Iridescence and Circular Dichroism in Cellulose Nanocrystal Thin Films Daniel James Hewson School of Physics University of Exeter A thesis submitted for the degree of Doctor of Philosophy September 2017 2 Structural Colouration in Cellulose Nanocrystal Thin Films Submitted by Daniel James Hewson to the University of Exeter as a thesis for the degree of Doctor of Philosophy in Physics July 2017 This thesis is available for Library use on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. I certify that all material in this thesis which is not my own work has been identified and that no material has previously been submitted and approved for the award of a degree by this or any other University. Signature: ………………………………………………………….. Date:………………………………………………………………... Abstract Only in recent times has the true potential of cellulose as a high-end functional and sustainable material been realised. As the world’s most abundant resource cellulose has been utilised by man throughout history for timber, paper and yarns. It is found in every plant as a hierarchical material and can be extracted and converted into fibres which are of great use, especially in the form of nanofibrous materials. This thesis has focused on the utilisation and ability of cellulose nanocrystals (CNCs) to generate structural colour in fabricated thin films. This has been achieved in two ways: Firstly, the natural morphology of CNCs and their ability to form a suspension have been applied to a layer-by-layer (LbL) regime to produce tunable Bragg reflecting thin films. Secondly, a novel technique combining profilometry and spectroscopy has been developed to estimate the distribution of CNCs within EISA thin films and correlate this with the optical properties of the film. This thesis reports the successful fabrication of synthetic CNC LbL Bragg reflecting thin films. The film was compiled using an additive layer-by-layer technique which allowed the construction of a multi-layered thin film and control over individual layer thicknesses and refractive index. Also reported is the discovered reflection of both left and right handed circularly polarised light (CPL) from CNC EISA thin films. These reflections were found to correlate with CNC distribution within the thin films. The distribution of CNCs was estimated using a novel technique which combined spectroscopically measured film absorbance as a function of the volume of the film area under investigation. The specific volumes were calculated using profilometry measurements and the beam spot size used in the spectroscopy measurements. Acknowledgements I have been very much inspired by the incredible focus, enthusiasm and the dedication to science of my supervisors, Professor Stephen Eichhorn and Professor Peter Vukusic. Without their guidance and readily shared experience the work in this thesis could not have been achieved; so, to them I express my sincere gratitude. I owe a great debt to my wife and children whose support; patience and encouragement have been unfailing and have spurred me on in times of difficulty and discouragement. I would also like to thank my parents for their temporal support and for providing much needed holidays! I must thank Dr Sebastian Mouchet for taking the time to cross the t’s and dot the i’s of this thesis and for his homeland, Belgium for making good things to eat. Thank you to Dr Luke McDonald, Dr Tim Starkey and Dr Matt Nixon for their amusing banter, curry nights and no less important photonics advice and support. I have had the privilege of working with two research groups, The Cellulose and Natural Materials Research group and the Natural Photonics Research group. The collected experience of the two groups has greatly influenced my work; the friendships I have developed within each group have made my life as a PhD student all the more enjoyable and rewarding. I am very grateful for the expertise and collaborative efforts of Professor Jamie Grunlan and Dr Ping Tzeng at the Polymer Nanocomposites Laboratory and Dr Christian Hacker at the Bio-Imaging Suite here at the University of Exeter. I must also thank Nick Cole for the skill and effort he puts into everything he builds and particularly thank him for building (from scratch) the goniometer used to obtain data presented in this thesis. To keep me going I have eaten much cake that has been lovingly provided by family members, cake in which I have enjoyed with David Richards, a good friend who went on before me to complete his thesis and whose valuable advice has helped me and my efforts to present my own. ii Contents Contents .............................................................................................................................. i List of Figures ................................................................................................................... iv List of Tables ................................................................................................................... xii List of Abbreviations ...................................................................................................... xiii Chapter 1 Introduction ........................................................................................................... 1 Chapter 2 Theory of Structural Colour .................................................................................. 5 2.1 Structural Colour ......................................................................................................... 5 2.1.1 A familiar Phenomenon ........................................................................................ 5 2.1.2 Biological Photonic Crystals ................................................................................ 7 2.1.3 Broadband reflectors........................................................................................... 12 2.1.4 Synthetic analogues ............................................................................................ 13 2.2 Thin Film Interference ............................................................................................... 15 2.3 Multilayer Interference .............................................................................................. 20 2.3.1 Factors affecting reflectance from Multilayer systems ...................................... 21 2.4 Cholesteric phases and chiral reflection .................................................................... 25 2.4.0 Pseudo Bragg Reflections ................................................................................... 29 2.4.1 Birefringence ...................................................................................................... 30 2.4.2 Circular Dichroism ............................................................................................. 32 2.4.3 Rotatory Power ................................................................................................... 35 2.5 Colourimetry using CIE values ................................................................................. 36 Chapter 3 Cellulose ............................................................................................................. 41 3.1 Synthesis and structure .............................................................................................. 41 3.2 Cellulose Nanocrystal Synthesis and Characterisation ............................................. 43 3.3 Mechanics of drying droplets .................................................................................... 49 3.4 CNC Thin Films ........................................................................................................ 53 Chapter 4 Layer by Layer Assembly ................................................................................... 59 4.1 The development of Layer by Layer Assembly ........................................................ 59 4.2 Electrostatic Self-Assembly ...................................................................................... 62 i 4.3 LbL: A Universal Technique ..................................................................................... 64 4.4 A Method for Fabricating Bragg Stacks ................................................................... 66 Chapter 5 Experimental Methods ........................................................................................ 69 5.1 Synthesis of CNC photonic structures ...................................................................... 69 5.1.1 Synthesis of LbL Bragg stacks ........................................................................... 69 5.1.2 EISA thin films ................................................................................................... 77 5.2 Structural Characterisation ........................................................................................ 80 5.2.1 Profilometry ....................................................................................................... 80 5.2.2 Scanning electron microscopy ........................................................................... 82 5.2.3 Transmission electron Microscopy .................................................................... 83 5.2.4 Polarised Optical Microscopy ............................................................................ 84 5.3 Optical characterisation ............................................................................................. 84 5.3.1 Angle resolved spectrophotometry....................................................................