Environmentally responsive smart textiles - I

Thermo-regulated textiles using microencapsulated PCMs shall enable humans to achieve new dimensions of success in their respective fields of professions even with extreme-environment, say Ashwini K Agrawal, Manjeet Jassal, Ninad S Save, S Periyasamy, Arnab K Ghosh, K R T Ramasubramani, Amrish Vishnoi, M Palanikkumaran & Kishor K Gupta

Conventional textiles are used to cover 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 utilise their flexible and strong structure. However, 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. At IIT Delhi, we have been working in making smart textile materials which respond to changes in the immediate environment.

(i) Thermo-regulated textile, which removes heat when temperature goes up and releases heat when temperature drops, has been jointly developed with Defence Bioengineering and Electromedical Laboratory (DEBEL), DRDO, Bangalore, using phase change materials with transition at near the body temperature (30øC). These can be effectively used to keep the microclimate near the body buffered from sudden cyclic changes in the environment temperature. (ii) Shape changing fibers, yarns and fabrics have also been developed using a novel class of stimuli sensitive - copolymers which can be readily processed and chemically integrated to textile substrate. These allow development of new composite yarns and fabrics that change shape reversibly by capturing moisture from the environment depending on the environment temperature and pH. Such materials may be used for making environmentally responsive textile, such as fabrics that alters water-vapor transmission or porosity with change in environment conditions. There are numerous products that may be developed using such materials both for apparel and technical applications.

Shape changing fibers, yarns and fabrics

A. Temperature-responsive textiles

The stimuli-sensitive polymers are smart materials being researched all over the world. They find applications in the medical field and have potential applications in separation, artificial muscles, molecular separation and enzyme-activity control.

The temperature sensitive polymers (TSPs) show transition at their Lower Critical Solution Temperature (LCST) called the transition temperature. These polymers have both hydrophilic and hydrophobic groups in their structure. Below LCST, the hydrophilic interactions dominate and polymer becomes soluble in water, while above this temperature hydrophobic interactions dominate and polymer becomes insoluble in water. These polymers find applications when polymerised in gel form. The gels change shape by swelling in water below transition temperature and deswell above transition temperature.

The major drawbacks of the current stimuli-sensitive polymer-gel structures are their weak mechanical properties and poor transitional response. Processing of these materials into thin shapes and their integration to textile materials is likely to solve the current problems and also develop responsive textile materials for smart textile. As suggested above, at the Indian Institute of Technology, Delhi (IIT, Delhi), we have attempted to solve these limitations by processing a suitably designed TSP into various forms such as structurally strong thin films, fibers, coatings, and chemically integrated TSP with yarns and fabrics. The successful processing of TSP as mentioned above involved various critical investigative steps as enumerated below

• Synthesis of a suitable TSP system which had transitional response tunable in a wide range of temperature.
• Production of TSP in high molecular weight, suitable for its conversion into high strength thin structures.
• Development of a method for stabilising processed forms, for example, possibility of carrying out chemical bonds among polymer chains, and with textile substrates after processing that would not compromise responsive behavior of the resultant fibers.

A series of TSP copolymer ‘poly (N-tert-butylacrylamide-ran-acrylamide) (PNTBA)’ was synthesised in both gel (using N, N-methylenebisacrylamide during the process of polymerisation) and linear form using free radical polymerisation with controlled dosing of comonomers. The solution of linear copolymer (27:73) containing polycarboxylic acid crosslinker and catalyst was processed into thin films (10 to 200 mm), fibers (30 to 50 mm), and coatings on to cellulosic yarns and fabrics. The processed forms were dried and cured at 150 to 200øC for 5 to 25 minutes. The responsiveness of the processed structures immersed in water bath was determined by measuring the swelling percentage and swelling kinetics with change in the temperature of the water bath.

The TSP synthesised in conventional gel forms could be cut with difficulty into 2mm thick disks. On the other hand TSP could be readily converted into finer forms and subsequently stabilised through crosslinking. The crosslinks (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 processed forms were found to change shape as the temperature of the water bath was altered across their transition temperature of 21øC. The structures could be repeatedly swollen and deswollen by decreasing and increasing the temperature of the water bath, respectively. The TSP prepared in conventional gel form as a 2 mm thick gel disks, showed a swelling of 490 %, and took 90 minutes to attain 70 % swelling, while the deswelling took 50 minutes.

Structurally strong thin films

The films of TSP (10 to 200 mm) showed a significantly enhanced magnitude of response depicted by an increase of 4 to 6 times in the swelling ratio compared to the 2 mm thick gel-disks. The response time reduced drastically to 1 minute for swelling and a few seconds for deswelling.

Responsive breathable fabric

A breathable fabric was prepared by integrating the TSP onto a cotton fabric with 23% add-on. The coating on the fabric showed a swelling ratio of around 800% and a response time of 20 minutes to equilibrium swelling. The water-vapor transmission rate (WVTR) values of the TSP integrated breathable fabric were measured as a percentage of control uncoated substrate. The transmission percentage at 20% relative humidity for TSP-fabrics were found to change across the transition temperature from 58% to 94% compared to a comparative non-responsive breathable fabric (made using poly (acrylamide) coated fabric), which changed only from 70% to 94%. The difference in percentage transmission, due to change in the environment temperature, shows the responsive (smart) behavior of the TSP-fabrics.