Accepted Manuscript Title: Processing and Characterization of PMMA/PI Composites Reinforced With Surface Functionalized Hexagonal Boron Nitride Author: Garima Mittal Kyong Y. Rhee Soo Jin Park PII: DOI: Reference:
Please cite this article as: Garima Mittal, Kyong Y.Rhee, Soo Jin Park, Processing and Characterization of PMMA/PI Composites Reinforced With Surface Functionalized Hexagonal Boron Nitride, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.10.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
International Symposium on Novel and Nano Materials (ISNNM), Budapest, Hungary, July 3~July 8, 2016 Processing and Characterization of PMMA/PI Composites Reinforced With Surface Functionalized Hexagonal Boron Nitride Garima Mittal1$, Kyong Y. Rhee1*, and Soo Jin Park2# 1
Department of Mechanical Engineering, College of Engineering, Kyung Hee University, Yongin 446-701, Korea 2
Department of Chemistry, Inha University, Incheon 402-751, Korea $
Surface functionalization of hBN particles was done with silane coupling agent, 3APTS. Functionalized hBN particles showed less stacking and better dispersion into PMMA/PI matrix. Thermal and tribological properties of the PMMA/PI/Si-hBN composites were analyzed. PMMA/PI/Si-hBN composites displayed increment in thermal and tribological properties.
Abstract Poly(methyl methacrylate) (PMMA) is acknowledged as a conventional polymer matrix because of its light weight, low friction, optical clarity, and environmental stability, with properties including UV resistance and moisture resistance. In the present study, PMMA/polyimide (PI)/hexagonal boron nitride (hBN) composites were processed by incorporating PI and hBN powder into the PMMA matrix. To augment the dispersion, the surface of hBN particles were functionalized with 3-aminopropyltriethoxysilane (3-APTS), which serves as a coupling agent. Two cases of composites were considered: one with as-received hBN and another one with silanized hBN. For validation of changes after silane treatment, X-ray diffraction, and Fourier transform infra-red were performed. The changes in morphology after surface treatment were analyzed through field-emission scanning electron microscope and high-resolution transmission electron microscope. The effects of hBN functionalization on the thermal properties of the composites were analyzed by thermo-gravimetric analysis. The tribological properties of the composites were studied by friction and wear tests and the morphology of the wear track was investigated using a surface profilometer and field-emission scanning electron microscope. The outcomes of these investigations indicated that the composite with silanized hBN exhibited superior tribological properties in comparison to the composites with as-received hBN. KEYWORDS Surface functionalization; Hexagonal boron nitride; Polymer composite; Friction; Wear.
1. Introduction Currently, the polymer composites are immensely in demand, particularly in automobile and aerospace engineering, because of their light weight and effortless processing [1, 2]. The properties of polymer composites, along with the different fillers like montmorillonite, graphene, carbon nanotubes, silica, basalt fiber, glass fiber, etc., can be regulated to get the desired synergistic outcomes for diverse commercial applications [3,4]. It is widely known that organicinorganic composites perform better by exhibiting favorable properties like thermal and mechanical stability, while maintaining easy processability, flexibility, and corrosion stability from the organic matrix [5]. Hexagonal boron nitride (hBN), also known as white graphite, is a layered ceramic compound that is an isoelectric analog to graphite, with an identical hexagonal lattice structure [1,6]. Due to
its very close similarity with graphite, it shows remarkable thermal and chemical stability (~1000 °C) along with the same favorable mechanical properties that facilitate applications in high temperature equipment [7]. Among all the boron nitrides, the hexagonal form is the most stable and is soft enough to be used in lubrication. The lamellar structure of hBN is maintained by the van der Waals’ force that allows the layers to slide across one another. Consequently, hBN functions as a solid lubricant even in vacuum conditions [8], making it useful for space applications. However, there are two major issues with using hBN to reinforce into the polymer matrix. The first problem is the poor interfacial interaction between hBN particles and the polymer matrix and the second is inhomogeneous dispersion into the matrix [9,10]. Generally, hBN particles have the tendency to settle down to the bottom of the polymer matrix. Consequently, potential applications of the composites are hindered. To evade this issue, many researches have been published [9,11]. For instance, Li et al. used mechanical milling, which decreased the thickness of the hBN flakes from hundreds of nanometers to a few nanometers by peeling [11]. Unfortunately, modest destruction to the in-planer structure and other defects were generated due to the mechanical milling process. As an alternative, different chemical approaches are used to avoid structural damage and increase the interfacial surface reactivity [12, 13]. In the present study, an oxidation process was used to enhance the interfacial surface interactions and to increase the interplaner distance between the hBN sheets, allowing maximization of matrix interactions. After oxidation, the surface of the hBN layers was functionalized using a silane coupling agent. Surface modification using organo-silane enhances the interactions between reinforced hBN and the polymer matrix. In addition, better dispersion of the reinforced nano or micro particles is also achieved, consequently improving the thermal and mechanical properties [14]. Our work is focused on PMMA/PI polymer composites reinforced
with silane-functionalized hBN powder. Amidst all the polymers, cost-effective thermoplastic polymer PMMA possesses a special position because of very good environmental stability like, moisture resistance, UV resistance, scratch resistance etc. [15, 16]. Along with these PMMA possesses good mechanical properties, optical clarity, low friction coefficient. On the other hand, polyimide (PI) is considered as high performance thermoplastic material because it owns unique structure due to the formation of charge transferring complex within the molecules that gives rise to ordered intermolecular stacking [17]. Consequently, it displays outstanding thermal and mechanical properties along with the good chemical and radiation resistance and electronic properties [18]. Although considerable research has been carried out associated with hBN particles, surface functionalization of hBN and the corresponding effects on tribological and thermal properties are yet to be done. Accordingly, X-ray diffraction (XRD), Fourier transform infra-red (FTIR), thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), field-emission scanning electron microscope (FE-SEM), and high-resolution transmission electron microscope (TEM) were performed to validate the changes after silane functionalization of hBN. Later, thermal and tribological properties were examined through TGA, DSC and friction and wear tests, respectively. 2. Experimental 2.1. Materials Required PMMA (avg Mw = ~120000, d= 1.188 g/mol at 25 °C, ACS grade), hBN powder (98%, 1µm size) and the coupling agent 3-aminopropyltriethoxysilane ((3-APTS), ≥ 98%) were purchased from Sigma-Aldrich Co., St. Louis, MO 63178, USA. N,N-Dimethylformamide (DMF) was purchased from Daejung Chemicals, Gyeonggi-do, Korea. Polyimide powder was purchased from Alfa Aesar Co., Ward Hill, MA 01835, USA.
2.2. Surface functionalization of hBN 5 grams of hBN powder were mixed with 4.2 grams of sodium nitrate and 200 ml of concentrated sulfuric acid and stirred for 7 hours at 200 °C. Afterwards, the mixture was filtered and washed with DI water until a neutral pH was achieved. The resulting powder was dried at 80 °C overnight in the oven. Later, this acid-treated hBN powder was mixed with 5 ml of 3-APTS, 375 ml of ethanol and 120 ml of DI water and stirred continuously at 130 °C for 8 hours. The resultant mixture was filtered and washed with DI water. The obtained hBN powder is referred to silanized-hBN powder (Si-hBN). 2.3. Synthesis of PMMA/PI/hBN or Si-hBN composites 1 gram (10%) of hBN was dispersed into the 50 ml DMF through sonication, followed by mixing with the desired amount of PMMA (8g) and PI (1g) powders. The mixer was kept stirring for 6 hours to get a homogenous solution and then held at room temperature for 15 minutes for degassing. Afterwards, the resulting solution was poured into the teflon mold and left in a vacuum oven for 48 hours at 80 °C to evaporate the solvent. The obtained composite sheet was cut into specimens for different characterizations. PMMA/PI/Si-hBN composites were fabricated in an identical manner. 2.4. Characterization XRD analysis was performed using a copper cathode for X-ray radiation (λ=0.1542 nm) with a 40 kV voltage and 100 mA current. The data was collected from 3° angle to 90° angle in 0.02° angle steps. FT-IR spectroscopy was performed over wavelengths ranging from 4000 cm-1 to 450 cm-1. FE-SEM and TEM were performed to analyze the morphology of the hBN after the silane treatment. Thermal properties of the composites were studied through thermogravimetric analysis (TGA, SQT 600 model) up to 800 °C with the temperature increment rate of 10 °C/min
in the presence of nitrogen gas (100 mL/min). Friction and wear tests were carried out by using a wear test machine (MPD friction & wear tester, Neoplus Inc., Korea), using the ball-on-disk method in the presence of a constant load of 5 kgf at room temperature. A zirconia ball-type tip was used for the test, with a track radius of 11.5 mm, a rotation speed of 30 rev/min, over a total time of 14400 sec. The size of the specimen was 30 mm x 30 mm. To check the reproducibility of the results, the tests were performed three times for each composite. Detailed analysis of wear track depth was accomplished through a surface profilometer. The tests were run for 100 seconds with a 1 mg force at 0.167 µm/sample resolutions within the 524 µm measurement range. This technique gives the cross-sectional view of the wear track, from which the wear depth can be calculated. Additionally, comparison of the surface morphology and the wear pattern of all the composites were performed using FE-SEM. 3. Results and discussion 3.1. XRD and FT-IR XRD was performed to analyze the structural changes into the hBN structure after surface functionalization. Figure 1 (a) shows the XRD patterns of hBN and Si-hBN powders. Both of the samples show an intense peak at around 2θ=26° (002) and other small peaks around 41°, 54° and 75°, which correspond to Miller indices of (100), (004) and (112), respectively [19]. It was found that after functionalization, the peak intensity increases because the attachment of 3-APTS ions leads to disoriented stacking of hBN layers that changes the scattering factors and consequently leads to changed intensity. To validate the attachment of chemical moieties of the silane coupling agent onto the hBN surface during silanization, FT-IR was performed. Figure 1 (b) represents the FT-IR spectra of hBN and Si-hBN powders.
Both powders show the
characteristic peaks of hBN as follows: one at 1375 cm-1 for B-N-B in-plane stretching vibration
and a peak at around 816 cm-1 that is due to out-of-plane B-N-B vibrations [12]. After silane treatment, some new peaks were also seen. A new peak at 2930 and 2833 cm-1 can be ascribed to symmetric and asymmetric vibrations of the -CH2- group of 3-APTS. A typical O-Si-O peak was observed at around 1065 cm-1. These newly originated peaks validate that after functionalization, 3-APTS ions of the coupling agent are attached to the oxidized hBN layers. <> 3.2. FE-SEM and TEM Figure 2 represents the FE-SEM (a & b) and TEM (c & d) images of hBN (a & c) and Si-hBN (b & d) powders, respectively. It can be seen that before silane functionalization, hBN layers have the tendency to form stacks together, and the thickness of the stacks is around a few hundred nanometers. However, during surface functionalization, 3-APTS ions reacted with the attached – OH groups on the surface of hBN. Therefore, due to the intercalation of big 3-APTS ions, interlayer distance between the hBN layers increased and less stacking was seen. Also, the thickness of stacked layers decreased up to a few nanometers. Similarly, the TEM images show multilayered stacking before functionalization, along with aggregated distributions of the hBN particles and a smaller interlayer distance [20], whereas after silanization, the layers of hBN were well-dispersed, validating the observations of surface modification of the hBN layers. After treatment, exfoliation occurred, meaning that the interlayer spacing also increased. This was confirmed with an SAED (Selective area electron diffraction, a crystallographic experimental technique) pattern (inset), which shows changes (disoriented stacking) after surface modification. <> 3.3. Thermal properties Comparison of the thermal properties of hBN powders before and after surface functionalization
was done through TGA. Figures 3 (a) and (b) represent the TGA and DSC curves of hBN and SihBN, respectively. Both of the samples exhibit a very small initial weight loss due to the evaporation of physically adsorbed moisture. Moreover, in comparison to hBN, the Si-hBN powder shows a well-defined further weight loss step (between 150 °C to 350 °C), which is ascribed to the thermal decomposition of the grafted chemical moieties of silane coupling agent (3-APTS). For the case of hBN, the total weight loss % was 0.85% while for Si-hBN, it was 0.63%. Similarly, by comparing the DSC curves of both samples, it was found that after silane treatment, the center of the main exothermic peak was shifted to a higher temperature (from 408 °C to 419 °C). All these obtained results confirm the attachment of the silane coupling agent onto the surface of the hBN layers. Figures 3 (c) and (d) represent the thermograms of PMMA/PI/hBN and PMMA/Pi/Si-hBN composites. It was found that the PMMA/PI/hBN composite shows higher weight loss (89.78%) as compared to the PMMA/PI/Si-hBN composite (81.75%). This means that the rate of thermal degradation was also different for both composites. The first step of weight loss was due to the evaporation of physically adsorbed moisture onto the composites and the release of carbon monoxide [21], the second step was because of the degradation of unsaturated end groups and the side chains of the PMMA and PI polymers [22], the third step was due to degradation of the main backbone of PMMA, and the last step was due to the degradation of PI chains. The major weight loss occurred between 400 °C and 800 °C. The PMMA/PI/hBN composite shows 3.66%, 13.10%, 39.35%, and 89.78% weight loss at each step, respectively. In contrast, the PMMA/PI/Si-hBN composite shows 2.76%, 12.31%, 25.56%, and 81.75% weight loss, respectively. Similarly, the DSC curves also show the different heat flow patterns for both of the composites. It was found that the exothermic peaks shifted to a higher temperature for Si-hBN
based composites. The reason behind the significant increase in thermal stability is the incorporation of surface functionalized hBN particles into the polymer matrix. Because of the attached functional groups of 3-APTS, the hBN layers exfoliate and disperse more homogeneously. Furthermore, the attachment of the 3-APTS ions converts hBN layers from hydrophilic to hydrophobic which enhances the compatibility between hydrophobic polymer matrix and hBN layers. Consequently, the interfacial interaction increases between the polymer matrix and the hBN layers. These homogenously dispersed Si-hBN layers act as heat barrier and provide rigidity by confining the movements of polymer chains during the thermal degradation process [23]. Therefore, it can be concluded that the PMMA/PI/Si-hBN composites are thermally more stable in comparison to the PMMA/PI/hBN composites. <> 3.4. Tribological properties Figure 4 shows the friction coefficients (CoF) of PMMA/PI/hBN and PMMA/PI/Si-hBN composites, which were 0.65 and 0.55, respectively. It can be concluded that the friction between the zirconia ball and the composite decreases (~18%) after reinforcement of surfacefunctionalized hBN particles. When hBN particles were reinforced into the PMMA/PI matrix, the particles acted as a binder by filling the voids, making the matrix more compact. Consequently, the surface became smoother in comparison to the PMMA/PI composite. Moreover, the graphite-like lamellar structure of hBN provided good lubricating properties [24]. When the layered structure of hBN comes into contact with the zirconia ball, the layers, weakly bound with the van der Waals’ force, start to slide and reduce the friction. When 3-APTS functionalized hBN particles were incorporated into the matrix due to compatibility and improved interfacial properties, the homogenous dispersion of Si-hBN particles into the matrix
occurred. Consequently, the friction coefficient of the PMMA/PI/Si-hBN composite became lower than that of the PMMA/PI/hBN composite. Furthermore, detailed analysis of wear properties were performed using the surface profilometer. The cross-sectional wear track patterns (b), depth and width (c), wear volume (d), and wear rate (e) are shown in Figure 4 and table 1. It can be observed from the wear pattern that the final wear depth and width were lower for the PMMA/PI/Si-hBN composite. Moreover, the wear volume (V) was calculated using the following formula: 𝑉 = 2𝜋rA,
(1)
where r is the wear track radius and A is the cross-sectional area of the wear track. As the wear track of PMMA/PI/hBN was greater in both width and depth, it therefore showed a higher crosssectional area (A). Consequently, a greater wear volume (42.9 mm3) was observed in comparison to the PMMA/PI/Si-hBN composite (30.2 mm3). Furthermore, the wear rate (k) was calculated through Archard equation [25], i.e.: 𝑉 = 𝑘. 𝐹. 𝑆,
(2)
where V is the wear volume, k is the wear rate, F is the normal load, and S is the sliding distance. The composite with a higher volume showed a higher wear rate, corresponding to greater wear loss. As can be seen in table 1 that PMMA/PI/hBN composite showed a greater wear rate (0.033mm3/Nm) compared to the PMMA/PI/Si-hBN composites (0.023mm3/Nm). The reason behind this is that the improved dispersion and enhanced interfacial interactions between the matrix and Si-hBN particles lead to better wear resistivity. Moreover, the sliding of weakly bound hBN layers due to the applied shear force provided additional smoothness to the wear track. The changes in morphology and the wear tract due to the functionalization of hBN are shown in Figure 5. Figure 5 (a) and (b) show the surface morphology of the hBN and Si-hBN
based composites, respectively while figure (c) and (d) represent the wear track morphology of the hBN and Si-hBN based composites, respectively. Aforementioned Si-hBN based composites showed smoother surface and less wear cracks due to the improved interfacial properties and homogenous dispersion of hBN particles into the matrix. <> <> 4. Conclusion In the present study, the surface functionalization of hBN particles was accomplished and their effects on the thermal and tribological properties of the PPMA/PI/hBN composites were also studied. A silane coupling agent, 3-APTS, was used for the surface functionalization, and the PMMA/PI/hBN composites were synthesized through solution casting method. It was found from the FTIR data that the silane moieties were attached onto the hBN particles after surface treatment. From FE-SEM and HR-TEM images, it was observed that multilayer stacking was reduced after functionalization, and the TGA-DSC curve also validated the attachment of the silane moieties. Additionally, the thermal and tribological properties of the composites were compared. It was found that after silanization, the interactions between hBN and the polymer matrix increased, yielding more homogeneous dispersion into the polymer matrix, giving rise to the superior thermal and tribological properties of PMMA/PI/hBN composites.
Acknowledgement This work was supported by the Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology (project number: 2016R1A2B4016034).
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Table List: Table 1: Different tribological parameters of PMMA/PI/hBN and PMMA/PI/Si-hBN composites are summarized.
Figure List: Fig. 1. Diffractograms (a) and FT-IR spectra (b) of hBN and Si-hBN powders. Fig. 2. FE-SEM (a & b) and TEM (c & d) images of hBN and Si-hBN powders, respectively. Fig. 3. Thermograms of hBN (a), Si-hBN (b), PMMA/PI/hBN (c), and PMMA/PI/Si-hBN (d). Fig. 4. Friction coefficient (CoF) (a), wear pattern (cross section) (b), Wear depth and width (c), wear volume (d), and wear rate (e) of PMMA/PI/hBN and PMMA/PI/Si-hBN composites. Fig. 5. FE-SEM images of surfaces (a & b) and wear tracks (c & d) of PMMA/PI/hBN (a & c) and PMMA/PI/Si-hBN (b & d) composites, respectively.
