Nanocellulose-Stabilized Pickering Emulsions As Designer Platforms For Polymeric Hybrid Materials

Nanocellulose is often used as filler particles to improve the mechanical properties,crystallization properties,thermal stability,etc.of polymers,due to its renewability,degradability,high strength,easy modification,and non-toxic.However,due to its molecular structure characteristics and large specific surface area,it is difficult to disperse nanocellulose evenly into the polymer matrix by traditional processing methods.In this project,firstly the nanocellulose is pre-dispersed on the polymer microspheres through the nanocellulose-stabilized Pickering emulsion template（in which the polymer solution is used as the oil phase）,which avoids the agglomeration of nanocellulose during the freeze-drying process.Then by combining with traditional melting process,such as hot-pressing molding and twin-screw blending,the grid-like and random distribution of nanocellulose in the polymer matrix could be obtained,which satisfies different application scenarios.Finally,the use of nanocellulose as a functional carrier to construct functional polymer composites is explored,which expands the application of composites.The main research contents and conclusions are as follows:Polylactic acid（PLA）has received extensive attention due to its ease of processing,degradability,and biocompatibility.However,some defects such as poor crystallinity and brittleness limit the application of PLA.In this chapter,the PLA composites were constructed using the cellulose nanocrystal（CNC）-stabilized Pickering emulsion template combined with the direct hot-pressing method,in which CNC was prepared by the hydrochloric acid vapor method.The CNC prepared by this method has not been modified,and still maintains the excellent oil-water two-phase wettability of cellulose,and then has good emulsifying performance.The properties of the composites were comprehensively analyzed.The results show that CNC had a good dispersion in the PLA matrix and form a grid-like structure.In the high temperature rheological test,the addition of CNC increased the storage modulus and viscosity of the PLA melt.Compared with neat PLA,the crystallinity of the composite was significantly improved.The crystallinity of PLA-5%-CNC was increased by 93.2%.In the dynamic thermomechanical analysis test,the storage modulus of the PLA-15%-CNC composite reached 0.67 GPa at a test temperature of 70°C,an increase of about 379%.After adding 5 wt%CNC in the PLA matrix,the flexural strength of the composites increased from 64 MPa to 76 MPa.Pickering emulsion method provides a new route for the preparation of nanocellulose/polymer composites.PLA foams have the potential to replace petroleum-based polymer foams,but their defects such as low melt strength,poor heat resistance,and brittleness limit the development and utilization of PLA foams.CNC can effectively improve the CO₂foaming behavior of PLA.However,the PLA/CNC composites prepared by the Pickering emulsion method had a high CNC content and CNC showed a grid-like distribution in the PLA matrix,which affected the morphologies of the foams.In this chapter,the above Pickering emulsion method is improved by adding a twin-screw blending process to achieve low CNC contents and randomly uniform dispersion of CNC in PLA matrix.This method is called the Pickering emulsion masterbatch method.The cross-sections of the composites were observed by scanning electron microscopy（SEM）,and no CNC aggregates were found.The tensile strength of PLA/CNC composites was improved at low CNC content.The foaming behavior of the composites was studied in detail,and the results showed that the cell morphology of the composites was significantly improved（the cell diameter became smaller and the cell density increased）.The effects of foaming temperature and foaming time on the foaming behavior of PLA/CNC composites were investigated,respectively.For the given saturation pressure（40 bar）and saturation time（6 hours）,foaming at 40℃for 5 s,PLA/CNC foams had the lowest mean cell size（～18μm）and highest cell density（～11.7×10⁶ cells/cm³）.The nanocellulose-stabilized Pickering emulsion system provides a new route for the preparation of functional polymer composites.Based on the Pickering emulsion method,carbon nanotube（CNT）was introduced into the stereocomplex polylactic acid（sc-PLA）matrix by using the non-covalent interaction between regenerated cellulose（RC）and CNT.RC was prepared by dissolving and regenerating cellulose with low-temperature phosphoric acid.Through transmission electron microscope（TEM）observation,it was found that RC was a network nanofibrous structure with a length of several micrometers and a diameter of several tens of nanometers,which belongs to nanocellulose.Compared with CNC,RC has a larger aspect ratio and can"capture"more CNT.It is found that the RC/CNT hybrid particles still have good emulsifying properties.The sc-PLA/RC/CNT composite exhibited good melt stability.After three thermal cycles,the complete stereo-complexation can still be maintained.In addition,the addition of CNT significantly improved the electrical conductivity of the composites.The electrical conductivity of the sc-PLA/6%RC/12%CNT composites reached 178.3 S/m.RC was dyed with reactive dyes,and then colored polymethyl methacrylate（PMMA）composites were prepared by colored RC（CRC）-stabilized Pickering emulsion template.The morphology of the RC before and after dyeing remained unchanged by comparing the TEM images before and after dyeing.It is found that CRC still has good emulsifying properties,and only 0.05 wt%CRC suspension can prepare stable Pickering emulsion.Due to the better dispersion of CRC,the colored PMMA/CRC composites still maintained high transparency.The color depth of the colored PMMA composites can be adjusted by changing CRC contents.Color composites have a good color fastness.Both the flexural strength and modulus of the composites were improved due to the addition of RC.CRC-stabilized Pickering emulsion systems can also be used to color other polymers.