Fibres and coatings with unique optical, magnetic and electrical properties are being widely used in research for both military and commercial applications and these dynamic colour responsive "chameleon" fibre systems will have wide application in a variety of new textile products, enlighten P Sudhakar and N Gobi."

Technology is getting smaller and faster and we all know the speed of this development is increasing. We see an on-going miniaturisation and production of materials equipped with special properties. It is possible to integrate properties of sensitivity, information and intelligence into single materials. 'First generation' textiles refer to one of humanity's early technologies, the spinning and weaving of natural fibres. 'Second generation textiles' were developed as alternatives to natural fibres and include synthetic, petroleum-based fibres like nylon, polyester and polypropylene. Most recently, we have moved towards 'third generation' textiles, enabled by the latest advances in material and biological sciences, nanotechnology and intelligent systems.

Intelligent textiles represent the next generation of fibres, fabrics and articles produced from them. They can be described as textile materials that think for themselves. Many intelligent textiles already feature in advanced types of clothing, principally for protection and safety and for added fashion or convenience. One of the main reasons for the fast development of intelligent textiles is due to the importance given to the military applications. This is because they are used in different projects such as extreme winter condition jackets or uniforms that change colour so as to improve camouflage effects.

Intelligent textiles provide ample evidence of the potential and enormous wealth of opportunities still to be realised in the textile industry, in the fashion and clothing sector, as well as in the technical textiles sector. Moreover, these developments will be the result of active collaboration between people from a whole variety of backgrounds and disciplines like engineering, science, design, process development, and business and marketing.

Here are four quite similar explanations of intelligent textiles:

They are materials that react to impulses without the need for us to control them.
They are able to respond to its environment.
In garment they react to impulses coming from outside or inside.
They react automatically to some kind of stimuli.

Intelligent textiles are fibres and fabrics with a significant and reproducible automatic change of properties due to defined environmental influences. Colour sells textiles. Traditionally, the industry has been concerned not only with colour hue and intensity, but also with maintaining the colour regardless of environmental influences. Yet there are numerous exciting and important markets, which require materials, which alter their colour on demand. Chameleon fibres will allow for the creation of value-added products
in traditional industry markets, as well as entry into entirely new areas. It is the objective of this article to provide the textile and fibre industry with a series of new "smart" materials that can quickly change their colour hue, depth of shade, or optical transparency by the application of an electrical or magnetic field.

Methodology for production of chameleonic textiles

The concept of producing textiles that readily vary in colour has long been an anathema to the textile colourist for whom achieving permanency of colour has been a primary goal stretching back into antiquity. Consequently, colourant manufacturers have striven for many years to develop fast colouring matters by
hunting for dyes and pigments that are chemically inert and physically unresponsive, once they have been applied to a substrate.

Some methods are:

pH changes: Molecules can change colour dramatically in the presence of acids and bases, but these reagents and the solvent required to transport them make this method extremely difficult to implement in the applications.

Oxidation state changes: This method is also highly effective, but it requires the migration of ions. The response time can be fast in solvents, but this complicates the device. Gel-type devices might also be possible, though physical robustness, oxygen stability and response times represent serious engineering challenges. A device built on this principle would be similar to a polymer LED.

Bond breaking/making: There are a number of systems that undergo reversible bond-breaking, bond forming processes that result in dramatic colour changes. Most commonly, these are light initiated processes.

Mechanochromism: Certain compounds have been shown to undergo colour changes as a result of applied stress. And a mechano-chromic system is constructed by surface modification of conducting polymers.

Electric or magnetic field effects: Some highly polaraisable systems have been observed to change colour in the presence of electric or magnetic fields.

Chromic materials

Chromic materials are also called chameleon fibres, because they can change their colour according to external conditions. These materials have mostly been used in fashion, to create funny colour changing designs. Because of this, some people fear that the chromic materials will be a short boom. But the accuracy and endurance of the materials are all the time being improved. Chromic materials are the general term referring to materials which radiate the colour, erase the colour or just change it because its induction caused by the external stimuli, as "chromic" is a suffix that means colour. So we can classify chromic materials depending on the stimuli affecting them:

Photo chromic: External stimuli energy is light.
Thermochromic: External stimuli energy is heat.
Electro chromic: External stimuli energy is electricity.
Piezorochromic: External stimuli energy is pressure.
Solvate chromic: External stimuli energy is liquid.
Carsolchromic: External stimuli energy is electron beam.

pH Changes

A cheaper, albeit less sensitive, thermochromic alternative that has been applied to textiles does not actually employ chromophores that are themselves thermo-chromic, but which are instead sensitive to some variable other than temperature. For example, pH sensitive colourants (Figure 1) known as colour formers, such as Crystal Violet lactone, can be used as the basis of such a system in which the variable is acidity.

Functional dyes of this type tend to ring open from a colourless to a coloured state when placed in contact with acid. For example, Crystal Violet lactone turns from colourless to a deep blue upon protonation. The trick to inducing thermochromic effects using colour formers is to design a system in which the acidity experienced by the colourant varies with heating and cooling, which in turn leads to the colour of the system changing in response to differences in temperature.

One means of achieving this requires the combined use of three components: the colour former, an acidic colour developer (typically a phenolic material) and a non-polar co-solvent medium (often a low-melting point, long-chain alkyl compound) that controls the interaction
between the first two ingredients of the formulation. When the components are heated and mixed together in the correct proportions so that the colour former and developer are dissolved in the co-solvent and the solution then cooled, the solid composite formed is intensely coloured.

Heating the composition above its melting point (determined largely by that of the co-solvent) results in complete colour loss. The colour change is reversible, the point at which it occurs corresponding closely to the range of temperatures at which the formulation melts.

Since these systems involve changes in phase between coloured solid and colourless liquid states, applications must generally employ microencapsulation or lamination to protect the composites and safeguard their thermochromic properties. For textile applications, the former approach is used. This restricts the techniques available for applying the thermochromic material to textiles to that of pigment printing of fabrics or incorporation into synthetic fibres during their manufacture.

Identification of chromophores

The coloured substances owe their colour to the presence of one or more saturated groups responsible for electronic absorption. Examples: C = C, C ≡ C, C = N, C ≡ N, C=O, N = N, etc, they all absorb intensely at short wavelength end of the spectrum, but some of them (example carbonyl) have less intense bands at higher wavelength owing to the participation of n electrons.

The part (atom or group of atoms) of a molecular entity (Table 1) in which the electronic transition responsible for a given spectral band is approximately localised. The term arose in the dyestuff industry, referring originally to the groupings in the molecule that are responsible for the dye's colour. The molecular entity is the any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer, etc, identifiable as a separately distinguishable entity.

Molecular entity is used in this Compendium as a general term for singular entities, irrespective of their nature, while chemical species stands for sets or ensembles of molecular entities. Note that the name of a compound may refer to the respective molecular entity or to the chemical species, eg, methane, may mean a single molecule of CH4 (molecular entity) or a molar amount, specified or not (chemical species), participating in a reaction. The degree of precision necessary to describe a molecular entity depends on the context. For example 'hydrogen molecule' is an adequate definition of a certain molecular entity for some purposes, whereas for others it is necessary to distinguish the electronic state and/or vibrational state and/or nuclear spin, etc of the hydrogen molecule. A chemical group capable of selective light electromagnetic radiation that has a wavelength in the range from 4,000 (violet) to about 7,700 (red) angstroms and may be perceived by the normal unaided human eye. Electromagnetic radiation of any wavelength absorption results in the colouration of certain organic compounds.

Synthesis of new polymers

Redox Polymers as Chromophores: Many electrically conducting polymers and even smaller oligomers built from the same monomers, undergo distinct colour changes when oxidised and reduced. By tuning the substitutions on the polymer backbone, we can adjust the colours that a particular system displays. This work is designed to correlate structure with chromatic properties, switching speed, oxygen stability, etc. PPV molecules are "tunable" by small structural variations.

PPV oligomers emit at different wavelengths for different molecular weights.

PPV' s with kinked chains have different emission spectra than non-kinked chains.

PPV' s with kinked chains have enhanced solubility.

Therefore, the constructing and developing synthetic routes to oligomers of PPV with well defined length and structure shown in Figure 2.

From here in (Figure 3), a variety of polymers can be prepared.

Processing of electrically conducting polymers

Electrically conducting organic polymers are required to generate the electric fields for Chameleon Materials. Stable, homogeneous fields can only be achieved from high quality, homogeneous polymers. Unfortunately, the development of processing technology in this field has lagged far behind the exploratory synthesis. Polyaniline is one of several candidates being explored as electrode materials. Solution processing of PANI is well established, but the possibility of thermal processing has yet to be fully explored.

The Leucoemeraldine base (LEB) form of polyaniline is the fully reduced form. In order to become conducting, the polymer must be partially oxidised and protonated. The LEB form is somewhat oxygen sensitive, particularly at higher temperatures, so these studies were performed either under nitrogen or vacuum. Powders of LEB were prepared by deprotonation and reduction of the as grown salt. The material shows a Tg of 170-2000C and a strong endothermic at 365-3950 C.

As it can be seen from the TGA thermal gravimetric analysis (Graphs 1 & 2), this event is not associated with a mass loss. Heating the sample below the melting transition results in a dramatic increase in the crystallinity of the powder as evidenced by X-ray diffraction. The conductivity of the sample is not adversely affected by the annealing process and the LEB can be converted to its conducting form either before or after heat treatment.

Surface attachment of chromophores to conducting polymers

A major thrust is to develop of methods to attach chromophores to the surface of films and fibres. And particularly interested in attachment to electrically conducting polymers, as this give us a method to alter the chromophores oxidation state or to subject it to an electrical field.

The optical response of a chromophore to an electric field is highly dependent upon its molecular orientation. One way to insure consistent sharp colour change is to assemble the chromophores on or very near the surface of the film or fibre. Thiol is powerful nucleophiles that might serve as anchors for attaching molecular devices to conducting polymers. The mechanism and extent of the nucleophilic attack will determine how useful this process will be in a given system. The following figure suggests how a chromophoric array, might be assembled into an organized "near-monolayer" on the surface of a polymer film or fibre shown in Figure 4 - Chromophoric Array. Films of four conducting polymers (while doped to their cationic, conducting forms) were immersed for one hour in a THF solution of an alkanethiol or a fluoroalkane-thiol. The films were then rinsed thoroughly to insure that any adsorbed thiol was removed. The extent of surface modification was measured by contact angle analysis. The raw data was analysed by the method of Wu.

The surface was found to be significantly changed by this process, as indicated by the change in surface energies and polarities. Table 2 shows surface energies and polarities.

The modified surface of the electro active polymer was further studied by atomic force microscopy. This was done by making "forcedistance"measurements on both modified and unmodified polymer surfaces. These measurements involve pressing the atomic force microscopy AFM cantilever into the polymer surface, and then measuring the amount of force required to pull it away. Since the polymer surface is very rough on the atomic force microscopy AFM size scale, the cantilever was modified by attaching a 5 micro silica sphere. This sphere could also be derivatised to give chemical sensitivity to the probe. Figure 5 shows the modification of surface of the Electro active polymer.

The results (Graph 3) show that the films were homogeneously modified and that the changes observed in the surface energies were a result of the thiol, not some other processing parameter. Further, they show that the density of thiol packing is very high, nearly equivalent to that seen on a glass surface.

Additional spectroscopic and electrochemical work has shown (see references for details) that the mechanism of thiol attack varies from polymer to polymer. For example, polyaniline (Figure 6) can reform an electro active polymer after thiol attack, while polypyrrole cannot.

Current work is focused on the attachment of chromophoric species to the surfaces of the conducting polymers. For example, porphyrins and porphyrin arrays have been attached and their spectro electro chemistry (Figure 7) has been explored.

Conclusion

The creation of field-responsive fibres, chameleon fibres, is a multi-disciplinary endeavour. In addition to chromophores, polymeric materials can generate a uniform, stable field for excitation of the colour
change processes. This may lead to micro devices encapsulated in fibres
for a variety of technical applications. These intelligent clothes are
worn like ordinary clothing providing help in various situations according to the designed application. Though lots of new products have come into existence, still there is
vast scope to utilise new technologies such as chameleonic textiles.

References:

1. Cha C, Hardaker S Setal: New Approaches to the Study of Polyaniline, Journal of Synthetic
Metal, 84(1), 743 (1997).

2. R Gregory et al: Chameleon Fibres, US National Textile Centre Annual Report M-98 C01.

3. S Nato et al: Temperature Sensitive Colouring Materials, Kikai Gakkaishi (1989), 42 (9), 435 - 439.

4. T Hongu et al: New Fibres, New York, 1990.

5. Liu T and R J Samuels et al: Novel Method for Optical Characterisation of Films, Proceedings of Interpak 2001, Hawai, July 8 - 13, 1 - 5, (2001).

6. P Leitch et al: New Material in New Millennium, Journal of Industrial Textile 2000, 29 (3), 173  191.

7. R Greygory et al: Evolution of an Isotropic Structure of Poly (Phenylene Vinylene) Films with Stretching, SPE ANTEC, 46, 1449 (2000)._

About the author:

P. Sudhakar is lecturer in the Department of Textile Technology K S Rangasamy College of Technology, Tiruchengode. Email: sudhakaren_p@rediffmail.com

N. Gobi is a lecturer in the Department of Textile Technology, K S Rangasamy College of Technology, Tiruchengode. E-mail: gobsn@yahoo.co.in

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