Article pubs.acs.org/Macromolecules
A MWCNT/Polyisoprene Composite Reinforced by an Effective Load Transfer Reflected in the Extent of Polymer Coating Junchun Yu,† Bounphanh Tonpheng,† Gerhard Gröbner,‡ and Ove Andersson*,† †
Department of Physics, Umeå University, 901 87 Umeå, Sweden Department of Chemistry, Umeå University, 90187 Umeå, Sweden
‡
S Supporting Information *
ABSTRACT: Tensile and microstructural properties of multiwall carbon nanotube (MWCNT)/polyisoprene (PI) composites have been investigated after cross-linking achieved purely by simultaneous high-pressure high-temperature treatment. The method enables gradual increase of the cross-link density without interference of vulcanization chemicals, and the results suggest a link between an interfacial PI layer wrapped/coated on the MWCNTs and reinforcement in carbon nanotube (CNT)/PI composites. The interfacial layer, which is augmented by high-pressure treatment, was detected indirectly in swelling experiments and also reflected in results of atomic force microscopy. The results imply more efficient load transfer and mechanical reinforcement by CNTs with improved interfacial layer and that changes in the layer can be probed by swelling measurements.
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Moniruzzaman et al.’s15 results of more than 2 times higher σUTS for fibers made of nylon-6, 10, and 1 wt % alkyl acid chloride group functionalized SWCNTs (SWCNT-(CH2)n-COCl) than fibers of nylon-6, 10, and 1 wt % nonfunctionalized SWCNTs, which implies increased reinforcement due to covalent bonds between the nylon matrix and the functionalized CNTs. Another method to obtain good load transfer is to use a cross-linkable polymer that can form a rigid matrix with good polymer−CNT interfacial contact between the entangled CNTs and the crosslinked polymer matrix. In fact, such a matrix even reinforce a fiber made of only CNTs and thus gives higher σUTS for the composite fiber than for the neat CNT fiber.16 For example, SWCNT/ poly(vinyl alcohol) and SWCNT/epoxy composite fibers showed σUTS of ca. 1.0 and 1.25 GPa, respectively, which are 40% and 80% stronger than neat SWCNT fibers (0.7 GPa). In general, film or bulk composites show less improvement of mechanical properties than those in the form of fibers. The difference between these is well demonstrated by Sui and Wagner’s17 study of poly(methyl metacrylate) (PMMA) based SWCNT and MWCNT composites. A 1.5 wt % MWCNT/ PMMA composite in the form of fibers showed 160% higher σUTS and 220% higher E than pure PMMA fibers, whereas the strength of thin film samples remained largely unchanged by MWCNTs. The difference was attributed to, e.g., the different sizes of the samples (fiber diameters of 500 nm and film thicknesses of 100 μm) and the alignment of the CNTs and polymer chains in the fibers. This is corroborated by a study of Young et al.,18 which
INTRODUCTION The filler/polymer composite materials of today have a long history of development.1,2 Various fillers such as carbon black, carbon fibers, silica, and clays, etc.,3 have been tested and proven favorable for mechanical and thermal reinforcement of polymers. But the recent discovery of new nanostructured materials and their extraordinary properties have raised expectations of major further improvements.4 In particular, carbon nanotubes (CNTs)5,6 and graphene7 have mechanical and structural properties which are superior to those of the most commonly used fillers.8 For example, assuming that CNTs in the perimeter of a bundle carried all the load, Yu et al.9 obtained a tensile strength (σUTS) and a Young’s modulus (E) of ∼22 and ∼1050 GPa, respectively, for a single-wall carbon nanotube (SWCNT) in a 19 nm bundle. Moreover, a 13 nm in diameter multiwall carbon nanotube (MWCNT) shows σUTS of 28 GPa and E in the range from ∼270 to 950 GPa,10 or an E average of 1.8 TPa, as reported by Treacy et al.11 In general, theoretical calculations for SWCNTs and MWCNT indicate even higher strength and typically show σUTS and E values of order of ∼1 TPa.8,12 Besides the excellent mechanical properties, the unique structure with high aspect ratio (∼103) and low density make CNTs particularly interesting for the purpose of fabricating lightweight composites with high strength and stiffness.1,13 In order to obtain CNT-based polymer composites with much improved mechanical performance, it is essential that the microstructure provides an efficient load transfer between the polymer matrix and the CNTs.14 Covalent functionalization of CNTs is one method that opens the possibility of strong bonding between the otherwise essentially inert CNTs and a polymer matrix. The capability of this method is shown by © 2012 American Chemical Society
Received: November 30, 2011 Revised: February 19, 2012 Published: March 13, 2012 2841
dx.doi.org/10.1021/ma202604d | Macromolecules 2012, 45, 2841−2849
Macromolecules
Article
HP&HT Treatment of PI and Composite Samples. The highly viscous pastelike sample was loaded in a cylindrically shaped, custommade, Teflon cell containing a type K thermocouple. The cell was mounted into a piston cylinder apparatus and the assembly transferred to a press, as described in more detail previously.22,23 The pressure was generated by a hydraulic press and calculated from load/area with an empirical correction for friction, which had been established using the pressure dependence of a Manganin wire. The pressure inaccuracy was estimated as ±40 MPa at 1 GPa. The MWCNT/PI mixtures, and pure PI samples, were treated for 4 h at 513 K and a pressure in the range 0.25−1.5 GPa. The pressure and temperature were controlled using two proportional-integral-derivative (PID) controllers, which kept the pressure and temperature constant to within ±1 MPa and ±1 K during the 4 h HP&HT treatments. Tensile Measurement. The HP&HT treated samples were removed from the Teflon cell as circular plates with thicknesses between 1 and 2 mm. A custom-made dog-bone-shaped die (16 mm in length and 7 mm in width) was used to cut the synthesized samples into dog bones for tensile testing. The exact dimensions of the samples were measured by means of a Nikon workshop microscope, and these values were used in the calculations of σUTS and E (in the range 1−5% strain). Tensile testing was performed at room temperature with a testing speed of 10 mm/min using an Instron 3343 tensile tester equipped a 500 N (or 1000 N) load cell. To enable the best comparison, the measurement procedure and analysis, e.g. testing speed and calculation of E, were the same as used previously.21 To ensure accuracy and repeatability, at least two and occasionally three test pieces from the same sample were tested. (The preparation and analysis of one batch of a PI composite took about 7 days, which precluded a more extensive sample production.) The results of the different sample pieces normally differed less than 15%, as also found previously,21 and occasional larger difference could be attributed to cracks in the sample plate. The stated σUTS and E are for the strongest dog-bone piece, which typically was the one cut from the center of the sample plate. Nuclear Magnetic Resonance (NMR). The grinded solid (or highly viscous liquid) material was densely packed into a zirconia MAS rotor with Macor inserts free of carbon. Measurements were carried out at room temperature on a 400 MHz Infinity spectrometer (Chemagnetics) with a 4 mm double resonance cross-polarization magic angle spinning (CP MAS) probe. 13C CP MAS NMR experiments under proton decoupling were acquired by using a contact time of 5 ms duration and a repetition delay of 3 s at a MAS speed of 6 kHz; direct polarization 13C MAS NMR experiments under proton decoupling were performed at a MAS speed of 6 kHz. All 13C NMR spectra were assigned using adamantane as an external reference. 1H MAS NMR experiments were acquired by using a standard Hahn-echo pulse (π/2−τ−π−τ− acquisition) of 50 μs for each τ spacing and a repetition delay of 3 s at a magic angle spinning speed of 10 kHz.24 Spectra were recorded with 120 transients and referenced by setting the CH signal of PI to 5.2 ppm.25 The intensity of the echo signal relative to the initial signal could be described by exp(−2τ/T2), where τ was varied between x and y, with T2 describing the spin−spin relaxation time.24 Characterization of Cross-Link Density. Cross-link density was determined using the swelling method (υswell), E measurements (υE), and NMR measurements (υNMR). In the swelling method, the samples were swelled in n-heptane or toluene at room temperature for 48 h.21 A composite with 9.1 wt % P-MWCNT was also swelled further for 48 h with an additional weight increase of less than 1%, which confirms that 48 h swelling time is enough to reach near-equilibrium. The υswell was estimated by the Flory−Rehner equation (see Supporting Information).21,26,27 Results for E were used to calculate cross-link density (υE) or molecular weight between cross-links (Mc) from27,28
showed that the strength of SWCNT/poly(vinyl alcohol) fibers increased strongly with decreasing fiber diameter. It thus appears that most previous successful reinforcements by CNTs concern composites in the form of fibers and/or rather hard matrixes such as PMMA, nylon, epoxy, etc.19 Considering instead soft matrixes, then a 8.3 wt % MWCNT/nature rubber composite film showed ∼60% higher σUTS and ∼900% higher E than the pure polymer.20 Although the relative improvement is good and better than those shown in many other studies of rubbery or soft materials, the highest σUTS and E only reached 7 and 10 MPa, respectively, which means that the composite still remained relatively weak and soft. However, we have shown that this can be changed by highpressure high-temperature (HP&HT) treatment, which induces cross-links in the polymer matrix, polyisoprene PI, without the use of vulcanization chemicals.21 In particular, a composite with 5 wt % SWCNT synthesized at 1.5 GPa, 513 K showed 115% higher σUTS (17 MPa), 137% higher E (220 MPa), and longer extension at break than PI processed at identical conditions (σUTS = 7.9 MPa and E = 93 MPa). An intriguing effect of the SWCNTs was an increased cross-link density as measured by the swelling method and shown concomitantly as the composites’ strength increased.21 The origin of this effect and its apparent relation with the reinforcement remained uncertain. In this work, we have studied the mechanical and microstructural properties of MWCNT−polymer composites in elastic states ranging from relatively soft to hard, which were obtained by tuning the cross-link density via HP&HT treatment. We find that a MWCNT/PI composite synthesized at the highest pressure of 1.5 GPa not only show better mechanical properties than lowpressure counterparts but that this also correlates with much higher cross-link densities as measured by the swelling method. As nuclear magnetic resonance (NMR) results instead show fewer chemical cross-links in the composites than in pure PI suggests that the reduced swelling (high cross-link density) observed for PI with MWCNTs is due to an interface layer of strongly bound PI. The layer, which is also reflected in AFM results, is augmented by HP&HT treatment and may act as an active bridge for load transfer.
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EXPERIMENTAL SECTION
Materials. Liquid cis-1,4-polyisoprene (PI) made from natural rubber, with a number-average relative molecular mass of 38 000, was purchased from Sigma-Aldrich. Solid high molecular weight (800 000, 97% cis) polyisoprene (LPI) was purchased from Scientific Polymer Products, Inc. MWCNTs produced by catalytic chemical vapor deposition were purchased from Nanocyl (3150 and 3151). Nanocyl 3150 (P-MWCNT: nonfunctionalized MWCNT) and Nanocyl 3151 (F-MWCNT: functionalized with less than 4% of −COOH) have a stated diameter of 10 nm, carbon purity of >95%, and a metal oxide content of
