Introduction

Ever since the observation of lower critical solution temperature (LCST) phenomenon  [1] for poly(N-isopropylacrylamide) (PNIPA), a lot of efforts have been expended, which have resulted in commercialization of the stimuli-sensitive polymers (temperature, pH, electric field, and ionic sensitive) particularly in the biomedical field [2]. These polymers also find potential applications in the separation and recovery of chemicals [3-5], development of artificial muscles, sensors and actuators (i.e. robotics). An simple but innovative application has been the development of clear/cloud glazing application, where the solar energy transmission in the room is regulated by the transparency of the solubilized polymer, below the transition temperature, and opacity of the precipitated polymer, above the transition temperature [6].

Transition point of the polymer may be varied by changing the molecular structure. For example temperature sensitive poly(N-acroylpiperidine) shows transition at 4-6 oC, poly(N-t-butylacrylamide) at 25 oC, poly(N-piperidylmethacrylamide) at 42 oC [7], and poly(N-isopropylacrylamide) at 32oC.

However, chemical modification by copolymerization is a simpler route for tuning the transition point to a desired value. In case of temperature sensitive polymers, incorporation of hydrophilic comonomer leads to an increase in the transition temperature, whereas incorporation of hydrophobic comonomer leads to a decrease. For example copolymerization of N-isopropylacrylamide, with butylmethacrylate, which is a hydrophobic monomer in a feed ratio of 96:4 mol%, gives a transition temperature of 29oC, while copolymerization with acrylamide, which is a hydrophilic comonomer in a similar feed ratio gives a transition temperature of 35oC [8].

Similarly, copolymerization may develop tunable response to pH, electric field, magnetic fields, and ionic medium.

Challenges

Only a few polymer systems have been studied using copolymerization approach and in temperature sensitive applications most of them are based on N-isopropylacrylamide (NIPA) [9-14]. NIPA has a drawback, that it is commercially unviable because it is synthesized in low yields.

Therefore, more polymer systems need to be developed keeping the commercial applications in mind.

Another major challenge in using stimuli-sensitive materials is their processability into desirable forms such as coatings, films and fibers. Because stimuli-sensitive polymers are water soluble below the transition point, they are requires to be polymerized in gel form. Often gels are thick, with poor mechanical strength, and slow diffusion of water in and out of gel at the transition transition. This leads to poor utilization of functional sites present in the gel in applications such as bioseparation, purification, and slow response in biomemetic applications.

In order to obtain fast transitions, high functional efficiency and good durability, it is therefore, extremely important that such stimuli sensitive polymer systems should be developed which are processable into structurally strong, thin desirable shapes. These polymers should also contain chemical site that may be used for subsequent crosslinking.

This is a challenging task, because one is required to incorporate functionality while maintaining the transition behaviour of the final system.

Recent Results

Our group has developed a series of stimuli sensitive copolymers based on substituted acrylamide, which can be processed into films and fibres. In the processed forms, the quickness of response and extent of transition in these systems could be increased several orders of magnitude. Using these materials, intelligent textile materials are being developed in our laboratory.

References

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