Structure And Properties Of Cellulose/poly(?-hydroxybutyrate) Composites

Poly(?-hydroxybutyrate)(PHB)is biodegradable and can be decomposed by microorganisms.Eventually PHB decomposes into carbon dioxide and water without causing any pollution to the environment.Substances commonly used to modify PHB are carbon materials,inorganic materials,metal materials,biomass materials,polymers and high molecular weight or low molecular weight diluents.Compared with other materials,biomass materials show great application prospects in many fields because of their light weight,biodegradability,good biocompatibility and renewable characteristics.PHB/cellulose composite is a fully biodegradable material.By now,the research on PHB/cellulose composites mainly focused on the effect of cellulose fibers on the mechanical properties of PHB,the compatibility of cellulose derivatives with PHB,the influence of cellulose derivatives on the orientation of PHB lamellae,and the effect of cellulose on the crystallization behavior of PHB.Previous studies have not discussed the effect of cellulose on the isothermal and non-isothermal crystallization kinetics of PHB in detail,and there are few studies about the influence of crystal size,surface functional groups,crystalline or amorphous structure of cellulose on the crystallization kinetics and spherulite morphology of PHB.The effects of cellulosic materials on the nucleation rate,spherulite growth rate and mechanical properties of PHB have not been studied in detail.PHB is a highly crystalline polymer,crystallization process affects the final properties of PHB.Therefore,it is important to systematically study the crystallization process of PHB.Based on the above issues,PHB/microcrystalline cellulose(MCC)composites and PHB/nanocrystalline cellulose(CNC)composites were first prepared by melt blending in this thesis.The effects of the size of cellulose crystals on the crystallization kinetics and mechanical properties of PHB were studied.On this basis,the effects of grafting lactic acid on the surface of nanocrystalline cellulose on the crystallization kinetics and mechanical properties of PHB were further studied.The poor interfacial adhesion between the hydrophilic cellulose crystals and the hydrophobic PHB matrix affects the mechanical properties of PHB/cellulose crystal composites.In order to improve the mechanical properties of PHB,a thermoplastic cellulose derivative ethyl cellulose(EC)and a tougher poly(?-caprolactone)(PCL),were used to blend with PHB.Effect of EC on the crystallization kinetics and mechanical properties of PHB was studies,as well as the effects of cellulose crystals and EC on the phase morphology and mechanical properties of PHB/PCL blend.The main findings of this thesis are listed below:1.Effect of MCC on the Structure and Properties of PHBPHB/MCC composites were first prepared by melt blending.The structure of MCC was characterized by SEM,FT-IR and WAXD.The dispersion of MCC in PHB was characterized by POM.The effect of MCC with different contents on the isothermal crystallization of PHB was studied,as well as the influence of MCC on the band space of PHB spherulites.The nucleation of MCC in isothermal crystallization of PHB was further studied by Avrami kinetic model and Lauritzen-Hoffman-Miller secondary nucleation theory.Results show that MCC has heterogeneous nucleation effect on the crystallization process of PHB.DSC curves show that MCC shortens the isothermal crystallization time of PHB,and the crystallization rate of PHB becomes larger with the increase of MCC content.From the SEM photographs it can be seen that MCC reduces band space of PHB spherulites.The Avrami index calculated by Avrami equation is between 2 and 3,indicating that the addition of MCC does not change the crystallization mechanism of PHB.Nucleation constant and folding surface free energy calculated by Lauritzen-Hoffman-Miller theory are reduced,further demonstrating the heterogeneous nucleation effect of MCC on PHB.2.Effect of Cellulose Crystal Size on the Structure and Properties of PHBPHB/MCC composites and PHB/CNC composites were first prepared by melt blending.The FT-IR and WAXD were used to characterize the structure of MCC and CNC.The dispersion of MCC and CNC in the system was characterized by POM.The nucleation effect of MCC on the isothermal crystallization of PHB was studied by using secondary nucleation theory.To estimate the differences in the nucleation abilities of MCC and CNC during the crystallization process of the PHB,the spherulite morphology of PHB/MCC and PHB/CNC systems was further investigated.As the results shown,the morphology of PHB spherulites transforms from bundle-like to ring banded and finally evolves into a random or zigzag pattern,with the increase of temperature.The periodic distortion of the radial crystallites in the growth direction leads to the formation of ring banded spherulites.Similar to PHB,PHB/MCC and PHB/CNC composites form non-banded spherulites at lower and higher crystallization temperatures and forms ring banded spherulites in the middle range.The band space of the spherulites becomes larger as the crystallization temperature rises.MCC has heterogeneous nucleation effect on the crystallization process of PHB,which increases the crystallization rate of PHB and reduces the free energy of surface folding.However,the addition of MCC does not change the crystallization mechanism of PHB.CNC has the same nucleation effect on PHB,and the nucleation capability of MCC and CNC is temperature-responsive.With the addition of cellulose crystals,the size of PHB spherulites becomes smaller,the number and the annular zone height difference of the spherulites increases,the band space decreases,and the ring banded structure degrades to a certain degree.This is due to the increased unbalanced stress and lamellar torsional frequency during crystallization.3.Effect of Cellulose Surface Modification on the Structure and Properties of PHBBy above knowable,it can be seen that MCC and CNC have heterogeneous nucleation effect on the crystallization process of PHB.Similar to other hydrophobic particles,the hydroxyl groups on the surface of CNC can be used for surface modification.To further study the influence of surface modification of CNC on the crystallization behavior and mechanical properties of PHB,-LA groups are grafed on the surface of CNC by ring-opening polymerization in ionic liquid[AMIM]Cl and CNC-g-PLA is obtained.PHB/CNC and PHB/CNC-g-PLA composites are prepared by melt blending.The structure of CNC-g-PLA is characterized by FT-IR and 1H NMR.The dispersion of CNC and CNC-g-PLA in PHB are measured by TEM.The structures of PHB,PHB/CNC and PHB/CNC-g-PLA composites are characterized using XPS and WAXD.Furthermore,the effects of CNC and CNC-g-PLA on the non-isothermal crystallization kinetics and lamellar structure of PHB are compared by DSC and SAXS.The results of the study indicate that the affinity of CNC-g-PLA with PHB is better than that of pure CNC.The improved interfacial adhesion and modified chemical structure alter the role of CNC in the crystallization of PHB.Pure CNC can act as an effective nucleating agent to promote the crystallization process of PHB,while CNC-g-PLA acts as an anti-nucleating agent hindering the crystallization process of PHB.Mo equation can well describe the non-isothermal crystallization process of all samples.The obtained kinetic data confirm the different role that CNC and CNC-g-PLA played in the crystallization of PHB.The modification of CNC has no effect on the lamellae structure and molecular chain diffusion of PHB,nucleation plays a dominant role in the crystallization process of PHB.Compared with pure PHB,CNC-g-PLA hinders the relaxation of adjacent PHB chains and inhibits the non-isothermal crystallization of PHB.The surface modification destroys the chain structure of CNC,making CNC-g-PLA have better compatibility with PHB,resulting in enhanced mechanical properties of PHB/CNC-g-PLA composites.4.Effect of Amorphous Cellulose on the Structure and Properties of PHBAmorphous cellulose derivatives such as EC can partially be compatible with biodegradable aliphatic polyester chain,and the dilution effect causes the decrease of crystallization temperature of the matrix.EC affects the crystal morphology and crystallization rate of the matrix,and even changes the compatibility and interfacial properties of incompatible system.Ultimately,the structure and properties of the matrix are regulated.PHB/EC blends are prepared by melt blending.Phase morphology of PHB/EC composites is characterized by SEM.Surface tension and interfacial tension of PHB and EC are calculated by equations.Effects of EC on the dynamic storage modulus and loss modulus of PHB are studied.Also,the effect of EC on the morphology and band space of PHB spherulite is studied.Furthermore,the effect of EC on the crystallization of PHB is further evaluated by calculating the nucleation constant.Lauritzen-Hoffman-Miller equation is also used to investigate the nucleation mechanism of PHB/EC system.At last,the effects of cellulose crystals and EC on the non-isothermal crystallization and mechanical properties of PHB are compared.Studies show that melt-blended PHB/EC system is a typical incompatible system.EC lowers the crystallinity of PHB,as well as the growth rate of spherulites.The rheological test results show that the addition of EC significantly increases the modulus of PHB in low frequency region.The presence of EC greatly increases the viscosity of the system,resulting in the increase of folding free energy of PHB molecular chain,thus the crystallinity of PHB decreases.The effect of EC on the non-isothermal crystallization process of PHB is entirely different from that of cellulose crystals.EC retards the crystallization process of PHB and has no nucleation effect.At the presence of EC,PHB molecular chains consume more energy to form spherulites.The morphology of PHB spherulites becomes more perfect at a lower growth rate.EC increase the tensile strength and impact strength of the blend to different extent.5.Effect of Cellulose on the Structure and Properties of PHB/PCL BlendsBiodegradable cellulose particles have been used as fillers to prepare fully green composites,but the relationship between cellulose particles and phase morphology as well as mechanical properties of PHB/PCL system has not been reported.Therefore,MCC and EC,which are incompatible with PHB and PCL,are mixed with PHB/PCL respectively.The phase morphology and mechanical properties of PHB/PCL/MCC and PHB/PCL/EC composites are studied.Phase morphology of PHB/PCL system is characterized by SEM.Surface tension and interfacial tension of PHB and PCL are calculated by Young's equation,harmonic equation and geometric equation.The interfacial tension between PHB and PCL is further verified by embedded fiber retraction.In order to clarify the effect of MCC and EC on the phase morphology and mechanical properties of the system,the selective localization of MCC and EC,as well as the interfacial and mechanical properties of PHB/PCL/MCC and PHB/PCL/EC ternary systems are discussed in detail.Studies have shown that PHB/PCL system is a typical thermodynamic incompatible system.PHB70/PCL30 system presents a "sea-island" structure while PHB50/PCL50 system presents a bicontinuous phase structure.MCC and EC particles have a great influence on the phase morphology and mechanical properties of PHB/PCL.MCC is incompatible with PHB and PCL,MCC does not promote the adhesion of the phase interface.The mechanical properties of the MCC-filled PHB70/PCL30 system are inferior to the PHB70/PCL30 system.However,for the bicontinuous PHB50/PCL50 system,MCC decreases the domain size and increases the tensile and impact strength of the system.EC tends to disperse in the PCL phase and the interface of PHB/PCL.The selective localization of EC increases the tensile strength of the PHB70/PCL30 system,but weakens the impact strength.The addition of EC greatly improves the mechanical properties of the bicontinuous PHB50/PCL50 system.MCC and EC can be used as the third component to regulate the phase morphology and mechanical properties of biodegradable aliphatic polyester blends.MCC and EC have a strong influence on the cyclical tensile behavior of PHB/PCL system,and the nonlinear "stress-strain" response can be used as a probe to monitor possible changes in the phase structure.