Macro-RAFT Agent Assisted Grafting Modification Of Cellulose Nanocrystals And Their Potential Application

As a natural polymer,cellulose has wide sources and shows biodegradability,its abundant hydroxyl groups impart peculiar chemical and physical properties to cellulose material.With the problem of energy shortage and environmental deterioration on our planet,these features result in huge potential application of cellulose in various aspects.Cellulose nanocrystals,which is a product from acid hydrolysis of cellulose raw material,has some excellent properties such as its nano-size,high degree of crystallinity and good mechanical properties.The widest application of cellulose nanocrystals is as reinforcing material to add into polymer matrix to improve the mechanical properties.However,the potential application of cellulose nanocrystals is much more than that.For example,its high specific surface area makes it a candidate for absorbent,or flocculant and so on.As a nano-sized material with a rod shape,cellulose nanocrystals has the ability to self-assembled into chiral nematic liquid crystals structure,which shows the potential in photonic application.In the application of cellulose nanocrystals,some can use it directly,but some needs to do some modification to suit the application field.With respect to the modification of cellulose nanocrystals,free radical polymerization grafting is one of the frequently used methods.However,as cellulose nanocrystals is highly crystalline,which decreases the reactivity of its hydroxyl groups.As a results,in the many cases of cellulose nanocrystals grafting modification,grafted chains are deadly terminated in the early stage,which greatly reduce the grafting amount.With the anticipation of increase the grafting amount of polymer chains on modified cellulose nanocrystals,this study designed a macro-RAFT agent assisted grafting strategy,which adds extra amount of water-soluble macromolecular RAFT agents into the grafting reaction of cellulose nanocrystals modification,through effective chain transfer to reduce deadly chain termination.In this way,the grafting amount of polymer chains is expected to increase.This graftingstrategywasusedinthegraftingofpolyacrylamideand poly?N,N-dimethylacrylamide?from cellulose nanocrystals,and carefully studied the grafting process.Furthermore,the modified cellulose nanocrystals were used as flocculants,and their self-assemble ability were carefully studied.1.Four water-soluble macro-RAFT agents were designed through RAFT polymerization,including three styrene-acrylic acid block copolymers with different composition(St₇AA₁₄RAFT,St₅AA₁₄RAFT and St₁AA₁₄RAFT)and one poly?acrylic acid?homopolymer(AA₁₄RAFT).With the same reaction condition?acrylamide=1M,macro-RAFT=0.01M?,these four macro-RAFTs were used in the grafting of polyacrylamide from cellulose naocrystals and compared with the ones without adding any macro-RAFTs,to study their influence on grafting polymerization.After that,the concentration of acrylamide was increased to 2M.In order to study the influence of macro-RAFT concentration,different concentration of the four macro-RAFTs was added in the grafting process,corresponding grafting amount in modified cellulose nanocrystals was characterized.Polyacrylamide grafted cellulose nanocrystals?CNC-g-PAM?with St₇AA₁₄RAFT as macro-RAFT that showed the highest graft amount was subjected to acid hydrolysis to cleave the grafted polyacrylamide chains;the structure was fully characterized.2.In order to increase the understanding of the graft process in grafting modification of cellulose nanocrystals by macro-RAFT assisted strategy,another water-soluble monomer,N,N-dimethylacrylamide,was used in the grafting polymerization of cellulose nanocrystals with St₇AA₁₄RAFT as macro-RAFT agent.The structure of poly?N,N-dimethylacrylamide?grafted cellulose nanocrystals?CNC-g-PDMA?was fully characterized.Results showed that this strategy works well for the grafting of poly?N,N-dimethylacrylamide?and showed high graft amount of side chain.Moreover,in order to understand the graft process and mechanism,the evolution of graft amount and monomer conversion with time,and the molecular weight of grafted chains and free homopolymers in solution in both polyacrylamide and poly?N,N-dimethylacrylamide?grafting were fully characterized.Based on these results,a three stage process mechanism,which corresponds to distribution and competition of three different kinds of grafted chains,was proposed.Finally,after extensive practices on application of macro-RAFT assisted strategy under different condition,four key operation steps were summarized as reference for future application.3.As cellulose nanocrystals has high specific surface area plus the characteristic of polyacrylamide and poly?N,N-dimethylacrylamide?,the two modified cellulose naocrystals,CNC-g-PAM and CNC-g-PDMA,was used as flocculants for kaolin suspension.The change in turbidity with different flocculant dosage and different settling time,and the floc size change with different amount of flocculant dosage were carefully characterized.Furthermore,through the perspective of flocculation speed,flocculants dosage and so on,the two flocculants were loosely compared with cellulose-based flocculants reported in literature.4.Cellulose nanocrystals shows the ability to self-assembled into chiral nematic liquid crystal under certain condition.Many factors influence the self-assemble behaviour of cellulose nanocrystals.They can be loosely divided into external ones,such as ionic strength,the species and structure of additives?such as surfactant,polymer?,ultrasonic dispersion and so on,and internal ones such as aspect ratio,charge amount,functional group species.The essence of these factors is their influences on the interaction force between cellulose nanocrystals,which finally affect the self-assemble process.For the two kinds of modified cellulose nanocrystals synthesized previously,the introduction of hydrophilic polymer chains might influence the interaction between cellulose nanocrystals and thus influence the self-assemble behaviour.CNC-g-PAM and CNC-g-PDMA with different reaction time was adjusted to different concentration and left to settle,and carefully studied their self-assemble behaviour.Furthermore,using vacuum assisted self-assemble method,we prepared modified cellulose nanocrystals based film.The structure of the films was characterized.