei Chem., Japan. Octadecylamine (mol wt = 269.51 g/mol, mp = 51[degrees]C and bp = 349[degrees]C) was purchased from Sigma-Aldrich, USA. Methanol (SK Chem., Korea) was used as a solvent to prepare alkylamine/methanol solutions for the alkylation of the GNPs.
Preparation of Alkyl-GNPs
To graft alkyl chains to the neat GNPs, 0.3 g of the neat GNPs was added to an alkylamine/methanol solution (0.1 M, 50 mL) and the solution was stirred at room temperature for 4 h [9,35]. When the grafting reaction was finished, the solution was filtered and then washed with methanol followed by drying in a vacuum oven at 60[degrees]C for 24 h. The resulting alkylated GNPs with residual oxygen-containing functional groups are shown in Fig. 1. As shown in the figure, during the process of preparing the alkyl-GNPs the physically adsorbed alkylamines are covalently bonded to the GNPs via amidation and nucleophilic epoxide ring opening reaction. The alkylated GNPs with residual oxygen-containing functional groups were reduced as shown in the second step of Fig. 1 by treating them in an aqueous hydrazine monohydrate solution (1 M, 50 mL) at 90[degrees]C for 1 h. When the reducing reaction was finished, the solution was filtered and then washed with methanol followed by drying at 60[degrees]C for 24 h in a vacuum oven to obtain the alkyl-GNPs (hexyl-GNPs, dodecyl-GNPs, and octadecyl-GNPs) in Fig. 1. It was considered that only the functional groups on the GNPs' surfaces would take part in the grafting and reducing reactions because it would be hard for the alkylamines to get in between graphene layers.
Analysis of the Alkyl-GNPs
Fourier transform infrared (FTIR) spectroscopy (Nicolet IR200, Thermo Scientific, USA) was used to confirm the alkylation of the neat GNPs. KBr powder was mixed with a GNPs sample at 99:1 weight ratio then a disc-shaped pellet was prepared for the FTIR analysis. The FTIR spectra of the neat GNPs and the alkyl-GNPs were measured from 400 to 4,000 [cm.sup.-1] accumulating 16 scans per spectrum at 4 [cm.sup.-1] resolution.
Preparation of PP/alkyl-GNP Nanocomposites
At first the PP granules and all the GNPs were dried in a vacuum oven at 60[degrees]C for 24 h to remove moisture from the materials. PP/alkyl-GNP nanocomposite panels with hexyl-GNPs, dodecyl-GNPs or octadecyl-GNPs 0.5 phr (parts per hundreds of the resin) were respectively prepared by melt-blending (Haake Rheomix 600 mixer, 170[degrees]C, 15 min, 60 rpm) followed by compression molding (Carver hydraulic hot press, 180[degrees]C, 1,000 psi, 6 min). A neat PP panel was also prepared by the same process to compare its physical properties with the physical properties of the PP/alkyl-GNP nanocomposites. Specimens for the thermal and mechanical properties measurements were prepared by cutting the panels into desired sizes.
Characterization of the PP/Alkyl-GNP Nanocomposites
The flexural properties of the neat PP and the PP/alkyl-GNP nanocomposites were measured by three point bending method according to ASTM D790 using a universal testing machine (LR30 K, Lloyd, UK) loaded with 1 kN cell at a cross-head speed of 6.5 mm/min. The dimensions of the specimens were 80 x 10 x 4 mm. The average value of six specimens' data was taken to report for each sample.
The impact tests of the notched neat PP and PP/alkyl-GNP nanocomposites specimens were carried out using an izod impact tester (SJI-103, Sungjin Co., Korea) according to ASTM D256. The dimensions of the specimens were 50 x 13 x 4 mm. The average value of six specimens' data was taken to report for each sample.
Dynamic mechanical analysis (DMA 2980, TA instruments, USA) was used to measure the dynamic mechanical properties of the neat PP and the PP/alkyl-GNP nanocomposites. Each specimen was subjected to a sinusoidal deformation of 20 [mu] at 1 Hz under a static loading force of 0.05 N. The DMA measurement was performed from room temperature to 180[degrees]C at 5[degrees]C/min. The specimens had dimensions of 36 x 12.8 x 3.2 mm.
Differential scanning calorimetry (DSC 2910, TA instruments, USA) was used to investigate the melting behavior of the neat PP and the PP/alkyl-GNP nanocomposites. Each dynamic DSC scan was carried out under [N.sub.2] atmosphere from room temperature to 200[degrees]C at 5[degrees]C/min. The percent crystallinities ([X.sub.c]) of the samples were calculated by [X.sub.c] = [DELTA][H.sub.f]/([DELTA][H.sup.0.sub.f]w) 100 (%), where, [DELTA][H.sub.f] is the heat of fusion for each sample, [DELTA][H.sub.f] [degrees] the standard heat of fusion for 100% crystalline PP (137.9 J/g) and w the weight fraction of PP.
The fracture surfaces images of the impact-tested neat PP and the PP/alkyl-GNPs nanocomposites were investigated using a scanning electron microscope (SEM, JEM-840A, JEOL Co., Japan) after coating the fracture surfaces with gold by sputtering.
RESULTS AND DISCUSSION
Characteristics of the Alkyl-GNPs
Figure 2 shows the FTIR spectra of the neat GNP and the three alkyl-GNPs. Because the neat GNPs have some oxygen content (<1%), oxygen atoms would exist as oxygen-containing functional groups on the surfaces and edges of the neat GNP as illustrated in Fig. 1. The FTIR spectrum of the neat GNP shows two distinct IR absorption peaks due to O-H and C=O groups. When compared with the neat GNP, all the three alkyl-GNPs show new IR absorption peaks at 1,600 and 2,950 [cm.sup.-1] resulted from the bending vibrations of the amide N-H bonds and the stretching vibrations of the covalently bonded alkyl C-H bonds respectively. The other strong IR absorption peaks for the alkyl-GNPs at 1,700 [cm.sup.-1] were considered due to the stretching vibrations of the amide C=O bonds. The broad IR absorption peaks for the alkyl-GNPs at 3,400-3,500 [cm.sup.-1] must be mainly due to the stretching vibrations of the amide N-H bonds. The FTIR spectra confirmed that each alkylamine could be covalently bonded to the neat GNPs by the reactions between the functional groups of the alkylamine and the neat GNPs.
Characteristics of the PP/Alkyl-GNP Nanocomposites
Figure 3 shows the flexural properties and impact strengths of the neat PP and the PP/alkyl-GNP nanocomposites. The effect of the grafted alkyl chain length on the flexural properties and impact strengths of the nanocomposites appeared apparently. The flexural moduli and impact strengths of the nanocomposites were much higher than those of the neat PP and increased with increasing alkyl chain length though the increase in the flexural strength was marginal. The improvements in the mechanical properties of the PP/alkyl-GNP nanocomposites were considered due to the enhanced interfacial adhesion between the alkyl-GNPs and the PP matrix. The alkyl chains grafted to the GNPs' surfaces would be compatible to the PP matrix due to van der Waals forces between the alkyl chains and PP chains resulting in stronger interfacial bonding. The effective stress transfer at the interfaces that results in increased mechanical properties would happen when GNPs are dispersed well in a polymer matrix with strong interfacial bonding . Besides, the improvements in the mechanical properties of the PP/alkyl-GNP nanocomposites with increasing alkyl chain length were considered due to the increased interface thickness between the GNPs and the PP matrix because a longer alkyl chain would penetrate deeper into the PP matrix than a shorter alkyl chain. Therefore, the PP/octadecyl-GNP nanocomposite showed the best mechanical properties among the nanocomposites and its flexural modulus and impact strength were 31 and 117% higher, respectively than those of the neat PP.
Figure 4 shows the dynamic storage modulus of the neat PP and the PP/alkyl-GNP nanocomposites as a function of temperature. The storage moduli of the PP/alkyl-GNP nanocomposites at 30[degrees]C (1,712 MPa for hexyl-GNP, 1,880 MPa for dodecyl-GNP, and 1,985 MPa for octadecyl-GNP) were much higher than that of the neat PP (1,324 MPa) and increased with alkyl chain length, as like as the flexural moduli shown in Fig. 3. The increase in the storage modulus was considered due to the improved interfacial adhesion strength between the alkyl-GNPs and the PP matrix as well as the improved dispersion and distribution of the GNPs in the PP matrix. Well dispersed and distributed GNPs with good interfacial bonding to the polymer matrix could enhance the modulus of the nanocomposite significantly because they could restrict the segmental motions of the polymer chains . Besides, it was simply predicted that the storage modulus of the nanocomposite would be higher than the neat PP because the GNPs have much higher modulus than the PP matrix. The crystalline melting temperature ([T.sub.m]) of each sample listed in Table 1 could be determined from the storage modulus curves by taking the temperature of the most drastic storage modulus decrease. The PP/alkyl-GNP nanocomposites showed marginally higher [T.sub.m] than the neat PP. From the mechanical properties shown in Fig. 3 as well as the storage moduli shown in Fig. 4, it was considered that the alkyl-GNPs could effectively enhance the performance of the PP/alkyl-GNP nanocomposites and the positive effect of the alkyl-GNPs increased with increasing alkyl chain length.
Crystalline melting behaviors of the neat PP and the PP/alkyl-GNP nanocomposites investigated by DSC are shown in Fig. 5. Table 1 includes the melting temperature ([T.sub.m]) and crystallinity ([X.sub.c]) of the neat PP and the PP/alkyl-GNP nanocomposites obtained from the DSC thermograms. It was evident that the crystallization of the PP matrix was affected by the presence of the alkyl-GNPs. As listed in Table 1, the crystallinity of the neat PP (57.6%) was slightly higher than those of the nanocomposites (52.2-56.7%). This result means that the alkyl-GNPs could inhibit the crystallization process of the PP matrix. The PP/alkyl-GNP nanocomposites showed marginally higher [T.sub.m]s than the neat PP as like as the [T.sub.m] data obtained from DMA. It was observed in this study that incorporating the alkyl-GNPs to the PP matrix resulted in the slight decrease in crystallinity and the slight increase in [T.sub.m]. The same trends on the crystallinity and [T.sub.m] of the filled PP composites have been also reported [38,39].
Figure 6 displays the images taken by SEM for the fracture surfaces of the impact-tested neat PP and PP/alkyl-GNP nanocomposites. The fracture surface of the neat PP is quite clean and smooth looking like fracture morphology due to typical brittle failure. However, the fracture surfaces of the PP/alkyl-GNP nanocomposites are quite rough looking like fracture morphology due to typical ductile failure. All the SEM images for the PP/alkyl-GNP nanocomposites show that each GNP indicated by arrows is dispersed and distributed well in the PP matrix and also embedded fairly well in the PP matrix without remarkable pull-outs indicating good interfacial interactions between the PP matrix and the alkyl-GNPs . The average height of the pull-outs of the alkyl-GNPs seems to decrease slightly with increasing alkyl chain length from hexyl (b) to octadecyl (d). These SEM images could explain the reason why the flexural modulus and impact strength of the PP/octadecyl-GNP nanocomposite were much higher than those of other nanocomposites by showing the strongest interfacial bonding due to the longest alkyl chain.
The preparation of the alkyl-GNPs was confirmed by the FTIR spectra of the alkyl-GNPs that showed the C-H stretching and NH bending peaks due to the alkyl chains grafted to the GNPs' surfaces. The flexural moduli and impact strengths of the PP/alkylGNP nanocomposites were much higher than those of the neat PP and increased with the alkyl chain length. The flexural modulus, impact strength and storage modulus of the PP/octadecyl-GNP nanocomposite were 31, 117, and 50% higher, respectively than those of the neat PP. The PP/alkyl-GNP nanocomposites showed slightly higher [T.sub.m]s than the neat PP. The crystallinity of the neat PP was slightly higher than those of the nanocomposites because the alkyl-GNPs could inhibit the crystallization process of the PP matrix. The SEM images for the fracture surfaces were helpful in understanding the reason why the flexural modulus and impact strength of the PP/octadecyl-GNP nanocomposite were much higher than those of other nanocomposites.
Sae Mi Park, Dae Su Kim [iD]
Department of Chemical Engineering, Chungbuk National University, Chungdaero 1, Seowongu, Cheongju, Chungbuk, 28644, South Korea
Correspondence to: D. S. Kim; e-mail: email@example.com
Contract grant sponsor: National Research Foundation of Korea; contract grant number: BK+2016.
Published online in Wiley Online Library (wileyonlinelibrary.com).
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Caption: FIG. 1. The procedure used in this study to prepare alkyl-GNPs.
Caption: FIG. 2. FT IR spectra of the neat GNPs and alkyl-GNPs with different alkyl chain lengths.
Caption: FIG. 3. Flexural properties and impact strengths of the neat PP and the PP nanocomposites with different alkyl-GNPs.
Caption: FIG. 4. Storage moduli of the neat PP and the PP nanocomposites with different alkyl-GNPs.
Caption: FIG. 5. DSC thermograms of the neat PP and the PP nanocomposites with different alkyl-GNPs.
Caption: FIG. 6. SEM images for the fracture surfaces of the neat PP (a) and PP nanocomposites with different alkyl-GNPs (b = hexyl-, c = dodecyl-, d = octadecyl).
TABLE 1. Mechanical and thermal properties of the neat PP and PP/GNPs nanocomposites. Flexural Flexural Sample modulus (MPa) strength (MPa) neat PP 864 [+ or -] 99 42.8 [+ or -] 6.6 PP/hexyl-GNP 1,007 [+ or -] 170 41.8 [+ or -] 5.1 PP/dodecyl-GNP 1,041 [+ or -] 197 40.4 [+ or -] 4.5 PP/octadecyl-GNP 1,131 [+ or -] 81 44.6 [+ or -] 2.9 Storage modulus at Impact 30[degrees]C Sample strength (J/m) (MPa) neat PP 15.0 [+ or -] 3.3 1,324 PP/hexyl-GNP 17.6 [+ or -] 1.7 1,712 PP/dodecyl-GNP 23.5 [+ or -] 4.0 1,880 PP/octadecyl-GNP 32.6 [+ or -] 7.3 1,985 [T.sub.m] [T.sub.m] [X.sub.c] by DMA by DSC by DSC Sample ([degrees]C) ([degrees]C) (%) neat PP 171.6 164.5 57.6 PP/hexyl-GNP 172.1 166.2 52.2 PP/dodecyl-GNP 172.2 166.2 55.1 PP/octadecyl-GNP 172.9 166.0 56.7
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|Author:||Park, Sae Mi; Kim, Dae Su|
|Publication:||Polymer Engineering and Science|
|Date:||Apr 1, 2019|
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