The versatility and applicability of thermoresponsive polymeric systems have led to great interest and a multitude of publications. Of particular significance, multifunctional poly(N‐isopropylacrylamide) (PNIPAAm) systems based on PNIPAAm copolymerized with various functional comonomers or based on PNIPAAm combined with nanomaterials exhibiting unique properties. These multifunctional PNIPAAm systems have revolutionized several biomedical fields such as controlled drug delivery, tissue engineering, self‐healing materials, and beyond (e.g., environmental treatment applications). Here, we review these multifunctional PNIPAAm‐based systems with various cofunctionalities, as well as highlight their unique applications. For instance, addition of hydrophilic or hydrophobic comonomers can allow for polymer lower critical solution temperature modification, which is especially helpful for physiological applications. Natural comonomers with desirable functionalities have also drawn significant attention as pressure surmounts to develop greener, more sustainable materials. Typically, these systems also tend to be more biocompatible and biodegradable and can be advantageous for use in biopharmaceutical and environmental applications. PNIPAAm‐based polymeric nanocomposites are reviewed as well, where incorporation of inorganic or carbon nanomaterials creates synergistic systems that tend to be more robust and widely applicable than the individual components. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48770.

Monomers containing biphenyl moieties were employed to create two sets of covalently crosslinked polymers that displayed noncovalent interactions in their 3‐dimensional network. The biphenyls (precursors) used were 2‐phenylphenol, 4‐phenylphenol and 4,4′‐dihydroxybiphenyl, and their acrylated forms were synthesized and named as 2‐phenylphenolmonoacrylate, 4‐phenylphenolmonoacrylate, and 4,4′‐dihydroxybiphenyldiacrylate, respectively. These were characterized by differential scanning calorimetry, nuclear magnetic resonance, and Fourier transform infrared spectroscopy to confirm the successful acrylation reaction. Polymers were synthesized via free radical polymerization reactions with varying crosslinker contents, and their network properties were characterized using swelling studies and compressive modulus tests. Interestingly, swelling studies did not show the expected decreasing swelling ratio with increasing crosslinker content, while compression testing indicated the expected trend of increasing modulus with increasing crosslinking density. The unexpected swelling results are hypothesized to result from the intramolecular interactions between the biphenyl side groups that result in noncovalent crosslinks.

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Poly(N‐isopropylacrylamide) was polymerized with comonomers containing a biphenyl moiety to create a unique thermoresponsive physically crosslinked system due to the presence of π–π 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 mol% and with initiator concentrations of 0.1 and 1.0 wt%. The resulting polymers were characterized using various techniques, such as gel permeation chromatography (GPC), swelling studies and mechanical testing. A decrease in the average molecular weight of the polymers due to the increase in the concentration of initiator was confirmed from GPC results. Swelling studies confirmed the expected temperature‐dependent swelling properties and explored the impact of the biphenyl comonomers. These studies indicated that with an increase in biphenyl comonomer content, the physical crosslinking increases which leads to a decrease in the swelling ratio. The results from the mechanical tests also indicate 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. © 2021 Society of Industrial Chemistry.