Study On Cellulose Nanocrystals Modified Eucommia Ulmoide Gum Based On Molecular Level Crosslinking/Nanoparticle Percolation Integrated Double Network Strategy

Cellulose nanocrystals(CNCs),which are rod-like nanomaterials with high crystalline and high aspect ratio,have been intensively investigated due to their excellent mechanical properties,chemical activities,renewability and abundance,especially as nanoreinforcer in the nanocomposites.When a relatively low loading level of CNCs was introduced,the mechanical properties of the nanocomposites could be significantly enhanced.However,with the matrix being nopolar and hydrophobic,the abundant hydroxyl groups on the surface of CNCs which were polar and hydrophilic,often resulted in poor interfacial compatibility and weak interaction,leading to deterioration in the mechanical properties of the nanocomposites.To improve the compatibility,physical or chemical modifications of CNCs were applied,such as the introduction of chemical active sites to enhance the interfacial interaction between CNCs and matrix.Eucommia ulmoide gum(EUG),an isomer of natural rubber(NR)with a molecular structure of trans-1,4-polyisoprene,is a potential hydrophobic biobased elastomer,but its application is limited by its high crystallinity due to its trans-structure.With the double bonds in the EUG polymer chains,its intermolecular force and cohesive energy density are far less than pertroleum-based materials.Traditionally,EUG could be vulcanized by sulfur or sulfur compounds,which could be processed into elastomers.Inorganic and organic particles were also mixed into EUG to adjust the mechanical properties and to control the crystalline behavior of EUG.However,the strength and the stretchability of EUG exhibited a trade-off effect,especially in a heavily cross-linked degree or with high filler content,resulting in high strength but low toughness.Besides,most particles came from petroleum material and they might act as the heterogeneous nucleation,which often resulted in the increase of EUG crystallinity.Thus,developing a method to control the crystalline behavior of EUG effectively at low filler content is still of great public value.Faced with such problems,we integrated both the molecular level cross-linking strategy and percolating strategy in EUG with a thiol functionalized biomass nanocrystal to control the crystalline behavior of EUG and to eliminate the trade-off effect between strength and stretchability.The specific details are as follows:(1)The cCNCs were obtained by sulfuric acid hydrolysis cotton liner,and the thiol functionalized cellulose nanocrystals(mcCNCs)were obtained through silane coupling reaction.Based on the thiol-ene click reaction,the molecular level of crosslinking between EUG and mcCNC was constructed and the mcCNC with high aspect ratio formed the percolation network between cCNC and cCNC(whose percolation value was 4.7vol%).Based on the synergistic effects of double network,the segment movement of EUG around cCNC was limited and the heterogeneous nucleating ability of cCNC was decreased.As a result,the strength and elongation at break of EUG-based nanocomposite maximally increased to 16.2 MPa and 498%,respectively,which were 2 times and 2.5times higher than pure EUG.(2)Based on the above work,the molecular level crosslinking network between EUG and cCNC was controlled by adjusting the surface thiol-functionalization of cCNC.We found that the surface thiol substitution could be controlled by optimizing the modification method.Thus,we tried mcCNCs with varied degree of thiol substitution values to reinforce EUG.Through adjusting the surface thiol degree and the mcCNC content,the crosslinking network could be effectively controlled.The synergistic effects of reinforcing and chemical crosslinking as the mcCNC-EUG interface effectively improve the mechanical properties of EUG.As a result,the maximum elongation at break of the nanocomposites could be improved to 600%.(3)Tunicate cellulose nanocrystals(tCNC)with the length of 1-2 ?m and the aspect ratio of 80 were obtained from marine sources and cCNCs with the length of 100-200 nm and aspect ratio of 18 from cotton(cCNC)were presented for comparison.Thiol groups were introduced into tCNCs surface via similar reaction(mtCNC)to enhance the interfacial interaction between tCNCs and EUG.Based on the percolation/crosslinking network,we investigated whether tCNC with low percolation threshold value could reduce the restriction of the movement of EUG chains and maintain the reinforcement effect of the percolation of nanoparticles,so as to further improve the synchronous strengthening and toughening effect of the nanocomposites.Compared with cCNC,tCNC with longer length and larger aspect ratio could facilitate filler entanglements,which lead to higher reinforcing effect.As a result,the maximum elongation at break of the mtCNCreinforced nanocomposite could reach 1000%,which was 3.2 times than EUG with a relatively lower mtCNC content(5wt%).