Controllable Surface Modification Of Cellulose Nanocrystals And Its Modified Polyester Materials

Biomass-based materials from renewable resources have attracted increasingly attention due to the decreasing of conventional fossil raw and the increasingly serious environmental problems caused by their products.Because of the rod-like morphology,high modulus,low density,biocompatibility,high surface area-to-volume ratio and biodegradability,cellulose nanocrystals?CNCs?are often used as nanoreinforcer for the production of nanocomposites with improved properties.However,the abundant hydroxyl groups on the CNC surface and its highly hydrophilic polarity resulted in a poor interfacial miscibility and interaction between CNC and hydrophobic polyester matrix,and leaded to a significant deterioration in the mechanical properties of the nanocomposites,which greatly restricted the use of CNC in the nanocomposite filed.Therefore,surface modification was considered as an ideal way to adjust the chemical structure and hydrophobic/hydrophilic properties on the surface of CNCs,and then the dispersability of nanofillers within matrix and the miscibility as well as the interfacial interaction between nanofillers and matrix were improved.In this work,the controlled surface modification of CNC was achieved via esterification reaction,and then the CNCs before and after surface modification,as nanofillers,were respectively introduced into polyester matrix to prepare biodegradable nanocomposites with excellent performance.The main contents of this work are as follows:?1?CNC was modified by acetic anhydride via esterification reaction,and acetylated cellulose nanocrystals?ACNs?with a gradient degree of substitution?DS?was obtained by controlling the reaction conditions;?2?ACNs with various DS were respectively introduced into poly?3-hydroxybutyrate-co-4-hydroxybutyrate??PHB?to prepare nanocomposites,and the impact of loading content and DS of ACN on the properties of PHB nanocomposites and the interaction mechanism on nanofiller/matrix interface were investigated;?3?the surface of CNC was chemically modified by grafting anhydride presenting different structures and lengths via esterification reaction,and the similar DS of surface-modified CNC was obtained by controlling the reaction conditions;?4?both unmodified CNC and functionalized CNC with different anhydride were introduced into poly?lactic acid??PLA?to prepare nanocomposites,and to characterize the structure and properties.The main conclusions were as follows:?1?The surface of CNC was modified with acetic anhydride via adjusting the molar ratio between acetic anhydride and the surface hydroxyl groups of CNC.The results showed that the surface acetylation gradient of CNC was achieved;the DS was ranged from 11.1%to 63.2%,and the gradient was controlled ca.10%;after modification,ACN remained the rod-like morphology and crystal structure of original CNC,and the hydrophobicity improved as the DS increased.?2?PHB nanocomposites reinforced with ACN or CNC were prepared.As the DS of ACN increased,the mechanical properties of PHB nanocomposites were significantly improved,especially when the DS increased from 0 to 62.8%,the tensile strength and elongation at break of PHB/ACN?IV?-15 increased by 43.3%and 12.6%,respectively,compared with those of PHB/CNC-15 nanocomposites.The results of Raman spectroscopy and scanning electron microscopy showed that as the DS increased,ACN dispersed more uniform in PHB matrix,and the miscibility between nanofiller and matrix was improved.The DMA and dynamic rheological tests suggested that as the DS increased,the interaction between ACN and PHB was enhanced,and ACN with high DS dispersed more uniform in PHB matrix.?3?The surface of CNC was chemically modified by grafting anhydride presenting different structures and lengths via esterification reaction.The results showed that the surface modification was successful,and the DS values were controlled between 25%and 30%.The modified CNC not only maintained the rod-like morphology and crystal structure of original CNC,but also improved the hydrophobic and dispersion inorganic solvent.?4?PLA nanocomposites filled with both unmodified CNC and modified CNC,which was modified with various anhydride,were prepared,and the mechanical properties of resulted nanocomposites were significantly improved.The DMA results showed that the glass transition temperature?T_g?of PLA nanocomposites first increased and then decreased with increasing of cellulose nanoparticles,and the T_g of nanocomposites filled with SACN or DDSACN was higher than that of nanocomposites filled with ACN or DACN.The DSC results suggested that the addition of cellulose nanoparticles decreased the glass transition temperature(T_g,min)and melting temperature?T_m?of PLA nanocomposites,but the T_g,min of nanocomposites filled with SACN or DDSACN was higher than that of nanocomposites filled with ACN or DACN.This work enriched the surface chemical modification methods of CNC and strengthened its controllability.Furthermore,CNC both before and after modification was introduced into polyester matrix to prepare nanocomposites based on the reinforcement of CNC,and then investigated the structure and properties of the nanocomposites,and the interaction mechanism on nanofiller/matrix interface.This work will provide an important basis for investigating the structure and degree of substitution of CNC after surface modification impact on the interface behavior and properties of nanocomposites.And it will provide valuable reference for the surface controlled chemical modification of CNC and the other polysaccharide nanocrystals.