Ionically Cross-Linked Poly(allylamine) as a Stimulus-Responsive Underwater Adhesive: Ionic Strength and pH Effects

Abstract

Gel-like coacervates that adhere to both hydrophilic and hydrophobic substrates under water have recently been prepared by ionically cross-linking poly(allylamine) (PAH) with pyrophosphate (PPi) and tripolyphosphate (TPP). Among the many advantages of these underwater adhesives (which include their simple preparation and low cost) is their ability to dissolve on demand when exposed to high or low pH. To further analyze their stimulus-responsive properties, we have investigated the pH and ionic strength effects on the formation, rheology and adhesion of PAH/PPi and PAH/TPP complexes. The ionic cross-linker concentrations needed to form these adhesives decreased with increasing pH and ionic strength (although the complexes ceased to form when the parent solution pH exceeded ca. 8.5; i.e., the effective pKa of PAH). Once formed, their ionic cross-links were most stable (as inferred from their relaxation times) at near-neutral or slightly alkaline pH values (of roughly 6.5-9) and at low ionic strengths. The decrease in ionic cross-link stability within complexes prepared at other pH values and at elevated (150-300 mM) NaCl concentrations diminished both the strength and longevity of adhesion (although, under most conditions tested, the short-term tensile adhesion strengths remained above 10(5) Pa). Additionally, the sensitivity of PAH/PPi and PAH/TPP complexes to ionic strength was demonstrated as a potential route to injectable adhesive design (where spontaneous adhesive formation was triggered via injection of low-viscosity, colloidal PAH/TPP dispersions into phosphate buffered saline). Thus, while the sensitivity of ionically cross-linked PAH networks to pH and ionic strength can weaken their adhesion, it can also impart them with additional functionality, such as minimally invasive, injectable delivery, and ability to form and dissolve their bonds on demand.

Linear viscoelasticity of complex coacervates

“…Data supportive of this hypothesis has been reported by various groups [61,79,103]. However, the opposite trend has also been reported.…”

“…In numerous examples, the elastic behavior dominates (i.e., the storage modulus (G') dominates) [8,61,137,138,171,175,[177][178][179][180], while in other coacervates the viscous or liquid-like behavior (i.e., the loss modulus (G") dominates) [95,103,171]. Meanwhile, there are other systems where a crossover is observed between the two regimes, with G" dominating at low frequencies, and G' dominating at higher frequencies [61,71,72,79,111,130,162,171]. Considering these observations in the context of the expected characteristic responses for polymeric materials suggests that the differences in the observed material properties are the result of experimental limitations and thermodynamic factors that limit experimental access to the full range of frequency behavior, rather than an inherent characteristic of the material itself.…”

“…The self-assembly of these materials is driven by entropy, where the initial electrostatic attraction between oppositely-charged macro-ions results in the release of small, bound counter-ions and the restructuring of water molecules [1][2][3][4]. Complex coacervates have a long history of use in the food [5][6][7][8][9][10][11][12][13] and personal care [14,15] industries, and have found increasing utility as a platform for drug and gene delivery [1][2][3][4], as well as underwater adhesives [5][6][7][8][9][10][11][12][13][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62]. Coacervation has also recently been implicated in the formation of various biological assemblies [1,[14][15][16]55,…”

Improved Concrete Materials with Hydrogel-Based Internal Curing Agents

“…HPC can easily achieve compressive strengths as high as 110 MPa, in contrast to the more common compressive strengths of (30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40) MPa [4]. The decreased porosity as well as disconnected pore network makes HPC quite resistant to corrosive fluid ingress and therefore extremely durable even in harsh environments [5].…”

“…Polymer chains can also self-assemble into ionically crosslinked gels when exposed to charged surfactants [35]. Such self-assembly of polyelectrolytes with oppositely charged ionic species is also reversible under the correct pH conditions [36]. Comparatively less research that examines the influence of aluminum on hydrogels has been done, although recent molecular dynamics simulations have shown that charged polymer chains will coil around trivalent cations much more tightly than around mono-and divalent cations [37].…”