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Mussels can affix themselves to a variety of wet surfaces under harsh marine conditions by secreting liquid mussel foot proteins (mfps) as superglues. Inside the mussel, the superglues are fluid-like and are kept at low pH, i.e. pH 3. Upon secretion into seawater at pH 8, the superglues are cured efficiently through crosslinking of the catecholic amino acid 3, 4-dihydroxyphenylalanine (DOPA). Currently, a lot of efforts have been devoted to developing functional materials using biomimetic polymers, although the crosslinking chemistry of the catechols in DOPA is still under debate. The goal of this thesis is twofold : 1) better understanding of crosslinking chemistry of catechols; 2) the design and synthesis of catechol-containing functional materials.
To gain better understanding of the crosslinking mechanism, we summarized the possible crosslinking mechanisms of catechols that have been proposed in the last few decades. We rationalize the parameters that may affect the crosslinking pathways and kinetics such as pH, temperature, types of oxidant and so on. Motivated by the open questions as discussed in the literatures, we investigate the crosslinking mechanism of catechols with amines using model compounds 4-methyl catechol (4MC) and propylamine (PA) in aqueous media. From the spectroscopic and chromatographic studies, we find that the reaction between 4MC and PA is very fast and complicated. In the first five minutes, more than 60 products have already been formed. These products are mainly formed via three pathways, i.e., Michael-type addition, Schiff-base reaction, and phenol-phenol coupling. Among these products, the majority are amine-catechol adducts formed by Michael-type addition.
In addition, we also designed catechol-containing functional copolymers. We designed a copolymer containing both functionalities-amines and catechols: poly(dopamine acrylamide-co-2-aminoethyl methacrylamide hydrochloride). We synthesized the copolymer in aqueous medium using free radical polymerization. The polymer is pH-responsive and meets two important requirements for a binder that can be used in water-borne coatings: (1) it is water-soluble at acidic pH during storage; (2) during drying and curing at basic pH, it becomes water-insoluble. The aqueous solubility switch is ascribed to the crosslinking reactions between catechols and amines at basic pH. Besides fulfilling the requirement of solubility switch, the coatings should also have proper adhesion properties. Therefore, we synthesize five copolymers with different amount of catechols from free radical polymerization of N-(3,4-dihydroxyphenethyl)methacrylamide (DMA) and 2-methoxyethyl methacrylate (MEA) with different compositions. We find that, under dry and wet conditions, an optimal composition for the best adhesion is achieved at 5 mol% of DMA. Polymers with a higher concentration of DMA show little adhesion, which is attributed to the high stiffness of the material, resulting in poor contact with the probe.
|Qualification||Doctor of Philosophy|
|Award date||12 Jan 2016|
|Place of Publication||Wageningen|
|Publication status||Published - 2016|
- polymer chemistry