Macromolecular design for viscoelasticity and toughness in polyelectrolyte complexes

Alexei Dmitrievitsj Filippov

Research output: Thesisinternal PhD, WU

Abstract

The majority of human-made materials are represented by  plastics and elastomers, polymeric solids synthesized from petrochemically obtained feedstock. When compared to materials that are constructed from the polymers produced in cells, human-made synthetics are substantially less adapted to function with or within living organisms. The adaptation of materials to fit a set of requirements can be described in terms of their character in the dimensions of hydrophilicity, viscoelasticity, and toughness. Polymers used by plants and animals, such as in tissues, are always built from water-soluble compounds, and are therefore predominantly hydrophilic, unlike most synthetic materials. The viscoelasticity, a property which encompasses both the time-dependent compliant (viscous) as well as the elastic (solid) response of a material, of natural materials is diverse and can be often described as solid with a dissipating component. Finally, natural materials present a toughness, the fracture resistance of a material to high deformations, that is adapted to their function in the organisms that the materials serves. While hydrophilicity, viscoelasticity, and toughness can be tuned in synthetic materials to some extent, the majority of available materials do not offer independent control of either.

The adoption of synthetic materials for use in the human body is highly attractive for the medical field: novel materials will allow to better rehabilitate tissues or even take over their function. However, chemical platforms do not offer the repertoire that billions of years of evolution allowed to perfect in biology. In this thesis, I present complex coacervates as a material class that offers untapped possibilities to bridge the property gap between natural and synthetic materials.
Complex coacervates, also termed polyelectrolyte complexes or saloplastics when high stiffnesses are reached, are phases that form on reversible association of oppositely charged polyelectrolytes in water. Complex coacervates have a strongly salt-depended viscoelasticity (saloplasticity), with more liquid-like phases forming at higher concentrations of added salt. Coacervates are highly hydrophilic due to their constituents being water-soluble, yet are insoluble in water. The latter property is unique for non-crosslinked solids, and as such coacervates represent a platform of enormous interest for future biomaterials.

Nonetheless, the saloplasticity of complex coacervates lacks two degrees of control to allow fabrication of a material with given specification. First, addition of salt not only reduces the
terminal relaxation time, but also softens the complex. Second, salt modifies the response of the complex to large deformations, thus the fracture resistance or toughness. The central mission of this thesis is to provide chemical tools to control the viscoelasticity and toughness of complex coacervates other than the salt concentration.

First, we show that the stiffness of coacervates can be effectively controlled by introducing metal-ligand bonds into the coacervate that form transient crosslinks between the polyelectrolyte chains. Choosing metals with shorter or longer relaxation times allows an additional layer of control over the viscoelasticity. Thus, hybrid metal-ligand complex coacervates can be made at the desired stiffness and terminal relaxation time. Moreover, we show that metal-ligand complexation increases the resistance of coacervate to fracture at high deformation amplitudes. Thus, key mechanical properties are disengaged from the salt concentration.

Furthermore, we demonstrate a synthesis route towards polyelectrolyte with the bottlebrush
architecture. A bottlebrush polymer comprises a backbone onto which a dense array of side-chains is grafted. The side-chains stretches the backbone, which results in a markedly different phase behaviour of complexes with linear, oppositely charged polyelectrolytes.
Finally, we present an improved synthesis of poly(acrylic acid), a ubiquitous polyanion.
We demonstrate the necessity of using a quantitative de-esterification method when
polyanions are synthesized from their esterified precursors. Specifically, the self-assembly
behaviour of thermoaggregating triblock copolymers is shown to be disabled when
non-quantitative deprotection has left hydrophobic impurities in the polymers. Our improved
methodology effectively prevents issues of non-quantitative ester cleavage.

In short, this thesis provide chemical handles on the mechanical behaviour of complex
coacervates. The mechanical targets of viscoelasticity and toughness are addressed with metal-ligand
complexation, changes to architecture of the polyelectrolytes from linear to densely grafted, and improvements to the structural fidelity of the polyelectrolytes themselves.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Sprakel, Joris, Promotor
  • Kamperman, M., Co-promotor
Award date16 Nov 2021
Place of PublicationWageningen
Publisher
Print ISBNs9789463959247
DOIs
Publication statusPublished - 16 Nov 2021

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