Environmentally responsive smart textiles - II

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

Similar results were obtained for other relative humidity conditions.

(a) Responsive breathable sample made from TSP coated cotton fabric and (b) Non-responsive breathable sample made from polyacrylamide coated cotton fabric.

Shape changing fabric

Another responsive model fabric was fabricated using TSP coated yarns. 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.

Shape changing fibers

The TSP was also successfully converted into a shape changing textile fiber of fine diameter. The fiber underwent change both diameter and length with change in temperature. 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 fiber, while the change in shape (swelling ratio) of the fibers increased by 36 times.

B. pH-responsive textiles

Similar to the TSPs, pH-sensitive polymer (PSP) structures reported in the literature also suffer with the slow response time, poor magnitude of response, and weak mechanical properties. Since these materials are prime candidates for making artificial-muscles, sensors and actuators; thin fiber shapes with enhanced transitional properties are desirable. In one of the recently reported approach, oxidised polyacrylonitrile (PAN) fibers are hydrolysed to give composite structures containing both acrylic acid and oxidised cyclic PAN moieties. Acrylic acid moieties provide the pH response while oxidised PAN regions provide the strength and structural integrity. However, the response of such structures has not been optimised with regards to conditions of oxidation and hydrolysis.

At IIT Delhi, we have attempted to optimise these parameters to produce pH-responsive fibers with enhanced response. However, these fibers still suffer with major drawbacks of being black (due to oxidation) in colour, brittle, and high cost of production. In order to overcome the above drawbacks, another attempt using a novel approach was made to solution spin pH sensitive fibers from a specially designed copolymer of acrylic acid and acrylonitrile. Unlike oxidised PAN fibers, these fibers are white in color, have high impact strength and show even higher response. The interesting feature of the newly designed fibers is that it is not chemically crosslinked.

Rather the physical structure of the fiber has been tuned to give both responsiveness and structural stability. The pH sensitive fibers 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.

Thermo-regulated textile

Thermo-regulated textile is another very important area of research to make environmentally responsive textile. 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 a pilot’s uniform in a fighter plane, a 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. One of the main applications where IIT Delhi jointly with DEBEL, Bangalore is placing a particular emphasis is the development of thermo-regulated clothing for pilots of fighter planes. The pilot has to go through extreme change in temperature during a flight. Their imperious anti-gravity suits and the high temperature of the cockpit during the initial flight period put enormous thermal induced stress on their alertness and analytical capability. Clothing with the encapsulated phase change materials (PCM) can help to retain a constant temperature buffer and provide better comfort.

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. At IIT Delhi, dozens of PCM were scanned for their potential application at low temperature, at near the body temperature and at high temperature. Out of several selected for near the body temperature application, octadecane was found to be the most suitable for the above mentioned application. It has a high enthalpy value of 240 J g -1 and its melting temperature is near to the human body temperature that is around 28°C.

Octadecane was encapsulated using two different methods. The in-situ polymerisation and interfacial polymerisation method. These two were found suitable among the various techniques known for encapsulations. In the first process, a prepolymer of melamine-formaldehyde was prepared to which emulsifier and water was added. Thereafter, the octadecane was added slowly, while the mixture was stirred at a high rpm. In-situ polymerisation was carried out in the prepared emulsion by slowly raising the temperature from 40°C to 68°C for a predetermined time.

In the second method, PCM was encapsulated by interfacial polymerisation technique. The oil phase was prepared by mixing n-octadecane (PCM), the core monomer toluene-2, 4-diisocyanate (TDI), the cyclohexane. The oil phase emulsified in an aqueous phase. The bulk monomer diethylene triamine (DETA) was added to the emulsion. The encapsulation was carried out at 60°C for 1.5 hr. The focal point of the study was to increase the core content, stability, and efficiency of encapsulation.

Conclusions

The research at IIT Delhi has made several successful attempts in developing truly smart textile materials in the last few years. These materials respond actively to the changes in the environment. For the first time, shape changing fibers, yarns and fabrics have been produced with the help of suitably designed stimuli sensitive copolymers. Textiles that respond quickly and reversibly to small changes in temperature and pH have been successfully demonstrated. The processed fine structures were able to overcome the major drawbacks, such as, slow response, poor efficiency, and poor mechanical properties exhibited by stimuli-sensitive hydrogels reported in the literature. Similarly, encapsulated PCM have been developed with very high core content and high encapsulation efficiency. These are stable to high temperature and multiple cycling.

(The authors are with the Department of Textile Technology, Indian Institute of Technology, Delhi)