Study On The Regulation Of Cellulose Spherulite Crystallization Behavior By Heterogeneous Nucleation

As the most abundant biomass raw material in nature,cellulose has become the focus of research in the fields of materials and chemicals in recent years.The strong hydrogen bonding effect determined by the molecular structure makes the melting temperature of cellulose materials much higher than the decomposition temperature,so "dissolutionregeneration" has become the main processing method.It is self-evident that the crystal morphology after regeneration has an important influence on the properties of cellulose materials.Natural cellulose is dissolved in a solvent and regenerated by a large number of anti-solvents such as water and ethanol.The regenerated cellulose obtained in a relatively short crystallization time usually does not show a characteristic regular appearance.In recent years,the research on the green solvent ionic liquids of cellulose has attracted extensive attention.It has been found that dissolving cellulose in an ionic liquid and slowly regenerating it in the presence of an anti-solvent such as water results in the formation of cellulose spherulites with a higher degree of regularity.Cellulose spherulites have been successfully prepared in cellulose/ionic liquid systems by vapor-diffusion precipitation.However,there are problems such as slow crystallization speed in this cellulose/ionic liquid system.Heterogeneous nucleation of polymers is an important means to speed up the crystallization of polymers.In this thesis,we first investigated the crystallization behavior of homogeneously nucleated cellulose spherulites,and exfoliated the spherulites to explore the application of cellulose spherulites.Subsequently,ionic liquid Amim Cl and cellulose nanospheres were used as nucleating agents to induce cellulose crystallization,and their effects on the crystallization behavior of cellulose spherulites were investigated.The main research contents are as follows:1.Firstly,the crystallization behavior of homogeneously nucleated cellulose spherulites was investigated by vapor diffusion precipitation method.The diameter of cellulose spherulites has obvious temperature dependence and increases with the increase of crystallization temperature.In the study,the temperature of spherulite growth was selected as 60 °C.When the temperature was 60 °C and the humidity was 25% RH,the spherulites began to crystallize on the 3rd day,and the time for complete crystallization was 16 days.The morphology of the fully grown spherulites is negative banded spherulites with a diameter of about 120 ?m.The cellulose spherulites were successfully stripped from the cellulose/ionic liquid membrane by water stripping,and the spherulites before and after stripping were characterized.It was found that the diameter of the exfoliated spherulites decreased from 120 ?m to 50 ?m after freeze-drying,the ring band disappeared,and the positive and negative optical properties disappeared.In addition,the morphology observation showed that pores and microporous structures appeared on the surface of freeze-dried spherulites after peeling.2.The effect of ionic liquid nucleating agent on the crystallization behavior of cellulose spherulites was investigated.First,the crystallization behaviors of different ionic liquids were compared: when crystallized at-20 °C,Amim Cl crystallized into prismatic crystals with a diameter of 300 ?m,Bmim Cl crystallized into spherical crystals with a diameter of 300 ?m,and Emim Ac crystallized into fine grains of 5 ?m-10 ?m.Since cellulose only crystallized into spherulites in Amim Cl,Amim Cl was finally used as a nucleating agent to control the crystallization behavior of cellulose spherulites.The crystallization time of Amim Cl nucleated cellulose crystals was shortened by 4 days,the number of crystals in the observed range increased by 3 times,and the average crystal diameter increased from 120 ?m to 350 ?m compared with the samples without nucleating agent added.In addition,the heterogeneous nucleation induced the growth of eye-like annular spherulites,which consisted of three regions: a broadband region(eye-like region),a narrow-band region,and a transition region between wide and narrow bands.Compared with unregulated cellulose spherulites,Amim Cl-nucleated cellulose spherulites exhibit unique thermal behavior.At room temperature,ILNCS is a mixed optical eye-like spherulite.When heated to 80 ?,the interference color reversal occurs at the periphery of the II/IV quadrant.With the further increase of temperature,when the temperature reaches85?,the spherulite The eye-shaped area of ILNCS is rotated by 20°clockwise.When the temperature reaches 95°C and higher,the spherulite annulus becomes clearly visible,and the II/IV initial birefringence region transforms into a negative refraction region,resulting in an irreversible wide annulus region with negative birefringence spherulites,indicating negative fibers.Plain spherulites are relatively more stable than positive cellulose.3.The effect of cellulose nanocrystals on the crystallization behavior of cellulose spherulites was investigated.Firstly,the effect of nanocrystal morphology on the crystallization behavior of cellulose spherulites was compared: for cellulose nanorods,the regulated cellulose crystals coexisted with spherulites and fine irregular grains,while the regulated cellulose nanospheres coexisted with spherulites and fine irregular crystals.The cellulose crystals are completely cellulose spherulites.Finally,cellulose nanospheres were selected as nucleating agents to control the crystallization behavior of cellulose spherulites.The growth rate of cellulose spherulites was successfully increased through the control of cellulose nanospheres.It was observed that crystals appeared immediately after adding cellulose nanospheres to the cellulose/Amim Cl solution,and the average diameter of the spherulites changed from 120 ?m when the crystallization was complete When reduced to55 ?m,the morphology of the spherulites is feather-like spherulites.The spherulites nucleated by cellulose nanospheres exhibited different thermal behaviors compared to the spherulites grown by homogeneous nucleation.The melting temperature of cellulose spherulites increased by 11 °C,and the melting sequence of the spherulite quadrants during the heating process was opposite to that of the unregulated cellulose spherulites.