arXiv subjects

Richard M. Parker

Publications and source records attributed to Richard M. Parker.

5 recordsLinked to original sources

arXiv2025

Tailoring the Morphology of Cellulose Nanocrystals via Controlled Aggregation

Cellulose nanocrystals (CNCs) are elongated nanoparticles derived from natural cellulose, with potential applications ranging from rheological modifiers and emulsion stabilizers to photonic pigments and sensors. For most applications, precise control over CNC morphology and surface chemistry is essential, but the relationship between process parameters, CNC characteristics, and their resulting behavior is poorly understood. Here, we investigate the impact of centrifugation and ionic strength on CNC morphology after dialysis using transmission electron microscopy, small-angle X-ray scattering and scanning electron diffraction. We find that the centrifugation step commonly applied during CNC purification promotes the formation of compact composite nanoparticles made of aligned crystallites, referred to as 'bundles', that are associated preferentially along their hydrophobic faces. In stark contrast, transient exposure to high ionic strength leads to fractal-like, irregular composite nanoparticles. We then examine the consequence of these morphological differences on the cholesteric self-organization of the CNCs: aligned bundles reduce the cholesteric pitch in suspension, causing a blue-shift in the color of dish-cast photonic films, while misaligned particles promote gelation, producing colorless films. This study reveals the importance of sample history, in particular, the often-disregarded purification steps, on CNC characteristics and their ensemble behavior, thereby unlocking new routes for tailoring this promising nanomaterial.

cond-mat.soft↗

arXiv2025

Tuning the morphology of aerosolised cellulose nanocrystals via controlled aggregation

Cellulose nanocrystals (CNCs) are polycrystalline, rod-shaped nanoparticles isolated from cellulose, which have attracted increasing attention for a wide variety of applications. While there has been significant research into CNCs in suspensions, hydrogels and films, there have been remarkably few studies that investigated their properties during and after aerosolisation. Here, we studied how aerosolisation impacts the size and morphology of different CNCs suspensions with different surface functionalities. By building a new experimental setup, we observed that colloidally-metastable aqueous CNC suspensions, achieved by carboxylation of the surface hydroxy groups or by exposure to high intensity ultrasonication, yield large particulates upon aerosolisation under ambient temperatures. In contrast, aqueous suspensions of unfunctionalised CNCs tend to produce, upon aerosolisation, smaller particulates, despite suffering from poor colloidal stability in liquid suspension. Our results demonstrate that both the aerosolisation process itself and the properties of the CNC suspension play a crucial role in determining the final particle size and morphology of CNC-based particles, highlighting the need to consider colloidal stability and surface functionality when designing CNC-based materials for applications involving aerosol delivery or spray drying.

cond-mat.soft↗

arXiv2022

Cellulose Photonic Pigments

When pursuing sustainable approaches to fabricate photonic structures, nature can be used as a source of inspiration for both the nanoarchitecture and the constituent materials. Although several biomaterials have been promised as suitable candidates for photonic materials and pigments, their fabrication processes have been limited to the small to medium-scale production of films. Here, by employing a substrate-free process, structurally coloured microparticles are produced via the confined self-assembly of a cholesteric cellulose nanocrystal (CNC) suspension within emulsified microdroplets. Upon drying, the droplets undergo multiple buckling events, which allow for greater contraction of the nanostructure than predicted for a spherical geometry. This buckling, combined with a solvent or thermal post-treatment, enables the production of dispersions of vibrant red, green, and blue cellulose photonic pigments. The hierarchical structure of these pigments enables the deposition of coatings with angular independent colour, offering a consistent visual appearance across a wide range of viewing angles.

cond-mat.soft↗

arXiv2022

Chiral Self-Assembly of Cellulose Nanocrystals is Driven by Crystallite Bundles

The transfer of chirality across length-scales is an intriguing and universal natural phenomenon. However, connecting the properties of individual building blocks to the emergent features of their resulting large-scale structure remains a challenge. In this work, we investigate the origins of mesophase chirality in cellulose nanocrystal suspensions, whose self-assembly into chiral photonic films has attracted significant interest. By correlating the ensemble behaviour in suspensions and films with a quantitative morphological analysis of the individual nanoparticles, we reveal an inverse relationship between the cholesteric pitch and the abundance of laterally-bound composite particles. These bundles thus act as colloidal chiral dopants, analogous to those used in molecular liquid crystals, providing the missing link in the hierarchical transfer of chirality from the molecular to the colloidal scale.

cond-mat.soft↗

arXiv2020

Angular Independent Photonic Pigments via the Controlled Micellization of Amphiphilic Bottlebrush Block Copolymers

Photonic materials with angular independent structural colour are highly desirable because they offer the broad viewing angles required for application as colorants in paints, cosmetics, textiles or displays. However, they are challenging to fabricate as they require isotropic nanoscale architectures with only short-range correlation. In this article, porous microparticles with such a structure are produced in a single, scalable step from an amphiphilic bottlebrush block copolymer. This is achieved by exploiting a novel controlled micellization self-assembly mechanism within emulsified toluene-in-water droplets. By restricting water permeation through the droplet interface, the size of the pores can be precisely addressed, resulting in structurally coloured pigments. Furthermore, the reflected colour can be tuned to reflect across the full visible spectrum using only a single polymer (Mn = 290 kDa) by altering the initial emulsification conditions. Such photonic pigments have several key advantages over their crystalline analogues, as they provide isotropic structural coloration that suppresses iridescence and improves colour purity without the need for either refractive index matching or the inclusion of a broadband absorber.

cond-mat.soft↗