Structure And Properties Of Biodegradable Composites Based On Cellulose Nanocrystals

In recent years,cellulose nanocrystal(CNC)has attracted huge interest in the nanotechnology and nanomaterial fields due to their intrinsic properties such as nanoscale dimensions,high surface area,low density and high mechanical strength.Therefore,a wide variety of applications,such as adhesives,reinforcements in nanocomposites,specialized barrier films and drug carriers,have been explored.Accordingly,it has been used as the new kind of nanofiller increasingly to reinforce polymers,and up to now utilized to fabricate a wide range of polymer nanocomposites successfully.But abundant hydroxyl can easily form intra-molecular and intermolecular hydrogen bond,limit its application in the hydrophobic polymer materials.Some surface modification methods have been made to improve dispersion of CNC in hydrophobic polymers.The existing modification process of CNC is tedious,so it is profound to establish a high efficiency and energy saving modification process.There are a large amount of researches involved polymer/cellulose nanocrystal composites,but only very few of them were associated with biodegradable composites.These current work were more focused on direct study of the structure and related properties of the composites contained modified cellulose nanocrystal.However,the intrinsic factors on the impact of the modification on the properties,interface evaluation,the relationship between structure and performance study is not deep enough.In this work,new method of preparation and modification of CNC has been established.In order to clarify the influence and control factors for CNC in the structure and properties of polymer composite,the structure and properties of polymer/CNC composites were researched systematically.The preliminary conclusions are as follows.(1)Cellulose nanocrystal was successfully produced by sulfuric acid hydrolysis.The acid hydrolysis conditions have been optimized by yield and the size of CNC.A 'continuous route' was then developed in this work for the chemical modification of cellulose nanocrystal.It combines several separated steps,including extraction of CNC,surface acetylation of CNC(aCNC),and final composite preparation,into a continuous process,without traditional freeze drying.FT-IR,1H NMR and dispersion test proved that CNC was successfully modified and the degrees of substitution of aCNC was calculated.(2)PLA/CNC composites were obtained by solution casting,the phase interface structure,rheological and mechanical properties were further studied.The results reveal that surface acetylation of CNC can improve its affinity to PLA evidently.The thickened interfacial layer makes the system filled with aCNC show lower percolation threshold than the one filled with pristine CNC.The phase interface adhesion also plays an important role in the mechanical behavior of PLA/CNC composites,which is further revealed by the nanomechanical analysis using atom force microscopy.Both CNC and aCNC have good nucleation activity during cold crystallization of PLA,but also highly impede transport of adjacent chain segments to the growing surface,acting as the role of physical barrier in the glassy bulk.During melt crystallization,nucleation is the dominant role as the systems crystallize from the melts.Thus,the presence of CNC and aCNC promotes melt crystallization of PLA.Besides,the surface acetylation of CNC improves its nucleation ability during melt crystallization of PLA,and as a result,the composite with aCNC has denser fold surfaces relative to the one with CNC.(3)Poly(?-caprolactone)composites containing the acetylated cellulose nanocrystal(aCNC)with various degrees of substitution(DS)were prepared for the phase compatibility study.An interesting phenomenon around the alteration of crystallization temperature(Tc)was reported here.Tc of PCL increases evidently in the presence of pristine CNC(DS=0)due to its nucleation effect.But for the composites with aCNCs,Tc decreases monotonously with increasing DSs of aCNCs,and is even lower than that of the neat PCL at higher DSs.This is attributed to the improved compatibility between the matrix and particles,which is further evaluated by the Flory-Huggins parameters.Therefore,the alteration of Tc can be used as the probe to detect the compatibility between two phases in aliphatic polyester composites with chemically modified CNCs.From another perspective,crystallization of aliphatic polyesters can be controlled using aCNCs with different DSs.