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Cellulose wet adhesives are applied to enhance the wet strength of paper products by binding individual paper fibers together. However, the recycling of the wet strength paper is a challenge as the fibers are hard to re-disperse in water. This project demonstrates new strategies for developing cellulose wet adhesives with controllable degradability, facilitating the recycling of wet strength papers. In this project, regenerated cellulose membranes were used to simulate paper fibers. In adhesion measurements, two wet cellulose membranes were laminated with a thin layer of adhesive (1–30 mg/m2), and the 90-degree wet-peel was used as a measure of cellulose wet adhesion. It was shown that the wet-peel was a simple and reliable method to evaluate the wet adhesives for paper products. Cellulose wet adhesives, in the form of microgels or linear polymers, were synthesized by incorporation of hydrazide, amine or azetidinium functional groups that can form covalent bonds to cellulose surfaces. Two strategies to design degradable adhesives were demonstrated in this project. 1) Reductant-responsive microgel adhesives were created by introducing cleavable disulfide linkages, either in the polymer chains tethering adhesive groups or as the microgel crosslinks. More than 70% reduction in wet adhesion was achieved after exposure to a reductant. 2) Degradable polymer cohesive bonds were used to “switch off” the cellulose wet adhesion. This adhesive was created by introducing labile boronate-dextran complexes to the PVAm adhesive layer between cellulose surfaces. The introduction of this new interaction between PVAm chains enhanced the cellulose wet adhesion. In response to subtle pH changes or the presence of monosaccharides, the wet adhesion decreased by 60%. / Thesis / Doctor of Philosophy (PhD) / Wet strength is important for paper products such as paper towels and paper packaging. In paper manufacturing, cellulose wet adhesives are applied to enhance the strength of wet papers by “gluing” together individual cellulose fibers. However, the recycling of wet strength papers is a challenge because the current adhesives prevent the easy disintegration of waste paper back to a suspension of discrete cellulose fibers. As an important part of the bio-based economy, the next generation of paper products are required to be both strong in water and easy to recycle. This thesis explores new designs for wet-strength adhesives that will facilitate recycling. Both nanoparticles and linear polymers were synthesized in this study as cellulose wet adhesives. Many important properties of wet adhesives were probed, including the size of nanoparticles, the pre-treatment of cellulose surfaces, the dosage of adhesives and the choice of adhesive chemistries. A few types of novel cellulose wet adhesives with controllable degradability were synthesized and evaluated. I demonstrated that the cellulose wet adhesion can be “switched off” in response to subtle pH changes, reducing agents or sugars, showing a promising start for the recycling of wet strength papers.

Wax anti-settling additives (WASA) are used to mitigate against the problems caused by the settling of n-alkane wax crystals, which crystallise from petroleum diesel. This can result in the blocking of fuel filters and hence vehicle failure. However, the mode of action for such additives is not currently known and two mechanisms have been proposed: they reduce the wax crystal size to such an extent that they settle very slowly; or they induce gelation in the wax suspension. This project aims to elucidate the mechanism of WASA within the diesel system. A room temperature crystallising model diesel (10 wt% n-alkanes in dodecane) has been developed. This model system has given a good response to the additives, with the wax crystals reduced in size, and is hence suitable for mechanistic studies. Differential scanning calorimetry and infra-red spectroscopy both suggest that the WASA is incorporated in or onto the wax crystal. DSC shows that small amounts of WASA suppress the wax crystallisation temperature and change the shape of the heat flow curve. FT-IR shows the WASA amide stretch present within filtered and dried wax crystals. Intriguingly, electrophoresis experiments show that the WASA imparts a positive charge to the wax crystals, suggesting an electrostatic role in the WASA action. Rheological experiments show the presence of a weak gel in the WASA doped model diesel. However, the gel strength is not altered by the presence of an organic salt and thus cannot be purely electrostatic in origin. Small angle neutron scattering has been conducted to help locate the WASA in the system. It has shown that in solution WASA shows a collapsed polymer coil structure with a single molecule occupying a 28 Å diameter sphere and multiple WASA molecules forming a 2400 Å diameter sphere. In the presence of the wax the WASA scatter does not significantly change suggesting that the WASA is on the surface of the wax crystal. By combining these results, a mechanism of WASA action is proposed as WASA cations interactions bridging between the wax crystals causing a weak bridging flocculation gel with electrostatic and steric effects contributing to stabilisation. The WASA charges are partially dissociated thus giving the electrophoretic effect and the long chains on the cations can contribute to stability via steric stabilisation.

This thesis presents an investigation into the factors affecting the morphology of hybrid inorganic/organic photoactive layers used in photovoltaic cells. Although optimisation of the organic (polymer) phase has received substantial attention, research into the morphology of the inorganic phase (semiconducting nanocrystals) remains limited. It is believed that there is a strong link between the morphology of the final photoactive film and the quality of the initial nanocrystal dispersion. To this end, two nanocrystal systems were investigated; zinc oxide (ZnO) and lead sulphide (PbS). ZnO nanocrystals were synthesised and found to possess reproducible characteristics. It was determined that colloid stability was initially dependent upon the presence of acetate groups bound to the surface, which in turn required a small quantity of methanol to be present in the organic dispersant. It was also discovered that while methanol evaporated readily from the surface of the nanocrystals, another molecule, 1-propylamine (1-PA), did not. Further investigations showed that while methanol only weakly physisorbed to the surface of ZnO nanocrystals, 1-PA formed strong, dative covalent bonds with Zn2+, preventing evaporation despite a low boiling point. Subsequent investigations into the effects of different ligands upon colloid stability found that amine-based groups typically possessed superior stabilising capabilities compared to alcohol-based analogues. The characteristics of nanocrystal / polymer blends were also investigated. It was determined that the nanocrystal dispersion became unstable at higher concentrations of polymer due to depletion aggregation. Films of nanocrystal / polymer blends were cast from dispersions containing either alcohol or amine-based ligands, and it was observed that dispersions stabilised with 1-PA possessed smooth morphologies on the micrometer scale. Investigations at the nanometer scale, however, revealed aggregates large enough to favour recombination.The latter half of this thesis regards the characterisation of PbS nanocrystals and investigations into triggered aggregation. It was determined that while PbS nanocrystals possessed reproducible characteristics, the stabilising molecule, oleic acid (OA) was insulating. The effects of exchanging the OA groups for a shorter ligand, butylamine (BA) were investigated.Finally, PbS nanocrystals were treated with a bidentate ligand, 1,2-ethanedithiol (EDT) to induce triggered aggregation. It was observed that the system was highly sensitive to the concentration of EDT in dispersion, forming small, relatively dispersed aggregates at low [EDT], and micrometer-sized crystalline structures at high [EDT]. The characterisation and entrapment of these nanocrystal structures within semi-conducting polymer films is also discussed.