Flash welding of conducting polymer nanofibres

LETTERS

nature materials | VOL 3 | NOVEMBER 2004 | www.nature.com/naturematerials 783

Flash welding of conducting polymer nanofi bresJIAXING HUANG AND RICHARD B. KANER*Department of Chemistry and Biochemistry and California NanoSystems InstituteUniversity of California, Los Angeles, California 90095-1569, USA*e-mail: [email protected]

Published online: 24 October 2004; doi:10.1038/nmat1242

The absorption of light by a material generates heat through non-radiative energy dissipation and exothermic photochemical reactions1. In nanostructured materials, the heat generated

through photothermal processes will be confi ned within the individual nanostructures when heat transfer to neighbouring nanostructures and the environment is slow. Th is leads to unprecedented photothermal eff ects that cannot be observed in bulk materials, especially when a strong, pulsed light source is used2,3. Here we demonstrate an enhanced photothermal phenomenon with conducting polymer nanofi bres in which a camera fl ash causes instantaneous welding. Under fl ash irradiation, polyaniline nanofi bres ‘melt’ to form a smooth and continuous fi lm from an originally random network of nanofi bres. Th is photothermal eff ect can be used to form asymmetric nanofi bre fi lms, to melt-blend polymer–polymer nanocomposites rapidly and to photo-pattern polymer nanofi bre fi lms.

Polyaniline is a deeply coloured conjugated polymer that has been extensively studied during the past twenty-fi ve years for its electrical properties4. In powder form, undoped polyaniline is dark blue, changing to a deep green conducting form when doped by acids5. As polyaniline has extremely low luminescence effi ciency, it converts most of the energy absorbed from light into heat6–8. We have now discovered that when exposed to a camera fl ash up close (within several centimetres), fi ne powders of polyaniline nanofi bres9,10 in either doped or undoped form respond audibly with distinct popping sounds and the concomitant formation of agglomerates within the exposed area. Transmission electron microscopy (TEM) images show that the nanofi bres appear to have melted and merged together (Fig. 1). This inspired us to explore welding and fi lm formation using polyaniline nanofi bres exposed to a camera fl ash.

a b

1 mµ 1 mµ

Figure 1 Typical TEM images showing polyaniline nanofi bres before and after exposure to a camera fl ash. a, Before exposure; b, after exposure.

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© 2004 Nature Publishing Group

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First, we found that a random network of nanofi bres can be fl ash welded to create a continuous fi lm. Polyaniline nanofi bres were cast from a water suspension onto fl at substrates such as silicon wafers. On drying, a fi lm containing a random mat of nanofi bres was obtained (Fig. 2a,b). After exposure for milliseconds to a camera fl ash, the fi lm became smooth and shiny (Fig. 2c,d). The change in surface roughness can be clearly seen with the naked eye. Optical microscope images in refl ectance mode show a distinct contrast in refl ectivity between the welded (Fig. 2b) and unwelded

(Fig. 2c) areas of a nanofi bre fi lm. Scanning electron microscopy (SEM) images reveal that the nanofi bres on the surface (Fig. 2a) are welded together to create a continuous fi lm (Fig. 2d). The pinholes correspond to the free volume in the nanofi bre fi lm before welding. The difference in surface roughness before and after welding also alters the wettability of the fi lm. For example, before fl ash welding, a porous, undoped nanofi bre fi lm, although hydrophobic, absorbs water droplets like fi brous fi lter paper (Fig. 2e). However, after fl ash welding, the less porous surface repels water with a contact angle of 99.0° because of its bulk hydrophobic nature (Fig. 2f).

Surprisingly, fl ash-welded fi lms can be readily removed from their substrates by water. They can also be peeled off by touching a corner to cellophane tape. The nanofi bres on the exposed side are welded together, whereas those on the unexposed side remain intact as seen in an SEM image (Fig. 3). Fourier-transform infrared spectroscopy studies (see Supplementary Information) on both sides of the fi lm indicate that the polyaniline nanofi bres are chemically cross-linked on fl ash irradiation11–14. This leads to a marked change in solubility. For example, a cross-linked polyaniline nanofi bre fi lm is no longer soluble in N,N-dimethylformamide (DMF), which is a good solvent for polyaniline nanofi bres (Fig. 3, inset). Therefore, fl ash welding appears to be a convenient method for making an asymmetric fi lm (Fig. 3, scheme). Asymmetric fi lms are particularly useful in many applications including separation membranes15,16, chemical sensors and actuators17–20. Such fi lms are usually made through multiple, relatively time-consuming steps15–20. Photothermally induced welding offers a rapid route to create free-standing asymmetric fi lms.

A great advantage of fl ash welding is its ability to weld certain areas selectively on a nanofi bre fi lm using a pre-designed photomask. This enables a pattern defi ned by the mask to be imprinted into the fi lm. A demonstration is shown in Fig. 4, in which a well-defi ned pattern is developed on a nanofi bre fi lm that is identical to the mask used. The areas exposed to the camera fl ash (the squares) are smooth and therefore appear bright gold because of the refl ectance of incident light. The protected areas (the lines) remain as green nanofi bres and appear darker because the rough surface scatters light and yields low refl ectance. The high contrast between the resulting patterns is due to

a b c d

e f

1 mµ 1 mµ

Figure 2 A polyaniline nanofi bre fi lm before and after welding. a, b, before welding; c, d, after welding. a, d, SEM images (scale bar: 1 µm) of the nanofi bre fi lm before and after welding. b, c, Optical microscope images (x250 magnifi cation) showing the refl ectance contrast of the unwelded and welded areas. e, f, Photographs showing water contact angles on the nanofi bre fi lm before and after welding.

Flashwelding

Peeloff

hν

1 mµ

Figure 3 SEM image showing the cross-section of an asymmetric free-standing nanofi bre fi lm produced by fl ash welding. The schematic diagrams above the SEM image illustrate the process. The inset picture shows that the cross-linked nanofi bre fi lms become insoluble in DMF. The pale blue colour is due to partial dissolution of uncross-linked polyaniline.



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nature materials | VOL 3 | NOVEMBER 2004 | www.nature.com/naturematerials 785

a change in refl ectivity induced by fl ash welding. Scanning electron microscopy studies confi rm the morphological differences comparable to Fig. 2a and d. Because many properties of polyaniline (conductivity, surface area, optical absorption, permeability and thermal stability) change after crosslinking21–25, fl ash welding through a mask represents a convenient way to fabricate polymer fi lms into pre-designed patterns, which could be especially useful for organic micro- or nano-devices.

The heat generated during a camera fl ash can be used to weld polyaniline to another polymer, offering a rapid and clean optical technique for polymer welding. This concept can be demonstrated by fl ash welding polyaniline nanofi bres onto polystyrene spheres as follows: First, polyaniline nanofi bres and polystyrene microspheres (~1 μm in diameter) were mixed in water. Then fi lms of a random

polyaniline–polystyrene mixture were made by drop casting. Such fi lms look white because of strong light scattering (Fig. 5, inset), as there are a signifi cant number of polystyrene beads on the surface (confi rmed by an SEM image; Fig. 5a). The blue hue is due to the underlying colour of undoped polyaniline. After irradiation by a camera fl ash, the nanofi bres and microspheres fuse together, leading to a dark-blue-coloured fi lm (Fig. 5b). This suggests that fl ash welding can be used for making polymer–polymer blends and offers a new way to embed guest polymer particles into a host matrix. Even polytetrafl uoroethylene (Tefl on) particles can be welded using polyaniline nanofi bres.

One can speculate on the origin of the heat generated by a camera fl ash. A photographic fl ash produces a relatively high intensity light

a b

Figure 4 Optical microscope images showing that fl ash welding through a copper grid (a) reproduces the grid pattern on a polyaniline nanofi bre fi lm (b). Scale bars: 100 μm.

a b

2 mµ 2 mµ

Figure 5 SEM images showing a mixture of polyaniline nanofi bres and polystyrene spheres before (a) and after (b) fl ash welding. Scale bars: 2 μm. The inset shows the visual colour contrast between un-exposed (bluish white) and exposed (dark blue) areas.



LETTERS

786 nature materials | VOL 3 | NOVEMBER 2004 | www.nature.com/naturematerials

within a short pulse (~1 ms)2,3. Although the energy is insuffi cient to melt the bulk polymer, local hot spots are likely. In single-walled carbon nanotubes and silicon nanowires, local hot spots of above 1,500 °C have been suggested on fl ash irradiation2,3,26,27. In polyaniline nanofi bres, hot spots are likely to form around the chromophores on the polyaniline chains as they absorb visible light owing to highly effi cient photothermal conversion of polyaniline6–8, which may further initiate and propagate exothermic crosslinking reactions between polymer chains. Because polyaniline has a low thermal conductivity (~10–1 W m–1 K–1)6,7, the heat from photothermal conversion and crosslinking reactions could accumulate within the nano-sized domains leading to the welding of nanofi bres. In contrast to carbon nanotubes and silicon nanowires, slight burning of the nanofi bres occurs only when the fl ash is very close (within 0.5 cm) and is visible as smoke coming off the powders. Melting of the polymer nanofi bres may act as a benign way to drain the pulsed heat away, avoiding complete structural breakdown or combustion2,3,26,27, and thus permitting many potentially useful applications.

Flash welding is not limited to pure nanofi bre powders; camera fl ash treatment of fi ne conventional polyaniline powders also produces a photoacoustic response that is probably due to the presence of a small number of nano-sized features. Such nanostructures become ‘smoothed’ out after fl ash welding. Preliminary tests on polyaniline derivatives and other conducting polymers such as polypyrrole and polythiophene indicate that camera fl ash irradiation is capable of welding them when nanostructures smaller than 100 nm are present. Flash welding should be a general phenomenon for materials with a high absorbance (deeply coloured), high photothermal effi ciency (low emission), low thermal conductivity and small size. It holds great promise for making asymmetric membranes, photo-patterning nanostructured fi lms and welding polymer–polymer and polymer–inorganic nanocomposites.

METHODSPolyaniline nanofi bres were synthesized using interfacial polymerization as described elsewhere9,10.

Thin fi lms of nanofi bres were made by drop casting a water dispersion of nanofi bres onto a fl at sub-

strate (for example, silicon wafer). Polystyrene spheres (Polyscience) and Tefl on particles (Aldrich)

were used as received. Flash-welding experiments were carried out using a photographic camera fl ash.

Morphological changes in the samples were observed by transmission electron microscopy

(JEOL 100CX), scanning electron microcopy (JEOL 6700, Philips XL 30) and optical microscopy

(Zeiss Axiotech 100). The wettability of the fi lms was studied using a contact angle analyser (First

Ten Angstroms, FTA 125). Fourier-transform infrared spectra of the nanofi bre fi lms were taken in

attenuated total refl ectance mode (Nicolet Avatar 360).

Received 13 August 2004; accepted 8 September 2004; published 24 October 2004.

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AcknowledgementsWe thank C.-W. Chu for help with FTIR measurements. We acknowledge the fi nancial and program

support of the Microelectronics Advanced Research Corporation (MARCO) and its Focus Center on

Function Engineered NanoArchitectonics (FENA).

Correspondence and requests for materials should be addressed to R.B.K.

Supplementary Information accompanies the paper on www.nature.com/naturematerials

Competing fi nancial interestsThe authors declare that they have no competing fi nancial interests.



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Flash welding of conducting polymer nanofibres