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Effects of montmorillonite and compatibilizer on the mechanical and thermal properties of dispersing intercalated PMMA nanocomposites
International Communications in Heat and Mass Transfer 67 (2015) 21–28
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International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Effects of montmorillonite and compatibilizer on the mechanical and thermal properties of dispersing intercalated PMMA nanocomposites☆ Ming-Kuen Chang ⁎, Hsiu-Chuan Lee Department of Safety Health and Environment Engineering, National Yunlin University of Science and Technology (NYUST), Yunlin 64002, Taiwan, ROC
a r t i c l e
i n f o
Available online 29 June 2015 Keywords: Nanoclays Coupling agent Nanocomposites Mechanical properties Thermal properties
a b s t r a c t We added montmorillonite (MMT) to improve the mechanical properties and thermal stability of poly (methyl methacrylate) (PMMA). We used a twin-screw extruder to produce standard specimens for examinations. We examined the layer distance of MMT and mechanical properties of the nanocomposites containing various amounts of MMT. The chemical structure and morphology of specimens were characterized with wide-angle powder X-ray diffraction, scanning, and transmission electron microscopy techniques. It was observed that by increasing the amount of MMT, layer distance increased from 2.16 to 3.87 nm for the composite containing 5 wt% MMT. In the tensile and impact properties examinations, the composite containing 1% MMT revealed the best result with increase of 1.81%. The shore hardness and wear resistance properties were also significantly improved. The beginning of thermal decomposition of nanocomposites slightly shifted to a higher temperature, compared with the thermogram of pure PMMA. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Recently, layered materials such as smectite clay (montmorillonite, MMT) have attracted intense interest for the preparation of polymerclay nanocomposite (PCN) materials. PCN usually demonstrates unique properties superior to those of conventional composites. In general, they combine both the characteristics of inorganic nanofillers and organic polymers at the molecular level. Currently, the PCN has exhibited as a promising system consisting of high aspect ratio and platy morphology. It can be used to boost the thermal stability [1], fire resistance [2], gas barrier [3], and corrosion protection [4] of polymers. Enhancements in the mechanical properties are particularly significant to be studies for various applications and development of PCN materials. Kim and White [5] studied a variety of organically modified MMTs to understand the contribution of the organophilicity of organoclay in the formation of the polymer/clay nanocomposite. Poly methyl methacrylate (PMMA) is a relatively strong and lightweight thermoplastic polymer. It has a density of 1.17–1.20 g/cm3, less than half that of glass. It also has good impact strength, better than glass and polystyrene; however, its impact strength is still significantly lower than that of polycarbonate and other engineering polymers. PMMA ignites at 460 °C and burns, forming carbon dioxide, water, carbon monoxide, and some low molecular weight compounds, including formaldehyde. The glass transition temperature of PMMA ranges from 85 °C to 165 °C, a range so wide due to its vast number of ☆ Communicated by W.J. Minkowycz ⁎ Corresponding author. E-mail address: [email protected] (M.-K. Chang).
http://dx.doi.org/10.1016/j.icheatmasstransfer.2015.06.016 0735-1933/© 2015 Elsevier Ltd. All rights reserved.
compositions. The forming of PMMA starts at the glass transition temperature and goes on above that. All common molding processes can be used on PMMA, including injection molding, compression molding, and extrusion. The best quality of PMMA sheets is produced through cell casting in which the polymerization and molding steps occur concurrently. The cast PMMA sheet is stronger than the molding grades owing to its extremely high molecular mass. Rubber toughening has been used to improve the toughness of PMMA, which has a brittle behavior in response to applied loads at room temperature. PMMA swells and dissolves in many organic solvents and shows poor resistance to many other chemicals on account of its easily hydrolysable ester groups. Nevertheless, its environmental stability is superior to most other plastics such as polystyrene and polyethylene. Therefore, PMMA is often the material of choice for outdoor applications [6]. Recently, several studies have been reported on the mechanical properties of PMMA–clay nanocomposite materials. Yeh et al. [7] reported the evaluation of the anticorrosive effect for the nonconjugated polymer-clay nanocomposite coatings, PMMA–clay, with a quaternary alkylphosphonium salt instead of a commonly used quaternary alkylammonium salt as the intercalating agent. The PCN materials in the form of coatings with low clay loading on cold-rolled steel were found to be superior in anticorrosion to those of bulk PMMA on the basis of a series of electrochemical measurements of corrosion potential, polarization resistance, corrosion current, and impedance spectroscopy in 5% aqueous NaCl electrolyte. According to literature [8,9], PCN materials prepared via in situ emulsion polymerization have the following characteristics: PMMA has a much higher value of MW and a better dispersion of clay nanolayers in the polymer framework. Galgali et al. [10] reported the creep behavior of molten polypropylene nanocomposites
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with organically modified clay in which the creep resistance of compatibilized hybrids was significantly better than that of uncompatibilized hybrids, which increased with annealing time. Moreover, wear resistance values of PET, nylon 6, PS, and PP nanocomposite materials have been reported by Schadler et al. [11] and Liu et al. [12–15]. However, wear resistance of PMMA–clay nanocomposite materials has seldom been studied. In this paper, we present a first evaluation of wear resistance of PMMA–clay nanocomposites and a comparative study on the effects of organoclay (MMT) on impact strength, shore hardness, and tensile strength of PMMA–clay nanocomposites.
2. Experimental 2.1. Materials and instrumentations Commercial PMMA (CM-211, melt flow index = 1.6 g/min, specific gravity = 1.19) was purchased as pellets from Chimei Corporation in Taiwan. The montmorillonite clay (Cloisite 30B) was purchased from Southern Clay Products, Inc. in USA. Wide-angle X-ray diffraction (WAXRD) study of the samples was performed on a Rigaku D/MAX-3C OD-2988 N X-ray diffractometer with a copper target and Ni filter at a scanning rate of 4°/min. The samples for the transmission electron microscopy (TEM) examination were prepared by putting the membrane of PCN materials into the low-viscosity embedding media epoxy resin capsules with four ingredients (ERL4206 5.0 g, DER736 3.0 g, NSA 13.0 g, and DMAE 0.15 g) and curing the resin at 100 °C for 24 h in a vacuum oven. Then, the cured epoxy resin containing PCN samples were microtomed with a Reichert-Jumg Ultracut-E into 60–90 nm slices. Subsequently, 10 nm carbon was deposited on these slices on mesh 100 copper nets for TEM observations on a JEOL-200FX, with an acceleration voltage of 120 kV. A Centrifugal Ball Mill (Retsch S100) was used to reduce the size of organophilic clay oarticles. A twin roll mill (Kobelco, model KXY-30) was used to melt mix the organoclay and PMMA. A Plastograph-Mix and a hot press machine manufactured by Brabender Machine Company (Germany, model PLE-331, co-rotating type, nonintermeshing, maximum screw speed of 120 rpm, screw L/D ratio of 32, test temperature from 20 °C to 400 °C, chamber maximum volume of 50 g) and Long-Chang Company (Taiwan, model FC-60 TON) were used to make the samples in the form according to ASTM D 3039 standard method. A scanning electron microscopy (SEM) model of Hitachi S-4100 FE-SEM was used to examine the surface morphology of materials. The thermal properties and decomposition temperatures of nanocomposites were determined using differential scanning calorimetry (DSC) (USA, Waters, model Q10), and thermogravimetric analysis (TGA) (USA, Waters, model Q50) techniques.
Fig. 1. X-ray diffraction patterns of neat PMMA, clay, and PMMA–MMT.
Fig. 2. TEM micrographs of PMMA–MMT composites containing (a) 1% and (b) 5% MMT (50,000×).
The tensile examinations were carried out with a universal testing instrument manufactured by Hung-Ta Company (Taiwan, model HT-9102). The impact strength examinations were performed using an Izod impact tester manufactured by Hung-Ta Company (Taiwan, model HT-8041B). The shore hardness tests were run on a Shore
Fig. 3. Variations of tensile strength of PMMA–MMT nanocomposites with clay content.
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2.5. Thermal properties analyses
Table 1 Materials designation and compositions. Designation
Composition
Parts (wt%)
PMMA PMMA1 PMMA3 PMMA5
PMMA PMMA/MMT PMMA/MMT PMMA/MMT
100 99/1 97/3 95/5
Hardness Test Machine by Excellence Company (Japan, Model D) for high and low hardness samples under an indentation time of 10 seconds at 25 °C. The wear resistance tests were performed on a wearing test machine manufactured by Taber Company (model 5130 Abraser).
2.2. Preparation of nanocomposites via melt intercalation Before blending, PMMA and clay were dried at 90 °C inside a vacuum oven for two hours (Baking the universe can drop aqueous vapor to under 0.01% within one hour). The PMMA compounds, containing 1, 3, and 5% of MMT, were prepared in a Plastograph-Mixer machine separately at 220 °C and screw speed of 65 rpm for 6 min. Then the screw speed was changed to 75 rpm, and blending was continued for another 4 min period. We designed a mold for the hot-pressing samples, according to the test standard methods. The compression molding was conducted after preheating the press for 10 min at 220 °C with a pressure of 2000 kg/cm2 for 1 min. Before opening the mold, it was cooled down with air to 25 °C. 2.3. Mechanical properties examinations Tensile strength is one of the most frequently examined mechanical properties that are performed based on standard methods. We examined at least five specimens in each group according to ASTM D 638 standard, calculated the average, and recorded the greatest strength values. The impact test determines the energy absorbed by the broken specimen and shows the toughness value of the material. This impact test was conducted using an Izod testing machine and specimens were made according to the ASTM D 256 standard method. Before the test, we made notch on the specimens.
2.4. Surface hardness and wear examination We used the Japan Shore Hardness Testing Machine type D to conduct hardness test according to ASTM D 790 standard method. The machine was simple to operate, and its assembly was less costly and also thin specimens could be used in the examination [16]. Wear resistance is one of the basic properties of materials in mechanical applications. When one object is in contact with the other ones for a long time, it will cause wearing. Thus, it is necessary to perform a long period of wearing–tearing test to evaluate materials performance in such applications. In this experiment, wearing–test machines was used to determine the material’s roll wearing and slip wearing performances according to the ASTM D 4060 standard method.
Thermal properties of materials were determined using differential scanning calorimetry (DSC) technique in which samples were heated from 30 °C to 300 °C at the rate of 4 °C/min. Thermal stability was examined with thermogravimetric analysis (TGA) technique at a heating rate of 10 °C/min from 30 °C to 800 °C under the N2 atmosphere.
3. Results and discussion 3.1. Microstructure characterization Fig. 1 shows the WAXRD patterns of raw MMT and a series of PCN materials. The outcome of annealing was determined using XRD measurements through monitoring the position, full-width-at-halfmaximum (FWHM), and intensity of the (001) basal reflection of the PCN samples. For raw clay (MMT), a diffraction peak at 2θ = 4.10° (d-spacing = 2.16 nm) was observed, as opposed to the diffraction peak when the amount of organoclay increased to 5 wt%. In Fig. 1, the small peak at 2θ = 2.28° is corresponding to a d-spacing of 3.87 nm, whereas the peak of composite sample containing 1% organoclay revealed a small peak at 2θ = 2.48°, corresponding to a d-spacing of 3.56 nm. This observation implies that there was a small amount of organoclay that could not be exfoliated in the PMMA matrix and existed in the form of a disordered intercalated layer structure. The finite layer expansion associated with intercalated structures results in a new basal reflection that corresponds to the larger gallery height of the intercalated hybrid. An increase in the degree of coherent layer stacking (i.e., a more ordered system) produces a relative decrease in the FWHM of the basal reflections upon hybrid formation. On the other hand, a decrease in the degree of coherent layer stacking results in peak broadening and intensity loss [17]. Regarding the microstructure, the clay particles are dispersed in the polymer matrix in either intercalated or exfoliated states. Intercalated nanocomposites are in general obtained when the polymer is located between the silicate layers. Even though the layer spacing increases, there are still attraction forces between the silicate layers to stack them evenly. Exfoliated nanocomposites are formed when the layer spacing increases to the point where there are no longer sufficient attractions between the silicate layers to maintain a uniform layer spacing [18]. In addition to XRD investigation, the internal structures of polymer/ clay nanocomposites were also examined using TEM technique, which can directly visualize the expanded layered structures in the nanocomposites and the dispersion pattern of the silicate layers in the polymer matrix [19]. As shown in Fig. 2a, we found that the sample containing 1% MMT had a good dispersion of particles in the PMMA matrix, where the dark line represents clay platelets and the gray/white areas represent the PMMA matrix. Furthermore, the composite containing 5% MMT also shows a good dispersion of particles in the polymer matrix. However, an overdose of MMT loading embedded in the composites may cause the observable aggregation of MMT clusters in the polymer matrix, as shown in Fig. 2b. It is clear that lamellar nanocomposites have a mixed morphology with major intercalated and minor unintercalated dispersion in the PMMA matrix.
Table 2 Relations of composition of PMMA–MMT nanocomposites with mechanical properties. Compound composition
Tensile strength (MPa)
Impact energy absorption (Joule/m)
Shore hardness (Hs)
Wear loss (after 1000 rpm) (mg)
PMMA PMMA1 PMMA3 PMMA5
5.53 5.63 [+1.81%] 4.77 [–13.74%] 4.45 [–19.53%]
110.91 112.92 [+1.81%] 112.58 [+1.51%] 111.58 [+0.60%]
84.80 89.66 [+5.73%] 94.60 [+11.56%] 91.27 [+7.63%]
278.1 182.7 [–34.30%] 116.4 [–58.14%] 163.4 [–41.24%]
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Fig. 4 shows the load-displacement measurements of neat PMMA and its corresponding PMMA–MMT nanocomposites. Originally, PMMA was a hard and brittle material, but the addition of MMT allowed it to absorb more elastic strain energy. Adding 1% of MMT resulted in the
Fig. 4. Variations of the tensile load of PMMA–MMT nanocomposites with displacement.
The different dispersion morphologies of MMT in the polymer matrix could lead to composites with different mechanical properties. The mechanical properties can be categorized into two types: bulk (tensile and impact strengths) and surface mechanical properties (wear resistance and shore hardness).
3.2. Bulk mechanical properties 3.2.1. Tensile strength Relationships between tensile properties and MMT loading, obtained from the tensile examinations on the standard dumbbell-shaped nanocomposites, are shown in Fig. 3. As the results indicate, upon the addition of MMT into PMMA, the tensile strength of nanocomposites increased to 5.63 MPa for PMMA1, decreased to 4.77 MPa for PMMA3, and decreased to 4.45 MPa for PMMA5 samples. Furthermore, an overdose of MMTs loading (3 and 5% for PMMA–MMT) in nanocomposites was found to decrease the tensile strength of as-prepared nanocomposites, summarized in Tables 1 and 2. This indicates that the introduction of MMTs into polymer matrix does effectively affect the tensile properties of pristine PMMA. However, the decrease in tensile strength for PMMA3 and PMMA5 may be resulted from the aggregation of MMTs in nanocomposites.
Fig. 5. Impact energy absorption of PMMA–MMT nanocomposites as a function of clay content.
Fig. 6. SEM images of the typical impact surfaces of (a) pure PMMA, (b) PMMA1, (c) PMMA3, and (d) PMMA5 nanocomposites.
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characteristic. That is to say, adding more organoclay enhances the shock resistant intensity. For example, the maximum impact energy absorption of PMMA1 was 112.92 J/m, and the minimum value for PMMA was 110.91 J/m.
Fig. 7. Shore-hardness of PMMA–MMT nanocomposites as a function of clay content.
best tensile strength and elastic modulus because of the strong mechanical properties of MMT particles. When increased to 5% loading, the phenomenon of aggregation of MMT particles made the nanocomposite weaker and more brittle. Although the hardness increased, tensile yield point and elongation at break values decreased. The absorption of elastic strain energy also decreased, and the materials were easily cracking. Similar trends were also observed in the impact strength. 3.2.2. Impact strength The impact strength test of PCN materials was performed according to ASTM D 256 standard method. The maximum capacity of impact instrument was 250 ft-lb and the maximum impact speed was 17 ft/s. The impact energy absorption of PMMA–MMT and LDPE-TPO-MMT nanocomposites increased when the organoclay loading increased, as shown in Fig. 5. The angle of the hammer shock increased from 142.9° to 143.3°. The larger the angle of the hammer strike corresponds to smaller the impact absorbed energy. Therefore, the shock resistant intensity was relatively worse because the neat PMMA matrix is relatively hard and tough. Therefore, the addition of organoclay into the matrix improved its bond strength and demonstrated better dispersion in the matrix. However, the addition of organoclay still impelled the nanocomposites when struck by the external force causing stress concentration. After that, the nanocomposites turned into macroscopic stress fractures, resulting in increasing its already existing shock resistant
Fig. 8. Effects of clay content on wear loss of PMMA–MMT nanocomposites.
Fig. 9. Wear SEM images of organoclay loadings for neat PMMA and PMMA–MMT nanocomposites at 200× magnification, clay content (a) 0%, (b) 1%, (c) 3%, and (d) 5%.
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Moreover, the impact strength of composites can be further evaluated via visual observations of samples after the test. Fig. 6 shows the SEM micrographs of neat PMMA (a), PMMA1 (b), PMMA3 (c), and PMMA5 (d). For example, the minimum impact energy absorption of PMMA was 110.91 J/m, and the maximum impact energy absorption of PMMA1 was 112.92 J/m. The SEM micrograph of PMMA (Fig. 6a) revealed a tear ridge and tongue protrusion surface morphology, indicating that the composites were transgranular brittle (cleavage) fracture materials. These tongue protrusions were due to the cleavage progress process that causes twin deformation. Moreover, after the integration of MMT particles into the PMMA matrix, the surface morphology of PMMA1 (Fig. 6b) composite displayed a river pattern, indicating that the composites were quasi-cleavage fracture materials. These SEM observations are consistent with the previous studies related to the impact strength of neat PMMA and corresponding composites. 3.3. Surface mechanical properties 3.3.1. Shore hardness The shore hardness is an important parameter for evaluating the performance or designating the materials. The hardness data of the PMMA–MMT nanocomposites in the form of a standard shape at various material compositions are given in Fig. 7 and Tables 1 and 2. The results indicate that as the MMT content increased, the corresponding shore hardness value (Hs) of composites increased up to 5% of MMT loading in composites. For example, the neat PMMA showed a relatively low surface hardness of 84.80 Hs. Once MMT was added to PMMA, the surface hardness increased to 89.66 Hs for PMMA1, 94.60 Hs for PMMA3, and 91.27 Hs for PMMA5. These observations indicate that the surface hardness of the composites increases with increasing the MMT content. Thus, the integration of MMTs into the PMMA matrix indeed significantly changes the surface characteristics of the composite materials based on the surface mechanical property studies. 3.3.2. Wear resistance Test specimens with the dimensions of 100 × 100 × 2.8 mm were prepared. After that, we used wear testing machine with a load of 1000 g and measured wear mass loss (the influence factor) through operating the wearing speeds of 500 and 1000 rpm to probe the wear resistance properties of composites. It has already been known that wear mass loss is the main effective factor in hardness and heat conduction and other characteristics. Fig. 8 shows the wearing mass loss of neat PMMA and PMMA–MMT composites. When operated at faster wearing test of 1000 rpm on composites, the wear mass loss amount significantly decreased. For
example, the amounts of wear mass loss at 1000 rpm were found to severely decrease from the original value of 278.1 mg for PMMA to 116.4 mg for PMMA3. It indicates that the wear mass loss values of composites severely decrease as the amount of MMTs in the composite increases at high wearing speed of 1000 rpm (Tables 1 and 2). This observation implies that the larger MMT loading (for instance, 3%) in composites lead to an obvious decrease in wear mass loss value at faster wearing test, reflecting that the incorporation of MMT into PMMA might show best wear resistance property in the composites. Moreover, the wear resistance of composites can be further evaluated through the visual observation of composites after the wearing test. Fig. 9 shows the SEM images of neat PMMA (a), PMMA1 (b), PMMA3 (c), and PMMA5 (d) after wear examination. The micrograph of neat PMMA morphology revealed an uneven and rough surface, indicating that PMMA, with a wear mass loss value of 278.1 mg, had soft surface structure as compared with the PS [20] (of 128.74 mg), PC [21] (of 1.25 mg), PP [14] (of 27 mg), and LDPE [22] (of 0.43 mg). After the wear test, the structure of PMMA can be easily degraded and removed from the surface of sample, as shown in Fig. 9a. However, the surface morphology of PMMA–MMT composites displayed a relatively smooth pattern compared with the neat PMMA, as illustrated in Fig. 9b–d. This implies that the incorporation of MMTs into PMMA might effectively enhance the wear resistance of PMMA. This conclusion obtained from the SEM observations is consistent with the previous studies related to the wear examinations of PMMA and its composites. 3.4. Thermal properties 3.4.1. DSC analysis The melting point (Tm) and the enthalpy of PMMA and PMMA–MMA group are shown in Fig. 10. In the twin-screw extrusion process of PMMA and clay, the clay particles produce an aggregation phenomenon leading to crystallization as a result of decrease in the endothermic crystallization enthalpy and because of the uniform dispersion of clay in polymer and impeding the polymer. In addition, the enthalpy of PMMA–MA group was also relatively lower, indicating that MMT would embed easier in polymer by MA grafts, so that it improves clay dispersion in the polymer matrix. There was no difference in melting temperature between samples, as shown in Fig. 10. This behavior was similar to the previous results reported in literature [23]. This may be attributable to the huge interfacial action between the polymer and the clay nanoparticles. Because clay is a better thermal barrier material, the decomposition temperature increases with higher clay content. Fig. 10 shows the DSC thermograms and melting temperatures of PMMA–MMT nanocomposites.
Fig. 10. DSC thermograms and melting temperatures of PMMA–MMT nanocomposites.
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Fig. 11. TGA thermograms of PMMA–MMT nanocomposites.
3.4.2. Thermal stability The thermal stabilities of materials were examined using TGA technique. In the TGA thermograms of composites (Fig. 11), only composite with clay content of 1% revealed thermal decomposition temperature (Td) (352.78 °C) below that of PMMA (358.34 °C). In PMMA–MA group, when clay contents were 3% and 5%, the Td points (370.95 and 374.66 °C) were above that of PMMA. By comparison, the Td of PMMA–MA group was almost 10 °C above that of PMMA group. The Td points of other samples at different clay contents revealed no significant difference, which is in agreement with the observations reported in the literature [24–26]. It appears that dispersion degree and thermal stability increased because of the presence of the MA grafts. In general, one major stage of weight loss starts at 352 °C and ends at 374 °C. This corresponds to the structural decomposition of polymer. For PCN materials, clay has been found to enhance the thermal stability of polymer [26]. We observed that the beginning of thermal decomposition in the nanocomposites shifted slightly to a higher temperature than that of neat PMMA. When MA is used to modify PMMA prepolymer, the MMT layer interval reveals greater expansion. Using a suitable MMT content, it can disperse evenly in order of nanograde of layer in the PMMA matrix. Because of this characteristic, the tensile strength, Young’s modulus, and storage modulus of PMMA composites increase as the amount of MA modified PMMA prepolymer increases and the glass transition temperature relatively improves. Although the mechanical performance of neat PMMA relatively improves, pharmaceutical performance is obvious while existing without modifier [15].
4. Conclusions In this paper, PCN materials consisting of commercial PMMA and organoclay platelets were successfully prepared through a melt intercalation approach. Morphology for the dispersion of MMTs in the PMMA matrix was subsequently characterized using XRD and TEM techniques. We found good dispersion of MMT particles in the PMMA matrix in the 1% MMT containing composite based on the TEM examinations. Moreover, 5% of MMT loading also showed good dispersion in the PMMA matrix. However, some domains of the TEM micrographs also revealed the presence of MMT clusters. In the studies of surface mechanical properties, the integration of MMT particles showed a distinct increasing trend on shore hardness up to 3% of MMT loading in composites. The enhancement of wear resistance of composites was identified relative to neat PMMA. In the bulk mechanical properties examinations, the composite containing 1% of
MMT exhibited the best performance for tensile and impact strengths, and both properties increased by 1.81%. The observations imply that for this type of composition, MMT exhibited good compatibility with the PMMA matrix. Furthermore, at higher MMT loading (5%), the raw MMT particles were found to be aggregated in the polymer matrix, as observed in the TEM micrographs, leading to an obvious decrease in tensile and impact strengths. In the shore hardness and wear resistance tests, 3% MMT strengthened the properties by 11.56% and 58.14%, respectively. It can be concluded that MMT particles could embed in polymer easier through grafting with MA, which cause the clay particles to disperse better in the matrix. In PMMA–MA group, when clay content was 3% and 5%, the Td was observed higher than Td of neat PMMA, respectively at 370.95 and 374.66 °C. Acknowledgments The financial support of this research was provided by the National Science Council, Taiwan, ROC. This support (Subsidy NSC 97-2221-E231-021) is gratefully acknowledged. The authors appreciate Dr. Saeed Doroudiani for editing the manuscript and providing useful suggestions. References [1] H.L. Tyan, C.M. Leu, K.H. Wei, Effect of reactivity of organics-modified montmorillonite on the thermal and mechanical properties of montmorillonite/polyimide nanocomposites, Chem. Mater. 13 (2001) 222–226. [2] J.W. Gilman, C.L. Jackson, A.B. Morgan, R.J. Harris, E. Manias, E.P. Giannelis, M. Wuthenow, D. Hilton, S.H. Phillips, Flammability properties of polymer-layeredsilicate nanocomposites. Polypropylene and polystyrene nanocomposites, Chem. Mater. 12 (2000) 1866–1873. [3] T. Lan, P.D. Kaviratna, T.J. Pinnavaia, On the nature of polyimide–clay hybrid composites, Chem. Mater. 6 (1994) 573–575. [4] Y.H. Yu, J.M. Yeh, S.J. Liou, C.L. Chen, D.J. Liaw, H.Y. Lu, Preparation and properties of polyimide–clay nanocomposite materials for anticorrosion application, J. Appl. Polym. Sci. 92 (2004) 3573–3582. [5] Y. Kim, J.L. White, Formation of polymer nanocomposites with various organoclays, J. Appl. Polym. Sci. 96 (2005) 1888–1896. [6] M. Ezrin, Plastics failure guide: cause and prevention, Hanser, Verlag, 1996, ISBN 1-56990-184-8. 168. [7] J.M. Yeh, S.J. Liou, C.Y. Lin, C.Y. Cheng, Y.W. Chang, K.R. Lee, Anticorrosively enhanced PMMA–clay nanocomposite materials with quaternary alkylphosphonium salt as an intercalating agent, Chem. Mater. 14 (2002) 154–161. [8] X. Huang, W.J. Brittain, Synthesis and characterization of PMMA nanocomposites by suspension and emulsion polymerization, Macromolecules 34 (2001) 3255–3260. [9] C.S. Chern, C.H. Lin, Particle nucleation loci in emulsion polymerization of methyl methacrylate, Polymer 41 (2000) 4473–4481. [10] G. Galgali, C. Ramesh, A. Lele, A rheological study on the kinetics of hybrid formation in polypropylene nanocomposites, Macromolecules 34 (4) (2001) 852–858.
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[11] P. Bhimaraj, D.L. Burris, J. Action, W.G. Sawyer, C.G. Toney, R.W. Siegel, L.S. Schadler, Effect of matrix morphology on the wear and friction behavior of alumina nanoparticle/poly(ethylene) terephthalate composites, Wear 258 (2005) 1437–1443. [12] S.P. Liu, S.S. Hwang, J.M. Yeh, C.C. Hung, Mechanical properties of polyamide-6/ montmorillonite nanocomposites—prepared by twin-screw extruder mixed technique, Int. Commun. Heat Mass Transfer 38 (1) (2011) 37–43. [13] S.P. Liu, I.J. Hwang, K.C. Chang, J.M. Yeh, Mechanical properties of polystyrene– montmorillonite nanocomposites—prepared by melt intercalation, J. Appl. Polym. Sci. 115 (2010) 288–296. [14] S.P. Liu, C.W. Liang, Preparation and mechanical properties of polypropylene/montmorillonite nanocomposites—after grafted with hard/soft grafting agent, Int. Commun. Heat Mass Transfer 38 (4) (2011) 434–441. [15] S.P. Liu, Studies on morphology and mechanical properties of dispersing intercalated silane montmorillonite in polypropylene matrix, Polym. Compos. 32 (9) (2011) 1389–1398. [16] T.W. Ebbesen, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Electrical conductivity of individual carbon nanotubes, Nature 382 (1996) 54–56. [17] R.A. Vaia, E.P. Giannelis, Polymer melt intercalation in organically-modified layered silicates: model predictions and experiment, Macromolecules 30 (25) (1997) 8000–8009. [18] T.H. Kim, L.W. Jang, D.C. Lee, H.J. Choi, M.S. Jhon, Synthesis and rheology of intercalated polystyrene/Na+-montmorillonite nanocomposites, Macromol. Rapid Commun. 23 (2002) 191–195. [19] B.J. Park, T.H. Kim, H.J. Choi, J.H. Lee, Emulsion polymerized polystyrene/ montmorillonite nanocomposite and its viscoelastic characteristics, J. Macromol. Sci., Part B: Phys. 46 (2007) 341–354.
[20] S.P. Liu, W.L. Hsu, K.C. Chang, J.M. Yeh, Enhancement of the surface and bulk mechanical properties of polystyrene through the incorporation of raw multiwalled nanotubes with the twin-screw mixing technique, J. Appl. Polym. Sci. 113 (2009) 992–999. [21] S.P. Liu, S.S. Hwang, J.M. Yeh, K.W. Pan, Enhancement of surface and bulk mechanical properties of polycarbonate through the incorporation of raw MWNTs—using the twin-screw extruder mixed technique, Int. Commun. Heat Mass Transfer 37 (7) (2010) 809–814. [22] S.P. Liu, L.C. Tu, Studies on mechanical properties of dispersing intercalated silane montmorillonite in low density polyethylene matrix, Int. Commun. Heat Mass Transfer 38 (7) (2011) 879–886. [23] J. Morawiec, A. Pawlak, M. Slouf, A. Galeski, E. Piorkowska, N. Krasnikowa, Preparation and properties of compatibilized LDPE/organo-modified montmorillonite nanocomposites, Eur. Polym. J. 41 (5) (2005) 1115–1122. [24] J. Zhang, C.A. Wilkie, Preparation and flammability properties of polyethylene–clay nanocomposites, Polym. Degrad. Stab. 80 (1) (2003) 163–169. [25] G. Malucelli, S. Ronchetti, N. Lak, A. Priola, N.T. Dintcheva, F.P.L. Mantia, Intercalation effects in LDPE/o-montmorillonites nanocomposites, Eur. Polym. J. 43 (2007) 328–335. [26] N. García, M. Hoyos, J. Guzmán, P. Tiemblo, Comparing the effect of nanofillers as thermal stabilizers of LDPE, Polym. Degrad. Stab. 94 (1) (2009) 39–48.
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Effects of montmorillonite and compatibilizer on the mechanical and thermal properties of dispersing intercalated PMMA nanocomposites☆ Ming-Kuen Chang ⁎, Hsiu-Chuan Lee Department of Safety Health and Environment Engineering, National Yunlin University of Science and Technology (NYUST), Yunlin 64002, Taiwan, ROC
a r t i c l e
i n f o
Available online 29 June 2015 Keywords: Nanoclays Coupling agent Nanocomposites Mechanical properties Thermal properties
a b s t r a c t We added montmorillonite (MMT) to improve the mechanical properties and thermal stability of poly (methyl methacrylate) (PMMA). We used a twin-screw extruder to produce standard specimens for examinations. We examined the layer distance of MMT and mechanical properties of the nanocomposites containing various amounts of MMT. The chemical structure and morphology of specimens were characterized with wide-angle powder X-ray diffraction, scanning, and transmission electron microscopy techniques. It was observed that by increasing the amount of MMT, layer distance increased from 2.16 to 3.87 nm for the composite containing 5 wt% MMT. In the tensile and impact properties examinations, the composite containing 1% MMT revealed the best result with increase of 1.81%. The shore hardness and wear resistance properties were also significantly improved. The beginning of thermal decomposition of nanocomposites slightly shifted to a higher temperature, compared with the thermogram of pure PMMA. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Recently, layered materials such as smectite clay (montmorillonite, MMT) have attracted intense interest for the preparation of polymerclay nanocomposite (PCN) materials. PCN usually demonstrates unique properties superior to those of conventional composites. In general, they combine both the characteristics of inorganic nanofillers and organic polymers at the molecular level. Currently, the PCN has exhibited as a promising system consisting of high aspect ratio and platy morphology. It can be used to boost the thermal stability [1], fire resistance [2], gas barrier [3], and corrosion protection [4] of polymers. Enhancements in the mechanical properties are particularly significant to be studies for various applications and development of PCN materials. Kim and White [5] studied a variety of organically modified MMTs to understand the contribution of the organophilicity of organoclay in the formation of the polymer/clay nanocomposite. Poly methyl methacrylate (PMMA) is a relatively strong and lightweight thermoplastic polymer. It has a density of 1.17–1.20 g/cm3, less than half that of glass. It also has good impact strength, better than glass and polystyrene; however, its impact strength is still significantly lower than that of polycarbonate and other engineering polymers. PMMA ignites at 460 °C and burns, forming carbon dioxide, water, carbon monoxide, and some low molecular weight compounds, including formaldehyde. The glass transition temperature of PMMA ranges from 85 °C to 165 °C, a range so wide due to its vast number of ☆ Communicated by W.J. Minkowycz ⁎ Corresponding author. E-mail address: [email protected] (M.-K. Chang).
http://dx.doi.org/10.1016/j.icheatmasstransfer.2015.06.016 0735-1933/© 2015 Elsevier Ltd. All rights reserved.
compositions. The forming of PMMA starts at the glass transition temperature and goes on above that. All common molding processes can be used on PMMA, including injection molding, compression molding, and extrusion. The best quality of PMMA sheets is produced through cell casting in which the polymerization and molding steps occur concurrently. The cast PMMA sheet is stronger than the molding grades owing to its extremely high molecular mass. Rubber toughening has been used to improve the toughness of PMMA, which has a brittle behavior in response to applied loads at room temperature. PMMA swells and dissolves in many organic solvents and shows poor resistance to many other chemicals on account of its easily hydrolysable ester groups. Nevertheless, its environmental stability is superior to most other plastics such as polystyrene and polyethylene. Therefore, PMMA is often the material of choice for outdoor applications [6]. Recently, several studies have been reported on the mechanical properties of PMMA–clay nanocomposite materials. Yeh et al. [7] reported the evaluation of the anticorrosive effect for the nonconjugated polymer-clay nanocomposite coatings, PMMA–clay, with a quaternary alkylphosphonium salt instead of a commonly used quaternary alkylammonium salt as the intercalating agent. The PCN materials in the form of coatings with low clay loading on cold-rolled steel were found to be superior in anticorrosion to those of bulk PMMA on the basis of a series of electrochemical measurements of corrosion potential, polarization resistance, corrosion current, and impedance spectroscopy in 5% aqueous NaCl electrolyte. According to literature [8,9], PCN materials prepared via in situ emulsion polymerization have the following characteristics: PMMA has a much higher value of MW and a better dispersion of clay nanolayers in the polymer framework. Galgali et al. [10] reported the creep behavior of molten polypropylene nanocomposites
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with organically modified clay in which the creep resistance of compatibilized hybrids was significantly better than that of uncompatibilized hybrids, which increased with annealing time. Moreover, wear resistance values of PET, nylon 6, PS, and PP nanocomposite materials have been reported by Schadler et al. [11] and Liu et al. [12–15]. However, wear resistance of PMMA–clay nanocomposite materials has seldom been studied. In this paper, we present a first evaluation of wear resistance of PMMA–clay nanocomposites and a comparative study on the effects of organoclay (MMT) on impact strength, shore hardness, and tensile strength of PMMA–clay nanocomposites.
2. Experimental 2.1. Materials and instrumentations Commercial PMMA (CM-211, melt flow index = 1.6 g/min, specific gravity = 1.19) was purchased as pellets from Chimei Corporation in Taiwan. The montmorillonite clay (Cloisite 30B) was purchased from Southern Clay Products, Inc. in USA. Wide-angle X-ray diffraction (WAXRD) study of the samples was performed on a Rigaku D/MAX-3C OD-2988 N X-ray diffractometer with a copper target and Ni filter at a scanning rate of 4°/min. The samples for the transmission electron microscopy (TEM) examination were prepared by putting the membrane of PCN materials into the low-viscosity embedding media epoxy resin capsules with four ingredients (ERL4206 5.0 g, DER736 3.0 g, NSA 13.0 g, and DMAE 0.15 g) and curing the resin at 100 °C for 24 h in a vacuum oven. Then, the cured epoxy resin containing PCN samples were microtomed with a Reichert-Jumg Ultracut-E into 60–90 nm slices. Subsequently, 10 nm carbon was deposited on these slices on mesh 100 copper nets for TEM observations on a JEOL-200FX, with an acceleration voltage of 120 kV. A Centrifugal Ball Mill (Retsch S100) was used to reduce the size of organophilic clay oarticles. A twin roll mill (Kobelco, model KXY-30) was used to melt mix the organoclay and PMMA. A Plastograph-Mix and a hot press machine manufactured by Brabender Machine Company (Germany, model PLE-331, co-rotating type, nonintermeshing, maximum screw speed of 120 rpm, screw L/D ratio of 32, test temperature from 20 °C to 400 °C, chamber maximum volume of 50 g) and Long-Chang Company (Taiwan, model FC-60 TON) were used to make the samples in the form according to ASTM D 3039 standard method. A scanning electron microscopy (SEM) model of Hitachi S-4100 FE-SEM was used to examine the surface morphology of materials. The thermal properties and decomposition temperatures of nanocomposites were determined using differential scanning calorimetry (DSC) (USA, Waters, model Q10), and thermogravimetric analysis (TGA) (USA, Waters, model Q50) techniques.
Fig. 1. X-ray diffraction patterns of neat PMMA, clay, and PMMA–MMT.
Fig. 2. TEM micrographs of PMMA–MMT composites containing (a) 1% and (b) 5% MMT (50,000×).
The tensile examinations were carried out with a universal testing instrument manufactured by Hung-Ta Company (Taiwan, model HT-9102). The impact strength examinations were performed using an Izod impact tester manufactured by Hung-Ta Company (Taiwan, model HT-8041B). The shore hardness tests were run on a Shore
Fig. 3. Variations of tensile strength of PMMA–MMT nanocomposites with clay content.
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2.5. Thermal properties analyses
Table 1 Materials designation and compositions. Designation
Composition
Parts (wt%)
PMMA PMMA1 PMMA3 PMMA5
PMMA PMMA/MMT PMMA/MMT PMMA/MMT
100 99/1 97/3 95/5
Hardness Test Machine by Excellence Company (Japan, Model D) for high and low hardness samples under an indentation time of 10 seconds at 25 °C. The wear resistance tests were performed on a wearing test machine manufactured by Taber Company (model 5130 Abraser).
2.2. Preparation of nanocomposites via melt intercalation Before blending, PMMA and clay were dried at 90 °C inside a vacuum oven for two hours (Baking the universe can drop aqueous vapor to under 0.01% within one hour). The PMMA compounds, containing 1, 3, and 5% of MMT, were prepared in a Plastograph-Mixer machine separately at 220 °C and screw speed of 65 rpm for 6 min. Then the screw speed was changed to 75 rpm, and blending was continued for another 4 min period. We designed a mold for the hot-pressing samples, according to the test standard methods. The compression molding was conducted after preheating the press for 10 min at 220 °C with a pressure of 2000 kg/cm2 for 1 min. Before opening the mold, it was cooled down with air to 25 °C. 2.3. Mechanical properties examinations Tensile strength is one of the most frequently examined mechanical properties that are performed based on standard methods. We examined at least five specimens in each group according to ASTM D 638 standard, calculated the average, and recorded the greatest strength values. The impact test determines the energy absorbed by the broken specimen and shows the toughness value of the material. This impact test was conducted using an Izod testing machine and specimens were made according to the ASTM D 256 standard method. Before the test, we made notch on the specimens.
2.4. Surface hardness and wear examination We used the Japan Shore Hardness Testing Machine type D to conduct hardness test according to ASTM D 790 standard method. The machine was simple to operate, and its assembly was less costly and also thin specimens could be used in the examination [16]. Wear resistance is one of the basic properties of materials in mechanical applications. When one object is in contact with the other ones for a long time, it will cause wearing. Thus, it is necessary to perform a long period of wearing–tearing test to evaluate materials performance in such applications. In this experiment, wearing–test machines was used to determine the material’s roll wearing and slip wearing performances according to the ASTM D 4060 standard method.
Thermal properties of materials were determined using differential scanning calorimetry (DSC) technique in which samples were heated from 30 °C to 300 °C at the rate of 4 °C/min. Thermal stability was examined with thermogravimetric analysis (TGA) technique at a heating rate of 10 °C/min from 30 °C to 800 °C under the N2 atmosphere.
3. Results and discussion 3.1. Microstructure characterization Fig. 1 shows the WAXRD patterns of raw MMT and a series of PCN materials. The outcome of annealing was determined using XRD measurements through monitoring the position, full-width-at-halfmaximum (FWHM), and intensity of the (001) basal reflection of the PCN samples. For raw clay (MMT), a diffraction peak at 2θ = 4.10° (d-spacing = 2.16 nm) was observed, as opposed to the diffraction peak when the amount of organoclay increased to 5 wt%. In Fig. 1, the small peak at 2θ = 2.28° is corresponding to a d-spacing of 3.87 nm, whereas the peak of composite sample containing 1% organoclay revealed a small peak at 2θ = 2.48°, corresponding to a d-spacing of 3.56 nm. This observation implies that there was a small amount of organoclay that could not be exfoliated in the PMMA matrix and existed in the form of a disordered intercalated layer structure. The finite layer expansion associated with intercalated structures results in a new basal reflection that corresponds to the larger gallery height of the intercalated hybrid. An increase in the degree of coherent layer stacking (i.e., a more ordered system) produces a relative decrease in the FWHM of the basal reflections upon hybrid formation. On the other hand, a decrease in the degree of coherent layer stacking results in peak broadening and intensity loss [17]. Regarding the microstructure, the clay particles are dispersed in the polymer matrix in either intercalated or exfoliated states. Intercalated nanocomposites are in general obtained when the polymer is located between the silicate layers. Even though the layer spacing increases, there are still attraction forces between the silicate layers to stack them evenly. Exfoliated nanocomposites are formed when the layer spacing increases to the point where there are no longer sufficient attractions between the silicate layers to maintain a uniform layer spacing [18]. In addition to XRD investigation, the internal structures of polymer/ clay nanocomposites were also examined using TEM technique, which can directly visualize the expanded layered structures in the nanocomposites and the dispersion pattern of the silicate layers in the polymer matrix [19]. As shown in Fig. 2a, we found that the sample containing 1% MMT had a good dispersion of particles in the PMMA matrix, where the dark line represents clay platelets and the gray/white areas represent the PMMA matrix. Furthermore, the composite containing 5% MMT also shows a good dispersion of particles in the polymer matrix. However, an overdose of MMT loading embedded in the composites may cause the observable aggregation of MMT clusters in the polymer matrix, as shown in Fig. 2b. It is clear that lamellar nanocomposites have a mixed morphology with major intercalated and minor unintercalated dispersion in the PMMA matrix.
Table 2 Relations of composition of PMMA–MMT nanocomposites with mechanical properties. Compound composition
Tensile strength (MPa)
Impact energy absorption (Joule/m)
Shore hardness (Hs)
Wear loss (after 1000 rpm) (mg)
PMMA PMMA1 PMMA3 PMMA5
5.53 5.63 [+1.81%] 4.77 [–13.74%] 4.45 [–19.53%]
110.91 112.92 [+1.81%] 112.58 [+1.51%] 111.58 [+0.60%]
84.80 89.66 [+5.73%] 94.60 [+11.56%] 91.27 [+7.63%]
278.1 182.7 [–34.30%] 116.4 [–58.14%] 163.4 [–41.24%]
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Fig. 4 shows the load-displacement measurements of neat PMMA and its corresponding PMMA–MMT nanocomposites. Originally, PMMA was a hard and brittle material, but the addition of MMT allowed it to absorb more elastic strain energy. Adding 1% of MMT resulted in the
Fig. 4. Variations of the tensile load of PMMA–MMT nanocomposites with displacement.
The different dispersion morphologies of MMT in the polymer matrix could lead to composites with different mechanical properties. The mechanical properties can be categorized into two types: bulk (tensile and impact strengths) and surface mechanical properties (wear resistance and shore hardness).
3.2. Bulk mechanical properties 3.2.1. Tensile strength Relationships between tensile properties and MMT loading, obtained from the tensile examinations on the standard dumbbell-shaped nanocomposites, are shown in Fig. 3. As the results indicate, upon the addition of MMT into PMMA, the tensile strength of nanocomposites increased to 5.63 MPa for PMMA1, decreased to 4.77 MPa for PMMA3, and decreased to 4.45 MPa for PMMA5 samples. Furthermore, an overdose of MMTs loading (3 and 5% for PMMA–MMT) in nanocomposites was found to decrease the tensile strength of as-prepared nanocomposites, summarized in Tables 1 and 2. This indicates that the introduction of MMTs into polymer matrix does effectively affect the tensile properties of pristine PMMA. However, the decrease in tensile strength for PMMA3 and PMMA5 may be resulted from the aggregation of MMTs in nanocomposites.
Fig. 5. Impact energy absorption of PMMA–MMT nanocomposites as a function of clay content.
Fig. 6. SEM images of the typical impact surfaces of (a) pure PMMA, (b) PMMA1, (c) PMMA3, and (d) PMMA5 nanocomposites.
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characteristic. That is to say, adding more organoclay enhances the shock resistant intensity. For example, the maximum impact energy absorption of PMMA1 was 112.92 J/m, and the minimum value for PMMA was 110.91 J/m.
Fig. 7. Shore-hardness of PMMA–MMT nanocomposites as a function of clay content.
best tensile strength and elastic modulus because of the strong mechanical properties of MMT particles. When increased to 5% loading, the phenomenon of aggregation of MMT particles made the nanocomposite weaker and more brittle. Although the hardness increased, tensile yield point and elongation at break values decreased. The absorption of elastic strain energy also decreased, and the materials were easily cracking. Similar trends were also observed in the impact strength. 3.2.2. Impact strength The impact strength test of PCN materials was performed according to ASTM D 256 standard method. The maximum capacity of impact instrument was 250 ft-lb and the maximum impact speed was 17 ft/s. The impact energy absorption of PMMA–MMT and LDPE-TPO-MMT nanocomposites increased when the organoclay loading increased, as shown in Fig. 5. The angle of the hammer shock increased from 142.9° to 143.3°. The larger the angle of the hammer strike corresponds to smaller the impact absorbed energy. Therefore, the shock resistant intensity was relatively worse because the neat PMMA matrix is relatively hard and tough. Therefore, the addition of organoclay into the matrix improved its bond strength and demonstrated better dispersion in the matrix. However, the addition of organoclay still impelled the nanocomposites when struck by the external force causing stress concentration. After that, the nanocomposites turned into macroscopic stress fractures, resulting in increasing its already existing shock resistant
Fig. 8. Effects of clay content on wear loss of PMMA–MMT nanocomposites.
Fig. 9. Wear SEM images of organoclay loadings for neat PMMA and PMMA–MMT nanocomposites at 200× magnification, clay content (a) 0%, (b) 1%, (c) 3%, and (d) 5%.
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Moreover, the impact strength of composites can be further evaluated via visual observations of samples after the test. Fig. 6 shows the SEM micrographs of neat PMMA (a), PMMA1 (b), PMMA3 (c), and PMMA5 (d). For example, the minimum impact energy absorption of PMMA was 110.91 J/m, and the maximum impact energy absorption of PMMA1 was 112.92 J/m. The SEM micrograph of PMMA (Fig. 6a) revealed a tear ridge and tongue protrusion surface morphology, indicating that the composites were transgranular brittle (cleavage) fracture materials. These tongue protrusions were due to the cleavage progress process that causes twin deformation. Moreover, after the integration of MMT particles into the PMMA matrix, the surface morphology of PMMA1 (Fig. 6b) composite displayed a river pattern, indicating that the composites were quasi-cleavage fracture materials. These SEM observations are consistent with the previous studies related to the impact strength of neat PMMA and corresponding composites. 3.3. Surface mechanical properties 3.3.1. Shore hardness The shore hardness is an important parameter for evaluating the performance or designating the materials. The hardness data of the PMMA–MMT nanocomposites in the form of a standard shape at various material compositions are given in Fig. 7 and Tables 1 and 2. The results indicate that as the MMT content increased, the corresponding shore hardness value (Hs) of composites increased up to 5% of MMT loading in composites. For example, the neat PMMA showed a relatively low surface hardness of 84.80 Hs. Once MMT was added to PMMA, the surface hardness increased to 89.66 Hs for PMMA1, 94.60 Hs for PMMA3, and 91.27 Hs for PMMA5. These observations indicate that the surface hardness of the composites increases with increasing the MMT content. Thus, the integration of MMTs into the PMMA matrix indeed significantly changes the surface characteristics of the composite materials based on the surface mechanical property studies. 3.3.2. Wear resistance Test specimens with the dimensions of 100 × 100 × 2.8 mm were prepared. After that, we used wear testing machine with a load of 1000 g and measured wear mass loss (the influence factor) through operating the wearing speeds of 500 and 1000 rpm to probe the wear resistance properties of composites. It has already been known that wear mass loss is the main effective factor in hardness and heat conduction and other characteristics. Fig. 8 shows the wearing mass loss of neat PMMA and PMMA–MMT composites. When operated at faster wearing test of 1000 rpm on composites, the wear mass loss amount significantly decreased. For
example, the amounts of wear mass loss at 1000 rpm were found to severely decrease from the original value of 278.1 mg for PMMA to 116.4 mg for PMMA3. It indicates that the wear mass loss values of composites severely decrease as the amount of MMTs in the composite increases at high wearing speed of 1000 rpm (Tables 1 and 2). This observation implies that the larger MMT loading (for instance, 3%) in composites lead to an obvious decrease in wear mass loss value at faster wearing test, reflecting that the incorporation of MMT into PMMA might show best wear resistance property in the composites. Moreover, the wear resistance of composites can be further evaluated through the visual observation of composites after the wearing test. Fig. 9 shows the SEM images of neat PMMA (a), PMMA1 (b), PMMA3 (c), and PMMA5 (d) after wear examination. The micrograph of neat PMMA morphology revealed an uneven and rough surface, indicating that PMMA, with a wear mass loss value of 278.1 mg, had soft surface structure as compared with the PS [20] (of 128.74 mg), PC [21] (of 1.25 mg), PP [14] (of 27 mg), and LDPE [22] (of 0.43 mg). After the wear test, the structure of PMMA can be easily degraded and removed from the surface of sample, as shown in Fig. 9a. However, the surface morphology of PMMA–MMT composites displayed a relatively smooth pattern compared with the neat PMMA, as illustrated in Fig. 9b–d. This implies that the incorporation of MMTs into PMMA might effectively enhance the wear resistance of PMMA. This conclusion obtained from the SEM observations is consistent with the previous studies related to the wear examinations of PMMA and its composites. 3.4. Thermal properties 3.4.1. DSC analysis The melting point (Tm) and the enthalpy of PMMA and PMMA–MMA group are shown in Fig. 10. In the twin-screw extrusion process of PMMA and clay, the clay particles produce an aggregation phenomenon leading to crystallization as a result of decrease in the endothermic crystallization enthalpy and because of the uniform dispersion of clay in polymer and impeding the polymer. In addition, the enthalpy of PMMA–MA group was also relatively lower, indicating that MMT would embed easier in polymer by MA grafts, so that it improves clay dispersion in the polymer matrix. There was no difference in melting temperature between samples, as shown in Fig. 10. This behavior was similar to the previous results reported in literature [23]. This may be attributable to the huge interfacial action between the polymer and the clay nanoparticles. Because clay is a better thermal barrier material, the decomposition temperature increases with higher clay content. Fig. 10 shows the DSC thermograms and melting temperatures of PMMA–MMT nanocomposites.
Fig. 10. DSC thermograms and melting temperatures of PMMA–MMT nanocomposites.
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Fig. 11. TGA thermograms of PMMA–MMT nanocomposites.
3.4.2. Thermal stability The thermal stabilities of materials were examined using TGA technique. In the TGA thermograms of composites (Fig. 11), only composite with clay content of 1% revealed thermal decomposition temperature (Td) (352.78 °C) below that of PMMA (358.34 °C). In PMMA–MA group, when clay contents were 3% and 5%, the Td points (370.95 and 374.66 °C) were above that of PMMA. By comparison, the Td of PMMA–MA group was almost 10 °C above that of PMMA group. The Td points of other samples at different clay contents revealed no significant difference, which is in agreement with the observations reported in the literature [24–26]. It appears that dispersion degree and thermal stability increased because of the presence of the MA grafts. In general, one major stage of weight loss starts at 352 °C and ends at 374 °C. This corresponds to the structural decomposition of polymer. For PCN materials, clay has been found to enhance the thermal stability of polymer [26]. We observed that the beginning of thermal decomposition in the nanocomposites shifted slightly to a higher temperature than that of neat PMMA. When MA is used to modify PMMA prepolymer, the MMT layer interval reveals greater expansion. Using a suitable MMT content, it can disperse evenly in order of nanograde of layer in the PMMA matrix. Because of this characteristic, the tensile strength, Young’s modulus, and storage modulus of PMMA composites increase as the amount of MA modified PMMA prepolymer increases and the glass transition temperature relatively improves. Although the mechanical performance of neat PMMA relatively improves, pharmaceutical performance is obvious while existing without modifier [15].
4. Conclusions In this paper, PCN materials consisting of commercial PMMA and organoclay platelets were successfully prepared through a melt intercalation approach. Morphology for the dispersion of MMTs in the PMMA matrix was subsequently characterized using XRD and TEM techniques. We found good dispersion of MMT particles in the PMMA matrix in the 1% MMT containing composite based on the TEM examinations. Moreover, 5% of MMT loading also showed good dispersion in the PMMA matrix. However, some domains of the TEM micrographs also revealed the presence of MMT clusters. In the studies of surface mechanical properties, the integration of MMT particles showed a distinct increasing trend on shore hardness up to 3% of MMT loading in composites. The enhancement of wear resistance of composites was identified relative to neat PMMA. In the bulk mechanical properties examinations, the composite containing 1% of
MMT exhibited the best performance for tensile and impact strengths, and both properties increased by 1.81%. The observations imply that for this type of composition, MMT exhibited good compatibility with the PMMA matrix. Furthermore, at higher MMT loading (5%), the raw MMT particles were found to be aggregated in the polymer matrix, as observed in the TEM micrographs, leading to an obvious decrease in tensile and impact strengths. In the shore hardness and wear resistance tests, 3% MMT strengthened the properties by 11.56% and 58.14%, respectively. It can be concluded that MMT particles could embed in polymer easier through grafting with MA, which cause the clay particles to disperse better in the matrix. In PMMA–MA group, when clay content was 3% and 5%, the Td was observed higher than Td of neat PMMA, respectively at 370.95 and 374.66 °C. Acknowledgments The financial support of this research was provided by the National Science Council, Taiwan, ROC. This support (Subsidy NSC 97-2221-E231-021) is gratefully acknowledged. The authors appreciate Dr. Saeed Doroudiani for editing the manuscript and providing useful suggestions. References [1] H.L. Tyan, C.M. Leu, K.H. Wei, Effect of reactivity of organics-modified montmorillonite on the thermal and mechanical properties of montmorillonite/polyimide nanocomposites, Chem. Mater. 13 (2001) 222–226. [2] J.W. Gilman, C.L. Jackson, A.B. Morgan, R.J. Harris, E. Manias, E.P. Giannelis, M. Wuthenow, D. Hilton, S.H. Phillips, Flammability properties of polymer-layeredsilicate nanocomposites. Polypropylene and polystyrene nanocomposites, Chem. Mater. 12 (2000) 1866–1873. [3] T. Lan, P.D. Kaviratna, T.J. Pinnavaia, On the nature of polyimide–clay hybrid composites, Chem. Mater. 6 (1994) 573–575. [4] Y.H. Yu, J.M. Yeh, S.J. Liou, C.L. Chen, D.J. Liaw, H.Y. Lu, Preparation and properties of polyimide–clay nanocomposite materials for anticorrosion application, J. Appl. Polym. Sci. 92 (2004) 3573–3582. [5] Y. Kim, J.L. White, Formation of polymer nanocomposites with various organoclays, J. Appl. Polym. Sci. 96 (2005) 1888–1896. [6] M. Ezrin, Plastics failure guide: cause and prevention, Hanser, Verlag, 1996, ISBN 1-56990-184-8. 168. [7] J.M. Yeh, S.J. Liou, C.Y. Lin, C.Y. Cheng, Y.W. Chang, K.R. Lee, Anticorrosively enhanced PMMA–clay nanocomposite materials with quaternary alkylphosphonium salt as an intercalating agent, Chem. Mater. 14 (2002) 154–161. [8] X. Huang, W.J. Brittain, Synthesis and characterization of PMMA nanocomposites by suspension and emulsion polymerization, Macromolecules 34 (2001) 3255–3260. [9] C.S. Chern, C.H. Lin, Particle nucleation loci in emulsion polymerization of methyl methacrylate, Polymer 41 (2000) 4473–4481. [10] G. Galgali, C. Ramesh, A. Lele, A rheological study on the kinetics of hybrid formation in polypropylene nanocomposites, Macromolecules 34 (4) (2001) 852–858.
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M.-K. Chang, H.-C. Lee / International Communications in Heat and Mass Transfer 67 (2015) 21–28
[11] P. Bhimaraj, D.L. Burris, J. Action, W.G. Sawyer, C.G. Toney, R.W. Siegel, L.S. Schadler, Effect of matrix morphology on the wear and friction behavior of alumina nanoparticle/poly(ethylene) terephthalate composites, Wear 258 (2005) 1437–1443. [12] S.P. Liu, S.S. Hwang, J.M. Yeh, C.C. Hung, Mechanical properties of polyamide-6/ montmorillonite nanocomposites—prepared by twin-screw extruder mixed technique, Int. Commun. Heat Mass Transfer 38 (1) (2011) 37–43. [13] S.P. Liu, I.J. Hwang, K.C. Chang, J.M. Yeh, Mechanical properties of polystyrene– montmorillonite nanocomposites—prepared by melt intercalation, J. Appl. Polym. Sci. 115 (2010) 288–296. [14] S.P. Liu, C.W. Liang, Preparation and mechanical properties of polypropylene/montmorillonite nanocomposites—after grafted with hard/soft grafting agent, Int. Commun. Heat Mass Transfer 38 (4) (2011) 434–441. [15] S.P. Liu, Studies on morphology and mechanical properties of dispersing intercalated silane montmorillonite in polypropylene matrix, Polym. Compos. 32 (9) (2011) 1389–1398. [16] T.W. Ebbesen, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Electrical conductivity of individual carbon nanotubes, Nature 382 (1996) 54–56. [17] R.A. Vaia, E.P. Giannelis, Polymer melt intercalation in organically-modified layered silicates: model predictions and experiment, Macromolecules 30 (25) (1997) 8000–8009. [18] T.H. Kim, L.W. Jang, D.C. Lee, H.J. Choi, M.S. Jhon, Synthesis and rheology of intercalated polystyrene/Na+-montmorillonite nanocomposites, Macromol. Rapid Commun. 23 (2002) 191–195. [19] B.J. Park, T.H. Kim, H.J. Choi, J.H. Lee, Emulsion polymerized polystyrene/ montmorillonite nanocomposite and its viscoelastic characteristics, J. Macromol. Sci., Part B: Phys. 46 (2007) 341–354.
[20] S.P. Liu, W.L. Hsu, K.C. Chang, J.M. Yeh, Enhancement of the surface and bulk mechanical properties of polystyrene through the incorporation of raw multiwalled nanotubes with the twin-screw mixing technique, J. Appl. Polym. Sci. 113 (2009) 992–999. [21] S.P. Liu, S.S. Hwang, J.M. Yeh, K.W. Pan, Enhancement of surface and bulk mechanical properties of polycarbonate through the incorporation of raw MWNTs—using the twin-screw extruder mixed technique, Int. Commun. Heat Mass Transfer 37 (7) (2010) 809–814. [22] S.P. Liu, L.C. Tu, Studies on mechanical properties of dispersing intercalated silane montmorillonite in low density polyethylene matrix, Int. Commun. Heat Mass Transfer 38 (7) (2011) 879–886. [23] J. Morawiec, A. Pawlak, M. Slouf, A. Galeski, E. Piorkowska, N. Krasnikowa, Preparation and properties of compatibilized LDPE/organo-modified montmorillonite nanocomposites, Eur. Polym. J. 41 (5) (2005) 1115–1122. [24] J. Zhang, C.A. Wilkie, Preparation and flammability properties of polyethylene–clay nanocomposites, Polym. Degrad. Stab. 80 (1) (2003) 163–169. [25] G. Malucelli, S. Ronchetti, N. Lak, A. Priola, N.T. Dintcheva, F.P.L. Mantia, Intercalation effects in LDPE/o-montmorillonites nanocomposites, Eur. Polym. J. 43 (2007) 328–335. [26] N. García, M. Hoyos, J. Guzmán, P. Tiemblo, Comparing the effect of nanofillers as thermal stabilizers of LDPE, Polym. Degrad. Stab. 94 (1) (2009) 39–48.