Defence of doctoral thesis: Hsin-Chen Chen – Cellulose Nanocrystals-stabilized Polyhydroxyurethane and Polyacrylate Latexes: Design, Functionalization, and Performance

Cellulose nanocrystals (CNCs) have emerged as highly effective solid stabilizers for Pickering emulsions and latexes, offering a sustainable alternative to conventional surfactants. Their amphiphilic nature allows partial wettability at the oil-water interface, creating robust colloidal stabilization in fully waterborne systems. Because CNCs originate from renewable lignocellulosic biomass and can be surface-modified through abundant hydroxyl groups, they provide a versatile platform for designing green, bio-based polymer composites with tailored interfacial and mechanical properties.

This thesis develops and investigates CNC-stabilized waterborne non-isocyanate polyurethane (WNIPU) and polyacrylate latexes with the goal of achieving high colloidal stability and excellent material performance. The work focuses on optimizing latex synthesis, engineering CNC surface functionalities, elucidating CNCs-polymer matrix interactions, and assessing the structural and mechanical properties of the resulting latexes and composites.

In the first part, pristine CNCs were utilized to prepare WNIPUs through CNC-stabilized suspension polymerization, eliminating hazardous isocyanates, volatile organic compounds (VOCs), and external surfactants. In Chapter 2, CNC-mediated Pickering stabilization of WNIPU latexes was successfully achieved for the first time, with systematic optimization of CNCs concentration, monomer composition and reaction parameters. The obtained WNIPUs also exhibited florescence, indicating potential for sensing or anti-counterfeiting applications. Chapter 3 demonstrated the dual role of CNCs as both stabilizer and reinforcing agent for the WNIPU latex. The incorporation of 17 wt% CNCs increased probe tack adhesion strength by 680% and lap-shear strength by 340%, enabled by their uniform dispersion within the WNIPU matrix.

The second part of the thesis focused on functionalized CNCs as advanced stabilizers for waterborne latexes. In Chapter 4, CNCs were covalently modified with octylamine (oCNCs) to increase surface hydrophobicity. The oCNCs-stabilized WNIPU showed improved stabilization efficiency and smaller sizes in monomer emulsion droplets. The resulting WNIPU/oCNC composites showed up to 34% and 57% increases in probe tack adhesion strength and work of adhesion, respectively, compared to pristine CNC-stabilized WINPU. Chapter 5 introduced a non-covalent surface modification using charge coupling method, which rendered CNCs sufficiently hydrophobic to stabilize polyacrylate latexes with solid contents up to 20 wt%. The resulting polyacrylate/CNC composites demonstrated coating performance comparable to surfactant-stabilized references.

Overall, this thesis establishes CNC-stabilized WNIPU latexes as a new class of Pickering polymer systems and extends the concept to polyacrylate matrices. It elucidates how CNC surface chemistry governs dispersion stability, interfacial stabilization, and composite performance. These findings provide a basis for designing scalable, surfactant-free, and sustainable waterborne polymer systems, advancing the use of functionalized nanocellulose in environmentally friendly coatings and adhesives.

urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-373035