US 20070053825 A1
A process for the modification of carbon fibers and/or carbon nanotubes, including 1-dimensional nanowire and nanofiber structures. In the process, certain polymeric materials are crystallized on a surface of the carbon fibers and/or carbon nanotubes. The crystallization process may be carried out under controlled conditions to produce a variety of useful modifications, including modifications at discrete intervals, as well as functional modifications. Also disclosed are carbon fibers, carbon nanotubes and nano-hybrid structures made by the modification process of the present invention.
1. A method for the modification of carbon fibers or carbon nanotubes comprising the step of:
crystallizing a polymeric material on a surface of the carbon fibers or carbon nanotubes.
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1. Field of the Invention
The present invention relates to the provision of modified carbon nanotubes. More particularly, the present invention relates to the use of controlled polymer crystallization to modify carbon nanotubes, including 1-dimensional nanowire and nanofiber structures, and to the resultant modified carbon nanotubes.
2. Description of the Related Technology Due to their extraordinary mechanical, electrical and optical properties, carbon nanotubes (hereinafter, “CNTs”) have attracted significant attention in recent years.[1-3] In order to transfer their outstanding properties from nano- to micro-scale, one essential step involves assembly and processing of CNTs,[4, 5] which is hindered by their intrinsic poor solubility and processibility.[6-10] It has been recognized that, to disperse CNTs for processing purposes, their surface has to be modified. This has led to the emergence of the field of CNT functionalization. [11, 12] Both chemical functionalization techniques and non-covalent wrapping methods have been employed for this purpose.[13-20]
The chemical functionalization technique involves covalently linking a functional group directly to the CNT surface or, to sites of surface defects. One example of this is CNT-bound carboxylic acids.[13-16] The non-covalent wrapping method involves using surfactants, oligomers, biomolecules and/or polymers to “wrap” CNTs in order to enhance their solubility. A number of surfactants, as well as a number of rigid, conjugated molecules, have been successfully used to modify CNT surface chemistry in this manner. It is postulated that these molecules form strong π-stacking with CNT'S, thereby resulting in CNT's coated by surfactant/functional molecules. In this manner, CNT surface properties can be altered. Smalley et al. proposed the use of water-soluble polymers such as polyvinyl pyrrolidone (PVP) and polystyrene sulfonate (PSS) to enhance the solubility of CNT's in water.
In another parallel research field, carbon materials, in various forms, have been known to induce polymer crystallization. [21-27] Epitaxial growth of polymers such as polyethylene (PE), Nylon 6, and similar materials has been observed, for example, on graphite surfaces using scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) techniques.[21-23] Carbon fibers (CFs), which are about 10 μm in diameter, have been used to induce polymer crystallization (transcrystallization).[24-27] It has also been demonstrated that a number of polymers (such as polypropylene, PE, Nylon 6,6, and poly(phenylene sulfide), can epitaxially grow on the surface of CFs.[24-27]
Accordingly, it is an object of certain embodiments of the present invention to provide an alternative method for the surface modification of carbon fiber's and/or carbon nanotubes.
It is also an object of certain embodiments of the present invention to provide surface modified carbon fibers and/or carbon nanotubes having improved processability and/or solubility.
In a first aspect, the present invention relates to a process for the modification of carbon fibers and/or carbon nanotubes, including 1-dimensional nanowire and nanofiber structures. In the process, certain polymeric materials are crystallized on a surface of the carbon fibers and/or carbon nanotubes. The crystallization process may be carried out under controlled conditions to produce a variety of useful modifications, including modifications at discrete intervals, as well as functional modifications.
In a second aspect, the present invention relates to carbon fibers, carbon nanotubes and nano-hybrid structures made by the modification process of the present invention.
In a first aspect, the present invention relates to a method for the surface modification of CFs and CNTs, including 1-dimensional nanowire and nanofiber structures. In the method, the technique of controlled polymer crystallization is employed. It is envisaged that if CNT induced polymer crystallization occurs, such crystallized polymers would form a crystalline layer on CNTs. The formed polymer layer would “wrap” the CNTs and therefore provide a form of surface modification. By controlling crystallization conditions, polymer single crystals can be associated with both single-walled and multi-walled CNTs (SWNTs and MWNTs). Moreover, the polymer crystals can be associated with the CNTs at periodic intervals along the surface of the CNTs in order to provide additional benefits.
The location of the polymer single crystals on the surface of the CNTs can be controlled to within about 20-70 nm using the techniques of certain embodiments of the present invention. Polymer nanocomposites with controlled tube-to-tube distance can also be prepared using the techniques of certain embodiments of the present invention. Since polymeric material can be easily removed by etching/dissolution, the method of the present invention provides a route for introducing multiple functionalities onto individual CNTs at well-defined intervals.
In order to achieve controlled modification of CNTs, a polymer solution crystallization technique is employed. CNTs are unbundled, preferably in a dilute polymer solution using any suitable technique, such as ultrasonication. Before reagglomeration of the unbundled CNTs occurs, the CNT/polymer solution is brought to the crystallization temperature (Tc) of the polymer resulting in crystallization of the polymer on the surface of the CNTs to produce CNT/polymer hybrid nanostructures.
Any suitable polymer that crystallizes onto a carbon substrate can be used in the present invention. Preferably, polymers that form single crystals upon solution crystallization are employed. Also, the preferred polymers are hydrophilic. Suitable polymers, include, but are not limited to, polypropylene, polyethylene, Nylon 6,6, polyethylene oxide and poly(phenylene sulfide).
A distinct morphology formed in polymer solution crystallization is known as “polymer shish-kebab”. Polymer crystals consist of a central elongated stem, around which many disc-shaped objects are grown. The central stem is PE single crystals formed by stretched polymer chains and the disc-shaped objects are folded PE lamellar crystals.
One typical example is crystallizing polyethylene/p-xylene solution (about 5%) under stirring. First, under external flow field, polymer chains might undergo a coil-to-stretch transition, which leads to the formation of stretched polymers. The stretched polymers then nucleate and form fibrous crystals. Coil polymer chains could migrate and epitaxially crystallize upon the stretched polymer chains forming folded lamellae. For a typical PE shish-kebab morphology, the folded lamellar crystals periodically grow along the central stem. The periodicity is about 110 nm and the central fiber is about 20 nm. The lateral size of the kebabs depends on the crystallization time and it varies from a few tens of nanometers to a few micrometers.
The formation of the “nano hybrid shish-kebab” (hereinafter “NHSK”) structures is attributed to the controlled polymer crystallization on the surface of the CNTs. The nucleation process begins after the CNTs and polymer are dispersed in a solvent at a relatively high temperature (120° C.). The temperature of the mixture consisting of unbundled, unmodified CNTs and free polymer chains, is then lowered to the polymer crystallization temperature for crystallization.
Agglomeration of the unbundled CNTs into bundled CNTs typically occurs within about 0.5 hrs for pure CNT in a p-xylene solvent because unbundled CNTs are not stable. With additional polymeric material in the system, however, three processes could occur: (1) polymer crystallization, via homogeneous nucleation; (2) polymer crystallization on the surface of the CNTs via secondary nucleation; and (3) CNT agglomeration. Since CNTs have external surface area which can function as a polymer nucleation site and, at relatively high Tc and for certain polymeric materials, secondary nucleation is energetically more favorable than homogeneous nucleation. Furthermore, if polymer crystallization kinetics is faster than that of the CNT agglomeration, CNTs will be wrapped by polymer single crystals. The polymer crystals then provide a steric hindrance thereby hindering or preventing CNT agglomeration.
In this type of polymer shish-kebab structure, it has been recognized that the kebab structures are epitaxially grown on the surface and that the polymer chains forming both the “shish” and the “kebab” structures are parallel to each other. As shown in
Nylon 6,6 was also used to modify CNT surfaces to tune the surface chemistry of the resulting NHSKs.
In order to demonstrate that the controlled polymer crystallization method also works for different types of CNTs, HiPco SWNT was also used for PE crystallization.
Fractions of the SWNTs forming with PE crystallized around the bundle captured the “state of the CNT agglomeration” in p-xylene solution. This image therefore can be considered as the direct visualization of the “degree of CNT exfoliation/agglomeration” in CNT/PE/p-xylene solution. Compared to unmodified CNTs, these modified NHSKs are easier to disperse in a polymer matrix.
The size of the kebab structures can be controlled by tuning the crystallization conditions.
Addition of more Nylon 6,6 to a Nylon 6,6/CNT NHSK suspension and further crystallization at 185° C. leads to the formation of Nylon 6,6 spherulites as shown in
Preliminary results demonstrate that both PE and Nylon 6,6 grow single crystals on MWNTs and CNT modification can be achieved. CNT concentration is related to NHSK formation since CNTs have to be exfoliated before crystallization of the polymer. Solubility of CNTs in organic solvent is low. For instance, one of the best solvents for SWNT is 1,2-dichlorobenzene, which is only able to dissolve 95 mg SWNT in 1 liter of solvent. Formation of PE crystals on individual MWNT suggests that the CNTs have been successfully exfoliated before crystallization. Increasing CNT concentration leads to more nucleation sites and possibly a higher yield of PE/CNT NHSKs. CNT concentration can be varied, for example, from (w/w) 0.001%-1% (Table 1).
As CNT concentration increases, nanotubes will not be exfoliated and phase separation will occur. As CNTs start to aggregate together with increasing concentration, two scenarios might occur: 1) CNTs might form uniform bundles with each bundle consisting of a few tubes and 2) phase separation occurs and the CNT/solvent dispersion might consist of large agglomeration of CNT as well as individual CNTs. In case one, controlled polymer crystallization will lead to NHSKs with CNT bundles forming the shish structure while in case two, CNT agglomeration and NHSK with single CNTs forming the shish structure will be formed.
Polymer concentration is another factor that influences the solution crystallization process. High polymer concentrations lead to dense nucleation/growth of polymer on the CNTs and spherulite structures might also be formed. The entire CNTs will thus be coated with spherulites, resulting in structures similar to those shown in
Crystallization temperature (Tc) may also influence the crystal structure and morphology. As Tc decreases, for solution crystallization, crystallization kinetics increases. Fast crystallization kinetics might lead to the crystal morphology change from single crystals to spherulites. Thickness of the lamellar crystals is known to depend on Tc.[49, 50] Therefore, NHSKs with different kebab thickness' can be obtained at different Tc. Furthermore, varing Tc may lead to different temperature gradients at the nucleation front and the periodicity (n) may also be altered.
A wide range of crystallization temperatures (Table 1) may be used in the process of the present invention. Generally, the crystallization temperature can be varied up to about plus or minus 10% from the actual Tc of the specific polymer employed.
Another parameter that may influence crystallization is the crystallization time. Upon quenching to Tc, there is a period of induction after which polymer crystal growth will start. After nucleation, lamellar crystal grows laterally and the size of the final kebab structure depends, at least in part, on the crystallization time. In order to achieve different kebab structure sizes, crystallization times from about 0.1 hr to about 10 hrs may be employed.
Table 1 lists some exemplary conditions for fabricating polymer/CNT NHSKs.
In summary, for the first time, polymer single crystals have been successfully grown on the surface of CNTs. It is evident that CNTs can induce polymer secondary nucleation and the resulting NHSK structures possess a controllable periodicity of the polymer modification, e.g. the polymer single crystals forming kebab structures.
This makes it possible to modify CNTs in an ordered and controlled manner. Both SWNT as well as MWNT. have been successfully modified with PE and Nylon 6,6, implying that this is likely a generic method for CNT modification. As demonstrated herein, periodicities of 20-120 nm can be achieved.
The NHSK structures can also be used to fabricate polymer/CNT nanocomposites with a controllable tube-to-tube distance. In many cases, the size of the kebab structure dictates the tube-to-tube distance. Polymerization conditions can be controlled to provide the desired size of kebab structure by, for example, using a longer crystallization time and/or lower crystallization temperature. Also, addition of uncrystallized polymer may provide additional structure growth, such as spherulites, which can be mixed with the CNT's to adjust the CNT contents of the NHSKs.
Furthermore, by coupling crystalline polymers with functional groups, ordered multi-functionality can be realized on individual CNTs. For example, Au or CdSe quantum dots can be coupled with the CNTs in this manner.
In addition, polymeric materials can be employed that can be removed by application of heat or solvent to allow further manipulation of the NHSKs by heat or solvent treatment. Polypyrrole can be synthesized on PE/CNT NHSKs by in situ surfactant-directed chemical oxidative polymerization. In a typical synthesis, surfactant cetyl trimethylammonium bromide and NHSK are mixed in deionized water and sonicated for over two hours to obtain well-dispersed suspensions. The mixture is then cooled to 0-5° C. A pre-cooled pyrrole monomer and an ammonium persulfate/deionized water solution are added sequentially to the suspension. The reaction mixture is ultrasonicated for about 2 minutes and then allowed to stand for about 10-20 hours. After filtration, polypyrrole coated NHSK is obtained. At this stage, CNTs are modified with both polyethylene and polypyrrole. Polyethylene can then be removed by hot p-xylene, resulting in polypyrrole functionalized CNTs.
The modification method of the present invention can be applied to carbon fibers, carbon nanotubes, carbon nanowires and similar devices.
The modified CNTs of the present invention can be used, for example, to provide multi-functional nano-materials and enable the use of one-dimensional nanostructures in nano-electronics, photovoltaic cells and fuel cells, for example.
Purified HiPco SWNTs were purchased from Carbon Nanotechnolgies Inc. Multi-Walled Carbon Nanotubes (MWNTs) were purchased from Aldrich and washed with 2.4 M nitric acid for 0.5 hrs. The resulting MWNTs were then centrifuged, collected and dried in a vacuum oven.
Nylon 6,6 (Mn=10,000 g/mol) was supplied by Dupont Company. Linear polyethylene (MFI=12 g/10 min) and glycerin were purchased from Aldrich and used as received.
Nano hybrid shish-kebabs (“NHSKs”) were obtained via solution crystallization using CNTs as seeds. For PE crystallization (see scheme 1), p-xylene was used as the solvent. 0.1 mg MWCNT was dispersed in 1 g p-xylene and ultrasonicated for 1-2 hrs and then added to 4 g (w/w) of 0.01% PE/p-xylene solution. The mixture was then quenched to the preset Tc. The crystallization time was controlled to be 0.5 hrs. The crystallization temperature for MWNT was 103° C. and it was 104° C. for SWNT. The crystallization time was about 0.1-3 hrs. The sample was then isothermally filtered to remove the uncrystallized materials before microscopy observation.
For Nylon 6, 6, solution crystallization, glycerin was used as a solvent and the concentration was about (w/w) 0.01%. Nylon 6,6 was dissolved in glycerin at 240° C. 0.1 mg MWNT was dispersed in 1 g glycerin and ultrasonicated for 1-2 hrs at 40° C. and then added to a 9 g (w/w) 0.01% Nylon 6, 6/glycerin solution. The mixture was then quenched to the preset Tc. The crystallization time was controlled to be about 0.5-3 hrs. The sample was isothermally filtered after crystallization to remove the uncrystallized materials.
TEM experiments were conducted using a JEOL-2000FX microscope with an accelerating voltage of 120 kV. SEM experiments were carried out using a FEI/Phillips XL30 field emission environmental SEM and the acceleration voltage was 15 kV.
A straight, isolated MWNT can be clearly seen in
To demonstrate the generality of this PCCF method, different kinds of CNTs were employed in Examples 5-7, as shown in Table 2.
First, MWNTs synthesized by arc discharge and CVD methods were used. The diameters of these MWNTs were reported as 5-15 and 20-30 nm, respectively. They are thus denoted as MWNT-10 and MWNT-25.
To further demonstrate the generality of this method, much larger size CNFs were used. CNFs are attractive from the practical point of view due to their relatively low cost. These CNFs have an outer diameter of 100-300 nm, a hollow core, and length on the order of 30-100 m.
TEM and SEM experiments were conducted to study the detailed lamellar orientation on the CNT/CNF surface.
Because of the decorating polymer crystals, NHSK can be easily dispersed in solution. To increase the efficiency of functionalization, the initial concentration of SWNTs in the PE solution was increased. HiPco SWNT samples were used for this study. DCB is one of the best solvents for SWNTs (95 mg/L). We thus conducted the PE crystallization experiment in DCB. In this case, 1 mg SWNT was dissolved in 1 g of DCB by ultrasonication for about 3 hours to prepare the premix (note that the SWNT concentration is much higher than that inp-xylene). This premix was added into 4 g of PE/DCB solution at 120 C, and the final PE concentration is 0.01 wt %. After equilibration for 10 min, the solution was transferred to a preset Tc Three different Tc's (91, 89, and 87° C.) were used. All three samples have the same SWNT concentration.
In sample 1 (comparative example A), black precipitates were observed within 1 hour after crystallization, presumably due to agglomeration of CNTs. On the other hand, samples 2 and 3 (Examples 8-9, respectively) appeared to be stable, and did not precipitate 3 months after being prepared. This phenomenon suggested that 91° C. was too high for PE crystallization in DCB; NHSK was not formed in sample 1. In samples 2 and 3, the stable suspension of SWNT indicates that SWNTs were decorated by PE crystals and NHSK structures prevented SWNTs from precipitation. TEM experiment observations were conducted to confirm this hypothesis, and the results are shown in
A lower concentration of SWNTs was used to achieve more complete exfoliation between SWNTs.
DCB was also used for a MWNT NHSK study;
The formation temperatures of NHSK depend on the CNT structures. The suitable Tc values for SWNT, MWNT-10, MWNT-25, and CNFs in p-xylene are 104, 103, 97, and 97° C., respectively. Because CNTs that do not form NHSK easily precipitate while NHSK is stable in the solvent, PCCF thus also provides a unique opportunity to achieve CNT separation. To demonstrate this concept, MWNT-10 and MWNT-25 were mixed together in p-xylene, and the solution was further mixed with PE and crystallized at 103° C. It was anticipated that at this Tc, only MWNT-10 NHSK could be formed. It was observed that after 3 hours of crystallization, part of the CNT precipitated while the rest was stable in the solution. The supernatant and the precipitant were collected, and SEM was used to study their structure; the results are shown in
To further prove the separation result, Raman spectra were obtained from both the supernatant and precipitant as well as the pristine CNTs as shown in
It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.