Environmentally Responsive (Smart) Textiles

Environmentally Responsive (Smart) Textiles

Introduction to Smart Textiles

What are Smart Textiles?

Conventional textiles are used to cover the human body and function as a protective layer for the body from dust, sunlight, wind, and other contaminants present in the normal living environment. It is also used for carrying out technical functions which utilize their flexible and strong structure. The textiles may be used for additional function specific to an adverse or extreme climate, job-environment or profession to enhance adaptability and/or productivity of the user. When textile assumes an additional function over and above the conventional purpose as mentioned above, it may be regarded as Smart Textile. And if this additional functionality changes with change in use conditions, then textile may be regarded as Active Smart or Intelligent Textile.

Mother Nature is the main source of inspiration for such innovations. Human skin is an excellent example of smart textile with functional capabilities which can perform protection, sensing and actuation. Some other examples seen in nature are - Chameleons change colour according to the environmental situation, leaves of Mimosa pudica collapse suddenly when touched, sunflower turn towards the Sun.

By understanding the nature’s design concepts, researchers are trying to bridge the disparity and gap between synthetic and natural materials.

The mechanism of functionality expected in smart textile can be understood from Figure 1.

Figure 1. Mechanism of Smart Textiles

Smart Textile or Responsive Textile can be broadly classified based on the active responses exhibited by them:

Change of Shape Reversible or one way Increase or decrease of dimensions, bending etc. Storage and release Heat Drugs, chemicals etc. Electronic functionality Communication, entertainment, monitoring

In this chapter we would learn about (A).  Shape changing fibres and (B).  Thermo-regulated (heat storage and release) textiles

Shape changing Textiles (fibres, yarns and fabrics)

Shape changing fibres, yarns and fabrics are developed using stimuli sensitive polymers (SSPs). These polymers show a reversible transformation from one state to another as a response to various stimuli from the environment. These polymers are also known as smart-polymers or intelligent-polymers. Across the transition temperature, the linear SSP’s change from soluble (clear) to insoluble form (turbid). However, in the gel form they swell and deswell because of the presence of cross-links by absorbing or releasing water. For most applications these are generally used in the gel or cross-linked form. The change may occur in their configuration, dimension or physical properties with a small change in appropriate stimuli.

Figure 2. Smart breathable textiles using temperature sensitive polymers

Among the various stimuli-sensitive polymers, the temperature-sensitive polymers are the most widely studied polymers. Besides the temperature stimulus, the other chemical and physical stimuli which bring about a reversible transition in such polymers include electric field, solvent composition, light, pressure, sound, stress, magnetic field, and chemical and bio-chemical stimuli (i.e. pH and ions). These polymers provide a big opportunity for creating intelligent materials.

Figure 3. Schematic representation of different types of stimuli

Smart or intelligent materials that respond to their environment by changing their shape or properties are useful for various critical applications in biomedical and engineering fields such as controlled drug delivery, separation, enzyme-activity control, tissue culture and now recently for artificial muscles and smart textiles.

Figure 4. Application areas for Smart Polymers

As these hydrogels are mostly synthesized using crosslinkers (like -  N,N' - methylenebisacrylamide or ethylene glycol dimethacrylate) during polymerization, this cross-linked physical structure results in a number of drawbacks

The stimuli-sensitive polymer-gel structures are weak /have poor mechanical properties and therefore are required to be used as thick structures. However, in thick gel structures, the transitional response is poor due to slow diffusion.

The simplest way to improve the response time is to make hydrogels thinner, smaller and stronger.

The processing of these materials into thin shapes and their stabilization or integration to textile materials is likely to solve result in strong structures, which can be used in thinner dimensions and would show higher magnitude of response in short span of time. Such responsive textile structures would show enhanced performance compared to gel structures.

Smart or intelligent textile structures can be developed using the following approach:

Step 1.  Preparation of suitable linear polymers with tuned response behaviour

Step 2. Shaping these linear polymers into desired thin structures (by solution spinning or coating)

Step 3.  Stabilization of shape by cross-linking (chemical or physical)

Some of the approaches that are used for imparting smart functionality to textile are shown in Figure 5.

Figure 5. Schematic representation of different approaches used for imparting smart functionality

By appropriate selection of responsive polymer containing reactive functional groups for integration or stabilization, shape changing textile structures with very fast response can be developed for various applications. In the next lecture we would study about temperature responsive textiles.

Temperature Responsive Textiles

Temperature responsive textiles are developed using temperature sensitive polymers (TSPs). These polymers exhibit transition known as Lower Critical Solution Temperature (LCST).These polymers have both hydrophilic and hydrophobic groups in their structure.

TSP’s present a fine hydrophobic-hydrophilic balance in their structure, and small temperature changes around the critical temperature, makes the chains to collapse or to expand responding to the new adjustments of the hydrophobic and hydrophilic interactions between the polymeric chains and the aqueous media.

Some examples of TSP’s and their transition temperature are shown in the Table 1

Table 1. List of Temperature Sensitive Polymers with their LCSTs

Class Polymer Name/Chemical repeat unit LCST,°C poly (N-alkyl substituted acrylamides) poly N isopropyl acrylamide(PNIPAm) 32 poly(N-n-propylacrylamide) 16-19 poly(N-isopropylmethacrylamide) 40 poly(N-cyclopropylacrylamide) 4-6 poly (N-vinylalkylamides) poly(N-vinylcaprolactam) 32-35

Poly-N-isopropyl acrylamide (PNIPAm) is the most widely studied thermo-responsive polymer. It has hydrophobic backbone and a pendant group which has a hydrophilic amide moiety and a hydrophobic isopropyl moiety.

Figure 6. Hydrophilic and hydrophobic moieties in PNIPAm polymer(Transition temp. 32 °C)

Below LCST, the hydrophilic interactions (the hydrogen bonds formed between water molecules and N-H or C=O groups of PNIPAm) dominate and polymer becomes soluble in water, while above this temperature hydrophobic interactions dominate and polymer becomes insoluble in water.

The transition temperature of these TSP’s can be tuned by changing the ratio of hydrophilic and hydrophobic groups, incorporation of additives, or by changing the nature of polymer system. As mentioned above the polymer gels prepared from these thermo responsive monomers change shape by swelling in water below transition temperature and deswell above transition temperature.

As explained before, the response of these materials can be enhanced by processing a suitably designed TSP into structurally strong thin films, fibres, coatings, and chemically integrated TSP with yarns and fabrics using the three steps. Using a copolymer based on N-tert butylacrylamide and acrylamide, temperature responsive smart textiles have been reported. The chemical structure of the temperature responsive copolymer and the chemical reaction showing its integration with textile are shown below. Here, N-tert butylacrylamide is a responsive monomer and acrylamide is a reactive monomer for attachment or cross-linking reaction in presence of suitable cross-linkers.

(a)

(b)

Figure 7. (a) Preparation of temperature responsive polymer, (b) Integration of polymer with substrate

The TSP synthesized in conventional gel forms could be cut with difficulty into 2 mm thick discs. This gel disc showed a swelling of 490%, and took 90 minutes to attain 70% swelling, while the deswelling took 50 minutes.

Shape Changing Fabric

Shape changing model fabric prepared by using cotton yarns coated with temperature responsive polymer and subsequent cross-linking. The cross-links (i.e., covalent bonding) were formed using polycarboxylic acid between amide side-groups of the copolymer and hydroxyl-group of the cellulosic substrate (in case of coatings).

The transition properties and the response time of the TSP in the different processed forms in comparison to polymer-gel discs are given in Table 2.

The responsive coated yarn shows very high degree of swelling and deswelling across the transition temperature in a very short span of time. For 70% of equilibrium swelling, it takes 3 minutes compared to 90 minutes for gels.

Table 2. Comparison of transition properties of TSP’s in different processed forms

Sample type Thickness Swelling (change in shape %) Time for 70% of equilibrium swelling Time for complete deswelling Gel disc (conventional) 2 mm 490 90 min 50 min Coated on Yarn 39 µ m 4500 3 min 10 sec Fibre 30 µ m 17800 <5 div="" sec=""> <1 div="" sec="">

The optical photographs of the model fabric at different temperatures are shown in Figure 8. At temperatures lower than transition temperature the yarns are highly swollen, while above the transition temperature, the yarns deswell by releasing all the water. The percentage cover of the model fabric (immersed in a water bath) changed from 0% at 6 °C, to 39% at 30 °C, and 57% at 80 °C. The change was completely reversible for several cycles. The change in the porosity (percentage cover) with temperature can be clearly seen in optical microphotographs given in Figure 8.

Figure 8. (a) Optical micrographs of model fabric at different temperature (b) Corresponding percentage cover of model fabric at different temperatures

Shape Changing Fibres

The TSP was also successfully converted into a shape changing textile fibre of fine diameter. The fibre underwent change in both diameter and length with change in temperature as shown in the optical micrographs (178 % change in volume ) Figure 9.

Figure 9: The optical micrograph of thermo-responsive shape changing fibre at 100 X (i) Deswollen fibre in water at 80 °C (ii) Swollen fibre in water at 6 °C.

The time for 70% transition (swelling) was found to reduce dramatically from 90 minutes for the 2 mm gel disc to less than 5 seconds for the TSP fibre, while the change in shape (swelling ratio) of the fibres increased by 36 times (shown in Table 2 ).

The rate and extent of swelling (equilibrium swelling) can be tuned by varying the degree of cross-linking on rate of transition and equilibrium swelling of the fibre is shown in the Figure 10.

Figure 10. Rate of transition of shape changing fibres at two different crosslinking levels (a) lower and (b) higher

Figure 11. Reversibility & cyclability of SSP coated fabric immersed in water at two different crosslinking levels (a) lower and (b) higher

pH-Responsive Textiles

What are pH responsive polymers?

Polymers that can sense the pH of their environment as a signal, judge the magnitude of the signal and change their properties accordingly are known as pH responsive hydrogels. In this case, the key element of the system is the presence of ionizable weak acidic or basic moieties attached to a hydrophobic backbone. The functional groups include ionisable acidic pendant groups such as carboxylic and sulfonic acids or basic groups like amine that can accept and donate protons in response to the environmental change in pH. As the environmental pH changes, the degree of ionization in pendant groups undergo dramatic change at a specific pH called pKa. This rapid change in the net charge of pendant groups causes an alternation of the hydrodynamic volume of the polymer chains. This results in a transition from collapsed hydrophobic state to soluble hydrophilic state of the polymer.

Figure 12. Effect of pH on chemical structure (a) Poly(acrylic acid), (b) Poly (N,N diethylamino ethylmethacrylate)

Some systems based on acidic carboxylic acid group co-monomers or basic amino group containing co-monomers are shown in Table 3 . The hydrogel swelling and deswelling properties depend on several factors including their hydrophobic- hydrophilic nature, crosslink density (elasticity), charge density and pKa.

Table 3. Commonly used synthetic monomers and polymers for pH sensitive hydrogel

Monomer Responsive group Responsive to Acrylic Acid & its derivatives -COOH Alkaline pH Vinyl monomers with sulphonic acid -SO3H N-Vinyl pyrrolidone NH2 Or Substituted Amino Acidic pH N,N`-diethyl amino ethyl methacrylate

These hydrogels are an attractive alternative for artificial muscles. However, the limiting factor is the poor mechanical properties, which are consequence of high water content of the hydrogels. Contrary to hydrogels, polymeric fibres exhibit very good mechanical properties as a result of high degree of orientation and crystallinity. Therefore for making artificial-muscles, sensors and actuators; thin fibre shapes with enhanced transitional properties are desirable.

Copolymers of acrylonitrile and acrylic acid are expected to result in gel fibres with lateral organisation of crystallites in oriented structure as well as pH sensitivity. Such fibres have been reported by two routes:

modification of commercial polyacrylonitrile fibre copolymerization route

In first approach, shape changing gel fibres with a fine diameter were prepared from commercially available polyacrylonitrile by preoxidation and subsequent saponification. During the preoxidation step some of the pendant nitrile groups form crosslink, while in the subsequent hydrolysis step the uncrosslinked nitrile groups can be converted to carboxamide and carboxylic acid groups.

Figure 13. Modification of Acrylic Fibre

Acrylic acid moieties provide the pH response while oxidized PAN regions provide the strength and structural integrity. This approach results in mechanically strong pH sensitive gel fibres suitable for artificial muscle applications. These fibres show muscle-like expanding and contraction behaviour stimulated by pH change in the environment. Strong alkaline solution (2N NaOH) and strong acidic solution (2N HCl), induced a more rapid change in length. Equilibrium was achieved in about 1-2 seconds. Compared to swelling, the deswelling was still faster and the entire change occurred spontaneously. Shown in Figure 14, excellent reversibility was obtained indicating the stability of structure. And the transition is sharp. However, these fibres suffer with major drawbacks of being black (due to oxidation) in colour, brittle, and high cost of production.

Figure 14. Cyclability behaviour of pH responsive fibre produced by modification of acrylic

In order to overcome the above drawbacks, another novel approach was to solution spin pH sensitive fibres from a specially designed copolymer of acrylic acid and acrylonitrile (Figure 15). Unlike oxidized PAN fibres, these fibres are white in colour, have high impact strength and show even higher response. The interesting feature of the newly designed fibres is that it is not chemically crosslinked. Rather the physical structure of the fibre has been tuned to give both responsiveness and structural stability.

These pH sensitive fibres exhibited increase in size at pH 10 in the range of ~1300% and decrease in size at pH 2 to near the original volume (range of 120-180 %) during the first two cycles; however in the subsequent cycles the increase is about 3300 % while nearly same shape and size was obtained at lower pH. The increase in the swelling ratio from the third cycle onwards could be due to opening-up of the structure. The response was reversible and stable in subsequent cycles. The swelling deswelling behaviour has been shown in Figure 15 b.

Figure 15. (a) Controlled Radical Co-polymerization, (b) Swelling De-swelling cycles

Thermo-regulated textiles

Thermo-regulated textile can be also classified as an environmentally responsive textile. As the name suggests, the thermo-regulated textile means textile that can help in regulating the body temperature. The most comfortable skin temperature for human body is 33.4 °C.

The human body itself regulates the body temperature by controlling the release of heat by blood vessel dilatation or constriction, muscle and sweat gland activity etc.

To keep the skin temperature between 30.4 -36.4 °C, we need to put on or take off clothings according to external temperature. However, if clothing could automatically change its thermal resistance according to temperature, it can control the speed of heat release and regulate the inner temperature.

The thermal comfort can be increased by use of certain materials known as Phase Change Materials (PCM). PCMs are the materials which undergo a phase change from solid to liquid by absorbing certain amount of heat and a phase change from liquid to solid by releasing certain amount of heat. Because these materials have to exist as liquids in one of the transition states, they need to be encapsulated to protect them from leaking out of the clothing during a phase change.

Phase Change Materials

Figure 16. Mechanism of PCM

Clothing with the encapsulated phase change materials (PCM) can help to retain a constant temperature buffer and provide better comfort.

There are numerous situations where these can be beneficial and find applications. These include professions where the person has to undergo extreme changes in immediate climate. For example pilot’s uniform in a fighter plane, soldier’s uniform in extreme climate zones, uniforms for workers working at extreme temperatures, fire fighters, tents and temporary structures in extreme climates, automobiles, etc.

Phase change materials store energy when they change from solid to liquid and dissipate it when they change back from liquid to solid. It would be most ideal, if the excess heat a person produces could be stored intermediately somewhere in the clothing system and then, according to the requirement, activated again when it starts to get chilly.

Figure 17. Schematic of Microcapsules

The key parameters of microencapsulated PCMs are particle size and uniformity, high core-to-shell ratio, thermal and chemical stability, and resistance to mechanical action. Most early studies of latent heat storage focused on the dehydration and hydration of inorganic salt hydrates due to their high energy storage density and high thermal conductivity. However, inorganic salt hydrates are corrosive and are incompatible with several materials, exhibit supercooling and phase separation during transition under thermal cycling. In an effort to avoid some of the problems inherent in inorganic PCMs, the research interest had turned toward the characteristics of numerous organic substances and their mixtures as novel PCMs, as well as the enhancement of heat capacity, thermal stability, thermal conductivity and durability of the composites enclosing PCMs.

For clothing applications, the appropriate PCMs are the ones that exhibit phase transition in a temperature range from 18 °C to 35 °C. Paraffin waxes, especially n-eicosane, n-octadecane and n hexadecane are mostly preferred for textile applications due to their high latent heat and the phase change temperature interval. Characteristics of PCMs, the composition of shell and the amount of microcapsules added into the structure are the main factors that control the thermal management in the textile product. When a sufficient amount of micro PCM is added for coating of fabrics, a suitable thermal management in the textile product, meeting the specified end-use need can be provided.

These are encapsulated using two different methods:

In-situ polymerization and Interfacial polymerization method.

Figure 18. Optical micrograph of the PCM capsules at the magnification of 500×

Figure 19. Application of microcapsules to textiles.