Chung Research Group Sustainable and Biomedical Polymers
Functional Adhesives Synthesis
Adhesives are nonmetallic polymer materials that can join two independent surfaces by chemical and/or physical bonding. Despite the importance of adhesives to complex and advanced applications, most existing adhesive products are single purpose, single material-based, and singular in function. The Chung group studies novel synthetic methods and structure-property relationships of multi-functional adhesives that exceed conventional role of adhesives. The adhesive research includes three types of adhesives: 1) electrically conductive adhesives (ECA), 2) biomedical adhesives, and 3) stimulus responsive adhesives. We use the properties of the marine organisms inspired 3,4-dihydroxy-L-phenylalanine (DOPA) integrated into various artificial polymers to overcome the limitations of current artificial adhesives.
The conventional ECA includes heterogeneous metallic and carbon-based additives in an organic adhesive polymer matrix. However, these heterogeneous ECAs with additives have serious issues with their aggregation, heavy weight, large filler content, rapid aging, toxicity, low adhesion/conductivity, poor mechanical properties, and high price. The new ECA solved these issues by employing a new molecular structure of flexible anionic copolymer complex to polythiophene (Figure 1). The following 5 areas will receive significant and direct benefit from the novel ECAs: battery binders, photovoltaics, electronic sensors, displays, and coating technologies.
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Figure 1. (a) Chemical structure of new ECA, (b) High conductivity; LED light on by connection with the ECA. The ECA was flexible and it operates well after bending. (c) Strong adhesion; 12mg of adhesive holds 15 lbs bowling ball.
Biomedical adhesives are a rapidly growing field because of the potential applications in tissue adhesion, wound closure, hemostatic agents, wound dressing, and drug delivery. However, it is very hard to find a single chemical structure that possesses the multifunctionality required for biomedical adhesives (e.g., strong internal organ tissue adhesion and controlled drug delivery). The newly synthesized biomedical adhesives include 1) zwitterionic polymer-based adhesives that can provide liquid-tight sealing to prevent intestinal anastomosis leakage (Figure 2) and 2) glucose-based adhesives that can form strong biological/tissue adhesion via the non-toxic copper free click reaction. Those novel polymers provide excellent biomedical adhesion on animal’s internal organs with high biocompatibility. In addition, the new biomedical polymers show adhesion strength controllability which is rare in present biomedical adhesives. The ongoing follow up study includes small molecules (e.g., drugs, imaging agents, etc.) in the deliverable biomedical adhesives for internal organs.
Figure 2. (a) Synthesis of new biomedical adheisve, (b) Ex vivo liquid leakage prevention test on porcine intestine; top - prevented leakage by adhesive, bottom - content leakage without adhesive.
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The first example of stimulus responsive adhesive is an anthracene containing terpolymer that undergoes photo-mediated [4+4] cycloaddition (Figure 3). The photoreaction results in the crosslinking of individual polymer chains after the interfacial adhesion between substrates and adhesive polymer occur. This external stimulus (photoirradiation) leads to 10-fold improvement on adhesion property, and the adhesion strength can also be conveniently controlled with the duration of UV irradiation. The second stimulus responsive adhesive possesses a diacrylate crosslinker that includes o-nitro benzyl groups which are cleavable under UV radiation. The crosslinking bonds can be cleaved, freeing individual linear polymers and thereby weakening the cohesion. The weakened cohesion directly reduces adhesion. The crosslinking can be systemically controlled to adjust the adhesion strength. This degradable adhesive can be applied for temporary purposes, in the fields such as the biomedical and industrial adhesives.
Figure 3. Anthracene containing photo-responsive adhesives; photo-crosslinking of anthracene segment results significant enhancement on adhesion with precise control.
