Development of temperature-responsive polymeric gels with physical crosslinking due to intermolecular 𝜋-𝜋 interactions.

Poly(N-isopropylacrylamide) PNIPAAm was polymerized with co-monomers containing a biphenyl moiety to create a unique thermoresponsive physically crosslinked system due to the presence of pi-pi interactions between the biphenyl moieties. The biphenyl monomers used were 2-phenylphenol monoacrylate (2PPMA) and 4-phenylphenol monoacrylate (4PPMA). These monomers were utilized to synthesize a set of polymers with biphenyl monomer (2PPMA/4PPMA) content from 2.5 to 7.5 mole percent and with initiator concentrations from 0.1 and 1.0 weight percent. The resulting polymers were characterized by various techniques, such as gel permeation chromatography (GPC), swelling studies and mechanical testing. The decrease in the average molecular weight of the polymers due to the increase in the concentration of initiator was confirmed by GPC results. Swelling studies confirmed the expected temperature dependent swelling properties and explored the impact of the biphenyl comonomers. These studies indicated that with the increase in biphenyl comonomers, the physical crosslinking increases which leads to decrease in the swelling ratio. The results from the mechanical tests also depict the effect of the concentration of biphenyl comonomers. These physically crosslinked polymeric systems with their unique properties have potential applications spanning environmental remediation/sensing, biomedicine, etc.