Abstract/Summary

Polymer adhesives and polymeric inks are used widely within industrial, biomedical, and packaging sectors; however, their presence poses issues as the inability to remove them from surfaces can hamper the recycling and reuse of polymer materials. Therefore, the introduction of debondable adhesives which break down into monomeric and oligomeric species provides new markets for polymer adhesive use by offering a route for their removal from surfaces. For example, in single use packaging the presence of polymeric adhesives during the recycling process introduces contaminants which can decrease the quality of the recycled materials, therefore, their removal from the surfaces is important for the development of a circular polymer economy. Mechanical strain on adhered joints also poses a problem, as over extended periods of time the weakening of the adhesive can result in the spontaneous detachment of the two bonded surfaces. Therefore, adhesives which can be repeatably reused without loss of their adhesive strength will allow for the extension of a products usable life cycle. The objective of this research was to design new stimuli responsive adhesives capable of debonding from a range of substrates upon exposure to external stimuli, namely specific chemical reagents and heat. The integration of such polymers into inkjet formulations would allow for the generation of well-resolved images which can be rapidly removed from a variety of surfaces upon exposure to specific stimuli. The application of such polymers could ultimately lead to the generation of a more circular polymer economy. The work towards this goal is summarised below. Chapter 1 provides an introduction to the fields of printing polymer-based inks, self-immolative chemistries, and polymer recycling – three of the main topics featured in this PhD thesis. Chapter 2 reports the design, evaluation, and implementation of a base susceptible chain-extender for use in chain-extended polyurethane (CEPU) adhesives. Evaluation of model small molecule analogues of the self-immolative system provided mechanistic and rate analysis of the base (sodium hydroxide (NaOH) and tetra-butylammonium fluoride (TBAF)) triggered degradation of the commercially available 2,2’-sulfonyldiethanol chain-extender. Detailed solution state degradation analysis of the CEPUs via 1H NMR spectroscopy and gel permeation chromatography before and after depolymerisation indicated that rapid and effective degradation of the polymers can be achieved upon exposure to basic conditions. The loss of physical and mechanical properties post-solid state degradation were observed using tensile testing, rheological analysis, and differential scanning calorimetry (DSC). Finally, the adhesive properties of the CEPUs were evaluated on glass and aluminium substrates over five re-adhesion cycles ̶ losses in shear strength were observed as a result of chain scission, with the base triggered debonding of the CEPUs revealing the desired debond-on-demand nature of the self-immolative polymer systems. Chapter 3 reports the further exploration of the use of 2,2’-sulfonyldiethanol as a self-immolative chain-extender within a series of CEPUs which feature variations in their polyol backbone functionality to engineer the physical, mechanical, and adhesive properties of the CEPUs for use in inkjet formulations. The expansion of the model small molecules susceptibility to base indicated 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) also provided rapid degradation. Varying of the polyol backbone functionality (i.e. from alkyl to ether or ester) allowed for the tailoring of the viscoelastic transition of the CEPUs providing excellent adhesion to a variety of different surfaces at lower temperatures. The base triggered debonding of the CEPU adhesives indicated the rapid loss in adhesive strength could be achieved after exposure to NaOH, TBAF, or DBU for only 30 minutes. Formulation of the CEPUs into continuous inkjet (CIJ) printer inks generated well-resolved images with excellent adhesion on a variety of substrates. Finally, a CEPU was intentionally degraded to its prepolymer form and recycled, providing a new recycled CEPU which possessed comparable adhesive and debonding characteristics to its pristine counterpart. Chapter 4 reports the design and synthesis of supramolecular hydrogen bonding and π-π stacking motifs in the form of aniline and nitroaniline derived end-caps to obtain self-healing and re-adhesive supramolecular polyurethanes (SPUs). By exploring the effects of ortho-methyl substituents relative to the urea and/or nitro functional groups of the end-caps, SPUs with enhanced physical, thermal, and mechanical properties to analogous SPUs without methyl substituents in the end caps could be achieved. Variations in the end-cap functionality also enabled the varying of the SPUs adhesive strength and the temperature required for adhesion; the use of supramolecular interactions of the type described also created materials capable of full recovery of the adhesive shear strength over five cycles. Furthermore, the self-healing of the SPUs was investigated ̶ recovery of the mechanical properties of the pristine SPU materials was achieved upon exposure to thermal stimuli. Finally, Chapter 5 further explores the effects of hydrogen bonding and π-π stacking recognition motifs have on the physical properties of supramolecular comb polymers (SCPs) through the use of aromatic meta-/para-nitro-urea isomeric recognition units and variation of the loading of these associative supramolecular monomers. Two series of SCPs comprising 0-12.5 mol% of either novel meta-nitro or para-nitro monomers were synthesised, and their physical and mechanical properties were investigated. Increases in supramolecular monomer loadings were shown to enhance the phase separation and concomitant physical properties of the resulting SCPs. Investigation of the adhesive characteristics of the SCPs on glass and aluminium substrates showed that the loading of the recognition units can affect the adhesive and re-adhesive properties. Finally, an investigation into the thermally triggered self-healing capabilities of the SCPs revealed that the mechanical properties of these supramolecular polymer networks could be recovered post-damage.