Ever since the observation of lower
critical solution temperature (LCST) phenomenon  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 . 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 .
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 , and poly(N-isopropylacrylamide)
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 .
Similarly, copolymerization may develop
tunable response to pH, electric field, magnetic fields, and ionic medium.
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.
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.
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