Evaporation-induced Self-assembly Of Carboxylated Cellulose Nanocrystals And Their Optical Properties

Cellulose nanocrystal(CNC)is the highly crystalline part of cellulose,which can be obtained through physical and chemical treatments.CNCs are widely applied in various fields due to their unique chiral nematic structure,such as anti-counterfeiting,display devices and mechanical enhancement.Compared with the sulfated CNCs hydrolyzed by the one-step method with sulfate acid,carboxylated CNCs prepared by the two-step method with HCl hydrolysis and TEMPO oxidation could get controllable negative surface charge and aspect ratio respectively.In addition,the modifiability and active reactivity of carboxyl groups of carboxylated CNCs provide the possibility to develop novel CNC-based functional systems.It is meaningful to carry out research on the assembly of carboxylated CNCs and their optical properties.At present,the research on the chiral nematic structure of carboxylated CNCs remains under-exploited and there are a few reports on the main influencing factors on its assembly.Our work changes different parameters like aspect ratio,carboxyl content,and surface charge density to explore the chiral nematic assembly of carboxylated CNCs.The influence of the above intrinsic properties of carboxylated CNCs and assembly conditions are adjusted to show the relationship between the mesostructures and parameters.The nematic structure,partially twisted structure and chiral nematic structure films are prepared.The related chiral nematic structure films are shown the ability to modulate incident light and auto-emission fluorescence,where the circularly polarized fluorescence asymmetry factors up to-0.51.The multi-nematic structure film with excellent optical depolarization properties was successfully constructed,exhibiting angle-independent optical depolarizing effects.By integrating the renewable,biocompatible,and low-cost characteristics of cellulose nanocrystals,carboxylated CNCs are expected to play a role in the field of optical depolarization devices.The paper focuses on the assembly of carboxylated CNCs and their optical properties,in which the factors related to the chiral nematic structure are important.The chiral nematic structure could modulate the circularly polarized light and the multinematic has a depolarizing effect,which promotes the CNC-based advanced materials.This thesis is divided into five chapters:Chapter 1: First,the introduction discusses the basic properties of photonic crystals and their special optical adjusting capabilities.Second,the basic physical and chemical properties,assembly abilities,and application prospects of CNCs are introduced.The significance of the research on the chiral nematic structure of CNCs films and their corresponding optical properties have been introduced at the same time.Finally,the reports about the present research and application of carboxylated CNCs have been summarized and the difficulty in assembling the chiral nematic structure is discussed.The purpose and significance of the research on the assembly and properties of carboxylated CNCs are introduced at the end of the chapter.Chapter 2: the self-assembly process and structure of carboxylated CNCs are studied and discussed.By changing the aspect ratio and carboxyl content(surface charge density)of carboxylated CNCs,the chiral nematic structure of carboxylated CNCs is formed and its photonic bandgap can be adjusted by external conditions(like ultrasonic,ionic strength and suspension p H).At the same time,CNCs with the same aspect ratio but different carboxyl content shows the ability to form different kinds of mesoscopic structures.It is found that carboxylated CNCs with low oxidation degrees tend to produce multi-nematic structure films.The chapter demonstrates the assembly ability of carboxylated CNCs to form chiral nematic structures and shows their richness and controllability of the assembly mesoscopic structure.Chapter 3: the chiral nematic structure films prepared by carboxylated CNCs with high oxidation degrees are studied to show their ability to regulate circularly polarized light and circularly polarized luminescence.Their chiral nematic structure film could selectively reflect left-handed circularly polarized light and let right-handed circularly polarized light through.At the same time,their chiral nematic structure could act as a carrier to mix with the luminescence object to show a selective emission of right-handed circularly polarized luminescence.The chapter demonstrates the potential of carboxylated CNCs as one kind of chiral nematic film to regulate circularly polarized light and circularly polarized luminescence.Chapter 4: multi-nematic structure films prepared by carboxylated CNCs with low oxidation degrees were studied on the depolarizing effect of linearly and circularly polarized light.Because the orientation and size of the multi-nematic structure are different and random in the carboxylated CNCs films,the polarized light could be scrambled into different kinds of states,which achieved the depolarizing effect of polarized light.The chapter demonstrates the excellent regulatory ability of carboxylated CNCs in the field of optical depolarization and its potential in the field of color-coded image and anti-counterfeiting.Chapter 5: the chapter demonstrates the conclusions and prospects.