Surface Modification Of Microcrystalline Cellulose For The Reinforcement Of Polymer Via Reversible Crosslinking

Thermoplastic elastomer(TPE)has excellent mechanical properties and can be repeatly thermos-processed.TPE has been widely applied in electronics,automotive manufacturing,medical devices and other fields.Traditional thermoplastic polyurethanes rely on rigid segment molecular chain microphase separation to achieve "physical crosslinking",which has the characteristics of traditional TPEs.However,the heat chain of polyurethane molecules is easily decomposed,and the molecular weight will decrease,which will affect the product performance.At present,thermosetting polyurethane occupies an absolute advantage in the field of application.In this study,through the design of molecular chain structure,the reversible covalent bond was applied to the polyurethane.Moreover,the microcrystalline cellulose was grafted on the surface,as a crosslinker and reinforcing agent,to prepare the crosslinked thermoplastic polyurethane withDiels-Alder(D-A)cycloaddition and phenolic hydroxyl blocked isocyanate.The crosslinking bond in the polyurethane can produce the reverse D-A reaction and the phenolic hydroxyl release reaction,which not only gives the thermoplastic of the crosslinked polyurethane material,but also realizes the reinforcement of the modified natural cellulose to the polyurethane.The main contents are as follows:D-A chain extender for polyurethane was prepared by the reaction of the maleimide phenol(HPM)and furfuryl alcohol.The results of FTIR,NMR and DSC showed that the products contained D-A cycloaddition structure and phenolic hydroxyl group.A thermoplastic polyurethane(TPU)with reversible crosslinking chemical bonds was prepared by the reaction of D-A chain extender and isocyanate terminated polyurethane prepolymer.The structure,solubility and thermoplasticity of the polyurethane were studied,which showed that the prepared TPU material has thermoplasticity owing to the reverse D-A reaction and the phenolic hydroxyl release reaction.After three times of thermo-reprocessing,the tensile strength of the TPU is almost unchanged,maintained at 7.1 MPa,and has repeatly thermos-processed.Modified microcrystalline cellulose(m-MCC)with D-A ring addition structure and phenol hydroxyl group was prepared by modification on the surface of microcrystalline cellulose(MCC),and the structure was confirmed by FTIR and elemental analysis.Modified microcrystalline cellulose/polyurethane(m-MCC/TPU)composites were prepared using m-MCC and D-A chain extenders,and the formula was explored.The structural characterization,thermal analysis and thermoplasticity of the composites were studied.The results showed that m-MCC/TPU not only had D-A cycloaddition structure,but also had the structure of phenol hydroxyl blocked isocyanate.When the mass of m-MCC:D-A chain extender is 1:9,the mechanical properties of m-MCC/TPU composite is the best,which tensile strength is about 150%higher than that of non cellulose reinforced TPU.The thermo-processability of m-MCC/TPU is better than the unmodified MCC reinforced polyurethane(MCC/TPU).Moreover,m-MCC/TPU demonstrates shape memory property and self healing property upon heating.