Preparation Of Tunicate Cellulose Nanocrystals/Epoxidized Natural Rubber Composites And Study On Their Structure And Properties

Cellulose nanocrystals（CNs）have attracted numerous attention due to their wide source,renewability,degradability and excellent mechanical properties,etc.Meawhile,abundant active hydroxyl groups on the surface of CNs are beneficial for the surface modification.At present,many researchers have focused their attention on the research of CNs extracted from terrestrial plant resources.As the only known marine animal to produce cellulose so far,tunicate（such as Halocynthia roretzi Drasche,etc.）cellulose nanocrystals（t-CNs）have unique advantages such as large aspect ratio and excellent mechanical properties.However,at present,there are few studies on t-CNs used in polymers,especially in rubber composites.Therefore,to make full use of marine biomass,this thesis takes the lead in incorporating t-CNs into rubber matrix.After obtaining the reinforcing effect and mechanism of t-CNs on epoxidized natural rubber（ENR）,surface modification of t-CNs was further carried out,and the reinforcement behavior and mechanism were investigated as well.Then,based on the hydrogen bonds between rubber chains and rubber/t-CNs interface,and taking full advantage of the surface modification and large aspect ratio,the potential applications of t-CNs in self-healing rubber with high strength,reprocessable rubber and anisotropic rubber composites were also investigated.The specific research contents and main results are as follows:（1）Taking marine biomass termed tunicates as the source of cellulose,and extracting t-CNs from the tunic of tunicates via commonly used sulfuric acid hydrolysis method.The comparison with c-CNs extracted from cotton showed that both of them showed diameters of10²0 nm,but their length were 0.5²μm and 150³00 nm,respectively.Thus,resulting aspect ratios were⁷5 and²0,respectively.Then,rubber composites were prepared by mixing it with ENR latex,which was also derived from biological basis.Because of the good interfacial interaction between them,t-CNs showed good reinforcing effect on ENR.For example,by adding 10 phr of t-CNs,tensile strength and stress at 300%strain of the composite were 57%and 6.8 times higher than that of neat ENR.Meanwhile,two different methods,latex casting and mixing,were used to prepare composites.The results indicated that due to the different dispersion state,the sample prepared by casting method showed higher strength and modulus,while the sample prepared by mixing method showed longer elongation at break.（2）Based on the above research,maleic anhydride（MAH）was first used to introduce carboxyl groups on the surface of t-CNs.And covalent bonds were constructed via the reaction between carboxyl and epoxy groups.The results indicated that dual networks of hydrogen bonds and covalent bonds were constructed in the composites after modification by MAH,in which hydrogen bonds can consume energy by serving as sacrificial bonds,while covalent bonds can bear higher external force.Thus,the samples with modified t-CNs showed higher tensile strength and elongation at break.For example,tensile strength and toughness of sample with 5 phr modified t-CNs were increased by 52%and 45%compared with unmodified sample.Then,one-pot method,namely NR latex was first mixed with t-CNs and then epoxy reaction was carried out,was also used to prepare composites.For comparison,two-step method involved epoxidizing NR and then mixed with t-CNs was also conducted.The results indicated that due to the strong acidity and oxidation during one-pot reaction,the surface of t-CNs may go a certain etching effect,which was beneficial for absorbing more rubber chains.Therefore,the interfacial interaction was also improved for samples prepared by one-pot method.For example,the sample with 5 phr t-CNs prepared by one-pot method showed comparative or even higher mechanical properties than the sample with 8 phr t-CNs prepared by two-step method.（3）Based on the hydrogen bonds between the oxygen-containing functional groups on the ENR chains（mainly epoxy groups,a small amount of carboxyl groups,hydroxyl groups,etc.）and hydroxyl groups on the surface of t-CNs,together with the strong mobility of ENR chains by controlling rubber matrix on a moderate curing level,the self-healing behavior of ENR/t-CNs composites by constructing t-CNs doped hydrogen bonds network were studied.The results indicated that the incorporation of t-CNs not only improved mechanical strength,but also improved self-healing efficiency.For example,with 20 phr t-CNs,tensile strength of the sample increased by 2.5 times compared with neat ENR,and self-healing efficiency also increased from 50%of neat ENR to 80%.The main self-healing mechanism is considered as the diffusion of rubber chains,the reversible hydrogen bonds between rubber chains and ENR/t-CNs interface.（4）A large amount of carboxyl groups were introduced onto the surface of t-CNs（TOCNs）by TEMPO-mediated oxidation,and hydroxyl ester covalent bonds were constructed at interface via the reaction between carboxyl groups and the epoxy groups,which achieved reinforcement and covalent cross-linking of TOCNs simultaneously.When adding 20 phr t-CNs,tensile strength and elongation at break of the sample increased by 5.5 times and 106%compared with neat ENR,respectively.Meanwhile,based on the transesterification reaction of interfacial hydroxyl esters,the rearrangement nature of the network can be achieved by exchange reaction at evaluated temperatures.The composites also exhibited impressive reprocessibility and self-healing capacity at high temperatures.The results indicated that the efficiency of reprocessing and self-healing can reach 80%.（5）Considering the high aspect ratio and large surface area of t-CNs,taking t-CNs as bio-template,Fe₃O₄@t-CNs magnetic nanohybrids were constructed by in-situ co-precipitation method,where Fe₃O₄ nanoparticles were loaded on the surface of t-CNs.The saturation meanetization of the hybrid nanoparticles was 42.9 emu/g.Then,the nanohybrids suspension was mixed with ENR solution,and during solvent evaporation process,weak external magnetic field was utilized to induce the parallel alignment of nanohybrids along the length direction of t-CNs.SEM and TEM were performed to study the orientation behavior,which was further verified theoretically by Halpin-Tsai model.Finally,rubber composites with anisotropic behavior were obtained.With the addition of 5 phr nanohybrids,tensile strength of the samples with parallel and perpendicular alignments were 3.72 MPa and 3.05 MPa,respectively.（6）When using rubbers or elastomers to tough polylactic acid（PLA）,the increase in toughness requires high content of toughening agent,which is often accompanied by a rapid decrease in rigidity.Considering the excellent reinforcing effect of t-CNs on polymers,t-CNs were further introduced into dynamic vulcanized PLA/ENR thermoplastic vulcanizates（TPVs）.The results indicated that the addition of t-CNs could improve the crystallinity of PLA and strengthen the cross-linked rubber network,which resulted in improved strength and rigidity of the TPVs and elongation at break could still remain at high level,simultaneously.The research on shape memory behaviors showed that the shape fixity ratios of all the TPVs were above 99%,and shape recovery ratios were above 80%.Moreover,the addition of t-CNs strengthened rubber network,which increased the shape recovery ratios.