Small Angle Neutron Scattering Study of Complex Coacervate Micelles and Hydrogels Formed from Ionic Diblock and Triblock Copolymers

Abstract

A complex coacervate is a fluid phase that results from the electrostatic interactions between two oppositely charged macromolecules. The nature of the coacervate core structure of hydrogels and micelles formed from complexation between pairs of diblock or triblock copolymers containing oppositely charged end-blocks as a function of polymer and salt concentration was investigated. Both ABA triblock copolymers of poly[(allyl glycidyl ether)-b-(ethylene oxide)-b-(allyl glycidyl ether)] and analogous poly[(allyl glycidyl ether)-b-(ethylene oxide)] diblock copolymers, which were synthesized to be nearly one-half of the symmetrical triblock copolymers, were studied. The poly(allyl glycidyl ether) blocks were functionalized with either guanidinium or sulfonate groups via postpolymerization modification. Mixing of oppositely charged block copolymers resulted in the formation of nanometer-scale coacervate domains. Small angle neutron scattering (SANS) experiments were used to investigate the size and spacing of the coacervate domains. The SANS patterns were fit using a previously vetted, detailed model consisting of polydisperse core-shell micelles with a randomly distributed sphere or body-centered cubic (BCC) structure factor. For increasing polymer concentration, the size of the coacervate domains remained constant while the spatial extent of the poly(ethylene oxide) (PEO) corona decreased. However, increasing salt concentration resulted in a decrease in both the coacervate domain size and the corona size due to a combination of the electrostatic interactions being screened and the shrinkage of the neutral PEO blocks. Additionally, for the triblock copolymers that formed BCC ordered domains, the water content in the coacervate domains was calculated to increase from approximately 16.8% to 27.5% as the polymer concentration decreased from 20 to 15 wt %.

Phase behavior of electrostatically complexed polyelectrolyte gels using an embedded fluctuation model

“…Computing such quantities experimentally would be challenging and normally requires performing multi-parameter fits on SANS data or the use of cryo-TEM. 72 Thus, we expect our predictions will be of practical use in interpreting experimental data, formulating and validating new fitting models for small angle scattering data.…”

Linear viscoelasticity of complex coacervates

“…While the bulk of the discussion in this review has focused on the viscoelastic characterization of macrophase separated coacervate materials, a wide variety of hierarchically-structured coacervate-based materials have also been reported [63,68,69,101,102,107,[184][185][186][187][188][189]. In these materials, coupling of a polyelectrolyte to a neutral, water-soluble polymer such as poly(ethylene glycol) facilitates the creation of a molecular interface, and drives microphase separation.…”

“…In these materials, coupling of a polyelectrolyte to a neutral, water-soluble polymer such as poly(ethylene glycol) facilitates the creation of a molecular interface, and drives microphase separation. Diblock copolymer systems have been reported to form micellar and vesicular structures [69,101,102,187,188], while triblock copolymer systems can form flower-like micelles under dilute conditions, and structured, hydrogel-like materials at higher polymer concentrations [63,68,[184][185][186]189]. …”

PRISM-Based Theory of Complex Coacervation: Excluded Volume versus Chain Correlation

“…[1][2][3][4][5][6] Historical and current investigations into complex coacervates have studied the formation of such phases in natural polymers, such as proteins or polysaccharides, which are currently widely used as food additives. [7][8][9][10][11][12][13][14][15][16][17][18] Despite the utility of these systems, there has only recently been a resurgence of interest in their molecular behavior, in particular for its promise as a powerful route to self-assembled materials such as micelles, 3,[19][20][21][22] block copolymers, [23][24][25][26] and layer-bylayer assembly. [27][28][29][30] This recent activity in the field is concomitant with a desire to emulate the molecular features observed in a number of biological materials, such as underwater adhesives in mussels and the matrix adhesive holding together the dwelling of a sandcastle worm.…”

“…2,[31][32][33] The novelty of these materials is their extreme stability (yet responsiveness) with respect to environmental ionic conditions, even at high salt concentrations, as well as their reversible assembly behavior. [34][35][36][37][38] Complex coacervates are used for microencapsulation, drug delivery, 3,19 biomaterials, [39][40][41] and underwater adhesives, 2,31,32 where their self assembly and functionality relies 19,[23][24][25][26] on the relationship between charged species. Thus, the molecular features of these charged species is crucial to designing function.…”