NANO LETTERS
Stepped Polymer Morphology Induced by a Carbon Nanotube Tip
2004 Vol. 4, No. 8 1439-1443
A. H. Barber,† S. R. Cohen,‡ and H. D. Wagner*,† Department of Materials and Interfaces and Chemical Research Support, Weizmann Institute of Science, RehoVot 76100, Israel Received May 13, 2004; Revised Manuscript Received June 8, 2004
ABSTRACT Stepped conical structures have been produced at the surface of poly(ethylene glycol) by contacting a single, relatively short carbon nanotube attached to an AFM tip with the molten polymer surface, followed by polymer cooling. Cooling of the polymer melt in the nanotube vicinity is the most likely mechanism for these ziggurat-like structural formations. Simple heat transfer calculations confirm the effect of the nanotube length on the propensity for local solidification of the polymer.
The formation of solid structures from the liquid state, induced by solid surfaces, has been observed for a large variety of polymers and other materials. Various causes for these solid polymer formations have been invoked, such as the nucleating ability of the surface substrate with respect to the polymer,1,2 or the lower temperature of the solid surface cooling the polymer melt.3 Polymer structures induced by the presence of the solid are often morphologically different than those of the bulk polymer,4 with properties such as thermal stability5 or mechanical behavior6 undergoing a modification due to this polymer morphology change. Carbon nanotubes show promise as reinforcements in polymer composites due to their high tensile strength and stiffness.7 Various studies8-10 have proposed that the presence of a nanotube in a semicrystalline polymer can modify and preferentially nucleate crystal polymer formations. These studies have shown that, while the crystallization kinetics favor nucleation of the polymer at the nanotube surface, this has not been directly observed thus far. This is in contrast with typical engineering fibers, where nucleation of a semicrystalline polymer at the fiber surface leads to an observable “transcrystalline layer”,1,2,4-6 which may or may not have the same crystal structure as the bulk polymer. In this work we extend experiments probing the interaction between single carbon nanotubes and a surrounding polymer11-13 to show that individual carbon nanotubes can induce the formation of structures in poly(ethylene glycol) (PEG), with a morphology that is distinct from that of the bulk polymer. Pellets of PEG (Sigma-Aldrich, Germany, molecular weight ) 4000) were dissolved in ethanol to produce a 10 * Corresponding author. † Department of Materials and Interfaces. ‡ Chemical Research Support. 10.1021/nl049281p CCC: $27.50 Published on Web 07/02/2004
© 2004 American Chemical Society
Figure 1. DSC characterization of PEG, molecular weight ) 4000.
wt % polymer solution. This solution was then spun onto a clean glass microscope slide. A polymer film formed upon evaporation of the solvent. The thermal behavior of the polymer was examined using differential scanning calorimetry (DSC, Mettler), with the resultant DSC curve showing a double peak with a melt range from 50 to 60 °C (Figure 1), consistent with the literature.14 Single multiwalled carbon nanotubes (MWCNTs, Nanolab) prepared by a chemical vapor deposition (CVD) method were attached to AFM tips (Nanosensors, Switzerland) using previously described methods.15 The nanotube tips used in this study are classified as either long or short tips, as indicated in Figure 2. The surface of the PEG film was imaged with the resultant nanotubeAFM tips using an AFM with an integral heating stage (NTMDT, Russia) in dynamic mode. The PEG film was then melted in situ, with the nanotube-AFM tip retracted from the polymer surface, up to a temperature of 55(2 °C, i.e., at the onset of the polymer melting. The nanotube-AFM tip was carefully lowered toward the polymer melt using the
Figure 2. Scanning electron microscope micrograph of (a) a long single MWCNT and (b) a short single MWCNT attached to an AFM tip (inset) prior to contact with a PEG melt.
z-piezo of the AFM while monitoring the deflection of the cantilever. Contact of the nanotube with the polymer melt was discernible by a sudden bending of the AFM cantilever toward the polymer melt surface, corresponding to the wetting of the nanotube-AFM tip with the melt. The nanotube was then pushed into the melt to a fixed distance of approximately 30 nm, corresponding to a small applied force of no more than 70-80 nN. The heater was then disabled so that the polymer melt cooled to close to room temperature, a process that occurred within a time span of ca. 5 min. The position of the nanotube in the cooling polymer was maintained using the feedback of the AFM. After cooling, the nanotube-AFM tip was pulled away from the polymer surface and a small pullout force was recorded, corresponding to the small embedded length of the nanotube within the polymer. Nanotube-AFM tips were examined after each experiment in a high-resolution scanning electron microscope and showed no attached polymer remnants, thus indicating that there were no artifacts in the AFM imaging of the polymer surface. Images of the polymer surface after experiments using a long nanotube-AFM tip (Figure 3) reveal that the PEG surface has undergone topographical changes due to the thermal treatment. This indicates that polymer flow occurred during the experiment, as would be expected when melting the polymer. A small ‘indent’ on the polymer surface is observed (Figure 3b, as indicated by the arrow) corresponding to the position where the long nanotube was pulled out 1440
Figure 3. 3-D AFM topography picture of a PEG surface (a) before and (b) after a relatively long MWCNT was pushed into the PEG melt and then pulled from the cooled polymer. Scan size is approximately 1µm × 1µm.
from the polymer surface. The depth of the pullout hole is only 20-30 nm, which indeed corroborates the small pullout force measured from retracting the small embedded length. Experiments using short nanotube-AFM tips with a similarly short embedded length also reveal changes in the polymer surface topography, as shown in Figure 4. However, as opposed to experiments using long nanotube-AFM tips, a ziggurat-like cone structure is now present, with the center of this structure corresponding to where the nanotube was pulled out from the polymer. Such stair-stepped structures have recently been predicted to arise as singularities in the dynamics of crystal surface formation.16 Interestingly, such stepped structures can also be observed in planetary geology, at a much larger scale.17 While previous studies18-20 have also produced single-crystal structures from a melt or solution, the ziggurat-like stepped structures in this work are uniquely formed around the perimeter of the contacting nanotube and, as such, share its circular symmetry. Figure 5 shows the height profile along a side of the conical structure, Nano Lett., Vol. 4, No. 8, 2004
Figure 5. 2-D AFM topography picture of a step-like circular PEG structure. A line scan along one of the structure faces highlights the recurring steps (left), as indicated with arrows. In this picture, each step is approximately 40 nm wide and 15 nm high.
Figure 4. 3-D AFM topography picture of a PEG surface (a) before and (b) after a relatively short MWCNT was pushed into the PEG melt and then pulled from the cooled polymer. Scan size is approximately 1µm × 1µm.
Figure 6. Plot of the height and width dimensions of the nanotubeinduced steps, taken from AFM topography pictures.
with each step indicated with an arrow. The steps are fairly regular in their geometry and occurrence, with morphology clearly different from that of the bulk PEG surface. The dimensions of these stepped structures could be ascertained from the AFM imaging data. In particular, the height and width of the steps can be measured, with the dimensions taken from five ziggurat-like areas shown in Figure 6. The height of the steps generally ranges from 5 to 35 nm, with highest frequency of heights at around 15 nm. The step widths varied from 20 to 120 nm but were always approximately four times larger than the corresponding height values. It is interesting to note that the step heights shown in Figure 6 are similar to those observed for single PEG crystals,18 indicating that the steps themselves could be single lamellae ordered around the nanotube. To understand the formation of these ziggurat-like structures, an initiation mechanism was deduced based on the difference in free lengths between the long and short nanotube AFM tips used in our experiments. Nucleation of
ziggurat-like structures due to chemical effects at the end of the nanotube tip, such as dangling bonds, was discounted as the nanotubes were selected from an unmodified carbon powder and were not cut in any way. To examine the possibility that these structures may indeed be induced by tip cooling, the role of the nanotube length on the polymer melt surface temperature was considered. Previous work3 analyzed the effect of an AFM tip on the melting of a poly(ethylene oxide) (PEO) film. This work analyzed radiative cooling of a small area under a tip during heating, depending on the distance between the AFM tip and the polymer surface. Thus, a small crystallite could be observed directly under the tip whereas the surrounding bulk polymer melted. In our experiments the free length of the nanotube dictates the distance between the AFM tip and the polymer melt. As the nanotube volume is small, cooling of the polymer melt upon nanotube-AFM tip contact will originate primarily from the large bulk of the AFM tip. Therefore, the local polymer surface melt temperature can be calculated3 using:
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Dpolkg Th - Tpol ) Th - Ttip Dpolkg + Dflkpol
(1)
where Th is the temperature of the heated substrate, Ttip is the AFM tip temperature, Tpol is the polymer surface temperature, Dpol is the thickness of the PEG film, Dfl is the spacing (nanotube free length) between the AFM tip and the polymer surface, kg is the thermal conductivity of the air gap between the AFM tip and the polymer surface, and kpol is the thermal conductivity of the PEG film. Electron microscopy was used to measure Dpol and the length of the nanotubes (essentially Dfl, as shown in Figure 2). Thermal conductivity values were obtained from the literature21 and the temperatures of the heated substrate and cantilever were taken as the selected heating plate temperature of 55 °C and room temperature (25 °C), respectively. Using eq 1, surface temperatures of Tpol ) 49 °C and Tpol ) 45 °C for the long and short nanotube-AFM tips, respectively, are obtained. These values should be compared with the DSC curve in Figure 1, which shows that the tip-induced cooling for the short nanotube is sufficient to shift the local temperature out of the range of the melting phase transition. In contrast, the cooling for long nanotubes is smaller and still on the shoulder of the melting transition. Circular symmetry is implied by such cooling around the nanotube perimeter. For long carbon nanotube-AFM tips the cooling is not sufficient to cause the PEG to solidify around the nanotube, and the PEG in the region around the nanotube remains as a melt. While the initiation step can be described in terms of the polymer cooling around the nanotube surface, the growth of a ziggurat-like structure cannot be directly observed during this AFM experiment because the imaging nanotube-AFM tip is held in the polymer melt during the cooling process. However, the dimensions and nature of the polymer steps require further comment. Figure 7 (inset) shows the frequency of various step heights as measured from AFM topography, with an average step height of 13 nm. An estimation of the PEG lamella thickness l* can be made by using22: l* )
4σeTm ∆hf∆T
In conclusion, single MWCNTs attached to an AFM tip have been lowered into a melt of PEG. The polymer melt was air cooled to close to room temperature and the nanotube-AFM tip removed from the solid polymer. Imaging of the polymer surface with relatively long carbon nanotubes revealed a small nanotube pullout hole, whereas shorter nanotubes caused the formation of easily distinguishable circular ziggurat-like structures. A probable mechanism for this formation is more rapid local cooling of the polymer upon contact with the nanotube-AFM tip. Simple calculations suggest that the observed steps in the ziggurat-like PEG structures could be single lamellae. Acknowledgment. This project was supported by the (CNT) Thematic European network on “Carbon Nanotubes for Future Industrial Composites” (EU), the Minerva Foundation, the G. M. J. Schmidt Minerva Centre of Supramolecular Architectures, and by the Israeli Academy of Science. H.D.W. is the recipient of the Livio Norzi Professorial Chair and wishes to acknowledge the inspiring assistance of B. Goodman, L. Hampton, and G. Krupa.
(2)
where σe is the surface tension of the polymer surface, Tm is the polymer melt temperature, ∆hf is the heat of fusion of the polymer, and ∆T is the degree of cooling. A plot of the variation in the expected lamella thickness with the degree of cooling using eq 2 is shown in Figure 7. For our experimentally observed average step size of 13 nm, the degree of required cooling ∆T is 15 °C, as indicated by the dashed lines in Figure 7. Such degree of cooling is in good agreement with the calculated temperature depression (10 °C) of the polymer melt upon contact with the short carbon nanotube-AFM tip, as calculated from eq 1. It is thus not implausible that each step could indeed be a single PEG lamella. Each ziggurat-like cone would therefore be highly crystalline as indicated by the presence of these steps. 1442
Figure 7. (Main) Plot of the degree of cooling ∆T against average lamellae thickness l*. (Inset) Plot of frequency of steps against step height, with a Gaussian fit showing an average step height of 13 nm as indicated by the dashed line.
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