Microstructure and water vapor transport properties of functionalized carbon nanotube-reinforced dense-segmented polyurethane composite membranes.
Since the discovery of C60 by Kroto in 1985 (1) and its large scale synthesis in 1990 (2), carbon nanotubes (CNT) have attracted considerable attention as reinforcing fillers in polymer matrices (3-5) due to their unique physical and mechanical properties, including high-axial Young's modulus and high-specific surface area (6). However, one approach to a successful CNT polymer composite is to achieve homogeneous dispersion of CNTs in the polymer matrix through strong interfacial interactions. Because of their small size and hydrophobic nature, CNTs are difficult to disperse in the polymer matrix. Attachment of small organic molecules (functionalization) helps to disperse the nanotubes in organic solvent. Research studies revealed that functionalized C60 is a more effective reinforcing filler than unfunctionalizcd C60 (3), (7-9). Moreover, theoretical studies suggested that the functionalization of CNTs did not greatly decrease their strength (7). It was observed that the amino group of aniline readily reacts with carbon nanotubes. Dissolution of nanotubes in aniline could be observed by the color change of the solution after a short period of refluxing (10). The solubility of single wall carbon nanotubes in aniline is up to 8 mg/ml. This aniline-nanotube solution could be easily diluted with other solvents such as acetone, tetrahydrofuran (THF), and dimethylformamide (DMF).
Segmented polyurethanes (SPU) can present microphase-separated structure due to the thermodynamic incompatibility between the constituent segments (11). Phase transition at glass transition ([T.sub.g]) or soft segment crystal melting point ([T.sub.ms]) temperature accompanies a great change in thermomechanical property of the SPU. In addition to the change of thermomechanical property of SPU, it was observed that SPU also has a large change in moisture permeability above and below the [T.sub.g]/[T.sub.ms] (12), (13). This significant change of water vapor transport behavior will be useful for SPU-coated textile that could provide thermal insulation at cold temperature and high permeability above [T.sub.g]/[T.sub.ms]. In application of waterproof breathable coating/lamination, the temperature dependency of water vapor permeability is an important aspect as far as comfort is considered. If the textiles materials coated/laminated with SPU would use in clothing applications, they could provide greater comfort in both cold and warm climates. Carbon nanotubes have many attractive properties, such as excellent conductivity (14), UV blocking (15), and high axial strength: therefore, the MWNT-containing SPU-coated or laminated fabric could find applications in advanced textile area.
It is well known that the additives such as plasticizer and filler soften or harden polymer-additive systems, respectively (16). A polymer-plasticizer system could depress the [T.sub.g], which leads to an enhanced segmental motion as a result of large permeability. On the other hand, the polymer-filler system often raises the [T.sub.g] value, which is indicative of the restriction of segmental motion between the polymer chains due to the strong polymer-filler interactions. The permeability of dense membranes decreases with increasing filler content. However, in our knowledge, there are no such literatures on water vapor transport properties on CNT-SPU composite membranes. Therefore, it will be interesting to study the structure and mass-transfer properties of carbon nanotube filler-reinforced polyurethane composite. In this study, we have investigated the influence of MWNT in SPU microstructure and water transport properties. SPU based on polytetra methylene glycol (PTMG) (number-average molecular weight 2900 g [mol.sup.-1]; PTMG 2900), 4,4'-diphenylmethane diisocyanate (MDI), and 1,4-butane diol (1,4-BDO) was modified with polyethylene glycol (PEG) (number-average molecular weight 3400 g [mol.sup.], PEG 3400) to enhance the permeability. SPUs were reinforced with 0.25, 0.50, 1, and 2.5% (calculate based on the solid content of the polymer) of the functionalized (with aniline) multiwall carbon nanotube (MWNT). The structures of SPU-MWNT were studied by wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), transmission electron microscopy (TEM), and water vapor transport properties were measured by equilibrium sorption and water vapor permeability measurements.
All chemicals and MWNT used in this study were obtained from Aldrich. Diameter and length of the MWNT were 2-15 nm and 1-10 [micro]m, respectively. The multiwall carbon nanotube core is surrounded by a fused carbon shell, and it has ~5-20 graphitic layers. PTMG 2900 was vacuum oven dried at 80[degrees]C for 12 h, and PEG 3400 was vacuum oven dried at 80[degrees]C for 4 h before use. 1,4-BDO and DMF were dried over 4 [Angstrom] molecular sieve. MDI was used as received.
SPU was synthesized from bifunctional diisocyanate (MDI), and diol such as, PTMG 2900 and PEG 3400, and chain extender (1,4-BDO), by two-step polymerization process. To get linear polymer, the mole ratio of CNO to OH was kept at 1.0:1.0. The detail procedure for synthesis of SPU and preparation of functionalized MWNT-SPU solution were described elsewhere (15).
SPU nanotube solutions were prepared with four different concentration of MWNT such as 0.25, 0.50, 1.0, and 2.5 wt/wt% (weight percent with respect to polymer solid content). Functionalized and diluted MWNT solutions were added with the previously synthesized SPU, and then the mixture was homogenized with high-speed stirring at room temperature for 12 h. The final SPU-MWNT solution concentration was about 15% (w/w). Membranes were cast from further diluted SPU-MWNT solution (concentration about 5% w/v) in DMF on Teflon-coated steel plate. To get the defect free membrane, solvent was evaporated slowly at 60[degrees]C for 12 h, and the final residual solvent was removed under vacuum at 80[degrees]C for another 12 h. Subsequently, Teflon plates were taken out from the vacuum oven and kept at room temperature for 2 h. After 2 h, membranes were pilled out from the Teflon plate. Thickness of the membranes for thermomechanical testing was about 0.2 mm and about 90 [micro]m for water vapor transport property measurements.
Wide Angle X-ray Diffraction
X-ray data were recorded by using Philips Analytical X-Ray (Philips Xpert XRD System) at voltage of 40 kV, 40 mA current, and a radiation of wavelength 1.542 [Angstrom]. Diffraction patterns were obtained at Bragg's angle of 20 = 10.01-50[degrees]. The scan speed was 0.03 s per step.
Differential Scanning Calorimetry
PerkinElmer DSC 7 was used to measure the heat of fusion ([DELTA]H), soft segment crystal melting temperature ([T.sub.ms]), and crystallization temperature. Each samples having weight from 5 to 10 mg was scanned from --50 to 120[degrees]C at a scanning rate of 10[degrees]C [min.sup.-1] under dry nitrogen purge.
Dynamic Mechanical Thermal Analysis
Dynamic mechanical thermal analysis was performed in a tension mode by a dynamic mechanical thermal analyzer (PerkinElmer Diamond DMA Lab System) over the temperature from--150 to 120[degrees]C at a frequency of 2 Hz under liquid [N.sub.2] purging and a heating rate of 2[degrees]C [m.sup.-1].
Transmission Electron Microscopy
TEM images of SPU-MWNT membranes and pure MWNT were taken with Phillips CM 120. Sample was prepared by dispersing the SPU-MWNT solution on deionized water and putting the copper grid on the thin polymer layer on water at room temperature. Girds were taken out with films carefully and dry. Ru[O.sub.4] was used for staining the samples. Image of pure MWNT was taken without staining. All the TEM images were taken at an accelerating voltage of 120 kV.
Scanning Electron Microscopy
The dense surface morphology of film was observed with a scanning electron microscopy (SEM) made with a Leica Stereoscan 440 equipped with an Oxford energy dispersive X-ray system, operating at 20 kV.
Water Vapor Transport Property
Water vapor transport properties of membranes were measured by equilibrium sorption measurements at 10, 15, 25, 35, and 45[degrees]C and water vapor permeability (WVP) (ASTM E 96-80 B) measurements. Water vapor permeability test was performed by the cup method at 12, 18, 25, 35, and 45[degrees]C, and at 70% of relative humidity. Detail procedure for the equilibrium sorption and WVP measurements were described elsewhere (17).
RESULTS AND DISCUSSIONS
Wide Angle X-ray Diffraction
Wide angle X-ray diffraction (WAXD) results for all SPU-MWNT samples and pure soft segment (PTMG 2900) are shown in Figs. 1 and 2, respectively. Pure PTMG 2900 shows two sharp diffraction peaks, which signify the presence of crystal structure. Because of their long polymer chain and order structure, soft segment (polyol) could only form crystalline structure in the phase-separated SPU nanocomposite and pristine SPU. However, all resulted SPU-MWNT samples show broad halo at 19.11[degrees] -20.38[degrees], which may be due to the presence of small crystallites or amorphous region or by diffraction within the large crystals (18). Interestingly, DSC heating curve shows endothermic peak due to the melting of crystalline structure (see DSC results section) for SPU-MWNT samples. Therefore, the broadening of diffraction maxima is due to the small crystallites exists in the polymer matrix, which are scattered throughout the polymer matrix and could not be detectable by WAXD. A new peak appeared with the films for MWNT composite near 26.4[degrees]. The peak at 2[theta] = 26.4[degrees] was totally absent in SPU without SPU-MWNT. Zhou et al. revealed that the pristine nanotubes exhibits an intense (002) Bragg diffraction at 2[theta] = 26.1 corresponding to the intershell spacing (d = 3.4 [Angstrom]) of concentric cylinders of graphite carbon (19). Presence of this new peak in SPU-MWNT samples suggested that the interaction between the functionalized MWNT and SPU was occurred; thus, as a result of layer structure for the MWNT-reinforced SPU domain.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Differential Scanning Calorimetry
Some of the DSC results of this study have been published in our previous work (15). The differential scanning calorimetry results for pure soft segment (pure PTMG 2900), SPU, and SPU-MWNT are summarized in Table 1. From the table, we can see that all tested samples have crystal structure. Soft segment could form crystal structure in phase-separated SPU due to their long polymer chains and order structure (17). However, heat of fusion due to the melting of soft segment crystal for all SPU-MWNT samples are lower than that of the heat of fusion, calculated on the percent quantity of pure PTMG 2900 in the SPU. Presence of hard segment (1,4-BDO and MDI) acts as reinforcing filler and hinders the crystallization of soft segment in SPU/SPU-MWNT matrix. Heat of fusion of the nanocomposite films (PU-0.25) with lowest quantity of MWNT (0.25%) is slightly higher than that of sample without MWNT, suggesting that slightly higher molecular packing was obtained at 0.25 wt% of MWNT in SPU domain. Crystallization in polymer involved three steps viz primary nucleation, relatively rapid spherulitic growth, followed by a slow kinetically difficult improvement in crystal perfection (18). Little amount of nano particles could act as nucleus for the crystal growth and increase the overall order structure. Little decrease of crystallization temperature (the temperature corresponding to the maximum of the crystallization exotherm) for the SPU-MWNT composite film with 0.25 wt% of MWNT indicating that the ease of crystallization by nanotube at lower concentration of MWNT (20). With further increase of MWNT decreases the heat of fusion of SPU-MWNT due to the lower amount of crystal structure exists in the SPU. The increasing MWNT content, which act as filler and suppressed the mobility of soft segment, therefore, decreases the crystallinity of soft segment in the micro-phase-separated SPU-MWNT composites. From the DSC-cooling results, we can see that the pure PTMG 2900 (see Fig. 3) as well as all SPU/SPU-MWNT shows exothermic peaks ([T.sub.c]), which signifies the crystallization of soft segment in the DSC cooling cycle.
[FIGURE 3 OMITTED]
TABLE 1. Thermal properties of pure soft segment, SPU, and SPU-MWNT composite membrances. Samples MWNT (wt%) [DELTA]H (J [T.sub.ms] [g.sup-1] ([degrees]C) (a) PU 0 23.670 22.200 (a) PU - 0.25 0.25 25.515 22.366 PU - 0.50 0.50 24.318 22.700 (a) PU - 1.00 1.00 22.175 21.533 (a) PU - 2.50 2.50 21.857 20.700 PTMG 2900 (soft segment) -- 42.77,32.61 27.8,36.7 Samples [DELTA][H.sub.c] [g.sup.-1] (J [T.sub.c] ([degrees]C) (a) PU 29.196 -15.140 (a) PU - 0.25 29.025 -16.137 PU - 0.50 26.700 -15.034 (a) PU - 1.00 24.890 -14.875 (a) PU - 2.50 24.257 -15.150 PTMG 2900 (soft segment) 70.8 7.47 [DELTA]H, heat of fusion; [T.sub.ms], soft segment crystal melting; [DELTA][H.sub.c], heat of crystallization; [T.sub.c], crystallization temperature. (a) (reproduced with permission, ref. 15).
Dynamic Mechanical Thermal Analysis
DMTA experiments were performed to study the visco-elastic properties of MWNT-SPU composite membranes. Figures 4 and 5 show the storage modulus and loss tangent with temperature of various SPU-MWNT samples. Storage modulus (E') of the composite SPU decreases rapidly whereas tan ([delta]) goes through a maximum when the polymer is heated through the glass transition temperature. The sudden decrease of E' at glass transition temperature ([T.sub.g]) can be observed in all cases. The modulus changes with temperature as the molecular motions within the polymer chains changes. Storage modulus increases with the increasing MWNT content in the polymer matrix; this behavior is due to the strong interaction between the functionalized MWNT filler and SPU polymer matrix. Larger the interfacial area between the nanotubes and SPU, stronger the hindrance of polymer chains movement and higher would be the storage modulus. Properties of the SPU composite would depend on a number of factors; among them, interfacial adhesion between the polymer matrix and nanotubes is important. Increase of storage modulus with the increasing MWNT content signifies the uniform dispersion of functionalized carbon nanotubes in the polymer matrix. Therefore, when the force is applied to the polymer composite, MWNT could shear the load together with polymer molecules. Functionalized MWNT makes strong adhesion in polymer matrix due to better reinforcement; therefore, the increase of storage modulus is due to the stiffening effect possessed by nanotubes. In addition, presence of MWNT also enables the SPU matrix to sustain a high modulus value at higher temperature.
[FIGURE 4 OMITTED]
Figure 5 shows the tan ([delta]) results of SPU and SPU nanocomposite samples as a function of temperature. tan ([delta]) peak height and shape provides the information about the freedom of molecular mobility of polymer chains. Because the tan ([delta]) peaks are not sharp, therefore, it is difficult to measure the exact value of glass transition temperature ([T.sub.g]) for SPU-MWNT samples. However, we could get some idea about [T.sub.g] from the tan [delta] peaks. The peaks position moved to slightly higher temperature with the increasing nanotube content, showing that the [T.sub.g] of the nanocomposite films increases due to the hindrance of polymer chains movement with the addition of nanotubes (21). Therefore, we can say that the MWNT not only influences the crystalline region (see DSC results) but also the amorphous region of SPU matrix. One possible explanation for this difference is the kinetics of the glass transitions, which would alter in the presence of nanotubes. MWNT is able to impart stiffness to the polymer matrix. The tan ([delta]) peak broadens significantly for SPU having 2.5 wt% of MWNT indicating that the functionalized nanotubes could effectively act as reinforcing filler because of their much larger surface area and stronger interaction with the polymer matrix. Improvement of the interfacial adhesion is also suggested by the decrease in the [(tan [delta]).sub.max] value with increasing MWNT content in the polymer matrix (3). Broadening of tan ([delta]) peak of SPU with 2.5 wt% of MWNT indicates that chain mobility has been decreased by MWNT. Therefore, the crystallization of SPU with 2.5 wt% of MWNT would become less difficult, which also supported the DSC results. tan ([delta]) value suddenly increases with 0.25 wt% of MWNT content; higher loss tangent value mean the material is viscous. This result is bit strange and not easily understandable. One possible reason may be the little amount of MWNT increases polymer chain movement of nanoparticle-polymer matrix interface, resulting in a more rapid overall transition. This explanation may be a plausible explanation for a shift of tan ([delta]) value.
[FIGURE 5 OMITTED]
Transmission Electron Microscopy
Figures 6 and 7 show the TEM images of pristine nanotubes and SPU-MWNT composite films, respectively. Dark lines in Fig. 7 are interactions of the functionalized nanotube layer with polymer matrix, and other region is the SPU matrix. It can be seen that the nanotubes are effectively exfoliated in the SPU matrix (22). Average diameter of the crude nanotube is about 20 nm (see Fig. 6). For the samples of SPU-MWNT (see Fig. 7). it can be observed that some parts of tubes are clothed with the polymer layers (23). These MWNTs, with a diameter much higher than the 20 nm; therefore, thicker than pristine MWNTs. The difference in diameter suggested that some SPU chains have interacted with the amino groups on the SPU-MWNT and attached with nanotubes (3). This observation suggests that the functionalization may have likely occurred along the MWNT length. Although, it is believed that higher degree of functionalization would occur at the end of the MWNTs where reactivity is much higher.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Water Vapor Transport Properties
Water vapor transport properties of original SPU- and MWNT-reinforced SPU membranes were measured by equilibrium sorption and water vapor permeability measurements. Table 2 shows that the presence of impermeable MWNT at 1 and 2.5 wt% concentration in SPU matrix slightly reduces the equilibrium water sorption. Increase of mean free path of the water molecules to pass through the matrix of SPU-MWNT seems to be the reason for reduced water swelling (24). Hydrophilicity/hydrophobicity characteristic of the polymer membrane is major parameter determining the equilibrium sorption of polymer matrix. In all cases, equilibrium water sorption increases with increasing temperature due to the increase of free volume. There were significant changes of equilibrium sorption at [T.sub.ms]. Increased water swelling of sample PU-0.25 above [T.sub.ms] was higher than the other samples. Amount of crystalline structure content in the PU-0.25 was little higher than the other samples. Therefore, the crystal melting provides more space for water molecule for PU-0.25. The desorption phenomena of SPU samples, PU-0.25 and PU-0.50 above 35[degrees]C could be explained by the lower critical sorption temperature phenomenon for PEG 3400 chain (25).
TABLE 2. Eqilibrium sorption (w/w%) data of SPU and SPU-MWNT. Sample 10 15 25 35 45 [degrees]C [degrees]C [degrees]C [degrees]C [degrees]C PU 17.65 18.50 22.15 20.45 19.15 PU-0.25 17.80 18.64 23.28 21.20 20.67 PU-0.50 17.50 18.40 22.50 20.68 19.50 PU-1.00 16.88 17.50 21.35 21.75 22.15 PU-2.50 16.50 17.15 20.85 21.28 21.95 occur at the end of the MWNTs where reactivity is much higher.
Figure 8 shows the dense surface structure of the SPU-MWNT membrane. From the SEM micrograph, we can see that the nanoparticles were completely entrapped by the polymer matrixes. SPU-MWNT is a multiphase system in which the coexistence of phases with different permeabilities could cause complex transport phenomena (26). In fact, SPU copolymer itself could be considered as two-phase domain viz. soft segment and hard segment rich phase domain structure, and permeability through the hard segment is poor. Permeation of small molecules through the nonporous polymer membrane (Table 3) is enhanced when their solubility and diffusivity in polymer would increase. The presence of MWNT content expected to decrease in permeability due to the more tortuous path for the diffusing molecules that must bypass through impermeable nanoparticles. At 0.50 wt% of MWNT content, it has no significant influence on the permeability. In contrast, the permeability of membrane with 0.25 wt% of MWNT increased slightly above the [T.sub.ms] (Table 3), due the presence of more crystalline structure and their melting enhances the permeability. Crystal melting is able to completely plasticize the polymer chains and the presence of water molecules would increase the mobility between the polymer chains, which would significantly enhance the water vapor permeability through the nonporous membranes. Permeability of the composite films decreases at 1 and 2.5 wt% of MWNT content due to the increased stiffness between the polymer chains (see DMTA results). Increased stiffness of the polymer chains obviously would prevent the passage of water molecules through the polymer matrix.
[FIGURE 8 OMITTED]
TABLE 3. Water Vapor permebility (g/[m.sup.2] 24 h) data of SPU and SPU-MWNT. Sample 12 18 25 35 45 [degrees]C [degrees]C [degrees]C [degrees]C [degrees]C PU 290 395 545 806 1270 PU-0.25 296 408 552 846 1285 PU-0.50 295 404 547 420 1277 PU-1.00 285 365 530 795 1265 PU-2.50 266 349 518 781 1258
The functionalized multiwall nanotubes increased the modulus of the SPU matrix. Crsytallinity slightly increased with 0.25 wt% of MWNT, and further increase of MWNT hinders the crystallization process of SPU due to the reinforcing effect. Glass transition temperature increases with increasing MWNT content, which suggested that the MWNT not only influences the crystalline region but also the amorphous region. TEM and WAXD show interaction between the MWNT and SPU matrix. TEM showed that the functionalized MWNT particles were exfoliated and dispersed in the soft and hard matrix of SPU. Soft segment crystal melting enhances the water vapor transport properties of the dense membrane. At soft segment crystal melting temperature, the water vapor transport properties of the sample with 0.25 wt% of MWNT content increase when compared with the virgin SPU. Further increase of MWNT hinders the water vapor transport properties due to the stiffening effect of the polymer chains by the functionalized MWNT. Finally, we could conclude that the reinforcement of carbon nanotube into SPU films would be applicable to improve other functional properties such as UV blocking (15) and so on, without affecting its water molecules transport properties.
The authors acknowledge the financial support of the International Postgraduate Scholarship of The Hong Kong Polytechnic University, Hong Kong.
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Correspondence to: S. Mondal: e-mail: firstname.lastname@example.org
S. Mondal, J.L. Hu
Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
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|Author:||Mondal, S.; Hu, J.L.|
|Publication:||Polymer Engineering and Science|
|Article Type:||Technical report|
|Date:||Sep 1, 2008|
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