Demonstration of thermo-sensitive tetra-gel with implication for facile and versatile platform for a new class of smart gels

A tertiary branched poly(N-isopropylacrylamide) with controlled molecular weight, distribution and the end amino-functionalization (tetra-PNIPAAm-NH2) was studied for the ability to form a gel via in situ chain-end reaction with a counterpart tertiary branched poly(ethyleneglycol) bearing N-hydroxysuccinimide end groups (tetra-PEG-NHS), a well-documented class of building block to yield the tetra-gel. Some of these polymers, both comparable and distinct (relative to the counterpart) extended chain length pairs, provided a self-standing and macroscopically homogeneous gel, which was capable of undergoing thermo-sensitive and reversible change in hydration in line with the nature of PNIPAAm. Phantom network model based calculation indicated that a half molar fraction of the polymer chains in the network remained unreacted, revealing further room for optimizing the reaction condition. Since such tetra-PNIPAAm based motif can be readily tailored to a variety of other physicochemical stimuli-responsive analogues, our finding may give important insight into a platform for 'smart' tetra-gels with exceptional mechanical properties and potentially highly controllable molecular cut-off capability.