Structure–Property Relationships in Polymer Systems: From Functional Microgels to Dynamic Polymer Solutions and Melts
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Abstract
Structure–property relationships in polymer materials connect microscopic parameters to macroscopic quantities. In this thesis, three studies are presented to better understand the structure–property relationships in stimuli-responsive and dynamic polymer compounds.
In a first study, the dynamic covalently cross-linked polydimethylsiloxane (PDMS) networks that are known to display molecular rearrangement in the presence of anionic end groups are investigated. PDMS networks are prepared by anionic ring-opening polymerization. Oscillatory shear rheology measurements show that the amount of initiators and cross-linkers used during synthesis directly correlates to the mechanical strength of the resulting material. Probing stress relaxation and self-healing properties with a purpose-built force-monitoring piercing device show fast self-healing rates at ambient temperature. Piercing test force measurements determine an elastic energy storage within the material during piercing that is suggested to be a prerequisite for the self-healing process.
In a second study, volume and interfacial interaction changes during the volume phase transition of poly(N-isopropylacrylamide) (pNIPAAm) hydrogels are decoupled from one another. For this purpose, thermosensitive pNIPAAm cores are encapsulated inside non thermosensitive poly-acrylamide (pAAm) shells to create core-shell microgels. Both pNIPAAm cores and the pNIPAAm-AAm core-shell microgels are templated via droplet-based microfluidics. The distribution of particle sizes is narrow and well defined. The core–shell microgels Young’s modulus at the microgel’s surface at temperatures above and below the volume phase transition temperature (VPT) of pNIPAAm is determined by colloidal probe force mapping using atomic force microscopy. Stiffening of the surface’s Young’s modulus upon deswelling of the pNIPAAm core, accompanied by a small but reproducible size reduction of the pAAm-shell, above the VPT is found. It is attributed to an interconnection between the core- and the shell network in the microgels that drags the shell toward the center of the core upon deswelling. Interfacial interactions that changes during the volume phase transition of pNIPAAm are shown to remain constant, which opens pathways for the rational design assemblies of microgel layers or aggregate-substrates with the ability to independently regulate their elasticity.
A third study investigates the interplay of supramolecular and macromolecular dynamics of linear associating supramolecular polymers in semi-dilute solution. For this purpose, linear poly(ethylene glycol) functionalized with terpyridine moieties that can form transient metallo-supramolecular bonds are synthesized. Oscillatory shear rheology experiments show that shorter precursors and stronger transient bonds form longer and more stable chain assemblies, whereas longer precursors and weaker transient bonds form shorter and less stable ones. The upper boundary of the supramolecular chain extension is estimated by covalent chain extension with a photocrosslinkable unit. Viscosity calculations reveal that the assemblies display Rouse-type relaxation behavior, with assemblies having a strong metallo–supramolecular bond deviating more towards reptative motion than those with a weaker transient bond. Data is modelled with the time marching algorithm to calculate number-average degrees of chain extension, number-average molar masses, effective relaxation times and dissociation constants as much as six orders of magnitude faster than the free metal–ligand complex. This acceleration depends on the length of the polymer precursor chain the complex is attached to with longer chain segments yielding faster dissociation times, which indicates that metal–ligand bond destabilization is caused by the dynamic activity of the polymer chain itself.