909408027304 Layer by Layer Assembly

NANOCOMPOSITES,LAYER-BY-LAYERASSEMBLYIntroduction

Advanced materials from inorganic nanoparticles (NPs) and other nanocolloidssuch as single wall carbon nanotubes (SWNT) are currently one of the most dy-namic areas of todays science. They represent signicant fundamental and com-mercial interest with a wide range of applications including the next generationoptics, electronics, and sensors (16). Synthetic methods of colloidal chemistryafford manipulation of their size, surface structure, and hence, their properties(7). In optical, electrical, and magnetic devices, nanocolloids will be mostly usedas thin lms. Currently, such lms are typically made by spin coating, spraying,or sometimes by simple painting of nanoparticlematrix mixtures. The layer-by-layer assembly (LBL) is one of the most promising new methods of thin lm de-position, which is often used for oppositely charged polyelectrolytes (PE) (8,9). Ithas also been successfully applied to thin lms of nanocolloids (1033). One of themajor advantages of LBL is simplicity: the process requires neither sophisticatedhardware nor high purity of the components. At the same time under optimal con-ditions, the method produces high quality of coatings with thickness controllableat the nanometer level (26). This deposition technique is also quite universal: forvirtually any aqueous dispersion of NPs, one can nd a matching polyelectrolyteand deposition conditions yielding a steady lm buildup (34). The lateral packingof the NPs and SWNTs in individual adsorption layers can also be controlled bydifferent means producing densely packed coatings (15,27,34). The LBL coatingsare also highly homogeneous unlike the composite polymer/NP coatings obtainedby other means (35), where phase separation may occur (36). Last but not theleast, LBL affords combining NPs with other functional materials often used by

470Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

Vol. 10 NANOCOMPOSITES LAYER-BY-LAYER ASSEMBLY 471

this technique such as dyes, proteins, and DNAs, leading to a palette of multi-functional nanostructured materials (37). In essence, LBL can be a convenientmethod for processing NPs in thin lms, which opens broad perspectives for thistechnique, both in research and in industry. This article attempts to summarizethe current status of understanding of the LBL deposition of NPs, the structureof produced lms and the future applications for these materials.

Layer-By-Layer Assembly

Layer-by-layer assembly can be described as the sequential adsorption of posi-tively and negatively charged species, say A and B, by dipping a substrate alterna-tively into their solutions. Rinsing with water between adsorption steps removesthe excess of the previous solution and leaves a thin layer of charged species onthe surface, thereby preparing the surface for the next adsorption step. The dy-namic development of LBL that has been seen recently was sparked mainly byits great success with assembly of polyelectrolytes (9,30,3840); However, simi-lar ideas were previously used (41) for the assembly of colloids, for production ofsemiconductor lms (42,43), and for the assembly of NP/PE magnetic lms (10).The description of experimental details of the technique can be found in the orig-inal publications of the respected authors (see also COLLOIDS; POLYELECTROLYTES;LANGMUIR-BLODGETT FILMS).

From the analysis of the abundant literature on LBL, one can say that Aand B are preferentially chosen to be of relatively high molecular weight. Theexperimental work on polymers with different chain lengths indicates that theincrease of the molecular mass of species promotes stable LBL growth (15,4446),which is related to the diverse nature of intermolecular interactions involved inthe process (see SWNT/Polyelectrolyte Composites). Although for relatively lowmolecular weight species such as multicharged metal ions (4751) and moleculardyes (5254), the LBL assembly was also reported, and their tendency to leach outshould be accentuated (55). Heavy mass and the multiple points of attachment ofA and B render the absorption sufciently irreversible to allow for the depositionof the next layer. Either A or B is almost always a PE, while the other LBL partnercan be a dispersion of NPs, clay sheets, proteins, dyes, vesicles, DNA, viruses orother species. The omnipresence of PEs in LBL assemblies acquired popularity inthe 1990s is explained by their ability to cover irregularities owing to the rod-likeconformation of the charged macromolecules in aqueous solutions (9,56).

Ultrastrong Materials

The massstrength ratio of materials is of exceptional importance for many dif-ferent applications. The critical parts of different vehicles and crafts depend onstrength and toughness of the materials they are made of, while strict limitationson the weight of the different components are placed by the modern air and space-craft technology. SWNT presents signicant potential as a basic material for spaceapplications. Exceptional mechanical properties of SWNT (5762) have prompted

472 NANOCOMPOSITES LAYER-BY-LAYER ASSEMBLY Vol. 10

intensive studies of SWNT composites. These qualities can also be used in a va-riety of other technologies from automotive to military and medical. However,the present composites have shown only a moderate strength enhancement whencompared to other hybrid materials (5365). Although substantial advances havebeen made (66), mechanical characteristics of SWNT-doped polymers are notice-ably below their highly anticipated potential. Pristine SWNTs are well known forpoor solubilization, which leads to phase segregation of composites. Severe struc-tural inhomogeneities result in the premature failure of the hybrid SWNT/polymermaterials. Connectivity with and uniform distribution within the matrix are es-sential structural requirements for strong SWNT composites (6769). A new pro-cessing approach based on sequential layering of chemically-modied nanotubesand polyelectrolytes can greatly diminish the phase segregation and render SWNTcomposites highly homogeneous. Combined with chemical cross-linking, this pro-cessing leads to drastically improved mechanical properties. Tensile strength ofthe composites is several times higher than that of SWNT composites made viamixing; it approaches values seen for hard ceramics. The universality of the lay-ering approach applicable to a wide range of functional materials makes possiblesuccessful incorporation of SWNT into a variety of composites imparting to themrequired mechanical properties.

The thin lm membranes that are obtained as a result of the layer-by-layerprocess can be used as an intermediate or as a component of ultrastrong laminates.At the same time, the prepared membranes can also be utilized in the as-preparedform for space and biomedical technologies because they combine the strength andmultiple functionality of the SWNT membranes.

SWNT composites are typically prepared by blending, in situ polymerization,and extrusion. After extensive surface modication, such as grafting or polymerwrapping (6870), the phase segregation from a macromolecular matrix is smallerthan for pristine SWNT, but still remains high owing to the high afnity of SWNTsto each other. Vastly different molecular mobilities of both components also con-tribute to widespread phase separation. Very intense research on appropriatesurface modication of SWNT is currently under way in many groups around theworld. Nevertheless, most common loadings of nanotubes in the polymer matrixare within the 115 wt% range, whereas more than 50% of the SWNT content isneeded for materials with special mechanical performance without compromis-ing the homogeneity of the composite at the nanometer level. This high loadingof the nanotubes is particularly important when both electrical and mechanicalqualities of the nanotubes are going to be utilized.

The phase segregation between dissimilar materials can be circumvented byapplying a layer-by-layer assembly (LBL) (71). The immobilization of the macro-molecular compounds and strong interdigitation of the nanometer-thick lm al-lows for the close-to-perfect molecular blending of the components (72,73).

SWNT/Polyelectrolyte Composites. The SWNT/polyelectrolyte com-posites produced in this study were assembled onto a solid support via alter-nate dipping of a solid substrate (glass slides, Si wafers) into dispersions ofSWNT and polyelectrolyte solutions (7476). The individual assembly steps, ieadsorption of SWNT and polyelectrolyte monolayers, were interlaced by rinsingsteps to remove the excess of assembling materials. When the LBL procedure


was complete, the multilayer lms were lifted off the substrate to obtain uni-form free-standing membranes, which can be handled as regular composites(77). Such lms make possible straightforward testing of their mechanicalproperties.

SWNT were produced by laser ablation and subsequently puried via acidtreatment. Single-wall nanotubes were manufactured by laser vaporization ofcarbon rods doped with Co, Ni and FeS in an atmosphere of Ar:H2. StandardSWNT products made by HiPCO and other methods contain signicant amountof sooth, graphite akes, and remnants of the catalyst, which need to be removedprior to the assembly. The quality of the dispersion directly affects the mechanicalperformance of the resulting composite.

A suspension of SWNT raw material was reuxed in 65% HNO3 and sub-sequently puried by centrifugation. Supplemented by sonication, this treatmentresults in the partial oxidation of ca 5% of the total number of carbon atoms bothin caps and walls of SWNT (78). A similar type of dispersion can also be madefollowing other recipes such as polymer wrapping the nanotubes, and chemicalderivatization. Optimization of the aqueous nanotube dispersions should be con-sidered as one of the most critical direction of the optimization of the carbonnanotube composites and speed and quality of their processing in the compos-ites. Dispersions made by other methods are expected to be also applicable forthe preparation of LBL lms. The strength and other mechanical properties willdepend though on energy of attractive interactions between the coating agent andthe matrix as well as between SWNT and the coating agent.

In slightly oxidized nanotubes, the presence of carboxylic acid groups affordsthe preparation of metastable SWNT dispersions after 1 min sonication in deion-ized water without any additional surfactant. Thus prepared negatively chargedSWNT with zetapotential of 0.08 V can be layer-by-layer assembled with posi-tively charged polyelectrolyte, such as branched poly(ethyleneimine),Mw = 70,000(PEI) (Fig. 1). Since the overall negative charge of the SWNT used here was fairlysmall, after every 5th deposition cycle, a layer of SWNT was replaced with a layerof poly(acrylic acid), Mw = 450,000 (PAA) (Fig. 1). These additional layers improve

Fig. 1. Common polyelectrolytes, used for LBL process.


the linearity of the deposition process and present a convenient chemical anchorfor subsequent chemical modication. For the same reasons, a single PEI/PAAbilayer was deposited on a bare glass or Si substrate prior to the SWNT assembly.The assembly conditions of the entire procedure (pH, ionic strength, concentra-tions, etc) were optimized so that the dipping cycles can be repeated as many timesas needed with linear growth of the multilayers (Fig. 2a). This enables the prepa-ration of lms with any desirable thickness and architecture tailored to differentapplications. The ionic conditions of LBL assembly were the following: 1% solutionof PEI at pH 8.5; 1% PAA at pH 6 (pH 3 for wafer coating); SWNT at pH 6.8. Allsolutions were made in 18 M deionized (DI) water without addition of any extrasalt or other low molecular weigh electrolyte. DI water was also used for rinsing atpH 8.5 adjusted by NaOH. Wafers/glass slides were cleaned in piranha solution,rinsed with DI water, sonicated for 15 min and again thoroughly rinsed with DIwater. After that, they were coated with a precursor layer: PEI (10 min) + PAA(15 min, pH 3), followed by the deposition of (PEI/SWNT)5. The layer sequenceof (PEI/PAA)(PEI/SWNT)5 was repeated until the desirable thickness is obtained.Exposure times of 10 and 60 min was used for polylectrolytes and SWNT baths,respectively.

Multilayer stacks with a cumulative structure of ((PEI/PAA)(PEI/SWNT)5)6and ((PEI/PAA)(PEI/SWNT)5)8 containing 30 and 40 (PEI/SWNT) bilayers, re-spectively, were typically used in this study.

Currently, other methods are being investigated for dispersing SWNT in wa-ter to avoid excessive damage to the SWNTwall such aswrapping of the nanotubeswith copolymers, which can equally well work on the SWNT and multiwall carbonnanotubes (MWNT). For some applications, MWNTs can be a preferred materialsdue to lower cost.

Similarly to other polyelectrolyte LBL systems (71), a submonolayer ofSWNT is deposited in each deposition cycle. The nal morphology of the mul-tilayers can be described as a mixture of individual carbon nanotubes and their49 nm bundles intricately interwoven together in a ne fabric (Fig. 2c) (74,75).Two important structural characteristics should be pointed out. SWNT uniformlycover the entire surface of the substrate without any evidence of phase separa-tion. Also, the presence of oxidized at graphite sheets and other forms of carboncolloids in the experiments was very small. Both these factors contributed to themechanical properties of the composites. The quality of the nanotube material wasalso assessed by Raman spectroscopy. (Raman measurements were performed ina backscattering conguration with a 50 mW of 514.5 nm laser light was inci-dent on the samples.) The characteristic Raman peaks for SWNTs, eg the radialbreathing mode at 182 cm1 and the tangential CC stretching modes located at1560 cm1 (G1 mode) and 1583 cm1 (G2 mode), were very sharp and narrowindicating the high uniformity of the SWNT and low level of impurities present inthe lms. A barely visible peak at 1340 cm1 (D mode) revealed the presence ofresidual amounts of disordered carbon structures. Using the correlation betweenthe frequency of the radial breathing mode, , and the SWNT diameter, d, ex-pressed as d = 223.75/ (79), a value of d = 1.2 nm is obtained, which is in a goodagreement with the SWNT diameters obtained from AFM images of many indi-vidual nanotubes. From these images, the length of the nanotubes was estimatedto be 27 m.


Fig. 2. Structural characterization of SWNTmultilayers. (a) SequentialUVvis spectra ofa glass substrate in the course of the LBL deposition of SWNT. The spectra were taken for atotal number of (PEI/SWNT) bilayers indicated in the graph. (b) Raman scattering spectraof SWNT dispersion (1), LBL lm on a glass substrate (2) and free-standing lm (3). (c) Tap-ping mode AFM image (DI, multimode IIIA) of a Si wafer bearing (PEI/PAA)(PEI/SWNT)5.See Refs. 74,75.


PEI was utilized as the LBL partner of SWNT because of terminal NH2and backbone NH groups in the main chain and branches suitable for thesubsequent chemical modication of the composite (80). The PEI chains can becross-linked (1) with each other and (2) with carboxyl groups on SWNT and PAA.Chemical stitching increases the connectivity of the polyelectrolyte matrix withSWNT, and therefore, load transfer in the composite (80). The combination ofboth modication pathways was used. SWNT/PEI/PAA composite was heated to130C after the deposition of each layer. According to the study by Sullivan andco-workers (81), this treatment should result in amide bonds between a varietyof protonated and nonprotonated functional groups of PEI, PAA, and SWNT com-plementing the intrinsic ionic cross-linking of the LBL lms (81). Subsequently,the lm was exposed to glutaraldehyde at room temperature. This treatment iswell known to produce covalent bonds with primary amine groups present in thepolyelectrolyte. Samples were cross-linked in 0.5% glutardialdehyde solution inphosphonate buffer (0.054M Na2HPO4, 0.013M NaH2PO4, pH 7.4) for 1 h at roomtemperature. To remove unreacted glutaraldidehyde, the lm was rinsed with tapwater for 3 10 min and then with DI water the same number of times. Thisreaction produces a tight network of polymeric chains and nanotubes connectedby dialdehyde linkages. It was found that if only 1% of all carbon atoms of SWNTare chemically bonded to the polymer matrix, such as cross-linking drasticallyincreases the shear between them by an order of magnitude (80). Therefore, a 5%density of COOH groups on the SWNT surface cited above should be sufcientto obtain good connectivity with the polyelectrolyte matrix. Note these groups arenot completely utilized at the moment, because of the relatively low temperatureof amide bond cross-linking step.

Mechanical Properties and Testing

The mechanical properties of the LBL-assembled SWNT thin lms were studiedin their free-standing form prepared by the chemical delamination from the sub-strate (77). SWNT multilayers were separated from the silicon wafers by immer-sion into 0.5% aqueous HF for 3 min. The Raman scattering spectrum of the sepa-rated lm is almost identical to that of the supported lm and original nanotubes(Fig. 2b), demonstrating that the structure of SWNT remains mostly unalteredduring the cross-linking and delamination. The breathing mode frequency shiftsfrom 185 cm1 in the assembled lm to 182 cm1 in the cross-linked self-standinglms indicating a small expansion of the tube diameters.

The delaminated thin lms (Fig. 3a), can be easily handled in a variety ofways. They can be made of any desirable size or shape determined only by thedimensions of the substrate. The lms that we routinely prepare in this studywere ca 1 3 cm. Assemblies with a structure of ((PEI/PAA)(PEI/SWNT)5)6 and((PEI/PAA)(PEI/SWNT)5)8 displayed anSWNTcontent of 50 5wt%as calculatedfrom carbon and nitrogen EDAX peak integrals. Previously reported compositesmade with modied SWNT revealed strong inhomogeneities even at SWNT load-ings as low as 68% (64,65). The cross-sectional image of the free-standing lm(Figs. 4a and 4b) clearly demonstrates the absence of micrometer-scale inhomo-geneities although the occasional inclusion of a round 3060 nm particles can be


Fig. 3. Electron microscopy of the rupture region in SWNT multilayers. (a) SEM imageof the surface and broken edges of ((PEI/PAA)(PEI/SWNT)5)8. (b) and (c) TEM images ofruptured areas of the freestanding lms. The arrows indicate the likely stubs of the brokennanotubes bundles. They were identied as such because (1) the diameter of both of them isequal to theat of the actual SWNT bundle bridging the gap and (2) their mutual positioningpresents a virtually perfect match with the expected location of the ends of a bundle brokenduring gap opening.

seen (possibly dust). The slight variations in the gray scale contrast between dif-ferent strata show the actual variations in SWNT distribution within the sample.They originate from small deviations in SWNT adsorption conditions, such as dis-persion concentration and pH, during the buildup procedure. In SEM microscopy(Fig. 3a), the surface of the sample also appears smooth and continuous. Typically,the separation of single/multiwall carbon nanotubes and their bundles in mixedpolymer composites can be observed as whiskers clearly visible in TEM and SEMimages (60,82). The TEM examination of the initial stages of rupturing showedthat virtually no ber pullout occurs in the LBL multilayers (Fig. 3b). This canbe contrasted by extensive nanotube pullout reported before by several groups(60,82). For many TEM images obtained in different areas of the self-standinglms, we were able to observe only one SWNT bundle bridging the break region


Fig. 4. TEM examination of the homogeneity of the SWNT LBL lm. Survey (a) andcloseup (b) TEM images of SWNT lm cross sections. The top and botom sides of thelm are slightly different in roughness: the one that was adjacent to the at substrate issmoother than the growth surface of the lm. See Refs. 74,75.

(Fig. 3c). The same image also shows two broken carbon ber stubs imbedded inthewalls of the crack (marked by arrows inFig. 3c). In total, themicroscopy resultsindicate efcient load transfer in the LBL composite. Currently, similar lms areprepared from multiwall carbon nanotubes utilizing polymer wrapping (Fig. 5).

The mechanical properties of the layered composites were tested on acustom-made thin lm tensile strength tester (McAllister Inc.) recording the dis-placement and applied force by using pieces cut from ((PEI/PAA)(PEI/SWNT)5)6and ((PEI/PAA)(PEI/SWNT)5)8 free-standing lms. The tester was calibratedon similar pieces made from cellulose acetate membranes and Nylon threads.((PEI/PAA)(PEI/SWNT)5)6 and ((PEI/PAA)(PEI/SWNT)5)8 samples had an aver-age TEM thickness of 0.75 and 1.0 m, respectively. Their typical stress ( ) vsstrain () curves differed quite markedly from stretching curves seen previously

Fig. 5. Optical photograph of the freestanding LBL lms made from multiwall carbonnanotubes.


Fig. 6. Typical tensile strength curves of the SWNT LBL lms. Stressstrain dependencefor (a) ((PEI/PAA)(PEI/SWNT)5)8 and (b) a similar free-standing multilayer lm madesolely from polyelectrolytes. The dependence of the mechanical properties of the cross-linked LBL composites on humidity was tested in the range of relative humidity of 30100%, T = 298C, and was found to be negligible See Refs. 74,75.

for SWNT composites (66) and for LBL lms made solely from polyelectrolytes,(PEI/PAA)40, obtained by the same assembly procedure (Fig. 6b). They displayeda characteristic wave-like pattern, gradual increase of d /d derivative, and thecomplete absence of the plateau region for high strains corresponding to plasticdeformations (Fig. 6a). The latter correlateswell with the enhanced connectivity ofSWNT with the polymer matrix (Fig. 3). Other mentioned stretching features in-dicate the reorganization of the layered composite under stress. A process similarto the sequential breakage of cross-linked parts of coiled molecules (see AFM im-age in Figure 2) observed in natural nanocomposites, such as seashells and bones(83,84), is likely to be responsible for the wave-like pattern and the increase of thestretching curve slope.

Considering the complexity of the deformation process, the assessmentof elastic and inelastic behavior in each part of the curve will be done upondetailed microscopy investigation. Meanwhile, the values of d /d exceeding50 GPa should be noted.

The comparison with stretching curves for polyelectrolytes (Fig. 6b) showsthat the incorporation of nanotubes in the LBL structure resulted in the trans-fer of the SWNT strength to the entire assembly. The stretching curves of theSWNT multilayers display a clear break point. The ultimate tensile strength, T,was found to be 220 40 MPa with some readings being as high as 325 MPa.This is several times to an order of magnitude greater than the tensile strength of


strong industrial plastics with T = 2066 MPa (85). It is also substantially higherthan the tensile strength of carbon ber composites made by mixing: polypropy-lene lled with 50 vol% Carbon Fibers (qv) has T = 53 MPa (86). A recent studyon SWNT/poly(vinyl alcohol) ribbons with axially aligned nanotubes reported atensile strength of 150 MPa (66). The T values obtained for SWNT LBL lmsare, in fact, close to those of ultrahard ceramics and cermets such as tungstenmonocarbide, T = 340 MPa, silicon monocarbide, T = 300 MPa, and tantalummonocarbide, T = 290 MPa (85). Such strength and failure strain greater than incermets (ca >1% in SWNT LBL vs. 0.20.6% in carbides) displayed by an organiccomposite is quite remarkable.

The tensile strength of single carbon nanotubes was experimentally deter-mined to be between 13 and 50 GPa (82,87). The lower values obtained for theSWNT multilayers should be mainly attributed to the contribution of polyelec-trolytes and some uncertainty in the actual cross-sectional area at the break pointand a degree of cross-linking. The mixing law predicts that a polyelectrolyte ma-trix with T = 9 MPa makes a negligible contribution to the strength of the com-posite while taking about 50% of its volume fraction (SWNT is d = 1.14 g/cm3).Since the density of the polyelectrolytes used for the preparation of the multi-laters, ie PDDA d = 1.04 g/cm3, PAA d = 1.14 g/cm3 is almost the same, thevolume fraction of SWNT in the composite can be considered to be equal to themass fraction). Additionally, the decrease of the mechanical strength of the nan-otubes in the process of ionic functionalization (estimate 15%) (88) should alsobe considered as a factor affecting the strength of these composites. These issuesare pointed out as means of further optimization of the multilayers. Tuning oftheir molecular structure and composition should lead to vast improvement oftheir mechanical properties that could possibly approach those of pristine carbonnanotubes.

It is also interesting to compare the T values for SWNT composite lmsto those obtained for other LBL lms made with other inorganic compo-nents such as montmorillonite platelets, M, and nanoparticles, NP, for instance810 nm magnetite nanoparticles. The free-standing lms (PDDA/NP)40 and(PDDA/NP/PDDA/M)40 made according to Reference (77) revealed T equal to 40and 72MPa, respectively. In conjunctionwith the tensile strength data (see above),it can be concluded that inorganic or SWNT components act as a molecular armorin the layered composites signicantly reinforcing them. The molecular organiza-tion of the material made possible the transfer of a part of their strength to theentire assembly.

The high structural homogeneity and interconnectivity of the structural com-ponents of the LBL lms combined with high SWNT loading leads to signicantincrease of the strength of SWNT composites, being somewhat weaker than someother organic and carbon ber materials but at the same time being far beyondtheir potential as ultrastrong composites. The described technique minimizes thestructural defects originating from phase segregation and opens a possibility forthe molecular design of layered hybrid structural materials from different poly-mers and other nanoscale building blocks. The prepared free-standingmembranescan serve as a unique component for a variety of technologies. One of the greatadvantages of it in respect to other technologies is the ability to prepare ultrathin,ultrastrong membranes with minimal heterogeneity.


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NICHOLAS A. KOTOVARIF A. MAMEDOVUniversity of MichiganDIRK M. GULDIUniversity of Notre DameMAURIZIO PRATOUniversita` di TriesteJAMES WICKSTEDOklahoma State UniversityANDREAS HIRSCHUniversitat Erlangen-Nurnberg

909408027304 Layer by Layer Assembly

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