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polyetherimide nanocomposite membrane
Solid State Sciences 12 (2010) 2155e2162
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Surfactant dispersed multi-walled carbon nanotube/polyetherimide nanocomposite membrane P.S. Goh, B.C. Ng, A.F. Ismail*, M. Aziz, S.M. Sanip Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2010 Received in revised form 12 August 2010 Accepted 19 September 2010 Available online 13 October 2010
Carbon nanotube based nanocomposite membranes have been fabricated through solution casting by embedding multi-walled carbon nanotubes (MWCNTs) within polyetherimide (PEI) polymer host matrix. In order to achieve fine dispersion of nanotubes and facilitate strong interfacial adhesion with the polymer matrix, the nanotubes were first treated with surfactants of different charges, namely anionic sodium dodecyl chloride, cationic cetyl trimethyl ammonium chloride and non-ionic Triton X100, prior to the dispersion in the PEI dope solution. Dispersion of MWCNTs in N-methyl-2-pyrrolidone solvent showed that the agglomeration and entanglement of the nanotubes were greatly reduced upon the addition of Triton X100. Scanning electron microscopy and atomic force microscopy examination has evidenced the compatibility of Triton X100 dispersed MWCNTs with the polymer matrix in which a promising dispersion and adhesion has been observed at the MWCNT-PEI interface. The increase in both thermal stability and mechanical strength of the resulting Triton X100 dispersed MWCNT/PEI nanocomposite indicated the improved interaction between MWCNTs and PEI. This study demonstrated the role of Triton X100 in facilitating the synergetic effects of MWCNTs and PEI where the resulting composite membrane is anticipated to have potential application in membrane based gas separation. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Carbon nanotubes Polymer matrix composite Surface treatments
1. Introduction The field of carbon nanotubes (CNTs) research has attracted a great deal of worldwide attention driven by their remarkable potential and unique properties which lead to many promising applications. A great focus is devoted to their exceptional thermal, electrical and mechanical properties which facilitate the viability of this material to be served as compatible filler for the development of a new generation of nanostructured CNT-based composite material. The current interests in this area have exceptionally blossomed within the past decade and the most well established composite material is most probably CNT reinforced polymer nanocomposites [1e4]. A substantial improvement in the desired properties can be achieved through the synergetic effects resulted from the intimately mixed polymer matrix and the CNT fillers in which the resulting materials possess the unique combination of physical and mechanical properties and as multifunctional systems with great potential that are not present in conventional polymer matrix composite. In fact, CNT-based polymer composites have spurred enormous interest in the community of materials primarily due to
* Corresponding author. Tel.: þ6075535592; fax: þ6075535925. E-mail address: [email protected] (A.F. Ismail). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.09.017
their potential to demonstrate remarkable increase in strength and thermal stability when compared to other conventional and typical carbon black or silica-based polymer composite [5,6]. In order to obtain a hybrid system with excellent and superior properties, several parameters have to be taken into account during the fabrication process, including the size of the filler particles, the degree of dispersion in the matrix, as well as the degree of adhesion with the polymer chains [7,8]. The full compatibility of the CNT filler with evenly distributed particle size and the polymer matrix will in turn result in high quality and homogeneously dispersed CNT-polymer nanocomposite [9]. Furthermore, the strong interfacial interaction will give rise to an effective load transfer from the polymer matrix to the CNTs. To exploit the utility of CNTs for this purpose, uniform dispersion of CNTs in the polymer matrix is required. Unfortunately, the use of CNTs in this application has been largely limited by their poor processability and dispersibility [10e13]. Besides the presence of intrinsic van der Waals force, their high surface area and aspect ratio of CNTs have rendered to the formation of highly stabilized bundles which in turn resulting in the formation of tight bundles and hollow ropes. In addition to that, the relatively smooth CNT surfaces which lack of interfacial bonding have also restricted the effective load transfer [14,15]. There have been several ways to produce CNT-polymer nanocomposites. The most commonly used is probably through polymer
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solution casting which possesses few advantages over the in-situ polymerization and melt processing. Preparation through polymer solution casting allows the use of polymers which were previously not suitable for in-situ polymerization [16,17]. During the polymer dissolving and agitating process, the most crucial part is the dispersion of CNT agglomerations before the evaporation stage. However, the economical scale up of CNT nanocomposite manufacturing remains hurdle as it is usually difficult to achieve homogeneous dispersion of CNTs throughout the polymer matrix. To date, the efforts made towards the controlling of CNT aggregation remain as a challenge. The existing challenges of poor dispersion and interfacial bonding in the CNT-polymer matrix are being addressed with some forms of improvement through the modification of CNT surfaces in order to create stronger CNT arrayepolymer interface adhesion. A surface modification of CNTs is generally classified in chemical and physical means. The former is known to be an effective method as it allows high quality dispersions to be obtained at high CNT loading in polymer matrix but unfortunately it always leads to remarkable damages and distortion to the structure of CNTs due to the oxidation or surface interaction [18,19]. The latter modification is therefore becoming an attractive mean of CNT surface modification. The surface modification of CNTs is carried out using third phase dispersing agent such as surfactants, with the assistance of sonication for a period of time to debundle the CNTs [20e22]. Unlike the harsh reaction condition experienced by the chemical modification, this physical modification is relatively mild and only involves the non-covalent bonding without disturbing the overall structure of the graphite sheets, hence maintaining the pristine properties of the CNTs. CNTs subjected to ultrasonication appeared shorter due to the sonication-induced nanotube cutting and the tips opening of the CNTs. At the same time, the surfactant molecules are uniformly built up on the tube walls. After the adsorption of surfactant molecules on the nanotube surface, ultrasonication for minutes or hours may helps the surfactant to disperse the nanotube bundles by steric or electrostatic repulsion. A recent report [23] has also underlined the ability of the surfactant coated nanotubes to maintain high quality dispersions at concentration approaching 1 mg/mL with reasonably large populations of individual singlewalled carbon nanotubes (SWCNTs). According to Shvartzman [24], mechanical exfoliation of the bundles prior to surface treatment must occur in order to obtain individual CNTs. The stabilization of aqueous suspension of CNTs using surfactants gives rise to the introduction of surface charges onto the CNTs depends on the nature of surfactant, which is either cationic, anionic or non-ionic properties. A considerable amount of works related to the dispersion of CNTs with different types of surfactants have been carried out [25e27]. In most of the cases, ultrasonication has been applied to disperse the highly entangled carbon nanotubes. In particular, the utilization of anionic sodium dodecyl chloride (SDS) as stabilizer has been widely studied due to its excellent CNT stabilization and separation capabilities. Yu et al. [25] have demonstrated the dispersion of MWCNTs in aqueous SDS solution. The surfactant molecules were found adsorbed on the surface of nanotubes and hence prevent the re-aggregation by electrostatic repulsion for several months. In a recent study by Karousis et al. [26], aqueous SDS has been applied to induce a stable aqueous colloidal dispersion of MWCNTs and significant solubility of the modified MWCNTs in polar solvent was also observed. Nonionic surfactant such as Triton X350 has also been used to functionalize CNTs in aqueous solution as it was found to interact strongly with graphite surfaces [11]. Although a vast number of papers dealing with dispersion of CNTs using surfactants have been published to date, the selection of suitable surfactant to disperse the as-grown CNTs prior to the
preparation of CNT-polymer nanocomposite has been scarcely reported. It is an important aspect of consideration as the proper choice of the surfactant is essential to ensure an effective surface wrapping of CNTs. The influence of the surfactant molecules is closely related to their ionic and non-ionic characteristics. To address the current issue, the objective of this work is to prepare a surfactant dispersed MWCNTs hence to evaluate the effects of varying types of surfactant on the dispersion property of the CNTs. The surfactant dispersed MWCNTs were subsequently used as the nanofiller for the realization of CNTs based polyetherimide (PEI) membrane through polymer solution casting. The purpose of varying the type of surfactant is to investigate the dispersion behaviour rendered by the surfactants and hence correlate to the physical properties of the resulting nanocomposite. 2. Experimental 2.1. Materials MWCNTs were prepared by catalytic decomposition of acetylene. The commercial available alumina hydrate powder purchased from Fluka was used as the support of metallic acetate (Fe/Co) catalyst for the synthesis of MWCNTs. In the catalytic processes, nitrogen was passed through the quartz tube as the furnace was heated to reach 700 C. The as-grown MWCNTs were collected as loosely aggregated fluffy black powders. The surfactants possessing different charges, namely anionic sodium dodecyl chloride (SDS) purchased from Merck, cationic cetyl trimethyl ammonium chloride (CTAB) purchased from Fluka and non-ionic Triton X100 supplied by J.T. Baker were used as dispersing agent of MWCNTs. PEI was used as polymer material, supplied by General Electric Co. (USA) under the trade name ‘Ultem 1000’. N-methyl-2-pyrrolidone (NMP) supplied by Sigma Aldrich was used as solvents for PEI. 2.2. Dispersion of MWCNTs The concentration of each type of surfactant in aqueous solution was fixed at 1 wt%. All solutions were prepared by mixing 0.5 g of MWCNTs with 100 cm3 of aqueous surfactant solution. After sonication for 60 min, the surfactant treated MWCNT was filtered from the solution in order to remove excess surfactants. The samples were then dried overnight in oven at 60 C. 2.3. Preparation of MWCNT/PEI nanocomposites To fabricate MWCNT/PEI nanocomposite, the surfactant dispersed MWCNTs were blended with PEI in NMP to obtain a polymer concentration of 25 wt%. The temperature and stirring speed were controlled so that the PEI dissolved completely to form a yellowish clear solution. Appropriate amount of surfactant dispersed MWCNTs was added to obtain a fixed CNT loading of 1 wt % in the resulting polymer composite to avoid agglomeration of CNTs that likely to occur at high concentration. The mixture was then stirred to obtain a homogeneous suspension. Flat sheet membranes were prepared according to the dry/wet phase separation process. The polymer solution was cast on a clean glass plate using a simple pneumatically-controlled membrane casting system under a fixed casting speed. The membranes were then immersed in water that acts as coagulation medium to allow wet phase inversion process. The resulting MWCNTs/PEI flat sheet membranes, with thickness of 200 mm, were left in water immersion for overnight to enable complete solvent exchange before drying in atmosphere for 24 h.
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2.4. Characterizations Morphological study of the surfactant dispersed MWCNTs and the resulting nanocomposite membranes was carried out using field emission electron scanning electron microscope (FESEM) and transmission electron microscope (TEM). A thin layer of gold was coated on the specimens before scanning with the FESEM (Phillips SEMEDAX, XL40, PW6822/10) while the specimens for TEM observation were prepared by ultrasonicating MWCNT in alcohol and dropped on a copper grid. Surface roughness of the resulting composite membrane was examined through JEOL, JSPM-5200TM atomic force microscope (AFM). Thermal analysis was done through thermogravimetric analysis (TGA) in the temperature range of 30e800 C in air at a heating rate of 10 C/min. The dispersion property of the surfactant dispersed MWCNTs were studied by observing the settling speed of the sample powders in NMP solvent. Approximately 0.05 g of surfactant dispersed MWCNTs were added to the vials containing 15 ml of NMP solvent. The tensile strength was measured using mechanical testing system, Universal testing machine (Llyod EZ 20). The testing was conducted with tensile rate of 5 mm/min and 5 measurements were performed and the results were quoted as average values. The correlation between the attached surface surfactant molecules and dispersion of the MWCNTs in the PEI polymer host matrix are discussed. 3. Results and discussion 3.1. Physical characterizations of surfactant/MWCNTs The FESEM micrographs of the resulting surfactant dispersed MWCNTs are demonstrated in Fig. 1. No remarkable physical damages or shortening of the MWCNTs are observed for all samples, which indicates that treatment with surfactants is a mild and physical surface modification that conserved the morphology structure of the nanotubes. The as-grown MWCTs (Fig. 1a) consist
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of nanotubes that are folded and agglomerations of tubes can be obviously seen. The modification with surfactants was thought to contribute to the debundling of the nanotubes. However, the morphology examinations pointed out that not all surfactants could play the same role during the dispersion of MWCNTs in the surfactant solution. It was noticed that the extent of nanotube debundling is surfactant dependant. The surfactants used for the surface modification possessed different affinity towards the surfaces of MWCNTs, which in turn resulted in the different degree of effectiveness in reducing the agglomeration of MWCNTs. It is in common consent that it is very difficult to entirely unfold and debundle all nanotubes into individual tubes, but to the least extent, it has been particularly observed that the Triton X100 dispersed MWCNTs appeared to be less bundled, which implied that Triton X100 is the most promising surfactant to debundle the MWCNTs. The thickening of the graphite walls has been observed through the high resolution scanning. The TEM micrograph of Triton X100 dispersed MWCNTs shown in Fig. 2 reveals that the surfaces appeared to be coarser than the raw MWCNTs due to the coating of surfactant molecules along the axial of the nanotubes. The TGA weight loss curves of raw MWCNTs and surfactant dispersed MWCNTs are depicted in Fig. 3. The dependence of weight loss and heat evolution on the temperature program of the samples was compared and served as an indicator to the amount of surfactant molecules that have been attached onto the sidewalls of MWCNTs, in which the greater the weight loss, the greater the amount of carbon materials or the organic functional groups present in the modified samples [28]. In general, greater weight loss has been observed for surfactant coated MWCNTs indicated that more carbonaceous compound existed in the samples. For all the surfactant treated samples, the initial stage of weight loss occurred in the temperature range of 100e300 C which corresponded to water evaporation as well as the decomposition of volatile compound. As-grown MWCNTs showed a relatively simple monotonic degradation pattern due to the lack of surface functional
Fig. 1. The FESEM micrographs (magnification: 50,000) of the a) raw MWCNT, b) SDS-MWCNTs, c) CTAB-MWCNTs and d) Triton X100-MWCNTs. The morphological structures of the nanotubes are conserved upon the mild physical surfactant dispersion.
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Fig. 2. The TEM micrograph of (a) raw MWCNTs and (b) Triton X100-MWCNTs. The comparison of the morphology revealed that the outer layer of CTAB dispersed nanotubes was apparently rougher due to the coating of CTAB molecules.
groups. The unusually weight gain demonstrated by as-grown MWCNTs is mainly due to the oxidation of metal catalyst into solid oxides [29]. The weight loss in the range of 300e500 C corresponds to the decomposition of amorphous carbon and the hydrocarbon functional groups that have been attached onto the walls of MWCNTs. All samples demonstrated significant weight loss in the temperature range of 400e600 C, which is mainly due to the degradation of graphene walls and oxidation of the MWCNTs into CO2. Due to the difference in amount of carbonaceous compounds that present in the surfactants, the pattern of the weight loss is found greatly differed from each other. However, the multidecomposition stages demonstrated in the thermogram indicates that the physical modification with surfactants does not involve the structure changes and the thermal properties of both nanotubes and surfactants were preserved. 3.2. Dispersion behaviour of surfactant/MWCNTs in NMP The dispersion test was conducted in NMP solvent as NMP appears to be one of the best solvents to disperse CNT and it is a commonly used solvent for the preparation of polymer based composite. Fig. 4 shows the settling pictures of raw MWCNTs and surfactant treated MWCNTs in NMP solvent. Visual characteristic of the dispersion differed drastically depending on the types of surfactant that dispersed onto the MWCNT surfaces. Sedimentation in NMP solvent can be obviously observed for the CTAB-MWCNTs and raw MWCNTs. The particles were agglomerated and caused the
Fig. 3. The TGA weight loss curves of a) raw MWCNT, b) SDS-MWCNTs, c) CTABMWCNTs and d) Triton X100-MWCNTs. All surfactant treated MWCNTs showed weight loss regions that corresponded to the organic compounds that dispersed onto MWCNTs.
solid precipitates to coexist with an almost transparent liquid supernatant. Raw MWCNTs have poor wetting and dispersion behaviour due to the strong van der Waals interaction between the sidewalls of the MWCNTs meanwhile the dispersion with CTAB did not show any effect on the dispersibility of the MWCNTs. MWCNTs modified with SDS were found apparently dispersed in the NMP solvent, but the suspension was not been well maintained as the sedimentation can be clearly observed after some days. Noticeable improvement has been observed for the suspension of Triton X100 dispersed MWCNTs in the solvent. During the surfactant modification of MWCNTs, ultrasonication helped to attach the Triton X100 molecules on the surface of the nanotubes and prevent the further aggregation that takes place due to the presence of van der Waal attraction. The suspension has demonstrated better dispersion and the colloidal stability can be maintained up to 30 days. When the bottom of vial was carefully checked, no sedimentation of black powder was observed. The results obtained with Triton X100 treated MWCNTs clearly showed that the surfactant played a significant role for the dispersion of the nanotubes in NMP solvent. The presence of dispersing CTAB on MWCNTs has repressed the effective dispersion meanwhile SDS did not contribute to long last stability of the suspension. To explain this different behaviour, it is necessary to imply the interfacial characteristic based on two factors: i) the surface interaction of Triton X100 with MWCNTs. Non-ionic surfactant seems to be rather promising for the stabilization of CNTs as this type of surfactant was evidenced to interact strongly with the graphite surface and hence enhanced the suspension stability [30]. In this case, the surfactant molecules adsorbed onto the exterior CNT surfaces along the axial length by their hydrophobic segments, leaving the hydrophilic tail directed to the exterior that eventually resulting in nanotube repulsion and leading to enhanced dispersion. At the surfactant concentration up to the critical micelle concentration (CMC) which is 0.2 mM for Triton X100, several surfactant monolayers were formed near the MWCNT surfaces and subsequently contribute to an outstanding MWCNT suspension stabilization [10]; ii) the interaction of surfactant coated MWCNTs with NMP solvent. The similar polarity between the polyethylene glycol parts and solvent having polar hydrophilic characters might act in an attractive manner and facilitate much stronger effective interaction between the MWCNTs and solvent compared to other surfactants modified MWCNTs. During the formation of CNT-based polymer nanocomposite, it is crucial to achieve good dispersion to take advantage of the high surface area of nanofillers [13]. It is interesting to note that, the dispersion of different types of surfactant on the surface of asgrown MWCNT can be differed, depending on the morphology of
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Fig. 4. Settling picture of a) raw MWCNT, b) SDS-MWCNTs, c) CTAB-MWCNTs and d) Triton X100-MWCNTs in NMP solvent. Triton X100-MWCNTs has demonstrated colloidal stability that can be maintained up to 30 days.
the MWCNTs. The dispersion test demonstrated that, by properly choosing a suitable surfactant with good dispersion, the segregation of MWCNTs in the organic solvent can be greatly suppressed. In this study, in order to obtain further confirmation on the roles of Triton X100 in facilitating better CNT/polymer interface compared to the other, different surfactant modified MWCNTs have been used for the preparation of CNT-based PEI nanocomposite membrane. 4. Physical characteristics of MWCNTs/PEI nanocomposite membrane 4.1. Morphological studies The morphology of the resulting MWCNTs/PEI nanocomposite membrane was characterized with FESEM and AFM. The FESEM cross-section micrographs of the membranes embedded with different surfactant treated MWCNTs are shown in Fig. 5. At low magnification of the membrane cross-section, a layer of spongy finger-like structure can be clearly observed. This indicates that the addition of MWCNTs has not significantly affected the macroscopic characteristic of the resulting membranes. The presence of teardrop shaped macrovoids with different sizes in the polymer sublayer can be related to the solvent-non solvent exchange rate during the phase inversion process. However, the discussion on the formation of these macrovoids is not in the scope of this study. When higher magnification of FESEM scanning was applied, it is clearly observed that the morphology structures demonstrated by different surfactant modified MWCNT/PEI membranes were greatly differed from each other. For raw MWCNTs/PEI, the nanotubes were found intercalated within the polymer matrix due to the agglomeration of the unmodified MWCNTs arising from the existence of the repulsive forces that retarded stable dispersion. The pretreatment of MWCNTs with both anionic SDS and cationic CTAB surfactants did not result in further improvement in the dispersion of nanotubes during the formation of the nanocomposite, as presented in Fig. 5(b) and (c). High density MWCNT regions are observed within the PEI matrix, however the nanotubes are closely entangled with one another and agglomerated clusters are clearly observed. The result obtained can be explained by the poor dispersion of these MWCNTs within the polymer matrix due to the failure of these surfactants in facilitating good adhesion at the nanotube/polymer interfaces. Therefore, FESEM examination has
verified that the treatment with these surfactants is not effective for the compatible interaction of the MWCNTs and PEI as no significant improvement of the nanotubes dispersion is observed from the micrographs. In contrast, the Triton X100 modified MWCNTs/PEI has exhibited reduced agglomeration of the nanotubes within the polymer matrix as shown in Fig. 5(d). The nanotubes are found dispersed within the PEI polymer matrix, thus confirming the quality of the dispersion as evidenced in the dispersion test discussed in the earlier section. Despite that, a promising adhesion between nanotubes and the polymer matrix can also be seen which indicates that the presence of Triton X100 surfactant has favoured the dispersion of the nanotubes. The complementary information on the distribution of the MWCNTs within the PEI matrix is obtained through the AFM topography images as illustrated in Fig. 6. The surface roughness observed upon the addition of the MWCNTs into the polymer matrix is an evidence of the spreading over of the nanotubes on the surface of the nanocomposite membranes. Large scale dispersion of Triton X100 treated MWCNTs homogeneously distributed at the top layer of the membrane can be easily identified compared to the raw as well as the SDS and CTAB- treated MWCNTs. The relatively rougher surface of the Triton X100 dispersed MWCNTs/PEI suggested that the distribution of MWCNTs per unit area is greater than other surfactant modified MWCNTs. Undoubtedly, the significant improvement in the MWCNT dispersion can be attributed to the effectiveness of the non-ionic Triton X100 surfactant to render a great contribution in the wetting of MWCNTs, hence resulting in the formation of an MWCNT/PEI nanocomposite that demonstrated desired CNT-polymer interaction. From a chemistry point of view, good dispersion indicates that polymer matrix tends to interact with the nanotubes without the formation of voids at the CNT-polymer interface and hence it is an evidence of good bonding [31]. The behaviour of the surfactant dispersed MWCNTs in polymer matrix has demonstrated the dependency of the resulting properties of a CNT/polymer nanocomposite on the types of surfactant manipulated during the pretreatment process. 4.2. Thermal properties The thermal behaviour of the neat PEI membranes and the different surfactant dispersed MWCNTs/PEI membranes were studied by TGA analysis. The results obtained are tabulated as
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Fig. 5. The FESEM cross-section micrographs of A) raw MWCNTs/PEI, B) SDS-MWCNTs/PEI, C) CTAB-MWCNTs/PEI and D) Triton X100-MWCNTs/PEI at low magnification (250) and (aed) at high magnification (50,000). A promising adhesion between nanotubes and the polymer matrix can be observed in Triton X100-MWCNTs/PEI.
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Fig. 6. AFM images a) neat PEI, b) raw MWCNTs/PEI, c) SDS-MWCNTs/PEI, d) CTAB-MWCNTs/PEI and e) Triton X100-MWCNTs/PEI. The degree of PEI polymer surface roughness increased upon the addition of MWCNTs.
shown in Table 1. The onset decomposition temperature of all samples was observed around 550 C. However, all the MWCNTs embedded nanocomposite membranes have exhibited the onset decomposition temperature that was slightly higher than that of without MWCNTs. This observation shows that the presence of MWCNTs helps to enhance the thermal degradation stability of the resulting MWCNTs/PEI membranes. It has been reported that MWCNTs improved the thermal stability of a nanocomposite by providing high heat-resistant effect and mass transport barrier for the volatile substances produced during the thermal decomposition [32]. However, the magnitude of change observed was found depending on the type of MWCNTs incorporated into the PEI matrix. A more remarkable increase has been observed for MMMs Table 1 The TGA decomposition and glass transition temperature of different surfactant dispersed MWCNTs/PEI membranes. Membrane
TGA decomposition temperature ( C)
Tensile strength (MPa)
Neat PEI Raw-MWCNTs/PEI SDS-MWCNTs/PEI CTAB-MWCNTs/PEI Triton X100-MWCNTs/PEI
549 552 556 558 558
16.2 15.7 14.6 17.4 18.3
embedded with CTAB and Triton X100 treated MWCNTs implying that the thermal stability of the composite membrane was improved due to the enhanced compatibility of the nanotubes within the polymer composite. 4.3. Mechanical strength Thermal properties alone could not be used as the standards to distinguish the dispersion ability of MWCNTs dispersed with different surfactants. Therefore, mechanical test has been adopted to verify the dispersion property of the MWCNTs reinforcement filler within the polymer composite. The tensile strength of the polymer composites incorporated with different MWCNTs is listed in Table 1. The addition of as-grown and SDS modified MWCNTs as reinforcement material has negatively affected the mechanical properties, which undoubtedly due to the tendency of these MWCNTs to agglomerate and hence resulted in non-homogeneous distribution within the polymer matrix. A considerably improvement has been demonstrated by the CTAB dispersed MWCNT/PEI while greater tensile strength was recorded for Triton X100 dispersed MWCNT/PEI. The observation clearly indicated that, the interaction between nanotubes and polymer, which governed the mechanical strength of the CNT-polymer composite was enhanced, attributed to the better
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dispersion of MWCNTs during the fabrication and hence contributed towards good load transfer characteristic. 5. Conclusion This study elucidated the importance of the interfacial interaction between the CNT surface and the surfactant, as well as with the solvent in playing a crucial fundamental role for the load transfer between the CNT fillers and polymer matrix. Triton X100 has turned out to be that most promising surfactant for the dispersion of hydrophobic MWCNT in NMP. The stability of the Triton X100 dispersed MWCNTs in the solvent was found to be promising for the preparation of nanocomposite membrane using MWCNTs as the filler. The morphological studies of the resulting MWCNT/PEI membranes conducted through FESEM and AFM confirmed the well distribution of the Triton X100 dispersed MWCNTs within the PEI polymer matrix. TGA analysis has suggested that the thermal stability of the nanocomposite membranes has been improved upon the addition of the nanotubes. Further complementary evaluation on the mechanical strength has proven the capability of the Triton X100 dispersed in enhancing the mechanical stability that related to the better dispersion of the nanotubes. The characterizations concluded that the nanotubeepolymer binding is very dependent on the surfactants used to facilitate enhanced dispersion of MWCNTs. This study has highlighted the important role played by the surfactant in which with the proper selection of dispersing agent, it is possible to achieve excellent dispersion for the preparation of MWCNTs embedded nanocomposite membrane with desired properties. MWCNT/PEI composite membrane prepared in this study has demonstrated promising interaction of MWCNT fillers with the polymer host matrix which can be attributed to the surface modification that has resolved the nanotube aggregation and subsequently facilitates a considerably well dispersion in the polymer matrix. It is anticipated that the resulting Triton X100-MWCNTs/PEI membrane has great potential for application in gas separation imputed by the presence of MWCNTs that offers abundant microchannels that favour the gas permeability by decreasing the gas diffusion resistance. The feasibility of CNT-based polymer membrane for this purpose is currently evaluated and will be reported in the near future.
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Surfactant dispersed multi-walled carbon nanotube/polyetherimide nanocomposite membrane P.S. Goh, B.C. Ng, A.F. Ismail*, M. Aziz, S.M. Sanip Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2010 Received in revised form 12 August 2010 Accepted 19 September 2010 Available online 13 October 2010
Carbon nanotube based nanocomposite membranes have been fabricated through solution casting by embedding multi-walled carbon nanotubes (MWCNTs) within polyetherimide (PEI) polymer host matrix. In order to achieve fine dispersion of nanotubes and facilitate strong interfacial adhesion with the polymer matrix, the nanotubes were first treated with surfactants of different charges, namely anionic sodium dodecyl chloride, cationic cetyl trimethyl ammonium chloride and non-ionic Triton X100, prior to the dispersion in the PEI dope solution. Dispersion of MWCNTs in N-methyl-2-pyrrolidone solvent showed that the agglomeration and entanglement of the nanotubes were greatly reduced upon the addition of Triton X100. Scanning electron microscopy and atomic force microscopy examination has evidenced the compatibility of Triton X100 dispersed MWCNTs with the polymer matrix in which a promising dispersion and adhesion has been observed at the MWCNT-PEI interface. The increase in both thermal stability and mechanical strength of the resulting Triton X100 dispersed MWCNT/PEI nanocomposite indicated the improved interaction between MWCNTs and PEI. This study demonstrated the role of Triton X100 in facilitating the synergetic effects of MWCNTs and PEI where the resulting composite membrane is anticipated to have potential application in membrane based gas separation. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Carbon nanotubes Polymer matrix composite Surface treatments
1. Introduction The field of carbon nanotubes (CNTs) research has attracted a great deal of worldwide attention driven by their remarkable potential and unique properties which lead to many promising applications. A great focus is devoted to their exceptional thermal, electrical and mechanical properties which facilitate the viability of this material to be served as compatible filler for the development of a new generation of nanostructured CNT-based composite material. The current interests in this area have exceptionally blossomed within the past decade and the most well established composite material is most probably CNT reinforced polymer nanocomposites [1e4]. A substantial improvement in the desired properties can be achieved through the synergetic effects resulted from the intimately mixed polymer matrix and the CNT fillers in which the resulting materials possess the unique combination of physical and mechanical properties and as multifunctional systems with great potential that are not present in conventional polymer matrix composite. In fact, CNT-based polymer composites have spurred enormous interest in the community of materials primarily due to
* Corresponding author. Tel.: þ6075535592; fax: þ6075535925. E-mail address: [email protected] (A.F. Ismail). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.09.017
their potential to demonstrate remarkable increase in strength and thermal stability when compared to other conventional and typical carbon black or silica-based polymer composite [5,6]. In order to obtain a hybrid system with excellent and superior properties, several parameters have to be taken into account during the fabrication process, including the size of the filler particles, the degree of dispersion in the matrix, as well as the degree of adhesion with the polymer chains [7,8]. The full compatibility of the CNT filler with evenly distributed particle size and the polymer matrix will in turn result in high quality and homogeneously dispersed CNT-polymer nanocomposite [9]. Furthermore, the strong interfacial interaction will give rise to an effective load transfer from the polymer matrix to the CNTs. To exploit the utility of CNTs for this purpose, uniform dispersion of CNTs in the polymer matrix is required. Unfortunately, the use of CNTs in this application has been largely limited by their poor processability and dispersibility [10e13]. Besides the presence of intrinsic van der Waals force, their high surface area and aspect ratio of CNTs have rendered to the formation of highly stabilized bundles which in turn resulting in the formation of tight bundles and hollow ropes. In addition to that, the relatively smooth CNT surfaces which lack of interfacial bonding have also restricted the effective load transfer [14,15]. There have been several ways to produce CNT-polymer nanocomposites. The most commonly used is probably through polymer
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solution casting which possesses few advantages over the in-situ polymerization and melt processing. Preparation through polymer solution casting allows the use of polymers which were previously not suitable for in-situ polymerization [16,17]. During the polymer dissolving and agitating process, the most crucial part is the dispersion of CNT agglomerations before the evaporation stage. However, the economical scale up of CNT nanocomposite manufacturing remains hurdle as it is usually difficult to achieve homogeneous dispersion of CNTs throughout the polymer matrix. To date, the efforts made towards the controlling of CNT aggregation remain as a challenge. The existing challenges of poor dispersion and interfacial bonding in the CNT-polymer matrix are being addressed with some forms of improvement through the modification of CNT surfaces in order to create stronger CNT arrayepolymer interface adhesion. A surface modification of CNTs is generally classified in chemical and physical means. The former is known to be an effective method as it allows high quality dispersions to be obtained at high CNT loading in polymer matrix but unfortunately it always leads to remarkable damages and distortion to the structure of CNTs due to the oxidation or surface interaction [18,19]. The latter modification is therefore becoming an attractive mean of CNT surface modification. The surface modification of CNTs is carried out using third phase dispersing agent such as surfactants, with the assistance of sonication for a period of time to debundle the CNTs [20e22]. Unlike the harsh reaction condition experienced by the chemical modification, this physical modification is relatively mild and only involves the non-covalent bonding without disturbing the overall structure of the graphite sheets, hence maintaining the pristine properties of the CNTs. CNTs subjected to ultrasonication appeared shorter due to the sonication-induced nanotube cutting and the tips opening of the CNTs. At the same time, the surfactant molecules are uniformly built up on the tube walls. After the adsorption of surfactant molecules on the nanotube surface, ultrasonication for minutes or hours may helps the surfactant to disperse the nanotube bundles by steric or electrostatic repulsion. A recent report [23] has also underlined the ability of the surfactant coated nanotubes to maintain high quality dispersions at concentration approaching 1 mg/mL with reasonably large populations of individual singlewalled carbon nanotubes (SWCNTs). According to Shvartzman [24], mechanical exfoliation of the bundles prior to surface treatment must occur in order to obtain individual CNTs. The stabilization of aqueous suspension of CNTs using surfactants gives rise to the introduction of surface charges onto the CNTs depends on the nature of surfactant, which is either cationic, anionic or non-ionic properties. A considerable amount of works related to the dispersion of CNTs with different types of surfactants have been carried out [25e27]. In most of the cases, ultrasonication has been applied to disperse the highly entangled carbon nanotubes. In particular, the utilization of anionic sodium dodecyl chloride (SDS) as stabilizer has been widely studied due to its excellent CNT stabilization and separation capabilities. Yu et al. [25] have demonstrated the dispersion of MWCNTs in aqueous SDS solution. The surfactant molecules were found adsorbed on the surface of nanotubes and hence prevent the re-aggregation by electrostatic repulsion for several months. In a recent study by Karousis et al. [26], aqueous SDS has been applied to induce a stable aqueous colloidal dispersion of MWCNTs and significant solubility of the modified MWCNTs in polar solvent was also observed. Nonionic surfactant such as Triton X350 has also been used to functionalize CNTs in aqueous solution as it was found to interact strongly with graphite surfaces [11]. Although a vast number of papers dealing with dispersion of CNTs using surfactants have been published to date, the selection of suitable surfactant to disperse the as-grown CNTs prior to the
preparation of CNT-polymer nanocomposite has been scarcely reported. It is an important aspect of consideration as the proper choice of the surfactant is essential to ensure an effective surface wrapping of CNTs. The influence of the surfactant molecules is closely related to their ionic and non-ionic characteristics. To address the current issue, the objective of this work is to prepare a surfactant dispersed MWCNTs hence to evaluate the effects of varying types of surfactant on the dispersion property of the CNTs. The surfactant dispersed MWCNTs were subsequently used as the nanofiller for the realization of CNTs based polyetherimide (PEI) membrane through polymer solution casting. The purpose of varying the type of surfactant is to investigate the dispersion behaviour rendered by the surfactants and hence correlate to the physical properties of the resulting nanocomposite. 2. Experimental 2.1. Materials MWCNTs were prepared by catalytic decomposition of acetylene. The commercial available alumina hydrate powder purchased from Fluka was used as the support of metallic acetate (Fe/Co) catalyst for the synthesis of MWCNTs. In the catalytic processes, nitrogen was passed through the quartz tube as the furnace was heated to reach 700 C. The as-grown MWCNTs were collected as loosely aggregated fluffy black powders. The surfactants possessing different charges, namely anionic sodium dodecyl chloride (SDS) purchased from Merck, cationic cetyl trimethyl ammonium chloride (CTAB) purchased from Fluka and non-ionic Triton X100 supplied by J.T. Baker were used as dispersing agent of MWCNTs. PEI was used as polymer material, supplied by General Electric Co. (USA) under the trade name ‘Ultem 1000’. N-methyl-2-pyrrolidone (NMP) supplied by Sigma Aldrich was used as solvents for PEI. 2.2. Dispersion of MWCNTs The concentration of each type of surfactant in aqueous solution was fixed at 1 wt%. All solutions were prepared by mixing 0.5 g of MWCNTs with 100 cm3 of aqueous surfactant solution. After sonication for 60 min, the surfactant treated MWCNT was filtered from the solution in order to remove excess surfactants. The samples were then dried overnight in oven at 60 C. 2.3. Preparation of MWCNT/PEI nanocomposites To fabricate MWCNT/PEI nanocomposite, the surfactant dispersed MWCNTs were blended with PEI in NMP to obtain a polymer concentration of 25 wt%. The temperature and stirring speed were controlled so that the PEI dissolved completely to form a yellowish clear solution. Appropriate amount of surfactant dispersed MWCNTs was added to obtain a fixed CNT loading of 1 wt % in the resulting polymer composite to avoid agglomeration of CNTs that likely to occur at high concentration. The mixture was then stirred to obtain a homogeneous suspension. Flat sheet membranes were prepared according to the dry/wet phase separation process. The polymer solution was cast on a clean glass plate using a simple pneumatically-controlled membrane casting system under a fixed casting speed. The membranes were then immersed in water that acts as coagulation medium to allow wet phase inversion process. The resulting MWCNTs/PEI flat sheet membranes, with thickness of 200 mm, were left in water immersion for overnight to enable complete solvent exchange before drying in atmosphere for 24 h.
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2.4. Characterizations Morphological study of the surfactant dispersed MWCNTs and the resulting nanocomposite membranes was carried out using field emission electron scanning electron microscope (FESEM) and transmission electron microscope (TEM). A thin layer of gold was coated on the specimens before scanning with the FESEM (Phillips SEMEDAX, XL40, PW6822/10) while the specimens for TEM observation were prepared by ultrasonicating MWCNT in alcohol and dropped on a copper grid. Surface roughness of the resulting composite membrane was examined through JEOL, JSPM-5200TM atomic force microscope (AFM). Thermal analysis was done through thermogravimetric analysis (TGA) in the temperature range of 30e800 C in air at a heating rate of 10 C/min. The dispersion property of the surfactant dispersed MWCNTs were studied by observing the settling speed of the sample powders in NMP solvent. Approximately 0.05 g of surfactant dispersed MWCNTs were added to the vials containing 15 ml of NMP solvent. The tensile strength was measured using mechanical testing system, Universal testing machine (Llyod EZ 20). The testing was conducted with tensile rate of 5 mm/min and 5 measurements were performed and the results were quoted as average values. The correlation between the attached surface surfactant molecules and dispersion of the MWCNTs in the PEI polymer host matrix are discussed. 3. Results and discussion 3.1. Physical characterizations of surfactant/MWCNTs The FESEM micrographs of the resulting surfactant dispersed MWCNTs are demonstrated in Fig. 1. No remarkable physical damages or shortening of the MWCNTs are observed for all samples, which indicates that treatment with surfactants is a mild and physical surface modification that conserved the morphology structure of the nanotubes. The as-grown MWCTs (Fig. 1a) consist
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of nanotubes that are folded and agglomerations of tubes can be obviously seen. The modification with surfactants was thought to contribute to the debundling of the nanotubes. However, the morphology examinations pointed out that not all surfactants could play the same role during the dispersion of MWCNTs in the surfactant solution. It was noticed that the extent of nanotube debundling is surfactant dependant. The surfactants used for the surface modification possessed different affinity towards the surfaces of MWCNTs, which in turn resulted in the different degree of effectiveness in reducing the agglomeration of MWCNTs. It is in common consent that it is very difficult to entirely unfold and debundle all nanotubes into individual tubes, but to the least extent, it has been particularly observed that the Triton X100 dispersed MWCNTs appeared to be less bundled, which implied that Triton X100 is the most promising surfactant to debundle the MWCNTs. The thickening of the graphite walls has been observed through the high resolution scanning. The TEM micrograph of Triton X100 dispersed MWCNTs shown in Fig. 2 reveals that the surfaces appeared to be coarser than the raw MWCNTs due to the coating of surfactant molecules along the axial of the nanotubes. The TGA weight loss curves of raw MWCNTs and surfactant dispersed MWCNTs are depicted in Fig. 3. The dependence of weight loss and heat evolution on the temperature program of the samples was compared and served as an indicator to the amount of surfactant molecules that have been attached onto the sidewalls of MWCNTs, in which the greater the weight loss, the greater the amount of carbon materials or the organic functional groups present in the modified samples [28]. In general, greater weight loss has been observed for surfactant coated MWCNTs indicated that more carbonaceous compound existed in the samples. For all the surfactant treated samples, the initial stage of weight loss occurred in the temperature range of 100e300 C which corresponded to water evaporation as well as the decomposition of volatile compound. As-grown MWCNTs showed a relatively simple monotonic degradation pattern due to the lack of surface functional
Fig. 1. The FESEM micrographs (magnification: 50,000) of the a) raw MWCNT, b) SDS-MWCNTs, c) CTAB-MWCNTs and d) Triton X100-MWCNTs. The morphological structures of the nanotubes are conserved upon the mild physical surfactant dispersion.
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Fig. 2. The TEM micrograph of (a) raw MWCNTs and (b) Triton X100-MWCNTs. The comparison of the morphology revealed that the outer layer of CTAB dispersed nanotubes was apparently rougher due to the coating of CTAB molecules.
groups. The unusually weight gain demonstrated by as-grown MWCNTs is mainly due to the oxidation of metal catalyst into solid oxides [29]. The weight loss in the range of 300e500 C corresponds to the decomposition of amorphous carbon and the hydrocarbon functional groups that have been attached onto the walls of MWCNTs. All samples demonstrated significant weight loss in the temperature range of 400e600 C, which is mainly due to the degradation of graphene walls and oxidation of the MWCNTs into CO2. Due to the difference in amount of carbonaceous compounds that present in the surfactants, the pattern of the weight loss is found greatly differed from each other. However, the multidecomposition stages demonstrated in the thermogram indicates that the physical modification with surfactants does not involve the structure changes and the thermal properties of both nanotubes and surfactants were preserved. 3.2. Dispersion behaviour of surfactant/MWCNTs in NMP The dispersion test was conducted in NMP solvent as NMP appears to be one of the best solvents to disperse CNT and it is a commonly used solvent for the preparation of polymer based composite. Fig. 4 shows the settling pictures of raw MWCNTs and surfactant treated MWCNTs in NMP solvent. Visual characteristic of the dispersion differed drastically depending on the types of surfactant that dispersed onto the MWCNT surfaces. Sedimentation in NMP solvent can be obviously observed for the CTAB-MWCNTs and raw MWCNTs. The particles were agglomerated and caused the
Fig. 3. The TGA weight loss curves of a) raw MWCNT, b) SDS-MWCNTs, c) CTABMWCNTs and d) Triton X100-MWCNTs. All surfactant treated MWCNTs showed weight loss regions that corresponded to the organic compounds that dispersed onto MWCNTs.
solid precipitates to coexist with an almost transparent liquid supernatant. Raw MWCNTs have poor wetting and dispersion behaviour due to the strong van der Waals interaction between the sidewalls of the MWCNTs meanwhile the dispersion with CTAB did not show any effect on the dispersibility of the MWCNTs. MWCNTs modified with SDS were found apparently dispersed in the NMP solvent, but the suspension was not been well maintained as the sedimentation can be clearly observed after some days. Noticeable improvement has been observed for the suspension of Triton X100 dispersed MWCNTs in the solvent. During the surfactant modification of MWCNTs, ultrasonication helped to attach the Triton X100 molecules on the surface of the nanotubes and prevent the further aggregation that takes place due to the presence of van der Waal attraction. The suspension has demonstrated better dispersion and the colloidal stability can be maintained up to 30 days. When the bottom of vial was carefully checked, no sedimentation of black powder was observed. The results obtained with Triton X100 treated MWCNTs clearly showed that the surfactant played a significant role for the dispersion of the nanotubes in NMP solvent. The presence of dispersing CTAB on MWCNTs has repressed the effective dispersion meanwhile SDS did not contribute to long last stability of the suspension. To explain this different behaviour, it is necessary to imply the interfacial characteristic based on two factors: i) the surface interaction of Triton X100 with MWCNTs. Non-ionic surfactant seems to be rather promising for the stabilization of CNTs as this type of surfactant was evidenced to interact strongly with the graphite surface and hence enhanced the suspension stability [30]. In this case, the surfactant molecules adsorbed onto the exterior CNT surfaces along the axial length by their hydrophobic segments, leaving the hydrophilic tail directed to the exterior that eventually resulting in nanotube repulsion and leading to enhanced dispersion. At the surfactant concentration up to the critical micelle concentration (CMC) which is 0.2 mM for Triton X100, several surfactant monolayers were formed near the MWCNT surfaces and subsequently contribute to an outstanding MWCNT suspension stabilization [10]; ii) the interaction of surfactant coated MWCNTs with NMP solvent. The similar polarity between the polyethylene glycol parts and solvent having polar hydrophilic characters might act in an attractive manner and facilitate much stronger effective interaction between the MWCNTs and solvent compared to other surfactants modified MWCNTs. During the formation of CNT-based polymer nanocomposite, it is crucial to achieve good dispersion to take advantage of the high surface area of nanofillers [13]. It is interesting to note that, the dispersion of different types of surfactant on the surface of asgrown MWCNT can be differed, depending on the morphology of
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Fig. 4. Settling picture of a) raw MWCNT, b) SDS-MWCNTs, c) CTAB-MWCNTs and d) Triton X100-MWCNTs in NMP solvent. Triton X100-MWCNTs has demonstrated colloidal stability that can be maintained up to 30 days.
the MWCNTs. The dispersion test demonstrated that, by properly choosing a suitable surfactant with good dispersion, the segregation of MWCNTs in the organic solvent can be greatly suppressed. In this study, in order to obtain further confirmation on the roles of Triton X100 in facilitating better CNT/polymer interface compared to the other, different surfactant modified MWCNTs have been used for the preparation of CNT-based PEI nanocomposite membrane. 4. Physical characteristics of MWCNTs/PEI nanocomposite membrane 4.1. Morphological studies The morphology of the resulting MWCNTs/PEI nanocomposite membrane was characterized with FESEM and AFM. The FESEM cross-section micrographs of the membranes embedded with different surfactant treated MWCNTs are shown in Fig. 5. At low magnification of the membrane cross-section, a layer of spongy finger-like structure can be clearly observed. This indicates that the addition of MWCNTs has not significantly affected the macroscopic characteristic of the resulting membranes. The presence of teardrop shaped macrovoids with different sizes in the polymer sublayer can be related to the solvent-non solvent exchange rate during the phase inversion process. However, the discussion on the formation of these macrovoids is not in the scope of this study. When higher magnification of FESEM scanning was applied, it is clearly observed that the morphology structures demonstrated by different surfactant modified MWCNT/PEI membranes were greatly differed from each other. For raw MWCNTs/PEI, the nanotubes were found intercalated within the polymer matrix due to the agglomeration of the unmodified MWCNTs arising from the existence of the repulsive forces that retarded stable dispersion. The pretreatment of MWCNTs with both anionic SDS and cationic CTAB surfactants did not result in further improvement in the dispersion of nanotubes during the formation of the nanocomposite, as presented in Fig. 5(b) and (c). High density MWCNT regions are observed within the PEI matrix, however the nanotubes are closely entangled with one another and agglomerated clusters are clearly observed. The result obtained can be explained by the poor dispersion of these MWCNTs within the polymer matrix due to the failure of these surfactants in facilitating good adhesion at the nanotube/polymer interfaces. Therefore, FESEM examination has
verified that the treatment with these surfactants is not effective for the compatible interaction of the MWCNTs and PEI as no significant improvement of the nanotubes dispersion is observed from the micrographs. In contrast, the Triton X100 modified MWCNTs/PEI has exhibited reduced agglomeration of the nanotubes within the polymer matrix as shown in Fig. 5(d). The nanotubes are found dispersed within the PEI polymer matrix, thus confirming the quality of the dispersion as evidenced in the dispersion test discussed in the earlier section. Despite that, a promising adhesion between nanotubes and the polymer matrix can also be seen which indicates that the presence of Triton X100 surfactant has favoured the dispersion of the nanotubes. The complementary information on the distribution of the MWCNTs within the PEI matrix is obtained through the AFM topography images as illustrated in Fig. 6. The surface roughness observed upon the addition of the MWCNTs into the polymer matrix is an evidence of the spreading over of the nanotubes on the surface of the nanocomposite membranes. Large scale dispersion of Triton X100 treated MWCNTs homogeneously distributed at the top layer of the membrane can be easily identified compared to the raw as well as the SDS and CTAB- treated MWCNTs. The relatively rougher surface of the Triton X100 dispersed MWCNTs/PEI suggested that the distribution of MWCNTs per unit area is greater than other surfactant modified MWCNTs. Undoubtedly, the significant improvement in the MWCNT dispersion can be attributed to the effectiveness of the non-ionic Triton X100 surfactant to render a great contribution in the wetting of MWCNTs, hence resulting in the formation of an MWCNT/PEI nanocomposite that demonstrated desired CNT-polymer interaction. From a chemistry point of view, good dispersion indicates that polymer matrix tends to interact with the nanotubes without the formation of voids at the CNT-polymer interface and hence it is an evidence of good bonding [31]. The behaviour of the surfactant dispersed MWCNTs in polymer matrix has demonstrated the dependency of the resulting properties of a CNT/polymer nanocomposite on the types of surfactant manipulated during the pretreatment process. 4.2. Thermal properties The thermal behaviour of the neat PEI membranes and the different surfactant dispersed MWCNTs/PEI membranes were studied by TGA analysis. The results obtained are tabulated as
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Fig. 5. The FESEM cross-section micrographs of A) raw MWCNTs/PEI, B) SDS-MWCNTs/PEI, C) CTAB-MWCNTs/PEI and D) Triton X100-MWCNTs/PEI at low magnification (250) and (aed) at high magnification (50,000). A promising adhesion between nanotubes and the polymer matrix can be observed in Triton X100-MWCNTs/PEI.
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Fig. 6. AFM images a) neat PEI, b) raw MWCNTs/PEI, c) SDS-MWCNTs/PEI, d) CTAB-MWCNTs/PEI and e) Triton X100-MWCNTs/PEI. The degree of PEI polymer surface roughness increased upon the addition of MWCNTs.
shown in Table 1. The onset decomposition temperature of all samples was observed around 550 C. However, all the MWCNTs embedded nanocomposite membranes have exhibited the onset decomposition temperature that was slightly higher than that of without MWCNTs. This observation shows that the presence of MWCNTs helps to enhance the thermal degradation stability of the resulting MWCNTs/PEI membranes. It has been reported that MWCNTs improved the thermal stability of a nanocomposite by providing high heat-resistant effect and mass transport barrier for the volatile substances produced during the thermal decomposition [32]. However, the magnitude of change observed was found depending on the type of MWCNTs incorporated into the PEI matrix. A more remarkable increase has been observed for MMMs Table 1 The TGA decomposition and glass transition temperature of different surfactant dispersed MWCNTs/PEI membranes. Membrane
TGA decomposition temperature ( C)
Tensile strength (MPa)
Neat PEI Raw-MWCNTs/PEI SDS-MWCNTs/PEI CTAB-MWCNTs/PEI Triton X100-MWCNTs/PEI
549 552 556 558 558
16.2 15.7 14.6 17.4 18.3
embedded with CTAB and Triton X100 treated MWCNTs implying that the thermal stability of the composite membrane was improved due to the enhanced compatibility of the nanotubes within the polymer composite. 4.3. Mechanical strength Thermal properties alone could not be used as the standards to distinguish the dispersion ability of MWCNTs dispersed with different surfactants. Therefore, mechanical test has been adopted to verify the dispersion property of the MWCNTs reinforcement filler within the polymer composite. The tensile strength of the polymer composites incorporated with different MWCNTs is listed in Table 1. The addition of as-grown and SDS modified MWCNTs as reinforcement material has negatively affected the mechanical properties, which undoubtedly due to the tendency of these MWCNTs to agglomerate and hence resulted in non-homogeneous distribution within the polymer matrix. A considerably improvement has been demonstrated by the CTAB dispersed MWCNT/PEI while greater tensile strength was recorded for Triton X100 dispersed MWCNT/PEI. The observation clearly indicated that, the interaction between nanotubes and polymer, which governed the mechanical strength of the CNT-polymer composite was enhanced, attributed to the better
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dispersion of MWCNTs during the fabrication and hence contributed towards good load transfer characteristic. 5. Conclusion This study elucidated the importance of the interfacial interaction between the CNT surface and the surfactant, as well as with the solvent in playing a crucial fundamental role for the load transfer between the CNT fillers and polymer matrix. Triton X100 has turned out to be that most promising surfactant for the dispersion of hydrophobic MWCNT in NMP. The stability of the Triton X100 dispersed MWCNTs in the solvent was found to be promising for the preparation of nanocomposite membrane using MWCNTs as the filler. The morphological studies of the resulting MWCNT/PEI membranes conducted through FESEM and AFM confirmed the well distribution of the Triton X100 dispersed MWCNTs within the PEI polymer matrix. TGA analysis has suggested that the thermal stability of the nanocomposite membranes has been improved upon the addition of the nanotubes. Further complementary evaluation on the mechanical strength has proven the capability of the Triton X100 dispersed in enhancing the mechanical stability that related to the better dispersion of the nanotubes. The characterizations concluded that the nanotubeepolymer binding is very dependent on the surfactants used to facilitate enhanced dispersion of MWCNTs. This study has highlighted the important role played by the surfactant in which with the proper selection of dispersing agent, it is possible to achieve excellent dispersion for the preparation of MWCNTs embedded nanocomposite membrane with desired properties. MWCNT/PEI composite membrane prepared in this study has demonstrated promising interaction of MWCNT fillers with the polymer host matrix which can be attributed to the surface modification that has resolved the nanotube aggregation and subsequently facilitates a considerably well dispersion in the polymer matrix. It is anticipated that the resulting Triton X100-MWCNTs/PEI membrane has great potential for application in gas separation imputed by the presence of MWCNTs that offers abundant microchannels that favour the gas permeability by decreasing the gas diffusion resistance. The feasibility of CNT-based polymer membrane for this purpose is currently evaluated and will be reported in the near future.
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