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Materials Science, Coatings & Films
Polydopamine (PDA), inspired by the adhesive mussel foot proteins, is widely applied in chemical, biological, medical, and material science due to its unique surface coating capability and abundant active sites. Recently, PDA was introduced into Energetic Materials for the modification of crystal phase stability and the interfacial bonding effect, and, as a result, to enhance the mechanical, thermal, and safety performances.
- polydopamine
- self-polymerization
- energetic materials
1. Introduction
In 2007, Lee et al. proposed the surface chemistry for multifunctional coatings, which is inspired by the main component of adherent proteins in mussel, namely, dopamine [1,2]. Dopamine is able to form adhesive polydopamine (PDA) thin films on various material surfaces by oxidative self-polymerization in an alkaline aqueous medium. Fascinatingly, PDA films can also provide an important platform for secondary reactions since there are many reactive groups, such as catechol, amine, and imine, on its surface, which can serve as the starting points for covalent modification with desired molecules [3].
Due to its unique surface-coating capability and abundant active sites, unsurprisingly, PDA was rapidly applied in many fields across the chemical, biological, medical, and material sciences [4]. In barely more than a decade, the physicochemical properties of PDA were extensively studied, covering its biocompatibility and biodegradation, electrical conductivity, metal ions chelating and redox activities, and other potential properties. Among all these attractive features, its adhesive property and chemical reactivity are closely related, while they are also the basis of the vast majority of applications [5,6]. The adhesive properties of different mussel foot proteins with diverse compositions and contents of 3,4-dihydroxyphenyl-L-alanine (DOPA, a derivative of PDA) were investigated, and it was concluded that possible interaction mechanisms between DOPA and substrates involve electrostatic, hydrogen bonding, hydrophobic interactions, cation–π, π–π stacking, and metal complexation [7]. Further, the roughness of the PDA nanomembrane was also observed as a factor deciding its adhesion [8]. The abovementioned generalizable theories and trends were critical for PDA coating and surface functionalization in engineering practices.
Energetic materials (EMs) are very important power resources for civilian and military applications, covering explosives, propellants, and pyrotechnics, which release energy in the forms of combustion and explosion. PDA was introduced into EMs until very recently. Lin et al. fabricated a very compact PDA film on the surface of an explosive crystal, octogen (HMX), which not only improved the stability of the crystal phase, but also synergistically enhanced the mechanical, thermal, and safety performances of HMX-based polymer-bonded explosives (PBXs) [9]. Meanwhile, Yan’s group was interested in the control of the reactivity of metastable intermixed composites (MICs) by constructing a PDA interfacial layer between the nanoscale fuel and oxidizer [10], which resulted in an increased energy release and reduced sensitivity.
2. Self-Polymerization Mechanisms and Catalytic Reactions of PDA
The self-polymerization of dopamine and deposition of PDA is a very facile but time-consuming process. A typical procedure may need 12 h including the dissolution and polymerization of the dopamine monomer (commercially, dopamine hydrochloride is typically used) in an alkaline aqueous solution (Tris-HCl buffer with pH 8.5 is typically used) without any sophisticated operations, presenting a significant color change from colorless to deep brown.
Although PDA can be facilely and mildly fabricated, its reaction mechanisms remain controversial due to the complex redox process and related intermediates during polymerization. The polymerization process was speculated as the oxidation of dopamine to dopamine-quinone, followed by intramolecular cyclization, oxidization, and rearrangement to form 5,6-dihydroxyindole (DHI) [11]. Furthermore, DHI and its oxide can eventually form a cross-linked polymer through the reverse dismutation reaction between catechol and o-quinone. Although the cyclized, nitrogenous species such as the indole- or indoline-type structures were also confirmed by other researchers, a distinct model was proposed that PDA was considered to be an aggregate of monomers cross-linked primarily via strong, noncovalent forces [12]. Hong et al. suggested that the formation of PDA is the combination of noncovalent self-assembly and covalent polymerization [13]. They identified a dual-path formation process, in which both paths form the oxidative product of dopamine, DHI, since a physical, self-assembled trimer of (dopamine)2/DHI was observed by HPLC. Instead, Alfieri et al. suggested that PDA’s polymerization mechanisms might have an alternative pathway besides the conventional DHI-oligomers as the essential intermediate [14]. Their experiments found that dissolved DHI cannot form PDA; rather, dopamine polymerization, on mechanism-based analysis, may arise by quinone-amine conjugation leading to polycyclic systems with extensive chain breakdown. It seems that the molecular mechanisms behind the polymerization and deposition of PDA are quite complicated and still unclear.
One part is for certain, though; oxidation plays a decisive role in PDA formation, which provides meaningful inspiration for accelerating this process. Several methods were excogitated to overcome the drawback of the low polymerization rate, for instance, UV irradiation, electrochemical actuation, and oxidant promotion [15,16,17,18,19]. Among all this research, Zhang et al. used CuSO4/H2O2 to induce the polymerization of dopamine and accelerate the deposition rate of PDA [20,21]. Their works obtained a uniform PDA thin film with a thickness of 30 nm in 0.7 h, which is a considerably rapid deposition rate compared to other works, as shown in Table 1. Another attempt to achieve rapid PDA formation included the rising reaction temperature from the perspective of kinetics, which fabricated a PDA film in 0.5 h and obtained similar properties as those polymerized by the conventional method in 24 h [22], but the surface of the PDA obtained by this method was relatively rough due to the shambolic deposition of PDA nanoparticles under vigorous stirring. Interestingly, the self-polymerization of dopamine normally occurs in alkaline conditions; however, some researchers also explored the possibility of PDA formation in an acidic environment [23,24], on the basis of understanding the relevance of oxidation.
Table 1. Thickness and deposition rate of PDA coatings with different methods.
Understanding the potential mechanisms of accelerating the polymerization and deposition rates of PDA is purposeful to engineering practices, not limited to EMs. However, given that most of EMs having oxidability, PDA formation on the surface of EMs seems to be rational and logical. For example, Ammonium perchlorate (AP), which is a commonly used oxidant in composite solid propellants, was employed to induce polymerization of dopamine in Wei et al.’s work [18]. Further, Lin et al. also realized the limitation of the low polymerization rate and inaccurate kinetic control of PDA in EMs, and they investigated the kinetics of PDA formation under different conditions, including concentrations of dopamine and oxygen, as well as environment temperatures, when studying the PDA coating on an insensitive high explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) [25].
This entry is adapted from the peer-reviewed paper 10.3390/cryst13060976
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